Precoding codebook adaptation in a communication network

Abstract

A communication device (12) is configured for use in a communication network (10). The communication device (12) obtains a base precoding codebook (15B). The communication device (12) also receives control signaling (18) that indicates an inter-subarray spacing (17-SP) according to which adjacent subarrays (17S) of a transmit antenna array (17) at a radio network node (14) are spaced apart from one another. The communication device (12) derives, from the base precoding codebook (15B), a precoding codebook (15D) tailored to this inter-subarray spacing (17-SP). The communication device (12) then determines, based on the derived precoding codebook (15D), channel state information (22) describing a state of a downlink channel between the communication device (12) and the radio network node (14). The communication device (12) reports this channel state information (22) to the communication network (10).

Description

[0001]PRECODING CODEBOOK ADAPTATION IN A COMMUNICATION NETWORK TECHNICAL FIELD The present application relates generally to a communication network, and relates more particularly to adaptation of a precoding codebook in such a network. BACKGROUND Downlink precoding in a communication network involves a radio network node adjusting the signal that it transmits from its antenna array to a communication device, in order to optimize signal reception at that communication device. To facilitate this, the communication device measures characteristics of the downlink channel between the radio network node and the communication device, and reports so-called channel state information (CSI) to the radio network node. The reported CSI may include a precoding matrix, also referred to as a precoder, for the radio network node to use for precoding in a way that takes into account the downlink channel characteristics. In codebook-based CSI reporting, the communication device selects the precoding matrix to report from a limited set of possible precoding matrices referred to as a precoding codebook. The radio network node may then use the reported precoder (or a different one that the radio network node selects) in order to precode a signal for transmission to the communication device. In doing so, the precoder adapts the amplitude and phase of the signal transmitted from each antenna element of the radio network node’s antenna array, in such a way as to focus the transmission energy towards the communication device and reduce interference from other signals. This precoding effectively "pre-corrects" the signal to compensate for the distortions and path losses that occur during transmission, ensuring that the communication device receives a clearer and stronger signal. Downlink precoding thereby improves overall link reliability and efficiency, which in turn helps maintain high data rates and robust communication in the communication network. As communication networks evolve, larger antenna arrays are increasingly used to enhance performance and capacity, particularly in the 6-15GHz bands. Antenna arrays may for example be enlarged to have hundreds of antenna elements. However, individually feeding and controlling each antenna element’s amplitude and phase in such arrays can overwhelm digital interfaces. Even with fully digital arrays, the bandwidth required to supply separate baseband samples to each antenna element may be impractical. To address this, antenna elements can be partitioned into subarrays, where the antenna elements in each subarray share the same baseband sample stream. Subarrays can be managed either individually or in groups using analog or digital beamforming. In some 6-15GHz / mmWave implementations, adjacent elements may be bundled together in radio frequency (RF), e.g., with a static RF tilt. This bundling may be done in the vertical dimension, which is effective in deployments where the channel angular spread is narrower in elevation than in azimuth. Network vendors will likely require flexibility in how to partition their antenna arrays into subarrays, in order to optimize performance and adapt to diverse deployment scenarios. This flexibility would allow vendors to tailor their designs to specific network needs and environmental conditions, enhancing overall efficiency and coverage. Codebook-based CSI reporting should therefore accommodate a wide range of antenna array designs and configurations to ensure design flexibility for network vendors. Known approaches described in International Patent Application Publication No. WO2018031082A1 and United States Patent Number 10129906B2 attempt to capture additional codebook design flexibility to accommodate different antenna array designs. Problematically, though, these and other approaches to increasing design flexibility fail to adequately accommodate for different subarray designs, burden the communication device with processing complexity that undesirably scales with increasing antenna array design flexibility, and threaten to significantly increase signaling overhead for CSI reporting, especially for implementations that use spatial oversampling. SUMMARY According to some embodiments herein, a communication network equips a communication device with information about the inter-subarray spacing according to which adjacent subarrays of a transmit antenna array at a radio network node are spaced apart from one another. The communication device exploits its knowledge of the inter-subarray spacing to derive a precoding codebook that is tailored to that inter-subarray spacing. By using this tailored codebook for channel state information (CSI) reporting, rather than a more generic codebook that includes precoders appropriate for many possible inter-subarray spacings, the communication device limits the precoders that it considers as candidates for use to those that are actually appropriate for the radio network node’s inter-subarray spacing. This not only improves precoder selection performance but also advantageously reduces the processing complexity imposed on the communication device for CSI reporting, as the communication device need not evaluate precoders that are not appropriate for the radio network node’s inter-subarray spacing. Moreover, with fewer precoders considered as candidates for CSI reporting, some embodiments advantageously reduce the signaling overhead for CSI reporting by limiting the CSI reporting signaling space to the precoders that are actually appropriate for the radio network node’s inter-subarray spacing. More particularly, embodiments herein include a method performed by a communication device configured for use in a communication network. The method comprises obtaining a base precoding codebook. The method also comprises receiving, from the communication network, control signaling that indicates an inter-subarray spacing according to which adjacent subarrays of a transmit antenna array at a radio network node are spaced apart from one another. In some embodiments, each subarray comprises a set of adjacent antenna elements that are driven by the same stream of baseband samples. The method also comprises deriving, from the base precoding codebook, a precoding codebook tailored to the inter-subarray spacing indicated by the control signaling. The method also comprises determining, based on the derived precoding codebook, channel state information describing a state of a downlink channel between the communication device and the radio network node. The method also comprises reporting the channel state information to the communication network. Other embodiments herein include a method performed by a radio network node configured for use in a communication network. The method comprises transmitting, to a communication device, control signaling that indicates an inter-subarray spacing according to which adjacent subarrays of a transmit antenna array at the radio network node are spaced apart from one another, wherein each subarray comprises a set of adjacent antenna elements that are driven by the same stream of baseband samples. The method also comprises receiving, from the communication device, channel state information describing a state of a downlink channel between the communication device and the radio network node, wherein the channel state information is based on a precoding codebook derived, from a base precoding codebook, to be tailored to the inter-subarray spacing indicated by the control signaling. Other embodiments herein include a communication device configured for use in a communication network. The communication device is configured to obtain a base precoding codebook. The communication device is also configured to receive, from the communication network, control signaling that indicates an inter-subarray spacing according to which adjacent subarrays of a transmit antenna array at a radio network node are spaced apart from one another, wherein each subarray comprises a set of adjacent antenna elements that are driven by the same stream of baseband samples. The communication device is also configured to derive, from the base precoding codebook, a precoding codebook tailored to the inter-subarray spacing indicated by the control signaling. The communication device is also configured to determine, based on the derived precoding codebook, channel state information describing a state of a downlink channel between the communication device and the radio network node. The communication device is also configured to report the channel state information to the communication network. Other embodiments herein include a radio network node configured for use in a communication network. The radio network node is configured to transmit, to a communication device, control signaling that indicates an inter-subarray spacing according to which adjacent subarrays of a transmit antenna array at the radio network node are spaced apart from one another, wherein each subarray comprises a set of adjacent antenna elements that are driven by the same stream of baseband samples. The radio network node is also configured to receive, from the communication device, channel state information describing a state of a downlink channel between the communication device and the radio network node, wherein the channel state information is based on a precoding codebook derived, from a base precoding codebook, to be tailored to the inter-subarray spacing indicated by the control signaling. Still other embodiments herein include corresponding computer programs and carriers of those computer programs. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a communication network configured to provide communication service to a communication device according to some embodiments. Figure 2 is a block diagram of a transmit antenna array in a uniform planar array with antenna elements arranged uniformly in both horizontal and vertical dimensions according to some embodiments. Figure 3A-3C illustrate inter-subarray spacing according to various embodiments. Figure 4 is a block diagram of a communication network configured to provide communication service to a communication device according to other embodiments. Figure 5 is a block diagram of precoding codebook derivation tailored to inter- subarray spacing according to particular embodiments. Figure 6 is a block diagram of an exemplary array with uniform antenna elements. Figure 7 is a logic flow diagram of a method performed by a communication device configured for use in a communication network in accordance with particular embodiments. Figure 8 is a logic flow diagram of a method performed by a communication device configured for use in a communication network in accordance with other particular embodiments. Figure 9 is a logic flow diagram of a method performed by a radio network node configured for use in a communication network in accordance with particular embodiments. Figure 10 is a block diagram of a communication device according to some embodiments. Figure 11 is a block diagram of a radio network node according to some embodiments. Figure 12 is a block diagram of a communication system in accordance with some embodiments. Figure 13 is a block diagram of a UE in accordance with some embodiments. Figure 14 is a block diagram of a network node in accordance with some embodiments. Figure 15 is a block diagram of a virtualization environment in accordance with some embodiments. DETAILED DESCRIPTION Figure 1 shows a communication network 10 configured to provide communication service to a communication device 12, e.g., a user equipment (UE). The communication network 10 in this regard includes a radio network node 14 that provides radio access to the communication network 10. The communication network 10 in some embodiments is a 5G or 6G network, e.g., whereby the radio network node 14 may be a gNB. In the example of Figure 1, the radio network node 14 is equipped with a transmit antenna array 17. The transmit antenna array 17 is an array of antenna elements 15 (also simply referred to as antennas) at the radio network node 14 that is usable for transmission from the radio network node 14. The transmit antenna array 17 may be a linear array with antenna elements 15 arranged in a single dimension (e.g., horizontal or vertical), a planar array with antenna elements 15 arranged in multiple dimensions (e.g., horizontal and vertical), a circular array with antenna elements 15 arranged in a circle, or any other type of array with antenna elements 15 spaced from one another according to a desired arrangement. The transmit antenna array 17 may be a uniform array with antenna elements 15 spaced at equal distances from one another, at least in a given dimension, or a non-uniform array with antenna elements 15 spaced at unequal distances from one another. The transmit antenna array 17 may or may not be dedicated for transmission, as the radio network node 14 may or may not have a separate receive antenna array that is usable for reception at the radio network node 14. According to embodiments herein, the antenna elements 15 of the transmit antenna array 17 are notably partitioned into subarrays 17S. Each subarray 17S includes two or more antenna elements 15. Antenna elements 15 belonging to the same subarray 17S are driven by the same stream of baseband samples, (i.e., the same baseband port input). This may be the case even if the electrical driving signals on individual antenna contacts are different. Partitioning the transmit antenna array 17 into subarrays 17S in this way advantageously avoids having to supply separate baseband samples to each antenna element 15 individually, so as to reduce digital interface bandwidth demands and accommodate a large number (e.g., hundreds) of antenna elements 15 in the array 17. In some embodiments, the partitioning of antenna elements 15 into subarrays 17S is statically fixed, such that how many and which antenna elements 15 belong to which subarrays 17S remains the same over time. The partitioning may for example be statically fixed so as to be specific to the deployment environment of the radio network node 14. In other embodiments, though, the partitioning of antenna elements 15 into subarrays 17S may be semi-statically or dynamically adapted, e.g., to address any variations in the radio network node’s environment over time. In this case, then, how many and which antenna elements 15 belong to which subarrays 17S may be configurable so as to vary over time. Whether the partitioning into subarrays 17S is static or dynamic, adjacent subarrays 17S are spaced apart from one another according to an inter-subarray spacing 17-SP. In one embodiment, this inter-subarray spacing 17-SP represents some measure of a distance (e.g., in terms of a number of wavelengths) between a reference point of one subarray 17S and the same reference point of an adjacent subarray 17S. Figure 1 shows the reference point as being the center point of a subarray 17S, such that the inter-subarray spacing 17- SP represents some measure of a distance between the respective center points of adjacent subarrays 17S, but the reference point may be any other point such as the start point or the end point of adjacent subarrays 17S. Note, too, that the inter-subarray spacing 17-SP may have one or more components, depending on the uniformity and / or dimensionality of the transmit antenna array 17. For example, in embodiments where the transmit antenna array 17 is a linear array with antenna elements 15 arranged in a single dimension (e.g., horizontal or vertical), the inter-subarray spacing 17-SP may represent some measure of the distance between respective reference points of adjacent subarrays 17S in that single dimension. And, in embodiments wherein the where the linear array is a uniform array with antenna elements 15 arranged uniformly in the single dimension, the inter-subarray spacing 17-SP is uniform (i.e., the same) for each pair of adjacent subarrays 17S in the transmit antenna array 17. As another example shown in Figure 2, in embodiments where the transmit antenna array 17 is a uniform planar array with antenna elements 15 arranged uniformly in both horizontal and vertical dimensions, the inter-subarray spacing 17-SP may have both a horizontal spacing component 17-H and a vertical spacing component 17-V. In this case, the horizontal spacing component 17-H may represent some measure of the distance between respective reference points of adjacent subarrays 17S in the horizontal dimension. And the vertical spacing component 17-V may represent some measure of the distance between respective reference points of adjacent subarrays 17S in the vertical dimension. Note, though, that the horizontal spacing component 17-H and the vertical spacing component 17-V may be the same or different. In still another example, in embodiments where the transmit antenna array 17 is a non-uniform array, the non-uniformity of the spacing between antenna elements 15 may create non-uniformity in the spacing between subarrays 17S. In this case, the inter-subarray spacing 17SP may have multiple components to capture the non-uniformity of the spacing between subarrays 17S. Generally, then-, the inter-subarray spacing 17SP may include one or more components, as needed to comprehensively represent- how adjacent subarrays 17S are spaced from one another. In this context, the communication network 10 according to some embodiments herein equips the communication device 12 with information about the intersubarray- spacing 17-SP according to which adjacent subarrays 17S of the transmit antenna array 17 at the radio network node 14 are spaced apart from one another. The communication network 10 as shown in this regard transmits control signaling 18 to the communication device 12. The control signaling 18 may be transmitted by the radio network node 14 itself, e.g., on a serving cell of the communication device 12. Or, the control signaling 18 may be transmitted by another network node in the communication network 10. Either way, the control signaling 18 indicates the intersubarray- spacing 17-SP of the subarrays 17S in the transmit antenna array 17 of the radio network node 14. Figures 3A-3C illustrate two simple examples for how the control signaling 18 may indicate the inter-subarray spacing 17-SP according to different embodiments. As shown in the embodiment of Figure 3A, the control signaling 18 may indicate the inter-subarray spacing 17-SP as a number 17-W of wavelengths (λ) between adjacent subarrays 17S. This number may be indicated explicitly as an integer, a fraction, or a decimal. In the example of Figure 3A, for instance, the control signaling 18 explicitly indicates the inter-subarray spacing 17-SP as being 3.5 wavelengths (λ). Or, in other embodiments, the control signaling 18 may implicitly indicate the inter-subarray spacing 17-SP as an index that is mapped to a certain integer, fraction, or decimal, with different indices mapped to different integers, fractions, or decimals. Generally, though, the control signaling 18 may indicate the inter-subarray spacing 17-SP as a multiple of any type of spacing unit, such as a nominal λ / 2 spacing unit. As a further example, Figure 3B shows that the control signaling 18 may indicate the inter-subarray spacing 17-SP as a multiple 17-M of an inter-element spacing according to which adjacent antenna elements 15 are spaced apart from one another. In the example of Figure 3B, for instance, the control signaling 18 explicitly indicates the inter-subarray spacing 17-SP as being 3 times the inter-element spacing. The inter-element spacing of which the inter-subarray spacing 17-SP is a multiple may be the actual inter-element spacing according to which adjacent antenna elements 15 are actually spaced apart from one another, e.g., at a certain time. Or, the inter-element spacing of which the inter-subarray spacing 17-SP is a multiple may be some nominal inter-element spacing according to which adjacent antenna elements 15 are nominally spaced apart from one another, e.g., by default or upon initialization. Figure 3C shows yet other embodiments where the control signaling 18 implicitly indicates the inter-subarray spacing 17-SP in a given dimension X of the transmit antenna array 17, by indicating an inter-element spacing 17-E and a subarray size 17-Z in that dimension X. More particularly, in this regard, the control signaling 18 may indicate the inter- element spacing 17-E in the dimension X as being the spacing according to which adjacent antenna elements 15 are spaced apart from one another in that dimension X. The control signaling 18 may also indicate the subarray size 17-Z in the dimension X as being the size of each subarray 17S formed in that dimension X, i.e., how many antenna elements 15 belong to each subarray 17S in the dimension X. The inter-subarray spacing 17-SP in these embodiments may be derivable from the inter-element spacing 17-E and the subarray size 17-Z. For example, the inter-subarray spacing 17-SP in dimension X may equal to the ratio between the subarray size 17-Z in dimension X and the inter-element spacing 17-E in dimension X. For example, if the inter-subarray spacing 17-SP in the horizontal dimension is four (i.e., there are four antenna elements 15 in each subarray 17S in the horizontal dimension) and the inter-element spacing 17-E in the horizontal dimension is 0.5 wavelengths (λ), then the inter-subarray spacing 17-SP in the horizontal dimension is equal to 2 wavelengths (λ). The control signaling 18 may accordingly implicitly indicate an inter- subarray spacing 17-SP with a value of 2 wavelengths (λ) by indicating an inter-element spacing 17-E of 0.5 wavelengths (λ) and a subarray size 17-Z of 4. If either one of these parameters is fixed, preconfigured, or known, though, the control signaling 18 may just indicate the other of the parameters. For example, if the inter-element spacing 17-E is fixed or otherwise known to be 0.5 wavelengths (λ), the control signaling 18 may implicitly indicate an inter-subarray spacing 17-SP of 2 wavelengths (λ) by indicating a subarray size 17-Z of 4. If the inter-subarray spacing 17-SP has multiple components, such as may be needed to reflect non-uniformity and / or multi-dimensionality of the transmit antenna array 17, the control signaling 18 may indicate the inter-subarray spacing 17-SP by indicating one or more of those components. For example, if the inter-subarray spacing 17-SP has a horizontal component and a vertical component that are different from one another, the control signaling 18 may indicate the inter-subarray spacing 17-SP as described above in any of Figures 3A-3C, for each of those components. In some embodiments, though, if the horizontal component and the vertical component are the same as one another, the control signaling 18 may indicate the inter-subarray spacing 17-SP as described above in any of Figures 3A-3C, for only one of those components, with the other component understood to be the same. Such may thereby reduce signaling overhead. Alternatively or additionally, the communication network 10 in some embodiments selectively transmits the control signaling 18 only if the inter-subarray spacing 17-SP differs from a predefined default inter-subarray spacing 17-SP. Such may operate on a component by component basis of the inter-subarray spacing 17-SP as well. For example, the communication network 10 may selectively transmit control signaling 18 indicating the vertical component of the inter-subarray spacing 17-SP if the vertical component of the inter- subarray spacing 17-SP differs from a predefined default inter-subarray spacing 17-SP in the vertical dimension. Similarly, the control signaling 18 may selectively indicate the inter-subarray spacing 17-SP in only the dimension(s) within which the subarrays 17S are defined. In embodiments where subarrays 17S may be dynamically defined and configured on an as-needed basis, this may mean that the communication network 10 adapts the control signaling 18 to the dimension(s) within which the subarrays 17S are defined. In one or more embodiments, for instance, the communication network 10 determines in which dimension(s) the subarrays 17S are defined and then generates the control signaling 18 to selectively indicate the inter- subarray spacing 17-SP in the determined dimension(s). In one such embodiment, the communication network 10 determines whether subarrays 17S are defined in a certain dimension by evaluating the spacing between neighboring groups of antenna elements 15 constituting a signal port. Assuming a regular array, for instance, if the spacing between any neighboring group of antenna elements 15 that are oriented horizontally to each other is half of a wavelength, there are no subarrays 17S defined in the horizontal dimension of the array 17, but there are subarrays 17S defined in the vertical dimension of the array 17. In this case, the communication network 10 only signals the inter-subarray spacing 17-SP in the vertical dimension. In another example, again assuming a regular array, if the spacing between any neighboring group of antenna elements 15 that are oriented vertically or horizontally to each other is different than half of a wavelength, there are subarrays 17S defined in both the horizontal and vertical dimensions of the array 17. In this case, the communication network 10 signals the inter-subarray spacing 17-SP in both the vertical and horizontal dimensions. In some embodiments, the control signaling 18 is dedicated Radio Resource Control (RRC) signaling. In other embodiments, the control signaling 18 may be System Information, e.g., that is broadcast using System Information Block (SIB) signaling such as in SIB #1 (SIB1) or another SIBn, where each SIB is a structured set of System Information for network access or configuration. In these and other embodiments, the control signaling 18 may be transmitted to the communication device 12 during or as part of access by the communication device 12 to the communication network 10, e.g., when the communication device 12 initially connects to the communication network 10 or when the communication device 12 (re-)establishes or resumes an RRC connection. Especially in embodiments where the definition of the subarrays 17S is static or semi-static, at least for a given deployment, so as to remain stable over time, the control signaling 18 may be transmitted to the communication device 12 only once per connected session, such as only at initial access or RRC connection establishment. In other embodiments, though, where the definition of the subarrays 17S is dynamically reconfigurable so as to potentially vary over shorter time periods, the control signaling 18 may be transmitted to the communication device 12 even after establishment of an RRC connection or session, as needed to update the communication device 12 about any dynamic changes to the inter-subarray spacing 17-SP. In some embodiments, the control signaling 18 and the indicated inter-subarray spacing 17-SP is cell-specific. In such a case, the control signaling 18 may be transmitted on the cell to which it is specific. Or, the control signaling 18 may be transmitted on one cell which indicates the inter-subarray spacing 17-SP specific to another cell, e.g., where the cells may be configured for Cell Joint Transmission (CJT) or Distributed Multiple-Input Multiple-Output (D-MIM). Irrespective of how or when the communication network 10 indicates the inter- subarray spacing 17-SP to the communication device 12, some embodiments herein exploit the communication device’s knowledge of the inter-subarray spacing 17-SP for channel state information (CSI) reporting. The communication device 12 as shown in this regard determines CSI 22 based on the inter-subarray spacing 17-SP indicated by the control signaling 18. The CSI 22 describes a state of a downlink channel between the communication device 12 and the radio network node 14. The CSI 22 may for example include a channel quality indicator (CQI) that reflects a quality of the downlink channel, e.g., in terms of a modulation and coding scheme (MCS) appropriate given the downlink channel quality. The CSI 22 may alternatively or additionally include a rank indicator (RI) that indicates a transmission rank supported by the downlink channel, where the transmission rank reflects the number of independent spatial layers supported by the downlink channel. Regardless, the communication device 12 reports the CSI 22 to the communication network 10 in a CSI report 20, e.g., via an uplink channel such as a Physical Uplink Control Channel (PUCCH) or a Physical Uplink Shared Channel (PUSCH). Equipped with such CSI 22, the radio network node 14 adapts a transmission 24 from the transmit antenna array 17 based on the CSI 22. More particularly in this regard, the radio network node 14 precodes the transmission 24 from the transmit antenna array 17 based on the CSI 22 reported by the communication device 12. The radio network node 14 does so by selecting, based on the CSI 22, a precoder 18 to apply to the transmission 24. Application of such a precoder 24 may for instance independently control the phase and / or amplitude of signal(s) for the transmission 24 that are transmitted from the antenna elements 15 of the transmit antenna array 17, e.g., so as to "pre-correct" the transmission 24 to compensate for the distortions and path losses on the downlink channel and / or to focus energy of the transmission in select direction(s) so as to perform the transmission 24 on a transmit beam corresponding to the precoder 24. In these and other embodiments, then, the CSI 22 reported by the communication device 12 assists the communication network 10 in determining which precoder to select for applying to the transmission 24. For example, the CSI 22 may indicate a precoder that the communication device 12 selects as being preferred or recommended, e.g., by conveying a precoding matrix indicator (PMI). The communication device 12 may for instance select a precoder that will maximize a performance or channel quality metric for the transmission 24 on the downlink channel, e.g., in terms of mutual information, received power, signal-to- noise ratio (SNR), signal-to-interference-plus-noise ratio (SINR), etc. By way of this CSI 22, then, the communication device 12 effectively recommends the indicated precoder, e.g., which may correspond go to a recommended beam or combination of beams. The communication network 10 can then take the indicated precoder into account when selecting which precoder to actually apply to the transmission 24. Alternatively or additionally, the communication device 12 may determine any other aspect of the CSI 22, such as a CQI and / or rank indicator, based on the transmission 24 being precoded with the determined precoder. According to embodiments herein, though, CSI reporting by the communication device 12 is codebook-based. With codebook-based CSI reporting, the precoders that are possible for the communication device 12 to consider for determining the CSI 22 are limited to those in a precoding codebook. A precoding codebook in this regard includes a limited set of candidate precoders that are allowed to be used for determining the CSI 22. Notably, according to embodiments herein, the precoding codebook based on which the communication device 12 determines the CSI 22 is tailored to the inter-subarray spacing 17-SP of the radio network node’s transmit antenna array 17. As shown in Figure 4, the communication device 12 derives such a tailored precoding codebook 15D as a function of the inter-subarray spacing 17-SP and a base precoding codebook 15B. The base precoding codebook 15B may be obtained by the communication device 12 in any number of ways, such as by retrieving it from memory (in case the base precoding codebook 15B is predefined) or by receiving signaling from the communication network 10 that explicitly or implicitly indicates the base precoding codebook 15B. Either way, the base precoding codebook 15B may be a precoding codebook that is generic or agnostic as to the transmit antenna array’s inter-subarray spacing 17-SP. In fact, the base precoding codebook 15B may include candidate precoders appropriate for multiple possible inter-subarray spacings, not just those appropriate for the inter-subarray spacing 17-SP of the transmit antenna array 17. Or, in embodiments where the base precoding codebook 15B is specific to just a single inter-subarray spacing, referred to as a base inter-subarray spacing, that inter- subarray spacing may be different from the transmit antenna array’s inter-subarray spacing 17-SP. But, from the base precoding codebook 15B, the communication device 12 derives a precoding codebook 15D that is indeed tailored to the inter-subarray spacing 17-SP of the radio network node’s transmit antenna array 17. The derived / tailored precoding codebook 15D may be tailored to the inter-subarray spacing 17-SP in the sense that, while the base codebook 15B is not directly appropriate for precoding selection for the given transmit antenna array 17, the precoders belonging to the codebook 15D are converted and limited to contain phase terms / coefficients that are specific to or appropriate for the inter-subarray spacing 17-SP of the transmit antenna array 17. For example, in some embodiments, all candidate precoders in the derived precoding codebook 15D are a function of the inter- subarray spacing 17-SP indicated by the control signaling 18. By using this tailored codebook 15D for determining the CSI 22 to report, rather than a codebook that includes precoders generically appropriate for multiple possible inter- subarray spacings or precoders appropriate for some other inter-subarray spacing, the communication device 22 limits the precoders that it considers as candidates for use to those that are actually appropriate for the radio network node’s inter-subarray spacing 17-SP. For example, in embodiments where the CSI 22 indicates a precoder selected by the communication device 12, the communication device 12 selects the precoder from among candidate precoders in the tailored precoding codebook 15D. The communication device 12 may for instance select whichever of the candidate precoders in the tailored precoding codebook 15D maximizes a performance or channel quality metric for transmission on the downlink channel, e.g., maximizes mutual information, received power, SNR, SINR, etc. This may involve performing measurements on non-precoded reference signals (RSs) received from the radio network node 14. In one or more such embodiments where the non-precoded RSs are non-precoded CSI-RS, each CSI-RS resource may corresponds to a logical antenna port at the radio network node 14, with each logical port in turn physically corresponding to an antenna element 15 or subarray 17S. In any event, the communication device 12 may use the derived precoding codebook 15D to estimate the performance or channel quality resulting from applying different candidate precoders in the derived precoding codebook 15D at the radio network node 14. Here, then, the different candidate precoders in the derived precoding codebook 15D correspond to different precoding hypotheses, with each hypothesis corresponding to a weight set in the derived precoding codebook 15D. By limiting the precoders considered as candidates for precoder selection, some embodiments improve precoder selection performance, e.g., by minimizing the possibility that an inappropriate precoder is selected and reported. Moreover, this advantageously reduces the processing complexity imposed on the communication device 12 for CSI reporting, as the communication device 12 need not evaluate precoders that are inappropriate for the radio network node’s inter-subarray spacing 17-SP. For example, the communication device 10 need not calculate performance or channel quality metrics for precoders that are inappropriate for the radio network node’s inter-subarray spacing 17-SP. Further in this regard, with the candidate precoders being tailored to the inter-subarray spacing 17-SP, some embodiments may obviate the need for a higher resolution codebook via oversampling. The base precoding codebook 15B may accordingly be non-oversampled, so as to further reduce the number of candidate precoders that the communication device 12 must consider and evaluate. Furthermore, with fewer precoders considered as candidates for CSI reporting, some embodiments advantageously reduce the signaling overhead for CSI reporting. Some embodiments for example effectively limit the CSI reporting signaling space to the precoders that are actually appropriate for the radio network node’s inter-subarray spacing 17-SP. The communication network 10 in receipt of the CSI 22 may in turn interpret the CSI 22 as pertaining to the tailored precoding codebook 15D. In particular, the radio network node 14 may apply the preferred precoder to the transmission ports whose physical spacing / structure corresponds to the previously signaled inter-subarray spacing 17-SP. Figure 5 now shows additional details of some embodiments for how the communication device 12 derives the tailored precoding codebook 15D from the base precoding codebook 15B. As shown, the base precoding codebook 15B includes candidate precoders W1…WK, e.g., corresponding to respective precoder indices 1…K. Each of the candidate precoders W1…WKis constructed from one or more sets 28 of beamforming weights, e.g., where each set 28 may be a vector of beamforming weights. Precoding may be interpreted as multiplying the signal to be transmitted by these set(s) of beamforming weights on the antenna ports prior to transmission. In order to derive the tailored precoding codebook 15D from this base precoding codebook 15B, the communication device 12 in these embodiments applies phase offsets 30 to at least some beamforming weights in each set 28. That is, for each set 28 of beamforming weights, the communication device 12 applies phase offsets 30 to at least some beamforming weights in that set 28, to obtain a modified set 28’ of beamforming weights in the derived precoding codebook 15D. The communication device 12 may for example apply phase offsets 30 to all of the beamforming weights in a set 28 except for a first one of the beamforming weights, e.g., the first beamforming weight may remain unchanged. Either way, the phase offsets 30 in some embodiments vary linearly across sequential beamforming weights to which the phase offsets 30 are applied, i.e., the phase offsets 30 are linear phase offsets. Moreover, the phase offsets 30 are notably a function of the inter-subarray spacing 17-SP. The phase offsets 30 in these and other embodiments may adapt the beamforming weights in each set 28 to account for the inter-subarray spacing 17-SP. Note that the phase offsets 30 may be applied in aggregate across multiple dimensions of the transmit antenna array 17, or may be applied individually on a dimension by dimensions basis. For example, in one embodiment, the phase offsets 30 are included in an aggregate phase correction set, e.g., that aggregates the phase offsets 30 for multiple dimensions. The communication device 12 in this case may apply the aggregate phase correction set, via element-wise multiplication, to each set 28 of beamforming weights. In another example, though, the phase offsets 30 may be distributed among one or more dimension-specific phase correction sets, one for each dimension of the transmit antenna array 17 into which its antenna elements 15 are partitioned into subarrays 17S. The dimension-specific phase correction set for a given dimension thereby includes phase offsets that are a function of the inter-subarray spacing 17-SP in that dimension. For each of these dimension(s), the communication device 12 may apply the corresponding dimension- specific phase correction set, via element-wise multiplication, to the each 28 set of beamforming weights. Consider a simple example where the transmit antenna array 17 is a single-polarized uniform linear array (ULA) with ^^ antennas. In this case, the base precoding codebook 15B may include candidate precoders W1…WKthat are each a so-called Discrete Fourier Transform (DFT) precoder. Each candidate precoder W1…WKmay be defined as: é^^^ଶగ⋅^⋅^ ைே ù ^ ú ú , úwhere ^^ ൌ 0,1, … ^^^^ െ 1 is integer oversampling factor. ^^^is also referred as a one- with beam index ^^. If the antenna elements 15 of the ULA are arranged along the horizontal dimension, each DFT beam points to an azimuth direction. If the antenna elements 15 of the ULA are arranged along the vertical dimension, each DFT beam points to an elevation direction. Each precoder corresponds to a so-called DFT beam. In this example, each precoder is formed from a single set ^^^of beamforming weights, where this single set ^^^of beamforming weights exemplifies a single set 28 of beamforming weights in Figure 5. As such, the communication device 12 in this example applies a phase correction vector ^^^to at least some beamforming weights in the set ^^^of beamforming weights. Here, the phase correction vector ^^^may be defined as: ^^^ ൌ ^^^^ … ^^ௌ^^ ,∈ ℂ^^ୗ^where ^^^^ corresponds to the number of subarrays 17S. In this case, then, thecommunication device 12 may apply the phase correction vector ^^^to the set ^^^of beamforming weights via element-wise multiplication, resulting in a modified set ^^ᇱ^ of beamforming weights that accounts for the inter-subarray spacing 17-SP: ^^ᇱ ൌ ^^ ்^ ௌ ∗ ^^^The result is that the derived includes precoders: ^ Consider next an example where the transmit antenna array 17 is a two- dimensional uniform planar array (UPA) with ^^^antenna elements 15 in one dimension and ^^ଶantenna ports in another dimension. In this case, the base precoding codebook 15B may include candidate precoders W1…WKthat are each defined as follows, e.g., consistent with 3GPP TS 38.214 V17.6.0: మഏ^మഏ^^ಿ షభ் ^^ ^^^^ ൌ ^^ ൌ ^^ ^భ^ଶ^ ^,^ ^^^ ^^ ೀభಿభ^^^ ⋯ ^^ೀభಿభ ^^^^, In the above, ^^ with ^^ and ^^ଶ,^,^as (2-D) DFT characterized by two beam ^^^,^^^, one in each dimension. Each such vector ^^ ^^^ ൌ 0, … ,^^^^^^ െ 1; ^^ … ,^^ଶ^^ଶ െ 1^ corresponds to a 2D DFTIn this example, each precoder is formed from a single set ^^^,^of beamforming weights, where this single set ^^ of beamforming weights a single set 28 beamforming weights in 5. As such, the communication device 12 in this example applies a phase correction ^^^to at least some beamforming weights in the set ^^^,^of beamforming weights. Here, the phase correction vector ^^^may be defined as: ^^^ ൌ ^^^^ … ^^ௌ^^ ,∈ ℂ^^ୗ^where ^^^^ corresponds to the number of subarrays 17S. In this case, then, thecommunication device 12 may apply the phase correction vector ^^^to the set ^^^,^of beamforming weights via element-wise multiplication, resulting in a modified ^^′^,^of beamforming weights that accounts for the inter-subarray spacing 17-SP: ^^′ ்^,^ ൌ ^^ௌ ∗ ^^^,^The result is that the derived precoding codebook 15D includes precoders: ^^′ଶ^^^^^ ൌ ^^′^,^Consider next yet another example where the transmit antenna array 17 is a dual- polarized UPA, as shown in Figure 6. The transmit antenna array 17 in this case is described as having a number of antenna ports, ^^^, in a first dimension (e.g., the horizontal dimension), a number of antenna ports, ^^ଶ, in the second dimension perpendicular to the first dimension (e.g., the vertical dimension), and a number of polarizations ^^^. The totalnumber of antenna ports is thus ^^ ൌ ^^^^^ଶ^^^. The concept of an antenna port is non-limiting in the sense that it can refer to any virtualization (e.g., linear mapping) to the physical antenna elements 15. For example, pairs of physical antenna elements 15 could be fed the same signal, and hence share the same virtualized antenna port. The example in Figure 6shows a 4 ൈ 4 (i.e., ^^^ ൈ ^^ଶ,) array with dual-polarized antenna elements (i.e., ^^^ ൌ 2).Here, the 2-D DFT vectors for dual-polarized UPA may be extended such that the base precoding codebook 15B includes candidate precoders W1…WKthat are each defined as follows: 1 ^^ ^^^^^ ^,^^,^,^ ൌ^^^^^^^,^^^^^ ൌ ^^^గ^ where / ଶ is a co- may from an M-ary Phase ShiftKeying (M-PSK) alphabet such as quadrature PSK (QPSK) with ^^ ൌ 0, 1, 2, 3, and ^^^ௌூିோௌ isa number of CSI-RS ports. This is the base precoding codebook 15B for single layer CSI report with ^^^ௌூିோௌports. In this example, each precoder is formed from a set ^^^,^of beamforming weights, where this set ^^^,^of beamforming weights exemplifies 28 of beamforming weights in Figure 5. As the communication device 12 in this applies a phase correction vector ^^^to at least some beamforming weights in the set ^^^,^of beamforming weights. Here, the phase correction vector ^^^may be defined ^^^ ൌ ^^^^ … ^^ௌ^^ ,∈ ℂ^^ୗ^where ^^^^ corresponds to the number of subarrays 17S. In this case, then, thecommunication device 12 may apply the phase correction vector ^^^to the set ^^^,^of beamforming weights via element-wise multiplication, resulting in a modified ^^′^,^of beamforming weights that accounts for the inter-subarray spacing 17-SP: ^^′ ்^,^ ൌ ^^ௌ ∗ ^^^,^The result is that the derived precoding codebook 15D includes precoders: 1 ^^′ 1்^^′^^^ ^,^^^^ௌ ∗ ^^ ^^,^ ൌ ^,^^,^^ ൌ^ ^ ^^^^^^் an array used for a multi- layer transmission. Here, the precoder matrix for a multi-layer transmission from a dual- polarized UPA may be created by appending columns of 2-D DFT vectors. For an example 2-layer transmission, then, the base precoding codebook 15B may include candidate precoders W1…WKthat are each defined as follows: 1 ^^ ^^^ଶ^ ^,^^^^`,^`^,^`,^,^`,^ ൌ^ ^^2^^^^^^^^,^ –^^^^^^`,^` Such DFT-based precoders may be used for instance in New Radio (NR) Type I CSI feedback, where each layer is associated with a 2-D DFT beam, e.g., consistent with clause 5.2.2.2.1 of 3GPP TS 38.214 V18.2.0. In this example, each precoder is formed from two sets of beamforming weights, ^^^,^and ^^^`,^`, where these sets ^^^,^and ^^^`,^`exemplify sets 28 of beamforming weights in device 12 in this example applies a phase some beamforming weights in each of the sets ^^^,^and ^^^`,^`. Here, the phase correction vector ^^^may be defined as: ^^^ ൌ ^ ௌ^^ ,∈ ℂ^^ୗ^ ^^^ … ^^where ^^^^ corresponds to this case, then, the communication device 12 may ^^^to each of the sets ^^^,^and ^^^`,^`via element-wise multiplication, resulting in modified sets ^^′^,^and ^^′^`,^` account for the inter-subarray spacing 17-SP: ் ^^′^,^ ൌ ^^ௌ ∗ ^^^,^The result is that the derived includes precoders: ^ଶ^^′ ^^^ ^,^ ^`,^`^,^`,^,^`,^ ൌ^ ^ Generally, one a can the one constructed according to 3GPP TS 38.214 V18.2.0. Below is an example structure for ranks ^^ ൌ∈^1,2,5,6,7,8^ and ^^^ௌூିோௌ ൌ∈ ^4,8,12,16,24,32^:1 ^^ … are mapped to the array second row correspond to the second polarization, which are mapped to the next ^^^^^ଶantenna elements of the transmit antenna array 17. Moreover, each column denotes a layer and ^^^ ൌ ^^^గ^ / ଶ, corresponds to a co-phasing factor corresponding to either a Binary PSKconstellation (for rank 1) or QPSK constellation for others. Here, the overall beamforming vector ^^^,^is defined via the vertical beamforming vector ^^^as follows: ^^^ ൌ ^^1 ^^^ଶగ^ ைమேమ ⋯ ^^^ଶగ^^ேమି^^ைమேమ^ ^^^ଶ ^ 11் where ^^^is the number of antenna ports in the horizontal dimension, ^^ଶis the number of antenna ports in the vertical dimension, ^^^and ^^ଶare the respective oversampling factors. As can be seen, the codebook matrix is directly associated with the structure of the transmit antenna array 17, as it depends on the array’s port structure given by ^^^and ^^ଶ. In one embodiment, though, the base precoding codebook 15B contains two or more precoding weight vectors, corresponding to respective two or more DFT beam directions / configurations with non-oversampled or oversampled granularity. The base precoding codebook 15B may be formulated for λ / 2 antenna element spacing or for other spacing assumptions. The base precoding codebook 15B may be provided separately for the azimuth and elevation dimensions, or as a joint codebook for both dimensions in a planar array. In this regard, the base precoding codebook 15B may be modified in some embodiments on a dimension by dimension basis, e.g., so as to be applied separately to codebooks or codebook dimensions. For example, in some embodiments, to adjust the beamforming weight vectors in a given dimension of a planar array, a phase correction vector (^^^) depending on the number of steerable elements (i.e. subarrays or other portentities in the dimension of interest, ^^^^ ൌ ^^^ or ^^ଶ) is applied:^^^ ൌ ^^^^ … ^^ௌ^^ ,∈ ℂ^^ୗ^For the horizontal dimension, the phase correction vector ^^^can be denoted as ^^^భ ൌ ^^^^^ … ^^ேభ^^ ,∈ ℂ^^ேభand for the vertical ^^^మ ൌ … ,∈. The modification terms in ^^^may be computed based మon the antenna element 17-SP indicated by the control signaling 18. In one example, if the antenna element spacing in the horizontal dimension is provided as ^^^λ / 2-units, the subarray spacing is provided as ^^^times the antenna element spacing, and the base precoding codebook 15B is formulated assuming λ / 2 port spacing, the phase correction vector ^^^భfor horizontal spacing may be expressed as ^^^ ଶగ^^ௗ ^^ଶగ^^ேభି^^^ௗ ^ ି^^^భ ൌ ^1 ^^ைభேభభ భି ^^… ^^ைభேభ భ భ ^Similarly, the may be derived via a similar on parameters ^^ଶ, ^^ଶ, ^^ଶ, and ^^ଶ: ^^^ଶగ^ ^ଶగ^^ேమି^^^ௗ ^ ି^^^మ ൌ ^1 ^^ைమேమ^ௗమ^మି^^… ^^ைమேమ మ మ ^ .In another 15B may be defined to allow oversampled beam space evaluation but a minimum-size, non-oversampled beam hypothesis set is desired after accounting for actual spacing parameters, the phase correction vectors may be expressed as ^^^ଶగ^ ^ ^ ^భ ൌ ^1 ^^ேభ^ௗభ^భି ைభ^ ^ଶగ^^ேభି^^… ^^ேభ^ௗభ^భି ைభ^ ^,^As codebook 15B (accounting for ் ^^^,^ ൌ ^^^^ ଶగ^ ^^^ ைభேభ^^^ … ^^^ଶగ^^ேைభି^^భேభ^^ ൨,∈ ℂேభேమ^^^The modification element- wise multiplication with an aggregate phase correction vector ^^^ ∈ ℂ భேమ, where ^^^,^are obtained as the new adjusted weights that account for the different or sub- array spacings: ^^^ௗ^^,^ ൌ ^^ ்^ ∗ ^^^,^For horizontal spacing correction, the aggregate phase correction vector may beformed as ^^^ ൌ ^^^భ, an ^^ଶ-times group repetition of ^^^భ elements.^^^భ ൌ ^^^^^^ ⋯^^^^^ ^^^ଶ^ ⋯^^ଶ^^ ⋯ ^^^ேభ^ ⋯^^ேభ^൧^ .For vertical vector ^^^ ൌ ^^^మ may be formed as an ^^^- ^^^మ, ^^^మ ൌ ^^^^మ ^^^మ ⋯ ^^^మ൧.For simultaneous correction in both dimensions, it may be formed as an element-wise product of the horizontal and vertical constructions, ^^^ ൌ ^^^భ ∗ ^^^మ.Other mathematically equivalent or similar phase correction formulation may be used in the alternative. In some examples, when the antenna element spacing and the inter-subarray spacing 17-SP are provided explicitly, corresponding oversampling parameters may be omitted, and assumed to be equal to one. In other embodiments, derivation of the tailored precoding codebook 15D may further include removal of superfluous precoding weight set entries, keeping only entries corresponding to relevant non-oversampled beam representation. Alternatively, derivation of the tailored precoding codebook 15D may include adding additional precoding weight set entries corresponding to oversampled beam representation at a desired resolution. Note, too, that rules for derivation of the tailored precoding codebook 15D may be specified by a communications standard, e.g., in a 3GPP TS, to ascertain unambiguous interpretation of the derivation by both the communication device 12 and the communication network 10. Note, too, that in some embodiments there may be multiple different possible base precoding codebooks 15B that are usable for CSI reporting. For example, different possible base precoding codebooks 15B may be defined for different radio network node types, different transmit antenna array types (e.g., with high-level / large-scale difference), different cell environments, different propagation conditions, etc. In such embodiments where there are multiple base precoding codebooks 15B defined, the communication device 12 may receive configuration information from the communication network 10 to indicate which base precoding codebook 15B to use, via e.g. a codebook index. Note further that although some embodiments herein have focused on derivation of the tailored precoding codebook 15D as a function of the inter-subarray spacing 17-SP, the tailored precoding codebook 15D may also be derived as a function of one or more other modification parameters, such as an inter-element spacing. Generally, then, to adapt the base precoding codebook 15B to match the specific transmit antenna array configuration at the radio network node 14, the communication device 12 may receive one or more modification parameters indicating how the array-specific codebook 15D to be used for CSI reporting is to be derived from the base precoding codebook 15B. In one embodiment, the modification parameter(s) indicate the antenna element spacing and the inter-subarray spacing 17-SP. The antenna element spacing parameter may be provided as a multiple of the wavelength lambda, or a multiple of lambda / 2 spacing units. It may be a fractional / decimal value, or an index mapping to a fractional / decimal value. The inter-subarray spacing parameter may be provided as a multiple of the antenna element spacing value, or if that is not explicitly provided, a multiple of the nominal lambda / 2 spacing unit. It may be an integer value, since the subarray grouping typically includes multiple antenna elements in a uniform grid. To support array architectures where port reduction via subarray grouping will be used, there is thus a need for a codebook adaptation approach where a base codebook can be adjusted to support a range of expected antenna array panel designs, including those employing sub-arrays, e.g., by signaling a limited number of parameters, in particular, where the inter-element and inter-subarray spacing can be accounted for simultaneously. The resulting scenario-specific codebook should also not include unnecessary or irrelevant entries, so that the number of hypotheses a communication needs to evaluate is minimized and device energy consumption degradation due to testing irrelevant hypotheses is avoided. Certain aspects of the disclosure and their embodiments may provide solutions to these or other challenges. According to some embodiments, a base DFT codebook is defined, assuming e.g. λ / 2 spacing between antenna ports. The radio network node 14 configures the communication device 12 with an array scaling factor or spacing parameter to account for the subarray spacing, possibly a multiple of the element spacing, and optionally for an actual antenna element spacing, possibly other than λ / 2. In a general setup, the spacing information may be signaled separately for the vertical and horizontal array dimensions. The spacing information for each dimension may include 2 types of antenna spacings: between elements of a sub-array 17S and between sub-arrays 17S. In a simplified setup, the scaling / spacing parameter may be limited to e.g. vertical inter-subarray spacing. The communication device 12 uses the spacing information to apply appropriate codebook modifications, e.g. linear phase shifts, to the per-element entries in the base precoding codebook 15B when evaluating multiple codebook hypotheses. A mathematical framework for applying the spacing parameters to the base codebook is provided, to derive the minimum set of hypotheses to be tested for the given transmit antenna array configuration. The communication device 12 reports to the communication network 10 CSI 22 estimated corresponding to the modified codebook. Multiple base codebooks may be defined, to cover different radio network node types. In addition to the inter-element or -subarray information, the radio network node 14 may also signal to the communication device 12 a base codebook index. Notably, the communication network 10 in some embodiments signals the (1) inter- subarray spacing 17-SP and (2) the inter-element distance in multiple (vertical and horizontal) angle dimensions. A radio network node 14 with a regular array geometry can be fully characterized by these 4 parameters, thus they are sufficient to tell how exactly the communication device 12 should modify its base precoding codebook 15B to be able to scan over a minimum set of relevant beam candidates. This approach may thereby prove advantageous over just signalling two oversampling factors, which would not be sufficient to determine how the communication device 12 should modify its codebook to be able to scan only over relevant beam candidates and which would result in a large number of beam candidates, some of which would not be valid / relevant for the current transmit antenna array panel structure. Also notable, the communication device 12 in some embodiments may build the modified / tailored precoding codebook 15D based on the received inter-element spacing and inter-subarray spacing 17-SP (vertical and / or horizontal dimensions of the array) based on deterministic mathematical constructs, where the modified codebook contains only relevant precoders / beam candidates. Certain embodiments may provide one or more of the following technical advantage(s). In some embodiments, the number of codebooks, and potentially codebook sizes, that need to be standardized are significantly reduced, e.g., as compared to standardizing full, oversampled codebooks for all antenna and subarray spacing options. Alternatively or additionally, some embodiments reduce signaling despite conveying more information (e.g., including both inter-subarray and antenna-element spacing). This may be the case especially in embodiments where the time-invariant scaling factor of the transmit antenna array 17 is only signaled once, and / or where the codebook is non-oversampled. Network resource usage may thus be lower and the impact on data traffic may be reduced. Alternatively or additionally, some embodiments may provide reduced device processing, since the number of precoder / beam hypotheses to be tested is reduced, compared to testing a more general codebook comprising precoder / beam hypotheses that are not relevant for the given array spacing (as in WO 2018031082 A1). Some embodiments thus lead to lower device processing complexity and resulting device energy savings. Alternatively or additionally, some embodiments provide better beam selection performance since only valid beam candidate hypotheses are tested. According to some standard-related embodiments, a communications standard specification defines a base precoding codebook 15B and a set of modification parameters. The set of modification parameters may include parameters that indicate inter-subarray distance and inter-antenna distance. The communication device 12 may thereby be configured by the communication network 10 to apply modifications to the base precoding codebook 15B regarding inter-subarray distance and inter-antenna distance in vertical and / or horizontal (elevation and azimuth) angle dimensions. In one embodiment, these 4 parameters are used since they can fully characterize the transmit antenna array 17 geometry in planar regular arrays. They are thus sufficient to indicate how exactly the communication device 12 should modify the base precoding codebook 15B to be able to scan over all relevant precoder / beam candidates. The modified / derived precoding codebook 15D then contains a minimum necessary set of relevant precoder / beam candidates. Based on the modification parameters, the communication device 12 builds a modified codebook based on the received inter-antenna and inter-subarray distances. After it performs CSI-RS measurements, it processes the measurements based on the modified codebook to find the best radio network node transmit beam. Figure 7 shows a high-level flow diagram of some embodiments from the communication device perspective. The method comprises obtaining a base precoding codebook 145B (Block 100). The method also comprises receiving, from the communication network 10, one or more modification parameters, comprising at least an inter-subarray spacing parameter (Block 110). The inter-subarray spacing parameter indicates an inter- subarray spacing 17-SP of a transmit antenna array at a radio network node 14. The method also comprises reporting, to the communication network 10, CSI 22 estimated corresponding to a modified codebook 14D, where the modified codebook is based on the modification parameter(s) (Block 120). In some embodiments, the modification parameters comprise an antenna element spacing parameter. In this case, the radio network node 14 thus signals both inter-subarray spacing and the inter-element spacing to the communication device 12. Alternatively or additionally, obtaining the base precoding codebook 15B may comprise receiving from the radio network node 14 a codebook index mapped to the base precoding codebook 15B. Alternatively or additionally, the reporting comprises performing non-precoded CSI- RS measurements and estimating CSI 22 for one or more entries of the modified codebook. Here, weights of the base precoding codebook entries are modified based on the spacing parameters. Alternatively or additionally, the weight modification may comprise adding linear phase shifts to base precoding codebook entries. Alternatively or additionally, separate spacing parameters may be provided for different array dimensions, e.g., the vertical and horizontal array dimensions. For example, all such separate spacing parameters may be provided for every instance of the method. Or, only some parameters may be provided for an instance of the method, namely those that are different than a half wavelength (lambda / 2 is the default assumption). Alternatively or additionally, the spacing parameters for each dimension may include an antenna element spacing parameter between antenna elements in a subarray 17S and / or may include an inter-subarray spacing parameter between sub-arrays 17 in an antenna panel. Alternatively or additionally, the modification parameters may be provided once per connection, e.g., the spacing index may be reused from a previous execution of the method. Alternatively or additionally, the base precoding codebook 15B may be non- oversampled. Note that the approach described herein may be extended to other codebook variations and options supported in any given codebook framework, e.g. multi-panel setups, etc. In view of the modifications and variations herein, Figure 8 depicts a method performed by a communication device 12 configured for use in a communication network 10 in accordance with particular embodiments. The method includes obtaining a base precoding codebook 15B (Block 800). The method also comprises receiving, from the communication network 10, control signaling 18 that indicates an inter-subarray spacing 17- SP according to which adjacent subarrays 17S of a transmit antenna array 17 at a radio network node 14 are spaced apart from one another (Block 810). In some embodiments, each subarray 17S comprises a set of adjacent antenna elements 15 that are driven by the same stream of baseband samples. The method also comprises deriving, from the base precoding codebook 15B, a precoding codebook 15D tailored to the inter-subarray spacing 17-SP indicated by the control signaling 18 (Block 820). The method also comprises determining, based on the derived precoding codebook 15D, channel state information 22 describing a state of a downlink channel between the communication device 12 and the radio network node 14 (Block 830). The method also comprises reporting the channel state information 22 to the communication network 10 (Block 840). In some embodiments, the control signaling 18 indicates the inter-subarray spacing 17-SP as a multiple of an actual or nominal inter-element spacing 17E according to which adjacent antenna elements 15 of the transmit antenna array 17 are actually or nominally spaced apart from one another. In other embodiments, the control signaling 18 indicates the inter-subarray spacing 17-SP as a number of wavelengths between adjacent subarrays 17S. In some embodiments, the control signaling 18 implicitly indicates the inter-subarray spacing 17-SP by indicating, for each of one or more dimensions of the transmit antenna array 17, a size of each subarray 17S of the transmit antenna array 17 in the dimension, and an inter-element spacing 17E according to which adjacent antenna elements 15 of the transmit antenna array 17 are spaced apart from one another in the dimension. In some embodiments, for each of one or more dimensions of the transmit antenna array 17, the control signaling 18 indicates the inter-subarray spacing 17-SP in that dimension. In some embodiments, the base precoding codebook 15B comprises candidate precoders that are each constructed from one or more sets of beamforming weights 28, and said deriving comprises, for each set of beamforming weights 28, applying phase offsets 24 to at least some beamforming weights 28 in the set. In some embodiments, the phase offsets 24 vary linearly across the beamforming weights 28 to which the phase offsets 24 are applied and are a function of the intersubarray spacing 17-SP. In some embodiments, applying the phase offsets 24 comprises applying an aggregate phase correction set, via element-wise multiplication, to the set of beamforming weights 28. In some embodiments, the aggregate phase correction set comprises the phase offsets 24. In other embodiments, applying the phase offsets 24 comprises for each of one or more dimensions of the transmit antenna array 17, applying a dimension-specific phase correction set, via element-wise multiplication, to the set of beamforming weights 28. In some embodiments, the dimension- specific phase correction set comprises phase offsets 24 that are a function of the intersubarray spacing 17-SP in that dimension. In some embodiments, all candidate precoders in the derived precoding codebook 15D are a function of the inter-subarray spacing 17-SP indicated by the control signaling 18. In some embodiments, the base precoding codebook 15B is non-oversampled. In some embodiments, said determining comprises selecting, from among candidate precoders in the derived precoding codebook 15D, a candidate precoder maximizing a performance or channel quality metric for transmissions on the downlink channel to the communication device 12. In some embodiments, the channel state information 22 indicates the selected candidate precoder. In some embodiments, the base precoding codebook 15B is defined for an antenna array partitioned into subarrays 17S that are spaced from one another according to a base inter-subarray spacing. In some embodiments, the base inter-subarray spacing differs from the inter-subarray spacing 17-SP according to which adjacent subarrays 17S of the transmit antenna array 17 at the radio network node 14 are spaced apart from one another. In some embodiments, the control signaling 18 is dedicated Radio Resource Control, RRC, signaling or System Information broadcast using System Information Block, SIB, signaling. In some embodiments, receiving the control signaling 18 comprises receiving the control signaling 18 upon establishment or resumption of a radio resource control, RRC, connection with the communication network 10. In other embodiments, receiving the control signaling 18 comprises receiving the control signaling 18 alternatively or additionally once per RRC connection with the communication network 10. In some embodiments, obtaining the base precoding codebook 15B comprises receiving, from the communication network 10, signaling indicating the base precoding codebook 15B. In some embodiments, the signaling either explicitly or implicitly indicates the base precoding codebook 15B. Figure 9 depicts a method performed by a radio network node 14 configured for use in a communication network 10 in accordance with other particular embodiments. The method includes transmitting, to a communication device 12, control signaling 18 that indicates an inter-subarray spacing according to which adjacent subarrays 17S of a transmit antenna array 17 at the radio network node 14 are spaced apart from one another (Block 900). In some embodiments, each subarray 17S comprises a set of adjacent antenna elements 15 that are driven by the same stream of baseband samples. The method also comprises receiving, from the communication device 12, channel state information 22 describing a state of a downlink channel between the communication device 12 and the radio network node 14 (Block 910). In some embodiments, the channel state information 22 is based on a precoding codebook 15D derived, from a base precoding codebook 15B, to be tailored to the inter-subarray spacing 17-SP indicated by the control signaling 18. In some embodiments, the control signaling 18 indicates the inter-subarray spacing 17-SP as a multiple of an actual or nominal inter-element spacing 17E according to which adjacent antenna elements 15 of the transmit antenna array 17 are actually or nominally spaced apart from one another. In other embodiments, the control signaling 18 indicates the inter-subarray spacing 17-SP as a number of wavelengths between adjacent subarrays 17S. In some embodiments, the control signaling 18 implicitly indicates the inter-subarray spacing 17-SP by indicating, for each of one or more dimensions of the transmit antenna array 17, a size of each subarray of the transmit antenna array 17 in the dimension, and an inter- element spacing 17E according to which adjacent antenna elements 15 of the transmit antenna array 17 are spaced apart from one another in the dimension. In some embodiments, for each of one or more dimensions of the transmit antenna array 17, the control signaling 18 indicates the inter-subarray spacing 17-SP in that dimension. In some embodiments, the base precoding codebook 15B comprises candidate precoders that are each constructed from one or more sets of beamforming weights 28, and the derived precoding codebook 15D is derived by, for each set of beamforming weights 28, applying phase offsets 24 to at least some beamforming weights 28 in the set. In some embodiments, the phase offsets 24 vary linearly across the beamforming weights 28 to which the phase offsets 24 are applied and are a function of the inter-subarray spacing 17- SP. In some embodiments, applying the phase offsets 24 comprises applying an aggregate phase correction set, via element-wise multiplication, to the set of beamforming weights 28. In some embodiments, the aggregate phase correction set comprises the phase offsets 24. In other embodiments, applying the phase offsets 24 comprises for each of one or more dimensions of the transmit antenna array 17, applying a dimension-specific phase correction set, via element-wise multiplication, to the set of beamforming weights 28. In some embodiments, the dimension-specific phase correction set comprises phase offsets 24 that are a function of the inter-subarray spacing 17-SP in that dimension. In some embodiments, all candidate precoders in the derived precoding codebook 15D are a function of the inter-subarray spacing 17-SP indicated by the control signaling 18. In some embodiments, the base precoding codebook 15B is non-oversampled. In some embodiments, the channel state information 22 indicates, from among candidate precoders in the derived precoding codebook 15D, a candidate precoder maximizing a performance or channel quality metric for transmissions on the downlink channel to the communication device 12. In some embodiments, the base precoding codebook 15B is defined for an antenna array partitioned into subarrays 17S that are spaced from one another according to a base inter-subarray spacing. In some embodiments, the base inter-subarray spacing differs from the inter-subarray spacing 17-SP according to which adjacent subarrays 17S of the transmit antenna array 17 at the radio network node 14 are spaced apart from one another. In some embodiments, the control signaling 18 is dedicated Radio Resource Control, RRC, signaling or System Information Broadcast, SIB, signaling. In some embodiments, transmitting the control signaling 18 comprises transmitting the control signaling 18 upon establishment or resumption by the communication device 12 of a radio resource control, RRC, connection with the communication network 10. In other embodiments, transmitting the control signaling 18 comprises transmitting the control signaling 18 alternatively or additionally once per RRC connection between the communication device 12 and the communication network 10. In some embodiments, the method further comprises transmitting, to the communication device 12, signaling indicating the base precoding codebook 15B. In some embodiments, the signaling either explicitly or implicitly indicates the base precoding codebook 15B (Block 920). Embodiments herein also include corresponding apparatuses. Embodiments herein for instance include a communication device 12 configured to perform any of the steps of any of the embodiments described above for the communication device 12. Embodiments also include a communication device 12 comprising processing circuitry and power supply circuitry. The processing circuitry is configured to perform any of the steps of any of the embodiments described above for the communication device 12. The power supply circuitry is configured to supply power to the communication device 12. Embodiments further include a communication device 12 comprising processing circuitry. The processing circuitry is configured to perform any of the steps of any of the embodiments described above for the communication device 12. In some embodiments, the communication device 12 further comprises communication circuitry. Embodiments further include a communication device 12 comprising processing circuitry and memory. The memory contains instructions executable by the processing circuitry whereby the communication device 12 is configured to perform any of the steps of any of the embodiments described above for the communication device 12. Embodiments moreover include a user equipment (UE). The UE comprises an antenna configured to send and receive wireless signals. The UE also comprises radio front- end circuitry connected to the antenna and to processing circuitry, and configured to condition signals communicated between the antenna and the processing circuitry. The processing circuitry is configured to perform any of the steps of any of the embodiments described above for the communication device 12. In some embodiments, the UE also comprises an input interface connected to the processing circuitry and configured to allow input of information into the UE to be processed by the processing circuitry. The UE may comprise an output interface connected to the processing circuitry and configured to output information from the UE that has been processed by the processing circuitry. The UE may also comprise a battery connected to the processing circuitry and configured to supply power to the UE. Embodiments herein also include a radio network node 14 configured to perform any of the steps of any of the embodiments described above for the radio network node 14. Embodiments also include a radio network node 14 comprising processing circuitry and power supply circuitry. The processing circuitry is configured to perform any of the steps of any of the embodiments described above for the radio network node 14. The power supply circuitry is configured to supply power to the radio network node 14. Embodiments further include a radio network node 14 comprising processing circuitry. The processing circuitry is configured to perform any of the steps of any of the embodiments described above for the radio network node 14. In some embodiments, the radio network node 14 further comprises communication circuitry. Embodiments further include a radio network node 14 comprising processing circuitry and memory. The memory contains instructions executable by the processing circuitry whereby the radio network node 14 is configured to perform any of the steps of any of the embodiments described above for the radio network node 14. More particularly, the apparatuses described above may perform the methods herein and any other processing by implementing any functional means, modules, units, or circuitry. In one embodiment, for example, the apparatuses comprise respective circuits or circuitry configured to perform the steps shown in the method figures. The circuits or circuitry in this regard may comprise circuits dedicated to performing certain functional processing and / or one or more microprocessors in conjunction with memory. For instance, the circuitry may include one or more microprocessor or microcontrollers, as well as other digital hardware, which may include digital signal processors (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as read-only memory (ROM), random-access memory, cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory may include program instructions for executing one or more telecommunications and / or data communications protocols as well as instructions for carrying out one or more of the techniques described herein, in several embodiments. In embodiments that employ memory, the memory stores program code that, when executed by the one or more processors, carries out the techniques described herein. Figure 10 for example illustrates a communication device 12 as implemented in accordance with one or more embodiments. As shown, the communication device 12 includes processing circuitry 1010 and communication circuitry 1020. The communication circuitry 1020 (e.g., radio circuitry) is configured to transmit and / or receive information to and / or from one or more other nodes, e.g., via any communication technology. Such communication may occur via one or more antennas that are either internal or external to the communication device 12. The processing circuitry 1010 is configured to perform processing described above, e.g., in Figure 8, such as by executing instructions stored in memory 1030. The processing circuitry 1010 in this regard may implement certain functional means, units, or modules. Figure 11 illustrates a network node 14 as implemented in accordance with one or more embodiments. As shown, the network node 14 includes processing circuitry 1110 and communication circuitry 1120. The communication circuitry 1120 is configured to transmit and / or receive information to and / or from one or more other nodes, e.g., via any communication technology. The processing circuitry 1110 is configured to perform processing described above, e.g., in Figure 9, such as by executing instructions stored in memory 1130. The processing circuitry 1110 in this regard may implement certain functional means, units, or modules. Those skilled in the art will also appreciate that embodiments herein further include corresponding computer programs. A computer program comprises instructions which, when executed on at least one processor of an apparatus, cause the apparatus to carry out any of the respective processing described above. A computer program in this regard may comprise one or more code modules corresponding to the means or units described above. Embodiments further include a carrier containing such a computer program. This carrier may comprise one of an electronic signal, optical signal, radio signal, or computer readable storage medium. In this regard, embodiments herein also include a computer program product stored on a non-transitory computer readable (storage or recording) medium and comprising instructions that, when executed by a processor of an apparatus, cause the apparatus to perform as described above. Embodiments further include a computer program product comprising program code portions for performing the steps of any of the embodiments herein when the computer program product is executed by a computing device. This computer program product may be stored on a computer readable recording medium. Figure 12 shows an example of a communication system 1200 in accordance with some embodiments. In the example, the communication system 1200 includes a telecommunication network 1202 that includes an access network 1204, such as a radio access network (RAN), and a core network 1206, which includes one or more core network nodes 1208. The access network 1204 includes one or more access network nodes, such as network nodes 1210a and 1210b (one or more of which may be generally referred to as network nodes 1210), or any other similar 3rdGeneration Partnership Project (3GPP) access nodes or non-3GPP access points. Moreover, as will be appreciated by those of skill in the art, a network node is not necessarily limited to an implementation in which a radio portion and a baseband portion are supplied and integrated by a single vendor. Thus, it will be understood that network nodes include disaggregated implementations or portions thereof. For example, in some embodiments, the telecommunication network 1202 includes one or more Open-RAN (ORAN) network nodes. An ORAN network node is a node in the telecommunication network 1202 that supports an ORAN specification (e.g., a specification published by the O-RAN Alliance, or any similar organization) and may operate alone or together with other nodes to implement one or more functionalities of any node in the telecommunication network 1202, including one or more network nodes 1210 and / or core network nodes 1208. Examples of an ORAN network node include an open radio unit (O-RU), an open distributed unit (O-DU), an open central unit (O-CU), including an O-CU control plane (O- CU-CP) or an O-CU user plane (O-CU-UP), a RAN intelligent controller (near-real time or non-real time) hosting software or software plug-ins, such as a near-real time control application (e.g., xApp) or a non-real time control application (e.g., rApp), or any combination thereof (the adjective “open” designating support of an ORAN specification). The network node may support a specification by, for example, supporting an interface defined by the ORAN specification, such as an A1, F1, W1, E1, E2, X2, Xn interface, an open fronthaul user plane interface, or an open fronthaul management plane interface. Moreover, an ORAN access node may be a logical node in a physical node. Furthermore, an ORAN network node may be implemented in a virtualization environment (described further below) in which one or more network functions are virtualized. For example, the virtualization environment may include an O-Cloud computing platform orchestrated by a Service Management and Orchestration Framework via an O-2 interface defined by the O-RAN Alliance or comparable technologies. The network nodes 1210 facilitate direct or indirect connection of user equipment (UE), such as by connecting UEs 1212a, 1212b, 1212c, and 1212d (one or more of which may be generally referred to as UEs 1212) to the core network 1206 over one or more wireless connections. Example wireless communications over a wireless connection include transmitting and / or receiving wireless signals using electromagnetic waves, radio waves, infrared waves, and / or other types of signals suitable for conveying information without the use of wires, cables, or other material conductors. Moreover, in different embodiments, the communication system 1200 may include any number of wired or wireless networks, network nodes, UEs, and / or any other components or systems that may facilitate or participate in the communication of data and / or signals whether via wired or wireless connections. The communication system 1200 may include and / or interface with any type of communication, telecommunication, data, cellular, radio network, and / or other similar type of system. The UEs 1212 may be any of a wide variety of communication devices, including wireless devices arranged, configured, and / or operable to communicate wirelessly with the network nodes 1210 and other communication devices. Similarly, the network nodes 1210 are arranged, capable, configured, and / or operable to communicate directly or indirectly with the UEs 1212 and / or with other network nodes or equipment in the telecommunication network 1202 to enable and / or provide network access, such as wireless network access, and / or to perform other functions, such as administration in the telecommunication network 1202. In the depicted example, the core network 1206 connects the network nodes 1210 to one or more host computing systems, such as host 1216. These connections may be direct or indirect via one or more intermediary networks or devices. In other examples, network nodes may be directly coupled to hosts. The core network 1206 includes one more core network nodes (e.g., core network node 1208) that are structured with hardware and software components. Features of these components may be substantially similar to those described with respect to the UEs, network nodes, and / or hosts, such that the descriptions thereof are generally applicable to the corresponding components of the core network node 1208. Example core network nodes include functions of one or more of a Mobile Switching Center (MSC), Mobility Management Entity (MME), Home Subscriber Server (HSS), Access and Mobility Management Function (AMF), Session Management Function (SMF), Authentication Server Function (AUSF), Subscription Identifier De-concealing function (SIDF), Unified Data Management (UDM), Security Edge Protection Proxy (SEPP), Network Exposure Function (NEF), and / or a User Plane Function (UPF). The host 1216 may be under the ownership or control of a service provider other than an operator or provider of the access network 1204 and / or the telecommunication network 1202. The host 1216 may host a variety of applications to provide one or more service. Examples of such applications include live and pre-recorded audio / video content, data collection services such as retrieving and compiling data on various ambient conditions detected by a plurality of UEs, analytics functionality, social media, functions for controlling or otherwise interacting with remote devices, functions for an alarm and surveillance center, or any other such function performed by a server. As a whole, the communication system 1200 of Figure 12 enables connectivity between the UEs, network nodes, and hosts. In that sense, the communication system may be configured to operate according to predefined rules or procedures, such as specific standards that include, but are not limited to: Global System for Mobile Communications (GSM); Universal Mobile Telecommunications System (UMTS); Long Term Evolution (LTE), and / or other suitable 2G, 3G, 4G, 5G standards, or any applicable future generation standard (e.g., 6G); wireless local area network (WLAN) standards, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards (WiFi); and / or any other appropriate wireless communication standard, such as the Worldwide Interoperability for Microwave Access (WiMax), Bluetooth, Z-Wave, Near Field Communication (NFC) ZigBee, LiFi, and / or any low-power wide-area network (LPWAN) standards such as LoRa and Sigfox. In some examples, the telecommunication network 1202 is a cellular network that implements 3GPP standardized features. Accordingly, the telecommunications network 1202 may support network slicing to provide different logical networks to different devices that are connected to the telecommunication network 1202. For example, the telecommunications network 1202 may provide Ultra Reliable Low Latency Communication (URLLC) services to some UEs, while providing Enhanced Mobile Broadband (eMBB) services to other UEs, and / or Massive Machine Type Communication (mMTC) / Massive IoT services to yet further UEs. In some examples, the UEs 1212 are configured to transmit and / or receive information without direct human interaction. For instance, a UE may be designed to transmit information to the access network 1204 on a predetermined schedule, when triggered by an internal or external event, or in response to requests from the access network 1204. Additionally, a UE may be configured for operating in single- or multi-RAT or multi-standard mode. For example, a UE may operate with any one or combination of Wi-Fi, NR (New Radio) and LTE, i.e. being configured for multi-radio dual connectivity (MR-DC), such as E- UTRAN (Evolved-UMTS Terrestrial Radio Access Network) New Radio – Dual Connectivity (EN-DC). In the example, the hub 1214 communicates with the access network 1204 to facilitate indirect communication between one or more UEs (e.g., UE 1212c and / or 1212d) and network nodes (e.g., network node 1210b). In some examples, the hub 1214 may be a controller, router, content source and analytics, or any of the other communication devices described herein regarding UEs. For example, the hub 1214 may be a broadband router enabling access to the core network 1206 for the UEs. As another example, the hub 1214 may be a controller that sends commands or instructions to one or more actuators in the UEs. Commands or instructions may be received from the UEs, network nodes 1210, or by executable code, script, process, or other instructions in the hub 1214. As another example, the hub 1214 may be a data collector that acts as temporary storage for UE data and, in some embodiments, may perform analysis or other processing of the data. As another example, the hub 1214 may be a content source. For example, for a UE that is a VR device, display, loudspeaker, or other media delivery device, the hub 1214 may retrieve VR assets, video, audio, or other media or data related to sensory information via a network node, which the hub 1214 then provides to the UE either directly, after performing local processing, and / or after adding additional local content. In still another example, the hub 1214 acts as a proxy server or orchestrator for the UEs, in particular if one or more of the UEs are low energy IoT devices. The hub 1214 may have a constant / persistent or intermittent connection to the network node 1210b. The hub 1214 may also allow for a different communication scheme and / or schedule between the hub 1214 and UEs (e.g., UE 1212c and / or 1212d), and between the hub 1214 and the core network 1206. In other examples, the hub 1214 is connected to the core network 1206 and / or one or more UEs via a wired connection. Moreover, the hub 1214 may be configured to connect to an M2M service provider over the access network 1204 and / or to another UE over a direct connection. In some scenarios, UEs may establish a wireless connection with the network nodes 1210 while still connected via the hub 1214 via a wired or wireless connection. In some embodiments, the hub 1214 may be a dedicated hub – that is, a hub whose primary function is to route communications to / from the UEs from / to the network node 1210b. In other embodiments, the hub 1214 may be a non-dedicated hub – that is, a device which is capable of operating to route communications between the UEs and network node 1210b, but which is additionally capable of operating as a communication start and / or end point for certain data channels. Figure 13 shows a UE 1300 in accordance with some embodiments. The UE 1300 presents additional details of some embodiments of the UE 1212 of Figure 1. As used herein, a UE refers to a device capable, configured, arranged and / or operable to communicate wirelessly with network nodes and / or other UEs. Examples of a UE include, but are not limited to, a smart phone, mobile phone, cell phone, voice over IP (VoIP) phone, wireless local loop phone, desktop computer, personal digital assistant (PDA), wireless cameras, gaming console or device, music storage / playback device, wearable terminal device, wireless endpoint, mobile station, tablet, laptop, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), an Augmented Reality (AR) or Virtual Reality (VR) device, wireless customer-premise equipment (CPE), vehicle, vehicle-mounted or vehicle embedded / integrated wireless device, etc. Other examples include any UE identified by the 3rd Generation Partnership Project (3GPP), including a narrow band internet of things (NB- IoT) UE, a machine type communication (MTC) UE, and / or an enhanced MTC (eMTC) UE. A UE may support device-to-device (D2D) communication, for example by implementing a 3GPP standard for sidelink communication, Dedicated Short-Range Communication (DSRC), vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), or vehicle- to-everything (V2X). In other examples, a UE may not necessarily have a user in the sense of a human user who owns and / or operates the relevant device. Instead, a UE may represent a device that is intended for sale to, or operation by, a human user but which may not, or which may not initially, be associated with a specific human user (e.g., a smart sprinkler controller). Alternatively, a UE may represent a device that is not intended for sale to, or operation by, an end user but which may be associated with or operated for the benefit of a user (e.g., a smart power meter). The UE 1300 includes processing circuitry 1302 that is operatively coupled via a bus 1304 to an input / output interface 1306, a power source 1308, a memory 1310, a communication interface 1312, and / or any other component, or any combination thereof. Certain UEs may utilize all or a subset of the components shown in Figure 13. The level of integration between the components may vary from one UE to another UE. Further, certain UEs may contain multiple instances of a component, such as multiple processors, memories, transceivers, transmitters, receivers, etc. The processing circuitry 1302 is configured to process instructions and data and may be configured to implement any sequential state machine operative to execute instructions stored as machine-readable computer programs in the memory 1310. The processing circuitry 1302 may be implemented as one or more hardware-implemented state machines (e.g., in discrete logic, field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), etc.); programmable logic together with appropriate firmware; one or more stored computer programs, general-purpose processors, such as a microprocessor or digital signal processor (DSP), together with appropriate software; or any combination of the above. For example, the processing circuitry 1302 may include multiple central processing units (CPUs). In the example, the input / output interface 1306 may be configured to provide an interface or interfaces to an input device, output device, or one or more input and / or output devices. Examples of an output device include a speaker, a sound card, a video card, a display, a monitor, a printer, an actuator, an emitter, a smartcard, another output device, or any combination thereof. An input device may allow a user to capture information into the UE 1300. Examples of an input device include a touch-sensitive or presence-sensitive display, a camera (e.g., a digital camera, a digital video camera, a web camera, etc.), a microphone, a sensor, a mouse, a trackball, a directional pad, a trackpad, a scroll wheel, a smartcard, and the like. The presence-sensitive display may include a capacitive or resistive touch sensor to sense input from a user. A sensor may be, for instance, an accelerometer, a gyroscope, a tilt sensor, a force sensor, a magnetometer, an optical sensor, a proximity sensor, a biometric sensor, etc., or any combination thereof. An output device may use the same type of interface port as an input device. For example, a Universal Serial Bus (USB) port may be used to provide an input device and an output device. In some embodiments, the power source 1308 is structured as a battery or battery pack. Other types of power sources, such as an external power source (e.g., an electricity outlet), photovoltaic device, or power cell, may be used. The power source 1308 may further include power circuitry for delivering power from the power source 1308 itself, and / or an external power source, to the various parts of the UE 1300 via input circuitry or an interface such as an electrical power cable. Delivering power may be, for example, for charging of the power source 1308. Power circuitry may perform any formatting, converting, or other modification to the power from the power source 1308 to make the power suitable for the respective components of the UE 1300 to which power is supplied. The memory 1310 may be or be configured to include memory such as random access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), magnetic disks, optical disks, hard disks, removable cartridges, flash drives, and so forth. In one example, the memory 1310 includes one or more application programs 1314, such as an operating system, web browser application, a widget, gadget engine, or other application, and corresponding data 1316. The memory 1310 may store, for use by the UE 1300, any of a variety of various operating systems or combinations of operating systems. The memory 1310 may be configured to include a number of physical drive units, such as redundant array of independent disks (RAID), flash memory, USB flash drive, external hard disk drive, thumb drive, pen drive, key drive, high-density digital versatile disc (HD-DVD) optical disc drive, internal hard disk drive, Blu-Ray optical disc drive, holographic digital data storage (HDDS) optical disc drive, external mini-dual in-line memory module (DIMM), synchronous dynamic random access memory (SDRAM), external micro-DIMM SDRAM, smartcard memory such as tamper resistant module in the form of a universal integrated circuit card (UICC) including one or more subscriber identity modules (SIMs), such as a USIM and / or ISIM, other memory, or any combination thereof. The UICC may for example be an embedded UICC (eUICC), integrated UICC (iUICC) or a removable UICC commonly known as ‘SIM card.’ The memory 1310 may allow the UE 1300 to access instructions, application programs and the like, stored on transitory or non-transitory memory media, to off-load data, or to upload data. An article of manufacture, such as one utilizing a communication system may be tangibly embodied as or in the memory 1310, which may be or comprise a device-readable storage medium. The processing circuitry 1302 may be configured to communicate with an access network or other network using the communication interface 1312. The communication interface 1312 may comprise one or more communication subsystems and may include or be communicatively coupled to an antenna 1322. The communication interface 1312 may include one or more transceivers used to communicate, such as by communicating with one or more remote transceivers of another device capable of wireless communication (e.g., another UE or a network node in an access network). Each transceiver may include a transmitter 1318 and / or a receiver 1320 appropriate to provide network communications (e.g., optical, electrical, frequency allocations, and so forth). Moreover, the transmitter 1318 and receiver 1320 may be coupled to one or more antennas (e.g., antenna 1322) and may share circuit components, software or firmware, or alternatively be implemented separately. In the illustrated embodiment, communication functions of the communication interface 1312 may include cellular communication, Wi-Fi communication, LPWAN communication, data communication, voice communication, multimedia communication, short-range communications such as Bluetooth, near-field communication, location-based communication such as the use of the global positioning system (GPS) to determine a location, another like communication function, or any combination thereof. Communications may be implemented in according to one or more communication protocols and / or standards, such as IEEE 802.11, Code Division Multiplexing Access (CDMA), Wideband Code Division Multiple Access (WCDMA), GSM, LTE, New Radio (NR), UMTS, WiMax, Ethernet, transmission control protocol / internet protocol (TCP / IP), synchronous optical networking (SONET), Asynchronous Transfer Mode (ATM), QUIC, Hypertext Transfer Protocol (HTTP), and so forth. Regardless of the type of sensor, a UE may provide an output of data captured by its sensors, through its communication interface 1312, via a wireless connection to a network node. Data captured by sensors of a UE can be communicated through a wireless connection to a network node via another UE. The output may be periodic (e.g., once every 15 minutes if it reports the sensed temperature), random (e.g., to even out the load from reporting from several sensors), in response to a triggering event (e.g., when moisture is detected an alert is sent), in response to a request (e.g., a user initiated request), or a continuous stream (e.g., a live video feed of a patient). As another example, a UE comprises an actuator, a motor, or a switch, related to a communication interface configured to receive wireless input from a network node via a wireless connection. In response to the received wireless input the states of the actuator, the motor, or the switch may change. For example, the UE may comprise a motor that adjusts the control surfaces or rotors of a drone in flight according to the received input or to a robotic arm performing a medical procedure according to the received input. A UE, when in the form of an Internet of Things (IoT) device, may be a device for use in one or more application domains, these domains comprising, but not limited to, city wearable technology, extended industrial application and healthcare. Non-limiting examples of such an IoT device are a device which is or which is embedded in: a connected refrigerator or freezer, a TV, a connected lighting device, an electricity meter, a robot vacuum cleaner, a voice controlled smart speaker, a home security camera, a motion detector, a thermostat, a smoke detector, a door / window sensor, a flood / moisture sensor, an electrical door lock, a connected doorbell, an air conditioning system like a heat pump, an autonomous vehicle, a surveillance system, a weather monitoring device, a vehicle parking monitoring device, an electric vehicle charging station, a smart watch, a fitness tracker, a wearable for tactile augmentation or sensory enhancement, a water sprinkler, an animal- or item-tracking device, a sensor for monitoring a plant or animal, an industrial robot, an Unmanned Aerial Vehicle (UAV), and any kind of medical device, like a heart rate monitor or a remote controlled surgical robot. A UE in the form of an IoT device comprises circuitry and / or software in dependence of the intended application of the IoT device in addition to other components as described in relation to the UE 1300 shown in Figure 13. As yet another specific example, in an IoT scenario, a UE may represent a machine or other device that performs monitoring and / or measurements, and transmits the results of such monitoring and / or measurements to another UE and / or a network node. The UE may in this case be an M2M device, which may in a 3GPP context be referred to as an MTC device. As one particular example, the UE may implement the 3GPP NB-IoT standard. In other scenarios, a UE may represent a vehicle, such as a car, a bus, a truck, a ship and an airplane, or other equipment that is capable of monitoring and / or reporting on its operational status or other functions associated with its operation. In practice, any number of UEs may be used together with respect to a single use case. For example, a first UE might be or be integrated in a drone and provide the drone’s speed information (obtained through a speed sensor) to a second UE that is a remote controller operating the drone. When the user makes changes from the remote controller, the first UE may adjust the throttle on the drone (e.g. by controlling an actuator) to increase or decrease the drone’s speed. The first and / or the second UE can also include more than one of the functionalities described above. For example, a UE might comprise the sensor and the actuator, and handle communication of data for both the speed sensor and the actuators. Figure 14 shows a network node 1400 in accordance with some embodiments. As used herein, network node refers to equipment capable, configured, arranged and / or operable to communicate directly or indirectly with a UE and / or with other network nodes or equipment, in a telecommunication network. Examples of network nodes include, but are not limited to, access points (APs) (e.g., radio access points), base stations (BSs) (e.g., radio base stations, Node Bs, evolved Node Bs (eNBs) and NR NodeBs (gNBs)), O-RAN nodes or components of an O-RAN node (e.g., O-RU, O-DU, O-CU). Base stations may be categorized based on the amount of coverage they provide (or, stated differently, their transmit power level) and so, depending on the provided amount of coverage, may be referred to as femto base stations, pico base stations, micro base stations, or macro base stations. A base station may be a relay node or a relay donor node controlling a relay. A network node may also include one or more (or all) parts of a distributed radio base station such as centralized digital units, distributed units (e.g., in an O- RAN access node) and / or remote radio units (RRUs), sometimes referred to as Remote Radio Heads (RRHs). Such remote radio units may or may not be integrated with an antenna as an antenna integrated radio. Parts of a distributed radio base station may also be referred to as nodes in a distributed antenna system (DAS). Other examples of network nodes include multiple transmission point (multi-TRP) 5G access nodes, multi-standard radio (MSR) equipment such as MSR BSs, network controllers such as radio network controllers (RNCs) or base station controllers (BSCs), base transceiver stations (BTSs), transmission points, transmission nodes, multi-cell / multicast coordination entities (MCEs), Operation and Maintenance (O&M) nodes, Operations Support System (OSS) nodes, Self-Organizing Network (SON) nodes, positioning nodes (e.g., Evolved Serving Mobile Location Centers (E-SMLCs)), and / or Minimization of Drive Tests (MDTs). The network node 1400 includes a processing circuitry 1402, a memory 1404, a communication interface 1406, and a power source 1408. The network node 1400 may be composed of multiple physically separate components (e.g., a NodeB component and a RNC component, or a BTS component and a BSC component, etc.), which may each have their own respective components. In certain scenarios in which the network node 1400 comprises multiple separate components (e.g., BTS and BSC components), one or more of the separate components may be shared among several network nodes. For example, a single RNC may control multiple NodeBs. In such a scenario, each unique NodeB and RNC pair, may in some instances be considered a single separate network node. In some embodiments, the network node 1400 may be configured to support multiple radio access technologies (RATs). In such embodiments, some components may be duplicated (e.g., separate memory 1404 for different RATs) and some components may be reused (e.g., a same antenna 1410 may be shared by different RATs). The network node 1400 may also include multiple sets of the various illustrated components for different wireless technologies integrated into network node 1400, for example GSM, WCDMA, LTE, NR, WiFi, Zigbee, Z- wave, LoRaWAN, Radio Frequency Identification (RFID) or Bluetooth wireless technologies. These wireless technologies may be integrated into the same or different chip or set of chips and other components within network node 1400. The processing circuitry 1402 may comprise a combination of one or more of a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application-specific integrated circuit, field programmable gate array, or any other suitable computing device, resource, or combination of hardware, software and / or encoded logic operable to provide, either alone or in conjunction with other network node 1400 components, such as the memory 1404, to provide network node 1400 functionality. In some embodiments, the processing circuitry 1402 includes a system on a chip (SOC). In some embodiments, the processing circuitry 1402 includes one or more of radio frequency (RF) transceiver circuitry 1412 and baseband processing circuitry 1414. In some embodiments, the radio frequency (RF) transceiver circuitry 1412 and the baseband processing circuitry 1414 may be on separate chips (or sets of chips), boards, or units, such as radio units and digital units. In alternative embodiments, part or all of RF transceiver circuitry 1412 and baseband processing circuitry 1414 may be on the same chip or set of chips, boards, or units. The memory 1404 may comprise any form of volatile or non-volatile computer- readable memory including, without limitation, persistent storage, solid-state memory, remotely mounted memory, magnetic media, optical media, random access memory (RAM), read-only memory (ROM), mass storage media (for example, a hard disk), removable storage media (for example, a flash drive, a Compact Disk (CD) or a Digital Video Disk (DVD)), and / or any other volatile or non-volatile, non-transitory device-readable and / or computer-executable memory devices that store information, data, and / or instructions that may be used by the processing circuitry 1402. The memory 1404 may store any suitable instructions, data, or information, including a computer program, software, an application including one or more of logic, rules, code, tables, and / or other instructions capable of being executed by the processing circuitry 1402 and utilized by the network node 1400. The memory 1404 may be used to store any calculations made by the processing circuitry 1402 and / or any data received via the communication interface 1406. In some embodiments, the processing circuitry 1402 and memory 1404 is integrated. The communication interface 1406 is used in wired or wireless communication of signaling and / or data between a network node, access network, and / or UE. As illustrated, the communication interface 1406 comprises port(s) / terminal(s) 1416 to send and receive data, for example to and from a network over a wired connection. The communication interface 1406 also includes radio front-end circuitry 1418 that may be coupled to, or in certain embodiments a part of, the antenna 1410. Radio front-end circuitry 1418 comprises filters 1420 and amplifiers 1422. The radio front-end circuitry 1418 may be connected to an antenna 1410 and processing circuitry 1402. The radio front-end circuitry may be configured to condition signals communicated between antenna 1410 and processing circuitry 1402. The radio front-end circuitry 1418 may receive digital data that is to be sent out to other network nodes or UEs via a wireless connection. The radio front-end circuitry 1418 may convert the digital data into a radio signal having the appropriate channel and bandwidth parameters using a combination of filters 1420 and / or amplifiers 1422. The radio signal may then be transmitted via the antenna 1410. Similarly, when receiving data, the antenna 1410 may collect radio signals which are then converted into digital data by the radio front-end circuitry 1418. The digital data may be passed to the processing circuitry 1402. In other embodiments, the communication interface may comprise different components and / or different combinations of components. In certain alternative embodiments, the network node 1400 does not include separate radio front-end circuitry 1418, instead, the processing circuitry 1402 includes radio front-end circuitry and is connected to the antenna 1410. Similarly, in some embodiments, all or some of the RF transceiver circuitry 1412 is part of the communication interface 1406. In still other embodiments, the communication interface 1406 includes one or more ports or terminals 1416, the radio front-end circuitry 1418, and the RF transceiver circuitry 1412, as part of a radio unit (not shown), and the communication interface 1406 communicates with the baseband processing circuitry 1414, which is part of a digital unit (not shown). The antenna 1410 may include one or more antennas, or antenna arrays, configured to send and / or receive wireless signals. The antenna 1410 may be coupled to the radio front-end circuitry 1418 and may be any type of antenna capable of transmitting and receiving data and / or signals wirelessly. In certain embodiments, the antenna 1410 is separate from the network node 1400 and connectable to the network node 1400 through an interface or port. The antenna 1410, communication interface 1406, and / or the processing circuitry 1402 may be configured to perform any receiving operations and / or certain obtaining operations described herein as being performed by the network node. Any information, data and / or signals may be received from a UE, another network node and / or any other network equipment. Similarly, the antenna 1410, the communication interface 1406, and / or the processing circuitry 1402 may be configured to perform any transmitting operations described herein as being performed by the network node. Any information, data and / or signals may be transmitted to a UE, another network node and / or any other network equipment. The power source 1408 provides power to the various components of network node 1400 in a form suitable for the respective components (e.g., at a voltage and current level needed for each respective component). The power source 1408 may further comprise, or be coupled to, power management circuitry to supply the components of the network node 1400 with power for performing the functionality described herein. For example, the network node 1400 may be connectable to an external power source (e.g., the power grid, an electricity outlet) via an input circuitry or interface such as an electrical cable, whereby the external power source supplies power to power circuitry of the power source 1408. As a further example, the power source 1408 may comprise a source of power in the form of a battery or battery pack which is connected to, or integrated in, power circuitry. The battery may provide backup power should the external power source fail. Embodiments of the network node 1400 may include additional components beyond those shown in Figure 14 for providing certain aspects of the network node’s functionality, including any of the functionality described herein and / or any functionality necessary to support the subject matter described herein. For example, the network node 1400 may include user interface equipment to allow input of information into the network node 1400 and to allow output of information from the network node 1400. This may allow a user to perform diagnostic, maintenance, repair, and other administrative functions for the network node 1400. In some embodiments providing a core network node, such as core network node 108 of FIG.12, some components, such as the radio front-end circuitry 1418 and the RF transceiver circuitry 1412 may be omitted. Figure 15 is a block diagram illustrating a virtualization environment 1500 in which functions implemented by some embodiments may be virtualized. In the present context, virtualizing means creating virtual versions of apparatuses or devices which may include virtualizing hardware platforms, storage devices and networking resources. As used herein, virtualization can be applied to any device described herein, or components thereof, and relates to an implementation in which at least a portion of the functionality is implemented as one or more virtual components. Some or all of the functions described herein may be implemented as virtual components executed by one or more virtual machines (VMs) implemented in one or more virtual environments 1500 hosted by one or more of hardware nodes, such as a hardware computing device that operates as a network node, UE, core network node, or host. Further, in embodiments in which the virtual node does not require radio connectivity (e.g., a core network node or host), then the node may be entirely virtualized. In some embodiments, the virtualization environment 1500 includes components defined by the O-RAN Alliance, such as an O-Cloud environment orchestrated by a Service Management and Orchestration Framework via an O-2 interface. Virtualization may facilitate distributed implementations of a network node, UE, core network node, or host. Applications 1502 (which may alternatively be called software instances, virtual appliances, network functions, virtual nodes, virtual network functions, etc.) are run in the virtualization environment Q400 to implement some of the features, functions, and / or benefits of some of the embodiments disclosed herein. Hardware 1504 includes processing circuitry, memory that stores software and / or instructions executable by hardware processing circuitry, and / or other hardware devices as described herein, such as a network interface, input / output interface, and so forth. Software may be executed by the processing circuitry to instantiate one or more virtualization layers 1506 (also referred to as hypervisors or virtual machine monitors (VMMs)), provide VMs 1508a and 1508b (one or more of which may be generally referred to as VMs 1508), and / or perform any of the functions, features and / or benefits described in relation with some embodiments described herein. The virtualization layer 1506 may present a virtual operating platform that appears like networking hardware to the VMs 1508. The VMs 1508 comprise virtual processing, virtual memory, virtual networking or interface and virtual storage, and may be run by a corresponding virtualization layer 1506. Different embodiments of the instance of a virtual appliance 1502 may be implemented on one or more of VMs 1508, and the implementations may be made in different ways. Virtualization of the hardware is in some contexts referred to as network function virtualization (NFV). NFV may be used to consolidate many network equipment types onto industry standard high volume server hardware, physical switches, and physical storage, which can be located in data centers, and customer premise equipment. In the context of NFV, a VM 1508 may be a software implementation of a physical machine that runs programs as if they were executing on a physical, non-virtualized machine. Each of the VMs 1508, and that part of hardware 1504 that executes that VM, be it hardware dedicated to that VM and / or hardware shared by that VM with others of the VMs, forms separate virtual network elements. Still in the context of NFV, a virtual network function is responsible for handling specific network functions that run in one or more VMs 1508 on top of the hardware 1504 and corresponds to the application 1502. Hardware 1504 may be implemented in a standalone network node with generic or specific components. Hardware 1504 may implement some functions via virtualization. Alternatively, hardware 1504 may be part of a larger cluster of hardware (e.g. such as in a data center or CPE) where many hardware nodes work together and are managed via management and orchestration 1510, which, among others, oversees lifecycle management of applications 1502. In some embodiments, hardware 1504 is coupled to one or more radio units that each include one or more transmitters and one or more receivers that may be coupled to one or more antennas. Radio units may communicate directly with other hardware nodes via one or more appropriate network interfaces and may be used in combination with the virtual components to provide a virtual node with radio capabilities, such as a radio access node or a base station. In some embodiments, some signaling can be provided with the use of a control system 1512 which may alternatively be used for communication between hardware nodes and radio units. Although the computing devices described herein (e.g., UEs, network nodes) may include the illustrated combination of hardware components, other embodiments may comprise computing devices with different combinations of components. It is to be understood that these computing devices may comprise any suitable combination of hardware and / or software needed to perform the tasks, features, functions and methods disclosed herein. Determining, calculating, obtaining or similar operations described herein may be performed by processing circuitry, which may process information by, for example, converting the obtained information into other information, comparing the obtained information or converted information to information stored in the network node, and / or performing one or more operations based on the obtained information or converted information, and as a result of said processing making a determination. Moreover, while components are depicted as single boxes located within a larger box, or nested within multiple boxes, in practice, computing devices may comprise multiple different physical components that make up a single illustrated component, and functionality may be partitioned between separate components. For example, a communication interface may be configured to include any of the components described herein, and / or the functionality of the components may be partitioned between the processing circuitry and the communication interface. In another example, non-computationally intensive functions of any of such components may be implemented in software or firmware and computationally intensive functions may be implemented in hardware. In certain embodiments, some or all of the functionality described herein may be provided by processing circuitry executing instructions stored on in memory, which in certain embodiments may be a computer program product in the form of a non-transitory computer- readable storage medium. In alternative embodiments, some or all of the functionality may be provided by the processing circuitry without executing instructions stored on a separate or discrete device-readable storage medium, such as in a hard-wired manner. In any of those particular embodiments, whether executing instructions stored on a non-transitory computer- readable storage medium or not, the processing circuitry can be configured to perform the described functionality. The benefits provided by such functionality are not limited to the processing circuitry alone or to other components of the computing device, but are enjoyed by the computing device as a whole, and / or by end users and a wireless network generally.

Claims

CLAIMS 1. A method performed by a communication device (12) configured for use in a communication network (10), the method comprising: obtaining (800) a base precoding codebook (15B); receiving (810), from the communication network (10), control signaling (18) that indicates an inter-subarray spacing (17-SP) according to which adjacent subarrays (17S) of a transmit antenna array (17) at a radio network node (14) are spaced apart from one another, wherein each subarray (17S) comprises a set of adjacent antenna elements (15) that are driven by the same stream of baseband samples; deriving (820), from the base precoding codebook (15B), a precoding codebook (15D) tailored to the inter-subarray spacing (17-SP) indicated by the control signaling (18); determining (830), based on the derived precoding codebook (15D), channel state information (22) describing a state of a downlink channel between the communication device (12) and the radio network node (14); and reporting (840) the channel state information (22) to the communication network (10).

2. The method of claim 1, wherein the control signaling (18) indicates the inter-subarray spacing (17-SP) as: a multiple of an actual or nominal inter-element spacing (17E) according to which adjacent antenna elements (15) of the transmit antenna array (17) are actually or nominally spaced apart from one another; or a number of wavelengths between adjacent subarrays (17S).

3. The method of claim 1, wherein the control signaling (18) implicitly indicates the inter-subarray spacing (17-SP) by indicating, for each of one or more dimensions of the transmit antenna array (17): a size of each subarray (17S) of the transmit antenna array (17) in the dimension; and an inter-element spacing (17E) according to which adjacent antenna elements (15) of the transmit antenna array (17) are spaced apart from one another in the dimension.

4. The method of any of claims 1-3, wherein, for each of one or more dimensions of the transmit antenna array (17), the control signaling (18) indicates the inter-subarray spacing (17-SP) in that dimension.

5. The method of any of claims 1-4, wherein the base precoding codebook (15B) comprises candidate precoders that are each constructed from one or more sets of beamforming weights (28), and wherein said deriving comprises, for each set of beamforming weights (28), applying phase offsets (24) to at least some beamforming weights (28) in the set, wherein the phase offsets (24) vary linearly across the beamforming weights (28) to which the phase offsets (24) are applied and are a function of the intersubarray spacing (17-SP).

6. The method of claim 5, wherein applying the phase offsets (24) comprises: applying an aggregate phase correction set, via element-wise multiplication, to the set of beamforming weights (28), wherein the aggregate phase correction set comprises the phase offsets (24); or for each of one or more dimensions of the transmit antenna array (17), applying a dimension-specific phase correction set, via element-wise multiplication, to the set of beamforming weights (28), wherein the dimension-specific phase correction set comprises phase offsets (24) that are a function of the inter-subarray spacing (17-SP) in that dimension.

7. The method of any of claims 1-6, wherein all candidate precoders in the derived precoding codebook (15D) are a function of the inter-subarray spacing (17-SP) indicated by the control signaling (18).

8. The method of any of claims 1-7, wherein the base precoding codebook (15B) is non-oversampled.

9. The method of any of claims 1-8, wherein said determining comprises selecting, from among candidate precoders in the derived precoding codebook (15D), a candidate precoder maximizing a performance or channel quality metric for transmissions on the downlink channel to the communication device (12), wherein the channel state information (22) indicates the selected candidate precoder.

10. The method of any of claims 1-9, wherein the base precoding codebook (15B) is defined for an antenna array partitioned into subarrays (17S) that are spaced from oneanother according to a base inter-subarray spacing, wherein the base inter-subarray spacing differs from the inter-subarray spacing (17-SP) according to which adjacent subarrays (17S) of the transmit antenna array (17) at the radio network node (14) are spaced apart from one another.

11. The method of any of claims 1-10, wherein the control signaling (18) is dedicated Radio Resource Control, RRC, signaling or System Information broadcast using System Information Block, SIB, signaling.

12. The method of any of claims 1-11, wherein receiving the control signaling (18) comprises receiving the control signaling (18): upon establishment or resumption of a radio resource control, RRC, connection with the communication network (10); and / or once per RRC connection with the communication network (10).

13. The method of any of claims 1-12, wherein obtaining the base precoding codebook (15B) comprises receiving, from the communication network (10), signaling indicating the base precoding codebook (15B), wherein the signaling either explicitly or implicitly indicates the base precoding codebook (15B).

14. A method performed by a radio network node (14) configured for use in a communication network (10), the method comprising: transmitting (900), to a communication device (12), control signaling (18) that indicates an inter-subarray spacing (17-SP) according to which adjacent subarrays (17S) of a transmit antenna array (17) at the radio network node (14) are spaced apart from one another, wherein each subarray (17S) comprises a set of adjacent antenna elements (15) that are driven by the same stream of baseband samples; and receiving (910), from the communication device (12), channel state information (22) describing a state of a downlink channel between the communication device (12) and the radio network node (14), wherein the channel state information (22) is based on a precoding codebook (15D) derived, from a base precoding codebook (15B), to be tailored to the inter-subarray spacing (17-SP) indicated by the control signaling (18).

15. The method of claim 14, wherein the control signaling (18) indicates the inter-subarray spacing (17-SP) as:a multiple of an actual or nominal inter-element spacing (17E) according to which adjacent antenna elements (15) of the transmit antenna array (17) are actually or nominally spaced apart from one another; or a number of wavelengths between adjacent subarrays (17S).

16. The method of claim 15, wherein the control signaling (18) implicitly indicates the inter-subarray spacing (17-SP) by indicating, for each of one or more dimensions of the transmit antenna array (17): a size of each subarray (17S) of the transmit antenna array (17) in the dimension; and an inter-element spacing (17E) according to which adjacent antenna elements (15) of the transmit antenna array (17) are spaced apart from one another in the dimension.

17. The method of any of claims 14-16, wherein, for each of one or more dimensions of the transmit antenna array (17), the control signaling (18) indicates the inter-subarray spacing (17-SP) in that dimension.

18. The method of any of claims 14-17, wherein the base precoding codebook (15B) comprises candidate precoders that are each constructed from one or more sets of beamforming weights (28), and wherein the derived precoding codebook (15D) is derived by, for each set of beamforming weights (28), applying phase offsets (24) to at least some beamforming weights (28) in the set, wherein the phase offsets (24) vary linearly across the beamforming weights (28) to which the phase offsets (24) are applied and are a function of the inter-subarray spacing (17-SP).

19. The method of claim 18, wherein applying the phase offsets (24) comprises: applying an aggregate phase correction set, via element-wise multiplication, to the set of beamforming weights (28), wherein the aggregate phase correction set comprises the phase offsets (24); or for each of one or more dimensions of the transmit antenna array (17), applying a dimension-specific phase correction set, via element-wise multiplication, to the set of beamforming weights (28), wherein the dimension-specific phase correction set comprises phase offsets (24) that are a function of the inter-subarray spacing (17-SP) in that dimension.

20. The method of any of claims 14-19, wherein all candidate precoders in the derived precoding codebook (15D) are a function of the inter-subarray spacing (17-SP) indicated by the control signaling (18).

21. The method of any of claims 14-20, wherein the base precoding codebook (15B) is non-oversampled.

22. The method of any of claims 12-21, wherein the channel state information (22) indicates, from among candidate precoders in the derived precoding codebook (15D), a candidate precoder maximizing a performance or channel quality metric for transmissions on the downlink channel to the communication device (12).

23. The method of any of claims 14-22, wherein the base precoding codebook (15B) is defined for an antenna array partitioned into subarrays (17S) that are spaced from one another according to a base inter-subarray spacing, wherein the base inter-subarray spacing differs from the inter-subarray spacing (17-SP) according to which adjacent subarrays (17S) of the transmit antenna array (17) at the radio network node (14) are spaced apart from one another.

24. The method of any of claims 14-23, wherein the control signaling (18) is dedicated Radio Resource Control, RRC, signaling or System Information Broadcast, SIB, signaling.

25. The method of any of claims 14-24, wherein transmitting the control signaling (18) comprises transmitting the control signaling (18): upon establishment or resumption by the communication device (12) of a radio resource control, RRC, connection with the communication network (10); and / or once per RRC connection between the communication device (12) and the communication network (10).

26. The method of any of claims 14-25, further comprising transmitting (920), to the communication device (12), signaling indicating the base precoding codebook (15B), wherein the signaling either explicitly or implicitly indicates the base precoding codebook (15B).

27. A communication device (12) configured for use in a communication network (10), the communication device (12) configured to:obtain a base precoding codebook (15B); receive, from the communication network (10), control signaling (18) that indicates an inter-subarray spacing (17-SP) according to which adjacent subarrays (17S) of a transmit antenna array (17) at a radio network node (14) are spaced apart from one another, wherein each subarray (17S) comprises a set of adjacent antenna elements (15) that are driven by the same stream of baseband samples; derive, from the base precoding codebook (15B), a precoding codebook (15D) tailored to the inter-subarray spacing (17-SP) indicated by the control signaling (18); determine, based on the derived precoding codebook (15D), channel state information (22) describing a state of a downlink channel between the communication device (12) and the radio network node (14); and report the channel state information (22) to the communication network (10).

28. The communication device (12) of claim 27, configured to perform the method of any of claims 2-13.

29. A radio network node (14) configured for use in a communication network (10), the radio network node (14) configured to: transmit, to a communication device (12), control signaling (18) that indicates an inter-subarray spacing (17-SP) according to which adjacent subarrays (17S) of a transmit antenna array (17) at the radio network node (14) are spaced apart from one another, wherein each subarray (17S) comprises a set of adjacent antenna elements (15) that are driven by the same stream of baseband samples; and receive, from the communication device (12), channel state information (22) describing a state of a downlink channel between the communication device (12) and the radio network node (14), wherein the channel state information (22) is based on a precoding codebook (15D) derived, from a base precoding codebook (15B), to be tailored to the inter-subarray spacing (17-SP) indicated by the control signaling (18).

30. The radio network node (14) of claim 29, configured to perform the method of any of claims 15-26.

31. A computer program comprising instructions which, when executed by at least oneprocessor of a communication device (12), causes the communication device (12) to perform the method of any of claims 1-13.

32. A computer program comprising instructions which, when executed by at least one processor of a radio network node (14), causes the radio network node (14) to perform the method of any of claims 14-26.

33. A carrier containing the computer program of any of claims 31-32, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

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