Extension of beams selection

Abstract

A method comprising: at a first apparatus, select a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor for each dimension; select at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension; and transmit, to the second apparatus, channel state information indicating the first beam and the at least one second beam.

Description

EXTENSION OF BEAMS SELECTION CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of IN provisional application No. 202441011495, filed February 19, 2024, the content of which are hereby incorporated by reference in their entirety. FIELDS

[0002] Various example embodiments of the present disclosure generally relate to the field of telecommunication and in particular, to methods, devices, apparatuses and computer readablestorage medium for extension of beam selection.BACKGROUND

[0003] Multiple-Input-Multiple-Output (MIMO) refers to the type of wireless transmission and reception scheme where both a transmitter and a receiver employ more than one antenna. MIMO allows for spatial diversity to transmit data by use of a plurality of antennas in both uplink (UL) and downlink (DL) directions. Currently, new radio (NR) MIMO features are further enhanced. One of the objectives is to extend support for channel state information (CSI) reporting to a larger number of antenna ports than previously supported, up to 128 ports. SUMMARY

[0004] In a first aspect of the present disclosure, there is provided a first apparatus. The firstapparatus comprises at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first apparatus at least to: select a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor for each dimension; select at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension; and transmit, to the second apparatus, channel state information indicating the first beam and the at least one second beam.

[0005] In a second aspect of the present disclosure, there is provided a second apparatus. Thesecond apparatus comprises at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second apparatus at least to: receive, from a first apparatus, channel state information indicating a first beam and at least one secondbeam; determine, based on the channel state information, a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor in each dimension; and determine, based on the channel state information, at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in one dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension.

[0006] In a third aspect of the present disclosure, there is provided a method. The methodcomprises: selecting, at a first apparatus, a first beam from a codebook obtained by oversamplinga set of beams in two dimensions with an oversampling factor for each dimension; selecting at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension; and transmitting, to the second apparatus, channel state information indicating the first beam and the at least one second beam.

[0007] In a fourth aspect of the present disclosure, there is provided a method. The methodcomprises: receiving, at a second apparatus from a first apparatus, channel state information indicating a first beam and at least one second beam; determining, based on the channel state information, a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor in each dimension; and determining, based on the channel state information, at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in one dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension.

[0008] In a fifth aspect of the present disclosure, there is provided a first apparatus. The firstapparatus comprises means for selecting a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor for each dimension; means for selecting at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension; and means for transmitting, to the second apparatus, channel state information indicating the first beam and the at least one second beam.

[0009] In a sixth aspect of the present disclosure, there is provided a second apparatus. Thesecond apparatus comprises means for receiving, from a first apparatus, channel state information indicating a first beam and at least one second beam; means for determining, based on the channelstate information, a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor in each dimension; and means for determining, based on the channel state information, at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in one dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension.

[0010] In a seventh aspect of the present disclosure, there is provided a computer readable medium.The computer readable medium comprises instructions stored thereon for causing an apparatus to perform at least the method according to the third aspect.

[0011] In an eighth aspect of the present disclosure, there is provided a computer readable medium.The computer readable medium comprises instructions stored thereon for causing an apparatus to perform at least the method according to the fourth aspect.

[0012] It is to be understood that the Summary section is not intended to identify key or essentialfeatures of embodiments of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Some example embodiments will now be described with reference to the accompanying drawings, where:

[0014] FIG. 1 illustrates an example communication environment in which example embodimentsof the present disclosure can be implemented;

[0015] FIG.2 illustrates a schematic diagram of an example Grid of Beams (GoB);

[0016] FIG. 3 illustrates a schematic diagram of a maximal orthogonal set for a given beam in thecase of GoB(4,2,4,4);

[0017] FIG. 4 illustrates a schematic diagram of a set of beams orthogonal to a given beam in thecase of GoB(4,2,4,4);

[0018] FIG. 5 illustrates a signaling chart for CSI reporting according to some example embodiments of the present disclosure;

[0019] FIG. 6A illustrates a schematic diagram of a two-beam combination indication for Type-Icodebook Mode 1;

[0020] FIG. 6B illustrates a schematic diagram of a two-beam combination indication for Type-Icodebook Mode 2;

[0021] FIG.6C illustrates a schematic diagram of a two-beam combination indication for Type-IIregular codebooks;

[0022] FIG. 7 illustrates a schematic diagram of example beam orthogonality for 8 portconfiguration according to some example embodiments of the present disclosure;

[0023] FIG. 8 illustrates a schematic diagram of a new two-beam indication from an extended orthogonal set for Type-I codebook (CB) with Mode 1 according to some example embodiments of the present disclosure;

[0024] FIG.9 illustrates a schematic diagram of a new two-beam group indication from an extendedorthogonal set, for Type-I CB with Mode 2 according to some example embodiments of the present disclosure;

[0025] FIG.10 illustrates a schematic diagram of a new indication of a beam combination from an extended orthogonal set for Type-II CBs according to some example embodiments of the present disclosure;

[0026] FIG.11 illustrates a schematic diagram of an example of subset restriction applied to the extended orthogonal set according to some example embodiments of the present disclosure;

[0027] FIG. 12 illustrates a flowchart of a method implemented at a first device according to someexample embodiments of the present disclosure;

[0028] FIG. 13 illustrates a flowchart of a method implemented at a second device according tosome example embodiments of the present disclosure;

[0029] FIG. 14 illustrates a simplified block diagram of a device that is suitable for implementingexample embodiments of the present disclosure; and

[0030] FIG.15 illustrates a block diagram of an example computer readable medium in accordancewith some example embodiments of the present disclosure.

[0031] Throughout the drawings, the same or similar reference numerals represent the same or similar element. DETAILED DESCRIPTION

[0032] Principle of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. Embodiments described herein can be implemented in various manners other than the ones described below.

[0033] In the following description and claims, unless defined otherwise, all technical and scientificterms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

[0034] References in the present disclosure to “one embodiment,” “an embodiment,” “an example embodiment,” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0035] It shall be understood that although the terms “first,” “second,”…, etc. in front of noun(s)and the like may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another and they do not limit the order of the noun(s). For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and / or” includes any and all combinations of one or more of the listed terms.

[0036] As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements.

[0037] As used herein, unless stated explicitly, performing a step “in response to A” does not indicate that the step is performed immediately after “A” occurs and one or more intervening steps may be included.

[0038] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and / or “including”, when used herein, specify the presence of stated features, elements,and / or components etc., but do not preclude the presence or addition of one or more other features,elements, components and / or combinations thereof.

[0039] As used in this application, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry) and (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and / or digital hardware circuit(s) withsoftware / firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

[0040] This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

[0041] As used herein, the term “communication network” refers to a network following any suitable communication standards, such as New Radio (NR), Long Term Evolution (LTE), LTE-Advanced (LTE-A), Wideband Code Division Multiple Access (WCDMA), High-Speed Packet Access (HSPA), Narrow Band Internet of Things (NB-IoT) and so on. Furthermore, the communications between a terminal device and a network device in the communication network may be performed accordingto any suitable generation communication protocols, including, but not limited to, the firstgeneration (1G), the second generation (2G), 2.5G, 2.75G, the third generation (3G), the fourth generation (4G), 4.5G, the fifth generation (5G), the sixth generation (6G) communication protocols, and / or any other protocols either currently known or to be developed in the future. Embodiments of the present disclosure may be applied in various communication systems. Given the rapid development in communications, there will of course also be future type communication technologies and systems with which the present disclosure may be embodied. It should not be seen as limiting the scope of the present disclosure to only the aforementioned system.

[0042] As used herein, the term “network device” refers to a node in a communication network via which a terminal device accesses the network and receives services therefrom. The network device may refer to a base station (BS) or an access point (AP), for example, a node B (NodeB or NB), an evolved NodeB (eNodeB or eNB), an NR NB (also referred to as a gNB), a Remote Radio Unit (RRU), a radio header (RH), a remote radio head (RRH), a relay, an Integrated Access and Backhaul (IAB) node, a low power node such as a femto, a pico, a non-terrestrial network (NTN) or non-ground network device such as a satellite network device, a low earth orbit (LEO) satelliteand a geosynchronous earth orbit (GEO) satellite, an aircraft network device, and so forth, depending on the applied terminology and technology. In some example embodiments, radio access network (RAN) split architecture comprises a Centralized Unit (CU) and a Distributed Unit (DU) at an IAB donor node. An IAB node comprises a Mobile Terminal (IAB-MT) part that behaves like a UE toward the parent node, and a DU part of an IAB node behaves like a base station toward the next-hop IAB node.

[0043] The term “terminal device” refers to any end device that may be capable of wireless communication. By way of example rather than limitation, a terminal device may also be referred to as a communication device, user equipment (UE), a Subscriber Station (SS), a Portable Subscriber Station, a Mobile Station (MS), or an Access Terminal (AT). The terminal device may include, but not limited to, a mobile phone, a cellular phone, a smart phone, voice over IP (VoIP) phones, wireless local loop phones, a tablet, a wearable terminal device, a personal digital assistant (PDA), portable computers, desktop computer, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, vehicle- mounted wireless terminal devices, wireless endpoints, mobile stations, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), USB dongles, smart devices, wireless customer-premises equipment (CPE), an Internet of Things (IoT) device, a watch or other wearable, a head-mounted display (HMD), a vehicle, a drone, a medical device and applications (e.g., remote surgery), an industrial device and applications (e.g., a robot and / or other wireless devices operating in an industrial and / or an automated processing chain contexts), a consumer electronics device, a device operating on commercial and / or industrial wireless networks, and the like. The terminal device may also correspond to a Mobile Termination (MT) part of an IAB node (e.g., a relay node). In the following description, the terms “terminal device”, “communication device”, “terminal”, “user equipment” and “UE” may be used interchangeably.

[0044] As used herein, the term “resource,” “transmission resource,” “resource block,” “physical resource block” (PRB), “uplink resource,” or “downlink resource” may refer to any resource for performing a communication, for example, a communication between a terminal device and a network device, such as a resource in time domain, a resource in frequency domain, a resource in space domain, a resource in code domain, or any other combination of the time, frequency, space and / or code domain resource enabling a communication, and the like. In the following, unless explicitly stated, a resource in both frequency domain and time domain will be used as an example of a transmission resource for describing some example embodiments of the present disclosure. It is noted that example embodiments of the present disclosure are equally applicable to other resources in other domains.

[0045] As used herein, the term “beam” may refer to a spatial-domain basis vector, i.e. a vector from a codebook of vectors, for example associated with a precoder matrix.

[0046] As used herein, the term “beam cluster” or short as “cluster” may refer to a plurality of beams which are adjacent in a grid of beams. In other words, a beam in a beam cluster may have an index in a dimension adjacent to an index of another beam in the same beam cluster. For example, a beam cluster may correspond to a beam before oversampling.

[0047] FIG. 1 illustrates an example communication environment 100 in which example embodiments of the present disclosure may be implemented. As shown in FIG. 1, the communication network 100 may include a first apparatus 110 and a second apparatus 120. The first apparatus 110 may communicate with the second apparatus 120. The first apparatus 110 maymeasure a channel between the first apparatus 110 and the second apparatus 120, and report CSIto the second apparatus 120.

[0048] It is to be understood that the number of second apparatus and first apparatus shown inFIG. 1 is given for the purpose of illustration without suggesting any limitations. The communication network 100 may include any suitable number of second apparatus and first apparatus.

[0049] In some example embodiments, the first apparatus 110 may comprise a terminal device (forexample, a UE), and the second apparatus 120 may comprise a network device (for example, a gNB). In the following, some example embodiments may be described by taking the UE and gNB as an example.

[0050] In the following, for the purpose of illustration, some example embodiments are describedwith the first apparatus 110 operating as a terminal device and the second apparatus 120 operating as a network device. However, in some example embodiments, operations described in connection with a terminal device may be implemented at a network device or other device, and operations described in connection with a network device may be implemented at a terminal device or other device.

[0051] In some example embodiments, if the first apparatus 110 is a terminal device and the secondapparatus 120 is a network device, a link from the second apparatus 120 to the first apparatus 110 is referred to as a downlink (DL), and a link from the first apparatus 110 to the second apparatus 120 is referred to as an uplink (UL). In DL, the second apparatus 120 is a transmitting (TX) device (or a transmitter) and the first apparatus 110 is a receiving (RX) device (or a receiver). In UL, the first apparatus 110 is a TX device (or a transmitter) and the second apparatus 120 is a RX device (or a receiver).

[0052] Communications in the communication environment 100 may be implemented according toany proper communication protocol(s), comprising, but not limited to, cellular communication protocols of the first generation (1G), the second generation (2G), the third generation (3G), the fourth generation (4G), the fifth generation (5G), the sixth generation (6G), and the like, wireless local network communication protocols such as Institute for Electrical and Electronics Engineers (IEEE) 802.11 and the like, and / or any other protocols currently known or to be developed in the future. Moreover, the communication may utilize any proper wireless communication technology, comprising but not limited to: Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Frequency Division Duplex (FDD), Time Division Duplex (TDD), Multiple-Input Multiple-Output (MIMO), Orthogonal Frequency Division Multiple (OFDM), Discrete Fourier Transform spread OFDM (DFT-s-OFDM) and / or any other technologies currently known or to be developed in the future.

[0053] In Release (Rel)-19 NR, a new work item was approved, called NR MIMO Phase 5, toenhance NR MIMO features. One of the objectives is to extend support for CSI reporting to a larger number of antenna ports than previously supported, up to 128 ports. More precisely, the objective is to specify CSI support for up to 128 CSI-RS ports, targeting frequency range 1 (FR1).

[0054] Regarding Type-I codebook, an extension of legacy Discrete Fourier Transform (DFT)-based codebooks that includes codebook design enhancements is allowed. Conversely, theextension for Type-II codebook excludes codebook enhancements other than a simple parameter extension. An extension mechanism may be based on CRI (CSI-RS resource indicator), i.e., the possibility to select a subset of the configured ports and report a CSI on the selected port subset.

[0055] Type-I codebook was introduced early in Rel-15 of NR and has not seen enhancements in subsequent releases. Rel-19 NR offers the opportunity to introduce improvements in the codebook design, particularly in the beam selection, whilst extending support to larger antenna arrays with up to 128 CSI-RS ports.

[0056] Hereafter, some solutions are provided on massive MIMO in NR and the codebook-basedspatial beam selection mechanism used in precoding matrix indicator (PMI) calculation, which is part of CSI reporting by a UE to the gNB.

[0057] The current generation wireless systems are faced with the several challenges such as very high spectral efficiency, enhanced coverage and seamless connectivity. Advent of the massive MIMO technology has equipped the current generation wireless systems to meet these challenges by providing high multiplexing gains and efficient inter-user interference suppression.

[0058] Beamforming is the core part of the transmission / reception technique in MIMO systems, which harnesses performance benefits through large antenna arrays. Beamforming is a procedure where the signals at the antenna elements are multiplied with complex weights (beforetransmission / after reception) with an objective to optimize the desirable performance metrics suchas directivity, interference suppression, diversity gain and multiplexing gain. These performancemetrics, in turn, translate into system-level quality measures, such as long-distance coverage, highdata rates and system throughput.

[0059] In the downlink (DL) of cellular networks involving multi-rank transmissions (for example,multi-user-MIMO (MU-MIMO) or multi-layer in single user-MIMO (SU-MIMO)), the objective ofbeamforming at the base station (BS) is to achieve a high degree spatial multiplexing by exploiting the spatial dimension offered by wireless channels with rich scattering environment. To enable spatial multiplexing, data symbols intended towards multiple UEs (and multiple layers of a single UE) are weighted independently and combined through multiantenna such that they are simultaneously transmitted to multiple UEs present in different locations in a beam-formed manner, while, at the same time, ensuring the suppression of interference due to inter-user (inter-layer) data streams. This procedure is termed as transmitter precoding (specifically, linear precoding).

[0060] The choice of a suitable transmitter precoder matrix (TPM) depends on the downlink channel to the scheduled UEs. In practice, BS transmits the channel state information-reference signals (CSI-RS). Upon receiving the CSI-RS, UEs perform channel measurements and estimate the best TPM. This is fed back to the BS. Ideally, TPMs are obtained by the UEs as an analytical solution to the optimization problem of maximizing the UE throughput or an appropriate objective function. However, such a design procedure can be practically infeasible due to two factors: complexity considerations at the UE in real-time computation of TPMs, and large feedback overhead incurred in reporting the best TPM.

[0061] Due to the considerations mentioned above, a codebook-based approach is taken to determine and report TPM. In this approach, a predetermined set of different TPMs are collected to form a precoder codebook. This codebook is known a priori to UE and BS. Each TPM in the codebook is identified by a set of indices which uniquely maps to the corresponding TPM. The UE, upon receiving the CSI-RS, makes measurements and predicts the best TPM based on the channel, and then feeds back the corresponding index to the transmitter. This index correspondingto the best TPM is called the Precoder-Matrix Indicator (PMI) and PMI is one of the indicatorscontained in a CSI report. The size of the codebook determines the effectiveness of the precoder as well as the feedback overhead. The codebook-based design of TPMs primarily hinges on the concept of Grid of Beams (GoB) which is a collection of beams obtained by appropriately oversampling a set of 2D-DFT beams.

[0062] Before describing how the beam selection mechanism can be enhanced in oversampled DFT codebooks, in the following section, the construction and important properties of GoB arebriefly described. In the practical case where the BS is equipped with a uniform rectangular array(URA), GoB is constructed by oversampling a set of 2D-DFT beams with appropriately chosenoversampling factors both in the vertical plane and the horizontal plane.

[0063] As an example, a BS with URA of dimension ^^ × ^^ is considered, where ^^ denotesthe number of antennas in the horizontal plane, and ^^ denotes the number of antennas in thevertical plane. The URA is virtually mapped to an ^^ × ^^ dimensional array of virtual ports, where^^ denotes the number of ports in the horizontal plane (^^ ≤ ^^), and ^^ denotes the numberof ports in the vertical plane (^^ ≤ ^^). The total number of ports is given by ^^ = ^^^^.

[0064] Let ^^, ^^ be the oversampling factors in the horizontal and vertical planes respectively.Then, the GoB comprises a total of ^^ = ^^^^^^, beams with each beam being a ^^ lengthcomplex vector. That is, GoB can be represented as:

[0065] Now an example of GoB (4,2,4,4) is described. Let ^^ = 4, ^^ = 2, ^^ = ^^ = 4. Then,the GoB consists of ^^ = 128 beams of length 8. Such a GoB is diagrammatically illustrated inFIG.2.

[0066] In traditional beam selection scheme, a maximal orthogonal set of beams is employed. In^^^(^^, ^^, ^^, ^^), there are ^ = ^^^^ maximal orthogonal sets. Maximal orthogonal sets inan oversampled codebook are disjoint sets, i.e., a maximal orthogonal set is a set of orthogonal elements such that this set cannot be a subset of any other orthogonal set.

[0067] Each maximal orthogonal set consists of ^^ = ^^^^ beams. Each beam is a member ofone and only one maximal orthogonal set. The GoB is a disjoint union of all its constituent maximalorthogonal sets. For given beam, the corresponding maximal orthogonal set of beams is illustratedin FIG. 3.

[0068] PMI determination in 3GPP is as below. Consider a base station with ^^^^transmitantennas, let ^^ (^^ ≤ ^^) be the rank of the DL transmission, i.e., total number of layers to betransmitted to multiple UEs in the downlink. Therefore, the dimension of the precoder matrixemployed at the BS is ^^ × ^^.

[0069] A ^^−rank PMI is obtained by appropriately sampling ^^beams from GoB: ^= ^^^^ , ^^^ , … , ^^^^ ^ ; ^^^ ∈ ^^^.

[0070] In the 3GPP based approach, the following steps are taken. At step 1, the best beam isdetermined based on received power measurements. Let ^^^ ∈ ^^^ be the best beam. At step2, the maximal orthogonal set corresponding to ^^^ is found. Let ^^^^^^^ denote thecorresponding set. At step 3, the subsequent best orthogonal beams ^^^^ , … , ^^^^ ^ is searchedfrom

[0071] In general, the precoding matrix, ^, is expressed as a linear combination: ^ = ^^^^,where ^^ contains ^ orthogonal beams selected from the spatial-domain DFT codebook and ^^contains the combination coefficients. In Type-II codebooks, ^^compression is performed acrosssubbands in frequency such that ^^ additionally decomposed as ^^ = ^^^^^, where ^^contains ^ orthogonal frequency-domain basis vectors selected from a frequency-domain DFTcodebook, and the Type-II precoding matrix reads: ^ =

[0072] Note that in Rel-15 Type-I codebook, each ^^beam, also referred to as spatial-domain basis vector, selected from the DFT codebook supports up to two MIMO layers, thanks tocophasing across polarisations operated by ^^. For rank 1, ^^ = 1, a single DFT beam (^ = 1)is selected up to two ^^ orthogonal beams (^ = 2) are selected for rank 2 (^^ = 2),two ^^ orthogonal beams (^ = 2) are selected for ranks 3 and 4 (^^ = 3,4). Three orthogonalbeams (^ = 3) are selected for rank 5-6 and four beams are selected for rank 7-8.

[0073] In the present disclosure, the construction of PMI codebooks for transmit beamforming inthe downlink (DL) for MIMO cellular network is considered. The current PMI codebook designapproach restricts the search for optimal PMI from predefined beam sets, where each beam set isrestricted to contain as many mutually orthogonal beams as the dimension of the CSI-RS ports ofa single polarization, i.e., ^^ × ^^. Given that so many mutually orthogonal beams are not needed,the current selection mechanism fails to efficiently exploit the numerous beam selection options available in the oversampled Grid of Beams (OS-GoB), that is an oversampled 2D-DFT codebook. Work Principle and Example Signaling for Communication

[0074] According to some example embodiments of the present disclosure, there is provided a solution for selecting orthogonal beams from an oversampled codebook, for example, an oversampled DFT codebook. In this solution, candidate beams available for selection are extended. An oversampled codebook is obtained by oversampling a set of beams in two dimensions with an oversampling factor for each dimension. The two dimensions may be the azimuth and elevationdimensions. After a first beam is selected from the oversampled codebook, at least one secondbeam is selected from a set of orthogonal beams of the first beam. At least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension. In other words, atleast one orthogonal beam in the set may be a beam orthogonal to the first beam and is not a member of a maximal orthogonal set for the first beam.

[0075] As such, candidate beams are not restricted to the maximal orthogonal set and extended to other orthogonal beams. In the present disclosure, this set of orthogonal beams is also referred toas extended orthogonal set of beams. Analysis and simulations show that the new beam selectionmechanism for PMI calculation yields substantial gains over the legacy solutions with very modest increase in feedback overhead.

[0076] Before describing the example embodiments, the core concept of the present disclosure is first described. The main idea behind the present disclosure is the design of an improved method to determine the PMI, and the beam selection in ^^, in particular, by allowing beam search from larger sets of potential candidate beams. It is noted that typically for DL or UL receive beamforming,the maximal orthogonal set of beams may not be needed. In particular, in DL, with Type-I codebook,only two orthogonal beams are needed to support up to rank 4. Note that rank 1 to 4 are the most common ranks used in CSI reporting because of the limited number of receive antennas that a handheld device can accommodate.

[0077] For Rel. 16 eType-II codebook, L=2, 4 or 6 orthogonal beams are needed to support ranks1 to 4, which are used as the basis for generating the precoding vectors as linear combinations of^ codebook beams. The typical number of CSI-RS ports used for massive MIMO systems is atleast 8, and 16 or 32 ports are widely deployed in NR systems. This trend of increasing the numberof CSI-RS ports even beyond 32 is likely to remain in the future as transmit antenna arrays growlarger in number of antennas and with Rel. 19 MIMO introducing CSI support for up to 128 ports.Thus, a lot more than the maximal set of orthogonal beams is available, hence the search pool for two orthogonal beams can be extended significantly.

[0078] A PMI construction method which efficiently utilizes the structure of the oversampled GoBis proposed to improve the performance. To better understand example embodiments describedbelow, some observations on the structure of GoB that is key to the idea of the present disclosureare presented now.

[0079] For any given beam ^ ∈ ^^^(^^, ^^, ^^, ^^), the set of beams orthogonal to ^ is given by^(^) = {^ ∈ ^^^: ^^^ = 0}.

[0080] It is observed that, for any given beam ^ ∈ ^^^(^^, ^^, ^^, ^^), there are ^^ beamswhich are orthogonal to ^, where

[0081] Beams orthogonal to a given beam ^^ for GoB(4,2,4,4) are illustrated in FIG. 4. Note that^^ = 37. In other words, FIG. 4 shows the extended orthogonal set for the beam ^^.

[0082] Example embodiments of the present disclosure will be described in detail below.

[0083] FIG. 5 illustrates a signaling chart 500 for CSI reporting according to some exampleembodiments of the present disclosure. The signaling chart 500 involves the first apparatus 110and the second apparatus 120 in FIG. 1. For purpose of illustration, the signaling chart 500 will bedescribed with respect to FIG. 1. For purpose of discussion, some example embodiments aredescribed by taking UE as an example of the first apparatus 110 and gNB as an example of thesecond apparatus 120.

[0084] In operation, the first apparatus 110 may select (505) a first beam from a codebook obtainedby oversampling a set of beams in two dimensions with an oversampling factor for each dimension.The oversampled codebook may have a first oversampling factor (which may be represented byin a first dimension and a second oversampling factor (which may be represented by ^^) in asecond dimension.

[0085] Once the first beam is selected, the first apparatus 110 may select (510) at least one second beam from a set of orthogonal beams of the first beam. At least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam whichis not zero or multiple of the oversampling factor for the dimension. In other words, the at leastone second beam may be selected from the extended orthogonal set of beams for the first beam. Depending on the codebook type and mode, different selection scheme for the second beam may be applied. Such example embodiments will be described in detail below.

[0086] Then, the first apparatus 110 may transmit (515), to the second apparatus 120, CSI indicating the first beam and the at least one second beam, which are collectively referred to asselected beams. That is, the CSI report may include information about the first beam and the atleast one second beam. For example, PMI in the CSI report may indicate the first beam and the at least one second beam.

[0087] Correspondingly, the second apparatus 120 may receive (520) the CSI from the firstapparatus 110. The second apparatus 120 may determine (525) the first beam based on thereceived CSI. Once the first beam is determined, the second apparatus 120 may determine (530),based on the channel state information, at least one second beam from the extended orthogonalset of beams for the first beam.

[0088] In the present disclosure, the candidate beams for the second beam are extended ascompared to legacy beam selection. Given that, the signaling overhead for indicating the selectedbeams shall be considered and handled so as to use as less overhead as possible. Some aspectsregarding the signaling overhead are now described.

[0089] It is observed that after picking the first best beam, the search space for at least one secondbeam is restricted to a subset of ^^ − 1 beams in the GoB. On the other hand, it is known fromthe above descriptions that, once the best beam is picked, there are ^^ ≥ ^^ − 1 beams fromwhich the second-best beam can be picked. Taking the GoB (4,2,4,4) as an example, using thelegacy approach, the second-best beam can be selected from a set limited to only 7 beams, forexample those beams in the maximal orthogonal set as shown in FIG. 3. By contrast, in theexample embodiments of the present disclosure, there are actually ^^ = 37 candidate beams forthe second-best beam, as shown in FIG. 4.

[0090] Table 1 shows the number of second orthogonal beams available according to the exampleembodiments, assuming oversampling factors ^^ = ^^ = 4, when compared to legacy, where thechoice is restricted to the maximal orthogonal set. For 2-3 extra bits, there is significant increase in the number of beams, and these beams may be quite adjacent to the first chosen beam. From field results and from 3GPP design, it is known that the adjacent multipaths are expected to be the stronger ones. Table 1 # second # second orthogonal orthogonal beams beams # bits needed # bits needed Extra bits# ports ^^ ^^(legacy) (enhanced) (legacy) (enhanced) needed8 2 2 3 15 2 4 28 4 1 3 12 2 4 216 4 2 7 37 3 6 316 8 1 7 28 3 5 232 4 4 15 87 4 7 332 8 2 15 81 4 7 364 8 4 31 187 5 8 364 16 2 31 169 5 8 3128 8 8 63 399 6 9 3128 16 4 63 387 6 9 3

[0091] In practice, in legacy codebook-based CSI reporting, the choice of orthogonal beams andtheir indication is done in two different ways for Type-I and Type-II codebooks. In Type-I codebook,two indicators, ^^,^and ^^,^, indicate the first beam (for codebook Mode 1) or the first beam cluster (for codebook Mode 2). A second beam (for Mode 1) or beam cluster (for Mode 2) is selected forrank 2, 3 or 4 and reported by the indicator ^^,^. The selection of the second beam or second beamcluster is done in a restricted subset of the same maximal orthogonal set of size ^^^^, such thatthe bit width of the indicator ^^,^ is, at most, 2 bits, indicating a pair of offset values (^^, ^^). Inthis way, the second beam or beam cluster is identified by the indices ^^,^ + ^^ and ^^,^ + ^^. Incase of beam clusters, the actual beams are selected from the clusters per subband such that orthogonality is preserved.

[0092] FIG.6A illustrates an example of an existing indication for a 2-beam selection in the caseof Type-I codebook Mode 1. Beam ‘a’ is selected for layer 1 and beam ‘a’, ‘b’, ‘c’, or ‘d’ is selectedfor layer 2. In the example, ^^,^ indicates the offset pair (^^, ^^) = (2,0), such that the selectedsecond beam is beam ‘d’. It is noted that the selection of the second beam is restricted to a subsetof the maximal orthogonal set of the first beam of size 4 < ^^^^ = 8, which includes the firstselected beam. This restriction is done to reduce the bit width for the second beam indication tojust 2 bits. Note also that the second beam may be the same as the first beam.

[0093] FIG.6B illustrates an example of an existing indication for a 2-beam-cluster combination with Type-I codebook Mode 2. Cluster ‘a’ is selected at wideband for layer 1 and cluster ‘a’, ‘b’, ‘c’,or ‘d’ is selected for layer 2. In the example, ^^,^ indicates the offset pair (^^, ^^) = (2,0), suchthat the selected second beam cluster is cluster ‘d’.

[0094] According to legacy beam selection, in Type-II regular codebooks, ^ orthogonal beams areselected from the same maximal orthogonal set identified by the indicator ^^,^ = [^^ ^^] of bitwidth⌈log^(^^^^)⌉. The selected beam combination is indicated by a combinatorial indicator, ^^,^, of bitwidth^log^Such an example is illustrated in FIG. 6C, where ^ = 2 as an example.

[0095] In the following, some example embodiments of the present disclosure are described.Example embodiments propose an enhanced selection of the spatial-domain basis vectors, i.e., spatial beams from an oversampled DFT codebook, to form thecomponent of a Type-I or Type- II PMI.

[0096] In general, let ^^ be strongest (first selected) beam and ^(^^) be the set of codebookbeams orthogonal to ^^ . For any two given beams ^^, ^^ , let ^(^^, ^^) be the set of beamsorthogonal to both ^^ and ^^ . It can be noted that ^(^^, ^^) = ^(^^)⋂^(^^) = {^ ∈

[0097] Similarly, for any given set of codebook beams ^^, ^^, … , ^^ , let ^(^^, ^^, … , ^^) be theset of beams orthogonal to the set {^^, ^^, … , ^^}. Then,

[0098] FIG. 7 illustrates a table showing the orthogonality of PMI beams for 8 port split-configuration (^^, ^^) = (2,2) and oversampling factor (^^, ^^) = (4,4). In FIG. 7, entries withthe number “1” show that a beam in row is orthogonal to a beam in column and the rest are non-orthogonal. This table may be used to calculate the orthogonal set ^(^) for each beam ^ that isreferred for precoder determination for the present disclosure.

[0099] Now some examples of how the selection of ^ orthogonal beams from a non-maximalorthogonal set can be signalled are provided. Some desirable objectives in the signalling designare to ensure that the indication has the same bit width size, independently of the selectedcombination, otherwise new indicators would be needed in Part 1 CSI to determine the payloadsize of Part 2 CSI; to reduce the feedback overhead as much as possible; scalability to ^ > 2beams; to reuse legacy indicators for the beam selection as much as possible.

[0100] In general, it is observed that, in the extended orthogonal set, only one beam can beselected in each cluster of ^^^^beams, as illustrated in FIGS.6A and 6C, for Type-I and Type-IICBs, respectively. Hence, the legacy beam indicators can be reused to identify the ^^ × ^^clusters where the selected beams are found. A new indicator can be introduced to identify, foreach of the other ^ − 1 beams selected after a first beam, the position within the respective^^ × ^^ cluster. The allowed beams within a ^^ × ^^ cluster are determined by the intersectionof the cluster with the extended orthogonal set associated with the first beam. As shown in FIG.4,there are at most (^^ + ^^ − 1) candidate beams that can be selected in any of the ^^ × ^^beam clusters of size ^^ × ^^ . In some example embodiments, these eligible beams can befurther restricted in a technical specification and / or by a RRC configuration from the network, forexample, to reduce the bitwidth of the new indicator.

[0101] For Type-I codebook, the first beam indicator, (^^,^, ^^,^), can still be used to locate theposition of the first beam. Examples of Type-I codebook enhancements and relative signalling aredescribed in more details in the following with reference to FIGS. 8 and 9.

[0102] For Type-II codebooks, the indicatorscan also be reused. For example, theindicator ^^,^ contains the indices (^^, ^^) which, in legacy codebooks, identify a maximalorthogonal set of size ^^ × ^^ . The indicator ^^,^ is a combinatorial indicator of bitwidth^log^that can be reused to indicate the ^ clusters of size ^^ × ^^ where the ^ selectedbeams are located. In legacy beam selection, the intersection between these ^ clusters and themaximal orthogonal set, indicated by ^^,^, identify the ^ selected beams. In the enhanced spatial-domain basis vector selection according to embodiments of the present disclosure, the indices(^^, ^^), indicated by ^^,^, identify the location of a “reference” beam. The reference beam maycorrespond to the first beam in a predetermined order, e.g., in order of increasing codebook index,the strongest beam, or be identified in some other manner. For this beam, an additional indicatoris not needed, similarly to the first beam in Type-I, because its position is found by the intersectionof the maximal orthogonal set and one of the ^ clusters indicated by the combinatorial indicator,^^,^. The cluster corresponding to the reference beam may be referred to as reference cluster. Anew indicator can be introduced to locate the remaining ^ − 1 selected beams within each of theremaining ^ − 1 selected cluster of size ^^ × ^^. Because the combinatorial indicator indicatesa combination of ^ clusters without an order, the reference cluster needs to be identified. Thereference cluster could be specified, for example, as the “first” cluster, for example, associatedwith the smallest of the ^ indices. Alternatively, the reference cluster may be indicated by a log^ ^bit indicator or identified by the strongest coefficient indicator, ^^,^,^of a predefined layer, for example the first layer. An example of Type-II codebook enhancements and relative signalling isdescribed in more details in the following with reference to FIG. 10.

[0103] Some example embodiments regarding beam selection extension and reporting for Type-I codebook with mode 1 are now described with reference to FIG.8. In some example embodiments,the at least one second beam may be selected from at least two orthogonal beams of the firstbeam, and the at least two orthogonal beams are comprised in a same beam cluster in thecodebook. For example, as shown in FIG.8, the first beam is selected as beam “a” and the secondbeam may be selected from beams in the beam cluster 801 orthogonal to the beam “a”. In thisexample, there are 4 candidate beams for selecting as the second beam. In the following, the firstbeam is also referred to as beam 0 and the second beam is referred to as beam 1.

[0104] In some example embodiments, a size of the beam cluster in a first dimension is equal to afirst oversampling factor in the first dimension, and a size of the beam cluster in a seconddimension is equal to a second oversampling factor in the second dimension. In other words, thebeam cluster may have a size of ^^ × ^^. For example, the beam cluster 801 has a size of 4 × 4.

[0105] Regarding reporting of the selected beams, CSI report, for example PMI, may comprise: afirst indicator and a second indicator for identifying the first beam, a third indicator for identifying a beam cluster in the codebook comprising a second beam, and a fourth indicator for identifying alocation of the second beam in the identified beam cluster. For example, the legacy indicators(^^,^, ^^,^) may be used as the first and second indicators for identifying the first beam. The thirdindicator ^^,^and the fourth indicator ^^,^may be used in combination to identify the second beam.

[0106] In some example embodiments, a first index of a second beam is determined by applying afirst offset to a first index of the first beam, and the first offset is identified based on the thirdindicator and the fourth indicator. An example is shown in Table 4 and will be described below.

[0107] FIG. 8 illustrates an example of the new extended two-beam indication for Type-I CB withMode 1. The legacy indicators (^^,^, is used to identify beam 0 and ^^,^ is used to identify thecluster of a size ^^ × ^^ for beam 1. In the example, the indicator ^^,^ = 3 and it is mapped tothe pair (^^, ^^) = (2^^, 0), following legacy Type-I specifications. A new two-bit indicator, ^^,^is used to locate the beam 1 as one of the four possible beams of the extended orthogonal setfound within the respective cluster of the size ^^ × ^^.

[0108] As an example of possible index mapping, when ^^ = ^^ = 4, the value set of ^^,^ can bedefined as {0,1,2,3} and ^^,^ is mapped to an offset pair (^^,^, ^^,^). The bitwidth of the newindicator, in this case is log^(max(^^, ^^)). The mapping can be defined such that, if ^^ ≠ 0and ^^ = 0, then ^^,^ = 0 and ^^,^ = ^^,^ − (^^,^^^^ ^^). If ^^ = 0 and ^^ ≠ 0, thenbeam 1 is the same as beam 0, thenthe two-bit indicator ^^.^ is reported but not used. Beam 1 from theextended orthogonal set is identified by the pair of indices+ ^^ + ^^,^, ^^,^ + ^^ + ^^,^). Inthe example, ^^,^) = (7,2), (^^, ^^) = (2^^, 0) and ^^,^ = 3. Hence, ^^,^ is mapped to theoffset pair ^^,^^ = (0,1), such that beam 1 is identified by (15,3).

[0109] As an example, how the second beam extension of the example of FIG.8 can be specifiedby reusing the existing description of beam selection for Rel-15 Type-I, Mode 1 is now described.The current specifications define a mapping table between the indicator ^^,^ and the azimuth andelevation indices of the second beam. The second beam is selected from the maximal orthogonalset to which the first selected beam, identified by ^^,^ and ^^,^, belongs. However, to reduce thebit-width of ^^,^ to a maximum of 2 bits, only four of the N1N2 beams in the maximal orthogonal setcan be selected when N1N2>4. Table 2. Legacy mapping of ^^,^to ^^and ^^for 2-layer CSI reporting ^^ > ^^ > 1 ^^ = ^ ^ = 2, ^ = 1 ^ > 2, ^ = 1^^ ^ ^ ^ ^^,^^^^^^^^^^^^^^^^^0 0 0 0 0 0 0 0 01 ^^ 0 ^^ 0 ^^ 0 ^^ 02 0 ^^ 0 ^^ 2^^ 03 2^^ 0 ^^ ^^ 3^^ 0Table 3. Codebook for 2-layer CSI reporting using antenna ports 3000 to 2999+PCSI-RScodebookMode = 1^^,^^^,^ ^^0,1, … , ^ ^ − 1 0, … , ^ ^ − 1 ^(^) ^^ ^ ^ 0,1 ^^,^,^^,^^^^,^^,^,^^,^^^^,^^^ ^^,^^^^,^^where ^ (^) ^,^^,^,^^,^ =^^^CSI-RS^ ^^^^,^−^^^^^,^^^. and the mapping from ^^,^to ^^and ^^is given in Table 5.2.2.2.1-3.

[0110] In the extended orthogonal set for the second beam selection, the new indicator ^^,^ ∈{0,1,2,3}, is introduced and is used to shift the azimuth index, ^^, or the elevation index, ^^ withina ^^ × ^^ cluster, as illustrated in FIG. 8. Table 4 shows the new mapping table for the indices^^and ^^identifying the second beam. The same codebook definition as legacy, shown in Table3 can be used with the new mapping.Table 4. New mapping of ^^,^ and ^^,^ ∈ {0,1,2,3} to to ^^ and ^^ for 2-layer CSI reporting^^^^^^^ > ^^ > 1 ^^ = ^^ = 2, ^^ > 2, ^^^,^= 1 = 1^^^^^^^^^^^^^^^^0 0 0 0 0 0 0 0 01 ^^ ^^^^^,^, ^^,^^ ^^ ^^^^^,^, ^^,^^ ^^ 0 ^^ 02 ^^,^ ^^ ^^^^^,^, ^^,^^ ^^ 2^^ 03 2^^ ^^^^^,^, ^^,^^^^+ ^ ^ 3^ 0 ^^^ , ^ ^ ^ ^^,^ ^,^where

[0111] As can be seen from Table 4, the offset ^^or ^^may be determined based on the indicators^^,^ and ^^,^. In some example embodiments, the offset ^^ may be determined further based onthe indicator ^^,^, and the offset ^^may be determined further based on the indicator ^^,^.

[0112] Some example embodiments regarding beam selection extension and reporting for Type-I codebook with mode 2 are now described with reference to FIG.9. In some example embodiments,the first beam may be comprised in a first beam group (which is denoted as beam group 0), and asecond beam may be comprised in a second beam group (which is denoted as beam group 1).The second beam group is selected from at least two beam groups in a beam cluster, and each beam in the at least two beam groups is orthogonal to a corresponding one in the first beam group.For example, as shown in FIG.9, the beam group “a” is selected as the first beam group, and the second beam group may be selected from the beam cluster 901, specifically, from the beam group “d” and the beam group below the beam group “d”.

[0113] Similarly to described above, in some example embodiments, a size of the beam cluster ina first dimension is equal to a first oversampling factor in the first dimension, and a size of the beam cluster in a second dimension is equal to a second oversampling factor in the seconddimension. In other words, the beam cluster may have a size of ^^ × ^^. For example, the beamcluster 801 has a size of 4 × 4.

[0114] Regarding reporting the selected beam, in some example embodiments, the channel stateinformation, such as PMI, may comprise: a first indicator and a second indicator for identifying thefirst beam group, and a third indicator for identifying a beam cluster comprising the second beam group, and a fourth indicator for identifying a location of the second beam group in the identifiedbeam cluster. For example, the legacy indicators (^^,^,may be used as the first and secondindicators for identifying the first beam group. The third indicator ^^,^and the fourth indicator ^^,^may be used in combination to identify the second beam group.

[0115] In some example embodiments, a second index of the second beam group is determined byapplying a second offset to a second index of the first beam group, and the second offset isidentified based on the third indicator and the fourth indicator. This is similar to the determinationof the offset in the case of Type-I codebook with mode 1.

[0116] illustrates an example of the new extended two-beam group indication for Type-I codebookwith Mode 2. The legacy indicators (^^,^, ^^,^) are reused to indicate the 2x2 wideband beam groupfor beam 0 and the indicator ^^,^ is used to identify the ^^ × ^^ cluster where the 2x2 widebandbeam group for beam 1 is located. In the example, the indicator ^^,^ = 2 and it is mapped to thepair (^^, ^^) = (2^^, 0), following legacy Type-I specifications. A new one-bit indicator ^^,^ isused to locate the 2x2 wideband beam group for beam 1 as one of two beam groups of theextended orthogonal set found within the respective ^^ × ^^ cluster.^

[0117] As an example of possible index mapping, when^^^^ = ^= 2, the value set of ^^,^ can bedefined as {0,1} and mapped to an offset pair (^^,^, ^^,^). The bitwidth of the new indicator, inthis case is logThe mapping can be defined such that, if ^^ ≠ 0 and ^^ = 0,then ^^,^ = ^^,^for beam 1 is thesame as that for beam 0, then ^^^,^, ^^,^^ = (0,0) and the one-bit index ^^.^ is reported but notused. The 2x2 wideband beam group for beam 1 from the extended orthogonal set is identified bythe pair of indices (2^^,^ + ^^ + 2^^,^, 2^^,^ + ^^ + 2^^,^). In the example,= (3,1),(^^, ^^) = (2^^, 0) and ^^,^ = 0. Hence, ^^,^ is mapped to the offset pair ^^^,^, ^^,^^ = (0, −1),such that the 2x2 wideband beam group for beam 1 is identified by (14,0).

[0118] Some example embodiments regarding beam selection extension and reporting for Type-II codebook with are now described with reference to FIG.10. In some example embodiments, thechannel state information, such as PMI, may comprise: a combinatorial indicator for identifying atleast two beam clusters. Each beam of the first beam and the at least one second beam iscomprised in a beam cluster of the at least two beam clusters. In other words, the combinatorialindicator is used to indicate the beam clusters including the selected beams. For example, in theexample of FIG.10, the combinatorial indicator may be used to identify the beam cluster 1001 and the beam cluster 1002.

[0119] In some example embodiments, the at least two beam clusters comprise a first beam clusterand at least one second beam cluster. The channel state information further comprises: an indicator for identifying a reference beam in the codebook. A location of the first beam in the firstbeam cluster is indicated by a location of the reference beam in a reference beam cluster. Thechannel state information further comprises an indicator for identifying respective location of the at least one second beam in the at least one second beam cluster.

[0120] illustrates an example of the new extended ^-beam indication for Type-II codebooks. Thelegacy indicators ^^,^ = [^^ ^^] and ^^,^ may be used to indicate the maximal orthogonal set (i.e.,the position of the reference beam inside the reference cluster of size ^^ × ^^) and the ^ selectedclusters, respectively. In this example, the reference cluster is the first, i.e. the cluster of lowestindex, with the ^-th cluster linear index given by: ^(^) =arethe horizontal and vertical coordinates,∈ {0, … , ^^ − 1}, ^(^) ^∈ {0, … , ^^ − 1} and ^ =0, … , ^ − 1. For each of the ^ − 1 beams other than the first beam, a new index, ^^,^ , may beintroduced with ^ = 1, … ^ − 1, of bitwidth ⌈log^(^^ + ^^ − 1)⌉, which determines the positionof beam ^ within the respective ^^ × ^^ cluster. Note that the possible positions are, at most,^^ + ^^ − 1 to ensure orthogonality with the previously selected beams.

[0121] The new indices ^^,^ can be reported by a new indicatorwithbitwidth (^ − 1)⌈log^(^^ + ^^ − 1)⌉. For example, for ^^ = ^^ = 4, and ^ = 2, at most 3additional bits are needed, whereas for ^ = 4, at most 9 additional bits are needed.

[0122] To further reduce the reporting overhead, in some example embodiments, the set oforthogonal beams for selecting the second beam may be restricted to a subset of beamsorthogonal to the first beams. A subset restriction mechanism applied to the extended orthogonalset can be used to reduce the bitwidth of the additional indicator as in the example of FIG.11. As shown, only the shaded beams can be selected by a UE and thus a single bit is used for each ^^,^.

[0123] The subset restriction may be predefined (for example, in a technical specification), or configured by the network. In some example embodiments, the subset restriction may beconfigured by the network. For example, the first apparatus 110 may receive, from the secondapparatus 120, a configuration indicating a restriction rule for determining the subset. Then, thesecond apparatus 120 may determine the subset for selection based on the configuration.

[0124] In some example embodiments described above, a beam cluster may be considered forbeam selection. In these embodiments, the number of beam clusters may equal to the number of channel state information-reference signal (CSI-RS) ports per polarization measured for the CSI.

[0125] FIG. 12 shows a flowchart of an example method 1200 implemented at a first device inaccordance with some example embodiments of the present disclosure. For the purpose ofdiscussion, the method 1200 will be described from the perspective of the first apparatus 110 inFIG.1.

[0126] At block 1210, the first apparatus selects a first beam from a codebook obtained byoversampling a set of beams in two dimensions with an oversampling factor for each dimension.

[0127] At block 1220, the first apparatus selects at least one second beam from a set of orthogonalbeams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension.

[0128] At block 1230, the first apparatus transmits, to the second apparatus, channel stateinformation indicating the first beam and the at least one second beam.

[0129] In some example embodiments, the channel state information comprises: a first indicatorand a second indicator for identifying the first beam, a third indicator for identifying a beam cluster in the codebook comprising a second beam, and a fourth indicator for identifying a location of the second beam in the identified beam cluster.

[0130] In some example embodiments, a first index of a second beam is determined by applying afirst offset to a first index of the first beam, and the first offset is identified based on the third indicator and the fourth indicator.

[0131] In some example embodiments, the at least one second beam is selected from at least twoorthogonal beams of the first beam, and the at least two orthogonal beams are comprised in a same beam cluster in the codebook.

[0132] In some example embodiments, a size of the beam cluster in a first dimension is equal to afirst oversampling factor in the first dimension, and a size of the beam cluster in a second dimension is equal to a second oversampling factor in the second dimension.

[0133] In some example embodiments, the first beam is comprised in a first beam group, and asecond beam is comprised in a second beam group, and the channel state information comprises: a first indicator and a second indicator for identifying the first beam group, and a third indicator for identifying a beam cluster comprising the second beam group, and a fourth indicator for identifying a location of the second beam group in the identified beam cluster.

[0134] In some example embodiments, a second index of the second beam group is determined byapplying a second offset to a second index of the first beam group, and the second offset is identified based on the third indicator and the fourth indicator.

[0135] In some example embodiments, the first beam is comprised in a first beam group, and asecond beam is comprised in a second beam group, and the second beam group is selected from at least two beam groups in a beam cluster, and each beam in the at least two beam groups is orthogonal to a corresponding one in the first beam group.

[0136] In some example embodiments, a size of the beam cluster in a first dimension is equal to afirst oversampling factor in the first dimension, and a size of the beam cluster in a second dimension is equal to a second oversampling factor in the second dimension.

[0137] In some example embodiments, the channel state information comprises: a combinatorialindicator for identifying at least two beam clusters, wherein each beam of the first beam and the at least one second beam is comprised in a beam cluster of the at least two beam clusters.

[0138] In some example embodiments, the at least two beam clusters comprise a first beam clusterand at least one second beam cluster, and the channel state information further comprises: an indicator for identifying a reference beam in the codebook, a location of the first beam in the first beam cluster be indicated by a location of the reference beam in a reference beam cluster, and an indicator for identifying respective location of the at least one second beam in the at least one second beam cluster.

[0139] In some example embodiments, the number of beam clusters equals to the number ofchannel state information-reference signal (CSI-RS) ports per polarization measured for the CSI.

[0140] In some example embodiments, the set of orthogonal beams are a subset of beamsorthogonal to the first beams.

[0141] In some example embodiments, the method 1200 further comprises: receiving, from thesecond apparatus, a configuration indicating a restriction rule for determining the subset.

[0142] FIG. 13 shows a flowchart of an example method 1300 implemented at a second device inaccordance with some example embodiments of the present disclosure. For the purpose of discussion, the method 1300 will be described from the perspective of the second apparatus 120 in FIG.1.

[0143] At block 1310, the second apparatus receives, from a first apparatus, channel stateinformation indicating a first beam and at least one second beam.

[0144] At block 1320, the second apparatus determines, based on the channel state information, a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor in each dimension.

[0145] At block 1330, the second apparatus determines, based on the channel state information, at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in one dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension.

[0146] In some example embodiments, the channel state information comprises: a first indicatorand a second indicator for identifying the first beam, a third indicator for identifying a beam cluster in the codebook comprising the second beam, and a fourth indicator for identifying a location of the second beam in the identified beam cluster.

[0147] In some example embodiments, a first index of a second beam is determined by applying afirst offset to a first index of the first beam, and the first offset is determined based on the third indicator and the fourth indicator.

[0148] In some example embodiments, the first beam is comprised in a first beam group, and asecond beam is comprised in a second beam group, and the channel state information comprises: a first indicator and a second indicator for identifying the first beam group, and a third indicator for identifying a beam cluster comprising the second beam group, and a fourth indicator for identifying a location of the second beam group in the identified beam cluster.

[0149] In some example embodiments, a second index of the second beam group is determined byapplying a second offset to a second index of the first beam group, and the second offset is determined based on the third indicator and the fourth indicator.

[0150] In some example embodiments, the channel state information comprises: a combinatorialindicator for identifying at least two beam clusters, wherein each beam of the first beam and the at least one second beam is comprised in a beam cluster of the at least two beam clusters.

[0151] In some example embodiments, the at least two beam clusters comprises a first beamcluster and at least one second beam cluster, and the channel state information further comprises: an indicator for identifying a reference beam in the codebook, a location of the first beam in the first beam cluster be indicated by a location of the reference beam in a reference beam cluster,and an indicator for identifying respective location of the at least one second beam in the at least one second beam cluster.

[0152] In some example embodiments, the number of beam clusters in the codebook equals to thenumber of channel state information-reference signal (CSI-RS) ports per polarization measured for the CSI.

[0153] In some example embodiments, the set of orthogonal beams are a subset of beamsorthogonal to the first beams.

[0154] In some example embodiments, the method 1300 further comprises: transmitting, to the firstapparatus, a configuration indicating a restriction rule for determining the subset.

[0155] In some example embodiments, a first apparatus capable of performing any of the method1200 (for example, the first apparatus 110 in FIG. 1) may comprise means for performing therespective operations of the method 1200. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module. The first apparatusmay be implemented as or included in the first apparatus 110 in FIG. 1.

[0156] In some example embodiments, the first apparatus comprises means for selecting a firstbeam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor for each dimension; means for selecting at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension; and means for transmitting, to the second apparatus, channel state information indicating the first beam and the at least one second beam.

[0157] In some example embodiments, the channel state information comprises: a first indicatorand a second indicator for identifying the first beam, a third indicator for identifying a beam clusterin the codebook comprising a second beam, and a fourth indicator for identifying a location of thesecond beam in the identified beam cluster.

[0158] In some example embodiments, a first index of a second beam is determined by applying afirst offset to a first index of the first beam, and the first offset is identified based on the third indicator and the fourth indicator.

[0159] In some example embodiments, the at least one second beam is selected from at least twoorthogonal beams of the first beam, and the at least two orthogonal beams are comprised in a same beam cluster in the codebook.

[0160] In some example embodiments, a size of the beam cluster in a first dimension is equal to afirst oversampling factor in the first dimension, and a size of the beam cluster in a second dimension is equal to a second oversampling factor in the second dimension.

[0161] In some example embodiments, the first beam is comprised in a first beam group, and asecond beam is comprised in a second beam group, and the channel state information comprises: a first indicator and a second indicator for identifying the first beam group, and a third indicator for identifying a beam cluster comprising the second beam group, and a fourth indicator for identifying a location of the second beam group in the identified beam cluster.

[0162] In some example embodiments, a second index of the second beam group is determined byapplying a second offset to a second index of the first beam group, and the second offset is identified based on the third indicator and the fourth indicator.

[0163] In some example embodiments, the first beam is comprised in a first beam group, and asecond beam is comprised in a second beam group, and the second beam group is selected from at least two beam groups in a beam cluster, and each beam in the at least two beam groups is orthogonal to a corresponding one in the first beam group.

[0164] In some example embodiments, a size of the beam cluster in a first dimension is equal to afirst oversampling factor in the first dimension, and a size of the beam cluster in a second dimension is equal to a second oversampling factor in the second dimension.

[0165] In some example embodiments, the channel state information comprises: a combinatorialindicator for identifying at least two beam clusters, wherein each beam of the first beam and the at least one second beam is comprised in a beam cluster of the at least two beam clusters.

[0166] In some example embodiments, the at least two beam clusters comprise a first beam clusterand at least one second beam cluster, and the channel state information further comprises: an indicator for identifying a reference beam in the codebook, a location of the first beam in the first beam cluster be indicated by a location of the reference beam in a reference beam cluster, and an indicator for identifying respective location of the at least one second beam in the at least one second beam cluster.

[0167] In some example embodiments, the number of beam clusters equals to the number ofchannel state information-reference signal (CSI-RS) ports per polarization measured for the CSI.

[0168] In some example embodiments, the set of orthogonal beams are a subset of beamsorthogonal to the first beams.

[0169] In some example embodiments, the first apparatus further comprises: means for receiving,from the second apparatus, a configuration indicating a restriction rule for determining the subset.

[0170] In some example embodiments, the first apparatus further comprises means for performingother operations in some example embodiments of the method 1200 or the first apparatus 110. In some example embodiments, the means comprises at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause theperformance of the first apparatus.

[0171] In some example embodiments, a second apparatus capable of performing any of themethod 1300 (for example, the second apparatus 120 in FIG. 1) may comprise means forperforming the respective operations of the method 1300. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.The second apparatus may be implemented as or included in the second apparatus 120 in FIG. 1.

[0172] In some example embodiments, the second apparatus comprises means for receiving, froma first apparatus, channel state information indicating a first beam and at least one second beam; means for determining, based on the channel state information, a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor in each dimension; and means for determining, based on the channel state information, at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in one dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension.

[0173] In some example embodiments, the channel state information comprises: a first indicatorand a second indicator for identifying the first beam, a third indicator for identifying a beam cluster in the codebook comprising the second beam, and a fourth indicator for identifying a location of the second beam in the identified beam cluster.

[0174] In some example embodiments, a first index of a second beam is determined by applying afirst offset to a first index of the first beam, and the first offset is determined based on the third indicator and the fourth indicator.

[0175] In some example embodiments, the first beam is comprised in a first beam group, and asecond beam is comprised in a second beam group, and the channel state information comprises: a first indicator and a second indicator for identifying the first beam group, and a third indicator for identifying a beam cluster comprising the second beam group, and a fourth indicator for identifying a location of the second beam group in the identified beam cluster.

[0176] In some example embodiments, a second index of the second beam group is determined byapplying a second offset to a second index of the first beam group, and the second offset is determined based on the third indicator and the fourth indicator.

[0177] In some example embodiments, the channel state information comprises: a combinatorialindicator for identifying at least two beam clusters, wherein each beam of the first beam and the at least one second beam is comprised in a beam cluster of the at least two beam clusters.

[0178] In some example embodiments, the at least two beam clusters comprises a first beamcluster and at least one second beam cluster, and the channel state information further comprises:an indicator for identifying a reference beam in the codebook, a location of the first beam in the first beam cluster be indicated by a location of the reference beam in a reference beam cluster, and an indicator for identifying respective location of the at least one second beam in the at least one second beam cluster.

[0179] In some example embodiments, the number of beam clusters in the codebook equals to thenumber of channel state information-reference signal (CSI-RS) ports per polarization measured for the CSI.

[0180] In some example embodiments, the set of orthogonal beams are a subset of beamsorthogonal to the first beams.

[0181] In some example embodiments, the second apparatus further comprises: means fortransmitting, to the first apparatus, a configuration indicating a restriction rule for determining the subset.

[0182] In some example embodiments, the second apparatus further comprises means forperforming other operations in some example embodiments of the method 1300 or the second apparatus 120. In some example embodiments, the means comprises at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the performance of the second apparatus.

[0183] FIG. 14 is a simplified block diagram of a device 1400 that is suitable for implementingexample embodiments of the present disclosure. The device 1400 may be provided to implementa communication device, for example, the first apparatus 110 or the second apparatus 120 asshown in FIG.1. As shown, the device 1400 includes one or more processors 1410, one or more memories 1420 coupled to the processor 1410, and one or more communication modules 1440 coupled to the processor 1410.

[0184] The communication module 1440 is for bidirectional communications. The communication module 1440 has one or more communication interfaces to facilitate communication with one or more other modules or devices. The communication interfaces may represent any interface that is necessary for communication with other network elements. In some example embodiments, the communication module 1440 may include at least one antenna.

[0185] The processor 1410 may be of any type suitable to the local technical network and may include one or more of the following: general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 1400 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

[0186] The memory 1420 may include one or more non-volatile memories and one or more volatile memories. Examples of the non-volatile memories include, but are not limited to, a Read Only Memory (ROM) 1424, an electrically programmable read only memory (EPROM), a flash memory, a hard disk, a compact disc (CD), a digital video disk (DVD), an optical disk, a laser disk, and other magnetic storage and / or optical storage. Examples of the volatile memories include, but are not limited to, a random access memory (RAM) 1422 and other volatile memories that will not last in the power-down duration.

[0187] A computer program 1430 includes computer executable instructions that are executed by the associated processor 1410. The instructions of the program 1430 may include instructions for performing operations / acts of some example embodiments of the present disclosure. The program 1430 may be stored in the memory, e.g., the ROM 1424. The processor 1410 may perform any suitable actions and processing by loading the program 1430 into the RAM 1422.

[0188] The example embodiments of the present disclosure may be implemented by means of the program 1430 so that the device 1400 may perform any process of the disclosure as discussedwith reference to FIG. 6 to FIG. 13. The example embodiments of the present disclosure may alsobe implemented by hardware or by a combination of software and hardware.

[0189] In some example embodiments, the program 1430 may be tangibly contained in a computerreadable medium which may be included in the device 1400 (such as in the memory 1420) or other storage devices that are accessible by the device 1400. The device 1400 may load the program 1430 from the computer readable medium to the RAM 1422 for execution. In some example embodiments, the computer readable medium may include any types of non-transitory storage medium, such as ROM, EPROM, a flash memory, a hard disk, CD, DVD, and the like. The term“non-transitory,” as used herein, is a limitation of the medium itself (i.e., tangible, not a signal) asopposed to a limitation on data storage persistency (e.g., RAM vs. ROM).

[0190] FIG. 15 shows an example of the computer readable medium 1500 which may be in form ofCD, DVD or other optical storage disk. The computer readable medium 1500 has the program 1430 stored thereon.

[0191] Generally, various embodiments of the present disclosure may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, and other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device. Although various aspects of embodiments of the present disclosure are illustrated and described as block diagrams, flowcharts, or using some other pictorial representations, it is to be understood that the block, apparatus, system, technique or method described herein may be implemented in, as non-limitingexamples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0192] Some example embodiments of the present disclosure also provide at least one computer program product tangibly stored on a computer readable medium, such as a non-transitory computer readable medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target physical or virtual processor, to carry out any of the methods as described above. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

[0193] Program code for carrying out methods of the present disclosure may be written in any combination of one or more programming languages. The program code may be provided to a processor or controller of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program code, when executed by the processor or controller, cause the functions / operations specified in the flowcharts and / or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

[0194] In the context of the present disclosure, the computer program code or related data may becarried by any suitable carrier to enable the device, apparatus or processor to perform various processes and operations as described above. Examples of the carrier include a signal, computer readable medium, and the like.

[0195] The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable medium may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the computer readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

[0196] Further, although operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, although several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Unless explicitly stated, certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, unless explicitly stated, various features that are described in the context of a single embodiment may also be implemented in a plurality of embodiments separately or in any suitable sub-combination.

[0197] Although the present disclosure has been described in languages specific to structural features and / or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

WE CLAIM:

1. A first apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first apparatus at least to: select a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor for each dimension; select at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension; and transmit, to the second apparatus, channel state information indicating the first beam and the at least one second beam.

2. The first apparatus of claim 1, wherein the channel state information comprises: a first indicator and a second indicator for identifying the first beam, a third indicator for identifying a beam cluster in the codebook comprising a second beam, and a fourth indicator for identifying a location of the second beam in the identified beam cluster.

3. The first apparatus of claim 2, wherein a first index of a second beam is determined by applying a first offset to a first index of the first beam, and the first offset is identified based on the third indicator and the fourth indicator.

4. The first apparatus of claim 1, wherein the at least one second beam is selected from at least two orthogonal beams of the first beam, and the at least two orthogonal beams are comprised in a same beam cluster in the codebook.

5. The first apparatus of claim 4, wherein a size of the beam cluster in a first dimension is equal to a first oversampling factor in the first dimension, and a size of the beam cluster in a second dimension is equal to a second oversampling factor in the second dimension.

6. The first apparatus of claim 1, wherein the first beam is comprised in a first beam group, and a second beam is comprised in a second beam group, andthe channel state information comprises: a first indicator and a second indicator for identifying the first beam group, and a third indicator for identifying a beam cluster comprising the second beam group, and a fourth indicator for identifying a location of the second beam group in the identified beam cluster.

7. The first apparatus of claim 6, wherein a second index of the second beam group is determined by applying a second offset to a second index of the first beam group, and the second offset is identified based on the third indicator and the fourth indicator.

8. The first apparatus of claim 1, wherein the first beam is comprised in a first beam group, and a second beam is comprised in a second beam group, and the second beam group is selected from at least two beam groups in a beam cluster, and each beam in the at least two beam groups is orthogonal to a corresponding one in the first beam group.

9. The first apparatus of claim 1, wherein a size of the beam cluster in a first dimension is equal to a first oversampling factor in the first dimension, and a size of the beam cluster in a second dimension is equal to a second oversampling factor in the second dimension.

10. The first apparatus of claim 1, wherein the channel state information comprises: a combinatorial indicator for identifying at least two beam clusters, wherein each beam of the first beam and the at least one second beam is comprised in a beam cluster of the at least two beam clusters.

11. The first apparatus of claim 10, wherein the at least two beam clusters comprise a first beam cluster and at least one second beam cluster, and the channel state information further comprises: an indicator for identifying a reference beam in the codebook, a location of the first beam in the first beam cluster be indicated by a location of the reference beam in a reference beam cluster, and an indicator for identifying respective location of the at least one second beam in the at least one second beam cluster.

12. The first apparatus of any of claims 4, 6 or 8, wherein the number of beam clusters equals to the number of channel state information-reference signal (CSI-RS) ports per polarization measured for the CSI.

13. The first apparatus of claim 1, wherein the set of orthogonal beams are a subset of beamsorthogonal to the first beams.

14. The first apparatus of claim 13, wherein the first apparatus is further caused to: receive, from the second apparatus, a configuration indicating a restriction rule for determining the subset.

15. A second apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second apparatus at least to: receive, from a first apparatus, channel state information indicating a first beam and at least one second beam; determine, based on the channel state information, a first beam from a codebook obtained by oversampling a set of beams in two dimensions with an oversampling factor in each dimension; and determine, based on the channel state information, at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in one dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension.

16. The second apparatus of claim 15, wherein the channel state information comprises: a first indicator and a second indicator for identifying the first beam, a third indicator for identifying a beam cluster in the codebook comprising the second beam, and a fourth indicator for identifying a location of the second beam in the identified beam cluster.

17. The second apparatus of claim 16, wherein a first index of a second beam is determined byapplying a first offset to a first index of the first beam, and the first offset is determined based on the third indicator and the fourth indicator.

18. The second apparatus of claim 15, wherein the first beam is comprised in a first beam group, and a second beam is comprised in a second beam group, and the channel state information comprises: a first indicator and a second indicator for identifying the first beam group, and a third indicator for identifying a beam cluster comprising the second beam group, anda fourth indicator for identifying a location of the second beam group in the identified beam cluster.

19. The second apparatus of claim 18, wherein a second index of the second beam group is determined by applying a second offset to a second index of the first beam group, and the second offset is determined based on the third indicator and the fourth indicator.

20. The second apparatus of claim 15, wherein the channel state information comprises: a combinatorial indicator for identifying at least two beam clusters, wherein each beam of the first beam and the at least one second beam is comprised in a beam cluster of the at least two beam clusters.

21. The second apparatus of claim 20, wherein the at least two beam clusters comprises a first beam cluster and at least one second beam cluster, and the channel state information further comprises: an indicator for identifying a reference beam in the codebook, a location of the first beam in the first beam cluster be indicated by a location of the reference beam in a reference beam cluster, and an indicator for identifying respective location of the at least one second beam in the at least one second beam cluster.

22. The second apparatus of claim 15, wherein the number of beam clusters in the codebook equals to the number of channel state information-reference signal (CSI-RS) ports per polarization measured for the CSI.

23. The second apparatus of claim 15, wherein the set of orthogonal beams are a subset of beams orthogonal to the first beams.

24. The second apparatus of claim 23, wherein the second apparatus is further caused to: transmit, to the first apparatus, a configuration indicating a restriction rule for determining the subset.

25. A method comprising: selecting, at a first apparatus, a first beam from a codebook obtained by oversampling a set ofbeams in two dimensions with an oversampling factor for each dimension; selecting at least one second beam from a set of orthogonal beams of the first beam, at least one orthogonal beam of the set of orthogonal beams has an index in a dimension with an offset to an index of the first beam which is not zero or multiple of the oversampling factor for the dimension; andtransmitting, to the second apparatus, channel state information indicating the first beam and the at least one second beam.

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