Method and network node for precoding of sets of subcarriers to be transmitted on radio branches

Abstract

There is provided techniques for precoding of subcarriers to be transmitted on radio branches of at least one transmission point. A method is performed by a network node. The method comprises precoding a first set of the subcarriers for transmission via the radio branches. Less than all transmission power, as available for the first set of the subcarriers according to a power headroom per each of the radio branches, is allocated to the first set of the subcarriers on at least one of the radio branches. The method comprises precoding a second set of the subcarriers for transmission via the radio branches. The second set of the subcarriers is associated with the same at least one time resource as the first set of the subcarriers. Remaining transmission power on each of the radio branches from the precoding of the first set of the subcarriers is, per radio branch, reallocated to the second set of the subcarriers in accordance with the power headroom.

Description

[0001] P111640W001 1

[0002] PRECODING OF SUBCARRIERS

[0003] TECHNICAL FIELD

[0004] Embodiments presented herein relate to a method, a network node, a computer program, and a computer program product for precoding of subcarriers to be transmitted on radio branches of at least one transmission point.

[0005] BACKGROUND

[0006] In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.

[0007] For example, output power is a key resource in cellular networks, where more power typically improves the network performance in terms of coverage, capacity, and user throughput. On the other hand, more power also means products with increased size, weight, cost and energy consumption. Hence, deciding the output power of radio equipment is a non-trivial exercise, and it is vital to be able to efficiently use all available power to reach the full performance potential of the radio equipment.

[0008] Multi-antenna technologies, such as beamforming, null-forming, and spatial multiplexing, are fundamental in most modern communication systems. These technologies make use of spatial and temporal properties of the radio channel to enhance the performance. This is achieved by a process referred to as precoding, in which the transmitted signals are mapped to the different radio branches of an antenna array. In some communication systems, such as orthogonal frequency -division multiplexing (OFDM) based communication systems, the phases and amplitudes of the signals can be adjusted individually for each radio branch and OFDM subcarrier to achieve constructive and / or destructive addition of signals in different directions.

[0009] A multi-antenna technology capable radio comprises several radio branches, also referred to as radio branches, or just branches for short. At transmission, a radio branch converts a low-power baseband signal to a high-power radio frequency (RF) signal that can be transmitted over the air. A radio branch contains various radio processing, such as digital signal processing (e.g. crest factor reduction and digital pre-distortion), digital-to-analog conversion, power amplification and RF fdtering.

[0010] The physical layer of OFDM based communication systems (e.g., fourth, fifth, and (likely) sixth generation telecommunication systems) uses the so-called time-frequency resource grid for layer 1 resource management. Different channels, or signals, can be time and / or frequency multiplexed onto the resource grid. One example of time-frequency resource grid 100 is illustrated in Fig. 1, where each subcarrier in each OFDM symbol, referred to as a resource element (RE), carries a modulated symbol containing information bits. P111640W001 2

[0011] For multi-antenna OFDM transmissions, precoding is used to generate signals for the different radio branches. In principle, each RE can be multiplied by an individual complex weight vector, where each weight vector element corresponds to one radio branch. Also, spatial multiplexing offers a third dimension to the resource grid, where different data streams can be multiplexed onto the same REs and transmitted using different weight vectors. An example of this is illustrated in Fig. 2. In more detail, in Fig. 2 is illustrated a mapping 200 of frequency-layers to the frequency-space resource grid. This is for one specific time instance, i.e. time is omitted from the time -frequency -space resource grid. Fig. 2 thus shows how different layers, or streams, for different subcarriers are mapped to different radio branches via a precoding matrix, or vector, Wk. The illustration is for one time instance. The precoding described here is often referred to as frequency -domain precoding (or frequency -domain beamforming) in which the precoding is done in the frequency domain and can be done per (groups of) REs independently. There exist different beamforming architectures, and two commonly used beamforming architecture types are frequency -domain beamforming and time-domain beamforming, where the latter is performed in timedomain after application of an inverse Fast Fourier Transformation to the signal at the transmitter side.

[0012] Different (groups) of REs can be assigned to different users and / or physical channels, and therefore different REs may use different precoding. Also, different REs assigned to the same user equipment (UE), or physical channel, can use different precoding (cf., frequency-selective precoding). In short, different REs may use different precoding.

[0013] According to the above, precoding implies that the amplitudes and phases of the streams, or layers, are adjusted individually for each radio branch (and RE-group) to achieve a given objective of the multiantenna algorithm(s). As first example, for single user (SU) single layer transmission, the objective of the precoding is typically to maximize the received signal power. As a second example, for SU multiple-input multiple-output (MIMO) communication, or spatial multiplexing, the objective of the precoding is typically to not only maximize the received signal powers of the multiple layers, but also to reduce interference between the layers. However, the latter aspect (i.e., inter-layer interference consideration) is often less important compared to maximizing the signal power as the receivers are equipped with multiple receive antennas that can be used to mitigate the inter-layer interference. As a third example, for multiple user (MU) MIMO, or spatial multiplexing, the objective of the precoding can be the same as for SU- MIMO, but typically, mitigating the interference between the layers is more important, as the receivers do not have enough receive antennas to mitigate interference from all layers and / or are unaware of the interfering layers. This means that the precoding uses so-called null-forming to mitigate the interference.

[0014] In any case, it could be desirable to use as much as possible of the available transmission power.

[0015] Hence, there is still a need for improved power management in the context of precoding. P111640W001 3

[0016] SUMMARY

[0017] An object of embodiments herein is to address the above shortcomings with respect to power underutilization.

[0018] For many techniques, such as reciprocity -based SU-MIMO and / or MU-MIMO transmission, precoding will lead to different amplitude values for weights associated with different radio branches (and layers and REs). This in turn leads to different transmit powers for the different antennas.

[0019] It is also possible to construct precoding weights with the same amplitude, and power, for all the antennas. Examples include the codebooks defined for channel state information (CSI) feedback in communication systems based on the new radio (NR) air interface and typical grid-of-beams codebooks with precoding vectors corresponding to different transmit directions. Such precoders may be suitable for SU-MIMO transmission, but are typically less suitable for MU-MIMO or coordinated beamforming as nulls may not always be placed where needed.

[0020] Another way to construct precoding with the same amplitude (and power) for all antennas is to calculate the precoding weights according to a given algorithm in a first step. Then, in a second step, the phases of the precoding weights are kept, but the amplitudes are rescaled so that all the amplitudes are the same for all antennas (and most often all layers). As the nulls change when the amplitudes are rescaled, this is typically not used in cases where null-forming is important, such as MU-MIMO and coordinated beamforming (for example interference sensing for reducing interference generated to UEs served in other cells).

[0021] The radio branches, or radio branches, are to a large extent independent of each other, and it is not possible to reallocate power from one radio branch to another (i.e., from the power amplifier (PA) of one radio branch to the PA of another radio branch). This impacts beamforming, or precoding, where no radio branch can use more than its pre -allocated power budget (which is a per-PA power constraint). This is not an issue for precoding schemes yielding equal amplitude for all antennas. However, for algorithms that do not necessarily ensure the same amplitude on all precoding weights, this implies that not all available transmission power is utilized.

[0022] For some cases, such as SU-MIMO, it is sufficient to keep only the phase information of the precoder weights, whilst the amplitude of each weight can be scaled independently to ensure full power utilization for each radio branch. Scaling the amplitudes of the precoder weights independently will impact the nulls associated with the precoder, but as SU-MIMO is not so sensitive to the nulls, this scaling procedure can be used.

[0023] In other cases, such as MU-MIMO and coordinated beamforming (e.g., in terms of interference sensing ), which to a high degree rely on accurate nulling, scaling the amplitudes independently is not an option. In such cases, all weights (also referred to as the complex elements of the precoding vector) of the precoder P111640W001 4 need to have the same scale factor, and this factor needs to be dictated by the radio branch with the highest power (after summing over all subcarriers of the carrier) to ensure that this radio branch does not violate its power constraint.

[0024] Hence, in these latter cases, many of the radio branches may use less than their allocated power budget, resulting in a power underutilization. Studies and measurements indicate that a power loss of 2-3 dB is common for cases where beamforming using MU-MIMO and / or inter-cell interference mitigation are used. In other words, with transmission schemes that are sensitive to precoder amplitude information, such as MU-MIMO or coordinated beamforming, all available power of all antennas is not used, implying that not all available transmission power is utilized.

[0025] The inventors of the enclosed embodiments have through a combination of practical experimentation and theoretical derivation discovered that if the unused transmission power (being a result of the power underutilization) could be utilized, it could be used to improve performance in terms of robustness, reliability, and / or coverage, capacity and user throughput. A particular object is therefore to enable all (or at least more) of the available transmission power to be used when precoding different sets of subcarriers.

[0026] According to a first aspect there is presented a method for precoding of subcarriers to be transmitted on radio branches of at least one transmission point. The method is performed by a network node. The method comprises precoding a first set of the subcarriers for transmission via the radio branches. Less than all transmission power, as available for the first set of the subcarriers according to a power headroom per each of the radio branches, is allocated to the first set of the subcarriers on at least one of the radio branches. The method comprises precoding a second set of the subcarriers for transmission via the radio branches. The second set of the subcarriers is associated with the same at least one time resource as the first set of the subcarriers. Remaining transmission power on each of the radio branches from the precoding of the first set of the subcarriers is, per radio branch, reallocated to the second set of the subcarriers in accordance with the power headroom.

[0027] According to a second aspect there is presented a network node for precoding of subcarriers to be transmitted on radio branches of at least one transmission point. The network node comprises processing circuitry. The processing circuitry is configured to cause the network node to precode a first set of the subcarriers for transmission via the radio branches. Less than all transmission power, as available for the first set of the subcarriers according to a power headroom per each of the radio branches, is allocated to the first set of the subcarriers on at least one of the radio branches. The processing circuitry is configured to cause the network node to precode a second set of the subcarriers for transmission via the radio branches. The second set of the subcarriers is associated with the same at least one time resource as the first set of the subcarriers. Remaining transmission power on each of the radio branches from the precoding of the first set of the subcarriers is, per radio branch, reallocated to the second set of the subcarriers in accordance with the power headroom. P111640W001 5

[0028] According to a third aspect there is presented a network node for precoding of subcarriers to be transmitted on radio branches of at least one transmission point. The network node comprises a first precode module configured to precode a first set of the subcarriers for transmission via the radio branches. Less than all transmission power, as available for the first set of the subcarriers according to a power headroom per each of the radio branches, is allocated to the first set of the subcarriers on at least one of the radio branches. The network node comprises a second precode module configured to precode a second set of the subcarriers for transmission via the radio branches. The second set of the subcarriers is associated with the same at least one time resource as the first set of the subcarriers. Remaining transmission power on each of the radio branches from the precoding of the first set of the subcarriers is, per radio branch, reallocated to the second set of the subcarriers in accordance with the power headroom.

[0029] According to a fourth aspect there is presented a computer program for precoding of subcarriers to be transmitted on radio branches of at least one transmission point. The computer program comprises computer code which, when run on processing circuitry of a network node, causes the network node to perform actions. One action comprises the network node to precode a first set of the subcarriers for transmission via the radio branches. Less than all transmission power, as available for the first set of the subcarriers according to a power headroom per each of the radio branches, is allocated to the first set of the subcarriers on at least one of the radio branches. One action comprises the network node to precode a second set of the subcarriers for transmission via the radio branches. The second set of the subcarriers is associated with the same at least one time resource as the first set of the subcarriers. Remaining transmission power on each of the radio branches from the precoding of the first set of the subcarriers is, per radio branch, reallocated to the second set of the subcarriers in accordance with the power headroom.

[0030] According to a fifth aspect there is presented a computer program product comprising a computer program according to the fourth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

[0031] Advantageously, these aspects enable all (or at least more) of the available transmission power to be used when precoding different sets of subcarriers.

[0032] Advantageously, this in turn increases the system performance in terms of robustness, reliability, and / or coverage and user throughput for users, or channels, that benefit from higher transmit power.

[0033] Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

[0034] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of P111640W001 6 the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

[0037] Fig. 1 schematically illustrates a time-frequency resource grid according to an example;

[0038] Fig. 2 schematically illustrates mapping of frequency -layers to a frequency-space resource grid according to an example;

[0039] Fig. 3 is a schematic diagram illustrating a communication network according to embodiments;

[0040] Fig. 4 is a block diagram of a transmission point according to an embodiment;

[0041] Fig. 5 schematically illustrates subcarriers provided along an operating bandwidth according to an embodiment;

[0042] Fig. 6 is a flowchart of methods according to embodiments;

[0043] Fig. 7 schematically illustrates allocation of transmission power to subcarriers per radio branch according to embodiments;

[0044] Fig. 8 is a schematic diagram showing structural units of a network node according to an embodiment;

[0045] Fig. 9 is a schematic diagram showing functional modules of a network node according to an embodiment; and

[0046] Fig. 10 shows one example of a computer program product comprising computer readable storage medium according to an embodiment.

[0047] DETAILED DESCRIPTION

[0048] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional. P111640W001 7

[0049] Fig. 3 is a schematic diagram illustrating a communication network 300 where embodiments presented herein can be applied. The communication network 300 comprises a network node 310 configured to control the operation of a transmission (and reception) point 320. The network node 310 and the transmission point 320 could collectively form a (radio) access network node, radio base station, base transceiver station, node B (NB), evolved node B (eNB), gNB, access point, access node, or integrated access and backhaul (IAB) node, or the like. The transmission point 320 is configured to serve, and thus provide network access to, users 330a:330N, such as user equipment, portable wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, smartphones, laptop computers, tablet computers, network equipped vehicles, sensors, and Internet of Things devices. As the skilled person understands, the communication network 300 may comprise a plurality of transmission (and reception) points 320, where the network node 310 may be configured one or more of the plurality of transmission (and reception) points 320.

[0050] In Fig. 4 is provided a block diagram of a transmission point 400 according to embodiments. The transmission point 400 comprises radio branches 410a:410NAthat are connected between a baseband unit 440 and antennas 430a:430NA, with one radio branch 410a:410NAper antenna 430a:430NA. Each radio branch 410a:410NAhas a power amplifier 420a:420NAby means of which the transmission power of each radio branch 410a:410NA(and thus each antenna 430a:430NA) can be set. Each radio branch 410a:410NAmay also have its own phase shifter.

[0051] As noted above, there is still a need for improved power management in the context of precoding.

[0052] The embodiments disclosed herein therefore relate to techniques for precoding of subcarriers to be transmitted on the radio branches 410a:410NAof at least one transmission point 320, 400. In order to obtain such techniques there is provided a network node 310, a method performed by the network node 310, a computer program product comprising code, for example in the form of a computer program, that when run on a network node 310, causes the network node 310 to perform the method.

[0053] Fig. 5 schematically illustrates subcarriers 500 provided along an operating bandwidth. The subcarriers 500 are to be transmitted on radio branch / , where 1 < i < NA. It is assumed that all antennas 430a:430NAhave the same allowed maximum power and that the power of a given antenna 430a:430NAis obtained by summing over all its subcarriers. In Fig. 5 is illustrated a typical case with a constant power spectral density (PSD) across the subcarriers, i.e., where all subcarriers have the same power. For illustrative purposes, the subcarriers 500 have been divided into two sets 520a, 520b. In this respect, it is understood that the subcarriers 500 can be divided into more sets. Different ways in which the subcarriers 500 can be divided into sets will be disclosed below.

[0054] At least some of the herein disclosed embodiments are based on classifying precoding of the subcarriers 500 into (at least) two sets of subcarriers. In some examples, one of the sets of subcarriers is subject to power-sensitive precoding and another set of subcarriers is subject to power-insensitive precoding. P111640W001 8

[0055] Remaining power after power-sensitive precoding of the first set of subcarriers can be reallocated to the second set of subcarriers employing power-insensitive precoding, but without exceeding the available power headroom for any radio branch. This ensures that all transmission power can be used, which results in better performance.

[0056] Fig. 6 is a flowchart illustrating embodiments of methods for precoding of subcarriers 500 to be transmitted on radio branches 410a:410NAof at least one transmission point 320, 400. The methods are performed by the network node 310. The methods are advantageously provided as computer programs 1020.

[0057] The method is based on utilizing all available transmission power, where available transmission power left per radio branch 410a:410NAafter precoding a first set of subcarriers is reallocated to increase the transmission power of a second set of subcarriers (but without exceeding available power headroom 510 per radio branch 410a:410NA). In further detail, each precoder is assumed to have an associated power budget that is semi-statically set by configuration (either explicitly or implicitly). A typical precoder power budget constraint is a per-PA constraint, and for the common case where all PAs are equal (i.e., are capable of yielding the same transmission power across all radio branches), this can be viewed as all precoders weight vector elements must have magnitude less than or equal to unity. If all precoders weight vector elements have an amplitude value equal to 1, then full precoder power utilization is achieved, but if any of the precoder’s weight vector elements has an amplitude value less than 1, then the precoding is underutilizing the available transmission power. The same holds also for the situation where, for example, different radio branches have different power constraints (e.g. different PA capability for different radio branches). In such situations, each precoder has an associated amplitude constraint element vector that can be either absolute or relative to the maximum constraint over all elements.

[0058] In some examples, before transmission, all subcarriers are per radio branch combined into one timedomain signal. Here, the total power per antenna branch is equal to the sum of the powers of the precoded symbols of the subcarriers for the corresponding antenna. Thus, if the amplitude of one set of the subcarriers for a given radio branch 41 Oi is reduced (i.e., scaled down), then the amplitude of another set of the subcarriers for the same given radio branch 41 Oi can be increased (i.e., scaled up) so that the full transmission power of the given radio branch 41 Oi can be utilized. This translates into a reallocation per radio branch of the transmission power from one set of the subcarriers to another set of the subcarriers, as in steps S106 and SI 10.

[0059] SI 06: The network node 310 precodes a first set of the subcarriers for transmission via the radio branches 410a:410NA. Less than all transmission power, as available for the first set of the subcarriers according to a power headroom 510 per each of the radio branches 410a:410NA, is allocated to the first set of the subcarriers on at least one of the radio branches 410a:410NA. P111640W001 9

[0060] This implies that, for at least one of the radio branches 410a:410NAthere is some unused, and thus still available, transmission power. This transmission power can, per radio branch 410a:410NA, be reallocated to a second set of the subcarriers (different from the first set of the subscribers), as in step SI 10.

[0061] SI 10: The network node 310 precodes a second set of the subcarriers for transmission via the radio branches 410a:410NA. The second set of the subcarriers is associated with the same at least one time resource as the first set of the subcarriers. Remaining transmission power on each of the radio branches 410a:410NAfrom the precoding of the first set of the subcarriers is, per radio branch 410a:410NA, reallocated to the second set of the subcarriers in accordance with the power headroom 510.

[0062] In this respect, not all remaining power per radio branch 410a:410NAneeds to be reallocated to the second set of the subcarriers. Further, and as will be disclosed below, the reallocation might either be implemented separately at the end of the precoding of the second set of the subcarriers, or as an integral part of precoding the second set of the subcarriers. Further, and as will be disclosed below, the reallocation might either be equal or unequal among the second set of the subcarriers per radio branch 410a:410NA.

[0063] Embodiments relating to further details of precoding of subcarriers 500 to be transmitted on radio branches 410a:410NAof at least one transmission point 320, 400 as performed by the network node 310 will now be disclosed with continued reference to Figs. 3, 4, 5, and 6.

[0064] Only re-scaling the precoders associated with the second set of subcarriers does not immediately lead to improved performance in terms of more transmitted information bits (i.e., it does not immediately yield an improved throughput). It makes, however, the transmissions more robust (e.g., in terms of reduced block error rate), which may, for example, improve user throughput by facilitating less retransmissions. It is also beneficial for TCC applications.

[0065] Further in this respect, to further benefit from increased transmission power and thereby improved signal- to-noise ratios (SNRs) for the second set of subcarriers, also the link adaptation can be adjusted and more information bits may need to be allocated. Hence, in some embodiments, the network node 310 is configured to perform (optional) step SI 02.

[0066] SI 02: The network node 310 performs a scheduling decision for the subcarriers. The scheduling decision is based on the power headroom 510 and the reallocation of the transmission power among the subcarriers per radio branch 410a:410NA.

[0067] In this respect, as illustrated in Fig. 6, the scheduling decision can be adaptively decided based on the precoding of the subcarriers. That is, the scheduling can be re-evaluated and updated based on the allocation and reallocation of the transmission power in steps S106 and SI 10. P111640W001 10

[0068] Power restrictions and power reallocation may thereby be taken into consideration when scheduling and link adaptation decisions are made. By knowing that there will be excess transmission power available after precoding the first set of subcarriers (resulting in better signal-to-interference-and-noise ratios (SINRs) for the second set of subcarriers), more information data can be allocated to the second set of subcarriers. For example, more aggressive modulation and coding schemes (MCSs) can be used for the second set of subcarriers, resulting in more transmitted data. This results in better user throughput, application coverage and network capacity. Hence, in some embodiments, the network node 310 is configured to perform (optional) step SI 04.

[0069] S104: The network node 310 selects transmission parameters (e.g., link adaptation adjustments, such as deciding modulation order and coding rate) for the second set of the subcarriers based on reallocation of the transmission power among the subcarriers per radio branch 410a:410NA.

[0070] In this respect, as illustrated in Fig. 6, the transmission parameters can be adaptively selected based on the precoding of the subcarriers. That is, the transmission parameters can be re-selected and updated based on the allocation and reallocation of the transmission power in steps S106 and SI 10.

[0071] In some embodiments, the first set of the subcarriers is subject to power-sensitive precoding according to which the transmission power of at least one of the radio branches 410a:410NAis less than its power headroom 510 for at least one of the subcarriers in the first set of the subcarriers. That is, the transmission power associated with at least one of the subcarriers for at least one of the radio branches is scaled down, which will translate into less transmission power than the power headroom 510 being used for this at least one radio branch. In general terms, power-sensitive precoding can be defined as any precoding where amplitude relations between different precoder weight elements matter across different radio branches 410a:410NA, and hence need to be kept intact. In this case, if any power scaling is applied, then all precoder weight elements need to be scaled in the same way (i.e., by the same scale factor). Whether the precoding is power-sensitive or not is decided by the precoder algorithm objectives.

[0072] In some embodiments, the second set of the subcarriers is subject to either power-insensitive precoding or power-sensitive precoding, and the transmission power of this at least one of the radio branches 410a:410NAis scaled up for at least one subcarrier in the second set of the subcarriers when reallocating remaining transmission power on each of the radio branches 410a:410NAfrom the precoding of the first set of the subcarriers. Further, the amount of which the transmission power is scaled up per radio branch 410a:410NAfor the at least one of the subcarriers in the second set of the subcarriers might be equal to the amount of which the transmission power is scaled down per radio branch 410a:410NAfor this at least one of the subcarriers in the first set of the subcarriers. In general terms, power-insensitive precoding can be defined as any precoding where the amplitude relations between different precoder weight elements can be changed without significantly affecting the algorithm performance (i.e., how well the objective of the algorithm is attained). Hence, in this case it is acceptable to apply different amplitude scaling to different P111640W001 11 precoding weight elements without significantly impacting the algorithm performance negatively.

[0073] Whether the precoding is power-insensitive or not is decided by the precoder algorithm objectives.

[0074] In some examples, the transmission power available for a radio branch after (amplitude sensitive) precoding for MU-MIMO on a first set of subcarriers is used to increase transmission power on the same radio branch for (amplitude insensitive) precoding for SU-MIMO on a second set of subcarriers, where MU-MIMO on the first set of subcarriers is used in parallel to SU-MIMO on the second set of subcarriers.

[0075] Different precoding algorithms can be derived, or selected, based on different algorithm objectives. For example, algorithm objectives could be to maximize the own received signal power, maximize the own received signal power whilst taking inter-layer interference into consideration, maximize the own received signal whilst minimizing interference in relevant directions towards neighboring cells, etc. Algorithm objectives can also include aspects such as potential power constraints.

[0076] In some aspects, the precoders used for precoding the first set of the subcarriers are determined based on algorithm objectives. Scaling of the transmission power can then be applied such that no radio branch violates the per-radio branch power headroom constraint, and such that amplitude relations are kept intact. Hence, in some embodiments, the network node 310 is configured to perform (optional) steps SI 06-2 and SI 06-4 as part of precoding the first set of the subcarriers in step SI 06.

[0077] SI 06-2: The network node 310 selects a first precoding algorithm based on a first precoding objective for the first set of the subcarriers.

[0078] SI 06-4: The network node 310 selects the transmission power of the first set of the subcarriers according to the first precoding algorithm whilst not exceeding the power headroom 510 and whilst retaining amplitude relations, as given by the first precoding algorithm, between the radio branches 410a:410NAfor the first set of the subcarriers.

[0079] Examples of precoder algorithms that typically are power sensitive include, but are not limited to, multilayer SU-MIMO precoding and often also multi-layer MU-MIMO precoding. Hence, in some embodiments, the first precoding objective pertains to either creating nulls (such as for MU-MIMO precoding) or performing interference reduction (such as for inter-cell interference-aware precoding).

[0080] In some aspects, the leftover transmission power per radio branch is determined. Therefore, in some embodiments, the network node 310 is configured to perform (optional) step S108.

[0081] S108: The network node 310 determines leftover transmission power per radio branch 410a:410NAwith respect to the power headroom 510 due to using less than the power headroom (510) when selecting the transmission power of the first set of the subcarriers. P111640W001 12

[0082] Different embodiments of precoding the second set of the subcarriers in step SI 10 will be disclosed next. In general terms, a first option is to first perform the precoding and then add a post-processing step of scaling the precoded output, whereas a a second option is to perform the precoding and the scaling jointly. These two options will be disclosed next, starting with the first option.

[0083] In some aspects, the precoders for the second set of subcarriers are first determined based on algorithm objectives. Thereafter, the amplitudes of the precoders’ weights for the different radio branches can be scaled according to the leftover transmission power per radio branch after precoding the first set of the subcarriers. Therefore, in some embodiments, the network node 310 is configured to perform (optional) steps SI 10-2a and SI 10-4a as part of precoding the second set of the subcarriers in step SI 10.

[0084] SI 10-2a: The network node 310 selecting SI 10-2a a second precoding algorithm based on a second precoding objective for the second set of the subcarriers.

[0085] SI 10-4a: The network node 310 selects the transmission power of the second set of the subcarriers according to the second precoding algorithm. Further, the transmission power of the second set of the subcarriers is increased according to the leftover transmission power per radio branch 410a:410NAwhilst not exceeding the power headroom 510 per radio branch 410a:410NA.

[0086] One advantage of this embodiment is that it has little impact on legacy implementations. All precoders can be determined based on legacy methods. Only an additional post-processing step of scaling the precoders associated with the second set of the subcarriers needs to be added (as in step SI 10-4a).

[0087] In other aspects, the power reallocation is part of the derivation, or selection, of the precoder for the second set of the subcarriers. That is, the precoders for the second set of the subcarriers can be determined whilst taking the leftover transmission power from precoding of the first set of the subcarriers into consideration. Therefore, in some embodiments, the network node 310 is configured to perform (optional) steps SI 10-2b and SI 10-4b as part of precoding the second set of the subcarriers in step SI 10.

[0088] SI 10-2b: The network node 310 selects SI 10-2b a second precoding algorithm based on a second precoding objective for the second set of the subcarriers and whilst taking into consideration the leftover transmission power per radio branch.

[0089] SI 10-4b: The network node 310 selects the transmission power of the second set of the subcarriers according to the second precoding algorithm and whilst not exceeding the power headroom 510. In other words, the network node 310 applies the adjusted precoder weights (complex weights) that comply with the power headroom 510.

[0090] One advantage of this embodiment is that more optimal precoders can be used, as compared to using a pure post-scaling procedure. One illustrative example where a slightly improved precoder can be obtained P111640W001 13 is for multi-layer SU-MIMO, where, although rather power insensitive, different radio branches could benefit from keeping some power constraints intact in order to reduce intra-layer interference.

[0091] Examples of precoder algorithms that typically are power sensitive include, but are not limited to, MU- MIMO precoding and inter-cell interference-aware precoding. Hence, in some embodiments, the second precoding objective pertains to either single layer SU-MIMO precoding or multi-layer SU-MIMO precoding.

[0092] There can be different ways in which the scaling is applied to the second set of the subcarriers in steps S110-4a and S110-4b.

[0093] In some aspects, equal power distribution is applied, according to which all subcarriers associated with the second set of subcarriers are equally scaled (per radio branch 410a:410NA). That is, in some embodiments, the leftover transmission power per radio branch 410a:410NAis equally distributed among all subcarriers in the second set of subcarriers.

[0094] In some aspects, equal power distribution is applied, according to which the power can be distributed considering some specific objective. Thus, all leftover transmission power may be distributed only among a subset of the subcarriers in the second set of subcarriers. That is, in some embodiments, the leftover transmission power is unequally distributed among the subcarriers in the second set of subcarriers.

[0095] Different criteria according to which the leftover transmission power can be unequally distributed will be disclosed next. In some embodiments, the second set of subcarriers comprises subcarriers carrying first type of information and subcarriers carrying second type of information, and the leftover transmission power is only distributed among the subcarriers in the second set of subcarriers carrying the first type of information. Here, the first type of information may be high priority information and the second type of information may be low priority information, where thus the high priority information has higher priority than the low priority information. For example, the first type of information may contain time critical communication (TCC) type of application data whereas the second type of information does not contain any TCC type of application data. Still further, the second set of subcarriers may be allocated to UEs with poor coverage that could benefit from a power boost.

[0096] Here, whether some of the subcarriers in the second set of subcarriers should, for a given radio branch, be allocated more transmission power than other subcarriers in the second set of subcarriers for the same given radio branch or not may be decided by the scheduler.

[0097] Once the subcarriers have been precoded, optionally followed by further signal processing, the subcarriers can be transmitted. Hence, in some embodiments, the network node 310 is configured to perform (optional) step SI 12. P111640W001 14

[0098] SI 12: The network node 310 initiates transmission from the transmission point 400 of the first set of the subcarriers and the second set of the subcarriers in the same at least one time resource.

[0099] In this respect, whilst the network node 310 initiates the transmission, the actual transmission is carried out by the transmission point 400.

[0100] Further implementational aspects of the transmission point 400 will be disclosed next.

[0101] In some aspects, the power amplifiers 420a:420NAare multi-band power amplifiers and the left-over power from pre-coding one frequency band is used when precoding the other frequency bands.

[0102] In further aspects, power amplifiers 420a:420NAon different radio branches 410a:410NAhave different nominal transmit power (e.g., due to hardware impairments, design choices, or other reasons). In such configurations, the power headroom 510 might thus vary among the power amplifiers 420a:420NAand / or among the radio branches 410a:410NA.

[0103] In some embodiments, the at least one transmission point 320, 400 comprises a digital unit and a radio unit, and reallocation of the transmission power is performed either in the digital unit (such as in a baseband unit or a scheduler unit) or in the radio unit.

[0104] An illustrative example of precoding of subcarriers 500 to be transmitted on radio branches 410a:410NAof at least one transmission point 320, 400 based on at least some of the preceding embodiments will now be disclosed with reference to Fig. 7. In Fig. 7 is schematically illustrated the transmission power allocated per set of subcarriers per radio branch. For simplicity, the transmission point is assumed to comprise three radio branches with equal power headroom (e.g., same PA capability and configured power). The configured power is spread evenly over the operating bandwidth, meaning that all subcarriers spread over all radio branches have the same PSD. In this setup, the power headroom allocated to each precoder can be described in a relative fashion, where the default amplitude scaling parameter for each element (i.e., each radio branch) in the precoder weight vector for each precoder equals unity (i.e., has an amplitude value equal to 1) for the case with single-layer transmission. For multiple layers, each precoder can be represented with a matrix whose columns are the precoding weight vectors of the individual layers, and in this case the precoder is scaled so that the power (i.e., the sum of the squared amplitudes) summed over all layers has a value equal to 1. In the present example, the subcarriers have been classified into two categories (with one set of the subcarriers per category); one that is subject to power-sensitive precoding and another that employs power-insensitive precoding. Precoding is performed across the radio branches. In the present example, there are four different wideband precoders, denoted “Precoder 1”, “Precoder 2”, “Precoder 3”, and “Precoder 4”, used for different parts of the bandwidth (and hence, for the subcarriers), as in Fig. 7(a), illustrating the configured bandwidth classified into two subcarrier categories. The four precoders could, for example, correspond to four different users. P111640W001 15

[0105] After precoding of the power-sensitive subcarriers with appropriate scaling of precoding weights, the power (or amplitude) scaling distribution as a function of subcarriers for the different radio branches is according to Fig. 7(b), illustrating the PSD per radio branch after power-sensitive precoding. For subcarriers associated with Precoder 1, the scaling has been reduced on radio branches 2 and 3 due to that precoding scaling resulted in full power utilization on radio branch 1 (scaling equal to 1) and reduced power utilization (scaling less than one) on radio branches 2 and 3. Similarly, for subcarriers associated with Precoder 2, the power (or amplitude) scaling has been reduced on radio branches 1 and 3 due to that precoding scaling resulted in full power utilization (scaling equal to 1) on radio branch 2 and reduced power utilization (scaling less than one) on radio branches 1 and 3. The herein disclosed embodiments provide techniques for efficiently using the unused transmission power. This unused transmission power corresponds to the areas marked with a question mark (i.e., marked with the symbol “?”) in Fig. 7(b).

[0106] According to the herein disclosed embodiments, the unused transmission power after performing powersensitive precoding is (re-)allocated to the remaining subcarriers that are subject to power-insensitive precoding. This can be performed in many ways, for example, as illustrated in Fig. 7(c) for equal power distribution and in Fig. 7(d) for unequal power distribution, respectively. The example in Fig. 7(c) corresponds to distributing the remaining transmission power equally over all remaining (allocated) subcarriers (associated with Precoders 3 and 4). The example in Fig. 7(d) corresponds to allocating all remaining power equally over only the subcarriers associated with Precoder 4. In any case, the power reallocation is done per radio branch; transmission power cannot be transferred from one radio branch to another. For example, for the case in Fig. 7(c), a power (or amplitude) scaling factor that is larger than 1 is used for all precoder weight vector elements associated with Precoders 3 and 4, and the factor is equal for both Precoder 3 and 4, but different for different radio branches since the magnitude of the scaling factor depends on the unused power remaining after power-sensitive precoding per each individual radio branch.

[0107] Fig. 8 schematically illustrates, in terms of a number of structural units, the components of a network node 800 according to an embodiment. Processing circuitry 810 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1010 (as in Fig. 10), e.g. in the form of a storage medium 830. The processing circuitry 810 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0108] Particularly, the processing circuitry 810 is configured to cause the network node 800 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 830 may store the set of operations, and the processing circuitry 810 may be configured to retrieve the set of operations from the storage medium 830 to cause the network node 800 to perform the set of operations. The set of operations may be provided as a set of executable instructions. P111640W001 16

[0109] Thus the processing circuitry 810 is thereby arranged to execute methods as herein disclosed. The storage medium 830 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The network node 800 may further comprise a communications (comm.) interface 820 at least configured for communications with other entities, functions, nodes, and devices, as in Fig. 3. As such the communications interface 820 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 810 controls the general operation of the network node 800 e.g. by sending data and control signals to the communications interface 820 and the storage medium 830, by receiving data and reports from the communications interface 820, and by retrieving data and instructions from the storage medium 830. Other components, as well as the related functionality, of the network node 800 are omitted in order not to obscure the concepts presented herein.

[0110] Fig. 9 schematically illustrates, in terms of a number of functional modules, the components of a network node 900 according to an embodiment. The network node 900 of Fig. 9 comprises a number of functional modules; a first precode module 910 configured to perform step S106, and a second module 918 configured to perform step SI 10. The network node 900 of Fig. 9 may further comprise a number of optional functional modules, such as any of a first select module 912 configured to perform step SI 06-2, a first scale module 914 configured to perform step SI 06-4, a first determine module 916 configured to perform step SI 08, a second select module 920 configured to perform step S110-2a, a second scale module 922 configured to perform step S110-4a, a third select module 924 configured to perform step SI 10-2b, a third scale module 926 configured to perform step SI 10-4b, a schedule module 928 configured to perform step SI 02, a fourth select module 930 configured to perform step SI 04, and an initiate module 932 configured to perform step SI 12.

[0111] In general terms, each functional module 910:932 may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 830 which when run on the processing circuitry makes the network node 800 perform the corresponding steps mentioned above in conjunction with Fig 9. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 910:9320 may be implemented by the processing circuitry 810, possibly in cooperation with the communications interface 820 and / or the storage medium 830. The processing circuitry 810 may thus be configured to from the storage medium 830 fetch instructions as provided by a functional module 910:932 and to execute these instructions, thereby performing any steps as disclosed herein.

[0112] The network node 310, 800, 900 may be provided as a standalone device or as a part of at least one further device. For example, the network node 310, 800, 900 may be provided in a node of the radio access network or in a node of the core network. Alternatively, functionality of the network node 310, P111640W001 17

[0113] 800, 900 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time. Thus, a first portion of the instructions performed by the network node 310, 800, 900 may be executed in a first device, and a second portion of the of the instructions performed by the network node 310, 800, 900 may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the network node 310, 800, 900 may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a network node 310, 800, 900 residing in a cloud computational environment. Therefore, although a single processing circuitry 810 is illustrated in Fig. 8 the processing circuitry 810 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 910:932 of Fig. 9 and the computer program 1020 of Fig. 10.

[0114] Fig. 10 shows one example of a computer program product 1010 comprising computer readable storage medium 1030. On this computer readable storage medium 1030, a computer program 1020 can be stored, which computer program 1020 can cause the processing circuitry 810 and thereto operatively coupled entities and devices, such as the communications interface 820 and the storage medium 830, to execute methods according to embodiments described herein. The computer program 1020 and / or computer program product 1010 may thus provide means for performing any steps as herein disclosed.

[0115] In the example of Fig. 10, the computer program product 1010 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 1010 could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1020 is here schematically shown as a track on the depicted optical disk, the computer program 1020 can be stored in any way which is suitable for the computer program product 1010.

[0116] The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

P111640W001 18CLAIMS1. A method for precoding of subcarriers (500) to be transmitted on radio branches (410a:410NA) of at least one transmission point (320, 400), wherein the method is performed by a network node (310, 800, 900), and wherein the method comprises: precoding (S106) a first set of the subcarriers for transmission via the radio branches (410a:410NA), wherein less than all transmission power, as available for the first set of the subcarriers according to a power headroom (510) per each of the radio branches (410a:410NA), is allocated to the first set of the subcarriers on at least one of the radio branches (410a:410NA); and precoding (SI 10) a second set of the subcarriers for transmission via the radio branches (410a:410NA), wherein the second set of the subcarriers is associated with same at least one time resource as the first set of the subcarriers, and wherein remaining transmission power on each of the radio branches (410a:410NA) from the precoding of the first set of the subcarriers is, per radio branch (410a:410NA), reallocated to the second set of the subcarriers in accordance with the power headroom (510).

2. The method according to claim 1, wherein the first set of the subcarriers is subject to powersensitive precoding according to which the transmission power of at least one of the radio branches (410a:410NA) is less than its power headroom (510) for at least one of the subcarriers in the first set of the subcarriers.

3. The method according to claim 1 or 2, wherein the second set of the subcarriers is subject to either power-insensitive precoding or power-sensitive precoding, and wherein the transmission power of said at least one of the radio branches (410a:410NA) is scaled up for at least one subcarrier in the second set of the subcarriers when reallocating said remaining transmission power on each of the radio branches (410a:410NA) from the precoding of the first set of the subcarriers.

4. The method according to claim 2 and 3, wherein an amount of which the transmission power is scaled up per radio branch (410a:410NA) for said at least one of the subcarriers in the second set of the subcarriers is equal to an amount of which the transmission power is scaled down per radio branch (410a:410NA) for said at least one of the subcarriers in the first set of the subcarriers.

5. The method according to any preceding claim, wherein precoding the first set of the subcarriers comprises: selecting (SI 06-2) a first precoding algorithm based on a first precoding objective for the first set of the subcarriers; and selecting (SI 06-4) the transmission power of the first set of the subcarriers according to the first precoding algorithm whilst not exceeding the power headroom (510) and whilst retaining amplitudeP111640W001 19 relations, as given by the first precoding algorithm, between the radio branches (410a:410NA) for the first set of the subcarriers.

6. The method according to claim 5, wherein the first precoding objective pertains to either MU- MIMO precoding or inter-cell interference-aware precoding.

7. The method according to claim 5 or 6, wherein the method further comprises: determining (S108) leftover transmission power per radio branch (410a:410NA) with respect to the power headroom (510) due to using less than the power headroom (510) when selecting the transmission power of the first set of the subcarriers.

8. The method according to claim 7, wherein precoding the second set of the subcarriers comprises: selecting (SI 10-2a) a second precoding algorithm based on a second precoding objective for the second set of the subcarriers; and selecting (SI 10-4a) the transmission power of the second set of the subcarriers according to the second precoding algorithm, wherein the transmission power of the second set of the subcarriers is increased according to the leftover transmission power per radio branch (410a:410NA) whilst not exceeding the power headroom (510) per radio branch (410a:410NA).

9. The method according to claim 7, wherein precoding the second set of the subcarriers comprises: selecting (SI 10-2b) a second precoding algorithm based on a second precoding objective for the second set of the subcarriers and whilst taking into consideration the leftover transmission power per radio branch; and selecting (SI 10-4b) the transmission power of the second set of the subcarriers according to the second precoding algorithm and whilst not exceeding the power headroom (510).

10. The method according to claim 8 or 9, wherein the second precoding objective pertains to either single layer SU-MIMO precoding or multi-layer SU-MIMO precoding.

11. The method according to claim 8, 9, or 10, wherein the leftover transmission power per radio branch (410a:410NA) is equally distributed among all subcarriers in the second set of subcarriers.

12. The method according to claim 8, 9, or 10, wherein the leftover transmission power is unequally distributed among the subcarriers in the second set of subcarriers.

13. The method according to claim 12, wherein the second set of subcarriers comprises subcarriers carrying first type of information and subcarriers carrying second type of information, and wherein theP111640W001 20 leftover transmission power is only distributed among the subcarriers in the second set of subcarriers carrying the first type of information.

14. The method according to any preceding claim, wherein there is a power amplifier (420a:420NA) along each of the radio branches (410a:410NA), and wherein the power headroom (510) varies among the power amplifiers (420a:420NA) and / or among the radio branches (410a:410NA).

15. The method according to any preceding claim, wherein the at least one transmission point (320, 400) comprises a digital unit and a radio unit, and wherein reallocation of the transmission power is performed either in the digital unit or in the radio unit.

16. The method according to any preceding claim, wherein the method further comprises: performing (SI 02) a scheduling decision for the subcarriers, wherein the scheduling decision is based on the power headroom (510) and reallocation of the transmission power among the subcarriers per radio branch (410a:410NA).

17. The method according to any preceding claim, wherein the method further comprises: selecting (SI 04) transmission parameters for the second set of the subcarriers based on reallocation of the transmission power among the subcarriers per radio branch (410a:410NA).

18. The method according to any preceding claim, wherein the method further comprises: initiating (SI 12) transmission from the transmission point (400) of the first set of the subcarriers and the second set of the subcarriers in said same at least one time resource.

19. A network node (310, 800, 900) for precoding of subcarriers (500) to be transmitted on radio branches (410a:410NA) of at least one transmission point (320, 400), the network node (310, 800, 900) comprising processing circuitry (810), the processing circuitry being configured to cause the network node (310, 800, 900) to: precode a first set of the subcarriers for transmission via the radio branches (410a:410NA), wherein less than all transmission power, as available for the first set of the subcarriers according to a power headroom (510) per each of the radio branches (410a:410NA), is allocated to the first set of the subcarriers on at least one of the radio branches (410a:410NA); and precode a second set of the subcarriers for transmission via the radio branches (410a:410NA), wherein the second set of the subcarriers is associated with same at least one time resource as the first set of the subcarriers, and wherein remaining transmission power on each of the radio branches (410a:410NA) from the precoding of the first set of the subcarriers is, per radio branch (410a:410NA), reallocated to the second set of the subcarriers in accordance with the power headroom (510).P111640W001 2120. A network node (310, 800, 900) for precoding of subcarriers (500) to be transmitted on radio branches (410a:410NA) of at least one transmission point (320, 400), the network node (310, 800, 900) comprising: a first precode module (910) configured to precode a first set of the subcarriers for transmission via the radio branches (410a:410NA), wherein less than all transmission power, as available for the first set of the subcarriers according to a power headroom (510) per each of the radio branches (410a:410NA), is allocated to the first set of the subcarriers on at least one of the radio branches (410a:410NA); and a second precode module (918) configured to precode a second set of the subcarriers for transmission via the radio branches (410a:410NA), wherein the second set of the subcarriers is associated with same at least one time resource as the first set of the subcarriers, and wherein remaining transmission power on each of the radio branches (410a:410NA) from the precoding of the first set of the subcarriers is, per radio branch (410a:410NA), reallocated to the second set of the subcarriers in accordance with the power headroom (510).

21. The network node (310, 800, 900) according to claim 19 or 20, further being configured to perform the method according to any of claims 2 to 18.

22. A computer program (1020) for precoding of subcarriers (500) to be transmitted on radio branches (410a:410NA) of at least one transmission point (320, 400), the computer program comprising computer code which, when run on processing circuitry (810) of a network node (310, 800, 900), causes the network node (310, 800, 900) to: precode (S106) a first set of the subcarriers for transmission via the radio branches (410a:410NA), wherein less than all transmission power, as available for the first set of the subcarriers according to a power headroom (510) per each of the radio branches (410a:410NA), is allocated to the first set of the subcarriers on at least one of the radio branches (410a:410NA); and precode (SI 10) a second set of the subcarriers for transmission via the radio branches (410a:410NA), wherein the second set of the subcarriers is associated with same at least one time resource as the first set of the subcarriers, and wherein remaining transmission power on each of the radio branches (410a:410NA) from the precoding of the first set of the subcarriers is, per radio branch (410a:410NA), reallocated to the second set of the subcarriers in accordance with the power headroom (510).

23. A computer program product (1010) comprising a computer program (1020) according to claim 22, and a computer readable storage medium (1030) on which the computer program is stored.

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