Methods and apparatuses for performing crest factor reduction

Abstract

Embodiments described herein relate to methods and apparatuses for a method for performing crest factor reduction of a combined input signal, x / xm, to determine an output signal, y / ym, for transmission to a plurality of wireless devices, wherein the combined input signal is derived from a plurality of individual input signals wherein each individual input signal comprises data intended for one of the plurality of wireless devices. A method comprises generating a high amplitude portion of the combined input signal, xhigh / xm high, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest level of clipping distortion allowed across the individual input signals, for one or more respective individual input signals of the plurality of individual input signals: generating a respective compensation signal based on the respective individual input signal, by utilizing a respective factor, αq derived from a respective level of clipping distortion allowed for the respective individual input signal; and determining the output signal by summing the combined input signal and the respective compensation signals, and subtracting the high amplitude portion of the input signal.

Description

[0001] METHODS AND APPARATUSES FOR PERFORMING CREST FACTOR REDUCTION

[0002] TECHNICAL FIELD

[0003] Embodiments described herein relate to methods and apparatuses for performing crest factor reduction of a combined input signal to determine an output signal for transmission to a plurality of wireless devices.

[0004] BACKGROUND

[0005] Power amplifiers (PAs) are inherently nonlinear; in particular, when they are operating with high efficiency. Therefore, to provide a reasonably efficient PA, the PA may be required to be linearized to meet linearity requirements. Prior to linearizing a PA using, for example, digital predistortion (DPD), reducing the dynamic range of the PA input signal enables better linearity. Input signal dynamic range may be reduced by reducing the peak-to-average-power-ratio (PAPR). One, and arguably the most common, technique for PAPR reduction is to clip high amplitude peaks and filter the resulting signal to reduce clipping noise spread across higher frequencies. Clip and filter based PAPR reduction however results in introducing in-band distortion quantified by error vector magnitude (EVM). The EVM resulting from the PAPR reduction dominates the EVM budget in the entire chain. Another PAPR reduction technique is peak cancellation (PC) (see for example US 2018 / 0331873 B1). where a pulse with a specific amplitude replaces peaks that exceed that amplitude while their phases are preserved by being the phases of that pulse. The replacing pulse is designed to introduce no out of band distortion while the in band distortion is caused by the difference between the replacing pulse and original peaks. It has been shown that both clip and filter and PC introduces the same or similar level of EVM.

[0006] A downlink carrier may typically be composed of multiple physical resource blocks (PRBs) each being assigned for different wireless devices (e.g. user equipment’s (UEs)). Usually, those different wireless devices require or allow (or can be served with) different levels of EVM. As an example, a cell edge user may be communicating with a low modulation order such as QPSK, 16 QAM or 64 QAM while other users may use high modulation order such as 256 QAM or even higher order modulation such as 1024 QAM Currently, a clipping threshold is set to fulfill the most stringent EVM requirements of all of the users. However, this selection of the most stringent EVM requirement will result in "over-protective” transmission and therefore an "under- performing” Crest Factor Reduction ( CFR), module.

[0007] In other words, conventionally, PAPR is reduced to fulfill the most stringent EVM requirements across the entire carrier. This leads to limitation of the attainable PAPR level which could be further reduced if clipping distortion is shaped based on different users’ EVM requirements.

[0008] US 8,817,900 proposes a technique to perform frequency domain PAPR reduction when different OFDM sub-carriers are differentiated by their EVM budget. This solution requires performing a Fast Fourier T ransform (FFT) for different sets of subcarriers and applying filtering. Thereafter, an Inverse FFT (IFFT) is performed, and the algorithms keep jumping between frequency and time domains until it converges which implies significant implementation complexity.

[0009] 5. Gdkceli, T. Levanen, T. Riihonen, M. Renfors and M. Valkama, "Frequency-Selective PAPR Reduction for OFDM," in IEEE Transactions on Vehicular Technology, vol. 68, no.

[0010] 6, pp. 6167-6171 , June 2019proposes frequency selective PAPR reduction with similar approach to US 8,817,900 but a frequency mask is defined which either allows or disallows clipping noise to exist in a specific set of subcarriers. This significantly reduces the partiality of such approach. Similar to US 8,817,900, this solution involves iterating FFT and IFFT operations.

[0011] SUMMARY

[0012] In embodiments described herein signals in different PRBs are clipped differently in accordance to different required EVM. This enables the PAPR to be reduced to the minimum level that fulfills different EVM requirements across different PRBs sets within the carrier. Similar to different requirements across frequency, different users at different spatial locations may require different EVM levels and hence signals can be clipped differently instead of fulfilling the most stringent EVM across the whole space which leads to less effective CFR.

[0013] Another realization of requiring clipping noise differentiation (i.e. , colored clipping noise) is when there is a non-contiguous PRB assignment and no or very little clipping noise is allowed to fall in the unused PRBs. Sub-band full duplex (SBFD) is a system where there is a non-contiguous PRB assignment for DL to free some resources for UL reception within a conventional DL slot in a TDD system aiming at enhancing UL latency and capacity. SBFD was introduced in Qualcomm, " On NR full Duplex, Perspectives for Release 18", RWS-210026 3GPP TSG RAN Rel-18 Workshop and it is now being investigated extensively in the cellular industry. The needed "extensively colored” clipping noise is more apparent in multi-carrier SBFD setup.

[0014] It will be appreciated that, from implementation point of view, a filter that follows all allowed clipping noise levels would be a very lengthy filter which would be difficult to design, and would introduce a long delay that may violate latency requirements of the system or for the latency requirements of some users.

[0015] According to some embodiments there is provided a method for performing crest factor reduction of a combined input signal, x / xm, to determine an output signal, y / ym, for transmission to a plurality of wireless devices, wherein the combined input signal is derived from a plurality of individual input signals wherein each individual input signal comprises data intended for one of the plurality of wireless devices. The method comprises generating a high amplitude portion of the combined input signal, Xhigh / Xm high, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest level of clipping distortion allowed across the individual input signals. The method further comprises for one or more respective individual input signals of the plurality of individual input signals: generating a respective compensation signal based on the respective individual input signal, by utilizing a respective factor, aqderived from a respective level of clipping distortion allowed for the respective individual input signal. The method further comprises determining the output signal by summing the combined input signal and the respective compensation signals and subtracting the high amplitude portion of the input signal.

[0016] According to some embodiments there is provided a crest factor reduction, CFR, module for performing CFR of a combined input signal (x, xm) to determine an output signal for transmission to a plurality of wireless devices, wherein the combined input signal is derived from a plurality of individual input signals wherein each individual input signal comprises data intended for one of the plurality of wireless device. The CFR module comprises an input (401) configured to receive the combined input signal, x / xmand a amplitude compensation module (402) configured to generate a high amplitude portion of the combined input signal, Xhigh / xmhigh, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest level of clipping distortion allowed across the individual input signals. The CFR module further comprises one or more compensation branches (403i to 403i_), wherein each compensation branch (403i) is configured to, for a respective individual input signal of the plurality of individual input signals: generate a respective compensation signal, Ci, based on the respective individual input signal, by utilizing a respective factor, Qi derived from a respective level of clipping distortion allowed for the respective individual input signal; and a combination element (404) configured to determine the output signal by summing the combined input signal and the respective compensation signals, and subtracting the high amplitude portion of the input signal.

[0017] According to some embodiments there is provided Crest Factor Reduction, CFR, module comprising processing circuitry and memory. The memory contains instructions executable by the processing circuitry whereby the CFR module is operable to: generate a high amplitude portion of the combined input signal, Xhigh / xmhigh, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest level of clipping distortion allowed across the individual input signals, for one or more respective individual input signals of the plurality of individual input signals: generate a respective compensation signal based on the respective individual input signal, by utilizing a respective factor, aqderived from a respective level of clipping distortion allowed for the respective individual input signal; and determine the output signal by summing the combined input signal and the respective compensation signals, and subtracting the high amplitude portion of the input signal.

[0018] According to some embodiments there is provided computer program, comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out any of the methods described above.

[0019] According to some embodiments there is provided computer program product comprising non transitory computer readable media having stored thereon a computer program as described above.

[0020] According to some embodiments there is provided computer-readable medium comprising instructions that, when executed on at least one processor, cause the at least one processor to perform any of the methods described above. According to some embodiments there is provided a base station comprising a CFR module as described above.

[0021] Aspects and examples of the present disclosure thus provide a method and apparatuses that perform PAPR reduction based on dynamic distortion requirements in either frequency or spatial domain, or in both domains. The embodiments described herein provide a way to reduce both implementation complexity and latency compared to utilizing a complex filter to reshape the clipping noise.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For a better understanding of the embodiments of the present disclosure, and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0024] Figure 1 illustrates how different clipping thresholds can be set in different UEs targeting different EVMs;

[0025] Figure 2 illustrates a method for performing crest factor reduction, CFR, of a combined input signal;

[0026] Figure 3 is a flowchart illustrating a method for generating the respective compensation signal;

[0027] Figure 4 illustrates a crest factor reduction, CFR, module, 400, for performing CFR of a combined input signal;

[0028] Figure 5 illustrates an example implementation of the CFR module 400 of Figure 4;

[0029] Figure 6 illustrates a block diagram of an AAS system;

[0030] Figure 7a illustrates an example implementation of the CFR module 400 of Figure 4;

[0031] Figure 7b illustrates an example implementation of the CFR module 400 of Figure 4; Figure 8 illustrates the clipping noise spectrum when the embodiment described with reference to Figure 5 is used which achieved PAPR of -7.15 dB;

[0032] Figure 9 illustrates other simulation results for a CFR output using the embodiment described with reference to Figure 5 applied to a dual-carrier SBFD configuration where each carrier is 100 MHz split as 40 / 20 / 40 MHz being DL / UL / DL sub-bands;

[0033] Figure 10 illustrates the PDF of both the first and final stages of the simulation of Figure 9;

[0034] Figure 11 illustrates simulation results when using the embodiment described with reference to Figure 7a;

[0035] Figure 12 illustrates an CFR module 1200 comprising processing circuitry (or logic).

[0036] DETAILED DESCRIPTION

[0037] Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and / or is implied from the context in which it is used. All references to a / an / the element, apparatus, component, means, step, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any methods disclosed herein do not have to be performed in the exact order disclosed, unless a step is explicitly described as following or preceding another step and / or where it is implicit that a step must follow or precede another step. Any feature of any of the embodiments disclosed herein may be applied to any other embodiment, wherever appropriate. Likewise, any advantage of any of the embodiments may apply to any other embodiments, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following description.

[0038] The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and / or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and / or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer- readable memory, such as (ROM, EEPROM, Flash memory, a memory disc, RAM etc.) solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

[0039] Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and / or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

[0040] Certain aspects of the present disclosure and their embodiments may provide solutions to these or other challenges. Particular embodiments are described more fully with reference to the accompanying drawings. Other embodiments, however, are contained within the scope of the subject matter disclosed herein. The disclosed subject matter should not be construed as limited to only the embodiments set forth herein; rather, these embodiments are provided by way of example to convey the scope of the subject matter to those skilled in the art.

[0041] Embodiments described herein may be considered to provide two stages. The first stage is an ordinary clip and filter stage which could also be a peak cancellation or any other PAPR reduction technique that introduces in-band distortion. This first stage clips and filters the signal based on the most lenient clipping distortion requirement based on the highest permittable EVM across the carrier or space. A second stage is added which utilizes parallel compensation paths where each compensation path is responsible for determining the clipping noise introduced by the first stage into each user’s signal. Finally, the clipping noise into each frequency / space block is controlled by removing a fraction of it based on EVM / distortion requirements.

[0042] For the ease of explanation, the following illustrates a method for determining a level of clipping distortion in a particular frequency domain (e.g. sub-band) in terms of EVM.

[0043] The same principals may be applied to a spatial domain differentiable EVM as will be shown later.

[0044] The PAPR of signal x is obtained as: ll% ll2ro

[0045] PAPR = (1) where ||-||^ and || - H2 denote infinite-norm and norm 2 receptively. And N is the number of signal samples.

[0046] Conventionally, PAPR is reduced for the entire carrier to satisfy: where 8 is the clipping noise introduced when the signal x is clipped using the clipping threshold y as:

[0047] Thereafter, 5(n) is filtered out to its in band only components as

[0048] 6 = h * 6 (4) where h is a time domain impulse response for the filter used to limit the clipping noise to in-band only components and * denotes convolution operator.

[0049] Accordingly, the EVM may be determined as:

[0050] And the clipping threshold y is set to achieve a certain EVM as

[0051] According to central limit theorem, adding many independent signals (which we do in radios), results in a Gaussian distribution of the amplitudes of the combined signal consisting of a sum of several sub-carriers. Therefore, EVM may be found as:

[0052] Consider a time domain signal x that is a downlink signal where L UEs share the carrier bandwidth and the Z-th UE signal xtis transmitted using the PRBs centered at frequency fi such that:

[0053] (It will be appreciated that for the spatial multiplexing context, Equation (8) may be replaced by the equation (14) below).

[0054] Now if the targeted EVM (e.g. the allowed level of clipping distortion) is different for different UEs, then the clipping threshold for each UE may be set differently (i.e., colored) across different PRB sets as:

[0055] Yi = r EVMf) (9)

[0056] The minimum number of sub-carriers used in LTE and NR is 12 which corresponds to a single PRB. Therefore, the Gaussian-ty of the amplitude distribution holds for all possible number of assigned PRBs for each UE, including single PRB, Accordingly Equation (10) shows how the EVM (e.g. the level of clipping distortion) on the I - th frequency sub-band is determined by the respective I - th clipping threshold. Hence, the clipping noise within the I - th frequency sub-band, ||5(H is found as:

[0057] It is therefore clear from equations (10) and (11) that the clipping threshold ytto apply in any particular sub-band can be designed given a known allowed level of clipping distortion (e.g. EVMi) in that sub-band.

[0058] Figure 1 illustrates how different clipping thresholds can be set in different UEs targeting different EVMs based on equation (10).

[0059] Figure 2 illustrates a method for performing crest factor reduction, CFR, of a combined input signal, x or xm, to determine an output signal, y or ym, for transmission to a plurality of wireless devices, wherein the combined input signal is derived from a plurality of individual input signals wherein each individual input signal comprises data intended for one of the plurality of wireless devices.

[0060] The method of Figure 2 may be performed by a network node, e.g. a base station or radio access network, RAN, node. The network node may comprise an open RAN (O- RAN) node.

[0061] The method as described with reference to Figure 2 may be applied in a frequency multiplexing context, in which case the combined input signal may comprise, as will be described in more detail with reference to Figure 5, a plurality of PRBs, where each individual input signal comprises the frequency components (e.g. PRBs) intended for a particular wireless device. In other words, each respective individual input signal may be comprised within or associated with a respective frequency band. Each individual input signal may fall within one of one or more downlink portions of a carrier band.

[0062] The method as described with reference to Figure 2 may also be applied in a spatial multiplexing context (e.g. for use in digital beamforming or the digital part of a hybrid beamforming), in which case the combined input signal may comprise, as will be described in more detail with reference to Figure 7a, a respective branch signal, xm, (e.g. associated with the index, m) derived by applying a precoding matrix to a respective plurality of user signals, s1,... , sL, intended for the respective plurality of wireless devices (associated with the indices 1 to L). Each individual input signal, may therefore comprise a component of the combined input signal xmthat contains data derived from the signal stintended to be received by the user associated with the index I. It will be appreciated that the signals sxto sLmay be input into the precoding matrix l / Vto generate the signals xm. In other words, each respective individual input signal may comprise a component of the combined input signal which is dependent on a respective user signal, sy, intended for a respective wireless device.

[0063] In other words, xmmay be written as: xm ~ ^ii = islwlm

[0064] Where wtmis the weight from the precoding matrix associated with the indices l, m. stis the user signal intended for the wireless device associated with the index I.

[0065] And an individual input signal, mmay then be expressed as:xl,m ~slwlm

[0066] It will be appreciated that the method of Figure 2 may be applied to each branch signal output by the precoding matrix. The output signal, ymin these examples may be for coupling to a first antenna of an active antenna system (as illustrated in Figure 7a).

[0067] In step 201 the method comprises generating a high amplitude portion of the combined input signal, xmhigh, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest amount of clipping distortion allowed across the individual input signals. In other words, each of the individual input signals may be associated with respective wireless devices. Each individual input signal may therefore be associated with an allowed level of clipping distortion. The highest allowed level of clipping distortion (e.g. the most lenient level of clipping distortion) is therefore used to determine where the amplitude threshold should be.

[0068] It will be appreciated that step 201 may comprise clipping the combined input signal (e.g. utilizing the amplitude threshold as the clipping threshold) to remove the high amplitude components of the combined input signal. In some examples, step 201 may then further comprise removing the clipped combined input signal from a copy of the combined input signal to generate the high amplitude portion of the combined input signal.

[0069] In this example, the amplitude threshold of step 201 may be the clipping threshold that would be used to produce highest of the allowed levels of clipping distortion across the individual input signals.

[0070] In other examples, step 201 may comprise performing peak cancellation (e.g. utilizing the amplitude threshold to determine which peaks to cancel - those peaks that are higher than the amplitude threshold would be cancelled) on the combined input signal to generate a peak cancelled version of the combined input signal. Step 201 may then further comprise subtracting the peak cancelled version of the combined input signal from a copy of the combined input signal.

[0071] It will be appreciated that the method may in some examples comprise filtering the high amplitude portion of the combined input signal to filter out frequencies outside of a carrier band (e.g. to remove frequency components not contained within the combined input signal).

[0072] In step 202 the method then comprises for a respective individual input signal of the plurality of individual input signals: generating a respective compensation signal, cL, based on the respective individual input signal, by utilizing a respective factor, aLderived from a respective level of clipping distortion allowed for the respective individual input signal.

[0073] Step 202 may be performed for each of the plurality of individual input signals other than the individual input signal associated with the highest amount of allowed clipping distortion.

[0074] The respective factor, atmay comprise a scaler multiplier a (0 < at< 1) that controls the level of clipping distortion falling within the PRBs assigned for user I.

[0075] It will be appreciated that the clipping noise allowed for a respective individual input signal may be determined using equation (10) above. As an example, when no clipping noise is permittable for a particular individual input signal, the value of atmay be set to 1.

[0076] The clip noise power that may need to be compensated for within any particular individual input signal for may have a power of ||5 H2 - ||<5(H2 with ||5 H2 being the clipping noise introduced by step 201 , and ||<5(H2 being the clipping noise allowed for a respective (e.g. I - th) individual input signal. Accordingly, to calculate the respective factor ahfollowing equality may be utilised for the I - th individual input signal:

[0077] I|5 |IM«! II2 = EVM2\\xl\\2- EVM2\\xl\\2= a2■ EVM2pl\\xl\\2(12)

[0078] Where EVM is the most lenient permissible EVM across the carrier , EVMtis the level of EVM allowed for the I - th individual input signal, and ptis a value representative of a respective power level in the I - th individual input signal.

[0079] Where p is a constant equivalent to the ratio of power within the I - th individual input signal (e.g. that the I - th branch of the second stage as illustrated in Figures 4 or 5 below outputs) and may be calculated as \\2df where Btis the width of the I - th sub-band and Hi(f) is the I - th filter’s transfer function in frequency domain.

[0080] In spatial embodiment, the high amplitude portion is a linear combination of the total L signals and the ratio of power contribution from the I - th signal to the total power in the m-th branch. Therefore,

[0081] It will be appreciated therefore, that the respective factor atfor the I - th individual input signal may be calculated as:

[0082] For example, as for the frequency multiplexing context, step 202 may comprise generating the respective compensation signal based on a respective component specific signal comprising frequency components of the high amplitude portion of the combined input signal that fall within the respective frequency band. For example, step 202 may be performed by performing the processing steps as described with reference to Figure 3 on the high amplitude portion of the combined input signal.

[0083] In some examples, the method of Figure 2 may further comprise generating respective compensation signals for zero amplitude frequency portions of the combined input signal.. For example, a zero amplitude frequency portion of the combined input signal may comprise an uplink portion of a carrier band.

[0084] It will be appreciated that the processing steps performed in Figure 3 may be performed in any suitable order.

[0085] Step 301 comprises obtaining signal components within the first frequency band. For example, step 301 may comprise frequency shifting 301a to shift the respective frequency band of the high amplitude portion of the combined input signal to be centred at 0 Hz; and baseband filtering 301b. The base band filtering may then filter out all components that were not in the respective frequency band. Step 301c may then comprise frequency shifting the output of the base band filtering to ensure the compensation signal is within respective frequency band.

[0086] Step 302 comprises multiplying by the respective factor. Step 302 may be performed on the output of step 301c, may be performed before step 301c, or even before step 301a.

[0087] However, in the spatial multiplexing context, (as will be described in more detail later with reference to Figure 5) step 202 may be comprise multiplying the first individual input signal, wy,msy. by the first factor, ayand this may then be scaled with the amplitude portion needed to compensate for, for example, (|xmhigh| / |xm| ) to generate the respective compensation signal:

[0088] Cy=Wy,m Sy, ay|Xm high| / |Xm |

[0089] Returning to Figure 2, in step 203 the method then comprises determining the output signal by summing the combined input signal and the respective compensation signals, and subtracting the high amplitude portion of the input signal. It will be appreciated that, in the frequency multiplexing context, the output signal, y, may therefore comprise different clipping levels within each frequency band, where the clipping levels are tailored to the respective wireless devices receiving the different frequency bands.

[0090] It will also be appreciated that, in the spatial multiplexing context, the output signal ymis one branch of the spatial multiplexing (as will be described in more detail with reference to Figure 5). The output signal comprises clipped versions of the individual input signals that have been clipped according to the respective wireless devices that are due to receive the data in that particular individual input signal.

[0091] Figure 4 illustrates a crest factor reduction, CFR, module, 400, for performing CFR of a combined input signal x, xmto determine an output signal, y, ym, for transmission to a plurality of wireless devices. The CFR module 400 may be configured to perform the method of Figure 2 as described above. The CFR module 400 may be comprised within a network node, e.g. a base station or radio access network, RAN, node. The network node may comprise an open RAN (O-RAN) node.

[0092] As described above with reference to Figure 2, the combined input signal may be derived from a plurality of individual input signals wherein each individual input signal comprises data intended for one of the plurality of wireless devices.

[0093] The CFR module 400 comprises an input 401 configured to receive a combined input signal, x / xm.

[0094] The CFR module 400 further comprises an amplitude compensation block 402 configured to configured to generate a high amplitude portion of the combined input signal, Xhigh, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from the highest amount of clipping distortion allowed across the individual input signals. For example, the amplitude compensation block 402 may be configured to perform step 201 of Figure 2 as described above.

[0095] It will be appreciated that the amplitude compensation block 202 may be recursive (as in conventional CFR) to clip re-generated peaks due to filtering. However, since the compensation branches (as will be described above) may obtain the clipping noise introduced by the first stage, these may not be implemented recursively. The CFR module 400 further comprises one or more compensation branches (403i to 403L). Each compensation branch is configured to, for a respective individual input signal generate a respective compensation signal, Ci to CL, based on the respective individual input signal, by utilizing a respective factor, Qi derived from a respective level of clipping distortion allowed for the respective individual input signal. For example, each compensation branch may be configured to perform step 202 of Figure 2 as described above.

[0096] The CFR module 400 further comprises a combination element 404 configured to determine the output signal, y or ym, by summing the combined input signal, x or xm, and the respective compensation signals Ci to CL, and subtracting the high amplitude portion of the input signal, Xhigh or xmhigh The combination element 404 may be configured to perform step 203 of Figure 2 as described above.

[0097] Figure 5 illustrates an example implementation of the CFR module 400 of Figure 4. In particular Figure 5 is an implementation of the CFR module 400 in a frequency multiplexing context. Corresponding elements to those of Figure 4 have been given the same reference numbers.

[0098] The amplitude compensation block 402 comprises a clipping module 501 configured to clip the combined input signal, x to remove the high amplitude components of the combined input signal.

[0099] The amplitude compensation block 402 then further comprises a subtraction block 502 configured to remove the clipped combined input signal from a copy of the combined input signal to generate the high amplitude portion of the combined input signal.

[0100] In this example, the amplitude compensation block 402 further comprises a filter 503 configured to filter the high amplitude portion of the combined input signal, Xhigh to filter out frequencies outside of the combined input signal (e.g. outside of a carrier band).

[0101] In this example, the amplitude compensation block 402 is configured to clip the combined input signal, x. It will be appreciated that peak cancellation could equally be utilized in embodiments described herein. For example, the amplitude compensation block 402 may instead comprise a peak cancellation block configured to perform peak cancellation on the combined input signal to generate a peak cancelled version of the combined input signal; and a subtraction module configured to subtract the peak cancelled version of the combined input signal from a copy of the combined input signal.

[0102] In this example, each compensation branch 403i to 403L receives the high amplitude portion of the combined input signal, Xhigh / Xm high as an input.

[0103] Each compensation branch 403i to 403L may then be configured to perform the method of Figure 3.

[0104] Taking the first compensation branch 403i as an example, the first compensation branch 403i may therefore comprise a first frequency shifter 504i to shift the respective frequency band of the high amplitude portion of the combined input signal, Xhigh to be centred at 0 Hz. The first frequency shifter 504i may comprise for example a numerically controlled oscillator, (NCO). The first frequency shifter 504i may be configured to perform step 301a of Figure 3.

[0105] The first compensation branch 403i may further comprise a base-band filter (e.g. a ow pass filter) 505i.The base band filter effectively takes the part of the high amplitude portion of the combined input signal that was in the first frequency band. The bass-band filter 505i may be configured to perform step 301b of Figure 3.

[0106] The first compensation branch 403i may then further comprise a second frequency shifter 506i configured to ensure that the first compensation signal is within the first frequency band. Again, the second frequency shifter 505i may comprise an NCO. The second frequency shifter 506i may be configured to perform step 301c of Figure 3.

[0107] Together, the first frequency shifter 504i the base-band filter 505i, and the second frequency shifter 506i may be configured to perform step 301 of Figure 3.

[0108] The first compensation branch 403i further comprises a multiplier 507i configured to multiply by the first factor Qi. The multiplier 507i may be configured to perform step 302 of Figure 3. It will be appreciated that the multiplier 507i may be implemented to multiply by the first factor in the signal chain before the second frequency shifter 506i, before the first frequency shifter 504i or before the base-band filter 505i.

[0109] It should be noted that the first and second frequency shifters 504i and 506i are used herein to ease the filter design. However, these frequency shifters are optional. If the frequency shifters are removed, the base-band (e.g. low pass) filter may be replaced by a band-pass filter centered on the first frequency band.

[0110] The other compensation branches may be configured similarly to the first compensation branch.

[0111] As described with reference to Figure 4, each compensation branch 403i outputs a respective compensation signal. The compensation signals are provided, along with the high amplitude portion of the combined input signal, and the combined input signal, to the combination block 404.

[0112] It will be appreciated that there may be zero amplitude frequency portions of the combined input signal x which contain no data for a receiving wireless device. A zero amplitude frequency portion may for example comprise an uplink portion of a carrier band. However, these zero amplitude frequency portions may still comprise noise caused by the clipping of the amplitude compensation block 402. Therefore, there may be provided compensation branches for the zero amplitude frequency portions of the signal x, and compensation signals may be generated for these zero amplitude frequency portions.

[0113] In other words, the CFR module 400 may comprise one or more further compensation branches configured to generate respective compensation signals for zero amplitude frequency portions of the combined input signal.

[0114] The combination block 404 then determines the output signal, y as:

[0115] It will be appreciated that the compensation branches of the second stage may be applied (e.g. enabled) for individual input signals for UEs that have EVM requirements that differ from the most lenient EVM across the entire carrier. In other words, corresponding compensation branches to the UEs with an EVM requirement equivalent to the most lenient EVM requirement may be disabled.

[0116] It is also worth mentioning that the proposed embodiments clip Orthogonal Frequency Division Multiplexed (OFDM) symbols based on EVM requirements set by modulation order on Physical Downlink Shared Channel (PDSCH) signal as Physical Downlink Control Channel (PDCCH) and Demodulation Reference Signal (DMRS) signals are sent using lowest possible modulation order which is for example QPSK or BPSK. Subsequently, PDCCH and DMRS are clipped alike.

[0117] In an active antenna system (AAS) L users signals, call them stwhere 1 < I < L are precoded using a precoding matrix W into M antenna branches where L « M and. Figure 6 illustrates a block diagram of an AAS system. At each branch, peaks are generated from precoding multiple users’ beams and a CFR module 400 is (placed at each branch) that applies clipping and filtering of those peaks as described below with reference to Figure 7a.

[0118] It will be appreciated that processing steps may be performed on each output signal, ym, before coupling to the antenna in the AAS, for example DPD, Digital to Analog conversion, and a power amplification.

[0119] Figure 7a illustrates an example implementation of the CFR module 400 of Figure 4. In particular Figure 7a is an implementation of the CFR module 400 in a spatial multiplexing context. Corresponding elements to those of Figure 4 have been given the same reference numbers.

[0120] In this Figure the CFR module 400 is illustrated only for one branch. It will be appreciated that this module may be replicated on each branch of the AAS (e.g. as illustrated in Figure 6). This embodiment may be utilised to reduces the PAPR based on different EVM requirements for different users located differently in space.

[0121] In this example, the combined input signal comprises a signal on an m - th branch of an output of a precoding matrix and is denoted as xm. The amplitude compensation module 402 is configured similarly to as described with reference to Figure 5.

[0122] In this example, each compensation branch 403i to 403L is configured to the m - th row of the precoding matrix as an input (e.g. wimto wi_m), and the user signals Si to SL. It will be appreciated, that in some examples the CFR module 400 may not receive the user signals as an input, and may instead derive the user signals by applying an inverse of the precoding matrix to the outputs of the precoding matrix xi to XM.

[0123] Taking the first compensation branch 403i as an example, the compensation branch receives the element from the precoding matrix wimand the user signal Si. The first compensation branch 403i further comprises a multiplier 701 configured to multiply together the precoding matrix element wim, the user signal Si, the amplitude of the high amplitude portion |xmhigh|, the inverse of the amplitude of the combined input signal, |xm|, and the first factor ai to determine the first compensation signal Ci. The arrows for |xmhigh| and the inverse of |xm| are only illustrated in the first compensation branch for clarity. It will however be appreciated that the same inputs may be received in the other compensation branches.

[0124] Each compensation branch is configured similarly to the first compensation branch.

[0125] As described with reference to Figure 4, each compensation branch outputs a respective compensation signal, q. The compensation signals are provided, along with the high amplitude portion of the combined input signal, and the combined input signal, to the combination block 404.

[0126] The combination block 404 then determines the output signal, y as: mathematical explanation of the spatial domain EVM differentiator CFR is shown below

[0127] L xm ~ 'sqwqm (14) q = l

[0128] For the I - th user contribution, c?iWimis the clip noise inserted to the I - th user signal that may need to be reduced to attain a lower EVM than the first stage CFR achieves. Consequently a should be proportional to the signal portion clipped from stwhile precoding weights should also be preserved. This implies that:

[0129] It is appreciated that since each of the combined signal xmand combined high amplitude portion xmhigh is a linear combination of all individual signals with the same weights, then the ratio of the amplitude of high amplitude portion corresponding to si contribution to the amplitude of si is equivalent to the combined high amplitude portion xmhigh and the combined amplitude xm. Therefore |o-(| / |s(| = |<5m| / |xm| which implies that: a = aiWimSi - lj5—m j-l (19)

[0130] Figure 7b illustrates an example implementation of the CFR module 400 of Figure 4.

[0131] In this example, spatially multiplexed individual signals are further frequency multiplexed to be transmitted into a plurality of wireless devices located within the same beam angular coverage.

[0132] It will therefore be appreciated that, Each of the spatially multiplexed individual signals, Si is composed of a plurality of frequency multiplexed dividual signals, xik. In the example of Figure 7b therefore, the amplitude compensation module 402 performs clipping to fulfil the most lenient EVM requirement across all individual signals in both spatial and frequency domains.

[0133] The compensation branches 403i in this embodiment are then divided into two parts, a first part deals with the spatial multiplexing and the second part deals with the frequency multiplexing.

[0134] For each individual signal, Si, a compensation signal, Ci, is generated by a first parts of the compensation branches 403i to 403L similarly to the compensation branches described with reference to Figure 7a. In other words, the first parts of the compensation branches 403i to 403L output signals ,

[0135] This compensation signal, , may then be subtracted from the high amplitude portion, xm, high to generate a specific high amplitude portion for the individual signal in space. The specific high amplitude portion for the individual signal in space may then be fed into a second parts of the compensation branches 403k, i which may operate as described with reference to the compensation branches 403i to 403L in Figure 5.

[0136] The second parts of the compensation branches therefore provide second compensation signals, ct kfor the individual signals multiplexed in both space and frequency. These are provided to the combination element 404.

[0137] Experimental Results

[0138] To verify the embodiments described herein, simulations were carried out. In one of the simulations a 40 MHz carrier was assumed with two values of required EVM across the carrier.

[0139] 10 MHz centered around the carrier frequency are assigned for users requiring 3% EVM while the rest of the carrier is assigned for users requiring 7% EVM.

[0140] Figure 8 illustrates the clipping noise spectrum when the embodiment described with reference to Figure 5 is used which achieved PAPR of -7.15 dB. On the other hand, a conventional method of clipping to fulfil 3% EVM across the entire carrier over protects users with 7% EVM requirements as shown by the figure but resulting PAPR is -7.5 dB. Figure 9 illustrates other simulation results for a CFR output using the embodiment described with reference to Figure 5 applied to a dual-carrier SBFD configuration where each carrier is 100 MHz split as 40 / 20 / 40 MHz being DL / UL / DL sub-bands. The solution achieves ~7.5 dB PAPR where the Probability Distribution Function (PDF) of both first and final stages are shown by Figure 10.. In this SBFD configuration, the objective of the compensation paths 403 is to remove the noise introduced by the generation of the high amplitude portion where the NCOs 504 center the high amplitude portion around the UL sub-bands while the filters 505 retrieve the high amplitude portion within the UL sub-bands and the NCOs 506 shift back the high amplitude pportion within UL sub-bands to their original locations in frequency. The values of at(I = 1 ,2) are set to — according PI to equation (13) which implies a full compensation within the UL sub-band that counteracts the filters 505 effects within the UL sub-band.

[0141] To evaluate the benefit of utilising the embodiments described herein, one would need to consider a single filter with 4096 taps that would be needed to implement the dynamic clipping described with reference to (Figure 9). This is compared to the 100 taps required for the first stage of filtering and two parallel filters with 256 taps each for the second stage required for the embodiments described herein. If one considers the effects of such a filtering operation in terms of delay, then the single filter with 4096 taps would cause a delay that is an order of magnitude higher than the embodiments described herein. The benefit of the proposed embodiments is even greater when there are more frequency regions with different EVM / distortion requirements, as a single filter would need to be even longer to cope with the number of different EVM / distortion requirements.

[0142] Figure 11 illustrates simulation results when using the embodiment described with reference to Figure 7a with 4 users in different locations. In this realization, the user located at ~6°can afford ~7.5% EVM while the other three users require EVM at a threshold of 3.0%. User directions are shown by dashed lines while the solid line curve is the radiated (normalized) power. The scale of the plot shows the EVM for the respective beams depicted by the rings.

[0143] Figure 12 illustrates an CFR module 1200 comprising processing circuitry (or logic) 1201. The processing circuitry 1201 controls the operation of the CFR module 1200 and can implement the method described herein in relation to an CFR module 1200. The processing circuitry 1201 can comprise one or more processors, processing units, multicore processors or modules that are configured or programmed to control the CFR module 1200 in the manner described herein. In particular implementations, the processing circuitry 1201 can comprise a plurality of software and / or hardware modules that are each configured to perform, or are for performing, individual or multiple steps of the method described herein in relation to the CFR module 1200. It will be appreciated that the CFR module 1200 may comprise one or more virtual machines running different software and / or processes. The CFR module 1200 may therefore comprise, or be implemented in or as one or more servers, switches and / or storage devices and / or may comprise cloud computing infrastructure that runs the software and / or processes.

[0144] Optionally, the CFR module 1200 may comprise a memory 1203. In some embodiments, the memory 1203 of the CFR module 1200 can be configured to store instructions (e.g. program code) executable by the processing circuitry 1201 of the CFR module 1200 whereby the CFR module is operable to perform the method as described herein with reference to Figure 2.

[0145] Alternatively or in addition, the memory 1203 of the CFR module 1200, can be configured to store any requests, resources, information, data, signals, or similar that are described herein. The processing circuitry 1201 of the CFR module 1200 may be configured to control the memory 1203 of the CFR module 1200 to store any requests, resources, information, data, signals, or similar that are described herein.

[0146] In some embodiments, the CFR module 1200 may optionally comprise a communications interface 1202. The communications interface 1202 of the CFR module 1200 can be for use in communicating with other nodes, such as other virtual nodes. For example, the communications interface 1202 of the CFR module 1200 can be configured to transmit to and / or receive from other nodes requests, resources, information, data, signals, or similar. The processing circuitry 1201 of CFR module 1200 may be configured to control the communications interface 1202 of the CFR module 1200 to transmit to and / or receive from other nodes requests, resources, information, data, signals, or similar. The communications interface 1202 can use any suitable communication technology.

[0147] The CFR module 1200 may be configured operate in the manner described herein in respect of an CFR module. There is also provided a computer program comprising instructions which, when executed on a least one processor (such as the processing circuitry 1201 of the CFR module 1200 described earlier), cause the processor to carry out at least part of the method(s) described herein. According to some embodiments there is provided a carrier containing the computer program. In some embodiments, the carrier can be any one of an electronic signal, an optical signal, an electromagnetic signal, an electrical signal, a radio signal, a microwave signal, or a computer-readable medium. There is also provided a (for example, tangible and / or non-transient) computer-readable medium comprising instructions which, when executed by at least one processor, cause the at least one processor to perform at least part of the method(s) described herein.

[0148] Embodiments described herein introduce a CFR algorithm that differentiate different EVM / distortion requirements across the frequency domain and / or a spatial domain.

[0149] In particular, embodiments described herein reduce the complexity and latency of the CFR due to a shorter filter length cascaded with a filter bank having short filter length instead of utilizing a (very) lengthy single filter capable of following the changes required by different inter and intra -carrier out-of-band emission (OoBE) requirements as well as the different EVM requirements across the total carrier bandwidth.

[0150] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims

CLAIMS1 . A method for performing crest factor reduction of a combined input signal, x / xm, to determine an output signal, y / ym, for transmission to a plurality of wireless devices, wherein the combined input signal is derived from a plurality of individual input signals wherein each individual input signal comprises data intended for one of the plurality of wireless devices, the method comprising: generating a high amplitude portion of the combined input signal, Xhigh / Xm high, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest level of clipping distortion allowed across the individual input signals, for one or more respective individual input signals of the plurality of individual input signals: generating a respective compensation signal based on the respective individual input signal, by utilizing a respective factor, aqderived from a respective level of clipping distortion allowed for the respective individual input signal; and determining the output signal by summing the combined input signal and the respective compensation signals, and subtracting the high amplitude portion of the input signal.

2. The method as claimed in claim 1 wherein generating the high amplitude portion of the combined input signal comprises: clipping the combined input signal to remove the high amplitude components of the combined input signal, removing the clipped combined input signal from a copy of the combined input signal to generate the high amplitude portion of the combined input signal.

3. The method as claimed in claim 1 or 2 wherein the step of deriving the output signal from the combined input signal further comprises: filtering the high amplitude portion of the combined input signal to filter out frequencies outside of the combined input signal.

4. The method as claimed in claim 1 wherein the step of generating a high amplitude portion of the combined input signal, Xhigh / Xmhigh, comprises: performing peak cancellation on the combined input signal to generate a peak cancelled version of the combined input signal; and subtracting the peak cancelled version of the combined input signal from a copy of the combined input signal.

5. The method as claimed in 1 to 4, wherein the respective individual input signal is associated with a respective frequency band, and the step of generating the compensation signal comprises: generating the respective compensation signal based on a respective component specific signal comprising frequency components of the high amplitude portion of the combined input signal that fall within the respective frequency band.

6. The method as claimed in claim 5 further comprising generating respective compensation signals for zero amplitude frequency portions of the combined input signal. .

7. The method as claimed in claim 5 or 6, wherein the step of generating the respective compensation signal further comprises: performing the following processing steps on the high amplitude portion of the combined input signal: obtaining signal components within the respective frequency band; and multiplying by the respective factor.

8. The method as claimed in claim 7, wherein the step of obtaining signal components within the respective frequency band comprises the following processing steps: frequency shifting to centre the high amplitude portion of the combined input signal on the respective frequency band; and baseband filtering; and frequency shifting to ensure the compensation signal is within respective frequency band.

9. The method as claimed in any one of claims 1 to 4, wherein the combined input signal comprises: a first branch signal, xm, derived by applying a precoding matrix to a respective plurality of user signals, Si , ... ,SL, intended for the respective plurality of wireless devices.

10. The method as claimed in claim 9 wherein the output signal is for coupling to a first antenna of an active antenna system.

11. The method as claimed in claim 9 or 10, wherein the respective individual input signal comprises a component of the combined input signal which is dependent on a respective user signal, Si, intended for a respective wireless device.

12. The method as claimed in claim 11 wherein the step of generating the respective compensation signal comprises: multiplying the respective individual input signal, wy,msy, by the respective factor, ay.

13. The method as claimed in any preceding claim wherein the clipping distortion comprises an error vector magnitude.

14. The method as claimed in any preceding claim wherein the respective factor is determined as:EVM2— EVMZa = , where EVM is the highest amount of clipping distortion, EVMi is thelevel of clipping distortion allowed for the respective individual input signal, and pi is a value representative of a respective power level in the respective individual input signal.

15. A crest factor reduction, CFR, module for performing CFR of a combined input signal (x, xm) to determine an output signal for transmission to a plurality of wireless devices, wherein the combined input signal is derived from a plurality of individual input signals wherein each individual input signal comprises data intended for one of the plurality of wireless device, the CFR module comprising:an input (401) configured to receive the combined input signal, x / xma amplitude compensation module (402) configured to generate a high amplitude portion of the combined input signal, Xhigh / Xm high, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest level of clipping distortion allowed across the individual input signals; and one or more compensation branches (403i to 403i_), wherein each compensation branch (403i) is configured to, for a respective individual input signal of the plurality of individual input signals: generate a respective compensation signal, Ci, based on the respective individual input signal, by utilizing a respective factor, Qi derived from a respective level of clipping distortion allowed for the respective individual input signal; and a combination element (404) configured to determine the output signal by summing the combined input signal and the respective compensation signals, and subtracting the high amplitude portion of the input signal.

16. The CFR module as claimed in claim 15 wherein the amplitude compensation module comprises: a clipping module (501) configured to clip the combined input signal to remove the high amplitude components of the combined input signal, and a subtraction block (503) configured to remove the clipped combined input signal from a copy of the combined input signal to generate the high amplitude portion of the combined input signal, Xhigh / Xm high.

17. The CFR module as claimed in claim 15 or 16, further comprising a filter (504) configured to filter the high amplitude portion of the combined input signal to filter out frequencies outside of the combined input signal.

18. The CFR module as claimed in claim 15 wherein the amplitude compensation module comprises : a peak cancellation block configured to perform peak cancellation on the combined input signal to generate a peak cancelled version of the combined input signal; anda subtraction module configured to subtract the peak cancelled version of the combined input signal from a copy of the combined input signal.

19. The CFR module as claimed in claim 15 to 18 wherein the respective individual input signal is associated with a respective frequency band, and each compensation branch is configured to: generate the respective compensation signal based on a respective component specific signal comprising frequency components of the high amplitude portion of the combined input signal that fall within the respective frequency band.

20. The CFR module as claimed in claim 19 further comprising one or more further compensation branches configured to generate respective compensation signals for zero amplitude frequency portions of the combined input signal.

21. The CFR module as claimed in claim 19 or 20, wherein each compensation branch generates the respective compensation signal from the high amplitude portion of the combined input signal utilizing one or more of: a first frequency shifter to shift the respective frequency band of the high amplitude portion of the combined input signal to be centred at Ohz; a baseband filter; a second frequency shifter ensure the compensation signal is within respective frequency band; and a multiplier configured to multiply by the respective factor.

22. The CFR module as claimed in any one of claims 15 to 18 wherein the combined input signal comprises a first branch signal, xm, derived by applying a precoding matrix to a respective plurality of user signals, Si , ... , SL, intended for the respective plurality of wireless devices.

23. The CFR module as claimed in claim 22 wherein the output signal is for coupling to a first antenna of an active antenna system.

24. The CFR module as claimed in claim 22 to 23, wherein the respective individual input signal comprises a component of the combined input signal which isdependent on a respective user signal, Si, intended for a respective wireless device, I.

25. The CFR module as claimed in any one of claims 22 to 24, wherein each compensation branch is configured to generate the respective compensation utilizing a multiplier configured to multiply the respective individual input signal by the respective factor.

26. The CFR module as claimed in any one of claims 15 to 25, wherein the clipping distortion level comprises an error vector magnitude.

27. The CFR module as claimed in any one of claims 15 to 26, wherein the respective factor is determined as:EVM2— EVMZa = , where EVM is the highest amount of clipping distortion, EVMi is thelevel of clipping distortion allowed for the respective individual input signal, and pi is a value representative of a respective power level in the respective individual input signal.

28. A Crest Factor Reduction, CFR, module comprising processing circuitry and memory, the memory containing instructions executable by the processing circuitry whereby the CFR module is operable to: generate a high amplitude portion of the combined input signal, Xhigh / Xm high, by applying an amplitude threshold to the combined input signal, wherein the amplitude threshold is derived from a highest level of clipping distortion allowed across the individual input signals, for one or more respective individual input signals of the plurality of individual input signals: generate a respective compensation signal based on the respective individual input signal, by utilizing a respective factor, aqderived from a respective level of clipping distortion allowed for the respective individual input signal; and determine the output signal by summing the combined input signal and the respective compensation signals, and subtracting the high amplitude portion of the input signal.

29. The CFR module as claimed in claim 28 wherein the memory further contains instructions executable by the processing circuitry whereby the CFR module is operable to perform the method as claimed in any one of claims 2 to 14.

30. A computer program, comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out a method according to any of claims 1 to 14.

31. A computer-readable medium comprising instructions that, when executed on at least one processor, cause the at least one processor to perform the method according to any of claims 1 to 14.

32. A computer program product comprising non transitory computer readable media having stored thereon a computer program according to claim 31.

33. A base station comprising a CFR module as claimed in any one of claims 15 to 29.

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