WO2020014825A1

WO2020014825A1 - Processing of reference signals in precoding

Info

Publication number: WO2020014825A1
Application number: PCT/CN2018/095813
Authority: WO
Inventors: Xuning SHAO; Frederick Vook; Eugene Visotsky
Original assignee: Nokia Shanghai Bell Co., Ltd.; Nokia Solutions And Networks Oy; Nokia Technologies Oy
Priority date: 2018-07-16
Filing date: 2018-07-16
Publication date: 2020-01-23
Also published as: CN112425079B; CN112425079A

Abstract

Embodiments of the present disclosure relate to a device, method and computer readable storage medium for processing of reference signals in precoding. In example embodiments, a first set of transmission layers are selected at a transmitter from a plurality of transmission layers via a plurality of antennas. Both feedback processing and feedforward processing are performed on a first set of reference signals to be transmitted on the first set of transmission layers. The first set of reference signals are transmitted on the first set of transmission layers. Moreover, a second set of transmission layers are selected from the plurality of transmission layers. For the second set of transmission layers, only the feedforward processing is applied to the reference signals. In this way, interference measurement and cancelling may be implemented at a receiver side on the selected layers, and therefore the receiver gain and the system performance may be improved.

Description

PROCESSING OF REFERENCE SIGNALS IN PRECODING

FIELD

Embodiments of the present disclosure generally relate to the field of signal processing, and in particular, to devices, methods and computer readable storage media for processing of reference signals in precoding.

BACKGROUND

Non-linear precoding (NLP) technology is proposed to improve the Multi-User Multiple Input Multiple Output (MU-MIMO) performance in New Radio (NR) . Compared with the linear precoding technology, the NLP technology may potentially achieve significantly higher throughput, especially in the case that the channels of users are highly correlated in a space domain.

Tomlinson-Harashima Precoding (THP) technology is a suboptimal variant of NLP, which is attractive due to its low complexity. In THP, interferences among different simultaneous transmission layers may be avoided by the feedback processing. For example, on a downstream layer, a signal to transmit is pre-distorted by the feedback processing to cancel the interferences from signals to be transmitted on one or more upstream layers. In the context of the present disclosure, an upstream layer refers to one of a plurality of transmission layers on which a signal is first processed for transmission, and a downstream layer refers to one of the transmission layers on which a signal is subsequently processed for transmission. The signals on the upstream and downstream layers are simultaneously transmitted via a plurality of antennas.

In order to enable either the non-linear or linear precoding, a network device such as a NR nodeB (or gNB) needs the knowledge of the channel states so that the network device can design a precoding matrix to compensate the effect of the channels on signal transmission. The channel states may be acquired from the channel state information (CSI) feedback of a terminal device such as user equipment (UE) , for example. As another example, the UE may transmit sounding reference signals (SRSs) to the network device. With channel reciprocity, the network device can obtain the channel state based on the SRSs. The UE also needs the CSI for the receiving process. The UE can measure the CSI from a demodulation reference signal (DMRS) which is known to both the gNB (as a transmitter) and the UE (as a receiver) . In NLP, reference signals require to be processed specially due to the use of the feedback processing for mitigating the inter-layer interferences.

SUMMARY

In general, example embodiments of the present disclosure provide devices, methods and computer readable storage media for processing of reference signals in precoding.

In a first aspect, a device is provided. The device comprises at least one processor and at least one memory including computer program code. The at least one memory and the computer program code configured to, with the at least one processor, cause the device to: select, at a transmitter, a first set of transmission layers from a plurality of transmission layers via a plurality of antennas; perform both feedback processing and feedforward processing on a first set of reference signals to be transmitted on the first set of transmission layers; and transmit the first set of reference signals on the first set of transmission layers. Moreover, the device is caused to select, at the transmitter, a second set of transmission layers other than the first set of transmission layers from the plurality of transmission layers via the plurality of antennas; perform the feedforward processing on a second set of reference signals to be transmitted on the second set of transmission layers from the plurality of transmission layers while avoiding the feedback processing on the second set of reference signals; and transmit the second set of reference signals on the second set of transmission layers simultaneously with the first set of reference signals.

In a second aspect, there is provided a method at a transmitter. In the method, a first set of transmission layers are selected from a plurality of transmission layers via a plurality of antennas. Both the feedback processing and the feedforward processing are performed on a first set of reference signals to be transmitted on the first set of transmission layers. Then, a second set of transmission layers other than the first set of transmission layers are selected from the plurality of transmission layers. The feedforward processing is performed on a second set of reference signals to be transmitted on the second set of transmission layers while avoiding the feedback processing on the second set of reference signals. The first set of reference signals are transmitted on the first set of transmission layers, and the second set of reference signals are transmitted on the second set of transmission layers, simultaneously.

In a third aspect, there is provided a device comprising at least one processor and at least one memory including computer program code. The at least one memory and the computer program code configured to, with the at least one processor, cause the device to: receive an indication of a first reference signal type of a first set of transmission layers to indicate that both feedback processing and feedforward processing have been performed on a first set of reference signals on the first set of transmission layers; receive an indication of a second reference signal type of a second set of transmission layers to indicate that the feedforward processing has been performed on a second set of reference signals without the feedback processing on the second set of reference signals on the second set of transmission layers; receive the first and second set of transmission layers simultaneously; and decode the first and second set of transmission layers in accordance with the first and second indications.

In a fourth aspect, there is provided a method comprising: receiving an indication of a first reference signal type of a first set of transmission layers to indicate that both feedback processing and feedforward processing have been performed on a first set of reference signals on the first set of transmission layers; receiving an indication of a second reference signal type of a second set of transmission layers to indicate that the feedforward processing has been performed on a second set of reference signals without the feedback processing on the second set of reference signals on the second set of transmission layers; receiving the first and second set of transmission layers simultaneously; and decoding the first and second set of transmission layers in accordance with the first and second indications.

In a fifth aspect, there is provided a computer readable storage medium that stores a computer program thereon. The computer program, when executed by a processor, causes the processor to perform the method according to the second or fourth aspect.

It is to be understood that the summary section is not intended to identify key or essential features of embodiments of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1 illustrates example architecture of a THP-enabled communication system;

FIG. 2 illustrates an example environment in which embodiments of the present disclosure can be implemented;

FIG. 3 illustrates a flowchart of an example method in accordance with some embodiments of the present disclosure;

FIG. 4 illustrates an example process of division of a plurality of transmission layers in accordance with some other embodiments of the present disclosure;

FIG. 5 illustrates a flowchart of an example method in accordance with some other embodiments of the present disclosure;

FIG. 6 illustrates an example signaling diagram between the transmitter and the receiver in accordance with some embodiments of the present disclosure;

FIG. 7 illustrates an example comparison of different schemes for processing the reference signals in the scenario of 8 UEs per cell in accordance with some embodiments of the present disclosure;

FIG. 8 illustrates an example comparison of different schemes for processing the reference signals in the scenario of 12 UEs per cell in accordance with some embodiments of the present disclosure;

FIG. 9 illustrates example power ratio between the full DMRS and data in accordance with some embodiments of the present disclosure; and

FIG. 10 illustrates a simplified block diagram of a device that is suitable for implementing embodiments of the present disclosure.

Throughout the drawings, the same or similar reference numerals represent the same or similar element.

DETAILED DESCRIPTION

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

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

As used herein, the term “transmitter” refers to a device capable of transmitting a signal. As used herein, the term “receiver” refers to a device capable of receiving a signal. The transmitter or receiver may be implemented by or as a part of any suitable device, including, for example, a network device or a terminal device.

As used herein, the term “network device” refers to any suitable device at a network side of a communication network. The network device may include any suitable device in an access network of the communication network, for example, including a base station (BS) , a relay, an access point (AP) , a node B (NodeB or NB) , an evolved NodeB (eNodeB or eNB) , a gigabit NodeB (gNB) , a Remote Radio Module (RRU) , a radio header (RH) , a remote radio head (RRH) , a low power node such as a femto, a pico, and the like.

As used herein, the term “terminal device” refers to a device capable of, configured for, arranged for, and/or operable for communications with a network device or a further terminal device in a communication network. The communications may involve transmitting and/or receiving wireless signals using electromagnetic signals, radio waves, infrared signals, and/or other types of signals suitable for conveying information over air. In some embodiments, the terminal device may be configured to transmit and/or receive information without direct human interaction. For example, the terminal device may transmit information to the network device on predetermined schedules, when triggered by an internal or external event, or in response to requests from the network side.

Examples of the terminal device include, but are not limited to, user equipment (UE) such as smart phones, wireless-enabled tablet computers, laptop-embedded equipment (LEE) , laptop-mounted equipment (LME) , and/or wireless customer-premises equipment (CPE) . For the purpose of discussion, some embodiments will be described with reference to UEs as examples of the terminal devices, and the terms “terminal device” and “user equipment” (UE) may be used interchangeably in the context of the present disclosure.

As used herein, the term “circuitry” may refer to one or more or all of the following:

(a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and

(b) combinations of hardware circuits and software, such as (as applicable) : (i) a combination of analog and/or digital hardware circuit (s) with software/firmware and (ii) any portions of hardware processor (s) with software (including digital signal processor (s)) , software, and memory (ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and

(c) hardware circuit (s) and or processor (s) , such as a microprocessor (s) or a portion of a microprocessor (s) , that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

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

As used herein, the singular forms “a” , “an” , and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “includes” and its variants are to be read as open terms that mean “includes, but is not limited to” . The term “based on” is to be read as “based at least in part on” . The term “one embodiment” and “an embodiment” are to be read as “at least one embodiment” . The term “another embodiment” is to be read as “at least one other embodiment” . Other definitions, explicit and implicit, may be included below.

For THP in NR, two types of processing of the DMRSs have been proposed, which will be described below with reference to FIG. 1. FIG. 1 shows example architecture of a THP-enabled communication system 100. In the system 100, a feedforward filter (F) 105 is designed to derive a lower triangular estimated effective channel

where K represents the number of transmission layers (or streams) ,

represents an expected receive filter, and

represents the estimated physical channel based on ordering of layers (or streams) . The lower triangular effective channel

may ensure that there is no interference from a downstream layer to an upstream layer.

For each resource element (RE) , the raw symbols

are processed by a feedback filter (B-I) 110 to avoid the interferences from an upstream layer to a downstream layer and processed by a modulo device 115 to limit the transmit power. The B matrix is a scaled version of the effective channel. It is assumed that

where the diagonal element b _i, i represents the channel of the ith layer after scaling and is normalized to 1. The pre-distorted symbol of the ith layer after the feedback processing and the modulo processing is represented as s _i = x _i -∑ _j＜i b _i, js _j + p _i, where each upstream layer j corresponds to a pre-distortion shift -b _i, js _j. p _i represents the modulo shift of layer-i, and p _i = 0 if the pre-distorted symbol is within the constellation boundary before modulo. The feedback and modulo processing can be expressed as X + P- (B-I) S= S, or equivalently S = B ^-1 (X + P) , where

B ^-1 represents an equivalent linear filter of the feedback processing if there is no modulo.

Ideally, if the base station has the perfect knowledge of H, the UE will apply the expected receive filter (120) and scaling (125) . The effective channel after scaling will exactly be represented by the B matrix, and the pre-distortion shift -b _i, js _j may perfectly cancel the interference from the jth layer. Furthermore, a RX modulo device 130 will reverse the modulo shift. In this case, each layer will have no interference.

In practice, the receive filter and scaling weights at the UE side are calculated by the channel and interference estimation (135) based on the DMRS. The estimated interference power may also be used in demodulation (140) . As shown, in THP, the DMRS may be inserted just before the feedforward filter 105 and precoded only by the feedforward filter 405, which is referred to as “feedforward DMRS” . Alternatively, the DMRS may be precoded by both the feedback filter 110 and the feedforward filter 105, which is referred to as “full DMRS” .

In a real-world network, the channel state information (CSI) at the base station is generally non-ideal. The imperfection of CSI may be caused by delay between CSI measurement and data transrnission, limited resolution of CSI reports, CSI quantization errors, and the like. Residual interferences are inevitable in the case with the non-ideal CSI, including the interference from a downstream layer to an upstream layer when the effective channel H ^eff is not lower triangular, and the interference from an upstream layer to a downstream layer when the pre-distortion cannot fully cancel the interference.

For the linear precoding, the UE can estimate the residual interferences based on the DMRS. For example, the DMRS is usually precoded in the same way as the data in the linear precoding. The channel and interference estimation from the DMRS is relatively accurate. Accordingly, the interferences may be mitigated with the receiver filter. The estimated interference power may also be used for calculating the log-likelihood ratio (LLR) during demodulation.

However, the interference measurement at the receiver side becomes very difficult for NLP due to the modulo device 115 introduced for power limitation. In NLP, the additional modulo shift of the modulo device is applied to the data, but not to the DMRS. The pre-distortion of the DMRS is different from the pre-distortion of the data, and therefore the additional modulo shift is unknown to the receiver side.

For example, for the full DMRS, no modulo is applied for the DMRS. In the case with the ideal CSI, the interference estimation may be accurate since no interference exists after the receive filtering. The interference estimation is also relatively accurate if the modulo shift of the modulo device 115 is not much, that is, most of the pre-distorted symbols are within the constellation boundary before modulo. However, there is a gap between the DMRS and the data when the CSI is non-ideal and the modulo shift is nontrivial. For data transmission, the RX modulo device 120 can only reverse the TX modulo shift of the target layer, but cannot reverse the modulo shift of the interfering layer. The modulo shift partially cancels the pre-distortion shift for data transmission, and changes the residual interferences. Similar to the interference estimation, the channel estimation from the DMRS is also impacted by the pre-distortion and the modulo shift, and is inaccurate in the case with non-ideal CSI.

Another critical issue with the full DMRS is that the TX power of the DMRS becomes higher than the data since there is no modulo device to limit the increased power of the pre-distorted DMRS. Since there is a power reduction to the data by the modulo processing, the power of the DMRS needs to back off as well. The desired power back-off may be dynamic since the channel estimation may deteriorate when there is too much power back-off. For the estimation of the channel, the UE has to know the dynamic power back-off value which may be difficult to be informed to the UE.

For the feedforward DMRS, only the feedforward processing but not the feedback processing is applied to the DMRS. The DMRS may be suffered to all the upstream to downstream interferences even if the interferences for data transmission may be cancelled by the pre-distortion. In this case, the measured interference from the DMRS is incorrect even with the ideal-CSI. Furthermore, just a Maximal Ratio Combining (MRC) receiver may be used, and only the noise may be taken as the input for the LLR calculation. As a result, the feedforward DMRS cannot benefit from the interference measurement at the receiver side.

Some studies are carried out on the performance of the full DMRS and the feedforward DMRS. In the case with a single receive antenna per UE and the ideal CSI at the base station, the potential benefit of the full DMRS from the interference measurement may not be achieved. The main drawback of the full DMRS results from the increased power of the pre-distorted DMRS. If the same power reduction is applied to the data and the DMRS, the full DMRS may have a bad performance, which means that the feedforward DMRS is better than the full DMRS.

Some approaches focus on reducing a power gap between the full DMRS and the data to improve the performance of the full DMRS. One approach is to apply a phase rotation to the feedforward precoding vector of each layer, and to optimize the phase rotations to minimize the TX power of the DMRS. The optimal phase rotations are dependent on the raw symbols of the DMRS. This approach is especially effective if there is only one resource element (RE) of the DMRS for a physical resource block (PRB) where the same feedforward matrix should be used. However, multiple DMRS REs (typically one OFDM symbol) per PRB are defined in NR to improve the channel estimation quality and to provide orthogonal DMRS ports. In this case, it is very difficult to find a set of phase rotations which may effectively reduce the power of all DMRS REs.

The inventor finds that no approach is focused on the interference measurement issue with the non-ideal CSI, especially the impact from the modulo processing. In addition, for the full DMRS, there is no effective and efficient approach to signal the power offset between the DMRS and the data to the receiver

Embodiments of the present disclosure provide a scheme of adaptively processing reference signals. According to this scheme, both the feedback processing and the feedforward processing are applied to reference signals to be transmitted on a set of transmission layers selected from a plurality of transmission layers via a plurality of antennas. On the rest of the transmission layers, the reference signals may be subject to only the feedforward processing without the feedback processing.

This scheme ensures that both the feedback and feedforward processing are performed on the selected set of transmission layers. Further, interference measurement and cancelling may be implemented at a receiver side, and therefore the receiver gain and the system performance (for example, throughput) may be improved.

FIG. 2 shows an example environment 200 in which embodiments of the present disclosure can be implemented. The environment 200, which is a part of a communication network, includes a transmitter 210 and two receivers 220-1 and 220-2 (collectively referred to as a receiver 220) . It is to be understood that one transmitter and two receivers are shown only for the purpose of illustration without suggesting any limitation to the scope of the present disclosure. The environment 200 may include any suitable number of transmitters and receivers adapted for implementing embodiments of the present disclosure.

The transmitter 210 and the receiver 220 can be implemented by or as a part of any suitable device. In some embodiments, the transmitter 210 may be implemented at a network device, and the receiver 220 may be implemented at a terminal device. In the embodiments where the environment 200 is a part of a relay communication network. In this example, the transmitter 210 may be implemented at a network device, and the receiver 220 may be at a relay, and vice versa. In some other embodiments, the transmitter 210 and the receiver 220 may be both implemented at terminal devices in device-to-device (D2D) communications, which may be alternatively referred to as sidelink, or vehicle to everything (V2X) .

In various embodiments, the transmitter 210 is equipped with a plurality of transmitting antennas. The transmitter 210 can communicate via the plurality of antennas with the receiver 220 which may have one or more antennas. The communication may follow any suitable communication standards or protocols such as Universal Mobile Telecommunications System (UMTS) , long term evolution (LTE) , LTE-Advanced (LTE-A) , the fifth generation (5G) NR, Wireless Fidelity (Wi-Fi) and Worldwide Interoperability for Microwave Access (WiMAX) standards, and employs any suitable communication technologies, including, for example, Multiple-Input Multiple-Output (MIMO) , Orthogonal Frequency Division Multiplexing (OFDM) , time division multiplexing (TDM) , frequency division multiplexing (FDM) , code division multiplexing (CDM) , Bluetooth, ZigBee, and machine type communication (MTC) , enhanced mobile broadband (eMBB) , massive machine type communication (mMTC) and ultra-reliable low latency communication (uRLLC) technologies.

Between the transmitter 210 and the receiver 220, a plurality of transmission layers are enabled on which reference signals are transmitted from the transmitter 210 to the receiver 220. The reference signal may be any suitable signal or training sequence that is known at both the transmitter 210 and the receiver 220. For example, the reference signals may include the DMRSs, sounding reference signals (SRSs) , and the like.

In various embodiments of the present disclosures, on a set of transmission layers selected from the plurality of transmission layers, the reference signals are subject to both the feedback and feedforward processing. As such, these reference signals may be used for interference cancellation at the receiver 220 to improve the receiver performance.

FIG. 3 shows a flowchart of an example method 300 in accordance with some embodiments of the present disclosure. The method 300 can be implemented at the transmitter 210 as shown in FIG. 2. For the purpose of discussion, the method 300 will be described with reference to FIG. 2.

At block 305, a set of transmission layers (referred to as a first set of transmission layers) are selected from a plurality of transmission layers via a plurality of antennas for both the feedback and feedforward processing. The selection may consider any suitable factors, such as channel correlation, line-of-sight or non-line-of-sight channels, channel changing speeds, and other channel conditions. For example, if the channels associated with certain transmission layers have relatively high correlation in the space domain, these transmission layers may not be selected to avoid too high increased powers of the pre-distorted references signals from the feedback and feedforward processing. As another example, the transrnission layers corresponding to line-of-sight channels or slow-changing channels may not be selected since the interferences may be easily detected and then cancelled at the receiver 220.

In some embodiments, a set of reference signals (referred to “first set of reference signals” ) to be transmitted on the selected transmission layers may have power reduction to back off for the increased powers of the data from the feedback and feedforward processing on the first set of transmission layers. In these embodiments, the first set of transmission layers may be selected such that the increased powers of the first set of reference signals to be transmitted on the first set of transmission layers are equal to or less than the reduced powers of the first set of reference signals to satisfy the overall power restriction. As such, the first set of reference signals may be transmitted with the restricted power, and the data may not be affected, for example. In this way, the system performance may benefit from the interference measurement at the receiver 220 with the reference signals pre-distorted based on the feedback and feedforward processing.

Upon the selection of the first set of transmission layers, at block 310, both the feedback and feedforward processing are performed on the first set of reference signals to be transmitted on the first set of transmission layers. As described above, the powers of the first set of reference signals may be reduced to offset the increasing of the data powers due to the pre-distortion from the feedback and feedforward processing. In some embodiments, each of the powers of the first set of reference signals may be reduced by a predetermined power level. The predetermined power level may be associated with the number of layers, the channel conditions, network deployment or scheduling, and the like.

By way of example, a semi-static power back-off value may be defined at the transmitter 210 for the first set of reference signals, and then indicated to the receiver 220. The indication of the power back-off value may be transmitted in any suitable signaling or message, such as in dedicated control information (DCI) , radio resource control (RRC) signaling, Media Access Control (MAC) control element (CE) , and the like. The use of the semi-static power back-off value may effectively and efficiently reduce the signaling overhead compared with the dynamical power back-off values.

At block 315, a further set of transmission layers (referred to “asecond set of transmission layers” ) are selected from plurality of transmission layers for only the feedforward processing without the feedback processing. Similar to the selection of the first set of transmission layers, the selection of the second set of transmission layers may consider any suitable factors, such as channel correlation, line-of-sight or non-line-of-sight channels, channel changing speeds, and other channel conditions. In some embodiments, the second set of transmissions may be the remaining layers of the plurality of transmission layers.

At block 320, the feedforward processing is performed on a set of reference signals (referred to as “a second set of reference signals” ) to be transmitted on the second set of transmission layers while the feedback processing is avoided.

An example process of dividing the plurality of transmission layers into the first and second sets of transmission layers will be described below with reference to FIG. 4. In this example, the reference signals are implemented by the DMRSs, and all of the transmission layers are selected to include in the first set from the start. As shown in FIG. 4, the process 400 starts at block 405. Then, the following operations (1) - (7) are performed at block 410.

(1) Define the power back-off value of α dB.

(2) Schedule K layers.

(3) For each subband (for example, each subcarrier) , calculate the feedforward filter F and the feedback filter B for the data.

(4) Set K ₁=K.

(5) For each subband, set

(6) For each subband, set B _DMRS=B.

(7) Define I _K as a K*K identity matrix.

For the DMRS, the feedforward precoding vector of the i-th layer is the i-th column of F _DMRS. The applying of the power back-off (for example, α dB) to the layer for the full DMRS is equivalent to scaling its feedforward precoding vector by

as indicated in the operation (5) . Thei-th row of B _DMRS contains the feedback weights for thei-th layer. The feedback weights are set to 0 for the feedforward DMRS.

At block 415, the power restriction is checked by comparing the DMRS power to the data power. For example, for each subband, the power restriction is decided by ||F|| ², where ||F|| is the Frobenius norm of the feedforward matrix of the subband. The DMRS power is decided by

since the feedback processing without the modulo processing is equivalent to a linear filter of

The full DMRS and the feedforward DMRS are precoded together by applying the filter

In this case, the DMRS power and the data power are compared by determining whether Sum (||F _DMRSB ^-1 _DMRS|| ²) ＜=Sum (||F|| ²) as shown.

If the DMRS power is higher than the data power, the process 400 proceeds to block 420 where the lowest downstream layer is switched from the full DMRS layer to the feedforward DMRS layer. For example, the following operations (8) - (10) may be performed for the switching as shown.

(8) For each subband, set F _DMRS (: , K ₁) = F (: , K ₁) to assign the K ₁-th column of F to the K ₁-th column of F _DMRS.

(9) For each subband, set B _DMRS (K ₁, : ) = I _K (K ₁, : ) to assign the K ₁-th line of B to the K ₁-th line of B _DMRS.

(10) Set K ₁=K ₁-1 to switch the lowest downstream layer from the full DMRS layer to the feedforward DMRS layer.

Then, the process 400 returns to block 415 to initiate the next iteration to check the power restriction. If it is determined at block 415 that the DMRS power is not higher than the data power, the power restriction is satisfied. Then, the process 400 proceeds to block 425 where K ₁ upstream layers are selected. For example, the following operations (11) - (13) may be performed as shown.

(11) For each subband, precode the DMRS with

(12) Set the DMRS type of the

layers

1, 2, ..., K ₁ as the full DMRS.

(13) Set the DMRS type of the layers K ₁+1, K ₁+2, ..., K as the feedforward DMRS.

The process 400 ends at block 430. In this way, the first K ₁ upstream layers are selected as the full DMRS layers while it is ensured that K ₁ is maximized under the power restriction. As such, the upstream layers usually slightly impacted by the modulo processing may be selected for the full DMRS to get accurate channel and interference estimations at the receiver. The downstream layers severely impacted by the modulo processing and needing more pre-distortion shifts may be selected for the feedforward DMRS.

In some embodiments, an indication of the reference signal type for a specific transmission layer may be transmitted to the corresponding receiver. For example, an indication of a reference signal type (referred to as “a first reference signal type” ) on the first set of transmission layers rnay be transmitted to indicate that both the feedback processing and the feedforward processing have been performed on the first set of reference signals. Moreover, an indication of a second reference signal type (referred to as “a second reference signal type” ) on the second set of transmission layers may be transmitted to indicate that the feedforward processing has been performed on the second set of reference signals while the feedback processing is not performed.

In the embodiments where the predetermined power level for the reduction of the powers of the first set of reference signals is indicated to the receiver, the indications of the predetermined reduced power level and the reference signal type may be jointly encoded and then indicated in one bit to further reduce the overhead. For example, for one layer, it is indicated whether the full DMRS is enabled with the power back-off value or the feedforward DMRS is enabled with no power back-off.

Still with reference to FIG. 3, upon the feedback and feedforward processing, at block 325, the first set of reference signals are transmitted on the first set of transmission layers, and the second set of reference signals are transmitted on the second set of transmission layers, simultaneously. Accordingly, at the receiver side, using the pre-distorted reference signals from the feedback and feedforward processing, the interference measurement may be performed for the corresponding transmission layers.

FIG. 5 shows a flowchart of an example method 500 in accordance with some other embodiments of the present disclosure. The method 500 can be implemented at the receiver 220 as shown in FIG. 2.

At block 505, an indication of the first reference signal type of the first set of transmission layers is received to indicate that both the feedback processing and feedforward processing have been performed on the first set of reference signals on the first set of transmission layers. At block 510, an indication of the second reference signal type of the second set of transmission layers is received to indicate that the feedforward processing has been performed on the second set of reference signals without the feedback processing on the second set of reference signals on the second set of transmission layers.

At block 515, the first and second set of transmission layers are received simultaneously. At block 520, the first and second set of transmission layers are decoded in accordance with the first and second indications.

The decoding may be implemented in any suitable way. In some embodiments, the channel estimation and interference estimation may be performed for the first set of layers based on the first set of reference signals. The channel estimation may be performed for the second set of layers based on the second set of reference signals while the interference estimation is avoided. Then, the first set of layers is decoded using a minimum mean square error receiver. The second set of layers is decoded using a maximum ratio combining receiver. In some embodiments, a pre-defined power back-off may be reversed for the first set of transmission layers while avoiding reversing the pre-defined power back-off for the second first set of transmission layers.

It is also to be understood that all operations and features related to the method 300 as described above with reference to FIGS. 2-4 are likewise applicable to the method 500 and have similar effects. For the purpose of simplification, the details will be omitted.

FIG. 6 shows an example signaling diagram between the transmitter and the receiver according to some embodiments of the present disclosure. In this example, the transmitter 210 is implemented at the gNB, the receiver 220 is implemented at the UE. The reference signal is implemented by the DMRS.

As shown in FIG. 6, at block 605, the gNB first defines the power back-off value of α dB for the full DMRS and sends the value to the UE via downlink signalling, such as RRC configuration, MAC CE or DCI.

During scheduling, at block 610, the gNB may design the feedforward and feedback filters and apply the feedforward and feedback filters and modulo to the data. Moreover, the gNB selects either the full DMRS or the feedforward DMRS for each layer. The gNB applies the feedforward and feedback filters and the power back-off to the full DMRS. The gNB applies the feedforward filter to the feedforward DMRS.

At block 615, the DMRS type is signalled to the UE in the control channel via the DMRS type indicator. At block 620, the gNB transmits the DMRS and data to the UE.

At block 625, since for the layers with the full DMRS, the UE performs the channel and interference measurement based on the full DMRS and reverses the power back-off for the desired channel. Then, the UE performs the Minimum Mean Square Error (MMSE) receiver processing and the LLR calculation based on the residual interference plus noise. This receiver processing is similar to the processing in the linear precoding. For the layers with the feedforward DMRS, the UE simply applies the Maximal Ratio Combining (MRC) receiver processing and noise based LLR calculation, which do not require any information about interferences.

Four transmission schemes are compared in simulation, including 1) the baseline scheme of linear zero forcing with the MMSE receiver, 2) THP with the feedforward DMRS and the MRC receiver, 3) THP with the full DMRS and the MMSE receiver, and 4) THP with adaptive DMRS and DMRS type dependent receiver. For the full DMRS, it is assumed that there is no power restriction for the DMRS. The simulation settings are shown in Table I.

Table I

The simulation graph 700 for 8 UEs per cell and the simulation graph 800 for 12 UEs per cell are shown in FIGS. 7 and 8, respectively. Both the LoS channel and NLoS channel are considered in each scenario. It can be seen from the simulation results that THP is generally better than the linear precoding even though the channel estimation and interference measurement are less accurate in THP. The comparison between the feedforward DMRS and the full DMRS is highly dependent on the scenario. The feedforward DMRS performs better in the LoS channel, while the full DMRS performs better in the NLoS channel. This is mainly because the NLoS channel is more dynamic, and interference suppression becomes more important in the NLoS channel. The feedforward DMRS have better performance in the high load (12 UEs) scenario, while the full DMRS is better in the relatively low load (8 UEs) scenario. The reason is that the impact from the modulo processing is significant for the last several layers in the high load scenario. Remarkably, the full DMRS is much better than the feedforward DMRS for NLoS in FIG. 7, while the feedforward DMRS is much better than the full DMRS for LoS in FIG. 8.

The proposed adaptive DMRS always achieves the best performance among all the THP schemes, since it reaches the desired trade-off between the gain from interference measurement and the loss from the lack of modulo in the DMRS. The power back-off for full DMRS is set as 3 dB in this example. A higher back-off value gives some margin for supporting additional full DMRS layers, and makes the performance of adaptive DMRS closer to the full DMRS. In contrast, adaptive DMRS with a low power back-off value is closer to the feedforward DMRS. It is possible to further optimize the DMRS type selection algorithm and the power back-off value for different scenarios. For example, the scheduler can assign the full DMRS to the NLoS UEs and the feedforward DMRS to the LoS UEs in a mixed LoS/NLoS scenario.

FIG. 9 shows a graph 900 of power ratio between the full DMRS and data when the feedforward and feedback filters from data are directly applied to the DMRS. It can be seen that the full DMRS needs much more power than the data in the LoS channel and the high load scenario. In the other word, there is more impact from the modulo processing on the data in this scenario, which means less accurate channel estimation and interference measurement from the full DMRS. It is assumed that no power restriction is applied for full DMRS in the simulation, but a power back-off has to be applied according to the power ratio in practice. It is difficult to signal the dynamic power ratio if the full DMRS is applied for all layers. In contrast, only a very limited back-off value of 3 dB is necessary for the adaptive DMRS, and the overhead is as small as one bit per layer in the control channel. As such, the adaptive DMRS improves the throughput by selecting a DMRS type for each layer. The joint signaling of the DMRS type and the power back-off information may further reduce the overhead.

In some embodiments, an apparatus capable of performing the

method

300 or 500 may comprise means for performing the respective steps of the

method

300 or 500. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

In some embodiments, the apparatus capable of performing the method 300 comprises: means for selecting, at a transmitter, a first set of transmission layers from a plurality of transmission layers via a plurality of antennas; means for performing both feedback processing and feedforward processing on a first set of reference signals to be transmitted on the first set of transmission layers; means for selecting, at the transmitter, a second set of transmission layers other than the first set of transmission layers from the plurality of transmission layers via the plurality of antennas; means for performing the feedforward processing on a second set of reference signals to be transmitted on the second set of transmission layers from the plurality of transmission layers while avoiding the feedback processing on the second set of reference signals; and means for transmitting the first set of reference signals on the first set of transmission layers and simultaneously transmit the second set of reference signals on the second set of transmission layers.

In some embodiments, the apparatus may further comprise means for transmitting an indication of a first reference signal type on the first set of transmission layers to indicate that both the feedback processing and the feedforward processing have been performed on the first set of reference signals.

In some embodiments, the apparatus may comprise means for transmitting an indication of a second reference signal type on the second set of transmission layers to indicate that the feedforward processing has been performed on the second set of reference signals without the feedback processing on the second set of reference signal.

In some embodiments, each of powers of the first second set of reference signals is reduced by a predetermined power level.

In some embodiments, the apparatus may further comprise means for transmitting an indication of the predetermined power level on the first set of transmission layers.

In some embodiments, the means for selecting the first set of transmission layers may comprise means for selecting the first set of transmission layers such that a power increase due to the feedforward and feedbackward processing of the first set of reference signals to be transmitted on the first set of transmission layers are equal to or less than a power reduction of the first set of reference signals.

In some embodiments, the apparatus capable of performing the method 500 comprises: means for receiving an indication of a first reference signal type of a first set of transmission layers to indicate that both feedback processing and feedforward processing have been performed on a first set of reference signals on the first set of transmission layers; means for receiving an indication of a second reference signal type of a second set of transmission layers to indicate that the feedforward processing has been performed on a second set of reference signals without the feedback processing on the second set of reference signals on the second set of transmission layers; means for receiving the first and second set of transmission layers simultaneously; and means for decoding the first and second set of transmission layers in accordance with the first and second indications.

In some embodiments, the means for decoding the first and second set of transmission layers may comprise: means for performing channel estimation and interference estimation for the first set of layers based on the first set of reference signals; means for performing channel estimation while avoiding interference estimation for the second set of layers based on the second set of reference signals; means for decoding the first set of layers using a minimum mean square error receiver; and means for decoding the second set of layers using a maximum ratio combining receiver.

In some embodiments, the means for decoding the first and second set of transmission layers may comprise: means for reversing a pre-defined power-backoff for the first set of transmission layers while avoiding reversing the pre-defined power-backoff for the second set of transmission layers.

FIG. 10 is a simplified block diagram of a device 1000 that is suitable for implementing embodiments of the present disclosure. The device 1000 can be implemented at or as at least a part of the transmitter 210 as shown in FIG. 2.

As shown, the device 1000 includes a processor 1010, a memory 1020 coupled to the processor 1010, a communication module 1030 coupled to the processor 1010, and a communication interface (not shown) coupled to the communication module 1030 having a a plurality of antennas (not shown) . The memory 1020 stores at least a program 1040. The communication module 1030 is for bidirectional communications. The communication interface may represent any interface that is necessary for communication.

The program 1040 is assumed to include program instructions that, when executed by the associated processor 1010, enable the device 1000 to operate in accordance with the embodiments of the present disclosure, as discussed herein with reference to FIGS. 2-9. The embodiments herein may be implemented by computer software executable by the processor 1010 of the device 1000, or by hardware, or by a combination of software and hardware. The processor 1010 may be configured to implement various embodiments of the present disclosure.

The memory 1020 may be of any type suitable to the local technical network and may be implemented using any suitable data storage technology, such as a non-transitory computer readable storage medium, semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one memory 1020 is shown in the device 1000, there may be several physically distinct memory modules in the device 1000. The processor 1010 may be of any type suitable to the local technical network, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 1000 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

All operations and features related to the transmitter 210 and the receiver 220 as described above with reference to FIGS. 2-9 are likewise applicable to the device 1000 and have similar effects. For the purpose of simplification, the details will be omitted.

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

The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the

method

300 or 500 as described above with reference to FIGS. 2-9. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

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

In the context of the present disclosure, the computer program codes or related data may be carried by any suitable carrier to enable the device, apparatus or processor to perform various processes and operations as described above. Examples of the carrier include a signal, computer readable media.

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

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

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

Various embodiments of the techniques have been described. In addition to or as an alternative to the above, the following examples are described. The features described in any of the following examples may be utilized with any of the other examples described herein.

Claims

A device comprising:

at least one processor; and

at least one memory including computer program code;

the at least one memory and the computer program code configured to, with the at least one processor, cause the device to:

select, at a transmitter, a first set of transmission layers from a plurality of transmission layers via a plurality of antennas;

perform both feedback processing and feedforward processing on a first set of reference signals to be transmitted on the first set of transmission layers;

select, at the transmitter, a second set of transmission layers other than the first set of transmission layers from the plurality of transmission layers via the plurality of antennas;

perform the feedforward processing on a second set of reference signals to be transmitted on the second set of transmission layers from the plurality of transmission layers while avoiding the feedback processing on the second set of reference signals; and

transmit the first set of reference signals on the first set of transmission layers and simultaneously transmit the second set of reference signals on the second set of transmission layers.
The device of claim 1, wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the device to:

transmit an indication of a first reference signal type on the first set of transmission layers to indicate that both the feedback processing and the feedforward processing have been performed on the first set of reference signals.
The device of claim 1 or 2, wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the device to:

transmit an indication of a second reference signal type on the second set of transmission layers to indicate that the feedforward processing has been performed on the second set of reference signals without the feedback processing on the second set of reference signals.
The device of any of claims 1-3, wherein each of powers of the first second set of reference signals is reduced by a predetermined power level.
The device of claim 4, wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the device to:

transmit an indication of the predetermined power level for the first set of transmission layers.
The device of claim 4 or 5, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the device to:

select the first set of transmission layers such that a power increase due to the feedforward and feedbackward processing of the first set of reference signals to be transmitted on the first set of transmission layers are equal to or less than a power reduction of the first set of reference signals.
A method comprising:

selecting, at a transmitter, a first set of transmission layers from a plurality of transmission layers via a plurality of antennas;

performing both feedback processing and feedforward processing on a first set of reference signals to be transmitted on the first set of transmission layers;

selecting, at the transmitter, a second set of transmission layers other than the first set of transmission layers from the plurality of transmission layers via the plurality of antennas;

performing the feedforward processing on a second set of reference signals to be transmitted on the second set of transmission layers from the plurality of transmission layers while avoiding the feedback processing on the second set of reference signals; and

transmitting the first set of reference signals on the first set of transmission layers and simultaneously transmitting the second set of reference signals on the second set of transmission layers.
The method of claim 7, further comprising:

transmitting an indication of a first reference signal type on the first set of transmission layers to indicate that both the feedback processing and the feedforward processing have been performed on the first set of reference signals.
The method of claim 7 or 8, further comprising:

transmitting an indication of a second reference signal type on the second set of transmission layers to indicate that the feedforward processing has been performed on the second set of reference signals without the feedback processing on the second set of reference signals.
The method of any of claims 7-9, wherein each of powers of the first second set of reference signals is reduced by a predetermined power level.
The method of claim 10, further comprising:

transmitting an indication of the predetermined power level for the first set of transmission layers.
The method of claim 10 or 11, wherein selecting the first set of transmission layers comprises:

selecting the first set of transmission layers such that a power increase due to the feedforward and feedbackward processing of the first set of reference signals to be transmitted on the first set of transmission layers are equal to or less than a power reduction of the first set of reference signals.
A computer readable storage medium storing a computer program thereon, the computer program, when executed by a processor, causing the processor to perform actions comprising:

selecting, at a transmitter, a first set of transmission layers from a plurality of transmission layers via a plurality of antennas;

performing both feedback processing and feedforward processing on a first set of reference signals to be transmitted on the first set of transmission layers;

selecting, at the transmitter, a second set of transmission layers other than the first set of transmission layers from the plurality of transmission layers via the plurality of antennas;

performing the feedforward processing on a second set of reference signals to be transmitted on the second set of transmission layers from the plurality of transmission layers while avoiding the feedback processing on the second set of reference signals; and

transmitting the first set of reference signals on the first set of transmission layers and simultaneously transmitting the second set of reference signals on the second set of transmission layers.
The computer readable storage medium of claim 13, wherein the actions further comprise:

transmitting an indication of a first reference signal type on the first set of transmission layers to indicate that both the feedback processing and the feedforward processing have been performed on the first set of reference signals.
The computer readable storage medium of claim 13 or 14, wherein the actions further comprise:

transmitting an indication of a second reference signal type on the second set of transmission layers to indicate that the feedforward processing has been performed on the second set of reference signals without the feedback processing on the second set of reference signals.
The computer readable storage medium of any of claims 13-15, wherein each of powers of the first second set of reference signals is reduced by a predetermined power level.
The computer readable storage medium of claim 16, wherein the actions further comprise:

transmitting an indication of the predetermined power level of the first set of transmission layers.
The computer readable storage medium of claim 16 or 17, wherein selecting the first set of transmission layers comprises:

selecting the first set of transmission layers such that a power increase due to the feedforward and feedbackward processing of the first set of reference signals to be transmitted on the first set of transmission layers are equal to or less than a power reduction of the first set of reference signals.
A device comprising:

at least one processor; and

at least one memory including computer program code;

the at least one memory and the computer program code configured to, with the at least one processor, cause the device to:

receive an indication of a first reference signal type of a first set of transmission layers to indicate that both feedback processing and feedforward processing have been performed on a first set of reference signals on the first set of transmission layers;

receive an indication of a second reference signal type of a second set of transmission layers to indicate that the feedforward processing has been performed on a second set of reference signals without the feedback processing on the second set of reference signals on the second set of transmission layers;

receive the first and second set of transmission layers simultaneously; and

decode the first and second set of transmission layers in accordance with the first and second indications.
The device of claim 19, wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the device to:

perform channel estimation and interference estimation for the first set of layers based on the first set of reference signals;

perform channel estimation while avoiding interference estimation for the second set of layers based on the second set of reference signals;

decode the first set of layers using a minimum mean square error receiver; and

decode the second set of layers using a maximum ratio combining receiver.
The device of claim 19 or 20, wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the device to:

reverse a pre-defined power back-off for the first set of transmission layers while avoiding reversing the pre-defined power back-off for the second set of transmission layers.
A method comprising:

receiving an indication of a first reference signal type of a first set of transmission layers to indicate that both feedback processing and feedforward processing have been performed on a first set of reference signals on the first set of transmission layers;

receiving an indication of a second reference signal type of a second set of transmission layers to indicate that the feedforward processing has been performed on a second set of reference signals without the feedback processing on the second set of reference signals on the second set of transmission layers;

receiving the first and second set of transmission layers simultaneously; and

decoding the first and second set of transmission layers in accordance with the first and second indications.
The method of claim 22, wherein decoding the first and second set of transmission layers comprises:

performing channel estimation and interference estimation for the first set of layers based on the first set of reference signals;

performing channel estimation while avoiding interference estimation for the second set of layers based on the second set of reference signals;

decoding the first set of layers using a minimum mean square error receiver; and

decoding the second set of layers using a maximum ratio combining receiver.
The method of claim 22 or 23, wherein decoding the first and second set of transmission layers comprises:

reversing a pre-defined power back-off for the first set of transmission layers while avoiding reversing the pre-defined power back-off for the second set of transmission layers.
A computer readable storage medium storing a computer program thereon, the computer program, when executed by a processor, causing the processor to perform actions comprising:

receiving an indication of a first reference signal type of a first set of transmission layers to indicate that both feedback processing and feedforward processing have been performed on a first set of reference signals on the first set of transmission layers;

receiving an indication of a second reference signal type of a second set of transmission layers to indicate that the feedforward processing has been performed on a second set of reference signals without the feedback processing on the second set of reference signals on the second set of transmission layers;

receiving the first and second set of transmission layers simultaneously; and

decoding the first and second set of transmission layers in accordance with the first and second indications.
The computer readable storage medium of claim 25, wherein decoding the first and second set of transmission layers comprises:

performing channel estimation and interference estimation for the first set of layers based on the first set of reference signals;

performing channel estimation while avoiding interference estimation for the second set of layers based on the second set of reference signals;

decoding the first set of layers using a minimum mean square error receiver; and

decoding the second set of layers using a maximum ratio combining receiver.
The computer readable storage medium of claim 25 or 26, wherein decoding the first and second set of transmission layers comprises:

reversing a pre-defined power back-off for the first set of transmission layers while avoiding reversing the pre-defined power back-off for the second set of transmission layers.