Network node and a method performed by the network node

Abstract

A network node, a method performed by a network node and a computer program configured to: · transmit a signal using at least two different precoders across N subbands, · pre-process the precoders across the N subbands before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.

Description

[0001] NETWORK NODE AND A METHOD PERFORMED BY THE NETWORK NODE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a network node and the method performed by said network node. It further relates to the field of subband precoding.

[0004] BACKGROUND OF THE INVENTION

[0005] One of the potential features to enhance the capacity of data transmission of a network node is precoding.

[0006] Precoding is a generalization of beamforming to support multi-stream, or multi-layer, transmission in multi-antenna wireless communications. In conventional single-stream beamforming, the same signal is emitted from each of the transmit antennas with appropriate weighting, phase and gain, such that the signal power is maximized at the receiver output. When the receiver has multiple antennas, single-stream beamforming cannot simultaneously maximize the signal level at all of the receive antennas. In order to maximize the throughput in multiple receive antenna systems, multi-stream transmission is generally required.

[0007] In point-to-point systems, precoding means that multiple data streams are emitted from the transmit antennas with independent and appropriate weightings such that the link throughput is maximized at the receiver output. In multi-user Multiple-input Multiple-output (MIMO), the data streams are intended for different users, known as Space-division multiple access (SDMA), and some measure of the total throughput, e.g., the sum performance or max-min fairness, is maximized.

[0008] Precoding is a technique used in MU-MIMO systems to eliminate multi-user interferences by applying specific algorithms at the transmitter side to enhance signal quality and reduce computational complexity.

[0009] In point-to-point systems, some of the benefits of precoding can be realized without requiring channel state information at the transmitter, while such information isessential to handle the inter-user interference in multi-user systems.

[0010] According to the above, precoding is a technique which exploits transmit diversity by weighting information stream, i.e. the transmitter sends the coded information to the receiver in order to the pre-knowledge of the channel. The receiver is a simple detector, such as matched filter, and does not have to know the channel side information. This technique will reduce the corrupted effect of communication channel.

[0011] By using pilot-assisted techniques, transmitter will have ability to predict the channel without feedback. It is called forward error control, and precoding is a technique of forward error control.

[0012] Considering a multi-antenna transmitter with NTXantenna ports simultaneously transmitting information to a receiver with NRXantenna ports in NSBsubbands, i.e., group of Physical Resource Block (PRBs), where the precoder Wve CN ×l×NjSapplied to the transmitted signal s e Cl×1×Nwith I being the number of transmission layers, and accordingly, the received signal y e CN×1×Ncan be expressed as (neglecting noise sources):

[0013] y = H wvs,

[0014] where H e CNTX X NRXX NSB jSthe channel. The goal for the transmitter is to apply the set of precoders Wvsuch that the norm of the effective channel || He||, where He= H Wve CNX1XNSB iSlarge, this norm represents the desired channel gain towards the receiver. Note that the same precoder Wvcan be applied across all the subbands corresponding to the channel H, known as wideband precoding or different precoders can be applied to different subbands corresponding to the channel H, known as subband precoding.

[0015] According to the above, H has dimension of NRX X NTXX NSB and W has dimension of NTXX I x NSB- SO, when we do the HW multiplication, we do matrix multiplicationalong first two dimensions NRX× NTX) for H and NTXx I) for W per subband (SB). Hence the final dimension is NRX× l × NSB.

[0016] The term "simultaneously transmitting" is understood in this application as transmitting the data from all the antenna ports at the same time and frequency resources.

[0017] Since a multipath radio channel varies with frequency, varying the precoder to be used for transmission along the channel bandwidth can better adapt to the channel, and therefore improves the performance of a radio link. The NR downlink (DL) supports subband or frequency-selective precoding using a DeModulation Reference Signal (DMRS) which is precoded to match the Physical Downlink Shared Channel (PDSCH), where the precoding should be held constant by the Next Generation Node B (gNB) over specified Physical Resource Block (PRBs) within Precoding Resource Block Groups (PRGs), wherein PRG consists of one or more PRBs.

[0018] By contrast, NR uplink (UL) does not support subband precoding, since at most one precoder, indicated for instance by a Transmit Precoder Matrix Indicator (TPMI), is used to schedule a Physical Uplink Shared Channel (PUSCH) for feedback-based codeboook precoding and because a PRG is not defined for the uplink. However, subband precoding is envisioned to be discussed for 6G also for UL to extract the gain from subband precoding seen in the DL.

[0019] Note that the receiver will estimate the channel experienced by the DMRS, which will be including the precoder: H Wv. Due to the Wvbeing unknown at the receiver this means that the receiver can only assume that the channel is smooth / constant within each subband, not across the subbands, unless precoding is wideband. This limits the freedom the channel estimator has in interpolation of the channel, and hence hurts performance.

[0020] The subband precoding involves applying different precoders to different subbands in the transmission bandwidth, which results in discontinuities at the subbandboundaries. These discontinuities effectively introduce artificial delay spread by making the effective channel Hehighly frequency selective. While, with ideal channel estimation at the receiver, there can be performance gain with subband precoding, the discontinuities result in a poor channel estimation adversely affecting the performance. Ideal channel estimation is understood as when it is perfectly known at the receiver.

[0021] In the state of the art, precoders are obtained either by:

[0022] o a Channel State Information (CSI) report or Transmit Precoder Matrix Indicator (TPMI) feedback from receiver, which can include the precoders per subband, or,

[0023] o using a reciprocity-based assumption, which include extracting precoder based on a reverse-link reference signal.

[0024] Though one solution is to perform channel estimation per subband, however, it results in smaller processing gain (PG) obtained from processing over a group of frequency units, due to smaller number of frequency units, i.e., subcarriers, in a subband compared to the wideband. This results in poor performance of channel estimation in the low Signal to Interference & Noise Ratio (SINR) regime.

[0025] SUMMARY OF THE INVENTION

[0026] It is an object of the invention a network node configured to:

[0027] • transmit a signal using at least two different precoders across N subbands,

[0028] • apply a pre-processing to the precoders across the N subbands before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries. It helps to minimize the effect of discontinuity at said subband boundaries.Signal is understood in this application as a vector of one or more complex numbers, where each complex number representing coded and modulated data bits. Output signal is the signal that comes out of the precoders.

[0029] A frequency band or subband is the range or interval of radio frequencies used to transmit a signal over a telecom network.

[0030] Specifically, the network node is configured such that the precoders applied across the subbands are firstly pre-processed to minimize the effect of discontinuity at the subband boundaries, i.e., the precoders are smoothen across the subbands, before the signal transmission.

[0031] According to the above, the network node object of the invention is configured to:

[0032] • transmit a signal in which at least two different precoders have been applied across N subbands,

[0033] • pre-process the precoders across the N subbands before applying the precorders to the signal, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.

[0034] Thus, in a first step the precoders are pre-processed and in a second step the signal is transmitted using the pre-processed precoders.

[0035] The invention is thus specifically directed to a network node configured for precoding smoothing. According to the invention, for a fixed transmission layer and transmit antenna pair, the vector obtained from the stacking the set of precoders, stacked in sequence over increasing frequency, is smooth, i.e., the discontinuity at subband boundary or limit is processed to minimize the addition of artificial frequency selectivity or delay spread to the effective channel. More specifically, the network node is configured such that it computes at least two precoders.Thus, the invention is directed to a network node obtaining frequency selective precoders such that the effective channel experienced by the receiver is smooth, i.e., effect of discontinuity at the subband boundaries on the effective channel is restricted, while preserving MIMO rank.

[0036] This minimizes the effect of discontinuity at the subband boundary of the effective channel, there by aiding the receiver algorithms and preventing performance loss.

[0037] For subband precoding, this invention describes precoder smoothening which produces a smooth effective channel even if precoder selection is not smooth in the first step by reducing the effect of discontinuities at the subband boundaries.

[0038] An example of such non-smooth precoder selection in the first step is Pre-coding Matrix Indicator (PMI) based subband precoding, where the claimed network node allows subband precoding to maintain single user / multiple users SU / MU-MIMO performance gains. The smoothness property of the network node of the claimed invention enables more efficient operation of the receiver algorithms, e.g. channel estimation algorithms.

[0039] In this invention the transmitter and the receiver can refer to a user equipment (UE) and the network, respectively, for PUSCH transmission, or the transmitter and the receiver can refer to the network and a UE, respectively, for PDSCH transmission. The invention is applicable in the uplink and in the downlink.

[0040] It is also an object of the present invention a first network node being configured to receive a signal transmitted by at least a second network node. The first network node being configured to:

[0041] receive a signal in which at least two different precoders have been used across N subbands and a pre-processing to the precoders across the N subbands has been applied before the signal is transmitted, the pre-processingconfigured to diminish the discontinuity of the output signals from the precoders at the subband boundaries, and

[0042] • use the received signal to generate a channel estimate.

[0043] It is also an additional object of the invention a method performed by a network node, the method comprising:

[0044] • transmitting a signal using at least two different precoders across N subbands,

[0045] • applying a pre-processing to the precoders across the N subbands before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.

[0046] It is also an additional object of the invention a method performed by a network node, the method comprising:

[0047] • receiving a signal with a precoding in the received signal so that at least two different precoders are applied across the different N subbands, and a preprocessing to the precoders across the N subbands has been used before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries, and

[0048] • using the received signal to generate a channel estimate.

[0049] It is also an object of the invention a computer program comprising instructions which when executed by a processing unit of a network node causes the network node to perform any of the previous methods.

[0050] A network nodes system may also be another object of the invention, comprising a first node and a second node, the first node being configured to transmit a signal andthe second node being configured to receive said signal, the first node being configured to apply a precoding to the transmitted signal so that at least two different precoders are configured to be applied across N subbands, wherein the first node is configured to apply a pre-processing to the precoders across the subbands before the signal is transmitted to the second node, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.

[0051] DESCRIPTION OF THE FIGURES

[0052] To complete the description and to provide a better understanding of the invention, drawings are provided. Said drawings form an integral part of the description and illustrate preferred embodiments of the invention. The drawings comprise the following figures.

[0053] Figure 1 shows a schematic representation of an embodiment of a subband precoding, where N PRBs are divided into NSBsubbands and different precoders are applied to each subband.

[0054] Figure 2 shows a chart showing the transmit precoders with and without smoothening for a transmit antenna and a transmission layer along the subcarrier dimension.

[0055] DETAILED DESCRIPTION OF THE INVENTION

[0056] As previously stated, is an object of the claimed invention a network node configured to:

[0057] • transmit a signal using at least two different precoders across N subbands,

[0058] pre-process the precoders across the N subbands before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.An example for the subband precoding is shown in Figure 1, where N PRBs are divided into NSBsubbands such that different N precoders can be applied to each NSBsubband.

[0059] In a first embodiment, to smoothen the precoders, the network node is configured to pre-process the precoders by:

[0060] - applying a linear transform to the precoders across the N subband domain, - zeroing out coefficients or samples which represents transitions along the subband boundary in the transformed precoders,

[0061] - applying an inverse linear transform to the precoders so that the precoders are brought back to the N subband domain, with inverse operation applied in the first step.

[0062] According to the above, the precoders across the subband domain are processed to transform to a different domain, such that the discontinuities at the subband boundary appears as components concentrated in specific regions that represent transitions, in an embodiment, rapid transitions.

[0063] A rapid transition refers to a significant and abrupt change in the estimated channel's behaviour, e.g., amplitude or phase, at the points where one subband ends, and the next begins, compared to estimated channel's behaviour within a subband.

[0064] Thus, a preprocessing is applied before the signal is emitted so that higher frequency components are removed in the transform domain and afterwards the precoders are brought back to the N subband domain.

[0065] More specifically, in a further embodiment, the linear transform is a Discrete Fourier Transform (DFT) or a Discrete Cosine Transform (DCT) along the N subbands to transform the precoder such that the discontinuities at the subband boundaries appear along the upper range of the transformed domain.Upper range refers to the higher indices in the transformed domain. For example, if the transformed domain consists of 16 indices, the 'upper' range could correspond to indices 10 through 16. The specific indices classified as 'upper' will depend on factors such as the channel characteristics, the number of subbands, and the nature of the applied transformation.

[0066] In a second embodiment, to smoothen the precoders, the network node is configured to pre-process the precoders by applying a linear combination of at least two precoders across the N subbands, where the linear combination is weighted based on a subband index which depends on number of subband over which transmission is done. For example, if there are four subbands, the index could be {1,2, 3, 4}, and correspondingly there can be four weights {wi, w2, w3, w4} to linearly combine the precoders.

[0067] More specifically, the network node may be configured to filter the precoders across the N subbands with a low pass filter such that the low pass filter reduces the impact of discontinuities at the subband boundaries.

[0068] In an example, the network node is configured to specifically filter using a Gaussian filter applied to the precoders along the N subbands. The Gaussian filter is parametrized by the standard deviation a which controls the level of smoothness and applied to the precoders along the subband dimension.

[0069] In a further example, the network node is configured to pre-process the precoders per each subband where same precoder is used for all subcarriers in a subband.

[0070] In another embodiment, the smoothening of precoders is processed across the subcarrier dimension after expanding the subband dimension along the subcarrier dimension.

[0071] A subband consists of certain number of PRBs, where each PRB has 12 subcarriers. For example, if a subband consists of 2 PRBs, then it will have 2*12 = 24 subcarriers. According to the above, each of the subcarriers in the subbands will apply the same pre-processes precoders. For example, consider 1536 subcarriers (or128 PRBs) are divided into subband size of 192 subcarriers (16 PRBs), such that one precoder per subband is computed. Accordingly, each precoder per subband is expanded along the subcarrier dimension, i.e., same precoder is applied to each 192 subcarriers, before applying the processing for precoder smoothening described in the previous embodiment along the subcarrier dimension.

[0072] Specifically, the network node is configured so that the precoders applied across the subbands are firstly pre-processed to minimize the effect of signal discontinuity at the subband boundary, i.e., the precoders are smoothen across the subbands, before the transmission. This has been illustrated later in Figure 2.

[0073] More specifically to illustrate the above embodiments, in Figure 2, the transmit precoders with and without smoothening for a transmit antenna and a transmission layer along the subcarrier dimension is shown, where the system bandwidth is 128 PRBs / 1536 subcarriers with subband size equal to 16 PRBs / 192 subcarriers.

[0074] Accordingly, the precoders are firstly computed per subband to maximize the channel gain per subband, followed by expanding the subband dimension to subcarrier dimension, where same precoder is used for all subcarriers in a subband. For precoder smoothening, the precoders along the subcarrier dimension are transformed using DFT operation, followed by discarding, zeroing out 16 samples at the upper range, higher frequency components. Finally, the precoders are transformed back to the original domain. It can be seen that with the proposed operation, the sharp discontinuity at the subband boundary is transformed to a smooth transition to minimize effect of signal discontinuity at subband boundary.

[0075] Precoders are orthogonal between layers, but during the preprocessing they lose orthogonality.

[0076] According to the above, in another embodiment, when the number of transmission layers is greater than one, the network node is configured to orthogonalize the pre-processed precoders, i.e., after smoothing, along the transmission layer dimension.The purpose is to obtain transmit precoders for any given subcarrier with orthogonal columns.

[0077] More specifically, the network node is configured to perform the orthogonalization with Gram- Schmidt orthogonalization or QR decomposition along the transmission layer dimension. QR decomposition means decomposing a matrix A as Q and R, i.e., A = QR. Q means an Orthogonal matrix and R means an upper triangular matrix.

[0078] Therefore, for a fixed transmission layer and transmit antenna pair, the vector obtained from the stacking the set of precoders, stacked in sequence over increasing frequency is smooth, i.e., the discontinuity at subband boundary is processed to minimize the addition of artificial frequency selectivity or delay spread to the effective channel. In addition, for each precoder, the columns, over antenna dimension, each corresponding to a transmission layer for any given frequency unit are orthogonal.

[0079] In an embodiment, the network node comprises at least an antenna transmitter configured to transmit the signal in N subbands of a transmission bandwidth.

[0080] In an embodiment, the network node is a user equipment, UE.

[0081] In an embodiment, the network node is further configured to obtain information and use the obtained information to determine whether to transmit the signal using at least two different precoders across N subbands, and / or apply a pre-processing to the precoders.

[0082] More specifically, the obtained or received information is transmitted by a first network node.

[0083] In a further embodiment the obtained information comprises

[0084] an indication whether to transmit the signal using at least two different precoders across different N subbands, and / or an indication whether to apply a pre-processing to the precoders, and / or an indication of parameters used for pre-preprocessing the precoders.More particularly, the parameters to use for smoothening the precoders are, when the transmitter smoothens the precoders by transforming the subband dimension to another domain, so that the receiver can signal the number of close-to-zero coefficients the transmitter needs to zero out to make the precoder smooth.

[0085] When the transmitter smoothens the precoder by applying a low-pass filter, the receiver can signal the filter parameters to the transmitter e.g. value of standard deviationσif a Gaussian filter is used.

[0086] As previously stated, it is also an object of the invention a first network node being configured to receive a signal transmitted by at least a second network node, wherein the first network node is configured to:

[0087] • receive a signal in which at least two different precoders has been used across different N subbands and a pre-processing to the precoders across the N subbands has been applied before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries, and

[0088] • use the received signal to generate a channel estimate.

[0089] In an embodiment, the first network node is further configured to transmit information comprising an indication whether to transmit the signal using at least two different precoders across different N subbands, and / or indication whether to apply a pre-processing to the precoders and / or an indication of parameters used for prepreprocessing the precoders.

[0090] As previously stated, it is also an object of the invention a method performed by a network node, the method comprising:

[0091] transmitting a signal using at least two different precoders across different N subbands,• applying a pre-processing to the precoders across the N subbands before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.

[0092] As previously stated, it is also an object of the invention a method performed by a network node, the method comprising:

[0093] • receiving a signal with a precoding in the received signal so that at least two different precoders are applied across the different N subbands, and a pre- processing to the precoders across the N subbands has been used before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries, and

[0094] • using the received signal to generate a channel estimate.

[0095] It is also an object of the claimed invention a computer program comprising instructions which when executed by a processing unit of a network node causes the network node to perform any of the previous methods.

Claims

CLAIMS1. A network node, wherein the network node is configured to:• transmit a signal using at least two different precoders across N subbands, • pre-process the precoders across the N subbands before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.

2. The network node of claim 1, wherein the network node is configured to pre-process the precoders by:- applying a linear transform to the precoders across the N subband domain, - zeroing out coefficients which represents transitions along the subband boundary in the transformed precoders,- applying an inverse linear transform to the precoders so that the precoders are brought back to the N subband domain.

3. The network node of claim 2, wherein the linear transform is a Discrete Fourier Transform (DFT) or a Discrete Cosine Transform (DCT) along the N subbands to transform the precoder.

4. The network node of claim 1, wherein the network node is configured to pre-process the precoders by applying a linear combination of at least two precoders across the N subbands, where the linear combination is weighted based on a subband index.

5. The network node of claim 4, wherein the network node is configured to filter the precoders across the N subbands with a low pass filter such that the low pass filter reduces the impact of discontinuities at the subband boundaries.

6. The network node of claim 5, wherein the network node is configured to filter using a Gaussian filter applied to the precoders along the N subbands.

7. The network node of claim 6, wherein the network node is configured to pre-process the precoders per each subband where same precoder is used for all subcarriers in a subband.

8. The network node of claim 1, wherein the network node is configured to pre-process the precoders across the subcarrier dimension after expanding the subband dimension along the subcarrier dimension.

9. The network node of claim 1, wherein when the number of transmission layers is greater than one, the network node is configured to orthogonalize the pre-processed precoders along the transmission layer dimension.

10. The network node of claim 9, wherein the network node is configured to perform the orthogonalization with Gram-Schmidt orthogonalization or QR decomposition along the transmission layer dimension.

11. The network node of any preceding claim, wherein the network node comprises at least an antenna transmitter configured to transmit the signal in N subbands of a transmission bandwidth.

12. The network node of any preceding claim, wherein the network node is a user equipment, UE.

13. The network node of any preceding claim, wherein the network node is further configured to obtain information and use the obtained information to determine whether to transmit the signal using at least two different precoders across different N subbands, and / or apply a pre-processing to the precoders.

14. The network node of claim 13 wherein the obtained information comprises an indication whether to transmit the signal using at least two different precoders acrossdifferent N subbands, and / oran indication whether to apply a pre-processing to the precoders, and / oran indication of parameters used for pre-preprocessing the precoders.

15. A first network node being configured to receive a signal transmitted by at least a second network node, wherein the first network node is configured to:• receive a signal in which at least two different precoders have been used across N subbands and a pre-processing to the precoders across the N subbands has been applied before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries, and• use the received signal to generate a channel estimate.

16. The first network node of claim 15 further configured to transmit information comprising an indication whether to transmit the signal using at least two different precoders across different N subbands, and / or indication whether to apply a preprocessing to the precoders and / or an indication of parameters used for prepreprocessing the precoders.

17. A method performed by a network node, the method comprising:• transmitting a signal using at least two different precoders across N subbands, • applying a pre-processing to the precoders across the N subbands before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries.

18. A method performed by a network node, the method comprising:• receiving a signal with a precoding in the received signal so that at least two different precoders are applied across the different N subbands, and a pre-processing to the precoders across the N subbands has been used before the signal is transmitted, the pre-processing configured to diminish the discontinuity of the output signal from the precoders at the subband boundaries, and• using the received signal to generate a channel estimate.

19. A computer program comprising instructions which when executed by a processing unit of a network node causes the network node to perform the method of any one of claims 17 and 18.

Images