WO2017167386A1

WO2017167386A1 - A transmitter for transmitting and a receiver for receiving a plurality of multicarrier modulation signals

Info

Publication number: WO2017167386A1
Application number: PCT/EP2016/057139
Authority: WO
Inventors: Xitao Gong; Qi Wang; Zhao ZHAO; Malte Schellmann
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2016-03-31
Filing date: 2016-03-31
Publication date: 2017-10-05
Also published as: CN109076042A

Abstract

The invention relates to a transmitter (101 ) for transmitting a plurality of MCM signals over a communication channel (150) to a receiver (121), wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the transmitter (101) comprises: a sampler (103) configured to sample each MCM signal at a plurality of sampling points in the frequency domain, wherein two subsequent sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1, a precoder (105) configured to precode the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoding matrix defined per sampling point of the plurality of sampling points in the frequency domain, and a plurality of transmit antennas (107) configured to transmit the plurality of precoded MCM signals over the communication channel (150) to the receiver (121).

Description

A TRANSMITTER FOR TRANSMITTING AND A RECEIVER FOR RECEIVING A PLURALITY OF MULTICARRIER MODULATION SIGNALS

TECHNICAL FIELD

In general, the present invention relates to the field of wireless communications. More specifically, the present invention relates to a transmitter for transmitting a plurality of multicarrier modulation signals over a communication channel, a receiver for receiving a plurality of multicarrier modulation signals over a communication channel as well as corresponding methods.

BACKGROUND

Multicarrier modulation (MCM) schemes, such as OFDM (orthogonal frequency-division multiplexing), have become one of the major transmission techniques in modern communication systems. By combining MCM transmission schemes with multiple antenna techniques at the transmitter and the receiver sides, such as MIMO (multiple input multiple output) techniques, the spectral efficiency of a cellular communication system could be significantly improved.

In MIMO-OFDM systems, precoding techniques have been widely adopted in order to achieve a higher spectral efficiency. In practical FDD systems, the channel state information (CSI) is measured and quantized at the receiver side and fed back into the reverse channel. Due to the capacity limit of the feedback channel, it is often the case that only a downsampled (e.g. in frequency domain) precoder matrix indicator (PMI) is fed back to the transmitter. Based on the block selective PMI, the precoder matrix is interpolated based on the subcarrier granularity using certain upsampling filters. At the receiver side, a corresponding equalization is applied to compensate the effective channels including the precoding effect. In TDD systems reverse-link estimation can be used.

Thus, in combining MIMO techniques with current multi-carrier transmission systems, e.g., OFDM or FBMC/OQAM (Filterbank Multicarrier transmission with offset quadrature- Amplitude Modulation), block-wise processing for precoding and equalization is widely applied. A first known solution is based on a quantized feedback and block-wise precoding/equalization for multi-carrier systems. For MIMO-OFDM systems the 3GPP LTE standard TS 36.21 1 defines that the transmitter will perform the precoding based on a matrix selected from the codebook indicated by the PMI to each individual subcarrier, and the PMI is reported to the transmitter on a per resource block (RB) granularity

(composed of several subcarriers, called (UE) selected subband PMI). In other words, the precoding matrix is identical for subcarriers belonging to the same precoding subband. This procedure implies the usage of a rectangular shaping window for upsampling to apply the PMI on the subcarrier level. For the equalization, the one-tap equalizer is applied at the receiver, which means one equalizer factor is used to compensate the distortion on one subcarrier. For the more general MCM transmission, the direct application of block-wise precoding and equalization procedure is feasible, but may introduce some challenges. A second known solution is based on an interpolation-based precoding and partial feedback for MIMO-OFDM systems. To reduce the amount of channel feedback and applying precoding to the subcarrier level, interpolation-based precoding and limited feedback was described for MIMO-OFDM systems in US 7676007. More specifically, the receiver sends information about a fraction of the CSI/beamforming vectors to the transmitter. Then, the transmitter reconstructs the beamforming vectors for all the subcarriers. This scheme can be considered as the special case of block-wise processing where the block granularity is one subcarrier.

There are two main drawbacks of block-wise processing schemes.

Firstly, block-wise processing fails to compensate for a severely selective channel fading. Specifically, the conventional block-wise processing requires the channel response to remain quasi-static within one block. For example, the per-subcarrier equalization system assumes a flat fading on subcarrier-level. If the maximum channel delay exceeds the cyclic prefix (CP) length of the OFDM system, this quasi-static assumption in the frequency domain does not hold any more and the system encounters severe

performance degradation.

Secondly, for general MCM schemes with relaxed orthogonality and the orthogonality being conditioned on some channel spectral selectivity requirements, block-wise processing leads to a discontinuity of the effective channels at the block boundary, thus resulting in violation of the orthogonality at the block boundaries, as will be explained in the context of an exemplary FBMC/OQAM system in the following. As already mentioned above, different from CP-OFDM systems, where a complex-valued orthogonality holds, FBMC/OQAM systems rely on the real-valued orthogonality (so called "relaxed

orthogonality"). More specifically, for neighbouring subcarriers, FBMC/OQAM attains the real-valued orthogonality by relying on the similarity of the channels on these two subcarriers. In other words, assuming the channel is locally "flat" within the two adjacent subcarriers, this orthogonality is preserved. However, if the conventional block-wise precoding scheme is applied, such as the one adopted by the LTE standard cited above, it effectively results in a non-flat (discontinuous) channel between the subcarriers at the block boundaries. When switching between different precoding matrices at the spectral resource blocks, the orthogonality between the two edge subcarriers can barely be maintained and therefore leads to substantial inter-block interference. MIMO processing techniques are widely applied in the currently known communication systems due to the potential spectral efficiency gain, diversity gain, etc. However, applying the state-of-the-art block-wise MIMO processing in MCM systems (e.g., OFDM, FMT (Filtered Multitone modulation), FBMC/OQAM) cannot effectively handle the case of severe frequency-selective fading. In addition, for the systems with relaxed orthogonality, it introduces the inter-block interference at the boundary of two successive blocks.

Thus, in the light of the above there is a need for improved devices and methods allowing, in particular, to compensate the performance degradation of MCM systems in severe frequency-selective scenarios and to alleviate inter-block interference of the block-wise transmission for the case of relaxed orthogonality.

SUMMARY

It is an object of the invention to provide improved devices and methods allowing, in particular, to compensate the performance degradation of MCM systems in severe frequency-selective scenarios and to alleviate inter-block interference of the block-wise transmission for the case of relaxed orthogonality.

The foregoing and other objects are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures. According to a first aspect, the invention relates to a transmitter for transmitting a plurality of MCM (multicarrier modulation) signals over a communication channel to a receiver, wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent, i.e. neighboring subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain. The transmitter comprises a sampler configured to sample each MCM signal at a plurality of sampling points in the frequency domain, wherein two subsequent, i.e. neighboring sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2, a precoder configured to precode the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain, i.e. at each sampling point of the plurality of sampling points in the frequency domain, on the basis of a precoding matrix defined per sampling point of the plurality of sampling points in the frequency domain, and a plurality of transmit antennas configured to transmit the plurality of precoded MCM signals over the communication channel to the receiver.

Thus, there is provided an improved transmitter allowing, in particular, to compensate the performance degradation of MCM systems in severe frequency-selective scenarios and to alleviate inter-block interference of the block-wise transmission for the case of relaxed orthogonality.

In a first possible implementation form of the transmitter according to the first aspect as such, the sampler comprises an upsampler and a filter bank configured to sample each MCM signal at the plurality of sampling points in the frequency domain.

In a second possible implementation form of the transmitter according to the first implementation form of the first aspect, the filter bank is a fast-convolution filter bank comprising an IFFT unit and an overlap-add unit or overlap-save unit and the precoder is arranged downstream of the IFFT unit and upstream of the overlap-add unit or the overlap-save unit.

In a third possible implementation form of the transmitter according to the first aspect as such or the first or second implementation for thereof, the precoder is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of channel state information associated with the communication channel.

In a fourth possible implementation form of the transmitter according to the third implementation form of the first aspect, the precoder is configured to obtain the channel state information associated with the communication channel on the basis of a pilot signal received from the receiver and/or the precoder is configured to obtain the channel state information associated with the communication channel from the receiver in response to a pilot signal transmitted to the receiver.

In a fifth possible implementation form of the transmitter according to the third or fourth implementation form of the first aspect, the channel state information is defined for the plurality of subcarriers, i.e. for the plurality of subcarrier frequencies, and the precoder is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain by interpolating the channel state information associated with the communication channel at the plurality of sampling points in the frequency domain, for which no channel state information is available, i.e. defined, and by determining the precoding matrix per sampling point of the plurality of sampling points in the frequency domain on the basis of the channel state information at the plurality of sampling points in the frequency domain.

In a sixth possible implementation form of the transmitter according to the fifth

implementation form of the first aspect, the precoder is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain by interpolating the channel state information associated with the communication channel at the plurality of sampling points in the frequency domain, for which no channel state information is available, on the basis of interpolation parameters provided by the receiver, in particular the oversampling factor K.

In a seventh possible implementation form of the transmitter according to the first aspect as such or the first or second implementation form thereof, the precoder is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoder matrix indicator provided by the receiver and wherein the precoder is configured to determine the precoding matrix at the plurality of sampling points in the frequency domain by selecting a precoding matrix from a predefined set, i.e. codebook, of precoding matrices on the basis of the precoder matrix indicator, wherein each precoding matrix of the predefined set of precoding matrices is defined at the plurality of subcarriers, i.e. at the plurality of subcarrier frequencies, and by interpolating the selected precoding matrix at the plurality of sampling points in the frequency domain, for which the precoding matrix is not defined, i.e. not available.

According to a second aspect, the invention relates to a receiver for receiving a plurality of MCM, i.e. multicarrier modulation signals over a communication channel from a transmitter, wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent, i.e. neighboring subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the receiver comprises: a plurality of receive antennas configured to receive the plurality of MCM signals over the communication channel; and an equalizer configured to equalize the plurality of MCM signals per sampling point of a plurality of sampling points in the frequency domain, i.e. at each sampling point of the plurality of sampling points in the frequency domain, on the basis of an equalization matrix defined per sampling point of the plurality of sampling points in the frequency domain, wherein two subsequent, i.e. neighboring sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2.

Thus, there is provided an improved receiver allowing, in particular, to compensate the performance degradation of MCM systems in severe frequency-selective scenarios and to alleviate inter-block interference of the block-wise transmission for the case of relaxed orthogonality.

In a first possible implementation form of the receiver according to the second aspect as such, the receiver is configured to provide channel state information associated with the communication channel to the transmitter on the basis of at least one pilot signal from the transmitter, wherein the channel state information allows the transmitter to determine a precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain. In a second possible implementation form of the receiver according to the first

implementation form of the second aspect, the receiver is further configured to provide interpolation parameters to the transmitter for interpolating the channel state information associated with the communication channel at the plurality of sampling points in the frequency domain.

In a third possible implementation form of the receiver according to the second aspect as such, the receiver is configured to determine a precoder matrix indicator on the basis of at least one pilot signal from the transmitter and to provide the precoder matrix indicator to the transmitter allowing the transmitter to determine the precoding matrix per sampling point of the plurality of sampling points in the frequency domain by selecting a precoding matrix from a predefined set, i.e. codebook, of precoding matrices on the basis of the precoder matrix indicator, wherein each precoding matrix of the predefined set of precoding matrices is defined at the plurality of subcarriers, i.e. at the plurality of subcarrier frequencies, and by interpolating the selected precoding matrix at the plurality of sampling points in the frequency domain, for which the precoding matrix is not defined.

According to a third aspect, the invention relates to a method for transmitting a plurality of MCM, i.e. multicarrier modulation signals over a communication channel to a receiver, wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent, i.e. neighboring subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the method comprises: sampling each MCM signal at a plurality of sampling points in the frequency domain, wherein two subsequent, i.e. neighboring sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an

oversampling factor K, wherein the oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2, precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoding matrix defined per sampling point of the plurality of sampling points in the frequency domain, and transmitting the plurality of precoded MCM signals over the communication channel to the receiver.

The method according to the third aspect of the invention can be performed by the transmitter according to the first aspect of the invention. Further features of the method according to the third aspect of the invention result directly from the functionality of the transmitter according to the first aspect of the invention and its different implementation forms.

According to a fourth aspect, the invention relates to a method for receiving a plurality of MCM, i.e. multicarrier modulation, signals over a communication channel from a transmitter, wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent, i.e. neighboring, subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the method comprises: receiving the plurality of MCM signals over the communication channel; and equalizing the plurality of MCM signals per sampling point of a plurality of sampling points in the frequency domain on the basis of an equalization matrix defined per sampling point of the plurality of sampling points in the frequency domain, wherein two subsequent, i.e.

neighboring sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the sampling point frequency spacing and the intercarrier frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2.

The method according to the fourth aspect of the invention can be performed by the receiver according to the second aspect of the invention. Further features of the method according to the fourth aspect of the invention result directly from the functionality of the receiver according to the second aspect of the invention and its different implementation forms. According to a fifth aspect the invention relates to a computer program comprising program code for performing the method according to the third aspect or the fourth aspect of the invention when executed on a computer.

The invention can be implemented in hardware and/or software.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, wherein: Fig. 1 shows a schematic diagram of a transmitter according to an embodiment in communication with a receiver according to an embodiment.

Fig. 2 shows a schematic diagram illustrating the steps of a method for transmitting a plurality of MCM signals according to an embodiment.

Fig. 3 shows a schematic diagram illustrating the steps of a method for receiving a plurality of MCM signals according to an embodiment. Fig. 4 shows a schematic diagram of a transmitter according to an embodiment in communication with a receiver according to an embodiment.

Fig. 5 shows the BLER performance of embodiments of the invention in comparison with the prior art for the case of a synchronous transmission; and

Fig. 6 shows the BLER performance of embodiments of the invention in comparison with the prior art for the case of a synchronous transmission.

In the various figures, identical reference signs will be used for identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present invention may be placed. It will be appreciated that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present invention is defined by the appended claims.

For instance, it will be appreciated that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a

corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. Moreover, in the following detailed description as well as in the claims embodiments with different functional blocks or processing units are described, which are connected with each other or exchange signals. It will be appreciated that the present invention covers embodiments as well, which include additional functional blocks or processing units that are arranged between the functional blocks or processing units of the embodiments described below.

Finally, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

Figure 1 shows a schematic diagram of a wireless communication system comprising a transmitter 101 according to an embodiment and a receiver 121 according to an embodiment configured to communicate via a communication channel 150. The transmitter 101 is configured to transmit a plurality of MCM (multicarrier modulation) signals over the communication channel 150 to the receiver 121 , wherein each MCM signal comprises a plurality of subcarriers and wherein two subsequent, i.e. neighboring subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain.

The transmitter 101 comprises a sampler 103 configured to sample each MCM signal at a plurality of sampling points in the frequency domain, wherein two subsequent, i.e.

neighboring sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an

oversampling factor K, wherein the oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2. For instance, in an exemplary embodiment, the intercarrier frequency spacing can be 15 kHz and the sampling point frequency spacing can be 3.75 kHz for an oversampling factor K equal to 4.

Moreover, the transmitter 101 comprises a precoder 105 configured to precode the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoding matrix defined per sampling point of the plurality of sampling points in the frequency domain. In other words, the precoder 105 is configured to precode the plurality of MCM signals at each sampling point of the plurality of sampling points in the frequency domain on the basis of a precoding matrix defined at each sampling point of the plurality of sampling points.

Finally, the transmitter 101 comprises a plurality of transmit antennas 107 configured to transmit the plurality of precoded MCM signals over the communication channel 150 to the receiver 121 .

Correspondingly, the receiver 121 is configured to receive a plurality of MCM signals over the communication channel 150 from the transmitter 101 . To this end, the receiver 121 comprises a plurality of receive antennas 127 configured to receive the plurality of MCM signals over the communication channel 150. Each MCM signal comprises a plurality of subcarriers, wherein two subsequent, i.e. neighboring subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain. Moreover, the receiver 121 comprises an equalizer 125 configured to equalize the plurality of MCM signals per sampling point of a plurality of sampling points in the frequency domain, i.e. at each sampling point of the plurality of sampling points in the frequency domain, on the basis of an equalization matrix defined per sampling point of the plurality of sampling points in the frequency domain. Two subsequent, i.e. neighboring sampling points of the plurality of sampling points define the sampling point frequency spacing in the frequency domain and the ratio between the intercarrier frequency spacing and the sampling point frequency is defined by the oversampling factor K, wherein the

oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2. Assuming M_u (M_u < N) subcarriers among N subcarriers are assigned to a user u being equipped with the receiver 121 , the sub-frequency-tone index in the oversampling domain can be denoted by I e {1,2, ... , KM_U + K - 1}, where K is the oversampling factor.

Denoting the number of spatial streams as N_s, for the synthesis filter bank output signal m e c^(WsXl) at the Z-th tone for the m-th symbol, a precoding matrix V_m can be applied using the precoder 105 at the transmitter 101 . Hence, for the Z-th tone, in an embodiment the following signal model holds:

sm.l ^~ Vm,lPm,l -

For codebook based transmission in the oversampled frequency domain, classical interpolation methods can be applied for oversampling the precoding matrix V_{m l} to be used by the precoder 105, e.g., (spherical) linear interpolation to keep the precoder's unitary condition, or spline interpolation. For the more general cases, if the CSI (potentially downsampled by the downsampling unit 139 of the receiver 121 ) is available at the transmitter 101 , spectral-spatial interpolation methods can be used to attain the channel response in the oversampling domain. In these cases, according to embodiments of the invention, more sophisticated precoding schemes, such as those using the ZF or MMSE criterion, can be applied by the precoder 105 of the transmitter 101 to each individual signal _m in the oversampling domain.

At the receiver 121 , the equalization can be performed by the equalizer 125 in the oversampling domain based on the effective CSI including the effect of precoding.

Assuming H_{M L} e C^{NR XNT} denotes the channel matrix at the Z-th tone and the m-th symbol, the input signal of the equalizer 125 in the frequency domain can be expressed as:

'τη,ί ^— ^m,l ^m,lPm,l ⁿm, where n_{m l} e C^{NR X1} is the additive white Gaussian noise vector on the Z-th tone and the m- th symbol. The effective channel, i.e. including precoding, on the Z-th tone and the m-th symbol can be considered as H_{M L} = H_{M L}V_{M L} denoting the channel matrix on the Z-th tone and the m-th symbol. Thus, the equalization is performed by the equalizer 125 also in the oversampling domain.

Figure 2 shows the steps of a method 200 according to an embodiment for transmitting a plurality of MCM signals over the communication channel 150 to the receiver 121 . Each MCM signal comprises a plurality of subcarriers, wherein two subsequent, i.e. neighboring subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain.

The method 200 comprises a first step 201 of sampling each MCM signal at a plurality of sampling points in the frequency domain, wherein two subsequent, i.e. neighboring sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2. The method 200 comprises a further step 203 of precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoding matrix defined per sampling point of the plurality of sampling points in the frequency domain.

The method 200 comprises a further step 205 of transmitting the plurality of precoded MCM signals over the communication channel 150 to the receiver 121 .

Figure 3 shows the steps of a method 300 according to an embodiment for receiving a plurality of MCM signals over the communication channel 150 from the transmitter 101 . Each MCM signal comprises a plurality of subcarriers, wherein two subsequent, i.e.

neighboring subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain. The method 300 comprises a first step 301 of receiving the plurality of MCM signals over the communication channel 150.

The method 300 comprises a further step 303 of equalizing the plurality of MCM signals per sampling point of a plurality of sampling points in the frequency domain on the basis of an equalization matrix defined per sampling point of the plurality of sampling points in the frequency domain, wherein two subsequent, i.e. neighboring sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the sampling point frequency spacing and the intercarrier frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 , in particular an integer equal to or greater than 2.

Further implementation forms, embodiments and aspects of the transmitter 101 , the receiver 121 as well as the methods 200 and 300 will be described in the following. Figure 4 shows further embodiments of the transmitter 101 and the receiver 121 .

In the embodiment shown in figure 4, the transmitter 101 comprises in addition to the sampler 103, the precoder 105 and the plurality of transmit antennas 107 (for the sake of clarity not shown in figure 4) a symbol mapping unit 109, a resource mapping unit 1 1 1 , an IFFT unit 1 13, an parallel-to-serial conversion unit 1 15, an interpolation unit 1 17 and a codebook 1 19. These components of the embodiment of the transmitter 101 shown in figure 4 will be described in more detail further below.

In the embodiment shown in figure 4, the receiver 121 comprises in addition to the plurality of receive antennas 127 (for the sake of clarity not shown in figure 4) and the equalizer 125 a serial-to-parallel conversion unit 135, a FFT unit 133, a resource demapping unit 131 , a filter bank 123b, a downsampler 123a and a symbol demapping unit 129. Moreover, the receiver 121 can comprise a channel (state information) estimation unit 137 and a downsampling unit 139. These components of the embodiment of the transmitter 121 shown in figure 4 will be described in more detail further below.

In the embodiment shown in figure 4, the sampler 103 of the transmitter 101 comprises an upsampler 103a and a filter bank 103b configured to sample each MCM signal at the plurality of sampling points in the frequency domain.

In an embodiment, the filter bank 103b can be implemented as fast-convolution filter bank, which could also include the IFFT unit 1 13 shown in figure 4 as well as an overlap-add unit or an overlap-save unit (not shown in figure 4). In such an embodiment, the precoder 105 would be arranged downstream of the IFFT unit 1 13 and upstream of the overlap-add unit or the overlap-save unit and, thus, could take advantage of the processing in the oversampling domain already implemented in a conventional fast-convolution filter bank.

The symbol mapping unit 109 of the transmitter 101 shown in figure 4 is implemented for channel coding and modulation by mapping from bit-to-symbol.

The resource mapping unit 1 1 1 of the transmitter 101 shown in figure 4 is implemented for mapping payload symbols together with reference symbols to a time-frequency resource grid for each transmission block. The (synthesis) filter bank 103b of the transmitter 101 shown in figure 4 is implemented for modulating and filtering the MCM signals in the frequency domain. In an embodiment, the synthesis filter bank 103b can be based on a frequency spreading structure.

The IFFT or IDFT unit 1 13 and the parallel-to-serial conversion unit 1 15 of the transmitter 101 shown in figure 4 are implemented for transferring the MCM signal from each antenna to the time domain and transforming from parallel to serial bit streams. As will be appreciated, according to embodiments of the present invention the IFFT or IDFT unit 1 13 will be configured to operate on the basis of a size KN, where K is the oversampling factor and N is the number of subcarriers.

The FFT or DFT unit 133 and the serial-to-parallel conversion unit 135 of the receiver 121 shown in figure 4 are implemented to transfer each signal from each antenna to the frequency domain and to transform from serial to parallel bit streams. As in the case of the IFFT or IDFT unit 1 13 of the transmitter, the FFT or DFT unit 133 operates on the basis of the size KN. The resource demapping unit 131 of the receiver 121 shown in figure 4 is implemented for demapping the symbols in the time-frequency resource grid back to each transport block. In the embodiment shown in figure 4, the receiver 121 is configured to extract pilot or reference signals at the resource demapping unit 131 for channel estimation using the channel estimation unit 137.

The (analysis) filter bank 123b of the receiver 121 shown in figure 4 is implemented for de-modulating and matched filtering the signals at the receiver side as well as combining and together with the downsampler 123a downsampling the signals for each subcarrier. In an embodiment, the analysis filter bank 123b can be based on a frequency spreading structure.

The symbol demapping unit 129 of the receiver 121 shown in figure 4 is the counterpart of the FEC coding and bit-to-symbol mapping in the transmitter 101 . In order to enable the adaptive precoding at the transmitter 101 , according to

embodiments of the invention the CSI (channel state information) is fed back from the receiver 121 to the transmitter 101 , such as in a FDD system. Alternatively, the transmitter 101 can be configured to directly estimate the CSI using uplink-downlink reciprocity, such as in a TDD system. Due to the limited capacity of the feedback channel, it is

advantageous to feed back only partial information. Thus, according to embodiments of the invention the CSI on a selected subcarrier can be quantized and/or down-sampled by the receiver 121 using, in particular the channel estimation unit 137 and the

downsampling unit 139. Alternatively or additionally, the receiver 121 can be configured to determine a suitable predictor matrix from a codebook on the basis of the CSI and to feed back a predictor matrix indicator (PMI) to the transmitter, as will be described in more detail further below. In an embodiment of the invention, the equalizer 125 of the receiver 121 is configured to use an interpolation based matrix calculation, such as the Laurent polynomial or other transforms, in order to decrease the numerical complexity of deriving the equalization matrix.

According to embodiments of the invention, the feedback mechanism implemented between the receiver 121 and the transmitter 101 shown in figures 1 and 4 can have one or more of the following features. In an embodiment, the CSI is measured by the receiver 121 based on the reference signals extracted from the components provided by the resource demapping unit 131 . In an embodiment, the CSI can be quantized and downsampled to match a required feedback channel capacity and performance requirement. It is important to note that in this case, the CSI is not averaged and quantized within one processing block (as is done in the prior art), but quantized and/or down-sampled for a selected subcarrier. For embodiments using a codebook based exchange of the CSI, the receiver 121 can be configured to select the precoding matrix indicator (PMI) on the basis of the possibly downsampled CSI. In an embodiment, both the PMI and interpolation parameters can be conveyed from the receiver 121 back to the transmitter 101 via the feedback channel 150a. For embodiments using a more general exchange of the CSI information, the receiver 121 can be configured to convey interpolation parameters as well as the possibly downsampled CSI to the transmitter 101 . In an embodiment, the transmitter 101 is configured to select on the basis of the PMI from the receiver 121 the corresponding precoding matrix from the codebook 1 19. In an embodiment, the transmitter 101 can be configured to interpolate the precoding matrix from the codebook 1 19 in the oversampled frequency domain, i.e. at each sampling point of the plurality of sampling points in the frequency domain, on the basis of the

interpolation parameters provided by the receiver 121 . In an embodiment, the interpolation parameters provided by the receiver 121 can include, in particular, the oversampling factor K used by the receiver 121 . Thus, more specifically, the exchange of CSI information between the transmitter 101 and the receiver 121 can be implemented according to the one or more of the embodiments described below.

In an embodiment, the transmitter 101 is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of channel state information associated with the communication channel.

In an embodiment, the transmitter 101 is configured to obtain the channel state information associated with the communication channel 150 on the basis of a pilot signal, which the transmitter 101 has received from the receiver 121 . In another embodiment, the transmitter 101 is configured to obtain the channel state information associated with the communication channel 150 from the receiver 121 , that has been determined by the receiver 121 on the basis of a pilot signal transmitted from the transmitter 101 to the receiver 121 .

In an embodiment, the transmitter 101 is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain by interpolating the channel state information associated with the communication channel 150 at the plurality of sampling points in the frequency domain, for which no channel state information is available, and by determining the precoding matrix per sampling point of the plurality of sampling points in the frequency domain on the basis of the channel state information at the plurality of sampling points in the frequency domain.

In an embodiment, the transmitter 101 is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain by interpolating the channel state information associated with the communication channel 150 at the plurality of sampling points in the frequency domain, for which no channel state information is available, on the basis of the interpolation parameters provided by the receiver 121 .

In an embodiment, the transmitter 101 is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of the precoder matrix indicator provided by the receiver 121 and further configured to determine the precoding matrix at the plurality of sampling points in the frequency domain by selecting a precoding matrix from a predefined set of precoding matrices defined in the codebook 1 19 on the basis of the precoder matrix indicator, wherein each precoding matrix of the predefined set of precoding matrices is defined at the plurality of subcarriers, and by interpolating the selected precoding matrix at the plurality of sampling points in the frequency domain, for which the precoding matrix is not defined.

In an embodiment, the receiver 121 is configured to provide the channel state information associated with the communication channel 150 to the transmitter 101 on the basis of at least one pilot signal from the transmitter 101 , wherein the channel state information allows the transmitter 101 to determine a precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain.

In an embodiment, the receiver 121 is further configured to provide interpolation parameters to the transmitter 101 for interpolating the channel state information associated with the communication channel at the plurality of sampling points in the frequency domain.

In an embodiment, the receiver 121 is configured to determine the precoder matrix indicator on the basis of at least one pilot signal from the transmitter 101 and to provide the precoder matrix indicator to the transmitter 101 allowing the transmitter 101 to determine the precoding matrix per sampling point of the plurality of sampling points in the frequency domain by selecting a precoding matrix from a predefined set of precoding matrices on the basis of the precoder matrix indicator, wherein each precoding matrix of the predefined set of precoding matrices is defined at the plurality of subcarriers, and by interpolating the selected precoding matrix at the plurality of sampling points in the frequency domain, for which the precoding matrix is not defined.

The performance of embodiments of the invention has been tested for an exemplary scenario on the basis of computer simulations, which have been run using the simulation parameters shown in the table below.

Number of bins after oversampling 180x4+(4-1 ) = 723

Interpolation Method apply spherical interpolation to the precoding/equalization matrices at resource blocks in order to obtain precoding/equalization matrix at each sub-bin

For this example, an FBMC-OQAM system with 256 subcarriers has been chosen. The user equipped with the receiver 121 according to an embodiment is allocated to a subband of 15 RBs (resource blocks), while each RB consists of 12 subcarriers. Thus, there are 180 used subcarriers. Assuming the prototype filter of the oversampling factor is K = 4, the signal vector in the oversampling domain consists of 723 tones. The performance has been evaluated with respect to the oversampling processing for the equalization. Specifically, given the fixed precoding matrix for each tone at the transmitter 101 and the equalization matrix at each subcarrier, spherical interpolation is performed in this example in order to obtain the equalization matrix for each tone.

Figure 5 shows the BLER performance of oversampling-based equalization provided by embodiments of the invention for a synchronous transmission scenario. For FBMC- OQAM, both a per-subcarrier (per SC) and a per-frequency-tone (per bin) equalization are performed. The state of the art scheme, i.e., conventional per-subcarrier equalization for OFDM, is also shown. The results validate that per-bin equalization provided by embodiments of the invention outperforms per subcarrier equalization in a scenario with severe frequency selective channels. This can be observed in the performance for the higher order modulation coding scheme (MCS), yielding 2-3 dB gain compared to the conventional per-SC equalization.

Figure 6 depicts the BLER performance of oversampling-based equalization provided by embodiments of the invention for the asynchronous transmission. Specifically, two user equipments (UEs) are transmitted with timing misalignment which introduces additional channel variation. As can be taken from figure 6, distortions caused by the timing misalignment of the two UEs as well as the corresponding channel frequency selectivity cannot be well-compensated by the conventional per-SC equalization, while this can be effectively achieved by the per-bin equalization as provided by embodiments of the invention. As already mentioned above, embodiments of the invention are generally applicable to MCM schemes. For multi-carrier system with a relaxed orthogonality (or at least where the orthogonality is conditioned on some frequency selectivity requirements posed to the channel), embodiments of the invention are remarkably beneficial, as the spectral-spatial domain smoothing is achieved by the continuous phase drift of the interpolated precoder, as exemplified for FBMC/OQAM systems.

Embodiments of the invention provide, for instance, for the following additional advantages. Since according to embodiments of the invention each tone is precoded and equalized individually, channel variations on each tone can be equalized, leading to a high robustness to high frequency selectivity. By interpolating the precoding matrix in the oversampling domain, inter-block interference at the precoded block boundaries can be completely eliminated. While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations or embodiments, such feature or aspect may be combined with one or more other features or aspects of the other implementations or embodiments as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "include", "have", "with", or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term "comprise". Also, the terms "exemplary", "for example" and "e.g." are merely meant as an example, rather than the best or optimal. The terms "coupled" and "connected", along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Although specific aspects have been illustrated and described herein, it will be

appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein.

Although the elements in the following claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.

Claims

1 . A transmitter (101 ) for transmitting a plurality of multicarrier modulation (MCM) signals over a communication channel (150) to a receiver (121 ), wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the transmitter (101 ) comprises: a sampler (103) configured to sample each MCM signal at a plurality of sampling points in the frequency domain, wherein two subsequent sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 ; a precoder (105) configured to precode the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoding matrix defined per sampling point of the plurality of sampling points in the frequency domain; and a plurality of transmit antennas (107) configured to transmit the plurality of precoded MCM signals over the communication channel (150) to the receiver (121 ).

2. The transmitter (101 ) of claim 1 , wherein the sampler (103) comprises an upsampler (103a) and a filter bank (103b) configured to sample each MCM signal at the plurality of sampling points in the frequency domain.

3. The transmitter (101 ) of claim 2, wherein the filter bank is a fast-convolution filter bank comprising an IFFT unit and an overlap-add unit or an overlap-save unit and wherein the precoder (105) is arranged downstream of the IFFT unit and upstream of the overlap- add unit or the overlap-save unit.

4. The transmitter (101 ) of any one of the preceding claims, wherein the precoder (105) is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of channel state information associated with the communication channel (150).

5. The transmitter (101 ) of claim 4, wherein the precoder (105) is configured to obtain the channel state information associated with the communication channel (150) on the basis of a pilot signal received from the receiver (121 ) or wherein the precoder (105) is configured to obtain the channel state information associated with the communication channel (150) from the receiver (121 ) in response to a pilot signal transmitted to the receiver (121 ).

6. The transmitter (101 ) of claim 4 or 5, wherein the channel state information is defined for the plurality of subcarriers and wherein the precoder (105) is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain by interpolating the channel state information associated with the communication channel (150) at the plurality of sampling points in the frequency domain, for which no channel state information is available, and by determining the precoding matrix per sampling point of the plurality of sampling points in the frequency domain on the basis of the channel state information at the plurality of sampling points in the frequency domain.

7. The transmitter (101 ) of claim 6, wherein the precoder (105) is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain by interpolating the channel state information associated with the communication channel (150) at the plurality of sampling points in the frequency domain, for which no channel state information is available, on the basis of interpolation parameters provided by the receiver (121 ).

8. The transmitter (101 ) of any one of claims 1 to 3, wherein the precoder (105) is configured to determine the precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoder matrix indicator provided by the receiver (121 ) and wherein the precoder (105) is configured to determine the precoding matrix at the plurality of sampling points in the frequency domain by selecting a precoding matrix from a predefined set of precoding matrices on the basis of the precoder matrix indicator, wherein each precoding matrix of the predefined set of precoding matrices is defined at the plurality of subcarriers, and by interpolating the selected precoding matrix at the plurality of sampling points in the frequency domain, for which the precoding matrix is not defined.

9. A receiver (121 ) for receiving a plurality of multicarrier modulation (MCM) signals over a communication channel (150) from a transmitter (101 ), wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the receiver (121 ) comprises: a plurality of receive antennas (127) configured to receive the plurality of MCM signals over the communication channel (150); and an equalizer (215) configured to equalize the plurality of MCM signals per sampling point of a plurality of sampling points in the frequency domain on the basis of an equalization matrix defined per sampling point of the plurality of sampling points in the frequency domain, wherein two subsequent sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 .

10. The receiver (121 ) of claim 9, wherein the receiver (121 ) is configured to provide channel state information associated with the communication channel (150) to the transmitter (101 ) on the basis of at least one pilot signal from the transmitter (101 ), wherein the channel state information allows the transmitter (101 ) to determine a precoding matrix for precoding the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain.

1 1 . The receiver (121 ) of claim 10, wherein the receiver (121 ) is further configured to provide interpolation parameters to the transmitter (101 ) for interpolating the channel state information associated with the communication channel (150) at the plurality of sampling points in the frequency domain.

12. The receiver (121 ) of claim 9, wherein the receiver (121 ) is configured to determine a precoder matrix indicator on the basis of at least one pilot signal from the transmitter (101 ) and to provide the precoder matrix indicator to the transmitter (101 ) allowing the transmitter (101 ) to determine the precoding matrix per sampling point of the plurality of sampling points in the frequency domain by selecting a precoding matrix from a predefined set of precoding matrices on the basis of the precoder matrix indicator, wherein each precoding matrix of the predefined set of precoding matrices is defined at the plurality of subcarriers, and by interpolating the selected precoding matrix at the plurality of sampling points in the frequency domain, for which the precoding matrix is not defined.

13. A method (200) for transmitting a plurality of multicarrier modulation (MCM) signals over a communication channel (150) to a receiver (121 ), wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the method (200) comprises: sampling (201 ) each MCM signal at a plurality of sampling points in the frequency domain, wherein two subsequent sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an oversampling factor K, wherein the oversampling factor K is greater than 1 ; precoding (203) the plurality of MCM signals per sampling point of the plurality of sampling points in the frequency domain on the basis of a precoding matrix defined per sampling point of the plurality of sampling points in the frequency domain; and transmitting (205) the plurality of precoded MCM signals over the communication channel (150) to the receiver (121 ).

14. A method (300) for receiving a plurality of multicarrier modulation (MCM) signals over a communication channel (150) from a transmitter (101 ), wherein each MCM signal comprises a plurality of subcarriers, wherein two subsequent subcarriers of the plurality of subcarriers define an intercarrier frequency spacing in the frequency domain, wherein the method (300) comprises: receiving (301 ) the plurality of MCM signals over the communication channel (150); and equalizing (303) the plurality of MCM signals per sampling point of a plurality of sampling points in the frequency domain on the basis of an equalization matrix defined per sampling point of the plurality of sampling points in the frequency domain, wherein two subsequent sampling points of the plurality of sampling points define a sampling point frequency spacing in the frequency domain and wherein the ratio between the intercarrier frequency spacing and the sampling point frequency spacing is defined by an

oversampling factor K, wherein the oversampling factor K is greater than 1 .

15. A computer program comprising program code for performing the method (200) of claim 13 or the method (300) of claim 14 when executed on a computer.