WO2024191330A1

WO2024191330A1 - Estimation of transmitted samples of mimo streams

Info

Publication number: WO2024191330A1
Application number: PCT/SE2023/050230
Authority: WO
Inventors: Christian CZEGLEDI
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2023-03-16
Filing date: 2023-03-16
Publication date: 2024-09-19
Anticipated expiration: 2025-09-16

Abstract

There is provided techniques for estimating transmitted samples of MIMO streams. A method is performed by a MIMO transceiver. The method comprises receiving a sequence of samples from each receive antenna containing combinations of the MIMO streams. The method comprises repeatedly, and for each block of samples of each MIMO stream, identifying, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points, and estimating the transmitted samples for the block of symbols of the MIMO stream as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream.

Description

ESTIMATION OF TRANSMITTED SAMPLES OF MIMO STREAMS

TECHNICAL FIELD

Embodiments presented herein relate to a method, a multiple-input multiple-output (MIMO) receiver, a computer program, and a computer program product for estimating transmitted samples of MIMO streams.

BACKGROUND

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

In general terms, phase (or frequency) noise is a noise signal that changes the instantaneous phase of the carrier away from its theoretical (ideal) value. Phase noise does not change the amplitude of the carrier. Amplitude noise affects the amplitude component of the signal only - it does not affect the phase of the signal.

Phase noise originating from local oscillators (LOs) used for up- and downconversion in radio transceivers is an impairment that should be compensated to ensure reliable communications. However, due to the need to demultiplex different streams in MIMO systems, phase noise becomes harder to compensate in MIMO systems than for single-input single output (SISO) systems.

Phase noise affect the process of, in the MIMO transceiver, estimating which sequence of samples that was transmitted. However, current MIMO transceivers cannot cope with high varying phase noise, when considering a reasonable computational complexity of the MIMO transceiver.

An algorithm is proposed in “Hardware-Efficient Coherent Digital Receiver Concept With Feedforward Carrier Recovery for M-QAM Constellations”, T Pfau, S.

Hoffmann, T. Noe, Journal of Lightwave technology, 2009 for phase noise tracking in fiber optical communication systems. The algorithm uses a discrete set of angles and chooses the one that minimizes the mean-squared error (MSE) over a block of symbols. However, the proposed algorithm applies only for SISO fiber optical communication systems.

Hence, there is still a need for MIMO transceivers that, with a reasonable computational complexity, can accurately estimate phase noise as part of estimating transmitted MIMO streams.

SUMMARY

An object of embodiments herein is to address the above issues.

According to a first aspect there is presented a method for estimating transmitted samples of MIMO streams. The method is performed by a MIMO transceiver. The method comprises receiving a sequence of samples from each receive antenna containing combinations of the MIMO streams. The method comprises repeatedly, and for each block of samples of each MIMO stream, identifying, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points, and estimating the transmitted samples for the block of symbols of the MIMO stream as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream.

According to a second aspect there is presented a MIMO transceiver for estimating transmitted samples of MIMO streams. The MIMO transceiver comprises processing circuitry. The processing circuitry is configured to cause the MIMO transceiver to receive a sequence of samples from each receive antenna containing combinations of the MIMO streams. The processing circuitry is configured to cause the MIMO transceiver to repeatedly, and for each block of samples of each MIMO stream, identify, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points, and estimate the transmitted samples for the block of symbols of the MIMO stream as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream.

According to a third aspect there is presented a MIMO transceiver for estimating transmitted samples of MIMO streams. The MIMO transceiver comprises a receive module configured to receive a sequence of samples from each receive antenna containing combinations of the MIMO streams. The MIMO transceiver comprises an identify module configured to, repeatedly, and for each block of samples of each MIMO stream, identify, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points. The MIMO transceiver comprises an estimate module configured to, repeatedly, and for each block of samples of each MIMO stream, estimate the transmitted samples for the block of symbols of the MIMO stream as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream.

According to a fourth aspect there is presented a computer program for estimating transmitted samples of MIMO streams. The computer program comprises computer code which, when run on processing circuitry of a MIMO transceiver, causes the MIMO transceiver to perform actions. One action comprises the MIMO transceiver to receive a sequence of samples from each receive antenna containing combinations of the MIMO streams. One action comprises the MIMO transceiver to repeatedly, and for each block of samples of each MIMO stream, identify, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points, and estimate the transmitted samples for the block of symbols of the MIMO stream as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream.

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

Advantageously, these aspects provide accurate estimations of transmitted samples of MIMO streams.

Advantageously, these aspects provide accurate estimations even in challenging situations with strong phase noise.

Advantageously, these aspects require only a reasonable computational complexity. Advantageously, implementations of these aspects can be highly parallelized.

Advantageously, the parameters used for the estimation can be easily tuned for different performance/complexity trade-offs.

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

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a schematic diagram illustrating communications network according to embodiments;

Fig. 2 is a flowchart of methods according to embodiments;

Figs. 3 and 4 schematically illustrate signal constellations according to embodiments;

Fig. 5 schematically illustrates an error function according to an embodiment;

Figs. 6, 7, 8, and 9 show simulation results according to embodiments;

Fig. 10 is a schematic diagram showing functional units of a MIMO transceiver according to an embodiment;

Fig. 11 is a schematic diagram showing functional modules of a MIMO transceiver according to an embodiment; and Fig. 12 shows one example of a computer program product comprising computer readable storage medium according to an embodiment.

DETAILED DESCRIPTION

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

Fig. i is a schematic diagram illustrating communications networks tooa, toob where embodiments presented herein can be applied. In Fig. 1(a) is illustrated a first communications network tooa representing wireless communication between a first network node noa and a second network node nob. The wireless communication is illustrated by MIMO streams 150a. Each network node 110a, 110b is provided with, comprises, is integrated with, or is operatively connected to, a respective MIMO transceiver 200. In Fig. 1(b) is illustrated a second communications network 100b representing wireless communication between a network node 110a and a user equipment 140. The wireless communication is illustrated by MIMO streams 150b. The network node 110a is provided with, comprises, is integrated with, or is operatively connected to, a MIMO transceiver 200. The network node 110a configured to provide network access to the UE 140. The network node 110a is operatively connected to a core network 120. The core network 120 is in turn operatively connected to a service network 130, such as the Internet. The UE 140 is thereby enabled to, via the network node 110a, access services of, and exchange data with, the service network 130.

Examples of network nodes 110a, 110b are radio access network nodes, radio base stations, base transceiver stations, Node Bs, evolved Node Bs, gNBs, TRPs, access points, access nodes, microwave transceivers, and integrated access and backhaul nodes. Examples of UEs 140 are wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, smartphones, laptop computers, tablet computers, network equipped sensors, network equipped vehicles, and so-called Internet of Things devices.

As noted above, there is still a need for MIMO transceivers that, with a reasonable computational complexity, can accurately estimate phase noise as part of estimating transmitted MIMO streams.

At least some of the embodiments disclosed herein are based on extend the above referenced algorithm from SISO systems to MIMO systems. This can be achieved by adapting the algorithm to involve carrying out a joint optimization over several test angles compensating for phase noise in the different MIMO branches.

The embodiments disclosed herein in particular relate to techniques for estimating transmitted samples of MIMO streams 150a, 150b. In order to obtain such techniques, there is provided a MIMO transceiver 200, a method performed by the MIMO transceiver 200, a computer program product comprising code, for example in the form of a computer program, that when run on a MIMO transceiver 200, causes the MIMO transceiver 200 to perform the method.

For non-limiting and illustrative purposes, assume a N_t x N_r MIMO system, comprised by N_t transmit antennas and N_r receive antennas, impacted by phase noise and additive white Gaussian noise (AWGN), represented by the following system model:

Y_k = R_kHT_kX_k + N_k = (P_k o H)X_k + N_k, where: k is the discrete time index,

X_k is a A_t-dimensional vector with transmitted samples,

Y_k is a N_r -dimensional vector with received samples,

N_k~N(0, ItTn) is the AWGN,

H is the LOS MIMO channel N_r x N_t matrix, T_k/R_kis the transmitter/receiver phase noise where each oscillator is modelled as a Wiener process,

P_k = R_klT_k (where 1 is an all-ones matrix) is a matrix that represents the combined phase noise from the transmitter and receiver, and the operator ° is the element -wise Hadamard product.

Assuming that the channel H_k is known, or at least can be estimated, one main object is to estimate X_k. As will be disclosed hereinafter, the enclosed embodiments are based on utilizing a discrete set of test angles, where the test angles that minimizes some distance, such as the MSE, over a block of N samples yield the estimate X_k.

Fig. 2 is a flowchart illustrating embodiments of methods for estimating transmitted samples of MIMO streams 150a, 150b. The methods are performed by the MIMO transceiver 200. The methods are advantageously provided as computer programs 1220.

S102: The MIMO transceiver 200 receives a sequence of samples from each receive antenna containing combinations of the MIMO streams 150a, 150b.

As disclosed above, the samples might be affected by transmitter/receiver phase noise as well as channel noise (such as AWGN). The MIMO transceiver 200 therefore needs to estimate the transmitted samples from the received sequence of samples. In general terms, the method is based on testing different combinations of test angles and finding the combination of test angles that yield minimum average error over a block of samples.

S104: The MIMO transceiver 200 repeatedly, and for each block of samples of each MIMO stream 150a, 150b, performs S104-2 and S104-4.

S104-2: The MIMO transceiver 200 identifies, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points. Thereby, for each MIMO stream t, the optimum angles a_mtropt ^can be found by trying all possible combinations of test angles (i.e., M^Nr combinations for a N_t x N_r MIMO system when using a set of M discrete angles).

In some aspects, the distance is the sum distance since the distances for each sample in the block of samples are summed, or at least averaged, together. The distance can be averaged over the block of N samples to average out the effects of noise.

S104-4: The MIMO transceiver 200 estimates the transmitted samples for the block of symbols of the MIMO stream 150a, 150b as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream 150a, 150b.

Embodiments relating to further details of estimating transmitted samples of MIMO streams 150a, 150b as performed by the MIMO transceiver 200 will now be disclosed.

In some aspects, the samples are in S104-2 not only rotated by the test angles but also scaled by some scale factors. In some examples, the scale factors are given by the MIMO equalizer tap values. That is, in some embodiments, the samples are, when rotated by each of the test angles, scaled by the MIMO equalizer tap values. In some examples, for block k, the samples as rotated by the test angles and scaled by the MIMO equalizer tap values are given by

where F_{t r} re MIMO equalizer tap values, y_{r n} are the samples from each receive antenna, a_{t r k-} are phase noise estimations for block k - 1, and a_mr are the test angles for the samples from each receive antenna of the MIMO transceiver 200. In other words, when the optimum test angles are found, the estimated transmitted samples are also found. These are the decoded samples, below denoted y_t,_n,_m m_r-, iⁿ the block of samples found for the set of optimum test angles. It is noted that due to the 4-fold rotational symmetry of some signal constellations (such as quadrature amplitude modulation; QAM), there can be a multiple of n/2 between the estimates and the true phase noise.

In some aspects, the nearest constellation point is given by the combination of test angles. This is since each combination of test angles is mapped to its nearest constellation point. Therefore, in some embodiments, which constellation points to select from the set of constellation points are given by each of the samples as rotated with the test angles. In some examples, for each combination of angles, a point given by

> is decoded, or sliced, to the constellation point y_t,n,_mi m_r- For ^each combination of test angles a_mi, a_m2, ... , a_mr, the point ^=1 ^t,ryr,n^e'^(aL'^r'^k~ ^+amr> will thus be decoded to the nearest constellation point

> _mr.

In some aspects, the optimum angles, or more precisely the indices of the optimum angles (since the number of test angles is a discrete value), are the ones that result in the minimum average distance, or error. In some embodiments, for a block of N samples, the (optimum) angles are identified according to

where m_{t l opt}, m_{t 2 opt}, , m_{t r opt} are indices of the identified angles, and y > 1. In case y = 2, this corresponds to calculating the squared Euclidian distance.

In general terms, the search for the optimal set of angles (e.g. using the argmin function) can be parallelized into parallel process, with one process for each stream t and set of test angles (a_mi, a_m2, ... , a_mr).

The method for finding the optimum angles, and thus also for estimating the transmitted samples of the MIMO streams 150a, 150b, can operate with feedback or without feedback.

In case feedback is used, the values are the phase noise estimations from the previous block (i.e., block k - 1) of samples. Further, when feedback is used, the phase noise estimations estimate the elements of the combined phase noise P_k. That is, in some embodiments, the phase noise estimations are estimates of combined transmitter and receiver phase noise. Based on the optimum angles a_mtropt, the phase noise estimations can then be updated as a_{t r k} = a_{t r k-} + a_mtrovt ^as part of a feedback loop. That is, in some embodiments, in presence of feedback, the phase noise estimations forblock k are updated as a_{t r k} = a_{t r k-} + a_mtropt.

In case feedback is not used, the values

can be set to zero. That is, in some embodiments, in absence of feedback, a_{t r k-} = 0. Further, when feedback is not used, the phase noise estimations without feedback, the elements of the combined phase noise P_k can estimated directly by optimum angles a_mtropt. That is, in some embodiments, the identified test angles are estimates of combined transmitter and receiver phase noise.

Aspects of the set of test angles will be disclosed next.

In some aspects, the set of test angles is selected based on the modulation format and phase noise. That is, in some embodiments, the set of test angles is selected based on modulation format of the transmitted samples, and expected phase noise at the MIMO transceiver 200.

In some aspects, more dense modulation formats require finer distribution of test angles. That is, in some embodiments, the modulation format has a density, and wherein the higher the density is, the larger the set of test angles is.

In some aspects, stronger phase noise requires a larger range for the test angles. That is, in some embodiments, the set of test angles is composed of test angles in a range of angles, and wherein the stronger the expected phase noise is, the larger the range is.

Further, with feedback, the angle range can be significantly reduced to a few degrees. When feedback is not used, the angle range could be 90°. Further, when feedback is not used, there is a quadrant ambiguity for some signal constellations (such as QAM) due to the 90-degree rotational symmetry. This can be resolved with pilots or differential encoding.

Aspects of the block of samples will be disclosed next. In general terms, the block of samples can be formed either on a block-by-block basis or a sliding window basis. That is, in some embodiments, the block of samples is composed of N > 1 samples, where the sequence of samples is shifted by any of 1 to N samples to form each block of N samples. A sliding window basis corresponds to the case where the sequence of samples is shifted by less than N samples. A block-by- block basis corresponds to the case where the sequence of samples is shifted by N samples.

In general terms, the value of N (i.e., the block size) is selected as a tradeoff between enabling tracking of fast phase noise (by selecting a comparatively low value of TV) and enabling the effects of noise to be averaged out (by selecting a comparatively high value of TV).

Example

An example that for illustrative purposes is for a SISO system will be disclosed next.

Assume a block of four quadrature phase shift keying (QPSK) samples that affected by AWGN and phase noise as shown in Fig. 3. In Fig. 3 is shown the QPSK constellation points (illustrated as “°”), that correspond to the possible transmitted samples, as well as the four received samples (illustrated as

“Real” represents the real axis and “Imag” represents the imaginary axis.

Test angles from a set of test angles are sequentially applied as illustrated in Fig. 4. In addition to what is shown in Fig. 3, in Fig. 4 is further illustrated the location of the test angles (illustrated as “■”) as well as the found optimum angles (illustrated as “+”). Again, “Real” represents the real axis and “Imag” represents the imaginary axis. In the present illustrative example, 31 test angles evenly spread from -45⁰ to +45⁰. For each test angle, all the samples are decoded, or sliced, to the nearest constellation point. The sum error is calculated as the MSE over the samples in the block.

The error function as calculated for every test angle for the example in Fig. 4 is shown in Fig. 5. The solid line in Fig. 5 is thus constructed from the 31 test angles. The test angle with the minimum error is -15⁰ thus correspond to the found optimum angles (illustrated as “+” in Fig. 4). Due to the 90-degree symmetry, the error function is periodic with 90-degree period (not shown in Fig. 5). Performance

Simulation results for a 2 x 2 MIMO system are shown in Figs. 6, 7, 8, and 9. All results presented are for forming the block of samples on a block-by-block basis. Fig. 6 and Fig. 8 compare the bit error rate (BER) for different angle resolutions. Fig. 7 and Fig. 9 compare the achievable rate (in terms of bits per channel use) for different angle resolutions. The simulation results in Fig. 6 and Fig. 7 are given for a 4096 QAM constellation, with a symbol rate of 50 Mbaud (megabaud), and with a blocks size of N = 31 samples per block, assuming a phase noise with -97 dBc (decibel per carrier) at 100 kHz. The simulation results in Fig. 8 and Fig. 9 are given for a 16384 QAM constellation, with a symbol rate of 50 Mbaud, and with a blocks size of N = 31 samples per block, assuming a phase noise with -97 dBc at 100 kHz.

Fig. 10 schematically illustrates, in terms of a number of functional units, the components of a MIMO transceiver 200 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1210 (as in Fig. 12), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry 210 is configured to cause the MIMO transceiver 200 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the MIMO transceiver 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The MIMO transceiver 200 may further comprise a communications (comm.) interface 220 at least configured for communications with other entities, functions, nodes, and devices, as in Fig. 1. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the MIMO transceiver 200 e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the MIMO transceiver 200 are omitted in order not to obscure the concepts presented herein.

Fig. 11 schematically illustrates, in terms of a number of functional modules, the components of a MIMO transceiver 200 according to an embodiment. The MIMO transceiver 200 of Fig. 11 comprises a number of functional modules; a receive module configured to perform step S102, an identify module 210b configured to perform step S104-2, and an estimate module 210c configured to perform step S104- 4. The MIMO transceiver 200 of Fig. 11 may further comprise a number of optional functional modules, as represented by functional module 2iod. In general terms, each functional module 210a: 2iod may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 230 which when run on the processing circuitry makes the MIMO transceiver 200 perform the corresponding steps mentioned above in conjunction with Fig 11. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 210a: 2iod maybe implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and/or the storage medium 230. The processing circuitry 210 may thus be configured to from the storage medium 230 fetch instructions as provided by a functional module 2ioa:2iod and to execute these instructions, thereby performing any steps as disclosed herein.

The MIMO transceiver 200 may be provided as a standalone device or as a part of at least one further device. For example, the MIMO transceiver 200 maybe provided in a node of a radio access network or in a node of a core network. Alternatively, functionality of the MIMO transceiver 200 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts. Thus, a first portion of the instructions performed by the MIMO transceiver 200 may be executed in a first device, and a second portion of the of the instructions performed by the MIMO transceiver 200 may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the MIMO transceiver 200 maybe executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a MIMO transceiver 200 residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 10 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 2ioa:2iod of Fig. 11 and the computer program 1220 of Fig. 12.

Fig. 12 shows one example of a computer program product 1210 comprising computer readable storage medium 1230. On this computer readable storage medium 1230, a computer program 1220 can be stored, which computer program 1220 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 1220 and/or computer program product 1210 may thus provide means for performing any steps as herein disclosed.

In the example of Fig. 12, the computer program product 1210 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 1210 could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1220 is here schematically shown as a track on the depicted optical disk, the computer program 1220 can be stored in any way which is suitable for the computer program product 1210. The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

1. A method for estimating transmitted samples of multiple-input multiple-output, MIMO, streams (150a, 150b), wherein the method is performed by a MIMO transceiver (200), and wherein the method comprises: receiving (S102) a sequence of samples from each receive antenna containing combinations of the MIMO streams (150a, 150b); repeatedly (S104), and for each block of samples of each MIMO stream (150a, 150b): identifying (S104-2), using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points; and estimating (S104-4) the transmitted samples for the block of symbols of the MIMO stream (150a, 150b) as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream (150a, 150b).

2. The method according to claim 1, wherein the samples when rotated by each of the test angles are scaled by MIMO equalizer tap values.

3. The method according to claim 1 or 2, wherein which constellation points to select from the set of constellation points are given by each of the samples as rotated with the test angles.

4. The method according to any preceding claim, wherein the set of test angles is selected based on modulation format of the transmitted samples, and expected phase noise at the MIMO transceiver (200).

5. The method according to claim 4, wherein the modulation format has a density, and wherein the higher the density is, the larger the set of test angles is.

6. The method according to claim 4, wherein the set of test angles is composed of test angles in a range of angles, and wherein the stronger the expected phase noise is, the larger the range is.

7. The method according to any preceding claim, wherein the block of samples is composed of N > 1 samples and wherein the sequence of samples is shifted by any of 1 to TV samples to form each block of N samples.

8. The method according to any preceding claim, wherein, for block k, the samples as rotated by the test angles and scaled by MIMO equalizer tap values are given by

where F_{t r} are MIMO equalizer tap values, y_{r n} are the samples from each receive antenna, a_{t r k-} are phase noise estimations for block k - 1, and a_mr are the test angles for the samples from each antenna of the MIMO transceiver (200).

9. The method according to claim 8, wherein, for each combination of angles, a point given by

is decoded, or sliced, to the constellation point

> _mr.

10. The method according to claim 9, wherein, for a block of N samples, the angles are identified according to

where m_{t l opt}, m_{t 2 opt}, ■■■ > ^mt,r,opt are indices of the identified angles, and y > 1.

11. The method according to any of claims 8 to 10, wherein, in presence of feedback, the phase noise estimations for block k are updated as a_{t r k} = a_{t r k-} +

12. The method according to any of claims 8 to 11, wherein the phase noise estimations are estimates of combined transmitter and receiver phase noise.

13- The method according to any of claims 8 to io, wherein, in absence of feedback, ^at,r,k-l ⁼ 0-

14. The method according to any of claims 1 to 10 or claim 13, wherein the identified test angles are estimates of combined transmitter and receiver phase noise.

15. A multiple-input multiple-output, MIMO, transceiver (200) for estimating transmitted samples of MIMO streams (150a, 150b), the MIMO transceiver (200) comprising processing circuitry (210), the processing circuitry being configured to cause the MIMO transceiver (200) to: receive a sequence of samples from each receive antenna containing combinations of the MIMO streams (150a, 150b); repeatedly, and for each block of samples of each MIMO stream (150a, 150b): identify, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points; and estimate the transmitted samples for the block of symbols of the MIMO stream (150a, 150b) as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream (150a, 150b).

16. A multiple-input multiple-output, MIMO, transceiver (200) for estimating transmitted samples of MIMO streams (150a, 150b), the MIMO transceiver (200) comprising: a receive module (210a) configured to receive a sequence of samples from each receive antenna containing combinations of the MIMO streams (150a, 150b); an identify module (210b) configured to, repeatedly, and for each block of samples of each MIMO stream (150a, 150b), identify, using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points; and an estimate module (210c) configured to, repeatedly, and for each block of samples of each MIMO stream (150a, 150b), estimate the transmitted samples for the block of symbols of the MIMO stream (150a, 150b) as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream (150a, 150b).

17. The MIMO transceiver (200) according to claim 15 or 16, further being configured to perform the method according to any of claims 2 to 14.

18. A computer program (1220) for estimating transmitted samples of multipleinput multiple-output, MIMO, streams (150a, 150b), the computer program comprising computer code which, when run on processing circuitry (210) of a MIMO transceiver (200), causes the MIMO transceiver (200) to: receive (S102) a sequence of samples from each receive antenna containing combinations of the MIMO streams (150a, 150b); repeatedly (S104), and for each block of samples of each MIMO stream (150a, 150b): identify (S104-2), using a set of test angles, which of the test angles that minimize a distance between each of the samples as rotated by the test angles and constellation points as selected from a set of constellation points; and estimate (S104-4) the transmitted samples for the block of symbols of the MIMO stream (150a, 150b) as a sequence of the constellation points as given by the identified test angles for the block of samples of the MIMO stream (150a, 150b).

19. A computer program product (1210) comprising a computer program (1220) according to claim 18, and a computer readable storage medium (1230) on which the computer program is stored.