Methods and apparatuses for interference suppression of wireless communication signals

Abstract

A method (300) in a transmitter node (110) for enabling interference suppression of wireless communication signals (121, 122). The transmitter node (110) comprises a number, equal to or larger than two, of transmitters (111, 112) configured to transmit respective wireless communication signals (121, 122) to a receiver node (130) over a wireless communication channel. The method comprises: embedding (310) a respective pilot symbol sequence in each of the wireless communication signals (121, 122), wherein each respective pilot symbol sequence comprises a number of pilot symbols (αn,1, αn2; bn1,bn,2) arranged to be transmitted within a coherence time of the wireless communication channel, and wherein the number of pilot symbols is equal to the number of transmitters (111, 112); and transmitting (320), to the receiver node (130), each of the wireless communication signals (121, 122) via respective transmitters of the number of transmitters (111, 112) such that the embedded respective pilot symbol sequences are transmitted within the coherence time.

Description

[0001] METHODS AND APPARATUSES FOR INTERFERENCE SUPPRESSION OF WIRELESS COMMUNICATION SIGNALS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates generally to the field of wireless communications. More particularly, the present disclosure relates to a method in a transmitter node for enabling interference suppression of wireless communication signals, and a transmitter node performing such a method. The present disclosure also relates to a method in a receiver node for interference suppression of wireless communication signals, and a receiver node performing such a method. The present disclosure also relates to a computer program for caring out the disclosed methods, and a computer program carrier carrying said computer program.

[0004] BACKGROUND

[0005] The capacity demand in wireless communication networks is growing rapidly. To meet the increasing capacity demand, one way to increase the spectral efficiency in backhaul microwave links is to use multiple-input multipleoutput (Ml MO), which is a technology utilizing multiple transmission antennas and multiple receiving antennas to increase capacity. As an example, in a 4 by 4 MIMO system, the transmitter node comprises four antennas and the receiver node comprises four antennas. The transmitter node transmits respective wireless communication signals from each of its antennas. Each transmitted wireless communication signal is received by all antennas of the receiver node. In this example, four MIMO layers are obtained. Each MIMO layer is typically associated with a respective encoded data stream. Each antenna of the receiver node receives four encoded streams comprised in a signal received by that particular antenna. To separate the encoded data streams comprised in the four received signals, the signals are digitized and processed using digital signal processing (DSP) individually and together using so-called MIMO equalization.

[0006] To be able to exploit the capacity gain in MIMO, it is important that the different MIMO layers do not cause interference to each other. When decoding one of the MIMO layers, bad estimates of the wireless communication channel or phase noise will reduce the possibility to suppress the other MIMO layers when performing the MIMO equalization, which will result in the MIMO system not reaching its full potential.

[0007] There is therefore a need for improved ways of enabling and performing interference suppression of wireless communication signals.

[0008] SUMMARY

[0009] It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least the above-mentioned problem. An object is to provide improved multi-antenna systems. In particular, an object is to provide improved ways of enabling and performing interference suppression of wireless communication signals.

[0010] According to a first aspect, the object is achieved by a method in a transmitter node for enabling interference suppression of wireless communication signals. The transmitter node comprising a number, equal to or larger than two, of transmitters configured to transmit respective wireless communication signals to a receiver node over a wireless communication channel. The method comprises embedding a respective pilot symbol sequence in each of the wireless communication signals. Each respective pilot symbol sequence comprises a number of pilot symbols arranged to be transmitted within a coherence time of the wireless communication channel. Furthermore, the number of pilot symbols is equal to the number of transmitters. The method further comprises transmitting, to the receiver node, each of the wireless communication signals via respective transmitters of the number of transmitters such that the embedded respective pilot symbol sequences are transmitted within the coherence time.

[0011] The disclosed method enables estimating phase rotations for surprising interference (such as Ml MO equalization) directly without using feedback loops when the transmitted wireless communication signals are received by a receiver node comprising a number of receivers equal to the number of transmitters in the transmitter node. As an example, if the encoded data stream comprised in a first wireless communication signal transmitted by first transmitter (Tx 0) of the number of transmitters is to be decoded by the receiver node, the respective pilot symbol sequences in each of the transmitted wireless communication signals enable estimations of respective phase rotation of the baseband-to-baseband channel for the first transmitter relative to all of the receivers (Rx 0, Rx 1 , Rx 2 etc.) in the receiver node, i.e. , the phase rotation estimate for the baseband-to-baseband channel for Tx 0 to Rx 0, for Tx 0 to Rx 1 , Tx 0 to Rx 2 etc. Using the estimated phase rotations, Ml MO equalization may be performed in the receiver node such that the encoded data stream comprised in the first wireless communication signal transmitted by the first transmitter can be decoded with the interference from the other transmitted wireless communication signals suppressed.

[0012] It is assumed that the pilot symbols of each pilot symbol sequence is known by the receiver node. Since the wireless communication signals are transmitted by the transmitter node within the coherence time of the wireless communication channel, each known pilot symbol of each pilot symbol sequence may be used together with each signal received by the respective receivers of the receiver node (as mentioned, each received signal comprises information of all transmitted wireless communication signals) to estimate phase rotations for MIMO equalization. Since the required phase rotations are estimated using information transmitted within the coherence time of the wireless communication channel, accurate estimations of the phase rotations are obtained. The disclosed method does not rely on information that has been transmitted outside of the coherence time of the wireless communication channel (e.g., previously transmitted pilot symbols). The disclosed method thereby enables improved performance of multi-antenna systems.

[0013] According to a second aspect, the object is achieved by a method in a receiver node for interference suppression of wireless communication signals. The receiver node comprises a number, equal to or larger than two, of receivers configured to receive respective signals from a transmitter node over a wireless communication channel. The method comprises receiving a respective signal by each receiver of the number of receivers within a coherence time of the wireless communication channel. Each received signal comprises one or more wireless communication signals transmitted by respective transmitters of a number of transmitters of a transmitter node. Each of the one or more wireless communication signals comprises a respective embedded pilot symbol sequence. Each respective pilot symbol sequence comprises a number of pilot symbols. Furthermore, the number of pilot symbols is equal to the number of transmitters. The method further comprises suppressing an interference of one of the one or more wireless communication signals by means of a phase rotation for each receiver of the number of receivers based on each received signal and on each embedded pilot symbol sequence.

[0014] The disclosed method surprises interference (such as MIMO equalization) directly without using feedback loops. The disclosed method does not rely on information that has been transmitted outside of the coherence time of the wireless communication channel (e.g., previously transmitted pilot symbols). The disclosed method thereby enables improved performance of multi-antenna systems.

[0015] According to a third aspect, the object is achieved by a transmitter node for enabling interference suppression of wireless communication signals. The transmitter node is associated with the above-discussed advantages. The transmitter node comprises a number, equal to or larger than two, of transmitters configured to transmit respective wireless communication signals to a receiver node over a wireless communication channel. The transmitter node further comprises a processing circuitry and a memory. The processing circuitry is configured to embed a respective pilot symbol sequence in each of the wireless communication signals. Each respective pilot symbol sequence comprises a number of pilot symbols arranged to be transmitted within a coherence time of the wireless communication channel. Furthermore, the number of pilot symbols is equal to the number of transmitters. The processing circuitry is further configured to transmit, to the receiver node, each of the wireless communication signals via respective transmitters of the number of transmitters such that the embedded respective pilot symbol sequences are transmitted within the coherence time.

[0016] According to a fourth aspect, the object is achieved by a receiver node for interference suppression of wireless communication signals. The receiver node is associated with the above-discussed advantages. The receiver node comprises a number, equal to or larger than two, of receivers configured to receive respective signals from a transmitter node over a wireless communication channel. The receiver node further comprises a processing circuitry and a memory. The processing circuitry is configured to receive a respective signal by each receiver of the number of receivers within a coherence time of the channel. Each received signal comprises one or more wireless communication signals transmitted by respective transmitters of a number of transmitters of a transmitter node, and wherein each of the one or more wireless communication signals comprises a respective embedded pilot symbol sequence. Each respective pilot symbol sequence comprises a number of pilot symbols. Furthermore, the number of pilot symbols is equal to the number of transmitters. The processing circuitry is further configured to suppress an interference of one of the one or more wireless communication signals by means of a phase rotation estimate for each receiver of the number of receivers based on each received signal and on each embedded pilot symbol sequence. According to a fifth aspect, the object is achieved by a computer program product comprising instructions which, when executed on at least one processing circuitry, cause the at least one processing circuitry to carry out the method according to the discussion above. The computer program is associated with the above-discussed advantages.

[0017] According to a sixth aspect, the object is achieved by a computer program carrier carrying a computer program product according to the discussion above, wherein the computer program carrier is one of an electronic signal, optical signal, radio signal, or computer-readable storage medium. The computer program carrier is associated with the above-discussed advantages.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] With reference to the appended drawings, below follows a more detailed description of embodiments of the present disclosure cited as examples. In the drawings:

[0020] Figure 1 is a schematic of a multi-antenna system;

[0021] Figure 2 shows an example of two respective pilot symbol sequences transmitted by two transmitters;

[0022] Figures 3-4 are flow charts illustrating methods;

[0023] Figure 5 schematically illustrates a transmitter node; and

[0024] Figure 6 schematically illustrates a receiver node.

[0025] DETAILED DESCRIPTION

[0026] The present disclosure is described below with reference to the accompanying drawings, in which certain aspects of the present disclosure are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments and aspects 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 present disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

[0027] It is to be understood that the present disclosure is not limited to the embodiments described herein and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.

[0028] Figure 1 illustrates a transmitter node 110 transmitting wireless communication signals to a receiver node 130. The transmitter node 110 and the receiver node 130 may form a multiple-input multiple-output (MIMO) system, which is a type of multi-antenna system. The transmitter node 110 of Figure 1 comprises a first transmitter 111 (also called Tx 0) and a second transmitter 112 (also called Tx 1). The first transmitter 111 is configured to transmit a first wireless communication signal 121 and the second transmitter 112 is configured to transmit a second wireless communication signal 122. The transmitter node 110 may, however, comprise any number of transmitters (the number being equal to or larger than two) configured to transmit respective wireless communication signals.

[0029] Each transmitter of the transmitter node 110 may comprise a respective antenna or be associated with a respective antenna element of an array antenna. In some embodiments, two different transmitters are associated with different polarizations of the same antenna or antenna element of an array antenna.

[0030] Each transmitter of the transmitter node 110 is configured to transmit a respected encoded data stream via the respective wireless communication signals. The transmitter node 110 may comprise a digital signal processing (DSP) unit providing digitized signals comprising the encoded data streams. Alternatively, each transmitter may comprise a respective DSP unit providing a respective digitized signal comprising a respective encoded data stream. Each transmitter may comprise a respective DAC configured to convert the respective digitized signals to the analog domain. Each transmitter may comprise an upconverter configured to upconvert the analog signals to a radio frequency (RF) carrier. In alternative embodiments, one or more DACs and / or one or more upconverters are shared between one or more transmitters. The alterative embodiments may, e.g., comprise homodyne upconversion and / or shared precoding of the encoded data streams. Prior to the upconversion, the signals are referred to as baseband signals.

[0031] The receiver node 130 of Figure 1 comprises a first receiver 131 (also called Rx 0) and a second receiver 132 (also called Rx 1). The first receiver 131 is configured to receive a first signal 123 and the second receiver 132 is configured to receive a second signal 124. The receiver node 130 may, however, comprise any number of receivers configured to receive respective signals, where the number of receivers is equal to the number of transmitters 111 , 112 of the transmitter node 110.

[0032] Each receiver of the receiver node 130 may comprise a respective antenna or be associated with a respective antenna element of an array antenna. In some embodiments, two different receivers are associated with different polarizations of the same antenna or antenna element of an array antenna.

[0033] Each receiver may comprise a respective downconverter configured to downconvert the respective received signals from the RF carrier. Furthermore, each receiver may comprise a respective analog-to-digital converter (ADC) configured to convert the respective downconverted signals to the digital domain. In alternative embodiments, one or more ADCs and / or one or more downconverters may be shared between one or more receivers. The alterative embodiments may, e.g., comprise homodyne downconversion. The receiver node 130 further comprises a DSP unit configured to decode the encoded data streams comprised in the received signals.

[0034] The first wireless communication signal 121 may be transmitted such that it received by both the first and the second receivers 131 , 132. Similarly, the first wireless communication signal 122 may be transmitted such that it received by both the first and the second receivers 131, 132. In Figure 1, the first wireless communication signal 121 is transmitted via a direct propagation path 121 a from the first transmitter 111 to the first receiver 131 , and via a cross propagation path 121 b from the first transmitter 111 to the second receiver 132. Similarly, the second wireless communication signal 122 is transmitted via a direct propagation path 122a from the second transmitter 112 to the second receiver 132, and via a cross propagation path 122b from the second transmitter 112 to the first receiver 131. In Figure 1 , the first signal 123 thus comprises encoded data streams of both the first and the second wireless communication signals 121 , 122, and the second signal 123 comprises encoded data streams of both the first and the second wireless communication signals 121, 122.

[0035] The receiver node 130 of Figure 1 comprises various DSP components configured to suppress interference components of the received signals. For example, suppressing the interference from Tx 1 when decoding the encoded data stream transmitted by Tx 0.

[0036] In Figure 1 , the first signal 123 is received by Rx 0. The first signal 123, as downconverted and digitized, thereafter reaches an equalizer module 141 , which is configured to equalize the signal. The equalizer module 141 may, e.g., be adapted on a slicer error, to suppress inter symbol interference and gain differences of the different signal paths. The equalized signal 103 is thereafter fed into a phase estimator module 144. As is also shown in Figure 1 , the second signal 124 is received by Rx 1. The second signal 124, as downconverted and digitized, thereafter reaches an equalizer module 142, which is configured to equalize the signal. The equalizer module 142 may, e.g., be adapted on a slicer error, to suppress inter symbol interference and gain differences of the different signal paths The equalized signal 104 is thereafter fed into the phase estimator module 144.

[0037] The phase estimator module 144 is configured to estimate a phase rotation 145 of the baseband-to-baseband channel from Tx 0 to Rx 0, which is denoted (pmainin Figure 1 , and a phase rotation 146 of the baseband-to- baseband channel from Tx 0 to Rx 1 , which is denoted <pmimoin Figure 1. Here, "main” in (pmainrepresents the encoded data stream of interest, and "mimo” in (pmimorepresents an undesired interference that is to be suppressed. The baseband-to-baseband channel represents the electric and propagation path for a signal and includes the upconversion in one transmitter, the propagation path over the air between the one transmitter and one receiver, and the downconversion in the one receiver.

[0038] In Figure 1 , pilot symbols an 0and an lare provided 101 to Tx 0, and are embedded into the first wireless communication signal 121 , and are thereby comprised (as affected by the baseband-to-baseband channel between Tx 0 and Rx 0) in the equalized signal 103 and comprised (as affected by the baseband-to-baseband channel between Tx 0 and Rx 1) in the equalized signal 104. Pilot symbols bn 0and bn lare provided 102 to Tx 1 , and are embedded into the second wireless communication signal 122, and are thereby comprised (as affected by the baseband-to-baseband channel between Tx 1 and Rx 0) in the equalized signal 103 and comprised (as affected by the baseband-to-baseband channel between Tx 1 and Rx 1) in the equalized signal 104. In the pilot symbols, “n” represents a time instance within a coherence time of the wireless communication channel. The coherence time of the wireless communication channel is discussed in more detail below. Furthermore, "0” and "1” represent a first and a second pilot symbol, respectively. A pilot symbol may e.g. be any symbol in an M-QAM encoding scheme, where M is an even power of two, and preferably equal to four. More generally, however, a pilot symbol may be any complex value.

[0039] The pilot symbols an 0, an l, bn 0, and bn lare known apriori by the receiver node 130. The phase estimator module 144 is provided with known pilot symbols by a pilot module 143. The phase rotations estimates <pmainand (Pmimoareestimated by the phase estimator module 144 by comparing the known pilot symbols to the equalized signals 103, 104.

[0040] The equalized signal 103 is shifted in phase by <pmainand is thereafter combined with the equalized signal 103 as shifted in phase by (pmtmo- Thus, the combined signal 105 comprises the encoded data stream comprised in the first wireless communication signal transmitted by Tx 0 where the interference from Tx 1 has been suppressed.

[0041] The dashed block 147 may comprise corresponding modules for obtaining a combined signal that comprises the encoded data stream comprised in the second wireless communication signal transmitted by Tx 1 where the interference from Tx 0 has been suppressed.

[0042] When developing the embodiments disclosed herein, it has been realized that if each transmitter transmits a respective pilot symbol sequence with a number of pilot symbols corresponding to the number of transmitters 111 , 112, if each pilot symbol sequence is transmitted within a coherence time of the wireless communication channel, and if the pilot symbol sequences are known by a receiver node 130 with a number of receivers 131 , 132 corresponding to the number of transmitter, the receiver node may estimate phase rotations of the baseband-to-baseband channels between any of the transmitters relative to all of the respective receivers. These estimated phase rotations may thereafter be used to suppress interference. In particular, MIMO equalization may be performed by the receiver node 130.

[0043] The coherence time of the wireless communication channel is a time duration in which the channel impulse response is considered to be not varying for the wireless communication channel. In some embodiments, the channel impulse response is considered to not vary if the phase noise introduced by the wireless communication channel is small enough to not rotate a transmitted symbol to another symbol value within the time of a pilot symbol sequence or the time between two consecutive pilot symbol sequences with data symbols in between. The coherence time of the wireless communication channel may be the smallest value of all baseband-to-baseband channels from any of the transmitters of the transmitter node 110 to any of the receivers of the receiver node 130.

[0044] As an example, if the transmitter node 110 comprises two transmitters and the receiver node 130 comprises two receivers (e.g. a 2 by 2 MIMO system), each transmitter transmits a respective pilot symbol sequence comprising two pilot symbols. In another example, the transmitter node 110 comprises four transmitters and the receiver node 130 comprises four receivers (e.g. a 4 by 4 MIMO system), where each transmitter transmits a respective pilot symbol sequence comprising four pilot symbols.

[0045] By transmitting such pilot symbol sequences, the phase rotations of the baseband-to-baseband channels can be estimated by solving an equation system, and long feedback loops are avoided. Thus, such strategy enables estimating phase rotations required for interference suppression (such as MIMO equalization) directly without using feedback loops. Furthermore, such strategy provides a multi-antenna system with phase tracking capabilities for highly spectral efficient communications.

[0046] The following equations show how the pilot symbols may be chosen for two transmitters (TxO and Tx1 , respectively). Below, the main Rx branch (Rx O in this case) is considered when signals 123, 124 from TxO and Tx1 are received. Looking at the pilot symbols in the main Rx branch, the following equations should be fulfilled when the phasesPmain and (Pmimo aro correct.

[0047] Equation 1el'<Pmam■ main0+ e1'^^0■ mimo0= a0

[0048] Equation 2 el'fPmain■ main-^ + el'Vmimo■ mimo = a

[0049] Here, the following notation is used: a0is the first pilot symbol (in a group of 2) of Tx 0 (note that “n" has been omitted from the subscript and is assumed to be the same value, such as 1, for all pilot symbols), a is the second pilot symbol (in a group of 2) of Tx 0,

[0050] (Pmain is the estimated phase rotation of the baseband-to-baseband channel from Tx 0 to Rx 0,

[0051] (Pmimo isthe estimated phase rotation of the baseband-to-baseband channel from Tx 0 to Rx 1 , main0is a first sample (e.g. first time instance within the coherence time of the wireless communication channel) of the first signal 123 main is a second sample (e.g. second time instance within the coherence time of the wireless communication channel) of the first signal 123 mimo0is a first sample (e.g. first time instance within the coherence time of the wireless communication channel) of the second signal 124 mimo1is a second sample (e.g. second time instance within the coherence time of the wireless communication channel) of the second signal 124

[0052] Since the pilot symbol sequences are transmitted within the coherence time of the wireless communication channel, it is assumed that <pmainand <pmtmoare the same for the two samples of main0, mai.n1and of mimo0, mlmo , respectively. To reiterate, main0comprises a0and b0as affected by the baseband-to-baseband channel from Tx 0 to Rx 0 and the baseband-to-baseband channel from Tx 1 to Rx 0, respectively, main comprises a and b as affected by the baseband-to-baseband channel from Tx 0 to Rx 0 and the baseband-to-baseband channel from Tx 1 to Rx 0, respectively, mimo0comprises a0and b0as affected by the baseband-to-baseband channel from Tx 0 to Rx 1 and the baseband-to-baseband channel from Tx 1 to Rx 1 , respectively, and mimo1comprises a and b as affected by the baseband-to-baseband channel from Tx 0 to Rx 1 and the baseband-to-baseband channel from Tx 1 to Rx 1 , respectively, may be estimated from

[0053] Equation

[0054] Equation

[0055] When using this type of feedforward approach, there is no limitation in loop delay, and the bandwidth of the phase estimation can be set much higher. The bandwidth corresponds to how fast the phase estimation can respond to changes in phase. As an example, in the methods disclosed herein, it is possible to implement a finite impulse response (FIR) filter for reducing additive white Gaussian noise without increasing loop latency and thus not limiting the bandwidth. Thus, the estimation of phase is improved. A penalty is an extra pilot symbol which reduces capacity. However, the improved estimation of phase rotations enables a higher-order modulation scheme, which increases the capacity.

[0056] Equations 1 and 2 may be generalized to the case when the transmitter node 110 comprises a number N transmitters and the receiver node 130 comprise the number N receivers (e.g. a N by N MIMO system), where N is any integer equal to or larger than two. In that case, the following equation system can be formed for the one of the Rx branches, which is denoted "main” (e.g. Rx 0)

[0057] Equation 5 R ■ A = P where

[0058] R is a matrix comprising samples from the signals received by the N receivers, where each row represents a respective receiver and the columns represent different time instances within the coherence time of the wireless communication channel, and may be in the form of A is a column vector comprising estimates of the phase rotations the baseband-to-baseband channel from Tx 0 to Rx 0, Rx 1, .... Rx N-1 , and may be in the form of

[0059] P is a column vector comprising the pilot symbols transmitted by one of the transmitters (such as Tx 0), and may be in the form of

[0060] The column vector A may be solved from Equation 5 since the column vector P is known.

[0061] It is desired to solve the equation system of Equation 5 so that A can be found with limited impact of noise. Preferably, each pilot symbol sequence is non-orthogonal to each of the other pilot symbol sequences. For example, a pilot symbol sequence of Tx 0, i.e., a1:... , aN-i), is non-orthogonal with respect to each of the respective pilot symbol sequences of Tx 1 , Tx 2, ... , Tx N-1 , e.g., (b0, b1:.... b^^ of Tx 1. In other words, the scalar product of a vector comprising the pilot symbols of one sequence and a vector comprising the pilot symbols of another sequence is not zero.

[0062] More preferably, the respective pilot symbol sequences may be selected such that the following equation is fulfilled.

[0063] Equation 6 where a0, alt... , aN-is the pilot symbol sequence for Tx 0 and b0, blt.... bp^ is the pilot symbol sequence for Tx 1 . Here, " ” denotes the complex conjugate. Furthermore, C is a complex value.

[0064] Preferably, C is selected A can be found with limited impact of noise. To do that the abs(real(C)) should preferably be equal abs(imag(C)). However, C may be selected for a limited impact on noise if a ratio of the magnitude of the real part of C and the magnitude of the imaginary part of C is equal to 0.1 -10, preferably 0.9- 1.1 , and more preferably 0.99-1.01. Furthermore, it is desired that abs(C) is as large as possible.

[0065] Equation 5 preferably holds between all of the respective pilot symbol sequences. In other words, a0, a ... , aN-±may represent the pilot symbol sequence for a first transmitter of the number of transmitters and b0, blt... , dW-i may represent the pilot symbol sequence for another transmitter of the number of transmitters different than the first transmitter.

[0066] Figure 2 shows an example of two symbol sequences transmitted by two transmitters, viz., by Tx 0 and Tx 1 , versus time. Each pilot symbol sequence comprises two pilot symbols. Around a first time instance n, Tx 0 transmits pilot symbols an 0and an lconsecutively in a pilot symbol sequence, around a second time instance n + 1, Tx 0 transmits pilot symbols an+1;0and an+1 1consecutively in a pilot symbol sequence, and around a third time instance, Tx 0 transmits pilot symbols an+2 0and an+2 1consecutively in a pilot symbol sequence. Around the first time instance n, Tx 1 transmits pilot symbols bn 0and bn lconsecutively in a pilot symbol sequence, around the second time instance n + 1, Tx 1 transmits pilot symbols dn+1;0and bn+lilconsecutively in a pilot symbol sequence, and around a third time instance, Tx 1 transmits pilot symbols bn+2 0and fcn+2;1consecutively in a pilot symbol sequence. At the time between transmitted pilot symbol sequences, non-pilots, such as data symbols, are transmitted.

[0067] The time instance n may be within a first coherence time of the wireless communication channel, the time instance n + 1 may be within a second coherence time of the wireless communication channel, and the time instance n + 2 may be within a third coherence time of the wireless communication channel. The first, second, and third coherence times represent time intervals where the channel impulse response is considered to be varying relative to each other. As an example, Tx 0 and Tx 1 may transmit non-pilots after transmitting their respective pilot symbol sequences up until the channel impulse has changed or is expected to change.

[0068] Each pilot symbol sequence of Figure 2 may comprise main pilot symbol and an auxiliary pilot symbol, wherein the auxiliary pilot symbol of each respective pilot symbol sequence is equal to, the negative of, and / or the complex conjugate of the corresponding main pilot symbol. The main pilot symbol may also be referred to as a "mother pilot symbol” and the auxiliary pilot symbol may be referred to as a "daughter pilot symbol”.

[0069] In example, how the auxiliary pilot symbol is selected based on the main pilot symbol alternates. In particular, the auxiliary pilot symbols may be selected as

[0070] Tx O: a at an even time instance n: Auxiliary pilot symbol is the conjugate of its mother a0at an odd time instance n: Auxiliary pilot symbol is the negative of its mother a0

[0071] Tx 1 : b at an even time instance n: Auxiliary pilot symbol is the negative of its mother b0b at an odd time instance n: Auxiliary pilot symbol is the complex conjugate of its mother

[0072] In other words, the transmissions from Tx 1 and Tx 2 can be written as:

[0073] Stream from Tx 0: an 0, an 0, data symbols, an+ii0, -an+i,o> data symbols, an+2 0,an+2 0, .

[0074] Stream from Tx 1 : bn 0,-bn 0, data symbols, dn+1;0, b^0, data symbols, bn+2 0,-bn+2 0, ..

[0075] Another example is:

[0076] Stream from Tx 0. ct-r^ Q, ct-r^ Q, data symbols, data symbols, ct-r^^2Q, ct-r^^2Q, Stream from Tx 1 : bnfi, bnfi, data symbols, bn+lfi,-bn+lfi, data symbols, bn+2.0, bn+2.0,

[0077] Yet another example is:

[0078] Stream from data symbols, 01^4-1 o> ^n+i o> data symbols, 01^4-2 o> ^714-2 o> ■ ■ ■ ■

[0079] Stream from Tx 1 : bni0, b~0, data symbols, bn+li0,bn+li0, data symbols, bn+2.0, b^0, ...

[0080] The alternating pattern of the pilot symbol sequences has shown to improve the interference suppression over time since any repetitive pattern may unintentionally be interpreted as noise.

[0081] The main pilot symbols may be generated as a pseudo-random binary sequence where the pilot symbol sequences for Tx 0 and Tx 1 are independent of each other.

[0082] In general, each respective pilot symbol sequence comprises a respective main pilot symbol and one or more auxiliary pilot symbols, wherein the one or more auxiliary pilot symbols of each respective pilot symbol sequence are equal to, the negative of, and / or the complex conjugate of the corresponding main pilot symbol.

[0083] Figure 3 illustrates a method 300 in a transmitter node 110 for enabling interference suppression of wireless communication signals 121 , 122. The transmitter node 110 comprises a number, equal to or larger than two, of transmitters 111 , 112 configured to transmit respective wireless communication signals 121, 122 to a receiver node 130 over a wireless communication channel. In other words, the transmitter node 110 comprises a plurality of transmitters 111, 112. The method 300 may, e.g., be performed by the transmitter node 110 shown in Figure 1. The receiver node 130 of the method 300 may be the receiver node 130 as shown in Figure 1. The method 300 may comprise a number of actions listed below.

[0084] Action 310. The method 300 comprises embedding a respective pilot symbol sequence in each of the wireless communication signals 121 , 122. Each respective pilot symbol sequence comprises a number of pilot symbols an l,an,2> bnilbn 2arranged to be transmitted within a coherence time of the wireless communication channel. Furthermore, the number of pilot symbols is equal to the number of transmitters 111 , 112.

[0085] Each pilot symbol of each pilot symbol sequence may be an M-QAM symbol, where M is an even power of two, and preferably equal to four. If a pilot symbol is an 4-QAM symbol, it may be selected as a value from 1 / V2(1, j); 1 / V2(-1, j); 1 / V2(-1, -j); 1 / V2(1, -j). In general, however, a pilot symbol may be any complex value.

[0086] Embedding a pilot symbol sequence in a wireless communication signal may mean to arrange the pilot symbols of the sequence in a stream of symbols to be transmitted by a transmitter of the number of transmitters 111 , 112. In other words, some of the symbols of the stream of symbols are the pilot symbols. The pilot symbol sequences may be embedded in a digitized signal that is to be converted to the analog domain and thereafter upconverted to an RF carrier before transmission over the air. The number of pilot symbols an l, an 2,' bn l, bn 2in each respective pilot symbol sequence may be arranged in a consecutive order. If the pilot symbols are embedded in a stream of symbols, the respective pilot symbols a pilot symbol sequence are arranged (i.e., embedded) after each other, with no data symbols in between. In this way, small variations of the wireless communication channel within the coherence time are reduced. Thus, an improved estimation of phase rotations for interference suppression is enabled.

[0087] Each respective pilot symbol sequence may be configured to be non-orthogonal to each of the other respective pilot symbol sequences. This reduces the impact noise has on the phase rotation estimates for interference suppression. Thus, an improved estimation of phase rotations for interference suppression is enabled. A first pilot symbol sequence is considered non-orthogonal to a second pilot symbol sequence if a scalar product of vector of the pilot symbols of the first pilot symbol sequence (in the order of transmission) and vector of the pilot symbols of the second pilot symbol sequence (in the order of transmission) is non-zero.

[0088] The respective pilot symbol sequences may be configured to fulfill the condition

[0089] ^1=0 a-ibi = C, where at is the i:th pilot of a pilot sequence of a first wireless communication signal 121 of the number of wireless communication signals, bt is the complex conjugate of the i:th pilot of a pilot sequence of one of the number of wireless communication signals 122 different than first wireless communication signal 121,

[0090] N is the number of pilots,

[0091] C is a complex value, wherein a ratio of the magnitude of the real part of C and the magnitude of the imaginary part of C is equal to 0.1- 10, preferably 0.9-1.1, and more preferably 0.99-1.01. This reduces the impact noise has on the phase rotation estimates for interference suppression. Thus, an improved estimation of phase rotations for interference suppression is enabled.

[0092] Each respective pilot symbol sequence may comprise a respective main pilot symbol and one or more auxiliary pilot symbols, wherein the one or more auxiliary pilot symbols of each respective pilot symbol sequence are equal to, the negative of, and / or the complex conjugate of the corresponding main pilot symbol. This provides a simple way of providing the pilot symbols with a relatively low peak to average power ratio (since repeated symbols may lead to high peaks in the signal). It is particularly preferably that the pilot symbols are 4-QAM symbols, and even more preferable that the pilot symbols are 4-QAM symbols and the condition 1 1)1ajbt = C (as discussed above) is fulfilled.

[0093] Each main pilot symbol of each respective pilot symbol sequence may be based on a pseudo random binary sequence. In this way, less periodicity of over time is provided. The number of transmitters 111 , 112 may be two. In that case, the auxiliary pilot symbols may be the negative of and the complex conjugate of, respectively, the corresponding main pilot symbols, or equal to and the complex conjugate of, respectively, the corresponding main pilot symbols.

[0094] Action 320. The method 300 comprises transmitting, to the receiver node 130, each of the wireless communication signals 121 , 122 via respective transmitters of the number of transmitters 111 , 112 such that the embedded respective pilot symbol sequences are transmitted within the coherence time.

[0095] The coherence time of the wireless communication channel may be the smallest value of all baseband-to-baseband channels from any of the transmitters of the transmitter node 110 to any of the receivers of the receiver node 130.

[0096] Each of the wireless communication signals 121 , 122 are transmitted by the transmitter node 110 over the air to the receiver node 130.

[0097] The method 300 enables surprising interference (such as MIMO equalization) directly without using feedback loops. The method 300 does not rely on information that has been transmitted outside of the coherence time of the wireless communication channel (e.g., previously transmitted pilot symbols). The method 300 thereby enables improved performance of multi-antenna systems.

[0098] The respective pilot symbol sequences may be respective primary pilot symbol sequences, and wherein the coherence time is a first coherence time. In that case, the method may comprise Actions 330 and 340 below.

[0099] Action 330. The method 300 may comprise embedding a respective secondary pilot symbol sequence in each of the wireless communication signals 121 , 122. Each respective secondary pilot symbol sequence comprises a number of pilot symbols an+i,ian+i,2>' bn+l lbn+12arranged to be transmitted within a second coherence time of the wireless communication channel. Furthermore, the number of pilot symbols is equal to the number of transmitters 111 , 112.

[0100] Action 340. The method 300 comprise transmitting, to the receiver node 130, each of the wireless communication signals 121 , 122 such that the embedded respective primary pilot symbol sequences are transmitted within the first coherence time and the embedded respective secondary pilot symbol sequences are transmitted within the second coherence time.,

[0101] In this way, phase rotations for interference suppression may be estimated as the wireless communication channel varies. Thus, an improved estimation of phase rotations for interference suppression is enabled.

[0102] Each secondary pilot symbol sequence is preferably different from a corresponding primary pilot symbol sequence. The method may comprise transmitting the respective primary and secondary pilot symbol sequences by respective transmitters in an alternating fashion, where data is transmitted between the pilot symbol sequences.

[0103] Figure 4 illustrates a method 400 in a receiver node 130 for interference suppression of wireless communication signals 121, 122. The receiver node 130 comprises a number, equal to or larger than two, of receivers 131 , 132 configured to receive respective signals 123, 124 from a transmitter node 110 over a wireless communication channel. In other words, the receiver node 130 comprises a plurality of receivers 131 , 132. The transmitter node 110 comprises an equal number of transmitters 111 , 112 as the number of receivers 131 , 132 of the receiver node 130. The method 400 may, e.g., be performed by the receiver node 130 shown in Figure 1. The transmitter node 110 of the method 400 may be the transmitter node 110 as shown in Figure 1. The method 400 may comprise a number of actions listed below.

[0104] Action 410. The method 400 comprises receiving a respective signal 123, 124 by each receiver of the number of receivers 131 , 132 within a coherence time of the wireless communication channel. Each received signal 123, 124 comprises one or more wireless communication signals 121 , 122 transmitted by respective transmitters of a number of transmitters 111 , 112 of a transmitter node 110. Each of the one or more wireless communication signals 121 , 122 comprises a respective embedded pilot symbol sequence. Each respective pilot symbol sequence comprises a number of pilot symbols an l, an 2\ bn l, bn 2. Furthermore, the number of pilot symbols is equal to the number of transmitters 111 , 112.

[0105] Each pilot symbol of each pilot symbol sequence may be an M-QAM symbol, where M is an even power of two, and preferably equal to four. If a pilot symbol is an 4-QAM symbol, it may be selected as a value from 1 / V2(1, j); 1 / V2(-1, j); 1 / V2(-1, -j); 1 / V2(1, -j). In general, however, a pilot symbol may be any complex value.

[0106] The embedded pilot symbol sequence in a wireless communication signal may mean that the pilot symbols of the sequence are arranged in a stream of symbols transmitted by a transmitter of the number of transmitters 111 , 112. In other words, some of the symbols of the stream of symbols are the pilot symbols. The pilot symbol sequences may be embedded in a digitized signal that has been converted to the analog domain and thereafter upconverted to an RF carrier before transmission over the air.

[0107] Action 420. The method 400 comprises suppressing an interference of one of the one or more wireless communication signals 121 by means of a phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131 , 132. The phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131 , 132 are based on each received signal 123, 124 and on each embedded pilot symbol sequence.

[0108] The method 400 surprises interference (such as Ml MO equalization) directly without using feedback loops. The method 400 does not rely on information that has been transmitted outside of the coherence time of the wireless communication channel (e.g., previously transmitted pilot symbols). The method 400 thereby enables improved performance of multi-antenna systems.

[0109] According to some embodiments, the method 400 comprises, prior to Action 420, estimating the phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131 , 132 based on each received signal 123, 124 and on each embedded pilot symbol sequence, where the phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131 , 132 is for suppressing interference of one of the one or more wireless communication signals 121 , 122. The phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131, 132 may be indicative of a baseband-to-baseband channel from one of the transmitters of the number of transmitters 111, 112 to the corresponding receiver of the number of receivers 131 , 132.

[0110] The phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131, 132 may be based on a sequence of samples of each received signal 123, 124, where each sequence of samples comprises a number of samples equal to the number of pilot symbols.

[0111] Furthermore, the phase rotation estimate <pmain, (pmimo may be obtained upon equalizing and synchronizing each received signal 123, 124 by the receiver node 130.

[0112] Action 421. The method 400 may comprise applying the respective phase rotation estimates <pmain, (pmimo to each received signal 123, 124 and thereafter combining each received signal 123, 124. Thereby, interference of one of the one or more wireless communication signals 121 is suppressed.

[0113] Figure 5 shows a schematic block diagram of embodiments of a transmitter node 110 for enabling interference suppression of wireless communication signals 121 , 122. The embodiments of the transmitter node 110 may be considered as independent embodiments or may be considered in any combination with each other. It should also be noted that, although not shown in Figure 5, the node may comprise known conventional features for such devices, such as a power source like a battery, mains connection, or an antenna arrangement.

[0114] The transmitter node 110 comprises a number, equal to or larger than two, of transmitters 111, 112 configured to transmit respective wireless communication signals 121, 122 to a receiver node 130 over a wireless communication channel.

[0115] The transmitter node 110 may comprise processing circuitry 510 and a memory 520. The processing circuitry 510 may comprise a receiving module 511, and a transmitting module 512 associated with the transmitters 111 , 112. The receiving module 511 and the transmitting module 512 may comprise radio frequency circuitry and baseband processing circuitry capable of transmitting and receiving wireless communication signals 121 , 122. The receiving module 511 and the transmitting module 512 may also form part of a single transceiver.

[0116] It should also be noted that some or all of the functionality described in the embodiments above as being performed by the transmitter node 110 may be provided by the processing circuitry 510 executing instructions stored on a computer-readable medium, such as, e.g. the memory 520 shown in Figure 5. Alternative embodiments of the transmitter node 110 may comprise additional components, such as, an embedding module 513, responsible for providing functionality to support the embodiments of the node described herein.

[0117] The transmitter node 110, the processing circuitry 510, or the embedding module 513 is configured to embed a respective pilot symbol sequence in each of the wireless communication signals 121 , 122. Each respective pilot symbol sequence comprises a number of pilot symbols an l, an 2; bn l, bn 2arranged to be transmitted within a coherence time of the wireless communication channel. Furthermore, the number of pilot symbols is equal to the number of transmitters 111 , 112. The transmitter node 110, the processing circuitry 510, or the transmitting module 512 is configured to transmit, to the receiver node 130, each of the wireless communication signals 121, 122 via respective transmitters of the number of transmitters 111 , 112 such that the embedded respective pilot symbol sequences are transmitted within the coherence time.

[0118] According to some embodiments, the number of pilot symbols an l, an 2; bn l, bn 2in each respective pilot symbol sequence are arranged in a consecutive order.

[0119] According to some embodiments, each respective pilot symbol sequence is configured to be non-orthogonal to each of the other respective pilot symbol sequences.

[0120] According to some embodiments, the respective pilot symbol sequences are configured to fulfill the condition atis the i:th pilot of a pilot sequence of a first wireless communication signal 121 of the number of wireless communication signals, bi is the complex conjugate of the i:th pilot of a pilot sequence of one of the number of wireless communication signals 122 different than first wireless communication signal 121 ,

[0121] N is the number of pilots,

[0122] C is a complex value, wherein a ratio of the magnitude of the real part of C and the magnitude of the imaginary part of C is equal to 0.1- 10, preferably 0.9-1.1 , and more preferably 0.99-1.01.

[0123] According to some embodiments, the respective pilot symbol sequences are respective primary pilot symbol sequences, and wherein the coherence time is a first coherence time. In that case, the transmitter node 110, the processing circuitry 510, or the embedding module 513 may be configured to embed a respective secondary pilot symbol sequence in each of the wireless communication signals 121 , 122. Each respective secondary pilot symbol sequence comprises a number of pilot symbols an+i,ian+i,2>' bn+l lbn+1 2arranged to be transmitted within a second coherence time of the wireless communication channel. Furthermore, the number of pilot symbols is equal to the number of transmitters 111, 112. The transmitter node 110, the processing circuitry 510, or the transmitting module 512 may further be configured to transmit, to the receiver node 130, each of the wireless communication signals 121, 122 such that the embedded respective primary pilot symbol sequences are transmitted within the first coherence time and the embedded respective secondary pilot symbol sequences are transmitted within the second coherence time.

[0124] According to some embodiments, each respective pilot symbol sequence comprises a respective main pilot symbol and one or more auxiliary pilot symbols, wherein the one or more auxiliary pilot symbols of each respective pilot symbol sequence are equal to, the negative of, and / or the complex conjugate of the corresponding main pilot symbol. According to some embodiments, each main pilot symbol of each respective pilot symbol sequence is based on a pseudo random binary sequence.

[0125] According to some embodiments, the number of transmitters 111 , 112 is two, and wherein the auxiliary pilot symbols are the negative of and the complex conjugate of, respectively, the corresponding main pilot symbols, or equal to and the complex conjugate of, respectively, the corresponding main pilot symbols.

[0126] According to some embodiments, each pilot symbol of each pilot symbol sequence is an M-QAM symbol, where M is an even power of two, and preferably equal to four.

[0127] Figure 6 shows a schematic block diagram of embodiments of a receiver node 130 for interference suppression of wireless communication signals 121 , 122. The embodiments of the receiver node 130 may be considered as independent embodiments or may be considered in any combination with each other. It should also be noted that, although not shown in Figure 6, the node may comprise known conventional features for such devices, such as a power source like a battery, mains connection, or an antenna arrangement.

[0128] The receiver node 130 comprises a number, equal to or larger than two, of receivers 131, 132 configured to receive respective signals 123, 124 from a transmitter node 110 over a wireless communication channel.

[0129] The receiver node 130 may comprise processing circuitry 610 and a memory 620. The processing circuitry 610 may comprise a transmitting module 612, and a receiving module 611 associated with the receivers 131, 132. The receiving module 611 and the transmitting module 612 may comprise radio frequency circuitry and baseband processing circuitry capable of transmitting and receiving wireless communication signals 121 , 122. The receiving module 611 and the transmitting module 612 may also form part of a single transceiver.

[0130] It should also be noted that some or all of the functionality described in the embodiments above as being performed by the receiver node 130 may be provided by the processing circuitry 610 executing instructions stored on a computer-readable medium, such as, e.g. the memory 620 shown in Figure 6. Alternative embodiments of the receiver node 130 may comprise additional components, such as, a suppressing module 613 and an estimating module 614, responsible for providing functionality to support the embodiments of the node described herein.

[0131] The receiver node 130, the processing circuitry 610, or the receiving module 611 is configured to receive a respective signal 123, 124 by each receiver of the number of receivers 131 , 132 within a coherence time of the channel. Each received signal 123, 124 comprises one or more wireless communication signals 121 , 122 transmitted by respective transmitters of a number of transmitters 1 11 , 112 of a transmitter node 110. Each of the one or more wireless communication signals 121, 122 comprises a respective embedded pilot symbol sequence. Each respective pilot symbol sequence comprises a number of pilot symbols an l, an 2bn l, bn 2. Furthermore, the number of pilot symbols is equal to the number of transmitters 111 , 112. The receiver node 130, the processing circuitry 610, or the suppressing module 613 is configured to suppress an interference of one of the one or more wireless communication signals 121 , 122 by means of a phase rotation estimate <pmain, (Pmimo for each receiver of the number of receivers 131 , 132 based on each received signal 123, 124 and on each embedded pilot symbol sequence.

[0132] According to some embodiments, the receiver node 130, the processing circuitry 610, or the estimating module 614 is configured to estimate a phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131 , 132 based on each received signal 123, 124 and on each embedded pilot symbol sequence.

[0133] According to some embodiments, the phase rotation estimate <pmain, (pmimo for each receiver of the number of receivers 131 , 132 is indicative of a baseband-to-baseband channel from one of the transmitters of the number of transmitters 111 , 112 to the corresponding receiver of the number of receivers 131 , 132.

[0134] According to some embodiments, the phase rotation estimate ((pmain, (pmimo) for each receiver of the number of receivers 131 , 132 is based on a sequence of samples of each received signal 123, 124, where each sequence of samples comprises a number of samples equal to the number of pilot symbols.

[0135] According to some embodiments, the phase rotation estimate <pmain, (Pmimo is obtained upon equalizing and synchronizing each received signal 123, 124 by the receiver node 130.

[0136] According to some embodiments, the receiver node 130 or the processing circuitry 610, or the suppressing module 613 is configured to apply the respective phase rotation estimates <pmain, (pmimo to each received signal 123, 124 and thereafter combining each received signal 123, 124.

[0137] According to some embodiments, each pilot symbol of each pilot symbol sequence is an M-QAM symbol, where M is an even power of two, and preferably equal to four.

[0138] The methods disclosed herein may be implemented through one or more processors, such as the processing circuitry 510 in the transmitter node 110 depicted in Figure 5 or the processing circuitry 610 in the receiver node 130 depicted in Figure 6, together with computer program code for performing the functions and actions of the embodiments herein. The program code may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code or code means for performing the embodiments herein when being loaded into the processing circuitry 510 in the transmitter node 110 or the processing circuitry 610 in the receiver node 130. The computer program code may e.g. be provided as pure program code in the transmitter node 110 or the receiver node 130, or on a server and downloaded to the transmitter node 110 or the receiver node 130. Thus, it should be noted that the modules of the transmitter node 110 or of the receiver node 130 may in some embodiments be implemented as computer programs stored in memory, e.g. in the memory 520 in Figure 5 or the memory 620 in Figure 5, for execution by processors or processing modules, e.g. the processing circuitry 510 of Figure 5 or the processing circuitry 610 of Figure 6. Those skilled in the art will also appreciate that the processing circuitry 510 and the memory 520 or the processing circuitry 610 and the memory 620 described above may refer to a combination of analog and digital circuits, and / or one or more processors configured with software and / or firmware, e.g. stored in a memory, that when executed by the one or more processors such as the processing circuitry 510 or 610 perform as described above. One or more of these processors, as well as the other digital hardware, may be included in a single application-specific integrated circuit (ASIC), or several processors and various digital hardware may be distributed among several separate components, whether individually packaged or assembled into a system-on-a-chip (SoC).

[0139] The description of the example embodiments provided herein have been presented for purposes of illustration. The description is not intended to be exhaustive or to limit example embodiments to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of various alternatives to the provided embodiments. The examples discussed herein were chosen and described in order to explain the principles and the nature of various example embodiments and its practical application to enable one skilled in the art to utilize the example embodiments in various manners and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products. It should be appreciated that the example embodiments presented herein may be practiced in any combination with each other.

[0140] It should be noted that the word "comprising” does not necessarily exclude the presence of other elements or steps than those listed and the words "a” or "an” preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the example embodiments may be implemented at least in part by means of both hardware and software, and that several "means”, "units” or "devices” may be represented by the same item of hardware.

[0141] It should also be noted that the various example embodiments described herein are described in the general context of method steps or processes, which may be implemented in one aspect by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

[0142] The embodiments herein are not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be construed as limiting.

Claims

CLAIMS1. A method (300) in a transmitter node (110) for enabling interference suppression of wireless communication signals (121 , 122), the transmitter node (110) comprising a number, equal to or larger than two, of transmitters (111 , 112) configured to transmit respective wireless communication signals (121 , 122) to a receiver node (130) over a wireless communication channel, the method comprising: embedding (310) a respective pilot symbol sequence in each of the wireless communication signals (121 , 122), wherein each respective pilot symbol sequence comprises a number of pilot symbolsbnbn 2) arranged to be transmitted within a coherence time of the wireless communication channel, and wherein the number of pilot symbols is equal to the number of transmitters (111 , 112); and transmitting (320), to the receiver node (130), each of the wireless communication signals (121, 122) via respective transmitters of the number of transmitters (111, 112) such that the embedded respective pilot symbol sequences are transmitted within the coherence time.

2. The method (300) according to claim 1, wherein the number of pilot symbols (an l, an 2; bn l, bn 2) in each respective pilot symbol sequence are arranged in a consecutive order.

3. The method (300) according to claim 1 or 2, wherein each respective pilot symbol sequence is configured to be non-orthogonal to each of the other respective pilot symbol sequences.

4. The method (300) according to any previous claim, wherein the respective pilot symbol sequences are configured to fulfill the condition^1=0 o-i^i = C, where atis the i:th pilot of a pilot sequence of a first wireless communication signal (121) of the number of wireless communication signals, bt is the complex conjugate of the i:th pilot of a pilot sequence of one of the number of wireless communication signals (122) different than first wireless communication signal (121),N is the number of pilots,C is a complex value, wherein a ratio of the magnitude of the real part of C and the magnitude of the imaginary part of C is equal to 0.1- 10, preferably 0.9-1.1 , and more preferably 0.99-1.01.

5. The method (300) according to any previous claim, wherein the respective pilot symbol sequences are respective primary pilot symbol sequences, and wherein the coherence time is a first coherence time, the method comprising: embedding (330) a respective secondary pilot symbol sequence in each of the wireless communication signals (121 , 122), wherein each respective secondary pilot symbol sequence comprises a number of pilot symbolsan+i,ian+i,2' bn+i,ibn+i,2> arranged to be transmitted within a second coherence time of the wireless communication channel, and wherein the number of pilot symbols is equal to the number of transmitters (111 , 112); and transmitting (340), to the receiver node (130), each of the wireless communication signals (121, 122) such that the embedded respective primary pilot symbol sequences are transmitted within the first coherence time and the embedded respective secondary pilot symbol sequences are transmitted within the second coherence time.

6. The method (300) according to any previous claim, wherein each respective pilot symbol sequence comprises a respective main pilot symbol and one or more auxiliary pilot symbols, wherein the one or more auxiliary pilot symbols of each respective pilot symbol sequence are equal to, the negative of, and / or the complex conjugate of the corresponding main pilot symbol.

7. The method (300) according to claim 6, wherein each main pilot symbol of each respective pilot symbol sequence is based on a pseudo random binary sequence.

8. The method (300) according to claim 6 or 7, wherein the number of transmitters (111 , 112) is two, and wherein the auxiliary pilot symbols are the negative of and the complex conjugate of, respectively, the corresponding main pilot symbols, or equal to and the complex conjugate of, respectively, the corresponding main pilot symbols.

9. The method (300) according to any previous claim, wherein each pilot symbol of each pilot symbol sequence is an M-QAM symbol, where M is an even power of two, and preferably equal to four.

10. A method (400) in a receiver node (130) for interference suppression of wireless communication signals (121 , 122), the receiver node (130) comprising a number, equal to or larger than two, of receivers (131, 132) configured to receive respective signals (123, 124) from a transmitter node (110) over a wireless communication channel, the method comprising: receiving (410) a respective signal (123, 124) by each receiver of the number of receivers (131 , 132) within a coherence time of the wireless communication channel, wherein each received signal (123, 124) comprises one or more wireless communication signals (121 , 122) transmitted by respective transmitters of a number of transmitters (111 , 112) of a transmitter node (1 10), and wherein each of the one or more wireless communication signals (121 , 122) comprises a respective embedded pilot symbol sequence, and wherein each respective pilot symbol sequence comprises a number of pilot symbols (an l, an 2; bn l, bn 2), and wherein the number of pilot symbols is equal to the number of transmitters (111 , 112); and suppressing (420) an interference of one of the one or more wireless communication signals (121) by means of a phase rotation estimate ((pmain, (pmimo) for each receiver of the number of receivers (131 , 132) based on each received signal (123, 124) and on each embedded pilot symbol sequence.11 . The method (400) according to claim 10, wherein the phase rotation estimate (g , (Pmimo) for each receiver of the number of receivers (131, 132) is indicative of a baseband-to-baseband channel from one of the transmitters of the number of transmitters (111 , 112) to the corresponding receiver of the number of receivers (131 , 132).

12. The method (400) according to claim 10 or 11 , wherein the phase rotation estimate (g , g>mimo) for each receiver of the number of receivers (131, 132) is based on a sequence of samples of each received signal (123, 124), where each sequence of samples comprises a number of samples equal to the number of pilot symbols.

13. The method (400) according to any of claims 10-12, wherein the phase rotation estimate (g)main, (Pmimo) is obtained upon equalizing and synchronizing each received signal (123, 124) by the receiver node (130).

14. The method (400) according to any of claims 10-13, wherein the suppressing (420) comprises applying (421) the respective phase rotation estimates (g)main, no) to each received signal (123, 124) and thereafter combining each received signal (123, 124).

15. The method (400) according to any of claims 10-14, wherein each pilot symbol of each pilot symbol sequence is an M-QAM symbol, where M is an even power of two, and preferably equal to four.

16. A transmitter node (110) for enabling interference suppression of wireless communication signals (121 , 122), the transmitter node (110) comprising a number, equal to or larger than two, of transmitters (111 , 112) configured to transmit respective wireless communication signals (121 , 122) to a receiver node (130) over a wireless communication channel, the transmitter node (110) further comprising a processing circuitry (510) and a memory (520), wherein the processing circuitry (510) is configured to: embed a respective pilot symbol sequence in each of the wireless communication signals (121 , 122), wherein each respective pilot symbol sequence comprises a number of pilot symbols (an l, an 2; bn l, bn 2) arranged to be transmitted within a coherence time of the wireless communication channel, and wherein the number of pilot symbols is equal to the number of transmitters (111 , 112); and transmit, to the receiver node (130), each of the wireless communication signals (121, 122) via respective transmitters of the number of transmitters (111, 112) such that the embedded respective pilot symbol sequences are transmitted within the coherence time.

17. The transmitter node (110) according to claim 16, wherein the number of pilot symbols (an l, an 2, bn l, bn,2) ineach respective pilot symbol sequence are arranged in a consecutive order.

18. The transmitter node (110) according to claim 16 or 17, wherein each respective pilot symbol sequence is configured to be non-orthogonal to each of the other respective pilot symbol sequences.

19. The transmitter node (110) according to any of claims 16-18, wherein the respective pilot symbol sequences are configured to fulfill the condition i o1 aibi = C, whereaLis the i:th pilot of a pilot sequence of a first wireless communication signal (121) of the number of wireless communication signals, bi is the complex conjugate of the i:th pilot of a pilot sequence of one of the number of wireless communication signals (122) different than first wireless communication signal (121),N is the number of pilots,C is a complex value, wherein a ratio of the magnitude of the real part of C and the magnitude of the imaginary part of C is equal to 0.1- 10, preferably 0.9-1.1, and more preferably 0.99-1.01.

20. The transmitter node (110) according to any of claims 16-19, wherein the respective pilot symbol sequences are respective primary pilot symbol sequences, and wherein the coherence time is a first coherence time, wherein the processing circuitry (510) is configured to: embed a respective secondary pilot symbol sequence in each of the wireless communication signals (121, 122), wherein each respective secondary pilot symbol sequence comprises a number of pilot symbols {an+i,ian+i,2>' bn+lilbn+1 2) arranged to be transmitted within a second coherence time of the wireless communication channel, and wherein the number of pilot symbols is equal to the number of transmitters (111, 112); and transmit), to the receiver node (130), each of the wireless communication signals (121, 122) such that the embedded respective primary pilot symbol sequences are transmitted within the first coherence time and the embedded respective secondary pilot symbol sequences are transmitted within the second coherence time.

21. The transmitter node (110) according to any of claims 16-20, wherein each respective pilot symbol sequence comprises a respective main pilot symbol and one or more auxiliary pilot symbols, wherein the one or more auxiliary pilot symbols of each respective pilot symbol sequence are equal to, the negative of, and / or the complex conjugate of the corresponding main pilot symbol.

22. The transmitter node (110) according to claim 21, wherein each main pilot symbol of each respective pilot symbol sequence is based on a pseudo random binary sequence.

23. The transmitter node (110) according to claim 21 or 22, wherein the number of transmitters (111, 112) is two, and wherein the auxiliary pilot symbols are the negative of and the complex conjugate of, respectively, the corresponding main pilot symbols, or equal to and the complex conjugate of, respectively, the corresponding main pilot symbols.

24. The transmitter node (110) according to any of claims 16-23, wherein each pilot symbol of each pilot symbol sequence is an M-QAM symbol, where M is an even power of two, and preferably equal to four.

25. A receiver node (130) for interference suppression of wireless communication signals (121, 122), the receiver node (130) comprising a number, equal to or larger than two, of receivers (131, 132) configured to receive respective signals (123, 124) from a transmitter node (110) over a wireless communication channel, the receivernode (130) further comprising a processing circuitry (610) and a memory (620), wherein the processing circuitry (610) is configured to: receive a respective signal (123, 124) by each receiver of the number of receivers (131, 132) within a coherence time of the channel, wherein each received signal (123, 124) comprises one or more wireless communication signals (121 , 122) transmitted by respective transmitters of a number of transmitters (111 , 112) of a transmitter node (110), and wherein each of the one or more wireless communication signals (121 , 122) comprises a respective embedded pilot symbol sequence, and wherein each respective pilot symbol sequence comprises a number of pilot symbols (an l, an 2; bn l, bn 2), and wherein the number of pilot symbols is equal to the number of transmitters (111 , 112); and suppress an interference of one of the one or more wireless communication signals (121 , 122) by means of a phase rotation estimate ((pmain, (pmimo) for each receiver of the number of receivers (131 , 132) based on each received signal (123, 124) and on each embedded pilot symbol sequence.

26. The receiver node (130) according to claim 25, wherein the phase rotation estimate (g , (pmimo) for each receiver of the number of receivers (131 , 132) is indicative of a baseband-to-baseband channel from one of the transmitters of the number of transmitters (111 , 112) to the corresponding receiver of the number of receivers (131, 132).

27. The receiver node (130) according to claim 25 or 26, wherein the phase rotation estimate (g , (pmimo) for each receiver of the number of receivers (131 , 132) is based on a sequence of samples of each received signal (123, 124), where each sequence of samples comprises a number of samples equal to the number of pilot symbols.

28. The receiver node (130) according to any of claims 25-27, wherein the phase rotation estimate (g , (Pmimo) is obtained upon equalizing and synchronizing each received signal (123, 124) by the receiver node (130).

29. The receiver node (130) according to any of claims 25-28, wherein the processing circuitry (610) is configured to apply the respective phase rotation estimates ((pmain, g>mimo) to each received signal (123, 124) and thereafter combining each received signal (123, 124).

30. The receiver node (130) according to any of claims 25-29, wherein each pilot symbol of each pilot symbol sequence is an M-QAM symbol, where M is an even power of two, and preferably equal to four.

31. A computer program product comprising instructions which, when executed on at least one processing circuitry (510, 610), cause the at least one processing circuitry (510, 610) to carry out the method (300) according to any of claims 1-9 or the method (400) according to any of claims 10-15.

32. A computer program carrier carrying a computer program product according to claim 31, wherein the computer program carrier is one of an electronic signal, optical signal, radio signal, or computer-readable storage medium.

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