WO2018100523A1

WO2018100523A1 - Communication method, apparatus and system for enhancing the spectral efficiency of alamouti coding by combining symbols on a specific antenna with a phase symbol

Info

Publication number: WO2018100523A1
Application number: PCT/IB2017/057524
Authority: WO
Inventors: Hongjun Xu; Narushan PILLAY
Original assignee: University Of Kwazulu-Natal
Priority date: 2016-11-30
Filing date: 2017-11-30
Publication date: 2018-06-07
Also published as: GB2557208A; ZA201904163B; GB201620288D0

Abstract

This invention relates to method and apparatus for transmitting data in a wireless communication system and a communication system. The method generally comprises receiving data and mapping the same to first, second and phase symbols. The method comprises receiving first, second, and phase symbols and using the same to generate, as a set of output symbols, the first symbol, a combination of the second symbol and the phase symbol, a negative complex conjugate of the second symbol, and a combination of the phase symbol and a complex conjugate of the first symbol. The method then entails transmitting two symbols from the output set over the first and second antennas in the first time-slot, and transmitting the other remaining two symbols from the output set of symbols in the second time-slot in a manner to preserve the orthogonality of the transmitted symbols. The apparatus and system typically implement the method.

Description

COMMUNICATION METHOD, APPARATUS AND SYSTEM FOR ENHANCING THE SPECTRAL EFFICIENCY OF ALAMOUTI CODING BY COMBINING SYMBOLS ON A SPECIFIC ANTENNA WITH A PHASE SYMBOL

FIELD OF INVENTION

THIS INVENTION relates to a communication method, an apparatus for communication, and a communication system, particularly a method of and apparatus for transmitting data in a wireless communication system.

BACKGROUND OF INVENTION

In recent times, there is an ever-growing demand for increased data rates and reliability in electrical communication systems. At the same time, the radio frequency spectrum, which is a natural resource, is apparently depleted. This introduces increased pressure on communication methods and systems to make efficient use of the frequency spectrum.

Two methods that show promise are Bell Laboratories layered space-time architecture (BLAST) and massive multiple-input multiple-output (MIMO). However, both methods demand large numbers of transmit and receive antennas, which impose several challenges, e.g. large form-factor, increased hardware or system complexity, complex receiver signal processing algorithms and difficulty in downlink (underdetermined) communication.

Space-time block codes (STBC), for example, the Alamouti space-time block code is a well-known and powerful communication technique which essentially employs two transmit antennas, which simultaneously transmit two message symbols over two consecutive transmission intervals. The transmission matrix maintains an orthogonal structure, which allows for simple linear maximum-likelihood (ML) detection in a quasi- static frequency-flat fading channel. STBC has been shown to achieve full-rate and full- diversity, while not requiring additional system resources. In comparison, to the aforementioned schemas of BLAST and massive MIMO, STBC does not impose such challenges, as mentioned earlier. However, it is desirable to be able to improve the spectral efficiency of STBC and many attempts have been made to do so. For example, some systems make use of two quadrature phase shift keyed (QPSK) constellations in STBC, allowing for an additional bit to be mapped to one of the constellations. Although this scheme maintains the simple decoupled, ML detector of the Alamouti STBC, the improvement is limited, since it is not generalized to more than two constellations.

In one approach, a rate-2 STBC based on field extensions is proposed for QPSK. However, the computational complexity imposed for ML detection is extremely high.

In another approach, a high rate STBC for QPSK was proposed, where the signal set is enlarged by considering a coset of the STBC transmission matrix. An additional bit is then mapped to one of the transmission signal sets. Optimum power scaling is further employed to ensure full-diversity; however, the achievable spectral efficiency is limited.

In yet another approach, high rate embedded Alamouti STBC (EAST) employs even numbers of transmit antennas up to 8; however, for 2 transmit antennas EAST reduces to a conventional Alamouti STBC, hence it is only full-rate. STBC for spatial modulation (STBC-SM) essentially improves the spectral efficiency of STBC by mapping additional message bits to transmit antenna pair combinations. A similar scheme, STBC-SM based on cyclic structure (STBCCSM), improves on the spectral efficiency of STBC-SM. Compared to STBC-SM, a larger set of codewords are employed by cycling through all possible transmit antenna pair combinations. Although only two transmit antennas are active per transmission interval, in the case of STBC-SM and STBC-CSM, more than two transmit antennas are required to facilitate the mapping of additional bits.

In view of the above, it is desirable to enhance the spectral efficiency of Alamouti STBC without substantial additional power or bandwidth being required and whilst preserving the orthogonal nature of the Alamouti STBC transmission matrix so that a simple decoupled ML detector, for quasi-static frequency-flat Rayleigh fading channels may be used.

SUMMARY OF INVENTION

According to a first aspect of the invention, there is provided method of transmitting data in a wireless communication system comprising: receiving a first symbol from a first symbol set, a second symbol from a second symbol set, and a phase symbol from a phase symbol set; transmitting two symbols from an output set of symbols over/via first and second spatially separate antennas in a first time-slot, wherein the output set of symbols comprise the first symbol, a combination of the second symbol and the phase symbol, a negative complex conjugate of the second symbol, and a combination of the phase symbol and a complex conjugate of the first symbol; and transmitting the other remaining two symbols from the output set of symbols over the first and second antennas in a second time-slot, wherein the symbols transmitted in the first and second time-slots maintain an orthogonal structure/relationship.

It will be appreciated that the method may comprise transmitting the symbols in a manner to preserve the orthogonal structure of the symbols both spatially between the first and second antennas and also between the first and second time-slots. In other words, the first and second antennas are spatially diverse and are configured to transmit symbols from the output set of symbols in a diverse fashion.

In addition, it will be understood that the first and second time-slots are temporally consecutive.

The method may comprise the step of generating the output set of symbols from the received symbols for transmission via the first and second antennas in the first and second time-slots, wherein the output set of symbols comprises, as a set of output symbols, the first symbol, the combination of the second symbol and the phase symbol, the negative complex conjugate of the second symbol, and the combination of the phase symbol and the complex conjugate of the first symbol.

The method may comprise transmitting the first symbol and the negative complex conjugate of the second symbol over the first antenna in the first and second time-slots, respectively.

The method may comprise transmitting the combination of the second symbol and the phase symbol and the combination of the phase symbol and the complex conjugate of the first symbol over the second antenna in the first and second time-slots, respectively. However, as alluded to above, the abovementioned transmission of symbols may be varied as long as the orthogonality of the transmitted output set of symbols is preserved. For example, the method may comprise transmitting the negative complex conjugate of the second symbol and the first symbol and over the first antenna in the first and second time-slots, respectively; and transmitting the combination of the phase symbol and the complex conjugate of the first symbol and the combination of the second symbol and the phase symbol over the second antenna in the first and second time-slots.

The first, second, and phase symbol sets may be selected from PSK (Phase Shift Keying) symbol sets. In one example embodiment, the first and second symbol sets may be selected from a MPSK (M-ary Phase Shift Keying) symbol set, whereas the phase symbol set may be selected from a NPSK (N-ary Phase Shift Keying) symbol set, wherein the symbol set from which the phase symbol set is selected is less than the symbol set from which the first and second symbol sets are selected, i.e., N is less than M as will be evident from the description which follows below. It will be understood by those skilled in the field of invention that the symbol sets may be constellations, and the symbols may thus be suitable constellation points of the aforementioned schema rotated by the phase symbol.

The method may comprise: receiving data to be transmitted; and mapping the received data to first, second and phase symbols from the first, second, and phase symbol sets.

In one example embodiment, the method may comprise partitioning the received data into three vectors, wherein the vector to be mapped to the phase symbol is of a different size than the vectors to be mapped to the first and second symbols.

It will be appreciated that the antennas are spatially separate.

According to a second aspect of the invention, there is provided an apparatus for wireless communication, wherein the apparatus comprises: an encoder module configured to: receive a first symbol from a first symbol set, a second symbol from a second symbol set, and a phase symbol from a phase symbol set; generate an output set of symbols from the received symbols for transmission via first and second antennas in first and second time-slots, wherein the output set of symbols comprises, as a set of output symbols, the first symbol, a combination of the second symbol and the phase symbol, a negative complex conjugate of the second symbol, and a combination of the phase symbol and a complex conjugate of the first symbol; generate a first output signal for transmitting over the first antenna in the first and second time-slots, wherein the first output signal comprises two symbols selected from the output set of symbols; and generate a second output signal for transmitting over the second antenna in the first and second time-slots, wherein the second output signal comprises the other remaining two symbols from the output set of symbols wherein the symbols for transmission in the first and second time-slots maintain an orthogonal structure.

According to a third aspect of the invention, there is provided an apparatus for wireless communication, wherein the apparatus comprises: an encoder module configured to: receive a first symbol from a first symbol set, a second symbol from a second symbol set, and a phase symbol from a phase symbol set; generate a first output signal for transmitting over a first antenna in first and second time-slots respectively, wherein the first output signal comprises two symbols for transmission over the first antenna in the first and second time-slots, wherein the two symbols for transmission over the first antenna is selected from an output set of symbols comprising, as the output set of symbols, the first symbol, a combination of the second symbol and the phase symbol, a negative complex conjugate of the second symbol, and a combination of the phase symbol and a complex conjugate of the first symbol; and generate a second output signal for transmitting over a second antenna, spatially separate from the first antenna, in the first and second time-slots respectively, wherein the second output signal comprises the remaining two symbols from the output set of symbols for transmission over the second antenna in the first and second time-slots respectively, wherein the symbols transmitted in the first and second time-slots, by the first and second antennas, respectively maintain an orthogonal structure.

It will be appreciated the remaining two symbols from the output set of symbols, is the two symbols remaining in the output set of symbols after the two symbols are selected for the first output signal. Thus in one example embodiment it will be appreciated that the output set of symbols may be a closed set of four symbols comprising i) the first symbol, ii) the combination of the second symbol and the phase symbol, iii) the negative complex conjugate of the second symbol, and iv) the combination of the phase symbol and the complex conjugate of the first symbol.

The first output signal may comprise the first symbol and the negative complex conjugate of the second symbol for transmission over the first antenna in the first and second time-slots, respectively. The second output signal may comprise the combination of the second symbol and the phase symbol and the combination of the phase symbol and the complex conjugate of the first symbol for transmission over the second antenna in the first and second time-slots, respectively. The apparatus may comprise a data mapping module configured to: receive data to be transmitted; and map the received data to first, second and phase symbols from the first, second, and phase symbol sets.

The encoder module may be configured to transmit the generated first and second signals to the first and second antennas for transmission in first and second time-slots.

The first, second, and phase symbol sets may be selected from PSK (Phase Shift Keying) symbol sets. In one example embodiment, the first and second symbol sets may be selected from an MPSK (M-ary Phase Shift Keying) symbol set, whereas the phase symbol set may be selected from an NPSK (N-ary Phase Shift Keying) symbol set, wherein the symbol set from which the phase symbol set is selected is less than the symbol set from which the first and second symbol sets are selected, i.e., N is less than M.

The data mapping module may be configured to partition the received data into three vectors, wherein the vector to be mapped to the phase symbol is of a different size than the vectors to be mapped to the first and second symbols.

According to a fourth aspect of the invention, there is provided a communication system comprising: an apparatus as described above; a first antenna; and a second antenna spatially separated from the first antenna.

The system may comprise a receiver antenna array comprising one or more second antennas and a suitable receiver apparatus configured to: receive the first and second signals transmitted by the first and second antennas in the first and second time-slots; decode the first and second signal.

The receiver apparatus may comprise an optimal ML (Maximum Likelihood) detector.

According to a fifth aspect of the invention, there is provided a method of transmitting data in a wireless communication system comprising: receiving data to be transmitted; mapping the received data to first, second and phase symbols from first, second, and phase symbol sets, respectively; receiving a first symbol from a first symbol set, a second symbol from a second symbol set, and a phase symbol from a phase symbol set; generating an output set of symbols from the received symbols for transmission via first and second spatially separate antennas in first and second time-slots, wherein the output set of symbols comprises, as a set of output symbols, the first symbol, a combination of the second symbol and the phase symbol, a negative complex conjugate of the second symbol, and a combination of the phase symbol and a complex conjugate of the first symbol; transmitting two symbols from the output set of symbols over the first and second antennas in the first time-slot; and transmitting the other remaining two symbols from the output set of symbols over the first and second antennas in the second time-slot, wherein the symbols transmitted in the first and second time-slots maintain an orthogonal structure. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a block diagram of a system in accordance with an example embodiment of the invention illustrating an apparatus in accordance with an example embodiment of the invention; Figure 2 shows a block flow diagram of a method in accordance with an example embodiment of the invention; Figure 3 shows simulation, and theoretical, results for the invention in accordance with an example embodiment; Figure 4 shows simulation, and theoretical, results for the invention in accordance with an example embodiment illustrating the effect of the factor N on error performance; Figure 5 shows simulation, and theoretical, results for the invention in accordance with an example embodiment for 2x4 MIMO with 8, 16, 32 and 64PSK; Figure 6 shows simulation, and theoretical, results for the invention in accordance with an example embodiment for 2x5 MIMO with 16, 32 and 64PSK; and Figure 7 shows a diagrammatic representation of a machine in the example form of a computer system in which a set of instructions for causing the machine to perform any one or more of the methodologies discussed herein, may be executed.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of an embodiment of the present disclosure. It will be evident, however, to one skilled in the art that the present disclosure may be practiced without these specific details. Referring to Figure 1 of the drawings where a system in accordance with an example embodiment of the invention is generally indicated by reference numeral 10. The system 10 illustrates a high level block diagram of a wireless data communication system which transmits data from a data source 12 to a remote or geographically spaced location in a wireless fashion.

The system 10 may be or may form part of a wireless cellular telecommunication system comprising a plurality of a plurality of radio communication stations or cellular base stations. To this end, the system 10 typically comprises a transmitter apparatus 14 which is typically located at a first communication station 16 and may form part of a transmitter side system of the first communication station 16.

The system 10 may further comprise a receiver apparatus 18 which may form part of a receiver side system of a second communication station 20, which second communication station 20 may be geographically spaced from the first communication station 16. The receiver apparatus 18 may be configured to receive transmitted radio signals from at least the first communication station 16. For ease of explanation and illustration, only the transmitter apparatus 14 of the first communication station 16 is illustrated, and the receiver apparatus 18 of the second communication station 20 is illustrated. However, it will be understood that the station 16 may have a suitable receiver apparatus similar to the receiver apparatus 18 forming part of its receiver side system. Similarly, the station 20 may have a suitable transmitter apparatus 14 forming part of its transmitter side system. Moreover, as alluded to above, though two stations 16, 20 are illustrated, it will be appreciated that a plurality of stations may be provided in the communications network.

In any event, it will be noted that the transmitter apparatus 14 comprises a data mapping module 22, an encoder module 24, and suitable transmit circuitry 26 to facilitate the apparatus 14 being coupled to a pair of antennas, particularly a first transmit antenna T1 and transmit antenna T2. Though not explained in detail, it will be understood that the circuitry 26 may be conventional electronics, drivers, processors, circuitry, etc. configured to control or communicate data carrying signals to the antennas T1 , T2 for on-sending wirelessly as radio frequency signals in a conventional fashion. The antennas T1 , T2 may be conventional antennas which are typically spatially separated as will be further discussed below.

The term "module" in the context of the specification will be understood to include an identifiable portion of code, computational or executable instructions, data, or computational object to achieve a particular function, operation, processing, or procedure. It follows that a module need not be implemented in software; a module may be implemented in software, hardware, or a combination of software and hardware. Further, the modules need not necessarily be consolidated into one device but may be spread across a plurality of devices to achieve the functionality described herein. The data mapping module 22 is typically configured to receive data to be transmitted from 12 data source in a conventional fashion, for example, in a serial stream of binary bits, etc. The data may be any variant of data, for example, voice data, multimedia data, text data, etc. By way of example, given r = log₂ M and s = log₂ N, the module 22 is configured to partition a (2r+ s)-tuple data message into two r-tuple vectors and a third vector, a s-tuple vector m . =

The module 22 is further configured to map the vector m₁ onto a first

symbol from a first symbol set, map vector m₂ onto a second symbol from a second symbol set, and map m₃ onto phase symbol from a third or phase symbol set. The first symbol and the second symbol may be first and second constellation points from an MPSK (M-ary Phase Shift Keying) symbol set or constellation set. The third or phase symbol may be a constellation point from an NPSK symbol set or constellation set. It follows that the module 22 may be configured to map the vectors m₁ and m₂ onto MPSK constellation points, or first and second symbols, x_q1and x_q2, respectively, in the Argand plane, where q₁, q₂ e [1: M]. Similarly, the module 22 may be configured to map the vector m₃ onto an NPSK constellation point, or phase symbol, in the Argand

plane. As mentioned above, the number of constellation points or symbols of the NPSK set may be less than the number of constellation points or symbols of the MPSK set. It will be appreciated that In addition, for

brevity, the terms "constellation point/s" may be used interchangeably with the term "symbol/s" in the specification.

From the above, it will be appreciated that the data mapping module 22 may comprise a suitable modulator to provide symbols, which are essentially data symbols to the module 24. To this end, it will be appreciated that the module 22 is communicatively coupled to module 24, for example, in a wired fashion to provide the symbols x,_qx_{1 q2} and χ_Ρθ thereto.

In a preferred example embodiment, the encoder module 24 is configured to space-time block code (STBC) the received symbols x_q1, x_q2, and χ_Ρθ, to this end, the module 24 may comprise an Alamouti STBC encoder 24 such that the system 10, particularly the apparatus 14 implements an NSTBC-MPSK (N-ary Space-time Block Code-M-ary Phase Shift Keying) scheme. It follows that in one example embodiment, the module 24 is configured to generate an output set of symbols from the received symbols x_qi, x_q2, and χ_Ρθ for transmission via first and second antennas T1 and T2 in first and second consecutive time-slots. The output set of symbols comprises, as a set of output symbols, the first symbol x_q,₁ a combination of the second symbol and the phase symbol Xq₂x_Pθ , a negative complex conjugate of the second symbol -x_q*₂, and a combination of the phase symbol and a complex conjugate of the first symbol x_q*₁x_Pθ - The combination of the phase symbol χ_Ρθ effectively introduces an additional dimension, or bit of information, in the form of a phase rotation.

Differently defined, in a mathematical matrix format, the output set of symbols may be represented as a transmission matrix:

To this end, the module 24 is further configured to generate a first output signal for transmitting over the first antenna T1 in the first and second time-slots, wherein the first output signal comprises the first symbol x„ which is transmitted by T1 during the first time- slot and the negative complex conjugate of the second symbol - x_q2 which is transmitted by T1 during the second time-slot.

The module 24 may be further configured to generate a second output signal for transmitting over the second antenna T2 also in first and second time-slots, wherein the second output signal comprises a combination of the second symbol and the phase symbol which is transmitted by T2 in the first time-slot and a combination of the phase symbol and a complex conjugate of the first symbol xx_{qP1 fj} in the second time-slot.

The output signals generated by the module 24 may be conveyed electronically via the circuitry 26 to the antennas T1 , T2, for on-sending in the form of radio frequency signals as is well understood in the field of invention.

Also well understood in the field of inventive is that, differently defined, the module 24 may be configured to generate signals which in the first time-slot controls T1 to transmit the symbol x_q1, and antenna T2 to transmit and in the second time-slot controls

T1 to transmit - x_q2, and T2 to transmit x_qx_1Pθ . Notwithstanding, it will be appreciated that the phase symbol χ_Ρθ is essentially added to the transmission matrix X(l) for transmission by the second antenna T2 in a manner which preserves the orthogonality and achieves the code matrix property of STBC whilst allowing the mapping of an additional log₂ N bits per two-time-slots. Consequently, the overall spectral efficiency of the apparatus 10 is δ_NSTBC__MPSK = which corresponds to a gain of compared to a

conventional Alamouti STBC.

In any event, referring to the transmission matrix X above (1 ), and assuming that

As mentioned above, the system 10 further comprises a receiver apparatus 18 located at the remote station 20. In the example under discussion, the apparatus 18 comprises suitable receive circuitry 28 which comprises a suitable electronics, circuitry etc. to be above to receive the signals transmitted wirelessly from T1 and T2 by way of antenna T3. In particular, T3 may be configured to receive a signal vector which may be defined as: (⁴)

in the first time-slot, and (5)

in the second time-slot, where is the average signal-to-noise ratio (SNR) at

each receive antennas, h₁ and h₂ represent the N_R x 1 (number of receive antennas - T3) channel gain vectors for transmit antennas T1 , T2, respectively. For the present example, it is assumed that quasi-static frequency-flat fading channels between antennas T1 and T2, and T2 and T3 are provided, such that channel effects are identical over the transmission interval, i.e. two consecutive time-slots. η_η, n 6 [1: 2] is an N_R x 1 additive white Gaussian noise (AWGN) vector. It will be noted that the entries of hi , h2, and η_η are independent and identically distributed (i.i.d) complex Gaussian random variables (RVs) distributed as CN(0,1).

In any event, the receiver apparatus 1 8 may comprise a suitable signal combiner module 30 communicatively/electrically coupled to the circuitry 28 and configured to combine the receive signal vectors (4) and (5). Given channel state information at the receiver apparatus 18 (CSIR), the module 30 may compute the following signals for the two time-slots: where

The apparatus 18 may further comprise an ML detector module 32 electrically coupled to the combiner module 30. The module 32 is based on the maximum-likelihood rule as is well understood in the field of invention so as to determine the symbols transmitted by the apparatus 14 as described above. To this end the optimal detector module 32 is configured to determine:

When considering the theoretical error performance of the system 10, the possibility of two types of errors exists at the receiver apparatus 18, viz. symbol error and phase error. Since the transmitter apparatus 14 is employing MPSK or NPSK, assuming L can take on values of M or N, then both of these errors are based on the average error probability of LPSK. In this regard, firstly, this error probability is derived assuming a frequency-flat Rayleigh fading channel, AWGN and an ML detector at the receiver apparatus 18. For LPSK with AWGN, the probability of symbol error is understood in the field of invention as:

where and γ is the SNR.

Employing a trapezoidal rule as is well understood in the field of invention, (13) may be expressed as: (14)

where g ^and c is a constant chosen for convergence.

Considering conventional maximum ratio combining (MRC), with N_R receive antennas, the probability density function of the combined SNR is given by:

Employing the moment generating function: (16)

where it can be shown that:

(17)

If the result in (17) is extended to the implementation of the system 10, particularly the NSTBC-MPSK scheme implemented by the apparatus 14: a) When χ_Ρθ is correctly detected at the receiver apparatus 18, and x_q1, x_q2 are incorrectly detected, then the error probability is that of a conventional Alamouti STBC with MPSK; b) When x_q1, x_q2 are correctly detected at the receiver apparatus 18 and χ_Ρθ is incorrectly detected, then the error probability is that of NPSK. It is assumed the two events are mutually exclusive, hence a) and b) represent two extremes. In the case of a), since (17) is valid for a single transmit antenna and NR receive antennas, slight modifications may be applied. Two transmit antennas T1 , T2 are employed in the transmission of x_q1, x_q2, hence we set Ύ =

Furthermore, to match the diversity of STBC, we set N_R = 2N_R. Hence, the average symbol error probability (ASEP), in terms of (17), is formulated as:

(18)

In the case of b), antenna T2 transmits x_Peover two time-slots to NR receive antennas T3. This is equivalent to a 1 x N_R MRC system in each time-slot, system in each time-slot. Hence, the ASEP in terms of (17) with L = N, is given as:

(19)

Finally, the overall average bit error probability (ABEP), assuming Gray coding, is given as:

From (17) and (18), it is evident that at high SNR, the diversity-order is N_R and

2N_R, respectively. This may be also validated by computing a codeword distance matrix as is well understood in the field.

Assume χ_Ρθ = e^jθ , the matrix X(l) as described above is transmitted and the matrix:

is detected the receiver apparatus 18 as described above. Then the codeword difference matrix for NSTBC-MPSK scheme as described herein is given as:

The codeword distance matrix is then computed as:

(23)

where

Considering the possibilities a) and b) as defined earlier, in a similar manner, for the case of = θ , (23) reduces to:

(24)

It is clear that (24) is full-rank; hence, the diversity-order is 2N_R.

For the case o the codeword distance matrix reduces to:

where

Clearly the matrix is rank-1 and the achieved diversity-order is N_R. Hence, the diversity-order of the NSTBC-MPSK scheme described herein varies between the extremities of 2N_R and N_R. The factors M and N control the switching between the extremities. For example, given M, it is clear upon inspection of (20) that if N < M is chosen, then (20) is dominated by the probability , it is clear that (20) is dominated by the probability

This is further indicated below with reference to Fig

^aures 3 to 6.

Turning now to Figure 2 of the drawings where a block flow diagram of a method in accordance with an example embodiment of the invention is generally indicated by reference numeral 40. The method 40 is described with reference to the system 10, particularly the apparatus 14 as described above but it will be appreciated by those skilled in the field of invention that the method 40 may be implemented by other systems not illustrated to achieve the NSTBC-MPSK scheme contemplated herein.

The method 40 comprises receiving, at block 42 by way of the encoder module 24, a first symbol x_q1 , a second symbol x_q2, and a third phase symbol χ_Ρθ. It will be appreciated that though not illustrated, the method 40 may comprise a prior step of receiving data to be transmitted from the data source 12, and partitioning the data into three message vectors m_i, m₂, m₃ by way of the module 22, wherein m₃ has a shorter length than the other two message vectors m₁ and m₂.

The method 40 then comprises mapping the aforementioned message vectors to constellation points, or symbols, of the MPSK constellation points and NPSK constellation point as described above, viz., the first symbol or constellation point x_q1, the second symbol or constellation point x_q2, and the third or phase symbol or constellation point χ_Ρθ, the last mentioned being from NPSK constellation set. It follows that the method 40 may thus comprise modulating the data received from the data source onto the MPSK/NPSK constellation points. The method 40 may comprise transmitting these generated symbols to the encoder 24 which is communicatively coupled thereto.

In any event, the method 40 comprises the step of, generating at block 44, an output set of symbols from the received symbols x_q1, x_q2, and χ_Ρθ for transmission via first and second antennas T1 and T2 in first and second consecutive time-slots by way of the module 24 as described above. To this end, it will be noted that the method 40 may comprise generating or determining the negative complex conjugate of the second symbol -x*₂ , the complex conjugate of the first symbol x,_q1 the combination of the phase symbol with the second symbol x_q2x_Pθ , and the combination the phase symbol with the complex conjugate of the first symbol x_q*₁x_Pθ so as to form the output set or matrix X as described by (1 ) above.

The method 40 may then comprise transmitting, at block 46 by way of the antenna T1 and T2, the first symbol x_q1 and the combination of the second symbol and the phase symbol x_q2x_Pθ , respectively in the first time-slot so as to maintain the orthogonality of the transmitted symbols. It will be understood that the phase component, particularly the phase symbol, is transmitted by the second antenna T2.

The method 40 may also then comprise transmitting, at block 48, by way of the antenna T1 and T2, the remaining symbols from the output symbol set or matrix i.e., the negative complex conjugate of the second symbol -x*₂ and the combination of the phase symbol and a complex conjugate of the first symbol x_q*₁x_Pθ, respectively, in the second consecutive time-slot such that orthogonality is also maintained for the second time-slot. Here as well in the second time-slot, the phase component, particularly the phase symbol, is transmitted by the second antenna T2.

It will be appreciated that the symbols of the output set may be transmitted wirelessly as radio frequency signals by the antennas T1 , T2 in a conventional fashion as is well understood in the field of invention. Thus it follows that the method 40 may comprise the step of providing the symbols to the antennas T1 , T2 in the form of suitable signals via the encoder 24 as described above.

Though not described in detail, it will be noted that method 40 may also extend to the receiving of the transmitted symbols in a manner as described by use of the receiver apparatus 18.

Turning now to Figures 3 to 6 of the drawings which illustrate simulation results for the NSTBC-MPSK scheme describe herein. The formulated ABEP for NSTBC-MPSK is evaluated for different SNRs and used to validate the simulation results (SNR versus average bit error rate (BER)). For the simulations, a fully known quasi-static frequency- flat Rayleigh fading channel and the presence of AWGN is assumed. Furthermore, Gray coded MPSK and NPSK constellation points are assumed. The notation STBC-(M,N_R)-5 is employed for a conventional Alamouti coding scheme, whereas the notation NSTBC- MPSK-(N,M,N_R)-δ is employed for the NSTBC-MPSK scheme described herein, where δ represents the spectral efficiency in b/s/Hz.

In Figure 3, simulation results demonstrated for various configurations of the NSTBC-MPSK scheme have been presented. The formulated ABEP of NSTBC-MPSK, given by (20), has also been evaluated. The results have been plotted against simulation results in Figure 3. It is immediately evident that the formulated ABEP agrees with the simulation results. The match is especially tight in the moderate-to-high SNR region.

Next, the effect of the value N on the NSTBC-MPSK scheme was evaluated. Figure 4, presents the results based on (20) for NSTBC-MPSK-(N,16,4)-5, where several values of N are considered. In Figure 4, the factors a = (log₂ M)^-1 and b = (log₂ N)^_1 are assumed. When N = 4, it is clear that (20) is dominated by aP_d, since the respective curves match. Hence, the diversity-order as explained above, is 2N_R, while bP_θ exhibits the lower diversity-order. For N = 8, the same behavior is demonstrated; the ABEP and aP_d have the same diversity-order of 2N_R. In the case of N = 16, it can be seen that (20) is now dominated by bP_θ, which has a diversity-order of N_R. The same behaviour is evident for N = 32. Hence, NSTBC-MPSK is only valid for N < M.

In order to evaluate the efficacy of the NSTBC-MPSK scheme described herein, in Figure 5, simulation results for a 2 X 4 configuration is presented and comparisons are drawn with conventional Alamouti STBC. For 2 X 4, 8PSK, the proposed scheme for 4 b/s/Hz matches STBC-(8,4)-3 very closely. When N = M = 8 for NSTBC-MPSK, a decrease in the diversity-order is evident (as abovementioned) and the error performance is degraded. For 2 X 4, 16PSK, similar behavior is demonstrated; compared to 4 b/s/Hz STBC, NSTBC-MPSK for 5 and 5.5 b/s/Hz match almost identically. Once again, for N = M = 16 degradation is evident. The lower two graphs in Figure 5 depict the results for 2 X 4, 32PSK and 64PSK; for 32PSK, NSTBC-MPSK matches the error performance of conventional Alamouti STBC right up till a spectral efficiency of 7 b/s/Hz. For 64PSK, even at a spectral efficiency of 8.5 b/s/Hz, there is negligible difference between the NSTBC-MPSK scheme described herein and conventional Alamouti STBC.

In Figure 6, simulation results for a 2X5 MIMO configuration with 16, 32 and 64PSK are presented. Similar behaviour as demonstrated earlier, is evident. For 2 X 5, 16PSK, several configurations of NSTBC-MPSK are included. For 5.5 b/s/Hz, the results of conventional Alamouti STBC and the NSTBC-MPSK scheme herein described are identical. For N > M, a decrease in the diversity-order is seen. As explained earlier, the NSTBC-MPSK scheme described herein is only valid for N < M. In the cases of 2 X 5, 32 and 64PSK, for N < M, the error performance results match very well; in the latter result, even for an increase of 2.5 b/s/Hz, error performance is preserved. Hence, NSTBC-MPSK scheme as described herein is capable of enhancing the spectral efficiency of STBC for N < M.

Figure 7 shows a diagrammatic representation of machine in the example of a computer system 100 within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed. In other example embodiments, the machine operates as a standalone device or may be connected (e.g., networked) to other machines. In a networked example embodiment, the machine may operate in the capacity of a server or a client machine in server-client network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated for convenience, the term "machine" shall also be taken to include any collection of machines, including virtual machines, that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein.

In any event, the example computer system 100 includes a processor 102 (e.g., a central processing unit (CPU), a graphics processing unit (GPU) or both), a main memory 104 and a static memory 106, which communicate with each other via a bus 108. The computer system 100 may further include a video display unit 1 10 (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)). The computer system 100 also includes an alphanumeric input device 1 12 (e.g., a keyboard), a user interface (Ul) navigation device 1 14 (e.g., a mouse, or touchpad), a disk drive unit 1 16, a signal generation device 1 18 (e.g., a speaker) and a network interface device 120.

The disk drive unit 16 includes a machine-readable medium 122 storing one or more sets of instructions and data structures (e.g., software 124) embodying or utilised by any one or more of the methodologies or functions described herein. The software 124 may also reside, completely or at least partially, within the main memory 104 and/or within the processor 102 during execution thereof by the computer system 100, the main memory 104 and the processor 102 also constituting machine-readable media.

The software 124 may further be transmitted or received over a network 126 via the network interface device 120 utilising any one of a number of well-known transfer protocols (e.g., HTTP).

Although the machine-readable medium 122 is shown in an example embodiment to be a single medium, the term "machine-readable medium" may refer to a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term "machine- readable medium" may also be taken to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present invention, or that is capable of storing, encoding or carrying data structures utilised by or associated with such a set of instructions. The term "machine-readable medium" may accordingly be taken to include, but not be limited to, solid-state memories, optical and magnetic media, and carrier wave signals.

The invention as described herein provides a different approach to conventional Alamouti STBC so as to increase the achievable spectral efficiency thereof. Specifically, by mapping additional message bits to a phase dimension introduced at the second transmit antenna in the manner described herein, increased spectral efficiency is achieved whilst the error performance of STBC is preserved. The system and methodology described herein maintains the orthogonal structure of STBC; hence, simple linear ML detectors may be used at the receiver side without an expensive variation of traditional equipment used in the communications system.

Claims

CLAIMS 1 . A method of transmitting data in a wireless communication system comprising: receiving a first symbol from a first symbol set, a second symbol from a second symbol set, and a phase symbol from a phase symbol set; transmitting two symbols from an output set of symbols over first and second spatially separate antennas in a first time-slot, wherein the output set of symbols comprises the first symbol, a combination of the second symbol and the phase symbol, a negative complex conjugate of the second symbol, and a combination of the phase symbol and a complex conjugate of the first symbol; and transmitting the other remaining two symbols from the output set of symbols over the first and second antennas in a second time-slot, wherein the symbols transmitted in the first and second time-slots maintain an orthogonal structure.

2. A method as claimed in claim 1 , wherein the method comprises generating the output set of symbols from the received symbols for transmission via the first and second antennas in the first and second time-slots, wherein the output set of symbols comprises, as a set of output symbols, the first symbol, the combination of the second symbol and the phase symbol, the negative complex conjugate of the second symbol, and the combination of the phase symbol and the complex conjugate of the first symbol.

3. A method as claimed in either claim 1 or claim 2, wherein the method comprises transmitting the first symbol and the negative complex conjugate of the second symbol over the first antenna in the first and second time-slots, respectively.

4. A method as claimed in any one of the preceding claims, wherein the method comprises transmitting the combination of the second symbol and the phase symbol and the combination of the phase symbol and the complex conjugate of the first symbol over the second antenna in the first and second time-slots, respectively.

5. A method as claimed in any one of the preceding claims, wherein the first, second, and phase symbol sets are selected from PSK (Phase Shift Keying) symbol sets.

6. A method as claimed in any one of the preceding claim, wherein the first and second symbol sets are selected from an MPSK (M-ary Phase Shift Keying) symbol set, the phase symbol set is selected from an NPSK (N-ary Phase Shift Keying) symbol set, and wherein the symbol set from which the phase symbol set is selected is smaller than the symbol set from which the first and second symbol sets are selected.

7. A method as claimed in any one of the preceding claims, wherein the method comprises: receiving data to be transmitted; and mapping the received data to first, second and phase symbols from the first, second, and phase symbol sets.

8. A method as claimed in claim 7, wherein the method comprises partitioning the received data into three vectors, wherein the vector to be mapped to the phase symbol is of a different size than the vectors to be mapped to the first and second symbols.

9. An apparatus for wireless communication, wherein the apparatus comprises: an encoder module configured to: receive a first symbol from a first symbol set, a second symbol from a second symbol set, and a phase symbol from a phase symbol set; generate a first output signal for transmission over a first antenna wherein the first output signal comprises two symbols selected from an output set of symbols generated from the received first, second and phase symbols, for transmission in first and second time-slots and comprising, as the output set of symbols, the first symbol, a combination of the second symbol and the phase symbol, a negative complex conjugate of the second symbol, and a combination of the phase symbol and a complex conjugate of the first symbol; and generate a second output signal for transmission over a second antenna spatially separated from the first antenna, wherein the second output signal comprises the remaining two symbols from the output set of symbols for transmission in the first and second time-slots respectively, wherein the symbols transmitted in the first and second time-slots, by the first and second antennas, respectively maintain an orthogonal structure.

10. An apparatus as claimed in claim 9, wherein the encoder module is configured to generate the output set of symbols from the received first, second, and phase symbols.

1 1 . An apparatus as claimed in either claim 9 or 10, wherein the first output signal comprises the first symbol and the negative complex conjugate of the second symbol for transmission over the first antenna in the first and second time-slots, respectively.

12. An apparatus as claimed in any one of claims 9 to 1 1 , wherein the second output signal comprises the combination of the second symbol and the phase symbol and the combination of the phase symbol and the complex conjugate of the first symbol for transmission over the second antenna in the first and second time-slots, respectively.

13. An apparatus as claimed in any one of claims 9 to 1 2, wherein the apparatus comprises a data mapping module configured to: receive data to be transmitted; and map the received data to first, second and phase symbols from the first, second, and phase symbol sets.

14. An apparatus as claimed in any one of claims 9 to 13, wherein the first, second, and phase symbol sets are selected from PSK (Phase Shift Keying) symbol sets.

15. An apparatus as claimed in any one of claims 9 to 14, wherein the first and second symbol sets are selected from an MPSK (M-ary Phase Shift Keying) symbol set, whereas the phase symbol set is selected from an NPSK (N-ary Phase Shift Keying) symbol set, wherein the symbol set from which the phase symbol set is selected is smaller than the symbol set from which the first and second symbol sets are selected.

16. An apparatus as claimed in claim 13, wherein the data mapping module is configured to partition the received data into three vectors, wherein the vector to be mapped to the phase symbol is of a different size than the vectors to be mapped to the first and second symbols.

17. A communication system comprising: an apparatus as claimed in any one of claims 9 to 16; a first antenna; and a second antenna spatially separated from the first antenna.

18. A communication system as claimed in claim 17, wherein the system comprises a receiver antenna array comprising one or more second antennas and a suitable receiver apparatus configured to: receive the first and second signals transmitted by the first and second antennas in the first and second time-slots; decode the first and second signal.

19. A communication system as claimed in claim 18, wherein the receiver apparatus comprises an optimal ML (Maximum Likelihood) detector.