Reconfigurable Intelligent Surface-Assisted Generalized Spatial Modulation Method with Phase Offset

Abstract

The present invention belongs to the field of information and communication technologies, and particularly relates to a reconfigurable intelligent surface-assisted generalized spatial modulation method with phase offset. The objective of the present invention is to propose a novel single radio-frequency chain MIMO scheme that solves the problem of the traditional system not fully utilizing the randomness of the phase dimension and improves the spectral efficiency. The technical solution of the present invention is to design a RIS-assisted generalized spatial modulation system to improve the spectral efficiency and the flexibility of transmit antennas combinations (TAC); meanwhile, according to the presence or absence of the direct path from the transmitter to the receiver, the present invention designs two phase offset schemes, thereby enhancing the beamforming ability of the RIS and the detection ability of the receiver for different TACs and beam patterns to improve the bit error rate performance. At the same time, in both cases, the simulation results show that the RIS-GSM-P scheme given by the present invention has a significant improvement in error performance compared with the existing traditional schemes.

Description

Technical Field

[0001] The present invention belongs to the field of information and communication technologies, and particularly relates to a reconfigurable intelligent surface-assisted generalized spatial modulation method with phase offset. Background Art

[0002] Spatial Modulation (SM) has the advantages of low complexity, low hardware cost, and low power consumption due to its single Radio Frequency (RF) structure, and has become an alternative solution for traditional Multiple-Input Multiple-Output (MIMO) wireless communication systems. SM maps information bits onto two dimensions, namely Amplitude-Phase-Modulation (APM) symbols and antenna indices, achieving a good balance between spectral efficiency and energy consumption. Mobile communication systems based on SM transmit information through the channel differences between antennas. However, traditional SM systems are limited because they cannot actively adapt to the wireless propagation environment. In recent years, Reconfigurable Intelligent Surface (RIS) has become a technology with great development prospects because it can adjust the wireless propagation environment by configurable reflection elements.

[0003] Considering that RIS can compensate for the inherent defects of SM, many researchers have begun to explore the combination of RIS and SM technologies. For example, "Q. Li, M. Wen and M. Di Renzo, 'Single-RF MIMO: From spatial modulation to metasurface-based modulation,' IEEE Wirel. Commun., vol. 28, no. 4, pp. 88-95, Aug. 2021" first adopted SM in an RIS-assisted communication system; meanwhile, the RIS-assisted Spatial Modulation (RIS-SM) mentioned in "T. Ma et al, 'Large intelligent surface assisted wireless communications with spatial modulation and antenna selection,' IEEE J. Select. Areas Commun., vol. 38, no. 11, pp. 2562-2574, Nov. 2020" combines RIS with transceiver spatial modulation, enabling the transceiver antenna indices to carry information simultaneously to further improve the spectral efficiency. However, its scheme still has certain defects. Under the assumption of no direct path from the transmitter to the receiver, RIS-SM only activates one transmit antenna. Since RIS cancels the channel phase between the selected transmit antenna and the selected receive antenna, the received signal power of the alternative receive antenna is enhanced, strengthening the detection ability of the alternative receive antenna index. However, since the channel between the selected antennas only has amplitude variations and loses the randomness of the phase dimension, the detection performance of the transmit antenna index deteriorates when there is no direct path; in addition, the RIS-VBLAST mode in "A. Khaleel and E. Basar, 'Reconfigurable intelligent surface-empowered MIMO systems,' IEEE Syst. J., vol. 15, no. 3, pp. 4358-4366, Sept. 2021" introduces VBLAST into the RIS-assisted SM system while utilizing the transceiver antenna indices, but it also has defects. The transceiver antenna index information selected by RIS does not propagate on the direct path, so not all diversity gains can be utilized. This is the development potential and challenges faced by the combination of RIS and SM technologies. Summary of the Invention

[0004] The objective of the present invention is to propose a novel single-RF-chain MIMO solution that addresses the problem of insufficient utilization of the randomness of the phase dimension in the RIS-SM system and improves the spectral efficiency, called Reconfigurable Intelligent Surface-Assisted Generalized Spatial Modulation with Phase Offset (RIS-GSM-P). The technical solution of the present invention proposes a RIS-assisted generalized spatial modulation (GSM) system, thereby improving the spectral efficiency and the flexibility of the Transmit Antennas Combinations (TAC); meanwhile, according to the presence or absence of the direct path from the transmitter to the receiver, two phase offset schemes are designed in the RIS-GSM-P system. Without losing the advantages of the traditional scheme, the two phase offset schemes enhance the beamforming ability of the RIS and the detection ability of the receiver for various TACs and beam patterns, and improve the bit error performance.

[0005] Consider the RIS-assisted MIMO system as Figure 1 、 Figure 2 shown. Communication between the transmitter and the receiver is assisted by the RIS, Figure 1 、 Figure 2 representing the two cases of the presence or absence of a direct path between the transmitter and the receiver, respectively. Specifically, the system consists of N T transmit antennas, N R receive antennas, and N S RIS reflection elements, and all transmit antennas are connected to a single RF link through a switch array. The TAC selector activates N T transmit antennas out of N A to form the combined pattern of the transmit antennas and send signals. In addition, it is assumed that the transmitter controls the RIS through a feedback link. The input bits of the entire system are divided into three parts: log2(M) bits are modulated into AMP symbols; bits are mapped to the index of the TAC; bits are used by the RIS for the index of the selected active transmit and receive antenna pairs, and the spectral efficiency of the entire system is

[0006]

[0007] The proposed RIS-GSM-P scheme will be elaborated below according to the two cases of the presence or absence of a direct path from the transmitter to the receiver.

[0008] (1) RIS-GSM-P scheme when there is no direct path from the transmitter to the receiver

[0009] As shown Figure 1 in the figure, the channels from the transmitter to the RIS and from the RIS to the receiver are represented by respectively. Therefore, the received signal y can be expressed as

[0010]

[0011] where x is the APM symbol vector, and x (k,l) = ρ (k,l) x is the transmission vector of length N T and has N A identical non-zero elements with value x (k,l) . ρ (k,l) is the phase offset with modulus 1, determined by the k-th TAC selected by the transmitter and the l-th activated antenna selected by the RIS, where l represents the index of the l-th activated antenna. is the noise vector, following the complex Gaussian distribution is a diagonal matrix, and its diagonal elements represent the reflection coefficients of the RIS units. is the normalization factor of the RIS incident power, where and h nm = [H] nm .

[0012] Assume Figure 1 the l-th activated antenna in n corresponds to the i-th transmitting antenna. To maximize the signal power from the i-th transmitting antenna to the j-th receiving antenna, each RIS unit cancels the channel phase between the antenna pairs. Specifically, θ S can be written as

[0013]

[0014] where and Meanwhile, the phase offset ρ (k,l) is defined as

[0015]

[0016] where phase min is the minimum phase difference between the transmitted APM symbols. Specifically, by equally dividing the minimum phase difference between the APM symbols, this design assigns a unique phase offset to each combination of the TAC and the selected activated antenna.

[0017] (2) The RIS-GSM-P scheme when there is a direct path from the transmitter to the receiver

[0018] As shownFigure 2 As shown, the link channel from the transmitter to the receiver is represented by . Therefore, the received signal y can be expressed as

[0019]

[0020] where s (j,l,i) is a transmission vector of length N T , and there are N A identical non-zero elements with value s (j,l,i) . The diagonal elements of the diagonal matrix represent the reflection coefficients of the RIS elements in this case. Different from the case without a direct path, the implementation of the phase shift in this scheme requires the joint phase rotation of the RIS and the transmitter. Specifically, s (j ,l ,i) should be expressed as

[0021]

[0022] where ω ji is the phase of the channel matrix element . Furthermore, n = 1,..., N S can be expressed as

[0023]

[0024] Thus, formula (5) can be rewritten as

[0025]

[0026] If the noise is ignored, the signal from the i-th transmitting antenna to the j-th receiving antenna can be expressed as

[0027]

[0028] That is, the joint phase shift makes the reflected signal and the signal from the direct path in the same phase.

[0029] On the one hand, the joint phase shift enhances the beamforming of the RIS for the signal in the direct path. The joint phase shift makes the direct and reflected signals from the i-th transmitting antenna to the j-th receiving antenna remain in the same phase, thus enhancing the beam power between the selected antennas. On the other hand, the introduction of the joint phase shift also makes it possible for the information transmitted by the active antenna index selected by the RIS to propagate on the direct path. For the RIS-SM system, although the introduction of the direct path helps to distinguish the transmitting antenna indices, it is not helpful for the detection of the selected receiving antenna indices. However, the RIS-GSM-P scheme with the introduced joint phase shift overcomes this shortcoming, thereby improving the bit error rate performance.

[0030] The beneficial effects of the present invention are as follows. For the problem that the RIS-SM system does not fully utilize the randomness of the phase dimension, the present invention provides a novel single-RF-chain MIMO scheme, namely the RIS-GSM-P scheme, to improve the bit error rate performance and obtain higher spectral efficiency. In the scenario of RIS-assisted communication, according to the two cases of whether there is a direct path or not, the phase shift method is designed respectively to improve the system bit error rate performance. At the same time, for the two cases, the simulation results show that the RIS-GSM-P scheme given by the present invention has a greater performance improvement in bit error rate performance compared with the existing traditional schemes such as RIS-VBLAST and RIS-SM. Description of the Drawings

[0031] Figure 1 It is a schematic diagram of the system model of the RIS-GSM-P scheme when the transmit→receive link is blocked.

[0032] Figure 2 It is a schematic diagram of the system model of the RIS-GSM-P scheme when the transmit→receive link is connected.

[0033] Figure 3 It is the bit error rate performance simulation diagram of the RIS-GSM-P scheme under different N A conditions, where (a) is without a direct path and (b) is with a direct path.

[0034] Figure 4 It is the comparative simulation diagram of the RIS-GSM-P scheme and the RIS-SM, RIS-VBLAST and SDR schemes under the same spectral efficiency, where (a) is without a direct path and (b) is with a direct path.

[0035] Figure 5 It is the comparative simulation diagram of the bit error rate performance of the RIS-GSM-P scheme and the RIS-SM and RIS-VBLAST schemes, where (a) is without a direct path and (b) is with a direct path. Detailed Implementation Manner

[0036] The steps and performance of the present invention will be described in detail below with reference to the drawings, so that those skilled in the art can better understand the present invention.

[0037] Figure 1 、 Figure 2 It is a general system schematic diagram applied in the present invention, and the corresponding scenarios are the cases where there is no direct path from the transmitter to the receiver and there is a direct path from the transmitter to the receiver. The purpose of this system is to further improve the bit error performance and spectral efficiency by designing the RIS-assisted GSM system and the phase shift on the RIS and the transmitting antenna. Under this channel model, the specific implementation steps of the present invention are as follows:

[0038] a) Input the channel matrices $\mathbf{H}$, $\mathbf{G}$, and $\mathbf{F}$ of the transmitter $\to$ RIS link, RIS $\to$ receiver link, and transmitter $\to$ receiver link respectively. Among them, the channel matrix $\mathbf{F}$ of the transmitter $\to$ receiver link only exists in the Figure 2 shown case;

[0039] b) Control the reflection coefficient matrices $\boldsymbol{\Theta}$ and $\boldsymbol{\Theta}$ of the RIS unit through the feedback link of the transmitter to control the RIS, un whose diagonal elements $\theta$ n and $n = 1,\ldots,N$ S are given by equations (3) and (7) respectively;

[0040] c) In the Figure 1 and Figure 2 shown cases, according to the $k$-th TAC selected by the transmitter and the $l$-th activated antenna selected by the RIS, add the phase offset $\rho$ (k,l) to the transmitted signal vector respectively.

[0041] (1) As shown in Figure 1 , when the transmit $\to$ receive link is blocked, the phase offset is calculated by equation (4), and the transmitted signal vector after addition is $\mathbf{x}$ (k,l) $=\rho$ (k,l) $\mathbf{x}$.

[0042] (2) As shown in Figure 2 , when the transmit $\to$ receive link is connected, the phase offset requires the joint phase rotation of the RIS and the transmitter, and the transmitted signal vector after addition is given by equation (6).

[0043] d) Use the ML detector to detect the received signal and output the detection result

[0044] Figure 3 gives the corresponding system bit error rate performance for different $N$ A . Specifically, Figure 3 at $N$ T $ = 4$, $N$ S $ = 64$, $N$ R $ = 2$, and $M = 4$, the bit error rate performances at $N$ A $ = 1,2,3,4$ are compared. As shown in Figure 3 , the bit error rate performances are different when the transmitter selects different numbers of activated antennas. Therefore, for the RIS-GSM-P scheme, the number of activated transmit antennas can be flexibly selected to carry more information in the TAC or achieve better bit error rate performance.

[0045] Figure 4 Compares the RIS-GSM-P, RIS-SM, RIS-VBLAST, and SDR schemes at the same spectral efficiency, $N$T ×N S ×N R The bit error rate performance under the condition of = 5×64×4. To ensure the fairness of comparison, the RIS-VBLAST scheme transmits the same symbols on all transmit antennas to meet the single RF chain constraint of RIS-GSM-P and RIS-SM. When BER = 10 -4 , the performance advantage of RIS-GSM-P over RIS-SM shrinks from more than 6 dB without a direct line-of-sight path to 4 dB with a direct line-of-sight path. This is because in the RIS-SM system without a direct line-of-sight path, the receiving end has poor detection performance for different transmit antenna indices. When BER = 10 -4 , RIS-GSM-P has a performance advantage of about 4 dB over RIS-VBLAST without a direct line-of-sight path. While with a direct line-of-sight path, the advantage expands to 6 dB. This is because when there is a direct line-of-sight path, RIS-GSM-P enhances the beamforming of RIS through joint phase shift, but RIS-VBLAST cannot achieve this effect. When BER = 10 -3 , the RIS-GSM-P scheme has gains of 6 dB and 8 dB respectively compared with the RIS-assisted communication scheme optimized based on SDR in the scenarios with and without a direct line-of-sight path. Because the latter only uses RIS-assisted communication and cannot carry more information bits through RIS in the single RF chain scenario, the RIS-GSM-P scheme has an advantage in spectral efficiency compared with the latter.

[0046] Figure 5 Compared the bit error rate performance of RIS-GSM-P, RIS-SM, and RIS-VBLAST schemes in N T ×N S ×N R = 4×64×4. As Figure 5 shown, in the case of no direct line-of-sight path, due to the introduction of the phase shift amount, RIS-GSM-P has a gain of more than 7 dB and 1 dB respectively compared with RIS-SM and RIS-VBLAST when N A = 4, BER = 10 -5 , and gains of more than 13 dB and 5 dB respectively when N A = 1, BER = 10 -5 ; as Figure 5 shown, in the case of having a direct line-of-sight path, the performance of RIS-SM has a large improvement, but the RIS-GSM-P scheme is still better than RIS-SM when N A = 4. Specifically, there is a gain of about 7 dB when N A = 1, BER = 10 -5 .

Claims

1. Reconfigurable intelligent surface-assisted generalized spatial modulation method with phase offset, where the RIS-assisted MIMO system is defined as consisting of N T transmit antennas, N R receive antennas, and N S RIS reflection elements. All transmit antennas are connected to a single RF link through a switch array. The transmit antenna combination selector activates N T transmit antennas out of N A transmit antennas to form a combination pattern of transmit antennas and send signals. The transmitter controls the RIS through a feedback link, characterized in that Including: When there is no direct path from the transmitter to the receiver, the signals from the transmitter to the RIS and the channel from the RIS to the receiver are defined as and Through the feedback link that controls the RIS from the transmitter, the reflection coefficient matrix Θ of the RIS unit is controlled, Define that the l-th active antenna corresponds to the i-th transmitting antenna. To maximize the signal power from the i-th transmitting antenna to the j-th receiving antenna, each RIS unit cancels the channel phase between the antenna pairs, so let: Among them, and α ni and represent the modulus value and phase corresponding to the channel coefficient of the i-th column in the n-th row of the channel matrix H, β jn and ψ jn represent the modulus value and phase corresponding to the channel coefficient of the n-th column in the j-th row of the channel matrix G. At the same time, the phase offset ρ (k,l) is defined as: Among them, is the number of bits used to map the transmit antenna combination, is the number of bits corresponding to the transmit antenna index selected and activated by RIS, phase min is the minimum phase difference between transmitted APM symbols; According to the k-th transmit antenna combination selected by the transmitter and the l-th activated antenna selected by the RIS, phase offsets ρ are respectively added to the transmit signal vector (k,l) , and the resulting transmit signal vector is x (k,l) = ρ (k,l) x. After passing through the channel, the received signal y is: where \(x\) is the APM symbol vector, \(x\) (k,l) =\(\rho\) (k,l) is a transmission vector of length \(N\) T and has \(N\) A identical non-zero elements with value \(x\) (k,l) ; \(\mathbf{n}\) is the noise vector, which follows a complex Gaussian distribution, and \(\rho\) is the normalization factor of the RIS incident power; Finally, an ML detector is used to detect the received signal and output the detection result When there is a direct path from the transmitter to the receiver, the link channel from the transmitter to the receiver is defined as Control the reflection coefficient matrix Θ of the RIS unit through the feedback link of the transmitter to control the RIS un , Let θ n , n = 1, ..., N S be: Among them, and α ni and represent the modulus value and phase corresponding to the channel coefficient of the \(i\)-th column and \(n\)-th row of the channel matrix \(H\), and \(\beta\) jn and \(\psi\) jn represent the modulus value and phase corresponding to the channel coefficient of the \(n\)-th column and \(j\)-th row of the channel matrix \(G\); according to the \(k\)-th transmit antenna combination selected by the transmitter and the \(l\)-th activated antenna selected by the RIS, phase offsets \(\rho\) (k,l) are respectively added to the transmit signal vector to obtain a transmit vector \(\mathbf{s}\) T of length \(N\) (j,l,i) : where ω ji is the phase of the channel matrix element ; The received signal y is obtained as: Ignoring the noise, the signal from the i-th transmitting antenna to the j-th receiving antenna is expressed as: Thus, the combined phase offset makes the reflected signal and the signal from the direct path be in the same phase. Finally, the ML detector is used to detect the received signal and output the detection result

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