MIMO backscatter communication system based on cyclic delay diversity technology

Abstract

The invention belongs to the technical field of wireless communication, and particularly relates to an MIMO backscatter communication system based on a cyclic delay diversity technology. The system comprises a radio frequency source with a # imgabs 0 # antenna, a receiver with a # imgabs 1 # antenna and a reflection device with a # imgabs 2 # antenna, the receiver receives a direct link signal from the radio frequency source and a reflection link signal from the reflection device at the same time, the direct link signal is set to be known, and for the reflection link signal, the direct link signal is set to be known. A radio frequency source is defined to send sine carrier signals to reflection equipment through a beam forming vector # imgabs3 #, and the reflection equipment adopts a single-carrier cyclic prefix transmission scheme and applies a cyclic delay diversity technology to carry out signal reflection.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communication, and in particular relates to a MIMO backscattering communication system based on cyclic delay diversity technology. Background Art

[0002] Passive IoT is a technology designed to leverage external environmental energy (including radio, light, heat, vibration, and more) to achieve low-power, low-cost connectivity. Nodes in a passive IoT do not rely on their own power supplies, but instead draw energy from the environment to power their computing, sensing, communication, and networking functions. This allows for signal transmission over a range of several meters with power consumption as low as 1 milliwatt, saving significant energy. In recent years, passive IoT technology, with its advantages of low cost, zero power consumption, and ease of deployment, has garnered significant attention from industry, academia, research, and application sectors, and has been widely adopted in industries such as warehousing and logistics, smart transportation, smart healthcare, and smart homes.

[0003] Backscatter communication is a key technology in the passive Internet of Things (IoT). It is an efficient transmission technology that effectively reduces power consumption and costs, and is expected to address the challenges of high manufacturing costs and power consumption of terminal hardware circuits brought about by the large-scale deployment of future IoT devices. A backscatter communication system typically consists of a dedicated radio frequency (RF) source, a backscatter device (BD), and a receiver. Backscatter communication enables wireless terminal nodes to passively transmit data. The RF source transmits a single-frequency sinusoidal carrier signal. The BD, activated by this sinusoidal signal, modulates the incident RF carrier signal by changing its antenna impedance to achieve data transmission, without generating a carrier signal itself. The BD then reflects the information to the receiver. The receiver receives the reflected signal from the BD and performs receive equalization to demodulate the information from the BD.

[0004] Another key technology is Multiple-Input Multiple-Output (MIMO). In MIMO, communication equipment is often equipped with multiple antennas, allowing signals to be transmitted simultaneously and received simultaneously at the receiving end, thereby improving communication quality and enhancing system performance. MIMO fully utilizes spatial resources, exponentially increasing system channel capacity and extending wireless system coverage without increasing spectrum resources or antenna transmit power. Furthermore, it can transmit multiple different or identical signals, further helping the system achieve spatial multiplexing and spatial diversity gains, thereby overcoming channel fading, reducing the bit error rate (BER), and increasing transmission rates.

[0005] Cyclic delay diversity (CDD) is a technique that effectively achieves transmit diversity in multi-antenna deployments. At a multi-antenna transmitter, each antenna simultaneously transmits a cyclically delayed replica of the same signal. The receiver can exploit equalization techniques to achieve multipath diversity gain and utilize enhanced frequency selectivity to further achieve transmit diversity without changing the receiver's structure. Summary of the Invention

[0006] Based on the existing technology, the present invention proposes a MIMO backscatter communication system based on cyclic delay diversity technology.

[0007] The technical solution adopted in the present invention is:

[0008] A MIMO backscatter communication system based on cyclic delay diversity technology, comprising RF source with antennas Receiver with antennas The receiver receives the direct link signal from the RF source and the reflected link signal from the reflecting device at the same time. The direct link signal is assumed to be known. For the reflected link signal, the RF source is defined as passing through the beamforming vector The sinusoidal carrier signal is sent to the reflection device. The reflection device adopts the single carrier cyclic prefix (SCCP) transmission scheme and applies the cyclic delay diversity technology to reflect the signal. Specifically, the symbols sent from the reflection device are divided into symbol blocks, each containing symbol, define the The symbol block is , all antennas of the reflector device simultaneously transmit symbol blocks with different cyclic delays , define The cyclic shift of the root antenna is expressed as , ,in is the cyclic delay between adjacent antennas, then The first antenna transmission The symbol block is ,in , is the loop delay matrix, It is a dimension A diagonal matrix with 1 as the diagonal element, It is a dimension A column vector of all zeros, After adding a cyclic prefix (CP), the signal is converted from parallel to serial and reflected to the receiver. The receiver converts the received symbols from each antenna into serial-to-parallel, removes the CP, and performs discrete Fourier transform (DFT). It then uses the maximum ratio combining (MRC) and minimum mean square error (MMSE) criteria to combine the received symbols from the antenna. The received signals from the three antennas are combined and processed, and then an inverse discrete Fourier transform (IDFT) is performed. The processed signals are converted from parallel to serial to realize the detection of the received signal.

[0009] Furthermore, the receiver is defined to convert the received symbols of each antenna into symbol blocks, and then remove the CP from each symbol block, defining the The first receiving antenna receives The signal blocks are:

[0010] ,

[0011] in, Indicates the distance from the RF source to the The direct link channel response of the root receive antenna, Representation Dimension A column vector of all ones, is a circulant matrix, , Indicates that from Root reflector device antenna to the The backward link channel response of the root receiver antenna is is the number of channel taps, For RF source to The channel response of the root reflecting device antenna, is zero mean and has a variance of complex Gaussian noise, where express The variance of a single element in ; is a known received signal related to the direct link, so it is removed from the received signal to obtain the new received signal:

[0012] ,

[0013] The circulatory matrix Decompose into ,in yes Point DFT matrix, It is a diagonal matrix whose diagonal elements are the channel frequency responses, which satisfies , ; According to the time domain cyclic shift is equivalent to the frequency domain phase shift, define ,in The diagonal elements of , for The frequency response of Transformed into the frequency domain:

[0014] ,

[0015] in, is a diagonal matrix, , is the received Gaussian white noise; definition ,get:

[0016] ,

[0017] in, , It corresponds to The equivalent channel vector of subcarriers is, express The product of Diagonal elements, further simplified by matrix multiplication to obtain ,in , thus obtaining:

[0018] ,

[0019] in, ;make No. List for:

[0020] ,

[0021] in, is the frequency domain signal to be estimated, for No. The receiver uses MRC and MMSE criteria to perform frequency domain equalization to estimate , MRC and MMSE estimators are:

[0022] ,

[0023] ,

[0024] in, is the MRC estimator, represents the forward reflection link from the RF source to the reflecting device, is the MMSE estimator, express The energy of Then we get :

[0025] ,

[0026] in ,right Perform IDFT to obtain the estimated signal in the time domain.

[0027] Furthermore, with the goal of minimizing the bit error rate, the beamforming vector The design method is that, for the signal detection scheme of the MRC criterion, minimizing the bit error rate is equivalent to minimizing , thus establishing the optimization problem as:

[0028] ,

[0029] in, represents the maximum transmit power; the solution to the optimization problem is:

[0030] Introducing new variables , transforming the original optimization problem into:

[0031] ,

[0032] Since the above problem is still a non-convex problem and a multi-ratio fractional programming (FP) problem, it can be transformed by using a unified quadratic transformation. In addition, the semidefinite relaxation (SDR) technique is used to remove This non-convex condition constraint. The optimization problem can be transformed into:

[0033] ,

[0034] in is an auxiliary variable. The optimization problem can be solved by iterative optimization. and Specifically, when When known, The optimal solution is ;when Given , this optimization problem is about is convex and can be solved using existing tools such as CVX. Then, the beamforming vector is randomized using Gaussian randomization. from Due to its high computational complexity, the present invention proposes a low-complexity algorithm:

[0035] definition And its first Rank The elements of the column are , for the diagonal elements, Sometimes, there are According to the law of large numbers, when When it satisfies:

[0036] ,

[0037] in satisfy The definition of , for non-diagonal elements is:

[0038] ,

[0039] because and exist Independent of each other, satisfying ,therefore ; From this we can see that when hour:

[0040] ,

[0041] in ; Therefore, when When , the optimization problem is approximately:

[0042] ,

[0043] Then, the closed-form solution is obtained by eigenvalue decomposition, and for The maximum eigenvalue of is the corresponding eigenvector, and the optimal solution is:

[0044] .

[0045] The present invention provides a MIMO backscatter communication system based on cyclic delay diversity technology. Within this system, a joint design scheme for RF source-side beamforming and receiver is proposed. A receiver signal detection scheme is designed using different receive equalization criteria, and theoretical bit error rate analysis is performed. Furthermore, by minimizing the bit error rate, the optimal RF source-side beamforming vector is designed, improving overall system performance and achieving both array gain and transmit diversity gain. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 The MIMO backscatter communication system of the present invention is shown;

[0047] Figure 2 The figure shows the working flow diagram of the modulator at BD in the present invention;

[0048] Figure 3 FIG. 4 shows a process of cyclic shift and CP addition processing of a signal sent at BD in the present invention;

[0049] Figure 4 shows a receiver signal processing flow chart in the present invention;

[0050] Figure 5 The figure shows the performance of the minimum BER of the algorithm proposed in the present invention as the signal-to-noise ratio changes;

[0051] Figure 6 The figure shows the performance of the minimum BER of the algorithm proposed in the present invention as a function of the signal-to-noise ratio under different numbers of transmitting antennas.

[0052] Figure 7 The figure shows the performance of the minimum BER of the algorithm proposed in the present invention as a function of the signal-to-noise ratio under different numbers of reflective and receiving antennas. DETAILED DESCRIPTION

[0053] The present invention will be described in detail below with reference to the accompanying drawings:

[0054] Figure 1 The MIMO backscatter communication system of the present invention is shown. The present invention considers that the dedicated RF source has Antenna, receiver has Antenna, BD has Antenna. represents the forward reflection link from the RF source to the BD, where For RF source to Channel response of BD antennas. Indicates that from BD antenna to the The backward link channel response of the root receive antenna is is the number of channel taps, and the average channel power satisfies , assuming that BD antenna to all receiving antennas equal. Indicates the distance from the RF source to the Direct link channel response for each receive antenna.

[0055] In order to obtain the transmit diversity gain brought by BD multiple antennas in the MIMO backscatter communication system, the present invention considers using CDD technology at the BD end. Figure 1 In the system model shown, the RF source passes through the beamforming vector ( ) sends a sinusoidal carrier signal to BD, where Indicates the maximum transmission power. The present invention considers using CDD technology at the BD end. BD adopts SCCP transmission scheme and applies CDD technology. The specific process is as follows Figure 2 As shown. The symbols sent at BD are first converted from serial to parallel and divided into symbol blocks, each containing symbols, and then the symbol block at each BD antenna is cyclically delayed, CP is added to avoid intra-block interference, and finally reflected to the receiver through parallel-serial conversion. Specifically, the first symbol sent at BD The symbol block is represented as When the CDD-SCCP transmission scheme is used, all BD antennas simultaneously transmit symbol blocks after different cyclic delays. . No. The cyclic shift of the root BD antenna can be expressed as , ,in is the cyclic delay between adjacent antennas, Figure 3 A simple processing example is given. The first BD antenna transmission The symbol block is ,in , is the cyclic delay matrix. Then, each symbol block after cyclic delay After joining CP Transmitted by a BD antenna.

[0056] The receiver processing flow involved in the present invention is as follows Figure 4 As shown. Different receiving antennas are processed by serial-to-parallel conversion to divide the received symbols into symbol blocks, and then remove the CP from each symbol block, and then perform DFT transformation on the symbol block, and then use MRC and MMSE criteria to analyze the received The received signals of the root antennas are combined and processed to achieve receiving equalization, and then the IDFT transformation is performed. The processed signals are converted from parallel to serial to realize the detection of the received signal.

[0057] Next, the present invention Figure 1 Taking the model shown in the figure as an example, the receiver signal detection scheme is specifically explained. Figure 4It can be seen that after the receiver performs serial-to-parallel conversion and removes the CP from each symbol block, the The first receiving antenna receives The signal block is

[0058]

[0059] in, Representation Dimension A column vector of all ones, is a circulant matrix, . is zero mean and has a variance of The first term in formula (1) is the received signal related to the direct link. Assuming that the direct link signal can be estimated and completely removed from the received signal, the received signal can be processed into the following form:

[0060]

[0061] because is a circulant matrix and can be decomposed into ,in yes Point DFT matrix, It is a diagonal matrix whose diagonal elements are the channel frequency responses, which satisfies , In addition, since the time domain cyclic shift is equivalent to the frequency domain phase shift, ,in The diagonal elements of , for The frequency response of . Therefore, by using DFT to transform formula (2) into the frequency domain, we have

[0062]

[0063] in, is a diagonal matrix, , is the received Gaussian white noise.

[0064] This paper proposes a joint design scheme for RF source-side beamforming and receivers. Specifically, the present invention considers using MRC and MMSE criteria for frequency-domain receive equalization at the receiver. Based on the selected receive equalization scheme, the present invention designs a signal detection scheme, analyzes the theoretical BER, and further designs RF source-side beamforming by minimizing the bit error rate.

[0065] make , we can get

[0066]

[0067] in, , It corresponds to The equivalent channel vector of subcarriers is, express The product of diagonal elements. According to matrix multiplication, it can be further simplified to ,in Therefore, we can get

[0068]

[0069] in, .

[0070] make No. Listed as , it can be seen that it contains frequency domain symbols ,therefore It can be expressed as

[0071]

[0072] in for No. To restore , the receiver needs to first estimate the frequency domain signal based on formula (6) , and the estimated Specifically, frequency domain equalization can be performed based on MRC and MMSE criteria to estimate ,and The relevant MRC and MMSE estimators are

[0073]

[0074]

[0075] Therefore, the estimated for

[0076]

[0077] in In order to obtain the estimated signal in the time domain, the receiver will The present invention uses BER minimization as an example to design an RF source beamforming transmission scheme. It should be noted that the design criteria of the beamforming vector are not limited to BER minimization, and other beamforming vector design criteria are also within the scope of protection of this patent.

[0078] The present invention will Taking quadrature phase shift keying (QPSK) modulation as an example for theoretical BER analysis, it is intuitive and easy to extend the solution to other modulation schemes. The BER is

[0079]

[0080] in, , Therefore, the entire symbol block The BER is When the MMSE criterion is used, formula (8) is substituted into formula (10). When the MRC criterion is considered, formula (7) is substituted into formula (10). The BER can be simplified to

[0081]

[0082] Combining formulas (10) and (11), it can be seen that the beamforming vector will affect The BER is Based on this finding, the present invention takes minimizing BER as an example to design beamforming vectors.

[0083] According to the above analysis, the signal detection scheme based on the MRC criterion has lower complexity, so the design of the beamforming transmission scheme in this invention will be carried out based on the signal detection scheme under MRC as an example, and other detection schemes are similar. Considering the The function is a monotonically decreasing function, so minimizing BER is equivalent to minimizing Therefore, consider the following optimization problem:

[0084] P1:

[0085]

[0086]

[0087] To solve P1, we introduce a new variable , the original optimization problem can be transformed into:

[0088] P2:

[0089]

[0090]

[0091]

[0092] Since P2 is still a non-convex problem and a multi-rate FP problem, it can be converted by using a unified quadratic transformation. In addition, the present invention removes This non-convex constraint, P2 can be transformed into:

[0093] P3:

[0094]

[0095]

[0096] in is an auxiliary variable. The optimization problem can be solved by iterative optimization. and Specifically, when When known, The optimal solution is ;when Given, P3 is about The optimization problem is convex and can be solved using existing tools such as CVX. Then, the beamforming vector is randomized using Gaussian randomization. from Since the computational complexity is relatively high, the present invention proposes a low-complexity algorithm.

[0097] The low complexity algorithm is introduced below. And define its Rank The elements of the column are , for the diagonal elements, Sometimes, there are According to the law of large numbers, when When it satisfies

[0098]

[0099] in satisfy For non-diagonal elements,

[0100]

[0101] because and exist When they are independent of each other, it satisfies ,therefore . From this we can see that when Sometimes, there are

[0102]

[0103] in Therefore, when When P1 can be approximated as

[0104] P1-LC:

[0105]

[0106]

[0107] Since P1-LC is a Rayleigh entropy optimization problem, its closed-form solution can be obtained by eigenvalue decomposition. Specifically, let for The maximum eigenvalue of is the corresponding eigenvector, then the optimal solution of P1-LC is

[0108]

[0109] Figure 5 The performance diagram of the minimum BER of the algorithm proposed in the present invention as a function of the signal-to-noise ratio is shown in FIG. , , , , , . In order to reflect the superiority brought by the optimized beamforming vector, the present invention considers the case where the RF source distributes energy to the BD antenna with equal power as a benchmark. It can be seen that the BER curve of the theoretical analysis is consistent with the simulation curve, which verifies the correctness of the theoretical BER derivation, and the performance under the MMSE criterion is better than MRC. Secondly, the FP-based and low-complexity beamforming design schemes proposed in the present invention are better than the benchmark scheme, and can achieve a lower BER and a steeper slope of the BER curve. The main reason for this phenomenon is that the optimized beamforming vector at the RF source can enhance the backscatter link and improve the reliability of backscatter communication. In addition, the low-complexity scheme can achieve performance comparable to that of the FP-based scheme.

[0110] Figure 6 The figure shows the performance of the minimum BER of the algorithm proposed in the present invention as a function of the signal-to-noise ratio under different numbers of transmitting antennas. , , , , unless otherwise specified, , the BER curve is generated under the receiving equalization scheme based on the MRC criterion. It can be seen that as the number of transmitting antennas at the RF source end increases, As the BER of the baseline solution remains unchanged, the BER of the FP-based solution decreases significantly, which is attributed to the enhanced BD incident signal on the forward link. Unlike the baseline solution, which cannot provide array gain on the forward link, the beamforming design proposed in this invention can effectively utilize the transmitting antennas of the RF source to obtain array gain proportional to the number of antennas. In addition, when When is larger, the BER performance of the low-complexity design scheme and the FP-based design scheme are consistent, which is consistent with the previous theoretical analysis results.

[0111] Figure 7 The figure shows the performance of the minimum BER of the algorithm proposed in the present invention as a function of the signal-to-noise ratio under different numbers of reflector antennas and receiving antennas. , , , , the BER curve is generated under the receiving equalization scheme based on the MRC criterion. It can be seen that when the number of fixed receiving antennas is When, with As the number of BD antennas increases, the BER decreases significantly and the BER curve becomes steeper. The main reason for this phenomenon is that when the number of BD antennas increases, more energy is reflected back to the receiver, enhancing the backscattered signal and providing array gain; secondly, the use of CDD technology at the BD allows the receiver to receive signals with different delays, thereby enhancing the frequency selectivity of the channel and providing additional frequency diversity for the backscattered signal. In addition, when , When This is because each subcarrier obtains array gain and receive diversity gain through maximum ratio combining at the receiving end.

Claims

1. A MIMO backscatter communication system based on cyclic delay diversity technology, comprising RF source with antennas Receiver with antennas The receiver simultaneously receives a direct link signal from a radio frequency source and a reflected link signal from the reflecting device, and the direct link signal is assumed to be known. For the reflection link signal, define the RF source through the beamforming vector Send a sinusoidal carrier signal to the reflection device, which adopts a single-carrier cyclic prefix transmission scheme and applies cyclic delay diversity technology to reflect the signal. Specifically, the symbols sent from the reflection device are divided into symbol blocks, each containing symbol, define the The symbol block is , all antennas of the reflector device simultaneously transmit symbol blocks with different cyclic delays , define The cyclic shift of the root antenna is expressed as , ,in is the cyclic delay between adjacent antennas, then The first antenna transmission The symbol block is ,in , is the loop delay matrix, It is a dimension A diagonal matrix with 1 as the diagonal element, It is a dimension A column vector of all zeros, After adding the cyclic prefix, it is reflected to the receiver through parallel-to-serial conversion; The receiver converts the received symbols from each antenna into serial-to-parallel, removes the cyclic prefix, and performs discrete Fourier transform, and then uses the maximum ratio combining and minimum mean square error criteria to combine the received symbols from each antenna. The received signals of the three antennas are combined and processed, and then the inverse discrete Fourier transform is performed. The processed signals are converted from parallel to serial to realize the detection of the received signal.

2. A MIMO backscatter communication system based on cyclic delay diversity technology according to claim 1, characterized in that: The receiver is defined to convert the received symbols of each antenna into serial and parallel signals. symbol blocks, and then remove the cyclic prefix from each symbol block to define the The first receiving antenna receives The signal blocks are: ， in, Indicates the distance from the RF source to the The direct link channel response of the root receive antenna, Representation Dimension A column vector of all ones, is a circulant matrix, , Indicates that from Root reflector device antenna to the The backward link channel response of the root receiver antenna is is the number of channel taps, For RF source to The channel response of the root reflecting device antenna, is zero mean and has a variance of complex Gaussian noise, where express The variance of a single element in ; is a known received signal related to the direct link, so it is removed from the received signal to obtain the new received signal: ， The circulatory matrix Decompose into ,in yes Point discrete Fourier transform matrix, It is a diagonal matrix whose diagonal elements are the channel frequency responses, which satisfies , ; According to the time domain cyclic shift is equivalent to the frequency domain phase shift, define ,in is a diagonal matrix whose diagonal elements are , for The frequency response of Transformed into the frequency domain: ， in, is a diagonal matrix, , is the received Gaussian white noise; definition ,get: ， in, , It corresponds to The equivalent channel vector of subcarriers is, express The product of Diagonal elements, further simplified by matrix multiplication to obtain ,in , thus obtaining: ， in, ;make No. List for: ， in, is the frequency domain signal to be estimated, for No. The receiver uses maximum ratio combining and minimum mean square error criteria to perform frequency domain equalization to estimate , the maximum ratio combining and minimum mean square error estimators are: ， ， in, is the maximum ratio combining estimator, represents the forward reflection link from the RF source to the reflecting device, is the minimum mean square error estimator, express The energy of Then we get : ， in , then Perform inverse discrete Fourier transform to obtain the estimated signal in the time domain.

3. The MIMO backscatter communication system based on cyclic delay diversity technology according to claim 2, characterized in that: To minimize the bit error rate, the beamforming vector The design method is that, for the signal detection scheme of the maximum ratio combining criterion, minimizing the bit error rate is equivalent to minimizing , thus establishing the optimization problem as: ， in, represents the maximum transmit power; the solution to the optimization problem is: definition And its first Rank The elements of the column are , for the diagonal elements, Sometimes, there are According to the law of large numbers, when When it satisfies: ， in satisfy The definition of , for non-diagonal elements is: ， because and exist Independent of each other, satisfying ,therefore ; From this we can see that when hour: ， in ; Therefore, when When , the optimization problem is approximately: ， Then, the closed-form solution is obtained by eigenvalue decomposition, and for The maximum eigenvalue of is the corresponding eigenvector, and the optimal solution is: 。

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