A signal receiving and transmitting method for an OFDM symbiotic wireless communication system

Abstract

The present invention belongs to the field of wireless communication technology, and specifically relates to a signal receiving and transmitting method of an OFDM symbiotic wireless communication system. The method proposed in the present invention uses a comb pilot to estimate the synthetic channel symbol by symbol in the process of receiving and demodulating the main signal, so that the main signal demodulation does not need to know the secondary information symbol. Compared with the traditional demodulation method, the method proposed in the present invention decouples the demodulation process of the primary and secondary signals, which greatly reduces the calculation complexity, and also reduces the changes to the OFDM main system caused by the introduction of the secondary system. At the same time, in the case where the direct link is very bad or even blocked, compared with the traditional method that cannot decouple the primary and secondary signals multiplied in the reflection link, the method proposed in the present invention can smoothly estimate and demodulate the primary and secondary signals, significantly expanding the application scenarios of OFDM symbiotic wireless communication. The present invention can achieve high-speed mobile communication performance improvement and high-spectral efficiency and high-energy efficiency low-speed Internet of Things communication, and has important application value and development potential.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communications, and in particular relates to a signal receiving and sending method of an OFDM symbiotic wireless communication system. Background Art

[0002] Symbiotic Radio (SR) is a new type of wireless communication technology based on cognitive radio and cooperative environmental backscatter communication technology with extremely high energy efficiency, high spectrum efficiency and high cost efficiency. It is considered to be particularly suitable for IoT applications. The symbiotic wireless communication system consists of a main system and a secondary system. Among them, the main system usually uses active communication equipment for data transmission; in the secondary system, the secondary transmitter (STx) is usually a passive device with a simple circuit structure. It can complete data collection, storage, backscatter transmission and other functions, and its cost is much lower than that of active communication equipment. Specifically, STx can use the received RF signal from the main system as a carrier, and modulate its own information onto the signal by changing the load impedance of the antenna, and send it to the receiver in the form of backscatter. At the same time, since the signal reflected by the secondary system contains the main signal component, it can be used as a beneficial multipath signal to enhance the main signal transmission of the main system.

[0003] On the other hand, as a multi-carrier transmission technology, Orthogonal Frequency Division Multiplexing (OFDM) technology can convert high-speed data signals into multiple parallel low-speed sub-data streams for modulation and transmission by dividing a whole spectrum into multiple mutually orthogonal sub-carriers. Since the signal bandwidth on each sub-channel is smaller than the channel coherence bandwidth, each sub-channel can be regarded as a flat fading channel, thereby eliminating inter-symbol interference. OFDM technology is adopted by a variety of wireless communication standards including wireless LAN IEEE 802.11a, 4G LTE and 5G NR. Symbiotic wireless communication can use various OFDM systems widely distributed in the environment as its main system components, which can not only promote the deployment and implementation of large-scale Internet of Things communications, but also assist high-speed mobile communication networks to achieve further performance improvements. Summary of the invention

[0004] The invention proposes a signal receiving and sending method of an OFDM symbiotic wireless communication system.

[0005] The technical solution of the present invention is:

[0006] A signal transceiving method of an OFDM symbiotic wireless communication system, defining that the OFDM symbiotic wireless communication system includes a single-antenna primary transmitter PTx, a single-antenna secondary transmitter STx, and a single-antenna cooperative receiver CRx, wherein PTx uses orthogonal frequency division multiplexing technology to modulate information and sends OFDM signals through an active radio frequency link to transmit primary information with CRx; STx modulates secondary information onto an incident signal from PTx and backscatters it to CRx, thereby realizing secondary information transmission; CRx receives two signals from PTx and STx, and recovers primary and secondary information of PTx and STx through a coherent reception algorithm; the signal transceiving method is specifically as follows:

[0007] PTx sends the main information symbol, adopts the comb pilot in OFDM technology, and sends the known pilot signal on the set subcarrier in each OFDM symbol;

[0008] STx sends secondary information symbols and defines that the secondary information symbol period sent by STx is equal to the OFDM symbol period. STx uses its built-in synchronization circuit to synchronize the start of the secondary information symbol to the start of the incident OFDM symbol. STx uses prefix pilot, that is, it sends pilot symbols at the frame header. The prefix pilot consists of two pilot symbols, which are the two symbols with the largest amplitude and opposite phases in the STx modulation symbol set.

[0009] After CRx samples the received signal, it first estimates the composite channel composed of the direct channel and the reflected cascade channel symbol by symbol through the comb pilot signal, so as to demodulate the main information symbol, wherein the direct channel refers to the channel from PTx to CRx, and the reflected cascade channel refers to the channel from STx to CRx; then, after accurately estimating the composite channel again using the pilot symbol and the demodulated main information symbol, the secondary information data frame is synchronized and positioned from the estimated composite channel information, and the direct channel and the reflected cascade channel are estimated respectively using the prefix pilot at the frame header, so as to estimate and demodulate the secondary information symbol.

[0010] The beneficial effect of the present invention is that the present invention proposes a signal receiving and transmitting method of an OFDM symbiotic wireless communication system, including a pilot structure of a primary and secondary system and a primary and secondary signal receiving and demodulating process. By using the OFDM system as the primary system to share its frequency band and energy, the secondary system information transmission can be realized without the use of additional frequency bands and energy. At the same time, the secondary information transmission can bring beneficial multipath gain to the primary system, thereby improving the main information transmission performance. The method proposed in the present invention uses a comb pilot to estimate the synthetic channel symbol by symbol in the process of receiving and demodulating the main signal, so that the main signal demodulation does not need to know the secondary information symbol. Compared with the traditional demodulation method, the method proposed in the present invention decouples the demodulation process of the primary and secondary signals, greatly reducing the computational complexity, and also reducing the changes to the OFDM main system caused by the introduction of the secondary system. At the same time, when the direct link between PTx and CRx is very bad or even blocked, compared with the traditional method that cannot decouple the multiplied primary and secondary signals in the PTx-STx-CRx reflection link, the method proposed in the present invention can smoothly estimate and demodulate the primary and secondary signals, significantly expanding the application scenarios of OFDM symbiotic wireless communication. Through simulation and experimental verification, the transceiver design method proposed in the present invention can achieve high-speed mobile communication performance improvement and high-spectrum efficiency and high-energy efficiency low-speed Internet of Things communication, and has important application value and development potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 : Schematic diagram of the system composition of the present invention;

[0012] Figure 2 : Schematic diagram of the pilot structure of the primary and secondary systems;

[0013] Figure 3 : Receiver demodulation flow chart;

[0014] Figure 4 :Bit Error Rate (BER) performance curve of the main system;

[0015] Figure 5 : Subsystem BER performance curve;

[0016] Figure 6 : BER performance curve of primary and secondary systems without direct link. DETAILED DESCRIPTION

[0017] The present invention is described in detail below with reference to the accompanying drawings and simulation examples.

[0018] The technical solution adopted by the present invention is an OFDM symbiotic wireless communication system: the composition structure is as follows Figure 1As shown, it consists of a single-antenna primary transmitter (PTx), a single-antenna secondary transmitter (STx), and a single-antenna cooperative receiver (CRx).

[0019] The basic working principle of the OFDM symbiotic wireless communication system is: PTx uses orthogonal frequency division multiplexing (OFDM) technology to modulate information, sends OFDM signals through an active radio frequency link, and transmits primary information with CRx. On the other hand, by periodically switching its antenna impedance, STx modulates secondary information on the incident signal from PTx and backscatters it to CRx, thereby realizing secondary information transmission. Therefore, CRx receives two signals from PTx and STx, and recovers the primary and secondary information of PTx and STx through a coherent reception algorithm.

[0020] The coherent transceiver design method for the OFDM symbiotic wireless communication system proposed in the present invention includes the primary and secondary system pilot structure and the primary and secondary signal receiving and demodulating process. It is stipulated that the secondary information symbol period is equal to the OFDM symbol period. At the same time, STx uses its built-in synchronization circuit to synchronize the start of its secondary information symbol to the start of the incident OFDM symbol. For this system, the present invention is designed as follows Figure 2 The pilot structure shown is as follows: the primary system adopts the comb pilot widely used in OFDM technology, that is, a known pilot signal is sent on a specific subcarrier in each OFDM symbol. The comb pilot of the primary system can be used to track the time-varying channel caused by the secondary information transmission in real time, helping CRx to estimate the composite channel. On the other hand, the secondary system adopts a prefix pilot, that is, a pilot symbol is sent at the frame header, which consists of two pilot symbols, which are the two symbols with the largest amplitude and opposite phase in the secondary system modulation symbol set. After the receiver receives and samples the signal, as shown in Figure 3 As shown, the primary and secondary information symbols are demodulated. Specifically, the composite channel composed of the direct channel and the reflected cascade channel can be estimated symbol by symbol through the comb pilot signal, so as to demodulate the primary information symbol. After the composite channel is accurately estimated again using the pilot symbol and the demodulated primary information symbol, the secondary information data frame is synchronized and positioned from the estimated composite channel information, and the direct channel and the reflected cascade channel can be estimated respectively using the prefix pilot at the frame header, so as to estimate and demodulate the secondary information symbol.

[0021] Specific:

[0022] In the nth OFDM symbol period, the signal required to be transmitted by PTx can be expressed as

[0023]

[0024] Where N is the number of OFDM subcarriers, W is the N-point Discrete Fourier Transform (DFT) matrix, S k (n) represents the information symbol sent by PTx on the kth subcarrier. After adding the cyclic prefix, PTx transmits with a power of P T Send the signal out.

[0025] On the other hand, the secondary information symbol period is equal to the OFDM symbol period, and STx uses its built-in synchronization circuit to synchronize the start of its secondary information symbol to the start of the incident OFDM symbol. In the nth OFDM symbol period, the signal that STx needs to send is c(n).

[0026] CRx receives and samples the signal, performs synchronization, estimates and eliminates frequency offset, removes CP, and transforms the time domain signal into a frequency domain signal through DFT. Therefore, in the nth OFDM symbol period, the frequency domain signal received by CRx is for

[0027]

[0028] in, for The diagonal matrix formed by is the frequency domain response of the direct channel from PTx to CRx, is the frequency domain response of the reflected cascade channel from PTx to STx to CRx, is the received noise in the frequency domain, H(n) = H d +c(n)H b The frequency response of the synthetic channel composed of the direct channel and the reflected cascade channel is specified as L.

[0029] For this system, the present invention is designed as follows Figure 2 The pilot structure shown in the figure: The main system adopts the comb pilot widely used in OFDM technology, that is, a specific N in each OFDM symbol p A known pilot signal is sent on each subcarrier. The comb pilot of the primary system can be used to track the time-varying channel caused by the secondary information transmission in real time, helping CRx to estimate the composite channel. On the other hand, the secondary system uses a prefix pilot, that is, a pilot symbol is sent at the frame header, which consists of two pilot symbols, which are the two symbols with the largest amplitude and opposite phases in the secondary system modulation symbol set. Here, the present invention stipulates c(0)=1 and c(1)=-1 as an example.

[0030] The receiver demodulation process is as follows Figure 3 First, using the pilot signal on the pilot subcarrier in the comb pilot, the synthetic channel impulse response can be estimated by the least squares (LS) method to obtain

[0031]

[0032] in, is the LS estimator, is the matrix consisting of the first L columns of the DFT matrix W, is the matrix F L The matrix consisting of the rows corresponding to the comb-shaped pilot subcarriers in , is the diagonal matrix of pilot signals, Y p (n) is the frequency domain received signal on the pilot subcarrier. Then, the frequency response of the synthetic channel can be obtained by DFT:

[0033]

[0034] Then, the main information symbol on each subcarrier can be estimated and demodulated by using maximum likelihood detection.

[0035]

[0036] Among them, A s is the main information symbol set, is a vector The kth element of Y k (n) is the frequency domain signal on the kth subcarrier received by CRx, S∈A s Possible values ​​for the main information symbol.

[0037] On the other hand, the synthetic channel frequency response can be estimated again using the pilot symbols and the estimated main information symbols. Compared with the estimation using the comb-shaped pilot symbols, the second estimation can further improve the estimation accuracy.

[0038]

[0039] in, is the LS estimator. Further, the subsystem frame header is cross-correlated with the estimated synthetic channel frequency response for frame synchronization, and the subsystem prefix pilot is used to estimate the direct channel and the reflected cascade channel frequency response.

[0040]

[0041] and The synthetic channel frequency response estimated at the prefix pilot when n = 0, 1. Finally, the secondary information symbol can be estimated and demodulated as

[0042]

[0043] c∈{-1,1} is the possible value of the secondary information symbol.

[0044] By further analyzing the system, when the primary and secondary systems use QPSK and BPSK for symbol modulation respectively, the bit error rates (BER) of the primary system corresponding to the reception and demodulation using the estimated channel information can be obtained as follows:

[0045]

[0046] On the other hand, since the demodulation process of the secondary information symbol depends on the demodulation of the primary information symbol, considering the analysis of the lower bound of the secondary system BER under the condition of perfect demodulation of the primary information symbol, we can get

[0047]

[0048] The beneficial effects of the present invention will be verified by simulation below. The simulation parameters are set as follows: the number of OFDM subcarriers N is 64, the CP length is 16, the number of OFDM pilots is 4, the main information symbol modulation mode is QPSK, and the secondary information symbol modulation mode is BPSK. Unless otherwise specified, the number of direct channel paths and the number of multipaths of the reflection cascade channel are both 4, and both are equal-power Rayleigh fading. It is stipulated that the direct channel path fading is fixed, and △γ is the ratio of the average signal-to-noise ratio of the reflection link to the average signal-to-noise ratio of the direct link. In order to illustrate the beneficial effects brought about by the present invention, simulations and comparisons are carried out for different △γ values ​​and the situation where the direct link is blocked (there is no direct link).

[0049] like Figure 4 As shown in the figure, it is first proved that the BER performance of the primary system simulated by this method is consistent with the theoretical BER performance obtained by analysis. In addition, compared with the case without secondary information transmission, the introduction of secondary information transmission can bring beneficial multipath gain to the primary information transmission. At the same time, with the increase of △γ (that is, the increase of the reflection cascade channel strength), the BER performance of the primary system is further improved.

[0050] like Figure 5 As shown in the figure, it is first proved that the subsystem BER lower bound obtained by simulation of this method is consistent with the theoretical BER lower bound obtained by analysis. In addition, the gap between the actual BER performance of the subsystem and the BER lower bound is small, and under high signal-to-noise ratio conditions, the actual BER performance and the BER lower bound obtained by analysis are close to convergence. At the same time, with the increase of △γ, the subsystem BER performance is further improved.

[0051] like Figure 6 As shown in the figure, it is proved that in the absence of a direct link, both the primary and secondary information symbols can be demodulated smoothly. At the same time, due to the diversity gain in the demodulation process of the secondary information symbol, the BER performance of the secondary information symbol is much better than the BER performance of the primary information symbol. In addition, since the BER performance of the primary information symbol is relatively limited under the secondary signal-to-noise ratio, the gap between the secondary system BER performance and the BER lower bound is quite obvious.

Claims

1. A signal transceiving method of an OFDM symbiotic wireless communication system, defining that the OFDM symbiotic wireless communication system includes a single-antenna primary transmitter PTx, a single-antenna secondary transmitter STx, and a single-antenna cooperative receiver CRx, wherein: PTx uses orthogonal frequency division multiplexing technology to modulate information and send OFDM signals through an active radio frequency link to transmit primary information with CRx; STx modulates secondary information onto the incident signal from PTx and backscatters it to CRx, thereby realizing secondary information transmission; CRx receives two signals from PTx and STx, and recovers the primary and secondary information of PTx and STx through a coherent reception algorithm; the signal transceiver method is specifically as follows: PTx sends the main information symbol, adopts the comb pilot in OFDM technology, and sends the known pilot signal on the set subcarrier in each OFDM symbol; STx sends secondary information symbols and defines that the secondary information symbol period sent by STx is equal to the OFDM symbol period. STx uses its built-in synchronization circuit to synchronize the start of the secondary information symbol to the start of the incident OFDM symbol. STx uses prefix pilot, that is, it sends pilot symbols at the frame header. The prefix pilot consists of two pilot symbols, which are the two symbols with the largest amplitude and opposite phases in the STx modulation symbol set. After CRx receives and samples the signal, it first estimates the composite channel composed of the direct channel and the reflected cascade channel symbol by symbol through the comb pilot signal, so as to demodulate the main information symbol, wherein the direct channel refers to the channel from PTx to CRx, and the reflected cascade channel refers to the channel from STx to CRx; then, after accurately estimating the composite channel again using the pilot symbol and the demodulated main information symbol, it synchronizes and locates the secondary information data frame from the estimated composite channel information, and estimates the direct channel and the reflected cascade channel respectively using the prefix pilot at the frame header, so as to estimate and demodulate the secondary information symbol; The specific method of CRx signal demodulation is: The synthetic channel is obtained by the least squares LS estimation based on the comb pilot signal: in, is the LS estimator, is the matrix consisting of the first L columns of the DFT matrix W, is the matrix F L The matrix consisting of the rows corresponding to the comb-shaped pilot subcarriers in , is the diagonal matrix of pilot signals, P T is the transmit power of PTx, Y p (n) is the frequency domain received signal on the pilot subcarrier; The frequency response of the synthetic channel obtained by DFT is: Then use maximum likelihood detection to estimate and demodulate the main information symbol on each subcarrier: Among them, A s is the main information symbol set, is a vector The kth element of Y k (n) is the frequency domain signal on the kth subcarrier received by CRx, S∈A s is the possible value of the main information symbol; Using the pilot symbols and the estimated main information symbols, the synthetic channel frequency response is estimated again: in, is the LS estimator, Y(n) is the frequency domain signal received by CRx; The frame synchronization is performed by cross-correlating the secondary information frame header with the estimated synthetic channel frequency response, and then the prefix pilot of the secondary information is used to estimate the frequency response of the direct channel and the reflected cascade channel respectively: in, is the direct channel frequency response, is the reflection cascade channel frequency response, and The synthetic channel frequency response estimated at the prefix pilot when n=0,1; The secondary information symbol is estimated to be demodulated as: Among them, c∈{-1,1} is the value of the secondary information symbol.

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