A backscatter communication transmission method based on multi-antenna cyclic modulation

Abstract

The present invention belongs to the field of wireless communication technology, and specifically relates to a backscatter communication transmission method based on multi-antenna cyclic modulation. The present invention includes a tag information modulation and a reader / writer receiving method. The tag controls the load impedance switching of multiple tag antennas by creating multiple cyclic shift copies, artificially creating multipath for the reader / writer's signal reception at the transmitting end, thereby compensating for fading channel losses and improving the probability of correct signal demodulation; on the other hand, the tag avoids the inter-symbol interference problem caused by the multipath effect by introducing a protection interval for each cyclic shift copy, so that the reader / writer can use signal equalization technology to complete the correct detection and judgment of the signal during the receiving and demodulation process. Compared with the traditional backscatter communication transmission scheme, the scheme proposed by the present invention can not only effectively counteract the influence of the multipath effect on signal demodulation, but also improve the transmission quality of the backscatter communication channel by utilizing the multi-antenna diversity at the tag.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communications, and in particular relates to a backscatter communication transmission method based on multi-antenna cyclic modulation. Background Art

[0002] Backscatter Communications (BackCom), an extremely low-power wireless communication technology, has been recognized as a key enabler for the large-scale interconnection of everything. Its core concept is that the wireless transmitter of an IoT tag modulates its own information onto the incident RF signal by adjusting its antenna load impedance and then reflects the modulated signal to the IoT reader. This eliminates the need for high-power components such as oscillators, thus achieving extremely low-power information transmission.

[0003] Because the reflection link in backscatter communication is affected by dual-path fading, the reception signal-to-noise ratio performance of reflection communication is very limited. Furthermore, when the transmission rate and coverage range of backscatter communication are improved, the signal received by the receiver will be affected by inter-symbol interference. To solve the problems of limited reception signal-to-noise ratio and inter-symbol interference, traditional active communication proposes technologies such as multiple antennas and receive equalization to improve signal demodulation performance. Inspired by this, the present invention proposes a multi-antenna cyclic modulation (MACM) technology that enables backscatter communication to simultaneously obtain multi-antenna gain and frequency domain diversity gain, thereby greatly improving its receiver performance. Summary of the Invention

[0004] The main content of the present invention is to propose a backscatter communication transmission scheme based on multi-antenna cyclic modulation, involving the system composition structure, working principle, tag information modulation algorithm, and reader receiving algorithm.

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

[0006] The present invention proposes a backscatter communication system based on cyclic modulation: the composition structure is as follows Figure 1 As shown in the figure, it consists of a single-antenna carrier generator (CE), a tag (Tag) with K antennas, and a single-antenna reader (Reader).

[0007] The backscatter communication method based on multi-antenna cyclic modulation is as follows: a carrier generator transmits a single-frequency sinusoidal signal into the environment; the tag is activated upon receiving this single-frequency signal and modulates the desired information onto the received single-frequency carrier. Specifically, the tag first maps the desired transmission bit string into transmission symbols, dividing the desired transmission symbols into blocks according to a pre-set signal block size; then, when modulating and transmitting each signal block, it is first cyclically shifted by a specific number of samples. Each tag antenna adds a guard interval (GI) based on the corresponding cyclically shifted signal block replica and switches its antenna load impedance accordingly, thereby modulating the information onto the received single-frequency carrier signal. In addition, the reader receives the reflected signal from the tag, performs corresponding receive equalization on it, and then demodulates the information the tag needs to send.

[0008] The multi-antenna cyclic modulation-based backscatter communication transmission scheme proposed in this paper includes a tag information modulation algorithm and a reader / writer reception algorithm. Using a single-frequency carrier signal emitted by a carrier generator, the tag modulates its desired transmission information onto the incident carrier signal and backscatters it to the reader / writer. During the information modulation process, the tag divides the desired transmission symbol into blocks, generates multiple cyclically shifted replicas, and adds guard intervals. The information is then modulated, loaded, and backscattered by switching the load impedance of multiple antennas. After the reader / writer receives the signal from the tag, it successfully extracts the tag information through signal equalization techniques.

[0009] The beneficial effects of the present invention are as follows: the present invention proposes a multi-antenna cyclically modulated backscatter communication transmission scheme, including a tag information modulation algorithm and a reader receiving algorithm. The tag controls the load impedance switching of multiple tag antennas by creating multiple cyclically shifted copies, artificially creating multipath for the signal reception of the reader at the transmitting end, thereby compensating for the fading channel loss and improving the probability of correct signal demodulation; on the other hand, the tag avoids the inter-symbol interference problem caused by the multipath effect by introducing a protection interval for each cyclically shifted copy, so that the reader can use signal equalization technology to complete the correct detection and judgment of the signal during the receiving and demodulation process. Compared with the traditional backscatter communication transmission scheme, the scheme proposed by the present invention can not only effectively counteract the influence of the multipath effect on signal demodulation, but also improve the transmission quality of the backscatter communication channel by utilizing the multi-antenna diversity at the tag. Through simulation verification, the backscatter communication transmission scheme proposed by the present invention can effectively reduce the probability of signal error demodulation under multipath channel conditions, while promoting the application of backscatter communication technology in the field of large-scale Internet of Things, and has important application value and development potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 : Schematic diagram of the system composition of the present invention;

[0011] Figure 2 : Tag information modulation block diagram;

[0012] Figure 3 :Cyclic modulation example diagram;

[0013] Figure 4 : Reader / writer information demodulation block diagram;

[0014] Figure 5 : Receiver bit error rate curves under different equalizers;

[0015] Figure 6 : Receiver bit error rate curves under different numbers of antennas. DETAILED DESCRIPTION

[0016] The present invention is described in detail below with reference to the accompanying drawings and simulation examples, and its beneficial effects are verified.

[0017] The present invention proposes a backscatter communication transmission scheme based on cyclic modulation technology. The system structure is as follows Figure 1 As shown in the figure, it consists of a single-antenna carrier generator (CE), a tag (Tag) with K antennas, and a single-antenna reader (Reader).

[0018] The operating principle of backscatter communication based on multi-antenna cyclic modulation is as follows: a carrier generator transmits a single-frequency sinusoidal signal into the environment; the tag is activated upon receiving this single-frequency signal and modulates the desired information onto the received single-frequency carrier. Specifically, the tag first maps the desired transmission bit string into transmission symbols, dividing the desired transmission symbols into blocks according to a pre-set signal block size; then, when modulating and transmitting each signal block, it is first cyclically shifted by a specific number of samples. Each tag antenna adds a guard interval (GI) based on the corresponding cyclically shifted signal block replica and switches its antenna load impedance accordingly, thereby modulating the information onto the received single-frequency carrier signal. In addition, the reader receives the reflected signal from the tag, performs corresponding receive equalization on it, and then demodulates the information the tag needs to send.

[0019] According to the above description, the single-frequency sinusoidal signal sent by the carrier generator can be expressed as

[0020]

[0021] Among them, f c is the frequency of the transmitted carrier, and Re{C} is the real part of the complex number C. At the kth tag antenna, the received single-frequency carrier signal is

[0022]

[0023] in, is the L1 tap multipath channel from the carrier generator to the kth tag antenna. At the same time, the L2 tap multipath channel from the kth tag antenna to the reader is defined as L from carrier generator to reader d The tapped multipath channel is After being activated by the signal, the tag modulates the information to be sent. The tag information modulation block diagram is as follows Figure 2 As shown. For the bit information stream to be transmitted, bit mapping is first performed to obtain a transmission symbol sequence. Then, the transmission symbols are divided into blocks according to the specified signal block size. The specified transmission signal block size is M (M≥K·L2), that is, each signal block contains M required transmission symbols. The required transmission signal blocks are cyclically shifted by (k·D) samples, where k=0, 1, …, K-1, A guard interval of length N (N ≥ L2) is then added to each cyclically shifted signal block replica. Each tag antenna switches its antenna impedance accordingly, thereby modulating the information onto the received single-frequency carrier signal. This invention specifically illustrates the use of a cyclic prefix (CP) as a guard interval. In practical systems, guard intervals in the form of zero padding (ZP) and known symbols, in addition to the CP, are also applicable to the present invention. Figure 3 An example of multi-antenna cyclic modulation is given when the number of tag antennas K = 4, the signal block size M = 8, the number of channel taps L2 = 2, the number of cyclic shift units D = 2 and the cyclic prefix length N = 4.

[0024] Therefore, for the nth signal block, its initial state and the kth cyclically shifted signal block copy can be expressed as

[0025]

[0026] On the other hand, the signal reflected by the kth tag antenna is c k (t)z k (t). Therefore, the time domain signal received at the reader is

[0027]

[0028] in, Represents the convolution symbol, u p (t) is the bandpass receiving noise. Then, the corresponding receiving baseband signal can be expressed as

[0029]

[0030] in, u(t) is the baseband receiving noise. After completing time and frequency synchronization and removing the guard interval, the nth received signal block can be expressed as follows

[0031] y(n)=H d 1 M×1 +H2c0(n)+u(n). (5)

[0032] Where H2 is the combined channel impulse response The circulant matrix, 1 M×1 represents an M-dimensional vertical vector whose elements are all 1, is the additive white Gaussian noise at the reader.

[0033] like Figure 4 As shown, after removing the cyclic prefix, the received signal block is subjected to a fast discrete Fourier transform (FFT), and the frequency domain received signal can be obtained as

[0034]

[0035] Among them, 0 (M -1 )×1 represents an (M-1)-dimensional vertical vector whose elements are all 0, F M is the M-dimensional Fourier transform matrix, is the frequency domain response of the combined channel, diag(h) is the diagonal matrix composed of vector h, This is the frequency domain receiving noise. Channel state information is used to equalize the received signal. Equalization methods include, but are not limited to, frequency domain minimum mean square error equalization and frequency domain zero-forcing equalization. The equalized frequency domain signal is then subjected to an inverse Fourier transform to restore the original transmitted signal block. Finally, symbol mapping is performed to obtain the original bit sequence.

[0036] The following simulation will verify the beneficial effects of the present invention. The simulation parameters are set as follows: the number of tag antennas is K = 4, the number of channel taps from the carrier generator to the reader is 5, the number of channel taps from the carrier generator to the tag is 3, and the number of channel taps from the tag to the reader is 4. The small-scale fading of the above-mentioned related channels is all Rayleigh fading model; the tag modulation method is binary phase shift keying, the size of each transmitted signal block is M = 64, and the protection interval adopts the cyclic prefix structure with a length of N = 16. The bit error rate result is 10 5 Average value under sub-channel realization.

[0037] like Figure 5As shown, the proposed transmission scheme first demonstrates that, in multipath channels, the reader can correctly receive and demodulate tag information. Within a given signal-to-noise ratio (SNR) range, both frequency-domain equalizers achieve good bit error rate (BER) performance. Furthermore, the frequency-domain minimum mean square error (MMSE) equalizer achieves multi-antenna diversity gain: when K = 4, the BER curve slope is 4. However, because the frequency-domain zero-forcing equalizer amplifies the received noise in deep fading channels, its diversity gain is only 1.

[0038] like Figure 6 As shown in the figure, the frequency-domain minimum mean square error equalizer can achieve corresponding diversity gain under different tag antenna conditions. Specifically, when the number of antennas is K = 2, 4, 8, and 16, the reader can achieve diversity gains of 2, 4, 8, and 16, respectively. Furthermore, due to the reflection of multiple antennas at the tag, the reader also achieves a corresponding signal-to-noise ratio gain. This phenomenon indicates that when more antennas are equipped at the tag, the reader can achieve lower bit error rate performance when the carrier generator transmits a single-frequency carrier at the same power.

Claims

1. A backscatter communication transmission method based on multi-antenna cyclic modulation, characterized in that: The backscatter communication system consists of a single-antenna carrier generator, a tag with K antennas, and a single-antenna reader. The carrier generator transmits a single-frequency sinusoidal signal into the environment. The tag is activated upon receiving the single-frequency signal, modulates the desired information onto the received single-frequency carrier, and then sends it to the reader. The reader receives the single-frequency signal and the reflected signal from the tag, performs receive equalization on them, and then demodulates the information sent by the tag. The tag modulates the information it needs to transmit onto the received single-frequency carrier by mapping the desired transmit bit string into transmit symbols, which are then divided into blocks according to a pre-set signal block size. Each signal block is cyclically shifted by a set number of samples before being modulated and transmitted. Each tag antenna then adds a guard interval to the corresponding cyclically shifted signal block replica and switches its antenna load impedance accordingly, thereby modulating the information onto the received single-frequency carrier signal. The single-frequency sinusoidal signal sent by the carrier generator is defined as: ， in, is the frequency of the transmitted carrier, is the real part of the complex number C; Then at the kth tag antenna, the received single-frequency carrier signal is: ， in, is the distance from the carrier generator to the kth tag antenna Tap multipath channel; let the kth tag antenna to the reader The tapped multipath channel is , carrier generator to reader The tapped multipath channel is ; The tag modulates the information to be sent, performs bit mapping first, obtains the sending symbol sequence, and defines the sending signal block size as , , each signal block contains M required transmission symbols, and each signal block is The cyclic shift of samples, where , , add a length of Each tag antenna switches its antenna impedance according to the protection interval, thereby modulating the information on the received single-frequency carrier signal. ; The signal reflected by the kth tag antenna is defined as , so the time domain signal received at the reader is: ， in, , represents the convolution symbol, is the bandpass receiving noise; the corresponding receiving baseband signal is expressed as: ， in, , is the baseband receiving noise; after completing time and frequency synchronization and removing the guard interval, the nth received signal block is expressed as: ， in, is the initial state of the nth signal block, The circulant matrix formed by the combined channel impulse response , represents an M-dimensional vertical vector whose elements are all 1, is the additive white Gaussian noise at the reader; After removing the cyclic prefix, the received signal block is subjected to a fast discrete Fourier transform, and the frequency domain received signal is obtained as follows: ， in, The representative elements are all 0 dimensional vertical vector, is the M-dimensional Fourier transform matrix, is the frequency domain response of the combined channel, is a diagonal matrix consisting of vectors h, The received signal is equalized using the channel state information, and then the equalized frequency domain signal is inverse Fourier transformed to restore the original transmitted signal block. Finally, after symbol mapping, the original bit sequence is obtained.

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