CN112954792B - Multi-reflection device joint positioning and communication method based on environment backscattering - Google Patents

Multi-reflection device joint positioning and communication method based on environment backscattering Download PDF

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CN112954792B
CN112954792B CN202110159173.8A CN202110159173A CN112954792B CN 112954792 B CN112954792 B CN 112954792B CN 202110159173 A CN202110159173 A CN 202110159173A CN 112954792 B CN112954792 B CN 112954792B
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reflection
radio frequency
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reflecting device
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CN112954792A (en
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梁应敞
朱贤明
杨刚
龙睿哲
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University of Electronic Science and Technology of China
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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Abstract

The invention belongs to the technical field of wireless communication, and particularly relates to a multi-reflector joint positioning and communication method based on environmental backscattering. The present invention uses a receiver to receive a signal that typically includes a direct link signal from an ambient radio frequency source and L reflecting device backscatter signals. And the simultaneous positioning of a plurality of reflecting devices is realized by a space domain signal processing technology. And meanwhile, the scheme of demodulating the reflected signal with low complexity under the OFDM radio frequency source is provided, and the radio frequency source information does not need to be demodulated. I.e. communication of the reflecting device with the receiver is achieved while the reflecting device is positioned. The reflecting device sends pilot frequency, the receiving end knows the corresponding relation between the pilot frequency and the reflecting device in advance, and the corresponding relation between the reflecting signal and the reflecting device can be judged by performing correlation operation on the pilot frequency known by the receiving end and the pilot frequency of the received signal, so that the information demodulation and the positioning of a certain reflecting device are finally realized.

Description

Multi-reflection device joint positioning and communication method based on environment backscattering
Technical Field
The invention belongs to the technical field of wireless communication, and particularly relates to a multi-reflector joint positioning and communication method based on environmental backscattering.
Background
The environmental backscattering technology is one of solutions of the internet of things proposed in recent years, and uses an existing radio frequency source in the environment as a signal source, and does not need to separately configure a radio frequency signal source like the traditional backscattering technology. The backscattering technology has the advantages of low power consumption and low cost, and on the basis of the application of the environmental backscattering technology, the cost is further reduced and the spectrum efficiency is improved due to the fact that a special radio frequency signal source is not required to be deployed.
With the development of science and technology, the demand for the simultaneous communication and positioning of the internet of things equipment is more and more extensive. The system capable of simultaneously realizing communication and positioning of a plurality of reflecting devices in the environment backscatter communication system is designed, and has wide application prospect. For example, in logistics management, the need for using a robot to replace manual operation is increasing, and when the robot knows the information and the position of a logistics item, corresponding actions can be performed, so that the logistics item using the internet of things device needs to have the information decoded correctly and also needs to know the position of the logistics item, and the robot and other devices can be used more conveniently to replace tedious manual operation.
However, since the environmental backscattering technology uses the existing radio frequency signal source in the environment, compared with the conventional backscattering technology, new challenges are brought about in the aspects of radio frequency source signal interference elimination, backscatter signal demodulation and the like. The OFDM technology is widely applied to commercial communication systems today, and therefore, OFDM signals are commonly used environmental signals for environmental backscatter communication systems, and have a cyclic prefix structure, and therefore, designing a multi-reflector joint positioning and communication system based on environmental backscatter according to the characteristics of the common radio frequency source signals also has important practical significance.
Disclosure of Invention
The invention mainly provides a multi-reflection device joint positioning and communication method based on environmental backscattering, which realizes the simultaneous positioning and communication of a plurality of reflection devices.
The present invention uses a receiver to receive a signal that typically includes a direct link signal from an ambient radio frequency source and L reflecting device backscatter signals. And the simultaneous positioning of a plurality of reflecting devices is realized by a space domain signal processing technology. The invention provides a scheme for demodulating the reflection signals with low complexity under an OFDM radio frequency source while realizing the positioning of a plurality of reflection signals, and does not need to demodulate the radio frequency source information. I.e. communication of the reflecting device with the receiver is achieved while the reflecting device is positioned. The reflecting device sends pilot frequency, the receiving end knows the corresponding relation between the pilot frequency and the reflecting device in advance, and the corresponding relation between the reflecting signal and the reflecting device can be judged by performing correlation operation on the pilot frequency known by the receiving end and the pilot frequency of the received signal, so that the information demodulation and the positioning of a certain reflecting device are finally realized.
The technical scheme of the invention is as follows:
a multi-reflection device joint positioning and communication method based on environment backscattering comprises an environment radio frequency source, L reflection devices and a receiver with M antennas, wherein the number of the reflection devices is 0,1r,yr) M is more than or equal to L + 1; characterized in that the joint positioning and communication method comprises the following steps:
s1, the environment radio frequency source emits radio frequency source signals, and the reflection equipment performs backscattering on the environment radio frequency source signals, wherein the waveform design method of the reflection equipment comprises the following steps:
assuming that the symbol period of the reflection device is K times of the symbol period of the OFDM radio frequency source, the binary information sent by each reflection device is Bl(m) represents, BlWhen one of the symbols b (m) in the transmission chip is equal to ± 1, (m) in the corresponding kth OFDM symbol period, the following reflection device waveform is designed:
Figure GDA0003476261240000021
wherein N iscIs the length of the cyclic prefix of an OFDM radio source symbol, N is the data portion length, N + NcK represents the serial number of the OFDM radio source symbol in the same symbol period of the reflection device;
when a certain symbol b (m) of the transmission chip is 1, the waveform of the reflection device is constant to 1;
s2, the receiver receives the signal, carries on DOA estimation, estimates the arrival angle theta of the direct link signal and multiple backscattering signalsdAnd thetaL＝[θ0,θ1,θ2,...,θl,...θL-1]；
S3, performing spatial filtering once on all estimated arrival angle signals, designing weight vectors to perform beam forming during each spatial filtering, processing received signals, only retaining signals of one angle, suppressing the power of other direction signals, and obtaining direction signals corresponding to all arrival angles;
s4, calculating the power of the signal z (N) after spatial filtering, and if the signal has N sampling points, then the power P of the signalzThe calculation method comprises the following steps:
Figure GDA0003476261240000031
s5, after obtaining the power corresponding to the direction signal of all arrival angles, distinguishing the power of the direct signal according to the characteristic that the backscatter signal will experience twice fading compared with the direct link signal
Figure GDA0003476261240000032
And reflected signal power
Figure GDA0003476261240000033
By direct signal power
Figure GDA0003476261240000034
Calculating the distance d from the radio frequency source to the receiverf：
Figure GDA0003476261240000035
Where n represents the attenuation coefficient of the environment, β is a constant determined by the carrier frequency and the environment, n and β are both available in actual measurements, psIs the transmit power;
and the rest signals are reflection link signals, and the product D of the distance from the radio frequency source to the reflection device and the distance from the reflection device to the receiver is estimated through power:
Figure GDA0003476261240000036
wherein,
Figure GDA0003476261240000037
the distance from the ambient radio source to the l-th reflecting device,
Figure GDA0003476261240000038
is the distance of the l reflecting device from the receiver, alphalThe reflection coefficient of the first reflection device;
s6, estimating the distance from the reflection device to the receiver by the geometric principle, specifically, establishing the following equation according to the cosine theorem of the triangle:
Figure GDA0003476261240000039
wherein Δ θ is an angle between the direct path and the reflection path, and Δ θ ═ θd-θlL is obtained by solving the equation
Figure GDA00034762612400000310
And
Figure GDA00034762612400000311
s7, utilizing the information theta of the angle of arrival of the backscatter signallAnd distance information of reflecting device to receiver
Figure GDA00034762612400000312
Calculating the coordinates (x) of the reflecting devicel,yl)：
Figure GDA0003476261240000041
Positioning is realized;
s8, demodulating the information of the reflected signal:
assuming that K is 1, i.e. the duration of one period of the reflection device waveform c (n) is the same as the duration of one OFDM symbol while ignoring noise, in one OFDM symbol period, we establish:
Figure GDA0003476261240000042
wherein L ishThe time delay expansion of the channel represents the number of samples polluted by intersymbol interference, and the samples are discarded; then there are:
u(m)＝|Kl|2|s(n)|2B(m)
wherein,
Figure GDA0003476261240000043
is a constant, p l1, s (n) is a radio frequency source signal, and the decision rule for b (m) during demodulation is as follows:
Figure GDA0003476261240000044
thereby recovering the symbols transmitted by the reflecting device;
s9, using the known pilot sequence to perform correlation operation with the pilot part of the decoded reflection signal, and determining the corresponding relationship between the signal and the known pilot when the correlation result between the demodulated signal and a certain known pilot sequence is the maximum, so as to determine which reflection device the signal belongs to while transmitting information.
The invention has the beneficial effects that: the invention provides a multi-reflection equipment combined positioning and communication method based on environmental backscattering, which is used for decoding information of reflection equipment on the basis of positioning a plurality of reflection equipment under the commonly used OFDM environmental radio frequency source scene, and has strong application value.
Drawings
FIG. 1 shows the system configuration of the present invention
FIG. 2 shows a waveform design of a reflective device contemplated by the present invention
FIG. 3 shows a signal structure description contemplated by the present invention
FIG. 4 shows the simulation result of the present invention considering the technical solution that the signal is correctly decided to the correct reflection device
FIG. 5 shows the simulation result of the information demodulation performance of the reflection device in the technical solution considered in the present invention
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings.
As shown in FIG. 1, the environmental backscatter system on which the present invention is based includes an environmental radio frequency source, L reflection devices, and a receiver having M antennas, where M is greater than or equal to L + 1; transmitting radio frequency source signal by environment radio frequency source
Figure GDA0003476261240000051
Wherein s (t) is a power normalized RF source baseband signal with a transmit power of ps. The channel is a strong LOS path, in the figure
Figure GDA0003476261240000052
Representing the channel parameters of the ambient radio source to the back scatterer,
Figure GDA0003476261240000053
representing the channel parameters of the ambient radio source to the receiver,
Figure GDA0003476261240000054
representing the channel parameters of the backscatter to the receiver. n represents the attenuation coefficient of the environment, which can be measured in a particular environment, and is typically between 2 and 4. dfDistance of ambient radio source to receiver, dhDistance of the ambient radio source to the back scatterer, dgIs the distance of the backscatter to the receiver.
Describing a model received by a receiver by taking a uniform linear array as an example, wherein the number of reflecting devices is L, the reflecting devices are numbered, L is 0,1dAnd the arrival angle of the reflected signal of the reflecting device numbered l is thetal。
The received signal may be represented as
Figure GDA0003476261240000055
L-1, wherein L is 0, 1.
Figure GDA0003476261240000056
Is the noise of the band-pass, and,
Figure GDA0003476261240000057
is the distance from the radio frequency source to the reflecting device numbered l,
Figure GDA0003476261240000058
is the distance from the reflecting device numbered l to the receiver. c. Cl(t) base band signal to be transmitted, alpha, for the reflecting device numbered llIs the reflection coefficient.
Figure GDA0003476261240000059
And
Figure GDA00034762612400000510
is a direction vector, wherein
Figure GDA00034762612400000511
Is a spatial phase, fcIs the carrier frequency, d is the antenna spacing, c is the speed of light, and λ is the carrier wavelength.
The digital baseband form of the received signal may be expressed as
Figure GDA0003476261240000061
Where ω (n) is complex baseband noise, subject to a circularly symmetric complex Gaussian distribution
Figure GDA0003476261240000062
σ2Is the noise power.
The invention designs the waveform of the reflected signal, and the specific waveform design scheme is as follows:
it is assumed that the reflection device symbol period is K times the OFDM radio source symbol period. Binary information from each reflecting device is Bl(m) represents, Bl(m) ± 1. When a certain symbol b (m) in the transmission chip is equal to-1, the following waveform of the reflection device is designed
Figure GDA0003476261240000063
NcIs the length of the cyclic prefix of an OFDM radio source symbol, N is the data portion length, N + NcConstituting the length of one complete OFDM symbol.
When one symbol b (m) of the transmission chip is 1, the reflection device waveform is constant at 1.
As shown in fig. 2, when transmitting-1, the waveform is designed to make a jump in the middle of one OFDM radio source symbol from 1 to-1, and when transmitting 1, the waveform is designed to have no jump.
L +1 angles of arrival can be estimated using the DOA estimation algorithm, here the Root-MUSIC algorithm (Barabell, "Improving the resolution performance of the eigen-based direction-defining algorithms," ICASSP'83.IEEE International Conference on Acoustics, Speech, and Signal Processing, Boston, Massachusetts, USA,1983, pp.336-339, doi:10.1109/ICASSP.1983.1172124) is used as an example estimation algorithm:
1. according to the set sampling number N, sampling for N times to obtain a sample sequence y (N), wherein
Figure GDA0003476261240000065
2. Calculating y (n) autocorrelation matrix R, R ═ E { y (n) yH(n) where the statistical average may be replaced by a time average, i.e.
Figure GDA0003476261240000064
3. The characteristic value decomposition is carried out on R, due to the target informationThe number of the numbers is L +1, so that the minimum normalized eigenvector u corresponding to the M-L-1 eigenvalues is obtainediWhere i ═ L +2, L +3 …, M.
ui＝[ui0,ui1,...ui(m-1)]T
4. Construct vector a (z), a (z) [1, z ]-1,...,z-(M-1)]T。
The following function is constructed:
Figure GDA0003476261240000071
Figure GDA0003476261240000072
multiplying by self conjugate transpose to obtain
Figure GDA0003476261240000073
i＝3,4,...,M。
5. Defining polynomial
Figure GDA0003476261240000074
Let it be 0, solve the equation. And obtaining the value of z.
6. The number of the target signals is L +1, the z value is subjected to modulo subtraction by 1, and then the absolute value is taken to be sorted from small to large. This would find 2L +2 roots closest to modulo 1 (a heavy root would appear). Since the signal reflected by the back scatterer undergoes two attenuations, its signal strength is much less than the direct signal. The root closest to the unit circle belongs to the direct link signal. And the other roots are subjected to de-duplication to obtain the arrival angle of the reflected link signal.
7. Since z is ejφPhi is the spatial angular frequency, phi ═ pi sin theta. Thus, it is possible to provide
Figure GDA0003476261240000075
And calculating to obtain the signal arrival direction estimation.
Obtaining an estimate of the angle of arrival of the direct signal
Figure GDA0003476261240000076
And reflected signal angle of arrival estimate
Figure GDA0003476261240000077
And then, spatial filtering is carried out. The aim is to only keep the signal in a certain direction and set the signals in other directions to zero.
There are many algorithms for spatial filtering, and one example here is a spatial filtering algorithm to retain
Figure GDA0003476261240000078
The reflected signal of the direction, the signal of other estimated directions is set to zero as an example to explain:
writing out a matrix
Figure GDA0003476261240000081
Representing the set of directions to force zero. And (3) an optimization problem is proposed:
Figure GDA0003476261240000082
s.t wHA＝0
the specific algorithm steps are as follows:
1. for the direction of the desired zero forcing, a matrix is constructed by the direction vector
Figure GDA0003476261240000083
2. Solving for BHA is 0 orHSolving for B-0
3.
Figure GDA0003476261240000084
Wherein B is+Is the M-P generalized inverse of the B matrix.
Here with only angle of arrival being preserved
Figure GDA0003476261240000085
The reflected signal of (2) is taken as an example, and after the spatial filtering is finished, the signal form is as follows:
z(n)＝wHy(n)
that is, the signal is:
Figure GDA0003476261240000086
Figure GDA0003476261240000087
wherein w is a weight vector designed in the spatial filtering process, p0＝wHa(θ0)，εl＝wHa(θl)εd＝wHa(θd) Representing the degree of suppression of the power of the respective signal component, depends on the accuracy of the DOA estimation. If the DOA estimate is accurate, p0≈1，εlAnd εdAre all very small values, about 0, and can be classified as noise, as discussed below based on the fact that p is0＝1，ε＝0。
Measurement z0(n) the power of the signal is
Figure GDA0003476261240000088
When p is01, reflection coefficient alpha0Given that n and β are known after the actual measurement, it can be estimated
Figure GDA0003476261240000089
I.e. the product D of the distance of the radio frequency source to the reflecting device and the distance of the reflecting device to the receiver.
Figure GDA00034762612400000810
Keeping the signals of the direct link, suppressing all reflected signals to estimate the distance from the radio frequency source to the receiver, and performing spatial filtering according to the similar method to obtain
Figure GDA0003476261240000091
In the formula pd＝wHa(θd)，pd≈1，εlIs a very small value, about 0, and can be classified as noise. Therefore, the signal z can be obtaineddPower of (n)
Figure GDA0003476261240000092
Calculate dfTo estimate the distance of the outgoing frequency source to the receiver.
Figure GDA0003476261240000093
The problem of localization of the reflected signal can therefore be modeled as a geometric problem as follows:
θdand theta0Can be obtained after performing DOA estimation, and dfBy keeping only the signal of the direct path one can estimate,
Figure GDA0003476261240000094
as the product of the two path distances can also be estimated. Then the cosine theorem of the triangle. The angle between the direct and reflected paths is delta thetad-θ0The following equation can be established:
Figure GDA0003476261240000095
in this equation, D and Δ θ and DfBoth equations are known, and can be solved
Figure GDA0003476261240000096
And
Figure GDA0003476261240000097
thus, the distance from the reflecting device to the receiver
Figure GDA0003476261240000098
Can be estimated and the arrival angle theta of the signal is known0The position of the reflecting device can be estimated.
The coordinate calculation method of the reflecting device is as follows, and the coordinate of the known receiver is set as (x)r,yr) The coordinates of the reflecting device are (x, y) and the angle of arrival is theta
Figure GDA0003476261240000099
And finally obtaining the positioning result of the reflected signal.
The method for locating and simultaneously demodulating the reflected signal is as follows:
for the sake of analysis convenience, the assumption that K is 1, i.e. the duration of one period of the reflection device waveform c (n) is the same as the duration of one OFDM symbol, does not affect the correctness of the theoretical derivation as well. I.e. one complete OFDM symbol corresponds to the symbol B of a reflecting devicel(m), i.e. a Bl(m) within a symbol period of N + NcC (n) samples.
The spatial filtered signal is as follows:
Figure GDA0003476261240000101
Figure GDA0003476261240000102
the model can be simplified to the following form:
z0(n)＝K0s(n)c0(n)+ω(n)
wherein
Figure GDA0003476261240000103
Is a constant number of times that the number of the first,
Figure GDA0003476261240000104
and
Figure GDA0003476261240000105
since the spatial filtering has been zeroed out, it is included in the ω (n) term.
As shown in fig. 3, assume that the delay spread of channel h is LhThen, a certain intersymbol interference will be caused to the CP symbol, and the length of the CP is greater than the delay spread, so that a part of CP symbols will not cause intersymbol interference, and the samples of the part without intersymbol interference are used for processing. I.e. ISI does not affect the accuracy of the derivation.
Due to the nature of the cyclic prefix:
s(n)＝s(n+N),n＝0,...,Nc-1
due to the nature of the designed waveform:
Figure GDA0003476261240000106
in one OFDM symbol period, i.e. one B0In (m) symbol periods, neglecting the effect of noise, as shown in FIG. 3, z0(n) the signal has the following properties: when n is equal to Lh-1,...,NcAt the time of-1
Figure GDA0003476261240000107
Processing the signal to cause:
Figure GDA0003476261240000111
then
Figure GDA0003476261240000112
ωcrossAnd (n) is a cross term generated in the conjugate multiplication operation and is also attributed to a noise term.
Therefore, ifNeglecting the noise, u (m) ═ Ko|2|s(n)|2B (m), i.e., u (m) after processing is the original transmission symbol b (m) multiplied by a constant.
The judgment rule for B (m) during demodulation is as follows:
Figure GDA0003476261240000113
thus, the symbols transmitted by the reflecting device can be recovered.
The known pilot sequence is used for carrying out correlation operation on the pilot part of the decoded reflection signal, and when the correlation operation result of the demodulated signal and a certain known pilot sequence is the maximum, the corresponding relation between the signal and the known pilot can be determined, so that the reflection device to which the signal belongs can be known while information is transmitted.
Simulation analysis:
the number of reflecting devices is 4, the positioning area is a space of 10 m × 10 m, the receiver is in the space, and the ambient radio frequency source is outside the space.
N of OFDM symbols of an ambient radio frequency sourcecSet to 16, N to 64, N + NcIs 80 and is the number of sampling points for one OFDM symbol. The reflection device symbol period is the same as the OFDM symbol period. A total of 80000 samples, i.e., 1000 symbol sequences, are collected for DOA estimation. The source transmission power is fixed to be 1, the source coordinate is set to be (-5,15), and the number of receiving antennas of the receiver is 8. The channel parameters β are set to 1 and the range attenuation coefficient is set to 2.5. The fixed coordinate of the receiver is (5,0), the arrival angle of the direct signal is-33.7 degrees, the distance from the source transmitter to the receiver is 18.03 meters, and the ratio of the signal-to-noise ratio of the direct link signal to the signal-to-noise ratio of the transmitted signal is delta gamma according to the previous signal modeld＝-31.4dB。
The reflection coefficient of the reflection device is set to α ═ 0.2+0.3 j. Coordinates of the reflecting device are fixed in the simulation, and coordinates of the reflecting device numbered 0 are (2,3), coordinates of the reflecting device numbered 1 are (4,7), coordinates of the reflecting device numbered 2 are (6,1), and coordinates of the reflecting device numbered 3 are (7, 9). I.e. 4 reflecting devices reach the receiver at distances of 4.24 meters, 7.07 meters, 1.41 meters and 9.22 meters, respectively, and the true directions of the 4 reflecting devices to the receiver are 45.0 °, 8.1301 °, -45.0 °, 12.53 °, respectively. The pilot sequence length of the reflecting device is set to 64 reflecting device symbols.
From the previous signal model, the ratio of the power of the reflected signal and the signal-to-noise ratio of the transmitted signal of the ith reflecting device at fixed coordinates can be calculated as:
Figure GDA0003476261240000121
under the simulation conditions, Δ γ of 4 reflecting devices can be knowniRespectively-53.1 dB, -57.1dB, -43.9dB, -61.2dB, the reflected signal power difference of the reflecting devices at different positions can be up to about 20dB.
Altering the power signal-to-noise ratio gamma of a transmitted signalsFrom 30dB to 60dB, the signal-to-noise ratio of the received signal of the direct link is gammas+ΔγdThe signal-to-noise ratio of the received signals of the 4 reflection devices is gammas+Δγi. And carrying out a plurality of Monte Carlo experiments in each round, and in each Monte Carlo experiment, counting the accuracy rate of the judgment of the reflection signals as corresponding reflection equipment and counting the error rate of each reflection equipment.
As shown in fig. 4, the signals of multiple reflection devices are simultaneously accessed, and the reflected signals are determined as simulation results of the reflection devices. It can be seen that the probability of correct decision is high when the signal-to-noise ratio of the transmitted signal is greater than 40 dB. Indicating that the scheme is valid. Meanwhile, the lowest decision probability is about 20%, because for a signal in one direction, assuming a completely random decision, it is possible to decide that 4 reflection devices reflect the signal or direct the signal, and the probability that the correct decision is exactly 20%.
Fig. 5 is a bit error rate simulation result of multi-reflector signal demodulation. It can be seen that the bit error rate performance is greatly affected by the signal-to-noise ratio of the received signal. Meanwhile, when the signal-to-noise ratio is high enough, the error rate of demodulation of the reflection device is also satisfactory, which indicates the correctness of the signal demodulation scheme.

Claims (1)

1. A multi-reflection device joint positioning and communication method based on environment backscattering comprises an environment radio frequency source, L reflection devices and a receiver with M antennas, wherein the number of the reflection devices is 0,1r,yr) M is more than or equal to L + 1; characterized in that the joint positioning and communication method comprises the following steps:
s1, the environment radio frequency source emits radio frequency source signals, and the reflection equipment performs backscattering on the environment radio frequency source signals, wherein the waveform design method of the reflection equipment comprises the following steps:
assuming that the symbol period of the reflection device is K times of the symbol period of the OFDM radio frequency source, the binary information sent by each reflection device is Bl(m) represents, BlWhen one of the symbols b (m) in the transmission chip is equal to ± 1, (m) in the corresponding kth OFDM symbol period, the following reflection device waveform is designed:
Figure FDA0003476261230000011
wherein N iscIs the length of the cyclic prefix of an OFDM radio source symbol, N is the data portion length, N + NcK represents the serial number of the OFDM radio source symbol in the same symbol period of the reflection device;
when a certain symbol b (m) of the transmission chip is 1, the waveform of the reflection device is constant to 1;
s2, the receiver receives the signal, carries on DOA estimation, estimates the arrival angle theta of the direct link signal and multiple backscattering signalsdAnd thetaL＝[θ0,θ1,θ2,...,θl,...θL-1]；
S3, performing spatial filtering once on all estimated arrival angle signals, designing weight vectors to perform beam forming during each spatial filtering, processing received signals, only retaining signals of one angle, suppressing the power of other direction signals, and obtaining direction signals corresponding to all arrival angles;
s4, calculating the power of the signal z (N) after spatial filtering, and if the signal has N sampling points, then the power P of the signalzThe calculation method comprises the following steps:
Figure FDA0003476261230000012
s5, after obtaining the power corresponding to the direction signal of all arrival angles, distinguishing the power of the direct signal according to the characteristic that the backscatter signal will experience twice fading compared with the direct link signal
Figure FDA0003476261230000021
And reflected signal power
Figure FDA0003476261230000022
By direct signal power
Figure FDA0003476261230000023
Calculating the distance d from the radio frequency source to the receiverf：
Figure FDA0003476261230000024
Where n represents the attenuation coefficient of the environment, β is a constant determined by the carrier frequency and the environment, n and β are both available in actual measurements, psIs the transmit power;
and the rest signals are reflection link signals, and the product D of the distance from the radio frequency source to the reflection device and the distance from the reflection device to the receiver is estimated through power:
Figure FDA0003476261230000025
wherein,
Figure FDA0003476261230000026
the distance from the ambient radio source to the l-th reflecting device,
Figure FDA0003476261230000027
is the distance of the l reflecting device from the receiver, alphalThe reflection coefficient of the first reflection device;
s6, estimating the distance from the reflection device to the receiver by the geometric principle, specifically, establishing the following equation according to the cosine theorem of the triangle:
Figure FDA0003476261230000028
wherein Δ θ is an angle between the direct path and the reflection path, and Δ θ ═ θd-θlL is obtained by solving the equation
Figure FDA0003476261230000029
And
Figure FDA00034762612300000210
s7, utilizing the information theta of the angle of arrival of the backscatter signallAnd distance information of reflecting device to receiver
Figure FDA00034762612300000211
Calculating the coordinates (x) of the reflecting devicel,yl)：
Figure FDA00034762612300000212
Positioning is realized;
s8, demodulating the information of the reflected signal:
assuming that K is 1, i.e. the duration of one period of the reflection device waveform c (n) is the same as the duration of one OFDM symbol while ignoring noise, in one OFDM symbol period, we establish:
Figure FDA0003476261230000031
wherein L ishThe time delay expansion of the channel represents the number of samples polluted by intersymbol interference, and the samples are discarded; then there are:
u(m)＝|Kl|2|s(n)|2B(m)
wherein,
Figure FDA0003476261230000032
is a constant, pl1, s (n) is a radio frequency source signal, and the decision rule for b (m) during demodulation is as follows:
Figure FDA0003476261230000033
thereby recovering the symbols transmitted by the reflecting device;
s9, using the known pilot sequence to perform correlation operation with the pilot part of the decoded reflection signal, and determining the corresponding relationship between the signal and the known pilot when the correlation result between the demodulated signal and a certain known pilot sequence is the maximum, so as to determine which reflection device the signal belongs to while transmitting information.
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