A system and receiver algorithm for integrated sensing and communication in environmental backscatter communication

Abstract

The application belongs to the technical field of wireless communication, and discloses a sensing and communication integrated system and a receiver algorithm in ambient backscatter communication, a radio frequency source, multiple reflection devices and a receiver in an environment; the reflection devices modulate signals in a frequency shift keying mode, and shift up or down a subcarrier of the OFDM signal from the radio frequency source according to the transmission signal "1" or "0"; the receiver is used for sensing the position and speed of the reflection devices, and simultaneously detecting the transmission information of the reflection devices. The application proposes a three-step receiver algorithm, estimates the arrival angle of the reflection devices through a root-MUSIC method, estimates the Doppler and the symbol of the reflection devices by using the null carrier of the OFDM signal, and finally estimates the time delay and the source symbol by using the inserted pilot. The scheme of the application is novel, realizes the joint sensing and signal detection of the reflection devices, and has strong application value.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology, and particularly relates to an integrated sensing system and receiver algorithm for environmental backscatter communication. Background Technology

[0002] Recently, ambient backscatter communication technology has been regarded as a low-power and high-spectral-efficiency Internet of Things (IoT) communication technology. The reflective device transmits information by passively modulating the existing radio frequency signals in the environment without generating a dedicated excitation signal, thus greatly improving the system's energy efficiency and spectral efficiency.

[0003] Most current research on environmental reflection communication focuses on demodulating signals from reflecting devices. However, in scenarios such as smart homes, smart factories, and intelligent transportation, the position and velocity information of sensing devices are also needed to support emerging applications. On the other hand, existing research on integrated sensing and communication mainly focuses on parameter estimation of scatterers in the environment. By analyzing changes in the received signal, it analyzes information such as the position and velocity of scatterers in the environment; the scatterers themselves do not transmit information.

[0004] It is noteworthy that in environmental backscatter communication systems, the received signal contains not only the transmission information of the reflecting device but also information such as the position and velocity of the reflecting device and scattering objects in the environment. This makes it possible to achieve integrated sensing and communication in environmental backscatter communication. Furthermore, accurate parameter estimation contributes to accurate channel estimation and reconstruction, which in turn helps in estimating the reflecting device signal through relevant methods.

[0005] Based on the above analysis, the existing technologies have the following problems and shortcomings: Most research on environmental reflection communication focuses primarily on demodulating the signals from reflecting devices, without considering the estimation of parameters such as the position and velocity of the reflecting devices. Furthermore, the demodulation of signals from reflecting devices often employs uncorrelated methods, limiting demodulation performance. On the other hand, in existing research on integrated sensing and communication, the sensed targets typically lack the ability to transmit information; the receiver only estimates the target's position, velocity, and other parameters based on the echo reflected from the target. This limits the application scenarios of integrated sensing and communication systems. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides an integrated sensing and communication system and receiver algorithm for environmental backscatter communication. This invention proposes an architecture for achieving integrated sensing and communication in an environmental backscatter communication system, designs the information modulation method for the reflecting device, and presents a joint estimation scheme for the receiver.

[0007] The present invention is implemented as follows: an integrated sensing system for environmental backscatter communication, the integrated sensing system for environmental backscatter communication includes an environmental radio frequency source (such as a WiFi AP), multiple backscatter devices (BDs) and a receiver.

[0008] The reflecting device transmits information by modulating OFDM signals from a radio frequency source; it uses frequency shift keying (FSK) modulation to shift the OFDM signal from the radio frequency source up or down by one subcarrier according to the transmitted signal "1" or "0".

[0009] The receiver is used to sense the position and speed of the reflecting device, and at the same time detect the transmission information of the reflecting device.

[0010] Furthermore, considering the OFDM signal transmitted by the radio frequency source, it is represented as...

[0011]

[0012] In the formula, K represents the subcarrier, Δf is the subcarrier spacing, and the system bandwidth is B = KΔf. The duration of an OFDM data block is... T and T g These represent the durations of an OFDM signal and a cyclic prefix, respectively, in seconds. k,n This represents the transmitted signal on the k-th subcarrier of the n-th OFDM signal. Represents a duration of A rectangular window.

[0013] Furthermore, the reflecting device employs a frequency shift keying modulation method, and the reflection coefficient is expressed as:

[0014]

[0015] In the formula, When c n,i When c = 1, the OFDM signal shifts up by one subcarrier. n,i When the value is -1, the OFDM signal shifts down by one subcarrier.

[0016] Furthermore, a uniform linear array of M antennas is deployed at the receiver, with the spacing between adjacent antennas being half the wavelength. The nth OFDM symbol received on the mth antenna of the receiver is represented as:

[0017]

[0018] In the formula, x n (t) represents the transmitted signal within the nth OFDM symbol, w n,m (t) represents additive white Gaussian noise.

[0019] Furthermore, at sampling intervals For y n,m After sampling and removing the cyclic prefix, the sampled signal is represented as:

[0020]

[0021] Representing the K samples in the above formula as a vector, i.e.

[0022]

[0023] in,

[0024]

[0025]

[0026]

[0027] It is a discrete Fourier matrix.

[0028] Furthermore, the signal reflected from the i-th reflecting device to the first antenna of the receiver is defined as:

[0029]

[0030] Define symbols The time-domain signal can then be represented in matrix form as follows:

[0031]

[0032] in,

[0033] Another objective of this invention is to provide a receiver algorithm for an integrated sensing system in environmental backscatter communication, specifically including the following steps:

[0034] Step 1: Estimate the angle of arrival of the reflecting device using the root-MUSIC method and separate the multipath signal in the angle domain;

[0035] Step 2: Estimate the Doppler and reflecting device symbols using a joint estimation algorithm based on empty carrier Doppler and reflecting device symbols;

[0036] Step 3: Estimate the time delay and the source signal.

[0037] Furthermore, in step one, the iterative root-MUSIC method is applied to estimate the angle of arrival of the reflecting device, and the estimated angle of arrival is represented as a set. The signal in the angular domain is obtained by calculating the projection of the received signal onto the estimated steering vector.

[0038] Furthermore, in step two, a cost function is defined.

[0039]

[0040] in, The fourth element of the Fourier transform matrix represents the... Okay, Ω nu Represents the set of empty carriers. This is an angle-domain signal.

[0041] The principle used in the above formula is that, after the frequency shift caused by Doppler is compensated, the position index of the empty subcarrier can only be... and This is because the frequency shift introduced by the reflecting device can only be Δf or -Δf. Therefore, for a given... and n, will and The minimum energy on the subcarrier is determined as the cost value, and the Doppler estimate is obtained by calculating the cost function value along the unit circle. After obtaining the Doppler estimate... After compensating for the frequency shift caused by Doppler, the frequency shift of each OFDM symbol is compared. and The energy on the subcarrier is used to estimate the symbol of the reflecting device.

[0042] Furthermore, in step three, comb-shaped pilots are inserted into the source signal. After compensating for the frequency shift caused by Doppler and the symbols of the reflecting device, the position of the pilots is determined. The time delay is estimated using conventional methods. Then, the channel gain is estimated using the signal on the pilot subcarriers through the least squares method. Subsequently, the channel is re-established, and the source symbols are detected through channel equalization.

[0043] Based on the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by this invention are as follows:

[0044] First, this invention proposes an integrated sensing and communication architecture in an environmental backscatter communication system. Achieving integrated sensing and communication in environmental backscatter communication presents numerous challenges. For example, the transmission of signals from the reflecting device introduces time-varying backscattering coefficients, while Doppler introduces phase rotation in the time domain. Therefore, the time-varying backscattering coefficients and the phase rotation caused by Doppler are coupled, making Doppler estimation and BD signal detection very difficult. To address this challenge, this invention proposes a novel signal modulation method for the reflecting device. The reflecting device uses frequency shift keying (FSK) to modulate the signal, shifting the OFDM signal from the radio frequency source up or down by one subcarrier based on the transmitted signal being "1" or "0". This information modulation method, combined with the receiver algorithm, effectively solves the aforementioned challenges and achieves joint estimation of the reflecting device signal and Doppler.

[0045] Secondly, this invention proposes a three-step receiver algorithm. First, the angle of arrival of the reflecting device is estimated using the root-MUSIC method. Then, an efficient estimation algorithm is proposed, utilizing the empty carriers of the OFDM signal to jointly estimate Doppler and the reflecting device symbol. Finally, the time delay and source symbol are estimated. This invention proposes a method for jointly estimating reflecting device information and Doppler using empty carriers, effectively solving the coupling problem between the time-varying backscattering coefficient and the phase rotation caused by Doppler. Furthermore, unlike most integrated inductive and sensor systems that ignore the assumption of inter-carrier interference caused by Doppler, this method uses a signal model that considers inter-carrier interference. Therefore, this method is applicable not only under low-speed conditions but also remains effective in high-speed environments.

[0046] Third, this invention offers a novel solution by designing an information modulation method for the reflecting device and proposing an efficient receiver algorithm to achieve joint detection of the angle of arrival, time delay, Doppler effect, and transmitted signal of the reflecting device. Since this invention does not require a dedicated radio frequency source, it can achieve real-time monitoring of the transmitted information and sensing parameters of multiple devices with low power consumption, demonstrating broad application prospects. Furthermore, while previous backscatter communication systems often used uncorrelated demodulation methods, this invention employs correlated demodulation, resulting in more accurate channel estimation and further improving the signal estimation performance of the reflecting device.

[0047] Fourth, the technical solution of this invention fills a technological gap in the domestic and international industry: existing research on environmental backscatter communication mainly focuses on the demodulation of signals from reflective devices. This invention establishes for the first time a system model that integrates sensing and communication in an environmental backscatter communication system, proposes a novel signal modulation scheme for reflective devices, and further proposes a three-step receiver algorithm, ultimately achieving joint detection of the position, Doppler effect, and signal of the reflective device. On the other hand, existing research on integrated sensing and communication often considers passive sensing targets without information transmission capabilities. In this invention, the application of backscattering technology endows the sensing target with information transmission capabilities, which inspires more emerging application scenarios in the future.

[0048] This invention enables joint sensing and information detection of low-power devices without requiring a dedicated radio frequency source, providing theoretical guidance for integrated sensing and communication in future low-power scenarios. After being adapted, this invention can be applied to multiple scenarios, such as industrial automation, smart healthcare, and smart homes, potentially bringing new services and generating greater revenue for operators. Attached Figure Description

[0049] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a schematic diagram of the integrated sensing system structure in environmental backscatter communication provided in an embodiment of the present invention;

[0051] Figure 2 This is a flowchart of the receiver algorithm for an integrated inductive system in environmental backscatter communication provided in an embodiment of the present invention;

[0052] Figure 3 This is a graph showing the relationship between the root mean square error of speed and the signal-to-noise ratio provided in an embodiment of the present invention;

[0053] Figure 4 This is a graph showing the relationship between the root mean square error of distance and the signal-to-noise ratio provided in an embodiment of the present invention;

[0054] Figure 5 This is a graph showing the relationship between the bit error rate and the signal-to-noise ratio of the reflection device signal provided in this embodiment of the invention.

[0055] Figure 6 This is a graph showing the relationship between the bit error rate and signal-to-noise ratio of the source signal provided in an embodiment of the present invention. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0057] Two specific embodiments of the present invention are as follows:

[0058] Example 1: Intelligent Cargo Management and Transportation in Warehousing and Logistics

[0059] 1) Scene setting:

[0060] In a smart warehouse, goods need to be sorted according to their freshness to automate and improve the efficiency of the inbound, outbound, and transportation processes.

[0061] 2) System Configuration:

[0062] Each cargo is equipped with a reflective device that can transmit information by adjusting the reflection coefficient. The receiver is equipped with multiple antennas to collect relevant information from the reflective device.

[0063] 3) Specific implementation:

[0064] Step 1: The radio frequency source in the warehouse sends OFDM signals.

[0065] Step 2: The reflective device installed on the goods passively reflects the OFDM signal and transmits relevant information, such as the freshness of fresh food, by adjusting the reflection coefficient.

[0066] Step 3: The receiver jointly estimates the position, speed parameters, and transmission information of the reflecting device.

[0067] Example 2: Patient Health Data Monitoring in Smart Healthcare

[0068] 1) Scene setting:

[0069] In remote monitoring scenarios for patients or the elderly, it is necessary to monitor the patient's health data in real time, such as blood pressure and heart rate, and also to detect the patient's location information in real time and transmit it to the remote monitoring center so that medical staff can provide timely assistance.

[0070] 2) System Settings:

[0071] In the monitored space, there is a radio frequency source, such as a WiFi access point. The person being monitored wears a reflective device, and there is a receiver in the space that estimates the state information of the person being monitored based on the received signal.

[0072] 3) Specific implementation:

[0073] Step 1: Monitor the radio frequency source in the environment to send OFDM signals.

[0074] Step 2: The reflective device worn by the monitored person transmits health data to the receiver by adjusting the reflectivity.

[0075] Step 3: The receiver estimates the health data and location and movement speed information of the monitored person.

[0076] The two embodiments provided by this invention demonstrate the flexibility and practicality of this invention in different application scenarios. This invention provides an integrated sensing system and receiver algorithm for environmental backscatter communication, realizing joint sensing and signal estimation of reflective devices in IoT scenarios.

[0077] To address the problems existing in the prior art, this invention provides an integrated sensing system and receiver algorithm for environmental backscatter communication.

[0078] like Figure 1 As shown in the figure, the present invention provides an integrated sensing system for environmental backscatter communication, which includes an environmental radio frequency source (such as a WiFi AP), multiple backscatter devices (BDs) and a receiver.

[0079] The reflecting device transmits information by modulating an OFDM signal from a radio frequency source. The receiver aims to sense the position and speed of the reflecting device while simultaneously detecting the transmitted information. This invention proposes that the reflecting device employs Frequency Shift Keying (FSK) modulation. Specifically, the reflecting device shifts the OFDM signal up or down by one subcarrier based on the transmitted information "1" or "0".

[0080] Considering that the OFDM signal transmitted by the radio frequency source contains K subcarriers with a subcarrier spacing of Δf, the system bandwidth is B = KΔf. T and T g These represent the durations of an OFDM signal and a cyclic prefix, respectively. The duration of an OFDM data block is expressed as... Assuming a data frame contains N OFDM symbols, the signal of a data frame transmitted from the radio frequency source can be represented as follows:

[0081]

[0082] In the formula, K represents the subcarrier, Δf is the subcarrier spacing, and the system bandwidth is B = KΔf. The duration of an OFDM data block is... T and T g These represent the durations of an OFDM signal and the cyclic prefix, respectively. (s) k,n This represents the transmitted signal on the k-th subcarrier of the n-th OFDM signal. Represents a duration of A rectangular window.

[0083] Each reflecting device transmits one bit of information within an OFDM symbol. The reflection coefficient of the i-th reflecting device within the n-th OFDM symbol is expressed as...

[0084]

[0085] In the formula, When c n,i When c = 1, the OFDM signal shifts up by one subcarrier. n,i When the value is -1, the OFDM signal shifts down by one subcarrier.

[0086] The channel modeling of the RF source and receiver consists of a direct link and multipath components reflected by reflecting devices. Each multipath component can be characterized by time delay, angle of arrival, Doppler effect, and attenuation coefficient. The symbol τ i θ i f D,i α i These represent the time delay, angle of arrival, Doppler effect, and attenuation coefficient of the i-th reflecting device path, respectively. Index i = 0 corresponds to the direct path.

[0087] Assume a uniform linear array of M antennas is deployed at the receiver, with the spacing between adjacent antennas being half the wavelength. The nth OFDM symbol received on the mth antenna of the receiver can be represented as...

[0088]

[0089] In the formula, x n (t) represents the transmitted signal within the nth OFDM symbol, w n,m (t) represents additive white Gaussian noise.

[0090] At sampling interval For y n,m After sampling and removing the cyclic prefix, the sampled signal can be represented as:

[0091]

[0092] From the above equation, we can see that Doppler f D,i and BD symbol c n,i Both introduce phase rotation into the received signal in the time domain, resulting in a frequency shift in the received signal.

[0093] Representing the K samples in the above formula as a vector, i.e.

[0094]

[0095] in,

[0096]

[0097]

[0098]

[0099] It is a discrete Fourier matrix.

[0100] The signal reflected from the i-th reflecting device to the first antenna of the receiver is defined as...

[0101]

[0102] Further define the symbols The sampled time-domain signal can then be written in matrix form as follows:

[0103]

[0104] in,

[0105] like Figure 2 As shown, this embodiment of the invention provides a receiver algorithm for an integrated sensing system in environmental backscatter communication, specifically including the following steps:

[0106] Step 1: Estimate the angle of arrival of the reflecting device using the root-MUSIC method and separate the multipath signal in the angle domain;

[0107] Step 2: Estimate the Doppler and reflecting device symbols using a joint estimation algorithm based on empty carrier Doppler and reflecting device symbols;

[0108] Step 3: Estimate the time delay and the source signal.

[0109] In step one, the angle of arrival is estimated. This can be viewed as NK samples of an M-dimensional antenna domain signal, with the number of samples being much larger than the number of paths. An iterative root-MUSIC method is applied to estimate the angle of arrival of the reflecting device. The estimated angles of arrival are represented as a set. The signal in the angular domain can be obtained by calculating the projection of the received signal onto the estimated steering vector. With a sufficient number of antennas, interference from other paths can be approximated as noise.

[0110] In step two, the joint estimation of the Doppler and reflecting device signals is performed. If the phase rotation caused by the Doppler and reflecting device symbols can be perfectly compensated for in the time-domain received signal, the energy on the empty carrier will still be zero. Alternatively, if only the frequency shift caused by the Doppler is compensated, the index of the empty subcarrier will either be... Either in Ω nu This represents the set of empty subcarriers. This is because the frequency offset caused by the reflecting device symbols is Δf or -Δf. Based on this fact, a cost function is defined.

[0111]

[0112] Doppler estimates can be obtained by calculating the cost function value along the unit circle. (The text then abruptly shifts to a seemingly unrelated topic: obtaining the Doppler estimate.) After compensating for the frequency shift caused by Doppler, it can be done by comparing the frequency shift of each OFDM symbol. and The energy on the subcarrier is used to estimate the reflecting device symbol. Note that the Doppler estimate obtained using the above method has discretization error. To eliminate the discretization error, after compensating for the frequency offset caused by the reflecting device symbol, an off-grid estimation method (e.g., root-MUSIC) can be used to refine the Doppler estimate.

[0113] In step three, time delay and source signal estimation are performed. A comb-shaped pilot signal is inserted into the source signal. After compensating for the frequency shift caused by Doppler and reflecting device symbols, the pilot signal's position can be determined. The signal on the pilot subcarrier can then have its time delay estimated using conventional methods such as MUSIC and orthogonal matched tracking. Then, using the signal on the pilot subcarrier, the channel gain is estimated using a least-squares method. Subsequently, the channel can be re-established, and the source symbol can be detected through channel equalization.

[0114] The simulation results are given below to verify the effectiveness and superiority of the above signal modulation method and receiver algorithm.

[0115] In the simulation, the number of subcarriers was set to K = 64, and the spacing between adjacent subcarriers was Δf = 30 kHz. The duration of the cyclic prefix was... Each data frame consists of N = 16 OFDM symbols. We set the number of empty carriers to J = 4 or J = 6. Doppler equals The velocity v is uniformly distributed in the range [-100, 100] m / s, and the carrier frequency f c =60GHz, where c represents the speed of light. The attenuation factor of the BD path is randomly generated within the range of -50dB to -45dB, while the attenuation factor of the direct path is fixed at -40dB. The RF source uses a QPSK modulation scheme. The simulation is based on 10,000 random locations.

[0116] Figure 3 and Figure 4The root mean square error (RMSE) of velocity and range was plotted against the signal-to-noise ratio (SNR). The RMS error of both velocity and range decreased with increasing SNR. For the same number of empty subcarriers, using more antennas resulted in better estimation performance because the angle of arrival estimate obtained with a larger antenna aperture was more accurate. Furthermore, for the same number of antennas, increasing the number of empty subcarriers helped reduce the RMS error of velocity. This is because the cost function can be calculated more accurately using more empty subcarriers.

[0117] Figure 5 and Figure 6 The bit error rate (BER) versus signal-to-noise ratio (SNR) for both the reflecting device signal and the source signal were plotted. These plots show that using a larger number of antennas results in a lower BER. Furthermore, with the same number of antennas, using a larger number of empty carriers yields better estimation performance.

[0118] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A sensor-integrated system for environmental backscatter communication, characterized in that, The integrated inductive system in the environmental backscatter communication includes an environmental radio frequency source, multiple reflective devices, and a receiver. The reflecting device transmits information by modulating OFDM signals from a radio frequency source; it employs frequency shift keying (FSK) modulation to shift the OFDM signal from the radio frequency source up or down by one subcarrier based on the transmitted signal "1" or "0". The receiver is used to sense the position and speed of the reflecting device, and at the same time detect the transmission information of the reflecting device; Consider an RF source transmitting an OFDM signal, represented as In the formula, Indicates subcarrier, The subcarrier spacing is and the system bandwidth is . ; The duration of an OFDM data block is , and These represent the duration of an OFDM signal and the cyclic prefix, respectively. Indicates the first The OFDM signal Transmission signals on each subcarrier Represents a duration of A rectangular window; The reflective device uses frequency shift keying modulation, and the reflection coefficient is expressed as: In the formula, ,when When, the OFDM signal shifts up by one subcarrier, when At that time, the OFDM signal shifts down by one subcarrier; Deployed at the receiver by A uniform linear array of antennas, with the spacing between adjacent antennas being half the wavelength, is used at the receiver. The first received on the antenna Each OFDM symbol is represented as: In the formula, For the first Transmitted signals within OFDM symbols It is additive white Gaussian noise; At sampling interval right After sampling and removing the cyclic prefix, the sampled signal is represented as: In the above formula Each sample is represented as a vector, i.e. in, It is a discrete Fourier matrix. .

2. The integrated sensing system in environmental backscatter communication as described in claim 1, characterized in that, From the The signal reflected by a reflecting device to the first antenna of the receiver is defined as: Define symbols , , , , , Then the time-domain signal can be represented in matrix form as follows: in, .

3. A receiver algorithm for an integrated sensing system in environmental backscatter communication as described in any one of claims 1 to 2, characterized in that, Specifically, the following steps are included: Step 1: Estimate the angle of arrival of the reflecting device using the root-MUSIC method and separate the multipath signal in the angle domain; Step 2: Estimate the Doppler and reflecting device symbols using a joint estimation algorithm based on empty carrier Doppler and reflecting device symbols; Step 3: Estimate the time delay and the source signal.

4. The receiver algorithm for the integrated sensing system in environmental backscatter communication as described in claim 3, characterized in that, In step one, the iterative root-MUSIC method is applied to estimate the angle of arrival of the reflecting device, and the estimated angles of arrival are represented as a set. The projection of the received signal onto the estimated steering vector is calculated to obtain the signal in the angular domain.

5. The receiver algorithm for the integrated sensing system in environmental backscatter communication as described in claim 3, characterized in that, Step two defines the cost function. in, , The fourth element of the Fourier transform matrix represents the... OK, Represents the set of empty carriers. It is an angle domain signal; The principle used in the above formula is that, after the frequency shift caused by Doppler is compensated, the position index of the empty subcarrier can only be... and This is because the frequency shift introduced by the reflecting device can only be 0. or Therefore, for a given and ,Will and The minimum energy on the subcarrier is determined as the cost value. The Doppler estimate is obtained by calculating the cost function value along the unit circle. After compensating for the frequency shift caused by Doppler, the frequency shift of each OFDM symbol is compared. and The energy on the subcarrier is used to estimate the symbol of the reflecting device.

6. The receiver algorithm for the integrated sensing system in environmental backscatter communication as described in claim 3, characterized in that, In step three, a comb-shaped pilot is inserted into the source signal. After compensating for the frequency shift caused by Doppler and the symbol of the reflecting device, the position of the pilot is determined. The time delay is estimated by conventional methods. Then, the channel gain is estimated by least squares method using the signal on the pilot subcarrier. Subsequently, the channel is re-established, and the source symbol is detected by channel equalization.

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