Subway train distance measuring method based on OFDM signal subcarrier phase

Abstract

The invention relates to a subway train distance measuring method based on OFDM signal subcarrier phases. The subway train distance measuring method comprises the following steps that OFDM radio frequency signals containing distance measuring symbols are received; performing signal down-conversion and sampling processing to obtain an OFDM discrete baseband signal; according to the self-correlation characteristic of the ranging symbol, cross-correlation operation is carried out by using the OFDM radio frequency signal containing the ranging symbol and the OFDM discrete baseband signal, and the symbol position of the ranging symbol is obtained; extracting the ranging symbol based on the symbol position of the ranging symbol, estimating a carrier frequency error in combination with the OFDM discrete baseband signal, and performing phase compensation to obtain a processed OFDM discrete baseband signal; and based on the processed OFDM discrete baseband signal, calculating the phase difference of adjacent subcarriers, and further calculating the distance between subway trains. Compared with the prior art, the method has the advantages of no influence of environmental factors, high ranging precision and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of wireless communication and rail transportation, and in particular to a subway train ranging method based on OFDM signal subcarrier phase. Background Art

[0002] Amidst the booming global development of intelligent rail transit, subways, as the main arteries of urban public transportation, continue to expand in scale and increase in frequency. High-precision train positioning technology has become a core element in ensuring safe and efficient subway operations. Accurate train positioning not only prevents accidents such as rear-end collisions and collisions, but also optimizes train scheduling, improving transportation efficiency and passenger travel experience.

[0003] Currently, the Global Navigation Satellite System (GNSS) is widely used in open environments such as the ground, thanks to its high precision, all-weather capabilities, and global coverage, providing reliable positioning services for various modes of transportation. However, in enclosed environments such as subway tunnels, GNSS signals face numerous challenges. Due to the closed structure and complex building materials of tunnels, GNSS signals are easily blocked, reflected, and absorbed during transmission, resulting in significant signal attenuation or even complete failure. For example, in urban subways, when trains travel in tunnels, GNSS positioning errors can reach several meters or even be completely incapable of positioning. This inherent flaw severely limits its application in subway positioning, forcing the industry to actively explore more reliable alternative positioning solutions.

[0004] In recent years, Orthogonal Frequency Division Multiplexing (OFDM) technology, with its unique technical advantages, has emerged as a key research focus in the convergence of communication and positioning. OFDM decomposes high-speed data signals into multiple parallel, lower-speed sub-data streams, which are transmitted via multiple orthogonal subcarriers. This effectively suppresses inter-symbol interference caused by multipath effects and improves the stability and reliability of signal transmission. Furthermore, OFDM boasts high spectral efficiency, making full use of limited spectrum resources and meeting the high-capacity data transmission requirements of subway communication systems. These characteristics make OFDM an ideal medium for integrating communication and positioning functions. In subway positioning, reliable position estimation relies on accurate distance measurement. Currently, research using OFDM for distance measurement is limited. Therefore, in-depth research on subway train ranging methods based on the subcarrier phase of OFDM signals is of great practical significance for promoting the development of subway positioning technology and enhancing the safety and intelligence of subway operations. Summary of the Invention

[0005] The purpose of the present invention is to provide a subway train ranging method based on OFDM signal subcarrier phase to improve the ranging accuracy of subway trains.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A subway train ranging method based on OFDM signal subcarrier phase includes the following steps:

[0008] receiving an OFDM radio frequency signal including a ranging symbol;

[0009] Performing signal down-conversion and sampling processing on the OFDM radio frequency signal containing the ranging symbol to obtain an OFDM discrete baseband signal;

[0010] According to the autocorrelation characteristics of the ranging symbol, a cross-correlation operation is performed using the OFDM radio frequency signal containing the ranging symbol and the OFDM discrete baseband signal to obtain the symbol position of the ranging symbol;

[0011] Extracting a ranging symbol based on the symbol position of the ranging symbol, estimating a carrier frequency error in combination with the OFDM discrete baseband signal, and performing phase compensation to obtain a processed OFDM discrete baseband signal;

[0012] Based on the processed OFDM discrete baseband signal, the phase difference between adjacent subcarriers is calculated, and the distance between subway trains is further calculated.

[0013] Furthermore, the OFDM radio frequency signal containing the ranging symbol is expressed as:

[0014]

[0015] Where r RF (t) is the received OFDM radio frequency signal containing the ranging symbol, f c is the carrier frequency, f ε is the carrier frequency error, L is the number of paths, α l and τ l are the attenuation coefficient and path delay corresponding to the lth multipath, n(t) is the ambient Gaussian noise, and s() is the transmitted OFDM ranging symbol.

[0016] Furthermore, the step of receiving an OFDM radio frequency signal including a ranging symbol includes:

[0017] The initial OFDM radio frequency signal sent by the transmitter is defined as:

[0018]

[0019] Where s(t) is the initial OFDM radio frequency signal sent by the transmitter, f c is the carrier frequency, N s is the number of subcarriers, X nis the modulated sequence on the nth subcarrier, f n =nΔf is the center frequency of the nth subcarrier, g(t) is the rectangular pulse function;

[0020] A transmission period of a ranging symbol is selected, wherein the transmission period of the ranging symbol satisfies:

[0021] T s <T c

[0022]

[0023] Where, T c is the coherence time, f m is the maximum Doppler shift;

[0024] The ranging symbol is included in the initial OFDM radio frequency signal, and the ranging symbol is represented as:

[0025]

[0026] Where s m (t) is the mth OFDM ranging symbol sent, M is the number of ranging symbols sent, x(n) is the sequence modulated on the nth subcarrier in the ranging symbol, which is a strong autocorrelation sequence, Δf is the subcarrier spacing, T s The periodic time for transmitting ranging symbols in the time domain;

[0027] The receiving end receives the initial OFDM radio frequency signal containing the ranging symbol by superposition of multipath signals, and obtains the final OFDM radio frequency signal containing the ranging symbol.

[0028] Furthermore, the step of signal down-conversion processing includes:

[0029] According to the local oscillation in the signal receiving process, two carrier frequencies are generated: c , a local carrier with a phase difference of 90°, multiplying the OFDM radio frequency signal containing the ranging symbol with the two local oscillators respectively to obtain:

[0030] r I (t) = r RF (t)·cos(2πf c t)

[0031] r Q (t) = r RF (t)·sin(2πf c t)

[0032] Where r I (t), r Q(t) is the I-path signal and Q-path signal extracted from the received signal, r RF (t) is the OFDM radio frequency signal containing the ranging symbol;

[0033] Use low-pass filter to remove r I (t) and r Q The high-frequency component of (t) is used to obtain the OFDM baseband signal, which is expressed as:

[0034]

[0035] Where r(t) is the OFDM baseband signal, f ε is the carrier frequency error, L is the number of paths, α l and τ l are the attenuation coefficient and path delay corresponding to the lth multipath, n(t) is the ambient Gaussian noise, and s() is the transmitted OFDM ranging symbol.

[0036] Furthermore, the OFDM discrete baseband signal is expressed as:

[0037]

[0038] Where r(p) is the OFDM discrete baseband signal, f ε is the carrier frequency error, N s Δf is the sampling frequency, is the sampling time, τ l is the path delay corresponding to the lth multipath, L is the number of paths, s() is the transmitted OFDM ranging symbol, and n(p) is the environmental noise.

[0039] Furthermore, the symbol position of the ranging symbol is expressed as:

[0040]

[0041] Among them, R(m) is the cross-correlation function, which is expressed as:

[0042]

[0043] Where, is the symbol position of the ranging symbol, m represents the displacement, r(p) is the OFDM discrete baseband signal, x * (p) is the complex conjugate of the copy of the OFDM radio frequency signal sequence containing the ranging symbol stored at the receiving end, and L is the length of the received sequence.

[0044] Furthermore, the step of estimating the carrier frequency error includes:

[0045] The phase difference between the ranging symbols of adjacent periods is calculated according to the ranging symbols in the OFDM discrete baseband signal, wherein the calculation expression of the phase difference is:

[0046] angle[r2(p)]-angle[r1(p)]≈2πf ε T S

[0047]

[0048] Where angle[·] is the phase of the solution signal, r1(p) and r2(p) are the ranging symbols of adjacent cycles, and f ε is the carrier frequency error, T s is the transmission cycle time of the ranging symbol in the time domain, is the sampling time, L is the number of paths, τ l is the path delay corresponding to the lth multipath, s() is the transmitted OFDM ranging symbol, and n(p) is the environmental noise;

[0049] Based on the phase difference, the carrier frequency error estimate is calculated as follows:

[0050]

[0051] Where, is the estimated value of carrier frequency error, Δf is the subcarrier spacing, N s is the number of sampling points in the ranging symbol.

[0052] Furthermore, the processed OFDM discrete baseband signal is expressed as:

[0053]

[0054] Where r ′ (p) is the processed OFDM discrete baseband signal, r(p) is the OFDM discrete baseband signal, is the estimated value of the carrier frequency error, is the sampling time.

[0055] Furthermore, before calculating the adjacent subcarrier phase difference, the method further includes:

[0056] The processed OFDM discrete baseband signal is demodulated using a discrete Fourier transform method to convert the processed OFDM discrete baseband signal into an OFDM frequency domain signal, wherein the OFDM frequency domain signal is represented as:

[0057]

[0058] Where r ′(k) is the OFDM frequency domain signal, N s is the number of subcarriers, r ′ (p) is the processed OFDM discrete baseband signal, is the rotation factor of the discrete Fourier transform.

[0059] Furthermore, the step of calculating the distance between subway trains includes:

[0060] Calculate the OFDM frequency domain signal subcarrier phase, where the OFDM frequency domain signal subcarrier phase is expressed as:

[0061]

[0062] Where, is the OFDM frequency domain signal subcarrier phase, r ′ (k) is the OFDM frequency domain signal, x(k) is the copy of the transmitted ranging symbol sequence saved by the receiving end;

[0063] Calculate the initial phase difference of adjacent subcarriers of the OFDM frequency domain signal, and the calculation expression is:

[0064]

[0065] Where, is the phase difference between adjacent subcarriers, N s is the number of subcarriers;

[0066] The initial phase difference is restricted to [-π,π], and the final phase difference is obtained, which is expressed as:

[0067]

[0068] The distance between subway trains is calculated based on the final phase difference, and the distance is expressed as

[0069]

[0070] Where, is the distance, c is the speed of light, and Δf is the subcarrier spacing.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] (1) The present invention performs signal down-conversion and sampling processing on the OFDM radio frequency signal containing the ranging symbol, and obtains an OFDM discrete baseband signal with low sensitivity to noise and interference and strong anti-interference ability. Thereafter, the ranging symbol in the signal is determined by obtaining the symbol position of the ranging symbol, and then the carrier frequency error is estimated and compensated by combining the OFDM discrete baseband signal, thereby improving the ranging accuracy of the subway train.

[0073] (2) The present invention uses the advantage that the phase difference between adjacent subcarriers tends to be constant under a certain time delay to estimate the straight-line distance between the transmitting and receiving ends, and reduces the interference of phase noise by increasing the number of subcarriers used for ranging. The frequency domain phase method is used to estimate the distance, which can ignore the resolution limitation of the signal bandwidth and achieve accurate distance measurement.

[0074] (3) The present invention suppresses multipath noise by averaging the frequency domain phase difference, and dynamically compensates for Doppler frequency deviation by combining the time domain phase difference, and uses existing communication signals to achieve centimeter-level ranging accuracy, avoiding complex hardware modifications and additional trackside equipment. It can be used in existing communication systems without the need for additional trackside equipment, thus reducing equipment costs. In addition, the present invention can achieve sub-meter-level positioning accuracy in subway tunnel scenarios through the integrated design of communication and positioning, providing highly reliable position perception for subway train control systems.

[0075] (4) The present invention does not require additional signal frequency bands and can effectively prevent frequency band interference in the tunnel. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 Schematic diagram of the method flow of the present invention;

[0077] Figure 2 This is a diagram of an IQ demodulation device of the present invention;

[0078] Figure 3 This is a time domain phase error diagram of an OFDM discrete baseband signal under carrier frequency error of the present invention;

[0079] Figure 4 A subcarrier phase diagram of an OFDM discrete baseband signal with time delay according to the present invention;

[0080] Figure 5 This is a graph showing the subcarrier phase ranging performance under different signal-to-noise ratios of the present invention;

[0081] Figure 6 This is a 3D model diagram of the tunnel of the present invention;

[0082] Figure 7 Schematic diagram of the placement of the transmitting and receiving antennas of the present invention, wherein (a) is a cross-sectional view of the tunnel and (b) is a top view of the tunnel. DETAILED DESCRIPTION

[0083] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0084] Example 1

[0085] This embodiment provides a subway train ranging method based on OFDM signal subcarrier phase. In this method, the transmitting antenna is placed on the train roof and the receiving antenna is placed on the tunnel wall. Figure 6-Figure 7 As shown, the OFDM system is deployed sequentially along the tunnel wall. The transmitting antenna continuously transmits an OFDM radio frequency signal containing ranging resources. After receiving the OFDM radio frequency signal, the receiving antenna downconverts and samples it to obtain an OFDM discrete baseband signal. Correlation operations are used to locate the symbol position of the ranging resource. The time-domain phase difference between adjacent ranging symbols is used to determine the magnitude of the carrier frequency error. Phase compensation is performed on the OFDM discrete baseband signal to offset the impact of the carrier frequency error. Subsequently, a discrete Fourier transform is performed to obtain an OFDM frequency-domain signal. After the modulated data is offset using a local sequence, the phase difference between adjacent subcarriers is used to estimate the distance between the transmitter and receiver, enabling train distance measurement.

[0086] Specifically, if Figure 1 As shown, the method includes the following steps:

[0087] S1. The train sends an OFDM signal containing ranging symbols through the onboard antenna:

[0088] The train sends OFDM radio frequency signals through the onboard antenna. The number of subcarriers of the OFDM radio frequency signal is N. s , the subcarrier spacing is Δf, and the carrier frequency is f c The ranging symbol occupies the entire bandwidth in the frequency domain and one OFDM symbol in the time domain. It is sent periodically, and the sequence modulated on the subcarrier is an autocorrelation sequence known to the transmitter and receiver.

[0089] Detailed process of transmitting OFDM radio frequency signals in S1:

[0090] (1) OFDM RF signal generation

[0091] The OFDM radio frequency signal can be expressed as:

[0092]

[0093] Among them, s(t) is the OFDM radio frequency signal, N s is the number of subcarriers, f c is the carrier frequency, X n Indicates the sequence modulated on the nth subcarrier. For existing OFDM communication systems, the number of subcarriers is usually 2048 or 4096.

[0094] The OFDM ranging symbol is contained in the OFDM radio frequency signal and can be expressed as:

[0095]

[0096] Among them, sm (t) represents the mth OFDM ranging symbol sent, x(n) represents the sequence modulated on the nth subcarrier in the ranging symbol, which is a strong autocorrelation sequence, Δf is the subcarrier spacing, N s is the number of subcarriers, T s is the transmission cycle time of the ranging symbol in the time domain, g(t) is a rectangular pulse, and M represents the number of ranging symbols transmitted.

[0097] (2) Selection of the transmission period of ranging symbols:

[0098] For high-speed train scenarios, it is necessary to ensure that the channel state of the symbols used for ranging remains essentially unchanged within a transmission cycle. The faster the train moves, the shorter the channel stabilization time, so it is necessary to consider the coherence time of the signal in ultra-high-speed mobile scenarios. In modern digital communications, the formula for calculating coherence time is defined as:

[0099]

[0100] where f m is the maximum Doppler shift. For digital signal systems, as long as the transmission period is less than T c , that is, T s <T c , the channel will not be distorted by terminal movement. For the 2.6GHz communication band, subway speeds reach 160km / h, and the maximum Doppler shift is approximately 1.1ms. Therefore, the measurement period can be selected as 0.5ms or 1ms.

[0101] S2. Use leaky cables or micro base stations to receive OFDM signals.

[0102] Detailed representation of the received OFDM radio frequency signal in S2:

[0103] During the transmission of wireless signals in a tunnel, they often experience reflection or scattering from the tunnel wall before reaching the receiving antenna. Therefore, the received OFDM radio frequency signal is the result of multipath superposition. Assume that there are L multipaths at the receiving end, and assume that the attenuation coefficient and path delay corresponding to the lth multipath are α l and τ l .

[0104] In addition to the effects of multipath, the received OFDM RF signal also suffers from carrier frequency offset (CFO) error, which represents the frequency error between the receiver oscillator and the transmitter. In subway tunnel scenarios, the high-speed movement of the subway inevitably introduces the Doppler effect, causing carrier frequency mismatches between the transmitter and receiver. This error further affects the baseband signal after downconversion. Furthermore, the receiver is also affected by spatial thermal noise n(t). Taking all of the above error factors into account, the OFDM RF signal at the receiver can be expressed as:

[0105]

[0106] Among them, r RF (t) is the received OFDM radio frequency signal, f ε is the carrier frequency error, and n(t) is the ambient Gaussian noise.

[0107] S3, down-convert and sample the received RF signal to obtain a discrete baseband signal:

[0108] The local oscillator at the receiving end generates two frequencies of f c , the local carrier with a phase difference of 90°, the received RF signal is multiplied by the two local oscillators respectively, and the high-frequency components are removed by low-pass filters to obtain the OFDM baseband signal. The OFDM baseband signal is sampled, and the sampling clock frequency is N s Δf, and obtain the OFDM discrete baseband signal.

[0109] Detailed process of signal down-conversion and sampling in S3:

[0110] (1) Down-conversion of OFDM RF signals

[0111] The local oscillator at the receiving end generates two frequencies of f c , the local carrier with a phase difference of 90°, the received OFDM radio frequency signal is multiplied by the two local oscillators respectively:

[0112] r I (t) = r RF (t)·cos(2πf c t)

[0113] r Q (t) = r RF (t)·sin(2πf c t)

[0114] Use low-pass filter to remove r I (t) and r Q The high-frequency components of (t), such as Figure 2 As shown, the OFDM baseband signal is obtained, which can be expressed as:

[0115]

[0116] (2) OFDM baseband signal sampling

[0117] Sample the OFDM baseband signal with a sampling frequency of N s Δf, the sampling time is Get the discrete signal:

[0118]

[0119] S4. Determine the symbol position of the ranging symbol by cross-correlation operation:

[0120] By utilizing the autocorrelation characteristics of the ranging symbol, the symbol position of the ranging symbol can be determined by performing a cross-correlation operation between the local sequence and the OFDM baseband discrete signal obtained above.

[0121] The symbol position is determined by the cross-correlation algorithm, which can be expressed as:

[0122]

[0123] Among them, R(m) is the cross-correlation function, x * (p) is the complex conjugate of the copy of the transmitted ranging symbol sequence stored at the receiving end, m represents the displacement, and L is the length of the received sequence. The symbol position is given by the following formula:

[0124]

[0125] That is, after moving m points, the cross-correlation function reaches a peak value, which represents the starting point of the ranging symbol.

[0126] S5. Estimation of time-domain phase change of ranging symbols and compensation of carrier error:

[0127] Imperfect oscillator hardware and Doppler shift caused by high-speed trains can lead to carrier frequency mismatches between the transmitter and receiver. This error persists in the baseband signal after the OFDM RF signal is down-converted, affecting the orthogonality between subcarriers. OFDM baseband discrete signals with carrier errors in the time domain experience additional time-domain phase rotation, which accumulates over time. The carrier frequency error is estimated by using the phase variations of ranging symbols in adjacent time domain cycles. Phase compensation is then performed in the time domain to produce the processed OFDM baseband discrete signal.

[0128] Detailed process of carrier frequency error estimation in S5:

[0129] Compared with the ideal case, the OFDM baseband discrete signal with carrier frequency error will have additional time domain phase rotation, and this phase will accumulate over time. The slope of the phase difference and time change is directly related to the error normalization coefficient and increases with the increase of the coefficient.

[0130] However, when channel state information is unknown, it is not possible to directly calculate phase rotation using time-domain phase differentiation. Therefore, this paper uses the time-domain phases of adjacent repeated ranging symbols for error estimation. For block fading channels, we can assume that the channel environment remains consistent within a measurement period that is less than the signal coherence time. The ranging symbols in adjacent periods are denoted as r1(p) and r2(p), respectively.

[0131]

[0132] The phase difference between adjacent symbols satisfies:

[0133] angle[r2(p)]-angle[r1(p)]≈2πf ε T S

[0134] Here, angle[·] represents the phase of the complex signal to be solved.

[0135] There are N ranging symbols s sampling points can be used to calculate the phase difference, and the estimated carrier frequency error is:

[0136]

[0137] The OFDM discrete baseband signal is phase compensated using the estimated carrier frequency error to obtain the processed OFDM discrete baseband signal r ＇ (p):

[0138]

[0139] S6. Discrete Fourier transform is converted into frequency domain sequence:

[0140] The processed OFDM discrete baseband signal is converted into an OFDM frequency domain signal using discrete Fourier transform (FFT).

[0141] Detailed process of OFDM signal demodulation in S6:

[0142] Perform Fourier transform on the compensated OFDM discrete baseband signal to convert it into frequency domain signal, and get r ＇ (k):

[0143]

[0144] S7. Calculate the distance between the transmitter and receiver by the frequency phase difference of adjacent subcarriers:

[0145] The local sequence is used to offset the modulated data information on the frequency domain signal subcarrier to obtain the frequency domain phase information. The transmission delay is calculated based on the frequency phase difference between adjacent subcarriers in the continuous subcarrier.

[0146] Detailed process of arrival delay estimation in S7:

[0147] The Fourier transform properties indicate that signal offsets in the time domain manifest as phase rotations in the frequency domain. For ranging symbols, after completing the FFT at the receiver, the resulting frequency domain signal is offset by the transmitted data sequence, yielding pure phase information. Using subcarrier phase for ranging can mitigate the resolution limitations imposed by signal bandwidth. However, high-position subcarriers may experience full-cycle phase ambiguity.

[0148] Therefore, the phase information on the subcarrier cannot be directly used for delay estimation. To avoid the phase ambiguity of the entire subcarrier cycle, the idea of ​​virtual carrier is referred to and the transmission delay is estimated in the form of subcarrier phase difference. The detailed steps are as follows:

[0149] (1) Calculate the subcarrier phase of OFDM frequency domain signal

[0150] The subcarrier phase of the OFDM frequency domain signal can be expressed as:

[0151]

[0152] Where x(k) is a copy of the transmitted ranging symbol sequence saved by the receiving end.

[0153] (2) Calculate the phase difference between adjacent subcarriers of OFDM frequency domain signal

[0154] Phase difference of adjacent subcarriers of OFDM frequency domain signal It can be expressed as:

[0155]

[0156] in, and They represent the phase information of the kth subcarrier and the k-1th subcarrier respectively.

[0157] In order to limit the phase change value of adjacent subcarriers to [-π,π], the differential phase can be modified by the following method:

[0158]

[0159] (3) Estimated distance

[0160] The estimated distance can be expressed as:

[0161]

[0162] in, is the estimated distance, and c is the speed of light.

[0163] The above method is tested in this embodiment using a subway train in a tunnel as an example. Figure 3 The simulation shown simulates the phase error of the time domain signal when there is a carrier frequency error (the error is 100Hz), that is, the phase difference between the signal with the error and the time domain signal without the error, which shows that the time domain phase will increase when there is a carrier frequency error. Figure 4 The figure shows the subcarrier phase diagram of the OFDM discrete baseband signal with time delay, indicating that the rotation angle of the subcarrier phase is different under a certain time delay of the OFDM signal. Figure 5 The following are the simulation results of subcarrier phase ranging performance under different signal-to-noise ratios. The experimental parameters include 2048 OFDM subcarriers and a subcarrier spacing of 30 kHz. By analyzing the ranging error under different signal-to-noise ratio environments, including the existence of carrier frequency error and the results of compensation after estimating the carrier error, it is found that the error is significantly reduced after compensation.

[0164] Example 2

[0165] This embodiment provides a subway train ranging system based on the subcarrier phase of an OFDM signal. The system includes an onboard signal generator, a signal receiver, a downconversion module, an analog-to-digital converter, a frequency error processing module, a signal demodulation module, and a ranging module. The system is characterized in that the signal generator generates and transmits the aforementioned OFDM radio frequency signal. The signal receiver receives the OFDM radio frequency signal. The downconversion module downconverts the OFDM radio frequency signal, and the analog-to-digital converter samples the OFDM baseband signal to obtain an OFDM baseband discrete signal. The frequency error processing module determines whether a frequency error exists and, if so, performs time-domain phase compensation to offset the frequency error. The signal demodulation module performs discrete Fourier transform of the OFDM baseband discrete signal, and the ranging module performs frequency-domain phase extraction and phase difference ranging of the OFDM baseband discrete signal.

[0166] The rest is the same as in Example 1.

[0167] If the above functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for enabling a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disk.

[0168] It will be understood by those skilled in the art that the embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of the present invention may be implemented in various computer languages, for example, the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0169] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowcharts and / or block diagrams, as well as combinations of processes and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowcharts and / or block diagrams. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0170] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0171] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0172] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0173] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A subway train ranging method based on OFDM signal subcarrier phase, characterized in that: The following steps are involved: receiving an OFDM radio frequency signal including a ranging symbol; Performing signal down-conversion and sampling processing on the OFDM radio frequency signal containing the ranging symbol to obtain an OFDM discrete baseband signal; According to the autocorrelation characteristics of the ranging symbol, a cross-correlation operation is performed using the OFDM radio frequency signal containing the ranging symbol and the OFDM discrete baseband signal to obtain the symbol position of the ranging symbol; Extracting a ranging symbol based on the symbol position of the ranging symbol, estimating a carrier frequency error in combination with the OFDM discrete baseband signal, and performing phase compensation to obtain a processed OFDM discrete baseband signal; Based on the processed OFDM discrete baseband signal, the phase difference between adjacent subcarriers is calculated, and the distance between subway trains is further calculated.

2. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: The OFDM radio frequency signal containing the ranging symbol is expressed as: Where r RF (t) is the received OFDM radio frequency signal containing the ranging symbol, f c is the carrier frequency, f ε is the carrier frequency error, L is the number of paths, α l and τ l are the attenuation coefficient and path delay corresponding to the lth multipath, n(t) is the ambient Gaussian noise, and s() is the transmitted OFDM ranging symbol.

3. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: The step of receiving an OFDM radio frequency signal including a ranging symbol comprises: The initial OFDM radio frequency signal sent by the transmitter is defined as: Where s(t) is the initial OFDM radio frequency signal sent by the transmitter, f c is the carrier frequency, N s is the number of subcarriers, X n is the modulated sequence on the nth subcarrier, f n =nΔf is the center frequency of the nth subcarrier, g(t) is the rectangular pulse function; A transmission period of a ranging symbol is selected, wherein the transmission period of the ranging symbol satisfies: T s <T c Where, T c is the coherence time, f m is the maximum Doppler shift; The ranging symbol is included in the initial OFDM radio frequency signal, and the ranging symbol is represented as: Where s m (t) is the mth OFDM ranging symbol sent, M is the number of ranging symbols sent, x(n) is the sequence modulated on the nth subcarrier in the ranging symbol, which is a strong autocorrelation sequence, Δf is the subcarrier spacing, T s The periodic time for transmitting ranging symbols in the time domain; The receiving end receives the initial OFDM radio frequency signal containing the ranging symbol by superposition of multipath signals, and obtains the final OFDM radio frequency signal containing the ranging symbol.

4. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: The step of signal down-conversion processing comprises: According to the local oscillation in the signal receiving process, two carrier frequencies are generated: c , a local carrier with a phase difference of 90°, multiplying the OFDM radio frequency signal containing the ranging symbol with the two local oscillators respectively to obtain: r I (t)=r RF (t)·cos(2πf c t) r Q (t)=r RF (t)·sin(2πf c t) Where r I (t), r Q (t) is the I-path signal and Q-path signal extracted from the received signal, r RF (t) is the OFDM radio frequency signal containing the ranging symbol; Use low-pass filter to remove r I (t) and r Q The high-frequency component of (t) is used to obtain the OFDM baseband signal, which is expressed as: Where r(t) is the OFDM baseband signal, f ε is the carrier frequency error, L is the number of paths, α l and τ l are the attenuation coefficient and path delay corresponding to the lth multipath, n(t) is the ambient Gaussian noise, and s() is the transmitted OFDM ranging symbol.

5. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: The OFDM discrete baseband signal is expressed as: Where r(p) is the OFDM discrete baseband signal, f ε is the carrier frequency error, N s Δf is the sampling frequency, is the sampling time, τ l is the path delay corresponding to the lth multipath, L is the number of paths, s() is the transmitted OFDM ranging symbol, and n(p) is the environmental noise.

6. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: The symbol position of the ranging symbol is expressed as: Among them, R(m) is the cross-correlation function, which is expressed as: Where, is the symbol position of the ranging symbol, m represents the displacement, r(p) is the OFDM discrete baseband signal, x * (p) is the complex conjugate of the copy of the OFDM radio frequency signal sequence containing the ranging symbol stored at the receiving end, and L is the length of the received sequence.

7. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: The step of estimating the carrier frequency error comprises: The phase difference between the ranging symbols of adjacent periods is calculated according to the ranging symbols in the OFDM discrete baseband signal, wherein the calculation expression of the phase difference is: angle[r2(p)]-angle[r1(p)]≈2πf ε T S Where angle[·] is the phase of the solution signal, r1(p) and r2(p) are the ranging symbols of adjacent cycles, and f ε is the carrier frequency error, T s is the transmission cycle time of the ranging symbol in the time domain, is the sampling time, L is the number of paths, τ l is the path delay corresponding to the lth multipath, s() is the transmitted OFDM ranging symbol, and n(p) is the environmental noise; Based on the phase difference, the carrier frequency error estimate is calculated as follows: Where, is the estimated value of carrier frequency error, Δf is the subcarrier spacing, N s is the number of sampling points in the ranging symbol.

8. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: The processed OFDM discrete baseband signal is expressed as: Where r ′ (p) is the processed OFDM discrete baseband signal, r(p) is the OFDM discrete baseband signal, is the estimated value of the carrier frequency error, is the sampling time.

9. The subway train ranging method based on OFDM signal subcarrier phase according to claim 1, characterized in that: Before calculating the adjacent subcarrier phase difference, the method further includes: The processed OFDM discrete baseband signal is demodulated using a discrete Fourier transform method to convert the processed OFDM discrete baseband signal into an OFDM frequency domain signal, wherein the OFDM frequency domain signal is represented as: Where r ′ (k) is the OFDM frequency domain signal, N s is the number of subcarriers, r ′ (p) is the processed OFDM discrete baseband signal, is the rotation factor of the discrete Fourier transform.

10. The subway train ranging method based on OFDM signal subcarrier phase according to claim 9, characterized in that: The step of calculating the distance between subway trains comprises: Calculate the OFDM frequency domain signal subcarrier phase, where the OFDM frequency domain signal subcarrier phase is expressed as: Where, is the OFDM frequency domain signal subcarrier phase, r ′ (k) is the OFDM frequency domain signal, x(k) is the copy of the transmitted ranging symbol sequence saved by the receiving end; Calculate the initial phase difference of adjacent subcarriers of the OFDM frequency domain signal, and the calculation expression is: Where, is the phase difference between adjacent subcarriers, N s is the number of subcarriers; The initial phase difference is restricted to [-π,π], and the final phase difference is obtained, which is expressed as: The distance between subway trains is calculated based on the final phase difference, and the distance is expressed as Where, is the distance, c is the speed of light, and Δf is the subcarrier spacing.

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