Radio relative positioning system based on OFDM (Orthogonal Frequency Division Multiplexing)

Abstract

The invention discloses a radio relative positioning system based on OFDM (Orthogonal Frequency Division Multiplexing). The radio relative positioning system comprises an antenna array, a radio frequency front end and a radio frequency signal processing board, the antenna array in the antenna array is an eight-array-element antenna array, the central array is used for communication transceiving, and the edge seven array elements are used as receiving array elements; the radio frequency front end comprises a receiving part and a transmitting part, wherein the receiving part comprises an anti-burning switch group, a filtering switch group, a low-noise amplifier switch group and an amplitude-phase self-correcting switch group of array signals; the OFDM-based radio relative positioning system has the beneficial effects that the OFDM-based radio relative positioning system is a system for cooperative formation flight and combat of airplanes; the system has the characteristics of formation situation awareness, formation flight collaboration, proximity prompt and the like, solves the problem that an aircraft cannot be reliably positioned under satellite rejection, can carry out dense and safe formation flight under complex weather conditions such as cloud crossing and night, effectively improves the formation flight capability of the aircraft under the satellite rejection and complex weather conditions, and improves the reliability of the aircraft. The flight safety is improved, and the working intensity of a pilot is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of avionics, and in particular relates to a radio relative positioning system based on OFDM. Background Art

[0002] With the continuous development of science and the continuous advancement of military technology, the modern battlefield environment has become increasingly complex. Therefore, it is no longer realistic to use only a single aircraft to complete the given tasks. At present, the coordination between aircraft is mainly carried out through control stations, which are coordinated by the upper-level or even several-level command organizations, and aircraft on the same front line of combat cannot achieve "face-to-face" direct information exchange, which reduces the timeliness of intelligence information exchange and affects the timeliness of coordination and cooperation, making it difficult to cope with fast-paced and highly confrontational air combat. In this context, we should focus on enhancing the real-time interactivity between aircraft, achieve more direct battlefield information interaction, combat support and action coordination, so as to achieve common formation and overall combat.

[0003] Currently, most aircraft use satellite navigation or inertial navigation systems to obtain the precise position of the aircraft during formation flight to achieve formation flight. However, the inherent fragility and limitations of satellite navigation systems mean that satellite navigation services have clear capacity deficiencies and defects. Once satellite navigation is interfered with or even paralyzed by an adversary, it will inevitably bring catastrophic consequences.

[0004] Aircraft formation flying requires reliable position information of each formation member, but in a satellite denial environment, reliable information cannot be obtained. Summary of the invention

[0005] The purpose of the present invention is to provide a radio relative positioning system based on OFDM, which integrates direction finding function and communication function, can realize networking and communication of multiple members, can realize the measurement of distance and position between members in the network, can adapt to member communication and positioning under high-speed movement, and can realize data interaction with a host computer.

[0006] To achieve the above object, the present invention provides the following technical solutions: a radio relative positioning system based on OFDM, comprising an antenna array, a radio frequency front end, and a radio frequency signal processing board;

[0007] The antenna array in the antenna array is an 8-element antenna array, the central array is used as the transmitting and receiving elements of the communication, and the 7 elements at the edge are used as the receiving elements;

[0008] The RF front end consists of two parts: receiving and transmitting. The receiving part includes array signal anti-burnout, filtering, low-noise amplification, and amplitude and phase self-calibration switch group, and the transmitting part includes power amplifier and RF switch.

[0009] The RF signal processing board includes a broadband RF transceiver array, FPGA, and DSP hardware resources. With the cooperation of software, it completes the amplitude and phase broadband correction, super-resolution two-dimensional direction finding, beam forming, and communication between members. Communication uses the communication subsystem, and super-resolution two-dimensional direction finding uses the direction finding subsystem.

[0010] The communication subsystem is used for wireless transmission and reception in this system, and adopts orthogonal frequency division multiplexing OFDM, which includes

[0011] Subcarrier: The OFDM transmitter divides the information stream into multiple symbol blocks and sends them in parallel on different subcarriers;

[0012] OFDM transmitter and OFDM receiver: They include modulation conversion from bit stream to symbol, signal mixing and filtering, and conversion between frequency domain and time domain through fast Fourier transform FFT and its inverse transform IFFT, respectively, to achieve efficient data transmission and reception;

[0013] Cyclic prefix insertion: A cyclic prefix is ​​inserted before each OFDM symbol to reduce or eliminate inter-symbol interference in a multipath environment;

[0014] The direction finding subsystem adopts the spatial smoothing MUSIC algorithm, including

[0015] Two-dimensional circular array antenna: A two-dimensional circular array consisting of seven elements, capable of measuring both elevation and azimuth angles simultaneously;

[0016] Amplitude and phase error calibration measures: including board-level dynamic amplitude and phase error calibration, RF front-end overall amplitude and phase error estimation, and external radiation source amplitude and phase error calibration.

[0017] As a preferred technical solution of the present invention, the OFDM transmitter receives a bit stream from a higher layer protocol and converts it into symbols using a selected modulation scheme.

[0018] As a preferred technical solution of the present invention, the OFDM receiver receives an input signal, mixes it with a complex conjugate copy of the original radio frequency, and filters the result, and then directs the signal to eight independent paths, mixing each of them with a complex conjugate copy of one of the original subcarriers.

[0019] As a preferred technical solution of the present invention, each subcarrier is modified by an arbitrary phase shift ψ before it reaches the receiver. To solve this problem, the OFDM transmitter adds reference symbols to the data stream with amplitude and phase defined in the relevant specifications. During the channel estimation process, the receiver measures the input reference symbols, compares them with those defined in the specifications, and uses the results to remove the phase shift of the input signal.

[0020] As a preferred technical solution of the present invention, the direction finding subsystem realizes weighted averaging of waves coming from different directions and suppresses clutter through digital beam synthesis.

[0021] As a preferred technical solution of the present invention, the calculation of beamforming weights is performed in DSP, wherein the covariance matrix used is calculated by the covariance calculation module of FPGA, and DSP performs calculation using the maximum signal-to-noise ratio MSNR criterion.

[0022] As a preferred technical solution of the present invention, the MSNR maximizes the ratio of the expected signal power to the noise power, and the data received by the receiving end array can be expressed as:

[0023] x(t)=x s (t)+x n (t)

[0024] Where: x s (t) = w H s0(t)a(θ0) is the desired signal component, s0(t) is the complex envelope of the signal, a(θ0) is the directional vector of the desired signal, x n (t) is the noise and interference component, and R s =E{x s (t) H x s (t)},R n =E{x n (t) H x n (t)}, at this time the output signal power of the beamformer is:

[0025] P s =E{|w H x s (t)| 2}=w H R s w

[0026] The output noise power is:

[0027] P n =E{|w H x n (t)| 2}=w H R n w

[0028] At this time, the output signal power to noise power ratio SNR is:

[0029]

[0030] Let the corresponding weight vector when the above formula takes the maximum value be wopt , then:

[0031]

[0032] If we do not consider interference and only consider noise, it can also be:

[0033]

[0034] Where: are the expected signal power and noise power respectively. According to Schwartz inequality, the following formula can be obtained:

[0035] w H a(θ0)a H (θ0)w≤||w|| 2 ||a(θ0)|| 2

[0036] When w=a(θ0), we get w H a(θ0)a H (θ0)w is the maximum value, at which point the array output SNR is maximum.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The OFDM-based radio relative positioning system is a system used for coordinated formation flying and combat of aircraft. It has the characteristics of formation situation awareness, formation flight coordination, and approach prompts, which solves the problem that aircraft cannot be reliably positioned under satellite denial. At the same time, the system can fly in dense and safe formations in complex weather conditions such as passing through clouds and at night, effectively improving the aircraft's formation flying capabilities under satellite denial and complex weather conditions, improving flight safety, and greatly reducing the workload of pilots. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a system architecture block diagram of the present invention;

[0040] Figure 2 It is a flowchart of the communication subsystem of the present invention;

[0041] Figure 3 OFDM is used for the present invention to divide the frequency band into a subcarrier map;

[0042] Figure 4 FIG. 1 is a processing step diagram of the OFDM transmitter of the present invention;

[0043] Figure 5 FIG. 1 is a processing step diagram of an OFDM receiver of the present invention;

[0044] Figure 6 The invention reduces the inter-symbol interference graph by transmitting on multiple sub-carriers;

[0045] Figure 7 An operation diagram for cyclic prefix insertion of the present invention;

[0046] Figure 8 is an operation diagram of a cyclic prefix on a single subcarrier of the present invention;

[0047] Fig. 9 This is a system block diagram of the direction finding subsystem of the present invention. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] See also Figure 1 ,A radio relative positioning system based on OFDM includes three parts: antenna array, RF front end and RF signal processing board, integrating direction finding function and communication function, and can realize super-resolution direction finding and real-time communication function in a variety of environments;

[0050] The antenna array in the antenna array is an 8-element antenna array, the central array is used for communication transmission and reception, and the 7 elements at the edge are used as receiving elements;

[0051] The RF front end consists of two parts: receiving and transmitting. The receiving part includes array signal anti-burnout, filtering, low-noise amplification, and amplitude and phase self-calibration switch group, and the transmitting part includes power amplifier and RF switch.

[0052] The RF signal processing board contains a wideband RF transceiver array, FPGA, and DSP hardware resources. With the cooperation of software, it can complete functions such as amplitude and phase broadband correction of members, super-resolution two-dimensional direction finding, beam forming, and communication between members.

[0053] 1. Communication subsystem:

[0054] See also Figure 2Communication subsystem flow chart, aircraft collaborative combat requires full use of intelligent advantages, different from the traditional formation, requiring real-time and ready information exchange between members within the formation, this system design is based on radio ranging and network communication technology, does not rely on satellites, can be applied to formation collaborative flight between multiple aircraft, can achieve position perception between aircraft formations, can achieve information collaborative flight coordination of formations by sending and responding to control commands; the system is based on digital radio communication technology to achieve independent self-organizing networking, this function is not limited by other sensors on the aircraft, automatically realizes the establishment of a network between multiple aircraft, and allows aircraft in the network to enter and exit the network according to their respective mission plans; after networking, the system uses the relative positions between members in the formation The system can realize the situation awareness of the formation by members within the network through high-precision and high real-time measurement, and share the members' measurement data and maneuver command information within the network through system links to realize information collaboration among formation members. The system realizes the unification of network time through high-precision time synchronization technology, and conducts network health monitoring and management on this basis. During the entire mission, the reference terminal manages the networking and denetworking processes of non-reference terminals in the system network in real time, has the ability to identify network members, and ensures the stability and reliability of network operation. The system can complete information collaboration among members within the network under networking conditions, and members within the network can broadcast the current aircraft attitude and other information that needs to be transmitted within the formation, and other members can accurately receive and display it.

[0055] One of the key technologies used for wireless transmission and reception in this system is called Orthogonal Frequency Division Multiplexing (OFDM). The principle of OFDM is:

[0056] 1)Subcarrier

[0057] like Figure 3 As shown, the OFDM transmitter needs to get the symbol block from the output information stream and send each symbol on a different radio frequency called a subcarrier. The bandwidth of each individual subcarrier is very small, so it can only support a low symbol rate. However, the subcarriers all occupy the same bandwidth as the traditional single-carrier system, and their collective symbol rate is the same if other issues remain unchanged.

[0058] In OFDM, the subcarrier spacing Δf is related to the symbol duration T on each individual subcarrier;

[0059] 2) OFDM Transmitter

[0060] Figure 4 The important parts of an OFDM transmitter are shown. The transmitter receives a bit stream from a higher layer protocol and converts it into symbols using the selected modulation scheme; for example, quadrature phase shift keying QPSK. The serial-to-parallel converter takes blocks of N symbols, 8 in this example, and directs them into N parallel substreams.

[0061] The transmitter mixes each substream with a subcarrier whose frequency is an integer multiple of 15kHz. By analogy with other modulators, it can be understood that each subcarrier acts as an intermediate frequency (IF), and its modulated signal will eventually be on the radio frequency (RF) carrier f c The subcarriers are placed at frequencies -60kHz, -45kHz, ..., +45kHz so that the RF carrier is approximately in the center of the transmit band. The symbol duration is the inverse of the subcarrier spacing, so the 15kHz subcarrier goes through one cycle during each 66.7μs symbol, while the subcarriers at 30kHz and 45kHz go through two and three cycles respectively;

[0062] Now there are eight sine waves, whose amplitude and phase represent the eight transmitted symbols; by adding these sine waves together and dividing by the scaling factor N, a single time domain waveform can be generated, which is the intermediate frequency representation of the signal we need to send. The only task left is to mix the waveform to the RF transmission. Therefore, the OFDM transmitter can be understood as a set of intermediate frequency modulators, each of which is tuned to the frequency offset of the corresponding subcarrier;

[0063] 3) OFDM Receiver

[0064] Figure 5 The following figure shows the important components of an OFDM receiver. By analogy with other demodulators, we can know how it works. The receiver receives the input signal, mixes it with a complex conjugate copy of the original radio frequency, and filters the result. It then directs the signal to eight separate paths, mixing each one with a complex conjugate copy of one of the original subcarriers, i.e. the information arriving on that subcarrier is now at zero frequency. By integrating the result for the duration of one symbol, the receiver can extract the amplitude and phase of the symbol arriving on that subcarrier, while rejecting any noise and interference.

[0065] Each subcarrier is modified by an arbitrary phase shift ψ before it reaches the receiver. To address this, the OFDM transmitter adds reference symbols to the data stream with amplitude and phase defined in the relevant specification. During the channel estimation process, the receiver measures the incoming reference symbols, compares them to those defined in the specification, and uses the result to remove the phase shift of the incoming signal.

[0066] exist Figure 5 In this paper, we assume that each subcarrier has the same phase shift of 30°. In the case of frequency-variant fading, the phase shift is a slowly varying function of frequency as well as time. To account for this problem, the reference symbols are scattered in the time and frequency domains in a certain way. The receiver can then measure the phase shift ψ as a function of frequency and can apply a different phase correction to each individual subcarrier.

[0067] After the phase removal stage, the receiver returns the symbols to their original order by means of a parallel-to-serial converter, recovering the transmitted bits. An OFDM receiver can therefore be understood as a set of intermediate frequency demodulators, each of which is tuned to the frequency offset of the corresponding subcarrier;

[0068] 4) Fast Fourier Transform

[0069] Two important issues in the receiver's processing chain are, first, after the low-pass filter, the data is the in-phase and quadrature components as a function of time, and second, after the integration stage, the data is the amplitude and phase of each subcarrier as a function of frequency. We can see that the mixing and integration steps have converted the data from a function of time to a function of frequency;

[0070] This transformation is actually the Discrete Fourier Transform DFT, and by using this technique we can hide Figure 5 Instead of the explicit mixing and integration steps shown, we can pass the time domain data through a discrete Fourier transform and extract the frequency domain data from the output. The transmitter converts the frequency domain data to time domain data in exactly the same way via an inverse discrete Fourier transform;

[0071] In turn, the discrete Fourier transform can be implemented using the Fast Fourier Transform FFT, which limits the computational load on both the transmitter and the receiver and allows both devices to be implemented in a computationally efficient way;

[0072] like Figure 5 As shown, the mixing and integration stages of the receiver can be expressed as follows:

[0073]

[0074] In formula (1), Z(f n ) is a complex number, indicating that at frequency f n The amplitude and phase of the nth output subcarrier are measured at the output of the integration stage, z(t k ) is another complex number representing the in-phase and quadrature components of the k-th input sample at time t_k, measured from the output of the low-pass filter, exp(-2πjf n t k ) describes the receiver's complex conjugate copy of the original carrier, while the sum describes the effect of the integration stage, and N is the number of samples per symbol incoming;

[0075] And t k and f n The value of is as follows, if the incoming data samples are uniformly used

[0076] Δt apart, then we can write the sampling time as follows:

[0077]

[0078] Where Δt is the sampling interval and T is the symbol duration. It is reasonable and practically correct to assume that the number of subcarriers should be equal to the number of samples per symbol. Therefore, the output of equation (1) carries the same amount of information as the input. Then, we can write the subcarrier frequency as follows, which we will discuss in the following section:

[0079]

[0080] Where Δf is the subcarrier spacing. Substituting these results into equation (1) yields the discrete Fourier transform:

[0081]

[0082] In the formula, z k = z(t k ), Z n = Z(f n ), in the same way, according to Figure 4 , we can write the mixing, adding, and scaling stages of the transmitter as follows:

[0083]

[0084] If we make the same substitutions as before, then we get the formula for the inverse discrete Fourier transform:

[0085]

[0086] As for the problem related to the selection of subcarrier frequency, in equation (2), we set the subcarrier at frequency nΔf, where n is 0 to N-1, which means that the eight subcarriers in our previous example are offset from the RF carrier by 0kHz, 15kHz, 30kHz, ..., 105kHz, which sounds similar to what we have in Figure 4 and Figure 5 The choices made are different, but in fact they are exactly the same. When eight subcarriers are used, the sampling interval of equation (1) is 8.33 μs, during which the 60 kHz subcarrier passes exactly half a cycle and is indistinguishable from the -60 kHz subcarrier passing half a cycle. With the same parameters, the subcarriers at 75 kHz, 90 kHz and 105 kHz are indistinguishable from the subcarriers at -45 kHz, -30 kHz and -15 kHz.

[0087] 5) Reduction of inter-symbol interference

[0088] High data rate transmission in a multipath environment leads to inter-symbol interference (ISI). If we share data on several different subcarriers by using OFDM, the symbol rate on each individual subcarrier is several times lower than before, so the duration of the symbol is several times that of before, which greatly reduces the degree of inter-symbol interference. Figure 6 Here is a simple example, where we have divided the original data stream between eight subcarriers, so the symbol rate on each subcarrier is now 15ksps, the symbol duration is 66.67μs, if the delay spread is kept at 3.33μs, then the symbols only overlap by 5%, which reduces the amount of ISI to 1 / 8 of the original and reduces the amount of error at the receiver. If the number of subcarriers is greater, then the ISI reduction is proportionally greater;

[0089] 6) Cyclic prefix insertion

[0090] Another technique allows us to completely eliminate intersymbol interference. The basic idea is to insert a guard interval CP before each symbol, in which no information is sent. If this guard interval is longer than the delay spread time, then the receiver can be sure that it reads information from only one symbol at a time, without any overlap with previous or following symbols.

[0091] Another slightly more complex technique is cyclic prefix insertion, see Figure 7 ,Here, the transmitter inserts a guard interval before each symbol as before, but it then copies data from the end of the following symbol in order to fill the guard interval. If the cyclic prefix is ​​longer than the delay spread time, the receiver can still be confident that it is reading information from only one symbol at a time;

[0092] Figure 8 The following is a diagram of the operation of the cyclic prefix on a single subcarrier of the present invention. The following is described in the diagram: The transmitted signal is a sine wave whose amplitude and phase vary from one symbol to the next. As mentioned above, each symbol contains an exact number of cycles of a sine wave. The amplitude and phase at the beginning of each symbol are equal to the amplitude and phase at the end. Because of this, the transmitted signal changes smoothly as we move from each cyclic prefix to the next symbol.

[0093] In a multipath environment, the receiver receives multiple copies of the transmitted signal at multiple arrival times. These are added together at the receiving antenna to produce a sine wave with the same frequency, but different amplitudes and phases. The received signal still changes smoothly from one cyclic prefix to the following symbol. There are a few minor issues, but these are only at the beginning of the cyclic prefix and the end of the symbol, where the preceding and following symbols begin to interfere.

[0094] The receiver processes the received signal in a window whose length is equal to the symbol duration and discards the rest; if the window is placed correctly, the received signal is transmitted correctly and is only subject to amplitude variations and phase offsets; but the receiver can use channel estimation techniques to compensate for these, so it can handle the cyclic prefix without any additional processing; although this system uses multiple subcarriers, the subcarriers do not interfere with each other and can be processed independently, so the presence of multiple subcarriers does not affect this statement at all.

[0095] 2. Direction Finding Subsystem Fig. 9 :

[0096] In the traditional array direction finding method, the angular resolution of the target is limited by the physical array size of the array, and it is difficult to overcome the error caused by the multipath effect, it is difficult to distinguish the incoming waves from multiple directions, and it is impossible to measure the azimuth and elevation angles at the same time;

[0097] Spatial spectrum estimation technology can realize super-resolution direction finding function, but neither MUSIC nor Capon can realize effective estimation of coherent signals. This is because when these two algorithms perform spectrum calculation, the former is through eigendecomposition of the covariance matrix, and the latter is through inversion of the covariance matrix; when coherent signals exist, the covariance matrix has rank loss, that is, the covariance matrix changes from a square matrix to a singular matrix, which makes it difficult to effectively perform eigendecomposition and inversion operations; therefore, when the received signal has multipath, the effective wave direction estimation algorithm based on coherent signals should be considered; this system uses the spatial smoothing MUSIC algorithm to realize the processing of coherent signals, and the forward spatial smoothing MUSIC method recovers the full rank covariance matrix. This is achieved by finding the mean of the covariance matrix of each sub-array. Within 0°~40°, the estimation deviation of MUSIC and Capon for signals is small, and effective estimation can be basically achieved. When multipath occurs, these two algorithms have large estimation deviations and even fail. At this time, smooth MUSIC can estimate multipath and signals more robustly. The omnidirectional low-elevation direction-finding system requires a huge amount of computation, and the hardware design needs to meet the needs of large-capacity storage devices. In terms of software, the data search compression algorithm based on the array structure is studied to greatly reduce the amount of system computation. The subspace direction-finding algorithm with multi-level search and multi-core parallel processing is used to further reduce the amount of computation and improve the processing speed, solving the problem of high-precision two-dimensional direction-finding system with large computation and low real-time performance.

[0098] The direction finding subsystem adopts a 7-element two-dimensional circular array, which has the ability of pitch and azimuth direction finding. However, due to the inconsistency of the amplitude and phase of the array antenna, the signal quality will be reduced, affecting the system's direction finding sensitivity. In order to ensure the amplitude and phase consistency to meet the overall index requirements of direction finding, this system has designed a complete set of amplitude and phase error calibration measures, including: board-level dynamic amplitude and phase error calibration, overall amplitude and phase error estimation including the RF front end, and external radiation source amplitude and phase error calibration. After correction, the amplitude and phase error disturbance is greatly reduced, and the direction finding accuracy is significantly improved, meeting the index requirements; the system uses the digital beam synthesis function to achieve weighted averaging of waves coming from different directions, suppress clutter, improve direction finding accuracy, and maintain high accuracy within the dynamic range during relatively high-speed movement; the calculation of beamforming weights is performed in DSP, and the covariance matrix used is calculated by the covariance calculation module of FPGA; then DSP uses the maximum signal-to-noise ratio MSNR criterion for calculation.

[0099] MSNR is to maximize the ratio of the expected signal power to the noise power. The data received by the receiving array can be expressed as:

[0100] x(t)=x s (t)+x n (t)

[0101] Where: x s (t) = w H s0(t)a(θ0) is the desired signal component, s0(t) is the complex envelope of the signal, a(θ0) is the directional vector of the desired signal, x n (t) is the noise and interference component, and R s =E{x s (t) H x s (t)},R n =E{x n (t) H x n (t)}, at this time the output signal power of the beamformer is:

[0102] P s =E{|w H x s (t)| 2}=w H R s w

[0103] The output noise power is:

[0104] P n =E{|w H x n (t)| 2}=w H R n w

[0105] At this time, the output signal power to noise power ratio SNR is:

[0106]

[0107] Let the corresponding weight vector when the above formula takes the maximum value be w opt , then:

[0108]

[0109] If we do not consider interference and only consider noise, it can also be:

[0110]

[0111] Where: are the expected signal power and noise power respectively. According to Schwartz inequality, the following formula can be obtained:

[0112] w H a(θ0)a H (θ0)w≤||w|| 2 ||a(θ0)|| 2

[0113] When w=a(θ0), we get w H a(θ0)a H (θ0)w is the maximum value, at which point the array output SNR is maximum.

[0114] In the direction finding subsystem, the antenna array has a horizontal omnidirectional gain of no less than -3.5dBi within a positive 60° pitch; and a 90° pitch gain of no less than -10dBi. Due to the obstruction of the airspace covered by the airborne communication antenna by the aircraft's vertical tail, wings, and fuselage, it is difficult for a single ordinary omnidirectional antenna or a broadband high-gain Quasi-Yagi end-fire antenna to meet the airspace coverage requirements, especially the airspace coverage requirements in the pitch dimension. In addition, the radiation characteristics of the omnidirectional antenna may cause strong multipath reflections in a complex installation environment, affecting the direction finding accuracy between aircraft. In view of the above practical problems, a antenna covering the horizontal plane (H plane)φ : -180°~180° angular range, elevation plane (E plane) θ: 0°~60° airspace range of wide lobe end-fire antenna, which meets the airspace coverage requirements of machine-to-machine communication, and meets the index requirements of +0°~60° elevation, the gain is not less than -3.5dB; 90° elevation gain is not less than -10dBi; at the same time, the end-fire antenna can suppress the multipath effect in machine-to-machine communication due to the characteristic that the direction of its maximum gain is located in the axial direction of the antenna. At the same time, the antenna and its feeding structure are integrated with the antenna cover, and the antenna reflector and the antenna mounting base are integrated into the design, so that the antenna has a compact appearance structure, which is conducive to installation.

[0115] In summary, the system is designed with good anti-interference performance, high communication rate, large direction finding range, high accuracy, good phase continuity, communication sensitivity up to -99dBm and ranging TOA accuracy less than 10ns, which can well meet the requirements of aircraft autonomous collaborative system.

[0116] Although the embodiments of the present invention have been shown and described, as detailed above, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A radio relative positioning system based on OFDM, characterized in that: Includes antenna array, RF front end, and RF signal processing board; The antenna array in the antenna array is an 8-element antenna array, the central array is used as the transmitting and receiving elements of the communication, and the 7 elements at the edge are used as the receiving elements; The RF front end consists of two parts: receiving and transmitting. The receiving part includes array signal anti-burnout, filtering, low-noise amplification, and amplitude and phase self-calibration switch group, and the transmitting part includes power amplifier and RF switch. The RF signal processing board includes a broadband RF transceiver array, FPGA, and DSP hardware resources. With the cooperation of software, it completes the amplitude and phase broadband correction, super-resolution two-dimensional direction finding, beam forming, and communication between members. Communication uses the communication subsystem, and super-resolution two-dimensional direction finding uses the direction finding subsystem. The communication subsystem is used for wireless transmission and reception in this system, and adopts orthogonal frequency division multiplexing OFDM, which includes Subcarrier: The OFDM transmitter divides the information stream into multiple symbol blocks and sends them in parallel on different subcarriers; OFDM transmitter and OFDM receiver: They include modulation conversion from bit stream to symbol, signal mixing and filtering, and conversion between frequency domain and time domain through fast Fourier transform FFT and its inverse transform IFFT, respectively, to achieve efficient data transmission and reception; Cyclic prefix insertion: A cyclic prefix is ​​inserted before each OFDM symbol to reduce or eliminate inter-symbol interference in a multipath environment; The direction finding subsystem adopts the spatial smoothing MUSIC algorithm, including Two-dimensional circular array antenna: A two-dimensional circular array consisting of seven elements, capable of measuring both elevation and azimuth angles simultaneously; Amplitude and phase error calibration measures: including board-level dynamic amplitude and phase error calibration, RF front-end overall amplitude and phase error estimation, and external radiation source amplitude and phase error calibration.

2. The OFDM-based radio relative positioning system according to claim 1, characterized in that: The OFDM transmitter receives a bit stream from a higher layer protocol and converts it into symbols using a selected modulation scheme.

3. The OFDM-based radio relative positioning system according to claim 1, characterized in that: The OFDM receiver receives an input signal, mixes it with a complex conjugate copy of the original radio frequency, filters the result, and then directs the signal into eight separate paths, mixing each one with a complex conjugate copy of one of the original subcarriers.

4. The OFDM-based radio relative positioning system according to claim 3, characterized in that: Each subcarrier is modified by an arbitrary phase shift ψ before it reaches the receiver. To solve this problem, the OFDM transmitter adds reference symbols to the data stream with amplitude and phase defined in the relevant specification. During the channel estimation process, the receiver measures the incoming reference symbols, compares them with those defined in the specification, and uses the result to remove the phase shift of the incoming signal.

5. The OFDM-based radio relative positioning system according to claim 1, characterized in that: The direction finding subsystem realizes weighted averaging of waves coming from different directions and suppresses clutter through digital beam synthesis.

6. The OFDM-based radio relative positioning system according to claim 5, characterized in that: The beamforming weights are calculated in the DSP, where the covariance matrix used is calculated by the covariance calculation module of the FPGA, and the DSP uses the maximum signal-to-noise ratio MSNR criterion for calculation.

7. The OFDM-based radio relative positioning system according to claim 6, characterized in that: The MSNR maximizes the ratio of the expected signal power to the noise power. The data received by the receiving array can be expressed as: x(t)=x s (t)+x n (t) Where: x s (t) = w H s0(t)a(θ0) is the desired signal component, s0(t) is the complex envelope of the signal, a(θ0) is the directional vector of the desired signal, x n (t) is the noise and interference component, and R s =E{x s (t) H x s (t)},R n =E{x n (t) H x n (t)}, at this time the output signal power of the beamformer is: P s =E{|w H x s (t)| 2 }=w H R s w The output noise power is: P n =E{|w H x n (t)| 2 }=w H R n w At this time, the output signal power to noise power ratio SNR is: Let the corresponding weight vector when the above formula takes the maximum value be w opt , then: If we do not consider interference and only consider noise, it can also be: Where: are the expected signal power and noise power respectively. According to Schwartz inequality, the following formula can be obtained: w H a(θ0)a H (θ0)w≤||w|| 2 ||a(θ0)|| 2 When w=a(θ0), we get w H a(θ0)a H (θ0)w is the maximum value, at which point the array output SNR is maximum.

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