Self-adaptive Chirp modulation method suitable for troposphere scatter communication

Abstract

The invention relates to a self-adaptive Chirp modulation method suitable for troposphere scatter communication, which is specifically implemented according to the following steps of: 1, establishing a troposphere scatter channel model by adopting a tapped delay line model and the like according to the fading characteristic of a troposphere scatter channel; 2, analyzing physical layer parameters according to the troposphere scattering channel model; step 3, through an adaptive modulation coding system, the transmitting end selects an optimal Chirp-BOK modulation or Chirp-BPSK modulation mode based on CSI information fed back by the receiving end, and receiving and feedback of an information sequence are completed at the receiving end; and 4, constructing a corresponding matched filter, and demodulating the obtained signal. According to the invention, the availability and reliability of the system are improved.

Description

Technical Field

[0001] The invention relates to the technical field of wireless communications, and in particular to an adaptive Chirp modulation method suitable for tropospheric scatter communications. Background Art

[0002] Tropospheric scatter communication is a method of over-the-horizon communication that uses the scattering effect of the medium in the atmosphere to propagate signals. Scattering communication has the advantages of large transmission capacity, long single-hop transmission distance, and strong anti-interference ability, and is widely used in military communications and emergency communications. Scattering channels are frequency-selective fading channels, which face adverse factors such as complex signal changes and multipath effects. Tropospheric scattering communication has fast fading characteristics, which is manifested as the change of the received signal strength after transmission through the scattering channel in seconds or shorter time, and the signal-to-noise ratio of the received signal also changes continuously. In this time-varying channel, the use of adaptive coding modulation can enhance the reliability of transmission. Linear frequency modulation is a spread spectrum modulation technology that does not require a pseudo-random coding sequence. Linear frequency modulation signals are also called Chirp signals. The bandwidth they occupy is much larger than the information bandwidth, so they can obtain a larger system processing gain. Chirp signals can significantly reduce the impact of multipath interference, and at the same time have the advantages of strong resistance to deep fading and frequency deviation. Summary of the invention

[0003] The purpose of the present invention is to take advantage of the multipath effect of tropospheric scatter channels, make full use of the advantages of linear frequency modulation, and propose an adaptive Chirp modulation method suitable for tropospheric scatter communication, which has the characteristics of high system availability.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] An adaptive Chirp modulation method suitable for tropospheric scatter communication is implemented in the following steps:

[0006] Step 1: According to the fading characteristics of the tropospheric scatter channel, a tapped delay line model is used to establish a tropospheric scatter channel model;

[0007] Step 2: Analyze physical layer parameters according to the tropospheric scatter channel model;

[0008] Step 3: Through the adaptive modulation and coding system, the transmitter selects the optimal Chirp-BOK modulation or Chirp-BPSK modulation based on the CSI information fed back by the receiver, and completes the reception and feedback of the information sequence at the receiver;

[0009] Step 4: Construct a corresponding matched filter and demodulate the obtained signal.

[0010] The present invention is also characterized in that: in step 1, a tapped delay line model (TDL) is used to model the tropospheric scatter channel, and the polynomial expression is:

[0011]

[0012] The tropospheric scatter channel model includes a tap line spacing T that affects a constant gain of the channel; It is expressed as the channel coefficient, and N represents the number of taps of the TDL model, that is, the number of multipaths in the scattering channel.

[0013] The present invention is also characterized in that: in step 2, the physical layer parameters include the number of channel paths and the power attenuation of each path.

[0014] The present invention is also characterized in that: in step 3, the Chirp signal time domain expression is:

[0015]

[0016] Where a(t) represents the Chirp signal envelope, usually a rectangular pulse, and T represents the pulse width; represents the phase of the Chirp signal; f0 represents the initial frequency; μ represents the linear frequency modulation slope of the Chirp signal; μ>0 represents up-sweep, and the frequency of the Chirp signal increases linearly during the sweep time. At this time, the Chirp signal is called an Up-Chirp signal, otherwise it is called a Down-Chirp signal.

[0017] The Chirp signal is uniquely determined by the initial frequency f0, the end frequency f1 and the symbol duration T:

[0018]

[0019] Where B represents the frequency modulation bandwidth of the Chirp signal, and the instantaneous frequency of the Chirp signal is:

[0020]

[0021] The Chirp-BOK modulation method described in step 3 is: at the transmitting end, Chirp-BOK modulation uses different sweep frequencies to represent different code elements within the symbol duration, the Up-Chirp signal represents the code element "1", and the Down-Chirp signal represents the code element "0", and the two Chirp signals corresponding to 1 and 0 are matched with each other. Mutually orthogonal waveforms represent different code elements, which can effectively suppress inter-symbol interference, but it is difficult to find completely mutually orthogonal waveforms in practical applications, and usually approximately orthogonal waveforms are used instead. At the receiving end, the autocorrelation and cross-correlation characteristics of the Chirp signal can be used to effectively demodulate the signal. Specifically, the same Chirp signal will produce a larger autocorrelation value during autocorrelation, while different Chirp signals are approximately orthogonal and have smaller autocorrelation values. Specifically, the receiving end can make a decoding decision as long as it uses the corresponding Chirp signal for matched filtering.

[0022] The Chirp-BPSK modulation method described in step 3 is: generate a Chirp signal of fixed length according to parameter configuration, including an Up-Chirp signal and a Down-Chirp signal; secondly, convert the original transmission signal into a BPSK signal, multiply the BPSK signal with the Up-Chirp signal to generate a modulation signal, and then modulate it into a radio frequency signal through the transmitting module for transmission. The receiving end link radio frequency receiving module converts the radio frequency signal into a baseband signal, multiplies the baseband signal with the Down-Chirp signal to obtain a despread signal, and finally inputs the despread data into the demodulation module for demodulation processing.

[0023] The adaptive modulation and coding system described in step 3 includes a signal-to-noise ratio estimation module, an adaptive modulation and coding module, and a control algorithm for the modulation and coding method at the transmitter. In the signal-to-noise ratio estimation module, the adaptive system selects the best modulation and coding method for this transmission based on the channel state information CSI; in the adaptive modulation and coding module, the transmitter selects the best modulation and coding method based on the CSI information, and different modulation and coding methods are defined by threshold values, with the system bit error rate as a measurement value; in the selection of the control algorithm for the modulation and coding method at the transmitter, the transmitter obtains the channel ratio estimation information, compares it with the preset threshold, and determines the modulation and coding method used for transmission based on the comparison result.

[0024] The transmitting end of the present invention first generates bit information, then performs coding and modulation mapping, and finally outputs it in frames. After channel transmission, the receiving end estimates the channel and obtains channel quality status information (CSI). This information is returned to the transmitting end through the feedback channel established at both ends of the transmitting and receiving ends. The transmitting end adaptively adjusts the code rate and modulation parameters according to the feedback channel characteristics. The receiving end performs CSI detection and feedback of the scattering channel, and performs soft demodulation and decoding soft decision output processing on the received data information.

[0025] The beneficial effect of the present invention is that a Chirp coding modulation method suitable for tropospheric scatter communication proposed by the present invention can adaptively adjust the coding rate and modulation parameters according to the channel quality status information fed back by the feedback channel established by the transmitting and receiving ends, thereby improving the reliability of system transmission.

[0026] The present invention establishes an adaptive coding modulation transmission model of a scattering communication system, estimates the signal-to-noise ratio of the receiving end, and informs the transmitting end of the signal-to-noise ratio in real time. Different signal-to-noise ratio thresholds correspond to different transmitting end coding modulation modes. The transmitting end sets the sending coding modulation mode according to the predicted signal-to-noise ratio, thereby improving the reliability of the transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is an overall system block diagram.

[0028] Figure 2 The present invention is a block diagram of a Chirp-BOK modulation and demodulation method for tropospheric scatter communications.

[0029] Figure 3 The present invention is a block diagram of a Chirp-BPSK modulation and demodulation method for tropospheric scatter communication. DETAILED DESCRIPTION

[0030] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] The present invention provides an adaptive coded Chirp modulation method for tropospheric scatter communication, such as Figure 1 As shown, the specific implementation steps are as follows:

[0032] Step 1: According to the fading characteristics of the tropospheric scatter channel, a tapped equal delay line (TDL) model is used to establish a tropospheric scatter channel model, which is expressed as follows:

[0033]

[0034] 1) The tap line spacing T affects the constant gain of the channel and can be the inverse of the channel bandwidth at most.

[0035] 2) It is expressed as the channel coefficient, and N represents the number of taps of the TDL model, that is,

[0036] The number of multipaths in a scattering channel.

[0037] Step 2: Analyze physical layer parameters according to the tropospheric scatter channel model; the physical layer parameters include the number of channel paths and the power attenuation of each path. The parameter settings are shown in Table 1.

[0038] Table 1

[0039] parameter value Number of tropospheric scatter channel paths 7 Path power attenuation (dB) [0-3-7-12-17-23-29] Delay of each path (ns) [0123456]×T

[0040] Step 3: Through the adaptive modulation and coding system, the transmitter selects the optimal Chirp-BOK modulation or Chirp-BPSK modulation based on the CSI information fed back by the receiver, and completes the reception and feedback of the information sequence at the receiver.

[0041] Chirp is a spread spectrum modulation technology that does not require a pseudo-random coding sequence. Chirp signals are also called linear frequency modulation signals, which refer to signals whose instantaneous frequency changes linearly with time. Chirp signals can significantly reduce the impact of multipath interference and Doppler interference.

[0042] The time domain expression of Chirp signal is:

[0043]

[0044] a(t) represents the Chirp signal envelope, usually a rectangular pulse, T b Indicates pulse width; represents the phase of the Chirp signal; f0 represents the initial frequency; μ represents the linear frequency modulation slope of the Chirp signal; μ>0 represents up-sweep, and the frequency of the Chirp signal increases linearly during the sweep time. At this time, the Chirp signal is called an Up-Chirp signal, otherwise it is called a Down-Chirp signal.

[0045] The Chirp-BOK modulation method is as follows: Chirp-BOK modulation uses different frequency sweeps to represent different code elements within the symbol duration, the Up-Chirp signal represents the code element "1", and the Down-Chirp signal represents the code element "0", and the two Chirp signals corresponding to 1 and 0 are matched with each other. Mutually orthogonal waveforms represent different code elements, which can effectively suppress inter-symbol interference, but in practical applications it is difficult to find completely mutually orthogonal waveforms, and usually approximately orthogonal waveforms are used instead. At the receiving end, the autocorrelation and cross-correlation characteristics of the Chirp signal can be used to effectively demodulate the signal. The same Chirp signal will produce a larger autocorrelation value when autocorrelated, while different Chirp signals are approximately orthogonal and have smaller autocorrelation values. Specifically, the receiving end can make a decoding decision as long as it uses the corresponding Chirp signal for matched filtering processing.

[0046] The Chirp-BPSK modulation method is as follows: a Chirp signal of fixed length is generated according to parameter configuration, including an Up-Chirp signal and a Down-Chirp signal. Secondly, the original transmission signal is converted into a BPSK signal, and then the BPSK signal is multiplied by the Up-Chirp signal to generate a modulation signal, which is then modulated into a radio frequency signal by a transmitting module and transmitted. The receiving end link radio frequency receiving module converts the radio frequency signal into a baseband signal, and the baseband signal is multiplied by the Down-Chirp signal to obtain a despread signal, and finally the despread data is input into a demodulation module for demodulation processing.

[0047] In the adaptive modulation and coding system, in order to achieve adaptive transmission of the current link, the transmitter needs to obtain channel quality status information (CSI) from the feedback channel. The transmitter obtains the CSI from the feedback channel and compares it with the optimal switching threshold corresponding to the modulation and coding to select the modulation and coding method most suitable for the current channel.

[0048] The signal-to-noise ratio interval of the adaptive modulation and coding system is divided into n snr +1 subinterval, assuming the interval [snr i ,snr i+1 ]The corresponding optimal modulation and coding method is MCS i , when the estimated signal-to-noise ratio received by the transmitter is snr, then the optimal adaptive modulation and coding method MCS op It can be expressed as

[0049]

[0050] Among them, when snr<snr1, the signal-to-noise ratio of the scattering channel is very low, and the bit error rate of the system is very high. At this time, channel coding can no longer correct errors, and the adaptive coding modulation coding system will no longer use the modulation coding method. At this time, the system will interrupt signal transmission. When the signal-to-noise ratio is high, a coding modulation method with a higher code rate and higher parameters is set, and when the signal-to-noise ratio is low, a coding modulation method with a lower code rate and lower parameters is set.

[0051] Step 4: Construct the corresponding matched filter and demodulate the obtained signal. The Chirp signal has good performance. The matched filter of the Up-Chirp signal is the corresponding Down-Chirp signal, and vice versa. For example, the real part of the Up-Chirp signal whose envelope is a rectangular pulse, its matched filter impulse response is:

[0052] h(t)=Ccos(2πf0t-πμt 2 ),0≤t≤T b

[0053] Where C is an arbitrary non-zero constant. The output of the matched filter g(t) can be approximately expressed as:

[0054]

[0055] From formula (3), we can see that g(t) is b Get the maximum value at This value is the matched filter gain. The first zero point is obtained at t = ±1 / B, and the signal energy is concentrated in the time interval Inside.

[0056] The Chirp-BOK demodulation method is as follows: at the receiving end, a matched filter is used to demodulate the Chirp-BOK modulated signal, and the autocorrelation and cross-correlation characteristics of the Chirp signal are utilized. The autocorrelation value of the same Chirp signal is large, and the autocorrelation value of different Chirp signals is approximately orthogonal, and the autocorrelation value is small; at the receiving end, a matched filter is used to demodulate the Chirp-BOK modulated signal. Figure 2 As shown, a Chirp-BOK modulation and demodulation system block diagram is provided.

[0057] The Chirp-BPSK demodulation method is as follows: the receiving end link RF receiving module converts the RF signal into a baseband signal, multiplies the baseband signal with the Down-Chirp signal to obtain a despread signal, and finally inputs the despread data into the demodulation module for demodulation processing. Figure 3 shown.

[0058] Chirp-BPSK modulation and demodulation includes a radio frequency module, a baseband modulation module and a baseband demodulation module. The radio frequency module includes a transmitting antenna module and a receiving antenna module; the baseband modulation module includes a BPSK modulation module and an Up-Chirp signal; the baseband demodulation module includes a data demodulation module, a BPSK demodulation module and a Down-Chirp signal; the transmitting antenna module is used to transmit data information, and uses an up-converter to convert the data information into radio frequency data for transmission; the receiving antenna module is used to receive radio frequency data and convert the radio frequency data into a baseband signal; the BPSK modulation module mainly converts the original data stream to be sent into a BPSK signal. The original information to be sent includes the header information (Header), the device address (Device Addr), the packet data (Payload) and the checksum (CRC). The original data is sent to the BPSK modulation module to generate a BPSK signal, and finally the BPSK signal is multiplied by the Up-Chirp signal to realize Chirp spread spectrum modulation; the data demodulation module multiplies the data after passing through the channel with the Down-Chirp signal to complete Chirp demodulation; the BPSK demodulation module is used to demodulate the signal after Chirp demodulation.

[0059] The principle of the adaptive Chirp coding modulation method suitable for tropospheric scatter communication of the present invention is as follows Figure 1 As shown, the transmitter first performs coding and modulation mapping. After channel transmission, the receiver estimates the channel and obtains channel quality status information (CSI) which is fed back to the transmitter. The transmitter adjusts the coding rate and modulation parameters at the physical layer based on the fed-back CSI information to improve transmission reliability.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. An adaptive Chirp modulation method suitable for tropospheric scatter communication, characterized in that: Follow the steps below to implement it: Step 1: According to the fading characteristics of the tropospheric scatter channel, a tapped delay line model is used to establish a tropospheric scatter channel model; Step 2: Analyze physical layer parameters according to the tropospheric scatter channel model; Step 3: Through the adaptive modulation and coding system, the transmitter selects Chirp-BOK modulation or Chirp-BPSK modulation based on the CSI information fed back by the receiver, and completes the reception and feedback of the information sequence at the receiver; Step 4: Construct a corresponding matched filter and demodulate the obtained signal.

2. The adaptive Chirp modulation method for tropospheric scatter communication according to claim 1, characterized in that: In step 1, the tapped delay line model (TDL) is used to model the tropospheric scatter channel, and the polynomial expression is: The tropospheric scatter channel model includes a tap line spacing T that affects a constant gain of the channel; It is expressed as the channel coefficient, and N represents the number of taps of the TDL model, that is, the number of multipaths in the scattering channel.

3. The adaptive Chirp modulation method for tropospheric scatter communication according to claim 1, characterized in that: In step 2, the physical layer parameters include the number of channel paths and the power attenuation of each path.

4. The adaptive Chirp modulation method for tropospheric scatter communication according to claim 1, characterized in that: In step 3, the time domain expression of the Chirp signal is: Where a(t) represents the Chirp signal envelope, usually a rectangular pulse, and T represents the pulse width; represents the phase of the Chirp signal; f0 represents the initial frequency; μ represents the linear frequency modulation slope of the Chirp signal; μ>0 represents up-sweep, and the frequency of the Chirp signal increases linearly during the sweep time. At this time, the Chirp signal is called an Up-Chirp signal, otherwise it is called a Down-Chirp signal.

5. The adaptive Chirp modulation method for tropospheric scatter communication according to claim 4, characterized in that: The Chirp signal is uniquely determined by the initial frequency f0, the end frequency f1 and the symbol duration T: Where B represents the frequency modulation bandwidth of the Chirp signal, and the instantaneous frequency of the Chirp signal is:

6. The adaptive Chirp modulation method for tropospheric scatter communication according to claim 4, characterized in that: The Chirp-BOK modulation method described in step 3 is: at the transmitting end, Chirp-BOK modulation uses different sweep frequencies to represent different code elements within the symbol duration, the Up-Chirp signal represents the code element "1", and the Down-Chirp signal represents the code element "0", and the two Chirp signals corresponding to 1 and 0 are matched with each other; at the receiving end, the signal is demodulated by using the autocorrelation and cross-correlation characteristics of the Chirp signal.

7. The adaptive Chirp modulation method for tropospheric scatter communication according to claim 4, characterized in that: The Chirp-BPSK modulation method described in step 3 is: generate a Chirp signal of fixed length according to the parameter configuration, including an Up-Chirp signal and a Down-Chirp signal; secondly, convert the original transmission signal into a BPSK signal, multiply the BPSK signal with the Up-Chirp signal to generate a modulation signal, and then modulate it into an RF signal through the transmitting module for transmission; the receiving end link RF receiving module converts the RF signal into a baseband signal, multiplies the baseband signal with the Down-Chirp signal to obtain a despread signal, and finally inputs the despread data into the demodulation module for demodulation processing.

8. The adaptive Chirp modulation method for tropospheric scatter communication according to claim 1, characterized in that: The adaptive modulation and coding system described in step 3 includes a signal-to-noise ratio estimation module, an adaptive modulation and coding module, and a control algorithm for the modulation and coding mode of the transmitting end.

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