GMSK signal receiving and demodulating method and system under complex channel conditions

Abstract

The present invention belongs to the field of communication signal processing technology, and particularly relates to a GMSK signal reception and demodulation method and system under complex channel conditions. First, a modulated signal transmitted by a receiving channel is received and matched filtering is performed on the signal. Then, parameter estimation and signal synchronization are performed on the matched filtering processed signal, and the GMSK signal demodulation signal under complex channel conditions is determined based on the GMSK signal eye diagram characteristics of different bandwidth period BT values. The eye diagram characteristics are: when one of the I / Q paths reaches the optimal sampling moment, the other path has a discrete distribution of values, and the discreteness varies with the size of the BT value. The present invention can realize the reception and demodulation of GMSK signals transmitted under complex channel conditions with strong aliasing and in-band interference, and is convenient for application in complex channel scenarios.

Description

Technical Field

[0001] The present invention belongs to the technical field of communication signal processing, and in particular relates to a GMSK signal receiving and demodulating method and system under complex channel conditions. Background Art

[0002] In recent years, atmospheric noise levels have continued to rise. Gaussian noise is superimposed with a significant amount of impulse noise, which is susceptible to seasonal and climatic factors, resulting in a low signal-to-noise ratio (SNR) in the channel. Furthermore, the transmission channel may contain multiple signals. When their carrier frequencies are similar, adjacent, or equal, they can severely interfere with the target signal, increasing the bit error rate (BER) required for demodulation and decision-making by the receiving system. Therefore, the channel environment in which signals are transmitted in practical applications is extremely complex. GMSK (Gaussian Filtered Minimum Shift Keying) modulation is a special case of continuous phase modulation technology. It exhibits excellent properties such as a constant envelope, continuous phase, concentrated power spectral density, and rapid spectral sidelobe decay. Therefore, GMSK signals have become an important target signal for non-cooperative communications, particularly in complex electromagnetic environments characterized by dense and variable interference signals, low SNR, and time-varying noise and fading.

[0003] Traditional GMSK signal reception and demodulation are primarily based on coherent detection algorithms, combined with a phase differential demodulation algorithm based on Viterbi detection and a 2-bit differential phase soft decision algorithm based on the Viterbi algorithm. Currently, a growing number of studies are working to improve these traditional demodulation algorithms, including research on GMSK signal modulation and demodulation methods for arbitrary information rates, adjustments to the calculation of Viterbi detection branch metrics, and improvements to Laurent decomposition-based suboptimal coherent detection techniques. Synchronization techniques, such as preamble-based forward synchronization algorithms and simplified open-loop synchronization algorithms based on the maximum likelihood criterion, have been proposed. By separating carrier synchronization and timing synchronization, these methods effectively improve synchronization efficiency and eliminate phase ambiguity. However, these studies primarily focus on signals transmitted in ideal white Gaussian noise channels and fail to fully consider the impact of harsh channel conditions, such as aliasing and strong in-band interference, on the demodulation algorithm. Experimental testing has shown that traditional reception and demodulation algorithms based on coherent reception and Viterbi phase path search are susceptible to time-varying noise and interference, and are not well suited to the needs of reception and demodulation processing in complex real-world channel conditions. Summary of the Invention

[0004] To this end, the present invention provides a GMSK signal reception and demodulation method and system under complex channel conditions, which can realize the reception and demodulation of GMSK signals transmitted under complex channel conditions with strong in-band interference, and is convenient for application in complex channel scenarios.

[0005] According to the design scheme provided by the present invention, a GMSK signal receiving and demodulating method under complex channel conditions is provided, which includes the following contents:

[0006] Receive the modulated signal transmitted by the channel and perform matched filtering on the signal;

[0007] Parameter estimation and signal synchronization are performed on the signal after matched filtering, and the GMSK signal demodulation signal under complex channel conditions is determined based on the GMSK signal eye diagram characteristics under different bandwidth period BT values. The eye diagram characteristics are: when one of the I / Q paths reaches the optimal sampling moment, the values of the other path have a discrete distribution, and the discreteness varies with the size of the BT value.

[0008] The present invention focuses on GMSK signals transmitted in complex electromagnetic environments, and focuses on resolving the contradiction between actual demodulation processing requirements and current GMSK signal demodulation processing technology. Specifically, the characteristics of actual complex channels, such as dense and variable interference signals, low signal-to-noise ratio, and time-varying noise and fading, make the traditional GMSK signal receiving and processing structure based on the correlation reception and Viterbi phase path search principle susceptible to time-varying noise and interference, resulting in failure to synchronize normally, excessive bit error rate of demodulation results, and weak information recovery effect. By optimizing and improving the receiving and demodulation scheme, the modulation signal recovery effect is improved.

[0009] Furthermore, based on the above method, the present invention also provides a GMSK signal receiving and demodulating system under complex channel conditions, comprising: a signal receiving module and a demodulation output module, wherein:

[0010] The signal receiving module is used to receive the modulated signal transmitted by the channel and perform matched filtering on the signal;

[0011] The demodulation output module is used to perform parameter estimation and signal synchronization on the signal after matched filtering processing, and to determine the GMSK signal demodulation signal under complex channel conditions based on the GMSK signal eye diagram characteristics of different bandwidth period BT values. The eye diagram characteristics are: when one of the I / Q channels reaches the optimal sampling time, the values of the other channel have a discrete distribution, and the discreteness changes with the size of the BT value.

[0012] Beneficial effects of the present invention:

[0013] The present invention analyzes the characteristics of GMSK signals, performs pre-filtering, parameter estimation and signal synchronization on the received modulated signals in sequence, and demodulates and determines the signals based on the eye diagram characteristics. It can achieve the best demodulation and determination effect for GMSK signals under complex channel conditions, and is convenient for application in practical scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a schematic diagram of the GMSK signal receiving and demodulating process under complex channel conditions in the embodiment;

[0015] Figure 2 Schematic diagram of the GMSK signal eye diagram under different BT values in the embodiment;

[0016] Figure 3 Schematic diagram of GMSK spectrum corresponding to different BT values in the embodiment;

[0017] Figure 4 Schematic diagram of the change of bit error rate with roll-off coefficient after filtering by root raised cosine and raised cosine filter in the embodiment;

[0018] Figure 5 Schematic diagram of the change of bit error rate with tail number after filtering by root raised cosine filter in the embodiment;

[0019] Figure 6 Schematic diagram of the bit error rate changing with the symbol rate after filtering by the root raised cosine filter in the embodiment;

[0020] Figure 7 Schematic diagram of the relationship between the bit error rate and BT after filtering by the root raised cosine filter in the embodiment;

[0021] Figure 8 FIG. 1 is a diagram illustrating the relationship between the bit error rate and the roll-off coefficient when the interference is large in the embodiment. DETAILED DESCRIPTION

[0022] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention is further described in detail below with reference to the accompanying drawings and technical solutions.

[0023] In GMSK signal modulation, the digital baseband signal expression is:

[0024]

[0025] Where T is the duration of the code element, a k is a sequence containing information, and b(t) is a baseband rectangular pulse shaping.

[0026] The frequency characteristics of the Gaussian filter are:

[0027]

[0028] The expression in the time domain is:

[0029]

[0030] in B represents the 3dB bandwidth of the filter.

[0031] The signal obtained after the baseband signal passes through the filter is set to x(t),

[0032]

[0033] g(t) is the response of the Gaussian filter to the rectangular pulse,

[0034]

[0035] make but

[0036]

[0037]

[0038]

[0039] From the above formula, we can see that g(t) contains the important parameter BT value for designing Gaussian filter, which affects the spectrum characteristics of GMSK signal.

[0040] The information of the CPM signal is stored in the phase. When the input symbol is 1, the phase increases by π / 2, and when the input symbol is 0, the phase decreases by π / 2. As the code element is input, the phase function can be obtained. Since the envelope of the CPM signal is constant, the GMSK baseband signal obtained by Simulink simulation is

[0041] The embodiment of the present invention provides a GMSK signal receiving and demodulating method under complex channel conditions. Figure 1 As shown in the figure, first, the modulated signal transmitted by the receiving channel is received and matched filtered. Then, parameter estimation and signal synchronization are performed on the matched filtered signal. The GMSK signal demodulation signal under complex channel conditions is determined based on the GMSK signal eye diagram characteristics of different bandwidth period (BT) values. The eye diagram characteristic is that when one of the I / Q paths reaches the optimal sampling time, the other path has a discrete distribution of values, and the discreteness varies with the BT value. Focusing on GMSK signals transmitted in complex electromagnetic environments, the paper focuses on resolving the contradiction between actual demodulation processing requirements and current GMSK signal demodulation processing technology. Specifically, the characteristics of actual complex channels, such as dense and variable interference signals, low signal-to-noise ratio, and time-varying noise and fading, make the traditional GMSK signal receiving and processing structure based on correlation reception and Viterbi phase path search principles susceptible to time-varying noise and interference, resulting in failure to synchronize properly, excessively high bit error rate of demodulation results, and weak information recovery. By optimizing and improving the receiving and demodulation scheme, the modulation signal recovery effect is improved.

[0042] The baseband GMSK signal x(t) is convolved with the pre-filter frequency response function to complete the matched filtering process. In this embodiment, a root raised cosine filter and an FIR filter can be selected as the pre-filter, and their parameters are designed according to the Matlab function. The filtered GMSK signal is obtained. Perform parameter estimation and signal synchronization.

[0043] In parameter estimation, the segmented spectral averaging method is used to divide the data obtained by signal sampling into several groups according to the segment length. The modulus, square or mth power of the signal data is subjected to FFT operation in each segment to obtain different spectral lines to obtain the corresponding features. The features obtained in each segment are added and averaged, and the average value is used as the final feature value.

[0044] Estimation of the optimal sampling time: For the modulation method that carries information in amplitude, signal shaping causes the instantaneous amplitude to change periodically. The periodicity can be extracted by using the discrete components in the power spectrum of the baseband signal. The bipolar baseband signal is modulated to obtain the unipolar baseband signal. The first non-zero discrete component obtained by taking the power spectrum is R B .

[0045] X(w)=FT(A(t))=FT(|s(t)|) (9)

[0046]

[0047] When demodulating, T B The data is sampled and judged for the period, so for w B =2πR B The single frequency component at The best sampling time can be obtained

[0048] Estimating frequency offset and phase offset: For signals that have completed timing synchronization, the following model can be simplified:

[0049]

[0050]

[0051] The effective information obtained after calculating the m-th power spectrum is shown in Equation (12), which can be transformed into a single frequency estimation problem. The peak position of the amplitude spectrum corresponds to the frequency offset, and the corresponding phase corresponds to the phase offset, as shown in Equation (13).

[0052]

[0053]

[0054] For GMSK, the carrier synchronization model becomes as shown in Equation (14). Although it is different from the theoretical model, it does not affect the basic principle of frequency offset and phase offset estimation.

[0055] For GMSK signals, since its information is stored in the phase, direct modulus calculation cannot observe the discrete components and R B Without the estimation of τ, timing synchronization becomes impossible. Therefore, in engineering, carrier synchronization of the GMSK signal is often performed first, followed by timing synchronization. In experiments, the received signal is segmented and analyzed using the fourth power spectrum to estimate the frequency offset. The phase offset is then calculated from the phase difference between the preceding and following segments. Based on the received signal, the frequency and phase offsets are removed to achieve carrier synchronization.

[0056] The experiment is based on the GMSK signal after carrier synchronization. It is set that 4 symbols need to be reserved before and after each symbol for interpolation. Therefore, the signal is divided into 8192 segments, and the overlap between segments is reserved for 8f. d The optimal sampling time τ is estimated using the first non-zero discrete spectral component. Timing synchronization is achieved through digital interpolation. After synchronization is achieved, the eye diagram characteristics are determined and the bit error rate is calculated.

[0057] In the GMSK signal eye diagram, when one of the I and Q channels reaches its optimal sampling moment, the values of the other channel exhibit a discrete distribution, and this discreteness increases as the BT value decreases. Therefore, the decision method can be designed as follows: for locations where the I channel reaches its optimal sampling moment, only the I channel's value is retained; for locations where the Q channel reaches its optimal sampling moment, only the Q channel's value is retained, resulting in values of 0, π / 2, π, or -π / 2. These phase values are used to derive the phase shift relationship between symbols: a phase shift of -π / 2 corresponds to bit 0, and a phase shift of π / 2 corresponds to bit 1.

[0058] Taking GMSK with BT=0.25 as an example, the signal-to-noise ratio (SNR) of the AWGN module is set to Eb / n0=5dB, and the modulated signal is simulated. On the one hand, it is output to the working area and filtered using a root raised cosine filter with a roll-off factor of 1 before judgment. On the other hand, it is directly input into the GMSK demodulation module for judgment to obtain the bit stream, and the difference in bit error rates between the two is calculated.

[0059] Since traditional demodulation algorithms cannot effectively address the impact of strong in-band interference on the algorithm, in this embodiment, pre-filtering is first considered to remove the impact of interference and noise; due to the existence of time delay in the channel and the clock difference between the transmitting and receiving sites, synchronization is then considered to remove the impact of frequency offset and phase offset; finally, considering that the Viterbi phase path search process is too complicated and easily affected by noise and interference, in this embodiment, the unique characteristics of the GMSK eye diagram are used as a judgment to achieve signal demodulation output.

[0060] Regarding the types of pre-filters, we consider studying the filtering effects of two types of filters: root raised cosine filter and FIR filter. Based on the functions used in Matlab programming, we discuss the four parameters of the root raised cosine filter, namely the roll-off coefficient, the number of tails, the number of sampling points per symbol, and the shape. For the FIR filter, we discuss the two parameters of the order and cutoff frequency.

[0061] Furthermore, based on the above method, an embodiment of the present invention also provides a GMSK signal receiving and demodulating system under complex channel conditions, comprising: a signal receiving module and a demodulation output module, wherein:

[0062] The signal receiving module is used to receive the modulated signal transmitted by the channel and perform matched filtering on the signal;

[0063] The demodulation output module is used to perform parameter estimation and signal synchronization on the signal after matched filtering processing, and to determine the GMSK signal demodulation signal under complex channel conditions based on the GMSK signal eye diagram characteristics of different bandwidth period BT values. The eye diagram characteristics are: when one of the I / Q channels reaches the optimal sampling time, the values of the other channel have a discrete distribution, and the discreteness changes with the size of the BT value.

[0064] To verify the effectiveness of this solution, the following is a further explanation based on experimental data:

[0065] Regarding the parameter setting of the pre-filter, taking the GMSK signal with BT=0.25 as the target signal, we first select the parameters of the filter itself, then consider the impact of the BT value in the GMSK signal on the filter parameters, and finally discuss the filtering effect and the changes in the pre-filter parameters when different types of interference signals are aliased in the GMSK signal to varying degrees. The specific design process is as follows:

[0066] Step 1: For the parameter setting of the root raised cosine filter, the control variable method can be used to discuss the change of the roll-off coefficient from 0.1 to 1, the change of the tail number from 0.5 to 50, and the number of sampling points per symbol f. d and 2f d , the bit error rate of the demodulation decision after filtering when the shape is root raised cosine and raised cosine;

[0067] Step 2: Calculate the cutoff frequency based on the main lobe width in the GMSK signal spectrum at high signal-to-noise ratio (SNR) using the FIR filter parameter settings. The resulting bit error rate (BER) after demodulation is further discussed when the filter order varies from 20 to 512.

[0068] Step 3: Simulate the GMSK signal when the BT value changes from 0.1 to 1, filter it using the filters obtained in steps 1 and 2, and obtain the demodulation decision bit error rate, thereby obtaining the basic design of the optimal receiving structure and the performance of the optimal receiving structure in demodulating the GMSK signal in a Gaussian white noise environment;

[0069] Step 4: Mix BPSK, QPSK, and 2FSK signals with the same sampling rate into the GMSK signal with BT = 0.25, filter them using the filter obtained in step 1, and analyze the impact of the interference signal on the filtering performance and filter parameters;

[0070] Step 5: Adjust the frequency deviation, bandwidth and amplitude of the interference signal relative to the target signal, and further obtain the performance of the optimal receiving structure to demodulate and process the GMSK signal under complex channel conditions.

[0071] See also Figure 2 and 3 As can be seen, the smaller the BT value, the greater the signal sampling distortion and zero-crossing distortion, the less noticeable the eye opening becomes, and the more chaotic the eye diagram. When the BT value increases from 0.2, the mainlobe width of the GMSK signal spectrum gradually increases, and the signal bandwidth becomes larger; however, the sidelobes decay more slowly, the sidelobes become larger in amplitude, and the number of sidelobes increases. Increasing the BT value can be viewed as increasing the roll-off bandwidth of a Gaussian filter. The effectiveness of the Gaussian filter decreases with increasing BT, resulting in slower sidelobe decay and an increase in the mainlobe width. Since the mainlobe width represents the degree of signal energy concentration, the demodulation bit error rate (BER) decreases as the mainlobe width increases. From a time domain perspective, increasing the signal spectrum width reduces the temporal width of the signal, reducing the overlap of signal tails and crosstalk between signals, thereby reducing the demodulation bit error rate. However, since sidelobes divert signal energy, excessively large sidelobe amplitudes and numbers are undesirable in practical applications, so the BT value cannot be increased indefinitely.

[0072] Depend on Figure 4 It can be seen that from the perspective of filter shape, the filtering effect of the raised cosine filter is obviously inferior to that of the root raised cosine filter, and its bit error rate differs from the reference limit (0.0129) by at least 0.013; from the perspective of roll-off coefficient, when the roll-off coefficient α = 0.1, the maximum bit error rate after filtering using the root raised cosine filter is 0.0216; when the roll-off coefficient α = 1, the minimum bit error rate is 0.0149, which is no more than 0.9% different from the reference limit, indicating that the effect of using root raised cosine filtering is excellent, and setting α = 1 can obtain the demodulation effect closest to the reference limit.

[0073] Depend on Figure 5It can be seen that the demodulation bit error rate is more severely affected by the tail number when the tail number is smaller, while it basically does not change when the tail number is greater than 20; on the other hand, as the tail number changes, the maximum bit error rate is 0.01713 and the minimum is 0.01436. Compared with the reference limit, the difference does not exceed 0.5%, indicating that even when the tail number setting is not considered, the root raised cosine filter can still achieve good filtering effect.

[0074] Depend on Figure 6 It can be seen that when the value of the number of sampling points per symbol meets R B When the symbol rate of OQPSK is just the same as the sampling rate, the minimum bit error rate can be obtained. In the embodiment of this case, in actual application, the roll-off coefficient can be 1, the tail number can be 4, and the number of sampling points per symbol can be 2f d A root raised cosine filter is used to obtain the best filtering performance.

[0075] Depend on Figure 7 As shown in Table 1, when the BT value changes from 0.1 to 1 at intervals of 0.05, the bit error rate (BER) of the demodulated signal after filtering with a root-raised cosine filter first decreases rapidly, then steadily increases, reaching a minimum BER value at BT = 0.35. For GMSK signals with BT < 0.2, the BER exceeds 2%. For GMSK signals with BT > 0.2, the BER increases with increasing BT value, but never exceeds 2%, demonstrating the suitability of the root-raised cosine filter. Since the BT value of GMSK signals in practical applications generally does not fall below 0.2, the implementation of this solution is not affected if the receiver cannot obtain the target GMSK signal's BT parameters. The modulated signal receiver can still demodulate the signal with performance very close to that of traditional processing structures.

[0076] Table 1: Bit error rate relationship after demodulation of different interference signals

[0077]

[0078] The demodulation effect of the best reception is greatly improved compared with the traditional method, and a lower and better bit error rate is obtained. In addition, there is no significant difference in the demodulation bit error rate of different types of interference signals. Therefore, it is speculated that the interference effects of MPSK and 2FSK signals on the target GMSK signal are similar.

[0079] Depend on Figure 8As shown in Table 2, after appropriately increasing the amplitude of the interference signal in the simulation, the bit error rate first slowly decreases and then rapidly increases as the roll-off coefficient increases, reaching a minimum bit error rate of 0.0268 at α = 0.46. In this case of severe interference, a larger roll-off coefficient increases the probability of retaining the interference signal during filtering, resulting in an increase in the demodulated bit error rate. Although reducing the roll-off coefficient reduces the filtering range and sacrifices some GMSK signals, it more thoroughly filters out the interference signal, thereby reducing the bit error rate.

[0080] Table 2: Bit error rate relationship when interference is strong

[0081]

[0082] When adjacent-band interference is strong, a root-raised cosine filter with a roll-off factor of 0.5 can achieve a lower demodulation bit error rate, which is more suitable for GMSK signals in actual harsh channels.

[0083] The test results of the reception and demodulation scheme in this case are shown in Table 3.

[0084] Table 3: Comparison of GMSK demodulation bit error rates for different situations

[0085] Signal form Optimal receiving structure Traditional structure Low signal-to-noise ratio simulation signal with no frequency offset or phase offset 0.0149 0.0129 Low signal-to-noise ratio simulation signal with large frequency offset and phase deviation 0.0276 0.3194 Low SNR simulation signal with no frequency deviation or phase deviation mixed with strong BPSK 0.0278 0.5016 Low SNR simulation signal with frequency deviation and phase deviation mixed with strong BPSK 0.0444 0.5009

[0086] Compared with the more ideal GMSK signal, the existence of frequency deviation and phase deviation affects the signal performance to a certain extent, reducing the demodulation bit error rate; however, for GMSK signals transmitted in harsh channels, the reception and demodulation scheme in this case not only improves the robustness performance and achieves stable synchronization compared to traditional reception, but also effectively solves the impact of in-band interference and strong noise, achieving a lower demodulation bit error rate.

[0087] Unless otherwise specifically stated, the relative steps, numerical expressions and values of the components and steps set forth in these embodiments do not limit the scope of the present invention.

[0088] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the systems disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0089] The units and method steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the components and steps of each example have been generally described in terms of function in the above description. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. A person of ordinary skill in the art may use different methods to implement the described functions for each specific application, but such implementation is not considered to be beyond the scope of the present invention.

[0090] Those skilled in the art will appreciate that all or part of the steps in the above method can be performed by a program instructing related hardware. The program can be stored in a computer-readable storage medium, such as a read-only memory, a magnetic disk, or an optical disk. Alternatively, all or part of the steps in the above embodiment can be implemented using one or more integrated circuits. Accordingly, each module / unit in the above embodiment can be implemented in the form of hardware or software functional modules. The present invention is not limited to any specific combination of hardware and software.

[0091] Finally, it should be noted that the above-described embodiments are only specific implementation methods of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the above-described embodiments, those skilled in the art should understand that any person skilled in the art can modify or easily conceive of changes to the technical solutions described in the above-described embodiments within the technical scope disclosed by the present invention, or replace some of the technical features therein with equivalents. Such modifications, changes, or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A GMSK signal receiving and demodulating method under complex channel conditions, characterized in that: Contains the following content: Receive the modulated signal transmitted by the channel and perform matched filtering on the signal; Parameter estimation and signal synchronization are performed on the matched filtered signal. The demodulated GMSK signal under complex channel conditions is determined based on the GMSK signal eye diagram characteristics of different bandwidth period (BT) values. The eye diagram characteristics are: when one of the I / Q channels reaches the optimal sampling time, the other channel has a discrete distribution of values, and the discreteness varies with the BT value. Parameter estimation involves estimating the optimal sampling moment and frequency offset and phase deviation. These are estimated using the segmented spectral averaging method. First, the sampled signal data is segmented according to segment length. Within each segment, Fourier transform operations are performed on the modulus, square, or mth power of the signal data to obtain different spectral lines. The spectral lines are then used to obtain the corresponding signal features. The signal features of each segment are summed and averaged, with this average value serving as the final feature data for parameter estimation. The process of determining the GMSK signal demodulation signal based on the GMSK signal eye diagram characteristics of different bandwidth period BT values includes the following contents: for the position where the I channel reaches the optimal sampling time, only the phase value of the I channel is retained; for the position where the Q channel reaches the optimal sampling time, only the phase value of the Q channel is retained, and the phase values are all one of 0, π / 2, π, and -π / 2; and the phase shift relationship between symbols is obtained from the phase values.

2. The GMSK signal receiving and demodulating method under complex channel conditions according to claim 1, characterized in that: In the matched filtering process, the GMSK modulated signal is convolved with the filter frequency response function to obtain the filtered GMSK signal.

3. The GMSK signal receiving and demodulating method under complex channel conditions according to claim 2, characterized in that: Filter parameters are set, and a root raised cosine filter is used to perform matched filtering on the signal, wherein the root raised cosine filter parameters include at least a roll-off factor, a tail number, a number of sampling points per symbol, and a shape.

4. The GMSK signal receiving and demodulating method under complex channel conditions according to claim 2, characterized in that: Set filter parameters and use an FIR filter to perform matched filtering on the signal, where the FIR filter parameters include at least an order and a cutoff frequency.

5. The GMSK signal receiving and demodulating method under complex channel conditions according to claim 1, characterized in that: In the optimal sampling time estimation, for bipolar baseband signals, first, the unipolar baseband signal is obtained by modulus calculation, and the periodicity is extracted using the discrete components in the power spectrum of the baseband signal. Then, the periodicity is used to sample the signal data and obtain the optimal sampling time.

6. The GMSK signal receiving and demodulating method under complex channel conditions according to claim 1 or 5, characterized in that: In frequency offset and phase offset estimation, first, signal timing synchronization is performed, and effective signal information is obtained through the signal m-th power spectrum; then, the frequency offset and phase offset estimation are obtained by using the frequency offset corresponding to the peak position of the amplitude spectrum.

7. The GMSK signal receiving and demodulating method under complex channel conditions according to claim 1, characterized in that: Signal synchronization includes: signal carrier synchronization that uses frequency offset and phase offset estimation to remove frequency offset and phase offset in the received signal, and timing synchronization that uses the best sampling moment to perform digital interpolation on the signal.

8. A GMSK signal receiving and demodulating system under complex channel conditions, characterized by: The method according to claim 1 is implemented, comprising: a signal receiving module and a demodulation output module, wherein: The signal receiving module is used to receive the modulated signal transmitted by the channel and perform matched filtering on the signal; The demodulation output module is used to perform parameter estimation and signal synchronization on the signal after matched filtering. It also determines the GMSK signal demodulation signal under complex channel conditions based on the GMSK signal eye diagram characteristics of different bandwidth period (BT) values. The eye diagram characteristics are: when one of the I / Q channels reaches the optimal sampling time, the other channel has a discrete distribution of values, and the discreteness varies with the BT value. Parameter estimation involves estimating the optimal sampling moment and frequency offset and phase deviation. These are estimated using the segmented spectral averaging method. First, the sampled signal data is segmented according to segment length. Within each segment, Fourier transform operations are performed on the modulus, square, or mth power of the signal data to obtain different spectral lines. The spectral lines are then used to obtain the corresponding signal features. The signal features of each segment are summed and averaged, with this average value serving as the final feature data for parameter estimation. The process of determining the GMSK signal demodulation signal based on the GMSK signal eye diagram characteristics of different bandwidth period BT values includes the following contents: for the position where the I channel reaches the optimal sampling time, only the phase value of the I channel is retained; for the position where the Q channel reaches the optimal sampling time, only the phase value of the Q channel is retained, and the phase values are all one of 0, π / 2, π, and -π / 2; and the phase shift relationship between symbols is obtained from the phase values.

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