Low-complexity MSK coded modulation system phase synchronization receiving system and method

Abstract

The invention discloses a low-complexity MSK coded modulation system phase synchronization receiving system and method, and relates to the technical field of continuous phase modulation. On one hand, an original message is coded and modulated by constructing a pre-coding mode, a receiving end realizes optimal sampling point extraction at the minimum intersymbol crosstalk moment after phase decoupling frequency conversion processing, MSK is converted into BPSK-like modulation, oversampling processing is not needed, modulation phase components of a receiving synchronization section can be offset, and the modulation efficiency of the receiving synchronization section is improved; therefore, accurate estimation of Doppler phase and frequency is realized, and the anti-Doppler capability is improved. On the other hand, coherent demodulation soft information is directly output after the real part of the signal subjected to Doppler frequency offset and phase offset compensation is obtained, the complexity of the demodulation algorithm is low, the performance of the demodulation algorithm is equivalent to that of BPSK, the demodulation algorithm is remarkably superior to that of an MSK optimal demodulation receiver in a low signal-to-noise ratio region, and the demodulation algorithm is particularly suitable for a bit interleaving coding modulation system combined with channel coding and has a wide application prospect. And reliable transmission under a low signal-to-noise ratio can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of continuous phase modulation, and particularly relates to a low-complexity phase synchronization receiving system and method for an MSK coding modulation system. Background Art

[0002] The statements in this section only provide background information related to the present disclosure and may not constitute prior art.

[0003] Minimum Shift Keying (MSK) is a special case of continuous phase modulation. Due to its high spectral efficiency and power efficiency, it can achieve a strong out-of-band suppression effect without filtering, while maintaining the original constant envelope characteristic, enabling it to operate in the non-linear saturation region of a high-power amplifier to achieve high-power amplification. Therefore, MSK has been widely studied in fields such as satellite navigation, satellite communication, and deep space communication where both power and bandwidth are limited.

[0004] Compared with non-coherent demodulation, the performance of the coherent demodulation algorithm is close to the theoretical value. Carrier synchronization is an important link in coherent reception processing, and the Doppler frequency and phase estimation accuracy affect the coherent demodulation performance of the communication system. For a high-dynamic MSK burst communication system, to overcome the short-term fast fading introduced by the time-frequency doubly selective channel, a data frame structure usually uniformly inserts several synchronization segments, and uses a data-aided open-loop synchronization method to quickly estimate the short-term Doppler frequency and phase changes to achieve carrier synchronization.

[0005] Due to the phase continuity of MSK, unknown data segments will introduce an ambiguous initial phase to subsequent synchronization segments, resulting in the inability to eliminate the MSK modulation phase of the synchronization segments and thus affecting the accurate estimation of the Doppler frequency. Although the frequency doubling method can be used as an effective solution to eliminate the influence of the ambiguous initial phase by integer multiple amplification of the phase and achieve accurate estimation of the frequency offset, the Doppler frequency estimation range will decrease proportionally, and its phase estimation range is reduced to -π / 4 to π / 4, and the frequency estimation range is reduced to ~ , which is not applicable to high-dynamic and large-Doppler scenarios. Summary of the Invention

[0006] The object of the present invention is to provide a phase synchronization receiving system and method for a low-complexity MSK coding and modulation system. By constructing a precoding method to encode the original message and then modulate it, the receiving end uses phase decoupling and frequency conversion processing to extract the optimal sampling points at the moment when the inter-symbol interference is minimized, transforms MSK into a BPSK-like modulation, decouples the phase between MSK symbols, and uses a data-aided open-loop synchronization method. Without oversampling processing, only the synchronization sequence is used to reconstruct the modulation phase of the optimal sampling point within each symbol to cancel the modulation phase component of the received synchronization segment, so as to achieve accurate estimation of the Doppler phase and frequency, effectively reduce the processing length of the sampling signal, and reduce the processing delay. The MSK phase estimation range of this method is increased to -π~π, and the frequency estimation range is increased to ~ , with strong anti-Doppler ability. The demodulation module directly outputs coherent demodulation soft information after taking the real part of the signal. Compared with the MSK optimal demodulation receiver, the proposed demodulation algorithm has low complexity, its performance is equivalent to that of BPSK, and it is significantly better than the MSK optimal demodulation receiver in the low signal-to-noise ratio region. It is especially suitable for the bit-interleaved coded modulation system combined with channel coding and can achieve reliable transmission at low signal-to-noise ratio.

[0007] The technical solution of the present invention is as follows: A phase synchronization receiving system for a low-complexity MSK coding and modulation system, including: The transmitting end encodes the service data and frames it with the synchronization sequence to generate a baseband pulse signal, which is output through the MSK precoding modulation and the radio frequency transmitting link; After the receiving end obtains the digital baseband signal through the radio frequency receiving link, it successively performs phase decoupling processing to eliminate the phase correlation between bits to generate a BPSK-like signal, frame synchronization detection, channel estimation and zero-forcing equalization based on the synchronization sequence, and BPSK-like demodulation to extract the coding soft information, and finally restores the service data through iterative decoding.

[0008] Further, the transmitting end includes: an LDPC coding module, a framing module, a precoding module, an MSK modulation module, a digital-to-analog conversion module, and an analog up-conversion module; The LDPC coding module outputs a coding sequence based on the service data; The framing module inserts a synchronization sequence before and after the coding sequence to generate a pulse signal; The precoding module performs precoding on all bits in the pulse signal and outputs an MSK modulation signal through the MSK modulation module; The digital-to-analog conversion module and the analog up-conversion module process the MSK modulation signal and radiate an MSK radio frequency signal.

[0009] Further, the receiving end includes: an analog down-conversion module, an analog-to-digital conversion module, a phase decoupling frequency conversion module, a signal detection module, a channel estimation module, a channel equalization module, a BPSK-like demodulation module, and an LDPC decoding module; The analog down-conversion module and the analog-to-digital conversion module process the MSK radio frequency signal after noise interference and output a received digital baseband signal; The phase decoupling frequency conversion module performs a matched filtering process on the received digital baseband signal, then performs a phase decoupling frequency conversion process on the filtered signal to eliminate the phase correlation between bits, and finally decimates the signal after the phase decoupling frequency conversion process to output a BPSK-like signal with a 2-fold bit sampling rate; The signal detection module performs a correlation detection on the BPSK-like signal and realizes frame synchronization through methods of peak search and threshold detection; The channel estimation module reconstructs the modulation phase of the optimal sampling point for each bit through the synchronization sequence and eliminates the MSK modulation phase of the received signal synchronization sequence; The channel equalization module directly cancels the channel fading response of the received signal pulse using the zero-forcing algorithm, and the equalized pulse signal outputs the soft information of the coded sequence through the BPSK-like demodulation module; The LDPC decoding module iteratively decodes the soft information of the coded sequence to recover the service data.

[0010] The present invention also proposes a phase synchronization receiving method for a low-complexity MSK coding and modulation system. Based on the above low-complexity MSK coding and modulation system phase synchronization receiving system, it includes: Step S1: The service data is encoded by the LDPC encoding module and outputs a coded sequence; Step S2: The framing module inserts a synchronization sequence before and after the coded sequence to generate a pulse signal; Step S3: All bits in the pulse signal are pre-coded by the pre-coding module and then output an MSK modulation signal through the MSK modulation module; Step S4: The MSK modulation signal is processed by the digital-to-analog conversion module and the analog up-conversion module and then radiates an MSK radio frequency signal; Step S5: The receiving platform processes the MSK radio frequency signal after noise interference through the analog down-conversion module and the analog-to-digital conversion module and outputs a received digital baseband signal; Step S6: The received digital baseband signal is subjected to a matched filtering process through the phase decoupling frequency conversion module, then a phase decoupling frequency conversion process is performed on the filtered signal to eliminate the phase correlation between bits, and finally the signal after the phase decoupling frequency conversion process is decimated to output a BPSK-like signal with a 2-fold bit sampling rate; Step S7: The signal detection module performs correlation detection on the BPSK-like signal, and realizes frame synchronization through peak search and threshold detection methods; Step S8: The channel estimation module reconstructs the modulation phase of the best sampling point of each bit through the synchronization sequence, eliminates the MSK modulation phase of the received signal synchronization sequence, and realizes accurate estimation of the Doppler phase of the received synchronization sequence; Step S9: The channel equalization module directly cancels the channel fading response of the received signal pulse by using the zero-forcing algorithm, and the equalized pulse signal outputs the soft information of the coding sequence through the BPSK-like demodulation module; Step S10: The LDPC decoding module iteratively decodes the soft information of the coding sequence to recover the service data.

[0011] Furthermore, the precoding method is as follows:

[0012] where is the i-th precoding bit within the pulse, is the i-th bit and takes values in [0, 1] and , is the exclusive OR operation; The precoding bit is bipolar encoded into and then MSK modulated, and the output MSK modulation signal is expressed as:

[0013] where is the bit energy, T is the bit time, is the phase of the i-th bit, is the bipolar encoding of the i-th bit, that is , and j is the imaginary part identifier.

[0014] Furthermore, the processing flow of the phase decoupling frequency conversion module in step S4 is as follows: Step S41: Perform matched filtering on the received digital baseband signal, that is:

[0015] where is the n-th sampling point of the matched filtering signal, is the m-th sampling point of the received digital baseband signal, is the -th coefficient of the matched filter; Step S42: Perform phase decoupling frequency conversion on the matched filtering signal, that is:

[0016] where is the signal after phase decoupling frequency conversion processing, T is the bit time, and Ts is the sampling time.

[0017] Step S43: Decimate the signal after phase decoupling frequency conversion processing to output a BPSK-like signal with a 2-fold bit sampling rate, that is:

[0018] In the formula, is the nth sampling point of the BPSK-like signal with a 2-fold bit sampling rate.

[0019] Furthermore, the channel estimation module is built-in with a Doppler frequency estimation algorithm and a phase estimation algorithm.

[0020] Furthermore, the Doppler frequency estimation algorithm includes: Step A: After frame synchronization, use the start moment within each MSK bit time as the moment with the least inter-symbol interference to extract the optimal sampling point, where the sampling time is the bit time, that is:

[0021] In the formula, is the nth optimal sampling point of the BPSK-like signal, is the sampling point at the frame header moment.

[0022] Step B: Extract the preamble sequence and the postamble sequence within the pulse signal, multiply them conjugate with the local sequence respectively, sum them up and take the phase to complete the phase estimation at the middle moment of the synchronization sequence, that is: Doppler phase estimation of the preamble sequence within the pulse :

[0023] In the formula is the phase-taking operation, and L is the symbol length of the synchronization sequence. is the nth bit value of the local synchronization sequence, that is:

[0024] Among them, is the nth original bit of the synchronization sequence within the pulse and its value is [0, 1]; Doppler phase estimation of the postamble sequence within the pulse :

[0025] In the formula, is the number of symbols between the start moment of the preamble sequence and the start moment of the postamble sequence; Step C: Since there is a 2π cycle ambiguity in the phase difference between the front and rear synchronization segments, it is necessary to correct the phase estimation values of the front and rear synchronization segments, that is: When , then ; When , then ; Step D: Utilize the phase difference between the estimated phases of the front and rear synchronization segments to achieve Doppler frequency estimation, that is: .

[0026] Furthermore, the phase estimation algorithm includes: According to the Doppler frequency estimation and the Doppler phase estimation of the preamble sequence , fit the channel Doppler phase of the pulse signal, that is:

[0027] In the formula, L is the synchronization sequence length, T is the bit time, Np is the number of pulse bits, and j is the imaginary part identifier.

[0028] Furthermore, the processing flow of the class BPSK demodulation module in step S9 is as follows: Step S91: After suppressing the channel fading response through the channel equalization module, extract the received equalized signal with a sampling period of the bit period, and T is the bit time, that is:

[0029] where Np is the number of pulse symbols, is the k-th bit observation information of the pulse; Step S92: Take out the real part of the equalized signal as the k-th bit soft information of the pulse signal, that is:

[0030] where is the real part extraction operation, is the k-th bit soft information of the pulse signal.

[0031] Compared with the existing technology, the beneficial effects of the present invention are: 1. Improve the anti-Doppler ability and reduce the receiving processing complexity: A low-complexity phase synchronization receiving system and method for an MSK coded modulation system are proposed. At the transmitting end, a precoding method is constructed. At the receiving end, after phase decoupling and frequency conversion, the optimal sampling point extraction is realized at the moment of minimum inter-symbol interference. The MSK is transformed into a BPSK-like modulation. Without oversampling processing, only the modulation phase of the optimal sampling point within each symbol is reconstructed using the synchronization sequence to cancel the modulation phase component in the receiving synchronization segment, so as to achieve accurate estimation of the Doppler phase and frequency, effectively reduce the processing length of the sampling signal, and reduce the processing delay and hardware overhead. Compared with the traditional frequency doubling method, the phase estimation range of this method is increased to -π~π, and the frequency estimation range is increased to ~ , with strong anti-Doppler ability, where is the time interval between two consecutive synchronization sequences. At the same time, the demodulation module only processes the optimal sampling point within the MSK symbol, and directly outputs the coherent demodulation soft information after taking the real part. Compared with the MSK optimal demodulation receiver, the complexity is significantly reduced.

[0032] 2. Improve the demodulation and decoding performance: At the transmitting end of this method, a precoding method is constructed. At the receiving end, after phase decoupling and frequency conversion, the optimal sampling point extraction is realized at the moment of minimum inter-symbol interference. The MSK is transformed into a BPSK-like modulation. The demodulation module directly outputs the coherent demodulation soft information after taking the real part of the signal. Its performance is equivalent to that of BPSK and is significantly better than the MSK optimal demodulation receiver in the low signal-to-noise ratio region. It is especially suitable for the bit-interleaved coded modulation system combined with channel coding, and can achieve correct decoding performance at low signal-to-noise ratio. Brief Description of the Drawings

[0033] Figure 1 is the schematic diagram of a low-complexity phase synchronization receiving system and method for an MSK coded modulation system; Figure 2 is the eye diagram of the MSK signal proposed by the present invention; Figure 3 is the schematic diagram of MSK phase decoupling proposed by the present invention; Figure 4 is the multi-Doppler frequency and phase estimation accuracy of the channel estimation module proposed by the present invention; Figure 5 is the demodulation performance of the coherent reception method of the MSK demodulation module proposed by the present invention; Figure 6 is the decoding performance of the MSK coded modulation system proposed by the present invention. Detailed Embodiments

[0034] It should be noted that relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element.

[0035] The features and performance of the present invention will be further described in detail below in conjunction with embodiments.

[0036] Embodiment 1 Please refer to Figure 1 , a phase synchronization receiving system for a low-complexity MSK coding modulation system, comprising: The transmitting end encodes service data and generates a baseband pulse signal by framing with a synchronization sequence, and outputs it through an RF transmitting link after MSK precoding modulation; After the receiving end obtains a digital baseband signal through an RF receiving link, it sequentially performs phase decoupling processing to eliminate the phase correlation between bits to generate a BPSK-like signal, frame synchronization detection, channel estimation and zero-forcing equalization based on the synchronization sequence, and BPSK-like demodulation to extract coding soft information, and finally restores the service data through iterative decoding.

[0037] In this actual example, specifically, the transmitting end includes: an LDPC coding module, a framing module, a precoding module, an MSK modulation module, a digital-to-analog conversion module, and an analog upconversion module; The LDPC coding module outputs a coding sequence based on the service data; The framing module inserts a synchronization sequence before and after the coding sequence to generate a pulse signal; The precoding module performs precoding based on all bits in the pulse signal and outputs an MSK modulation signal through the MSK modulation module; The digital-to-analog conversion module and the analog upconversion module process the MSK modulation signal and radiate an MSK RF signal.

[0038] In this actual example, specifically, the receiving end includes: an analog downconversion module, an analog-to-digital conversion module, a phase decoupling and frequency conversion module, a signal detection module, a channel estimation module, a channel equalization module, a BPSK-like demodulation module, and an LDPC decoding module; The analog down-conversion module and the analog-to-digital conversion module process the MSK radio frequency signal after noise interference and output a received digital baseband signal; The phase decoupling frequency conversion module performs matched filtering on the received digital baseband signal, then performs phase decoupling frequency conversion on the filtered signal to eliminate the phase correlation between bits, and finally decimates the signal after the phase decoupling frequency conversion process to output a BPSK-like signal with a 2-fold bit sampling rate; The signal detection module performs correlation detection on the BPSK-like signal and achieves frame synchronization through peak search and threshold detection methods; The channel estimation module reconstructs the modulation phase of the optimal sampling point for each bit through the synchronization sequence to eliminate the MSK modulation phase of the received signal synchronization sequence; The channel equalization module uses the zero-forcing algorithm to directly cancel the channel fading response of the received signal pulse, and the equalized pulse signal outputs the soft information of the coded sequence through the BPSK-like demodulation module; The LDPC decoding module iteratively decodes the soft information of the coded sequence to recover the service data.

[0039] This embodiment also proposes a phase synchronization receiving method for a low-complexity MSK coding modulation system. Based on the above low-complexity MSK coding modulation system phase synchronization receiving system, it includes: Step S1: The service data is encoded by the LDPC encoding module and outputs a coded sequence; Step S2: The framing module inserts a synchronization sequence before and after the coded sequence to generate a pulse signal; Step S3: All bits in the pulse signal are pre-coded by the pre-coding module and output an MSK modulation signal through the MSK modulation module; Step S4: The MSK modulation signal is processed by the digital-to-analog conversion module and the analog up-conversion module and radiates an MSK radio frequency signal; Step S5: The receiving platform processes the MSK radio frequency signal after noise interference through the analog down-conversion module and the analog-to-digital conversion module and outputs a received digital baseband signal; Step S6: The phase decoupling frequency conversion module performs matched filtering on the received digital baseband signal, then performs phase decoupling frequency conversion on the filtered signal to eliminate the phase correlation between bits, and finally decimates the signal after the phase decoupling frequency conversion process to output a BPSK-like signal with a 2-fold bit sampling rate; Step S7: The signal detection module performs correlation detection on the BPSK-like signal and achieves frame synchronization through peak search and threshold detection methods; Step S8: After frame synchronization, the starting moment within each MSK bit time is taken as the moment with the least inter-symbol interference for extracting the optimal sampling points, where the sampling time is the bit time. The channel estimation module reconstructs the modulation phase of each bit's optimal sampling point through the synchronization sequence, eliminates the MSK modulation phase of the received signal synchronization sequence, and realizes the accurate estimation of the Doppler phase of the received synchronization sequence; the phases of the synchronization sequences before and after the pulse signal are differentially processed to realize the Doppler frequency estimation, and the channel fading response is fitted out. Step S9: The channel equalization module directly cancels the channel fading response of the received signal pulse using the zero-forcing algorithm, and the equalized pulse signal outputs the soft information of the coding sequence through the BPSK-like demodulation module. Step S10: The LDPC decoding module iteratively decodes the soft information of the coding sequence to recover the service data.

[0040] In this embodiment, specifically, the precoding method is as follows:

[0041] where is the i-th precoding bit within the pulse, is the i-th bit and takes values in [0, 1] and , is the exclusive OR operation; The precoding bit is bipolar coded into and then MSK modulated, and the output MSK modulation signal is expressed as:

[0042] where is the bit energy, T is the bit time, is the phase of the i-th bit, is the bipolar coding of the i-th bit, that is , and j is the imaginary part identifier.

[0043] In this embodiment, specifically, the processing flow of the phase decoupling and frequency conversion module in step S4 is as follows: Step S41: Perform matched filtering on the received digital baseband signal, that is:

[0044] where is the n-th sampling point of the matched filtering signal, is the m-th sampling point of the received digital baseband signal, is the -th coefficient of the matched filter; Step S42: Perform phase decoupling and frequency conversion on the matched filtering signal, that is:

[0045] Wherein, is the signal after phase decoupling frequency conversion processing, T is the bit time, and Ts is the sampling time.

[0046] Step S43: Decimate the signal after phase decoupling frequency conversion processing to output a BPSK-like signal with a 2-fold bit sampling rate, that is:

[0047] Wherein, is the nth sampling point of the BPSK-like signal with a 2-fold bit sampling rate.

[0048] In this embodiment, specifically, the channel estimation module is built with a Doppler frequency estimation algorithm and a phase estimation algorithm.

[0049] In this embodiment, specifically, the Doppler frequency estimation algorithm includes: Step A: After frame synchronization, take the starting moment within each MSK bit time as the moment with the least inter-symbol interference for extracting the optimal sampling point, where the sampling time is the bit time, that is:

[0050] Wherein, is the nth optimal sampling point of the BPSK-like signal, is the sampling point at the frame header moment.

[0051] Step B: Extract the preamble sequence and the postamble sequence within the pulse signal, multiply them conjugate with the local sequence respectively and sum and take the phase to complete the phase estimation at the middle moment of the synchronization sequence, that is: Doppler phase estimation of the preamble sequence within the pulse :

[0052] Wherein is the phase-taking operation, and L is the symbol length of the synchronization sequence. is the nth bit value of the local synchronization sequence, that is:

[0053] Wherein, is the nth original bit of the synchronization sequence within the pulse and its value is [0, 1]; Doppler phase estimation of the postamble sequence within the pulse :

[0054] Wherein, is the number of symbols between the start time of the preamble sequence and the start time of the postamble sequence; Step C: Since there is a 2π periodic ambiguity phenomenon in the phase difference between the two synchronization segments before and after, it is necessary to correct the phase estimation values of the two synchronization segments before and after, that is: When then ; When then ; Step D: Using the phase difference between the estimated phases of the two synchronization segments before and after, Doppler frequency estimation is realized, that is:

[0055] In this embodiment, specifically, the phase estimation algorithm includes: According to the Doppler frequency estimation and the Doppler phase estimation of the preamble sequence , the channel Doppler phase of the pulse signal is fitted, that is:

[0056] In the formula, L is the synchronization sequence length, T is the bit time, Np is the number of pulse bits, and j is the imaginary part identifier.

[0057] In this embodiment, specifically, the processing flow of the class BPSK demodulation module in step S9 is as follows: Step S91: After suppressing the channel fading response through the channel equalization module, the received equalized signal is extracted, and the sampling period is the bit period, and T is the bit time, that is:

[0058] Among them, Np is the number of pulse symbols, is the observation information of the k-th bit of the pulse; Step S92: Take the real part of the equalized signal as the soft information of the k-th bit of the pulse signal, that is:

[0059] Among them, is the real part extraction operation, is the soft information of the k-th bit of the pulse signal.

[0060] Refer to Figure 2 and Figure 3 , from the eye diagram of the MSK signal of Figure 2 , it can be seen that the starting moment within the MSK bit time is the moment when the inter-symbol interference is the smallest. Refer to Figure 3, in this method, the transmitting end constructs a precoding method, and the receiving end realizes the extraction of the optimal sampling point at the moment of the minimum inter-symbol interference through phase decoupling and frequency conversion. Then, the phase at the starting moment of the signal after phase decoupling and frequency conversion within the bit time is the original bit information, realizing the transformation from MSK modulation to BPSK-like modulation.

[0061] See Figure 4 , the optimal sampling point is extracted at the starting moment of the signal after phase decoupling and frequency conversion within the bit time, where the sampling time is the bit time. The channel estimation module reconstructs the modulation phase of each bit's optimal sampling point through the synchronization sequence, eliminates the MSK modulation phase of the received signal synchronization sequence, and realizes the accurate estimation of the Doppler phase of the received synchronization sequence. The Doppler frequency is estimated by performing differential processing on the phases of the synchronization sequences before and after the pulse signal, and the channel fading response is fitted. When the interval length between the front and rear synchronization sequences is set to 20 μs and the duration of the synchronization segment is 2 μs, the frequency estimation range of the frequency doubling method is -6.25 kHz to 6.25 kHz, and the frequency estimation range of the algorithm proposed in this paper is -25 kHz to 25 kHz. As can be seen from the figure, when the Doppler frequency is 5 kHz, the frequency estimation accuracy of the algorithm in this paper is better than that of the frequency doubling method at low signal-to-noise ratios, and the accuracies of the two are comparable at high signal-to-noise ratios. When the Doppler frequency is 20 kHz, since it exceeds the estimation range of the frequency doubling method, the estimated frequency offset maintains a large error. However, the frequency estimation accuracy of the algorithm of the present invention still remains at a relatively high level. Therefore, the algorithm of the present invention can improve the Doppler frequency range and estimation accuracy and has strong anti-Doppler ability.

[0062] See Figure 5 , under ideal synchronization, the coherent reception method of the BPSK-like demodulation module proposed in the present invention for MSK is comparable to the theoretical performance of BPSK. At the same time, when there is a 20 kHz Doppler frequency, the coherent demodulation performance after being processed by the channel estimation module and the channel equalization module of the present invention shows a negligible performance degradation compared with the ideal synchronization performance. In addition, in the low signal-to-noise ratio region, the BPSK-like demodulation performance proposed in the present invention for MSK is significantly better than that of the MSK optimal demodulation receiver, and the receiving complexity is significantly reduced. The MSK optimal demodulation receiver requires 1 filter and 4 phase states for maximum likelihood sequence detection, but the demodulation module of the present invention does not require oversampling, and each symbol is only processed at the optimal sampling moment. After extracting the real part of the equalized signal, the coherent demodulation soft information is directly output, effectively reducing the hardware overhead and processing delay and being easy to implement in engineering.

[0063] See Figure 6, since the MSK demodulation performance proposed by the present invention is equivalent to the theoretical performance of BPSK, it is particularly suitable for a bit-interleaved coded modulation system combined with channel coding, and can achieve correct decoding performance at low signal-to-noise ratios. In the embodiment of the 1 / 4 code rate LDPC coding system of the present invention, the decoding performance of the MSK coding modulation system proposed by the present invention is significantly better than that of the maximum likelihood demodulation MSK coding modulation system, with a 2dB advantage at a BER of 10 -6 and the lower the code rate, the greater the advantage.

[0064] The above embodiments only represent the specific implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the protection scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the technical solution of the present application, several deformations and improvements can be made, and these all belong to the protection scope of the present application.

[0065] This background art section is provided to generally present the context of the present invention. The work of the currently named inventors, to the extent described in this background art section, and aspects of the work that are not prior art as of the time of filing of this application are neither expressly nor impliedly admitted to be prior art to the present invention.

Claims

1. A low-complexity MSK coded modulation system phase synchronization receiving system, characterized in that: include: The transmitter encodes the service data and frames it with the synchronization sequence to generate a baseband pulse signal, which is then modulated by MSK precoding and output through the RF transmission link. After the receiving end obtains the digital baseband signal through the RF receiving link, it performs phase decoupling processing in sequence to eliminate the phase correlation between bits to generate a BPSK-like signal, frame synchronization detection, channel estimation and zero-forcing equalization based on the synchronization sequence, and BPSK-like demodulation to extract the coded soft information, and finally restores the service data through iterative decoding.

2. The low-complexity MSK coding modulation system phase synchronization receiving system according to claim 1, characterized in that: The transmitting end includes: an LDPC encoding module, a framing module, a precoding module, an MSK modulation module, a digital-to-analog conversion module and an analog up-conversion module; The LDPC encoding module outputs a coding sequence based on the service data; The framing module inserts synchronization sequences before and after the coding sequence to generate a pulse signal; The precoding module performs precoding based on all bits in the pulse signal and then outputs an MSK modulated signal through the MSK modulation module; The digital-to-analog conversion module and the analog up-conversion module process the MSK modulated signal and radiate an MSK radio frequency signal.

3. The low-complexity MSK coding modulation system phase synchronization receiving system according to claim 2, characterized in that: The receiving end includes: an analog down-conversion module, an analog-to-digital conversion module, a phase decoupling frequency conversion module, a signal detection module, a channel estimation module, a channel equalization module, a BPSK-like demodulation module and an LDPC decoding module; The analog down-conversion module and the analog-to-digital conversion module process the MSK radio frequency signal after noise interference and output a received digital baseband signal; The phase decoupling frequency conversion module performs matched filtering on the received digital baseband signal, then performs phase decoupling frequency conversion on the filtered signal to eliminate the phase correlation between bits, and finally extracts the signal after the phase decoupling frequency conversion processing to output a BPSK-like signal with a bit sampling rate of 2 times; The signal detection module performs correlation detection on the BPSK-like signal and realizes frame synchronization through peak search and threshold detection methods; The channel estimation module reconstructs the modulation phase of the optimal sampling point of each bit through the synchronization sequence, and eliminates the MSK modulation phase of the synchronization sequence of the received signal; The channel equalization module uses a zero-forcing algorithm to directly offset the channel fading response of the received signal pulse, and the equalized pulse signal outputs the coding sequence soft information through a BPSK-like demodulation module; The LDPC decoding module iteratively decodes the soft information of the coding sequence to recover the service data.

4. A low-complexity phase synchronization receiving method for MSK coded modulation system, characterized in that: A low-complexity MSK coding modulation system phase synchronization receiving system according to any one of claims 1 to 3, comprising: Step S1: The service data is encoded by the LDPC encoding module and then output as a coding sequence; Step S2: The framing module inserts a synchronization sequence before and after the coding sequence to generate a pulse signal; Step S3: All bits in the pulse signal are pre-coded by the pre-coding module and then output as MSK modulated signals through the MSK modulation module; Step S4: radiate the MSK radio frequency signal after the MSK modulated signal is processed by the digital-to-analog conversion module and the analog up-conversion module; Step S5: The receiving platform receives the MSK radio frequency signal after noise interference through the analog down-conversion module and the analog-to-digital conversion module and outputs the received digital baseband signal; Step S6: performing matched filtering processing on the received digital baseband signal through the phase decoupling frequency conversion module, and then performing phase decoupling frequency conversion processing on the filtered signal to eliminate the phase correlation between bits, and finally extracting the signal after the phase decoupling frequency conversion processing to output a BPSK-like signal with a bit sampling rate of 2 times; Step S7: the signal detection module performs correlation detection on the BPSK-like signal, and realizes frame synchronization through peak search and threshold detection methods; Step S8: The channel estimation module reconstructs the modulation phase of the optimal sampling point of each bit through the synchronization sequence, eliminates the MSK modulation phase of the received signal synchronization sequence, and realizes accurate estimation of the Doppler phase of the received synchronization sequence; Step S9: The channel equalization module uses a zero-forcing algorithm to directly offset the channel fading response of the received signal pulse, and the equalized pulse signal outputs the coded sequence soft information through the BPSK-like demodulation module; Step S10: The LDPC decoding module iteratively decodes the soft information of the coding sequence to recover the service data.

5. The low-complexity phase synchronization receiving method of the MSK coding modulation system according to claim 4, characterized in that: The precoding method is as follows: In the formula, is the i-th precoded bit in the pulse, is the i-th bit and takes the value [0, 1] and , is an XOR operation; Precoding bits Dual polarization coding Then perform MSK modulation and output the MSK modulated signal, which is expressed as: In the formula, is the bit energy, T is the bit time, is the phase of the i-th bit, is the dual-polarization coding of the i-th bit, that is , j is the imaginary part identifier.

6. The low-complexity phase synchronization receiving method of the MSK coding modulation system according to claim 5, characterized in that: The phase decoupling frequency conversion module processing flow in step S4 is as follows: Step S41: Perform matched filtering on the received digital baseband signal, that is: In the formula, is the nth sampling point of the matched filter signal, To receive the mth sampling point of the digital baseband signal, The matched filter coefficients; Step S42: performing phase decoupling and frequency conversion processing on the matched filter signal, namely: In the formula, is the signal after phase decoupling and frequency conversion processing, T is the bit time, and Ts is the sampling time; Step S43: extract the signal after the phase decoupling frequency conversion processing to output a quasi-BPSK signal with a bit sampling rate of 2 times, that is: In the formula, The nth sampling point of a BPSK-like signal with a bit sampling rate of 2 times.

7. The low-complexity phase synchronization receiving method of the MSK coding modulation system according to claim 6, characterized in that: The channel estimation module has a built-in Doppler frequency estimation algorithm and a phase estimation algorithm.

8. The low-complexity phase synchronization receiving method of the MSK coding modulation system according to claim 7, characterized in that: The Doppler frequency estimation algorithm comprises: Step A: After frame synchronization, the starting time of each MSK bit time is used as the minimum time of inter-symbol interference to extract the best sampling point, where the sampling time is the bit time, that is: In the formula, is the nth best sampling point of the BPSK-like signal, is the sampling point at the frame header moment; Step B: Extract the pre-synchronization sequence and post-synchronization sequence in the pulse signal, conjugate and multiply them with the local sequence respectively, and take the phase to complete the phase estimation of the middle moment of the synchronization sequence, that is: Doppler phase estimation of intra-pulse preamble sequence : In the formula For phase operation, L is the synchronization sequence symbol length; The value of the nth bit of the local synchronization sequence, that is: in, is the nth original bit of the synchronization sequence in the pulse and its value is [0, 1]; Doppler phase estimation based on post-synchronization sequence in pulse : In the formula, is the number of symbols between the start time of the preamble sequence and the start time of the postamble sequence; Step C: Since the difference between the phases of the two synchronization segments has a 2π period ambiguity phenomenon, it is necessary to correct the phase estimation values ​​of the two synchronization segments, that is: when ,but ; when ,but ; Step D: Use the difference between the estimated phases of the two synchronization segments to estimate the Doppler frequency, that is: 。 9. The low-complexity phase synchronization receiving method of the MSK coding modulation system according to claim 8, characterized in that: The phase estimation algorithm comprises: Doppler frequency estimation and preamble sequence Doppler phase estimation , fit the channel Doppler phase of the pulse signal, that is: Where L is the synchronization sequence length, T is the bit time, Np is the number of pulse bits, and j is the imaginary part identifier.

10. The low-complexity phase synchronization receiving method of the MSK coding modulation system according to claim 9, characterized in that: The BPSK-like demodulation module processing flow in step S9 is as follows: Step S91: After suppressing the channel fading response through the channel equalization module, the received equalized signal is extracted. The sampling period is the bit period, and T is the bit time, that is: Where Np is the number of pulse symbols, is the k-th bit observation information of the pulse; Step S92: Equalize the signal The real part of is taken out as the k-th bit soft information of the pulse signal, that is: in, To take the real part operation, is the kth bit soft information of the pulse signal.

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