阅读说明：本技术 一种简化的msk扩频系统非相干解扩解调方法 (Simplified non-coherent despreading and demodulating method for MSK spread spectrum system ) 是由 蔡文波 杨飞 肖方可 赵珍祥 袁勇 于 2020-08-05 设计创作，主要内容包括：本发明公开了一种简化的MSK扩频系统非相干解扩解调方法,基于并行实现结构,在接收端不需要进行第二级载波解调,直接进行相关峰搜索,在保持了传统的解扩解调结构基础上,非常简便地实现非相干解调,大幅简化MSK-扩频系统的工程实现难度。通过本发明所述的解扩方法,有效避免了串行结构中复杂的转换滤波器设计,将传统并行结构中的波形解调和扩频序列相关相结合,变成了对一个复数解扩序列的构造,从而简化了接收机实现结构。(The invention discloses a simplified non-coherent de-spreading demodulation method of an MSK spread spectrum system, which is based on a parallel realization structure, does not need to carry out secondary carrier demodulation at a receiving end, directly carries out related peak search, very simply and conveniently realizes non-coherent demodulation on the basis of keeping the traditional de-spreading demodulation structure, and greatly simplifies the engineering realization difficulty of the MSK-spread spectrum system. The despreading method effectively avoids the design of a complex conversion filter in a serial structure, combines the waveform demodulation and the correlation of a spreading sequence in the traditional parallel structure, and changes the structure of a complex despreading sequence, thereby simplifying the realization structure of a receiver.)

1. A simplified non-coherent despreading and demodulating method for MSK spread spectrum system is characterized in that: the method is applied to the non-coherent receiver of the MSK spread spectrum system, and based on a parallel structure, a complex number de-spreading sequence is constructed to search a correlation peak.

2. The simplified method for noncoherent despreading and demodulation in an MSK spread spectrum system as set forth in claim 1, wherein constructing the complex despreading sequence comprises:

the MSK received signal model is:

nLT_b+lT_c＜t≤nLT_b+(l+1)T_c(0.1)

wherein A is the signal amplitude, f_cIs a carrier frequency point, a_nFor the nth symbol of the spread sequence, T_cFor the duration of the symbol period,

symbol z (t) before system spreading CN (0, N)₀) Period of T_bThe system spreading factor is L, i.e. T_b＝LT_c；

b_nFor the nth information sequence before spreading, p_lIs the first symbol in the spreading sequence, a_nL+l＝b_np_l；

The output end of the transmitting baseband is output by M times of interpolation, and the symbol period of the output of the transmitting baseband is assumed to be T_sAnd T is_c＝MT_s；

z (t) is channel noise, f_dDoppler frequency offset caused by relative motion of a receiving end and a transmitting end;

the receiving end has a pass frequency of f_cThe local oscillator of' performs carrier demodulation assuming that:

Δf＝f_c+f_d-f_c' (0.2)

when the crystal oscillator for the modulation and demodulation of the carrier wave at the receiving and transmitting end has high precision, the crystal oscillator has the condition that delta f is approximately equal to f_dAfter carrier demodulation and low-pass filtering, the signal is processed by T_sAfter periodic sampling, the digital signal obtained is written in complex form as:

for the above signals, the optimal sampling point is M ═ M, and there is M at the optimal sampling point

Further conversion to (1.4) formula

To obtain b_nSequence, construct the following despreading sequence

Wherein, P_l ¹,P_l ^-1The despreading sequences for the 1 sequence and the-1 sequence which are suitable for the MSK spread spectrum signal are respectively used for carrying out correlation operation on the received signal by utilizing the sequences, so that the despreading of the MSK spread spectrum signal is realized.

3. The simplified method for noncoherent despreading and demodulation in an MSK spread spectrum system as set forth in claim 2, wherein P is chosen to be the best sampling point_l ¹After complex correlation accumulation with y (n, l, M), the obtained correlation signal modulus is expressed as

At p_lWhen the probability of occurrence of. + -. 1 is equivalent, the following steps are performedWhen there is

In the same way, P is_l ^-1The correlation with y (n, l, M) is calculated by

4. The simplified method for noncoherent despreading and demodulation in an MSK spread spectrum system as set forth in claim 2, wherein b is_nThe value of (a) is determined based on the following formula:

5. the simplified method for noncoherent despreading and demodulation in an MSK spread spectrum system as set forth in claim 2, wherein the soft information for soft decoding is obtained based on the following equation:

6. the simplified method for demodulating noncoherent despreading of an MSK spread spectrum system as set forth in claim 1, wherein the complex despreading sequence is constructed by MSK modulating the spreading sequence, and the values of the real part and the imaginary part of the complex despreading sequence belong to a simple set of {1, -1,0 }.

Technical Field

The invention relates to the technical field of communication transmission, in particular to a simplified non-coherent despreading and demodulating method for an MSK spread spectrum system.

Background

A communication system based on an msk (minimum frequency Shift keying) spread spectrum modulation system is widely applied to a plurality of fields such as communication transmission, electronic countermeasure, data link, and the like. The MSK modulation signal has the characteristics of constant envelope, low carrier energy sidelobe and the like, is very suitable for a wireless communication system with high carrier dynamics, limited transmitting power and serious nonlinear distortion, and can ensure the normal communication of the whole system under the environment of high dynamics and strong interference when a combined transmission system formed by combining a spread spectrum system and a frequency hopping system.

The existing demodulation method of the MSK spread spectrum system can be divided into a parallel mode and a serial mode.

For the parallel modulation and demodulation method, the method is realized by 2-level quadrature modulation and demodulation. The receiving end needs to perform 2-level carrier demodulation before performing de-spreading processing, and the structure is complex when performing noncoherent demodulation. The parallel architecture implementation is shown in fig. 1.

For the serial modem method, the MSK system can be equivalent to a BPSK system and a special shaping filter. The method can make the modulation and demodulation of the MSK system equivalent to the structure of the BPSK system. However, the design of the shaping filter of the method is a convex optimization problem, the calculation is very complex, and the obtained result is only a numerical solution, and an analytic solution cannot be obtained.

The existing parallel implementation method needs to carry out carrier demodulation for 2 times and then de-spread, and when carrying out incoherent demodulation, the implementation structure is complex; the existing serial implementation method needs a complex conversion filter design. The technical defects are that only the transformation processing of signals is concerned, a transmitting end carries out the transformation processing of the signals, and a receiving end carries out the inverse transformation processing so as to accord with the traditional despreading and demodulation structure, and the transformation processing of a spread spectrum sequence is neglected.

Disclosure of Invention

Aiming at the problems of higher complexity and difficult engineering realization of a despreading and demodulating algorithm of the existing MSK spread spectrum system, the invention provides a simplified non-coherent despreading and demodulating method of the MSK spread spectrum system, which combines waveform demodulation and spread spectrum despreading so as to simplify the structure of a receiver and reduce the realization complexity of the receiver.

In order to achieve the above object, the present disclosure provides a simplified method for demodulating incoherent despreading of an MSK spread spectrum system, which is applied to an incoherent receiver of an MSK spread spectrum system, and constructs a complex despreading sequence based on a parallel structure to perform correlation peak search.

Optionally, constructing the complex despreading sequence comprises:

the MSK received signal model is:

wherein A is the signal amplitude, f_cIs a carrier frequency point, a_nFor the nth symbol of the spread sequence, T_cFor the duration of the symbol period,

in order to be the initial phase position,is (nL + l) T_cThe accumulated phase values of all transmitted symbols at a time,

symbol z (t) before system spreading CN (0, N)₀) Period of T_bThe system spreading factor is L, i.e. T_b＝LT_c；

b_nFor the nth information sequence before spreading, p_lIs the first symbol in the spreading sequence, a_nL+l＝b_np_l；

The output end of the transmitting baseband is output by M times of interpolation, and the symbol period of the output of the transmitting baseband is assumed to be T_sAnd T is_c＝MT_s；

z (t) is channel noise, f_dDoppler frequency offset caused by relative motion of a receiving end and a transmitting end;

the receiving end has a pass frequency of f_cThe local oscillator of' performs carrier demodulation assuming that:

Δf＝f_c+f_d-f_c' (0.2)

carrier modulation and demodulation crystal of transmitting and receiving endWhen the vibration precision is high, the existence of delta f is approximately equal to f_dAfter carrier demodulation and low-pass filtering, the signal is processed by T_sAfter periodic sampling, the digital signal obtained is written in complex form as:

for the above signals, the optimal sampling point is M ═ M, and there is M at the optimal sampling point

Further conversion to (1.4) formula

To obtain b_nSequence, construct the following despreading sequence

Wherein, P_l ¹,P_l ^-1The despreading sequences for the 1 sequence and the-1 sequence which are suitable for the MSK spread spectrum signal are respectively used for carrying out correlation operation on the received signal by utilizing the sequences, so that the despreading of the MSK spread spectrum signal is realized.

Alternatively, for the optimal sampling point, P is_l ¹After complex correlation accumulation with y (n, l, M), the obtained correlation signal modulus is expressed as

At p_lWhen the probability of occurrence of. + -. 1 is equivalent, the following steps are performedWhen there is

In the same way, P is_l ^-1The correlation with y (n, l, M) is calculated by

Alternatively, b_nThe value of (a) is determined based on the following formula:

optionally, the soft information for soft decoding is obtained based on:

optionally, the method for constructing the complex despreading sequence is to generate the spreading sequence through MSK modulation, and values of a real part and an imaginary part of the spreading sequence both belong to a simple set of {1, -1,0 }.

The invention has the beneficial effects that:

the simplified incoherent despreading and demodulating method of the MSK spread spectrum system is based on a parallel realizing structure, does not need to carry out secondary carrier demodulation at a receiving end, directly carries out related peak searching, very simply and conveniently realizes incoherent demodulation on the basis of keeping the traditional despreading and demodulating structure, and greatly simplifies the engineering realizing difficulty of the MSK spread spectrum system. The despreading method effectively avoids the design of a complex conversion filter in a serial structure, combines the waveform demodulation and the correlation of a spreading sequence in the traditional parallel structure, and changes the structure of a complex despreading sequence, thereby simplifying the realization structure of a receiver.

Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

Drawings

The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure without limiting the disclosure. In the drawings:

fig. 1 is a block diagram of a parallel structure transmitter and receiver implementation of an MSK spread spectrum system in the background art, where a is a block diagram of a parallel structure transmitter implementation of a spread spectrum system, and b is a block diagram of a parallel structure non-coherent receiver implementation of a spread spectrum system;

fig. 2 is a block diagram of an implementation of the transmission and reception of the MSK spread spectrum system according to the present invention, wherein c is a block diagram of an implementation of a transmitter in a parallel structure of the spread spectrum system, and d is a block diagram of an implementation of a non-coherent receiver in a parallel structure of the spread spectrum system;

fig. 3 is a diagram of the despreading and soft information mapping of the MSK spread spectrum system of the present invention;

fig. 4 is a simulation diagram of the error rate performance of the MSK spread spectrum system according to the present invention.

Detailed Description

The following detailed description of specific embodiments of the present disclosure is provided in connection with the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating the present disclosure, are given by way of illustration and explanation only, not limitation.

As shown in fig. 2, compared to the conventional parallel demodulation method, the construction of the complex despreading sequence according to the present invention includes the following steps:

the MSK receiving signal model related by the invention is as follows:

wherein A is the signal amplitude, f_cIs a carrier frequency point, a_nFor the nth symbol of the spread sequence, T_cFor the duration of the symbol period,

is an initialPhase (phase corresponding to time 0, which can be assumed in general)

Is (nL + l) T_cThe accumulated phase values of all transmitted symbols at a time,

symbol z (t) before system spreading CN (0, N)₀) Period of T_bThe system spreading factor is L, i.e. T_b＝LT_c。b_nFor the nth information sequence before spreading, p_lIs the first symbol in the spreading sequence, a_nL+l＝b_np_l. At the output end of the transmitting baseband, the signal is generally output by M times of interpolation, and the symbol period of the baseband output is assumed to be T_sAnd T is_c＝MT_s. z (t) is the channel noise, here assumed to be additive white Gaussian noise, f_dThe Doppler frequency offset caused by the relative motion of the receiving end and the transmitting end.

At the receiving end the pass frequency is f_cThe local oscillator of' performs carrier demodulation, assuming

Δf＝f_c+f_d-f_c' (0.13)

In general, if the crystal oscillator for modulation and demodulation of the carrier at the transmitting and receiving ends has high precision, Δ f ≈ f_d. The above signals, after carrier demodulation and low-pass filtering, are processed by T_sAfter periodic sampling, the obtained digital signal is written into a complex form (noise is ignored, and the signal amplitude is normalized), which can be expressed as

For the above signals, the optimal sampling point is M ═ M, and there is M at the optimal sampling point

For the above signals, it is desired to remove the influence of the spreading factorTo directly obtain b_nSequence, further transformation of the above formula

It can be found that b is desired to be obtained_nSequence, it is necessary to construct the following despreading sequences

Wherein, P_l ¹,P_l ^-1Respectively, despreading sequences for 1 sequence and-1 sequence, which are actually sequences obtained by msk modulation of spreading sequences, are applied to the msk spread signal. By using the sequence to carry out correlation operation with the received signal, the despreading of the msk spread spectrum signal can be directly realized.

Calculating the complex correlation accumulation according to the designed despreading sequence:

in the case of an alignment of the information bits, P is assigned to the best sampling point_l ¹The correlation signal modulus obtained after complex correlation accumulation with y (n, l, M) can be expressed as

At p_lWhen the probability of occurrence of. + -. 1 is equivalent, the following steps are performed

When there is

In the same way, P is_l ^-1The correlation with y (n, l, M) is calculated by

B is carried out according to the designed despreading sequence_nAnd (6) symbol judgment.

b_nThe value of (a) can be decided based on the following method:

if soft information is to be obtained for subsequent decoding, the soft information may be defined as

Based on the above method, the despreading and demodulation scheme of the MSK spread spectrum system is shown in fig. 3.

Through the method, the spread spectrum sequence is only required to be subjected to MSK modulation at the receiving end, the de-spreading of the MSK spread spectrum system can be changed into the same de-spreading process as that of the common BPSK/QPSK spread spectrum system, the symbol synchronization and the code synchronization can be obtained only by carrying out code search, and the soft information output can be carried out after the code synchronization is finished so as to carry out subsequent channel decoding.

When the spreading code is determined, the complex de-spreading sequence is determined and can be calculated and stored in advance without real-time calculation; compared with the traditional spread spectrum sequence search, the complexity is not increased;

the real part and the imaginary part of the complex de-spreading sequence constructed by the invention take values of { +1,0, -1}, and the complexity of correlation operation is low; for example, for a spreading sequence [1,1,0,0] with a spreading multiple of 4, the real part of the corresponding complex despreading sequence for the +1 sequence is [0, -1,0,1], and the imaginary part is [ -1,0, -1,0 ];

the method can be well used for incoherent demodulation and is suitable for burst communication;

compared with the traditional parallel structure, the method has the advantages that the first-level waveform construction process is reduced, the receiver structure is simplified, and the implementation complexity is reduced;

compared with a serial structure demodulation method, the method avoids complex conversion filter design and reduces demodulation complexity.

As shown in fig. 4, the method of the present invention was simulated for non-coherent demodulation performance under different spreading ratios. Under the AWGN channel condition, the error code performance of the spreading multiples of 16, 32 and 64 can well approach the theoretical performance of msk incoherent demodulation, and the correctness of the method is explained.

The preferred embodiments of the present disclosure are described in detail with reference to the accompanying drawings, however, the present disclosure is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solution of the present disclosure within the technical idea of the present disclosure, and these simple modifications all belong to the protection scope of the present disclosure.

It should be noted that, in the foregoing embodiments, various features described in the above embodiments may be combined in any suitable manner, and in order to avoid unnecessary repetition, various combinations that are possible in the present disclosure are not described again.

In addition, any combination of various embodiments of the present disclosure may be made, and the same should be considered as the disclosure of the present disclosure, as long as it does not depart from the spirit of the present disclosure.