CN113630358A - Simplified despreading and demodulating method for direct sequence spread spectrum of SOQPSK - Google Patents
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Abstract
本发明涉及一种用于SOQPSK的直接序列扩频的简化解扩解调方法，包括步骤：1)在每个码元周期内，将每个单极性原始信息与该码元周期内的扩频码组进行异或处理，获取扩频序列；2)将扩频序列进行非递归式预编码的SOQPSK调制，得到DSSS‑SOQPSK基带发射信号；3)对接收信号进行匹配滤波，得到滤波结果；4)根据已知的扩频码组计算每个码片的度量增量；5)通过累加每个码元期间的码片度量增量得到码元度量增量，利用Viterbi算法进行解调恢复每个单极性原始信息。现有技术相比，本发明具有降低解扩解调算法复杂度，降低硬件开销，保证性能等优点。
The present invention relates to a simplified despreading and demodulation method for direct sequence spread spectrum of SOQPSK, comprising the steps of: 1) in each symbol period, compare each unipolar original information with the spread in the symbol period The frequency code group is XORed to obtain the spread spectrum sequence; 2) the spread spectrum sequence is subjected to the SOQPSK modulation of non-recursive precoding to obtain the DSSS-SOQPSK baseband transmission signal; 3) the received signal is subjected to matched filtering to obtain the filtering result; 4) Calculate the metric increment of each chip according to the known spread spectrum code group; 5) Obtain the symbol metric increment by accumulating the chip metric increment during each symbol period, and use the Viterbi algorithm to demodulate and recover each chip. unipolar raw information. Compared with the prior art, the present invention has the advantages of reducing the complexity of the despreading and demodulation algorithm, reducing the hardware overhead, and ensuring the performance.
Description
Technical Field
The invention relates to the technical field of wireless communication, in particular to a simplified despreading and demodulating method for direct sequence spread spectrum of SOQPSK (quadrature phase shift keying).
Background
SOQPSK is a special continuous phase modulation technique, which is developed based on continuous phase modulation and offset quadrature phase shift keying, and not only has the characteristics of constant envelope and continuous phase, but also has the advantages of high spectrum utilization rate and high power utilization rate, thus gaining wide attention and being adopted by a plurality of international standards. The SOQPSK mainly has two standards of SOQPSK-MIL and SOQPSK-TG. SOQPSK-MIL adopts full response rectangular pulse to make the phase change smooth; and the SOQPSK-TG adopts a raised cosine frequency pulse shaping function, further limits the bandwidth, simultaneously inhibits side lobes and has good spectral characteristics.
The direct sequence spread spectrum technology has the characteristics of signal concealment, narrow-band interference resistance, multiple access communication and the like. The SOQPSK and the direct sequence spread spectrum technology are combined to obtain a better spread spectrum modulation scheme which can meet the modern requirements on communication interference resistance, interception resistance and high frequency spectrum efficiency. However, the traditional despreading and demodulation algorithm has too high complexity and too large hardware overhead, and is difficult to be directly used in the direct sequence spreading technique of SOQPSK.
Disclosure of Invention
It is an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing a simplified despreading and demodulation method for direct sequence spreading in SOQPSK.
The purpose of the invention can be realized by the following technical scheme:
a simplified despreading demodulation method for direct sequence spreading of SOQPSK, the method comprising the steps of:
s1: and in each code element period, carrying out exclusive OR processing on each unipolar original information and the spreading code group in the code element period to obtain a spreading sequence.
The calculation of the spreading sequence is:
in the formula, NcFor spreading code block length, alphan,jThe jth spreading sequence for the nth symbol period, cn,jIs the jth pseudo-random spreading code of the nth symbol period, dnFor the nth unipolar original information, all alpha's are addedn,jSplicing into a spread spectrum sequence alpha according to time sequencei。
S2: and performing non-recursive pre-coding SOQPSK modulation on the spread spectrum sequence to obtain a DSSS-SOQPSK baseband transmission signal.
The expression of the DSSS-SOQPSK baseband transmission signal s (t, d, c) is as follows:
in the formula (I), the compound is shown in the specification,
representing the process of SOQPSK-TG modulation, N being the total number of input symbols, TcIs the chip period, g (t) is the phase shaping pulse of SOQPSK-TG, h is the modulation order, betaiFor spreading the frequency sequence alphaiAnd calculating the obtained ternary symbols.
S3: and performing matched filtering on the received signal to obtain a filtering result. Specifically, the method comprises the following steps:
respectively obtaining the results y of the matched filters by K groups of matched filters with pulse functions of PAM decomposition as filter parametersk,i：
yk,i＝∫r(t)pk(t-iTc)k∈{0,1,...,K-1}
In the formula, TcIs a chip period, pk(t) is the pulse function of the PAM decomposition.
S4: the metric increment per chip is calculated from the known spreading code group. The metric increment per chip is calculated as:
in the formula, Zc(n, j) is the jth chip metric increment of the nth symbol, vk,iThe k-th pulse with the largest energy for the ith chip corresponds to the decomposed pseudo-symbol.
S5: and obtaining a symbol metric increment by accumulating the chip metric increment during each symbol, and demodulating by using a Viterbi algorithm to recover the original information of each single polarity. Specifically, the method comprises the following steps:
selecting the signal with the minimum Euclidean distance from the received signal r (t) according to the maximum likelihood sequence detection theory
Is the decision output, which is expressed as:
simplifying the above equation, is equivalent to maximizing the following equation:
and thus the recursive expression Λ (d) is obtained as:
to find the corresponding original symbol information d by the above formulanWill beIt is further pressed by TcDecomposing and developing to obtain:
in the formula, the symbol metric increment
And performing incremental accumulation calculation by using the chip metric obtained by calculation in the step S4, and performing path search by using a Viterbi algorithm to recover the symbol information.
Compared with the prior art, the simplified despreading and demodulating method for the SOQPSK direct sequence spread spectrum at least has the following beneficial effects:
1) the spread spectrum sequence is obtained by using a pseudo-random spread spectrum sequence XOR information code with a rate much higher than that of the information code through direct sequence spread spectrum, and then SOQPSK modulation is carried out on the spread spectrum sequence, so that the transmission bandwidth of signals is expanded; and further, spread spectrum gain can be introduced, and the method has the advantages of strong anti-interference and anti-noise capabilities and the like.
2) According to the invention, an optimal de-spreading demodulation algorithm based on Maximum Likelihood Sequence Estimation (MLSE) is deduced at a receiving end according to a DSSS-SOQPSK signal model, and the complexity of the de-spreading demodulation algorithm is reduced by introducing a PAM decomposition mode, so that the hardware cost is reduced, and the performance is ensured.
Drawings
FIG. 1 is a diagram of an SOQPSK-TG based direct sequence spread spectrum baseband transmitter in an embodiment;
FIG. 2 is a diagram of direct sequence spread spectrum in an embodiment;
FIG. 3 is a diagram of a despreading receiver in an embodiment;
fig. 4 is a flow diagram of a simplified despreading demodulation method for direct sequence spreading for SOQPSK in an embodiment.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, shall fall within the scope of protection of the present invention.
Examples
The invention relates to a simplified despreading and demodulating method for SOQPSK direct sequence spread spectrum, which firstly deduces a direct sequence spread spectrum signal expression based on SOQPSK; secondly, deducing a despreading and demodulation algorithm based on MLSE; and finally, combining PAM decomposition to reduce the complexity of a despreading and demodulation algorithm on the premise of ensuring certain despreading and demodulation performance, thereby reducing the hardware overhead.
This example illustrates the method of the present invention in conjunction with a SOQPSK-TG transmitter. As shown in fig. 1, a diagram of a SOQPSK-TG based direct sequence spread spectrum baseband transmitter is shown. As shown in fig. 4, the method of the present invention comprises the following steps:
step one, the transmitting terminal mainly generates a direct sequence spread spectrum signal based on SOQPSK-TG, as shown in figure 2, in each code element period TbEach unipolar original information dnWith the spreading code group c in this periodnCorresponding exclusive OR is carried out to obtain a spread spectrum sequence alphanWill all be alphan,jSplicing into a spread spectrum sequence alpha according to time sequencei，NcIs the spreading code group length.
In the present invention, i and j are two latitudes and represent different meanings, i represents the index of the chip and ranges from 0 to NNc-1, j denotes the spreading code index within each symbol, ranging from 0 to Nc-1. N represents the index of the symbol, ranging from 0 to N-1, where N is the total number of input symbols.
Step two, spreading sequence alphaiAnd carrying out SOQPSK-TG modulation, wherein the SOQPSK-TG modulation comprises two parts of precoding and CPM modulation. Wherein, the pre-coding adopts non-recursive pre-coding and combines the binary symbols alphaiThe ternary symbol beta is obtained by the following calculationi。
βi＝(-1)i+1αi-1(αi-αi-2)/2
Wherein alpha isi∈{-1,1}，βi∈{-1,1}。
Will ternary symbol betaiModulating a baseband DSSS-SOQPSK-TG signal by:
wherein s (t, d, c) represents a base band DSSS-SOQPSK-TG signal,
represents the process of SOQPSK-TG modulation, where N is the total number of input symbols, TcFor the chip period, g (t) is the phase shaping pulse of SOQPSK-TG, h is the modulation order of 1/2, and the phase shaping pulse g (t) of SOQPSK-TG can be expressed as:
g(t)＝n(t)×w(t)
where n (T) is a raised cosine function, w (T) is a window function, the amplitude a is used to normalize the pulse shape so that the phase shift caused by a single frequency pulse is pi/2, and four parameters p is 0.7, B is 1.25, T is n (T) and w (T)1＝1.5，T20.5. q (t) is the frequency shaping pulse of SOQPSK-TG, which is expressed as:
step three, for the receiving end, the received signal can be modeled as:
r(t)＝s(t,d,c)+wn(t)
in the formula, wn(t) is the power spectral density N0A complex white gaussian noise signal.
Fig. 3 shows a schematic diagram of a despreading receiver.
For the convenience of derivation, the synchronization is ideal. For the received signal, since SOQPSK-TG is a partial response signal, its correlation length L is 8, i.e. the phase state in 1 symbol period is affected by the frequency pulse with the time length of 8 symbol periods. The maximum likelihood detection can cause the state number of the demodulation grid diagram to be overlarge, further cause the demodulation complexity to be overlarge, and is difficult to be practically applied. In order to reduce the demodulation complexity, PAM decomposition is introduced, and the SOQPSK signal is decomposed into a linear accumulation sum of K pulses with the maximum energy:
wherein T is a code element period; p is a radical ofk(t) is the kth pulse function of the PAM decomposition; v. ofk,iThe pulse with the largest energy at the kth cycle corresponds to a decomposed pseudo symbol, which is only related to the input symbol; there are various alternatives for PAM decomposition.
Since DSSS-SOQ SPK can be viewed as a modification of the SOQPSK signal, p is availablekPerforming multiple groups of matched filtering on filter parameters to obtain yk,iThe result of the matched filter is expressed as:
yk,i＝∫r(t)pk(t-iTc)k∈{0,1,...,K-1}
step four, because the currently decomposed pseudo symbol is only related to the input symbol, and the spreading code group is known, the jth chip metric increment of the nth symbol can be expressed as follows:
step five, obtaining a symbol measurement increment Z by accumulating the chip measurement increments in each symbol perioddUsing Viterbi algorithm to demodulate and recover original information dn。
Selecting a signal having a minimum Euclidean distance from a received signal according to maximum likelihood sequence detection theory (MLSE)
To be a decision output, it can be expressed as:
that is, since SOQPSK-TG is a constant envelope signal, the above equation can be simplified to the following equation:
it can therefore be rewritten with Λ (d) as a recursive expression:
since the spreading code group of the receiving end is known, the corresponding original code information d can be obtained by the above formulan. It can be further pressed by TcDecomposing to obtain:
therefore, the symbol metric increment can be obtained by accumulating the chip metric increments obtained by the calculation in the fourth step, and can be further used for the Viterbi algorithm to carry out path search and recover the symbol information.
According to the invention, by directly using the pseudorandom spread spectrum sequence XOR information code with the rate higher than that of the information code in sequence spread spectrum, the spread spectrum sequence is obtained and then subjected to SOQPSK modulation, so that the transmission bandwidth of signals is expanded; and further, spread spectrum gain can be introduced, and the method has the advantages of strong anti-interference and anti-noise capabilities and the like. At a receiving end, an optimal despreading and demodulation algorithm based on MLSE is deduced according to a DSSS-SOQPSK signal model, and due to the fact that the complexity of the optimal despreading and demodulation algorithm based on maximum likelihood detection is too high, the complexity of the despreading and demodulation algorithm is reduced by introducing a PAM decomposition mode, so that hardware cost is reduced, and performance is guaranteed.
While the invention has been described with reference to specific embodiments, the invention is not limited thereto, and those skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope of the invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (6)
1. A simplified despreading demodulation method for direct sequence spreading of SOQPSK comprising the steps of:
1) in each code element period, carrying out exclusive or processing on each unipolar original information and the spread spectrum code group in the code element period to obtain a spread spectrum sequence;
2) performing non-recursive pre-coding SOQPSK modulation on the spread spectrum sequence to obtain a DSSS-SOQPSK baseband transmitting signal;
3) performing matched filtering on the received signal to obtain a filtering result;
4) calculating the measurement increment of each chip according to the known spread spectrum code group;
5) and obtaining a symbol metric increment by accumulating the chip metric increment during each symbol, and demodulating by using a Viterbi algorithm to recover the original information of each single polarity.
2. The simplified despreading demodulation method for SOQPSK direct sequence spreading according to claim 1, wherein the spreading sequence is calculated in step 1) by the following formula:
in the formula, NcFor spreading code block length, alphan，jThe jth spreading sequence for the nth symbol period, cn，jIs the jth pseudo-random spreading code of the nth symbol period, dnFor the nth unipolar original information, all alpha's are addedn，jSplicing into a spread spectrum sequence alpha according to time sequencei。
3. The simplified despreading demodulation method for SOQPSK direct sequence spreading according to claim 2, wherein in step 2), the expression of DSSS-SOQPSK baseband transmit signal s (t, d, c) is:
in the formula (I), the compound is shown in the specification,
representing the process of SOQPSK-TG modulation, N being the total number of input symbols, TcIs the chip period, g (t) is the phase shaping pulse of SOQPSK-TG, h is the modulation order, betaiFor spreading the frequency sequence alphaiAnd calculating the obtained ternary symbols.
4. The simplified despreading demodulation method for SOQPSK direct sequence spreading according to claim 3, wherein the specific content of step 3) is:
respectively obtaining the results y of the matched filters by K groups of matched filters with pulse functions of PAM decomposition as filter parametersk，i：
yk，j＝∫r(t)pk(t-iTc)k∈{0，1，...，K-1}
In the formula, TcIs a chip period, pk(t) is the pulse function of the PAM decomposition.
5. The simplified despreading demodulation method for SOQPSK direct sequence spreading according to claim 4, wherein in step 4), the metric increment per chip is calculated by:
in the formula, Zc(n, j) is the jth chip metric increment of the nth symbol, vk，iThe k-th pulse with the largest energy for the ith chip corresponds to the decomposed pseudo-symbol.
6. The simplified despreading demodulation method for SOQPSK direct sequence spreading according to claim 4, wherein the specific content of step 5) is:
selecting the signal with the minimum Euclidean distance from the received signal r (t) according to the maximum likelihood sequence detection theory
Is the decision output, which is expressed as:
simplifying the above equation, is equivalent to maximizing the following equation:
and thus the recursive expression Λ (d) is obtained as:
to find the corresponding original symbol information d by the above formulanFurther press it by TcDecomposing and developing to obtain:
in the formula, the symbol metric increment
The chip measurement increment obtained by calculation in the step 4) is accumulated and calculated, and path search is carried out by adopting a Viterbi algorithm to recover code element information.
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