CN114759949A - Fast code capture method in BPSK modulation direct sequence spread spectrum communication - Google Patents

Abstract

The invention relates to the field of wireless communication equipment, in particular to a fast code capturing method in BPSK modulation direct sequence spread spectrum communication. The invention can reduce the capture time, has no relation between the capture time and the distance uncertainty, improves the code capture efficiency, completes the main operation through the most commonly used FIR filter in the digital signal processing, and is easy to realize.

Description

A Fast Code Acquisition Method in BPSK Modulated Direct Sequence Spread Spectrum Communication

technical field

The invention relates to the field of wireless communication equipment, in particular to a fast code acquisition method in BPSK modulation direct sequence spread spectrum communication.

Background technique

Code (PN) acquisition technology is an important technology in spread spectrum communication. The so-called code acquisition is to align the PN code of the local receiver and the received PN code within a chip (T seconds period), when the symbol has been realized. After the acquisition and the code loop acquisition enters steady-state tracking, the synchronization is completed.

When designing a pseudo-code capture system for a spread spectrum receiver, many parameters should be considered. The uncertainty of frequency and time are the two most important parameters of the capture performance. The time uncertainty includes two aspects, one is the transmit clock and the receive clock. The second is the distance uncertainty. The greater the effective time uncertainty, the longer the time required to complete the acquisition. For the general slow acquisition method, the influence of noise is not considered. If the length of the PN code is L, and the duration of one chip is T, as long as the "hit" "The capture can be determined once, and the capture time is related to the uncertainty of the distance. The longest capture time is L*L*T, and its probability of occurrence is 1/L. If the influence of noise is considered, in order to reduce the false alarm probability of false capture, it must be M consecutive "hit" is determined as capture, then the longest capture time is M*L*L*T.

SUMMARY OF THE INVENTION

Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a fast code acquisition method in BPSK modulation direct sequence spread spectrum communication, which can reduce the acquisition time, and the acquisition time has nothing to do with the distance uncertainty, thereby improving the code acquisition efficiency.

In order to achieve the above objects, the present invention adopts the following technical solutions to achieve.

A fast code acquisition method in BPSK modulation direct sequence spread spectrum communication, comprising the following steps:

Step 1: The received signal is multiplied by the I-channel local carrier signal cos(ω ₁ t+θ ₁ ) generated by the local carrier NCO and the Q-channel local carrier signal sin(ω ₁ t+θ ₁ ) generated by the local carrier NCO, and then Mixing to obtain signal I ₀ and signal Q ₀ respectively;

Step 2, the signal I ₀ and the signal Q ₀ are respectively subjected to low-pass filtering to obtain the baseband signal I ₁ and the baseband signal Q ₁ ;

Step 3, filter and extract the baseband signal I ₁ and the baseband signal Q ₁ respectively, extract two sample points within the duration of each chip, and obtain the low-sampling signal I ₂ and the low-sampling signal Q ₂ respectively;

Step 4, the low sampling signal I ₂ and the low sampling signal Q ₂ are respectively input to the FIR filter to obtain the data I ₃ and the data Q ₃ respectively;

Step 5, for each pair of sample points of the data I ₃ and the data Q ₃ , do the sum of squares and then take the square root, and consider the frequency difference to obtain R ₀ , R ₁ , R ₂ .....R _i , i=0, 1,...,2L-1; let R _x be the maximum value among R ₀ , R ₁ , R ₂ .....R _i , and x be the serial number;

Step 6, compares the maximum value R _x with the capture threshold n, and sends an instruction signal to the code generator according to the comparison result;

Step 7, the code generator outputs the spread spectrum sequence according to the instruction signal.

Compared with the prior art, the beneficial effects of the present invention are: if the influence of noise is not considered, code acquisition can be realized within one symbol time L*Tc, and if the influence of noise is considered, the longest acquisition time is M*L*Tc , the longest acquisition time is only 1/L of the longest acquisition time of the general slow acquisition, which reduces the acquisition time; and the acquisition time has nothing to do with the distance uncertainty, which improves the code acquisition efficiency; the main operation is the most commonly used in digital signal processing. FIR filter to complete, easy to implement.

Description of drawings

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

FIG. 1 is a flow chart of the fast code acquisition method for BPSK modulation direct sequence spread spectrum communication according to the present invention.

Detailed ways

The embodiments of the present invention will be described in detail below with reference to the examples, but those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention.

Referring to FIG. 1, a method for fast code acquisition in BPSK modulation direct sequence spread spectrum communication includes the following steps:

Step 1: The received signal is multiplied by the I-channel local carrier signal cos(ω ₁ t+θ ₁ ) generated by the local carrier NCO and the Q-channel local carrier signal sin(ω ₁ t+θ ₁ ) generated by the local carrier NCO, and then Mixing to obtain signal I ₀ and signal Q ₀ respectively;

Specifically, for BPSK chip (PN code) modulation, the complex envelope form is as follows:

Among them, P is the power of the spread spectrum signal; P _T (t) is the unit pulse of T seconds, starting at t=0 and ending at t=T seconds; k is an integer, indicating how many T times the unit pulse delays, T is the unit pulse duration, that is, the pulse width; a _k is the coefficient of the direct spreading sequence chip sequence, a _k ∈ {1, -1};

For the convenience of discussion, PN(t) is usually used to represent the chip sequence, then we have

In addition, define the data sequence d(t) as

Among them, P _MT (t) is a unit pulse with a duration of MT, j in P _MT (t-jMT) is an integer, indicating how many MTs the unit pulse delays, and M indicates that each bit or coded bit has exactly M chips, T is the width of the unit pulse P _T (t); d _j is the discrete data symbol, which is 1 or -1; the probability of each bit d _j being 1 or -1 is 1/2, and Each d _j is statistically independent of each other;

When BPSK chip modulation uses BPSK data modulation, the complex envelope form can be abbreviated as:

The carrier modulation is performed on the signal v(t), and the form of the spread spectrum signal is obtained as:

Among them, P _t is the power of the spread spectrum signal;

The signal modulated by the carrier will produce attenuation and phase delay during transmission, so the received signal y(t) can be written as:

Among them, P is the power of the received signal and P<<P _t , T _p is the signal delay, the carrier phase θ ₀ is simulated by a uniform random variable, the distribution area is 0-2π radians, n(t) is noise, J (t) is an interfering signal;

Signal I ₀ is:

Signal Q0 is _:

Step 2, the signal I ₀ and the signal Q ₀ are respectively subjected to low-pass filtering to obtain the baseband signal I ₁ and the baseband signal Q ₁ ;

Specifically, the baseband signal I ₁ is:

_The baseband signal Q1 is:

Step 3, filter and extract the baseband signal I ₁ and the baseband signal Q ₁ respectively, extract two sample points within the duration of each chip, and obtain the low-sampling signal I ₂ and the low-sampling signal Q ₂ respectively;

Step 4, the low sampling signal I ₂ and the low sampling signal Q ₂ are respectively input to the FIR filter to obtain the data I ₃ and the data Q ₃ respectively;

Specifically, the value of the FIR filter coefficient is twice the sampling value of the spread spectrum code sequence PN(t); the length of the spread spectrum code sequence PN(t) is L, then the FIR filter coefficient length is 2L;

Data I ₃ is:

Data _Q3 is:

Among them, i=0, 1,..., 2L-1, T _b is the spread spectrum code sequence delay, and T _p is the signal delay.

Step 5, for each pair of sample points of the data I ₃ and the data Q ₃ , do the sum of squares and then take the square root, and consider the frequency difference to obtain R ₀ , R ₁ , R ₂ .....R _i , i=0, 1,...,2L-1; let R _x be the maximum value among R ₀ , R ₁ , R ₂ .....R _i , and x be the serial number;

specific,

)时，由于扩频码的自相关性，R_i最大。When the difference between T _b and T _p is less than half a chip period (ie

), R _i is the largest due to the autocorrelation of the spreading code.

Step 6, compares the maximum value R _x with the capture threshold n, and sends an instruction signal to the code generator according to the comparison result;

Specifically, if the maximum value R _x is greater than n, then judge whether to complete the capture; if the capture is not completed, output the capture indication signal and the sequence number x to the code generator; if the capture has been completed, then output the sequence number 0 to the code generator;

If the maximum value Rx _is not greater than η, then judge whether the acquisition is completed; if the acquisition is not completed, set the serial number x=0 and output it to the code generator; if the acquisition has been completed, then judge whether the maximum value Rx _is not greater than 3 consecutive times n, if it is, output the signal of loss of capture to the code generator, if not, output the serial number 0 to the code generator.

Step 7, the code generator outputs the spread spectrum sequence samples according to the instruction signal;

的2倍采样样点，即输出样点数目为2L个的扩频码序列a₀,a₀,a₁,a₁,a₂,a₂.....a_L-1,a_L-1，其中T_a为码发生器第0个样点的输出时间；同时复位FIR滤波器，返回步骤4；Specifically, if the code generator receives the loss-of-capture indication signal, the code generator is reset, and the code generator starts to output the spread spectrum code sequence sample points again from the 0th sample point a _0. At this time, Ta=Tb, the output spread spectrum code frequency code sequence

2 times the sampling points, that is, the spread spectrum code sequence a ₀ ,a ₀ ,a ₁ ,a ₁ ,a ₂ ,a ₂ .....a _L-1 ,a _{L- 1} , where T _a is the output time of the 0th sample point of the code generator; reset the FIR filter at the same time, and return to step 4;

的2倍采样样点，即输出样点数目为2L个的扩频码序列a₀,a₀,a₁,a₁,a₂,a₂.....a_L-1,a_L-1，；If the code generator only receives sequence number 0, the code generator does not reset and delay, and normally outputs the spread spectrum code sequence

2 times the sampling points, that is, the spread spectrum code sequence a ₀ ,a ₀ ,a ₁ ,a ₁ ,a ₂ ,a ₂ .....a _L-1 ,a _{L- 1} ,;

的时间后)，此时

输出扩频码序列

的2倍采样样点，即输出样点数目为2L个的扩频码序列a₀,a₀,a₁,a₁,a₂,a₂.....a_L-1,a_L-1，此时码发生器输出的扩频码序列和接收信号的伪随机码相位差小于半个码片周期

完成捕获。If the code generator receives the capture indication signal and the serial number x, the code generator resets first, and then delays x samples (that is, delays

time), at this time

Output spreading code sequence

2 times the sampling points, that is, the spread spectrum code sequence a ₀ ,a ₀ ,a ₁ ,a ₁ ,a ₂ ,a ₂ .....a _L-1 ,a _{L- 1} , the phase difference between the spread spectrum code sequence output by the code generator and the pseudo-random code of the received signal is less than half a chip period

Complete capture.

Although the present invention has been described in detail with general description and specific embodiments in this specification, some modifications or improvements can be made on the basis of the present invention, which will be obvious to those skilled in the art. Therefore, these modifications or improvements made without departing from the spirit of the present invention fall within the scope of the claimed protection of the present invention.

Claims (7)

1. A fast code acquisition method in BPSK modulated direct sequence spread spectrum communication, comprising the steps of:

step 1, receiving signals and local carrier signals cos (omega) generated by local carrier NCO respectively₁t+θ₁) And a Q-path local carrier signal sin (omega) generated by a local carrier NCO₁t+θ₁) Multiplying and mixing to obtain signals I₀Sum signal Q₀；

Step 2, signal I₀Sum signal Q₀Respectively low-pass filtering to obtain baseband signals I₁And baseband signal Q₁；

Step 3, for baseband signal I₁And baseband signal Q₁Filtering and extracting respectively, extracting two samples in each chip duration to obtain low-sampling signals I₂And low sampling signal Q₂；

Step 4, low sampling signal I₂And low sampling signal Q₂Respectively input into FIR filters to respectively obtain data I₃And data Q₃；

Step 5, data I is processed₃And data Q₃For each pair of samples, squaring and re-squaring, and taking frequency difference into account to obtain R₀,R₁,R₂.....R_iI-0, 1, ·, 2L-1; let R_xIs R₀,R₁,R₂.....R_iX is a serial number;

step 6, setting the maximum value R_xComparing with a capture threshold eta, and sending an indication signal to a code generator according to a comparison result;

and 7, outputting a spread spectrum sequence by the code generator according to the indication signal.

2. The method of claim 1, wherein in step 1, the received signal is:

Wherein P is the power of the received signal; pn (t) represents a chip sequence; d (t) represents a data sequence; t is_pDelaying the signal; theta₀The carrier phase is distributed in a uniform random variable with 0-2 pi radian; n (t) is noise; j (t) is an interference signal;

signal I₀Comprises the following steps:

signal Q₀Comprises the following steps:

3. the method of claim 1, wherein in step 2, the baseband signal I is used as the fast code acquisition in BPSK modulated direct sequence spread spectrum communications₁Comprises the following steps:

baseband signal Q₁Comprises the following steps:

4. the method of claim 1, wherein in step 4, the FIR filter coefficients are 2 times the sample values of the spreading code sequence pn (t); the length of the spreading code sequence PN (t) is L, and the coefficient length of the FIR filter is 2L;

data I₃Comprises the following steps:

data Q₃Comprises the following steps:

wherein i is 0,1, 2L-1, T_bDelaying the spreading code sequence, T_pThe signal is delayed.

5. The method of claim 1A fast code acquisition method in BPSK modulation direct sequence spread spectrum communication, characterized in that in step 5, R_iThe calculation method is as follows:

wherein, i is 0,1, 2L-1.

6. The method of claim 1, wherein step 6 is performed specifically if the maximum value R is R _xIf the current is greater than eta, judging whether the capture is finished; if the capture is not completed, a capture indication signal and a serial number x are output to a code generator; if the capture is finished, outputting a serial number 0 to a code generator;

if the maximum value R_xIf the current value is not greater than eta, judging whether the capture is finished or not; if the capture is not completed, the serial number x is made to be 0 and output to the code generator; if the capture is finished, judging whether the maximum value R is continuously carried out for 3 times_xIf not, outputting a capture losing indication signal to the code generator, otherwise, outputting a serial number 0 to the code generator.

7. The method of claim 1, wherein the code generator is reset if the code generator receives the mis-acquisition indication signal, and the code generator resets from the 0 th sampling point a₀Starting to re-output an output spread code sequence

2 times the number of sampling points, i.e. a spreading code sequence a with 2L output sampling points₀,a₀,a₁,a₁,a₂,a₂.....a_L-1,a_L-1(ii) a Resetting the FIR filter simultaneously and returning to the step 4;

if the code generator only receives the serial number 0, the code generator normally outputs the spreading code sequence

2 times the number of sampling points, i.e. a spreading code sequence a with 2L output sampling points₀,a₀,a₁,a₁,a₂,a₂.....a_L-1,a_L-1，；

If the code generator receives the capture indication signal and the serial number x, the code generator is first reset and then delayed

After time of (3), outputting the spread spectrum code sequence

2 times of sampling points, i.e. spreading code sequence a with 2L output samples₀,a₀,a₁,a₁,a₂,a₂.....a_L-1,a_L-1And completing the capture.

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