A fast code acquisition method in BPSK modulation direct sequence spread spectrum communication

Abstract

The present invention relates to the field of wireless communication equipment, and in particular to a fast code capture method in BPSK modulation direct sequence spread spectrum communication. The present invention can reduce the capture time, and the capture time is independent of distance uncertainty, improve the code capture efficiency, and the main operation is completed by the most commonly used FIR filter in digital signal processing, which is easy to implement.

Description

Technical Field

[0001] The invention relates to the field of wireless communication equipment, and in particular to a fast code capture method in BPSK modulation direct sequence spread spectrum communication. Background Art

[0002] Code (PN) capture technology is an important technology in spread spectrum communication. The so-called code capture is to align the PN code of the local receiver with the received PN code within one code chip (T second period). When the code element capture has been achieved and the code loop capture enters steady-state tracking, the synchronization is completed.

[0003] When designing a spread spectrum receiver pseudo code capture system, many parameters must be considered. Frequency and time uncertainties are the two most important parameters for capture performance. Time uncertainty includes two aspects: one is the actual uncertainty of the transmitting clock and the receiving clock, and the other is the distance uncertainty. The greater the effective time uncertainty, the longer it takes to complete the capture. For the general slow capture method, without considering the influence of noise, if the PN code length is L and the duration of a code chip is T, as long as it is "hit" once, it can be determined to be captured. The capture time is related to the distance uncertainty. 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 probability of false capture, it must be "hit" M times in a row to be determined as captured, then the longest capture time is M*L*L*T. Summary of the invention

[0004] In view of the problems existing in the prior art, the purpose of the present invention is to provide a fast code capture method in BPSK modulated direct sequence spread spectrum communication, which reduces the capture time, and the capture time is independent of distance uncertainty, thereby improving the code capture efficiency.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions to achieve it.

[0006] A fast code acquisition method in BPSK modulation direct sequence spread spectrum communication comprises the following steps:

[0007] Step 1, the received signal is multiplied by the I-channel local carrier signal cos(ω1t+θ1) generated by the local carrier NCO and the Q-channel local carrier signal sin(ω1t+θ1) generated by the local carrier NCO, respectively, and mixed to obtain signal I0 and signal Q0 respectively;

[0008] Step 2, the signal I0 and the signal Q0 are respectively subjected to low-pass filtering to obtain the baseband signal I1 and the baseband signal Q1 respectively;

[0009] Step 3, filtering and extracting the baseband signal I1 and the baseband signal Q1 respectively, extracting two sampling points within each chip duration, and obtaining the low sampling signal I2 and the low sampling signal Q2 respectively;

[0010] Step 4, the low sampling signal I2 and the low sampling signal Q2 are respectively input into the FIR filter to obtain data I3 and data Q3 respectively;

[0011] Step 5: For each pair of sample points of data I3 and data Q3, square the sum and then take the square root, and consider the frequency difference to obtain R0, R1, R2.....R i , i=0,1,...,2L-1; let R x R0, R1, R2.....R i The maximum value in , x is the serial number;

[0012] Step 6: Set the maximum value R x Compare with the capture threshold η, and send an indication signal to the code generator according to the comparison result;

[0013] Step 7: The code generator outputs a spread spectrum sequence according to the indication signal.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: if the influence of noise is not considered, code capture can be achieved within one code element time L*Tc; if the influence of noise is considered, the longest capture time is M*L*Tc, and the longest capture time is only 1 / L of the longest capture time of general slow capture, thereby reducing the capture time; and the capture time is independent of distance uncertainty, thereby improving the code capture efficiency; the main operations are completed by the most commonly used FIR filter in digital signal processing, which is easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0016] Figure 1 The present invention is a flowchart of a fast code acquisition method for BPSK modulation direct sequence spread spectrum communication. DETAILED DESCRIPTION

[0017] The embodiments of the present invention will be described in detail below with reference to 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 construed as limiting the scope of the present invention.

[0018] refer to Figure 1 , a fast code acquisition method in BPSK modulation direct sequence spread spectrum communication, comprising the following steps:

[0019] Step 1, the received signal is multiplied by the I-channel local carrier signal cos(ω1t+θ1) generated by the local carrier NCO and the Q-channel local carrier signal sin(ω1t+θ1) generated by the local carrier NCO, respectively, and mixed to obtain signal I0 and signal Q0 respectively;

[0020] Specifically, for BPSK chip (PN code) modulation, the complex envelope is as follows:

[0021]

[0022] Where, P is the power of the spread spectrum signal; P T (t) is a 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 is delayed, T is the unit pulse duration, i.e., the pulse width; a k is the coefficient of the direct spread spectrum code chip sequence, a k ∈{1, -1};

[0023] For the convenience of discussion, PN(t) is usually used to represent the chip sequence, then

[0024]

[0025] In addition, define the data sequence d(t) as

[0026]

[0027] Among them, P MT (t) is a unit pulse with a duration of MT, P MT In (t-jMT), j is an integer, which indicates the number of MTs of delay of the unit pulse, M indicates that there are exactly M chips per bit or coded bit, and T is the unit pulse P. T (t) width; d j is a discrete data symbol, with a value of 1 or -1; each bit d j The probability of taking 1 or -1 is 1 / 2, and each d j are all statistically independent of each other;

[0028] When BPSK chip modulation is used with BPSK data modulation, the complex envelope form can be abbreviated as:

[0029]

[0030] Carrier modulate the signal v(t) to obtain a spread spectrum signal in the form of:

[0031]

[0032] Among them, P t is the spread spectrum signal power;

[0033] The signal modulated by the carrier will produce attenuation and phase delay during the transmission process, so the received signal y(t) can be written as:

[0034]

[0035] Where P is the power of the received signal and P<<P t , T p is the signal delay, the carrier phase θ0 is simulated by a uniform random variable, the distribution range is 0-2π radians, n(t) is the noise, and J(t) is the interference signal;

[0036] Signal I0 is:

[0037]

[0038] The signal Q0 is:

[0039]

[0040] Step 2, the signal I0 and the signal Q0 are respectively subjected to low-pass filtering to obtain the baseband signal I1 and the baseband signal Q1 respectively;

[0041] Specifically, the baseband signal I1 is:

[0042]

[0043] The baseband signal Q1 is:

[0044]

[0045] Step 3, filtering and extracting the baseband signal I1 and the baseband signal Q1 respectively, extracting two sampling points within each chip duration, and obtaining the low sampling signal I2 and the low sampling signal Q2 respectively;

[0046] Step 4, the low sampling signal I2 and the low sampling signal Q2 are respectively input into the FIR filter to obtain data I3 and data Q3 respectively;

[0047] Specifically, the FIR filter coefficient is twice the sampling value of the spread spectrum code sequence PN(t); if the length of the spread spectrum code sequence PN(t) is L, then the length of the FIR filter coefficient is 2L;

[0048] Data I3 is:

[0049]

[0050] The data for Q3 is:

[0051]

[0052] Where i = 0, 1, ..., 2L-1, T b is the delay of the spreading code sequence, T p Delay for the signal.

[0053] Step 5: For each pair of sample points of data I3 and data Q3, square the sum and then take the square root, and consider the frequency difference to obtain R0, R1, R2.....R i , i=0,1,...,2L-1; let R x R0, R1, R2.....R i The maximum value in , x is the serial number;

[0054] Specifically,

[0055] When T b and T p The difference is less than half a chip period (i.e. ), due to the autocorrelation of the spreading code, R i maximum.

[0056] Step 6: Set the maximum value R x Compare with the capture threshold η, and send an indication signal to the code generator according to the comparison result;

[0057] Specifically, if the maximum value R x If it is greater than η, it is determined whether the capture is completed; if the capture is not completed, a capture indication signal and a sequence number x are output to the code generator; if the capture is completed, a sequence number 0 is output to the code generator;

[0058] If the maximum value R x If the capture is not greater than η, it is determined whether the capture is completed; if the capture is not completed, the sequence number x=0 is set and output to the code generator; if the capture is completed, it is determined whether the maximum value R is reached for three consecutive times. x Not greater than η, if so, then output the lost capture indication signal to the code generator, if not, then output the sequence number 0 to the code generator.

[0059] Step 7, the code generator outputs spread spectrum sequence samples according to the indication signal;

[0060] Specifically, if the code generator receives a lost capture indication signal, the code generator is reset and the code generator re-outputs the spread spectrum code sequence sample points starting from the 0th sample point a0. At this time, Ta = Tb, and the output spread spectrum code sequence The sampling points are twice as many as the spreading code sequence a0, a0, a1, a1, a2, a2.....a L-1 ,a L-1 , where T ais the output time of the 0th sample point of the code generator; at the same time, reset the FIR filter and return to step 4;

[0061] If the code generator only receives the sequence number 0, the code generator does not reset or delay, and outputs the spread spectrum code sequence normally. The sampling points are twice as many as the spreading code sequence a0, a0, a1, a1, a2, a2.....a L-1 ,a L-1 ,;

[0062] If the code generator receives the capture indication signal and the sequence number x, the code generator is first reset, and then after a delay of x samples (i.e., delay time), at this time Output spreading code sequence The sampling points are twice as many as the spreading code sequence a0, a0, a1, a1, a2, a2.....a L-1 ,a L-1 At this time, 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.

[0063] Although the present invention has been described in detail in general terms and in specific embodiments in this specification, it is obvious to those skilled in the art that some modifications or improvements may be made to the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection claimed by the present invention.

Claims

1. A fast code acquisition method in BPSK modulation direct sequence spread spectrum communication, characterized in that: The following steps are involved: Step 1, the received signal is multiplied by the I-channel local carrier signal cos(ω1t+θ1) generated by the local carrier NCO and the Q-channel local carrier signal sin(ω1t+θ1) generated by the local carrier NCO, respectively, and mixed to obtain signal I0 and signal Q0 respectively; Step 2, the signal I0 and the signal Q0 are respectively subjected to low-pass filtering to obtain the baseband signal I1 and the baseband signal Q1 respectively; Step 3, filtering and extracting the baseband signal I1 and the baseband signal Q1 respectively, extracting two sampling points within each chip duration, and obtaining the low sampling signal I2 and the low sampling signal Q2 respectively; Step 4, the low sampling signal I2 and the low sampling signal Q2 are respectively input into the FIR filter to obtain data I3 and data Q3 respectively; Step 5: For each pair of sample points of data I3 and data Q3, square the sum and then take the square root, and consider the frequency difference to obtain R1, R2, R3, ..., R i , ..., R 2L-1 , i=0,1,...,2L-1; let R x R1, R2, R3, ..., R i , ..., R 2L-1 The maximum value in , x is the serial number; Among them, R i The calculation method is: Where i = 0, 1, ..., 2L-1, P is the power of the received signal; PN(t) represents the chip sequence; d(t) represents the data sequence; T p is the signal delay; T b Delay for the spreading code sequence; When T b and T p The difference is less than When R i maximum; Step 6: Set the maximum value R x Compare with the capture threshold η, and send an indication signal to the code generator according to the comparison result; Step 7, the code generator outputs a spread spectrum sequence according to the indication signal; Specifically, in step 6, if the maximum value R x If it is greater than η, it is determined whether the capture is completed; if the capture is not completed, a capture indication signal and a sequence number x are output to the code generator; if the capture is completed, a sequence number 0 is output to the code generator; If the maximum value R x If the capture is not greater than η, it is determined whether the capture is completed; if the capture is not completed, the sequence number x=0 is set and output to the code generator; if the capture is completed, it is determined whether the maximum value R is reached for three consecutive times. x is not greater than η, if so, then output the missed capture indication signal to the code generator, if not, then output the sequence number 0 to the code generator; Step 7: Specifically, if the code generator receives a lost acquisition indication signal, the code generator is reset and the code generator re-outputs the spread spectrum code sequence starting from the 0th sample point a0. The sampling points are twice as many as the spreading code sequence a0, a0, a1, a1, a2, a2.....a L-1 ,a L-1 ; Reset the FIR filter at the same time and return to step 4; If the code generator only receives sequence number 0, the code generator outputs the spread spectrum code sequence normally. The sampling points are twice as many as the spreading code sequence a0, a0, a1, a1, a2, a2.....a L-1 ,a L-1 ; If the code generator receives the capture indication signal and the sequence number x, the code generator will first reset and then delay After a period of time, the spreading code sequence is output The sampling points are twice as many as the spreading code sequence a0, a0, a1, a1, a2, a2.....a L-1 ,a L-1 , the capture is completed; Wherein, in step 5 and step 7, L is the length of the spreading code sequence PN(t), is half a chip period; in step 7, k is an integer, indicating how many T the unit pulse delay is, a k is the coefficient of the direct spread spectrum code chip sequence, a k ∈{1,-1},P T is a unit pulse with a duration of T, T is the pulse width, that is, one chip period, and x is the number of sampling points.

2. The fast code acquisition method in BPSK modulation direct sequence spread spectrum communication according to claim 1, characterized in that: In step 1, the received signal is: Among them, θ0 is the carrier phase, and its distribution area is a uniform random variable of 0-2π radians; n(t) is noise; J(t) is the interference signal; The signal I0 is: The signal Q0 is:

3. The fast code acquisition method in BPSK modulation direct sequence spread spectrum communication according to claim 2, characterized in that: In step 2, the baseband signal I1 is: The baseband signal Q1 is:

4. The fast code acquisition method in BPSK modulation direct sequence spread spectrum communication according to claim 3, characterized in that: In step 4, 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, and the length of the FIR filter coefficient is 2L; the data I3 is: The data for Q3 is:

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