CN104811219A - Improved intermediate frequency digital matched filtering false code capturing method - Google Patents

Abstract

The invention discloses an improved intermediate frequency digital matching filter pseudo-code capture method. The system is composed of an improved digital matching filter, envelope detection, threshold judgment and a local pseudo-code generation circuit. The present invention uses the product of the reverse sequence of the square wave signal and bipolar pseudo-code identical with carrier signal period as the impulse response of matched filter, thereby the tap coefficient of matched filter becomes ± 1, uses T/R circuit ( Through/Reverse, pass through when the coefficient is 1, and reverse when the coefficient is -1) to replace the multiplier circuit in the traditional IF matched filter, and filter out the harmonic components in the output of the matched filter through a low-pass filter; then send Make a threshold judgment to the envelope detection circuit. When the maximum value is output, the matched filter successfully captures the pseudocode. This method can greatly save chip resources while maintaining the advantages of the traditional matched filter pseudo-code capture method. This advantage is more obvious when the pseudo-code sequence is longer and the sampling frequency is higher.

Description

An Improved IF Digital Matched Filter Pseudocode Acquisition Method

technical field

The invention belongs to the technical field of spread spectrum communication, in particular to an improved acquisition method of intermediate frequency digital matched filter pseudo-code.

Background technique

Direct Sequence Spread Spectrum (Direct Sequence Spread Spectrum) communication is a communication technology that uses pseudocode to broaden the spectrum of the transmitted signal so that the bandwidth it occupies is much larger than the minimum bandwidth required to transmit information. Direct-sequence spread spectrum communication has strong anti-interference ability, less interference to other communication systems, difficult to intercept, and multiple access multiplexing, etc. It is widely used in mobile communication, satellite communication, electronic countermeasures and other fields.

The longer the pseudo-code sequence, the greater the processing gain, and the stronger the multiple access capability, anti-interception and anti-jamming capabilities. Pseudo-code acquisition and tracking are the prerequisites for the normal operation of the spread spectrum communication system. Commonly used pseudo-code capture methods include sliding correlation method and matched filtering method. The circuit structure of sliding correlation method is simple, but the acquisition speed is too slow; the acquisition speed of matched filter method is fast, and can be realized by surface acoustic wave matched filter (SAW-MF) and digital matched filter (DMF). At present, the main factors that limit the application of SAW filters in pseudo code capture systems are limited by process conditions, it is difficult to manufacture SAW filters with large time-bandwidth products, the length of pseudo codes cannot be too long, and the configurability is poor. Compared with the traditional SAW matched filter, the digital matched filter has special advantages: (1) It can achieve extremely high processing gain; (2) It has strong programmability and can design PN code arbitrarily; (3) It does not There is inherent noise, and there is no problem of noise accumulation; (4) It is convenient to adopt digital signal processing technology. At present, digital matched filters are mainly implemented on Field Programmable Gate Array (FPGA) devices. However, when digital matched filtering is performed at the intermediate frequency, the detection signal often requires high sampling accuracy (a certain fraction of the carrier cycle), so the number of multipliers that need to be used will be large, which will occupy a large amount of resources on the chip.

Contents of the invention

In view of this, the purpose of the present invention is to provide an improved intermediate frequency digital matched filtering pseudo-code capture method, which can realize fast capture of long-sequence pseudo-codes, and has the advantage of saving chip resources.

To achieve the above object, the present invention provides the following technical solutions:

An improved intermediate frequency digital matched filter pseudo-code capture method, the system is composed of an improved digital matched filter, envelope detection, threshold judgment, and local pseudo-code generation circuit. The present invention uses the product of the reverse sequence of the square wave signal and bipolar pseudo-code identical with carrier signal period as the impulse response of matched filter, thereby the tap coefficient of matched filter becomes ± 1, uses T/R circuit ( Through/Reverse, straight-through/reverse) replace the multiplier circuit in the traditional matched filter, and filter out the harmonic component in the output of the matched filter through the low-pass filter; then send it to the envelope detection circuit for threshold judgment, when When outputting the maximum value, the matched filter successfully captures the pseudocode.

The beneficial effect of the present invention is: (1) use T/R circuit (Through/Reverse, through/reverse) to replace the multiplier circuit in the traditional matched filter, the circuit structure is simple; The number of multipliers saves chip resources, and this advantage is more obvious when the pseudo code sequence is longer and the sampling frequency is higher.

Description of drawings

In order to make the purpose, technical scheme and beneficial effect of the present invention clearer, the present invention provides the following drawings for illustration:

Fig. 1 is the structural block diagram of the improved IF digital matched filtering pseudo-code capture system;

Fig. 2 is a schematic diagram of a digital matched filter impulse response waveform;

Detailed ways

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 1 is a structural block diagram of the improved IF digital matched filter pseudo-code capture system shown in Fig. 2 . The system is composed of improved digital matched filter 1, envelope detection 2, threshold judgment 3, and local pseudocode generation circuit 4. Improved digital matcher 1 is made up of matched filter 12 and low-pass filter 13, and 121 and 131 among the figure represent delay unit T _s , and T _s is the cycle of sampling signal; 122 represents T/R circuit, and T/R circuit It is a straight-through or inversion circuit. When the coefficient is 1, the signal is straight-through. When the coefficient is -1, the signal is inverted; 123 and 133 represent summation operations; 132 represents traditional multiplier operations.

For any system, the output signal y ₂ (t) is the convolution of the input signal y ₁ (t) and the system impulse response h(t), namely:

${\mathrm{the\; y}}_2\left(t\right)={\mathrm{the\; y}}_1\left(t\right)\⊗h\left(t\right)$ (Formula 1)

Assuming that the input signal is BPSK modulation, in the absence of noise, the input spread spectrum signal can be expressed as:

(Formula 2)

In the above formula, P is the received signal power, and τ _d is the transmission delay. ω ₀ , f ₀ , is the carrier angular frequency, carrier frequency and phase, d(t) is the data, p(t) is the spreading code.

The traditional matched filter impulse response H _mc (t) is the product of the inverse sequence p _r (t) of the spreading code p (t) of the same length and the matched filter carrier x _c (t), which can be expressed by the following formula:

$\begin{array}{c}{h}_{\mathrm{mc}}\left(t\right)=\left\{\begin{array}{cc}{p}_r\left(t\right)\×x_c\left(t\right)& 0\≤t\≤T\\ 0& \mathrm{others}\end{array}\right.\\ =\left\{\begin{array}{cc}p(T-t)\×\cos \left({\mathrm{\ω}}_{c}t\right)& 0\≤t\≤T\\ 0& \mathrm{others}\end{array}\right.\\ =\left\{\begin{array}{cc}p(T-t)\×\cos \left(({\mathrm{\ω}}_0+\mathrm{\Δ\ω})t\right)& 0\≤t\≤T\\ 0& \mathrm{others}\end{array}\right.\end{array}$ (Formula 3)

In the formula, T is the period of the pseudo code sequence, p _r (t) is the inverse time sequence with the same length as the spreading code p(t), ω _c =2πf _c is the center angular frequency of the matched filter, Δω is the center of the matched filter The difference between the angular frequency ω _c and the input signal carrier angular frequency ω ₀ .

The square wave signal x _s (t) with the same cycle as the matched filter carrier x _c (t) is:

$x_{\mathrm{the\; s}}\left(t\right)=\left\{\begin{array}{cc}1& {\mathrm{kT}}_c-T_c/4\≤t\≤{\mathrm{kT}}_c+T_c/4\\ -1& {\mathrm{kT}}_c-T_c/4\≤t\≤{\mathrm{kT}}_c+3T_c/4\end{array}\right.$ (Formula 4)

T _c in the formula is the cycle of the matched filter carrier signal, T _c =2π/ω _c .

$\begin{array}{c}{x}_{\mathrm{the\; s}}\left(t\right)=\underset{k=1}{\overset{\∞}{\mathrm{\Σ}}}{x}_{\mathrm{the\; s}}^k\left(t\right)\\ =\left(\frac{4}{\mathrm{\π}}\right)[\cos \left({\mathrm{\ω}}_{c}t\right)-\frac{1}{3}\cos \left({3\mathrm{\ω}}_{c}t\right)+...+\frac{{(-1)}^{(k-1)}}{2k-1}\cos \left((2k-1){\mathrm{\ω}}_{c}t\right)+...]\end{array}$ (Formula 5)

Then the matched filter impulse response h _ms (t) of the present invention is the product of the inverse sequence p _r (t) of the spread spectrum code p (t) of the same length and the square wave x _s (t) corresponding to the matched filter carrier , which can be expressed by the following formula

$h_{\mathrm{ms}}\left(t\right)=\left\{\begin{array}{cc}{p}_r\left(t\right)\×x_{\mathrm{the\; s}}\left(t\right)& 0\≤t\≤T\\ 0& \mathrm{others}\end{array}\right.$ (Formula 6)

Obviously, the values of h _ms (t) are only +1 and -1. In matched filtering, when the coefficient is 1, the signal is passed through, and when the system is -1, the signal is inverted.

The present invention uses the product of the square wave signal x _s (t) having the same period as the carrier signal x _c (t) and the inverse sequence p _r (t) of the bipolar pseudo code p (t) as the impulse response of the matched filter h _ms (t), so that the tap coefficient of the matched filter becomes ±1, and the T/R circuit (Through/Reverse, straight-through/inverted) is used to replace the multiplier circuit in the traditional matched filter, and filtered by a low-pass filter Remove the high-order harmonic components in the output of the matched filter; then send it to the envelope detection circuit for threshold judgment. When the maximum value is output, the matched filter successfully captures the pseudocode.

The output of the matched filter is:

$\begin{array}{c}{\mathrm{the\; y}}_2\left(t\right)={\∫}_0^T{\mathrm{the\; y}}_1\left(\mathrm{\τ}\right)h_{\mathrm{ms}}(t-\mathrm{\τ})\mathrm{d\τ}={\∫}_0^T{\mathrm{the\; y}}_1\left(\mathrm{\τ}\right)\left(p_r(t-\mathrm{\τ})\underset{k=1}{\overset{\∞}{\mathrm{\Σ}}}{x}_{\mathrm{the\; s}}^k(t-\mathrm{\τ})\right)\mathrm{d\τ}\\ =\underset{k=1}{\overset{\∞}{\mathrm{\Σ}}}{\mathrm{the\; y}}_2^k\left(t\right)=\underset{k=1}{\overset{\∞}{\mathrm{\Σ}}}{\∫}_0^T{\mathrm{the\; y}}_1\left(\mathrm{\τ}\right)\left(p_r(t-\mathrm{\τ})x_{\mathrm{the\; s}}^k(t-\mathrm{\τ})\right)\mathrm{d\τ}\end{array}$ (Formula 7)

in the above formula can be written as:

      

(Formula 8)

Since the carrier frequency is much higher than the spreading code rate, The second item of is approximately 0; we can always find a certain moment when p(t) matches p(T-(t-τ)). So p(t)p(T-(t-τ)) is equal to 1 in the integration interval, assuming that the data changes slowly, d(τ-τ _d ) can be regarded as a constant d equal to ±1.

becomes:

      

(Equation 9)

After passing through the low-pass filter h _lf (t), high-order terms are filtered out Keep only the first item;

(Formula 10)

A specific example is used below to illustrate the resource saving situation of this method.

Assume that the code length of the spread spectrum code p(t) is L=15, the pseudo code rate f _p =1Mcps, the chip width T _p =1μs, the carrier frequency of the spread spectrum modulation signal is f ₀ =10MHz, and the sampling rate f of the system _s = 100MHz, sampling interval T _s = 0.01μs, matched filter carrier frequency f _c = 100MHz, and the corresponding square wave signal period is T _c = 0.01μs; then the number of coefficients of the matched filter is:

N= _LTp / _Ts = _Lfs / _fc =1500

The pass-band cut-off frequency of the low-pass filter is f _pass =15MHz, the stop-band cut-off frequency is f _stop =25MHz, the in-band ripple A _stop =1dB, and the out-of-band attenuation A _stop =30dB, then the number of coefficients of the low-pass filter M=11;

Traditional matched filters need to use N=1500 multipliers, and these L multipliers are replaced by N T/R circuits in the present invention (T/R is a straight-through or inverting circuit, which takes up very few resources), low The number of multipliers introduced by the pass filter LPF is M. Since N>>M, the improved digital matched filter greatly saves the multiplier resources of the chip. This advantage is more obvious when the length of the pseudocode is longer and the sampling rate of the system is higher.

In practical engineering, the low-pass filter can also be omitted, because the high-order term in y ₂ (t) Since the carrier angular frequency ω _c is relatively large, the high-order term This saves more resources.

Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that it can be described in terms of form and Various changes may be made in the details without departing from the scope of the invention defined by the claims.

Claims (1)

1. the intermediate frequency digital matched filtering method for acquiring pseudo code improved, it is characterized in that: the impulse response of product as matched filter using the opposite sequence of the square-wave signal identical with the carrier signal cycle and bipolarity pseudo-code, thus the tap coefficient of matched filter become ± 1, use T/R circuit (Through/Reverse, straight-through/negate) replace multiplier circuit in conventional matched-filter, the harmonic component in low pass filter filtering matched filter exports; Then be sent to envelope detection circuit, do threshold judgement, when exporting maximum, matched filter successfully catches pseudo-code.