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

Abstract

The invention discloses an improved intermediate frequency digital matched filtering false code capturing method. The system consists of an improved digital matched filer, envelope detection, threshold decision, and a local false code generating circuit. The method uses the product of the square signal having the same period as the carrier signal and the inverted sequence of the bipolar false code as the impulse response of the matched filter, so the tapping coefficient of the matched filter is changed to +1 and -1, the T/R circuit (Through/Reverse, through while the coefficient is 1, and reverse while the coefficient is -1) is used for replacing the multiplier circuit in the traditional intermediate frequency matched filter, and the harmonic component in the matched filter output is filtered by a low pass filter; and then the output is sent to the envelope detection circuit as the threshold decision, the matched filter successfully captures the false code while the output is the maximum value. The method is capable of greatly saving the chip resources while the advantages of the traditional matched filtering false code capturing method are maintained. The advantage is more obvious while the false code sequence is longer and the sampling frequency is higher.

Description

A kind of intermediate frequency digital matched filtering method for acquiring pseudo code of improvement

Technical field

The invention belongs to spread spectrum technic field, particularly relate to a kind of intermediate frequency digital matched filtering method for acquiring pseudo code of improvement.

Background technology

Direct sequence spread spectrum (Direct Sequence Spread Spectrum) communication utilizes pseudo-code to be transmitted the video stretching of signal, and the bandwidth making it occupy is far longer than a kind of communication technology of the minimum bandwidth needed for transmission information.Direct Sequence Spread Spectrum Communication has very strong antijamming capability, little to the interference of other communication system, not easily intercepts and captures, can the advantage such as multiple access multiplexing, is widely used in the fields such as mobile communication, satellite communication, electronic countermeasures.

Pseudo-code sequence is longer, and processing gain is larger, and multiple access capability, Anti TBIgG and antijamming capability are stronger.Acquiring pseudo code is the prerequisite that spread spectrum communication system normally works with following the tracks of.Conventional method for acquiring pseudo code has sliding correlation method and matching matrix.Sliding correlation method circuit structure is simple, but acquisition speed is too slow; Matching matrix acquisition speed is fast, and EMPLOYING THE SAW MATCHED FILTER (SAW-MF) and digital matched filter (DMF) can be used to realize.The principal element that current restriction SAW (Surface Acoustic Wave) filter is applied in acquiring pseudo code system limits by process conditions, the SAW filter difficulty that manufacturing time bandwidth product is larger, and pseudo-code length can not be oversize, and configurability is poor.Compared with traditional EMPLOYING THE SAW MATCHED FILTER, digital matched filter has special advantage: (1) can reach high processing gain; (2) programmability is strong, can design arbitrarily PN code; (3) there is not intrinsic noise, there is no noise accumulation problem; (4) be convenient to adopt Digital Signal Processing.At present, digital matched filter mainly at the scene programmable gate array (FPGA) device realizes.But when intermediate frequency carries out digital matched filtering, detection signal often need very high sampling precision (for carrier cycle some/mono-), need the multiplier number used will be a lot of like this, the ample resources of chip can be taken.

Summary of the invention

In view of this, the object of the present invention is to provide a kind of intermediate frequency digital matched filtering method for acquiring pseudo code of improvement, the fast Acquisition to long sequence pseudo-code can be realized, and there is the advantage of saving resources of chip.

For achieving the above object, the invention provides following technical scheme:

An intermediate frequency digital matched filtering method for acquiring pseudo code for improvement, system is made up of the digital matched filter improved, envelope detection, threshold judgement, local pseudo-code generation circuit.The product of the opposite sequence of the square-wave signal that use of the present invention is identical with the carrier signal cycle and bipolarity pseudo-code is as the impulse response of matched filter, 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.

Beneficial effect of the present invention is: (1) use T/R circuit (Through/Reverse, straight-through/negate) replaces the multiplier circuit in conventional matched-filter, and circuit structure is simple; (2) greatly saved the quantity of multiplier in matched filter, thus saved resources of chip, when pseudo-code sequence is longer, when sample frequency is higher, this advantage is just more obvious.

Accompanying drawing explanation

In order to make object of the present invention, technical scheme and beneficial effect clearly, the invention provides following accompanying drawing and being described:

Fig. 1 is the intermediate frequency digital matched filtering acquiring pseudo code system architecture diagram improved;

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

Embodiment

Below in conjunction with accompanying drawing, the preferred embodiments of the present invention are described in detail.

Fig. 1 is Fig. 2 is the intermediate frequency digital matched filtering acquiring pseudo code system architecture diagram improved.System is made up of the digital matched filter 1 improved, envelope detection 2, threshold judgement 3, local pseudo-code generation circuit 4.The digital adaptation 1 improved is made up of matched filter 12 and low pass filter 13, and 121 in figure and 131 represents delay cell T _s, T _sfor the cycle of sampled signal; 122 represent T/R circuit, and T/R circuit is straight-through or negate circuit, and when coefficient is 1, signal leads directly to, when coefficient is-1, and signal negate; 123 and 133 represent summation operation; 132 represent traditional multiplier computation.

For any system, output signal y ₂t () is input signal y ₁t () and system shock respond the convolution of h (t), that is:

$y_2\left(t\right)=y_1\left(t\right)\⊗h\left(t\right)$ (formula 1)

Suppose that input signal is BPSK modulation, under noise-free case, input spread-spectrum signal can be expressed as:

(formula 2)

In above formula, P is received signal power, τ _dfor propagation delay time.ω ₀, f ₀, for carrier angular frequencies, carrier frequency and phase place, d (t) is data, and p (t) is spreading code.

Traditional matched filter impulse response H _mct () is the opposite sequence p of spreading code p (t) of same length _r(t) and matched filter carrier wave x _cthe product of (t), available following formula represents:

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

In formula, T is the pseudo-code sequence cycle, p _rt () is and the inverse time series of spreading code p (t) with length, ω _c=2 π f _cfor matched filter center angular frequency, Δ ω is matched filter center angular frequency ω _cwith input signal carrier angular frequency ₀difference.

With matched filter carrier wave x _c(t) synperiodic square-wave signal x _s(t) be:

$x_s\left(t\right)=\left\{\begin{array}{cc}1& {\mathrm{kT}}_c-T_c/4\≤t\≤{\mathrm{kT}}_c+T_c/4\\ -1& {\mathrm{kT}}_c-T_c/4\≤t\≤{\mathrm{kT}}_c+3T_c/4\end{array}\right.$ (formula 4)

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

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

Then matched filter impulse response h of the present invention _mst () is the opposite sequence p of spreading code p (t) of same length _rt square wave x that () is corresponding with matched filter carrier wave _st the product of (), available following formula represents

$h_{\mathrm{ms}}\left(t\right)=\left\{\begin{array}{cc}{p}_r\left(t\right)\×x_s\left(t\right)& 0\≤t\≤T\\ 0& \mathrm{others}\end{array}\right.$ (formula 6)

Clearly, h _mst the value of () only has+1 and-1 two kind of situation, when matched filtering, when coefficient is 1, led directly to by signal, when system is-1, by signal negate.

Use of the present invention and carrier signal x _ct square-wave signal x that () cycle is identical _sthe opposite sequence p of (t) and bipolarity pseudo-code p (t) _rt the product of () is as the impulse response h of matched filter _ms(t), thus the tap coefficient of matched filter become ± 1, use T/R circuit (Through/Reverse, straight-through/negate) replaces the multiplier circuit in conventional matched-filter, the higher harmonic components 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.

The output of matched filter is:

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

In above formula can be written as:

(formula 8)

Owing to carrying wave frequency far above spread-spectrum code rate, the 2nd be approximately 0; Can look for sometime, p (t) mates with p (T-(t-τ)).So p (t) p (T-(t-τ)) equals 1 in integrating range, tentation data is slow change, d (τ-τ _d) can regard as and equal ± constant the d of 1.

become:

(formula 9)

By low pass filter h _lfafter (t), filtering high-order term only retain Section 1;

(formula 10)

Illustrate that the resource of this method saves situation with an instantiation below.

Suppose that the code length of spreading code p (t) is L=15, pseudo-bit rate f _p=1Mcps, chip width T _p=1 μ s, the carrier frequency of modulated spread spectrum signal is f ₀=10MHz, the sample rate f of system _s=100MHz, sampling interval T _s=0.01 μ s, matched filter carrier frequency f _c=100MHz, the corresponding square wave signal period is T _c=0.01 μ s; Then the coefficient number of matched filter is:

N＝LT _p/T _s＝Lf _s/f _c＝1500

The cut-off frequecy of passband f of low pass filter _pass=15MHz, stopband cut-off frequency is f _stop=25MHz, ripple A in band _stop=1dB, attenuation outside a channel A _stop=30dB, then the coefficient number M=11 of low pass filter is individual;

Traditional matched filter needs to use N=1500 multiplier, and this L multiplier has changed N number of T/R circuit into (T/R has been straight-through or negate circuit in the present invention, take resource few), the multiplier number that low pass filter LPF introduces is M, due to N>>M, the digital matched filter of improvement saves the multiplier resources of chip greatly.When pseudo-code length is longer, the sample rate of system is higher, and this advantage is just more obvious.

In Practical Project, low pass filter also can save, this is because y ₂high-order term in (t) in due to carrier angular frequency _ccomparatively large, high-order term the resource of such saving is more.

What finally illustrate is, above preferred embodiment is only in order to illustrate technical scheme of the present invention and unrestricted, although by above preferred embodiment to invention has been detailed description, but those skilled in the art are to be understood that, various change can be made to it in the form and details, and not depart from claims of the present invention limited range.

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.