CN101854189B - Soft signal identifying and spreading code catching method with frequency difference estimation - Google Patents

Abstract

The invention relates to a soft signal identifying and spreading code catching method with frequency difference estimation, which is conductive to increasing the probability of correctly identifying signals and catching spreading codes under the presence of frequency difference. The invention provides a soft SR/CA method adopting a dual decision threshold combined with frequency difference estimation, and the treatment on a sampling signal comprises the following processes: matching and relating the sampling signal with local spreading codes, calculating decision variables, and then comparing the decision variables with high and low thresholds so as to decide whether frequency difference correction is needed. On the basis of one path of SR/CA, the invention can carry out decision after further correcting the frequency difference by being combined with the frequency difference estimation method when the decision variable is between the high threshold and the low threshold via reasonably arranging the high and low dual decision thresholds, so the probability of signal identification can be increased under the condition that false-alarm probability is basically not changed, and the detection probability of the system can be further increased.

Description

A kind of soft signal identification and spreading code catching method with frequency difference estimation

Technical field

The present invention relates to the Resistant DS Spread Spectrum System simultaneous techniques in a kind of digital communicating field, particularly for adoptable synchronous method in the undesirable situation that contains certain frequency difference (being frequency difference in-less-than symbol speed), specifically identify and spreading code catching method with the soft signal of frequency difference estimation.

Background technology

Direct sequence spread spectrum (DS-SS) technology has anti-interference, anti-intercepting and capturing and the series of advantages such as ability of anti-multipath is strong, all have a wide range of applications at military and commercial field, second generation IS-95, third generation IMT-2000 standard as cell mobile communication systems have all adopted the CDMA scheme based on the DS-SS technology; Military jam-resistant communication system, satellite communication system also often adopt the DS-SS technology.In the DS-SS system, at first the data-signal that terminal will be recovered to transmit from receive signal must make local frequency expansion sequence synchronize with the frequency expansion sequence in the reception signal.

In the spread spectrum communication environment of reality, exist the effects such as multidiameter delay expansion, amplitude fading and Doppler frequency shift, must take corresponding measure to be overcome, inhibition and the elimination negative effect to the DS-SS systematic function.Traditional method is that Rake receives, and estimates the information such as time delay, the amplitude of fading and phase place in different paths by simultaneous techniques, and the multipath signal that separates is got up to reduce the impact of multipath fading by certain compatible rule merging.But the method is only applicable to the apparent in view situation of mulitpath, if when the mulitpath differentiation is not obvious, just is difficult to obtain reliable time delay and estimates.

In addition, carrier beat is to the time delay of DS-SS system and channel estimating and overall performance important all.Because the original signal to noise ratio of spread-spectrum signal is just lower, and carrier frequency missionary society causes and the loss of the correlation peak energy of receiving spread frequency signal and local spreading code makes correct identification correlation peak location very difficult.Even time delay is estimated correct, also can be due to the decline of received signal to noise ratio, and cause the deterioration of DS-SS receiver performance.Channel is also complicated due to the existence of carrier beat, and the estimation meeting of channel parameter is very difficult, and so, the design of respective equalizers and performance also can be affected.In addition, if there is strong Doppler effect, spread-spectrum code rate also can produce skew, and the spreading code time delay of this moment and channel estimating and Receiver Design just seem more complicated so.

Therefore, frequency difference, time delay and channel estimating are extremely important, are the key technologies in the DS-SS system.How to estimate frequency difference, time delay and channel parameter to fast and reliable, and how effectively to assess frequency difference to the impact of DS-SS systematic function, be all studying a question of meriting attention, and this application design and performance evaluation for the DS-SS system is significant.

Summary of the invention

The object of the invention is to for the deficiencies in the prior art, propose a kind of soft signal identification and spreading code catching (soft SR/CA) method with frequency difference estimation.Consider the DS-SS system when frequency difference in-less-than symbol speed, on the basis of one road SR/CA, by the height dual decision threshold rationally is set, make when judgment variables is between high low threshold, adjudicate again after can carrying out proofreading and correct frequency difference further combined with the frequency difference estimation method, so just can increase the probability that signal is identified in the situation that false alarm probability is substantially constant, and then can improve the detection probability of system.

Technical scheme of the present invention is:

A kind of identification of soft signal and spreading code catching method with frequency difference estimation, i.e. soft SR/CA, concrete steps are as follows:

Step 1: initialization---set height decision threshold λ _H, λ _L, chip period is T _s, frequency difference section correction time T _K, during frequency difference T=0 correction time, digital vco (NCO) is resetted;

Step 2: receiving terminal carries out down-converted with the signal s (t) that receives through NCO and obtains baseband signal r (t) signal,

Re[wherein] be to get real part, f _cBe the output frequency of NCO, baseband signal r (t) is passed through chip matched filter g _R(t) output, obtain continuous signal

Step 3: to the output of chip matched filter With spreading rate 1/T _sSampling obtains sampled signal

Then use local spreading code c ^*Mate and obtain a series of correlation e (h), can be expressed as:

$e\left(h\right)=\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}{c}^{*}\left(i\right)\tilde{r}(\mathrm{hL}+i)$

Wherein L is spreading code length;

Step 4: a series of correlation e (h) that will obtain through square, smoothly obtain z (h), wherein W is smoothing factor, z (h) is expressed as:

$z\left(h\right)=\underset{i=0}{\overset{W-1}{\mathrm{\Σ}}}{\left|e(h-i)\right|}^2$

Step 5: every T _sTime width is selected L level and smooth result { z ₀(h), z ₁(h) ..., z _L-1(h) } the maximum z in _max(h), its position is as catching position , be expressed as:

$z_{\max }\left(h\right)\stackrel{\mathrm{\Δ}}{=}\max \{z_0\left(h\right),z_1\left(h\right),...,z_{L-1}\left(h\right)\}$

$\widehat{\mathrm{\δ}}\left(h\right){|}_{z_{\widehat{\mathrm{\δ}}\left(h\right)}}=z_{\max }\left(h\right)$

Then with maximum z _max(h) with the individual smoothly mean value of results of other L-1

Ratio as judgment variables λ (h) output, be expressed as:

$\stackrel{\‾}{z}\left(h\right)\stackrel{\mathrm{\Δ}}{=}\frac{1}{L-1}\underset{l=0,l\≠\widehat{\mathrm{\δ}}\left(h\right)}{\overset{L-1}{\mathrm{\Σ}}}{z}_l\left(h\right)$

$\mathrm{\λ}\left(h\right)\stackrel{\mathrm{\Δ}}{=}{z}_{\max }\left(h\right)/\stackrel{\‾}{z}\left(h\right)$

Wherein

Expression " being defined as ";

Step 6: with λ (h) and high low threshold λ _H, λ _LCompare,

(a) if. λ (h)＞λ _H, there is H in decision signal ₁, namely receiving terminal detects signal, and this moment, soft SR/CA completed, output

Be the position that captures, end of identification;

(b) if. λ (h)≤λ _L, there is not H in decision signal ₀, need to continue execution in step two;

(c) if. λ _L＜λ (h)≤λ _H, execution in step seven;

Step 7: locking catching position

If, frequency difference T≤T correction time _K, beginning to carry out the frequency difference estimation method, NCO proofreaies and correct to digital vco, T=T+WLT _s, execution in step two; If T＞T _K, execution in step.

In step 1 of the present invention, described setting height decision threshold λ _H, λ _LMethod refer to by presetting initial false alarm probability P _FAWith final false alarm probability P _FAOccurrence, be designated as

, according to

With Calculate respectively λ _LAnd λ _H, wherein

Refer to the false alarm probability computing formula of soft SR/CA method.

In step 1 of the present invention, described frequency difference section correction time T _KThe longest frequency difference that refers to allow is proofreaied and correct duration, surpasses this duration and thinks that frequency difference is proofreaied and correct and lost efficacy.

In step 4 of the present invention, described with correlation e (h) through square, smoothly be at first with e (h) delivery square, the then square value summation that W is continuous.

In step 6 of the present invention, described frequency difference estimation method is double filter frequency difference estimation method.Local spreading code c in step 3 of the present invention ^*Setting at receiving terminal, is tens to several kilobits.

Beneficial effect of the present invention:

The present invention is with soft signal identification and the spreading code catching method of frequency difference estimation, for the receiving baseband signal in certain sliding time section, at first by the chip matched filter, then sample with spreading rate, then use local spreading code to mate to obtain a series of correlations; By serial to parallel conversion, squared magnitude, level and smooth and calculating, obtain signal identification judgment variables and maximum value position subsequently, judgment variables and high low threshold are compared, and then determine that whether needing to proofread and correct frequency difference re-starts signal identification and spread spectrum seizure.

The present invention is directed to the drawback of single decision threshold strategy in existing signal identification and spreading code catching method, the DS-SS system of (frequency difference in-less-than symbol speed) when considering to have certain frequency difference proposes a kind of soft signal identification and spreading code catching method with frequency difference estimation.The present invention is on the basis of one road SR/CA, by the height dual decision threshold rationally is set, make when judgment variables is between high low threshold, can proofread and correct frequency difference by the frequency difference estimation method, then re-start signal identification and spreading code catching, so just can increase the probability that signal is identified in the situation that false alarm probability is substantially constant, and then can improve the detection probability of system.

Description of drawings

Fig. 1 is that the present invention is with the soft signal identification of frequency difference estimation and the block diagram of spreading code catching (soft SR/CA) method.

Fig. 2 is the concrete block diagram of signal identification and spreading code catching SR/CA.

Fig. 3 is the block diagram of double filter frequency difference estimation method.

Fig. 4 is frequency difference Δ fT under awgn channel _sDetection probability and the average code sheet signal to noise ratio of=0,0.2,0.4 o'clock SR/CA, soft SR/CA method Graph of relation.

Fig. 5 is frequency difference Δ fT under the slow fading rayleigh channel of single footpath _sDetection probability and the average code sheet signal to noise ratio of=0,0.2,0.4 o'clock SR/CA, soft SR/CA method

Graph of relation.

Fig. 6 is detection probability and the frequency difference AfT of SR/CA under the awgn channel condition, soft SR/CA method _sGraph of relation.

Fig. 7 is detection probability and the frequency difference Δ fT of SR/CA, soft SR/CA method under the slow fading rayleigh channel condition of single footpath _sGraph of relation.

Fig. 8 is flow chart of the present invention.

Embodiment

The present invention is further illustrated below in conjunction with drawings and Examples.

A kind of soft signal identification and spreading code catching (soft SR/CA) method with frequency difference estimation, concrete steps are as follows:

Step 1: initialization---set height decision threshold λ _H, λ _L, frequency difference section correction time T _K, during frequency difference T=0 correction time, digital vco (NCO) is resetted, T _sIt is chip period;

Step 2: receiving terminal carries out down-converted with the signal s (t) that receives through NCO and obtains baseband signal r (t) signal,

Re[wherein] be to get real part, f _cBe the output frequency of NCO, baseband signal r (t) is passed through chip matched filter g _R(t) output obtains continuous signal

Step 3: to the output of chip matched filter

With spreading rate 1/T _sSampling obtains sampled signal

Then use local spreading code c ^*Mate and obtain a series of correlation e (h), can be expressed as:

$e\left(h\right)=\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}{c}^{*}\left(i\right)\tilde{r}(\mathrm{hL}+i)$

Wherein L is spreading code length;

Step 4: a series of correlation e (h) that will obtain through square, smoothly obtain z (h), wherein W is smoothing factor, z (h) is expressed as:

$z\left(h\right)=\underset{i=0}{\overset{W-1}{\mathrm{\Σ}}}{\left|e(h-i)\right|}^2$

Step 5: every T _sTime width is selected L level and smooth result { z ₀(h), z ₁(h) ..., z _L-1(h) } the maximum z in _max(h), its position is as catching position

, be expressed as:

$z_{\max }\left(h\right)\stackrel{\mathrm{\Δ}}{=}\max \{z_0\left(h\right),z_1\left(h\right),...,z_{L-1}\left(h\right)\}$

$\widehat{\mathrm{\δ}}\left(h\right){|}_{z_{\widehat{\mathrm{\δ}}\left(h\right)}}=z_{\max }\left(h\right)$

Then with maximum z _max(h) with the individual smoothly mean value of results of other L-1

Ratio as judgment variables λ (h) output, be expressed as:

$\stackrel{\‾}{z}\left(h\right)\stackrel{\mathrm{\Δ}}{=}\frac{1}{L-1}\underset{l=0,l\≠\widehat{\mathrm{\δ}}\left(h\right)}{\overset{L-1}{\mathrm{\Σ}}}{z}_l\left(h\right)$

$\mathrm{\λ}\left(h\right)\stackrel{\mathrm{\Δ}}{=}{z}_{\max }\left(h\right)/\stackrel{\‾}{z}\left(h\right)$

Wherein

Expression " being defined as ".

Step 6: with λ (h) and high low threshold λ _H, λ _LCompare,

(a) if. λ (h)＞λ _H, there is H in decision signal ₁, namely receiving terminal detects signal, and this moment, soft SR/CA completed, output

Be the position that captures, end of identification;

(b) if. λ (h)≤λ _L, there is not H in decision signal ₀, need to continue execution in step two.

(c) if. λ _L＜λ (h)≤λ _H, execution in step seven.

Step 7: locking catching position

If, frequency difference T≤T correction time _K, beginning to carry out the frequency difference estimation method, NCO proofreaies and correct to digital vco, T=T+WLT _s, execution in step two; If T＞T _K, execution in step.

During concrete enforcement:

Embodiment one:

Take binary system DS-SS/MPSK system as example, provided with the soft signal identification of frequency difference estimation and the concrete computational process of spreading code catching method, the DS-SS system parameters is: several L=31, W=8.The flow process of the present embodiment as shown in Figure 8, concrete computational process is as follows:

1. according to normal false alarm probability CFAR criterion, double threshold is set

Suppose initial false alarm probability P _FAWith final false alarm probability P _FAOccurrence be

, the definition auxiliary variable

Height decision threshold λ _L, λ _HComputing formula be

$P_{\mathrm{FA}}^B\≅1-F_{\mathrm{\λ}0}^{H_0}\left({\mathrm{\λ}}_L\right)$

$P_{\mathrm{FA}}^E\≅1-F_{\mathrm{\λ}0}^{H_0}\left({\mathrm{\λ}}_H\right)$

The initial p of soft SR/CA method _FAWith final P _FABe respectively:

Dual thresholding is set to: λ _L=2.5, λ _H=3.2, λ ₀=λ _H

2. the correlation after the calculating matched filtering

Suppose c ^*Represent local spreading code,

Expression receives signal through the chip matched filtering and the output sampling signal after sampling, and carrying out filtered correlation output with local spreading code can be expressed as:

$e\left(h\right)=\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}{c}^{*}\left(i\right)\tilde{r}(\mathrm{hL}+i)$

3. construct the decision-aided variable

E (h) through square, the output of a W symbol period after smoothly is respectively:

$z\left(h\right)=\underset{i=0}{\overset{W-1}{\mathrm{\Σ}}}{\left|e(h-i)\right|}^2$

Define following auxiliary variable:

$z_{\max }\left(h\right)\stackrel{\mathrm{\Δ}}{=}\max \{z_0\left(h\right),z_1\left(h\right),...,z_{L-1}\left(h\right)\}$

$\stackrel{\‾}{z}\left(h\right)\stackrel{\mathrm{\Δ}}{=}\frac{1}{L-1}\underset{l=0,l\≠\widehat{\mathrm{\δ}}\left(h\right)}{\overset{L-1}{\mathrm{\Σ}}}{z}_l\left(h\right)$

4. calculate catching position and judgment variables

$\widehat{\mathrm{\δ}}\left(h\right){|}_{z_{\widehat{\mathrm{\δ}}\left(h\right)}=z_{\max }\left(h\right)}$

$\mathrm{\λ}\left(h\right)\stackrel{\mathrm{\Δ}}{=}{z}_{\max }\left(h\right)/\stackrel{\‾}{z}\left(h\right)$

5. judgement

With λ (h) and high low threshold λ _H, λ _LCompare.

(1) if λ (h)＞λ _H, there is (H in decision signal ₁), this moment, soft SR/CA completed.

(2) if λ (h)≤λ _L, there is not (H in decision signal ₀), need to continue to carry out soft SR/CA.

(3) if λ _L＜λ (h)≤λ _H, activate frequency difference estimation and cor-rection loop, after proofreading and correct, frequency difference continues to carry out soft SR/CA.

For the ease of to comparing with the soft signal identification of frequency difference estimation and spreading code catching method and existing differential ference spiral method in the present invention, below in conjunction with the detection probability P of two kinds of methods in simulation result shows AWGN and rayleigh fading channel _DWith false alarm probability P _FASituation of change.

Fig. 4,5 respectively for awgn channel and single footpath slow fading rayleigh channel, has provided frequency difference Δ fT _sDetection probability and the average code sheet signal to noise ratio of=0,0.2,0.4 o'clock SR/CA, soft SR/CA method

Relation curve.As can be seen from the figure: at identical frequency difference Δ fT _sWith average code sheet signal to noise ratio Under condition, the detection probability P of soft SR/CA method _DWill be higher than the SR/CA method.When false dismissal probability be 0.001, when frequency difference is 0.4, soft SR/CA method has the performance gain of about 1dB.

Fig. 6,7 have provided respectively difference

The detection probability of two methods and frequency difference Δ fT under condition _sRelation curve.As can be seen from the figure: as frequency difference AfT _sRaise or average chip signal to noise ratio

During reduction, the detection probability P of SR/CA, soft SR/CA method _DAll reduce, but the performance of soft SR/CA method still obviously is better than traditional SR/CA method.

In the above-described embodiments, although research work is for binary system DS-SS/MPSK system, but can be used for equally M-ary Orthogonal Code Spread-Spectrum System (NOrth-MDPSK), because before with M-ary orthogonal spread spectrum transmission data, fully can be by binary system spread spectrum mode, even do not transmit any data and realize SR/CA.

The part that the present invention does not relate to all prior art that maybe can adopt same as the prior art is realized.

Claims (5)

1. the identification of soft signal and spreading code catching method with a frequency difference estimation, i.e. soft SR/CA is characterized in that concrete steps are as follows:

Step 1:Initialization---set the height decision threshold

, chip period is

, frequency difference section correction time , frequency difference correction time

The time, digital vco (NCO) is resetted;

Step 2:Receiving terminal is with the signal that receives

Carry out down-converted through NCO and obtain baseband signal

Signal,

, wherein To get real part,

The output frequency of NCO, with baseband signal

By the chip matched filter Output obtains continuous signal ,

Step 3:Output to the chip matched filter

With spreading rate Sampling obtains sampled signal

, then use local spreading code

Mate and obtain a series of correlations

, can be expressed as:

Wherein

Be spreading code length;

Step 4:With a series of correlations that obtain

Through square, smoothly obtain , wherein

Smoothing factor,

Be expressed as:

Step 5:Every

Time width is selected

Individual level and smooth result In maximum

, its position is as catching position

, be expressed as:

Then with maximum

With other

The mean value of individual level and smooth result

Ratio as judgment variables

Output is expressed as:

Wherein

Expression " being defined as ";

Step 6:Will

With high low threshold Compare,

(a) if.

, think that decision signal exists, and use The expression decision signal exists, and namely receiving terminal detects signal, and this moment, soft SR/CA completed, output

Be the position that captures, end of identification;

(b) if.

, think that decision signal does not exist, and use

The expression decision signal does not exist, and needs to continue execution in step two;

(c) if.

, execution in step seven;

Step 7:The locking catching position

If, frequency difference correction time

, beginning to carry out the frequency difference estimation method, NCO proofreaies and correct to digital vco,

, execution in step two; If

, execution in step;

In step 1, set the height decision threshold

Method refer to by presetting initial false alarm probability

With final false alarm probability

Occurrence, be designated as

,

, according to

With

Calculate respectively

With , wherein

Refer to the false alarm probability computing formula of soft SR/CA method.

2. soft signal identification and spreading code catching method with frequency difference estimation according to claim 1, is characterized in that in step 1 described frequency difference section correction time

The longest frequency difference that refers to allow is proofreaied and correct duration, surpasses this duration and thinks that frequency difference is proofreaied and correct and lost efficacy.

3. soft signal identification and spreading code catching method with frequency difference estimation according to claim 1, is characterized in that in step 4, and be described with correlation

Through square, smoothly be at first will

Delivery square, then will

Individual continuous square value summation.

4. soft signal identification and spreading code catching method with frequency difference estimation according to claim 1, is characterized in that in step 7, and described frequency difference estimation method is double filter frequency difference estimation method.

5. soft signal identification and spreading code catching method with frequency difference estimation according to claim 1, is characterized in that local spreading code in step 3

Setting at receiving terminal, is tens to several kilobits.

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