CN103036589A - Spread spectrum communication combination carrier synchronization method based on signal to noise ratio (SNR) - Google Patents

Abstract

The invention discloses a spread spectrum communication combination carrier synchronization method based on a signal to noise ratio (SNR), relates to a carrier synchronization method, and aims at realizing switchover between a phase locking loop and a frequency locking loop under the control of a receiving end carrier signal SNR and further realizing fast and accurate synchronization of spread spectrum communication carrier signals. According to the spread spectrum communication combination carrier synchronization method based on the SNR, a receiver receives a spread spectrum signal, captures the spread spectrum signal, estimates the SNR at the same time, and when the spread spectrum signal is successfully captured and the SNR is greater than a threshold value, frequency and a phase control word are initialized. In a carrier tracking loop, a phase discrimination mark word is set as 1, the phase locking loop starts to work, and the frequency locking loop stops. When a phase error exceeds the threshold value, the phase discrimination mark word is set as 0, the frequency locking loop starts to work while the phase locking loop stops, and therefore the switchover between the phase locking loop and the frequency locking loop is realized, and the carrier synchronization of spread spectrum communication is further realized. The spread spectrum communication combination carrier synchronization method based on the SNR is applicable to a spread spectrum communication system.

Description

A kind of spread spectrum communication united carrier method for synchronous based on SNR

Technical field

The present invention relates to a kind of carrier synchronization method.

Background technology

Carrier synchronization is one of key technology of spread spectrum communication system.Generally, if the performance of carrier synchronization is bad, signal despreading and demodulation all can not well be worked.High-performance, high-precision carrier synchronization system can improve and receive clever lightness.When the carrier phase error of double-sideband signal is Δ θ, receiving terminal signal to noise ratio (SNR) will be reduced to cos ²Δ θ doubly.Many factors all can cause carrier phase error, and are wherein topmost former because Doppler frequency shift.

Costas loop is normally used for carrier frequency acquisition and the tracking in the spread spectrum communication.Although in the antagonism white Gaussian noise good performance is arranged, this method can't be competent at dynamic Doppler frequency shift situation.If noise bandwidth of equal value increases, the corresponding increase of frequency-tracking scope meeting, but tracking accuracy has reduced.If only use FLL (FLL), its dynamic range can be larger than phase-locked loop (PLL).But because a FLL locking frequency, tracking phase shift not, its tracking accuracy is just very low.As a rule, these 2 of the precision of carrier frequency tracking and scopes are conflicting.In order to address this problem, we use FLL to follow the tracks of large frequency shift (FS) and little frequency or the phase deviation of phase-locked loop tracking.When the difference of the frequency of digital controlled oscillator (NCO) output frequency and reference signal was very large, we adjusted NCO with FLL and reduce this deviation.Can be caught rapidly less than the phase-locked loop interval time when deviation, at this moment frequency is finally locked.Generally speaking, carry out coarse adjustment by FLL, finally locked by phase-locked loop.Thus, carrier tracking loop can satisfy dynamic range and tracking accuracy simultaneously.

For the carrier synchronization of high speed transmission of signals system, the retention time that it need to be adjusted fast and grow.The adjustment time that this means FLL and PLL is very short, and the tracking time of PLL will be grown.But it is irregular working as the reception signal, i.e. in the situation of signal to noise ratio sudden change, PLL will keep error tracking and have a strong impact on the adjustment time.Therefore carrier synchronization need to be considered signal to noise ratio.This just need to be to receiving terminal carrier synchronization and SNR control.

Summary of the invention

The present invention is in order to realize the switching of receiving terminal carrier signal signal to noise ratio control phase-locked loop and FLL, and then realizes quick and precisely synchronous to the spread spectrum communication carrier signal, thereby a kind of spread spectrum communication united carrier method for synchronous based on SNR is provided.

A kind of spread spectrum communication united carrier method for synchronous based on SNR, it is realized by following steps:

Step 1, after the receiving terminal of spread spectrum communication system receives spread-spectrum signal, simultaneously execution in step two and step 3;

Step 2, the spread-spectrum signal that receives is caught; And execution in step 21;

Step 2 one, whether the acquisition success number of times in the step 2 is arrived preset value M1; M1 is positive integer; If the result is yes, then execution in step two or two; If the determination result is NO, then return execution in step two;

Step 2 two, frequency control word and phase control words are carried out initialization; And execution in step four;

Step 3, the spread-spectrum signal that receives is carried out signal-to-noise ratio (snr) estimation, obtain the signal-to-noise ratio (snr) estimation value; And execution in step four;

Whether the signal-to-noise ratio (snr) estimation value that obtains in step 4, the determining step three is less than predetermined threshold value M2, and M2 is positive number; If judged result is yes, then return execution in step two or two; If the determination result is NO, then execution in step five;

Step 5, in carrier-tracking loop, whether judge the phase demodulation banner word more than or equal to 1, if judged result is yes, execution in step May Day then; If the determination result is NO, then execution in step six;

Step May Day, the phase place of spread-spectrum signal is adjusted, and phase control words after obtaining to adjust;

Whether phase control words if judged result be yes, then returns execution in step five less than the first default threshold value after the adjustment of step 5 two, determining step acquisition on May Day; If the determination result is NO, then execution in step five or three;

Step 5 three, the phase demodulation banner word is set is 0, switches to FLL work and phase-locked loop dead status that is:; Realize the switching of phase-locked loop and FLL, finish once synchronously based on the spread spectrum communication united carrier of SNR, and return execution in step five;

Step 6, the frequency of spread-spectrum signal is adjusted, and the frequency control word after obtaining to adjust;

Whether frequency control word after the adjustment that step 6 one, determining step six obtain if the determination result is NO, then returns execution in step five less than the second default threshold value; If judged result is yes, then execution in step six or three;

Step 6 three, the phase demodulation banner word is set is 1, switches to phase-locked loop operation and FLL dead status that is:; Realize the switching of phase-locked loop and FLL, finish once synchronously based on the spread spectrum communication united carrier of SNR, and return execution in step five; Realization is based on the synchronous closed-loop control of the spread spectrum communication united carrier of SNR.

Described in the step 3 spread-spectrum signal that receives is carried out signal-to-noise ratio (snr) estimation, obtaining the signal-to-noise ratio (snr) estimation value is to adopt the estimation of snr computation module to obtain.

Carrier tracking technique in the present invention and a kind of radio communication, adopt frequency-locked loop under the large dynamic range and adopt phase-locked loop under the high tracking accuracy, finish FLL and phase-locked loop switching by the estimation snr of received signal, and then realize the carrier synchronization of spread spectrum communication.The present invention can be widely used in actual spread spectrum system and can be good at resisting Doppler frequency shift.The present invention can shorten the time that carrier loop catches and follows the tracks of.

Description of drawings

Fig. 1 is the signal processing flow schematic diagram in the specific embodiment of the invention one; Fig. 2 is the principle schematic of phase-locked loop or FLL; Fig. 3 is that the FPGA of digital controlled oscillator realizes principle schematic.

Embodiment

Embodiment one, in conjunction with Fig. 1 to Fig. 3 this embodiment is described, a kind of spread spectrum communication united carrier method for synchronous based on SNR, it is realized by following steps:

Step 1, after the receiving terminal of spread spectrum communication system receives spread-spectrum signal, simultaneously execution in step two and step 3;

Step 2, the spread-spectrum signal that receives is caught; And execution in step 21;

Step 2 one, whether the acquisition success number of times in the step 2 is arrived preset value M1; M1 is positive integer; If the result is yes, then execution in step two or two; If the determination result is NO, then return execution in step two;

Step 2 two, frequency control word and phase control words are carried out initialization; And execution in step four;

Step 3, the spread-spectrum signal that receives is carried out signal-to-noise ratio (snr) estimation, obtain the signal-to-noise ratio (snr) estimation value; And execution in step four;

Whether the signal-to-noise ratio (snr) estimation value that obtains in step 4, the determining step three is less than predetermined threshold value M2, and M2 is positive number; If judged result is yes, then return execution in step two or two; If the determination result is NO, then execution in step five;

Step 5, in carrier-tracking loop, whether judge the phase demodulation banner word more than or equal to 1, if judged result is yes, execution in step May Day then; If the determination result is NO, then execution in step six;

Step May Day, the phase place of spread-spectrum signal is adjusted, and phase control words after obtaining to adjust;

Method of adjustment is: phase error signal and the former phase control words of spread-spectrum signal added up, and the phase control words after obtaining to upgrade, thus control produces the phase signal after upgrading.

Whether phase control words if judged result be yes, then returns execution in step five less than the first default threshold value after the adjustment of step 5 two, determining step acquisition on May Day; If the determination result is NO, then execution in step five or three;

Step 5 three, the phase demodulation banner word is set is 0, switches to FLL work and phase-locked loop dead status that is:; Realize the switching of phase-locked loop and FLL, finish once synchronously based on the spread spectrum communication united carrier of SNR, and return execution in step five;

Step 6, the frequency of spread-spectrum signal is adjusted, and the frequency control word after obtaining to adjust;

Method of adjustment is: frequency error signal and the primary frequency control word of spread-spectrum signal added up, and the frequency control word after obtaining to upgrade, thus control produces the frequency signal after upgrading.

Whether frequency control word after the adjustment that step 6 one, determining step six obtain if the determination result is NO, then returns execution in step five less than the second default threshold value; If judged result is yes, then execution in step six or three;

Step 6 three, the phase demodulation banner word is set is 1, switches to phase-locked loop operation and FLL dead status that is:; Realize the switching of phase-locked loop and FLL, finish once synchronously based on the spread spectrum communication united carrier of SNR, and return execution in step five; Realization is based on the synchronous closed-loop control of the spread spectrum communication united carrier of SNR.

Described in the step 3 spread-spectrum signal that receives is carried out signal-to-noise ratio (snr) estimation, obtaining the signal-to-noise ratio (snr) estimation value is to adopt the estimation of snr computation module to obtain.Wherein: M1=20.M2=10。

Operation principle: the present invention proposes a kind of according to estimation snr of received signal switching phase-locked loop and FLL, and to finish the method for spread spectrum communication carrier synchronization, idiographic flow as shown in Figure 1.When receiver is received spread-spectrum signal, at first spread-spectrum signal is caught, simultaneously with snr computation module estimation signal to noise ratio, if acquisition success and signal to noise ratio are greater than threshold value, then initialization frequency and phase control words, and carry out next step carrier synchronization, otherwise proceed to catch.Normal operation, in the carrier-tracking loop, the phase demodulation banner word is 1, phase-locked loop (PLL) work and FLL (FLL) is stopped using.And when phase error surpasses threshold value, i.e. frequency deviation (f _Error) surpass the following range (f of PLL _Range) time, change phase demodulation banner word is 0, FLL work and PLL stops using.And the time of common PLL work is long, and receiver is cisco unity malfunction in the handoff procedure.The faster the better for this process time.In case it is undesired to detect the reception signal, the PLL signal should be stopped using at once and be switched FLL work.In fact when the reception signal was undesired, the bit error rate of reception significantly increased, and namely SNR is lower.Therefore, unusual if the quick interpretation of energy goes out SNR, can realize very soon that PLL is to the switching of FLL.Method is a given threshold value, if judge that receiving signal SNR is lower than threshold value, PLL stops using and enables FLL.

The operation principle of following paper PLL and FLL.Suppose that modulation system is DQPSK.Carrier tracking loop comprises FLL and PLL.This illustraton of model of two is identical, and difference is frequency or phase detector as shown in Figure 2.FLL or PLL comprise three parts: digital loop filters, NCO and frequency or phase detector.

Frequency or phase detector be used for to differentiate local carrier and the digital carrier that receives between frequency difference or differ.Digital loop filters is a low pass filter, is used to the noise in the filtering loop, because frequency difference or phase difference signal are low passes.NCO is the carrier wave that produces different frequency according to control information.

Phase detector: the effect of this part is that to make phase deviation be zero.System uses the decision-feedback loop, and method is as follows: suppose the intermediate-freuqncy signal I that receives _InAnd Q _InFor:

Addition after multiplying each other at them with by the local oscillation signal that NCO produces, I _OutAnd Q _OutFor (Δ ω is the frequency difference between intermediate-freuqncy signal and the local oscillation signal,

Differing between the two):

(2)

After the filtering, output signal I _ΣAnd Q _ΣFor:

The error controling signal S that phase detector produces _AFCFor:

S _AFC=Sign[Q _∑]I _∑-Sign[I _∑]Q _∑(4)

Frequency detector: final PLL makes and receives signal and NCO to produce between the signal frequency identical with phase place.Although PLL has realized higher tracking accuracy, its capture range is smaller.Although and FLL can keep fixing differing when realizing that frequency departure is zero, its capture range is very large.The frequency discrimination method is as follows:

When local carrier with receive frequency between signal when identical with phase place, output signal I in the equation (3) _ΣAnd Q _ΣBe rewritten as digital form:

I _∑(k)=A _∑(k)cosφ(k)

Q _∑(k)=A _∑(k)sinφ(k)(5)

Definition S _In(k):

S _in(k)=I _∑(k)+jQ _∑(K)=A _∑(k)e ^-jφ(k)(6)

S is arranged simultaneously _Out(k) satisfy:

$S_{\mathrm{out}}\left(k\right)=S_{\mathrm{in}}\left(k\right)S_{\mathrm{in}}^{*}(k-1)=A_{\mathrm{\Σ}}\left(k\right)A_{\mathrm{\Σ}}(k-1)e^{-j[\mathrm{\φ}\left(k\right)-\mathrm{\φ}(k-1)]}---\left(7\right)$

Definition:

Dot(k)=Re[S _out(k)]

Cross(k)＝Im[S _out(k)](8)

If modulation system is QPSK, have:

A _∑(k)=A _∑(k-1)=A _∑(9)

Wherein Dot (k) and Cross (k) can be expressed as:

$\mathrm{Dot}\left(k\right)=A_{\mathrm{\Σ}}^2\cos [\mathrm{\φ}\left(k\right)-\mathrm{\φ}(k-1)]$

$\mathrm{Cross}\left(k\right)=-A_{\mathrm{\Σ}}^2\sin [\mathrm{\φ}\left(k\right)-\mathrm{\φ}(k-1)]---\left(10\right)$

If have frequency deviation and differ, formula (10) is write and is done:

$\mathrm{Cross}\left(k\right)=-A_{\mathrm{\Σ}}^2\cos [{\mathrm{\Δ\φ}}_{\mathrm{mod}}-{\mathrm{\Δ\φ}}_e+\mathrm{\π}/4]---\left(11\right)$

Suppose that whole deviation is:

With:

Δφ _mod=φ(k)-φ(k-1)。

Δ φ in the QPSK situation _ModValue is: 0, and pi/2, π, 3 pi/2s. then the value of Dot (k) and Cross (k) is 0,1 and-1.Therefore, additional fixed phase drift is π/4.

Believe S by the error control that frequency detector produces _AFCFor:

S _AFC=Sign[Dot(k)]Cross(k)-Sign[Cross(k)]Dot(k)(12)

NCO: this is the oscillating signal by the accumulation of phase generation of high frequency clock.But because Nyquist's theorem, what carrier frequency must be than clock frequency is 1/2 little.As shown in Figure 3, NCO is by the phase information register, and phase accumulator and check table form.

Incoming frequency control word (FCW) in the phase accumulator, cumulative until in clock cycle, the accumulator meter is full and overflow.Accumulator output is equivalent to the signal phase of the such one-period of 0～2 π.When the phase bits numerical value of phase accumulator was set to 32, maximum was output as 2 ³²-1, and corresponding phase place is 2 π.The cumulative code value that can both obtain phase place output of each step, and code value can be converted into by tabling look-up the amplitude of signal.It is 2 that phase accumulator overflows Threshold ³²/ K, when FCW output K, the spill-over of phase accumulator meter goes out and finishes a circulation.Output frequency f ₀With clock frequency f _CLKBetween satisfy:

$f_0=\frac{K}{2^{32}}{f}_{\mathrm{CLK}}---\left(13\right)$

Digital loop filters: this is an order digital filter, can regulate passband width by adjusting two gain coefficients, improves the performance of whole loop.But this is the process of a very complex, because simple theoretical calculating can not be directly used in real system.Only have by continuous test and could obtain desired result.

Normal operation, in the carrier-tracking loop, PLL work and FLL stops using.And as frequency deviation (f _Error) surpass the following range (f of PLL _Range) time, at this moment, SNR is lower, is lower than a threshold values of setting, and FLL work PLL stops using.The concrete grammar that SNR calculates:

Signal I based on equation (5) _Σ(k), its absolute value I _Abs(k) be:

I _abs(k)=|I _∑(k)|(14)

The average I of N continuous point (N=2048 in the native system) signal _MeanFor:

$I_{\mathrm{mean}}=\frac{\underset{k=0}{\overset{2047}{\mathrm{\Σ}}}{I}_{\mathrm{abs}}\left(k\right)}{2048}---\left(15\right)$

Signal power S _iCan be expressed as:

S _i=I _mean×I _mean(16)

Signal I _Abs(k) and average I _MeanBetween deviation I _ErrorFor

I _err=I _abs(k)-I _mean (17)

Noise power N _iCan be expressed as

$N_i=\frac{\underset{i=0}{\overset{2047}{\mathrm{\Σ}}}{\left(I_{\mathrm{err}}\right)}^2}{2048}---\left(18\right)$

So, signal to noise ratio snr is

$\mathrm{SNR}=\frac{S_i}{N_i}---\left(19\right)$

In order to calculate signal to noise ratio, equation (15), (18) and (19) have all used division.During FPGA realized, division arithmetic took a lot of logical blocks.Because the SNR in the carrier loop only need to provide approximation, do not need exact value, division can be converted into add operation.In equation (15) and (18), the division to 2048 can be directly changed into the cut position computing, namely the 11-bit data of low level can directly be deleted, and are that division has become addition and directly cut out minimum 11, and keep other position.For equation (19), division arithmetic can utilize subtraction, namely judges S _iWith N _iDifference if difference greater than 0 o'clock, continues to calculate and the merchant adds 1, inputs to S to difference _i, and continue to calculate S _iWith N _iDifference is until difference stops computing less than 0.The result of final division is the value of the number of times-SNR of subtraction.Native system uses the FPGA-EP20K300EQC240 of ALTERA company.In addition, data I _AbsBuffering area needs the RAM of 2048 bytes.

The present invention relates to the carrier tracking technique in a kind of radio communication, adopt frequency-locked loop under the large dynamic range and adopt phase-locked loop under the high tracking accuracy, relate to and a kind ofly finish the method that FLL and phase-locked loop switch according to the estimation snr of received signal.Given first FPGA specific implementation principle and the method for FLL or phase-locked loop, then provided according to SNR and finished phase-locked loop to the method for the quick switching of FLL, concrete principle and the FPGA method of SNR estimation have been provided at last, and according to the characteristics of FPGA the multiplying in the algorithm has been converted to add operation, save the logical block of FPGA.This carrier tracking loop has been used to actual spread spectrum system and can have well resisted Doppler frequency shift.In the test, can further promote the efficient of signal to noise ratio method.It is simpler that the present invention realizes, to the resource occupation of FPGA still less, can shorten the time that carrier loop catches and follows the tracks of.

Claims (4)

1. spread spectrum communication united carrier method for synchronous based on SNR, it is characterized in that: it is realized by following steps:

Step 1, after the receiving terminal of spread spectrum communication system receives spread-spectrum signal, simultaneously execution in step two and step 3;

Step 2, the spread-spectrum signal that receives is caught; And execution in step 21;

Whether the acquisition success number of times in step 2 one, the determining step two arrives preset value M1; M1 is positive integer; If the result is yes, then execution in step two or two; If the determination result is NO, then return execution in step two;

Step 2 two, frequency control word and phase control words are carried out initialization; And execution in step four;

Step 3, the spread-spectrum signal that receives is carried out signal-to-noise ratio (snr) estimation, obtain the signal-to-noise ratio (snr) estimation value; And execution in step four;

Whether the signal-to-noise ratio (snr) estimation value that obtains in step 4, the determining step three is less than predetermined threshold value M2, and M2 is positive number; If judged result is yes, then return execution in step two or two; If the determination result is NO, then execution in step five;

Step 5, in carrier-tracking loop, whether judge the phase demodulation banner word more than or equal to 1, if judged result is yes, execution in step May Day then; If the determination result is NO, then execution in step six;

Step May Day, the phase place of spread-spectrum signal is adjusted, and phase control words after obtaining to adjust;

Whether phase control words if judged result be yes, then returns execution in step five less than the first default threshold value after the adjustment of step 5 two, determining step acquisition on May Day; If the determination result is NO, then execution in step five or three;

Step 5 three, the phase demodulation banner word is set is 0, switches to FLL work and phase-locked loop dead status that is:; Realize the switching of phase-locked loop and FLL, finish once synchronously based on the spread spectrum communication united carrier of SNR, and return execution in step five;

Step 6, the frequency of spread-spectrum signal is adjusted, and the frequency control word after obtaining to adjust;

Whether frequency control word after the adjustment that step 6 one, determining step six obtain if the determination result is NO, then returns execution in step five less than the second default threshold value; If judged result is yes, then execution in step six or three;

Step 6 three, the phase demodulation banner word is set is 1, switches to phase-locked loop operation and FLL dead status that is:; Realize the switching of phase-locked loop and FLL, finish once synchronously based on the spread spectrum communication united carrier of SNR, and return execution in step five; Realization is based on the synchronous closed-loop control of the spread spectrum communication united carrier of SNR.

2. a kind of spread spectrum communication united carrier method for synchronous based on SNR according to claim 1, it is characterized in that described in the step 3 spread-spectrum signal that receives being carried out signal-to-noise ratio (snr) estimation, obtaining the signal-to-noise ratio (snr) estimation value is to adopt the estimation of snr computation module to obtain.

3. a kind of spread spectrum communication united carrier method for synchronous based on SNR according to claim 1 is characterized in that M1=20.

4. a kind of spread spectrum communication united carrier method for synchronous based on SNR according to claim 1 is characterized in that M2=10.

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