CN103762977A - Method and device for improving tracking performance of phase-locked loop with low signal to noise ratio - Google Patents

Abstract

Provided is a device for improving tracking performance of a phase-locked loop with a low signal to noise ratio. A phase discriminator coefficient modification device is additionally arranged behind a phase discriminator of the phase-locked loop, the modification device divides the output result of the phase discriminator by the coefficient of the phase discriminator, and then the result obtained after division is completed is output to a loop filter. The coefficient of the phase discriminator is modified, the situation that a phase-locked loop design bandwidth part is out of work under the low signal to noise ratio is avoided, the design bandwidth of the phase-locked loop is an actually-effective bandwidth, and the influence of the Doppler effect within the capacity range of the phase-locked loop can be effectively resisted.

Description

Under low signal-to-noise ratio, phase-locked loop tracking performance improving one's methods and installing

Technical field

The present invention relates to communication technical field, be specifically related to improving one's methods and implement device of phase-locked loop tracking performance under a kind of low signal-to-noise ratio.

Background technology

Phase-locked loop, is the receiver for communicating by letter, and processes to received signal, utilizes FEEDBACK CONTROL to realize frequency and phase locked technology.In continuous communiction receiver, conventional phase-locked loop is realized carrier synchronization and tracking.In widely used digital communication system nowadays, all-digital phase-locked loop circuit is comprised of phase discriminator (PD), loop filter (LF) and these modules of digital controlled oscillator (NCO), the reference signal of input and the phase difference of feedback signal are differentiated and output error voltage by phase discriminator, loop filter through low pass character, form the control voltage of voltage controlled oscillator, the output frequency of oscillator is pulled to the frequency of input signal, until the two equates to be locking.

The key performance of phase-locked loop is phase noise and dynamic characteristic.Phase noise mainly comprises thermal noise σ _twith generator noise σ _v, θ _a, main noise source thermal noise wherein, relevant with PLL loop bandwidth and signal to noise ratio, bandwidth is larger, and noise is larger; Signal to noise ratio is lower, and noise is larger.Dynamic characteristic has reflected the ability of phase-locked loop opposing Doppler effect, relevant with phase-locked loop exponent number with PLL loop bandwidth.Exponent number is the phase-locked loop dynamic stress error of L, is directly proportional to the L order derivative of sending and receiving end sighting distance, is inversely proportional to the L power of loop bandwidth.

When not there is not Doppler effect, the phase jitter of phase-locked loop is mainly derived from thermal noise, and tracking signal-noise ratio threshold simulation value and the theoretical thresholding of phase-locked loop are very approaching, and along with the reducing of bandwidth, signal-noise ratio threshold also can reduce.And when having Doppler effect to exist, along with bandwidth reduces according to identical ratio with Doppler frequency deviation, the tracking signal-noise ratio threshold of phase-locked loop possibly cannot reduce, signal-noise ratio threshold simulation value and theoretical thresholding differ larger, that is, near theoretical thresholding, or perhaps under low signal-to-noise ratio, phase-locked loop can not effectively be followed the tracks of the Doppler frequency deviation in its design bandwidth limit of power, the design bandwidth partial failure of phase-locked loop.

Summary of the invention

In view of under low signal-to-noise ratio, owing to affected by Doppler effect, bandwidth of phase lock loop meeting partial failure, theoretical signal-noise ratio threshold can not accurately be indicated actual emulation signal-noise ratio threshold, the present invention proposes a kind of method and implement device of phase discriminator in phase-locked loop being done to coefficient correction, to improve the tracking performance of phase-locked loop under low signal-to-noise ratio.The present invention is not only applicable to BPSK modulation, is applicable to QPSK modulation yet.

Technical scheme provided by the invention is as follows:

In phase-locked loop, a modification method for phase discriminator coefficient, is characterized in that,

Phase discriminator is approximately to linear model within the scope of certain phase demodulation, and phase discriminator output phase can be expressed as:

φ _o=αφ _i+n

Wherein, φ _ifor the actual phase of phase discriminator input signal, α is the coefficient of phase discriminator linear approximation, is called for short phase discriminator coefficient, is inputted the noise effect in data, and α≤1 increases and increases gradually with signal to noise ratio, and n is that average is zero phase demodulation noise, and its variance is designated as

Due to phase discriminator factor alpha≤1, the actual effective bandwidth of phase-locked loop only has design bandwidth B _nα doubly, signal to noise ratio is lower, phase discriminator factor alpha is less, actual bandwidth is narrower, thus under low signal-to-noise ratio, phase-locked loop cannot accurately be followed the tracks of the impact of certain Doppler frequency deviation, signal-noise ratio threshold obviously worsens;

The Output rusults of phase discriminator, divided by phase discriminator factor alpha, is obtained:

${\widehat{\mathrm{\φ}}}_o=\frac{{\mathrm{\φ}}_o}{\mathrm{\α}}={\mathrm{\φ}}_i+n^{\′}$

Wherein, n' is phase demodulation noise, n'=n/ α, and revised phase discriminator coefficient (gain) becomes 1, and the actual effective bandwidth of phase-locked loop equals design bandwidth, but the variance of phase demodulation noise n' becomes

Before revising, although it is large that phase demodulation noise becomes, actual bandwidth also increases to some extent, phase-locked loop phase noise under low signal-to-noise ratio slightly increases, but for original Doppler frequency deviation losing efficacy of following the tracks of under low signal-to-noise ratio, after revising phase discriminator coefficient, can realize accurately and following the tracks of, the dynamic characteristic of phase-locked loop strengthens, and finally makes the tracking signal-noise ratio threshold of phase-locked loop decrease.

In described phase-locked loop, the modification method of phase discriminator coefficient, is characterized in that, the computational methods following (ginseng Fig. 1) of described phase discriminator factor alpha:

1), according to modulation system, determine that phase discriminator is approximately linear phase demodulation scope

when modulation system is BPSK, M=2, when modulation system is QPSK, M=4;

2) in phase demodulation scope

inside evenly get discretely N phase value x _i, i=1,2 ..., N, respectively as modulating data initial phase, obtains repeatedly identified result by Monte Carlo simulation having under noise conditions, then asks assembly average y _i;

3) by N point (x _i, y _i) fitting to straight line, the slope of this straight line is just as phase discriminator factor alpha.

Step 2) phase discriminator in can be any one conventional phase discriminator, comprises arc tangent phase discriminator, sinusoidal phase discriminator, decision-directed phase discriminator, IQ phase discriminator etc.

In described phase-locked loop, the modification method of phase discriminator coefficient, is characterized in that step 2) in, if the phase discriminator of selecting is responsive to modulation system, can not in phase demodulation, remove modulation intelligence, before phase demodulation, need to remove modulation intelligence.

The modification method of phase discriminator coefficient in described phase-locked loop, it is characterized in that, the method of described removal modulation intelligence is hard decision method: for BPSK, modulate, if modulating data real part is for just, the phase place that modulation intelligence produces is 0, if negative, the phase place that modulation intelligence produces is π, remove after this part phase place, data phase just only includes initial phase; For QPSK modulation, according to the positive and negative and order of magnitude of modulating data real part, imaginary part, determine the phase place that modulation intelligence produces, then remove this part phase place.

In described phase-locked loop, the modification method of phase discriminator coefficient, is characterized in that, the method for described removal modulation intelligence is: modulating data is carried out to M power, and the modulation intelligence in modulating data is removed.

In described phase-locked loop, the modification method of phase discriminator coefficient, is characterized in that, the method for asking fitting a straight line slope in step 3) is least square method.

The present invention provides improve one's methods (ginseng Fig. 2) of phase-locked loop tracking performance under a kind of low signal-to-noise ratio simultaneously, it is characterized in that,

The signal feeding back with digital controlled oscillator and input signal are done conjugate multiplication and are realized down-conversion;

Then through phase discriminator, obtain residual phase;

According to modulation system, modulating data signal to noise ratio and phase discriminator type, from calculate in advance phase discriminator factor alpha select corresponding value, by identified result divided by this coefficient;

Pass through again loop filter and the digital controlled oscillator of low pass character, produce sine, the cosine value of the phase place tracing into, feed back to phase-locked loop input.

The present invention also provides the modifying device (ginseng Fig. 3) of phase-locked loop tracking performance under a kind of low signal-to-noise ratio, it is characterized in that, after the phase discriminator of conventional phase locked loops, add phase discriminator coefficient correcting device, this correcting device divided by phase discriminator coefficient, then outputs to loop filter finishing the result obtaining after division by the Output rusults of phase discriminator.

Beneficial effect of the present invention:

The present invention, by the coefficient correction to phase discriminator, has improved the situation that under low signal-to-noise ratio, Design of PLL portions of bandwidth lost efficacy, and makes the design bandwidth of phase-locked loop be actual effectively bandwidth, can effectively resist the Doppler effect impact in its limit of power.

Accompanying drawing explanation

Fig. 1 is the flow chart that the present invention asks phase discriminator coefficient method.

Fig. 2 is the flow chart of improving one's methods of phase-locked loop tracking performance under low signal-to-noise ratio of the present invention.

Fig. 3 is the modifying device schematic diagram of realizing phase-locked loop tracking performance under low signal-to-noise ratio of the present invention.

Embodiment

MPSK modulating data, in k sampled value of receiving terminal is

$r_k=e^{j(\frac{2\mathrm{\π}{m}_k}{M}+{\mathrm{\φ}}_k)}+n_k,(m_k=\mathrm{0,1},...,M-1)$

Wherein, M=2 during BPSK modulation, M=4 during QPSK modulation, φ _kfor modulating data initial phase, n _kfor the multiple Gaussian random variable of independent identically distributed zero-mean additivity.

The method of asking phase discriminator factor alpha and carrying out the correction of phase discriminator coefficient of the present invention, embodiment is as follows:

The 1st step, according to modulation system, determines that phase discriminator does the phase demodulation scope of linear approximation.The rule of thumb of considering phase-locked loop tracking thresholding is, 3 times of phase error are no more than phase discriminator and draw in 14 of scope, and the present invention mainly pays close attention under low signal-to-noise ratio, near the phase-locked loop tracking performance theoretical thresholding namely, so select threshold value that the rule of thumb provides as the scope of phase discriminator linear approximation.For MPSK modulating data, it is 2 π M that phase discriminator is drawn in scope, and threshold value is π 2M, and phase discriminator is done the scope of linear approximation and is

The 2nd step,

in scope, evenly get N phase value x _i(i=1 ..., N), respectively as modulating data initial phase, under noisy condition, by Monte Carlo simulation, obtain repeatedly identified result, and ask corresponding assembly average y _i.

If the phase discriminator of selecting is here responsive to modulation system, can not in phase demodulation, remove modulation intelligence, before phase demodulation, need to increase the operation of removing modulation intelligence.Method is a hard decision, for BPSK modulation, if modulating data real part for just, the phase place that modulation intelligence produces is 0, if negative, the phase place of modulation intelligence generation is π, removes after this part phase place, data phase just only includes initial phase,

$r_k{e}^{-j\frac{2\mathrm{\π}{m}_k}{M}}=[e^{j(\frac{2\mathrm{\π}{m}_k}{M}+{\mathrm{\φ}}_k)}+n_k]\·e^{-j\frac{2\mathrm{\π}{m}_k}{M}}=e^{j{\mathrm{\φ}}_k}+n_k{e}^{-j\frac{2\mathrm{\π}{m}_k}{M}}$

Similarly, for QPSK modulation, can determine the phase place that modulation intelligence produces according to the positive and negative and order of magnitude of modulating data real part, imaginary part, phase demodulation again after removal.

Another kind is by modulating data r _kcarry out M power,

$r_k^M={[e^{j(\frac{2\mathrm{\π}{m}_k}{M}+{\mathrm{\φ}}_k)}+n_k]}^M=e^{j(2\mathrm{\π}{m}_k+M{\mathrm{\φ}}_k)}+{n_k}^{\′}=e^{\mathrm{jM}{\mathrm{\φ}}_k}+{m_k}^{\′}$

Wherein, n _k' be to remove in expansion

in addition all and, with n _krelevant.Modulation intelligence in modulating data is removed, then phase discriminator output phase now can be obtained to identified result divided by M.

The 3rd step, the N obtaining in previous action point (x _i, y _i) formed

phase demodulation curve in scope, becomes a straight line of crossing initial point by this section of curve, and its slope is phase discriminator linear approximation factor alpha.Approximation coefficient α can obtain by least square method:

$\mathrm{\α}=\frac{\underset{i}{\mathrm{\Σ}}(y_i-\stackrel{\‾}{y})(x_i-\stackrel{\‾}{x})}{\underset{i}{\mathrm{\Σ}}{(x_i-\stackrel{\‾}{x})}^2}$

Wherein, represent respectively x _i, y _imean value.

According to above-mentioned flow process, can try to achieve respectively under BPSK modulation and QPSK modulation system the phase discriminator factor alpha of different phase discriminators under different signal to noise ratios.Especially, phase discriminator factor alpha is independent of outside phase-locked loop and tries to achieve.

In improved phase-locked loop is realized, can be according to modulation system, phase discriminator type, modulating data signal to noise ratio, from the phase discriminator coefficient of trying to achieve in advance, select corresponding numerical value, the identified result of traditional phase discriminator, divided by this coefficient, has been completed to the coefficient correction of phase discriminator.

Emulation testing shows, no matter be BPSK modulation, or QPSK modulation system, the present invention is the tracking performance of improved PLL under low signal-to-noise ratio and Doppler effect impact effectively, makes the bandwidth of design can actual effectively follow the tracks of the Doppler frequency deviation in its limit of power.And phase discriminator factor alpha is independent of outside phase-locked loop tries to achieve, only need in traditional phase-locked loop, increase by one divided by the operation of α, workable, successful, has certain practical value.

Claims (9)

1. a modification method for phase discriminator coefficient in phase-locked loop, is characterized in that,

Phase discriminator is approximately to linear model within the scope of certain phase demodulation, and phase discriminator output phase can be expressed as:

φ _o=αφ _i+n

Wherein, φ _ifor the actual phase of phase discriminator input signal, α is the coefficient of phase discriminator linear approximation, is called for short phase discriminator coefficient, is inputted the noise effect in data, α≤1, and n is that average is zero phase demodulation noise, its variance is designated as

The Output rusults of phase discriminator, divided by phase discriminator factor alpha, is obtained:

${\widehat{\mathrm{\φ}}}_o=\frac{{\mathrm{\φ}}_o}{\mathrm{\α}}={\mathrm{\φ}}_i+n^{\′}$

Wherein, n' is phase demodulation noise, n'=n/ α, and revised phase discriminator coefficient becomes 1, and the actual effective bandwidth of phase-locked loop equals design bandwidth, but the variance of phase demodulation noise n' becomes

2. the modification method of phase discriminator coefficient in phase-locked loop as claimed in claim 1, is characterized in that, the computational methods of described phase discriminator factor alpha are as follows:

1), according to modulation system, determine that phase discriminator is approximately linear phase demodulation scope

when modulation system is BPSK, M=2, when modulation system is QPSK, M=4;

2) in phase demodulation scope

inside evenly get discretely N phase value x _i, i=1,2 ..., N, respectively as modulating data initial phase, obtains repeatedly identified result by Monte Carlo simulation having under noise conditions, then asks assembly average y _i;

3) by N point (x _i, y _i) fitting to straight line, the slope of this straight line is just as phase discriminator factor alpha.

3. the modification method of phase discriminator coefficient in phase-locked loop as claimed in claim 1, is characterized in that step 2) in, if the phase discriminator of selecting is responsive to modulation system, can not in phase demodulation, remove modulation intelligence, before phase demodulation, need to remove modulation intelligence.

4. the modification method of phase discriminator coefficient in phase-locked loop as claimed in claim 1, it is characterized in that, the method of described removal modulation intelligence is hard decision method: for BPSK, modulate, if modulating data real part is for just, the phase place that modulation intelligence produces is 0, if negative, the phase place that modulation intelligence produces is π, remove after this part phase place, data phase just only includes initial phase; For QPSK modulation, according to the positive and negative and order of magnitude of modulating data real part, imaginary part, determine the phase place that modulation intelligence produces, then remove this part phase place.

5. the modification method of phase discriminator coefficient in phase-locked loop as claimed in claim 1, is characterized in that, the method for described removal modulation intelligence is: modulating data is carried out to M power, and the modulation intelligence in modulating data is removed.

6. the modification method of phase discriminator coefficient in phase-locked loop as claimed in claim 1, is characterized in that, the method for asking fitting a straight line slope in step 3) is least square method.

7. the improving one's methods of phase-locked loop tracking performance under a low signal-to-noise ratio, is characterized in that,

The signal feeding back with digital controlled oscillator and input signal are done conjugate multiplication and are realized down-conversion;

Then through phase discriminator, obtain residual phase;

According to modulation system, modulating data signal to noise ratio and phase discriminator type, from calculate in advance phase discriminator factor alpha select corresponding value, by identified result divided by this coefficient;

Pass through again loop filter and the digital controlled oscillator of low pass character, produce sine, the cosine value of the phase place tracing into, feed back to phase-locked loop input.

8. as claimed in claim 7 improving one's methods, is characterized in that, the computational methods of described phase discriminator factor alpha are as follows:

1), according to modulation system, determine that phase discriminator is approximately linear phase demodulation scope

when modulation system is BPSK, M=2, when modulation system is QPSK, M=4;

2) in phase demodulation scope

inside evenly get discretely N phase value x _i, i=1,2 ..., N, respectively as modulating data initial phase, obtains repeatedly identified result by Monte Carlo simulation having under noise conditions, then asks assembly average y _i;

3) by N point (x _i, y _i) fitting to straight line, the slope of this straight line is just as phase discriminator factor alpha.

9. the modifying device of phase-locked loop tracking performance under a low signal-to-noise ratio, it is characterized in that, after the phase discriminator of conventional phase locked loops, add phase discriminator coefficient correcting device, this correcting device divided by phase discriminator coefficient, then outputs to loop filter finishing the result obtaining after division by the Output rusults of phase discriminator.

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