CN114785475A - Method for enhancing Gardner timing error detection - Google Patents
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Abstract
The invention discloses an enhancement method for detecting the timing error of Gardner, which directly removes or reduces self-noise interference from different sources in the traditional Gardner method. For BPSK/QPSK equivalent baseband signals filtered by adopting a low roll-off coefficient, when adjacent symbols have polarity hopping, firstly removing the influence of front and back 2 symbols which are adjacent to the adjacent symbols on a middle sample point, and then using the middle sample point for calculating timing errors; when there is no polarity jump in the adjacent symbol, the timing error is set to zero directly. By directly removing or reducing the self-noise interference of the band-limited signal, the invention not only can enlarge the band-limited range applicable to the traditional Gardner timing error detection method, but also is beneficial to improving the capture time of a timing loop and the anti-noise performance of the timing loop.
Description
Technical Field
The invention belongs to the technical field of digital communication synchronization, and particularly relates to a method for enhancing Gardner timing error detection.
Background
Timing synchronization is also sometimes referred to as symbol timing synchronization or symbol synchronization. As the name implies, the symbol timing synchronization is a process of synchronously aligning a symbol clock at a receiving end with a corresponding clock at a transmitting end, and aims to overcome the code sliding phenomenon which may occur in long-term operation due to clock crystal oscillator drift and other reasons at the receiving and transmitting ends; the second purpose is to make decision on the received symbol at the time when the signal-to-noise ratio is maximum so as to obtain the best receiving performance.
The traditional Gardner timing error detection method has a simple structure, only 2 sampling points (namely, a decision sampling point and an intermediate sampling point) are needed for each symbol, and the method is not sensitive to the carrier phase, so that the method is widely applied to a timing loop of a digital receiver, but is easily influenced by self-noise. Since currently in band-limited signal systems, raised cosine filters are commonly used to constrain the signal spectrum and control inter-symbol interference. However, as the roll-off coefficient of the raised cosine filter decreases, the jitter of the filtered signal between the optimal sampling points becomes large, which results in a large variance of the output result of the conventional Gardner timing error detection method. In this case, if the timing loop bandwidth is reduced in order to reduce the timing phase jitter, the lock time of the timing loop needs to be increased. Therefore, the rise cosine roll-off coefficient applicable to the traditional Gardner timing error detection method is between 0.4 and 1, which is not beneficial to improving the frequency band utilization rate.
Disclosure of Invention
The purpose of the invention is as follows: in order to overcome the defects in the prior art, the invention provides an enhancement method for detecting the timing error of the Gardner, which effectively reduces the self-noise interference caused by the reduction of the raised cosine roll-off coefficient.
The technical scheme is as follows: the application discloses a reinforcing method for detecting a Gardner timing error, for BPSK (Binary Phase Shift Keying) and/or QPSK (Quadrature Phase Shift Keying) equivalent baseband signals after raising cosine filtering by adopting a low roll-off coefficient, when adjacent symbols have polarity hopping, firstly removing the influence of front and back N symbols adjacent to the adjacent symbols on middle sample points, wherein N is an even number more than or equal to 2, obtaining corrected middle sample points, and then calculating the timing error according to the corrected middle sample points; when there is no polarity jump in the adjacent symbol, the timing error is set to zero directly.
Further, when there is a polarity jump in the adjacent symbol, the influence of the front and rear 2 symbols immediately adjacent to the adjacent symbol on the middle sample point is removed, that is, the influence of the front and rear 2 symbols spaced by 3/2 symbol periods on the two sides of the middle sample point on the middle sample point is removed, and then the middle sample point is used for calculating the timing error.
Further, let the decision samples of adjacent symbols be y (t)k-1) And y (t)k) And the front and back 2 symbols immediately adjacent to the adjacent symbol are y (t)k-2) And y (t)k+1) (ii) a Is adjacent toThe influence of the immediately preceding and following 2 symbols on the middle sample point is β (y (t)k-2)+y(tk+1) Where β represents a weight coefficient of the trailing interference.
Further, when there is a polarity jump in adjacent symbols of the in-phase branch, the timing error is
Wherein, the first and the second end of the pipe are connected with each other,
represents the real part of {. DEG }, y (t)k-1/2) Representing intermediate samples of adjacent symbols; otherwise, when the adjacent symbols of the same-phase branch have no polarity jump, the timing error e is forcedI(tk)＝0。
Further, when adjacent symbols of the orthogonal branch have polarity jump, the timing error
Wherein, the first and the second end of the pipe are connected with each other,
represents the imaginary part of {. cndot.); otherwise, when the adjacent symbols of the orthogonal branch have no polarity jump, the timing error e is forcedQ(tk)＝0。
Further, in the case that the roll-off coefficient is known, the weight coefficient β of the tail interference is calculated as g (3T/2)/g (0), where g (kt) is the impulse response of the raised cosine roll-off filter and T is the symbol period.
Further, the low cosine roll-off coefficient refers to a cosine roll-off coefficient less than or equal to 0.4.
Further, the timing error within the symbol period T is e (T)k)＝eI(tk)+eQ(tk)。
Wherein, yI[·]And yQ[·]Respectively representing BPSK/QPSK equivalent baseband signals y [ ·]The real and imaginary parts of (a) and (b),
representing a decision value or a sign function.
Further, the enhancement method is applied to a timing error detector of a digital phase-locked loop structure, and calibration is carried out according to the timing error.
Furthermore, the timing error detector adopts 7-stage shift memory to shift and store the BPSK/QPSK equivalent baseband signal samples input at 2 times the symbol rate.
Has the advantages that:
the invention can not only enlarge the band-limited range applicable to the traditional Gardner timing error detection method, but also is beneficial to improving the capture time of a timing loop and the anti-noise performance of the timing loop by directly removing or reducing the self-noise interference of the band-limited signal.
Drawings
The foregoing and/or other advantages of the invention will become further apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings.
Fig. 1 is a block diagram of a timing synchronization loop.
Fig. 2 is an eye diagram of a BPSK signal with a roll-off factor of 1.
Fig. 3 is an eye diagram of a BPSK signal with a roll-off factor of 0.5.
Fig. 4 is an eye diagram of a BPSK signal with a roll-off factor of 0.25.
Fig. 5 is a raised cosine pulse waveform for different roll-off coefficients.
Fig. 6 is a flow diagram of enhanced Gardner timing error detection.
Detailed Description
Embodiments of the present invention will be described below with reference to the accompanying drawings.
The method for enhancing the Gardner timing error detection provided by the embodiment of the application is applied to a band-limited signal system using a raised cosine filter, wherein the band-limited signal includes but is not limited to a BPSK/QPSK equivalent baseband signal. Such as in the exemplary digital phase-locked loop configuration of fig. 1, which includes an interpolator, a timing error detector, a loop filter, and a timing controller. An enhancement method of Gardner timing error detection provided by the embodiment of the present application is applied to the timing error detector, also referred to as enhanced Gardner timing error detector; the present invention does not modify other components except for the timing error detector, and is completely consistent with the conventional Gardner method.
Since the conventional Gardner timing error detection algorithm requires 2 samples per symbol, the output rate of the interpolator should be 2 times the symbol rate. Meanwhile, for the convenience of implementation, the input rate of the interpolator is generally set to be 2 times of the output rate, i.e., 4 times the symbol rate. In fig. 1, the input of the matched filter is r (t), and the output of the matched filter is y (t); sampling the matched filtered result y (t) at a fixed rate enables the input rate configuration of the interpolator.
In the timing error detector, 2 samples y (t) are input every timek-1/2)，y(tk) Calculate 1 time timing error e (t)k). Apparently, the timing error detector needs to definitely judge the sequential input order of the sampling points and the intermediate sampling points to correctly calculate the timing error. However, due to the automatic adjustment function of the feedback loop, even if the actual input sequence of the samples at the time of starting the loop does not coincide with the assumed input sequence of the timing error detector, the actual input sequence of the samples is automatically adjusted to the assumed input sequence of the timing error detector when the loop is finally locked.
The loop filter generally adopts an active proportional integral filter to correct the timing error e (t)k) Stepping after filtering for adjusting timing controllerThe value is obtained. The stepping base value of the timing controller is fixed at 0.5, and the input-output basic rate ratio of the interpolator is controlled. The timing error after loop filtering is taken as a step deviation value of the timing controller, and the module 1 down counter in the timing controller is updated at 4 times of the symbol rate by combining with the step base value. Each time an underflow of the counter occurs, a fractional time interval is calculated from the contents of the counter before the underflow. The interpolator calculates a corrected interpolation value based on the sampling time at which the counter underflows and the fractional time interval.
Conventional Gardner method calculates timing error e (t)k) The method comprises the following steps:
e(tk)＝{yI(tk-1)-yI(tk)}·yI(tk-1/2)+{yQ(tk-1)-yQ(tk)}·yQ(tk-1/2) (1)
wherein, yI(tk) And yQ(tk) BPSK/QPSK equivalent baseband signal samples y (t)k) Real and imaginary parts of, y (t)k-1) And y (t)k) Representing two adjacent decision samples, y (t)k-1/2) Are their intermediate samples. The conventional Gardner method generates directional information using decision samples and multiplies intermediate samples to obtain a measure of timing error. It can be seen that the essence of the conventional Gardner method is zero-crossing detection, and if the intermediate sampling point is interfered by noise, the accuracy of the timing error measurement is significantly reduced.
It is well known that the conventional Gardner method is susceptible to self-noise even without considering external noise. This is because at present in band-limited signal systems, raised cosine filters are usually used to constrain the signal spectrum and control inter-symbol interference, and the lower the roll-off coefficient of raised cosine filters, the higher the band-limited degree. But as the roll-off factor decreases, although there is no intersymbol interference at the optimal sampling points, the jitter of the signal between the optimal sampling points becomes large. As shown in the BPSK signal eye diagrams of different roll-off coefficients in fig. 2, 3 and 4. It can be seen that the smaller the roll-off coefficient, the greater the self-noise at the intermediate sample points. When the roll-off factor is reduced from 1 to 0.25, the intermediate samples are also gradually dispersed upwards and downwards from the initially converged 1 zero-crossing points, and the maximum amplitude can even reach 0.64 of the decision sample, so that the roll-off factor applicable to the conventional Gardner method is between 0.4 and 1.
Observing the raised cosine pulse waveforms of different roll-off coefficients in fig. 5, the smaller the roll-off coefficient is, the slower the trailing decay of the pulse waveform is. Consider when adjacent symbols y (t)k-1) And y (t)k) When no polarity jump exists, the middle zero-crossing point does not exist, and therefore timing error information cannot be provided. Only if the adjacent symbol y (t)k-1) And y (t)k) The middle zero crossing point can provide timing error information when a polarity jump occurs. At this time, the intermediate sampling point y (t) is offset due to the opposite polaritiesk-1/2) Does not come from both sides of the symbol y (t)k-1) And y (t)k) But from symbols other than the two-sided symbols. That is, the main lobe of the pulse waveform is either the main source of self-noise at the intermediate sample point, but there is no zero-crossing point at this time; or zero-crossing occurs but little self-noise is generated because of mutual cancellation. When the main lobes cancel each other at the intermediate sample point, the tail oscillations will be the main source of self-noise.
The embodiment of the application provides an enhancement method for detecting a Gardner timing error, which comprises the steps of firstly removing the influence of N symbols which are adjacent to adjacent symbols and are front and back to the adjacent symbols on a middle sample point when the adjacent symbols have polarity jump for BPSK/QPSK equivalent baseband signals filtered by a low cosine roll-off coefficient, wherein N is an even number which is more than or equal to 2, obtaining the corrected middle sample point, and then calculating the timing error according to the corrected middle sample point; when there is no polarity jump in the adjacent symbol, the timing error is set to zero directly.
In this embodiment, if only the tailing oscillation influence in the 1 st symbol period from both sides of the main lobe is considered, the influence of the front and rear 2 symbols immediately adjacent to the adjacent symbol on the middle sample point is removed first, that is, the influence of the front and rear 2 symbols spaced by 3/2 symbol periods on both sides of the middle sample point on the middle sample point is removed first, the middle sample point is corrected according to the influence on the middle sample point, and then the timing error is calculated according to the corrected middle sample point, the specific process includes the following steps:
step 1In the enhanced Gardner timing error detector provided in this embodiment, samples of BPSK/QPSK equivalent baseband signal input at 2 times symbol rate are subjected to 7-stage shift storage, and are respectively labeled as y (t) according to time sequencek-2)、y(tk-3/2)、y(tk-1)、y(tk-1/2)、y(tk)、y(tk+1/2) And y (t)k+1) There are 7 consecutive signal samples.
Step 3, removing 2 outermost judgment sampling points y (t)k-2) And y (t)k+1) To the intermediate sampling point y (t)k-1/2) Is multiplied by the middle 2 adjacent decision samples y (t)k-1) And y (t)k) The difference between the two values, the current timing error e (t) is calculatedk)。
e(tk)＝eI(tk)+eQ(tk)
Wherein, yI[·]And yQ[·]Respectively representing BPSK/QPSK equivalent baseband signals y [ ·]The real and imaginary parts of (a) and (b),
indicating a decisionValue or sign function, beta denotes the weight coefficient of the trailing interference, beta (y (t)k-2)+y(tk+1) I.e., the effect on the middle sample of the preceding and following 2 symbols spaced 3/2 symbol periods on either side of the middle sample. In the case where the roll-off coefficient is known, the weight coefficient β of the tail interference may be calculated in advance as g (3T/2)/g (0), where g (kt) is the impulse response of the raised cosine roll-off filter and T is the symbol period. The low cosine roll-off coefficient refers to a cosine roll-off coefficient less than or equal to 0.4. For example, when the roll-off coefficient is 0.25, it is easy to calculate the weighting coefficient β of the tail interference at this time to — 0.1856.
As with the conventional Gardner method, the enhanced Gardner method also requires 2 samples per symbol, i.e., 2 times the symbol rate input. But the difference is that each time 1 timing error is calculated, the enhancement method requires 4 decision samples and 1 intermediate sample at the same time. That is, at least 7 stages of shift memories should be used to shift and store the BPSK/QPSK equivalent baseband signal samples input at 2 times the symbol rate. Assume that, in chronological order, the symbols are respectively denoted by y (t)k-2)、y(tk-3/2)、y(tk-1)、y(tk-1/2)、y(tk)、y(tk+1/2) And y (t)k+1) There are 7 consecutive complex signal samples.
Taking out the samples labeled y (t) every 2 samples are inputk-2)、y(tk-1)、y(tk-1/2)、y(tk) And y (t)k+1) 5 samples. Wherein, y (t)k-2)、y(tk-1)、y(tk) And y (t)k+1) Are all taken as decision samples, y (t)k-1/2) As intermediate samples. Then removing 2 outermost decision sampling points y (t) according to the formula (2)k-2) And y (t)k+1) To the intermediate sampling point y (t)k-1/2) And calculates the current timing error e (t)k)。
The process flow for enhanced timing error detection is shown in fig. 6. Wherein, the input mark is used for controlling 2 sampling points to be input every time, and 1 time of timing error calculation is enabled. Although the input flag value is related to the sample point input order assumed by the timing error detector, whether the loop eventually converges or not is not related to the initial value of the input flag due to the auto-tuning function of the feedback loop. Certainly, in applications requiring fast convergence such as burst communication, other auxiliary methods can be discussed to distinguish the input sequence of the decision sample point and the intermediate sample point in advance, and then set the input flag value in particular, so as to accelerate the convergence speed of the timing synchronization loop.
While the present invention provides an enhanced method for Gardner timing error detection, and the method and means for implementing the same are numerous, the above description is only an embodiment of the present invention, and it should be noted that, for those skilled in the art, many modifications and amendments can be made without departing from the principle of the present invention, and these modifications and amendments should also be considered as the protection scope of the present invention. All the components not specified in this embodiment can be implemented by the prior art.
Claims (10)
1. A method of enhancing Gardner timing error detection, comprising: for BPSK/QPSK equivalent baseband signals filtered by low cosine roll-off coefficients, when adjacent symbols have polarity jump, the influence of front and back N symbols adjacent to the adjacent symbols on middle sample points is removed, N is an even number larger than or equal to 2, corrected middle sample points are obtained, and then timing errors are calculated according to the corrected middle sample points; when there is no polarity jump in the adjacent symbol, the timing error is directly set to zero.
2. An enhancement method of Gardner timing error detection according to claim 1, characterized in that: when the adjacent symbols have polarity jump, the influence of the front and back 2 symbols adjacent to the adjacent symbols on the middle sample point is removed, that is, the influence of the front and back 2 symbols which are positioned at two sides of the middle sample point and have an interval of 3/2 symbol periods on the middle sample point is removed, and then the middle sample point is used for calculating the timing error.
3. An enhancement method of Gardner timing error detection according to claim 2, characterized in that: note that the decision samples of adjacent symbols are y (t)k-1) And y (t)k) Immediately preceding an adjacent symbolThe last 2 symbols are y (t)k-2) And y (t)k+1) (ii) a The effect of the preceding and following 2 symbols immediately adjacent to the adjacent symbol on the middle sample point is β (y (t)k-2)+y(tk+1) Where β represents a weight coefficient of the trailing interference.
4. A method of enhancing Gardner timing error detection according to claim 3, wherein: timing error when adjacent symbols of the in-phase branch have polarity jumps
Wherein the content of the first and second substances,
represents the real part of {. DEG }, y (t)k-1/2) Representing intermediate samples of adjacent symbols; otherwise, when the adjacent symbols of the same-phase branch have no polarity jump, the timing error e is forcedI(tk)＝0。
5. An enhancement method of Gardner timing error detection according to claim 4, wherein: timing error when adjacent symbols of the orthogonal branch have polarity jumps
Wherein the content of the first and second substances,
represents the imaginary part of {. cndot.); otherwise, when the adjacent symbol of the orthogonal branch has no polarity jump, the timing error e is forcedQ(tk)＝0。
6. An enhancement method of Gardner timing error detection according to claim 5, wherein: in the case where the roll-off coefficient is known, the weight coefficient β of the tail interference is calculated as g (3T/2)/g (0), where g (kt) is the impulse response of the raised cosine roll-off filter and T is the symbol period.
7. An enhancement method of Gardner timing error detection according to claim 6, wherein: the low cosine roll-off coefficient refers to a cosine roll-off coefficient less than or equal to 0.4.
8. An enhancement method of Gardner timing error detection according to claim 7, wherein: the timing error within the symbol period T is e (T)k)＝eI(tk)+eQ(tk)。
9. An enhancement method of Gardner timing error detection according to claim 8, wherein: and the timing error detector is applied to a digital phase-locked loop structure and is used for calibrating according to the timing error.
10. An enhancement method of Gardner timing error detection according to claim 9, wherein: the timing error detector adopts a 7-stage shift memory to shift and store BPSK/QPSK equivalent baseband signal sampling points input at 2 times of symbol rate.
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| JPH1028149A (en) * | 1996-07-10 | 1998-01-27 | Matsushita Electric Ind Co Ltd | Clock recovery circuit |
| CN103457680A (en) * | 2013-08-20 | 2013-12-18 | 重庆邮电大学 | Satellite communication timing synchronization error detection method based on full-digital receiving |
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