CN112399551B - High-precision synchronization method for short-time burst signals - Google Patents
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Abstract
The invention provides a high-precision synchronization method for short-time burst signals. The method comprises the following steps: a frame synchronization capturing stage: signal detection and frame synchronization capture are completed by using a unique code differential matching mode; and a frequency difference estimation stage: acquiring a leader sequence from a captured burst signal according to a physical layer frame structure agreed by both communication parties, performing frequency difference estimation by using the leader sequence, driving an NCO (NCO) by using estimated frequency difference information, and performing frequency difference correction on the signal; symbol and phase synchronization stage: acquiring a unique code from the captured burst signal according to a physical layer frame structure agreed by both communication parties, and finding a maximum correlation point by using the unique code in a sliding correlation manner to acquire initial symbol synchronization information and initial phase synchronization information of the signal; symbol and phase synchronization tracking stage: and performing symbol synchronization tracking and phase synchronization tracking by using a phase-locked loop to acquire optimal sampling and decision information.
Description
Technical Field
The invention relates to the technical field of wireless communication, in particular to a high-precision synchronization method for short-time burst signals.
Background
Burst signals are widely used in wireless communication systems. Due to the fact that the relative motion of the receiving and sending terminals is inconsistent with the system clock and the like, the signals of the receiving end have frequency deviation and timing deviation, and accordingly carrier synchronization and timing synchronization are needed to recover the original information.
The burst signal oriented synchronization algorithm mainly solves the problems that: after a receiver receives a signal, how to quickly and accurately realize signal detection, carrier synchronization, timing synchronization and frame synchronization.
Common methods of signal detection are energy detection methods, high-order cumulant-based detection methods, and unique code-based autocorrelation methods. The implementation methods of carrier synchronization, timing synchronization and phase synchronization can be divided into closed-loop and open-loop algorithms according to the existence of a feedback loop. The closed-loop algorithm generally has low computational complexity, can achieve very high precision after the loop is successfully locked, but has narrow dynamic capture range and needs longer convergence time to be suitable for being applied to continuous signals. The accuracy of the open-loop algorithm is lower than that of a closed loop, the calculation complexity is generally higher, but the convergence time is not required, and the method is suitable for being applied to burst signals. There are generally two methods for frame synchronization: one is a signal self-synchronization method using the characteristics of data itself, and the other is a cross-correlation detection method based on a preamble sequence and a received signal.
Fig. 1 shows a conventional synchronization scheme, and as shown in fig. 1, each module generally adjusts different implementation sequences according to different application scenarios. As can be seen from fig. 1, the order of signal detection, frequency offset estimation, symbol synchronization, and phase estimation is relatively fixed, and frame synchronization can be achieved after signal detection or after symbol synchronization is completed.
In cooperative communication, a signal detection method based on unique code autocorrelation is often used. The principle is that the sampled data and known unique code are subjected to correlation operation, then the result is compared with a fixed threshold through modular squaring, and whether a burst signal arrives or not is judged. The basic principle is shown in fig. 2. This approach has certain limitations. Firstly, when a receiving signal has a large frequency offset, an autocorrelation peak value is reduced or even disappears; secondly, the set size of the decision threshold directly affects the false alarm rate and the false alarm rate of the system, if the decision threshold is fixed, the adaptability of the system is reduced, and if the decision threshold is adaptive to the dynamic threshold, the complexity of the system is improved.
Disclosure of Invention
The invention provides a high-precision synchronization method for short-time burst signals, aiming at the problem that the existing synchronization method is not suitable for large frequency offset scenes and the precision and complexity caused by threshold judgment cannot be compatible.
The invention provides a high-precision synchronization method for short-time burst signals, which comprises the following steps:
a frame synchronization capturing stage: signal detection and frame synchronization capture are completed by using a unique code difference matching mode;
and a frequency difference estimation stage: acquiring a leader sequence from a captured burst signal according to a physical layer frame structure agreed by both communication parties, performing frequency difference estimation by using the leader sequence, driving an NCO (NCO) by using estimated frequency difference information, and performing frequency difference correction on the signal;
symbol and phase synchronization stage: acquiring a unique code from the captured burst signal according to a physical layer frame structure agreed by both communication parties, and finding a maximum correlation point by using the unique code in a sliding correlation manner to acquire initial symbol synchronization information and initial phase synchronization information of the signal;
symbol and phase synchronization tracking stage: and performing symbol synchronization tracking and phase synchronization tracking by using a phase-locked loop to acquire optimal sampling and decision information.
Further, the physical layer frame structure comprises a guard interval field, a preamble sequence field, a unique code field and an information carrying field which are sequentially arranged from front to back.
Further, the frame synchronization capturing phase comprises: firstly, carrying out intersymbol difference on a received signal; then, acquiring a real part of the inter-symbol differential signal, and comparing and judging the real part with a threshold value 0; and finally, matching the judgment result with the differential sequence of the unique code to obtain frame synchronization capture information.
Further, in the frequency offset estimation stage, the performing of frequency offset estimation by using the preamble sequence includes: firstly, integrating a leader sequence with an integration period of N, and then carrying out intersymbol difference on the integrated sequence to obtain a phase difference; and finally, performing statistical average on the phase difference on the length of the preamble sequence to obtain frequency difference information.
Further, the value of the integration period N should satisfy: and in an integration period N, the phase difference obtained by carrying out the inter-symbol difference on the integrated sequence is less than pi/4.
Further, the symbol and phase synchronization stage comprises:
sampling the unique code by taking different time sampling points of a first code element of the unique code as starting points and taking a code element period as a sampling interval to obtain sequences corresponding to the different time sampling points;
respectively carrying out conjugate multiplication on sequences corresponding to sampling points at different moments and the unique code, and then accumulating to obtain accumulation results corresponding to the sampling points at different moments;
comparing the numerical values of the accumulation results of the sampling points at different moments, and taking the moment sampling point corresponding to the accumulation result with the maximum numerical value as the optimal sampling moment;
and calculating the argument of the accumulated result with the maximum numerical value, and calculating to obtain the initial phase difference according to the argument and the length of the unique code.
Further, in the symbol and phase synchronization tracking stage, a phase-locked loop is used to perform symbol synchronization tracking, specifically, a second-order phase-locked loop is used to perform symbol synchronization tracking, which includes:
and acquiring timing error information by adopting a delay gate, taking the timing error information as the input of a second-order phase-locked loop, and adjusting to obtain an optimal sampling point according to the output of the second-order phase-locked loop.
Further, in the symbol and phase synchronization tracking stage, the phase synchronization tracking using a phase-locked loop includes: firstly, judging a sampling symbol to obtain a standard constellation point; then calculating the phase difference between the actual sampling output and the standard constellation point; and finally, the phase difference is used as the input of a phase-locked loop, and the phase-locked loop is driven to correct the phase difference.
The invention has the beneficial effects that:
in the prior art, the unique code is only used in the signal detection phase of the burst signal in cooperative communication. Based on the above, the invention provides a high-precision synchronization method for short-time burst signals, which comprehensively considers the performance and complexity of an algorithm by reasonably utilizing a preamble sequence and a unique code in a transmission frame, and can realize accurate carrier synchronization and symbol synchronization in a high dynamic range.
The frame synchronization capturing technology provided by the invention can realize frame synchronization capturing under the condition of large frequency difference by adopting a unique code difference matching mode.
The frequency difference estimation technology provided by the invention can improve the precision of frequency difference estimation based on the integral difference cumulant of the leader sequence.
Drawings
FIG. 1 is a schematic diagram of a synchronization scheme provided by the prior art;
FIG. 2 is a diagram of signal detection based on unique code autocorrelation provided by the prior art;
fig. 3 is a schematic diagram of a processing flow of sender information according to an embodiment of the present invention;
fig. 4 is a schematic diagram of a processing flow of the receiver information according to an embodiment of the present invention;
fig. 5 is a schematic flowchart of a short-time burst signal oriented high-precision synchronization method according to an embodiment of the present invention;
FIG. 6 is a diagram illustrating a physical layer frame structure according to an embodiment of the present invention;
FIG. 7 is a diagram illustrating a frame synchronization capture phase according to an embodiment of the present invention;
fig. 8 is a schematic diagram of a frequency offset estimation stage according to an embodiment of the present invention;
fig. 9 is a schematic diagram of estimation thresholds corresponding to different preamble symbols under a given signal-to-noise ratio condition according to an embodiment of the present invention;
FIG. 10 is a schematic diagram of a symbol and phase synchronization phase (initial acquisition phase) provided in accordance with an embodiment of the present invention;
Fig. 11 is a schematic structural diagram of a second-order pll used for symbol synchronization tracking according to an embodiment of the present invention;
FIG. 12 is a schematic diagram of phase synchronization tracking provided by an embodiment of the present invention;
fig. 13 is a schematic overall flow diagram of a system receiving end according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions in the embodiments of the present invention will be described clearly below with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the present invention, the information processing flow of both the appointed transmitting and receiving parties is shown in fig. 3 and 4. Wherein DUC represents the digital up-conversion phase; DDC denotes the digital down-conversion stage; SRRC denotes a root raised cosine filter to reduce intersymbol interference.
As shown in fig. 5, an embodiment of the present invention provides a high-precision synchronization method for short-time burst signals, including:
a frame synchronization capturing stage: signal detection and frame synchronization capture are completed by using a unique code differential matching mode;
And a frequency difference estimation stage: acquiring a leader sequence from a captured burst signal according to a physical layer frame structure agreed by both communication parties, performing frequency difference estimation by using the leader sequence, driving an NCO (NCO) by using estimated frequency difference information, and performing frequency difference correction on the signal;
specifically, as shown in fig. 6, the physical layer frame structure in the embodiment of the present invention includes a guard interval field, a preamble sequence field, a unique code field, and an information carrying field, which are sequentially arranged from front to back; wherein: the guard interval field does not transmit a signal; the preamble sequence in the preamble sequence field is a single-tone sequence, the preamble sequence corresponds to a '0' symbol of the BPSK signal, and the length of the preamble sequence may be determined according to the accuracy to be achieved by the frequency offset estimation, which is not described herein again; the unique code on the unique code field is a fixed sequence, the unique code adopts BPSK modulation, the length is 32 symbols, and the code pattern is a pseudorandom sequence; and the information bearing field is used for bearing various modulation mode data.
Symbol and phase synchronization stage: acquiring a unique code from the captured burst signal according to a physical layer frame structure agreed by both communication parties, and finding a maximum correlation point by using the unique code in a sliding correlation manner to acquire initial symbol synchronization information and initial phase synchronization information of the signal;
A symbol and phase synchronous tracking stage: and performing symbol synchronization tracking and phase synchronization tracking by using a phase-locked loop to acquire optimal sampling and decision information.
Specifically, in fig. 5, NCO denotes a digital oscillator; SRRC represents a root raised cosine filter for reducing intersymbol interference; the capture indication signal is used to determine the frame header position of the input signal of the next stage (i.e., the output signal of the previous stage).
In the prior art, the unique code is only used in the signal detection phase of the burst signal in cooperative communication. Based on the above, the high-precision synchronization method for the short-time burst signal provided by the embodiment of the invention realizes precise carrier synchronization and symbol synchronization by reasonably utilizing the preamble sequence and the unique code in the transmission frame.
On the basis of the foregoing embodiments, an embodiment of the present invention provides a frame synchronization capturing method, as shown in fig. 7, where the frame synchronization capturing stage includes:
firstly, carrying out intersymbol difference on a received signal; then, acquiring a real part of the inter-symbol differential signal, and comparing and judging the real part with a threshold value 0; and finally, matching the judgment result with the differential sequence of the unique code to obtain frame synchronization capture information.
Specifically, symbolThe calculation formula of the difference between the numbers is as follows: d (t) ═ r (t) · r*(t-Ts) (ii) a Wherein d (t) represents an inter-symbol differential signal; r (t) represents a received signal, r*(t-Ts) Representing the signal r (T-T)s) The conjugated signal of r (T-T)s) Denotes r (T) passes through TsA delayed signal. The essence of the inter-symbol difference is to extract the phase changes of adjacent symbols. Under ideal conditions, the difference signal can be correctly recovered as long as the normalized frequency difference is between-0.5 and 0.5. In practice, performance is lost due to the presence of noise in the system. Simulation results show that for BPSK signals at 1M Hz, the signal-to-noise ratio>15dB(Es/N0) Frequency difference of<Probability of frame capture under 100KHz condition>99％。
In the prior art, a unique code autocorrelation algorithm is adopted as a signal detection method of a burst signal, and when the frequency difference is too large, the method can cause that the autocorrelation peak value is reduced or even disappears in the duration of the unique code due to the fact that the phase difference caused by the frequency difference is larger than 2 pi, so that the matched filtering performance is sharply reduced or even fails. Compared with the prior art, the frame synchronization capturing technology provided by the embodiment of the invention has the advantages that as can be seen from simulation results, the frame synchronization capturing can be realized under the condition of larger frequency difference by adopting a unique code difference matching mode.
On the basis of the foregoing embodiments, an embodiment of the present invention provides a frequency offset estimation method, as shown in fig. 8, in the frequency offset estimation stage, the performing frequency offset estimation by using the preamble sequence includes: firstly, integrating a leader sequence with an integration period of N, and then carrying out intersymbol difference on the integrated sequence to obtain a phase difference; and finally, performing statistical average on the phase difference on the length of the preamble sequence to obtain frequency difference information.
Specifically, the value of the integration period N should satisfy: and in an integration period N, the phase difference obtained by performing the inter-symbol difference on the integrated sequence is less than pi/4.
Under the wireless channel model of additive white gaussian noise, the frequency difference estimation obeys gaussian distribution, the estimation precision is related to the length of a leader sequence and the signal-to-noise ratio at the same time, and theoretically, the cramer-meror threshold of the frequency estimation is as follows:
where N is the preamble length (i.e., integration period) used for estimation, FsymIs a function of the rate of the symbols,
is the energy ratio of the signal to noise, i.e., the signal-to-noise ratio. According to the formula, the relation between the frequency estimation error and the length of the leader sequence can be obtained by setting the lower limit of the signal-to-noise ratio according to the actual symbol rate, and the length of the leader sequence can be further determined according to the estimation precision to be achieved. For example: the frequency estimation error versus preamble symbol length for a transmission mode at 20M symbol rate assuming a signal-to-noise ratio of 15dB is shown in fig. 9. From the simulation results, under the condition of 15dB snr, for the transmission mode with 20 mhud symbol rate, the number of preamble symbols needs to be greater than 256 to achieve ± 2KHZ estimation accuracy.
The embodiment of the invention also simulates the performance of frequency difference estimation. The simulation parameters are as follows: the signal-to-noise ratio is 20dB, the frequency difference is +/-45 KHz, the length of a leader sequence is 384 symbols, and double-precision floating point operation is adopted. The maximum error of the frequency offset estimation, the average error of the frequency offset estimation, and the mean square error of the frequency offset estimation are respectively taken for the results of the 1000 frequency offset estimation, and the simulation results are respectively shown in table 1, table 2, and table 3:
TABLE 1 frequency offset estimation Performance-Max error
TABLE 2 frequency offset estimation Performance-average error
TABLE 3 frequency offset estimation Performance-mean Square error
Simulation results show that for four typical transmission rates of 1M, 5M, 10M and 20M, the frequency difference estimation precision can be higher than one ten thousandth of the symbol rate under the condition that the frequency difference is +/-45 KHz.
Since the sampling clock of the signal after frequency difference correction is different from the source of the transmitting end, the sampling time is not necessarily the optimal sampling time, but there may be a certain phase difference and frequency difference, so that further symbol and phase synchronization is required to obtain the optimal sampling time. On the basis of the above embodiments, the embodiments of the present invention provide a symbol and phase synchronization method, which is divided into two stages, namely initial acquisition and synchronous tracking. As shown in fig. 10, the initial acquisition phase includes:
Sampling the unique code by taking different time sampling points of a first code element of the unique code as starting points and taking a code element period as a sampling interval to obtain sequences corresponding to the different time sampling points; respectively carrying out conjugate multiplication on sequences corresponding to sampling points at different moments and the unique codes and then accumulating to obtain accumulation results corresponding to the sampling points at different moments; comparing the numerical values of the accumulation results of the sampling points at different moments, wherein the symbol corresponding to the optimal sampling moment has the highest matching degree with the unique code, so that the moment sampling point corresponding to the accumulation result with the largest numerical value is used as the optimal sampling moment; and calculating the argument of the accumulated result with the maximum numerical value, and calculating to obtain the initial phase difference according to the argument and the length of the unique code.
It should be noted that, although only 4 sampling points at different time instants are illustrated in fig. 10, it is understood that a person skilled in the art may set several sampling points at different time instants according to actual needs, and the number is not limited to 4.
In order to increase the robustness of the system and reduce the influence of the residual frequency difference and the frequency difference generated by the disturbance of the wireless channel on the system, further tracking and locking of the symbol and the phase are required. On the basis of the above embodiments, the embodiments of the present invention provide a symbol and phase synchronization tracking method. Wherein:
The method for tracking the symbol synchronization by using the second-order phase-locked loop comprises the following steps: and acquiring timing error information by adopting a delay gate, taking the timing error information as the input of a second-order phase-locked loop, and adjusting to obtain the optimal sampling point according to the output of the second-order phase-locked loop.
Specifically, the early-late gate respectively takes two sampling points on the left and right of the sampling time, the magnitude of the amplitude of the two sampling points is compared, if the sampling time is the optimal sampling point, the two sides are in equal amplitude, if the sampling time is not the optimal sampling point, the amplitude of one side is larger than that of the other side, the difference of the amplitudes can indicate the deviation of the sampling phase, the sampling phase can be used as the input of a phase-locked loop, and the optimal sampling point is adjusted through the output of the phase-locked loop so as to achieve the optimal sampling. The loop structure of the second order pll is shown in fig. 11.
Phase synchronization tracking using a phase locked loop, as shown in fig. 12, includes: firstly, judging a sampling symbol to obtain a standard constellation point; then calculating the phase difference between the actual sampling output and the standard constellation point; and finally, the phase difference is used as the input of a phase-locked loop, and the phase-locked loop is driven to correct the phase difference.
In the basic line of the above embodiments, the embodiment of the present invention further provides an overall process of a system receiving end, as shown in fig. 13.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, and not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (7)
1. A high-precision synchronization method for short-time burst signals is characterized by comprising the following steps:
a frame synchronization capturing stage: method for differential matching using unique codesCompleting signal detection and frame synchronization acquisition; the method specifically comprises the following steps: firstly, carrying out intersymbol difference on a received signal; then, acquiring a real part of the inter-symbol differential signal, and comparing and judging the real part with a threshold value 0; finally, matching the judgment result with the differential sequence of the unique code to obtain frame synchronization capture information; the calculation formula of the inter-symbol differential signal is as follows:
(ii) a Wherein the content of the first and second substances,
representing an inter-symbol differential signal;
which is indicative of the received signal or signals,
representing signals 
The signal of the second signal is then converted into the second signal,
to represent
ThroughT s A delayed signal;
and a frequency difference estimation stage: acquiring a leader sequence from a captured burst signal according to a physical layer frame structure agreed by both communication parties, performing frequency difference estimation by using the leader sequence, driving an NCO (NCO) by using estimated frequency difference information, and performing frequency difference correction on the signal;
symbol and phase synchronization stage: acquiring a unique code from the captured burst signal according to a physical layer frame structure agreed by both communication parties, and finding a maximum correlation point by using the unique code in a sliding correlation manner to acquire initial symbol synchronization information and initial phase synchronization information of the signal;
a symbol and phase synchronous tracking stage: and performing symbol synchronization tracking and phase synchronization tracking by using a phase-locked loop to acquire optimal sampling and decision information.
2. The method of claim 1, wherein the physical layer frame structure comprises a guard interval field, a preamble sequence field, a unique code field and an information carrying field sequentially arranged from front to back.
3. The method of claim 1, wherein the frequency offset estimation using the preamble sequence in the frequency offset estimation stage comprises:
Firstly, integrating a leader sequence with an integration period of N, and then carrying out intersymbol difference on the integrated sequence to obtain a phase difference; and finally, carrying out statistical average on the phase difference on the length of the preamble sequence to obtain frequency difference information.
4. The method of claim 3, wherein the integration period N is selected to satisfy: in an integration period N, the phase difference obtained by performing inter-symbol difference on the integrated sequence is less than
。
5. The method of claim 1, wherein the symbol and phase synchronization phase comprises:
sampling the unique code by taking different time sampling points of a first code element of the unique code as starting points and taking a code element period as a sampling interval to obtain sequences corresponding to the different time sampling points;
respectively carrying out conjugate multiplication on sequences corresponding to sampling points at different moments and the unique code, and then accumulating to obtain accumulation results corresponding to the sampling points at different moments;
comparing the numerical values of the accumulation results of the sampling points at different moments, and taking the moment sampling point corresponding to the accumulation result with the maximum numerical value as the optimal sampling moment;
and calculating the argument of the accumulated result with the maximum numerical value, and calculating to obtain the initial phase difference according to the argument and the length of the unique code.
6. The method according to claim 1, wherein in the symbol and phase synchronization tracking stage, the symbol synchronization tracking is performed by using a phase-locked loop, specifically, a second-order phase-locked loop, and the method comprises:
and acquiring timing error information by adopting a delay gate, taking the timing error information as the input of a second-order phase-locked loop, and adjusting to obtain the optimal sampling point according to the output of the second-order phase-locked loop.
7. The method of claim 1, wherein the symbol and phase synchronization tracking phase uses a phase locked loop for phase synchronization tracking, comprising:
firstly, judging a sampling symbol to obtain a standard constellation point; then calculating the phase difference between the actual sampling output and the standard constellation point; and finally, the phase difference is used as the input of a phase-locked loop, and the phase-locked loop is driven to correct the phase difference.
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