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CN102185822B - OFDM/OQAM (Orthogonal Frequency Division Multiplexing/Offset Quadrature Amplitude Modulation) system and time frequency synchronization method thereof - Google Patents

OFDM/OQAM (Orthogonal Frequency Division Multiplexing/Offset Quadrature Amplitude Modulation) system and time frequency synchronization method thereof Download PDF

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CN102185822B
CN102185822B CN 201110142938 CN201110142938A CN102185822B CN 102185822 B CN102185822 B CN 102185822B CN 201110142938 CN201110142938 CN 201110142938 CN 201110142938 A CN201110142938 A CN 201110142938A CN 102185822 B CN102185822 B CN 102185822B
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胡苏�
陈浩
杨刚
武刚
李少谦
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University of Electronic Science and Technology of China
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Abstract

The invention discloses an OFDM/OQAM (Orthogonal Frequency Division Multiplexing/Offset Quadrature Amplitude Modulation) system and a time frequency synchronization method thereof. The time frequency synchronization method comprises the following steps of: a coarse synchronization estimating step for estimating an estimated frequency offset value of a received baseband receiving signal frame sequence; a frequency offset compensating step for compensating the frequency offset of the received base band receiving signal frame sequence by using the estimated frequency offset value obtained by the coarse synchronization estimating step; a fine synchronization estimating step for estimating an estimated time offset value of r2 (KTS) obtained by the frequency offset compensating step; and a time offset compensating step for compensating the time offset of the r2 (kTS) obtained by the frequency offset compensating step by using the estimated time offset value obtained by the fine synchronization estimating step so as to obtain r3 (kTS). In the invention, one-step synchronization is divided into two steps including coarse synchronization and fine synchronization; and the frequency offset of the baseband receiving signal frame sequence is first compensated by using the estimated frequency offset value obtained in the coarse synchronization and then the fine synchronization is carried out by using the compensated signal to obtain a final estimated time offset value so that the influence of CFO (Carrier Frequency Offset) during the fine synchronization is minimized and the time offset estimation accuracy and the acquisition probability are improved.

Description

OFDM/OQAM system and time-frequency synchronization method thereof
Technical Field
The invention belongs to the technical field of mobile communication, and particularly relates to an Orthogonal Frequency Division Multiplexing (OFDM/OQAM) system for staggered Orthogonal amplitude Modulation and a time-Frequency synchronization method thereof.
Background
Due to the increasing requirement of wireless users for transmission rate, multi-carrier modulation technology has become the mainstream modulation method of wireless communication at present, for example, the conventional CP-OFDM (Cyclic Prefix-Orthogonal Frequency Division Multiplexing) technology based on Cyclic Prefix is selected as the key technology in the LTE standard. Compared with CP-OFDM, OFDM/OQAM has higher frequency spectrum efficiency and better time-frequency focusing characteristic, and has good effect on overcoming the influence of Inter-Symbol Interference (ISI) and Inter-carrier Interference (ICI), and particularly the latter becomes one of the main advantages of OFDM/OQAM compared with CP-OFDM. The sending signal of the OFDM/OQAM system is real value, which is taken from the real part and imaginary part of the complex symbol to be transmitted, compared with the traditional orthogonal frequency division multiplexing system, the OFDM/OQAM system only meets the strict orthogonality condition in the real number domain; an Isotropic Orthogonal Transform (IOTA) function with good Time Frequency focusing (TFL) characteristics may be selected.
Fig. 1 shows a schematic diagram of a transmitting end of an OFDM/OQAM system using a conventional time-frequency joint estimation method. The device comprises a signal source module, a conventional signal processing module, an orthogonalization phase mapping module, an N-point IFFT module, a forming filtering module, a D/A conversion module and an up-conversion module.
For convenience of description, terms used therein are first introduced:
(1) shaping filter function g (T) in the interval T ∈ {0, Ts,...,(Ng-1)TsTake a nonzero value on, where Ng=ξT/TsIs the number of non-zero sampling points, xi is the number of taps of the shaping filter function, TsIs the system sample time interval and T is the symbol time interval.
(2) Frequency domain synchronous pilot frequency symbol by continuously transmitted NTROne OFDM symbol constitutes (N)TRIs at least 2 more than the tap number xi of the shaping filter function, thus ensuring that the time domain pilot frequency part modulated by the sending end has the repeated pilot frequency symbol), namely, the frequency domain repeated pilot frequency symbol is represented as al(p)=al TR,l=0,...,N-1,p=0,...,NTR-1, wherein N isDenotes the number of subcarriers, l denotes the subcarrier number, and p denotes the pilot symbol number. Time domain pilot signal s after modulation of transmitting endTR(kTs),k∈{0,1,2,…,NTRN-1 in transmission NgAfter N2 sample points, for an interval of time k e { N ∈ Ng-N/2,...,NTRN-N-1}, satisfies the relation sTR(kTs+NTs)=sTR(kTs)。
(3) The frame sequence of the baseband receiving signal is r (kT)s) K is equal to {0,1,2, … }, τ is equal to {0,1, …, N/4} is the real value of time bias,in order to estimate the time offset value,
Figure GDA00002896516800022
is a time bias experiment value, epsilon ∈ -<0.5,0.5>To normalize to the true value of the frequency offset over the subcarrier spacing,
Figure GDA00002896516800023
for the frequency offset estimate, a frequency offset estimate is obtained,
Figure GDA00002896516800024
in order to be a related sequence,for evaluating the phase angle operator, τmaxAnd D is the length of a sliding summation window for the maximum time delay of the multipath channel.
Firstly, initialization processing is carried out: the same transmit pilot sequence is stored in registers at the transmitting end (mobile station) and the receiving end (base station), and a time domain pilot sequence s is transmitted at the receiving endTR(kTs) ξ N +1 to (N)TR-1) storing N data as a transmission reference sequence, storing the same shaping filter function sequence at the transmitting end and the receiving end, and establishing a corresponding rule. The specific treatment steps are as follows:
step 11, the data bit generated by the signal source module is coded by the conventional signal processing moduleAfter obtaining complex data, Quadrature Amplitude Modulation (QAM), framing and adding a length N at the front endTROf the frequency domain repetitive pilot symbol al(p)=al TR,l=0,...,N-1,p=0,...,NTR-1, followed by real-imaginary separation;
step 12, respectively carrying out orthogonalization phase mapping on the real and imaginary parts of the data obtained in the step 11 through an orthogonalization phase mapping module;
step 13, performing Inverse Fast Fourier Transform (IFFT) on the data in step 12 by using an N-point IFFT module to complete multicarrier modulation;
step 14, completing signal forming and parallel-serial conversion by the data obtained in the step 13 through a forming filtering module;
and step 15, transmitting the signals by the data obtained in the step 14 through a D/A conversion module and an up-conversion module.
As a typical multi-Carrier modulation method, OFDM/OQAM has the commonality of a multi-Carrier modulation technique, i.e., is susceptible to Carrier Frequency Offset (CFO) and time Offset, and the main reason for CFO generation is that the oscillators at the transmitting and receiving ends are not stable and accurate enough, and the generated frequencies are deviated, thereby destroying the orthogonality between sub-carriers and further introducing ISI and ICI. Moreover, since no CP is added, once synchronization deviation occurs, the whole Discrete Fourier Transform (DFT) window is misaligned, which causes interference that is difficult to recover, and thus OFDM/OQAM is very sensitive to time offset.
In order to obtain better system performance, it is necessary to ensure that the time-frequency offset estimation has higher precision. T.Fusco, A.Petrella and M.Tanda propose MLS time-frequency joint estimation method based on baseband receiving signal frame sequence second-order autocorrelation in Data-Aided Symbol Timing and CFO Synchronization for Filter Bank MultiCarrier Systems [ J ]. IEEE trans.Wireless Commun, May2009,8(5): 2705-.
Fig. 2 is a schematic diagram of the operation of the receiving end of the OFDMA/OQAM system using the conventional MLS and TR2 time-frequency joint estimation, which includes a down-conversion module, an a/D conversion module, a synchronization estimation module, a time-frequency offset compensation module, a matched filtering module, an FFT module, a de-orthogonalization phase mapping module, and a conventional signal processing module. Assuming that the channel information of the receiving end is known, the demodulation step of the receiving end can be expressed as the following steps:
step 21, the received signal passes through a down-conversion module and an A/D conversion module to obtain a baseband received signal frame sequence r (kT)s),k∈{0,1,2,…};
Step 22, the baseband receiving signal frame sequence r (kT) obtained in step 21s) Estimating the time frequency offset by a synchronization module by subjecting a sequence of baseband received signal frames to a length of (N)TRThe processing method adopted by the sliding autocorrelation (MLS time-frequency joint estimation) method of-1-xi) N +1 can be specifically expressed as
Figure GDA00002896516800031
Wherein,
R ( &tau; ~ ) = &Sigma; k = N g - 1 N TR &CenterDot; N - N - 1 r * ( k T s + &tau; ~ ) r ( k T s + N T s + &tau; ~ ) , Q ( &tau; ~ ) = &Sigma; i = 1 2 &Sigma; k = N g - 1 N TR &CenterDot; N - N - 1 | r ( k T s + ( i - 1 ) N T s + &tau; ~ ) | 2 or a sequence of base band received signal frames r (kT)s) And transmitting the reference sequence sTR(kTs) The processing method adopted by the fourth-order sliding cross-correlation operation (TR 2 time-frequency joint estimation method) can be specifically expressed asWherein,
R ( &tau; ~ ) = &Sigma; k = N g - 1 N TR &CenterDot; N - N - 1 r * ( k T s + &tau; ~ ) s TR ( k T s ) r ( k T s + N T s + &tau; ~ ) s TR * ( k T s + N T s )
Figure GDA00002896516800036
obtaining a correlation sequence
Figure GDA00002896516800037
Selecting
Figure GDA00002896516800038
Time offset experimental value corresponding to peak value
Figure GDA00002896516800039
As an estimate of time bias
Figure GDA000028965168000310
Reuse of
Figure GDA000028965168000311
Further estimating frequency deviation estimated value
Figure GDA000028965168000312
Here, "·" denotes a multiplication operation, and "+" denotes a conjugate operation;
step 23, the time frequency offset compensation module compensates the baseband receiving signal frame sequence by using the time frequency offset and frequency offset estimation value obtained in the step 22;
step 24, the data obtained in the step 23 is processed by matched filtering through a matched filtering module;
step 25, the data passing the step 24 is processed by a Fast Fourier Transform (FFT) module to complete multicarrier demodulation;
step 26, subjecting the data obtained in the step 25 to de-orthogonalization phase mapping by a de-orthogonalization phase mapping module;
step 27, eliminating or reducing the influence of the multipath channel on the OFDM/OQAM signal by using the known channel information through an equalizer (such as zero-forcing equalization and the like) by the data after passing the step 26;
and 28, merging the real part and the imaginary part of the data after the step 27, carrying out QAM demodulation and corresponding decoding, and outputting data bits.
The OFDM/OQAM system using the conventional frequency-domain pilot-based time-frequency joint estimation method has the following disadvantages:
firstly, the receiving end performs only one time of correlation operation, and performs time-offset compensation immediately after estimating a time-offset estimation value, and it is noted that the time-offset estimation is performed before the frequency-offset estimation, that is, the influence of the frequency offset is not removed when performing the time-offset estimation, which also affects the capture probability of the time-offset estimation.
And secondly, directly selecting the time offset experimental value corresponding to the correlation sequence peak value obtained in the step 22 as a time offset estimation value. When the first path in the multi-path channel is not the path with the highest instantaneous power, the time offset estimation value obtained in this way will automatically lock onto the path with the highest instantaneous power, not the desired first path. This directly results in the demodulation operations of step 24 and step 25 being misaligned at the receiving end, which results in the receiving end failing to demodulate correctly.
Step 22 is used for two methods of time offset estimation. The first method adopts the second-order autocorrelation estimation time bias of a baseband receiving signal frame sequence, so that the obtained correlation value changes more smoothly and is more easily influenced by noise and CFO; the second method adopts the fourth order sliding cross correlation estimation time offset of the base band receiving signal frame sequence and the sending reference sequence, so that the obtained correlation value can obtain a higher correlation peak at a timing point, but the realization complexity is increased.
Step 22 is to select a sliding correlation window range (N) during the correlation operationTR1- ξ) N does not reach the maximum range that can be theoretically selected, that is to say there is still some room for improvement in the estimation accuracy of the time offset. And the frequency offset estimation value is calculated by utilizing the time offset estimation value, and the improvement space is provided.
Disclosure of Invention
The invention aims to solve the problem that the capturing probability of the time offset estimation of an OFDM/OQAM system adopting a time-frequency joint estimation method based on frequency domain pilot frequency is low, and provides an OFDM/OQAM system and a time-frequency synchronization method thereof.
The technical scheme of the invention is as follows: an OFDM/OQAM system, wherein a receiving end of the OFDM/OQAM system comprises:
a coarse synchronization estimation module for estimating a sequence r of received baseband received signal frames1(kTS) Frequency offset estimation of
A frequency deviation compensation module for estimating the frequency deviation value obtained by the coarse synchronization estimation module
Figure GDA00002896516800042
For a sequence of base band received signal frames r1(kTS) Performing frequency offset compensation to obtain a received symbol frame sequence r after frequency offset compensation2(kTS);
A fine synchronization estimation module for estimating r obtained by the frequency offset compensation module2(kTS) Time offset estimate of
Figure GDA00002896516800043
A time offset compensation module for compensating for the time offsetTime bias estimated value obtained by synchronous estimation module
Figure GDA00002896516800051
R obtained by frequency offset compensation module2(kTS) Performing time offset compensation to obtain a received symbol frame sequence r after time offset compensation3(kTS)。
The invention also provides a time-frequency synchronization method of the OFDM/OQAM system, which comprises the steps of
A coarse synchronization estimation step for estimating a sequence r of received baseband received signal frames1(kTS) Frequency offset estimation of
Figure GDA00002896516800052
A frequency offset compensation step for estimating the frequency offset value obtained in the coarse synchronization estimation step
Figure GDA00002896516800053
For received base band received signal frame sequence r1(kTS) Performing frequency offset compensation to obtain a received symbol frame sequence r after frequency offset compensation2(kTS);
A fine synchronization estimation step for estimating r obtained in the frequency offset compensation step2(kTS) Time offset estimate of
A time offset compensation step for estimating the time offset value obtained in the fine synchronization estimation step
Figure GDA00002896516800055
R obtained in the step of frequency offset compensation2(kTS) Performing time offset compensation to obtain a received symbol frame sequence r after time offset compensation3(kTS)。
As a preferred embodiment of the present invention, the rough synchronization estimation step specifically includes the following steps:
sequence of frames r of a baseband received signal1(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRXi-12) N to obtain a correlation sequence
Figure GDA00002896516800056
Is particularly shown asWherein R 1 ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 r 1 * ( k T s + &tau; ~ ) s TR ( k T s ) , Q ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 | s TR ( k T s ) | 2 , To pair
Figure GDA000028965168000510
Performing sliding summation operation with fixed window length D to obtain a summation sequence
Figure GDA000028965168000511
Where D > τmaxSelection of a sum sequence
Figure GDA000028965168000512
Time offset test value corresponding to peak value of (1)
Figure GDA000028965168000513
As an estimate of time biasReuse of
Figure GDA000028965168000515
Further estimating frequency deviation estimated value
Figure GDA000028965168000516
Wherein
Figure GDA000028965168000517
Wherein N is the number of subcarriers, NTRFor the frequency domain, repeating the number of pilot symbols, NgNumber of non-zero samples, T, for shaping filter functionsIs the sampling period.
As another preferred embodiment of the present invention, the detailed procedure of the fine synchronization estimation step is as follows:
r is obtained from the frequency deviation compensation step2(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRξ -1/2) N second order sliding cross-correlation to obtain a correlation sequenceIs particularly shown as
Figure GDA000028965168000519
Wherein R 2 ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 r 2 * ( k T s + &tau; ~ ) s TR ( k T s ) , Q ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 | s TR ( k T s ) | 2 , To pair
Figure GDA000028965168000522
Performing sliding summation operation with fixed window length D to obtain a summation sequence
Figure GDA00002896516800061
Where D > τmaxSelection of a sum sequence
Figure GDA00002896516800062
Time offset test value corresponding to peak value of (1)
Figure GDA00002896516800063
As an estimate of time biasWherein N is the number of subcarriers, NTRFor the frequency domain, repeating the number of pilot symbols, NgNumber of non-zero samples, T, for shaping filter functionsIs the sampling period.
The invention has the beneficial effects that: the method expands the one-step synchronization method of the time-frequency joint estimation method based on frequency domain pilot frequency in the traditional OFDM/OQAM system into two steps, namely, coarse synchronization estimation and fine synchronization estimation, firstly carries out frequency offset compensation on a baseband receiving signal frame sequence by using a frequency offset estimation value obtained in the coarse synchronization estimation, and then carries out fine synchronization estimation on a compensated signal to obtain a final time offset estimation value, so that the influence of CFO (computational fluid dynamics) in the fine synchronization is reduced to the minimum, and the precision and the capture probability of the time offset estimation are improved; meanwhile, the related range of the base band receiving signal frame sequence and the transmitting reference sequence is expanded, and the frequency offset estimation precision is improved.
The method and the system can effectively improve the capturing probability of the time offset estimation and the precision of the frequency offset estimation so as to achieve the aim of better meeting the requirement of high-speed mobile communication.
Drawings
Fig. 1 is a schematic diagram of a structure of a transmitting end of a conventional time-frequency joint estimation OFDM/OQAM system.
Fig. 2 is a schematic diagram of a receiving end structure of a conventional time-frequency joint estimation OFDM/OQAM system.
Fig. 3 is a schematic diagram of a receiving end structure of the OFDM/OQAM system of the present invention.
FIG. 4 is a schematic diagram comparing the time bias capture probability of the method of the present invention with that of TR2 and MLS methods.
Fig. 5 is a schematic diagram comparing the mean square error of the frequency offset estimation of the method of the present invention with that of TR2 and MLS method.
Detailed Description
The invention is further illustrated with reference to the figures and the specific examples.
The invention is mainly realized by the following technical means:
firstly, expanding a one-step synchronization method of a frequency-domain pilot frequency-based time-frequency joint estimation method in a traditional OFDM/OQAM system into two steps, namely coarse synchronization estimation and fine synchronization estimation, firstly performing frequency offset compensation on a baseband receiving signal frame sequence by using a frequency offset estimation value obtained in the coarse synchronization estimation, and then performing fine synchronization estimation on a compensated signal to obtain a final time offset estimation value;
in order to further improve the technical effect, the following measures are taken in the estimation process of the coarse synchronization and the fine synchronization:
secondly, the theoretical range of the cross correlation between the baseband receiving signal frame sequence and the transmitting reference sequence is expanded to the maximum when synchronization is carried out;
thirdly, the correlation operation of the receiving end is changed into second-order cross correlation operation of a baseband receiving signal frame sequence and a sending reference sequence;
and fourthly, performing further sliding window summation processing on the correlation sequence, and selecting the peak value of the modulus value of the obtained sum sequence as a time bias estimation value.
The following is a detailed description. As shown in fig. 3, the receiving end of the OFDM/OQAM system includes:
a coarse synchronization estimation module for estimating a sequence r of received baseband received signal frames1(kTS) Frequency offset estimation of
Figure GDA00002896516800078
A frequency offset compensation module for compensating for frequency offsetFrequency deviation estimated value obtained by coarse synchronization estimation module
Figure GDA00002896516800079
For received base band received signal frame sequence r1(kTS) Performing frequency offset compensation to obtain a received symbol frame sequence r after frequency offset compensation2(kTS);
A fine synchronization estimation module for estimating r obtained by the frequency offset compensation module2(kTS) Time offset estimate of
Figure GDA000028965168000710
A time bias compensation module for estimating the time bias estimated value obtained by the fine synchronization estimation module
Figure GDA000028965168000711
R obtained by frequency offset compensation module2(kTS) Performing time offset compensation to obtain a received symbol frame sequence r after time offset compensation3(kTS)。
The OFDM/OQAM system time-frequency synchronization method of the invention comprises the following steps:
a coarse synchronization estimation step for estimating a sequence r of received baseband received signal frames1(kTS) Frequency offset estimation of
Figure GDA000028965168000712
A frequency offset compensation step for estimating the frequency offset value obtained in the coarse synchronization estimation step
Figure GDA000028965168000713
For a received sequence of complex symbol frames r1(kTS) Performing frequency offset compensation to obtain a received symbol frame sequence r after frequency offset compensation2(kTS);
A fine synchronization estimation step for estimating r obtained in the frequency offset compensation step2(kTS) Time offset estimate of
Figure GDA000028965168000714
A time offset compensation step for estimating the time offset value obtained in the fine synchronization estimation step
Figure GDA000028965168000715
R obtained in the step of frequency offset compensation2(kTS) Performing time offset compensation to obtain a received symbol frame sequence r after time offset compensation3(kTS)。
Here, the estimation processes in the coarse synchronization estimation step and the fine synchronization estimation step may adopt MLS and TR2 methods in the background art, and as a preferred scheme of the time-frequency synchronization method of the present invention, the specific process of the coarse synchronization estimation step is as follows:
sequence of frames r of a baseband received signal1(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRξ -1/2) N second order sliding cross-correlation to obtain a correlation sequenceIs particularly shown as
Figure GDA00002896516800072
Wherein R 1 ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 r 1 * ( k T s + &tau; ~ ) s TR ( k T s ) , Q ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 | s TR ( k T s ) | 2 , To pair
Figure GDA00002896516800075
Performing sliding summation operation with fixed window length D to obtain a summation sequenceWhere D > τmaxSelection of a sum sequence
Figure GDA00002896516800077
Time offset test value corresponding to peak value of (1)
Figure GDA00002896516800081
As an estimate of time bias
Figure GDA00002896516800082
Reuse of
Figure GDA00002896516800083
Further estimating frequency deviation estimated value
Figure GDA00002896516800084
Wherein F ( &tau; ~ ) = &Sigma; k = N g N / 2 N TR &CenterDot; N - N - 1 r 1 ( k T s + N T s + &tau; ~ ) r 1 * ( k T s + &tau; ~ ) .
As another preferred embodiment of the time-frequency synchronization method of the present invention, the detailed procedure in the fine synchronization estimation step is as follows:
r is obtained from the frequency deviation compensation step2(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRξ -1/2) N second order sliding cross-correlation to obtain a correlation sequence
Figure GDA00002896516800086
Is particularly shown as
Figure GDA00002896516800087
Wherein R 2 ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 r 2 * ( k T s + &tau; ~ ) s TR ( k T s ) , Q ( &tau; ~ ) = &Sigma; k = N g - N / 2 N TR &CenterDot; N - N - 1 | s TR ( k T s ) | 2 , To pair
Figure GDA000028965168000810
Performing sliding summation operation with fixed window length D to obtain a summation sequenceWhere D > τmaxSelection of a sum sequence
Figure GDA000028965168000812
Time offset test value corresponding to peak value of (1)
Figure GDA000028965168000813
As an estimate of time bias
Figure GDA000028965168000814
It can be seen that here, the coarse synchronization estimation module and the fine synchronization estimation module can be used to implement the specific procedures of the coarse synchronization estimation step and the fine synchronization estimation step, respectively.
The method firstly expands the one-step synchronization method of the time-frequency joint estimation method based on frequency domain pilot frequency in the traditional OFDM/OQAM system into two steps, namely, coarse synchronization and fine synchronization, firstly carries out frequency offset compensation on a baseband receiving signal frame sequence by using a frequency offset estimation value obtained in the coarse synchronization, and then carries out fine synchronization on a compensated signal to obtain a final time offset estimation value, so that the influence of CFO (computational fluid dynamics) in the fine synchronization is minimized, and the precision of time offset estimation is improved; secondly, the theoretical range of cross-correlation between the baseband received signal frame sequence and the transmitted reference sequence is enlarged to the maximum when synchronization is carried out, namely (N)TRξ -1/2) N, so that the received data carrying time frequency offset information can be utilized to the maximum extent, and the estimation result is more accurate; the correlation operation of the receiving end is changed into second-order cross correlation of the baseband receiving signal frame sequence and the sending reference sequence, the timing precision of the process is higher than that of directly performing second-order self-correlation on the baseband receiving signal frame sequence, and the complexity of the process is lower than that of performing fourth-order cross correlation on the baseband receiving signal frame sequence and the sending reference sequence, and a better balance point is found between the estimation performance and the complexity of controlling and realizing; furthermore, the invention does not directly use the time offset test value corresponding to the peak value of the correlation sequence as the time offset estimation value, but carries out further sliding window summation processing on the correlation sequence, selects the peak value of the module value of the obtained summation sequence as the time offset estimation value, when the starting end of the sliding summation window is just aligned with the channel impact response of the first path, all path responses of the multipath channel are in the summation window, the obtained summation value is maximum, and when the first path is moved out of the summation window, the summation value is reduced to a certain extent, so the time offset test value corresponding to the peak value of the summation sequence is selected as the time offset estimation value, and the time offset estimation value can be obtainedLocking more accurately on the arrival time of the first path. Therefore, the timing error caused by locking the path with the strongest instantaneous power in the multipath channel in the traditional time offset estimation method on the path with the strongest instantaneous power under the condition that the path is not the first path can be effectively solved.
The present embodiment adopts the following system conditions: the number of sub-carriers of the OFDM/OQAM system is 256, the IOTA function is selected as the shaping filter function, the tap number xi of the filter is 8, and the repeat pilot length N is selectedTRAdopting 4-QAM modulation as 10 data symbols, and sampling frequency: 4M (mega) -samples per second, using a 6 path 802.22 typical channel with a channel delay of [ -3024711 [ -](unit is sampling time interval) and the path gain is [ -60-7-22-16-20 [ -](in dB), (carrier spacing) normalized Doppler spread of 1.5 x 10-3
In the simulation environment described above, the channel second path has the largest average gain. The simulation results are shown in fig. 4 and fig. 5, wherein the synchronization method of the present invention is referred to as the estimation method in the better scheme adopted in the coarse synchronization estimation step and the fine synchronization estimation step, as can be seen from fig. 4, the timing acquisition effect of the conventional MLS and TR2 methods is poor, the upper limit of the acquisition probability is only 0.05 and 0.17, respectively, because the power of the first path is less than that of the other paths in most cases, and the conventional method can only acquire the arrival time of the path with the largest gain; the time offset estimation method provided by the invention can capture the first path arrival time in most cases by using a sliding window summation mode, the upper limit of the capture probability is 0.78, and the performance is obviously improved. As can be seen from fig. 5, the frequency offset estimation accuracy of TR2, MLS and the synchronization method of the present invention are all significantly improved in sequence. At 1X 10-5Compared with the TR2 method, the method of the invention obtains 5dB performance improvement, and compared with the MLS method, the method of the invention obtains 3dB performance improvement.
It will be appreciated by those of ordinary skill in the art that the embodiments described herein are intended to assist the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited embodiments and examples. Those skilled in the art can make various other specific changes and combinations based on the teachings of the present invention without departing from the spirit of the invention, and these changes and combinations are within the scope of the invention.

Claims (4)

1. An OFDM/OQAM system, wherein a receiving end of the OFDM/OQAM system comprises:
a coarse synchronization estimation module for estimating a sequence r of received baseband received signal frames1(kTS) Frequency offset estimation of
Figure FDA000028965167000121
A frequency deviation compensation module for estimating the frequency deviation value obtained by the coarse synchronization estimation module
Figure FDA00002896516700011
For received base band received signal frame sequence r1(kTS) Performing frequency offset compensation to obtain a received symbol frame sequence r after frequency offset compensation2(kTS);
A fine synchronization estimation module for estimating r obtained by the frequency offset compensation module2(kTS) Time offset estimate of
Figure FDA00002896516700012
A time bias compensation module for estimating the time bias estimated value obtained by the fine synchronization estimation module
Figure FDA00002896516700013
R obtained by frequency offset compensation module2(kTS) Performing time offset compensation to obtain a received symbol frame sequence r after time offset compensation3(kTS);
The coarse synchronization estimation module is used for realizing the following processes:
sequence of frames r of a baseband received signal1(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRξ -1/2) N second order sliding cross-correlation to obtain a correlation sequence
Figure FDA00002896516700014
Is particularly shown as
Figure FDA00002896516700015
Wherein
Figure FDA00002896516700016
To pairTo fix the length of the windowD, obtaining a sum sequence by the sliding summation operation
Figure FDA00002896516700019
Where D > τmaxSelecting a sequence of the sum,
Figure FDA000028965167000110
time offset test value corresponding to peak value of (1)
Figure FDA000028965167000111
As an estimate of time bias
Figure FDA000028965167000112
Reuse of
Figure FDA000028965167000113
Further estimating frequency deviation estimated value
Figure FDA000028965167000114
Wherein
Figure FDA000028965167000115
Wherein N is the number of subcarriers, NTRFor the frequency domain, repeating the number of pilot symbols, NgNumber of non-zero samples, T, for shaping filter functionsFor a sampling period, τmaxThe maximum time delay of the multipath channel.
2. The OFDM/OQAM system according to claim 1, wherein the fine synchronization estimation module is configured to implement the following procedures:
r is obtained from the frequency deviation compensation step2(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRξ -1/2) N second order sliding cross-correlation to obtain a correlation sequenceIs particularly shown as
Figure FDA000028965167000117
Wherein
Figure FDA000028965167000118
Figure FDA000028965167000119
To pair
Figure FDA000028965167000120
Performing sliding summation operation with fixed window length D to obtain a summation sequence
Figure FDA00002896516700021
Where D > τmaxSelecting a sequence of the sum,
Figure FDA00002896516700022
time offset test value corresponding to peak value of (1)
Figure FDA00002896516700023
As an estimate of time bias
Figure FDA00002896516700024
3. A time-frequency synchronization method of an OFDM/OQAM system comprises the following steps:
a coarse synchronization estimation step for estimating a sequence r of received baseband received signal frames1(kTS) Frequency offset estimation of
Figure FDA00002896516700025
A frequency offset compensation step for estimating the frequency offset value obtained in the coarse synchronization estimation stepFor received base band received signal frame sequence r1(kTS) IntoPerforming line frequency offset compensation to obtain a received symbol frame sequence r after frequency offset compensation2(kTS);
A fine synchronization estimation step for estimating r obtained in the frequency offset compensation step2(kTS) Time offset estimate of
Figure FDA00002896516700027
A time offset compensation step for estimating the time offset value obtained in the fine synchronization estimation step
Figure FDA00002896516700028
R obtained in the step of frequency offset compensation2(kTS) Performing time offset compensation to obtain a received symbol frame sequence r after time offset compensation3(kTS);
The estimation process of the coarse synchronization estimation step is as follows:
sequence of frames r of a baseband received signal1(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRξ -1/2) N second order sliding cross-correlation to obtain a correlation sequence
Figure FDA00002896516700029
Is particularly shown as
Figure FDA000028965167000210
Wherein
Figure FDA000028965167000211
Figure FDA000028965167000212
To pair
Figure FDA000028965167000213
Performing sliding summation operation with fixed window length D to obtain a summation sequence
Figure FDA000028965167000214
Where D > τmaxSelection of a sum sequence
Figure FDA000028965167000215
Time offset test value corresponding to peak value of (1)As an estimate of time bias
Figure FDA000028965167000217
Reuse of
Figure FDA000028965167000218
Further estimating frequency deviation estimated value
Figure FDA000028965167000219
Wherein
Figure FDA000028965167000220
Wherein N is the number of subcarriers, NTRFor the frequency domain, repeating the number of pilot symbols, NgNumber of non-zero samples, T, for shaping filter functionsFor a sampling period, τmaxThe maximum time delay of the multipath channel.
4. The time-frequency synchronization method according to claim 3, wherein the estimation procedure of the fine synchronization estimation step is as follows:
r is obtained from the frequency deviation compensation step2(kTS) And transmitting the reference sequence sTR(kTs) Run length of (N)TRξ -1/2) N second order sliding cross-correlation to obtain a correlation sequence
Figure FDA000028965167000221
Is particularly shown as
Figure FDA000028965167000222
Wherein
Figure FDA00002896516700031
To pair
Figure FDA00002896516700033
Performing sliding summation operation with fixed window length D to obtain a summation sequenceWhere D > τmaxSelecting a sequence of the sum,
Figure FDA00002896516700035
time offset test value corresponding to peak value of (1)As an estimate of time bias
Figure FDA00002896516700037
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