CN1677909A - Frame Synchronization Method and System for Time Domain Synchronous Orthogonal Frequency Division Multiplexing Receiver - Google Patents

Abstract

Characters are that TDS - OFDM frame synchronization PN sequence adopted in the invention based on a group of displaced m sequence. Sliding correlation method is utilized to calculate correlativity between oversampled TDS - OFDM signal and local PN sequence. Peak detection is adopted to test correlative peak and location. Phase encoding for adjacent signal frames determines phase of PN sequence in each signal frame. Then, third correlative peak is utilized to determine whether signal is in real synchronization; if no, code capture program is restarted. Experimental verifies that after each frame is captured, timing error is ensured within minus or plus sampling interval / 2 in order to carry out timing track correctly.

Description

The frame synchornization method of time-domain synchronization OFDM receiver and system thereof

Technical field

The invention belongs to Digital Television reception technique field, particularly a kind of time-domain synchronization OFDM (Time DomainSynchronous OFDM, TDS-OFDM) frame synchornization method of digital television receiver and system thereof.

Background technology

At present, the ground digital television broadcast transmission standard mainly contains three kinds in the world:

1) United States advanced television systems committee (Advanced Television Systems Committee, ATSC) eight level vestigial side bands of Yan Fa lattice code (Trellis-Coded 8-Level Vestigial Side-Band, 8-VSB) modulating system.

2) European digital video broadcast-terrestrial (Digital Video Terrestrial Broadcasting-Terrestrial, DVB-T) Coded Orthogonal Frequency Division Multiplexing (COFDM) (Coded Orthogonal Frequency DivisionMultiplexing, the COFDM) modulating system of standard employing.

3) Japanese floor synthetic service digital broadcasting (Integrated Service Digital Broadcasting-Terrestrial, lSDB-T) the frequency band segment transmissions of Cai Yonging (Bandwidth Segmented Transmission, BST) modulating in OFDM system.

Since these three ground digital television broadcast transmission systems had been arranged, many countries and regions were all in the ground digital television broadcast transmission standard of formulating separately.

China also began the research work of high definition TV from 1994.And succeeded in developing Chinese first generation functional prototype of high definition TV in 1998.Organized the expert of China that Digital Television and digital high-definition television standard are formulated by radio and television responsible institution, SARFT(The State Administration of Radio and Television), (DigitalMultimedia Broadcasting for Terrestrial, DMB-T) host-host protocol is one of testing scheme to Tsing-Hua University's T-DMB.

The time-domain synchronization OFDM that adopts among the DMB-T of Tsing-Hua University (TDS-OFDM) modulation belongs to multi-transceiver technology, but it is different with the COFDM in Europe, in TDS-OFDM, do not insert the pilot tone signal, but utilized spread spectrum technic, inserted the PN sequence time domain carry out frame synchronization, Frequency Synchronization, regularly synchronously, channel transfer characteristic is estimated and follow the tracks of phase noise etc.Owing to used spreading code, make DMB-T have the superior function of spread spectrum communication, thereby the system that makes both had the advantage of COFDM, had avoided its shortcoming again.

Seeing grant number for details about the correlation circumstance of DMB-T, TDS-OFDM is that 00123597.4 " ground digital multimedia TV broad cast system " by name, grant number are 01115520.5 " time-domain synchronous orthogonal frequency division multiplex modulation method " by name, and grant number is the Chinese invention patent that 01124144.6 " protection fill method at interval in the orthogonal FDM modulation system " by name waits Tsing-Hua University to apply for.

The synchronous implementation method of DMB-T receiver frame proposed by the invention is at this scheme.

As long as initial frequency deviation is smaller, at first obtaining regularly before obtaining accurate phase place and frequency is more rational synchronously in digital television receiver.In order to guarantee to capture timing parameters in the scope than broad, and can access enough precision, generally regularly recovery process is divided into two stages: carry out thick symbol synchronization (frame synchronization) earlier, promptly catch the stage; After finishing, seizure enters thin symbol synchronization.Because ground TV broadcast is continuous data flow, so its thin symbol synchronization algorithm adopts the algorithm of feedback arrangement usually, to obtain tracking performance preferably.

In the frame structure of U.S. ATSC, length is data segment of code stream composition of 832, and preceding 4 symbol sections of being of each data segment are synchronous, and its symbol is 8 medium general values of level, as shown in Figure 1.Thick symbol synchronization algorithm is finished by detecting this segment sync signal usually.Because the section synchronization length is very short, to use long detection time for obtaining the higher detection accuracy.But generally also do not carry out Frequency Synchronization when catching, the frequency departure that is existed influences, and detection time can not be long, and this has just limited the accuracy that detects.Thin regularly synchronously for the 8-VSB system, the error of its feedback loop extract be with section synchronously as auxiliary data owing to only locate to extract error synchronously in section, other timing error constantly keeps.

Europe DVB-T adopts the modulation technique COFDM different with U.S. 8-VSB, and OFDM belongs to multi-carrier modulation technology.The frame structure of DVB-T as shown in Figure 2.The basic principle of OFDM is divided into a broad frequency band some subchannels (Sub Channel or Subcarrier) exactly, and the frequency spectrum of each subchannel is overlapping, but is quadrature again between them, the origin of its OFDM (OFDM) title that Here it is.Under the multipath channel situation, in order to keep the orthogonality between its each subchannel, must add protection at interval (Guard Interval, GI).DVB-T uses Cyclic Prefix, and (Cycle Padding, CP) protection copies to each code element front end with OFDM code element last part at interval exactly.

Finish synchronous task for auxiliary, DVB-T has placed a large amount of pilot signals at frequency domain, interts among data subcarrier, and sends with the power that is higher than data 3dB.As shown in Figure 3.These pilot signals can finish system-frame synchronously, carrier wave recovers, clock adjustment and channel estimating.Because changes in channel characteristics faster can be followed the tracks of and estimate to the use of scattered pilot.The purpose of the thick symbol synchronization of DVB-T is exactly correctly to find DFT the window's position of COFDM frame, and it uses the relevant of Cyclic Prefix (CP) to carry out usually.Utilize CP to carry out symbol synchronization and can avoid sending extra special training sequence, but the correlation properties of CP also have some other neighbor, thereby cause erroneous judgement not as the PN sequence near peak-peak, estimated accuracy is low.In addition in multidiameter fading channel, intersymbol interference (ISI) destroyed that causes by multipath effect the repeat property of Cyclic Prefix.In this case, the thick symbol synchronization method performance based on CP can not guarantee.Thin timing for the DVB-T system is synchronous, and the error of its feedback loop is extracted and carried out at frequency domain based on continuous pilot and scattered pilot signal.

For realize quick and stable synchronously, Tsing-Hua University's DMB-T transmission system has adopted hierarchical frame structure.The elementary cell of frame structure is called signal frame, as shown in Figure 4.253 signal frames are defined as a frame group, and 512 frame groups are defined as a superframe.The top layer of frame structure is called a day frame, is made up of superframe.Each signal frame among the frame group has unique frame number, and it is coded in the PN sequence of frame head.

The signal frame of DMB-T transmission system uses the OFDM modulation of Domain Synchronous, and perhaps being called with the PN sequence is protection OFDM modulation at interval.A signal frame is made up of frame synchronization and frame two parts, and they have identical baseband signalling rate 7.56MS/s (1/T).A signal frame can be used as an OFDM (OFDM) piece.An OFDM piece further is divided into a protection interval and an inverse discrete Fourier transform (IDFT) piece.For TDS-OFDM, frame synchronization sequence is as the protection interval of OFDM, and frame is as the IDFT piece, as shown in Figure 5.

Frame synchronization in the signal frame is made of preamble, synchronous three parts of 8 rank PN sequences and back, as shown in Figure 5.The PN sequence definition is 255 symbols, preamble and after be defined as the cyclic extensions of PN sequence synchronously, account for 512 symbols altogether with the PN sequence.8 rank PN sequence definitions are proper polynomial x ⁸+ x ⁶+ x ⁵The m sequence of+x+1, its initial condition will be determined the phase place of the m sequence that generated.The frame number of each signal frame determines the initial condition of its m sequence.Each signal frame among the signal frame group is assigned unique frame synchronizing signal, with the recognition feature as signal frame.

Based on above-mentioned DMB-T signal frame structure, the thick symbol synchronization of DMB-T system of Tsing-Hua University that the present invention proposes is to seek a phase place for local PN sequence, local PN sequence is alignd with transmission PN sequence phase, use for reference the noun of spread spectrum communication, claim this stage to catch (CA) (frame synchronization) again for sign indicating number.Will further make the phase error of two sequences littler at tracking phase, and automatically to keep this high-precision phase alignment state, this stage under the interference of various foeign elements be the symbol timing recovery (STR) that adopts closed circuit tracking technique.

Summary of the invention

The object of the present invention is to provide a kind of time-domain synchronization OFDM (Time Domain SynchronousOFDM, TDS-OFDM) frame synchornization method of receiver and system thereof

DS-OFDM frame synchronization PN sequence is based on the m sequence of one group of displacement, and the sign indicating number acquisition algorithm that the present invention proposes is finished by the relevant peaks that detects these PN sequences generations.Because the PN sequence self correlation can be good, (CP) compares with Cyclic Prefix, can obtain better relevant peaks.Especially under the multipath situation, the PN sequence correlator provides a relevant peaks to each distinguishable channel impulse pulse, only need catch the top and get final product, so performance is subjected to multi-path influence little.In order to improve the detection performance, the present invention has used four samplings to carry out associative operation, and finishes peak value by a simple comparator and detect.Timing error can be guaranteed after the frame-grab at ± T _sIn/2 scopes, Ts is the sampling interval, makes regularly to recover correctly to enter regularly to recover (STR) part.

The frame synchornization method of TDS-OFDM receiver of the present invention is characterized in that, it realizes that in digital circuit it contains following steps successively:

1) set: use TDS-OFDM, i.e. the frame synchronization PN sequence of time domain synchronous OFDM, it is defined as one 8 rank m sequence, realizes that by linear feedback shift register proper polynomial is x ⁸+ x ⁶+ x ⁵+ x+1, PN sequence period are K, and the frame synchronizing signal length overall is N;

Set: slip dependent threshold T _h

2) the TDS-OFDM signal that receives is carried out L ₀Times over-sampling, L ₀=4;

3) the TDS-OFDM signal behind the over-sampling is carried out relevant treatment with the local PN sequence that generates with the relevant method of sliding, seeks relevant peaks:

Mould when relevant output peak value | and R (k, m) | 〉=T _hThe time, this moment R (k, m) near peak value, R (k m) can be represented by the formula:

$R(k,m)=\frac{1}{K}\underset{n=0}{\overset{K-1}{\mathrm{\Σ}}}r\left(n\right)\·c^{*}(k-n-m)$

Wherein r (n) is for receiving TDS-OFDM frame synchronizing signal, c ^*(k-n-m) grip computing altogether for getting behind the PN sequence displacement m position that receives, k represents the sequence number of correlator output, and n is the sample value sequence number;

4) detect the position of relevant peaks corresponding points with following formula:

If at moment k, | and R (k, m) | 〉=T _hAnd d _c(k-1)＞0, d _c(k)≤0,

D wherein _c(k)=| R (k-1, m) |-| R (k+1, m) |+2 (| R (k-2, m) |-| R (k+2, m) |)

Then obtain relevant peaks, and the value of k ' is in the k ' moment:

5) determine the sequence number of current received signal frame in the frame group:

Phase deviation Δ phase=N with adjacent two signal frames _P-p-N determines the sequence number of current received signal frame in the frame group, N _P-pBe two relevant peak-to-peak sampling numbers;

The phase place of the PN sequence of each signal frame after determining in view of the above;

6) the relevant peaks of the 3rd signal frame and the threshold T of setting _hRelatively, the judgement system is in synchronously or desynchronizing state, and whether decision restarts above-mentioned frame swynchronization code acquisition procedure.

The frame synchronization system of TDS-OFDM receiver of the present invention is characterized in that: contain:

Interpolation filter, it has a TDS-OFDM signal receiving end;

Sampler, it is a 4/T times of octave filter, its input links to each other with the difference filter output;

Correlator, it is a multiplier, its two inputs link to each other with the output of sampler and the output of local PN sequence generator respectively;

Peak detector, it is made of comparison circuit and differential circuit series connection successively, presets slip dependent threshold T _hThe program of value, searching relevant peaks and the relevant corresponding points of detection, and the R of its input and correlator (k, m) signal output part links to each other;

The sequence number detection switch, it is a phase-detection switch, and the sequence number that prestores is determined program, and its input links to each other with the output of peak detector;

Phase matcher, it is to be made of a shift register, and its input links to each other with output behind the sequence number detection switch, and its timing of output output recovers enabling signal.

Because traditional ofdm system is bad based on the relevant code element coarse synchronizing method correlation properties of Cyclic Prefix (CP), the present invention is directed to TDS-OFDM and propose a kind of new frame swynchronization code catching method based on the PN sequence, and analyzed the detection characteristic of this method at the white Gaussian noise channel, this method capture velocity is fast, the precision height.Simulation result shows, compares with Cyclic Prefix (CP) method in traditional OFDM modulation, and the PN sequence capturing method relevant peaks of proposition is very steep, the Domain Synchronous uncertainty that does not have CP circular correlation to bring, and it is minimum to detect wrong probability.The acquisition algorithm that the present invention proposes can guarantee timing error after every frame-grab at ± T _sIn/2 scopes, satisfy the TDS-OFDM system requirements.

Major parameter shown in our the employing table 1 has carried out emulation to the frame synchronization performance under awgn channel.

The main simulation parameter of table 1

Occupied bandwidth 8MHz

Character rate 7.56MHz

Sample frequency Ts 30.42MHz

Subcarrier-modulated 64QAM

Subcarrier number 3780

Synchronous head length 512

Figure 10 (left figure) has provided the relevant peaks of PN correlator output in the sign indicating number acquisition algorithm that proposes, and two relatively low relevant peaks on its main peak both sides are obtained by preamble and back sync buffering.In order to contrast, provided the relevant thick synchronized algorithm relevant peaks of code element (right figure) simultaneously based on CP.The correlation properties of being not difficult to find CP are not as the PN sequence, and the PN relevant peaks is steeper, and the CP relevant peaks also has some other neighbor near peak-peak, thereby cause erroneous judgement easily.

When the SNR of system is 10dB, carried out 10000 emulation, Figure 11 is the relevant output of overlapping PN that obtains.After Figure 12 has shown that code element is caught, timing error is added up and the histogram that obtains.Be not difficult to find, relatively poor based on the accuracy that CP is relevant, be easy to judge by accident into neighbor.And the relevant basic inerrancy probability of PN takes place, and timing error can be guaranteed after every frame-grab at ± T _s/ 2.Just can correctly carry out STR like this after frame-grab regularly follows the tracks of.

Comprehensively above-mentioned, the present invention proposes a kind of new OFDM modulation code catching method based on the PN sequence at the TDS-OFDM signal frame, and analysis and Computer Simulation the detection characteristic of this method at the white Gaussian noise channel, compare with traditional Cyclic Prefix (CP) method, performance has clear superiority, satisfies system requirements.This TDS-OFDM receiver frame method for synchronous that the present invention proposes is applied in the model machine of field programmable gate array (FPGA) version of DMB-T system of Tsing-Hua University and application-specific integrated circuit (ASIC) (ASIC) version, performance index excellence in the actual test of laboratory tests and place of country's tissue is better than existing other ground system of digital television broadcast.

Description of drawings

Fig. 1 is the ATSC frame structure.

Fig. 2 is the DVB-T frame structure.

Fig. 3 is the DVB-T pilot signal.

Fig. 4 is the DMB-T hierarchical frame structure.

Fig. 5 is the structure chart of DMB-T signal frame.

Fig. 6 is the LFSR block diagram.

The realization block diagram of the frame synchronization that Fig. 7 proposes for the present invention (sign indicating number is caught) system.

Fig. 8 is for finding the solution the relevant peaks maximum of points.

Fig. 9 is 4K (left figure) and 8K (right figure) PN sequence auto-correlation function.

Relevant peaks that Figure 10 obtains for the proposition method (left side) and CP relevant peaks (right side).

Figure 11 catches the relevant output (1000 times) that obtains for the sign indicating number that proposes.

Figure 12 is the timing error histogram, the code capture method of a left side for proposing, and the right side is the CP correlation technique.

Figure 13 is the FPGA program flow chart.

Embodiment

Below in conjunction with accompanying drawing theory analysis of the present invention and specific embodiment are described in detail.

The TDS-OFDM frame synchronization PN signal that the frame synchronization that the present invention proposes (sign indicating number is caught) method will be used, it by a preamble cushion, sync buffering constitutes behind PN sequence and one.The PN sequence has 255 symbols, and the preamble buffering of frame synchronization and back sync buffering are defined as the cyclic extensions of PN sequence, and length overall N is 512.The PN sequence definition is one 8 rank m sequence, is realized by a Fibonacci molded lines feedback shift register (LFSR).Its proper polynomial is defined as x ⁸+ x ⁶+ x ⁵+ x+1.The LFSR square frame illustrates as Fig. 6, the phase place of the definite m sequence that is produced of initial value.One 8 rank m sequence is that one-period is the periodic sequence of K=255.Can produce the m sequence of 255 outs of phase based on the initial condition of the LFSR of Fig. 6.These m sequences are by its initial condition numbering in LFSR, and initial condition is by x ¹⁶ⁿNumbering.For example, the initial condition of the m sequence of sequence number 0 is 000000001, and it is x ^{16 * 0}=1.The initial condition of the m sequence of sequence number 1 is 11110011, and it is x ^{16 * 1}=x ¹⁶The initial condition of the m sequence of sequence number 2 is 11110100, and it is x ^{16 * 2}=x ³²The initial condition of the m sequence of sequence number 254 is 00010110, and it is x ^{16 * 254}=x ²³⁹Time power arbitrarily of x can transform to a state with proper polynomial, and promptly the LFSR of a character used in proper names and in rendering some foreign names Luo Hua form can produce each time power of x with continuous order.Arbitrary signal frame among frame group can be assert by its frame synchronization PN sequence.253 different m sequences are to being applied to 253 signal frames among the frame group, and the PN sequence can be made the specific characteristic of a signal frame and be used for identification like this, and the sign indicating number acquisition algorithm of proposition utilizes these PN sequence achieve frames synchronous exactly.

At awgn channel, when having frequency shift (FS) Ω, the synchronous head signal r (n) that receives can be expressed as:

r(n)＝c(n)·exp(j(nΩT+θ))+n(n) (1)

Wherein c (n) is the PN sequence that receives, and n (n) is a white Gaussian noise, and θ is a phase place, and T is the cycle.Carry out that we did not consider timing error when frame synchronization was caught, i.e. ε=0.And the putative signal waveform satisfies the Nyquist condition.Provide a sign indicating number detailed process of catching below, as shown in Figure 7, dotted line block representation recovered part regularly wherein, its specific algorithm is described in the another one patent application.

1) the relevant relevant peaks of seeking of slip

The TDS-OFDM signal frame uses the PN sequence as synchronous code, and the auto-correlation function of PN sequence is sequence period K when phase shift is zero, and when phase shift was not equal to zero, correlation function value was-1.The seizure of frame synchronization in the present invention realizes according to this characteristic, as long as detect the correlation that sends PN sequence and local sequence, can judge just whether the two phase place aligns.In order to reduce the correlator quantity of seizure, adopt the method for catching of slip scan, only need the correlator of length K=253 to get final product.The basic principle of slide catching is: received signal is carried out relevant (multiplication) processing continuously with local PN sequence, the correlated results in any moment all with a thresholding T _hCompare.Show the phase place and the receiving sequence Phase synchronization of carving copy ground sequence at this moment if surpassed thresholding.Local sequence is static, and correlated process is equivalent to received signal and slips over local sequence, and each all produces a correlated results constantly, when sliding to two sequence phase alignment, a correlation peak output (the PN sequence is at the auto-correlation district of zero phase-shift numerical value) must be arranged.The output R of correlator (k m) can be expressed as:

$R(k,m)=\frac{1}{K}\underset{n=0}{\overset{K-1}{\mathrm{\Σ}}}r\left(n\right)\·c^{*}(k-n-m)---\left(2\right)$

Wherein r (n) is for receiving synchronous head signal, c ^*(k-n-m) grip computing altogether for getting behind the PN sequence displacement m position that receives, k represents the sequence number of correlator output, and n is the digital samples sequence number.

Carry out sign indicating number when catching, receiver does not have phase information, and the output delivery of will being correlated with obtains | R (k, m) |.When | R (k, m) | 〉=T _hThe time, illustrate | and R (k, m) | near maximum.

Only survey once in each symbol period T as infructescence, correct hypothesis may be to accept to detect under the situation that the half symbols circular error is arranged, in this case, relevant output performance may reduce a lot, thereby causes several decibels performance loss (generally at 4-6dB).Therefore, the L that should in each symbol time, sample ₀Inferior, the data after the sampling are carried out L ₀Inferior detection, this moment, the loss of relevant output can be expressed as:

sin ²[π/(2L ₀)]/[π/(2L ₀)] ² (3)

L ₀The loss of relevant output in=2 o'clock is reduced to 0.9dB, for L ₀The loss of relevant output in=4 o'clock is reduced to 0.2dB.With that in mind, we adopt four times of over-sampling L ₀=4, i.e. T _s=T/4.After adopting four samplings, relevant output near relevant peaks some value generally all greater than T _h, to utilize this moment the method for differentiate to obtain the position of relevant peaks corresponding points.

The function differentiate is cumbersome problem in general, but under our situation, can carry out digital differentiate.The frequency domain response of numerical differentiation Be defined as:

$H_d\left(e^{\mathrm{j\ω}{T}_s}\right)=\mathrm{j\ω}{T}_s|\frac{\mathrm{\ω}}{2\mathrm{\π}}\≤\frac{\mathrm{\π}}{T_s}|---\left(4\right)$

Wherein Ts is the sampling interval, time-domain pulse response h _d(kT _s) can be by frequency domain response Inverse-Fourier transform obtains:

$h_d\left(k{T}_s\right)=\frac{T_s}{2\mathrm{\π}}\underset{-\mathrm{\π}/T_s}{\overset{\mathrm{\π}/T_s}{\∫}}{H}_d\left(e^{\mathrm{j\ω}{T}_s}\right)e^{\mathrm{j\ω}{T}_s}\mathrm{d\ω}---\left(5\right)$

Obtain by derivation

Signal can use the approximate derivative that obtains relevant output of following formula (7) through four samplings:

d _c(k)＝|R(k-1，m)|-|R(k+1，m)|+2(|R(k-2，m)|-|R(k+2，m)|) (7)

If at moment k, | and R (k, m) | 〉=T _hAnd d _c(k-1)＞0, d _c(k)≤0, can judge like this to obtain obtaining relevant peaks, and the value of k ' is in the k ' moment:

Find the solution relevant output peak point method as shown in Figure 8.

Use the slip correlation technique, shortened pull-in time widely, and only used a correlator.In time, all possible phase place of PN sequence is all searched one time at KT, has very high phase search speed.

2) phase matched

The front is mentioned, the PN sequence of each signal frame is based on the m sequence of one group of displacement, and like this when obtaining first relevant peaks, and what do not know to receive is the PN sequence of which frame, we then search for the relevant peaks of next frame, obtain two relevant peak-to-peak sampling number N _P-pDMB-T signal frame length is the N that fixes, and obtains the phase deviation Δ Phase=N of adjacent two signal frames like this _P-p-N.

In the TDS-OFDM system, the PN sequence of 253 signal frames among each frame group all is to determine to be unique with the phase deviation that guarantees the adjacent signals frame well in advance.By the Δ Phase that obtains, just can determine the sequence number of current received signal frame in the frame group fully like this, thus the phase place of the PN sequence of each signal frame after just obtaining.

In the top method, do not consider the influence of noise to correlator.In fact owing to noise jamming, the relevant peaks detector may be done the judgement that makes mistake.We provide false alarm probability (judgement is not for synchronous when phase place is alignd) and alarm dismissal probability (judgement is for synchronously when phase place is unjustified) below.When false dismissal took place, local sequence phase stopped search, and the synchronizing signal of mistake will make system be in desynchronizing state, can't operate as normal.Take place for fear of this situation, the present invention uses phase matched (Phase Match) circuit to verify.When obtaining second signal frame relevant peaks, do not think to have entered synchronous regime, need again relatively the threshold value of the relevant peaks of the 3rd signal frame and setting, whether see it greater than threshold value, come validation signal whether really to enter synchronously.If the validating circuit judgement shows sequence true synchronization for synchronously, can start the STR tracking circuit.If the validating circuit judgement is then restarted catching circuits for asynchronous.

After entering synchronously, only need upgrade local PN sequence phase during signal frame after receiving and just can continue to keep genlocing, and continuous frame synchronizing signal is provided, will use this frame synchronizing signal during the extraction of STR loop timing error, so that to greater than ± T _s/ 2 signal of timing error estimates that Ts is the sampling interval.In order under synchronous regime, to prevent the influence of false-alarm, to think that system is in desynchronizing state just we set when continuous three signal frames all occur synchronously, and restart sign indicating number and catch.Receiver carries out sign indicating number before Frequency Estimation catches, and this moment, may there be big frequency departure in the frequency and the transmitter of receiver, and this can influence a yard acquisition performance.The variance of putative signal c (k) is ${\mathrm{\σ}}_s^2=1,$ When having frequency shift (FS) Ω, the output R of correlator _Ω(k) be expressed as:

Wherein $n_c\left(k\right)=(1/K)\underset{i=0}{\overset{K-1}{\mathrm{\Σ}}}n\left(i\right)\·c^{*}(i-k),$ n _c(k) the relevant output of expression white noise, c ^*Gripping altogether of c got in expression.

We can see that frequency deviation has following influence to auto-correlation function from formula (9):

1) constant phase place is rotated (K-1) Ω T/2

2) decay sin (K Ω T/2)/(Ksin (Ω T/2)) ≈ sin c (K Ω T/2)

The secondary lobe of relevant output is relevant with sequence, and random process is as the criterion.When Ω was very big, sidelobe level can become very high (it increases with frequency shift (FS)).The carrier frequency that Digital Television is used as 800MHz about, the stability of supposing receiving terminal down-conversion crystal is 10ppm, Zui Da frequency offseting value is 80010 like this ⁶1010 ^-6=8KHz.Following Fig. 9 has provided the R when frequency offseting value is 4KHz and 8KHz _Ω(k) waveform.

As long as K Ω T is smaller, deterioration degree is just smaller.For example, when T=1/7.65 μ s, if frequency shift (FS) 8kHz, K=255, worsening coefficient is 0.7823, promptly-2.45dB.

In conjunction with the accompanying drawings specific embodiments of the invention are had been described in detail above, but the present invention is not restricted to the foregoing description, under the spirit and scope situation of the claim that does not break away from the application, those skilled in the art can make various modifications or remodeling.

Claims (2)

1, the frame synchronization method of time-domain synchronous OFDM receiver, it is characterized in that, it is realized in digital circuit, and it contains following steps successively: 1) Setting: Use TDS-OFDM, frame synchronous PN sequence of instant domain synchronous orthogonal frequency division multiplexing, which is defined as an 8-order m sequence, realized by a linear feedback shift register, and the characteristic polynomial is x ⁸ +x ⁶ +x ⁵ +x+1, the period of the PN sequence is K, and the total length of the frame synchronization signal is N; Setting: sliding correlation threshold T _h ; 2) Carry out L0 times oversampling to the received TDS-OFDM signal, L0=4; 3) Correlate the oversampled TDS-OFDM signal with the locally generated PN sequence using a sliding correlation method to find the correlation peak: When the modulus of the correlation output peak value |R(k, m)|≥T _h , R(k, m) is close to the peak value at this time, and R(k, m) can be expressed by the following formula: $\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; c</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>$ Wherein r (n) is to receive the TDS-OFDM frame synchronization signal, c ^* (knm) is to take the conjugate operation after shifting m bits of the PN sequence received, k represents the sequence number output by the correlator, and n is the sample value sequence number; 4) Use the following formula to detect the position of the corresponding point of the correlation peak: If at time k, |R( k, m)|≥T _h and d _c ( k-1) > 0, d _c ( k) ≤ 0, where d _c (k)=|R(k-1, m)|-|R(k+1, m)|+2(|R(k-2, m)|-|R(k+2, m )|) Then the correlation peak is obtained at time k', and the value of k' is: 5) Determine the serial number of the current received signal frame in the frame group: Use the phase offset Δphase=N _pp -N of two adjacent signal frames to determine the sequence number of the current received signal frame in the frame group, and N _pp is the number of sampling points between the two correlation peaks; Determine accordingly the phase of the PN sequence of each signal frame in the future; 6) Comparing the correlation peak of the third signal frame with the set threshold value T _h , judging that the system is in a synchronous or out-of-synchronization state, and deciding whether to restart the above frame synchronization code acquisition process. 2. The frame synchronization system of the time-domain synchronous OFDM receiver, characterized in that it contains: Interpolation filter, it has a TDS-OFDM signal receiving end; A sampler, which is a 4/T multiplier filter, whose input is connected to the output of the interpolation filter; A correlator, which is a multiplier whose two inputs are respectively connected to the output of the sampler and the output of the local PN sequence generator; The peak detector, which is composed of a comparison circuit and a differential circuit in series, presets the program of sliding the correlation threshold _Th value, searching for the correlation peak and detecting the corresponding point of the correlation, and its input terminal is connected with the R (k, m) signal of the correlator connected to the output; A serial number detection switch, which is a phase detection switch, prestores the serial number determination program, and its input terminal is connected with the output terminal of the peak detector; The phase matching device is composed of a shift register, its input end is connected with the output end of the serial number detection switch, and its output end outputs a timing recovery start signal.

Images