CN1937604B - Initial access frame synchronizing method and device for orthogonal frequency division multiplexing system - Google Patents

Abstract

The invention discloses an initial access frame synchronizing method and device in OFDM system. And the method comprises: a receiver buffers the received maximum frame length signal and makes power mutation detection on it, and records the power mutation and length of each signal; synchronizing mutation signals by decimal frequency and making FFT on the mutation signals; calculating the maximum likelihood ratio ... of each mutation signal, where the mutation signal with the maximum ... value is a precursor symbol, and making frequency correction and FFT on the precursor symbol again; calculating PN sequence of the precursor symbol; making frequency correction and FFT on one of the remaining mutation signals that has the maximum ... value and correlating with the PN sequence detected, and calculating the frame length and making initial access synchronization by timer. And it is simple to implement and has high processing efficiency.

Description

Initial access frame synchronizing method and device in the ofdm system

Technical field

The present invention relates to initial access frame synchronizing method and device in the mobile radio system, relate in particular to initial access frame synchronizing method and device in a kind of ofdm system.

Background technology

When carrying out reflex time at building, wall, furniture and other large-sized object, signal transmission can produce multipath effect.In this case, signal transmission is not to arrive receiver by single directapath, but has passed through a plurality of different paths.Every paths length that signal experiences from the reflector to the receiver is all different, so the delay of each signal is different.The signal that finally receives is actually repeatedly superposition and the signal that produces, and each superposition signal arrives receiver constantly in difference, and the intensity of each superposition signal is all not identical.This effect is commonly referred to intersymbol interference (ISI).Concerning receiver, can reduce intersymbol interference by T blanking time that increases transmission code.But can reduce transmission rate like this, this aim with high-speed transfer is refuted mutually.Orthodoxy Frequency Division Multiplex (OFDM) technology has appearred under this background.

As a kind of multi-carrier transmission pattern, OFDM is converted to the data flow of one group of low-speed parallel transmission by the data flow with a high-speed transfer, system is reduced greatly to the susceptibility of multidiameter fading channel frequency selectivity, in the broadband wireless transmission system, be with a wide range of applications.Can predict, the communication system take OFDM as core technology is a kind of system of broadband wireless communication that has development prospect.

Simultaneous techniques is one of key technology of ofdm system, and one of top priority that initially to access frame synchronization be user terminal (SS) must finish with base station (BS) signal post.In ofdm communication system, in order to realize the frame synchronization of descending reception, downlink frame often has following several characteristics:

1, the subcarrier of the symbol of the some fixed positions of downlink frame (being generally first symbol) is shone upon and modulation by a special sequence (being generally pseudo noise (PN) sequence) by certain way, claims that usually this symbol is leading symbol.

2, in order to make the leading symbol can be by reliable reception, leading symbol often adopts binary phase shift keying (BPSK) modulation, and data symbol adopts the leggy system modulation such as quaternary phase shift keying modulation (QPSK) or quadrature amplitude modulation (QAM) in order to improve spectrum efficiency.

3, leading symbol has certain power ascension with respect to data symbol.

For example, adopting OFDM is the OFDMA(OFDM of the IEEE802.16 standard of core technology) first symbol of downlink frame of agreement namely is the leading symbol with aforementioned feature.

As shown in Figure 1, be the frame structure commonly used that adopts Frequency Division Duplexing (FDD) (FDD) mode ofdm system among the figure, it namely has These characteristics; P represents leading symbol among the figure, and D represents data symbol.Ofdm system for time division duplex (TDD) mode; in order to protect the uplink/downlink frames non-interference, guarantee that radio system has the sufficient time to switch between transmitter and receiver, usually can keep up and down one section aperture time that does not send any data between the frame.As shown in Figure 2, namely be to adopt TDD mode ofdm system to have the frame structure commonly used in space.RTG represents the gap of uplink frame and downlink frame among the figure, and TTG represents the gap of downlink frame and uplink frame, and P represents leading symbol, and D represents data symbol.

With special sequence (being generally the PN sequence) shine upon with the subcarrier that is modulated to leading symbol on mode usually have two kinds.As shown in Figure 3, this modulation system is that special sequence is mapped on all subcarriers except left and right sides boundary belt subcarrier and direct current carrier wave; The subcarrier that represents special sequence among the figure with the arrow vertical line, zero expression, zero subcarrier.As shown in Figure 4, another kind of modulation system is on the subcarrier that equally spaced special sequence is mapped to except left and right sides boundary belt subcarrier and direct current carrier wave.Subcarrier mapping mode among Fig. 4 is mainly used in the cellular radio ofdm system, and in order to distinguish the leading symbol of neighbor cell (or sector), the leading symbol of neighbor cell (or sector) often adopts different PN sequences; And the PN sequence of adjacent several residential quarter (or sector) is mapped to respectively on the different sub carrier position of leading symbol; The subcarrier that represents special sequence among the figure with the arrow vertical line, zero expression, zero subcarrier.

When the user terminal initial access network, user terminal at first will keep frame synchronization with downlink frame.As shown in Figure 1 and Figure 2, for the ofdm system with leading symbol, if the user terminal receiver can detect leading symbol in the signal that receives, just can realize initially accessing frame synchronization.Therefore, the user terminal key that initially accesses frame synchronization is how to detect leading symbol from receive signal.The existing method that detects leading symbol mainly can be divided into following a few class:

The time domain waveform correlation method.The method be with length be maximum frame length reception OFDM time-domain signal with might leading symbol time domain waveform to pursue sampling point relevant, obtain the maximal correlation position.That is:

$\hat{n}=\underset{n,j}{\arg \max }\frac{{\left|\underset{l=0}{\overset{N-1}{\mathrm{\Σ}}}{y}^{*}(n+l)c_j\left(l\right)\right|}^2}{\underset{l=0}{\overset{N-1}{\mathrm{\Σ}}}{\left|y^{*}(n+l)c_j\left(l\right)\right|}^2}$

C in the formula _j(l) (j=0,1,2 ..., P-1, l=0,1,2 ..., N-1), it is the time domain sampling point of N, P possibility leading symbol for symbol lengths.The benefit of the method searches out the PN sequence that current area (sector) adopts when being can conducting frame synchronous.Yet the complexity of the method is too high.It needs M * N * N * P(M is largest frames internal symbol number) inferior complex multiplication.For example, for Cyclic Prefix (CP) length be 256, fast Fourier transform (FFT) is counted is 2048 IEEE802.16-OFDMA agreement, M is about 100, then needs altogether 6.0 * 10 ¹⁰Inferior complex multiplication.

Postpone correlation method.For the mapping modes that inserted two zero subcarriers in the frequency domain between per three subcarriers and reflect, the time domain of leading symbol shows pseudo-periodicity, and namely time domain leading symbol is approximately the signal of three sections repetitions.So can detect leading symbol by following formula:

When the interference of the leading symbol that does not have other residential quarters, this algorithm has preferably performance.Yet when user terminal is in cell boarder, receiver will be received the superposed signal of a plurality of leading symbols from different districts.At this moment, the reception signal of leading symbol will no longer have pseudo-periodicity, and this algorithm performance also worsens thereupon.

The power detection method.The method is to utilize the power of leading symbol to have the character of larger power ascension with respect to other data symbols, or to utilize the front of leading symbol be RTG, has the character of very high-power sudden change from RTG to the leading symbol.Yet because the impact of channel fading or power control, the false drop rate of power detection method or loss are too high.

Summary of the invention

For the existing problem and shortage of initial access frame synchronizing method in the above-mentioned existing ofdm system, the purpose of this invention is to provide a kind of initial access frame synchronizing method and device in the ofdm system simple, that efficient is high of processing.

The present invention is achieved in that initial access frame synchronizing method in a kind of ofdm system, may further comprise the steps:

(1) data-signal of the maximum frame length that receives of receiver buffer memory starts timer simultaneously, and this longest frame signal is carried out chugging detect, and the higher symbol of power in the longest described frame signal is namely recorded in each symbol and the position thereof of recording power sudden change;

(2) recording power sudden change symbol is carried out J decimal frequency bias and proofread and correct, then it is carried out fast Fourier transform (FFT), the chugging symbol transition in the time domain is arrived frequency domain; Here J is positive integer;

(3) to k frequency domain chugging symbol Y after the correction of j secondary frequencies _{K, j}(i), be calculated as follows maximum likelihood ratio Λ _{K, j}Value: ${\mathrm{\Λ}}_{k,j}=\frac{\underset{i=B_L}{\overset{N-3L-1-B_R}{\mathrm{\Σ}}}\left|\mathrm{Re}\right[Y_{k,j}\left(i\right){Y_{k,j}}^{*}(i+L){Y_{k,j}}^{*}(i+2L)Y_{k,j}(i+3L)\left]\right|}{\underset{i=B_L}{\overset{N-3L-1}{\mathrm{\Σ}}}\left|\mathrm{Im}\right[Y_{k,j}\left(i\right){Y_{k,j}}^{*}(i+L){Y_{k,j}}^{*}(i+2L)Y_{k,j}(i+3L)\left]\right|},$ Wherein, N is the FFT size, and L is the sub-carrier number of adjacent pilot frequencies subcarrier spacing, B _LAnd B _RBe respectively left and right sides boundary belt sub-carrier number, Y _{K, j} ^*The expression conjugated signal;

(4) obtain the Λ of each chugging symbol _{K, j}Value is got Λ=max (Λ _{K, j}), the chugging symbol with maximum Λ value is leading symbol, again this leading symbol is carried out decimal frequency bias estimation and correction, fast fourier transform;

(5) this leading symbol and all possible PN (pseudo noise) sequence are carried out relevant and detect the PN (pseudo noise) sequence of current leading symbol;

(6) the Λ value of remainder is calculated Λ=max (Λ _{K, j}), the chugging symbol that the maximum Λ of this new calculating is corresponding carries out decimal frequency bias estimation and correction, fast fourier transform, and with step (5) in detected PN (pseudo noise) sequence carry out relevantly, judge whether this chugging symbol is leading symbol; If it is repeating step (6), otherwise stop to judge next chugging symbol, and add up maximum frame length and receive leading symbol number S in the signal, thereby estimate frame length Len=MaxLen/S;

(7) find out the position Loc that occurs leading symbol in the data-signal of maximum frame length in the step (1) for the first time, when the counting of timer deducts Loc and is the Len of integral multiple, begin to start the initial access frame-synchronizing impulse that frame length is Len.

Wherein, in the described step (1) the longest described frame signal is carried out the chugging detection and is specially, two sliding window A (n), the B (n) of equal in length is set,

$A\left(n\right)=\underset{m=0}{\overset{\mathrm{WinAlen}-1}{\mathrm{\Σ}}}y(n-m)y^{*}(n-m)=\underset{m=0}{\overset{\mathrm{WinAlen}-1}{\mathrm{\Σ}}}{\left|y(n-m)\right|}^2,$

$B\left(n\right)=\underset{m=0}{\overset{\mathrm{WinBlen}-1}{\mathrm{\Σ}}}y(n+m)y^{*}(n+m)=\underset{m=0}{\overset{\mathrm{WinBlen}-1}{\mathrm{\Σ}}}{\left|y(n+m)\right|}^2,$

Wherein, y (n) is the longest described frame signal, y ^*(n) be its conjugation, WinAlen, WinBlen are respectively the length of sliding window A (n), B (n), m for caused by sliding window with y (n) phase difference, detect the chugging position that all surpass threshold value by following formula:

Wherein, J fractional frequency that the decimal emending frequency is five equilibrium-0.5～0.5 Δ f in the described step (2), wherein Δ f is subcarrier spacing.

Wherein, described Loc, Len and timer counting has identical chronomere.

Wherein, described sliding window length WinAlen, WinBlen are slot length.

Wherein, described sliding window length should be less than the length of leading symbol.

Initial access frame-synchronizing device in a kind of ofdm system includes: the receive data buffer is used for receiving external signal data and carrying out the long signal buffer memory of largest frames; The chugging detector is for detection of the symbol of the chugging in the long signal of described largest frames; Cyclic prefix estimator is for the length of estimating Cyclic Prefix; The decimal frequency bias estimator, the result who inputs according to described cyclic prefix estimator carries out the fractional frequency bias estimation to the chugging symbol; Frequency corrector, the fractional frequency bias estimation result who inputs according to described decimal frequency bias estimator carries out the decimal frequency bias correction to the chugging symbol; The fast Fourier transform (FFT) device carries out fast fourier transform to the chugging symbol after proofreading and correct through described frequency corrector; The maximum likelihood ratio calculator carries out maximum likelihood ratio to the chugging symbol after described fast fourier transform and calculates; The maximum likelihood ratio comparator compares and sorts each the chugging symbol maximum likelihood ratio that calculates; Sub-carrier demapper is to separating mapping for the chugging symbol of leading symbol; Correlator is used for carrying out relevantly to separating the chugging symbol after the mapping and all possible PN (pseudo noise) sequence that memory is stored, and obtains its PN (pseudo noise) sequence; Be used for storing the memory of all possible PN (pseudo noise) sequence; The first leading symbol position, frame length and timer count calculator are used for judging that initial access frame synchronization is to start corresponding pulses; Timer, receive data buffer start timing when receiving the longest frame; The frame-synchronizing impulse generator is used for receiving the enabling signal of described the first leading symbol position, frame length and timer count calculator to produce frame-synchronizing impulse.

Wherein, described chugging detector specifically includes two sliding window detectors, and this pair sliding window detector can be used for calculating the position of chugging signal.

Wherein, this device also comprises decimal frequency bias adjuster and the fast fourier transformer that is connected successively with described correlator, proofreaies and correct and fast fourier transform detected leading symbol is carried out decimal frequency bias; Described fast fourier transformer exports described the first leading symbol position, frame length and timer count calculator to.

Wherein, described two sliding window slot lengths equate.

The present invention at first carries out buffer memory to the longest frame that receives, and calculates first chugging symbolic component (leading symbol) in the longest frame, finally calculates frame length, then utilizes timer to finish the synchronous of access signal.Processing speed of the present invention is very fast, and realization is simple, net synchronization capability is good, the initial access frame synchronization on energy suitable cell border, and is compatible good.

Description of drawings

Fig. 1 is the ofdm communication system downlink frame structural representation of FDD mode;

Fig. 2 is the ofdm communication system downlink frame structural representation of TDD mode;

Fig. 3 is a kind of mapping mode of the subcarrier of leading symbol;

Fig. 4 is a kind of mapping mode of the subcarrier of leading symbol;

Fig. 5 is the schematic diagram of the two sliding window detection signals of the present invention;

Fig. 6 is the structural representation of apparatus of the present invention.

Embodiment

The present invention is described in further detail below in conjunction with accompanying drawing.

Method of the present invention specifically may further comprise the steps:

1, after receiver received downlink frame, buffer storage length was the reception of data signal of maximum possible frame length, and in beginning buffer memory receive data start-up time counter T;

2, the longest frame signal of buffer memory is carried out chugging and detect, the position of each symbol of recording power sudden change, the present invention namely records the higher signal of power in the longest frame signal, namely may be the signal of leading symbol; Concrete detection mode is: as shown in Figure 5, two sliding window A (n), the B (n) of equal in length in the time domain is set, order

$A\left(n\right)=\underset{m=0}{\overset{\mathrm{WinAlen}-1}{\mathrm{\Σ}}}y(n-m)y^{*}(n-m)=\underset{m=0}{\overset{\mathrm{WinAlen}-1}{\mathrm{\Σ}}}{\left|y(n-m)\right|}^2$

$B\left(n\right)=\underset{m=0}{\overset{\mathrm{WinBlen}-1}{\mathrm{\Σ}}}y(n+m)y^{*}(n+m)=\underset{m=0}{\overset{\mathrm{WinBlen}-1}{\mathrm{\Σ}}}{\left|y(n+m)\right|}^2$

In the formula, the longest frame signal of y (n) expression buffer memory, y ^*(n) be its conjugation, WinAlen, WinBlen are respectively the length of sliding window A (n), B (n), m for caused by sliding window with y (n) phase difference, detect the chugging position that all surpass threshold value Th by following formula:

Namely find out all chugging positions, each corresponding length in sudden change position of buffer memory is the data block signal of maximum possible symbol lengths.Here, the length of sliding window is preferably less than the length of leading symbol.

3, be that the data-signal (chugging data-signal) of maximum possible symbol lengths carries out the J(positive integer to length corresponding to each chugging position) inferior decimal frequency bias proofreaies and correct, wherein J decimal correction frequency deviation is five equilibrium-0.5～0.5 Δ f(Δ f subcarrier spacing) decimal frequency bias.Then all chugging data symbols are carried out the FFT conversion, it is transformed to frequency domain.

4, to k frequency domain chugging symbol Y after the correction of j secondary frequencies _{K, j}(i), be calculated as follows maximum likelihood ratio Λ _{K, j}Value: ${\mathrm{\Λ}}_{k,j}=\frac{\underset{i=B_L}{\overset{N-3L-1-B_R}{\mathrm{\Σ}}}\left|\mathrm{Re}\right[Y_{k,j}\left(i\right){Y_{k,j}}^{*}(i+L){Y_{k,j}}^{*}(i+2L)Y_{k,j}(i+3L)\left]\right|}{\underset{i=B_L}{\overset{N-3L-1}{\mathrm{\Σ}}}\left|\mathrm{Im}\right[Y_{k,j}\left(i\right){Y_{k,j}}^{*}(i+L){Y_{k,j}}^{*}(i+2L)Y_{k,j}(i+3L)\left]\right|},$ Wherein, N is the FFT size, and L is the sub-carrier number of adjacent pilot frequencies subcarrier spacing, B _LAnd B _RBe respectively left and right sides boundary belt sub-carrier number, Y _{K, j} ^*The expression conjugated signal.

5, obtain the Λ of each chugging symbol _{K, j}Value is got Λ=max (Λ _{K, j}), then the chugging symbol of judgement with maximum Λ value is leading symbol, again this leading symbol carried out decimal frequency bias estimation and correction, fast fourier transform.

6, the leading symbol that utilizes maximum Λ value to determine in the step 5 and all possible PN sequence are carried out relevant and detect the PN sequence of current leading symbol.

7, the Λ value of remainder recomputated Λ=max (Λ _{K, j}), the chugging symbol that the maximum Λ of this new calculating is corresponding carries out decimal frequency bias estimation and correction, fast fourier transform, and with step 6 in detected PN (pseudo noise) sequence carry out relevantly, judge whether this chugging symbol is leading symbol; If leading symbol, then repeating step 7, continue to judge whether next chugging symbol is leading symbol; Then stop to judge next chugging symbol if not leading symbol, and add up the leading symbol number S in the maximum frame length reception signal, thereby estimate frame length Len=MaxLen/S.MaxLen is the frame length (time domain) that maximum frame length receives signal.

8, for all leading symbols that find out, determine that leading symbol that occur the first time in the data-signal of maximum frame length, thereby determine the position Loc(time domain of first leading symbol); Calculate simultaneously the counting T of current time counter T _tWith the difference of Loc, as (T _t-when Loc) being the integral multiple of Len, begin to start the initial access frame-synchronizing impulse that frame length is Len.

Wherein, described Loc, Len and timer counting has identical chronomere; Described sliding window length WinAlen, WinBlen are slot length.

As shown in Figure 6, each device that the inventive system comprises connects by data-interface or signaling interface such as figure mode, and thick arrow line represents data flow among the figure, and thin arrow line represents signal stream.Wherein, receive data buffer 601 buffer storage lengths are the reception of data signal of maximum possible frame length, when beginning to receive maximum possible frame length signal, and timer 613 beginning timing.

When length is after the reception signal of maximum possible frame length receives, 602 pairs of the longest signals that receive of two sliding window detectors carry out two sliding windows and detect, one of every discovery surpasses threshold value Th(and rule of thumb sets) local peaking, just with the position n of this local peaking's sampling point _kBe kept at peak buffer 603.After all data are finished the sliding window detection in the reception data buffer 601, all position n _kCorresponding chugging data symbol is transported to frequency corrector 604 and is carried out J decimal and proofread and correct that (frequency is-0.5+j/J(j=0,1, ..J-1) frequency correction), then fast fourier transformer (FFT) 605 is carried out the FFT computing to the data after proofreading and correct respectively.The FFT result that 606 pairs of every kind of emending frequencies of Λ value calculator are corresponding carries out the Λ value and calculates, and the maximum Λ value that each chugging symbol is corresponding is exported to Λ value sorting unit 607.

After the calculating of the Λ of all chugging symbols value and ordering end, CP estimator 608 and decimal frequency bias estimator 609 are inputted the chugging symbol with maximum Λ value from receive data buffer 601, and carry out CP length and decimal frequency bias estimation according to this chugging symbol.Then frequency correction 604 is carried out frequency correction according to the decimal frequency bias estimated result to this chugging symbol.FFT605 carries out the FFT computing to the data after proofreading and correct, and FFT result is exported to sub-carrier demapper 610.Correlator 611 will be separated in subcarrier and the PN sequence memory 612 after the mapping all possible PN sequence and carry out relevantly, and select to have the PN sequence of maximum related value as the PN sequence that accesses the residential quarter.

When the PN Sequence Detection of access residential quarter out after, the chugging symbol that decimal frequency bias estimator 609 has time maximum Λ value from receive data buffer 601 input, and carry out the decimal frequency bias estimation according to this chugging symbol.Then frequency correction 604 is carried out frequency correction according to the decimal frequency bias estimated result to this chugging symbol.FFT605 carries out the FFT computing to the data after proofreading and correct, and FFT result is exported to sub-carrier demapper 610.Correlator 611 will separate after the mapping subcarrier with carry out relevantly when the PN sequence that access the residential quarter, judge whether this chugging symbol is leading symbol.If so, then input has next time that the chugging symbol of maximum Λ value determines whether leading symbol again, until it is not leading symbol.

When correlator 611 rules out not as behind the leading symbol, Loc and Len calculator 614 find out first leading symbol position Loc, calculate frame length Len, and as the timing output T of timer 613 _tSatisfy (T _t-when Loc) being the integral multiple of Len, starting the frame-synchronizing impulse 615 generation cycles of generator is the frame-synchronizing impulse of Len.

This device also comprises decimal frequency bias adjuster and the fast fourier transformer that is connected successively with described correlator, proofreaies and correct and fast fourier transform detected leading symbol is carried out decimal frequency bias; Described fast fourier transformer exports described the first leading symbol position, frame length and timer count calculator to.In apparatus of the present invention, described two sliding window slot lengths equate.

Certainly; the present invention also can have other various embodiments; in the situation that does not deviate from spirit of the present invention and essence thereof; those skilled in the art work as can make according to the present invention various corresponding changes and distortion, but these corresponding changes and distortion all should belong to the protection range of the appended claim of the present invention.

Claims (8)

1. initial access frame synchronizing method in the ofdm system is characterized in that the method may further comprise the steps:

(1) the longest frame signal that receives of receiver buffer memory starts timer simultaneously, and this longest frame signal is carried out chugging detect, each symbol and the position thereof of recording power sudden change;

(2) J decimal frequency bias correction carried out in the chugging symbol of record, then it is carried out fast Fourier transform (FFT), the chugging symbol transition in the time domain is arrived frequency domain; Here J is positive integer;

(3) to k frequency domain chugging symbol Y behind the j time frequency offset correction _{K, j}(i), be calculated as follows maximum likelihood ratio Λ _{K, j}Value: ${\mathrm{\Λ}}_{k,j}=\frac{\underset{i=B_L}{\overset{N-3L-1-B_R}{\mathrm{\Σ}}}\left|\mathrm{Re}\right[Y_{k,j}\left(i\right){Y_{k,j}}^{*}(i+L){Y_{k,j}}^{*}(i+2L)Y_{k,j}(i+3L)\left]\right|}{\left|\mathrm{Im}\right[Y_{k,j}\left(i\right){Y_{k,j}}^{*}(i+L){Y_{k,j}}^{*}(i+2L)Y_{k,j}(i+3L)\left]\right|},$ Wherein, N is the FFT size, and L is the sub-carrier number of adjacent pilot frequencies subcarrier spacing, B _LAnd B _RBe respectively left and right sides boundary belt sub-carrier number, Y _{K, j} ^*The expression conjugated signal;

(4) obtain the Λ of each chugging symbol _{K, j}Value is got Λ=max (Λ _{K, j}), then the chugging symbol of judgement with maximum Λ value is leading symbol, again this leading symbol carried out decimal frequency bias estimation and correction, fast fourier transform;

(5) this leading symbol and all possible PN (pseudo noise) sequence are carried out relevant and detect the PN (pseudo noise) sequence of current leading symbol;

(6) the Λ value of remainder is calculated Λ=max (Λ _{K, j}), the chugging symbol that the maximum Λ of this new calculating is corresponding carries out decimal frequency bias estimation and correction, fast fourier transform, and with step (5) in detected PN (pseudo noise) sequence carry out relevantly, judge whether this chugging symbol is leading symbol; If it is repeating step (6), otherwise stop to judge next chugging symbol, and add up maximum frame length and receive leading symbol number S in the signal, thereby estimating frame length Len=MaxLen/S, MaxLen is the frame length that maximum frame length receives signal;

(7) find out in the step (1) the position Loc that occurs leading symbol in the longest frame signal for the first time, when the counting of timer deducts Loc and is the integral multiple of Len, begin to start the initial access frame-synchronizing impulse that frame length is Len;

Wherein, in the described step (1) the longest described frame signal is carried out the chugging detection and is specially, two sliding window A (n), the B (n) of equal in length is set,

$A\left(n\right)=\underset{m=0}{\overset{\mathrm{WinAlen}-1}{\mathrm{\Σ}}}y(n-m)y^{*}(n-m)=\underset{m=0}{\overset{\mathrm{WinAlen}-1}{\mathrm{\Σ}}}{\left|y(n-m)\right|}^2,$

$B\left(n\right)=\underset{m=0}{\overset{\mathrm{WinBlen}-1}{\mathrm{\Σ}}}y(n+m)y^{*}(n+m)=\underset{m=0}{\overset{\mathrm{WinBlen}-1}{\mathrm{\Σ}}}{\left|y(n+m)\right|}^2,$

Wherein, y (n) is the longest described frame signal, y ^*(n) be its conjugation, WinAlen, WinBlen are respectively the length of sliding window A (n), B (n), m for caused by sliding window with y (n) phase difference, detect the chugging position that all surpass threshold value by following formula:

2. initial access frame synchronizing method in the ofdm system according to claim 1 is characterized in that, J fractional frequency that the decimal emending frequency is five equilibrium-0.5～0.5 Δ f in the described step (2), and wherein Δ f is subcarrier spacing.

3. initial access frame synchronizing method in the ofdm system according to claim 1 is characterized in that, described Loc, Len and timer counting have identical chronomere.

4. initial access frame synchronizing method in the ofdm system according to claim 1 is characterized in that, described sliding window length WinAlen, WinBlen are slot length.

5. initial access frame synchronizing method according to claim 1 or in the 4 described ofdm systems is characterized in that described sliding window length should be less than the length of leading symbol.

6. initially access frame-synchronizing device in an ofdm system, it is characterized in that this device includes: the receive data buffer is used for receiving external signal data and growing the frame signal buffer memory most; The chugging detector, the symbol of the chugging in the longest frame signal that receives for detection of described receive data buffer; Cyclic prefix estimator is used for the length according to the testing result estimation Cyclic Prefix of described chugging detector; The decimal frequency bias estimator, the result who inputs according to described cyclic prefix estimator carries out the fractional frequency bias estimation to the chugging symbol; Frequency corrector, the decimal frequency bias estimated result of inputting according to described decimal frequency bias estimator carries out the decimal frequency bias correction to the chugging symbol; The fast Fourier transform (FFT) device carries out fast fourier transform to the chugging symbol after proofreading and correct through described frequency corrector; The maximum likelihood ratio calculator carries out maximum likelihood ratio to the chugging symbol after described fast fourier transform and calculates; The maximum likelihood ratio comparator compares and sorts each the chugging symbol maximum likelihood ratio that calculates; Sub-carrier demapper is connected with described fast Fourier transform (FFT) device, is used for result of calculation according to described fast Fourier transform (FFT) device to separating mapping for the chugging symbol of leading symbol; Correlator is used for carrying out relevantly to separating the chugging symbol after the mapping and all possible PN (pseudo noise) sequence that memory is stored, and obtains its PN (pseudo noise) sequence; Memory is connected with described correlator, is used for storing all possible PN (pseudo noise) sequence; Timer is connected with the first leading symbol position, frame length and timer count calculator, is used for starting when data buffer receives the longest frame signal timing; The first leading symbol position, frame length and timer count calculator are connected with described correlator, are used for according to the result of described correlator output and the timing result of described timer output, judge that initial access frame synchronization is to start corresponding pulses; The frame-synchronizing impulse generator is used for receiving the enabling signal of described the first leading symbol position, frame length and timer count calculator to produce frame-synchronizing impulse.

7. initially access frame-synchronizing device in the ofdm system according to claim 6, it is characterized in that, described chugging detector specifically includes two sliding window detectors, and this pair sliding window detector can be used for calculating the position of chugging signal.

8. initial access frame-synchronizing device in the ofdm system according to claim 7 is characterized in that described two sliding window slot lengths equate.

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