CN106789807A - A kind of OFDM timing metric methods of estimation based on FIR filter - Google Patents

Abstract

It is as follows the step of the method the invention provides a kind of OFDM timing metric methods of estimation based on FIR filter：First, signal and its own time delayed signal will be received, conjugate multiplication obtains M₀Individual reception difference sequence；By M₀Individual reception difference sequence carries out FIR filtering, obtains M₀Road filtering output p (m, d)；By M₀Road filtering output p (m, d) carry out average, obtain average value P (d), and average value P (d) is normalized using OFDM receptions signal energy R (d), obtain normalization timing metric M (d) of the symbol timing synchronization method based on FIR structures；Finally, ask for normalizing by the d corresponding to the maximum of timing metric M (d) can obtain and obtain OFDM symbol timing estimation amountThe method is based on FIR filter, timing metric can be calculated by the modularized processing of the FIR filter of different coefficients in Project Realization, realize the real time high-speed treatment of reception signal stream in ofdm demodulator, effectively reduce implementation complexity, hardware resource is reduced, it is cost-effective.

Description

A kind of OFDM timing metric methods of estimation based on FIR filter

Technical field

The invention belongs to digital wireless communication transmission technique field, it is related to be filtered based on FIR in a kind of TTC ＆ DT Systems The Symbol Timing measurement of the ofdm demodulator of device estimates structure.

Background technology

OFDM (Orthogonal Frequency Division Multiplexing, OFDM) can be provided The information transfer of Large Copacity, and solve the problems, such as that radio channel performance is deteriorated during detection and control terminal transmitting high speed data, with frequency spectrum Utilization rate is high, the advantages of mitigating frequency-selective fading, is classified as integrated increase telemetry network of future generation by international remote measurement association One of technical scheme of plan.Under the complicated multipath channel of telemetry communication, OFDM can provide a kind of high speed, efficient and safety Data transmission scheme, effectively alleviate the frequency spectrum resource being becoming tight day, improve the antijamming capability of single carrier TT＆C system.

In OFDM receives demodulating process, it is necessary first to the window's position of FFT is determined by timing synchronization, protect Orthogonality between card subcarrier.Conventional timing synchronization algorithm such as document " Schmidl T, Cox D C.Robust Frequency and Timing Synchronization for OFDM[J].IEEE Transactions on Communications,1997,45(12):1613~1621. " S＆C algorithms are using two sections of identical leading characters before and after time domain in Number conjugation correlation carries out timing synchronization, but the Cyclic Prefix of OFDM symbol to cause timing synchronization position to exist fuzzy.In order to Improve the Timing Synchronization performance of S＆C algorithms, Ren is in document " Ren G L, Chang Y, Zhang H, et al.Synchronization Methods Based on a New Constant Envelope Preamble for OFDM Systems[J].IEEE Transactions on Broadcasting,2005,51(1):139~143. " in using it is pseudo- with Two sections of identical leading symbols are weighted before and after machine sequence pair, to two sections of conjugation before and after the leading symbol after weighting in demodulator Correlation carries out timing synchronization, but pseudo-random sequence can increase outside the band of ofdm signal power spectrum the weighting of OFDM leading symbols Leakage.

For the problem that above-mentioned conventional sign timing synchronization algorithm is present, document " Awoseyila A B, et al.Robust Time-domain Timing and Frequency Synchronization for OFDM Systems [J].IEEE Transactions on Consumer Electronics,2009,55(2):391~399. " and document " Ma Y S,Zhou S W,Yan C X,et al.Design of OFDM Timing Synchronization Based on Correlations of Preamble Symbol[C]//2016 IEEE 83rd Vehicular Technology Conference:Two kinds of symbol timing synchronization methods are proposed in VTC 2016-Spring, Nanjing. " respectively, can be not The timing metric with pulse characteristic is obtained in the case of influence ofdm signal power spectrum performance, multidiameter fading channel is effectively improved Lower OFDM symbol Timing Synchronization performance, it is adaptable to the TTC ＆ DT Systems under complicated multipath channel characteristic, but the timing degree of the two Measure computationally intensive, Project Realization complex structure, hardware resource occupancy high.By taking the latter as an example, during timing metric is calculated Need to calculate multiple difference correlation functionsObviously, each difference correlation function only in the step Calculating be just respectively necessary for N-m times and multiply again and summation operation, cause the computationally intensive of algorithm, realize complex structure.

The content of the invention

Technology solve problem of the invention is：Overcome the deficiencies in the prior art, propose a kind of OFDM based on FIR filter Symbol Timing measures method of estimation, solves the timing detection poor-performing of traditional timing synchronization algorithm, realizes complex structure, hardware The problems such as resources occupation rate is high.

Technical solution of the invention is：One kind is based on FIR filter OFDM symbol timing metric method of estimation, OFDM M is included in signal frame_sIndividual OFDM symbol, M_sIt is positive integer, M_s>=2, wherein, first OFDM symbol is known leading symbol, Each symbol includes N=N_s+N_cpIndividual sampled value, N_sIt is the IFFT/FFT transform sizes of OFDM symbol, N_cpBefore the circulation of OFDM Sew sample number, define for calculating the OFDM receiving sequence original position d of timing metric, and be initialized with 0, the method bag Include following steps：

(1) the signal r (n+d) for receiving, is divided into 1,2 ..., M₀Road, respectively postpone 1,2 ..., M₀After individual sampling clock To r (n+d-1), r (n+d-2) ..., r (n+d-M₀), n=m, 1 ..., N_t- 1,2≤N_t≤N_s, M₀≤N_t；

(2), will receive signal r (n+d) respectively with r (n+d-1), r (n+d-2) ..., r (n+d-M₀) conjugate multiplication is carried out, Obtain receiving difference sequence { y_m(n, d), n=m ..., N_t-1}；

(3) difference sequence { y will, be received_m(n, d), n=m ..., N_t- 1 }, it is respectively fed to M₀Individual corresponding N_tThe FIR of-m ranks Wave filter, through obtaining M after FIR filter filtering process₀Road exports p (m, d)：

In formula, { w_m(n)=c^*(n-m) c (n), n=m ..., N_t- 1 }, c (n) is leading symbol sequential sampling value；

(4), by M₀Output p (m, d) of path filter carries out averagely, obtaining average value P (d), and receive letter using OFDM Number energy R (d) is normalized to average value P (d), and the normalization for obtaining the symbol timing synchronization method based on FIR structures is determined When measurement M (d)；

(5), d is added 1, repeat step (1)~step (5) is calculated new normalization timing metric M (d), until d =M_s×(N_s+N_cp) -1, obtains frame in M altogether_s×(N_s+N_cp) individual timing metric, into step (6)；

(6), calculate a frame in and amount to M_s×(N_s+N_cp) individual timing metric maximum, the d corresponding to the value is OFDM Symbol Timing estimator

The computing formula of the step (2) is：

y_m(n, d)=r^*(n+d-m) r (n+d),

Wherein, m takes positive integer, and span is 1≤m≤M₀。

Use OFDM described in the step (5) receives the calculating that signal energy R (d) is normalized to average value P (d) Formula is：

Wherein, signal energy is received to be expressed as：

Compared with the prior art, the invention has the advantages that：

1), the OFDM symbol timing metric method of estimation based on FIR filter proposed by the invention employs FIR filtering Device calculates timing metric, has versatility in Project Realization；

2), the OFDM symbol timing metric method of estimation based on FIR filter proposed by the invention is by M₀Individual difference phase The FIR filter that pass value p (m, d) is used uniformly across different coefficients is calculated, and realizes modularized processing；

3), the OFDM symbol timing metric method of estimation based on FIR filter proposed by the present invention is obtained with pulse The timing metric of characteristic is consistent with the theoretical performance of timing estimation algorithms；

4), estimate that structure can be realized according to the OFDM symbol timing metric based on FIR filter proposed by the invention The real time high-speed treatment of signal stream is received in ofdm demodulator；

5), estimate that structure is effectively reduced according to the OFDM symbol timing metric based on FIR filter proposed by the invention Implementation complexity, reduces shared hardware resource, has saved Project Realization cost.

Brief description of the drawings

Fig. 1 is that OFDM timing metric of the present invention based on FIR filter estimates structure；

Fig. 2 is the timing metric value that embodiment of the present invention emulation is obtained.

Specific embodiment

Below in conjunction with the accompanying drawings and specific embodiment, specific embodiment of the invention is described in further detail.Following reality Example is not limited to use scope of the invention for illustrating the present invention.

By taking typical ofdm communication system as an example, OFDM modulators have following spy respectively with ofdm signal in demodulator Levy：

1), in OFDM modulators, by subcarrier maps by length be N_uMPSK/MQAM modulation multiple random sequence Length is mapped to for N_sOFDM frequency domain sequences, and N is carried out to the frequency domain sequence after mapping_sWhen the IFFT conversion of point obtains OFDM Domain symbol, extracts last N from OFDM time-domain symbols_cpIn OFDM time-domain symbols front end, final (Ncp is before circulating for individual point splicing Sew length) the OFDM time-domain symbols with protection interval are obtained, can be expressed as：

Wherein, X (k) is the data on k-th subcarrier, N_sIt is the size of IFFT/FFT transform sizes, N_cpBefore circulation Sew sample number.Each symbol includes N=N_s+N_cpIndividual sample value.

2), per M of the frame OFDM transmission signal comprising setting_sIndividual OFDM symbol, wherein, first OFDM symbol is known Leading symbol, when OFDM time-domain symbols are leading symbol C={ c (n), n=0 ..., N_sWhen -1 }, c (n)=x (n).

3), in ofdm demodulator, OFDM receives signal and generally there is the intersymbol interference introduced by channel multi-path time delay, Therefore receive signal and include symbol time offset and carrier frequency offset, it is expressed as

Wherein, ε is unknown symbol time offset, and v is normalized carrier frequency offset, and w (n) is that variance isZero Average multiple Gauss noise, h (m) is the impulse response of multidiameter fading channel, and L is channel memory length.

Ofdm signal feature based on said modulator Yu demodulator, for OFDM solutions under telemetry communication complexity multipath channel Symbol Timing measurement is estimated to realize complicated problem in adjusting device, the invention provides a kind of OFDM demodulation based on FIR filter The Symbol Timing measurement method of estimation of device.The method will receive signal and is divided into M first₀Circuit-switched data, postpones M respectively₀After individual clock, Conjugate multiplication will be carried out with current Received Signal respectively per circuit-switched data, the result on every road will then be sent into FIR filter, it is finally right M₀The output of path filter is carried out averagely, and average value is normalized using signal energy is received, and obtains normalization timing Measurement is estimated.

It is N to define for calculating the leading symbol length of timing metric estimation_t(2≤N_t≤N_s), N_tValue is bigger, timing Estimate that performance is better, work as N_sWhen=1024,128≤N can be set_t≤1024.It is proposed by the invention based on FIR filter OFDM symbol timing metric method of estimation has steps of：

(1) the signal r (n+d) for receiving, is divided into 1,2 ..., M₀Road, respectively postpone 1,2 ..., M₀After individual sampling clock To r (n+d-1), r (n+d-2) ..., r (n+d-M₀), the d=0,1 ..., M_s×(N_s+N_cp) -1, n=m, 1 ..., N_t- 1,2≤ N_t≤N_s, M₀It is positive integer, span is M₀≤N_t；

(2), will receive signal r (n+d) respectively with r (n+d-1), r (n+d-2) ..., r (n+d-M₀) conjugate multiplication is carried out, Obtain M₀Individual reception difference sequence { y_m(n, d), n=m ..., N_t- 1 }, specific computing formula is：

y_m(n, d)=r^*(n+d-m) r (n+d), n=m, 1 ..., N_t- 1,

Wherein, m takes positive integer, and span is 1≤m≤M₀, M₀For maximum difference is spaced, r^*(n+d-m) it is r (n+d-m) Conjugate operation；

(3), by M₀Individual reception difference sequence { y_m(n, d), n=m ..., N_t- 1 }, it is respectively fed to M₀Individual corresponding N_t- m ranks FIR filter, through obtaining M after FIR filter filtering process₀Road exports p (m, d)：

In formula, { w_m(n)=c^*(n-m) c (n), n=m ..., N_t-1}；C (n) is leading symbol sequential sampling value；

(4), by M₀Output p (m, d) of path filter carries out averagely, obtaining average value P (d), and receive letter using OFDM Number energy R (d) is normalized to average value P (d), and the normalization for obtaining the symbol timing synchronization method based on FIR structures is determined When measurement M (d)：

Wherein, signal energy is received to be expressed as：

(5), d is added 1, repeat step (1)~step (5) is calculated new normalization timing metric M (d), until d =M_s×(N_s+N_cp) -1, obtains frame in M altogether_s×(N_s+N_cp) individual normalization timing metric, into step (6)；

(6), calculate a frame in and amount to M_s×(N_s+N_cp) individual normalization timing metric maximum, the d corresponding to the value is It is OFDM symbol timing estimation amount

The timing estimation amountIt is the position at the timing metric peak value of a frame data, there was only 1 peak in this segment data Value, and d is all of data of the frame in：D=0,1 ..., M_s×(N_s+N_cp)-1。

Because normalization timing metric M (d) has pulse characteristic, one rational door of setting can also be used when solving d The mode of limit, when M (d) is more than default thresholding, it is believed that have found the peak value of M (d), and then obtain Symbol Timing estimator

Estimate that structure is as shown in Figure 1 according to the OFDM symbol timing metric based on FIR filter proposed by the invention.Figure In, the multiplier in multiplier, all FIR filters that each branch road difference conjugate multiplication is used is complex multiplier. Timing metric can be calculated by the modularized processing of the FIR filter of different coefficients in Project Realization, realize ofdm demodulator The middle real time high-speed treatment for receiving signal stream, effectively reduces implementation complexity, reduces shared hardware resource, saves Project Realization cost.

Embodiment：

There are two sections of leading symbols of repetitive structure for using time domain, can be only with wherein one section as known pilot Symbol, is now N for calculating the leading symbol length of timing metric estimation_t(N_t≤N_s/ 2), N_tThe lower limit of span can root Estimate that performance is set according to timing metric, work as N_sWhen=1024,128≤N is set_t≤512.In FPGA Project Realizations, if OFDM systems System IFFT sizes N_s=256, the symbol lengths for calculating timing metric estimation are N_t=128, because the number of leading symbol 256 According to that can be divided into two sections of identical, first paragraph 128 is equal with 128 data of second segment, the local phase used in timing estimation Pass sequence is 128 of wherein one section, so having two peak values during leading symbol timing estimation.Entered using two frame OFDM symbols Row emulation, is observed by Chipscope softwares and estimates structure using the timing metric based on FIR filter proposed by the invention The timing metric for obtaining is as shown in Figure 2.As shown in Figure 2, estimated using the timing metric based on FIR filter proposed by the invention The timing metric that meter structure is obtained has the peak value of impulse form, and peak amplitude is about 85000000, and bottom is made an uproar less than 11000000, I.e. peak amplitude is made an uproar 7 times of amplitude the bottom of more than.

Under a multipath fading channel, the OFDM timing metrics based on FIR filter estimate that structure is obtained with pulse spy The timing metric of property can effectively improve OFDM symbol Timing Synchronization performance under multidiameter fading channel, it is adaptable to complicated multipath channel TTC ＆ DT Systems under characteristic.

The above, only one specific embodiment of the present invention, but protection scope of the present invention is not limited thereto, and appoints What those familiar with the art the invention discloses technical scope in, the change or replacement that can be readily occurred in, all Should be included within the scope of the present invention.

The content not being described in detail in description of the invention belongs to the known technology of professional and technical personnel in the field.

Claims (3)

1. it is a kind of to be based on FIR filter OFDM symbol timing metric method of estimation, M is included in ofdm signal frame_sIndividual OFDM symbol, M_sIt is positive integer, M_s>=2, wherein, first OFDM symbol is known leading symbol, and each symbol includes N=N_s+N_cpIt is individual to adopt Sample value, N_sIt is the IFFT/FFT transform sizes of OFDM symbol, N_cpIt is the Cyclic Prefix sample number of OFDM, defines for calculating timing The OFDM receiving sequence original position d of measurement, and it is initialized with 0, it is characterised in that comprise the following steps：

(1) the signal r (n+d) for receiving, is divided into 1,2 ..., M₀Road, respectively postpone 1,2 ..., M₀R (n are obtained after individual sampling clock +d-1)、r(n+d-2)…、r(n+d-M₀), n=m, 1 ..., N_t- 1,2≤N_t≤N_s, M₀≤N_t；

(2), will receive signal r (n+d) respectively with r (n+d-1), r (n+d-2) ..., r (n+d-M₀) conjugate multiplication is carried out, connect Astigmat sub-sequence { y_m(n, d), n=m ..., N_t-1}；

(3) difference sequence { y will, be received_m(n, d), n=m ..., N_t- 1 }, it is respectively fed to M₀Individual corresponding N_tThe FIR filtering of-m ranks Device, through obtaining M after FIR filter filtering process₀Road exports p (m, d)：

$p(m,d)=\underset{n=0}{\overset{N_t-m-1}{\Σ}}{y}_m(n,d)\·w_m^{*}\left(n\right)$

In formula, { w_m(n)=c^*(n-m) c (n), n=m ..., N_t- 1 }, c (n) is leading symbol sequential sampling value；

(4), by M₀Output p (m, d) of path filter carries out averagely, obtaining average value P (d), and receive signal energy using OFDM R (d) is normalized to average value P (d), obtains the normalization timing metric M of the symbol timing synchronization method based on FIR structures (d)；

(5), d is added 1, repeat step (1)~step (5) is calculated new normalization timing metric M (d), until d=M_s× (N_s+N_cp) -1, obtains frame in M altogether_s×(N_s+N_cp) individual timing metric, into step (6)；

(6), calculate a frame in and amount to M_s×(N_s+N_cp) individual timing metric maximum, the d corresponding to the value is OFDM symbol Timing estimation amount

2. according to claim 1 a kind of based on FIR filter OFDM symbol timing metric method of estimation, its feature exists In：The computing formula of the step (2) is：

y_m(n, d)=r^*(n+d-m) r (n+d),

Wherein, m takes positive integer, and span is 1≤m≤M₀。

3. according to claim 1 a kind of based on FIR filter OFDM symbol timing metric method of estimation, its feature exists In：Use OFDM described in the step (5) receives the computing formula that signal energy R (d) is normalized to average value P (d) For：

$M\left(d\right)=\frac{P\left(d\right)}{{\left(R\right(d\left)\right)}^2}$

Wherein, signal energy is received to be expressed as：

$R\left(d\right)=\underset{n=0}{\overset{N_t-1}{\Σ}}|r(n+d){|}^2.$