CN101848180A - OFDM symbol timing synchronization method under multipath channel - Google Patents

Abstract

The invention belongs to the field of wireless or wired communication, and relates to an OFDM synchronization method, in particular to an OFDM symbol timing synchronization method under a multipath channel. The OFDM symbol timing synchronization method under the multipath channel comprises the following steps that: when OFDM training symbols are utilized to carry out symbol timing synchronization, a two-step type synchronization system structure is adopted, and bold symbol synchronization can limit first-path time delay within a less search region; the bold symbol synchronization utilizes global optimization and search performance of a genetic algorithm, and simultaneously carries out channel estimation and fine symbol timing; and the genetic algorithm utilizes convolution relation of receiving signals and sending the training symbols, and can correctly estimate the first-path time delay. Compared with the conventional method, the method of the invention obviously improves the estimation accuracy and timing flexibility of synchronous symbols, and can still estimate out the first-path time delay correctly even under the condition of very low signal-to-noise ratio. The method of the invention also can be used for the fields of other relevant projects.

Description

OFDM symbol timing synchronization method under a kind of multipath channel

Technical field

The invention belongs to wireless or the wire communication field, relate to the OFDM method for synchronous, the OFDM symbol timing synchronization method under particularly a kind of multipath channel.

Background technology

It is a kind of effective modulation technique of antagonism multipath fading that prior art discloses OFDM (OFDM).By inserting a Cyclic Prefix at the symbol front end, can effectively eliminate intersymbol interference (Inter-Symbol Interference, ISI).Owing to adopted the overlap technique of subcarrier, the availability of frequency spectrum of OFDM is very high simultaneously.It has been widely used in aspects such as digital audio broadcasting (DAB), digital video broadcasting (DVB), high definition TV (HDTV) and wireless lan (wlan), becomes a standard technique in many high speed data transfer field.Thought the major technique that next generation mobile communication (4G) most probable is selected for use by most of people, its standard is among formulating.

One of shortcoming of OFDM technology is more much higher than single-carrier system to the requirement of symbol timing and carrier frequency synchronization.Therefore, in the Base-Band Processing algorithm of receiving terminal, be a very crucial task synchronously.

OFDM is divided into timing synchronization and carrier frequency synchronization synchronously.Fig. 1 has shown the position of synchronization module in ofdm system.Under wireless multipath channel, the task of timing synchronization is exactly the time delay that estimates first footpath.

Usually, establishing N and be the FFT length in the ofdm system, also is the number of subcarrier simultaneously, N _gBe the circulating prefix-length of OFDM symbol, the length of complete OFDM symbol is N+N _gTraditional OFDM symbol timing synchronization method has two kinds:

1.) do not have data-aided blind synchronization method, this method utilizes the statistic correlation of the OFDM Cyclic Prefix copy part corresponding with it to carry out symbol regularly.Its estimation equation is (as shown in Figure 2):

Wherein

$\mathrm{\φ}\left(m\right)=\frac{1}{2}\underset{k=m}{\overset{m+N_g-1}{\mathrm{\Σ}}}{\left|r\left(k\right)\right|}^2+{\left|r(k+N)\right|}^2$

Under the multipath channel situation, the statistic correlation of Cyclic Prefix can be destroyed by multipath effect, and it is the time delay in the maximum footpath of energy that the symbol that estimates is regularly put, rather than the first footpath time delay.So estimation is the result have than mistake, is not suitable for the multipath channel environment.

2.) data-aided method for synchronous is arranged, this kind method for synchronous is that cost improves estimated performance to sacrifice certain bandwidth, is widely adopted in the OFDM of demblee form transmission system.Traditional data householder method is carried out timing synchronization by means of an identical training symbol of front and back two parts.Its estimation equation is as follows:

$M\left(d\right)=\frac{{\left|P\left(d\right)\right|}^2}{{\left(R\left(d\right)\right)}^2}---\left(3\right)$

Wherein

$P\left(d\right)=\underset{m=0}{\overset{L-1}{\mathrm{\Σ}}}\left(r_{d+m}^{*}{r}_{d+m+L}\right)$

$R\left(d\right)=\underset{m=0}{\overset{L-1}{\mathrm{\Σ}}}{\left|r_{d+m+L}\right|}^2$

L is half of training symbol length.There is a flat site in this estimator near symbol is regularly put, so estimation precision is not high.

Summary of the invention

The objective of the invention is to overcome the deficiencies in the prior art, the OFDM under a kind of multipath channel is provided symbol timing synchronization method, the present invention utilizes an OFDM training symbol of transmitting terminal transmission, can correctly estimate the time delay in first footpath.

The present invention proposes the OFDM symbol timing synchronization method under a kind of multipath channel.This method is applicable to demblee form OFDM transmission system.By adding the training symbol that receiving terminal is known in each OFDM packet front, receiving terminal utilizes acknowledge(ment) signal and training symbol then, can correctly estimate the first footpath time delay.

Particularly, the present invention utilizes a known training symbol of receiving terminal to carry out timing synchronization, the timing synchronization architecture that is proposed comprised for two steps: thick sign synchronization and smart sign synchronization, described thick timing synchronization utilization is based on the maximal possibility estimation of Cyclic Prefix, obtain regularly point of a thick symbol, and first footpath is postponed to be limited in the less region of search, described smart timing synchronization adopts genetic algorithm to finish, utilize received signal and the convolution property that sends signal to carry out channel estimating and the timing of smart symbol, choosing of region of search length can be compromised on speed and precision.

Method for synchronous proposed by the invention comprises thick sign synchronization and smart sign synchronization two parts, wherein, thick sign synchronization adopts traditional maximal possibility estimation algorithm based on Cyclic Prefix, this algorithm can estimate regularly point of a thick symbol, the first footpath time delay is limited in thick symbol regularly puts near the less region of search for smart sign synchronization use; Smart sign synchronization adopts genetic algorithm to carry out channel estimating and smart symbol regularly, by following method:

A) first point of getting in the region of search is starting point, the data block that length is identical with training symbol is with reference to received signal, genetic algorithm is attempted to look for one group of discrete channel impulse response by iteration and is allowed training symbol be result after the convolution with it then, with the reference received signal minimum mean square deviation is arranged;

B) all do above iterative search for each point in interval, obtain the mean square deviation of corresponding each starting point;

C) the first footpath time delay that will estimate exactly of the pairing timing point of lowest mean square difference, thus smart symbol finished regularly.

The inventive method comprises the steps:

One, transmitting terminal

Transmitting terminal utilizes anti-fast fourier transform that N data are modulated on the subcarrier of N quadrature, then the last N of each OFDM symbol _gIndividual data form Cyclic Prefix with the antagonism multipath effect before being inserted into symbol.Sending signal at last can be expressed as:

$x\left(n\right)=\frac{1}{\sqrt{N}}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}X\left(k\right)e^{j2\mathrm{\πnk}/N}$ n＝0，1，2，...，N-1 (5)

Two, multipath channel transmits

Ofdm signal is the process that convolution is done in and discrete channel impulse response by multipath channel.The present invention considers symbol timing error and The noise, and the signal that receiving terminal is seen can be expressed as:

$r\left(n\right)=\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}{h}_{l}x(n-l-\mathrm{\θ})e^{j2\mathrm{\π\ϵn}/N}+w\left(n\right)---\left(6\right)$

Wherein θ represents the first footpath time delay, h ₁The impulse response of expression discrete channel, w (n) is the sample function of a white Gaussian noise.

Three, receiving terminal

Receiving terminal utilizes known training symbol and received signal to carry out timing synchronization.The present invention in order to compromise, adopts the two-step structure simultaneously for the shortcut calculation complexity on speed and precision, promptly thick timing synchronization and smart timing synchronization (as shown in Figure 3), and concrete detailed algorithm is as follows:

A.) thick timing synchronization has adopted traditional maximal possibility estimation algorithm based on Cyclic Prefix, and this algorithm derives under additive white Gaussian noise channel.It utilizes the statistic correlation of the copy part that Cyclic Prefix is corresponding with it in the ofdm signal, designs a time estimator, regularly obtains maximum in the place of putting at symbol, finishes timing synchronization thus.But under multipath channel, the Cyclic Prefix that this algorithm relied on can part be polluted by multipath effect, and statistic correlation also can be destroyed.Its results estimated is the time delay in the maximum footpath of energy normally, rather than the first footpath time delay, so error can be bigger, the point of estimate symbol timing roughly.Described method is limited in thick symbol to the first footpath time delay and regularly puts near the less region of search for smart sign synchronization use, the compromise that has speed and precision of choosing in interval width, the search time of smart sign synchronization can be reduced in less interval, and the interval simultaneously possibility of the first footpath time delay that do not comprise can become big; Bigger interval can become big so that comprise the possibility of the first footpath time delay, but can expend the more time finishes search, and extreme case is not adopt thick timing synchronization and search in whole interval.

B.) smart timing synchronization adopts genetic algorithm to carry out channel estimation and smart timing synchronization simultaneously.

If the training symbol x that sends _tDiscrete impulse response h with channel ₁Be respectively:

$x_t=\left[\begin{array}{cccccc}{x}_1& x_2& x_3& x_4& L& x_{{N+N}_g}\end{array}\right]---\left(7\right)$

h _l＝[h ₁?h ₂?h ₂?h ₄?L h _L] (8)

Thick symbol is regularly put and is θ _Coarse, the region of search is [θ _Coarse-α, θ _Coarse+ α].For received signal, to get first point of the region of search and be starting point, length is N+N _gData block be reference signal x _rGenetic algorithm attempts to find one group of discrete channel impulse response by iterative search

, make training symbol x _tWith

Convolution results after removing a last L-1 data, with reference received signal x _rMinimum Mean Square Error is arranged, simultaneously With h _lMinimum mean square deviation (as shown in Figure 4) is also arranged.Concrete computing formula is as follows:

generate

convolution： $x_{\mathrm{temp}}={\hat{h}}_1\⊗x_t;$

take?partofx _temp?as?reference?signal：x _ref＝x _temp[1：N+N _g]； (9)

mean?square?error：MSE＝|(x _r-x _ref)| ²；

Above process is the process of a continuous iterative search, converge on a fixed value up to mean square deviation, and new iteration can not produce littler mean square deviation, and the search of genetic algorithm can stop.For each the timing point in the region of search, genetic algorithm is all done above iteration, obtains some different mean square deviations at last.Because received signal is a convolution relation with sending training symbol,, thereby finish smart timing synchronization so the timing point of lowest mean square difference correspondence is exactly the first footpath time delay.

When the present invention utilizes the OFDM training symbol to carry out timing synchronization, adopt the two-step synchronization architecture, thick sign synchronization can be limited to the first footpath time delay in the less region of search, smart sign synchronization has been utilized the global optimization and the search performance of genetic algorithm, carries out channel estimating and smart symbol simultaneously regularly; Genetic algorithm has been utilized received signal and has been sent the convolution relation of training symbol, can correctly estimate the first footpath time delay.

Among the present invention, the foundation of system is synchronously:

1.) utilizing the symbol timing synchronization method of training symbol is cost to sacrifice certain bandwidth, exchanging better net synchronization capability for, and is widely used in the OFDM transmission system of demblee form.

2.) though understand variation based on the performance of maximal possibility estimation under multipath channel of Cyclic Prefix, but can obtain regularly point of a thick symbol, and first footpath is postponed to be limited in the less region of search, therefore can be used as the thick symbol timing synchronization method in the synchronous system of two-step.

3.) genetic algorithm has excellent global optimization and search performance, in extensive use and the numerous engineerings field, and under multipath channel, statistical correlation characteristic part among the ofdm signal is destroyed by multipath effect, genetic algorithm is no longer utilized statistic correlation, finishes synchronously then utilize received signal and send the signal convolution property.

The present invention has the following advantages:

1, the symbol timing synchronization method of the present invention's proposition is than conventional method, owing to utilized received signal and the convolution property that sends signal, estimated accuracy greatly improves.

2, the present invention proposes the synchronization architecture of two-step,, can on speed and precision, compromise, improved the flexibility of symbol timing synchronization method according to the length of the region of search.

3, the present invention adopts genetic algorithm as smart signal-timing method, because genetic algorithm has excellent global search and optimizes performance, even under very low state of signal-to-noise, still can correctly estimate the first footpath time delay.

4,, can also use it in other respects because receiving terminal is known to training symbol.

Description of drawings

Fig. 1 is conventional ofdm system block diagram, and wherein synchronization module is positioned at after the receiving terminal down conversion module.

Fig. 2 is traditional maximal possibility estimation algorithm block diagram based on Cyclic Prefix,

Among the figure, r (n) is a received signal, and N is that the FFT of ofdm system counts Z ^NN sampled point of expression time-delay, () ^*Conjugate is got in expression, || it is a constant relevant with system signal noise ratio that absolute value, ρ are asked in expression, and moving sum represents to slide and adds up, and argmax () expression makes the function in the bracket obtain peaked variate-value, and is last

Expression first footpath postpones estimated value.

Fig. 3 is the block diagram of two-step symbol timing synchronization method proposed by the invention,

Among the figure, as can be seen, smart timing synchronization has been utilized thick synchronous estimated result, by each point in the thick region of search that produces synchronously, genetic algorithm all attempts to find one group of discrete channel to satisfy received signal and the convolution relation that sends signal by iterative search, and after iteration was finished each time, regularly point upgraded again and searches for next time, up to whole interval search is finished.

Fig. 4 is an optimization flow chart of regularly putting genetic algorithm at each,

Among the figure, P represents chromosomal quantity in each generation, and chromosome is the genetic algorithm term, be one group of discrete channel also, wherein, G represents the iterations of genetic algorithm, can influence the speed and the precision of algorithm, select training symbol and with reference to the mean square deviation of received signal as target function;

Fitness value also is the term of genetic algorithm, and its size has determined a probability that chromosome is survived in the next generation, determined by target function usually;

Selecting, intersecting and make a variation is the basic operator of genetic algorithm, its objective is that eliminating suboptimum chromosome keeps more excellent chromosome simultaneously.

Embodiment

Below in conjunction with concrete ofdm system, describe the inventive method in detail.

Embodiment 1

If FFT length is N=1024 in the ofdm system, circulating prefix-length is N _g=128.

One, transmitting terminal

Transmitting terminal sends a known training symbol of receiving terminal and uses for symbol timing synchronization method, and length is N+N _gWherein Cyclic Prefix uses as thick timing synchronization.

Two, receiving terminal

Receiving terminal receives the result after one group of training symbol and discrete channel are done convolution, and the Cyclic Prefix part is contaminated.Carry out the first footpath delay estimation according to the synchronous system of two-step then.Specifically, be exactly: at first with reference to formula (6),

$r\left(n\right)=\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}{h}_{l}x(n-l-\mathrm{\θ})e^{j2\mathrm{\π\ϵn}/N}+w\left(n\right)---\left(6\right)$

Wherein θ represents the first footpath time delay, h ₁The impulse response of expression discrete channel, w (n) is the sample function of a white Gaussian noise,

Utilize the statistic correlation of the copy part of Cyclic Prefix and its correspondence, adopt the maximal possibility estimation algorithm to estimate regularly point of a thick symbol, and the first footpath time delay is limited in the smaller region of search.Next enter smart symbol timing process, genetic algorithm is used as starting point to each point in the region of search, and intercepted length is that the data block of N+Ng is for reference to received signal in received signal then.According to formula (9),

generate

convolution： $x_{\mathrm{temp}}={\hat{h}}_1\⊗x_t;$

take?partofx _temp?as?reference?signal：x _ref＝x _temp[1：N+N _g]；(9)

mean?square?error：MSE＝|(x _r-x _ref)| ²；

Genetic algorithm is attempted to seek one group of discrete channel by iterative search and is satisfied training symbol and convolution relation with reference to received signal.

Obtain the mean square deviation of each point in one group of corresponding region of search at last, the timing point of lowest mean square difference correspondence i.e. the first footpath time delay.

Claims (3)

1. the OFDM symbol timing synchronization method under the multipath channel, it is characterized in that adopting the two-step synchronization architecture, by adding the training symbol that receiving terminal is known in each OFDM packet front, receiving terminal utilizes acknowledge(ment) signal and training symbol then, correctly estimates the first footpath time delay.

2. by the OFDM symbol timing synchronization method under the described multipath channel of claim 1, it is characterized in that described two-step synchronization architecture comprises thick sign synchronization and smart sign synchronization two parts; Wherein thick sign synchronization adopts the maximal possibility estimation algorithm based on Cyclic Prefix; Described smart sign synchronization adopts genetic algorithm to carry out channel estimating and regularly smart.

3. by the described method of claim 2, it is characterized in that described smart sign synchronization is by following step:

A) first point of getting in the region of search is starting point, the data block that length is identical with training symbol is with reference to received signal, genetic algorithm is looked for one group of discrete channel impulse response by iteration and is allowed training symbol be result after the convolution with it then, with the reference received signal minimum mean square deviation is arranged;

B) each point in interval is done iterative search, obtain the mean square deviation of corresponding each starting point;

C) the pairing timing point of lowest mean square difference is the first footpath time delay that will estimate, finishes smart symbol regularly.

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