CN1764177A - Timing synchronizing method for OFDM system - Google Patents

Abstract

A kind of OFDM (OFDM) system time synchronization method, regularly tolerance (timing metric) is defined as, in former and later two adjacent correlation lengths, the correlation function of training symbol is promptly regularly measured the degree of correlation that has characterized two interior training symbols of segment length.This method is applicable to the ofdm system that all allow to add at the Frame front end training symbol.It is characterized in that, comprise the steps: step 1: the length of selecting related operation (perhaps correlation window); Step 2: the energy that calculates received signal coherent signal in two adjacent correlation lengths; Step 3: the energy of asking for received signal in these two adjacent correlation lengths is long-pending; Step 4: set regularly synchronous decision threshold; Step 5: calculate the timing tolerance of received signal, provide regularly synchronized result according to preset threshold.

Description

Timing synchronizing method for OFDM system

Technical field

The present invention relates to a kind of OFDM (OFDM) system time synchronization method.(technical field)

Background technology

OFDM (OFDM) is the high-speed wideband transmission technology under a kind of wireless environment.Frequency selectivity response in view of wireless channel, the main thought of orthogonal frequency division multiplexi is in frequency domain institute to be divided into a plurality of subchannels to channel, use a subcarrier to modulate on each subchannel, subcarrier sequence parallel transmission, therefore the frequency response of subchannel is a relatively flat, and signal bandwidth has significantly reduced intersymbol interference less than the correlation bandwidth of channel; The subcarrier spectrum that covers improves the availability of frequency spectrum of channel because orthogonal property can not produce the phase mutual interference mutually.

Because the advantage of strong anti-channel distortion and high spectrum utilization, orthogonal frequency division multiplexi is used widely in wireless lan (wlan), digital audio broadcasting (DAB), digital video broadcasting (DVB) and ADSL (Asymmetric Digital Subscriber Line) many fields such as (ADSL), also may be the wireless access technology of cellular mobile network of future generation.

Synchronously for the digital communication system link that is absolutely necessary.And in the OFDM technology relate to synchronously frame synchronization, symbol regularly, many aspects such as carrier frequency synchronization and sample frequency tracking.Wherein regularly synchronously/and symbol regularly is to play crucial effects in whole synchronizing process, its accuracy directly affects the communication quality of whole system.

So far, synchronous for the timing of ofdm system, multiple thinking has been proposed in various documents.(MMSE) standard of Minimum Mean Square Error wherein and maximum likelihood (ML) standard are only applicable to the continuous type orthogonal frequency division multiplexi and use owing to there be not the rate of false alarm under the data symbol arrival situation very big.Maximal correlation standard (MC) worsens significantly in transmission of multipath Rayleigh fading channel or non-constant envelope modulation technology occasion personality.The time synchronization method that proposes with Schmidl and Cox is a comparison other emphatically, the performance of the time synchronization method that evaluation and test the present invention proposes.

Summary of the invention

The objective of the invention is to, a kind of OFDM (OFDM) system time synchronization method is provided, realize more accurate regularly synchronized, improve the integrated communication quality of system; With respect to existing time synchronization method, the method that the present invention proposes has better estimate variance performance when carrying out fixed cycle operator.

The present invention relates to a kind of OFDM (OFDM) system time synchronization method, regularly tolerance (timing metric) is defined as, in former and later two adjacent correlation lengths, the correlation function of training symbol is promptly regularly measured the degree of correlation that has characterized two interior training symbols of segment length.This method is applicable to the ofdm system that all allow to add at the Frame front end training symbol.It is characterized in that, comprise the steps:

Step 1: the length of selecting related operation or correlation window;

Step 2: the energy that calculates received signal coherent signal in two adjacent correlation lengths;

Step 3: the energy of asking for received signal in these two adjacent correlation lengths is long-pending;

Step 4: set regularly synchronous decision threshold;

Step 5: calculate the timing tolerance of received signal, provide regularly synchronized result according to preset threshold.

Wherein the length of related operation can equal the length of training symbol, also can be greater than the length of training symbol, but the positive integer that must equal the training symbol cycle is doubly.

Wherein calculate the energy of received signal coherent signal in two adjacent correlation lengths.

Wherein set regularly synchronous decision threshold, guarantee in the identical wrong report allowance that has under data symbol and the free of data symbol situation.

Wherein when the timing tolerance that receives data is greater than or equal to setting threshold, with distinguishable time of minimum as the timing synchronized result; Otherwise not thinking has data symbol to arrive.

If wherein suitably reduce the length of related operation, can be applicable to also that in this structure the thick synchronization frame of ofdm system detects less than the one-period length of training symbol.

Description of drawings

For further specifying technology contents of the present invention, below in conjunction with embodiment and accompanying drawing describes in detail as after, wherein:

Fig. 1 is average and the variogram of regularly measuring when data symbol is arranged;

Average that Fig. 2 regularly measures when being the free of data symbol and variogram

Embodiment

Accompanying drawing is the timing tolerance example under the multipath Ruili fading channel that obtains of basic ideas according to the present invention, and Fig. 1 and Fig. 2 are respectively to having data symbol and free of data symbol situation.Based on the IEEE802.11a standard, consider general indoor environment, the root mean square (RMS) of setting multipath channel expands to 100ns, and the white Gaussian noise of stack makes that the signal to noise ratio (snr) of received signal is 10dB.For the validity of verification algorithm, additional length is the white Gaussian noise of 100 sampling period length before normal frame signal, and its power equals to be superimposed upon that noise power equates on the normal signal.

The present invention relates to a kind of OFDM (OFDM) system time synchronization method, regularly tolerance (timing metric) is defined as, in former and later two adjacent relevant (being somebody's turn to do " being correlated with " is a kind of physical quantity of weighing similarity degree between two signals) length, the correlation function of training symbol is promptly regularly measured the degree of correlation that has characterized two interior training symbols of segment length.This method is applicable to the ofdm system that all allow to add at the Frame front end training symbol.It is characterized in that, comprise the steps:

Step 1: the length of selecting related operation (perhaps correlation window);

Step 2: the energy that calculates received signal coherent signal in two adjacent correlation lengths;

Step 3: the energy of asking for received signal in these two adjacent correlation lengths is long-pending;

Step 4: set regularly synchronous decision threshold;

Step 5: calculate the timing tolerance of received signal, provide regularly synchronized result according to preset threshold.

This method is applicable to the ofdm system that all allow to add at the Frame front end circuit training symbol.The length of related operation can equal the length of training symbol, also can be greater than the length of training symbol, but the positive integer that must equal the training symbol cycle is doubly.Calculate the energy of received signal coherent signal in two adjacent correlation lengths.The energy of asking for received signal in these two adjacent correlation lengths is long-pending.Set regularly synchronous decision threshold, and rule of thumb, guarantee substantially in the identical wrong report allowance that has under data symbol and the free of data symbol situation.When the timing tolerance that receives data is greater than or equal to setting threshold, with distinguishable time of minimum as the timing synchronized result; Otherwise not thinking has data symbol to arrive.If suitably reduce the length (can less than the one-period length of training symbol) of related operation, also be applicable to thick (the frame detection) synchronously of ofdm system in this structure.

Be that example describes with IEEE 802.11a wireless local area network (WLAN) system emphatically below, in transmitter, the power normalization complex baseband equivalent expression of radiofrequency signal is as giving a definition:

$x\left(n\right)=\frac{1}{\sqrt{N}}\underset{\mathrm{\κ}=0}{\overset{N-1}{\mathrm{\Σ}}}x\left(k\right)\mathrm{exp\; j}(2\mathrm{\πnk}/N),(n,k=\mathrm{0,1},K,N-1)$

Wherein, X (K) and x (n) are respectively frequency domain and time domain discrete time signal, and N is the size of IFFT/FFT, and j is a complex unit.

Send signal through after the multipath channel, the sampled signal of receiver can be expressed as:

$r\left(n\right)=\frac{1}{\sqrt{N}}\underset{i=0}{\overset{H-1}{\mathrm{\Σ}}}x(n-i)h\left(i\right)\exp \left(j2\mathrm{\πi\Δf}\right)+w\left(n\right),(n=\mathrm{0,1},K,N-1)$

Wherein, h (i) is the sampled value of channel impulse response, and H is the number of channel impulse response sampled value, and Δ f is the normalized frequency skew, and w (n) is the envelope sampled value of additive white Gaussian noise.

The coherent signal that defines two groups of adjacent L sampled signals is P (n):

$\left(n\right)=\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}{r}_{n+l}^{*}{r}_{n+l+L}$

And the energy R (n) of the sampled signal of length L, can be expressed as:

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

The regularly synchronous timing tolerance of define system is M (n):

$M\left(n\right)=\frac{{\left|P\left(n\right)\right|}^2}{R\left(n\right)\·R(n+L)}$

Might as well suppose that received signal is by signal component s _nWith noise component(s) w _nForm, and the average power of signal component and noise component(s) is respectively 2 σ _s ²With 2 σ ₀ ², promptly

$E\left[\mathrm{Re}{\left\{s_n\right\}}^2\right\}=E\left[\mathrm{Im}{\left\{s_n\right\}}^2\right]={\mathrm{\σ}}_s^2$

$E\left[\mathrm{Re}{\left\{w_n\right\}}^2\right]=E[\mathrm{Im}{\left\{w_n\right\}}^2={\mathrm{\σ}}_0^2$

Wherein Re{} and Im{} represent real and imaginary part, and E{} represents expectation of a random variable.Define signal to noise ratio in addition $\mathrm{SNR}={\mathrm{\σ}}_s^2/{\mathrm{\σ}}_0^2.$

In most practical application, frequency domain signal X (k) can be thought discrete uniformly distributed random variable, and MQAM and MPSK are typical examples.All frequency domain data X (k) therefore are N discrete independent uniformly distributed random variables from identical planisphere, and then time-domain sampling value x (n) is incoherent.In the IFFT algorithm, N time-domain sampling value is the linear combination of N discrete uniformly distributed random variable, therefore, by central-limit theorem as can be known, if N is enough big, that is to say that each phase plus item can not produce decisive influence for summation, can know the time-domain sampling value approximate Gaussian distribution of symbol by inference.So, the time-domain sampling value of symbol can be assumed to independent identically distributed Gaussian random variable, and plural Short Training symbol real part and imaginary part also are independent identically distributed Gaussian random variables, the variance that has zero-mean and equate.

According to central-limit theorem, the sampling instant n of data symbol is being arranged _Opt, Re{P (n _Opt) can think the stochastic variable of Gaussian Profile, its average is 2L σ _s ⁴, variance is $2L({2\mathrm{\σ}}_s^4+{2\mathrm{\σ}}_0^2{\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^4);$ And Im{P (n _Opt) also Gaussian distributed, average is zero, variance is $2L({2\mathrm{\σ}}_0^2{\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^4).$ So stochastic variable | P (n _Opt) | ²Average equal $4L[{\mathrm{L\σ}}_s^4+{({\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^2)}^2].$

The power of single sampled point | r _N+l| ²Can be expressed as $({\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^2){\mathrm{\χ}}_2^2$ Distribute, wherein x ₂ ²The x of expression average 2 variances 4 ²Distribute.According to central-limit theorem, power entry R (n _Opt) Gaussian Profile, average equals $2L({\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^2),$ Variance equals $4L{({\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^2)}^2.$ Because variable | P (n _Opt) | ²And R (n _Opt) R (n _Opt+ L) standard deviation is much smaller than separately average, then regularly tolerance at a n _OptAverage be expressed as:

$E\left[M\left(n_{\mathrm{opt}}\right)\right]=\frac{{\mathrm{L\σ}}_s^4+{({\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^2)}^2}{L{({\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^2)}^2}.$

If signal to noise ratio snr ∈ (0dB, 20dB), its variance is approximately equal to:

$\mathrm{var}\left[M\left(n_{\mathrm{opt}}\right)\right]=\frac{L^2-2L-1}{L^2{(L+1)}^2}\·\frac{{\mathrm{L\σ}}_s^4+{2\mathrm{L\σ}}_s^2{\mathrm{\σ}}_0^2+{\mathrm{\σ}}_0^4}{{({\mathrm{\σ}}_s^2+{\mathrm{\σ}}_0^2)}^3}$

And under the high s/n ratio condition, can obtain littler variance.Sampling instant n at the free of data symbol _Nop, received signal is simplified to noise component(s).Received signal is obeyed zero-mean σ ₀ ²The Gaussian Profile of variance, thereby variable R e{P (n _Nop) and Im{P (n _Nop) also be Gaussian Profile, and average is zero, variance is 2L σ ₀ ⁴| P (n _Nop) | ²Distribution be equal to 2L σ ₀ ⁴x ₂ ², average is 4L σ ₀ ⁴And variance is 16L ²σ ₀ ⁸At this moment, | r _N+l| ²Probability density function be have 2 the dimension degrees of freedom, average 2 σ ₀ ², variance 4 σ ₀ ⁴Center x ²Distribute, therefore, according to central-limit theorem, R (n _Nop) be Gaussian Profile, average 2L σ ₀ ²And variance is 4L σ ₀ ⁴At sampled point n _Nop, the standard deviation of molecule denominator of regularly measuring definition is all much smaller than average separately, and then regularly tolerance can be reduced to:

$E\left[M\left(n_{\mathrm{nop}}\right)\right]=\frac{1}{L}$

Variance is:

$\mathrm{var}\left[M\left(n_{\mathrm{nop}}\right)\right]=\frac{L^2-2L-1}{L^4}$

It is synchronous to suppose to carry out under the situation of 15dB signal to noise ratio timing, and the length of setting correlation computations is 32 sampling periods, i.e. the length of two Short Training symbols in the IEEE802.11a standard.According to aforesaid conclusion, when having data symbol to arrive, regularly the desired value of tolerance equals 0.9709, and standard deviation approximately is 0.0290.When not having data symbol to arrive, regularly the desired value of tolerance equals 0.0313, and the corresponding standard difference is 0.0302.If the synchronous decision threshold of setting of timing is 0.3.So, when having data symbol to arrive, the judgement allowance that wrong conclusion do not occur equals:

$\frac{0.9709-0.5}{0.0290}\≈16.24$

When not having data symbol to arrive, the allowance that wrong report occurs equals:

$\frac{0.5-0.0313}{0.0302}\≈15.52$

Emulation experiment is that finish on the basis with IEEE 802.11a standard.The length of correlation computations is set to 16, is a Short Training symbol period.Wireless channel is a model with the multipath Rayleigh fading channel.The implication of each curve is shown in legend among the figure.

From the contrast of Fig. 1 and Fig. 2, can see, no matter have or not data symbol to arrive, the average basically identical of two kinds of methods, difference is regularly synchronous variance aspect of performance.

When data symbol is arranged, in 0～40dB interval, choose uniformly-spaced on 5, the estimate variance mean value that the method that Schmidl method and the present invention propose obtains is respectively 0.0051 and 0.0035.During the free of data symbol, the Schmidl method is respectively 1.606,2.314,9.814,110.7 and 888.7 times with the estimate variance ratio of proposition method.

From above conclusion, the method that this paper proposes can realize accurate more regularly simultaneous operation.

Claims (6)

1, a kind of timing synchronizing method for OFDM system, regularly tolerance is defined as, in former and later two adjacent correlation lengths, the correlation function of training symbol, promptly regularly measure the degree of correlation that has characterized two interior training symbols of segment length, this method is applicable to the ofdm system that all allow to add at the Frame front end training symbol, it is characterized in that, comprises the steps:

Step 1: the length of selecting related operation or correlation window;

Step 2: the energy that calculates received signal coherent signal in two adjacent correlation lengths;

Step 3: the energy of asking for received signal in these two adjacent correlation lengths is long-pending;

Step 4: set regularly synchronous decision threshold;

Step 5: calculate the timing tolerance of received signal, provide regularly synchronized result according to preset threshold.

2, a kind of timing synchronizing method for OFDM system according to claim 1, it is characterized in that, wherein the length of related operation can equal the length of training symbol, also can be greater than the length of training symbol, but the positive integer that must equal the training symbol cycle is doubly.

3, a kind of timing synchronizing method for OFDM system according to claim 1 is characterized in that, wherein calculates the energy of received signal coherent signal in two adjacent correlation lengths.

4, a kind of timing synchronizing method for OFDM system according to claim 1 is characterized in that, wherein sets regularly synchronous decision threshold, guarantees in the identical wrong report allowance that has under data symbol and the free of data symbol situation.

5, a kind of timing synchronizing method for OFDM system according to claim 1 is characterized in that, wherein when the timing tolerance that receives data is greater than or equal to setting threshold, with distinguishable time of minimum as the timing synchronized result; Otherwise not thinking has data symbol to arrive.

6, a kind of timing synchronizing method for OFDM system according to claim 2, it is characterized in that, if wherein suitably reduce the length of related operation, can be applicable to also that in this structure the thick synchronization frame of ofdm system detects less than the one-period length of training symbol.

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