CN101692666B - Method for filtering time domain channel response noises - Google Patents

Abstract

The invention discloses a method for filtering time domain channel response noises. Based on time domain synchronous orthogonal frequency division multiplexing system, and on the basis of eliminating inter-symbol interferences and inter-carrier interferences between a pseudorandom sequence and a data frame body part, the method adopts a minimum mean square error algorithm and simultaneously combines the dynamic evaluation of the noise margin of a time domain channel response to accurately differentiate an effective path and an invalid path in a time domain channel impulse response, can filter most of channel noises and accurately estimate channel characteristics under various channel environments, and greatly improves the performance of a system receiver.

Description

A kind of method of filtering time domain channel response noises

Technical field

The present invention relates to a kind of method of filtering time domain channel response noises.

Background technology

In time-domain synchronization OFDM (TDS-OFDM) system; Signal frame in the transmitting terminal transmission can be divided into two parts of non-overlapping copies in time, and promptly pseudorandom PN sequence

and data division

are as depicted in figs. 1 and 2.Yet, because the existence of multipath effect receives signal at receiving terminal But the two parts by aliasing constitute:

With

Wherein

The linear convolution result of expression PN sequence and channel impulse response, promptly ${\left\{y_{i,k}\right\}}_{k=0}^{M+L-1}={\left\{c_{i,k}\right\}}_{k=0}^{M-1}*{\left\{h_{i,k}\right\}}_{k=0}^{L-1};$

The expression data division

With the linear convolution result of channel impulse response, promptly ${\left\{x_{i,k}\right\}}_{k=0}^{N+L-1}={\left\{s_{i,k}\right\}}_{k=0}^{N-1}*{\left\{h_{i,k}\right\}}_{k=0}^{L-1}.$ Like Fig. 3 and shown in Figure 4.

The aliasing that receives signal under the situation of not protecting prefix, will produce intersymbol interference ICI and intersymbol interference ISI owing to the multidiameter delay expansion causes.Therefore need take certain measure that interference is eliminated, realize receiving separating of signal

and

.

According to Fig. 3 and Fig. 4, the data

that can obtain the reception of i frame can be expressed as:

r _i，k＝u _i，k+n _i，k，0≤k≤M+N+L-2 (1)

Wherein,

$u_{i,k}=\left\{\begin{array}{cc}{x}_{i-1,k+N}+y_{i,k}& 0\≤k\≤L-2\\ y_{i,k}& L-1\≤k\≤M\\ x_{i,k-M}+y_{i,k}& M+1\≤k\≤M+L-1\\ x_{i,k-M}& M+L\≤k\≤N+M\\ x_{i,k-M}+y_{i+1,k-N-M}& M+N\≤k\≤N+M+L-2\end{array}\right.---\left(2\right)$

Therefore, in order to recover the initial data that transmitting terminal sends accurately, channel estimating is carried out channel compensation accurately.In order to obtain time domain channel impulse response accurately, adopt following handling process usually:

1), at first the PN sequence of i frame signal is separated with the Frame body portion; Obtain the linear convolution result of channel impulse response

of PN sequence

and the i frame of i signal frame, promptly eliminating i frame PN after intersymbol interference and the intersymbol interference, to receive signal

as shown in Figure 5.

2), the i frame PN after the above-mentioned elimination interference is received signal zero padding to 2048 point

3), receiver synchronously after, obtain i signal frame PN sequence zero padding to 2048 point

simultaneously

4), utilize the DFT conversion according to

of separation signal and the PN sequence

of i frame; Obtain initial time domain channel impulse response, promptly

${\hat{h}}_{i,k}=\mathrm{IDFT}\left\{\frac{\mathrm{DFT}\left({\left\{{\hat{y}}_{i,k}^{\′}\right\}}_{k=0}^{2047}\right)}{\mathrm{DFT}\left({\left\{c_{i,k}^{\′}\right\}}_{k=0}^{2047}\right)}\right\},$ 0≤k＜2047

(3)

The time domain channel impulse response that calculates according to formula (3) does not have the influence of consideration of noise, under the bigger situation of noise ratio, can have a strong impact on the performance of system receiver.In order to improve the receptivity of system, must take the method for effective filtering noise, to obtain relatively accurate time domain channel estimated value.

Summary of the invention

The objective of the invention is to propose a kind of method of filtering time domain channel response noises, technical problem to be solved is: can the most channel response noises of filtering.

Technical problem of the present invention solves through following technical scheme:

A kind of method of filtering time domain channel response noises is applied to may further comprise the steps under the time-domain synchronous orthogonal frequency-division multiplexing system framework,

(1) receives current frame signal, and pseudo random sequence is separated with the Frame body portion, eliminate corresponding intersymbol interference and intersymbol interference;

(2), and do Fourier transform with pseudo random sequence part zero padding to 2048 point that receives;

(3) receiver synchronously after, pseudo random sequence zero padding to 2048 point that the present frame that obtains is original, and do FFT;

(4) on the basis of the fourier transform results that step (2) and step (3) obtain, utilize least-mean-square error algorithm, obtain initial time domain channel impulse response;

(5) according to initial time domain channel impulse response, the noise power of assessment time domain channel response, and adaptive calculating optimum time domain channel thresholding;

(6), distinguish effective diameter and invalid footpath in the time domain channel response, filtering overwhelming majority noise according to best time domain channel thresholding.

In the optimized technical scheme,

Comprise that also step (7) all is changed to zero with the invalid footpath that obtains in the step (6), further eliminates the noise in the time domain channel response.

The beneficial effect of the present invention and prior art contrast is:

The present invention in conjunction with the self-adaptive estimation of time domain channel response noises, assesses the multipath length of time domain channel response through the frequency domain least-mean-square error algorithm accurately, can filtering overwhelming majority time domain channel response noises.Further, with invalid multipath zero setting, further eliminate the noise in the channel response of assessing.The method that the present invention proposes is handled simple, realize easily, and it is few to take resource, can improve the receptivity of receiver system greatly.

Description of drawings

Fig. 1 is the transmission signal frame sketch map in the background technology;

Fig. 2 is the Time Domain Decomposition sketch map of the transmission signal frame in the background technology;

Fig. 3 is the Time Domain Decomposition sketch map of the reception signal frame in the background technology;

Fig. 4 is the reception signal frame sketch map in the background technology;

Fig. 5 is that the i frame PN after the elimination in the background technology is disturbed receives signal;

Fig. 6 is a filtering time domain channel response noises flow chart in the embodiment of the invention;

Below through concrete execution mode and combine accompanying drawing that the present invention is explained further details.

Embodiment

Among the embodiment, the method for filtering time domain channel response noises comprises the steps:

The first step: receive current frame signal, and pseudo random sequence PN is partly separated with the OFDM frame, eliminate corresponding intersymbol interference and intersymbol interference;

Second step:, and do the FFT conversion with pseudo random sequence part zero padding to 2048 point that receives;

The 3rd step: obtain the original pseudo random sequence of present frame, same zero padding to 2048 point, and do the FFT conversion;

The 4th step: according to the time-domain and frequency-domain transformational relation of linear convolution relation, and utilize MMSE frequency domain filtering algorithm, effectively eliminate partial noise, obtain initial time domain channel impulse response;

The 5th step: according to initial time domain channel impulse response, the noise power of assessment time domain channel response, and adaptive calculating optimum time domain channel thresholding;

The 6th step: accurately judge the multipath length of time domain channel response, accurately distinguish effective diameter and invalid footpath in the time domain channel response simultaneously, remove the various noises in the time domain channel estimated value;

The 7th step: the invalid footpath zero setting in directly time domain channel being estimated, eliminate the most of noise in the time domain channel response, obtain final accurate relatively time domain channel response estimated value.

Wherein, why adopting 2048 points in second and third step, is through the preferred result of inventor.If adopt 1024 points, then count and do not reach effect very little; If adopt more the multiple spot number then cause operand to increase sharply.

Utilize least mean-square error MMSE algorithm in the 4th step, formula (3) is optimized, with the influence of filtering partial noise, promptly

${\hat{h}}_{i,k}=\mathrm{IDFT}\left\{\frac{\mathrm{DFT}\left({\left\{{\hat{y}}_{i,k}^{\′}\right\}}_{k=0}^{2047}\right)}{\mathrm{DFT}\left({\left\{c_{i,k}^{\′}\right\}}_{k=0}^{2047}\right)+\frac{1}{\mathrm{SNR}}}\right\},$ 0≤k＜2047

(4)

Wherein, SNR is for receiving the signal to noise ratio of signal.

Although the time domain channel response that formula (4) calculates has been eliminated partial noise, but still need do further noise filtering and handle.Generally speaking, actual time domain channel response and formula (4) relation of assessing the time domain channel response that obtains can be expressed as:

${\hat{h}}_{i,k}=\left\{\begin{array}{c}{h}_{i,k}+{\tilde{w}}_k,0\≤k\≤L-1\\ {\tilde{w}}_k,L\≤k\≤2047\end{array}\right.---\left(5\right)$

In above-mentioned, L representes the length of true multipath channel, Expression and signal are fully independently, the time domain average is zero, variance is σ ²Additive white Gaussian noise.

Since the residing wireless channel of terrestrial DTV be a broadband, at a high speed, the channel of (tens kilometers) on a large scale.Under abominable overland propagation environment; Its channel circumstance has very strong multipath interference characteristic; Radiofrequency signal is through various reflected by objects, diffraction, refraction; Influence each other after the different paths of process arrive receivers, thereby the various declines of formation signal spectrum cause the length variations of channel multi-path very big.

Therefore, need assess accurately,, therefore utilize adaptive dynamic multi-path thresholding to adapt with it because noise situations is dynamic change to the length of time domain channel response.Because the length of multipath must be less than the length of pseudorandom PN sequence, that is:

L≤N _cp＝M (6)

So, utilize formula (5) and formula (6) that the average power of noise is assessed accurately:

Through assessing the noise situations of above-mentioned time domain channel response, adopt certain decision threshold, accurately assess effective multipath length on the one hand; On the other hand; To be regarded as noise less than the smaller value of this thresholding, directly zero setting, thus effectively eliminate the noise in the time domain channel impulse response.Through this decision threshold, obtain more accurate time domain channel estimated value:

Therefore, how to obtain best time domain channel threshold value h _ThrBecome key.For the domain channel response mean square error minimum that assessment is obtained, promptly

Wherein, the signal to noise ratio snr and the β factor do

$\mathrm{SNR}=\frac{E\left[{\left|X_k\right|}^2\right]}{{\widehat{\mathrm{\σ}}}_n^2}---\left(10\right)$

β＝E[|X _k| ²]E[|X _k| ^-2] (11)

The β factor only depends on the constellation modulation system of employing.

Therefore, best threshold value can be set does

$h_{\mathrm{thr}}^{\mathrm{optimal}}=2\frac{\mathrm{\β}}{\mathrm{SNR}}=2E\left[{\left|X_k\right|}^{-2}\right]{{\widehat{\mathrm{\σ}}}_n}^2---\left(12\right)$

At last; Utilize formula (8) and formula (12), through the noise of dynamic evaluation time domain channel impulse response, according to state of signal-to-noise; The noise of adaptive filtering channel response; Thereby obtain more accurate time domain channel response, to eliminate the distortion effects of multipath effect, the well various declines of compensate for channel to initial data.

The present invention is based on the TDS-OFDM system; In the intersymbol interference (ISI) and inter-carrier interference (ICI) basis eliminated between PN sequence and the OFDM data division, adopt the MMSE filtering algorithm, combine the dynamic evaluation of the noise gate of time domain channel response simultaneously; Effective diameter and invalid footpath in the time domain channel impulse response have been distinguished accurately; Filtering most interchannel noises, estimate the characteristic of channel under the various channel circumstances accurately, greatly improved the performance of system receiver.

Above content is to combine concrete preferred implementation to the further explain that the present invention did, and can not assert that practical implementation of the present invention is confined to these explanations.For the those of ordinary skill of technical field under the present invention, under the prerequisite that does not break away from the present invention's design, can also make some simple deduction or replace, all should be regarded as belonging to protection scope of the present invention.

Claims (2)

1. the method for a filtering time domain channel response noises is applied to it is characterized in that under time-domain synchronization OFDM (TDS-OFDM) system framework: may further comprise the steps,

(1) receives current frame signal, and pseudo random sequence (PN) is separated with the Frame body portion, eliminate corresponding intersymbol interference and intersymbol interference;

(2), and do Fourier transform with pseudo random sequence part zero padding to 2048 point that receives;

(3) receiver synchronously after, the pseudorandom that the present frame that obtains is original (PN) sequence zero padding to 2048 point, and do FFT;

(4) on the basis of the fourier transform results that step (2) and step (3) obtain, utilize least-mean-square error algorithm, obtain initial time domain channel impulse response; The said step of utilizing least-mean-square error algorithm to obtain initial time domain channel impulse response is specially:

${\hat{h}}_{i,k}=\mathrm{IDFT}\left\{\frac{\mathrm{DFT}\left({\left\{{\hat{y}}_{i,k}^{\′}\right\}}_{k=0}^{2047}\right)}{\mathrm{DFT}\left({\left\{c_{i,k}^{\′}\right\}}_{k=0}^{2047}\right)+\frac{1}{\mathrm{SNR}}}\right\},$ 0≤k＜2047，

In the formula; The fourier transform results that

obtains for step (2); The fourier transform results that

obtains for step (3), SNR is for receiving the signal to noise ratio of signal;

(5) according to initial time domain channel impulse response, the noise power of assessment time domain channel response, and adaptive calculating optimum time domain channel thresholding;

Wherein, according to initial time domain channel impulse response, the step of the noise power of assessment time domain channel response is specially:

In the formula; The initial time domain channel impulse response that

obtains for step (4); M is the length of said pseudo random sequence (PN),

be the average power of the time domain channel response noises that obtains of assessment;

Wherein, the step of adaptive calculating optimum time domain channel thresholding is specially:

$h_{\mathrm{Thr}}^{\mathrm{Optimal}}=2\frac{\mathrm{\β}}{\mathrm{SNR}},$ Wherein, $\mathrm{SNR}=\frac{E\left[{\left|X_k\right|}^2\right]}{{\widehat{\mathrm{\σ}}}_n^2},$ β=E{|X _k| ²] E [| X _k| ^-2], in the formula, the β factor only depends on the constellation modulation system that is adopted,

Be the best time domain channel thresholding that calculates;

(6) based on best time domain channel thresholding, distinguish effective diameter and invalid footpath in the time domain channel response, filtering overwhelming majority noises, specifically promptly calculate final time domain channel estimated value and be:

In the formula,

Be the initial time domain channel impulse response that step (4) obtains, L representes the length of true multipath channel, and L≤N _Cp=M, h _ThrGet the best time domain channel thresholding that calculates in the step (5)

2. the method for filtering time domain channel response noises according to claim 1 is characterized in that: comprise that also (7) all are changed to zero with the invalid footpath that obtains in the step (6), further eliminate the noise in the time domain channel response.

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