CN101692666A - 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, the signal frame that transmits at transmitting terminal can be divided into two parts of non-overlapping copies in time, i.e. pseudorandom PN sequence { c _{I, k}} _K=0 ^M-1And data division { s _{I, k}} _K=0 ^N-1, as depicted in figs. 1 and 2.Yet, because the existence of multipath effect, at receiving terminal received signal (r _{I, k}} _K=0 ^M+N-1But the two parts by aliasing are constituted: { y _{I, k}} _K=0 ^M+L-1{ x _{I, k}} _K=0 ^N+L-1, { y wherein _{I, k}} _K=0 ^M+L-1The linear convolution result of expression PN sequence and channel impulse response, promptly

{ x _{I, k}} _K=0 ^N+L-1Expression data division { s _{I, k}} _K=0 ^N-1With the linear convolution result of channel impulse response, promptly

As shown in Figure 3 and Figure 4.

The aliasing of received 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 received signal (x _{I, k}} _K=0 ^N+L-1{ y _{I, k}} _K=0 ^M+L-1Separation.

According to Fig. 3 and Fig. 4, can obtain the data { r that the i frame receives _{I, k}} _K=0M+N+L-2Can 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, obtains the PN sequence { c of i signal frame _{I, k}} _K=0 ^M-1Channel impulse response with the i frame

The linear convolution result, promptly eliminate the i frame PN received signal after intersymbol interference and the intersymbol interference

As shown in Figure 5.

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

3), receiver synchronously after, obtain i signal frame PN sequence { c _{I, k}} _K=0 ^M-1, the point of zero padding to 2048 simultaneously c ' _{I, k}} _K=0 ²⁰⁴⁷

4), according to separation signal

With the PN sequence of i frame c ' _{I, k}} _K=0 ²⁰⁴⁷, utilize the DFT conversion, obtain initial time domain channel impulse response, promptly

${\hat{h}}_{i,k}=\mathrm{IDFT}\left\{\frac{\mathrm{DFT}\left({\left\{{\hat{y}}_{i,k}^{\&prime;}\right\}}_{k=0}^{2047}\right)}{\mathrm{DFT}\left({\left\{c_{i,k}^{\&prime;}\right\}}_{k=0}^{2047}\right)}\right\},0\&le;k<2047---\left(3\right)$

Do not consider The noise according to the time domain channel impulse response that formula (3) calculates, 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: the channel response noises of the energy filtering overwhelming majority.

Technical problem of the present invention is solved by 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 fast fourier transform;

(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 that the present invention is compared with the prior art 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 by the frequency domain least-mean-square error algorithm accurately, can filtering overwhelming majority time domain channel response noises.Further, with invalid multipath zero setting, the noise in the channel response is assessed in further elimination.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 schematic diagram in the background technology;

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

Fig. 3 is the Time Domain Decomposition schematic diagram of the received signal frame in the background technology;

Fig. 4 is the received signal frame schematic diagram in the background technology;

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

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

Also in conjunction with the accompanying drawings the present invention is described in further details below by concrete execution mode.

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

${\hat{h}}_{i,k}=\mathrm{IDFT}\left\{\frac{\mathrm{DFT}\left({\left\{{\hat{y}}_{i,k}^{\&prime;}\right\}}_{k=0}^{2047}\right)}{\mathrm{DFT}({\left\{c_{i,k}^{\&prime;}\right\}}_{k=0}^{2047}+\frac{1}{\mathrm{SNR}})}\right\},0\&le;k<2047---\left(4\right)$

Wherein, SNR is the signal to noise ratio of received 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, the relation of the time domain channel response that obtains of Shi Ji time domain channel response and formula (4) assessment can be expressed as:

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

In above-mentioned, L represents 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, through influencing each other behind the different paths arrival receivers, thereby form the various declines of 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:

By 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 considered as noise less than the smaller value of this thresholding, directly zero setting, thus effectively eliminate noise in the time domain channel impulse response.By 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

${\mathrm{\&epsiv;}}_{\min }=\min \left\{E\right[\underset{n=0}{\overset{N}{\mathrm{\&Sigma;}}}{|{\tilde{H}}_n-H_n|}^2\left]\right\}$

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

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

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 is

$h_{\mathrm{thr}}^{\mathrm{optimal}}=2\frac{\mathrm{\&beta;}}{\mathrm{SNR}}=2E\left[{\left|X_k\right|}^{-2}\right]{\mathrm{\&sigma;}}_n^2---\left(12\right)$

At last, utilize formula (8) and formula (12), noise by 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, while is in conjunction with the dynamic evaluation of the noise gate of time domain channel response, 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 be in conjunction with concrete preferred implementation to further describing that the present invention did, can not assert that concrete enforcement of the present invention is confined to these explanations.For the general technical staff of the technical field of the invention, without departing from the inventive concept of the premise, can also make some simple deduction or replace, all should be considered 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 fast fourier transform;

(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.

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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