CN101588330B - Joint channel estimation method for short-wave OFDM communication system - Google Patents

Abstract

The invention discloses a joint channel estimation method for short-wave OFDM communication system, belongs to receiver channel estimation field in OFDM communication system. The method is implementedon the base of prior short-wave OFDM communication system high-performance channel estimation calculation method, steps are as follows: obtaining channel information initial value of pilot subcarrier s on the base of least-squares criterion; proceeding delay subspace tracking to the initial value to implement separation of pilot position channel information and noise information; interpolating thede-noising pilot subcarriers and obtaining channel estimation value of OFDM signal by lowpass filters; and then proceeding IDFT when the channel estimation value is zeroized in frequency-domain direc tion to obtain channel impulse response, after impluse response correction de-noising of the channel, proceeding DFT to return to the frequency domain, at last using pilot subcarriers to compensate the data subcarriers and outputting the estimation date. The invention can improve channel estimatioin accuracy to maximum.

Description

A kind of combination channel estimating method for short-wave OFDM communication system

Technical field

The present invention relates to a kind of combination channel estimating method for short-wave OFDM communication system, belong to receiver channel estimation field in the ofdm communication system.

Background technology

The shortwave transmission is divided into sky wave and earthwave dual mode.For the sky wave transmission means, the loose channel of variable color when short wave channel is a kind of, it utilizes ionospheric reflection to transmit information.Because ionosphere is layering, inhomogeneous, anisotropy, random, medium that space-time is arranged, there are the series of complex phenomenons such as the white noise of multidiameter delay, decline, Doppler frequency shift, frequency displacement diffusion, approximate Gaussian distribution and station interference in channel, therefore, when receiving short-wave signal, channel is estimated it is an indispensable processing procedure.

The purpose of channel estimating is exactly time domain or the frequency domain response that estimates channel, the data that receive is proofreaied and correct and is recovered, to obtain certain performance gain.In wireless OFDM communication system, what usage comparison was many at present is to adopt based on pilot tone or training sequence (Pilot Symbol Assisted Modulation, PSAM) channel estimation methods.In data, insert time domain or the frequency domain given data (pilot tone) of some based on the channel estimation methods utilization of pilot tone and carry out channel estimating, so just can estimate by the sampled value of known point upper signal channel response the complete response of whole channel.At present in the short-wave OFDM communication system, high performance channel estimation method commonly used has following several: 1, postpone subspace adaptive tracing method (referring to " Pilot-Based ChannelEstimation for OFDM Systems by Tracking the Delay-Subspace " IEEE Trans on WirelessCommunication, vol.3); 2, low-pass filtering time domain interpolation method; 3, frequency domain DFT converter technique.But the estimated accuracy of above method is desirable not enough, and performance is also good not.

Summary of the invention

The technical problem to be solved in the present invention is: propose a kind of combination channel estimating method for short-wave OFDM communication system for the precision of estimating at shortwave noisy channels condition lower channel and performance issue.

Combination channel estimating method for short-wave OFDM communication system of the present invention comprises the steps:

(1) based on least square criterion, obtains the channel information initial value of pilot sub-carrier;

(2) postpone subspace tracking

Channel information with subspace amplitude tracking algorithm estimating pilot frequency subcarrier: the subspace dimension is low and variation is slow owing to postponing in the ofdm system, by the subspace projection method in the theory of subspace, pilot sub-carrier channel information initial value in the step (1) is postponed the subspace adaptive tracing, and carry out denoising;

(3) time domain interpolation low-pass filtering

To the pilot sub-carrier after the denoising in time orientation low pass filtering method interpolation: if pilot tone evenly distributes at each subchannel time orientation, the symbolic number of being separated by is N _t, the channel transfer functions sample value at pilot tone place is directly carried out N with interpolater _tTimes interpolation draws the OFDM symbol transfer function estimated value of whole time-frequency plane, and wherein: the filtering bandwidth of interpolater is determined by doppler spread;

(4) frequency domain DFT conversion

At frequency domain direction each OFDM symbol is done channel estimating based on DFT: the OFDM symbol transfer function estimated value that obtains in to step (3) is IDFT and is obtained channel impulse response after the frequency domain direction zero padding, then get back to frequency domain to being DFT after this channel impulse response correction denoising, by pilot sub-carrier the data subcarrier is done the rear delivery channel data estimator of compensation at last, wherein: IDFT is the inverse transformation of DFT.

The inventive method has been united delay subspace adaptive tracing, time domain interpolation low-pass filtering and three kinds of channel estimation methods of frequency domain DFT conversion in order, advance layer by layer, improve to greatest extent the precision that channel information is estimated in the short-wave OFDM communication system, and improved the performance of channel estimating.

Description of drawings

Fig. 1 is method flow diagram of the present invention.

Fig. 2 is DSP actual measurement channel impulse response schematic diagram after the inventive method frequency domain DFT conversion.

Fig. 3 is that the Performance Ratio of fir, sat-fir, fir-dft, four kinds of channel estimation scheme of sat-fir-dft is than simulation result figure.

Fig. 4 is that the Performance Ratio of sat-fir-dft, fir-dft-sat, three kinds of channel estimation scheme of fir-sat-dft is than simulation result figure.

Among Fig. 3 and Fig. 4: fir refers to low-pass filtering time domain interpolation method; Sat refers to postpone subspace adaptive tracing method; Dft refers to frequency domain DFT converter technique.

Embodiment

Be the combination channel estimating method flow chart for short-wave OFDM communication system of the present invention as shown in Figure 1, it comprises the steps:

(1) based on least square (LS) criterion, obtains the channel information initial value of pilot sub-carrier;

(2) postpone subspace tracking

Channel information with subspace amplitude tracking (Subspace Amplitudes Track) algorithm estimating pilot frequency subcarrier: the subspace dimension is low and variation is slow owing to postponing in the ofdm system, by the subspace projection method in the theory of subspace, to step

(1) the pilot sub-carrier channel information initial value in postpones the subspace adaptive tracing, and carries out denoising;

(3) time domain interpolation low-pass filtering

To the pilot sub-carrier after the denoising in time orientation low pass filtering method interpolation: if pilot tone evenly distributes at each subchannel time orientation, the symbolic number of being separated by is N _t, the channel transfer functions sample value at pilot tone place is directly carried out N with interpolater _tTimes interpolation draws the OFDM symbol transfer function estimated value of whole time-frequency plane, and wherein: the filtering bandwidth of interpolater is determined by doppler spread;

(4) frequency domain DFT conversion

At frequency domain direction each OFDM symbol is done channel estimating based on DFT: the OFDM symbol transfer function estimated value that obtains in to step (3) is IDFT and is obtained channel impulse response after the frequency domain direction zero padding, then get back to frequency domain to being DFT after this channel impulse response correction denoising, by pilot sub-carrier the data subcarrier is done the rear delivery channel data estimator of compensation at last, wherein: IDFT is the inverse transformation of DFT.

Be better explanation this method, the below is take an ofdm system that is used for short wave communication as example, and in conjunction with Fig. 1 specific implementation method of the present invention is described.

Have 36 subcarriers in this ofdm system.Pilot frequency sequence is s (k), and 1≤k≤36 are distributed in 3 adjacent OFDM symbols, and pilot density is 1/3, and repeats take 3 symbols as the cycle, and detailed process comprises the steps:

Step 1: postpone subspace tracking

Algorithm initialization: ${\mathrm{<figure-callout\; id="0"\; label="B"\; filenames="H2009100321622E00012.png"\; state="\{\{state\}\}">B</figure-callout>}}_0=\left[\begin{array}{c}{I}_{r_{\max }}\\ 0\end{array}\right];$ ${\mathrm{\&Theta;}}_0=I_{r_{\max }};$ A ₀＝0；b ₀＝0；0≤γ≤1；0＜μ＜2；

For n symbol, input pilot frequency locations H value: H _{LS, n}(12 pilot tones, the i.e. vector of 12*1)

Algorithm iteration process (referring to " Low-Rank Adaptive Filters " IEEE Trans on Signal Processing, vol.44, No.12, Dec.1996, pp.2932-2947):

$Z_n=B_{n-1}^H{\stackrel{\&OverBar;}{H}}_{\mathrm{LS},n}$

$A_n=\mathrm{\&gamma;}{A}_{n-1}{\mathrm{\&Theta;}}_{n-1}+(1-\mathrm{\&gamma;}){\stackrel{\&OverBar;}{H}}_{\mathrm{LS},n}{Z}_n^H$

A _n＝B _nR _n

${\mathrm{\&Theta;}}_n=B_{n-1}^H{B}_n$

${\stackrel{\&OverBar;}{U}}_n={\left\{B_n\right\}}_{1:r_{\max }}$

ε _n＝H _LS，n-B _nb _n-1

$b_n=b_{n-1}+\mathrm{\&mu;}{B}_n^H{\mathrm{\&epsiv;}}_n$

${\stackrel{\&OverBar;}{d}}_n={\left\{b_n\right\}}_{1:r_{\max }}$

Obtain H value: H behind the pilot frequency locations burbling noise _n=U _nd _n

Wherein: r _MaxB _nThe matrix column number should more than or equal to the footpath number of shortwave multidiameter fading channel, be got r here _Max=2; γ, μ are weight coefficients, and under different channel conditions, γ, μ have different optimum values, get γ=0.977 here, μ=0.97; Z _n, B _n, Θ _n, A _n, b _n, U _b, ε _n, d _nBe intermediate variable, do not have actual physical significance, its subscript n represents n symbol, and n is natural number; Formula ${\stackrel{\&OverBar;}{U}}_n={\left\{B_n\right\}}_{1:r_{\max }}$ With ${\stackrel{\&OverBar;}{d}}_n={\left\{b_n\right\}}_{1:r_{\max }}$ In subscript 1:r _Max1 to r of matrix is got in expression _MaxRow; [g] ^HThe expression conjugate transpose.

Step 2: time domain interpolation low-pass filtering

Filter coefficient is designed by MATLAB: the Blackman filter, get sample rate f s=53.33333Hz, cut-off frequency fc=fs/6=8.8889Hz, filter order order=48.

Step 3: frequency domain DFT conversion

As shown in Figure 2, take out the frequency domain value of 36 subcarriers of a symbol, zero padding to N=128 point obtains 128 channel estimation value H (k) (integer power of zero padding to 2 is in order conveniently to do the quick computing of IDFT and DFT), then is IDFT and obtains channel impulse response: $G\left(n\right)=\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}H\left(k\right)e^{j2\mathrm{\&pi;kn}/N},$ n＝0，1，...，N-1；

This impulse response is revised according to following rule, with the interference of noise reduction:

$K=\frac{1}{N}\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\left|G\left(n\right)\right|$

Wherein: K is thresholding, and value is the mean value of G (n) mould, M ₁, M ₂Be illustrated in the impulse response of N point one section of intercepting and compose null value reaching the effect of denoising, come value according to thresholding K and channel delay and doppler spread, here M ₁Value is 80, M ₂Value is 100.

To revised impulse response G _p(n) be the channel estimation value H that DFT can obtain all subcarriers _p(k), utilize channel estimation value that the data subcarrier after postponing is compensated rear delivery channel data estimator.

Step 3 is repeated several times as required, to improve precision.

Simulation result is as follows:

Fig. 3 be the Performance Ratio of fir, sat-fir, fir-dft, four kinds of channel estimation scheme of sat-fir-dft than simulation result figure, simulated conditions is 2 footpath fading channels, the 2nd footpath time-delay is 2ms, doppler spread 1Hz adopts 1/3 density pilot tone.This simulation result shows, signal to noise ratio less than the 25dB situation under, channel estimation scheme is better than only doing the time domain interpolation low-pass filtering, or postpones subspace tracking and time domain interpolation low-pass filtering, or three kinds of schemes of time domain interpolation low-pass filtering and frequency domain DFT conversion; Signal to noise ratio is only second to the scheme that postpones subspace tracking and time domain interpolation low-pass filtering greater than in the 25dB situation.

Fig. 4 be the Performance Ratio of sat-fir-dft, fir-dft-sat, three kinds of channel estimation scheme of fir-sat-dft than simulation result figure, simulated conditions is 2 footpath fading channels, the 2nd footpath time-delay is 2ms, doppler spread 1Hz adopts 1/3 density pilot tone.Simulation result shows, postpone subspace tracking, time domain interpolation low-pass filtering and three algorithm for estimating of frequency domain DFT conversion in the situation that all do, be better than other two kinds of orders according to the scheme that postpones subspace tracking, time domain interpolation low-pass filtering and frequency domain DFT conversion sequencing, especially more obvious under high s/n ratio.

Claims (1)

1. a combination channel estimating method that is used for short-wave OFDM communication system is characterized in that comprising the steps:

(1) based on least square criterion, obtains the channel information initial value of pilot sub-carrier;

(2) postpone subspace tracking

Channel information with subspace amplitude tracking algorithm estimating pilot frequency subcarrier: the subspace dimension is low and variation is slow owing to postponing in the short-wave OFDM communication system, by the subspace projection method in the theory of subspace, pilot sub-carrier channel information initial value in the step (1) is postponed the subspace adaptive tracing, and carry out denoising;

(3) time domain interpolation low-pass filtering

To the pilot sub-carrier after the denoising in time orientation low pass filtering method interpolation: if pilot tone evenly distributes at each subchannel time orientation, the symbolic number of being separated by is N _t, the channel transfer functions sample value at pilot tone place is directly carried out N with interpolater _tTimes interpolation draws the OFDM symbol transfer function estimated value of whole time-frequency plane, and wherein: the filtering bandwidth of interpolater is determined by doppler spread;

(4) frequency domain DFT conversion

At frequency domain direction each OFDM symbol is done channel estimating based on DFT: the OFDM symbol transfer function estimated value that obtains in to step (3) is IDFT and is obtained channel impulse response after the frequency domain direction zero padding, then get back to frequency domain to being DFT after this channel impulse response correction denoising, by pilot sub-carrier the data subcarrier is done the rear delivery channel data estimator of compensation at last, wherein: IDFT is the inverse transformation of DFT.

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