CN101188591B - A method for reducing subcarrier interference of OFDM system - Google Patents

Abstract

The invention relates to a method for reducing ICI in an OFDM system by utilizing precode transmission. Because of the time variability of an actual channel and the phase noise between a sender and a receiver, frequency departure is possibly caused, the departure of the carrier frequency can cause the damage to the orthogonality among sub carriers, therefore inter carrier interference (ICI) is generated. Through predetermining the demodulation algorithm of the receiving terminal, the invention adopts a zero forcing principle to construct a modulation matrix M, a sending signal is preprocessed through the modulation matrix, the channel influence is counteracted in advance at the sending terminal, therefore the effect of reducing the ICI is achieved. By adopting the method of the invention, not only bit error rate is greatly improved, but also the complexity of the OFDM system receiver is reduced, thereby the invention is especially suitable for the down chain of a TDD system.

Description

A kind of method that reduces the interference of ofdm system sub-carriers

Technical field

The present invention relates to wireless communication technology, relate in particular to the wireless transmission preconditioning technique.

Background technology

Ofdm system is more responsive more to carrier frequency shift than single-carrier system; Because the time variation of channel and the phase noise between the transmitter and receiver; All possibly cause frequency departure, the skew of carrier frequency can cause between the subcarrier orthogonality to be destroyed, thereby produces subcarrier interference ICI; If do not take measures ICI is not overcome, can bring very systematic function to have a strong impact on.

The method that reduces at present ICI mainly contains the frequency domain equalization method, the time domain equalization method, and the time-domain windowed method, ICI is from null method, coding/converter technique etc.The frequency domain equalization method is to utilize pilot signal and training sequence, and it can be good at the frequency departure of the system of estimating, thereby eliminates subcarrier interference ICI, and is complicated but it is realized, and needs unnecessary information overhead.When the impulse response length of channel is longer; Only relying on the length that prolongs Cyclic Prefix to eliminate the intersymbol interference of front and back OFDM will make the efficient of system that bigger reduction is arranged; Can adopt time-domain equalizer to reduce demodulator input end signal hangover length this moment at receiving terminal, but must combine with the frequency domain equalization method to eliminate ICI effectively.Through being carried out windowing, the OFDM symbol improves frequency offset issues; Mean that the amplitude that makes the symbol period edge carries out the transition to zero gradually; The length of window function is big more, thereby makes out-of-band radiation power descend soon more, but also can reduce the degrees of tolerance of OFDM symbol to the time delay expansion simultaneously.ICI is a kind of outstanding effect ICI removing method from null method, but this method has reduced frequency efficiency and bandwidth.Coding/converter technique makes signal obtain bigger coding gain and diversity gain through matrix/conversion of algebraic method design, thereby can tolerate the interference that part causes owing to multipath fading, but becomes quite complicated at the receiving terminal receiver.

Summary of the invention

The present invention is directed to the above-mentioned deficiency of prior art, solve the ICI problem, proposed a kind of method of utilizing the transfer pre-coding technology to eliminate ICI from new thinking.The transfer pre-coding technology is combined to reduce to disturb between subcarrier with the OFDM technology; Compel zero precoding processing through signal being transmitted at transmitting terminal; Not only reduce the ofdm system sub-carriers and disturb the influence of ICI, and reduced the complexity of ofdm system receiving terminal (like travelling carriage in the down link).

The technical scheme that the present invention addresses the above problem is to adopt transmission to compel zero method for precoding at transmitting terminal to signal and handle.Specifically comprise following processing procedure, at first, change a finite impulse response FIR channel into a convolution matrix channel H according to the Cyclic Prefix of ofdm system, with the H matrix carry out a channel equivalence of structure matrix H after FFT conversion, the IFFT conversion '=F ^HHF, this equivalence matrix H ' be a pair of angular moment battle array; Secondly, confirm the demodulation matrix D of receiving terminal; Then, transmitting terminal according to channel equivalence matrix H ', demodulation matrix D constructs a modulation matrix M; Through modulation matrix signal to be sent is carried out preliminary treatment at transmitting terminal, the signal after the processing is delivered to the recipient through channel.Through such scheme, reduced the ofdm system sub-carriers and disturbed ICI.

The present invention adopts the process of Zero Forcing structure modulation matrix M to be called transmission and compels zero method for precoding.In the process that adopts Zero Forcing structure modulation matrix,, adopt to make the minimized processing method of mean square error MSE according to the influence of transmitting filter gain factor.

After in ofdm system, adopting transfer pre-coding of the present invention to handle, ICI disturbs obviously and reduces in the system, and the error rate has greatly improved, and can not reduce frequency efficiency and bandwidth.In addition; Owing to before structure modulation matrix M, selected simple unit matrix earlier as demodulation matrix; Receiving terminal also no longer need carry out complicated channel estimating and equilibrium treatment simultaneously; Therefore reduced the complexity of receiver (like travelling carriage), so the present invention is particularly suitable for the down link of TDD (time division duplex) system.

Description of drawings

Fig. 1 is a transfer pre-coding ofdm system model structure theory diagram

Fig. 2 adopts the performance of transmission zero forcing algorithm precoding ofdm system down for the IMT-20003A channel

Fig. 3 adopts the performance of transmission zero forcing algorithm precoding ofdm system down for the COST207RA channel

Embodiment

Because the time variation of channel and the phase noise between the transmitter and receiver; All possibly cause frequency departure; The skew of carrier frequency can cause between the subcarrier orthogonality to be destroyed, thereby produces subcarrier interference ICI, and this interference meeting brings very systematic function and seriously influences.For overcoming above-mentioned influence, the present invention adopts transmission to compel zero method for precoding at transmitting terminal to signal, signal to be sent is carried out transfer pre-coding handle, and eliminates subcarrier interference.Above-mentioned processing procedure specifically may further comprise the steps:

Change a finite impulse response FIR channel into convolution matrix channel H at transmitting terminal according to the Cyclic Prefix of ofdm system; The transmitting terminal processor controls is called formula H '=F ^HHF is by FFT transformation matrix F, IFFT transformation matrix F ^HConvolution matrix channel H is carried out FFT conversion, IFFT conversion, promptly construct channel equivalence matrix H '; Secondly, confirm the demodulation matrix D of receiving terminal; Transmitting terminal is according to demodulation matrix, the channel equivalence matrix construction modulation matrix M of receiving terminal; Primary signal to be sent is carried out sending to receiving terminal through channel after the preliminary treatment through modulation matrix at transmitting terminal.

To accompanying drawing and specific embodiment enforcement of the present invention is specifically described below.

Transfer pre-coding ofdm system model structure theory diagram is as shown in Figure 1.

The transfer pre-coding technology is a kind of towards the linear preconditioning technique that receives; Its basic principle is the preferential demodulation method of confirming receiving terminal; According to the reciprocal characteristics of TDD system uplink downlink channel information; Carry out the channel information that channel estimating just can obtain down link in up link, this channel information can be expressed as matrix form, uses the H matrix notation; Transmitting terminal carries out corresponding transition structure channel equivalence matrix to the H matrix, according to the demodulation matrix D and the channel equivalence matrix H of receiving terminal ' construct modulation matrix M at transmitting terminal.

Simple for narrating, here a taking into account system disposes one one situation of receiving antenna, but the present invention can be generalized to many antennas situation such as mimo system easily.

(I) Cyclic Prefix according to ofdm system changes a finite impulse response FIR channel into a convolution matrix channel H, to H carry out conversion obtain channel equivalence matrix H '.

Utilize the reception information of up link to carry out channel estimating at transmitting terminal (like the base station), just can obtain according to the Cyclic Prefix of ofdm system the channel impulse response of down link: h=[h (0), h (1) ... h (l)] ^T

Suppose N data symbol d _n, n=1,2 ... N, this data symbol is taken from finite aggregate V={v ₁, v ₂..., v _M, if select the QPSK modulation system for use, V={1+j is just arranged ,-1+j, 1-j ,-1-j}.The vector form of data symbol is:

d＝(d ₁，d ₂…d _N) ^T (1)

Suppose that M is the modulation matrix of transfer pre-coding to be determined, after the preliminary treatment through this modulation matrix, signal d is modulated to the signal s of transmission, that is: s=Md (2)

Effect through the cyclic prefix CP in the ofdm system with a FIR channel h=[h (0), h (1) ... h (l)] ^TBecome a convolution matrix channel [H] _{I, j}:

[H] _i，j＝h((i-j)modN) (4)

We represent FFT conversion (Fourier transform) matrix of receiving terminal with F:

${\left[F\right]}_{n,k}=(1/\sqrt{N})\exp (-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="S2007100929291E00011.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;nk}/N)---\left(5\right)$

IFFT conversion (inverse fourier transform) matrix of transmitting terminal can be expressed as F so ^HAt transmitting terminal convolution matrix channel H is carried out FFT conversion, IFFT conversion, promptly controller calls formula:

H′＝diag(H(0)，...H(N-1))＝F ^HHF (6)

Through above-mentioned formula H is carried out conversion, can obtain like this channel equivalence matrix H '.This channel equivalence matrix H ' be a diagonal matrix, wherein

$H\left(n\right)=\underset{l=0}{\overset{L}{\mathrm{\&Sigma;}}}h\left(l\right)\exp (-\mathrm{<figure-callout\; id="2"\; label="jl"\; filenames="S2007100929291E00011.png"\; state="\{\{state\}\}">jl</figure-callout>}2\mathrm{\&pi;n}/N)---\left(7\right)$

Can be expressed as through receiving signal e after the equivalent matrix channel:

e＝H′s+n＝F ^HHFs+n＝F ^HHFMd+n (8)

(II) the demodulation matrix D of selective reception end

The demodulation matrix of selective reception end; The system of selection of demodulation matrix D has a variety of; Compel zero precoding processing because the present invention has carried out transmission at transmitting terminal to signal, receiving terminal also no longer need carry out complicated channel estimating and equilibrium treatment, can select simple demodulation matrix to carry out demodulation to received signal at receiving terminal; In order to reduce the complexity of receiver (travelling carriage), so need to select simple as far as possible demodulation matrix.For this reason, our matrix D=I of selection unit like this, only needs simple matched filter processing to get final product at receiving terminal.

Receiving terminal employing unit matrix D e to received signal carries out demodulation, so the data after the demodulation are:

$\hat{q}=\mathrm{De}=H^{\&prime;}s=H^{\&prime;}\mathrm{Md}---\left(9\right)$

Can find out that after the employing transfer pre-coding technology, separate timing at the down link receiving terminal, receiving terminal (portable terminal) need not carry out complicated channel estimating just can accomplish Data Detection.

(III) adopt Zero Forcing structure modulation matrix M at transmitting terminal

The present invention adopts Zero Forcing structure modulation matrix M, and the process of constructing said modulation matrix is transmission line zero forcing algorithm (TxZF), according to the structure of the modulation matrix M that is constructed, has promptly confirmed the transfer pre-coding processing method of transmitting terminal.TxZF is under the constraint of Zero Forcing, adopts and makes the minimized preprocess method of mean square error MSE, and we consider the influence of transmitting filter gain factor β in processing method, and the MSE of correction is:

$\mathrm{\&epsiv;}(M,\mathrm{\&beta;})=E\left[{\left|\right|d-{\mathrm{\&beta;}}^{-1}\hat{d}\left|\right|}_2^2\right]---\left(10\right)$

(9) formula can obtain above rewriting:

$\hat{q}=H^{\&prime;}\mathrm{Md}+n==\mathrm{diag}\left[H^{\&prime;}M\right]d+\stackrel{\&OverBar;}{\mathrm{diag}}\left[H^{\&prime;}M\right]d+\underset{\&OverBar;}{n}$ (11)

144442?44443?144442?44443 {

useful interference noise

First of following formula is useful signal, and second is interference, and the 3rd is noise.Disturb for eliminating, being provided with second is zero, has like this:

$\mathrm{\&epsiv;}(M,\mathrm{\&beta;})=E\left[{\left|\right|{\mathrm{\&beta;}}^{-1}n\left|\right|}_2^2\right]$ (12)

Because filter gain factor-beta and n, M independence, (M β) is equal to and minimizes β to minimize ε ^-2

Therefore have

$[M_{\mathrm{ZF}},{\mathrm{\&beta;}}_{\mathrm{ZF}}]=\arg \underset{M,\mathrm{\&beta;}}{\min }{\mathrm{\&beta;}}^{-2}$ s.t.:H′M＝βI (13)

Thereby the modulation matrix M that obtains the TxZF algorithm is:

M _ZF＝H′ ^H(H′H′ ^H) ^-1＝(F ^HHF) ^H((F ^HHF)(F ^HHF) ^H) ^-1 (14)

(IV) demodulation process

Modulation matrix M carries out transfer pre-coding to transmitting terminal primary signal d to be sent to be handled; Modulation generates modulating data s; Obtain receiving data e after modulating data s channel equivalence matrix H ' processing; Receiving terminal demodulation matrix D carries out demodulation to receiving data e, obtains restituted signal for

$\hat{d}=\mathrm{De}=H^{\&prime;}{M}_{\mathrm{ZF}}d=H^{\&prime;}{H}^{\&prime;H}{\left(H^{\&prime;}{H}^{\&prime;H}\right)}^{-1}d$ (15)

The transmitting terminal signal is carried out not having the ICI distracter in the above-mentioned restituted signal after transfer pre-coding handles, and ICI disturbs obviously minimizing like this; The error rate has greatly improved; Improved nonlinear precoding ofdm system performance, and realized simply not needing unnecessary information overhead.Because before structure modulation matrix M, considered the influence of demodulation matrix, receiving terminal no longer need carry out complicated channel estimating and equilibrium treatment, has therefore reduced the complexity of receiver (travelling carriage).So the present invention is particularly suitable for the down link of TDD system.

Below through concrete application example enforcement of the present invention and effect are described further.

Utilize MATLAB emulation TD-SCDMA3G system down link environment; Sample frequency 1.6e6Hz, 256 subcarriers, subcarrier spacing 6.25KHz; The actual carrier number 240 that uses (can supply 16 users to use; 15 data subcarriers of each user), OFDM symbol period length 160 μ s, circulating prefix-length 8.75 μ s.

Channel adopts COST207RA channel (channel impulse response effective length W=2) and IMT-20003A channel (W=4).Suppose that the base station knows the channel impulse response that is fit to a down link fully, promptly channel impulse response estimation is errorless, although this hypothesis is Utopian, it provides the UPS upper performance score that possibly reach.

Fig. 2 has compared ofdm system traditional under IMT-2000 3A channel and has adopted transmission to compel performance aspect the errored bit of zero precoding ofdm system, from figure, is not difficult to find out, and be 10 in the error rate ^-1The time improvement of general 3dB arranged.Fig. 3 has compared ofdm system traditional under COST 207 RA channels and has adopted transmission to compel performance aspect the errored bit of zero precoding ofdm system, from figure, is not difficult to find out, and be 10 in the error rate ^-3The time improvement of general 6dB arranged.Among the last figure, abscissa is an interchannel noise, and ordinate is the error rate, and wherein TxZF representes the transmission line zero forcing algorithm, and Noprecoding representes to adopt traditional ofdm system.

The above results shows that ICI disturbs obviously and reduces in the ofdm system behind the employing transfer pre-coding, and the error rate has greatly improved, and the influence that this explanation employing transfer pre-coding reduces ICI is feasible and effective.

Claims (4)

1. one kind is reduced the method that the ofdm system sub-carriers is disturbed, and it is characterized in that, changes finite impulse response FIR channel into convolution matrix channel H according to the Cyclic Prefix of ofdm system; Utilize the reception information of up link to carry out channel estimating at transmitting terminal, obtain the FIR channel impulse response of down link according to the Cyclic Prefix of ofdm system; Through the cyclic prefix CP in the ofdm system FIR channel impulse response is become a convolution matrix channel; The transmitting terminal processor controls is called formula H '=F ^HHF is by FFT transformation matrix F, IFFT transformation matrix F ^HTo convolution matrix channel H carry out FFT conversion, IFFT transition structure channel equivalence matrix H '; Confirm the demodulation matrix D of receiving terminal; Transmitting terminal adopts Zero Forcing according to channel equivalence matrix and receiving terminal demodulation matrix structure modulation matrix M; Modulation matrix M carries out transfer pre-coding to transmitting terminal primary signal to be sent to be handled, and generates modulating data, obtains data e after modulating data channel equivalence matrix H ' processing, and receiving terminal demodulation matrix D carries out demodulation to receiving data e, obtains restituted signal.

2. method according to claim 1 is characterized in that, said demodulation matrix D selection unit matrix.

3. method according to claim 1 is characterized in that, adopts Zero Forcing structure modulation matrix M at transmitting terminal, and said modulation matrix M is: M=(F ^HHF) ^H((F ^HHF) (F ^HHF) ^H) ^-1

4. method according to claim 3 is characterized in that, in the method that adopts Zero Forcing structure modulation matrix, according to the influence of transmitting filter gain factor β, makes the minimized principle of mean square error MSE.

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