彭章友,張 興,劉艷艷

（1.上海大學(xué)通信與信息工程學(xué)院,上海200072;2.上海大學(xué)特種光纖與光接入網(wǎng)省部共建重點實驗室,上海 200072）

高速移動OFDM系統(tǒng)信干比的低復(fù)雜差分計算

彭章友1,2,張 興1,2,劉艷艷1,2

（1.上海大學(xué)通信與信息工程學(xué)院,上海200072;2.上海大學(xué)特種光纖與光接入網(wǎng)省部共建重點實驗室,上海 200072）

針對高速移動正交頻分復(fù)用系統(tǒng),提出了迭代消除子載波干擾的信干比差分計算方法。該方法把前后兩次迭代的子載波干擾進(jìn)行差分運(yùn)算,差分運(yùn)算值的加權(quán)作為干擾,每次迭代消除子載波干擾后的信號作為有用信號,實現(xiàn)信干比的計算。誤差分析和仿真數(shù)據(jù)表明:采用這種方法計算信干比可以有效估算出系統(tǒng)信干比,同時算法復(fù)雜度較低。

正交頻分復(fù)用;迭代算法;差分計算;子載波干擾;信干比

1 引 言

隨著高速鐵路技術(shù)的快速發(fā)展,以高速鐵路為代表的地面超高速移動性O(shè)FDM寬帶無線通信系統(tǒng)的研究越來越受到人們的關(guān)注。然而,由于OFDM系統(tǒng)對定時和頻率偏差較為敏感,易受多普勒頻移、收發(fā)載波頻偏（CFO）、振蕩器相位噪聲等時變因素[1]的影響,使子載波間正交性遭到破壞,產(chǎn)生子載波間的干擾（ICI）,導(dǎo)致系統(tǒng)性能明顯下降。在高速移動環(huán)境下,多普勒頻移是影響系統(tǒng)性能的關(guān)鍵因素[2-3]。為此,許多學(xué)者提出了一些消除方法,包括迫零法（ZF）[4]、最小均方誤差法（MMSE）[5]、多普勒分集方法[6]等。近年來,低復(fù)雜度的ICI消除算法研究受到了越來越多的關(guān)注[7-9],尤其是低復(fù)雜度ICI迭代消除算法[10]。文獻(xiàn)[11]在STBC-OFDM系統(tǒng)中提出的迭代算法,初始值確定無需通過檢測器,降低了計算復(fù)雜度,提高了系統(tǒng)性能和ICI消除速度。但這些算法均采用誤碼率作為指標(biāo),而誤碼率在實際的ICI消除過程中是難以計算的。我們在分析了信干比與多普勒頻移的關(guān)系后,認(rèn)為可以采用信干比作為消除程度指標(biāo),而且要求信干比的計算復(fù)雜度也要低。為此,針對高速移動正交頻分復(fù)用系統(tǒng),本文提出了迭代消除子載波干擾的信干比差分計算方法。首先根據(jù)OFDM系統(tǒng)ICI產(chǎn)生模型,回顧了傳統(tǒng)的系統(tǒng)信干比計算方法;在此基礎(chǔ)上,提出了低復(fù)雜度的信干比計算方法,同時對該方法進(jìn)行了誤差分析。仿真結(jié)果表明,本文提出的信干比計算方法復(fù)雜度較低且能夠有效地估算系統(tǒng)信干比。

2 OFDM系統(tǒng)ICI模型與迭代算法

2.1 OFDM系統(tǒng)ICI模型

圖1 OFDM系統(tǒng)ICI模型Fig.1 ICI model of OFDM system

2.2 迭代算法

3 ICI消除信干比的低復(fù)雜度差分計算方法

3.1 差分計算方法

3.2 誤差分析

3.3 信干比差分計算復(fù)雜度分析

3.4 小頻偏下信干比的簡化差分算法

4 仿真

4.1 相對誤差@SIRn的仿真

相對頻偏分別為0.08和0.008時,相對誤差 @SIRn的仿真圖如圖2所示。從圖中可以得出:隨著迭代次數(shù)的增加,計算信干比的相對誤差越來越小;信道信噪比越大,計算信干比的相對誤差也越來越小。當(dāng) ε=0.08、信噪比大于10 dB時,或當(dāng) ε=0.008、信噪比大于6 dB時,3次迭代后信干比的相對誤差就能達(dá)到10-2以下。

圖2 相對頻偏分別為0.08和0.008時,相對誤差@SIRn仿真Fig.2 Relative error simu lation of@SIRn when ε=0.08,0.008

4.2 較小頻偏時簡化差分算法計算的相對誤差@SIRn仿真

圖3為相對頻偏在0.008下,簡化差分算法計算的相對誤差 @SIRn的仿真圖。比較圖2和圖3知,當(dāng) ε=0.008、信噪比大于6 dB時,3次迭代后信干比的相對誤差均能達(dá)到10-2以下,并且圖2和圖3的數(shù)值很接近,說明較小頻偏時可以采用差分計算的簡化方法。

圖3 相對頻偏ε=0.008時,@SIRn與信噪比的關(guān)系Fig.3 Relations between@SIRn and SNR when ε=0.008

4.3 噪聲項歸一化功率仿真

圖4為相對頻偏在0.08和0.15下,迭代次數(shù)與噪聲項歸一化功率的仿真圖。從圖4（a）中我們可以知道,當(dāng)相對頻偏為0.08的情況下,經(jīng)過6次迭代以后的噪聲項功率趨于收斂且能達(dá)到信道噪聲功率P{W}的97.9%。因此,當(dāng)系統(tǒng)SNR已知的情況下,通過迭代次數(shù)和頻偏變化可以近似地計算出噪聲項功率。同理,從圖4（b）知,當(dāng)相對頻偏為0.15的情況下,噪聲項的功率8次以后也基本上趨于收斂且能達(dá)到信道噪聲功率P{W}的92.8%。

圖4 相對頻偏分別為0.08和0.15時,迭代次數(shù)與噪聲項功率的關(guān)系曲線Fig.4 Relations between iterative numbers and noise power when ε=0.08,0.15

5 結(jié) 論

本文提出了一種迭代消除子載波干擾的信干比差分計算方法。該方法把前后兩次迭代的子載波干擾進(jìn)行差分運(yùn)算,差分運(yùn)算值的加權(quán)作為干擾,每次迭代消除子載波干擾后的信號作為有用信號,實現(xiàn)了信干比的計算。誤差分析和仿真數(shù)據(jù)表明:信噪比大于10 dB時,采用這種方法計算信干比與理論信干比的相對誤差小于10-2,其計算復(fù)雜度僅為O（N3）。但是,信道特點對誤差的影響在論文中沒有進(jìn)行詳細(xì)分析,希望在后續(xù)工作中進(jìn)行研究。

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Low-complexity Differential Calculation of SIR for High-Mobility OFDM System

PENG Zhang-you1,2,ZHANG Xing1,2,LIU Yan-yan1,2
（1.School of Communication and Information Engineering,Shanghai University,Shanghai 200072,China;2.Key Laboratory of Specialty Fiber Optics and Optical Access Networks,Shanghai University,Shanghai 200072,China）

A differential calculation method of Signal to Interference Ratio（SIR）based on iterative Inter-carrier Interference（ICI）cancellation is proposed for high-mobility orthogonal frequency division multiplexing（OFDM）system.This method performs differential calculation of ICI before and after iterations,takesweightings of differential calculation and signal of each ICI iterative cancellation separately as interference and signal,respectively so as to realize the calculation of SIR.Error analysis and simulation results show that this method can effectively estimate SIRand has low-complexity.

orthogonal frequency division multiplexing（OFDM）;iterative algorithm;differential calcu lation;inter-carrier interference（ICI）;signal to interference ratio（SIR）

The National Natural Science Foundation of China（No.61071109）;Shanghai Committee of Education Fund for Innovation（11YZ02）

TN911.4

A

10.3969/j.issn.1001-893x.2011.02.007

1001-893X（2011）02-0034-06

2010-12-28;

2011-02-18

國家自然科學(xué)基金資助項目（61071109）;上海市教委創(chuàng)新基金資助項目（11YZ02）

彭章友（1965-）,男,浙江人,博士,副教授,主要研究方向為寬帶無線通信技術(shù)、通信信號處理;

PENG Zhang-you was born in Zhejiang Province,in 1965.He is now an associate professor with the Ph.D.degree.His research interests include broadband wireless communication and communication information processing.

Email:zypeng@mail.shu.edu.cn

張 興（1986-）,男,安徽人,碩士研究生,主要研究方向為高速環(huán)境下低復(fù)雜ICI算法研究;

ZHANG Xing was born in Anhui Province,in 1986.He is now a graduate student.His research direction is low-complexity ICI cancellation algorithm for high-mobility.

Email:jiagnzhang@163.com

劉艷艷（1986-）,女,山東人,碩士研究生,主要研究方向為高速環(huán)境下OFDM系統(tǒng)ICI模型研究。

LIU Yan-yan was born in Shangdong Province,in1986.She is now a graduate student.Her research concerns ICI model of OFD M system for high-mobility.

Email:yfy.68@163.com