摘要
海洋水声信道是一种极其复杂多变的时-空-频变信道。信道带宽窄、多径干扰强、传输损耗大是水声通信中实现高速可靠传输的主要障碍。正交频分复用技术(OFDM)是一种多载波调制技术,有较高的频带利用率和传输速率,在不使用复杂的信道均衡技术的条件下可以较好地克服由多径效应带来的码间串扰(ISI)。但是,OFDM对同步误差非常敏感,当存在同步误差必然会引入子载波间干扰(ICI)或ISI,这将影响信道估计的准确度,进而影响OFDM的系统性能。OFDM系统要在接收端获得信道信息,实现实时而准确的相干解调,信道估计是必不可少的。本文针对这些问题研究了OFDM水声通信系统中的联合同步和信道估计技术,并进行了仿真和试验,结果表明系统有效,方法可行。全文的主要工作概括如下:
一、论文在分析水声信道的特性和OFDM系统的基本原理的基础上,介绍了OFDM水声通信系统的基本实现结构和系统参数的选择。
二、对OFDM系统中的信道估计技术进行研究。着重对基于频域导频的信道估计算法进行归纳和总结,在讨论传统导频分布格式优缺点的基础上,给出了基于块状导频直接判决和梳状导频联合跟踪的改进算法。最后,我们还介绍了基于CAZAC序列相关运算的信道估计算法。
三、在分析同步误差对信道估计性能影响的基础上,研究了基于最大似然准则和基于判决的联合同步和信道估计技术。通过仿真,对比分析了算法在AWGN信道和多径衰落信道下的同步、信道估计性能。最后通过实验室水池和海洋试验进一步验证了联合估计算法在实际系统中的性能。结果表明,本文介绍的联合估计算法在水声通信中是行之有效的。
Ocean channel is such a complex environment that it varies with time, frequency and space. Narrow available bandwidth, great transmission loss and severe multipath effect are the main barriers to the implementation of high speed reliable communication in an underwater acoustic (UWA) channel. Orthogonal Frequency Division Multiplexing (OFDM) is a technology of multicarrier modulation which has high utilization of the frequency spectrum and raises the transmission rate. It combats the inter-symbol interference (ISI) caused by multipath effect without complex channel equalization. However, OFDM is very sensitive to synchronization errors which may introduce much ICI and ISI that greatly degrade the performance of channel estimation and the BER performance of OFDM system. In order to realize real-time and accurate coherent demodulation, we must estimate the channel. This paper investigates combined synchronization and channel estimation technology for OFDM underwater communication system, and carries out simulation and experiment. The results show the feasibility of the scheme.
The main contents are summarized as follows:
1. Based on analysis on the characteristics of underwater acoustic channel andbasic principle of OFDM system, the parameter selection and basic structure of OFDM system are produced.
2. Channel estimation technology for OFDM underwater communication system is studied. This thesis emphasizes on induction and summarization on OFDM channel estimation algorithm which is based on frequency pilot. We not only discuss the advantages and disadvantages of traditional pilot distribution formats, but also improve the algorithm based on decision directed of block-type pilot and tracking of comb-type pilot. Finally, we introduce the channel estimation algorithm based on CAZAC sequence correlation.
3. We analyze how synchronization error effects on channel estimation. Thekey research is on combined synchronization and channel estimation technology based on maximum likelihood criterion and detected-directed. According to simulation and comparison, we analyze the synchronization and channel estimation performance under both AWGN channel and multipath fading channel. At last, we verify the performance of combined synchronization and channel estimation algorithm in practical system by pool and sea testing. The results show the feasibility of the scheme in underwater acoustic communication.
引文
[1]JJ Van de Beek,M.Sandell.ML estimation of timing and frequency offset in OFDM systems[J].IEEE Trans on Signal Processing,1997,45(7):1800-1805.
[2]U.Tureli,Liu H,M.D.Zoltowski.OFDM blind carrier offset estimation:ESPRIT[J].IEEE Trans on Commun,2000,48(9):1459-1461.
[3]F Daffara,A Chouly.Maximum likelihood frequency detectors for orthogonal multicarrier systems[C].ICC 93,1993,vol.2,766-771.
[4]J Echavarri,M E Woodward and S Kbarton.A comparison of time and frequency synchronization algorithms for the European DVB-T system[C].VTC,1999,vol.2,678-682.
[5]Moose P H.A technique for Orthogonal Frequency Division Multiplexing frequency offset correction[J].IEEE Trans.On Common,1994,42(10):2908-2914.
[6]T.M.Schmidl,D.C.Cox.Robust frequency and timing synchronization for OFDM[J].IEEE Trans.On Commun,1997,45,1613-1621.
[7]C.Y.Li,Roy.S.Subspace-based blind channel estimation for OFDM by exploiting virtual carriers[J].IEEE Trans on Wireless Commun.2003,2(1):141-150.
[8]X.D.Cai,A.N.Akansu.A subspace method for blind channel identification in OFDM systems[C].ICC 2000,vol.2,929-933.
[9]Jsn-Jaap van de Beek,Ove Edfors.On channel estimation in OFDM systems[A].IEEE VTC'95[C],1995.815-819.
[10]孙翠珍.正交频分复用(OFDM)系统中信道估计算法的研究[D].西安科技大学.2005
[11]Nilson R,Edfors O,Sandell M.An analysis of two-dimensional Pilot-symbol assisted modulation for OFDM[C].ICPWC 1997,71-74.
[12]R.Negi,J.Cioffi.Pilot tone selection for channel estimation in a mobile OFDM system.[J].IEEE Transactions on Consumer Electronics,1998,vol.44,pp:1122-1128.
[13]侯伟昆,叶梧,冯穗力.叠加训练序列OFDM系统的迭代最大似然信道估计[J].华南理工大学学报.2007(9):16-19
[14]Xiaoqiang Ma,Kobayashi H,Schwartz S.C.Joint Frequency Offset and Channel Estimation for OFDM[C].GLOCOM 2003,vol.1,pp:15-19.
[15]E.Larsson,G.Liu,J.Li,G.Giannakis.Joint symbol timing and channel estimation for OFDM based WLANS[J].IEEE Communications Letters,2001,5(8):325-327.
[16]Jian sun,Haitham M.A.Issa,Peiliang Qiu.Frequency and Timing Synchronization and Channel Estimation in Preamble Based OFDM System.vol.2.2002pp:1063-1068.
[17]M.Hlaing,et al.,"A robust timing and frequency synchronization for OFDM systems," IEEE Transactions on Wireless Communications,vol.2,pp.822-39,2003.
[18]M.Hlaing,et al.,"A combined timing and frequency synchronization and channel estimation for OFDM," 2004 IEEE International Conference on Communications(IEEE Cat.No.04CH37577),pp.872-6 Vol.2|cⅷ+4411,2004.
[19]赵丹,于全.OFDM系统中新的同步与新的联合估计算法[J].电子科技大学学报.2007(36)896-899.
[20]赵丹,于全,王建新.OFDM系统中面向判决的同步与信道联合估计算法[J].电子与信息学报.2008(10).2446-2449.
[21]D.Zhao and Q.Yu,"A Joint Detected-Directed Carrier Frequency Offset and Channel Estimation Technique for OFDM Systems," 2007 International Conference on Wireless Communications,Networking and Mobile Computing,Vols 1-15,pp.338-341,2007.
[22]常娟.OFDM水声通信中的频率同步与信道估计[D].西北工业大学,2007
[23]Paul C.Etter.水声建模与仿真(第三版)[M].北京:电子工业出版社,2003.
[24]郭华伟.OFDM技术在水声通信中地应用研究[D].厦门大学电子工程系.2002:
[25]张少联.OFDM技术在水声通信中的应用研究[D].哈尔滨工程大学.2007
[26]李国梁 水声相位相干通信与自适应均衡技术研究[D]西北工业大学 2007
[27]惠俊英.水下声信道[M].北京:国防工业出版社,2002.
[28]佟学俭.OFDM移动通信技术原理与应用[M].北京:人民邮电出版社,2003
[29]王永刚.OFDM水下通信信道估计与同步技术研究[D].哈尔滨工程大学.2007
[30]魏莉 OFDM水声通信系统FPGA实现初探及多普勒频移补偿研究[D]厦门大学 2008
[31]梁婷 OFDM系统中信道估计技术的研究[D]湖南大学.2007
[32]李子,蔡跃明,阚春荣,徐友.OFDM系统中的盲信道估计[J].信号处理,2005,21(5):508-514
[33]Hoeher,P,Kaiser,S.Two-dimensional pilot-symbol-aided channel estimation by Wiener filtering[C].ICASSP.1997.vol.3,1845-1848
[34]M.H.Hsieh,C.H.Wei.Channel estimation for OFDM systems based on comb-type pilot arrangement in frequency selective fading channels.IEEE Transactions on Consumer Electronics.1998 vol.44..
[35]Y.Li.Pilot-symbol-aided channel estimation for OFDM in wireless system.IEEE Transations on Vehicular Technology,vol.49 2000.
[36]Sinem Coleri.Channel estimation techniques based on pilot arrangement in OFDM systems[A].IEEE.Transaction on broadcasting.2002.223-229
[37]S.Boumard,"Novel noise variance and SNR estimation algorithm for wireless MIMO OFDM systems," Globecom'03:Ieee Global Telecommunications Conference,Vols 1-7,pp.1330-1334,2003.
[38]X.Huilin,et al.,"A novel SNR estimation algorithm for OFDM," 2005 IEEE 61st Vehicular Technology Conference.VTC2005-Spring(IEEE Cat.No.05CH37631),pp.3068-71 Vol.515 vol.(lⅸ+3206),2005.
[39]罗美玲.OFDM系统信噪比估计方法研究[D].西安电子科技大学.2007.
[40]O.Edfors,M.Sandeil,and J.J.van de Beek,"OFDM channel estimation by singular value decomposition,"IEEE Transactions on Communications,vol.46,pp.931-939,July1998.
[41]宋伯炜.OFDM无线宽带移动通信系统中信道估计与均衡技术研究[D].上海交通大学.2005
[42]张海滨.正交频分复用基本原理与关键技术[M].北京:国防工业出版社 2006.77-79
[43]Zhao Yuping,Huang Aiping.A novel estimation method for OFDM mobile communication systems based on pilot signals and transform-domain processing[A].IEEE VTC'97[C].19972089-2093.
[44]DOWLER A,NIX A,MCGEEHAN J.Data-derived iterative channel estimation with channel tracking for a mobile fourth generation wide area OFDM system[C]GLOBECOM'03.New York,USA:IEEE,2003:804-808.
[45]陈晖,陈晓光,汤晓黎.一种基于直接判决的OFDM快衰落信道估计新方法[J].信息与电子工程 2006(1).1672-2892.
[46]陈晖,陈晓光.基于直接判决和导频跟踪的OFDM系统快时变信道估计[J].通信学报.2006(27).1-13.
[47]D.C.Chu,"Polyphase codes with good periodic correlation properties," IEEE Transactions on Information Theory|IEEE Transactions on Information Theory,vol.IT-18,pp.531-2,1972.
[48]刘东生.基于CAZAC码的LTE上行导频结构的研究[D].西安电子科技大学.2007.
[49]Y.Che-Shen,et al.,"OFDM system channel estimation using time-domain training sequence for mobile reception of digital terrestrial broadcasting," IEEE Transactions on Broadcasting|IEEE Transactions on Broadcasting,vol.46,p.10.1109/11.892158,2000.
[50]Y.Che-Shen and L.Yinyi,"Channel estimation using pilot tones in OFDM systems," IEEE Transactions on Broadcasting|IEEE Transactions on Broadcasting,vol.45,p.10.1109/11.825535,1999.
[51]魏蔚.OFDM水声通信系统的时频同步技术研究(D).厦门大学.2008.
[52]井雅.正交频分复用无线通信系统中信道估计技术研究[D].东南大学.2007
[53]M.Speth,F.Classen and H.Meyr,"Frame synchronization of OFDM systems in frequency selective fading channels,"in Proc.IEEE VTC'97,Vol.3,1997,pp.1807-1811.
[54]M.Speth,S.A.Fechtel,G.Fock and H.Meyr,"Optimum receiver design for wireless broad-band systems using OFDM-part Ⅰ," IEEE Trans.Commun.,Vol.47,No.11,pp.1668-1677,Nov.1999.
[55]刘桓.无线OFDM系统中的同步和信道估计算法研究[D].北京交通大学 2008
[56]N.-L.Hung,et al.,"Joint channel estimation and synchronization with inter-carrier interference reduction for OFDM," 2007 IEEE International Conference on Communications,pp.2841-6,2008.
[57]曹雪虹.OFDM系统中新的同步与信道联合估计算法[J].电子学报.Vol.34 2006
[58]Fechtel S A.OFDM carrier and sampling frequency synchronization and its performance on stationary and mobile channels[J].IEEE Trans.Consumer Electronics,2000,46(8):438-441