基于IEEE802.11n协议的MIMO-OFDM系统中信道估计技术研究
摘要
随着无线局域网技术的快速发展,IEEE802.11已成为无线局域网的主流标准。目前IEEE已经成立802.11n工作小组,以制定一项新的高速无线局域网标准802.11n。新兴的802.11n在高吞吐量上有比较大的突破,是下一代的无线网络技术的标准,可提供支持对带宽最为敏感的应用所需的速率、范围和可靠性。802.11n结合了多种技术,其中包括MIMO、20和40MHz信道和双频带(2.4 GHz和5 GHz),以便形成很高的速率,同时又能与以前的IEEE 802.11b/g设备兼容。
由于MIMO-OFDM系统是高速传输系统,精确的信道估计对系统的性能提升是非常重要的。而且由于802.11n协议中对于信道估计的算法还没有明确标准化,如何设计训练序列以区别不同的天线,以及选择合适的信道估计方法,以实现精确信道估计具有很强的理论价值和实践意义。
本论文在IEEE802.11n草案框架下,提出一种改进的利用相移码(PS)的重叠导频设计方法进行信道估计。由于802.11n系统使用20MHz带宽,高速模式下使用了部分子载波传输导频信号,因此不能使用传统意义上的正交导频设计方法。本文在使用部分子载波的情况下提出了跳跃设计PS码,并且和传统的截短设计法进行了比较,经过理论推导和仿真验证,证明了跳跃设计法的优越性。改进部分还使用种子序列降低整个系统的峰均比。在仿真的过程中发现,估计的信道冲击响应长度的选择和信噪比是息息相关的,因此第一次提出了在802.11n系统中利用一种新的信道估计器。由于信道估计器的最终目的是改善信道估计性能,而不是简单估计出信道冲击响应长度,因此最理想的结果是估计出有实时噪声的情况下可以达到最小MSE的信道冲击响应长度。论文还将通过仿真比较利用不同信道估计器时系统的均方差,并且证明这种FCLI方法是最好的。
With the fast progress of wireless LAN (Local Area Network), IEEE802.11 has become the main flow standard in WLAN. Recently, 802.11n work group has been established to constitute a new high-speed wireless LAN standard, namely 802.11n, which has great break through in throughput and will be the standard of next generation WLAN without doubt. 802.11n could provide higher throughput, larger range and more reliability for application that is the most sensitive to bandwidth. Combined with many kinds of advanced telecommunication technologies, including spatial multiplexing MIMO (Multi-In Multi-Out), 20/40M bandwidth and dual frequency point (2.4 GHz and 5 GHz), 802.11n could support very high throughput and is compatible with previous IEEE 802.11b/g.
Due to the indefinite standardization of channel estimation algorithm in 802.11n, therefore, how to design a perfect training sequence to identify different antennas and choose a proper channel estimation method to estimation the channel information accurately is the main research orientation of this thesis. This thesis will introduce the technique characteristics of MIMO-OFDM and the whole system model firstly. Then the wireless fading channel model and its characteristics will be analyzed. After these works, the channel estimation algorithms based on OFDM system are reviewed, including LS (Least Square) channel estimation algorithm and MMSE (Minimum Mean Square Error) channel estimation algorithm. This thesis analyses the channel estimation implementation scheme in IEEE 802.11a in detail, such as design of test sequence, pilots, and algorithms. This thesis focuses on the research of channel estimation in MIMO-OFDM system, including current research status, algorithm systematic architecture and variance definition. The derivation of uniform time-frequency domain result using LS channel estimation and design of interleaving preamble and superposition preamble is emphasized. At last, an improved interleaving preamble using PS code is proposed, which is based on the channel model mentioned before. A traditional LS channel estimation algorithm is simulated on 802.11n system. The simulation result is compared with the algorithm proposed by this thesis, which shows that.The improved algorithm has much better performance. Besides, a novel channel estimation algorithm for 802.11n is proposed, which improves the performance of channel estimation greatly. It does not estimate the length of CIR, but the length of minimum MSE in situation of real time noise. More simulation and comparison using different channel estimation schemes are given at the end of this thesis.
All in all, the thesis focuses on channel estimation algorithms based on interleaving preamble and frequency LS method. In practical application, 802.11n employs training sequence to make channel estimation, therefore, we mainly research design of preamble by considering physical channel characteristics and frame structure of realistic system, and look for the optimum channel estimation scheme for 802.11n.
由于MIMO-OFDM系统是高速传输系统,精确的信道估计对系统的性能提升是非常重要的。而且由于802.11n协议中对于信道估计的算法还没有明确标准化,如何设计训练序列以区别不同的天线,以及选择合适的信道估计方法,以实现精确信道估计具有很强的理论价值和实践意义。
本论文在IEEE802.11n草案框架下,提出一种改进的利用相移码(PS)的重叠导频设计方法进行信道估计。由于802.11n系统使用20MHz带宽,高速模式下使用了部分子载波传输导频信号,因此不能使用传统意义上的正交导频设计方法。本文在使用部分子载波的情况下提出了跳跃设计PS码,并且和传统的截短设计法进行了比较,经过理论推导和仿真验证,证明了跳跃设计法的优越性。改进部分还使用种子序列降低整个系统的峰均比。在仿真的过程中发现,估计的信道冲击响应长度的选择和信噪比是息息相关的,因此第一次提出了在802.11n系统中利用一种新的信道估计器。由于信道估计器的最终目的是改善信道估计性能,而不是简单估计出信道冲击响应长度,因此最理想的结果是估计出有实时噪声的情况下可以达到最小MSE的信道冲击响应长度。论文还将通过仿真比较利用不同信道估计器时系统的均方差,并且证明这种FCLI方法是最好的。
With the fast progress of wireless LAN (Local Area Network), IEEE802.11 has become the main flow standard in WLAN. Recently, 802.11n work group has been established to constitute a new high-speed wireless LAN standard, namely 802.11n, which has great break through in throughput and will be the standard of next generation WLAN without doubt. 802.11n could provide higher throughput, larger range and more reliability for application that is the most sensitive to bandwidth. Combined with many kinds of advanced telecommunication technologies, including spatial multiplexing MIMO (Multi-In Multi-Out), 20/40M bandwidth and dual frequency point (2.4 GHz and 5 GHz), 802.11n could support very high throughput and is compatible with previous IEEE 802.11b/g.
Due to the indefinite standardization of channel estimation algorithm in 802.11n, therefore, how to design a perfect training sequence to identify different antennas and choose a proper channel estimation method to estimation the channel information accurately is the main research orientation of this thesis. This thesis will introduce the technique characteristics of MIMO-OFDM and the whole system model firstly. Then the wireless fading channel model and its characteristics will be analyzed. After these works, the channel estimation algorithms based on OFDM system are reviewed, including LS (Least Square) channel estimation algorithm and MMSE (Minimum Mean Square Error) channel estimation algorithm. This thesis analyses the channel estimation implementation scheme in IEEE 802.11a in detail, such as design of test sequence, pilots, and algorithms. This thesis focuses on the research of channel estimation in MIMO-OFDM system, including current research status, algorithm systematic architecture and variance definition. The derivation of uniform time-frequency domain result using LS channel estimation and design of interleaving preamble and superposition preamble is emphasized. At last, an improved interleaving preamble using PS code is proposed, which is based on the channel model mentioned before. A traditional LS channel estimation algorithm is simulated on 802.11n system. The simulation result is compared with the algorithm proposed by this thesis, which shows that.The improved algorithm has much better performance. Besides, a novel channel estimation algorithm for 802.11n is proposed, which improves the performance of channel estimation greatly. It does not estimate the length of CIR, but the length of minimum MSE in situation of real time noise. More simulation and comparison using different channel estimation schemes are given at the end of this thesis.
All in all, the thesis focuses on channel estimation algorithms based on interleaving preamble and frequency LS method. In practical application, 802.11n employs training sequence to make channel estimation, therefore, we mainly research design of preamble by considering physical channel characteristics and frame structure of realistic system, and look for the optimum channel estimation scheme for 802.11n.
引文
[1]侯晓林,宽带无线移动通信系统中信道估计的研究,北京邮电大学博士学位论文,2005。
[2]Y.(G.)Li,N.Seshadri,and S.Ariyavisitakul,"Channel estimation for OFDM systems with transmitter diversity in mobile wireless channels," IEEE J.Select.Areas Commun.,vol.17,pp.461-471,Mar.1999.
[3]Y.(G.)Li,"Optimum training sequences for OFDM systems with multiple transmit antennas," in Proc.IEEE GLOBECOM'00,vol.3,pp.1478-1482,27 Nov.-1 Dec.2000.
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[5]H.Minn,D.I.Kim,and V.K.Bhargava,"A reduced complexity channel estimation for OFDM systems with transmit diversity in mobile wireless channels," IEEE Trans.Commun.,vol.50,pp.799-807,May 2002.
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[9]W.Bai,C.He,L.G.Jiang,and X.X.Li,"Robust channel estimation in MIMO-OFDM systems,"IEE Electronics Letters,vol.39,pp.242-244,Jan.2003.
[10]J.Du and Y.(G.)Li,"MIMO-OFDM channel estimation based on subspace tracking," in Proc.IEEE VTC03-Spring,vol.2,pp.1084-1088,22-25 April 2003.
[11]D.M.Wang,B.Han,J.H.Zhao,X.Q.Gao,and X.H.You,"Channel estimation algorithms for broadband MIMO-OFDM sparse channel," in Proc.IEEE PIMRC'03,vol.2,pp.1929-1933,7-10Sept.2003.
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[20]W.G.Jeon,K.H.Paik,and Y.S.Cho,"Two-dimensional MMSE channel estimation for OFDM systems with transmitter diversity",in Proe.IEEE VTC01-Fall,vol.3,pp.1682-1685,7-11 Oct.2001.
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[25]D.Schafhuber,M.Rupp,G.Matz,and F.Hlawatsch,"Adaptive identification and tracking of doubly selective fading channels for wireless MIMO-OFDM systems," in Proc.IEEE SPAWC'03,pp.417-421,15-18 June 2003.
[26]J.Tong,Y.H.Gong,and S.X.Sun,"An adaptive channel tracking method for MIMO-OFDM systems," in Proc.IEEE ICCCAS'04,vol.1,pp.354-358,27-29 June 2004.
[27]D.Schafhuber,G.Matz,and F.Hlawatsch,"Kalman tracking of time-varying channels in wireless MIMO-OFDM systems," in Proc.the 37th Asilomar Conference on Signals,Systems &Computers,vol.2,pp.1261-1265,9-12 Nov.2003.
[28]H.Bolcskei,R.W.Heath,Jr.,and A.J.Paulraj,"Blind channel identification and equalization in OFDM-based multiantenna systems," IEEE Trans.Signal Processing,vol.50,pp.96-109,Jan.2002.
[29]W.Bai,C.He,L.Jiang,and H.Zhu,"Blind channel estimation in MIMO-OFDM systems," in Proc.IEEE GLOBECOM'02,vol.1,pp.317-321,17-21 Nov.2002.
[30]C.Gao,M.Zhao,S.Zhou,and Y.Yao,"Blind channel estimation algorithm for MIMO-OFDM systems," IEE Electronics Letters,vol.39,pp.1420-1422,Sept.2003.
[31]M.-S.Baek,M.-J.Kim,Y.-H.You,and H.-K.Song,"Semi-blind channel estimation and PAR reduction for MIMO-OFDM system with multiple antennas," IEEE Trans.Broadcasting,vol.50,pp.414-424,Dec.2004.
[32]C.Shin and E.J.Powers,"Blind channel estimation for MIMO-OFDM systems using virtual carriers," in Proc.IEEE GLOBECOM'04,vol.4,pp.2465-2469,29 Nov.-3 Dec.,2004.
[33]H.Fu,P.H.W.Fung,and S.Sun,"Semi-blind channel estimation for MIMO-OFDM," in Proc.IEEE PIMRC'04,vol.3,pp.1850-1854,5-8 Sept.2004.
[34]S.M.Kay,Fundamentals of Statistical Signal Processing:Estimation Theory,Englewood Cliffs,N.J.:Prentice Hall,1993.
[35]S.Haykin,Adaptive Filter Theory,4th Ed.,Beijing:Pubilishing House of Electronicas Industry,2002.
[36]Yang Tang,Branka Vucetic and Yonghui Li,"An FFT-based multiuser detectionf for asynchronous block-sqreading CDMA ultra wideband communication systems," in Proc.ICC'2005,vol.5,pp.2872-2876,16-20 May 2005.
[37]毛纲源,循环矩阵及其在分子振动中的应用,武汉:华中理工大学出版社,1995。
[38]C.Suh,C.-S.Hwang,and H.Choi,"Preamble design for channel estimation in MIMO-OFDM systems," in Proc.IEEE GLOBECOM'03,vol.1,pp.317-321,1-5 Dec.2003.
[39]L.Harjula,A.Mammela,and Z.Li,"Comparison of channel frequency and impulse response estimation for space-time coded OFDM systems," in Proc.IEEE VTC02-Fall,vol.4,pp.2081-2085,24-28 Sept.2002.
[40]W.G.Jeon,K.H.Paik,and Y.S.Cho,"Two-dimensional MMSE channel estimation for OFDM systems with transmitter diversity",in Proc.IEEE VTC01-Fall,vol.3,pp.1682-1685,7-11 Oct.2001.
[41]Y.Gong and K.B.Letaief,"Low rank channel estimation for space-time coded wideband OFDM systems," in Proc.IEEE VTC01-Fall,vol.2,pp.772-776,7-11 Oct.2001.
[42]F.W.Vook and T.A.Thomas,"MMSE multi-user channel estimation for broadband wireless communications," in Proc.IEEE GLOBECOM'01,vol.1,pp.470-474,25-29 Nov.2001.
[43]I.Barhumi,G.Leus,and M.Moonen,"Optimal Training Design for MIMO OFDM Systems in MobileWireless Channels," IEEE Transactions on Signal Processing,vol.51,pp.1615-1624,June 2003.
[44]Y.Li and H.Wang,"Channel estimation for MIMO-OFDM Wireless Communications," in Proc.IEEE PIMRC,vol.1,pp.535-539,Sept.2003.
[45]M.Morelli and U.Mengali,"A Comparison of Pilot-Aided Channel Estimation Methods for OFDM Systems," IEEE Transactions on Signal Processing,vol.49,pp.3065-3073,Dec.2001.
[46]E.G.Larsson and J.Li,"Preamble Design for Multiple-Antenna OFDMbased WLANs with Null Subcarders," IEEE Signal Processing Letters,vol.8,pp.285-288,Nov.2001.
[47]M.Golay,"A Class of Finite Binary Sequences with Alternate Autocorrelation Values Equal to Zero," IEEE Transactions on Information Theory(Correspondence),vol.18,pp.449-450,May 1972.
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[49]Marco Krondorf,Ting-Jung Liang,Improved Channel Estimation for Complexity-Reduced MIMO-OFDM Receiver by Estimation of Channel Impulse Response Length
[2]Y.(G.)Li,N.Seshadri,and S.Ariyavisitakul,"Channel estimation for OFDM systems with transmitter diversity in mobile wireless channels," IEEE J.Select.Areas Commun.,vol.17,pp.461-471,Mar.1999.
[3]Y.(G.)Li,"Optimum training sequences for OFDM systems with multiple transmit antennas," in Proc.IEEE GLOBECOM'00,vol.3,pp.1478-1482,27 Nov.-1 Dec.2000.
[4]Y.(G.)Li,"Simplified channel estimation for OFDM systems with multiple transmit antennas,"IEEE Trans.Wireless Commun.,vol.1,pp.67-75,Jan.2002.
[5]H.Minn,D.I.Kim,and V.K.Bhargava,"A reduced complexity channel estimation for OFDM systems with transmit diversity in mobile wireless channels," IEEE Trans.Commun.,vol.50,pp.799-807,May 2002.
]6]Y.(G.)Li,J.H.Winters,and N.R.Sollenberger,"MIMO-OFDM for wireless communications:signal detection with enhanced channel estimation," IEEE Trans.Commun.,vol.50,pp.1471-1477,Sept.2002.
[7]Y.(G.)Li and H.Wang,"Channel estimation for MIMO-OFDM wireless communications," in Proc.IEEE PIMRC'03,vol.3,pp.2891-2895,7-10 Sept.2003.
[8]I.Barhumi,G.Leus,and M.Moonen,"Optimal training design for MIMO OFDM systems in mobile wireless channels," IEEE Trans.Signal Processing,vol.51,pp.1615-1624,June 2003.
[9]W.Bai,C.He,L.G.Jiang,and X.X.Li,"Robust channel estimation in MIMO-OFDM systems,"IEE Electronics Letters,vol.39,pp.242-244,Jan.2003.
[10]J.Du and Y.(G.)Li,"MIMO-OFDM channel estimation based on subspace tracking," in Proc.IEEE VTC03-Spring,vol.2,pp.1084-1088,22-25 April 2003.
[11]D.M.Wang,B.Han,J.H.Zhao,X.Q.Gao,and X.H.You,"Channel estimation algorithms for broadband MIMO-OFDM sparse channel," in Proc.IEEE PIMRC'03,vol.2,pp.1929-1933,7-10Sept.2003.
[12]L.Hanzo,M.Miinster,B.J.Choi,and T.Keller,OFDM and MC-CDMA for Broadband Multi-User Communications,WLANs and Broadcasting,[England]West Sussex:John Wiley &Sons,2003.
[13]A.N.Mody and G.L.Stiiber,"Parameter estimation for OFDM with transmit receive diversity,"in Proc.IEEE VTC01-Spring,vol.2,pp.820-824,6-9 May 2001.
[14]S.B.Slimane,"Channel estimation for HIPERLAN/2 with transmitter diversity," in Proc.IEEE ICC'02,vol.1,pp.26-30,28 April-2 May 2002.
[15]J.Guo,D.Wang,and C.Ran,"Simple channel estimator for STBC-based OFDM systems",IEE Electronics Letters,vol.39,pp.445-447,Mar.2003.
[16]W.G.Jeon,K.H.Paik,and Y.S.Cho,"An efficient channel estimation technique for OFDM systems with transmitter diversity",in Proc.IEEE PIMRC'00,vol.2,pp.1246-1250,18-21 Sept.2000.
[17]C.Sub,C.-S.Hwang,and H.Choi,"Comparative study of time-domain and frequency-domain channel estimation in MIMO-OFDM systems," in Proc.IEEE PIMRC'03,vol.2,pp.1095-1099,7-10 Sept.2003.
[18]C.Sub,C.-S.Hwang,and H.Choi,"Preamble design for channel estimation in MIMO-OFDM systems," in Proc.IEEE GLOBECOM'03,vol.1,pp.317-321,1-5 Dec.2003.
[19]L.Harjula,A.Mammela,and Z.Li,"Comparison of channel frequency and impulse response estimation for space-time coded OFDM systems," in Proc.IEEE VTC02-Fall,vol.4,pp.2081-2085,24-28 Sept.2002.
[20]W.G.Jeon,K.H.Paik,and Y.S.Cho,"Two-dimensional MMSE channel estimation for OFDM systems with transmitter diversity",in Proe.IEEE VTC01-Fall,vol.3,pp.1682-1685,7-11 Oct.2001.
[21]Y.Gong and K.B.Letaief,"Low rank channel estimation for space-time coded wideband OFDM systems," in Proc.IEEE VTC01-Fall,vol.2,pp.772-776,7-11 Oct.2001.
[22]E W.Vook and T.A.Thomas,"MMSE multi-user channel estimation for broadband wireless communications," in Proc.IEEE GLOBECOM'01,vol.1,pp.470-474,25-29 Nov.2001.
[23]H.Zhang,J.Chen,Y.Tang,and S.Li,"Analysis of pilot-symbol aided channel estimation for MIMO-OFDM systems," in Proc.IEEE ICCCAS'04,vol.1,pp.299-303,27-29 June 2004.
[24]R.Chert and K.B.Letaief,"Channel estimation for space time coded OFDM systems in non-sample-spaced multipath channels," in Proc.IEEE WCNC'02,vol.1,pp.61-66,17-21March 2002.
[25]D.Schafhuber,M.Rupp,G.Matz,and F.Hlawatsch,"Adaptive identification and tracking of doubly selective fading channels for wireless MIMO-OFDM systems," in Proc.IEEE SPAWC'03,pp.417-421,15-18 June 2003.
[26]J.Tong,Y.H.Gong,and S.X.Sun,"An adaptive channel tracking method for MIMO-OFDM systems," in Proc.IEEE ICCCAS'04,vol.1,pp.354-358,27-29 June 2004.
[27]D.Schafhuber,G.Matz,and F.Hlawatsch,"Kalman tracking of time-varying channels in wireless MIMO-OFDM systems," in Proc.the 37th Asilomar Conference on Signals,Systems &Computers,vol.2,pp.1261-1265,9-12 Nov.2003.
[28]H.Bolcskei,R.W.Heath,Jr.,and A.J.Paulraj,"Blind channel identification and equalization in OFDM-based multiantenna systems," IEEE Trans.Signal Processing,vol.50,pp.96-109,Jan.2002.
[29]W.Bai,C.He,L.Jiang,and H.Zhu,"Blind channel estimation in MIMO-OFDM systems," in Proc.IEEE GLOBECOM'02,vol.1,pp.317-321,17-21 Nov.2002.
[30]C.Gao,M.Zhao,S.Zhou,and Y.Yao,"Blind channel estimation algorithm for MIMO-OFDM systems," IEE Electronics Letters,vol.39,pp.1420-1422,Sept.2003.
[31]M.-S.Baek,M.-J.Kim,Y.-H.You,and H.-K.Song,"Semi-blind channel estimation and PAR reduction for MIMO-OFDM system with multiple antennas," IEEE Trans.Broadcasting,vol.50,pp.414-424,Dec.2004.
[32]C.Shin and E.J.Powers,"Blind channel estimation for MIMO-OFDM systems using virtual carriers," in Proc.IEEE GLOBECOM'04,vol.4,pp.2465-2469,29 Nov.-3 Dec.,2004.
[33]H.Fu,P.H.W.Fung,and S.Sun,"Semi-blind channel estimation for MIMO-OFDM," in Proc.IEEE PIMRC'04,vol.3,pp.1850-1854,5-8 Sept.2004.
[34]S.M.Kay,Fundamentals of Statistical Signal Processing:Estimation Theory,Englewood Cliffs,N.J.:Prentice Hall,1993.
[35]S.Haykin,Adaptive Filter Theory,4th Ed.,Beijing:Pubilishing House of Electronicas Industry,2002.
[36]Yang Tang,Branka Vucetic and Yonghui Li,"An FFT-based multiuser detectionf for asynchronous block-sqreading CDMA ultra wideband communication systems," in Proc.ICC'2005,vol.5,pp.2872-2876,16-20 May 2005.
[37]毛纲源,循环矩阵及其在分子振动中的应用,武汉:华中理工大学出版社,1995。
[38]C.Suh,C.-S.Hwang,and H.Choi,"Preamble design for channel estimation in MIMO-OFDM systems," in Proc.IEEE GLOBECOM'03,vol.1,pp.317-321,1-5 Dec.2003.
[39]L.Harjula,A.Mammela,and Z.Li,"Comparison of channel frequency and impulse response estimation for space-time coded OFDM systems," in Proc.IEEE VTC02-Fall,vol.4,pp.2081-2085,24-28 Sept.2002.
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