窄带COFDM系统中的信道估计研究及其实现
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摘要
正交频分复用(OFDM)技术作为一种可以有效对抗多径衰落的高速数据传输技术,在无线通信领域得到了广泛应用。由于在移动通信中,信道具有时变特性,因此动态地对信道进行估计是十分必要的。本项目采用级联RS码与卷积码的信道编码技术,通过交织处理,保证了对一般信道误码进行纠错的能力。把外交织深度提高到34,达到了连续纠错272个字节的能力。根据接收的信号做信道估计和均衡处理,可以降低信道产生的误码,从而降低纠错的压力。因此,对高质量的视频传输系统而言,信道估计对系统的性能有着举足轻重的作用。
本文首先介绍了OFDM基本原理,进行了COFDM系统的概述,分析了各组成模块的功能和相关参数,并简述了数字上变频采用专用芯片的设计过程。
然后探讨了无线信道的多径衰落,比较了差分QPSK和QPSK子载波调制的性能,分析了基于导频的信道估计方法,根据信道的时变性和频率选择性,设计了离散导频插入的图案,并比较了几种信道估计的内插算法的性能,针对基于IDFT/DFT的内插算法的缺点进行了改进,并给出了仿真后的改进结果和改善的误码率曲线。
通过对基于导频信道估计理论的分析,选择在时间方向进行线性内插、频率方向二阶内插的信道估计方案。建立信道模型,搭建了系统的仿真平台,进行了误码率和星座点分析,验证了方案的可行性和有效性。
最后,根据设计的信道估计和均衡的方案,完成了各个模块的硬件设计。Modelsim仿真验证了每个模块设计的正确性。通过系统调试,验证了信道估计模块的性能。
To be a high-speed data transmission technique,orthogonal frequency division multiplexing(OFDM) has been widely used in wireless communication systems. Due to its capability to combat multi-path fading, in mobile communication systems,a dynamical estimation is necessary since the channel is frequency-selective and time-varying. The project employs the channel encoder method by cascading the RS encoder and convolution encoder. Through the interleave process, the capability of correcting normal channel error was guaranteed. Besides, this project increase the length of the outside interleave to 34 bytes. Because of that, 272 continuous errors can be corrected. It can decrease the error through channel estimation and equalization according to the received signal; thus, decrease the pressure of channel correcting. Therefore, channel estimation is very important to the performance in high-quality video transmission system.
Firstly, this dissertation introduces the OFDM theory and the coder OFDM system; analyze the function and parameters setting of each module, given the design process of digital upconverter briefly.
Secondly, the dissertation discuss the multi-path fading of wireless channel, compare the performance of DQPSK and QPSK, analyze pilot-based channel estimation, design the discrete pilot insert pattern according to the time-varying and frequency selective characters of the channel, compare the performance of several interpolation algorithms, improve the IDFT/DFT interpolation method, and the simulation result was given.
Thirdly, through the analysis of the pilot-based channel estimation principle, design the channel estimation method by doing linear interpolation in time and second-order interpolation in frequency. Through the simulation platform of the channel model, analyze the symbol error rate and the distribution of constellation points, which prove that the scheme in this dissertation can be implemented effectively.
Lastly, each module of channel estimation was implemented in FPGA according to the theory. Each module was tested and verified through the simulation in modelsim. Through debugging in FPGA, the performance of channel estimation was verified finally.
本文首先介绍了OFDM基本原理,进行了COFDM系统的概述,分析了各组成模块的功能和相关参数,并简述了数字上变频采用专用芯片的设计过程。
然后探讨了无线信道的多径衰落,比较了差分QPSK和QPSK子载波调制的性能,分析了基于导频的信道估计方法,根据信道的时变性和频率选择性,设计了离散导频插入的图案,并比较了几种信道估计的内插算法的性能,针对基于IDFT/DFT的内插算法的缺点进行了改进,并给出了仿真后的改进结果和改善的误码率曲线。
通过对基于导频信道估计理论的分析,选择在时间方向进行线性内插、频率方向二阶内插的信道估计方案。建立信道模型,搭建了系统的仿真平台,进行了误码率和星座点分析,验证了方案的可行性和有效性。
最后,根据设计的信道估计和均衡的方案,完成了各个模块的硬件设计。Modelsim仿真验证了每个模块设计的正确性。通过系统调试,验证了信道估计模块的性能。
To be a high-speed data transmission technique,orthogonal frequency division multiplexing(OFDM) has been widely used in wireless communication systems. Due to its capability to combat multi-path fading, in mobile communication systems,a dynamical estimation is necessary since the channel is frequency-selective and time-varying. The project employs the channel encoder method by cascading the RS encoder and convolution encoder. Through the interleave process, the capability of correcting normal channel error was guaranteed. Besides, this project increase the length of the outside interleave to 34 bytes. Because of that, 272 continuous errors can be corrected. It can decrease the error through channel estimation and equalization according to the received signal; thus, decrease the pressure of channel correcting. Therefore, channel estimation is very important to the performance in high-quality video transmission system.
Firstly, this dissertation introduces the OFDM theory and the coder OFDM system; analyze the function and parameters setting of each module, given the design process of digital upconverter briefly.
Secondly, the dissertation discuss the multi-path fading of wireless channel, compare the performance of DQPSK and QPSK, analyze pilot-based channel estimation, design the discrete pilot insert pattern according to the time-varying and frequency selective characters of the channel, compare the performance of several interpolation algorithms, improve the IDFT/DFT interpolation method, and the simulation result was given.
Thirdly, through the analysis of the pilot-based channel estimation principle, design the channel estimation method by doing linear interpolation in time and second-order interpolation in frequency. Through the simulation platform of the channel model, analyze the symbol error rate and the distribution of constellation points, which prove that the scheme in this dissertation can be implemented effectively.
Lastly, each module of channel estimation was implemented in FPGA according to the theory. Each module was tested and verified through the simulation in modelsim. Through debugging in FPGA, the performance of channel estimation was verified finally.
引文
[1]佟学俭,罗涛.OFDM移动通信技术原理与应用.北京:人民邮电出版社,2003.6:118-141
[2] J. G. Proakis.数字通信(第四版).北京:电子工业出版社,2006.9:800-887
[3] R. W. Chang. Synthesis of band-limited orthogonal signals for multichannel data transmission. Bell Systems Tech. J., Dec.1996, Vol.45:1775-1796
[4] M. H. Hsieh, C. H. Wei. Channel estimation for OFDM systems based on comb-type piot arrangement infrequency selective fading channels, IEEE Trans. On Consumer Electronics. Feb.1998, 44(1):217-225
[5] M. Sandell, O. Edfors. A comparative study of pilot-based channel estimators for wireless OFDM. Research Report Tulea 1996: 17, in signal Processing, Lulea University of Technology, Sept.1996:175-181
[6] P. Hoeher, S. Kaiser, P. Robertson. Two-dimensional pilot-symbol aided channel estimation by Wiener filtering. in Proc. 1997 IEEE Int. Conf. Acoustics, Speech, and Signal Processing,”Munich, Germany, Apr.1997:1845-1848
[7] Vittoria Mignone, Alberto Morello. CD3-OFDM: A novel Demodulation Scheme for Fixed and Mobile Receivers. IEEE Trans. On Comm. 1996, 44(9):1144-1151
[8] Ahmad Chini, Yiyan Wu, et al. Filtered Decision Feedback Channel Estimation for OFDM Based DTV Terrestrial Broadcasing System. IEEE Trans. On Broadcasting, Mar.1998, 44(1):2-10
[9] Necker. M. C, Stuber. G. L. Totally blind channel estimation for OFDM on fast varying mobile radio channels. IEEE Trans. On Wireless Communications, Sept.2004, 3(5):1514-1525
[10] Petropulu. A, Ruifeng. Zhang. Blind channel estimation for OFDM systems. Digital Signal Processing Workshop, 2002 and the 2nd Signal Processing Education Workshop. Proceedings of 2002 IEEE 10th, 13-16 Oct. 2002:366-370
[11] B. Muquest, M. de. Courville. Blind and semi-blind channel identification methods using second order statistics for OFDM systems, Proc. ICASSP 99, Jan.1999, Vol.5: 2745-2748
[12] Chaocheng Tu, Champagne. B. Subspace-based Blind Channel Estimation for MIMO-OFDM Systems: Reducing the Time Averaging Interval of the Correlation Matrix. Personal, Indoor and Mobile Radio Communications, 2007. PIMRC 2007. IEEE 18th International Symposium on 3-7 Sept.2007:1-5
[13] G. B. Giannakis. Filterbanks for blind channel identification and equalization. IEEE Trans. On Signal Processing, Jun.1997, 4(6):184-187
[14] Feifei Gao. Wenyuan Wu. Yonghong Zeng. Nallanathan. A. A Novel Blind Channel Estimation for CP-Based MIMO OFDM Systems. Communications, 2007. ICC 07. IEEE International Conference on 24-28, Jun.2007:2586-2591
[15] M. Tsatsanis, G. B. Giannakis. Transmitter induced cyclostationarity for blind channel equalization. IEEE Trans. On Signal Processing, Jul.1997, 45(1):1785-1794
[16] Shengli Zhou, Georgios. B. Giannakis. Finite-alphabet based channel estimation for OFDM and related multicarrier systems. IEEE Trans. On Comm. Aug.2001, 49(8):1402-1414
[17] Shin. C, Heath. R. W, Powers. E. J. Blind Channel Estimation for MIMO-OFDM Systems. IEEE Trans. On Vehicular Technology, Mar.2007, 56(2):670-685
[18] ETSI. EN 300 421. Digital Video Broadcasting (DVB). Framing structure, channel coding and modulation for 11/12 GHz satellite services, 1997.8
[19]尹长川,罗涛,乐光新.多载波宽带无线通行技术.北京:北京邮电大学出版社,2004:1-97
[20] S. Weinstein, P. Ebert. Data transmission by frequency-division multiplexing using the discrete Fourier transform. IEEE Trans. On Comm, Oct.1971, 19(5):628-634
[21] J. Bingham. Multicarrier modulation for data transmission: An idea whose time has come. IEEE Trans. On Comm, Mar.1990, 28(5):5-14
[22] Peled, A. Ruiz. Frequency domain data transmission using reduced computational complexity algorithms. Proc. IEEE Int. Conf. Acoust Speech Signal Processing, Denver, 1980: 964-967
[23]胡健栋.现代无线通信技术.北京:机械工业出版社.2003:1-84
[24]杨大成等著.现代移动通信中的先进技术.北京:机械工业外出版社.2005.7:3-48
[25]黄毅,方晓颖.尹秋艳.DVB-T中的COFDM调制技术.北京:现代电视技术.2003.12:92-93
[26] Bowei Song, Lin Gui, Wenjun Zhang. Comb type pilot aided channel estimation in OFDM systems with transmit diversity. IEEE Trans. On Broadcasting. Mar.2006, 52(1):50-57
[27] Jiachin Lin, Least-Squares Channel Estimation for Mobile OFDM Communication on Time-Varying Frequency-Selective Fading Channels. IEEE Trans, On Vehicular Technology, Nov.2008, 57(6):3538-3550
[28] LI Y, Seshadri N, Ariyavisitakul S. Channel estimation for OFDM systems with transmitter diversity in mobile wireless channels. IEEE Journal on Selected Areas in communications, 1999, 17(3):461-470
[29] LI Y. Simplified channel estimation for OFDM systems with multiple transmit antennas. IEEE Trans on Wireless Communication, Jan.2002, 1(1):67-75
[30] Minn H, Kim Di, Bhagava V K. A reduced complexity channel estimation for OFDM systems with transmit diversity in mobile wireless channels. IEEE Trans. On Comm. 2002, 50(5):799-801
[31] Foschini. G. J, Gans. M. J. On limits of wireless communication in a fading environment when using multiple antennas. Wireless Personal Comm, 1998, 6(3):311-335
[32]龚耀寰.自适应滤波(第二版).北京:电子工业出版社,2003.7:26-27
[33] L. J. Cimini. Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplexing. IEEE Trans. On Comm, Jul.1985, 33(7):665-675
[34] Edfors, M. Sandell, J. Jvande. Deek, S. K. Wilson, P. O. Borjesson. OFDM channel estimation by singular value decomposition. Proc.IEEE 46th Vehicular Technology Conference, Alanta, GA, USA, Apr.1996:923-927
[35] MOON. J. K, CHOI. S. I. Performance of channel estimation methods for OFDM systems in a multipath fading channels. IEEE Trans. On Consumer Electronics, 2000, 46(1):161-170
[36] Stephen. A. Dyer, Justin. S. Dyer. Cubic-Spline Interpolation: Part 1, Numbers, Mar 2001:44-66
[37] Stephen. A. Dyer, Xin He. Cubic-Spline Interpolation: Part 2, Numbers, Mar 2001:34-36
[38] Jae. Kyoung. Moon, Song. In. Choi. Performance of channel estimation methods for OFDM systems in a multipath fading channels. IEEE Trans. On Comm, Feb.2000, 46(1):161-170
[39] Baoguo Yang, Zhigang Cao, K B Letaief. Analysis of low-complexity windowed DFT-based MMSE channel estimator for OFDM systems. IEEE Trans. On Comm, Nov.2001, 49(11):1977-1987
[40] ETSI TR 101 112 V3.2.0,“Universal mobile telecommunications system (UMTS); selection procedures for the choice of radio transmission technologies of the UMTS (UMTS 30.03 version 3.2.0),”ETSI, Apr.1998
[41] Alim. O. A, Elboghdadly. N, Ashour. M. A, Elaskary. A. M. FPGA implementation for an optimized CORDIC module for OFDM system. Computer Engineering & Systems, ICCES 2008. International Conference. Nov.2008:21-26
[42] VOLDER. J. E. The CORDIC trigonometric computing technique. IEEE Trans. On Electronic Computers, 1959, EC-8(3):330-334
[43] B. Yang, K. B. Letaief, R. S. Chand, Z. Cao. Windowed DFT Based Pilot-Symbol-Aided Channel Estimation for OFDM Systems in Multipath Fading Channels. Proc. IEEE Vehicular Technology Conference 2000 (VTC’2000-Spring), Tokyo, Japan, May.2000:1480-1484
[44]李滔,韩秋月.基于流水线CORDIC算法的三角函数发生器.电子技术应用,1999, 25(6):45-49
[2] J. G. Proakis.数字通信(第四版).北京:电子工业出版社,2006.9:800-887
[3] R. W. Chang. Synthesis of band-limited orthogonal signals for multichannel data transmission. Bell Systems Tech. J., Dec.1996, Vol.45:1775-1796
[4] M. H. Hsieh, C. H. Wei. Channel estimation for OFDM systems based on comb-type piot arrangement infrequency selective fading channels, IEEE Trans. On Consumer Electronics. Feb.1998, 44(1):217-225
[5] M. Sandell, O. Edfors. A comparative study of pilot-based channel estimators for wireless OFDM. Research Report Tulea 1996: 17, in signal Processing, Lulea University of Technology, Sept.1996:175-181
[6] P. Hoeher, S. Kaiser, P. Robertson. Two-dimensional pilot-symbol aided channel estimation by Wiener filtering. in Proc. 1997 IEEE Int. Conf. Acoustics, Speech, and Signal Processing,”Munich, Germany, Apr.1997:1845-1848
[7] Vittoria Mignone, Alberto Morello. CD3-OFDM: A novel Demodulation Scheme for Fixed and Mobile Receivers. IEEE Trans. On Comm. 1996, 44(9):1144-1151
[8] Ahmad Chini, Yiyan Wu, et al. Filtered Decision Feedback Channel Estimation for OFDM Based DTV Terrestrial Broadcasing System. IEEE Trans. On Broadcasting, Mar.1998, 44(1):2-10
[9] Necker. M. C, Stuber. G. L. Totally blind channel estimation for OFDM on fast varying mobile radio channels. IEEE Trans. On Wireless Communications, Sept.2004, 3(5):1514-1525
[10] Petropulu. A, Ruifeng. Zhang. Blind channel estimation for OFDM systems. Digital Signal Processing Workshop, 2002 and the 2nd Signal Processing Education Workshop. Proceedings of 2002 IEEE 10th, 13-16 Oct. 2002:366-370
[11] B. Muquest, M. de. Courville. Blind and semi-blind channel identification methods using second order statistics for OFDM systems, Proc. ICASSP 99, Jan.1999, Vol.5: 2745-2748
[12] Chaocheng Tu, Champagne. B. Subspace-based Blind Channel Estimation for MIMO-OFDM Systems: Reducing the Time Averaging Interval of the Correlation Matrix. Personal, Indoor and Mobile Radio Communications, 2007. PIMRC 2007. IEEE 18th International Symposium on 3-7 Sept.2007:1-5
[13] G. B. Giannakis. Filterbanks for blind channel identification and equalization. IEEE Trans. On Signal Processing, Jun.1997, 4(6):184-187
[14] Feifei Gao. Wenyuan Wu. Yonghong Zeng. Nallanathan. A. A Novel Blind Channel Estimation for CP-Based MIMO OFDM Systems. Communications, 2007. ICC 07. IEEE International Conference on 24-28, Jun.2007:2586-2591
[15] M. Tsatsanis, G. B. Giannakis. Transmitter induced cyclostationarity for blind channel equalization. IEEE Trans. On Signal Processing, Jul.1997, 45(1):1785-1794
[16] Shengli Zhou, Georgios. B. Giannakis. Finite-alphabet based channel estimation for OFDM and related multicarrier systems. IEEE Trans. On Comm. Aug.2001, 49(8):1402-1414
[17] Shin. C, Heath. R. W, Powers. E. J. Blind Channel Estimation for MIMO-OFDM Systems. IEEE Trans. On Vehicular Technology, Mar.2007, 56(2):670-685
[18] ETSI. EN 300 421. Digital Video Broadcasting (DVB). Framing structure, channel coding and modulation for 11/12 GHz satellite services, 1997.8
[19]尹长川,罗涛,乐光新.多载波宽带无线通行技术.北京:北京邮电大学出版社,2004:1-97
[20] S. Weinstein, P. Ebert. Data transmission by frequency-division multiplexing using the discrete Fourier transform. IEEE Trans. On Comm, Oct.1971, 19(5):628-634
[21] J. Bingham. Multicarrier modulation for data transmission: An idea whose time has come. IEEE Trans. On Comm, Mar.1990, 28(5):5-14
[22] Peled, A. Ruiz. Frequency domain data transmission using reduced computational complexity algorithms. Proc. IEEE Int. Conf. Acoust Speech Signal Processing, Denver, 1980: 964-967
[23]胡健栋.现代无线通信技术.北京:机械工业出版社.2003:1-84
[24]杨大成等著.现代移动通信中的先进技术.北京:机械工业外出版社.2005.7:3-48
[25]黄毅,方晓颖.尹秋艳.DVB-T中的COFDM调制技术.北京:现代电视技术.2003.12:92-93
[26] Bowei Song, Lin Gui, Wenjun Zhang. Comb type pilot aided channel estimation in OFDM systems with transmit diversity. IEEE Trans. On Broadcasting. Mar.2006, 52(1):50-57
[27] Jiachin Lin, Least-Squares Channel Estimation for Mobile OFDM Communication on Time-Varying Frequency-Selective Fading Channels. IEEE Trans, On Vehicular Technology, Nov.2008, 57(6):3538-3550
[28] LI Y, Seshadri N, Ariyavisitakul S. Channel estimation for OFDM systems with transmitter diversity in mobile wireless channels. IEEE Journal on Selected Areas in communications, 1999, 17(3):461-470
[29] LI Y. Simplified channel estimation for OFDM systems with multiple transmit antennas. IEEE Trans on Wireless Communication, Jan.2002, 1(1):67-75
[30] Minn H, Kim Di, Bhagava V K. A reduced complexity channel estimation for OFDM systems with transmit diversity in mobile wireless channels. IEEE Trans. On Comm. 2002, 50(5):799-801
[31] Foschini. G. J, Gans. M. J. On limits of wireless communication in a fading environment when using multiple antennas. Wireless Personal Comm, 1998, 6(3):311-335
[32]龚耀寰.自适应滤波(第二版).北京:电子工业出版社,2003.7:26-27
[33] L. J. Cimini. Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplexing. IEEE Trans. On Comm, Jul.1985, 33(7):665-675
[34] Edfors, M. Sandell, J. Jvande. Deek, S. K. Wilson, P. O. Borjesson. OFDM channel estimation by singular value decomposition. Proc.IEEE 46th Vehicular Technology Conference, Alanta, GA, USA, Apr.1996:923-927
[35] MOON. J. K, CHOI. S. I. Performance of channel estimation methods for OFDM systems in a multipath fading channels. IEEE Trans. On Consumer Electronics, 2000, 46(1):161-170
[36] Stephen. A. Dyer, Justin. S. Dyer. Cubic-Spline Interpolation: Part 1, Numbers, Mar 2001:44-66
[37] Stephen. A. Dyer, Xin He. Cubic-Spline Interpolation: Part 2, Numbers, Mar 2001:34-36
[38] Jae. Kyoung. Moon, Song. In. Choi. Performance of channel estimation methods for OFDM systems in a multipath fading channels. IEEE Trans. On Comm, Feb.2000, 46(1):161-170
[39] Baoguo Yang, Zhigang Cao, K B Letaief. Analysis of low-complexity windowed DFT-based MMSE channel estimator for OFDM systems. IEEE Trans. On Comm, Nov.2001, 49(11):1977-1987
[40] ETSI TR 101 112 V3.2.0,“Universal mobile telecommunications system (UMTS); selection procedures for the choice of radio transmission technologies of the UMTS (UMTS 30.03 version 3.2.0),”ETSI, Apr.1998
[41] Alim. O. A, Elboghdadly. N, Ashour. M. A, Elaskary. A. M. FPGA implementation for an optimized CORDIC module for OFDM system. Computer Engineering & Systems, ICCES 2008. International Conference. Nov.2008:21-26
[42] VOLDER. J. E. The CORDIC trigonometric computing technique. IEEE Trans. On Electronic Computers, 1959, EC-8(3):330-334
[43] B. Yang, K. B. Letaief, R. S. Chand, Z. Cao. Windowed DFT Based Pilot-Symbol-Aided Channel Estimation for OFDM Systems in Multipath Fading Channels. Proc. IEEE Vehicular Technology Conference 2000 (VTC’2000-Spring), Tokyo, Japan, May.2000:1480-1484
[44]李滔,韩秋月.基于流水线CORDIC算法的三角函数发生器.电子技术应用,1999, 25(6):45-49