多载波调制中的关键问题及实现
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摘要
随着互联网的快速发展及全球信息化进程的加快,人们对通信的需求己经逐渐从传统的语音业务向高速Internet接入、视频点播、视频会议、远程医疗等宽带业务转换,因此对通信网的传输能力提出了越来越高的要求。正交频分复用(OFDM)多载波调制技术作为第四代宽带通信(4G)系统的备选关键技术之一,具有很好的抗频率选择衰落特性和较高的频带利用率,适合于当前多媒体通信需求。然而,由于OFDM技术多载波传输的特点,使得其对同步误差异常敏感。符号定时偏差和载波偏差都将破坏子载波的正交性,从而引入符号间干扰(ISI)和子载波间干扰(ICI),使得系统整体性能降低。这就要求高精度的符号定时和载波偏差估计算法。
本论文主要分为两部分:在理论上,针对OFDM系统严格的同步要求,提出了一种新型的基于循环平稳特性的OFDM系统时延和频偏盲估计算法;在实际应用中,针对课题需要,在瑞泰公司的ICETEK-C6713-A开发板上实现了基带OFDM系统。具体工作如下:
论文首先介绍了OFDM技术的基本原理,分析了其基带系统框图和数学模型,并讨论了系统具体实现时傅里叶变换(FFT)、傅里叶逆变换(IFFT)间的能量转换问题。
其次,对OFDM系统的同步问题展开研究。在分析了符号定时误差、载波误差和样值误差对OFDM系统性能影响的基础上,对两种典型同步算法进行了原理分析并给出了仿真结果。进一步,基于OFDM信号的循环平稳特性,提出了一种适用于频率选择衰落信道环境的OFDM系统时延和频偏盲估计算法:通过分析接收信号的循环平稳特性,分别利用两个不用的循环频率来估计时延和频偏。经理论分析和计算机仿真实验验证,这种算法构造的估计器具有对信噪比不敏感和时延、频偏估计互不影响的优点。
再次,为减少算法实现过程中调试的复杂度,提出了用Maltab Link for Code Composer Studio (CCSLink)辅助数字信号处理器(DSP)实现OFDM系统算法仿真的方法。同时,结合使用的硬件平台,给出了OFDM系统发送端、接收端的软件设计,包括多路缓冲串行口(McBSP)、增强型直接存储器访问(EDMA)的配置和音频模块的使用等,并给出了具体的软件流程。
最后,在瑞泰公司的ICETEK-C6713-A开发板上实现了整个基带OFDM系统,并对系统进行了测试。测试结果验证了系统的正确性,为今后的进一步研究提供了硬件平台。
With the rapid development of the Internet and the accelerated process of global information, the communications people needed have gradually shifted from voice services to high-speed internet access, video on demand, video conferencing, telemedicine and broadband services transformation. So it requires high transmission capacity of communication network. Orthogonal frequency division multiplexing (OFDM) has been regarded as a candidate for the coming 4G-wideband mobile communication systems. It has excellent resistance to frequency selective fading characteristics and high bandwidth efficiency, and is ideal for the current multimedia communications needs. However, OFDM is especially sensitive to synchronization errors due to the characteristic of multi-carrier transmission. The symbol timing and carrier frequency offsets both destroy the orthogonality among the subcarriers, thereby result in intersymbol interference (ISI) and intercarrier interference (ICI) and reduce the overall performance of the system. So high-precision timing and frequency offset estimation are the key factors to determine the performance of OFDM system.
As the strict requirements of synchronization in OFDM system and project needs, the thesis mainly discusses the blind estimation of timing offset and frequency offset in OFDM system, it also discusses the implementation of OFDM systems. The detailed works are as follows:
First of all, we introduce the basic principle of OFDM technology. Then we give the base-band system and the mathematical model. In addition, we analyze the energy conversion between the Fourier Transform (FFT) and Inverse Fourier Transform (IFFT) in the OFDM system implemention.
Secondly, the synchronization in OFDM system is investigated. After analyzing the performance of symbol errors, carrier errors and sample errors in the OFDM systems, we study two typical synchronization algorithms and provide computer simulation result. Furthermore, based on cyclostationary characteristics, we propose a blind synchronization algorithm for estimating carrier frequency and symbol offset in frequency-selective fading channels in OFDM systems. Through analyzing the cyclostationarity of the received signal, we select two different cycle frequencies for estimating the carrier frequency and symbol offset. Theoretical analysis and computer simulation results show that the proposed algorithm is robust to Signal-to-Noise (SNR) in wide-sense stationary noise, also the symbol timing estimator and the frequency estimator do not rely on each other.
Thirdly, to reduce the algorithm complexity in the process of debugging, we propose a method from using CCSLink to assist DSP implementing the algorithm in OFDM system. Meanwhile, based on hardware platform, we implement the software design and process of the transmitter and receiver, including: the configuration of multi-buffered serial port (McBSP) and enhanced direct memory access (EDMA); the use of AIC23 Coder and so on.
Finally, the base-band OFDM system is implemented in Skandis’s ICETEC-C6713-A evaluation board. The test results verify the correctness of the system, which provides a hardware platform for further research.
本论文主要分为两部分:在理论上,针对OFDM系统严格的同步要求,提出了一种新型的基于循环平稳特性的OFDM系统时延和频偏盲估计算法;在实际应用中,针对课题需要,在瑞泰公司的ICETEK-C6713-A开发板上实现了基带OFDM系统。具体工作如下:
论文首先介绍了OFDM技术的基本原理,分析了其基带系统框图和数学模型,并讨论了系统具体实现时傅里叶变换(FFT)、傅里叶逆变换(IFFT)间的能量转换问题。
其次,对OFDM系统的同步问题展开研究。在分析了符号定时误差、载波误差和样值误差对OFDM系统性能影响的基础上,对两种典型同步算法进行了原理分析并给出了仿真结果。进一步,基于OFDM信号的循环平稳特性,提出了一种适用于频率选择衰落信道环境的OFDM系统时延和频偏盲估计算法:通过分析接收信号的循环平稳特性,分别利用两个不用的循环频率来估计时延和频偏。经理论分析和计算机仿真实验验证,这种算法构造的估计器具有对信噪比不敏感和时延、频偏估计互不影响的优点。
再次,为减少算法实现过程中调试的复杂度,提出了用Maltab Link for Code Composer Studio (CCSLink)辅助数字信号处理器(DSP)实现OFDM系统算法仿真的方法。同时,结合使用的硬件平台,给出了OFDM系统发送端、接收端的软件设计,包括多路缓冲串行口(McBSP)、增强型直接存储器访问(EDMA)的配置和音频模块的使用等,并给出了具体的软件流程。
最后,在瑞泰公司的ICETEK-C6713-A开发板上实现了整个基带OFDM系统,并对系统进行了测试。测试结果验证了系统的正确性,为今后的进一步研究提供了硬件平台。
With the rapid development of the Internet and the accelerated process of global information, the communications people needed have gradually shifted from voice services to high-speed internet access, video on demand, video conferencing, telemedicine and broadband services transformation. So it requires high transmission capacity of communication network. Orthogonal frequency division multiplexing (OFDM) has been regarded as a candidate for the coming 4G-wideband mobile communication systems. It has excellent resistance to frequency selective fading characteristics and high bandwidth efficiency, and is ideal for the current multimedia communications needs. However, OFDM is especially sensitive to synchronization errors due to the characteristic of multi-carrier transmission. The symbol timing and carrier frequency offsets both destroy the orthogonality among the subcarriers, thereby result in intersymbol interference (ISI) and intercarrier interference (ICI) and reduce the overall performance of the system. So high-precision timing and frequency offset estimation are the key factors to determine the performance of OFDM system.
As the strict requirements of synchronization in OFDM system and project needs, the thesis mainly discusses the blind estimation of timing offset and frequency offset in OFDM system, it also discusses the implementation of OFDM systems. The detailed works are as follows:
First of all, we introduce the basic principle of OFDM technology. Then we give the base-band system and the mathematical model. In addition, we analyze the energy conversion between the Fourier Transform (FFT) and Inverse Fourier Transform (IFFT) in the OFDM system implemention.
Secondly, the synchronization in OFDM system is investigated. After analyzing the performance of symbol errors, carrier errors and sample errors in the OFDM systems, we study two typical synchronization algorithms and provide computer simulation result. Furthermore, based on cyclostationary characteristics, we propose a blind synchronization algorithm for estimating carrier frequency and symbol offset in frequency-selective fading channels in OFDM systems. Through analyzing the cyclostationarity of the received signal, we select two different cycle frequencies for estimating the carrier frequency and symbol offset. Theoretical analysis and computer simulation results show that the proposed algorithm is robust to Signal-to-Noise (SNR) in wide-sense stationary noise, also the symbol timing estimator and the frequency estimator do not rely on each other.
Thirdly, to reduce the algorithm complexity in the process of debugging, we propose a method from using CCSLink to assist DSP implementing the algorithm in OFDM system. Meanwhile, based on hardware platform, we implement the software design and process of the transmitter and receiver, including: the configuration of multi-buffered serial port (McBSP) and enhanced direct memory access (EDMA); the use of AIC23 Coder and so on.
Finally, the base-band OFDM system is implemented in Skandis’s ICETEC-C6713-A evaluation board. The test results verify the correctness of the system, which provides a hardware platform for further research.
引文
[1] Michael Speth, Stefan Fechtel, Gunnar Fock, and Heinrich Meyr. Optimum Receiver Design for OFDM-Based Broadband Transmission—Part II: A Case Study[C]. IEEE Trans. On Commun. 2001.
[2]吕卓.宽带无线通信中OFDM关键技术的研究[D].西安:西安电子科技大学,2005.
[3]王文博,郑侃.宽带无线通信OFDM技术[M].人民邮电出版社,2003.
[4]龙腾,John M.Cioffi,刘峰.xDSL技术与应用[M].北京:电子工业出版社,2002.
[5]许莉.基于OFDM技术的电力线通信系统中同步技术的研究与实现[D].江苏:东南大学,2003.
[6] Schmidl T M, Cox D C. Robust frequency and timing synchronization for OFDM [J]. IEEE Transition on Communication, 1997, 1613-1621.
[7] A J Coulson. Maximum likelihood synchronization for OFDM using a pilot symbol: Analysis [J]. IEEE J. Select. Areas Commun, 2001, 2495-2503.
[8] S. B. Weinstein, P. M. Ebert. Data Transmission by Frequency-division Multiplexing Using the Discrete Fourier Transform[C]. IEEE Trans. Commun. Tech. 1971.
[9]胡梅霞,张海林.基于循环平稳特性的OFDM盲同步算法[J].西安电子科技大学学报,2005,32(2):264-267.
[10] Bolcskei H. Blind estimation of Symbol Timing and Carrier Frequency Offset in Wireless OFDM System [J]. IEEE Transactions on Communication, 2001, 49(6):988-999.
[11] L.J. Cimini. Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplexing [J]. IEEE Transcations on Communications, 1985, 665-675.
[12] John G. Proakis, Masoud Salehi, Gerhard Bauch著,刘树棠译.现代通信系统(第二版)[M].北京:电子工业出版社,2005.
[13]傅祖云,信息论基础理论与应用[M].北京:电子工业出版社,2001.
[14]邱家涛.正交频分复用(OFDM)技术在无线通信中的应用研究及硬件实现[D].贵州:贵州大学,2005.
[15]刘超.基于DSP的OFDM调制解调器的设计与实现[D].江苏:东南大学,2007.
[16] Chen B, Wang H. Blind estimation of OFDM carrier frequency offset via oversampling [J]. IEEE Transmision on Signal Processing, 2004, 2047-2057.
[17] Andreas F. Molisch, Wideband Wireless Digital Communications [M].许希斌,赵明等译,北京:电子工业出版社,2002.
[18] Li Youming, Amir Leshem, and Fang Liming. Computationally Efficient Approximated Matrix Inversion with Application to Crosstalk Precoding in Downstream VDSL [J]. Proceedings of the International Conference on Wireless Communications and Mobile Computing, 2007, 429-433.
[19]郭里婷,朱近康.一种OFDM系统同步参数盲估计方法[J].通信学报,2005,26(9):35-40.
[20]司秉楠,卓东风,李艳武.一种基于PN序列和循环前缀的OFDM同步算法.山西:山西电子技术,2008.
[21] Dafara F, and Chouly A. Maximum-likelihood frequency detectors for orthogonal multi-carrier systems [J]. In Proc. Int. Conf., 1993, 766-771.
[22] W A Gardner. Exploitation of spectral redundancy in cyclostationary signals. IEEE ASSP Magazine[J], 1991, 8(4):14-36.
[23] Peter Hoeher .A Statistical Discrete-Time Model for the WSSUS Multipath Channel [J].IEEE Transactions on Vehicular Technology, 1992, 41(4):461-468.
[24]王娜,张邦宁.频偏条件下基于循环平稳的定时估计算法[J].电子科技大学学报,2008,37(2):191-194.
[25] O Simeone, Y bar-Ness, U Spagnolini. Pilot-based Channel Estimation for OFDM systems by Tracking the Delay-subspace [J]. IEEE Transactions on Communication, 2004, 3(1):315-325.
[26] Gardner W A. Exploitation of spectral redundancy in cyclostationary signals [J].IEEE Signals Processing Magazine, 1991, 8(4):14-36.
[27]丁建华.NMT物理样机的设计与实现[D].大连:大连海事大学, 2008.
[28] Biao Chen, Hao Wang. Blind Estimation of OFDM Carrier Frequency Offset via Oversampling [J]. IEEE Transactions on Signal Processing, 2004,52(7): 2047-2057.
[29]李方慧,王飞等. TMS320C6000系列DSPs原理与应用(第二版)[M].北京:电子工业出版社,2005.
[30] Texas Instruments. TMS320C67x DSP Library Programmer’s Referemce Guide. http://www.ti.com, 2003: 37-69.
[31]邹彦,唐冬,宁志刚等.DSP原理及应用(第4版)[M].北京:电子工业出版社,2006.
[32]流向宇.DSP嵌入式常用模块与综合系统设计实例精讲[M].北京:电子工业出版社,2009.
[33]赵加祥等.DSP系统设计和BIOS编程及应用实例—基于TMS320C67X系列DSP芯片[M],北京:机械工业出版社,2008.
[34]尹勇,欧光军等. DSP集成开发环境CCS开发指南[M].北京:北京航天航空大学出版社,2003.
[35]淘伟.DSP嵌入式无线通信系统开发实例精讲[M].北京:电子工业出版社,2009.
[36]胡晓磊,喻俊志.基于CCSLink的两栖机器人伺服系统级开发[J].微电脑信息,2009,25(9-2):154-156.
[37] MATLAB Link for Code Composer Studio Development Tools User’s Guide [DB]. The Math Works Inc. 2002-2003.
[38]余成波,陶红艳等.数字信号处理及matlab实现(第二版)[M],北京:清华大学出版社,2008.
[39]李真芳,苏涛,黄小宇.DSP程序开发:MATLAB调试及直接代码生成[M].西安:西安电子科技大学出版社,2003.
[40]段国强,陈月云.MATLAB辅助DSP设计的研究与实现[J].微电脑信息,2007,23(7-2):130-132.
[41] Texas Instruments.TLV320AIC23 Stereo Audio Codec, 8- to 96- kHz, with Integrated Headphone Amplifier Data Manual. Texts Instruments, INC. 2001.
[42]伍坚.基于数字中频技术的OFDM收发信机研究与实现[D].北京,北京交通大学,2006.
[43] Su Yongtao, Zhang Xianda, Gao Qiubin, Ding Zizhe. Particle-Based Iterative Blind Receiver for Orthogonal Frequency-Division Multiplexing with Frequency Offset [J]. TSINGHUA SCIENCE AND TECHNOLOGY, 12(6):678-683.
[44]孟捷.数字调幅广播系统的基带调制解调研究[D].江苏:东南大学,2006.
[45] O Simeone, Y bar-Ness, U Spagnolini. Pilot-based Channel Estimation for OFDM systems by Tracking the Delay-subspace [J]. IEEE Transactions on Communication, 2004, 3(1):315-325.
[46]范欣.基于OFDM技术的高速实时水声通信技术研究[D].哈尔滨:哈尔滨工程大学,2009.
[47]何平征.基于ARM+DSP的OFDM水下图像传输系统的研究与实现[D].厦门:厦门大学,2008年.
[48]花再军.基于DSP的OFDM中压电力线Modem关键技术研究与实现[D].北京:北京交通大学,2008.
[2]吕卓.宽带无线通信中OFDM关键技术的研究[D].西安:西安电子科技大学,2005.
[3]王文博,郑侃.宽带无线通信OFDM技术[M].人民邮电出版社,2003.
[4]龙腾,John M.Cioffi,刘峰.xDSL技术与应用[M].北京:电子工业出版社,2002.
[5]许莉.基于OFDM技术的电力线通信系统中同步技术的研究与实现[D].江苏:东南大学,2003.
[6] Schmidl T M, Cox D C. Robust frequency and timing synchronization for OFDM [J]. IEEE Transition on Communication, 1997, 1613-1621.
[7] A J Coulson. Maximum likelihood synchronization for OFDM using a pilot symbol: Analysis [J]. IEEE J. Select. Areas Commun, 2001, 2495-2503.
[8] S. B. Weinstein, P. M. Ebert. Data Transmission by Frequency-division Multiplexing Using the Discrete Fourier Transform[C]. IEEE Trans. Commun. Tech. 1971.
[9]胡梅霞,张海林.基于循环平稳特性的OFDM盲同步算法[J].西安电子科技大学学报,2005,32(2):264-267.
[10] Bolcskei H. Blind estimation of Symbol Timing and Carrier Frequency Offset in Wireless OFDM System [J]. IEEE Transactions on Communication, 2001, 49(6):988-999.
[11] L.J. Cimini. Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplexing [J]. IEEE Transcations on Communications, 1985, 665-675.
[12] John G. Proakis, Masoud Salehi, Gerhard Bauch著,刘树棠译.现代通信系统(第二版)[M].北京:电子工业出版社,2005.
[13]傅祖云,信息论基础理论与应用[M].北京:电子工业出版社,2001.
[14]邱家涛.正交频分复用(OFDM)技术在无线通信中的应用研究及硬件实现[D].贵州:贵州大学,2005.
[15]刘超.基于DSP的OFDM调制解调器的设计与实现[D].江苏:东南大学,2007.
[16] Chen B, Wang H. Blind estimation of OFDM carrier frequency offset via oversampling [J]. IEEE Transmision on Signal Processing, 2004, 2047-2057.
[17] Andreas F. Molisch, Wideband Wireless Digital Communications [M].许希斌,赵明等译,北京:电子工业出版社,2002.
[18] Li Youming, Amir Leshem, and Fang Liming. Computationally Efficient Approximated Matrix Inversion with Application to Crosstalk Precoding in Downstream VDSL [J]. Proceedings of the International Conference on Wireless Communications and Mobile Computing, 2007, 429-433.
[19]郭里婷,朱近康.一种OFDM系统同步参数盲估计方法[J].通信学报,2005,26(9):35-40.
[20]司秉楠,卓东风,李艳武.一种基于PN序列和循环前缀的OFDM同步算法.山西:山西电子技术,2008.
[21] Dafara F, and Chouly A. Maximum-likelihood frequency detectors for orthogonal multi-carrier systems [J]. In Proc. Int. Conf., 1993, 766-771.
[22] W A Gardner. Exploitation of spectral redundancy in cyclostationary signals. IEEE ASSP Magazine[J], 1991, 8(4):14-36.
[23] Peter Hoeher .A Statistical Discrete-Time Model for the WSSUS Multipath Channel [J].IEEE Transactions on Vehicular Technology, 1992, 41(4):461-468.
[24]王娜,张邦宁.频偏条件下基于循环平稳的定时估计算法[J].电子科技大学学报,2008,37(2):191-194.
[25] O Simeone, Y bar-Ness, U Spagnolini. Pilot-based Channel Estimation for OFDM systems by Tracking the Delay-subspace [J]. IEEE Transactions on Communication, 2004, 3(1):315-325.
[26] Gardner W A. Exploitation of spectral redundancy in cyclostationary signals [J].IEEE Signals Processing Magazine, 1991, 8(4):14-36.
[27]丁建华.NMT物理样机的设计与实现[D].大连:大连海事大学, 2008.
[28] Biao Chen, Hao Wang. Blind Estimation of OFDM Carrier Frequency Offset via Oversampling [J]. IEEE Transactions on Signal Processing, 2004,52(7): 2047-2057.
[29]李方慧,王飞等. TMS320C6000系列DSPs原理与应用(第二版)[M].北京:电子工业出版社,2005.
[30] Texas Instruments. TMS320C67x DSP Library Programmer’s Referemce Guide. http://www.ti.com, 2003: 37-69.
[31]邹彦,唐冬,宁志刚等.DSP原理及应用(第4版)[M].北京:电子工业出版社,2006.
[32]流向宇.DSP嵌入式常用模块与综合系统设计实例精讲[M].北京:电子工业出版社,2009.
[33]赵加祥等.DSP系统设计和BIOS编程及应用实例—基于TMS320C67X系列DSP芯片[M],北京:机械工业出版社,2008.
[34]尹勇,欧光军等. DSP集成开发环境CCS开发指南[M].北京:北京航天航空大学出版社,2003.
[35]淘伟.DSP嵌入式无线通信系统开发实例精讲[M].北京:电子工业出版社,2009.
[36]胡晓磊,喻俊志.基于CCSLink的两栖机器人伺服系统级开发[J].微电脑信息,2009,25(9-2):154-156.
[37] MATLAB Link for Code Composer Studio Development Tools User’s Guide [DB]. The Math Works Inc. 2002-2003.
[38]余成波,陶红艳等.数字信号处理及matlab实现(第二版)[M],北京:清华大学出版社,2008.
[39]李真芳,苏涛,黄小宇.DSP程序开发:MATLAB调试及直接代码生成[M].西安:西安电子科技大学出版社,2003.
[40]段国强,陈月云.MATLAB辅助DSP设计的研究与实现[J].微电脑信息,2007,23(7-2):130-132.
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