OFDM中的自适应调制技术研究
摘要
正交频分复用(OFDM)是一种新型的高效的多载波调制技术,它是由多载波调制MCM(Multi-Carrier Modulation)发展来的,能够有效地对抗多径传播,使受到干扰的信号能够可靠地接收。OFDM 各个子信道的正交调制和解调可以通过采用快速傅立叶反变换(IFFT)和快速傅立叶变换(FFT)来实现,而随着大规模集成电路技术和DSP 技术的发展,IFFT 和FFT 都是非常容易实现的。无线通信中,由于物体的运动或其他原因,信道是时变的。如果我们在设计通信系统时依据最坏的环境来设计,那么在信道好的时候,我们并没有完全利用信道容量。在无线频道资源日益紧张的今天,如何有效的利用有限的频段资源是一个比较重要的问题。使用自适应调制技术,在信道状况好的时候可以使用较高的调制,整个系统的频谱利用率得到大幅度的改善,自适应调制能够使系统容量翻番。
论文首先介绍OFDM 的基本结构和OFDM 技术的应用领域,然后论述了移动通信中的多径信道成因对信号传播的影响。本文主要研究自适应调制技术的基础:信道估计技术、MQAM 信噪比误比特率性能。在性能,实现复杂度等方面分析和比较了基于LS 准则和MMSE 准则和MMSE 的改进型三种信道估计算法;并讨论了三种信道估计内插算法:线性内插、二阶内插和DFT 内插;论文重点讨论了自适应OFDM-MQAM 的调制参数选择的问题,包括详细地推导了MPSK、矩形MQAM、圆形MQAM 调制在高斯信道和瑞利信道下的目标误比特率和比特信噪比的函数关系,在此基础上讨论了调制方式切换门限的两种确定方法:即时误比特率受限和平均误比特率受限。
在对OFDM自适应调制技术讨论的基础上,构建了Matlab/Simulink仿真模型,对OFDM 基带系统进行了仿真,并介绍了一种基于FPGA 的MQAM 调制器高效实现方法,主要是系数对称的CSD 编码滤波器和简化混频器的设计,使得系统中不需要乘法器。该方法可以提高系统的工作频率,而且消耗资源少,是较理想的MQAM 调制器设计方法。
Orthogonal frequency division multiplexing (OFDM) is a novel kind of multi-carriers modulating technique of high efficiency. It develops from MCM (Multi-Carriers Modulation) and it can effectively resist the multipath propagation. Thus we are able to receive the signal disturbed reliably. The quadrature modulation and demodulation of each subchannel can be realized with fast Fourier inverse transformation (IFFT) and fast Fourier transforms (FFT). IFFT and FFT can be easily implemented with the development of large scale integrated circuit technology and the DSP technology.
In the wireless communication, as a result of the object moving or other reasons, the channel is time-varying. If we design communications system according to the worst environment, we can not utilize the overall channel capacity completely when channel condition is good. It is a quite important problem to employ limited frequency band resources effectively today, when the wireless frequency resources is more and more sparse day and day. We can use a higher modulation by adaptive modulation technique when the channel condition is good. The overall system frequency utilization efficiency can be improved greatly, and the system capacity can be doubled.
Firstly, the basic structure and the application fields of OFDM is introduced, and then the reason of the mobile multipath channel’s forming and its influence on signals is elaborated. This letter mainly deals with the base of adaptive modulating technique: Channel estimation technology and the signal to noise ratio vs. error bit rate performance of MQAM. The three channels estimates algorithms: LS criterion, MMSE criterion and the improved version of MMSE are analyzed and compared in the performance and realization complexity. The paper also discusses three kinds of channel estimation interpolation algorithms: Linear interpolation, two steps interpolation and DFT interpolation. The emphasis of this paper is the parameter choice of the adaptive modulation and the detailed deduction of function relation between the signal to noise ratio and error bit rate of MPSK, rectangular MQAM and the circular MQAM modulation under the Gauss channel and the Rayleigh channel. And then it discussed two ways to choose the switch threshold, one is based on the limitation of immediately error bit rate and another is based on the limitation of average error bit rate.
On the base of the discussion of OFDM adaptive modulating technique ahead, the Matlab/Simulink simulation model is constructed and the simulation of OFDM baseband system is carried. This letter proposes an efficient approach to realize MQAM modulation based on FPGA, which makes use of the symmetry characteristic of FIR filter coefficients, canonic signed-digit and multiplexer to simplify the design of MQAM modulator. The system needs no multiplier. The result demonstrates the method has some improvements in size and internal fmax. It is an ideal method to design MQAM modulator.
论文首先介绍OFDM 的基本结构和OFDM 技术的应用领域,然后论述了移动通信中的多径信道成因对信号传播的影响。本文主要研究自适应调制技术的基础:信道估计技术、MQAM 信噪比误比特率性能。在性能,实现复杂度等方面分析和比较了基于LS 准则和MMSE 准则和MMSE 的改进型三种信道估计算法;并讨论了三种信道估计内插算法:线性内插、二阶内插和DFT 内插;论文重点讨论了自适应OFDM-MQAM 的调制参数选择的问题,包括详细地推导了MPSK、矩形MQAM、圆形MQAM 调制在高斯信道和瑞利信道下的目标误比特率和比特信噪比的函数关系,在此基础上讨论了调制方式切换门限的两种确定方法:即时误比特率受限和平均误比特率受限。
在对OFDM自适应调制技术讨论的基础上,构建了Matlab/Simulink仿真模型,对OFDM 基带系统进行了仿真,并介绍了一种基于FPGA 的MQAM 调制器高效实现方法,主要是系数对称的CSD 编码滤波器和简化混频器的设计,使得系统中不需要乘法器。该方法可以提高系统的工作频率,而且消耗资源少,是较理想的MQAM 调制器设计方法。
Orthogonal frequency division multiplexing (OFDM) is a novel kind of multi-carriers modulating technique of high efficiency. It develops from MCM (Multi-Carriers Modulation) and it can effectively resist the multipath propagation. Thus we are able to receive the signal disturbed reliably. The quadrature modulation and demodulation of each subchannel can be realized with fast Fourier inverse transformation (IFFT) and fast Fourier transforms (FFT). IFFT and FFT can be easily implemented with the development of large scale integrated circuit technology and the DSP technology.
In the wireless communication, as a result of the object moving or other reasons, the channel is time-varying. If we design communications system according to the worst environment, we can not utilize the overall channel capacity completely when channel condition is good. It is a quite important problem to employ limited frequency band resources effectively today, when the wireless frequency resources is more and more sparse day and day. We can use a higher modulation by adaptive modulation technique when the channel condition is good. The overall system frequency utilization efficiency can be improved greatly, and the system capacity can be doubled.
Firstly, the basic structure and the application fields of OFDM is introduced, and then the reason of the mobile multipath channel’s forming and its influence on signals is elaborated. This letter mainly deals with the base of adaptive modulating technique: Channel estimation technology and the signal to noise ratio vs. error bit rate performance of MQAM. The three channels estimates algorithms: LS criterion, MMSE criterion and the improved version of MMSE are analyzed and compared in the performance and realization complexity. The paper also discusses three kinds of channel estimation interpolation algorithms: Linear interpolation, two steps interpolation and DFT interpolation. The emphasis of this paper is the parameter choice of the adaptive modulation and the detailed deduction of function relation between the signal to noise ratio and error bit rate of MPSK, rectangular MQAM and the circular MQAM modulation under the Gauss channel and the Rayleigh channel. And then it discussed two ways to choose the switch threshold, one is based on the limitation of immediately error bit rate and another is based on the limitation of average error bit rate.
On the base of the discussion of OFDM adaptive modulating technique ahead, the Matlab/Simulink simulation model is constructed and the simulation of OFDM baseband system is carried. This letter proposes an efficient approach to realize MQAM modulation based on FPGA, which makes use of the symmetry characteristic of FIR filter coefficients, canonic signed-digit and multiplexer to simplify the design of MQAM modulator. The system needs no multiplier. The result demonstrates the method has some improvements in size and internal fmax. It is an ideal method to design MQAM modulator.
引文
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[2] [美]Clint Smith,Daniel Collins 著;李波卢树军张志龙孙敏琪等译;第3 代无线通信网络;北京:人民邮电出版社,2003,2
[3] 佟学俭罗涛编著; OFDM 移动通信技术原理与应用;北京:人民邮电出版社,2003.6
[4] 王文博郑侃编著; 宽带无线通信OFDM 技术;北京:人民邮电出版社,2003.11
[5] [美]Juha Heiskala ;John Terry 著杨晓春何建吾等译;OFDM 无线局域网;北京:电子工业出版社,2003.1
[6]Torabi, M.; Soleymani, M.R.;Adaptive modulation for OFDM systems using space-frequency block codes.Wireless Communications and Networking, 2003. WCNC 2003. 2003 IEEE ,Volume: 1 ,16-20 March 2003 .Pages:61 -65
[7] Cai, X.; Giannakis, G.B.; Error Probability Minimizing Pilots for OFDM With M-PSK Modulation Over Rayleigh-Fading Channels.Vehicular Technology, IEEE Transactions on , Volume: 53 , Issue: 1 , Jan. 2004 .Pages:146 -155
[8] 朱丽陈彦辉;无线局域网中的OFDM 技术;现代通信;2003 年第5 期
[9] 宋鹏鹏;OFDM 及其在蜂窝通信系统中的应用;西安电子科技大学硕士学位论文;2002年1 月
[10] Keller T,Hanzo L.Adaptive modulation techniques for duplex DFDM transmission [J].IEEE Transactionson Vehicular Technology,2000,49:1893-1906.
[11] Ye, S.; Blum, R.S.; Cimini, L.J., Jr.;Adaptive modulation for variable-rate OFDM systems with imperfect channel information.Vehicular Technology Conference, 2002. VTC Spring 2002. IEEE 55th , Volume: 2 , 6-9 May 2002.Pages:767 -771 vol.2
[12] Hyung Suk Chu; Chong Koo An;Bit and subcarrier allocation for OFDM transmission using adaptive modulation.Science and Technology, 2003. Proceedings KORUS 2003. The 7th Korea-Russia International Symposium on , Volume: 2 , June 28 -July 6, 2003.Pages:82 –85
[13] Souryal, M.R.; Pickholtz, R.L.;Adaptive modulation with imperfect channel information in OFDM.Communications, 2001. ICC 2001. IEEE International Conference on , Volume: 6 , 11-14 June 2001 .Pages:1861 -1865 vol.6
[14] Keller, T.; Hanzo, L.;Adaptive multicarrier modulation: a convenient framework for time-frequency processing in wireless communications.Proceedings of the IEEE , Volume: 88 , Issue: 5, May 2000 .Pages:611 –640
[15] 孟利民;宽带无线多媒体通信系统中自适应调制技术的研究;浙江大学博士学位论文;2002 年12 月
[16] 藏道华;无线信道中的自适应调制技术研究;南京理工大学硕士学位论文;2001.12.
[17] Wasantha, M.K.; Fernando, W.A.C.;QAM based adaptive modulation for OFDM-CDMA wireless networks.Wireless Personal Multimedia Communications, 2002. The 5th International Symposium on , Volume: 2 ,27-30 Oct. 2002 .Pages:715 -719 vol.2
[18] Kobayashi, H.; Fukuhara, T.; Hao Yuan; Takeuchi, Y.;Proposal of single carrier ofdm technique with adaptive modulation method.Vehicular Technology Conference, 2003. VTC 2003-Spring. The 57th IEEE Semiannual ,Volume: 3 ,April 22-25, 2003 ,Pages:1915 –1919
[19] 孟利民,杨扬,仇佩亮;基于DSP 技术的自适应调制器的设计;浙江工业大学学报;第29卷第4 期,2001 年12 月
[20] Morinaga N,Nakagawa M,Kohono R.New concepts and technologies for achieving highly reliable and high capacity multimedia wireless communications systems[J].IEEE Communications Maganizine,1997,(1):34-40.
[21] Goldsmith,A.J.;Soon-Ghee Chua;Variable-rate variable-power MQAM for fading channels. Communications, IEEE Transactions on , Volume: 45, Issue: 10, Oct. 1997 .Pages:1218-1230
[22] Ue, T.; Sampei, S.; Morinaga, N.; Symbol rate and modulation level controlled adaptive modulation/TDMA/TDD for personal communication systems.Vehicular Technology Conference, 1995 IEEE 45th , Volume: 1 , 25-28 July 1995 .Pages:306 -310 vol.1
[23] Webb, W.T.; Steele, R.; Variable rate QAM for mobile radio.Communications, IEEE Transactions on , Volume: 43 , Issue: 7 , July 1995 .Pages:2223 -2230
[24] Matsuoka, H.; Sampei, S.; Morinaga, N.; Kamio, Y.; Adaptive modulation system with punctured convolutional code for high quality personal communication systems.Universal Personal Communications. 1995. Record., 1995 Fourth IEEE International Conference on , 6-10 Nov. 1995.Pages:22 –26
[25] Yuanrun Teng; Nagaosa, T.; Mori, K.; Kobayashi, H.;Grouping adaptive modulation method for burst mode OFDM transmission system.TENCON '02. Proceedings. 2002 IEEE Region 10 Conference on Computers, Communications, Control and Power Engineering , Volume: 2 , 28-31 Oct. 2002 .Pages:929 -932 vol.2
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