宽带无线多媒体通信系统中自适应调制技术的研究
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摘要
自适应调制技术是移动宽带通信系统的核心技术之一。它的主要思想是通过研究无线信道的衰落程度、信道流量等参数动态地改变调制方式，在任何时刻都使信道容量达到最大或者使系统性能达到最佳，从而使移动无线信道频带日益拥挤的今天能得到较高的频谱利用率，有效地提高信息传输速率、从而提高传输系统的带宽。自适应调制技术是以当今计算机技术和数字信号处理技术高度发展为前提的，对自适应调制技术的算法和实现进行研究将对个人通信、卫星通信、多媒体无线通信、高速无线局域网和城域网的研究具有重要意义。目前，无线局域网和城域网是美国、欧洲和日本在无线通信领域研究的重点，它是继第三代移动通信后的高速无线多媒体通信网，具有广阔的应用前景，自适应调制技术就是其中要解决的核心技术之一。
自适应调制技术主要包括信号成形、信道估计和检测、全数字接收等方面的问题，由于自适应调制所涉及的信道是非常恶劣的无线传输信道，其调制与解调的算法和结构必须加以精心的设计和考虑，针对具体所假设的平衰落和选择性衰落的无线信道情况来设计自适应调制系统的算法和结构是本文研究的主要方向。
本文以QAM星座可调的自适应调制系统、符号率及调制电平可控的自适应调制系统为具体的研究对象。
在引入了慢速自适应调制和快速自适应调制的思想后，从宽带无线多媒体通信的需求角度论述了采用自适应调制的必要性，介绍了自适应调制技术在当前众多标准中的实际应用，并对自适应调制技术的研究背景和发展趋势作了一些扼要的介绍。然后，从理论上分析和推导了自适应调制系统的模型，给出了适合实际信道的三种信道模型结构并推导了脉冲响应矩阵方程。接着，对自适应调制系统中的信道估计问题难点，详细推导了平衰落信道条件下和选择性衰落信道条件下最大似然(ML)估计和最大后验概率(MAP)估计算法，针对平衰落信道，我们仿真了MAP估计和ML估计的方差与导频符号长度的关系，仿真结果表明，错误方差受多谱勒频率的变化影响最大，并且对实际的自适应调制系统，导频符号长度的取值超过20个符号长度时，MAP信道估计明显优于ML信道估计。
针对QAM星座可调的自适应调制系统，深入研究了系统的模型、帧结构、星型QAM星座和方型QAM星座的选择、实现的结构等，分析了方型星座下的判决门限，并给出了判决门限的仿真结果。着重分析了影响自适应调制系统性能的信道估计器，运用第三章推导的MAP信道估计算法，提出了导频强度估计器和导频信噪比估计器的具体实现结构，并给出了导频信噪比估计的性能仿真，结果表明：MAP估计的信噪比能动态地跟踪输入信噪比的变化；在小的多谱勒频率下，MAP信道估计的自适应调制系统与理想信道估计的自适应调制系统性能非常接近。
针对符号率及调制电平可控的自适应调制系统，深入研究了系统的模型、帧结构、判
决门限、信道测量方法、实现的结构等，将有限状态MORKOV信道模型应用到自适应调制
系统中，重点分析了调制参数选择规则，具体计算了载噪比判决门限和延时扩展判决l」限，
着重提出了延时扩展测量方法及算法、载噪比测量方法及算法以及信道脉冲响应的MAP估
计器结构。论文对符号率及调制电平可控的自适应调制系统的误比特率、数据吞吐量与固定
速率的系统进行了比较，表明自适应调制系统比固定速率调制系统在误比特率和数据吞吐量
性能上有明显的改进。
最后，给出了本论文的结论及进一步的研究方向。
Adaptive modulation technology is one of the crucial techniques for the mobile broadband systems. Its ideal is that modulation mode can be adaptively adjusted by estimating such wireless parameters as fading degree,channel traffic and etc. Channel capacity will achieve a maximum and system performance will be best anytime. By this way,although mobile wireless channel broadband is more and more congesting today,adaptive modulation can be provided preferable spectral efficiency and can be increased the information transmission rate. So the transmission system broadband will be effectively improved. Adaptive modulation technology is based on the rapidly development of computer technology and digital signal processing technology. Studying algorithm and implementation of the adaptive modulation technology have a signification for studying personal communications,satellite communications,multimedia communications,high wireless local area network and metro area network. At the present,wireless LAN and wireless MAN
is the most of the wireless communications field in America,Europe and Japan. Following the third-generation mobile communication,it is a high wireless multimedia communication network. It has extensively application foreground. Adaptive modulation technology is one of the key techniques that must be deal with.
Adaptive modulation mainly include several problem for example signal shape,channel estimation,all digital receiver and so on. Since the channel referring to adaptive modulation is very hostile in wireless transmission channel,its algorithms and architectures used for the modulation and demodulation should be carefully devised and considered. Designing appropriate algorithms and architectures of the adaptive modulation are main research goal of this paper to meet the need of flat fading channel and selective fading channel.
This paper,special research object is the QAM constellation controllable adaptive modulation system,symbol rate and modulation level controllable adaptive modulation system.
After introducing the ideals of the slow adaptive modulation and fast adaptive modulation,we demonstrated the need applying adaptive modulation from the aspects of broadband wireless multimedia communications. We have introduced the practice application of the adaptive modulation technology in many protocols nowadays. And we have briefly showed research background and developing trend of the adaptive modulation technology. Afterwards we theoretically analyze and conclude the model of the adaptive modulation system. We find out three kinds of channel model appropriated for actual cannels and draw out pulse response matrix equations. And then facing the problem of the channel estimation of the adaptive modulation system,we conclude out the channel estimation algorithms on maximum likelihood (ML) estimation and maximum a posteriori (MAP) estimation under the condition of flat fading channel and selective fading channel in detail. To meet flat fading channel,we simulate the relationship of the ratio between the
error covariance in MAP estimation and ML estimation and pilot symbol
message length. The conclusion can be drawn from these results. The error covariance can mainly be affected on Doppler spectrum. And considering the practical adaptive modulation,when the value of pilot symbol message length is more than twenty symbol message length,MAP channel estimation is obviously superior to ML channel estimation.
Facing the QAM constellation controllable adaptive modulation system,we go deep into the models,frame architecture,the constellation select for star QAM and square QAM,architecture implementation and so on. We elaborate on the slice decision of the square QAM. And we present the simulation result of the slice decision. We mainly stress the channel estimator that effect on the performance of adaptive modulation systems. Utilizing the MAP channel estimated algorithm we draw out from the third chapter,we put forward to the special implementation architecture for the pilot intensive estimator a
自适应调制技术主要包括信号成形、信道估计和检测、全数字接收等方面的问题，由于自适应调制所涉及的信道是非常恶劣的无线传输信道，其调制与解调的算法和结构必须加以精心的设计和考虑，针对具体所假设的平衰落和选择性衰落的无线信道情况来设计自适应调制系统的算法和结构是本文研究的主要方向。
本文以QAM星座可调的自适应调制系统、符号率及调制电平可控的自适应调制系统为具体的研究对象。
在引入了慢速自适应调制和快速自适应调制的思想后，从宽带无线多媒体通信的需求角度论述了采用自适应调制的必要性，介绍了自适应调制技术在当前众多标准中的实际应用，并对自适应调制技术的研究背景和发展趋势作了一些扼要的介绍。然后，从理论上分析和推导了自适应调制系统的模型，给出了适合实际信道的三种信道模型结构并推导了脉冲响应矩阵方程。接着，对自适应调制系统中的信道估计问题难点，详细推导了平衰落信道条件下和选择性衰落信道条件下最大似然(ML)估计和最大后验概率(MAP)估计算法，针对平衰落信道，我们仿真了MAP估计和ML估计的方差与导频符号长度的关系，仿真结果表明，错误方差受多谱勒频率的变化影响最大，并且对实际的自适应调制系统，导频符号长度的取值超过20个符号长度时，MAP信道估计明显优于ML信道估计。
针对QAM星座可调的自适应调制系统，深入研究了系统的模型、帧结构、星型QAM星座和方型QAM星座的选择、实现的结构等，分析了方型星座下的判决门限，并给出了判决门限的仿真结果。着重分析了影响自适应调制系统性能的信道估计器，运用第三章推导的MAP信道估计算法，提出了导频强度估计器和导频信噪比估计器的具体实现结构，并给出了导频信噪比估计的性能仿真，结果表明：MAP估计的信噪比能动态地跟踪输入信噪比的变化；在小的多谱勒频率下，MAP信道估计的自适应调制系统与理想信道估计的自适应调制系统性能非常接近。
针对符号率及调制电平可控的自适应调制系统，深入研究了系统的模型、帧结构、判
决门限、信道测量方法、实现的结构等，将有限状态MORKOV信道模型应用到自适应调制
系统中，重点分析了调制参数选择规则，具体计算了载噪比判决门限和延时扩展判决l」限，
着重提出了延时扩展测量方法及算法、载噪比测量方法及算法以及信道脉冲响应的MAP估
计器结构。论文对符号率及调制电平可控的自适应调制系统的误比特率、数据吞吐量与固定
速率的系统进行了比较，表明自适应调制系统比固定速率调制系统在误比特率和数据吞吐量
性能上有明显的改进。
最后，给出了本论文的结论及进一步的研究方向。
Adaptive modulation technology is one of the crucial techniques for the mobile broadband systems. Its ideal is that modulation mode can be adaptively adjusted by estimating such wireless parameters as fading degree,channel traffic and etc. Channel capacity will achieve a maximum and system performance will be best anytime. By this way,although mobile wireless channel broadband is more and more congesting today,adaptive modulation can be provided preferable spectral efficiency and can be increased the information transmission rate. So the transmission system broadband will be effectively improved. Adaptive modulation technology is based on the rapidly development of computer technology and digital signal processing technology. Studying algorithm and implementation of the adaptive modulation technology have a signification for studying personal communications,satellite communications,multimedia communications,high wireless local area network and metro area network. At the present,wireless LAN and wireless MAN
is the most of the wireless communications field in America,Europe and Japan. Following the third-generation mobile communication,it is a high wireless multimedia communication network. It has extensively application foreground. Adaptive modulation technology is one of the key techniques that must be deal with.
Adaptive modulation mainly include several problem for example signal shape,channel estimation,all digital receiver and so on. Since the channel referring to adaptive modulation is very hostile in wireless transmission channel,its algorithms and architectures used for the modulation and demodulation should be carefully devised and considered. Designing appropriate algorithms and architectures of the adaptive modulation are main research goal of this paper to meet the need of flat fading channel and selective fading channel.
This paper,special research object is the QAM constellation controllable adaptive modulation system,symbol rate and modulation level controllable adaptive modulation system.
After introducing the ideals of the slow adaptive modulation and fast adaptive modulation,we demonstrated the need applying adaptive modulation from the aspects of broadband wireless multimedia communications. We have introduced the practice application of the adaptive modulation technology in many protocols nowadays. And we have briefly showed research background and developing trend of the adaptive modulation technology. Afterwards we theoretically analyze and conclude the model of the adaptive modulation system. We find out three kinds of channel model appropriated for actual cannels and draw out pulse response matrix equations. And then facing the problem of the channel estimation of the adaptive modulation system,we conclude out the channel estimation algorithms on maximum likelihood (ML) estimation and maximum a posteriori (MAP) estimation under the condition of flat fading channel and selective fading channel in detail. To meet flat fading channel,we simulate the relationship of the ratio between the
error covariance in MAP estimation and ML estimation and pilot symbol
message length. The conclusion can be drawn from these results. The error covariance can mainly be affected on Doppler spectrum. And considering the practical adaptive modulation,when the value of pilot symbol message length is more than twenty symbol message length,MAP channel estimation is obviously superior to ML channel estimation.
Facing the QAM constellation controllable adaptive modulation system,we go deep into the models,frame architecture,the constellation select for star QAM and square QAM,architecture implementation and so on. We elaborate on the slice decision of the square QAM. And we present the simulation result of the slice decision. We mainly stress the channel estimator that effect on the performance of adaptive modulation systems. Utilizing the MAP channel estimated algorithm we draw out from the third chapter,we put forward to the special implementation architecture for the pilot intensive estimator a
引文
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[3] N.Morinaga, M.Nakagawa, R.Kohno. New concepts and technologies for achieving highly reliable and high capacity multimedia wireless communications systems. IEEE Communications Magazine, Jan 1997:34-40
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[9] S.Sampei, N.Morinaga, Y. Kamio. Adaptive modulation TDMA with a BDDFE for 2Mbit/s multimedia wireless communication system. 45th IEEE Vehic. Tech. Conf.,Chicago,IL,July 1995:311～315
[10] S.Markus, C.Sonke, G. Alexander. Dynamical channel allocation in hierachical cellular systems. IEEE 46th Vehicular technology conference. Vol.2, 1996:721～725
[11] T. Ikeda, S.Sampei, N.Morinaga. TDMA based adaptive modulation with dynamic channel assignment(AMDCA) for microcellular systems. Electronics letters. 20th June 1996,Vol.32 No.13:1175～1176
[12] E.Sourour, M.Nakagawa. Performance of orthogonal multi-carrier CDMA in a multipath fading channel. IEEE Trans. Comm., vol.44,no.3,1996:356～367
[13] M.Ohkawa, R.Kohno, H.Imai. Orthogonal multicarrier FH-CDMA schemes for frequency selective fading. IEICE Trans. Comm., vol.J77-B11,Nov.,1994:691～702
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