基于多载波传输系统的自适应技术研究

【作者】 亓中瑞；

【导师】 高振明；

【作者基本信息】 山东大学 ， 通信与信息系统， 2005， 硕士

【摘要】 目前的数字无线通信系统正在完成从语音通信到多媒体通信的转变。移动用户飞速增长的业务需求和日益紧张的频率资源要求更有效的传输技术来实现频谱资源的高效利用。从这一点出发,多元正交幅度调制(MQAM)、多载波调制(MCM)及自适应调制技术成为下一代移动无线系统中最有竞争力的几种候选技术。 作为一种新型高速的传输技术,MCM被人们广泛的应用于现代无线通信系统中。该技术相对于其它调制技术的显著特点是通过串行数据的并行传输实现在多径信道中信息的有效传递。众所周知,当无线信道的时延扩展远远小于发送符号的持续时间时,系统中符号间干扰(ISI)所带来的影响就可以忽略不计。MCM正是这样一种技术,通过发送端的串并转换及逆的快速傅立叶变换(IFFT)实现发送数据的多载波传输,有效增加发送符号的周期长度。实践证明,在频率选择性衰落信道中,MCM技术可以大幅度提高系统的误比特率(BER)性能,特别是在高速数据传输的场合。 在众多MCM的实现方法中,正交频分复用(OFDM)和滤波多音频调制(FMT)是最为典型的两种代表性技术。由于两种技术设计的出发点不同,OFDM和FMT被应用于不同的通信标准中。凭借其良好的抗符号间干扰能力和对信道时间弥散的鲁棒性,OFDM技术已被用于众多无线通信标准中,例如数字视频广播(DVB)、数字音频广播(DAB)、高性能局域网(HIPERLAN/2)及IEEE 802.11a等。然而由于OFDM存在频偏敏感性的缺点,在多普勒频移丰富,同步技术粗糙的环境中,FMT无疑是一个非常合理的选择。该技术以牺牲部分抗符号间干扰(ISI)能力为代价来换取系统抗信道间干扰(ICI)的性能,目前主要被应用于有线通信的标准当中。 为了最大限度的应用系统资源,自适应技术被广泛的应用到现代通信系统中来。自适应调制、自适应功率控制、自适应滤波、自适应均衡、自适应编码等技术已成为如今研究的热门话题。将自适应技术与MCM技术相结合亦成为当今通信发展的必然趋势。本文以多载波技术为基础,对部更多还原

【Abstract】 Digital wireless communication systems are now changing from voice communication to multimedia communication. Tremendous growth in demand of the users and limitation of our frequency resource needs more efficient transmission techniques to realize high efficiency of spectrum utilization. From this point, M-ary Quadrature Amplitude Modulation (MQAM), MultiCarrier Modulation (MCM) and adaptive modulation techniques are good candidates in next generation mobile radio system.As a new technique for high data rate transmission, MCM systems have been extensively used in high-speed data transmission applications. Their main characteristic over other modulation schemes is that they transmit data in parallel subcarriers across the channel. As we know, the effects of the Inter Symbol Interference (ISI) are negligible so long as the delay spread is significantly shorter than the duration of one transmitted symbol. This requirement can be met in MCM system by serial to parallel (S/P) conversion and IFFT at the transmitter. With MCM techniques, BER performance can be improved remarkably over frequency selective fading scenarios when high data rate is required.There are many approaches to carry out MCM. Orthogonal Frequency Division Multiplexing (OFDM) and Filtered MultiTone modulation (FMT) are two typical techqniues among them. Due to the different purposes of the two techniques, OFDM and FMT are employed in different communication standards. Because of the immunity to ISI and the robustness to the time dispersion of the channel, OFDM has been used in many communication standards, such as digital video broadcasting (DVB), digital audio broadcasting (DAB), high performance loacal area network (HIPERLAN/2), IEEE 802.11a and etc. For the disadvantage of OFDM in sensitivity to frequency offset, whereas, FMT is an idea choice for communication in the environment with abundant Doppler frequency shift and coarsesynchronization technique. More immunity to Inter Channel Interference (ICI) can be achieved by sacrificing the partly immunity to IS I in FMT. It has been widely used in wire communication standards. In order to utilize the system resources as much as possible, adaptive techniques are widely employed in the commuincation systems, including:1. Adaptive modulation and power control. Adaptive modulation is a typical one of adaptive techniques. It has been extensively used in wireless communication systems in order to enhance the frequency efficiency and the ability of resisting noise. Simulations are made over multicarrier modulation system. Methods advocated here can increase the frequency efficiency remarkably while maintaining the BER performance of the system. In addition, power control method is employed at the same time. Simulation results can prove the efficiency of this method.2. Adaptive bit and power loading. Wireless channel is separated into several sub-channels in MCM systems. One of the advantages is that we can distribute the transmitting power and bits flexibly and reasonably according to the channel scenarios. So we can see that, adaptive bit and power loading is a unique adaptive technique in MCM system. In order to reduce the complexity of the algorithm in MCM system, we modified the greedy algorithm and proposed a method based on sub-band instead of sub-channel. Simulation results in FMT prove the efficiency of this method.3. Adaptive image transmission. Adaptive image transmission can be a representative application of adaptive technique. Here we combined source division and adaptive sub-channel distribution techqnues to accomplish the adaptive image transmission over slow Rayleigh fading channel. We proposed two kinds of methods to carry it out. Simulation results and theory analysis demonstrate that we can improve the subjective quality of the received image remarkably and enhance the ability of resisting noise significantly.Because of the limiation of various impersonal reasons, partly simulation and analysis for adaptive technique in MCM system have been done in this paper. There are many topics, such as the application of adaptive technique inrestraining the Peak to Average Power Ratio (PAPR) in MCM and the application in Coded Modulation (CM), expected for the further exploration in theory. More study on these topics will be done in future work.更多还原