And the orthogonal frequency division multiplexing ( ofdm ) is given more and more attention in wireless area , because ofdm lessens the severe effects of frequency selective fading and improves the spectrum efficiency , and this technology has been indicated to become a key technology in the fourth moving communication 其中ieee802 . 11a协议中使用到的ofdm (正交频分复用)技术，由于其先天具有抗多径衰落及频谱使用率高等特性，成为当代无线通信领域研究的热点，并预示着为第四代移动通信的关键技术。
Because of the affection of noise and signal fading , in order to high data rate access and high quality , wireless communication need new technology to improve the link reliability and enhance the spectrum efficiency . mimo ( multiple input and multiple output ) can enhance the spectrum efficiency and increase channel capacity greatly , and reduce the multipath affection without spectrum band and power increasing . the 3th generation mobile communication wcdma is coming , with the purpose of putting mimo , stbc ( space time block coding ) , space diversity into practice of wcdma systems , in order to lay the foundation of 3g toward 4g , this dissertation has done some research under this background 在发射机和接收机使用多个天线进行数据传输的多输入多输出( mimo )技术,可以在不增加带宽和天线发送功率的条件下,成倍的提高频谱利用率,提高系统的信道容量,还可以抗多径干扰。第三代移动通信wcdma即将到来,为了将多天线技术( mimo ) ,空间分集技术,空时编码技术用到wcdma系统中去,为第三代移动通信向第四代移动通信系统平稳过渡奠定一定的基础,需要做一些理论上的研究。
Civil air defense alarm system was a system to dispense antiaircraft alarm sig - nal , hand hostile air attack informational communication to urbanite at wartime . it is a particular informational communication system of the civil air defense depart - ment , and can not be taken place . at the development trend of digitaliza - tion , unitization , integration of civil air defense communication , the civil air defense alarm system is developing too , for adaptation to the require of both future war and peaceful dual , and enhancing civil air defense communication system ' s directing automation . in recent years , development of different kinds of communication technology infuses new clearly energy into communication , especially spread spectrum communication , because of its strongly interference - free feature , invisibility best , realizable code division multiple access and antimultipath wane and so on merit , gets extensive use at wireless region ; the corresponding spread spectrum chips emerge as the times and get boom , which provides advantag e to realization of the circuit of spread spectrum communication . at the same time , in company with the advance of electronics level , the single chip microcomputer has a great development , world renowned chip manufacturer pushout respective products one after another , the kinds of which are too many to statiste 在人防通信的数字化、一体化、综合化必然的发展趋势下，人防警报系统也不断发展，以适应未来战争和和平时期的双重需要，提高人防通信系统指挥自动化。近年来，各种通信技术的发展给通信注入了活力，扩频通信更因其抗干扰能力强、隐蔽性好、可实现码分多址和抗多径衰落等优点在无线领域得到了广泛应用；其相应扩频处理芯片应运而生并得到迅速发展，这为扩频通信的电路实现提供便利。同时随着微电子工艺水平的提高，单片微型计算机有了飞跃发展，世界上著名的集成电路芯片制造商纷纷推出各自的产品，单片机型号之多，已达到难以统计的地步。
It is quite suitable to the lduwarc system demanding for covert communications while the signal - to - noise ratio ( snr ) is low . spread spectrum technique can fight against the intersymbol interferences caused by the multipaths propagation and fading , and it has the advantage of low probability of detection 由于它具有很好的抗多径、抗衰落和低的截获概率的优点，特别适合于处于低信噪比环境且保密性要求比较高的远程声遥控系统，所以我们采用扩频技术作为遥控系统的通信方式。
These key technologies include channel - coding which overcome white gaussian noise , rake receiver which resist time - selective and frequency - selective fading , multi - user detection which overcome multiple access interference , and power control which overcome near - far effect . they are effective in increasing the performance of resisting interference of cdma mobile communication system , and help to improve the reliability of transferring information 这些技术包括：主要用于抗白噪声的信道编码技术，特别是turbo码技术；主要用于抗多径干扰的rake接收技术；主要用于抗多址干扰的多用户检测技术；主要用于抗远近效应的功率控制技校术。
The videos , voices , data combined traffic and high speed data are becoming more and more predominent in the future , and how to efficiently transfer this multi - medium integrated - data is one of the researcher ' s interesting field . the cdma system , which has striking advantages of anti - multipath fading , soft capacities and large capacities over fdma and tdma systems , has been focused by all of the countries 未来信息社会中，图象，话音，数据相结合的多媒体业务和高速数据业务量越来越占主要地位，如何有效地传输这种多媒体数据，已经成为移动通信中的研究热点之一。扩频技术由于具有抗多径，软容量和时分多址与频分多址更大的容量和优势，而受到各国研究者的重视。
Ofdm system is a very promising technology since it has many advantages , such as its high - efficient spectrum and ici interference avoidance . it can also easily connect with mimo and access technologies like fdma , tdma , and cdma . because different subcarriers can adopt different coding and modulation methods , the capacity of the channel is able to reach the optimum value Ofdm多载波调制解调系统,具有抗多径干扰能力强,频谱利用率高,可以与mimo技术、不同的多址接入方式相结合,可使信道容量到达最优,所以基于ofdm的多载波调制解调系统越来越得到广泛的应用。
The direct - sequence ( ds / ss ) multiple access , also know as code - division access ( ds - cdma ) , has significant advantages in the areas of capacity , frequency planning , privacy , and resistance to multipath fading to cellular and personal communication system . it has been the important manner of multiple access in the 3rd generation mobile communication system 由于直接序列扩频多址接入技术（也叫ds - cdma ）在蜂房移动通信系统中显示出的良好性能：系统容量大，频率规划简单，频率复用系数高，抗多径能力强，保密性，抗干扰能力强等， ds - cdma在第三代移动通信系统中已经成为首选的多址接入方式。
Recently , multiple input multiple output ( mimo ) system which originates from the employment of multiple antennas at transmitter and receiver has many advantages such as large system capacity , high spectral efficiency . on the other hand , orthogonal frequency division multiplexing ( ofdm ) which bases on the orthogonal transmission of sub - carriers also has some obvious merits such as strong anti - multipath ability , high spectral efficiency . the combination of the two techniques mimo - ofdm , has become a hotspot recently and regarded as promising technique for future next - generation mobile communication systems 近来,在无线通信系统中的收发两端同时使用多根天线的多入多出( mimo , multiple - input - multiple - output )通信结构,具有系统容量大、频谱效率高等优点;基于正交的多路子载波并行传输的正交频分复用( ofdm , orthogonalfrequencydivisionmultiplexing )技术,具有抗多径能力强、频谱效率高等优点,二者的结合,即mimo - ofdm技术,被认为是构建下一代移动通信系统的框架技术。
