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In the 21st century , the mobile communication technology is moving forward at an unprecedented rate . As users of the various real-time multimedia services, increased demand and the rapid development of Internet technology , it can be expected that future mobile communications and information technology will have a higher transmission rate , provides users with greater convenience , the network structure fundamental change will occur .

第一代移动通信系统（1G）主要采用模拟和频分多址（FDMA）技术，由于受到传输带宽的限制，不能进行移动通信的长途漫游，只能是一种区域性的移动通信系统。

The first generation of mobile communication systems (1G) analog and main frequency division multiple access (FDMA) technology , due to transmission bandwidth limitations , can not be roaming mobile communication distance , only a regional mobile communication system . The first generation of mobile communications has many inadequacies , such as limited capacity , standard too , incompatible , poor security , call quality is not high , can not provide data services .

第二代移动通信系统(2G)主要采用的是数字的时分多址(TDMA)技术和码分多址(CDMA)技术。全球主要有GSM和CDMA两种体制。由于第二代采用不同的制式，移动通信标准不统一，用户只能在同一制式覆盖的范围内进行漫游，因而无法进行全球漫游。

Second generation mobile communication system (2G) is mainly used in digital time division multiple access (TDMA) technology and code division multiple access (CDMA) technology . The world's major institutions have both GSM and CDMA . Because the second generation of different format, a mobile communication standards are not unified , the user can in the same system to roam within the coverage , and can not be global roaming , since the second generation digital mobile communication system, limited bandwidth , limiting the data service application , can not be achieved a high rate of mobile multimedia services such as services.

Third generation mobile communication systems (3G) using (CDMA) technology , as opposed to second-generation mobile communication technology , the third generation mobile communication technology will have a wider bandwidth , the transmission speed minimum 384k / s, up to 2Mbit / s bandwidth up to 5MHz or more. Currently, there are three criteria , namely Europe 's WCDMA, CDMA2000 and of the United States put forward proposed TD-SCDMA. Third generation mobile communications to achieve high-speed data transmission and multimedia communications .

The fourth generation mobile communication systems (4G) research originated in the beginning of this century , although the third generation mobile communication system has reached a data transmission speed of 2Mbit / s, but still can not meet the requirements of multimedia communications , and we are now committed to the study's first four generations of mobile communication systems can reach 10Mbit / s to 20Mbit / s, to provide greater bandwidth to meet the requirements. ITU has begun to develop the next generation of mobile communication systems , with the 4G standards , OFDM 4G will be written as a mainstream technology standards .
By the multi-carrier OFDM modulation (MCM) evolved . U.S. military back in the 50s and 60s of last century to create the world's first MCM system , derived in 1970 using large scale and frequency of overlapping subcarrier OFDM system technology . But after a long period of time , OF本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。