Читать книгу Spatial Multidimensional Cooperative Transmission Theories And Key Technologies - Lin Bai - Страница 13

In 1992, with the birth of the first digital cellular mobile communication network, namely the global system for mobile (GSM) communications, mobile communication entered the second generation (2G). Due to its superior performance, GSM made rapid progress worldwide. In 1993, the first all-digital mobile phone GSM system was completed in China, and then both China Telecom and China Unicom adopted GSM. The GSM system has the following main characteristics: microcell structure, digitalization of voice signals, new modulation methods (GMSK, QPSK, etc.), the use of FDMA or time division multiple access (TDMA) technology, high spectrum utilization, high confidentiality, etc.

In 1995, Qualcomm proposed another digital cellular system technology solution using code division multiple access (CDMA), which is IS-95 CDMA. It is currently used in Hong Kong, South Korea, and North America with good reviews from users. The CDMA system mainly has the following characteristics: the user access mode adopting CDMA, soft capacity, soft cut-in, large system capacity, anti-multipath fading, voice activation, and diversity reception, etc.

Compared with the 1G system, 2G system has higher spectrum utilization, stronger security, and better voice quality. Up to now, the 2G system standards have become more and more perfect, and the technology is relatively mature. However, as people’s demand for data services continues to increase, the rate provided by the 2G system is no longer sufficient, and a stronger system is needed to support high-speed mobile communications.