The second generation of digital cellular mobile communication (GSM mobile communication) service adopts the advanced switching method between digital wireless transmission and wireless cellular, which has higher frequency utilization than the first generation of analog cellular system. At this time, the mobile station can transmit and receive voice, data, supplementary services and other information services in the service area. The second generation mobile communication system, also known as 2G communication technology, mainly includes the content of G sm900 / 1800 dual band work, immediate billing, support for optimal routing (so), customized application of mobile network Enhanced Logic (cmael) and enhanced voice coding and decoding technology, which improves the voice quality. There are two typical systems for different access modes of mobile communication in 2G era: TDMA and CDMA. GSM has the advantages of high spectrum efficiency and large system capacity. Due to the increase of transmission bandwidth of each channel from the first generation analog system 25kHz to the GSM channel bandwidth 200kHz, the introduction of GSM half rate speech coding technology and traffic allocation make the capacity of GSM system higher than TACs, high voice quality, high security, interconnection with ISDN, PSTN, etc., and roaming on the basis of SIM card. CDMA uses 800MHz and 1900MHz frequency bands, and uses narrow-band code division multiple access technology, mainly in North America and South Korea. Because narrowband CDMA technology is more mature than GSM technology, its application in the world is less extensive than GSM technology, but compared with FDMA and TDMA technology, CDMA also has many unique advantages. The key technology advantages of CDMA are as follows: