Code division multiple access (CDMA) literally translates into code division multiple access, which is developed on the basis of spread spectrum communication, a branch of mathematical communication technology. The so-called spread spectrum is simply spread spectrum. CDMA technology adopts direct sequence spread spectrum technology, which is the same modulation technology as FM and AM technology. It uses a code sequence (high speed) to modulate the original data information (low speed), so that the modulated information can be transmitted at high speed. Each user is assigned to a channel with the same time slot and frequency, and each user is distinguished by a pseudo-random orthogonal code (PN code) sequence. Unicom's CDMA system uses this system.

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GSM (Global System for Mobile Communications) is a unified standard introduced by european committee for standardization in 1992. It adopts digital communication technology and unified network standard, which ensures the communication quality and can develop more new services for users. The transmission speed of GSM mobile communication network is 9.6k/s. At present, there are more than 500 million GSM mobile users in the world, covering112 population. The proportion of GSM technology in the world digital mobile phone field has exceeded 70%. Because GSM is the second generation mobile communication technology compared with analog mobile communication technology, it is called 2G for short. At present, China has more than 80 million GSM users and is the largest operating network in the world.

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