One. Multiple access technology
Multiple access techniques so that many users share a common communication line. Is a signal multiplexing method for multiple access is achieved essentially three, which were used multiple access frequency, time or code partition, i.e. people often called frequency division multiple access (FDMA), time division multiple access ( TDMA), and code division multiple access (CDMA) three access mode. 0 means that these three methods is a simple concept with a model.
FDMA frequencies in a different communication channel realizations, TDMA is based on the different time slots for communication, CDMA code sequence in a different implement communication.
(A) Frequency Division Multiple Access
Frequency division, sometimes referred to as channelization, is, the entire allocated spectrum can be divided into a plurality of individual radio channel (carrier frequency for transmission and reception), each channel can transmit one voice or control information. Under the control of the system, any user can access any one of these channels. Analog cellular FDMA system is a typical example of the structure of digital cellular systems also may be employed FDMA, not only pure frequency-division manner, such as GSM and CDMA systems on the use of FDMA.
(B) Time Division Multiple Access
TDMA is a wireless carrier bandwidth, time (or time slots) into several time division channels, each user occupies one slot, closing only (or fat) within the designated time slot signal , it is known as Time division Multiple Access. This multiple access methods used in digital cellular systems, GSM system is also used in this way. TDMA is a more complex structure, the simplest case is a single carrier frequency is divided into a number of different time slots, each burst-type information transmission way. The TDMA on some of the key portion, each user is assigned to a time slot (at the start of a call assignment), synchronous communications between the user and the base station, the slot and counted. When the arrival of its own time slot, the mobile station starts the reception and demodulation circuit for burst-type information sent by the base station is decoded. Similarly, when the user wants to send a message, first information cached until the arrival of their own time slot. Slot after the start, and then transmit information at the rate doubling out, and then begin to accumulate the next burst transmission formula.
TDMA is a variation in a single frequency channel for transmission and reception, called time division duplex (TDD). In its simplest configuration is to use two time slots, a receive a hair. When the mobile station receives the transmission base station, the mobile station receives the base station transmitting, alternating. TDD TDMA structure having a number of advantages: the burst-type transmission, no reception shared antenna like device. Its main advantage is that may be implemented on a single carrier frequency to transmit and receive, without the need for both upstream and downstream carrier frequencies, no frequency switching, it is possible to reduce the cost. The main disadvantage is that TDD can not meet the capacity requirements of large-scale systems.
(C) Code Division Multiple Access
CDMA is a multiple access spread spectrum technology different code sequences formed by achieved. Unlike FDMA, TDMA as the user information is separated from the frequency and time, it can transmit information to a plurality of users on one channel, that is, to allow mutual interference between users. The key is information before transmission to be special codes, without losing the original information after mixing the encoded information. The number of code sequences orthogonal to each other, how many users can communicate simultaneously on one carrier. Each transmitter has its own unique code (pseudorandom code), while the receiver knows the code to be received, and use this code as a filter signal, the receiver can recover from all other signals in the background to the original information code (a process called despreading).