CDMA Code Division Multiple Access. is a channel access method Both data and voice are separated from signals using codes and then transmitted using a wide frequency range. CDMA is the preferred technology for the 3G generation.

FDMA: Division by frequency each pair of communicators is allocated part of the spectrum for all of the time TDMA: Division by time each pair of communicators is allocated all of the spectrum for part of the time CDMA: every communicator will be allocated the entire spectrum all of the time.

CODE TYPES PN codes:   Pseudo-random number codes (pseudo-noise or PN code) can be generated very easily. A feature of PN codes is that if the same versions of the PN code are time shifted, then they become almost orthogonal, and can be used as virtually orthogonal codes within a CDMA system. Truly orthogonal codes:   Two codes are said to be orthogonal if when they are multiplied together the result is added over a period of time they sum to zero. For example a codes 1 -1 -1 1 and 1 -1 1 -1 when multiplied together give 1 1 -1 -1 which gives the sum zero. An example of an orthogonal code set is the Walsh codes used within the IS95 / CDMA2000 system.

FEATURES Many users share the same frequency There is no absolute limit on the number of users . System performance is inversely proportional to the no. of users Channel data rates are very high in CDMA systems A RAKE receiver is used to improve reception by collecting time delayed versions of required signal. Self jamming is a problem The near-far problems at receiver

CDMA system showing use of spreading codes

CDMA spread spectrum CDMA spreading the data is multiplied using the XOR function. Each bit in the spreading sequence is called a chip

CDMA spread spectrum generation

CDMA spread spectrum decoding

CDMA spreading gain Spreading gain = W / R w - the bandwidth of the CDMA spread spectrum signal R - the input data bit length System performance is proportional to the spreading gain of CDMA spreading factor - chip rate/ data rate

ADVANTAGES using this CDMA technique, it is possible to transmit several sets of data independently on the same carrier and then reconstitute them at the receiver without mutual interference. In this way a base station can communicate with several mobiles on a single channel.

CDMA IS-95 Interim Standard 95 (IS-95) is the first CDMA-based digital cellular standard also known as TIA-EIA-95 It is described as a 3 layer stack. The speech coder is Qualcomm 9600 Code Excited Linear Predictive (QCELP)coder. That vocoder is detects voice activity, Reduces data rate to 1200 bps during silent periods

CDMA2000 CDMA2000 1X EV-DV known as IMT Multi‑Carrier (IMT‑MC) The set of standards includes:  CDMA2000 1X,  CDMA2000 EV-DO Rev. 0,  CDMA2000 EV-DO Rev. A,  CDMA2000 EV-DO Rev. CDMA2000 1X EV-DV 1x-meaning 1 times Radio Transmission Technology

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