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CDMA

IS-95 specifies a direct-sequence, spread spectrum CDMA system and does not follow the channelization principles of traditional cellular radio communications systems. Rather than dividing the allocated frequency spectrum into narrow bandwidth channels, one for each user, information is transmitted (spread) over a very wide frequency spectrum with as many as 20 mobile subscriber units using the same carrier frequency within the same frequency band. IS-95 is not asymmetrical as it specifies a different modulation and spreading technique for the forward (digital QPSK) and reverse (digital OQPSK) channels. On the forward channel, the base station simultaneously transmits user data from all current mobile units in that cell by using different spreading sequences (codes) for each user’s transmissions. A pilot code is transmitted with the user data at a higher power level, thus allowing all mobile units to use coherent detection. On the reverse link, all mobile units respond in an asynchronous manner (i.e. no time or duration limitations) with a constant signal level controlled by the base station. The speech coder used with IS-95 is the Qualcomm 9600-bps Code-Excited Linear Predictive (QCELP) coder. The vocoder converts an 8-kbps compressed data stream to a 9.6 kbps data stream. 

Advantages of CDMA:

Frequency diversity – frequency-dependent transmission impairments have less effect on signal 

Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation 

Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals 

Graceful degradation – system only gradually degrades as more users access the system

Limitations of CDMA

Self-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized 

Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away

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