CDMA

With IS-95, each mobile user within a given cell, and mobile subscribers in adjacent cells use the same radio-frequency channels. Tn essence , frequency reuse is available in all cells. This is made possible because 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 bandwith channels, one for each user, information is transmitted over a very wide frequency spectrum with as many as 20 mobile subscriber units simultaneousaly using the same carrier frequency within the same frequency band. Interference is incorporated into the system so that there is no limit to the number of subscribers that CDMA can support.

With CDMA unlike other cellular telephone standards, subscriber data change in real time , depending on the voice activity and requirements of the networks and other users of the network. IS-95 also specifies a different modulation and spreading technique for the forward and reverse 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 for each users 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 is an asynchronous manner 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 coder. The vocoders converts an 8-kbps compressed data stream to a 9.6-kbps data stream. the vocoders original design detects voice activity and automatically reduces the data rate to 1200bps during silent periods. Intermediate mobile user data rates of 2400 bps and 4800 bps are also used for special purposes. In 1995 , Qualcomm introduced a 14,400 bps vocoder that transmits 13.4 kbps of compressed digital voice information.