IS-95

This standard is being developed as the North American version of the Code Division Multiple Access (CDMA) standard. CDMA or Spread Spectrum Multiple Access is an access technique which, like TDMA and FDMA, allows several users to access the cellular network simultaneously. Unlike time and frequency schemes, CDMA users share all time and frequency resources simultaneously. This is accomplished by assigning each user a distinct Pseudo-Noise (PN) or user-unique digital code made up of "chips" (single bits within the PN code). This code is added to the information data and modulated onto the carrier. An identical PN code is used in the receiver which is used to correlate the two signals. The correlation process only passes data that matches the code sequence. In this way, non-valid signals (i.e. signals from other users) are not decoded and appear as noise [7]. Noise immunity is provided by means of the PN code spreading the spectrum off the information signal to a much wider bandwidth than is needed for baseband signal transmission. This ability of a CDMA system to reject unwanted signals allows for operation in high noise environments. IS-95 is based on this scheme which allows for several users to share the same channel for transmission. The basic user channel rate is 9.6 kb/s. This is spread to a channel chip rate of 1.2288 Mchips/s (a total spreading factor of 128) using a combination of techniques. The spreading process is different on the forward (base-to-mobile) and reverse links. On the forward link, the user data stream is encoded using a rate 1/2 convolutional code, interleaved, and spread by one of 64 convolutional spreading sequences (Walsh functions). Each mobile in a given cell is assigned a different spreading sequence, providing perfect separation among the signals from different users, at least for a single-path channel. [8] This form of multiple access offers the additional channel capacity and higher speech quality with the additional benefit of increased link reliability, variable rate speech coding, forward error correction coding, and reduced transmitter power requirement. However, this comes at the cost of increased system complexity and increased hardware costs.