In this method, which is used in mobile communications systems, all users occupy the same frequency range, but the user signal is encoded differently for each user. A different code is used to transmit the bits for user "A" than for user "B".
Coding is based on spreading the user data channel. In this process, the individual bits of a narrowband user signal are replaced by longer bit combinations. If one bit is replaced by a bit combination of ten bits, for example, spreading by a factor of 10 is achieved. Although a higher transmission bandwidth is required, the transmission channel can be used for more user channels at the same time. The data of the individual users can be clearly distinguished from one another in the transmission channel.
The CDMA method offers the advantage that the available bandwidth is better utilized and a larger number of subscribers can access the transmission medium than with Frequency Division Multiple Access( FDMA) or Time Division Multiple Access( TDMA). It has been standardized by the Telecommunications Industry Association( TIA) under IS-95. This standard is based on 1.25 MHz bands, over which 128 channels of 9.6 kbit/s can be transmitted. A Walsh code with a total of 64 different codes is used for signal spreading.