CDMA Channel Spreading

The IS-95 CDMA system uses a 1.23 MHz wide radio channel that is divided into 64 coded channels. Of these 64 channels, some are used for traffic (user voice and/or data) and some are used for control purposes (paging and the coordination of access to the system).

To create a wide digital radio channel from low-speed digital audio or control signals, each bit the information signal is multiplied (converted) into long sequence of bits called a spreading code. This spreading code effectively represents each bit of an information signal by multiple RF information signals (chips) over a frequency band that is much wider than the information signal. This is why it is called a wideband system.

By using different channel spreading codes for each information signal, multiple coded channels can co-exist at the same time on the same frequency. The CDMA system uses pseudo-random noise (PN) codes to create 64 uniquely coded channels for each CDMA radio channel. To receive each coded channel, radio receivers look for the pre-defined unique spreading code. When it finds a match to the channel code (a match to all the chips of an information bit), it converts (de-spreads) the code into its original information signal.

Each CDMA radio channel uses some of the coded channels for a signal timing reference pilot; channel timing synchronization, paging, and access control channels. Each of these channels is received by decoding (dispreading) the signals using the proper PN sequences.

This figure shows how IS-95 CDMA radio channels can provide multiple communication channels through the use multiple coded channels.  This diagram shows that a code pattern mask is used to decode each communication channel. The channel mask is shifted along the radio channel until the code chips (or a majority of the code chips) match the expected code pattern. When a match occurs, this produces a single bit of information (a logical 1 or 0). This example shows that the use of multiple code patterns (multiple masks in this example) allow multiple users to share the same radio channel.

CDMA Channel Spreading Diagram