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Channelization code and IS-95 forward channels

Short answer

A channelization code is a spreading code separating same-source channels by orthogonality. In IS-95 forward, 64 Walsh codes create 64 channels: pilot on W0 for reference, sync on W32, paging on W1–W7, and traffic on the remaining 55.

What is channelization code? Explain briefly forward channels in cdma IS-95.

Channelization code

A channelization code is a spreading code used to separate the different channels (users) transmitted from the same source on one carrier. In IS-95 the forward link uses 64 orthogonal Walsh codes of length 64 chips (1.2288 Mcps). Because Walsh codes are mutually orthogonal (zero cross-correlation when time-aligned), channels from one base station do not interfere with each other. Cells are separated by a different code – the short PN code offset – so channelization codes can be reused in every cell.

 Walsh (order 4): W0 = + + + +
                  W1 = + - + -
                  W2 = + + - -
                  W3 = + - - +
 W1·W2 = (1 −1 −1 +1) sum = 0 → orthogonal

Forward channels in IS-95

The 64 Walsh codes give up to 64 logical channels on each 1.25 MHz forward carrier:

ChannelWalsh codeFunction
PilotW0Unmodulated reference for timing, coherent detection, handoff measurement
SyncW321200 bps; system time, PN offset, paging channel rate
PagingW1–W74800/9600 bps; paging, system parameters, channel assignment
TrafficRemaining 55User data with power control subchannel (800 bps)

Traffic data (9600 bps) is convolutionally coded (r = 1/2), interleaved, scrambled by the long code, Walsh-spread and then quadrature-spread by the I/Q short PN codes before QPSK transmission.

✓ Answer checked by Claude Opus 5.5 (high effort)

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