Forward CDMA (IS-95) channel

The forward (downlink) channel of IS-95 carries signals from the base station to the mobiles on a 1.25 MHz carrier at 1.2288 Mcps. It is divided into 64 logical channels using 64-chip Walsh codes (orthogonal), so the downlink users do not interfere with each other within a cell. Each base station is told apart by the time offset of its short PN code.

Forward traffic channel processing

 speech 9600 bps (rate set 1, also 4800/2400/1200)
   |
 [Conv. encoder r=1/2, K=9] -> 19.2 ksps
   |
 [Symbol repetition] (for lower rates) -> 19.2 ksps
   |
 [Block interleaver, 20 ms]
   |
 (XOR)<--[Long code gen]->[Decimator 64:1] (user mask)
   |                         |
 [MUX] <--power ctrl bits 800 bps (decimated long code
   |       picks position)
 (XOR)<-- Walsh code Wn (64 chips) -> 1.2288 Mcps
   |
  +--> (x) I short PN -> [Baseband filter] -> cos wct
  +--> (x) Q short PN -> [Baseband filter] -> sin wct
                              |
                       (+) -> QPSK RF out

Steps

1. Vocoder: variable-rate speech coder (QCELP) gives 9600, 4800, 2400 or 1200 bps in 20 ms frames.
2. Convolutional coding: rate 1/2, constraint length 9 → 19.2 ksps; lower rates are repeated to keep 19.2 ksps.
3. Block interleaving over 20 ms to fight burst errors.
4. Data scrambling: XOR with the user's long PN code (2⁴² − 1), decimated to 19.2 ksps; gives privacy.
5. Power control subchannel: power control bits (800 bps) are punctured into the stream; they tell the mobile to raise or lower power by 1 dB.
6. Walsh spreading: each symbol is multiplied by 64-chip Walsh code → 1.2288 Mcps; gives orthogonal channelisation.
7. Quadrature spreading: the same chips are spread by I and Q short PN codes (2¹⁵ length, base-station specific offset).
8. Filtering and QPSK modulation onto the carrier; all channels are summed and transmitted.

Pilot channel: all-zero data spread by W0 – it lets the mobile acquire timing and do coherent detection and RAKE combining.