Reverse CDMA (IS-95) channel

The reverse (uplink) channel of IS-95 carries signals from the mobile to the base station on a 1.25 MHz carrier at 1.2288 Mcps. It has two kinds of channels: access channels (4800 bps, for call origination and replies to pages) and reverse traffic channels (user data). Users are separated by their long PN code (user-specific mask); since there is no pilot, the base station uses non-coherent 64-ary orthogonal (Walsh) modulation.

 data 9600/4800/2400/1200 bps
    |
 [Conv. encoder r=1/3, K=9] -> 28.8 ksps
    |
 [Symbol repetition] -> 28.8 ksps
    |
 [Block interleaver, 20 ms]
    |
 [64-ary orthogonal modulator]
   6 code symbols -> 1 Walsh word (64 chips)
   4.8 ksym/s -> 307.2 kcps
    |
 [Data burst randomizer] (gating for low rates)
    |
 (XOR)<--[Long code generator] (user mask)
    |      1.2288 Mcps
    +--(x) I short PN --[filter]--> cos wct --+
    |                                         (+)-> out
    +--(x) Q short PN-[1/2 chip delay]-[filter]
                         --> sin wct ---------+
                (OQPSK)

Steps

1. Variable-rate vocoder data in 20 ms frames.
2. Rate 1/3, K = 9 convolutional coding → 28.8 ksps (lower rates repeated).
3. Block interleaving over 20 ms.
4. 64-ary orthogonal modulation: each group of 6 symbols selects one of 64 Walsh codes → 307.2 kcps.
5. Data burst randomizer: at lower rates, only the needed power-control groups are sent (pseudo-random gating), saving power and reducing interference.
6. Long code spreading (42-bit, user mask from ESN) → 1.2288 Mcps; identifies the user.
7. Quadrature spreading by I and Q short PN codes, Q branch delayed by half a chip → OQPSK, which reduces envelope variations and suits the mobile's power amplifier.