Short note: CDMA design considerations

CDMA design considerations are the factors that decide the capacity and quality of a CDMA system, since all users share the same band at the same time.

1. Spreading codes – Codes must have sharp autocorrelation (for synchronisation and multipath resolution) and low cross-correlation (for low multiple-access interference). Walsh codes (orthogonal) and long/short PN codes (m-sequences, Gold codes) are used.
2. Processing gain – G = W/R (chip rate/bit rate). Higher G gives more interference rejection and more users; e.g. IS-95: 1.2288 Mcps / 9.6 kbps = 128 (21 dB).
3. Power control – Because of the near–far problem, a nearby mobile can swamp a distant one. Fast closed-loop and open-loop power control keep all signals arriving at equal power (IS-95: 800 updates/s).
4. Interference-limited capacity – Capacity N ≈ 1 + (W/R)/(Eb/N₀); so lowering required Eb/N₀ (good coding), voice activity (factor ≈ 2.5), sectorization and frequency reuse factor 1 increase capacity.
5. Synchronisation and code acquisition – Receivers must acquire and track the code phase within a fraction of a chip.
6. Multipath handling – RAKE receivers combine resolvable paths (separated by more than one chip).
7. Soft handoff – Mobile communicates with several base stations together; needs network support.
8. Bandwidth – Wide band needed; must suit spectrum available (1.25 MHz in IS-95).