8.5.6 Performance Characteristics
The performance of a CDMA system differs fundamentally from that of FDMA and TDMA because interference is intrinsic to its operation. Whereas frequency- and time-division systems attempt to prevent overlap, CDMA allows overlap and manages interference statistically through processing gain and power control.
As shown in Equation (8.11), if all users are received with approximately equal power and processing gain is Gp, the system capacity increases with processing gain, which is proportional to the spread bandwidth. Second, capacity decreases as more users become active, since each contributes additional interference.
Unlike FDMA, which has a fixed number of frequency channels, or TDMA, which has a fixed number of time slots per frame, CDMA does not impose a hard limit on user count. Instead, as the number of users increases, the interference level rises and the error probability gradually worsens. This behavior is often described as graceful degradation or soft capacity. Beyond a certain load, performance becomes unacceptable, but there is no abrupt cutoff.
CDMA systems are therefore typically interference-limited rather than bandwidth-limited or distortion-limited. Their achievable capacity depends on acceptable error probability, required data rate, and processing gain, as well as on the effectiveness of power control. If power control is imperfect, capacity can degrade sharply due to the near–far effect.
In environments with multipath propagation, wideband CDMA can offer additional robustness because delayed signal components may be combined constructively. This can improve energy efficiency relative to narrowband systems under similar channel conditions.
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