8.5.4 Power Control And The Near–Far Problem
Because CDMA allows all users to transmit simultaneously over the same time and frequency resources, separation depends critically on relative signal power levels at the receiver. If one user’s received power is significantly stronger than another’s, the stronger signal can dominate the correlation process and prevent reliable detection of the weaker signal. This phenomenon is known as the near–far problem.
To understand this effect, consider two users transmitting simultaneously. Suppose the received power of user 1 is P1, and that of user 2 is P1. Even if the spreading codes are ideally uncorrelated, the interference contributed by user 1 to user 2’s detection process is proportional to P1, and vice versa. If P1>> P2 then user 2’s signal may be submerged beneath the residual interference from user 1 after de-spreading.
In an idealized system with equal received powers and processing gain Gp, the signal-to-interference ratio for each user can be approximated as:
where K is the number of active users. This expression illustrates two key properties of CDMA. First, capacity depends directly on processing gain. Second, capacity decreases as the number of users increases, because each additional user contributes to the interference term.
However, this relationship assumes equal received powers. If one user transmits with higher received power, it increases interference for all other users without suffering equivalent degradation itself. Consequently, CDMA systems require that all users be received at approximately the same power level.
Power control mechanisms are therefore essential. A network controller or distributed algorithm adjusts each transmitter’s output so that the received power at the base station or central receiver is maintained within a narrow tolerance band. In mobile systems, this adjustment must respond rapidly to fading, mobility, and shadowing. In fixed systems, slower control loops may suffice.
Power control in CDMA serves two distinct purposes. First, it mitigates the near–far problem by equalizing received powers among users. Second, it limits overall interference growth, thereby stabilizing system capacity. Without effective power control, CDMA performance degrades rapidly as load increases.
This reliance on power control differentiates CDMA from FDMA and TDMA. In frequency- and time-division systems, unequal user power primarily affects individual link margins but does not intrinsically degrade other users’ channels, provided spectral or temporal separation is maintained. In CDMA, by contrast, power imbalance directly impacts all users because interference is shared.
Thus, while FDMA is often distortion-limited and TDMA is often synchronization-limited, CDMA is fundamentally interference-limited and power-control-limited.
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