8.5.3 Multiuser Operation And Multiple-Access Interference
In a CDMA system, the received signal is the linear superposition of all simultaneously transmitted spread signals plus noise. If there are K active users, the received waveform may be expressed conceptually as:
where each sk(t) represents a spread signal using a distinct code sequence.
At the receiver, detection of user i is performed by correlating r(t) with the spreading code assigned to user i. Ideally, cross-correlation between different users’ codes is zero, so that only the desired signal contributes significantly to the decision statistic. In practice, codes are only approximately orthogonal, and imperfect synchronization further degrades orthogonality. The residual contribution from other users is known as multiple-access interference (MAI).
Multiple-access interference is a defining characteristic of CDMA systems. Unlike FDMA and TDMA, where interference arises primarily from implementation imperfections such as nonlinear distortion or timing error, interference in CDMA is intrinsic to the multiple-access mechanism. Each additional user raises the interference floor experienced by all other users.
As the number of active users increases, the aggregate interference power grows approximately linearly, and the effective signal-to-interference-plus-noise ratio decreases. System capacity is therefore not defined by a fixed number of orthogonal channels, but by the maximum number of users that can be supported while maintaining acceptable error performance. This behavior is often described as soft capacity: rather than a hard limit, performance degrades gradually as user load increases.
The performance of CDMA depends strongly on the cross-correlation properties of the spreading sequences. Orthogonal codes can provide near-perfect separation when users are precisely synchronized, but in asynchronous systems orthogonality is partially lost. Pseudorandom sequences offer good average cross-correlation properties and are widely used in practice.
Because CDMA permits simultaneous transmission over identical time and frequency resources, system design must carefully manage interference accumulation. The most critical mechanism for doing so is power control, which is examined in the next section.
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