Outage probability of multi-code DS-CDMA with linear interference suppression

We examine the outage probability of a multi-code DS-CDMA system with minimum mean squared error (MMSE) linear interference suppression. Multi-code DS-CDMA allows different information rates by varying the number of spreading codes assigned to each user. Power control is used to achieve a set of target signal-to-interference-plus-noise ratios (SINRs), and an outage occurs if a user must increase the transmitted power above a maximum power limit. To compete the outage probability, we derive an accurate approximation for the variance of received SINR over the user population assuming MMSE detection with short spreading codes. Outage probability is then shown as a function of the number of codes assigned to each user. For a fixed outage probability, our results show that a multi-code scheme can provide approximately 2 dB gain in SINR relative to the corresponding single-code (variable processing gain) scheme