Fundamental BER-floor for long-code CDMA

In a discrete-time realisation of a long-code CDMA system, a non-zero probability of encountering a singular channel (linear dependency among spreading codes) leads to an inherent error floor at high SNR. In this paper we derive a lower bound on this error floor for a synchronous CDMA system in AWGN. The lower bound is found to correspond closely to the error floor encountered by the full-complexity maximum-likelihood detector.