Detection of the existence of a desired user within a cellular mobile communication system is considered. We assume that the effects of the interference by other users overwhelm those of other noise. The inter-user interference and user signals are modelled by the lognormal model. The observation model for this situation is proposed, the locally optimum test statistic is derived under the model, and the asymptotic performance of the locally optimum detector is compared with that of the envelope detector. We show that the locally optimum detector performs better than the conventional envelope detector.