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The probability of incorrect beam selection (PIBS) with a switched multibeam antenna system is theoretically analyzed under power-limited and interference-limited mobile communication environments. Periodic transmission of unique word sequence followed by information symbol sequence is assumed. The model of beam selection used in this paper is based upon a simple three-stage mechanism: 1) signal validation; 2) averaging; and 3) selection of the largest output. Complex correlation peaks corresponding to the unique words, detected by the matched filter for the signal validation, are averaged over several consecutive unique words, and a beam having the largest output is selected. Equally weighted noncoherent integration is assumed for the averaging process. The beam selection takes place frame-by-frame. The first half of this paper is devoted to the PIBS derivation for a simple switched two-beam antenna system. Numerical calculation results are then presented. The latter half of this paper investigates the impact of the incorrect beam selection on overall average signal-to-noise (SNR) power ratio and signal-to-interference (SIR) power ratio. The amount of the overall SNR (or SIR) improvement over the omnidirectional antenna depends on the propagation conditions. In general, larger improvements can be achieved by smaller values of PIBS. Index Terms - Digital cellular system, switched multibeam antenna, TDMA, time diversity, unique word detection.