This paper use a site-specific model to characterize the performance of millimeter wave BPSK system with single cochannel interference. Shooting and bouncing ray/image techniques are applied to compute the impulse responses for concrete-wall-partition rooms and plasterboard-wall-partition rooms. By using the impulse responses of these multipath channels, the BERs (Bit Error Rates) for high-speed BPSK (binary phase shift keying) systems with phase and timing recovery circuits are calculated. In addition, the carrier-to-interference ratio is also computed. Numerical results show that the interference for the plasterboard-wall-partition rooms is more severe than that for the concrete-wall-partition rooms.