In this paper, we propose an explicit rate change notification (ERCN) mechanism to improve the transmission control protocol (TCP) performance in the wideband code-division multiple-access (WCDMA) system. The base station in the WCDMA system dynamically adapts transmission data rates according to the wireless channel condition. Therefore, if the TCP sender can not be notified about the rate change in the wireless channel, congestion in the radio link between the base station and mobile terminal occurs quite often. In this work, we first derive the relation between the bandwidth in the physical layer and the maximum queue size in the TCP layer, and then obtain a closed-form expression for the average throughput in terms of average queue size, queue delay and channel capacity. We validate results by simulations. The analytical formula can facilitate the design of a cross-layer TCP-physical congestion control mechanism for the rate adaptive wireless system.