Abstract-We consider a single TCP session traversing a wireless channel, with a constant signal to noise ratio (SINR) at the receiver. We consider the problem of determining the optimal transmission energy per bit, to maximize TCP throughput. Specifically, in the case where direct sequence spread spectrum modulation is used over a fixed bandwidth channel, we find the optimal processing gain m that maximizes TCP throughput. Block codes applied to each packet for forward error correction can also be used, and in that case we consider the joint optimization of the coding rate to maximize TCP throughput. Finally, we discuss the issue of assigning target SINR values. In order to carry out our analysis, we obtain a TCP throughput formula in terms of the packet transmission error probability p and the transmission capacity C, which is of independent interest. In our TCP model, the window size is cut in half for each packet transmission loss, and also cut in half whenever the window size exceeds the transmission capacity C.