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. In the case where there is a high signal to noise ratio, we consider the scenario where adaptive modulation is used over a fixed bandwidth channel, and find the optimal symbol alphabet size $M$ to maximize 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 $C$. This formula is then used to characterize the optimal processing gain or the optimal symbol alphabet size as the solution of a simple fixed point equation that depends on the wireless channel parameters and the parameters of the TCP connection.