In contemporary wireless systems, simple ARQ is used above the physical 
layer in order to provide reliable data communication. As a result, 
there is a tradeoff between the physical layer transmitted rate and 
the probability of packet error/ARQ retransmissions. This tradeoff is 
non-trivial when the transmitter does not have knowledge of the 
instantaneous channel (e.g., high velocity mobiles in cellular systems). 
We determine the optimum transmitted rate for such systems, and 
characterize the dependence of the optimum on the average SNR and the 
time/frequency selectivity of the channel