  Adaptive transmission schemes are a crucial aspect of the radio design for
future wireless networks. The paper studies the performance of two classes of
adaptive transmission schemes in a cellular downlink. One class is based on
physical layer rateless codes with constant transmit power and the other uses
fixed-rate codes in conjunction with power adaptation. Using a simple
stochastic geometry model for the cellular downlink, the focus is to compare
the adaptivity of fixed-rate codes with power adaptation to that of physical
layer rateless codes only. The performance of both rateless and fixed-rate
coded adaptive transmission schemes are compared by evaluating the typical user
success probability and rate achievable with the two schemes. Based on both the
theoretical analysis and simulation results, it is clearly shown that
fixed-rate codes require power control to maintain good performance whereas
physical layer rateless codes with constant power can still provide robust
performance.
