  We consider single cell multi-user OFDMA downlink resource allocation on a
flat-fading channel such that average supply power is minimized while
fulfilling a set of target rates. Available degrees of freedom are transmission
power and duration. This paper extends our previous work on power optimal
resource allocation in the mobile downlink by detailing the optimal power
control strategy investigation and extracting fundamental characteristics of
power optimal operation in cellular downlink. We find that only a system wide
allocation of transmit powers is optimal rather than on link level. The
allocation strategy that minimizes overall power consumption requires the
transmission power on all links to be increased if only one link degrades.
Furthermore, we show that for mobile stations with equal channels but different
rate requirements, it is power optimal to assign equal transmit powers with
proportional transmit durations. To relate the effectiveness of power control
to live operation, we take the power model into consideration which maps
transmit power to supply power. We show that due to the affine mapping, the
solution is independent of the power model. However, the effectiveness of power
control measures is completely dependent on the underlying hardware and the
load dependence factor of a base station (instead of absolute consumption
values). Finally, we conclude that power control measures in base stations are
most relevant in macro stations which have load dependence factor of more than
50%.
