  Traditional communication theory focuses on minimizing transmit power.
However, communication links are increasingly operating at shorter ranges where
transmit power can be significantly smaller than the power consumed in
decoding. This paper models the required decoding power and investigates the
minimization of total system power from two complementary perspectives.
  First, an isolated point-to-point link is considered. Using new lower bounds
on the complexity of message-passing decoding, lower bounds are derived on
decoding power. These bounds show that 1) there is a fundamental tradeoff
between transmit and decoding power; 2) unlike the implications of the
traditional "waterfall" curve which focuses on transmit power, the total power
must diverge to infinity as error probability goes to zero; 3) Regular LDPCs,
and not their known capacity-achieving irregular counterparts, can be shown to
be power order optimal in some cases; and 4) the optimizing transmit power is
bounded away from the Shannon limit.
  Second, we consider a collection of links. When systems both generate and
face interference, coding allows a system to support a higher density of
transmitter-receiver pairs (assuming interference is treated as noise).
However, at low densities, uncoded transmission may be more power-efficient in
some cases.
