  This paper develops a sum-power minimized resource allocation (RA) algorithm
subject to a sum-rate constraint for cooperative orthogonal frequency division
modulation (OFDM) transmission with subcarrier-pair based opportunistic
decode-and-forward (DF) relaying. The improved DF protocol first proposed in
[1] is used with optimized subcarrier pairing. Instrumental to the RA algorithm
design is appropriate definition of variables to represent source/relay power
allocation, subcarrier pairing and transmission-mode selection elegantly, so
that after continuous relaxation, the dual method and the Hungarian algorithm
can be used to find an (at least approximately) optimum RA with polynomial
complexity. Moreover, the bisection method is used to speed up the search of
the optimum Lagrange multiplier for the dual method. Numerical results are
shown to illustrate the power-reduction benefit of the improved DF protocol
with optimized subcarrier pairing.
