  In relay-aided wireless transmission systems, one of the key issues is how to
decide assisting relays and manage the energy resource at the source and each
individual relay, to maximize a certain objective related to system
performance. This paper addresses the sum rate maximized resource allocation
(RA) problem in a point to point orthogonal frequency division modulation
(OFDM) transmission system assisted by multiple decode-and-forward (DF) relays,
subject to the individual sum power constraints of the source and the relays.
In particular, the transmission at each subcarrier can be in either the direct
mode without any relay assisting, or the relay-aided mode with one or several
relays assisting. We propose two RA algorithms which optimize the assignment of
transmission mode and source power for every subcarrier, as well as the
assisting relays and the power allocation to them for every {relay-aided}
subcarrier. First, it is shown that the considered RA problem has zero
Lagrangian duality gap when there is a big number of subcarriers. In this case,
a duality based algorithm that finds a globally optimum RA is developed.
Second, a coordinate-ascent based iterative algorithm, which finds a suboptimum
RA but is always applicable regardless of the duality gap of the RA problem, is
developed. The effectiveness of these algorithms has been illustrated by
numerical experiments.
