  This paper considers the weighted sum rate (WSR) maximized resource
allocation (RA) constrained by a system sum power in an orthogonal frequency
division multiple access (OFDMA) downlink transmission system assisted by
multiple decode-and-forward (DF) relays. In particular, multiple relays may
cooperate with the source for every relay-aided transmission. A two-step
algorithm is proposed to find the globally optimum RA. In the first step, the
optimum source/relay power and assisting relays that maximize the rate is found
for every combination of subcarrier and destination, assuming a sum power is
allocated to the transmission at that subcarrier to that destination in the
relay-aided transmission mode and the direct mode, respectively. In the second
step, a convex-optimization based algorithm is designed to find the globally
optimum assignment of destination, transmission mode, and sum power for each
subcarrier to maximize the WSR. Combining the RAs found in the two steps, the
globally optimum RA can be found. In addition, we show that the optimum RA in
the second step can readily be derived when the system sum power is very high.
The effectiveness of the proposed algorithm is illustrated by numerical
experiments.
