  In this letter, we propose a joint resource allocation algorithm for an
OFDM-based multi-user system assisted by an improved Decode-and-Forward (DF)
relay. We aim at maximizing the sum rate of the system by jointly optimizing
subcarrier pairing, subcarrier pair-user assignment, and power allocation in
such a single DF relay system. When the relay does not perform any transmission
on some subcarriers in the second phase, we further allow the source to
transmit new symbols on these inactive subcarriers. We effectively solve the
formulated mixed integer programming problem by using continuous relaxation and
dual minimization methods. Numerical results verify the theoretical analysis,
and illustrate the remarkable gains resulted from the extra direct-link
transmissions.
