  Two-way relaying can considerably improve spectral efficiency in
relay-assisted bidirectional communications. However, the benefits and flexible
structure of orthogonal frequency division multiplexing (OFDM)-based two-way
decode-and-forward (DF) relay systems is much less exploited. Moreover, most of
existing works have not considered quality-of-service (QoS) provisioning for
two-way relaying. In this paper, we consider the OFDM-based bidirectional
transmission where a pair of users exchange information with or without the
assistance of a single DF relay. Each user can communicate with the other via
three transmission modes: direct transmission, one-way relaying, and two-way
relaying. We jointly optimize the transmission policies, including power
allocation, transmission mode selection, and subcarrier assignment for
maximizing the weighted sum rates of the two users with diverse
quality-of-service (QoS) guarantees. We formulate the joint optimization
problem as a mixed integer programming problem. By using the dual method, we
efficiently solve the problem in an asymptotically optimal manner. Moreover, we
derive the capacity region of two-way DF relaying in parallel channels.
Simulation results show that the proposed resource-allocation scheme can
substantially improve system performance compared with the conventional
schemes. A number of interesting insights are also provided via comprehensive
simulations.
