  In two-way OFDM relay, carrier frequency offsets (CFOs) between relay and
terminal nodes introduce severe intercarrier interference (ICI) which degrades
the performance of traditional physical-layer network coding (PLNC). Moreover,
traditional algorithm to compute the posteriori probability in the presence of
ICI would incur prohibitive computational complexity at the relay node. In this
paper, we proposed a two-step asynchronous PLNC scheme at the relay to mitigate
the effect of CFOs. In the first step, we intend to reconstruct the ICI
component, in which space-alternating generalized expectationmaximization
(SAGE) algorithm is used to jointly estimate the needed parameters. In the
second step, a channel-decoding and network-coding scheme is proposed to
transform the received signal into the XOR of two terminals' transmitted
information using the reconstructed ICI. It is shown that the proposed scheme
greatly mitigates the impact of CFOs with a relatively lower computational
complexity in two-way OFDM relay.
