  This paper proposes a relay selection scheme that aims to improve the
end-to-end symbol error rate (SER) performance of a two-way relay network
(TWRN). The TWRN consists of two single-antenna sources and multiple relays
employing decode-and-forward (DF) protocol. It is shown that the SER
performance is determined by the minimum decision distance (DD) observed in the
TWRN. However, the minimum DD is likely to be made arbitrarily small by channel
fading. To tackle this problem, a phase rotation (PR) aided relay selection
(RS) scheme is proposed to enlarge the minium DD, which in turn improves the
SER performance. The proposed PR based scheme rotates the phases of the
transmitted symbols of one source and of the selected relay according to the
channel state information, aiming for increasing all DDs to be above a desired
bound. The lower bound is further optimized by using a MaxMin-RS criterion
associated with the channel gains. It is demonstrated that the PR aided
MaxMin-RS approach achieves full diversity gain and an improved array gain.
Furthermore, compared with the existing DF based schemes, the proposed scheme
allows more flexible relay antenna configurations.
