  In this paper, code pairs based on trellis coded modulation are proposed over
PSK signal sets for a two-user Gaussian multiple access channel. In order to
provide unique decodability property to the receiver and to maximally enlarge
the constellation constrained (CC) capacity region, a relative angle of
rotation is introduced between the signal sets. Subsequently, the structure of
the \textit{sum alphabet} of two PSK signal sets is exploited to prove that
Ungerboeck labelling on the trellis of each user maximizes the guaranteed
minimum squared Euclidean distance, $d^{2}_{g, min}$ in the \textit{sum
trellis}. Hence, such a labelling scheme can be used systematically to
construct trellis code pairs for a two-user GMAC to approach \emph{any rate
pair} within the capacity region.
