  In bidirectional relaying using Physical Layer Network Coding (PLNC), it is
generally assumed that users employ same modulation schemes in the Multiple
Access phase. However, as observed by Zhang et al., it may not be desirable for
the users to always use the same modulation schemes, particularly when
user-relay channels are not equally strong. Such a scheme is called
Heterogeneous PLNC. However, the approach in [1] uses the computationally
intensive Closest Neighbour Clustering (CNC) algorithm to find the network
coding maps to be applied at the relay. Also, the treatment is specific to
certain cases of heterogeneous modulations. In this paper, we show that, when
users employ heterogeneous but symmetric PSK modulations, the network coding
maps and the mapping regions in the fade state plane can be obtained
analytically. Performance results are provided in terms of Relay Error Rate
(RER) and Bit Error Rate (BER).
