Modulation design analysis for two-way relay channels

This paper deals with the joint modulation relaying (JMR) design for the two-way relay channel where two users exchange information via a half-duplex relay node. We studied in particular the broadcast mode where the relay node combines the bits received from the two users and retransmit the combined information to both users. Symmetric and asymmetric data rate transmissions are considered. We show that the performance of the JMR scheme that uses a higher order modulation is improved using asymmetric constellations. This was verified by calculation the capacity of the channel and by determining the bit error probability of an LDPC coded JMR scheme using Monte Carlo simulation. Moreover, increasing the degree of asymmetry will lead asymptotically to a lower order signal constellation as in a network coding scheme. Therefore, there´s no need for a higher order modulation at the relay node in order to retransmit data but only to apply network coding and modulation using the constellation of the stronger link.