Relay based wireless networks with joint modulation, power allocation and relay selection

In this paper we consider performance improvement of decode and forward (DF) relays via joint modulation at relays, power allocation across the relay nodes and relay selection. Initially, we consider a canonical relay network scenario consisting of three nodes, of which two of them constitute source and sink nodes and one of them would act as a relay. The source and sink nodes cannot reach each other directly and therefore rely on the relay for data forwarding. We assume all the three nodes use orthogonal frequency division multiplexing (OFDM) as the physical (PHY) layer transmission mechanism. Further it is assumed that, the relay node performs joint modulation on the data received from both the nodes as described in [1] (henceforth referred to as decode and forward joint modulation (DFJM)), thus creating a broadcast frame which is then transmitted to both the nodes. The performance of this DFJM at relay is compared against the simple DF technique in which relay does not perform any processing. Our simulation results suggest that the former outperforms the latter in terms of throughput. Our simulation results also suggest that the throughput improvement with DFJM against the simple DF technique was found to be much higher in networks that are asymmetric with respect to transmission rates. Next we consider the performance of DFJM scheme with relay selection and power allocation by considering a relay based network with more than one relay in it. Our simulation results on this aspect suggest that an equal power allocation among various relays gives better throughput values as against that of the scheme where relay selection is combined with optimal power allocation. This behavior is contrary to what was observed in a related work where simple analog relays were considered [2]. The relatively better performance of the DFJM scheme with equal power allocation as against that of the scheme with relay selection followed by optimal power allocation can be attributed largely to the following: (i) less number of transmission slots that a DFJM based scheme would require and (ii) the inherent diversity advantage of having all the relays transmit.