Physical layer network coding in relay assisted OFDM based wireless networks

Abstract

Relays have the potential to support transmissions over long distance ranges, much morethan what can be accomplished by relay-less wireless networks. The concept of network coding(NC) that was originally conceived for wired networks can also be used for efficientcommunication in wireless networks, owing to the ubiquitous nature of wireless transmissions.Earlier, NC was applied only to the higher layers of protocol stack. Only recently researchershave started exploring network coding at physical (PHY) layer in which, instead of bits, signalsare combined. In literature, PHY layer NC is also referred as symbol level network coding. Inthis thesis, we considered performance evaluation of PHY layer NC scheme in OFDM basedrelay assisted wireless networks. Specifically, we conducted simulation study by implementing acanonical scenario consisting of three nodes, of which two of them constitute source and sinknodes and one of them would act as a relay. The OFDM based PHY layer that we used is similarto the one that is used in IEEE 802.11a standard. The relay node uses the popular decode andforward joint modulation (DF-JM) technique, which as the name implies decode and forwardmechanism combined with joint encoding of data received from both the nodes. Theperformance of DF-JM has been compared with the conventional decode and forward (DF)technique. Our simulation results suggest that DF-JM based PHY layer NC outperforms theconventional DF scheme across different scenarios of interest. We also considered theperformance of DF-JM scheme with relay selection and power allocation. Our simulation resultssuggest that an equal power allocation among various relays gives better throughput values asagainst that of the scheme where relay selection is combined with optimal power allocation. Thisbehaviour is contrary to what was observed in a related work where simple analog relays wereconsidered. The relatively better performance of the equal power allocation based DF-JMscheme can be attributed largely to the less number of transmission slots that a DF-JM basedscheme would require, along with the diversity advantage of having all the relays transmit.