Outage-Capacity Based Adaptive Relaying in LTE-Advanced Networks

In this paper, we investigate the benefits of relaying by comparing the transmission rates of both direct transmission (DT) and relaying. It is shown that relaying achieves SNR (signal-to-noise power ratio) gain over DT due to less pathloss, but with several relaying penalties, including a lower multiplexing gain (due to half-duplex), a lower transmit power and a higher outage requirement at each hop (due to multi-hop). We determine the conditions over which relaying outperforms DT, where the SNR gain is greater than the loss due to relaying penalties. The result is applied to the LTE-advanced networks (LTE-A) where the relay nodes (RNs) are implemented to relay information between the user equipment (UE) and the evolutional NodeB (eNB). The major difference between LTE-A and a general relay system lies in that the UE-RN hop consists of multiple frequency-division access links, while the RN-eNB hop is a point-to-point link. By investigating the effects of diversity gain on the transmission rate, we propose an outage-capacity based adaptive relaying (OCA-R) scheme to replace the conventional same-carrier relaying (SC-R). It is shown that the transmission rates of both SC-R and OCA-R are one half of the harmonic means between the outage-capacities for two hops, where the advantage of OCA-R over SC-R comes from a higher diversity gain in the RN-eNB link.