The Potential of a Hybrid Fixed/User Relay Architecture -- A Performance Analysis

Paper in proceeding, 2012

Future wireless systems are likely to comprise a dense grid of fixed relay nodes (FRNs) and contain a large number of user terminals (UTs) that can act as user relay nodes (URNs) under certain circumstances. In this paper we argue that significant gains can be attained if FRNs and URNs act together under the umbrella of a hybrid architecture. To this end, we derive closed-form expressions for the outage probability (OP) and an ergodic capacity upper bound of a dual-hop system as a function of the number of FRNs and URNs. For this analysis single relay selection under decode-and-forward and Rayleigh fading are assumed. We show that in low signal-to-noise ratio (SNR), FRNs are much more effective in reducing OP than URNs. In high SNR, URNs are almost equally as effective as FRNs in reducing OP. In terms of ergodic capacity, FRNs achieve higher performance than URNs in all SNR regimes. We conclude that from a system design viewpoint, if the aim is to meet certain quality-of-service constraints, FRNs can be reserved to serve low SNR users, while URNs could be employed when channel conditions to destination UTs are more favorable.