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 perform an exact and asymptotic outage probability (OP) analysis of a dual hop system with single relay selection under Rayleigh fading. Furthermore, we derive an ergodic capacity upper bound as a function of the number of FRNs and URNs. We show that in low signal-to-noise ratio (SNR), FRNs are much more effective in reducing OP than URNs. Although it is always more beneficial to employ FRNs instead of 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.
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