We consider a two-hop downlink hybrid orthogonal frequency-division multiple-access [OFDM(A)] communication system in which a fixed infrastructure-based relay node is deployed to serve multiple user equipment beyond the coverage area of a base station. The key factor defining the performance and complexity of such systems is the relaying protocol, i.e., the interface between the OFDM point-to-point backhaul link and the OFDMA point-to-multipoint service link. We introduce and analyze a comprehensive set of OFDM(A) relaying protocols that are based on the following design choices: whether to perform time-domain, frequency-domain, resource block (RB)-wise, or symbol-wise processing; whether to apply amplify-and-forward (AF) or decode-and-forward (DF) retransmission; whether to implement random, fixed, or adaptive pairing of RBs in backhaul and service links; whether to introduce buffering over fading states or not; and how the time sharing between the backhaul and service links is controlled. The joint effect of all these design choices has not previously been evaluated in a unified framework or by means of closed-form performance expressions. The main contribution of this paper is to calculate the average ergodic system capacity when adopting each of the proposed relaying protocols. Consequently, the new expressions facilitate thorough numerical performance comparison between the different protocols in a typical suburban vehicular system.
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