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We study the physical-layer performance of a wideband multihop system in which a chain of amplify-and-forward relays is deployed for routing data from a source to a destination. The novelty of the system model w.r.t. prior work is in assuming frequency-selective multipath channels for all hops. We propose a new approach in which the channels are specified in terms of clustered exponentially decaying power-delay profiles (PDPs). Focusing on relays that apply time-domain signal processing, the distinctive form of the end-to-end source-destination PDP motivates us to introduce a new hypoexponential PDP and study it in detail. Then, we analyze the performance of orthogonal frequency division multiplexing (OFDM) transmission over the multihop relay link. The type of the adopted analysis is suitable mainly for investigating the typical time-domain issues of OFDM transmission such as excessive multipath delay spread, non-ideal time syncronization, and interference due to insufficient cyclic prefix duration. In particular, we derive new closed-form expressions for evaluating the signal-to-interference and noise ratio. Our analysis confirms that multihopping increases considerably the delay spread of the end-to-end PDP and makes the channel more frequency selective. This imposes significant effect on the design of receiver time synchronization and pilot structures, and on the choice of OFDM physical layer parameters.