I. Introduction
Multichannel-based transmission is an essential component of the physical-layer technology for next-generation wireless systems [1]. Correspondingly, future system designs are evolving toward the adoption of a multi-channel relaying architecture to allow broadband access and coverage improvement [2]. As opposed to a narrow-band single-channel relay, a multi-channel relay has access to multiple channels, e.g., the subcarriers (or subchannels) in an Orthogonal Frequency Division Multiplexing (OFDM) relaying system. Such multichannel system creates an additional frequency dimension for the relay to exploit, where it can process the incoming signals adaptively based on the strength of each channels for forwarding purpose. This capability is unique to relaying systems, and such exploitation can potentially improve the overall relay performance.