Cognitive radio (CR) has been considered a key technology to enhance overall spectrum utilization by opportunistic transmissions in CR transmitter-receiver link(s). However, CRs must form a cognitive radio network (CRN) so that the messages can be forwarded from source to destination, on top of a number of opportunistic links from co-existing multi-radio systems. Unfortunately, appropriate routing in CRN of coexisting multi-radio systems remains an open problem. We explore the fundamental behaviors of CR links to conclude 3 major challenges, and thus decompose general CRN into cognitive radio relay network, cognitive radio uplink relay network, cognitive radio downlink relay network, and tunneling (or core) network. Due to extremely dynamic nature of CR links, traditional routing to maintain end-to-end routing table for ad hoc networks is not feasible. We locally build up one-step forward table at each CR to proceed based on spectrum sensing to determine trend of paths from source to ...