  Cooperative relaying has been proposed as a promising transmission technique
that effectively creates spatial diversity through the cooperation among
spatially distributed nodes. However, to achieve efficient communications while
gaining full benefits from cooperation, more interactions at higher protocol
layers, particularly the MAC (Medium Access Control) and network layers, are
vitally required. This is ignored in most existing articles that mainly focus
on physical (PHY)-layer relaying techniques. In this paper, we propose a novel
cross-layer framework involving two levels of joint design---a MAC-network
cross-layer design for forwarder selection (or termed routing) and a MAC-PHY
for relay selection---over symbol-wise varying channels. Based on location
knowledge and contention processes, the proposed cross-layer protocol, CoopGeo,
aims at providing an efficient, distributed approach to select next hops and
optimal relays along a communication path. Simulation results demonstrate that
CoopGeo not only operates properly with varying densities of nodes, but
performs significantly better than the existing protocol BOSS in terms of
packet error rate, transmission error probability, and saturated throughput.
