  Communication reliability improving is one of most important research
requirements in cognitive networks, as 5G communications technology rapidly
develop nowadays. In this paper, we propose generalized optimal cloud-based
region relays subsets paring model in underlay dual-hop cognitive networks,
this unified model reveals three relays nodes characteristics of cloud-based
cooperative networks in the nearby area- A subset, only needs receiving for
first hop, B subset, just receive and forward, C subset, only forwarding. In
addition, this generalized model can be converted into various classical relay
selection algorithms when A, B and C subsets are taken as special selection
values [Table II]. Furthermore, we put forward optimal relays subsets pairings
and replacement algorithm flowchart to improve minimum outage probability (OP)
for communication reliability, and prove that optimal relays subsets pairings
(A, B, C) will better guarantee reliability of communication, comparing other
popular relays selection schemes. Simulation results show that optimal relays
paired subsets are exist and this generalized algorithm enormously reduces OP,
comparing other selection algorithms.
