Cooperative decode-and-forward relaying for secondary spectrum access

We propose a two-phase protocol based on cooperative decode-and-forward relaying for a secondary system to achieve spectrum access along with a primary system. The primary and secondary systems comprise of a transmitter-receiver pair, PT-PR and ST-SR, respectively. In the first transmission phase, PT transmits the primary signal to PR, which is also received by ST and SR, where it is decoded. At ST, the primary signal is regenerated and linearly combined with the secondary signal by assigning fractions alpha and (1 - alpha) of the available power to the primary and secondary signals respectively. This combined signal is then broadcasted by ST in the second transmission phase. We show that as long as ST is located within a critical radius from PT, there exists a threshold value for alpha above which the outage probability of the primary system will be equal to or lower than the case without spectrum sharing. We also determine the outage probability achieved by the secondary system. Theoretical and simulation results confirm the efficiency of the proposed spectrum sharing scheme.