6. Conclusions
COMAC is proposed as a cooperative MAC framework that implements distributed cooperator selection and power assignment (D-CSPA) algorithm for minimizing the energy costs of low mobility WSNs, while achieving reliability in terms of BER. In this framework, the relay nodes announce their cooperation decisions via ACO packets, which are coordinated by the ACO timer for the incremental implementation of D-CSPA. Four different ACO timers are designed for different cases, where different amount of channel state information and complexity are considered. It is shown through extensive simulations that when the average channel statistics and instantaneous channel state information are available at each relay node, the performance of the ACO timer (τ 4) turns out to be very close to the ideal timer (τ 1), providing at least two orders of magnitude improvement in the energy costs, at least a factor of five in throughput, and up to three orders of magnitude in delay, as compared to the standard Zigbee WSNs with direct transmission. The MAC overhead cost of COMAC is similar if not significantly smaller than the overhead of direct Zigbee, depending on the channel quality. Addition of a second ACO epoch and ACO collision resolution enhances the protocol performance when deployed challenging topologies. Despite slightly lower throughput and increased MAC overhead as compared to the ideal COMAC with sophisticated timers (τ 1, τ 4), COMAC with a simpler timer (τ 3) along with ACO collision resolution outperforms direct transmission in all aspects. With the cooperative sleep mode for the relay nodes, additional energy savings of up to 33% is possible while the throughput of the cooperative system remains unchanged. All these performance gains are achieved with almost no additional energy cost for computations, as computational energy is only 0.5% of the total energy cost.
