Joint Sensing Period and Transmission Time Optimization for Energy-Constrained Cognitive Radios

Under interference constraint and energy consumption constraint, to maximize the channel utilization, an opportunistic spectrum access (OSA) strategy for a slotted secondary user (SU) overlaying an unslotted ON/OFF continuous time Markov chain (CTMC) modeled primary network is proposed. The OSA strategy is investigated via a cross-layer optimization approach, with joint consideration of sensing period (related to PHY layer) and transmission time (related to MAC layer), which will affect both interference and energy consumption. Two access policies are investigated in this paper; that is, SU transmits only in "OFF slots" (i.e., the slots that the sensing results are OFF) and transmits in both "OFF slots" and "ON slots". The allocation of sensing period and transmission time for two access policies is investigated and analyzed by means of geometric methods. The closed form solutions are derived, which show that SU should transmit in "OFF slots" as much as possible, and that the proposed OSA strategy has low computational cost. Numerical results also show that with the proposed policies, SU can efficiently access the channel and meanwhile consume less energy and time to sense.