Which is better for opportunistic spectrum access: The duration-fixed or duration-variable MAC frame?

A media access control (MAC) frame for opportunistic spectrum access (OSA), which decides the scheduling of the sensing-Then-transmission time slots, has a major effect on the sensing quality, the interference to primary users (PUs) and the achievable throughput of secondary users (SUs). Considering channel handoff of SUs, there are two MAC frame structures for OSA: the duration-variable (DV) frame and the duration-fixed (DF) frame. Finding out which frame structure is better is the focus of this paper. We first formulate an improved frame structure optimization framework, with which the optimal DV and DF frames, each including a sensing slot and a transmission slot, are derived as closed forms, respectively. Then, the DV and DF frames are compared from different perspectives under the same sensing quality constraint and interference constraint (IC). It turns out that the DV frame provides slightly higher time efficiency and larger throughput for SUs than the DF frame. Simulation results confirm this theoretical analysis and show that the performance difference between both frames depends on the IC, the number of channels, and the traffic rates. In addition, the proposed optimization scheme protects PUs better than previous schemes while keeping the throughput of SUs enhanced or just reduced slightly.