Coordination of multi-link spectrum handoff in multi-radio multi-hop cognitive networks

In cognitive networks, spectrum handoff occurs when primary users reclaim their rights to access their licensed spectrum. When links perform spectrum handoff, their communication may be interrupted for a certain period and incurs spectrum handoff delay. Existing work only considered the problem of minimizing spectrum handoff delay of a single link in single-hop cognitive networks, referred to as the SH-SLSH problem. This paper studies a more challenging problem (referred to as the SH-MLMH problem) in which multiple links perform spectrum handoff in multi-hop cognitive networks. Assuming each node is equipped with multiple radios and multi-path routing is adopted, the SH-MLMH problem targets at maintaining the network connectivity and minimizing the Total Handoff Completion Time (THCT) by coordinating multiple links to perform spectrum handoff. THCT is defined as the time for all links to finish spectrum handoff. We can keep the communication of switching links uninterrupted by maintaining network connectivity and adopting multi-path routing. To the best of our knowledge, we are the first to study the SH-MLMH problem. We make following contributions in this paper. We prove that the SH-MLMH problem is NP-hard. We propose both centralized and distributed algorithm to solve the SH-MLMH problem. We prove that the centralized algorithm can achieve a logarithmic approximation ratio. The simulation results show that our proposed algorithms not only improve the network throughput but also reduce THCT compared with spectrum handoff without coordination.