Spectrum Handoff Without Forced Termination in Cognitive Radio Networks

In cognitive radio networks, when a primary user reclaims his/her frequency band, the secondary users of this band must stop using the channels of this band and attempt to use the other free channels. This process is known as spectrum handoff. The existing studies on spectrum handoff implicitly assume that each channel is assigned to one secondary user at a time. This implicit assumption results in an all-or-none outcome: if a secondary user is assigned a channel in spectrum handoff, he/she could exclusively use this channel; otherwise, he/she must terminate his/her ongoing communication and this phenomenon is known as forced termination. In this paper, we study an alternative approach to avoid forced termination in spectrum handoff. When there are not sufficient channels for the secondary users involved in spectrum handoff, multiple secondary users may be assigned to each available channel and they share this channel via multiple access. As a result, the secondary users would not suffer from forced termination but the per-user data rate is lower. This is a desirable tradeoff because forced termination is more inconvenient and troublesome than lower per-user data rate. We formulate a new spectrum handoff problem in which forced termination is avoided and the objective is to balance the traffic loads in the channels in spectrum handoff. We prove that this problem is NP-hard and design an efficient heuristic algorithm to tackle this problem. We present numerical results to demonstrate that the proposed algorithm is fast and effective.