In cognitive radio system, it was still a problem for a secondary user that how the delay and the number of spectrum handoff could be reduced so as to realize transmitting information of numerous data by utilizing spectrum aggregation when the primary user returned. A proactive handoff strategy was proposed and applied to the spectrum aggregation to solve this problem. According to the transition probability of the secondary user and the constraint of span, the strategy formulated a new regulation for proactive handoff of the secondary user to ensure smoothing handoff and to avoid the collision. Meanwhile,a backup channel selection method based on handoff probability was advised to cut down spectrum handoff delay and handoff number. Simulation results showed that when the average interval of PUs’ arrivals was higher than 50 slots and average holding time in a sub-channel of the SU was less than 100 slots, the handoff strategy of spectrum aggregation could keep the number of handoff below 5 times and shorten the delay below 30 slots. The strategy not only guaranteed the real-time of handoff but also reduced the cost of handoff as much as possible to promote the efficiency of system.
