  Powerful spectrum handover schemes enable cognitive radios (CRs) to use
transmission opportunities in primary users' channels appropriately. In this
paper, we consider the cognitive access of primary channels by a secondary
user. We evaluate the average detection time and the maximum achievable average
throughput of the secondary user when the sequential method for hand-over
(SMHO) is used. We assume that a prior knowledge of the primary users' presence
and absence probabilities are available. When investigating the maximum
achievable throughput of the secondary user, we end into an optimization
problem, in which the optimum value of sensing time must be selected. In our
optimization problem, we take into account the spectrum hand over due to false
detection of the primary user. We also propose a weighted based hand-over
(WBHO) scheme in which the impacts of channels conditions and primary users'
presence probability are considered. This Spectrum handover scheme provides
higher average throughput for the SU than the SMHO method. The tradeoff between
the maximum achievable throughput and consumed energy is discussed, and finally
an energy efficient optimization formulation for finding a proper sensing time
is provided.
