where N = Q/U˘ denotes the number of cooperative sensing
rounds, Er,i(t) is the residual energy at the ith SU at time slot
t, Ethd,i is the energy threshold for the ith SU, and Ec is the
capacity of the rechargeable battery of an SU. {ea(t), t = 1, 2,
…} is the harvesting energy for each SU at time slot t, which
is modeled as an independent and identically distributed
sequence of random variables with mean E[ea(t)] = –Ea.