In this article, we evaluate the collaborative sub-channel activity index (SAI) to characterize the primary users activity in cognitive radio network. The optimal transmit power allocation policy is then formulated based on SAI as an optimization problem with the objective of maximizing the total user rate and minimizing the energy consumption at the secondary base station (SBS), subject to various constraints at the primary system. The obtained results enable to achieve a tradeoff between contradicting energy efficiency (EE) and spectral efficiency (SE) objectives as a function of SAI which ultimately provides a quantitative insights on the system design. The simulations show the better tradeoff deal between EE and SE by tuning the sensing constraints.