This paper presents a delay-QoS-driven spectrum and energy efficiency optimization transmission technique. In particular, considering the cross-layer effective capacity (EC) model in Rayleigh fading channels, a spectrum- and energy-efficient power allocation strategy is proposed when maximum spectrum efficiency is achieved under minimum energy efficiency (EE) requirement. For this purpose, at a target delay-outage probability, the spectrum efficiency is measured by the EC. Further, the EE is formulated as the ratio of the EC to the total expenditure power. At first, the maximum achievable EE of the link at the target delay-outage probability is found. Then, the optimal power allocation strategy is obtained to maximize the EC subject to a minimum EE constraint set at a certain ratio of the maximum achievable EE. We prove that the optimization
problem is a concave maximization problem and develop the global optimal solution. The analytical results show that a considerable EC-gain can be achieved with a small sacrifice in EE. This gain increases considerably as the delay constraint becomes tight.