In this paper, we propose an analytical solution for the joint optimization of spectral efficiency and energy consumption of future deployed cellular networks based on small cell base stations (BSs). We derive an optimization problem that consists in minimizing the energy consumption under the constraint of meeting the throughput and spectral efficiency challenges of 5G - and upcoming 6G - networks. The relaxed problem is convex and therefore there is an optimal solution that can be found numerically and/or derived analytically. Comparative studies show a perfect accuracy of the analytical approach compared to the numerical solution. Particular attention is thus paid to the evaluation of the impact of the system parameters on the optimal metrics of energy consumption and spectral efficiency.