Spectrum efficiency limits in mobile cellular systems

In cellular mobile radio systems using coded modulations, the spectrum efficiency of the system is related to the cutoff rate and the cluster size. We evaluate the cutoff rate for M-ary QAM considering a coherent detection and a memoryless flat fading channel. Simulation results relate the cluster size to the signal-to-noise ratio and to the cutoff rate. We present the spectrum efficiency of the system as a function of the cutoff rate and show its limits in the classical cellular system. We apply the same procedure to examine the performance of systems using directional antennas with and without sectorization of the original cell surface. With our proposed representation, we can always determine the optimum spectrum efficiency of the modulation that maximizes the spectrum efficiencies of the different systems.