Optimum data rate in cellular systems

The spectrum efficiency in a cellular radio system is affected by the data rates used in the system. High data rates may result in low spectrum efficiency if the frequency reuse is low. The spectrum efficiency may be enhanced by using adaptive modulation with carefully chosen data rates. We present an analytical method, based on a lower bound on the BER, for finding the best data rates. The method results in the best code rates to be used with QPSK, 8-PSK, and 16-QAM in an adaptive fashion. The general conclusion is that a large number of carefully chosen data rates enhances the spectrum efficiency.