Energy Efficiency of Power-Adaptive Spatial Diversity Methods

The energy efficiency of power-adaptive multipleinput multiple-output (MIMO) diversity methods is studied in this paper. The considered diversity methods are antenna selection (AS), maximum ratio transmission (MRT), and equal gain transmission (EGT) at the transmitter and maximum ratio combining (MRC) at the receiver. The transmitter energy efficiency is evaluated using the average power amplifier (PA) efficiency and the transceiver energy efficiency is evaluated using the bit error rate (BER) as a function of average PA input signal-to-noise ratio (SNR), which is a new metric. With the new metric, the effect of the PA efficiency is taken into account in the performance evaluation. The analytical results are verified by Monte Carlo simulations. It is shown that larger diversity in the spatial or frequency domain improves the average PA efficiency in a system with channel inversion. The BER results show that the performance improvement from channel inversion diminishes due to the nonideal PA efficiency. Even though MRT is the received SNR maximizing transmitter diversity method, EGT requires less PA input energy per bit when a PA with nonideal efficiency is used. These conclusions could not have been reached using the traditional SNR metrics that do not measure the PA input energy.