Total Energy Efficiency of TR-MRC and FD-MRC Receivers for Massive MIMO Uplink

This paper proposes a detailed investigation on the uplink (UL) performance of massive multiple-input-multiple-output (maMIMO) systems employing maximum-ratio combining (MRC) at the receiver. While most papers in maMIMO literature assume orthogonal frequency-division multiplexing (OFDM), current standards such as long-term evolution (LTE) employ single-carrier (SC) waveform in the UL because of several benefits. We, thus, perform a systemic comparison between two fundamental schemes: the time-reversal MRC (TR-MRC) operating under SC, and the frequency-domain MRC (FD-MRC) employed with OFDM. On the one hand, it was recently shown that the TR-MRC outperforms the FD-MRC in terms of achievable rates, since SC systems do not require the cyclic prefix of OFDM. On the other hand, the computational complexity of the TR-MRC algorithm is higher than that of the FD-MRC algorithm, even when efficient solutions are employed (e.g., fast convolution with the overlap-and-add method). Hence, the best scheme for the UL maMIMO systems still remains an open question. The main contribution of this paper is the comparison of the total energy efficiency of both the TR-MRC and FD-MRC algorithms when used in the UL of maMIMO systems. Our results show that, for current typical system parameters, the FD-MRC/OFDM scheme achieves higher total energy efficiency than that achieved by the TR-MRC/SC scheme. However, if the cell radius is less than 300 m and/or the computational efficiency increases by 30% regarding the current processors, the TR-MRC algorithm under SC waveform becomes a much more attractive alternative for the UL of maMIMO systems.