PAPR Reduction Using Null Space in MIMO Channel for MIMO-OFDM Signals in Multiple-Antenna AF Relay Transmission

In the 5th generation mobile communication system and beyond, the importance of relay transmission will increase in order to provide wide coverage for high-speed high-quality data transmissions using a higher carrier frequency band such as the millimeter wave band. Assuming multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) transmission as the baseline for 5G systems, mitigating its high peak-to-average power ratio (PAPR) not only at the base station (BS) but also at the relay station (RS) transmitters is essential to achieve sufficient coverage enhancement from the RSs by minimizing the transmission power backoff levels at the nonlinear power amplifier. In this paper, we propose applying our previously reported adaptive PAPR reduction method using null space in a MIMO channel for MIMO-OFDM transmission to a RS. In this study, we assume an amplify-and-forward (AF)-type RS with multiple antennas. Computer simulation results show that the proposed method significantly improves the PAPR-vs.-throughput performance compared to that for the conventional one thanks to the reduced interference levels from the PAPR reduction signal observed at the user equipment (UE) receiver.