PAPR Reduction Using Null Space in MIMO Channel Based on Signal Processing at Base Station for Downlink AF-Based Relaying MIMO-OFDM Signals

In this paper, we propose a peak-to-average power ratio (PAPR) reduction method using the null space in a multiple-input multiple-output (MIMO) channel for downlink MIMO-orthogonal frequency division multiplexing (OFDM) signals with multiple-antenna amplify-and-forward (AF)-type relaying. Different from the conventional approach, the proposed method reduces the PAPR at the relay station (RS) using only the signal processing at the base station (BS). Mitigating the high PAPR of transmit MIMO-OFDM signals at the BS and at the RS transmitters is essential to achieve sufficient coverage enhancement from the RSs. On the other hand, performing complex signal processing for PAPR reduction at the AF-type RS is not possible in reality. In the proposed method, the RS does not require any signal processing for its PAPR reduction. The proposed method alternately repeats the generation of the signal that reduces the PAPR at the BS transmitter and the signal that reduces the PAPR at the RS transmitter, where both PAPR reduction signals are transmitted from the BS. Compared to the conventional method in which the PAPR at the RS transmitter is reduced using the signal processing at the RS utilizing the null space in the MIMO channel, the proposed method achieves almost the same PAPR reduction level at the BS and RS while requiring no signal processing for PAPR reduction at the RS.