PAPR Reduction Method Using Null Space in MIMO Channel for MIMO-OFDM Signals at Multiple-Antenna AF Relay Station Considering Direct Link from Base Station to User Equipment

This paper proposes 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 considering direct link from base station (BS) to user equipment (UE). Performing complex signal processing for PAPR reduction at the AF-type relay station (RS) is not acceptable in the real system. In the proposed method, the signal processing only at the BS 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 to the null space of the MIMO channel including both the direct and relay links. Compared to the previous method in which the direct link is not taken into account, the proposed method realizes more aggressive PAPR reduction.