Base Station-Driven PAPR Reduction Method Utilizing Null Space for MIMO-OFDM Systems With Amplify-and-Forward Relaying

This paper proposes a peak-to-average power ratio (PAPR) reduction method that utilizes 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. In order to achieve sufficient coverage enhancement using beamforming with multiple antennas and relaying, the PAPR not only at the base station (BS) but also at the relay station (RS) should be reduced to suppress the amount of input backoff (IBO) in the non-linear power amplifier. However, performing complex signal processing for PAPR reduction at the AF-type RS is impractical and leads to the concern that the channel capacity (throughput) will be reduced due to processing delay. In the proposed method, the BS alternately repeats the signal processing for generating the signal to reduce the PAPR at the BS and the signal to reduce the PAPR at the RS, where the RS does not require any signal processing for PAPR reduction. The generated PAPR reduction signals are projected onto the null space of the overall MIMO channel of the entire system for each frequency block. Computer simulation results using the non-linear power amplifier model show that the proposed method achieves higher throughput compared to the clipping and filtering (CF) method for the BS by reducing the PAPR at the RS utilizing the null space of the channel.