Out-of-Band Power Reduction in MIMO OFDM

One of the main disadvantages of orthogonal frequency-divi-sion multiplexing (OFDM) is its large peak-to-average power ratio (PAR). In case of nonlinear power amplification this fact causes undesired out-of-band radiation. In this paper PAR reduction schemes for multiple-antenna OFDM, in particular variants of selected mapping (SLM), are considered. Directed SLM (dSLM), where a joint PAR reduction over the anten-nas is performed, is reviewed and compared to the obvious application of SLM to each single antenna (oSLM). Due to the avoidance of the signal peaks, less out-of-band radiation is caused. It is shown that both, oSLM and dSLM, provide significant gains in the reduction of the out-of-band radiation compared to conventional OFDM. However, using the same complexity, dSLM provides significant extra gain over oSLM. The gain, which is clearly dependent on the type of nonlin-earity and the chosen power backoff, is assessed by means of numerical simulations