Spectral re-growth due to high power amplifier nonlinearities in MIMO-OFDM relaying channels

MIMO-OFDM combination is a very attractive solution for broadband wireless services. Thus the two prominent fourth-generation (4G) cellular systems, WiMAX and LTE-advanced, have both adopted MIMO-OFDM transmission at the physical layer. OFDM signal however suffer from nonlinear distortions when passed through high-power amplifier (HPA) at the RF stage. This nonlinear distortion introduces out-of-band spectral re-growth and in-band distortions in the transmitted signals. Cellular regulators world-wide place limits on allowable out-of-band emissions to insure that an operator is not significantly interfering with an adjacent spectrum licensed to a different operator. In this article, we characterize the out-of-band spectral broadening introduced by HPA when MIMO-OFDM signals are transmitted over multiple relaying channels. It is shown that due to the cumulative effect of multiple HPA in a MIMO link, and the cascade effect of many relaying HPA's in a multihop relaying channel, significant out-of-band emission results. We show that depending on the number of relays and the relaying configuration employed, it may happen that a transmitted MIMO-OFDM signal with spurious emissions initially within the allowable limits at the source node arrives at the destination with the cumulative spectral re-growth due to multihop relaying, beyond the set limit.