OFDM Sidelobe Suppression by Time Domain Extended Active Interference Cancellation

Abstract

In orthogonal frequency-division multiplexing (OFDM) systems, one of the main issues is the emitted out of band energy (OOBE), better known as the spectral sidelobes of the OFDM signal. Current methods, addressing this problem, the so called sidelobe-suppression- or out-of-band-emission- reduction-techniques, can roughly be divided into two main categories of approaches, time domain and frequency domain techniques. Neither of them reaches the level of sidelobe suppression, experienced with single channel modulation, with identical conditions like spectral efficiency and level of self- interference. Apparently, these approaches do not represent an optimal solution to the problem. For time domain techniques, the reason for this weakness is their undirected operation which only tries to handle the symptoms instead of avoiding the reasons. Frequency Domain techniques try to avoid the reason for the sidelobes, but they abandon the best suited degrees of freedom, by using only the strict corset of basic functions that are offered by the OFDM-system. This paper shows a new approach, that builds the bridge between time domain and frequency domain approaches and effectively reduces OOBE with a time domain signal that is generated depending on the OFDM-carrier content. It is similar with the known method of extended additive interference cancelation (EAIC) under the aspect of overlaying the current and its adjacent OFDM- symbols with a cancelation signal, but it is different by using the mentioned degrees of freedom with a more generic time domain approach. In detail, we show how to calculate the mentioned time domain cancelation signal by a convex optimization, and we show how it performs at the decisive figures like self- interference and power spectral density (PSD) and peak to average power ratio (PAPR).