On SNR degradation in OFDM systems with aliasing and a frequency offset

Abstract

In this work the effects caused by aliasing and a frequency offset in orthogonal frequency division multiplexing (OFDM) systems will be analysed. Specifically, scope of this paper is to provide an exact analysis of the disturbances originating from aliasing in combination with a frequency offset that cause a degradation in the signal to noise ratio (SNR). In OFDM systems due to the narrow frequency spacing of the utilized carriers it is difficult to practically avoid aliasing because the transition bandwidth, which is available for the anti-aliasing low-pass filter is quite small for a given sampling frequency and large FFT- lengths. We separately discuss the SNR degradation caused by aliasing and the effects caused by a tolerated clock frequency offset. The combined case is investigated and the reaction of the different effects is explained. Finally, exact formulas that give the SNR degradation according to the low-pass filter's characteristics are given, which provides an additional tool that should simplify the work of system design engineers while helping them to choose the appropriate low-pass filter and frequency oscillators as well as system parameters related to the SNR estimation such as the length of the measurement interval.