Edge Windowing for OFDM Based Systems

In this letter, we propose a new windowing method for efficient spectral shaping in orthogonal frequency division multiplexing (OFDM) based systems. Unlike the conventional windowing techniques where the same windowing is used for all the subcarriers within an OFDM symbol, in the proposed approach, windowing is heavily applied to the subcarriers located at the edges of the band; shorter windowing sizes are used for the inner subcarriers compared to edge subcarriers. Instead of introducing additional windowing interval, in the proposed approach, the windowing time is stolen from the cyclic extension size. Therefore, with the proposed approach, sidelobe suppression is achieved while maintaining the spectral efficiency. The proposed technique brings about a new degree of freedom in achieving spectrally efficient sidelobe suppression and introducing controllable inter-carrier-interference (ICI) and inter-symbol-interference (ISI). This degree of freedom can be exploited with subcarrier based adaptive approaches like adaptive modulation, power control, and scheduling to improve the capacity and performance of OFDM based wireless communication systems.

I Introduction

Efficient usage of the spectrum resources has gained an extra importance with the emerging concepts of cognitive radio and spectrum aggregation [1], [2]. Orthogonal frequency division multiplexing (OFDM) is one of the candidate signaling schemes for cognitive radios and spectrum aggregation due to the operability over the discontinuous subbands [3]. However, OFDM waveform is a combination of rectangular windowed orthogonal subcarriers. Since the rectangular windowed subcarriers correspond to sinc functions in frequency domain, OFDM suffers from high sidelobes, which results in adjacent channel interference (ACI).