Reduction of Out of Band Radiation Using Modified Constellation Expansion in OFDM based Cognitive Radios

Abstract

Out of band radiation (OOBR) is one of the major problems in OFDM based Cognitive Radios. In the spectrum sharing scenario, OOBR generates appreciable interference to the primary user, even if its spectrum band is not used. The basic cause of this OOBR is the rectangular shaped OFDM symbol. To mitigate the effects of the OOBR, a number of techniques have been proposed in the literature. These techniques shape the OFDM symbol either in time or in frequency domain. Time domain techniques achieve OOBR reduction by using smoother windows. But this reduces the overall throughput of the system. In frequency domain techniques, some of the subcarriers are either used as guard band or reserved for interference cancellation. These subcarriers do not carry any data and as such reduce the spectral efficiency of the system. In this paper, we use different variants of Constellation expansion (CE) technique to reduce OOBR. CE techniques do not suffer from throughput reduction or spectral efficiency loss, as the original symbols are mapped to a higher order constellation set and the symbol sequence is replaced by a new sequence that produces minimum OOBR. We study different mapping techniques based on single bit or two bit mapping of CE technique. We show through simulations that two bit mapping of CE technique gives appreciable improvement over the basic CE technique.