NARROWBAND INTERFERENCE AVOIDANCE TECHNIQUES IN COGNITIVE MB-OFDM UWB SYSTEMS

Abstract

Due to large bandwidth of UWB systems, it has to share the spectrum with already existing narrowband services. In order to coexist with existing systems MB-OFDM systems should have cognitive radio capabilities which can detect and avoid interference to the Primary User (PU). In this dissertation, our main focus is 011 interference cancellation techniques to protect PU. For MB-OFDM system, notch can be produced by iiulliug tones, Active Interference Can-cellation (AIC), Polynomial cancellation Coding (PCC) etc. Nulling tones can produce shallow notch (approx. 15dB for 5 iiulling tones) and AIC can produce much deeper notch (approx. 30dB for 2 AIC edge tones) which can be improved by using protecting tones. In this report irnprovcxrient in AIC is proposed by utilizing Guard Carriers which provides notch gain of around 8dB over AIC technique. Sidelobe suppression technique called Subcarrier Weighting (SW) is almost unexplored for inband interference cancellation. This report shows importance of SW to protect NB systems. This scheme can produce variable notch depth (20dB to 50dB) depending OH the constraints on weights. Increase in notch depth comes with sacrificing the BER perfor-mance. More variations like SW-Rectangle, SW-Circle, SW-Ellipse and joint SW-AIC are proposed to improve the performance of system. Two recent precodirig techniques named Orthogonal Projection Precoder (OOP) and Mask Complaint Precoder (MCP) are explored in this report for NB interference cancellation. We observed a very dccl) notch (approx. -160dB) using app but scheme distorts the sym-bols from original points on constellation which leads to degraded BER performance of system whereas MCP can provide variable notch depth by compensating with the BER performance. Improvement in OPP scheme is proposed using protecting subcarrier which is extendable to MCP as well. Finally a novel spectrum sharing scheme for STFBC MB-OFDM UWB system is pro-posed to create a notch in