ANALYSIS OF ACTIVE INTERFERENCE CANCELLATION FOR JOINT SUBCARRIER WEIGHTING AND SIDELOBE SUPPRESSION IN OFDM COMMUNICATION TECHNIQUES

The wireless communications with high data rates entails a substantial need for additional spectral resources through more flexible and efficient use of the available spectrum. Orthogonal frequency division multiplexing (OFDM) has been a prime candidate for spectrum pooling-based wireless transmission systems since subcarriers in the vicinity of the licensed users can be deactivated in order to minimize interference from the unlicensed user. Orthogonal frequency division multiplexing (OFDM) is a recognized transmission technique for Cognitive radio (CR) systems but one of the major drawbacks of OFDM scheme is its out-of-band (OOB) leakage due to high spectral sidelobes. The major challenge that OFDM has to deal with is high spectral sidelobes. Active interference cancellation (AIC) and subcarrier weighting (SW) are recognized techniques for sidelobe suppression. This paper proposes a joint SW-AIC scheme which has the ability to produce much deeper notch as compared to individual AIC and SW and system parameters can be adjusted depending on user's requirements. By using both the technique we will suppress the sidelobes. In AIC Technique, instead of turning off a large number of tones, we define two special tones at the edge of the interference band and these two tones can sufficiently cancel the interference in the band. The tone values can be arbitrarily determined without affecting the information tones due to the orthogonality relationship. We call these special tones Active Interference Cancellation (AIC) tones. We will discuss how to compute the AIC tones, and how to create the notch using the minimum number of tones, or, equivalently, how to maxime the spectrum efficiency. The Subcarrier Weighting (SW) Technique is based on the multiplication of the used subcarriers with subcarrier weights. The subcarrier weights are determined in such a way that the sidelobes of the transmission signal are minimized according to an optimization algorithm which allows several optimization constraints. As a result, sidelobe suppression by subcarrier weighting reduces OFDM sidelobes by more than 10 dB in the average without requiring the transmission of any side information. Active Interference Cancellation (AIC) scheme is suitable for the transmissions which can reduce throughput but will increase BER degradation, for e.g. important data losses and performance of software. On the other hand SW scheme is best suited for the systems which can reduce BER degradation but also reduce throughput of the system such as live video/audio streaming. But for a dynamic system where requirements such as BER and throughput of a cognitive user are changing with time, we need to have a scheme which can dynamically change its parameters to best serve the user needs. Motivated by this, we propose a SW-AIC scheme which provides us with various knobs for adjusting system parameters. Apart from this, interference reduction capability of SWAIC scheme is far better than individual AIC or SW.