Choose Your Subcarriers Wisely: Active Interference Cancellation for Cognitive OFDM

A novel low complexity active interference cancellation (AIC) scheme for primary user (PU) protection is presented for application to cognitive orthogonal frequency division multiplexing (OFDM) systems, in which out-of-band radiation spilling over the PU protected band is to be minimized. A set of cancellation subcarriers are modulated by appropriate linear combinations of the remaining data subcarriers. The combination weights are fixed so that, in contrast with previous AIC approaches, they need not be recomputed on a symbol-by-symbol basis. Weight optimization can thus be carried out offline, drastically reducing the online computational cost. In addition, it is shown that by carefully selecting the location of cancellation subcarriers, significant performance improvements can be achieved. Given that finding the optimal location is an intractable combinatorial problem, an heuristic approach is proposed, based on a greedy search which provides a good tradeoff. The proposed scheme is shown to outperform current AIC solutions both in terms of performance and computational cost, obtaining significant improvements in terms of notch depth, with almost 50 dB depth in typical settings, to protect a narrowband PU inside the secondary user OFDM band. Further, experimental measurements from the implementation of the proposed scheme on both professional and off-the-shelf hardware platforms validate its effectiveness.