Abstract
A subspace-based sidelobe suppression scheme is proposed for interference-avoidance in orthogonal frequency division multiplexing (OFDM) cognitive radio systems. Specifically, OFDM systems have often been employed in cognitive radio systems due to its flexibility and adaptivity in utilizing certain parts of the spectrum. That is, when primary or licensed users are sensed in certain frequency subbands, secondary or unlicensed users can simply turn off the corresponding subcarriers in these subbands to avoid interference. However, due to sidelobe radiation from neighboring subcarriers, this simple on-off operation is often insufficient to guarantee low interference to primary users and, thus, more advanced techniques must be employed. In this thesis, we propose a subspace-based sidelobe suppression scheme where, instead of utilizing all available subcarriers for data transmission, we map the data into a carefully chosen subspace in order to minimize the sidelobe radiation in the primary users' spectrum and possibly impose a active cancellation signals in its orthogonal subspace. We show that the general subspace-based sidelobe suppression scheme includes as special cases the carrier cancellation and the guard band schemes previously proposed in the literature. First of all, with the goal of minimizing the interference, we show that the active cancellation signal is not needed as long as the data is placed in a properly chosen subspace. This allows secondary users to utilize all its power for data transmission. Then, based on the chosen data-bearing subspace, we derive the optimal input covariance matrix by maximizing the system sum rate. The optimal solution is derived for the case employing only the AWGN channel model whereas suboptimal solutions are proposed for the case with fading channels to reduce the complexity of the problem. In the further work, it is worthwhile to note that, with the goal of maximizing the system sum rate, the use of suppression signals can now be beneficial. Furthermore, to increase the dimensions and the flexibility of the design, the proposed schemes will be then extended to the case where the data subspace and the cancellation signals are jointly optimized over multiple OFDM symbols in our future investigation.