Non-Contiguous OTFS

In cognitive radio (CR) networks, non-contiguous orthogonal frequency division multiplexing (NC-OFDM) is a widely adopted overlay-based CR technique. However, it suffers from serious performance degradation in high-speed mobile environments due to its susceptibility to high Doppler shifts, a vulnerability inherited from OFDM. To overcome this drawback, we introduce non-contiguous orthogonal time frequency space (NC-OTFS). Unlike OFDM, OTFS spreads data symbols across all sub-carriers using the inverse symplectic finite fourier transform (ISFFT), making it challenging to deactivate unwanted sub-carriers. To address this issue, we propose two types of NC-OTFS schemes: Type 1, which generates individual OTFS signals for each disjoint contiguous sub-carrier block, and Type 2, which creates a single OTFS signal across all available non-contiguous sub-carrier blocks to achieve the maximum frequency diversity. Furthermore, we present an enhanced time domain block minimum mean squared error (MMSE) demodulation algorithm to mitigate primary user (PU) interference in the proposed NC-OTFS receivers. In addition, we provide an implementation of Type 2 NC-OTFS transceiver by adding simple pre-processing and post-processing steps to the conventional OTFS transceiver, which allows reusing all the transceiver algorithms for the conventional OTFS. Numerical results demonstrate the remarkable superiority of the proposed NC-OTFS over the conventional NC-OFDM, especially in high Doppler environments.