Silent Subcarrier-Based Interference Rejection Combining for Mobile Black-Space Cognitive Radios

In this paper, we investigate the interdependence of sample covariance estimation structure and transceiver mobility in the black-space cognitive radio (BS-CR) context. Here the BS-CR utilizes multi-antenna interference rejection combing (IRC) schemes to enhance the reception. These BS-CR algorithms and schemes enable effective spectrum reuse of the primary user (PU) for CRs, especially over short distance. The BS-CR and the PU system apply orthogonal frequency division multiplexing (OFDM) scheme using equal subcarrier spacing and equal cyclic prefix (CP) length. Then, we can assume flat fading at the subcarrier level, which allows to use subcarrier-wise IRC processing. During silent time/frequency gaps in the CR transmission, the receiving CR station uses sample covariance matrix-based IRC adaptation. We focus on comparing the performance between our new approach, silent subcarriers, against the earlier-used silent time-domain gap method in case of relatively low mobility. We present an experimental study of how the CR link performance varies depending on the structure of the silent gaps used for covariance estimation, the path delay profile of the propagation channel, and the velocity of the mobile CR. The performance evaluation is provided assuming 4G LTE-like signal structure for the PU.