Channel Assignment and Opportunistic Spectrum Access in Two-tier Cellular Networks with Cognitive Small Cells

Cognitive small cells using opportunistic spectrum access (OSA) are envisioned to be a key technology for future generation multi-tier cellular wireless networks. In a multi-tier cellular network with universal frequency reuse, macrocell users may suffer performance degradation due to inter-tier interference caused by closed-access small cells. On the other hand, when the traffic load served by the macro network tier is high, the OSA performance of the small cells will be poor. However, similar to that for the macrocell users, the quality-of-service (QoS) performance of the small cell users need to be guaranteed. Therefore, efficient channel sharing mechanisms need to be developed for macrocell and small cell users. In this paper, we investigate on two extremal channel assignment techniques, namely, the random channel assignment (RCA) and the sequential channel assignment (SCA) techniques in the macro tier in order to accommodate the overlaid cognitive small cells with an acceptable OSA performance. We show that, for downlink transmission, while the RCA scheme offers a low intra-tier interference for macro users, it results in a degraded OSA performance (in terms of probability of successful channel access) for the small cells. On the other hand, the SCA scheme enhances the OSA performance of the small cells at the expense of increased intra-tier interference for macro users. To quantify the performance gain achieved due to OSA by the small cells for these two channel assignment schemes, we compare the coverage probability of a generic macro user when the small cells are cognitive and when the small cells aggressively use the spectrum.