Joint Transmission Mode and Tilt Adaptation in Coordinated Small-Cell Wireless Networks

We focus on downlink transmission in a wireless network consisting of small-sized cells, denoted as a small-cell network (SCN). We consider multi-antenna base stations (BSs) and single-antenna users, BS coordination, downlink training and channel estimation, 3D antenna patterns, distant-dependent pathloss, and fair scheduling. We investigate the impact of the variable parameter that controls the elevation angleof the BS antenna pattern, denoted as tilt, on the performance of the considered SCN when employing either a conventional uncoordinated transmission mode or a fully coordinated transmission mode. Using the results of this investigation, we propose a novel hybrid-mode transmission technique that can achieve a performance comparable to that of a fully coordinated transmission but with a significantly lower complexity and signaling requirement. The main idea is to divide the coverage area into two so-called vertical regions and jointly adapt the transmission mode and the tilt at the BSs when serving each region. A fair scheduler is used to share the time-slots between the vertical regions and among the users in each region. To make the performance analysis computationally efficient, analytical expressions for the user ergodic rates at different transmission modes are also derived.