Wireless optical convergence enables spectrum-energy efficient wireless networks

The most effective solution to simultaneously improve both spectrum efficiency (SE) and energy efficiency (EE) is a small-cell structured wireless network such as distributed antenna network (DAN). Instead of using centralized antennas at macro-cell base station (BS), antennas are spatially distributed and are connected via coherent optical links to signal processing center (SPC) (which is equivalent to macro-cell BS). Although DAN is actually small-cell structured, SPC and its distributed antennas form a virtual macro-cell. The channel between SPC and a user terminal can be treated as an equivalent wireless channel. Therefore, handover problem in a small-cell network can be replaced by antenna selection problem within a virtual macro-cell transceiver of SPC. The deployment density of SPCs can be made similar to the present macro-cell network. In this paper, we will discuss SE-EE tradeoff of a cellular network using frequency reuse and show the potential improvement of SE-EE tradeoff introduced by the small-cell structured network. Then, we will describe DAN concept forming virtual macro-cell and heterogeneous DAN overlaid by macro-cells.