1 Introduction
Over the last decade, we have witnessed a dramatic increase in mobile data traffic due to the large-scale deployment of increasingly powerful mobile devices and the resulting proliferation of mobile services/applications. This trend is expected to continue in the foreseeable future [5]. To deal with this explosive growth of mobile traffic, there have been numerous efforts to increase wireless link/network capacity by developing new physical-layer technologies and/or improving the existing design. However, these solutions are still unable to keep up with fast growing demands.