I. Introduction
With the exponential growth in the demand for mobile data, wireless systems in general are experiencing densification of the wireless network elements that provide mobile data access. A notable example of wireless systems that have followed this densification trend is cellular systems, in which the high demand for data has been addressed through the introduction of heterogeneous cellular networks (HCNs) [1]. HCNs are a paradigm shift in the deployment of cellular network infrastructure, moving away from expensive high-power macro base stations mounted on towers to less expensive lower-power small cells mounted on buildings and light poles. Small cells include microcells, picocells, femtocells as well as distributed antenna systems, all of which are distinguished by their transmit power, coverage areas, physical size, backhaul, and propagation characteristics. Macrocells are typically interconnected through high-speed fiber optics links, whereas small cells are backhaul-constrained due to deployment limitations, putting constraints on the cooperation mechanisms.