I. Introduction
As THE number of wireless data subscribers keep increasing world wide, the cellular operators have to cope with an increasing demand for providing ubiquitous coverage with high rates even while maintaining a low carbon footprint [1]. To meet target data rates, the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 10 [2] proposes to build wireless networks with planned macro base stations (BSs) and secondary base stations such as relays, picos and femtos [3]. Such a cellular network consisting of diverse transmitter nodes operating simultaneously in the same geographical area is called a heterogeneous network (HetNet).