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Orthogonal frequency division multiplexing (OFDM) technology, in combination with advanced features such as bit-interleaved coded modulation (BICM) \cite{cai98} and link resource adaptation (LRA) \cite{gold97} and resource allocation techniques (RA), have been identified as the building blocks for future wireless communication systems, in that they can meet the strict quality of service (i.e. high data rate, reliable communications) requirements of the users in the harsh propagation environment of the wireless multipath channel.
Due to the proliferation of advanced mobile devices (i.e., smartphones, tablets), a 20-fold increase in data traffic is expected over the next few years, compelling mobile operators to find new ways to significantly boost their network capacity, provide better coverage, and reduce network congestion. In this context, the idea of heterogeneous networks (HetNets), consisting of a mix of short-range and low-cost small cell base stations (SBSs) underlaying the existing macrocellular network, has recently emerged as a key solution for solving this capacity crunch problem.
The surest way to increase the system capacity of a wireless link is by getting the transmitter and receiver closer to each other, which creates the dual benefits of higher-quality links and more spatial reuse. In a network with nomadic users, this inevitably involves deploying more infrastructure, typically in the form of microcells, hot spots, distributed antennas, or relays.
The enormous gains reaped from smaller cell sizes arise from efficient spatial reuse of spectrum or, alternatively, higher area spectral efficiency and also from mitigate fading and reduce time dispersion on urban mobile radio channels.