NETWORK DEPLOYMENT
In heterogeneous networks, illustrated in Fig. 3,low power nodes complement the conventional macro-cellular network layout, micro-/pico-cells cover high traffic demand (hot spot) areas, while indoor base stations constitute femto-cells that provide broadband coverage to indoor users.
Furthermore, relay nodes are a cost efficient means to extend outdoor coverage, since expensive backhaul links are avoided. Such heterogeneous networks potentially promise energy
savings, since they shorten the propagation distance between nodes, and thereby reduce the required transmission power. On the other hand,
a deployment with only small cells may be uneconomical, due to the prohibitive number of low power, and therefore short range, base stations.Moreover, this increases the number of base stations operating at low loads, which may degrade
the overall energy efficiency. Hence, for each scenario (e.g., dense urban vs. rural) heterogeneous deployments with an optimal balance of macro-, micro-, pico-, and femto-cells must be found for a most energy efficient network layout.
Cooperative transmission from several base stations to one mobile device is a further enabler for enhanced energy efficient network operation; such inter-base station cooperation avoids interference, or can even turn interference into a useful signal. However, the improvement in energy efficiency at the air interface may be cancelled out by more complex signal processing and/or increased backhaul traffic. Fundamental tradeoffs among these factors need to be studied, providing the basis for designing energy efficient site cooperation protocols.