Energy Efficiency of Wireless Access Networks

Energy Efficiency of Wireless Access Networks

Energy efficiency in mobile radio networks has recently gained great interest due to escalating energy cost and environmental concerns. Rapidly growing demand for capacity will require denser and denser networks which further increase the energy consumption. In this regard, the deployment of small cells under macro-cellular umbrella coverage appears a promising solution to cope with the explosive demand in an energy efficient manner. The research team at KTH Royal Institute of Technology has investigate the impact of joint macro-and femtocell deployment on energy efficiency of wireless access networks, based on varying area throughput requirements. The team take into account the co-channel interference, fraction of indoor users, fem to base station density and backhaul power consumption. It is shown that utilizing indoor base stations provide signficant energy savings compared to traditional macro only network in urban areas with medium and high user demand where the gain increases up to 75 percent as more data traffic is offloaded to femtocells.

In recent years, operators are facing exponential growth in data traffic due to the rapid proliferation of smart-phones, laptops, and tablet PCs with built-in cellular access as well as flat rate tariff. This situation poses steeply reducing revenue per unit data consumed, and thus increases the significance of the cost effective solutions. Another important consequence of the data explosion is the rapid rise in energy consumption which is used to have minor impact on the operators operational expenditures (OPEX). However, nowadays, energy bill constitutes 20-50 percent of the OPEX depending on number of off-grid sites, unit energy costs,etc. To solve this issue, academia and industry are making great efforts to improve energy efficiency of the state-of-the-art wireless broadband networks at all levels.

Considering the fact that exponential increasing demand for high data rates will require the deployment of several orders of magnitude more base stations (BSs), using high power macroBSs is expected to be neither energy efficient nor very sound from a radiation perspective. In this regard, heterogeneous networks (Hetnets), i.e., strategically located large number of small BSs such as micro-, pico- and femto BSs, under the macro-cellular umbrella coverage is believed to enable energy savings due to their low transmit power requirements. Especially femto BSs, i.e., low-power, low-cost, user-deployed base stations operating in licensed spectrum, have attracted great interest in order to address coverage and capacity needs in residental or enterprise environments in which 60 percent of voice and 70-90 percent of data traffic are expected to be originated. However, co-channel operation with the existing macro-cellular network creates challenges due to the mutual interference generated between fem to cells and macrocells. This issue has been widely investigated in the literature and the findings indicated that co-channel deployment has a minor impact on the performance of macro-cellular network with the appropriate configurations.

On the contrary, despite its cost-effectiveness, joint macro-femto deployment is indicated to increase the energy consumption compared to traditional macro-only network for medium and high femto deployment densities and high number of supported users per macrocell. This difference is claimed to be further increased when idle mode operation for macro BSs is implemented. The energy efficiency of user-deployed femto BSs are also compared with outdoor Hetnet deployment,i.e., macro-micro, and it is stated that these strategies have similar energy consumption for a given capacity enhancement requirement. However, outdoor small cell deployment is proposed as a better solution for the operators due to their high convenience in terms of management and control.

There are two main reasons that the studies indicate contradictory conclusions. First, the power consumption of the network is modelled differently, i.e., transmit power only or total power consumption including idle and backhaul power consumption. Second, the energy efficiency comparison between different deployment strategies has been done either under a chosen performance constraint, e.g., average user throughput, network capacity, coverage, or only based on the total power consumption. However, in order to evaluate different solutions in the unbiased manner, network performance requirements by means of both coverage and capacity should be characterized.

The research team assess the energy efficiency of joint macro-femto deployment with respect to area power consumption for a given coverage and area through put requirement by considering backhaul power consumption which is often ignored in the literature. The results have been compared with traditional macro-only network offering service to both indoor and outdoor users and providing the same performance. Furthermore, the team investigate the impact of several factors affecting area throughput and energy efficiency, such as femto BS density, the fraction of the indoor users, and inter site distance (ISD). The results show that energy efficiency improvement through joint macro-femto deployments are highly related to area throughput requirement and the amount of mobile traffic being of floaded to femtocells, where up to 75% power savings is found to be feasible in urban areas with high capacity demand.

Conclusion

In this research, the team investigated the impact of femtocell deployment with varying density on the energy efficiency of wireless access networks. The team demonstrated how different area throughput targets and percentage of indoor users affect the area power consumption taking into account the impact of backhaul. The results show that traditional macro-cellular networks achieves the minimum area power consumption only when the capacity requirement is very low. However, the team observe that they quickly lose their efficiency with the increasing demand for high data rate and even becomes infeasible to fulfill the requirement. On the other hand, cochannel deployment of fem to BSs under the macro-cellular coverage is shown to increase the area spectral efficiency due to high spectral frequency reuse and minor mutual interference between macro- and femtocells with a relatively lower increase in power consumption. This results in significant energy savings compared to macro-only network for medium and high capacity demand where the gain increases up to 75% as more traffic is offloaded to femtocells. Considering the fact that orders of magnitude of more mobile data traffic will be generated at indoors in the near future, blasting the signals over the walls is shown to be neither energy efficient nor feasible to satisfy the growing capacity demand.

Courtesy of Sibel Tombaz, [email protected]

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