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Optimum Mix: Energy efficiency of optically backhauled LTE in different scenarios

May 23, 2016

The increase in the number of data users and average access data rates has resulted in the demand for a larger number of high capacity network terminals in the wireless backhaul network. Most of the conventional backhauling methods such as microwave links, time division multiplexing-based leased lines (E1/T1), asynchronous transfer mode and digital subscriber lines are not capable of providing high data rates to radio base stations in the order of Gbps, as being offered under advanced technologies such as long term evolution (LTE).

Meanwhile, optical access networks can provide high energy efficiency and very high bandwidth almost independent of distance. Therefore, a combination of mobile wireless access and optical access networks seems very promising for a sustainable and efficient next-generation ubiquitous access network. An integrated approach will create synergies and provide complementary features. While a wireless cellular network provides mobility and ubiquity, an optical access network provides high data rates, low power consumption and robustness.

There are several ways of combining wireless networks and optical fibre technologies. The most basic among these is radio over fibre, in which radio frequency signals are modulated directly on the optical carrier and transmitted through an optical fibre link. Different signals, frequency bands, modulation formats and coding schemes can be combined and transmitted over a single fibre using this technology. It can provide transparency in the optical domain in terms of wireless standards and services as well as high energy efficiency by minimising the number of components between a mobile user and the central office of the network provider. However, it suffers from signal impairments such as noise, distortion and dispersion. Moreover, the existing optical access infrastructure and devices do not support this technology.

Therefore, combining wireless access with a standard fibre-to-the-home technology such as gigabit passive optical network (GPON), Ethernet PON (EPON) or point-to-point optical Ethernet is the most likely option for achieving efficient and fast penetration of high speed LTE technology.

The following is a case study conducted by Slavia Aleksic of the Institute of Telecommunications, Vienna Institute of Technology, Austria, and Margot Deruyck and Wout Joseph of the Department of Information Technology, iMinds Ghent University, Belgium, to investigate various options for deploying LTE in urban areas with a particular emphasis on energy efficiency. For the purpose of evaluation, the researchers have applied an access network model to estimate both the power consumption and the achievable access data rates for various deployment scenarios in Vienna.

Network model

The authors considered two sub-models for the purpose of the study-cellular wireless and optical wireless backhaul. The input parameters of the cellular wireless sub-model include the area to be covered, the population density, the number of subscribers, the percentage of subscribers that are active at a particular time, and downstream and upstream data rates that are demanded by active users. Meanwhile, the input parameters for the sub-model of optical wireless backhaul are the outputs of the cellular sub-model such as the average number of active users per cell, the number of active base stations and average data rates (both upstream and downstream) per base station.

Further, the authors assumed four deployment scenarios for a radio access network (RAN) in Vienna. The key difference among these is the coverage and the choice of technology used for the RAN. The first is a baseline scenario, with the GSM/GPRS network in operation in parallel with the universal mobile telecommunications system (UMTS)/enhanced data gsm environment (EDGE) network and negligible LTE penetration. Microwave is the key backhaul technology used in this case. The second scenario envisages an LTE network roll-out with an optical backhaul using GPON technology while the UMTS/ EDGE network is phased out. Thus, there are coexisting GSM/GPRS and LTE networks, both with an optical backhaul. In the third scenario, the UMTS/EDGE network is still in operation in parallel with GSM/GPRS and LTE. The fourth scenario envisages a phase-out of both the GSM/GPRS and UMTS/EDGE networks, which are completely replaced with an optically backhauled LTE network.

Methodology

The estimation of energy efficiency is done in three steps. First, the design and configuration parameters are calculated on the basis of the number of customers, the area, technology peculiarities, aimed coverage, etc. Then, the aggregate traffic at each network element is determined and the intermediate results are exchanged between the network sub-models. Finally, average data rates and energy consumption are calculated in order to obtain the energy efficiency. The power consumption of network elements is determined by defining generic structures of each network element and summing up the values of power consumption at the component (sub-function) level.

Observations

The results of energy consumption and achievable average data rates for the four considered scenarios reveal that deploying a high LTE coverage network along with the existing and operating GSM/GPRS and UMTS/EDGE networks (Scenario 2) would unavoidably lead to an increase in the total network energy consumption. Even if all base station sites are optically backhauled, the energy consumption will increase by 25 per cent. In addition, high LTE coverage enables an increase in data rates by a factor of 3.6. If the UMTS/ EDGE infrastructure is completely replaced with LTE (Scenario 3) and an area-wide optical backhaul is provided, the total energy consumption can be reduced by around 6 per cent relative to the baseline scenario, while the average data rate increases by a factor of 5. Finally, if both GSM/GPRS and UMTS/EDGE networks could be phased out and replaced with a high-coverage LTE network as assumed in Scenario 4, the average data rate could be increased even further by a factor of 5.4 as compared to the baseline scenario, while at the same time reducing the total energy consumption by more than 20 per cent.

The authors then analysed the impact of the choice of technology for optical wireless backhaul on energy efficiency. For this, they considered Scenario 3 and calculated the achievable average data rates and the total energy consumption while using either GPON or EPON or a point-to-point active optical Ethernet for backhaul. It was observed that all the three backhaul options were able to provide the same average data rate of approximately 665 kbps, while the energy consumption differed only insignificantly. The maximum relative difference in energy consumption of the three considered backhaul options was below 2 per cent.

Conclusion

The above study concludes that while optical backhaul is generally able to provide noticeable improvements in energy efficiency as compared to conventional backhauling methods, the influence of the choice of optical technology for backhaul is insignificant. Meanwhile, a transition to 4G LTE with a phase-out of 2G and/or 3G networks would result in a significant improvement in the energy efficiency of the RAN infrastructure.

 
 

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