Energy and environmental aspects of mobile communication systems

The reduction of the energy consumptions of a Telecommunication Power System represents one of the critical factors of the telecommunication technologies, both to allow a sizeable saving of economic resources and to realize "sustainable" development actions. The consumption of about one hundred base stations for mobile phones were monitored for a total of over one thousand days, in order to study the energy consumption in relation to the environmental, electric and logistics parameters of the stations themselves. It was possible to survey, then, the role of the mobile communication systems in the general national energy framework and to plot the best areas of intervention for saving energy and improving the environmental impact, showing the role played by air conditioning and transmission equipments. Finally, new transmission algorithms and the use of renewable energy based techniques have been tested. Introduction Mobile Telecommunication applications have seen, in the last years, a remarkable increase in the number of installations and consequently there has been a relevant growth of energy consumptions. This is due to an ever-growing interest about new and reliable services in mobility calls with an increase of the BTS operational hours and traffic management, in order to guarantee the quality of the service anywhere and any time. In other words improving the energy efficiency of telecommunication networks is not just a necessary contribution towards the fight against global warming, but with the rapidly rising prices of energy, it is becoming also a financial opportunity [13]. From a technical and economical point of view, possible interventions in the field are: a) energetic auditing for radio-telecommunication stations in different operational contexts (urban and rural areas, different periods in the year, different working load, etc.); b) interventions of efficiency increase and energy saving such as reduction of transmission apparatus consumptions, optimization of air conditioning consumptions, efficiency in the temperature control system; c) evaluation and development of interventions and technical solutions based on the local production of a part of the consumed energy, through the use of photovoltaic cells on the infrastructures themselves; analyses of possible uses of other renewable sources (e.g. wind micro turbines) generating energy usable for systems located in areas not reached by the electricity network; d) analyses of the social and environmental advantages from the introduction of technologies based on renewable sources; e) environmental monitoring of the sites where prototypal solutions have been installed, aimed to compare the conditions before and after the intervention. The typical wireless network can be viewed as composed by three different sections: • the Mobile Switching Center (MSC): switching and interfacing to fixed network; • the Base Transceiver Station (BTS): which is used as interface between network and mobile terminals; • the Mobile Terminals, normally limited to the hand-held devices. The key elements are the BTSs because their contribution is the most relevant for the total energy consumption. Indeed, as the number of core network elements is low, the total energy consumption due to core network is low. Moreover, the power consumption of mobile terminals has been optimized in the years and is comparatively very low (a few Watts). The energy allocation per function within the BTS has been extensively studied [1,4-5]. More than 60% of the power is consumed by the radio equipment and amplifiers, 11% is consumed by the DC power system and 25% by the cooling equipment, an air conditioning unit. The Radio Equipment and the Cooling are the two major sections where the highest energy savings potential resides. Other strategies, involving rearrangement of the network topology, are depicted in [6] and references therein. Therefore it is very important to consider BTS Energy Savings Strategies applied both to the radio equipment, i.e. radio “standby” mode [7-9] and to the cooling, i.e. passive cooling, advanced climate control [10], and power equipment. Moreover one has to consider the aggregate effect that represents a further benefit [1]. Another way to reduce cost and CO2 emissions is the evaluation and development of interventions and technical solutions based on the production of a part of the energy used by radio-telecommunication apparatuses, through the use of renewable sources (e.g. photovoltaic cells, wind micro turbines or new alternative power based on fuel cells) installed on the infrastructures themselves usable both by grid-connected and by off-grid telecommunication power systems. The use of alternative energy sources has been studied in particular for sites that are beyond the reach of an electricity grid, or where the electricity supply is unreliable or sites that are remote enough to make the regular maintenance and refueling of diesel generators prohibitive [11-12]. The choice of alternative energy sources will depend on local conditions, BTS typology and energy consumption amount. In most cases an hybrid solution combining solar and wind is actually the most feasible solution for autonomous BTSs site. Anyway, the size of solar cell and wind turbine have to be defined based on the BTS load and on-site availability of sun and wind as we will see later on. In the following the results of on site experiments concerning energy consumption of BTSs, power saving actions and application of renewable energy supply for BTSs will be shown. Radio Base Station Energy consumptions. Energy auditing of a BTS is the most important step in the understanding of an energy management of wireless telecommunication power systems. With this aim we made a campaign of measurements for a radio-telecommunication apparatus starting from on-site measurements, performed in collaboration with Italian companies of mobile communications systems (Vodafone, H3G, Telecom and Wind). Thanks to this collaboration, it has been possible to retrieve data coming from a statistical sample of 95 Radio Base Stations spread on the whole Italian national territory, that corresponds to more than 1000 monitoring days. All the field measurements were performed using a specific monitoring system described in chapter II of [13]: it is composed of a series of Remote Terminal Units (one for every station) collecting all the data of interest and sending them periodically to a Site Energy Saving SW tool installed in the Control Center Server. Aim of our studies is to find statistical correlations between the energy consumption and the operational parameters of the BTS. Moreover we were interested to study both the energy consumption correlated to the transmission function of the apparatuses and the energy consumption related to the cooling of the equipments and infrastructures. To achieve this goal, we made extended statistical analysis (using the software "R" of the "R-Foundation for Statistical Computing" [14]). We considered separately the following characteristics of the systems: • Systems typologies (Shelter, Room, Outdoor); • Systems technologies (GSM, DCS, UMTS); • Localization (South, Centre and North Italy); and the following operating parameters: • Energy consumption (Wh); • Instant Power (W); • Internal temperature (°C); • External temperature (°C); • Phone traffic for cells (‘erlang’). A specific database has been built. The results are: • The average yearly consumption of a BTS is ca. 35500 kWh, considering that in Italy there are about 60.000 BTSs [15], the total average yearly consumption of the Italian BTS systems is ca. 2,1 TWh/year, which is the 0,6 % of the whole national electrical consumption (data source: TERNA 2007). In terms of economical and environmental impact, the data correspond to ca. 300M€ yearly energy costs and ca. 1,2 Mton of CO 2eq emitted in the atmosphere every year. • Carrying out analyses on the average energy consumptions associated to the different technologies, we note that the GSM energy consumptions are considerably higher than the UMTS technology as it is expected because of the different mode of operation of the two technologies (Table 1). Energy Consumption/technology Technology kWh/day kWh/year