Network densification by adding either more sectors per site or by deploying an overlay of small cells is always considered to be a key method for enhancing the RAN coverage and capacity. The impact of these two techniques on cellular network energy consumption is investigated in this chapter. The aim is to find an energy efficient deployment strategy when trading-off the order of sectorisation with the intensity of small cell densification. A new enhanced base station power consumption model is presented, followed by a novel metric framework for the evaluation of the RAN energy efficiency. The use of the power model and the proposed metrics is demonstrated by applying them to a RAN case study when the two techniques are used to improve the network capacity. In addition, the chapter evaluates the amount of network energy efficiency improvement when various adaptive sectorisation schemes are implemented. The results show that the strategy of adding more sectors is less energy efficient than directly deploying an overlay of small cells, even when adaptive sectorisation is implemented.
TopIntroduction
The unprecedented exponential growth in the demand for mobile data services is expected to continue over the next five years. The latest Cisco visual networking index reported that global mobile data traffic grew by 69 percent in 2014, and predicted about a tenfold increase in traffic from 2.5 EB to 24.3 EB between 2014 and 2019 (Cisco Visual Networking, 2015). This trend is mainly due to the proliferation of the use of smart devices and the expansion of mobile services in developing societies. In response, operators need to enhance the capacity of their networks to meet this growth in demand. There exist three main technical approaches for the operators to improve their radio access network (RAN) capacity. The first is to improve the link spectral efficiency by implementing more advanced signal processing techniques such as multiple-input multiple-output (MIMO) antennas and efficient modulation and coding schemes. The second approach is the addition of new radio spectrum while the third is to increase the geographic spectrum reuse by cell densification of the RAN.
Historically, the reuse of radio spectrum was the main reason for the growth of capacity in cellular mobile radio systems. According to Martin Cooper (Arraycomm.com, 2015), a 1600 fold increase during the last 45 years has come from the reuse of spectrum, whereas a 625 fold increase has resulted from adding more spectrum and implementing more advanced signal processing techniques to enhance the link capacity. The wireless channel link capacity is very close to its maximum Shannon limit. To significantly increase capacity by adding more spectrum, technologies such as millimetre wave radio (Rappaport et al., 2013) and visible light communications are being considered (Singh, O’Farrell, & David, 2014) and (Singh, O’Farrell, & David, 2013). Also, technology based on large antenna arrays such as Massive MIMO has the potential to significantly increase spectral efficiency without using more spectrum (Larsson, Edfors, Tufvesson, & Marzetta, 2014). However, the deployment of more cells (i.e. base station densification) will continue to be the key enabling technique to achieve high target capacities in the next generation of cellular mobile networks. Therefore, the trend of base station densification in cellular networks is expected to intensify in 5G RANs. However, such growth in the number of deployed base stations will be accompanied by an increase in the total network energy consumption, which represents a major concern for network operators for the following reasons:
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Economic Reasons: Reducing the network operating cost is an important target for the operators as the network energy consumption bill represents a significant cost for the operators (Li, Sheng, Yang, & Wang, 2013). For example, the cost of energy represents 30% of the total operating expenses of the networks in the urban areas of India, and around 50% in rural areas (Rakesh Kumar Bhatnagar & Panigrahi, 2013).
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Environmental Reasons: The information and communication technology (ICT) sector was responsible for about 2% of the global CO2 emissions in 2009 and such emissions are predicted to grow to about 2.8% in 2020 (Forster, Dickie, Maile, Smith, & Crisp, 2009).
Network densification can be accomplished by adding either more macro base station sites, more sectors per site or by deploying an overlay of small cells to form what is known as a heterogeneous network. Deploying more macro sites and adding more sectors have been the traditional methods that operators have used to expand the coverage and capacity of their existing networks. However, reducing the site to site distance between the macro sites cannot continue without limit due to the technical issues of inter cell interference and, more importantly, due to the cost of macro base stations coupled with the difficulty of finding new sites, especially in urban and dense urban areas.