Downlink Power Consumption of HetNets Based on the Probabilistic Traffic Model of Mobile Users Ali Rıza Ekti * , Muhammad Zeeshan Shakir † , Erchin Serpedin * and Khalid A. Qaraqe † * Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX, 77843–3128, USA † Department of Electrical and Computer Engineering, Texas A&M University at Qatar, Education City, 23874, Doha, Qatar E–mails: arekti@tamu.edu, muhammad.shakir@qatar.tamu.edu, serpedin@ece.tamu.edu, khalid.qaraqe@qatar.tamu.edu Abstract—Heterogeneous networks (HetNets) are considered as a standard part of the future generation of wireless networks where masses of low power, low cost smallcells (e.g., femtocells) are anticipated to support the existing macrocell networks. While HetNets are increasing the spectral efficiency and decreasing the over–the–air signaling and uplink power consumption compared to macro networks, the large scale deployment of many lightly loaded smallcells is expected to increase the downlink power consumption of the HetNets. This paper studies the impact of smallcell population on the downlink power consumption of the HetNets. In this context, we propose that the population of smallcells is strictly depending on the traffic load due to active mobile users, which is a random variable and time–varying. We derive the mathematical framework to calculate the required population of smallcells based on the probabilistic traffic models where the number of total mobile users and number of active mobile users have different probabilistic distributions such as different combinations of Binomial and Poisson distributions. The proposed method guarantees the reduction in downlink power consumption of HetNets by forcing the smallcells to turn on the sleeping mode under low and medium traffic load conditions. Several simulation results are included to illustrate the impact of traffic load dependent population of smallcells on the downlink power consumption of HetNets. Moreover, it is shown that the mathematical and simulation results are in perfect agreement. Index Terms—Heterogeneous networks; smallcells; downlink power consumption; traffic load; probability distributions I. I NTRODUCTION Information and communication technology (ICT) industry has grown rapidly and massively over the last decade and there are more than 4 millions base stations (BSs) [1]. In order to maintain the Quality of Service (QoS), i.e., expand coverage and increase data rate, the operators have deployed a large number of BSs. This has highly escalated the number of cell phone subscribers thus increasing the over–the–air signaling and power consumption for the whole network. Currently, ICT industry exhibits 0.5% of the total global energy consumption [2]. In order to overcome the power consumption problem while maintaining the desired QoS, telecommunication operators have been trying to find new cellular technologies to support the future generations of wireless networks, e.g., 4G and 5G networks. Heterogeneous networks (HetNets) are considered as an integral part of future generations of wireless networks where multiple low power, low cost smallcell (e.g., femtocell) BSs (SBSs) are deployed to complement the macrocell net- This publication was made possible by NPRP grant 09–341–2–128 and NPRP grant 4–1293–2–513 from the Qatar National Research Fund. works 1 . HetNets are proposed to increase the data rates and capacity to the residential areas by reducing traffic from the macrocell network [3]. Power consumption of a network can be divided into two subcategories: (i) Uplink power consumption, and (ii) down- link power consumption. The fundamental factors contributing to the increase in the uplink power consumption of mobile communication networks include an increased number of mo- bile subscribers which was 4.5 billions in 2012 and expected to reach 7.6 billions till 2020 and explosive mobile data traffic volume which attained 45 million TB/year in 2012 and expected to reach 623 million TB/year till 2020 [4]. In [5], it has been shown that the uplink power consumption can be reduced significantly by using link adaptation where mobile users are transmitting with adaptive transmit power and thereby establishing the green networks. The cellular BSs are the most intensive components in terms of downlink energy consumption in 4G/5G mobile communi- cation networks. At present, there are more than 4 million BSs serving mobile users, each consuming an average of 25 mega–Watt Hour (MWH) per year [1]. The cellular operators are facing a challenge to meet the escalated demands of mobile users by introducing smallcells into the network and thereby minimizing the capital and operational expenditures (CAPEX and OPEX) of the network. The population of smallcells is expected to be around 100 millions with 500 million mobile users in 2020 [4]. The power consumption of a smallcell today is around 6–10 W, and it can be assumed that a smallcell in 2020 will still consume approximately 5 W. Therefore, the 100 million smallcells in 2020 will consume approximately 4.4 TWH, i.e., an extra 5% on top of the energy consumption of existing BS infrastructure. To cope up with this issue, numerous remedies are currently under consideration [1], [6], [7] such as (i) employing power saving protocols such as BS sleeping which enables an inactive operation mode for BSs under low load conditions; (ii) improved power amplifier technology which makes the hardware design of a typical BS more energy efficient; (iii) cell size adjustment schemes such as cell–breathing and cell–zooming, where different cells adapt their size depending on the received interference or traffic load conditions; (iv) use of renewable energy sources such as solar and wind energy in place of diesel generators may also be useful in reducing the power consumption of BSs, in particular, those at the off–grid sites; and (v) deployment 1 In this study, Heterogeneous networks refers to cellular systems with different cell sizes. 978-1-4577-1348-4/13/$26.00 ©2013 IEEE 978-1-4577-1348-4/13/$31.00 ©2013 IEEE 2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC) 2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC) 2812