electronics Communication Stochastic Geometry-Based Analysis of Heterogeneous Wireless Network Spectral, Energy and Deployment Efficiency Jasmin Musovic 1 , Vlatko Lipovac 2 and Adriana Lipovac 2,* 1 Communications Regulatory Agency, 71000 Sarajevo, Bosnia and Herzegovina; [email protected] 2 Deparetmet of Electrical Engineering and Computing, University of Dubrovnik, 20000 Dubrovnik, Croatia; [email protected] * Correspondence: [email protected]; Tel.: +385-20445734 Abstract: For quite a while, it has been evident that homogeneous network architectures, based on cells with a uniform radiation pattern, cannot fulfill the ever increasing demand of mobile users for capacity and service quality while still preserving spectrum and energy. However, only with the introduction of the Fourth Generation mobile communication networks to deal with the surging data traffic of multimedia applications, have smaller cells been widely used to break down service zone areas of macro base stations into multiple tiers, thus improving network performance, reducing traffic congestion, and enabling better management of spectrum and energy consumption in a macro network. In this paper, we present an analytical model for assessing the efficiency of bandwidth and energy usage, as well as of network deployment, taking into account overall network investment and maintenance costs. This paves the way to the improved planning of network coverage, and its capacity and reliability, thus preserving its spectrum and energy, as well as the environment. The analysis considers the downlink of an arbitrary heterogeneous cellular network by using tools Citation: Musovic, J.; Lipovac, V.; of stochastic geometry that adopt the distribution of base stations in the form of a Poisson Point Lipovac, A. Stochastic Process. The proposed analytical model is verified by the according software simulations using the Geometry-Based Analysis of ns-3 network simulator. The obtained results closely match the theoretically predicted values and Heterogeneous Wireless Network boundaries, clearly indicating that, in all three analyzed aspects: spectral, energy, and deploymental, Spectral, Energy and Deployment the efficiency of small-cell networks was higher with respect to traditional large-cell networks and Efficiency. Electronics 2021, 10, 786. increased even further for heterogeneous (two-tier in our tests) networks. https://doi.org/10.3390/electronics 10070786 Keywords: heterogeneous cellular network; spectral efficiency; deployment efficiency; Poisson Point Academic Editor: Francisco Falcone Process; ns-3 Received: 9 February 2021 Accepted: 22 March 2021 Published: 26 March 2021 1. Introduction With the deployment of the Third Generation (3G) wireless systems, it became evident Publisher’s Note: MDPI stays neutral that homogeneous network architectures, based on cells with uniform radiation pattern with regard to jurisdictional claims in in service zones, could not fulfill the ever increasing demand of mobile users for capacity, published maps and institutional affil- service quality, and economically efficient energy consumption. This evidence has become iations. even more pronounced with the introduction of the Fourth Generation (4G) networks, when implementing smaller cells has become a way to improve network performance, reducing traffic congestion in macro networks. Hence, for example, plug-and-play installation of femto-cells is much simpler and more profitable than whatever can be achieved with macro Copyright: © 2021 by the authors. cells, due to reduced backhaul costs. The smaller the cell, the less power needs to be Licensee MDPI, Basel, Switzerland. transmitted, so small cells can serve small groups of users (UE), and thus can improve the This article is an open access article quality of service (QoS) [1]. distributed under the terms and Furthermore, complex Radio Access Systems (RAS) can comprise cells of various conditions of the Creative Commons classes, thus forming the so-called Heterogeneous Cellular Network (HCN), also commonly Attribution (CC BY) license (https:// referred to as HetNet, where the basic topology concept presumes layering two or more creativecommons.org/licenses/by/ groups of the same-class cells, called tiers [2,3]. 4.0/). Electronics 2021, 10, 786. https://doi.org/10.3390/electronics10070786 https://www.mdpi.com/journal/electronics Electronics 2021, 10, x FOR PEER REVIEW 2 of 13 Electronics 2021, 10, 786 2 of 13 Over the last decade, the rapid evolution of mobile telecommunication services fueledOver by growing the last decade,demand the for rapid multimedia evolution applications of mobile telecommunication has led to surging data services transmis- fueled sionby growing traffic, which demand (according for multimedia to annual applications reports of has major led to network surging dataoperators transmission and network traffic, equipmentwhich (according manufacturers) to annual increases reports of approximately major network ten operators times every and networkfive years, equipment to reach aboumanufacturers)t 12 Exabytes increases per month approximately—13 times more ten than times in every2017. five years, to reach about 12 ExabytesThe very per month—13direct consequence times more of such than data in 2017. traffic growth is the increasing demand for RAS nodes,The very which direct severely consequence affects energy of such efficiency. data traffic Hence growth, for isexample, the increasing the average demand an- nualfor RAS energy nodes, consumption which severely of a sin affectsgle base energy station efficiency. (BS) is about Hence, 25 forMWh example, [4], which the averagesignifi- cantlyannual contributes energy consumption to almost 2% of of a singlethe global base carbon station emission (BS) is about currently 25 MWh estimated [4], which to be generatedsignificantly by contributesICT industry. to almost 2% of the global carbon emission currently estimated to be generatedAlthough by HetNet ICT industry. cell planning has been in use even in the Second Generation (2G) Although HetNet cell planning has been in use even in the Second Generation (2G) mobile networks, it is only the Long Term Evolution (LTE) and its extension LTE Ad- mobile networks, it is only the Long Term Evolution (LTE) and its extension LTE Advanced vanced (LTE-A), commonly referred to as 4G standard, which fully define the concept for (LTE-A), commonly referred to as 4G standard, which fully define the concept for enhancing enhancing the HetNet performance. the HetNet performance. Such strategies for LTE/LTE-A radio performance planning are centered around re- Such strategies for LTE/LTE-A radio performance planning are centered around ducing the distance between the transmitter and receiver, i.e., improving the service signal reducing the distance between the transmitter and receiver, i.e., improving the service coverage of regions demanding more intensive traffic. Accordingly, in Figure 1, examples signal coverage of regions demanding more intensive traffic. Accordingly, in Figure1 , of a simplified HetNet with a multistandard 4-tier scenario (a), and 3-tier topology (b), are examples of a simplified HetNet with a multistandard 4-tier scenario (a), and 3-tier topology presented. (b), are presented. (a) (b) FigureFigure 1. 1. SimplifiedSimplified presentations presentations of of an an exemplar exemplar Heterogeneous Heterogeneous Cellular Cellular Network Network (HCN) (HCN),, also also commonlycommonly referred referred to to as as HetNet HetNet,, (HetNet (HetNet);: (a);) (ma)ultiple multiple-standard-standard radio radio access access HetNet; HetNet; (b) 3 (b-tier) 3-tier HetNetHetNet coverage coverage with with blue blue lines lines as as service service zones’ zones’ boundaries, boundaries, and and red red spots, spots, green green triangles triangles,, and and black squares as macro-cells, pico-cells, and femto-cells, respectively. black squares as macro-cells, pico-cells, and femto-cells, respectively. WeWe investigate networknetwork energy energy efficiency efficiency according accordin tog theto the following following metrics metrics proposed pro- posedfor wireless for wireless access access networks networks [5,6]: [5,6]: • energyenergy per per information information bit bit [J/b]; [J/b]; • averageaverage power power used used by by a device a device that that provides provides connectivity connectivity to a tocertain a certain covered covered area 2 [Wmarea [Wm]. 2]. BesideBeside service service provisioning, provisioning, the the European European Telecommunication Telecommunicationss Standards Institute (ETSI)(ETSI) proposes proposes the the following following QoS QoS-related-related metrics [7]: [7]: • ratioratio between thethe throughputthroughput ofof users users obtaining obtaining the the minimal minimal specified specified (service (service depen- de- pendent)dent) QoS QoS within within the served the served area, andarea, the and total the power total consumedpower consumed by the BSs by providing the BSs providingservice in thatservice area in [b/J]; that area [b/J]; • ratioratio between between the the number number of of UEs UEs obtaining obtaining the the minimal minimal specified specified QoS QoS within within the the servedserved area, area, and and the the total total energy energy consumed consumed by by the the BSs BSs providing providing service service in in that that area area duringduring the the observation observation time. time. TheThe essence ofof thethe proposedproposed metrics metrics is is improving improving HetNet