ITU Journal on Future and Evolving Technologies, Volume 1 (2020), Issue 1, 21 December 2020 ON THE EVOLUTION OF INFRASTRUCTURE SHARING IN MOBILE NETWORKS: A SURVEY Lorela Cano1, Antonio Capone2, Brunilde Sansò3 1,2Politecnico di Milano, Milan, Piazza Leonardo da Vinci, 32, 20133, Milano MI, Italy, 3Polytechnique Montréal, 2500 Chemin de Polytechnique, Montréal, QC H3T 1J4, Canada, NOTE: Corresponding author: Lorela Cano ([email protected]) Abstract – Infrastructure sharing for mobile networks has been a prolific research topic for more than three decades now. The key driver for Mobile Network Operators to share their network infrastructure is cost reduction. Spectrum sharing is often studied alongside infrastructure sharing although on its own it is a vast research topic outside the scope of this survey. Instead, in this survey we aim to provide a complete picture of infrastructure sharing both over time and in terms of research branches that have stemmed from it such as performance evaluation, resource management etc. We also put an emphasis on the relation between infrastructure sharing and the decoupling of infrastructure from services, wireless network virtualization and multi-tenancy in 5G networks. Such a relation reflects the evolution of infrastructure sharing over time and how it has become a commercial reality in the context of 5G. Keywords – 5G, infrastructure sharing, mobile networks, multi-tenancy, spectrum sharing, wireless network virtu- alization 1. INTRODUCTION among them and hence make their business more prof- itable. In these lines, infrastructure sharing has accom- Infrastructure sharing in mobile networks is a multi- panied the technology migrations from 2G to 3G and faceted problem involving not only academic and indus- from 3G to 4G due to the high upfront cost met by trial research entities but also national and international MNOs during these migrations. In turn, in 5G networks, regulatory entities [51, 52, 65, 66], standardization bod- infrastructure sharing, besides from being a means for ies [1–4] and vendors [45,107]. In essence, infrastructure cost-reduction, it is also an important pillar of the 5G sharing in mobile networks is the shared use of existing architecture. Another paradigm strongly linked to in- or jointly deployed network infrastructure among mul- frastructure sharing is spectrum sharing. The need for tiple Mobile Network Operators (MNOs). spectrum sharing comes from spectrum being an intrin- Based on which network elements (nodes) MNOs agree sically scarce resource, even more so in the context of to/can share, there are two main types of sharing: pas- 5G, given its target throughputs. However, spectrum sive and active, the latter comprising the former. Pas- sharing alone is a really vast research topic and will be sive sharing (also referred to as site sharing or co- outside the scope of this survey unless combined with location [49]) implies the sharing of the site physical infrastructure sharing. space and of the non-active elements on the site (such as shelter, cabinet, mast, etc. [49, 104]). Instead, active What’s more, in this paper we will also address some sharing extends to active elements of the Radio Access literature on Wireless Network Virtualization (WNV) Network (RAN) (such as antennas, Base Transceiver [91] and network slicing (enabling multi-tenancy) in the Stations/Base Station Controller for 2G, Node B/Radio context of 5G [6], since both are based on infrastruc- Network Controller for 3G, eNode B for 4G, and gN- ture and spectrum sharing. Conversely, WNV and net- odeB for 5G) and part of the core nodes (in fact, core work slicing can be seen as enablers for infrastructure node elements related to user billing and accounting are and spectrum sharing. Besides, another concept closely not shared). related to infrastructure and spectrum sharing is that of the decoupling of infrastructure from services, which The phenomenon of infrastructure sharing has disrupted was envisioned by some of the early literature on in- the business model of a conventional MNO, that is, an frastructure sharing (see Section 2). The concept has MNO which is by itself responsible for (i) purchasing a been further carried out in the context of WNV and spectrum license, (ii) deploying and managing the net- then in the context of network slicing. In fact, the dif- work infrastructure, (iii) tailoring services for their sub- ferent research efforts on introducing Software-Defined scribers (e.g., voice, data, etc.) and (iv) handling their Networking (SDN), virtualization in general and Net- billing and accounting. The main reason for MNOs to work Functions Virtualization (NFV) in particular into share infrastructure is to divide the infrastructure cost mobile networks seem to have converged into the 5G ©International Telecommunication Union, 2020 Some rights reserved. This work is available under the CC BY-NC-ND 3.0 IGO license: https://creativecommons.org/licenses/by-nc-nd/3.0/igo/. More information regarding the license and suggested citation, additional permissions and disclaimers is available at https://www.itu.int/en/journal/j-fet/Pages/default.aspx. ITU Journal on Future and Evolving Technologies, Volume 1 (2020), Issue 1, 21 December 2020 architecture as enablers for network slicing. 3GPP 3rd Generation Partnership Project BS Base Station Infrastructure sharing in this broader sense has been a C-RAN Cloud Radio Access Network very prolific research topic over the last three decades. DCS Digital Cellular System Samdanis et al. in [125] provide a compelling analysis of EDGE Enhanced Data rates for GSM Evolution the path from infrastructure sharing to multi-tenancy. GERAN GSM EDGE RAN However, to the best of our knowledge, our survey is GSM Global System for Mobile communications the first1 comprehensive study on how the infrastructure IaaS Infrastructure as a Service sharing topic in mobile networks has evolved over time, InP Infrastructure Provider i.e., with the advent of the different mobile network gen- IoT Internet of Things IP Integer Programming erations, and which research branches have spurred from JV Joint Venture this topic. Reviewing this evolution is particularly im- MLFG Multi-Leader-Follower Game portant now that networking slicing is being introduced mmWave millimiter Wave in 5G (from release 16 onwards) and operators are look- MNO Mobile Network Operator ing for models for sharing infrastructure costs and to MVNO Mobile Virtual Network Operator invest more in new services and applications, collaborat- NaaS Network as a Service ing with different players of vertical industrial sectors. NFV Network Functions Virtualization Moreover, it is becoming clear to the telecommunica- NSP Network Service Provider tions industry sector that some form of infrastructure OTT Over The Top sharing will be the common basis on which networks PRB Physical Resource Block will be deployed in different countries and services will QoS Quality of Service RAN Radio Access Network evolve and diversify, going beyond 5G and preparing the RRH Remote Radio Head ground for the next generation. SaaS Software as a Service This survey is organized in the following fashion. Due SDN Software-Defined Networking SINR Signal-to-Interference-plus-Noise Ratio to the change in the nature of problems studied over SLA Service Level Agreement time, we first make a broad chronological classification SP Service Provider of the literature into early works and recent and up-to- UMTS Universal Mobile Telecommunications System date works. For the latter, we further identify several VNO Virtual Network Operator research branches/categories. The overall picture of our VO Virtual Operator classification is depicted in Fig. 1. An overview ofthe W-CDMA Wideband Code Division Multiple Access early works on the topic is provided in Section 2. Fur- WNV Wireless Network Virtualization ther, in Section 3, we focus on the more recent and up- xG xth mobile network Generation to-date works. Then in Section 4 we make a critical discussion of the research area related to infrastructure Table 1 – Definitions of acronyms and abbreviations sharing and provide an outlook of future research direc- • state regulatory standpoints and provide guidelines tions. Finally, conclusions are drawn in Section 5. for the latter and For readers’ ease, in Table 1 we provide the definitions of the acronyms and abbreviations used in the paper. • conceive new paradigms for the mobile market. 2. EARLY WORKS In [118], which dates back to 1994, Ramsdale states that national roaming3 is part of the specifications of the Dig- [16,49,55,70,115,118,142] are among the earliest articles ital Cellular System at 1800 MHz (DCS 1800), unlike on infrastructure sharing (combined at times also with the Global System for Mobile Communications at 900 2 spectrum sharing). With the exception of [70] , these MHz (GSM 900), which supported international roam- articles have tended to: ing only. National roaming was introduced in the DCS 1800 to improve coverage due to smaller cell sizes at • address technical issues of different sharing alterna- 1800 MHz (as opposed to 900 MHz). tives, Instead, the work in [55] shows the positive impact of • assess the financial profitability through techno- infrastructure sharing in financial terms for the Univer- economic approaches, sal Mobile Telecommunications Systems (UMTS), espe- 1This survey is based on the PhD thesis of Lorela Cano [24]. cially for lowly populated areas in which network de- 2The study in [70] is an early work on the problem of schedul- ployment is dictated by coverage instead of capacity. ing users of multiple operators arising from the case when a 3G, facility-based MNO hosts several Mobile Virtual Network Oper- 3National roaming is an infrastructure sharing alternative that ators (MVNOs): the authors propose a non-pre-emptive priority allows users of an operator which does not provide coverage in queuing model for circuit-switched traffic applied through an ad- certain areas of a country to be served by the network of another mission control scheme.