# Bluetooth Low Energy Mesh Networks: A Survey
## 1 Introduction  
Bluetooth Low Energy (BLE), also marketed as Bluetooth Smart, has emerged as a major low-power wireless technology [1]. Leveraging a design that can reuse classic Bluetooth circuitry to a large extent, BLE has gained a dominant position in smartphones. This allows low-energy communication between the latter and other devices such as sensors, actuators, wearables, etc. [2]. Furthermore, the Internet Engineering Task Force (IETF) recently developed the adaptation layer to support Internet Protocol version 6 (IPv6) over BLE, thus facilitating the connectivity of BLE devices with the Internet of Things (IoT) [3]. While BLE is currently exhibiting high momentum, it is also facing significant challenges. A major drawback of a BLE network is limited coverage range since BLE was designed to follow the star network topology. For example, Wireless Home Automation Networks (WHANs) often require mesh topologies to enable communication between two end devices in a home. For this reason, technologies such as IEEE 802.15.4 (and thus Zigbee or Thread), Z-Wave, or Insteon, all of which support mesh networks, are being used in WHANs [4; 5]. However, in such a relevant domain, BLE can only be used for point-to-point, single-link applications. A similar problem can be found in any scenario (e.g., industrial, urban, agricultural, etc.) where direct connectivity between any two endpoints may not be possible [6; 7]. In order to cope with BLE network coverage limitations, two main approaches have been proposed by the community. The first one is based on reducing BLE Physical Layer signal bandwidth in order to increase link range while keeping the star topology network model, as in the recently published Bluetooth 5.0 specification [8]. However, this scheme still suffers from the hard coverage limitation of a star topology, i.e., extending the network coverage beyond one hop is not possible in such topology. Furthermore, a star topology network does not offer path diversity, which is a crucial property in wireless systems in order to cope with radio propagation impairments and node failures. The secondapproach relies on enabling a BLE mesh network. While this model involves the complexity of requiring mesh network mechanisms for end-to-end communication, it allows the coverage and path diversity limitations of a star topology to be overcome. These features have attracted the interest of academia, industry, and standards development organizations, which have developed or are developing numerous BLE mesh network solutions by following different techniques [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. However, to the best of our knowledge, the literature lacks a consolidated view on this emerging area. This paper comprehensively surveys BLE mesh network proposals. We provide a taxonomy for BLE mesh network solutions, whereby we have divided the identified solutions into three main categories, namely: standardized solutions, academic solutions, and proprietary solutions. The taxonomy, which is depicted in Figure 1, serves as the main structure for describing BLE mesh solutions in this paper. This structure provides the reader with homogeneous content in each main category, given the nature and the availability of information in each case. After reviewing the BLE mesh techniques considered, we critically discuss their main advantages and drawbacks and we present currently open issues. In order to carry out the survey, we have considered and analyzed all existing BLE mesh network proposals and products to the best of our knowledge, as of the date of manuscript submission. In order to explore and find BLE mesh network solutions, we used web search tools (e.g., Google) as well as academic archives and digital libraries such as Institute of Electrical and Electronics Engineers (IEEE), Association for Computing Machinery (ACM), Multidisciplinary Digital Publishing Institute (MDPI), and Elsevier. We believe that, as a result, this survey is exhaustive.