# RESEMBLE: A Real-Time Stack for Synchronized Mesh Mobile Bluetooth Low Energy Networks
## 2 Related Work  
Many works in the literature investigated the adoption of BLE for a wide range of applications, such as home automation [6; 7], Internet of Things (IoT) [8; 9; 10], passive human sensing [11], healthcare and wearable devices [12; 13]. Several works also investigated the case for BLE in industrial communications [14; 15; 16; 17; 18]. For instance, the work in [16] addressed BLE's suitability for industrial automation communications, while the work in [18] focused on Industrial IoT applications. However, the limited transmission range represents an issue for BLE's use in large industrial networks. To solve this issue, the work in [19] proposed extending the coverage of BLE clusters by leveraging LoRa technology. Another approach to solve this issue is to enable multihop transmissions over BLE. To this aim, the work in [20] proposed a connection-oriented approach for BLE networks that supports tree topologies. However, the approach does not support mobility and real-time communications, which are typical requirements for industrial applications. A solution, aiming to achieve real-time communications over BLE networks with mesh topologies, is proposed in [21]. The proposed protocol, called MRT-BLE, allows for real-time transmissions and obtains maximum delays from hundreds to thousands of milliseconds. However, MRT-BLE does not provide mobility support, as it works in a connection-oriented way.  
To increase the BLE coverage, the Bluetooth SIG defined the Bluetooth Mesh Profile [22], a stack operating on top of BLE that supports node mobility and mesh topologies. The work in [23] evaluated the end-to-end delays and the network-joining phase duration of Bluetooth Mesh (called provisioning) and proposed a lightweight provisioning protocol to speed up the network-joining time of the nodes. The work in [24] evaluated Bluetooth Mesh in a mobile robotic system for logistics and showed its suitability for this kind of applications. However, Bluetooth Mesh lacks the support for real-time communications, as it cannot guarantee bounded delays. An way to deal with Quality-of-Service (QoS) in Bluetooth Mesh is presented in [25], where a transmission backoff algorithm allows for the message priorities to be differentiated. However, this approach is not able to provide bounded delays.  
In industrial environments, both the timeliness of the communication and their robustness to noise and interference [26] are important to avoid retransmissions that may affect the end-to-end delays. For this reason, Bluetooth Mesh was assessed in [27] and the results demonstrated that, in dense networks, Bluetooth Mesh obtains a high packet error rate, mainly due to collisions. The adoption of dynamic traffic smoothing techniques inspired by those proposed in [28; 29] can help reduce the collision probability.  
As will be discussed in the following sections, the RESEMBLE stack that is proposed here fills a gap, as it introduces both real-time communications and node mobility over mesh topologies to BLE. Moreover, RESEMBLE can be implemented on COTS devices with no hardware modifications.