# HARE: Supporting Efficient Uplink Multi-Hop Communications in Self-Organizing LPWANs
## 1 Introduction  
In the coming years, electronic equipment will be interconnected and consequently almost every person and industry will become simultaneously data generators and consumers. The Internet of Things (IoT) paradigm is a key enabler of this vision by delivering machine-to-machine (M2M) and machine-to-person communications on a massive scale. As more and more things are connected to the Internet, low-cost and low-traffic devices are starting to be demanded. However, traditional cellular networks do not deliver a good combination of technical features and operational cost for those IoT applications that need wide-area coverage combined with relatively low bandwidth, long battery lifetime, low hardware and operating cost, and high connection density [1]. Low-power wide area networks (LPWANs) are intended to become the engine of long-range, low-bandwidth IoT applications (see Figure 1), which until now have been constrained by deployment costs and power issues. The goal of these networks is to deliver small amounts of data over long ranges, at rates of up to tens of kilobits per second (kbps), with a battery lifetime of up to several years, supporting thousands of devices connected to a base station, and facilitating online integration. LPWAN architecture is characteristically single-hop, where end devices are connected directly to the base station by following a star topology, greatly simplifying the network and endowing it with robustness and centralized control. However, this single-hop massive channel access sets out some inherent challenges: reliability, scalability, flexibility, and quality of service (QoS). In fact, the channel access mechanism of some LPWAN technologies resorts to the use of ALOHA [2; 3], a random medium access control (MAC) protocol in which end devices transmit without doing any carrier sensing to checkthe channel state in advance. Although simple, this uncontrolled medium access leads to interference or packet collisions among uncoordinated devices, acutely affecting reliability and scalability in dense networks [4; 5; 6; 7; 8; 9]. In addition, LPWAN devices located far away from the base station must make use of high transmission power levels, resulting in severe energy consumption and reduced battery lifetime [10]. In this article, the HARE protocol stack is proposed as a new LPWAN technology flexible enough to adopt uplink multi-hop communications when proving energetically more efficient than single-hop. A full set of advanced techniques belonging to different communication layers has been designed for this purpose, while ensuring data transmission reliability:  
* Network synchronization, with nodes being periodically set in time by means of beacons emitted from the base station. * Time division multiple access (TDMA)-like channel access for groups of contenders with multiple transmission opportunities. * Adaptive transmission power level. * Flexible and scalable network association process. * Energy-aware, adaptive and resilient routing protocol. * Regular use of deep-sleep states. A HARE protocol stack has been implemented and tested in real hardware platforms. Results evaluation from different network configurations (single-hop vs. multi-hop, use of different radio duty cycling protocols, channel error injection) shows very high reliability while maintaining low energy consumption (particularly in multi-hop topologies). Furthermore, a better overall system's behavior when using multi-hop topologies in non error-prone scenarios has been observed. The remainder of this paper is organized as follows: Section 2 presents the current state-of-the-art in LPWANs and their multi-hop approaches. Section 3 introduces the main requirements of feasible use cases for HARE. Next, Section 4 describes the general operation of the protocol stack and Section 5 provides detailed information of the developed mechanisms. Section 6 describes the system's implementation in three testbeds and Section 7 compiles the obtained results from different experiments.