Long Range Wide Area Networks (LoRaWANs) have recently emerged as a hot research topic for their capability to collect sporadic data from a great number of widely spread low power devices. By enabling low-cost low-traffic wireless communications at large scale, such networks can be adopted in many application domains, including smart agriculture, logistics, and emergency detection among others. LoRaWAN employs a pure ALOHA default medium access scheme, that limits the maximum achievable throughput to 18%. Increasing the number of terminals would lead to more frame collisions, and eventually to network collapse. The easiest way to allow bigger network sizes is to synchronize devices, so that they can discretize time into slots, and use them to schedule frame transmissions. Herein, a Time Division Multiple Access (TDMA) scheme on the top of LoRaWANs is highly desired. However, since keeping synchronization can lead to heavy energy consumption, the design of such scheme should enforce energy-efficiency. This contribution describes the technical issues related to the implementation of such mechanism, that as a matter of fact represents the cornerstone on which more sophisticated random access protocols or even scheduling techniques can be designed. In dense networks, the proposed scheme combined with a very simple slotted ALOHA mechanism has also been shown to outperform the default LoRaWAN access protocol in terms of expected energy consumption.

The Long Range (LoRa) modulation keeps gaining relevance in the landscape of low-power sensor networks. Most models used to evaluate the performances of LoRa deployments are based on the assumption that two colliding frames are necessarily lost. Recent findings have shown that the capture effect occurs in these networks, allowing the receiver to sometimes demodulate the frame featured with the highest signal power. This finding notably improves the overall throughput compared to expectations, but in turn decreases the network fairness. In this paper, we analyze the benefits and drawbacks of such an effect. We therefore provide new throughput models for LoRa networks operating Pure and Slotted ALOHA access schemes. For this purpose, an experimental testbed has been setup and used to measure the occurrence probabilities of capture events in several transmission scenarios. The resulting models are validated with real-life data gathered on the same setup. We additionally analyze the fairness in our deployment, showing that the devices featured with the highest average power at the receiver benefit from a higher success rate than others. By computing Jain's index, we show that this unfairness gets more pronounced as the traffic load increases.

The emerging LoRa technology is quickly becoming the de facto standard for Low Power Wide Area Networks upon unlicensed frequencies. Herein, the LoRaWAN medium access sets up a lightweight network architecture able to connect very low power devices to the Internet. Traffic flows in such deployments can be variable, or even unpredictable, depending on the needs of the monitoring applications using the network. As an example, to track air quality in cities, some applications can trigger an increased need of fine grained pollution data during the daytime. However, the network capacity is currently limited by the default LoRaWAN pure ALOHA access scheme. A time synchronized scheduled access would considerably improve the achievable throughput, at the cost of an increased power consumption for synchronization duties. In such a context, this contribution introduces the traffic-aware energy efficient Medium Access Control (TREMA) protocol for LoRa networks, capable of seamlessly switching between asynchronous and synchronous schemes according to the probed traffic variations. TREMA ultimately increases the maximum capacity of LoRa deployments while always selecting the most energy efficient access scheme.