# Energy Performance of LR-FHSS: Analysis and Evaluation
## 1 Introduction  
In the last decade, low-power wide Area networks (LPWANs) have emerged as a family of long-range, low-power communication technologies suitable for many Internet of Things (IoT) applications [1; 2; 3].  
LoRaWAN has arisen as one of the most popular LPWAN technologies, with around one billion devices predicted to use this technology in the near future [4; 5; 6]. In order to expand the capabilities of LoRaWAN, the LoRa Alliance recently introduced a new physical layer called long-range frequency hopping spread spectrum (LR-FHSS) [7]. Exploiting techniques such as intrapacket fragmentation, frequency diversity, and increased transmission redundancy, LR-FHSS is expected to enable network deployments with greater node density, robustness, and coverage [7; 8; 9; 10; 11; 12; 13; 14].  
One particularly promising use case for LR-FHSS is direct-to-satellite IoT (DtS-IoT), which is a field with significant momentum [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. DS-IoT provides communication means for IoT devices in remote areas, where terrestrial network infrastructure may not be feasible or practical to deploy. In DtS-IoT, when an IoT device is visited by a satellite, the former can transmit frames (e.g., carrying sensed data) to the latter, which can act as a gateway.  
Since the power grid is not available for many IoT devices in general, and especially for those using LR-FHSS, such devices often need to rely on a limited energy source, such as a simple battery or energy harvesting (based on either natural sources or wireless transmission systems [18]). Therefore, although energy efficiency is not the main objective of the mechanism, determining the energy performance of LR-FHSS is crucial. However, to our knowledge, the present paper is the first study that has specifically addressed this topic.  
In this paper, we provide a detailed analytical model of the current consumption of an LR-FHSS IoT device (end device, in LoRaWAN terminology), derived from measurements on real hardware within a complete LoRaWAN network (i.e., including an end device, gateway, and network server) supporting LR-FHSS. This represents the first such model in the academic literature. We also use the model to determine the lifetime of a battery-operated device and the energy efficiency of data transmission with LR-FHSS. In addition, we compare the energy performance of LR-FHSS with that of classic LoRaWAN physical layer alternatives. The evaluation results present trade-offs that depend on the data rate (DR), operational mode, and payload sizes for every performance metric. Among other findings, the results show that battery lifetime can approach 2.5 years with a 50 min notification interval when utilizing the proper configuration and can even reach a theoretical maximum of up to 16 years with a more infrequent interval of 1 day between messages for the most energy-efficient packet size.  
The remainder of the paper is organized as follows. Section 2 reviews related work. Section 3 provides background concepts on LoRaWAN and LR-FHSS. Section 4 presents our model, which is used in Section 5 to evaluate and discuss the current consumption, battery lifetime, and energy efficiency of an LR-FHSS end device. Finally, Section 6 concludes the paper.