Abstract
Long-range, low-power wireless technologies such as LoRa have been shown to exhibit excellent performance when applied in body-centric wireless applications. However, the robustness of LoRa technology to Doppler spread has recently been called into question by a number of researchers. This paper evaluates the impact of static and dynamic Doppler shifts on a simulated LoRa symbol detector and two types of simulated LoRa receivers. The results are interpreted specifically for body-centric applications and confirm that, in most application environments, pure Doppler effects are unlikely to severely disrupt wireless communication, confirming previous research, which stated that the link deteriorations observed in a number of practical LoRa measurement campaigns would mainly be caused by multipath fading effects. Yet, dynamic Doppler shifts, which occur as a result of the relative acceleration between communicating nodes, are also shown to contribute to link degradation. This is especially so for higher LoRa spreading factors and larger packet sizes.
Highlights
Introduction with software-defined radio (SDR)Sensors 2021, 21, 4049.In recent years, sub-GHz low-power wide-area network (LPWAN) technologies such as SigFox [1], NB-IoT [2], and LoRa [3] have played key roles in the development of the rapidly evolving Internet of Things (IoT)
A default preamble length of 12.25 symbols is considered. This preamble consists of eight up-chirps that constitute the pilot sequence, two so-called sync word symbols, which enable the user to distinguish packets from different LoRa networks, and 2.25 down-chirps, which make up the start-of-frame delimiter (SFD), used for packet synchronization
Whereas the authors of both papers consider the impact of Doppler effects on the full LoRa modulation protocol based on experimental data gathered with commercial transceiver modules, the present paper focuses on the fundamental performance impact of the Doppler effects on the symbol detector itself
Summary
LoRa is based on chirp spread spectrum (CSS) modulation [26], which uses wide-band frequency-modulated pulses to encode information. Increasing the spreading factor or the frequency swing impacts the sensitivity of LoRa receivers, as presented in [27]. A default preamble length of 12.25 symbols is considered This preamble consists of eight up-chirps that constitute the pilot sequence, two so-called sync word symbols, which enable the user to distinguish packets from different LoRa networks, and 2.25 down-chirps, which make up the start-of-frame delimiter (SFD), used for packet synchronization. The interleaving and decoding steps that are applied to data when encapsulated in a LoRa packet have been documented in [13] and are considered to be part of the data link layer. These are not discussed in this paper. LoRa and LoRaWAN can be found in [27,28]
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