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The LoRa Modulation Over Rapidly-Varying Channels Are the Higher Spreading Factors Necessarily More Robust?

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Document pages: 4 pages

Abstract: The chirp spread spectrum (CSS) modulation scheme is employed by the physicallayer of the Long Range (LoRa) communication technology. In this paper, weexamine the performance of CSS over time-varying channels whose gain may changeduring the reception of a LoRa frame. This is in contrast to the usuallyemployed model in the literature, which assumes the channel gain to be constantthroughout a frame. Specifically, we investigate the effects of exponentiallycorrelated Rayleigh fading on the frame-error rate of a CSS receiver in whichthe channel gain is estimated at the beginning of each frame. Our primaryobservation is that over rapidly-varying channels, the robustness benefits ofthe larger spreading factors tend to disappear as the payload size grows. Thisobservation, which is contrary to the common perception that higher spreadingfactors necessarily provide greater immunity against noise, highlights the needto consider channel characteristics and payload sizes in allocating thespreading factor for reliable and energy-efficient LoRa communications.

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