Modulation scheme is the mechanism by which waveforms are translated into 1s and 0s. Some common modulation schemes are phase shift (used by WiFi / Bluetooth), frequency shift (used in garage door openers), on-off keying (Morse Code). LoRa uses the chirp modulation scheme which uses the change in frequency over time to dictate a 1 or a 0. In the picture, there is an example of an upchirp in which the frequency increases linearly over time. Sometimes the frequency of upchirps increase exponentially over time. As with other spread spectrum methods, chirp spread spectrum uses its entire allocated bandwidth to broadcast a signal, making it robust to channel noise. Further, because the chirps utilize a broad band of the spectrum, chirp spread spectrum is also resistant to multi-path fading even when operating at very low power. However, it is unlike direct-sequence spread spectrum (DSSS) or frequency-hopping spread spectrum (FHSS) in that it does not add any pseudo-random elements to the signal to help distinguish it from noise on the channel, instead relying on the linear nature of the chirp pulse. Additionally, chirp spread spectrum is resistant to the Doppler effect, which is typical in mobile radio applications.
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