content: entry on lora

zk/LoRa_radio.md

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+---
+tags:
+  - radio
+  - LoRa
+---
+
+# LoRa radio
+
+LoRa (which stands for "long range") is a proprietary method of radio
+communication.
+
+It operates within the unlicenced ISM (Industrial, Scientific and Medical)
+bands. In Europe, the center frequency within the ISM for LoRa is 868MHz.
+
+It uses a specialised and characteristic technique to avoid interference in
+communication: **chirp-spread-spectrum modulation**. With this technique, radios
+"chirp", meaning they sweep over a range of frequencies over time. This is
+similar to a voice getting higher in pitch.
+
+This technique gives LoRa several beneficial properties.
+
+We know that in general, radio signals get weaker with distance. The weaker they
+become, the more susceptable they are to background noise. When a signal falls
+below the noise floor it is lost.
+
+Noise is noise, because it is random, without uniformity. A chirping signal
+however, has a clear pattern - it rises through both the frequency and time
+domain (it traverses different frequencies over a duration of time). This rising
+pattern can be explicitly filtered for which makes it detectable even amidst
+noise. This means even though a signal may be weak and far away, it can still be
+detected because of it's sweeping pattern, thus making LoRa long range.
+
+Moreover, a higher spreading rate results in a longer range of transmission and
+reception.
+
+Thus the key properties of LoRa are:
+
+- The signal is long range, travelling from 2-15km in urban areas and up to 50Km
+  in rural areas
+
+- Receivers can pick up very weak signals
+
+- There is good resistance to noise interference since the receiver is trained
+  to look for a pattern amidst the noise. Also, if there is interference at one
+  frequency this is discounted once the data moves to the next frequency in the
+  sweep.
+
+There are some trade-offs though.
+
+- A higher spreading rate results in a longer range but slower data transfer
+  since each bit takes longer to transmit because it is spread accross more
+  chirps.
+
+- A lower spreading rate results in faster data transfer (increased bandwidth)
+  but reduced range