RFChip vs. Chirp in LoRaWAN: Understanding the Difference
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This article explains the difference between “chip” and “chirp” within the context of LoRaWAN and LoRa technology. Both terms relate to the Chirp Spread Spectrum (CSS) modulation technique, which is central to LoRa’s functionality.
Introduction:
LoRa leverages Chirp Spread Spectrum (CSS) modulation. This involves using wideband linear frequency modulated “chirp” pulses to encode baseband information. CSS is favoured in scenarios like space and military communications due to its ability to achieve long range, low power transmission, and resilience to interference.
Symbol to chirps
Symbols, Spreading Factor, and Chips
In LoRa, chirps are cyclically shifted. Data encoding is determined by frequency jumps within these chirps. Let’s delve into how the spreading factor (SF) is used to encode messages.
Example:
Consider a symbol representing the decimal value 95, which in binary is 1011111. Here, the number of raw bits encoded by one symbol is 7. Therefore, the Spreading Factor (SF) is 7.
A symbol has 2SF values. For an SF of 7, the range spans from 0 to 127. The symbol value is encoded onto a sweep signal (an up-chirp). This sweep signal is divided into 27 = 128 chips.
LoRa modulated signal
As mentioned, the spreading factor defines:
- The number of raw bits encoded by a symbol.
- Each symbol contains 2SF chips.
LoRaWAN Spreading Factors and Characteristics
The following table details the various spreading factors supported by LoRaWAN. It includes the number of chips per symbol, SNR limit, time on air (for a 10-byte packet), and bit rate for each spreading factor, assuming a LoRa bandwidth of 125 kHz.
| Spreading factor | Chips/Symbol | SNR Limit | Time on air (10 byte packet) | Bit Rate (bps) |
|---|---|---|---|---|
| 7 | 128 | -7.5 | 56 ms | 5469 |
| 8 | 256 | -10 | 103 ms | 3125 |
| 9 | 512 | -12.5 | 205 ms | 1758 |
| 10 | 1024 | -15 | 371 ms | 977 |
| 11 | 2048 | -17.5 | 741 ms | 537 |
| 12 | 4096 | -20 | 1483 ms | 293 |
Key Difference: Chirp vs. Chip
- A symbol contains 2SF chips.
- Chirps refer to the frequency ramp itself: from a low frequency (Flow) to a high frequency (Fhigh) for an up-chirp, and from Fhigh to Flow for a down-chirp.
In simpler terms, the chirp is the actual signal sweep, while the chips are the subdivisions of that sweep used to encode the data within the LoRa signal.
