Terminology » IoT » Chip vs. Chirp in LoRaWAN: Understanding the Difference

Chip vs. Chirp in LoRaWAN: Understanding the Difference

lorawan chip chirp spread spectrum modulation

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 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 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 factorChips/SymbolSNR LimitTime on air (10 byte packet)Bit Rate (bps)
7128-7.556 ms5469
8256-10103 ms3125
9512-12.5205 ms1758
101024-15371 ms977
112048-17.5741 ms537
124096-201483 ms293

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.