LoRaWAN: Implicit vs. Explicit Headers Explained

This page explains the difference between implicit and explicit headers in the LoRaWAN packet structure. LoRaWAN is a Low Power Wide Area Network (LPWAN) technology known for its long range, low power consumption, and resistance to interference.

LoRa packets consist of three main elements:

• Preamble
• Header (optional)
• Payload

LoRa PHY structure

The figure above illustrates the general LoRa physical layer (PHY) structure as defined in the LoRaWAN specifications.

To be compliant with LoRaWAN, end devices must support the following LoRa physical layer settings:

• Preamble size: 8 symbols
• Sync Word: 0x34 (Public)
• Header: Explicit in uplink/downlink
• CRC presence: True (Uplink) and False (Downlink)
• Spreading Factor (SF) and Bandwidth (BW): As per region and defined by the data rate
• IQ Polarization: Not-inverted (uplink) and Inverted (downlink)

Explicit Header

Explicit header in LoRaWAN packet

The figure above depicts the explicit header mode used in the LoRa physical layer. In this mode, both the PHY Header (PHDR) and Header CRC are included in the packet.

The PHDR contains the following information:

• Payload length (in bytes)
• Coding Rate (CR)
• Presence of optional CRC for the payload

The integrity of the payload is protected by the CRC for uplink messages.

Implicit Header

Implicit header in LoRaWAN packet

The figure above depicts the implicit header mode. In this mode, neither the PHDR nor the PHDR_CRC are included in the packet.

LoRaWAN beacons utilize the implicit header mode with a fixed length for transmission.