Downlink payload format

• Downlink MHDR
• Downlink Metadata
  • Uplink ID + Downlink data-rate
    • Uplink ID
    • Data-rate
  • Downlink Frequency
  • Delay + TX Power
    • TX Power
    • Delay
• Relay ID
• LoRaWAN PHYPayload
• MIC

On downlink, the Border Gateway creates and re-transmits a Relay encapsulated to the Relay Gateway in the following format. The Relay encapsulation overhead is 15 bytes.

Bytes:

Downlink MHDR

Bits:

• MType = 111 (= Proprietary LoRaWAN MType)
• Payload type = 01 (Relayed downlink)
• Hop count = 000

Note: The hop count is incremented each time the downlink payload is relayed by an other Relay Gateway. As this changes the downlink payload, the MIC must be re-calculated.

Downlink Metadata

Bytes:

Uplink ID + Downlink data-rate

Bits:

Uplink ID

Unique identifier (to the Relay Gateway) identifying the uplink to which this downlink relates. E.g. the Relay Gateway uses this to retrieve the uplink counter value.

Data-rate

Downlink data-rate, unsigned integer with a minimum value of 0 and a maximum value of 15.

Downlink Frequency

Encoded as Frequency_Hz = Downlink Frequency x 100.

Delay + TX Power

Bits:

TX Power

The TX Power which must be used for sending the downlink.

Delay

The relative delay in seconds to the Uplink ID which must be used for the downlink transmission. Delay is an unsigned integer, encoded as:

Delay_Seconds = Delay + 1.

Relay ID

This contains the Relay ID that must relay the downlink to the End Device using the provided Downlink metadata.

LoRaWAN PHYPayload

The LoRaWAN PHYPayload that must be sent to the End-Device.

MIC

Message integrity code, used by other Relay gateways to check the data integrity of the packet. This is obtained by calculating the CMAC over the downlink payload (- MIC bytes), and using the first 4 bytes of the calculated CMAC as MIC.