LoRa Protocol Stack : Exploring the LoRa Physical Layer
The LoRa protocol stack, with its unique physical layer, is designed to support long-range, low-power wireless communication. The LoRa physical layer employs Chirp Spread Spectrum (CSS) modulation to achieve robust communication, even in challenging environments. Understanding the characteristics of the LoRa physical layer is key to leveraging the full potential of the LoRa protocol, enabling reliable data transmission over extended distances. LoRa Physical layers for different country wide ISM bands and LoRa MAC Layer including different MAC message formats and MAC commands have been explained.
As shown in the figure-1 below, LoRa protocol stack consists of following protocol layers:
• LoRa Application Layer
• LoRa MAC Layer
• LoRa PHY Layer
• LoRa RF Layer
LoRa frame consists of uplink messages and downlink messages. There are three type of classes supported in LoRa system. Based on these classes, LoRa frame structure varies. Uplink messages are transmitted from end devices to the server using one or more gateways.
Downlink message is transmitted from server to only one LoRa end device. This is done using single gateway connected with network server. Refer different LoRaWAN classes for more information on types of frames supported in LoRa system.
LoRa Physical Layer
Following are the functions of LoRa Physical Layer (PHY):
• Physical Layer constructs the frame in order to transmit payload from MAC layer over RF link.
• It inserts PHDR, PHDR_CRC, preamble and CRC for the entire frame.
CRC field is available in uplink message only.
• As a preamble specific constant sync words are used based on
modulation technique either LORA, GFSK or FSK.
This preamble will help in synchronization at the receiver as it is known to the receiver.
• PHY layer uses specific RF bands as per countrywide requirement.
➨Refer LoRaWAN Physical Layer >>.
LoRa MAC Layer
Following MAC messages are used in LoRa for establishing communication between end device and server.
• Join request (From End device to Server)
• Join accept (from network server to End device)
• Beacon frame (from gateway to End device) for scheduling slot for reception by
End devices.
• Confirmed Data Up/Down (This messages are to be acknowledged by LoRa receiver)
• Unconfirmed Data Up/Down (This messages do not require any ack).
Here Up stands for uplink transmission and Down stands for downlink transmission.
➨Refer LoRa MAC layer inside>>.
Following table mentions LoRa MAC message fields as depicted in the figure-2 above.
| LoRa MAC message field | Description |
|---|---|
| MHDR | MAC Header, Single octet long |
| MAC Payload | Data from upper layer |
| MIC | Message Integrity Code, 4 octet long |
| FHDR | Frame Header |
| FPort | Optional port field |
| FRMPayload | Optional Frame Payload field |
| Devaddr | Device address |
| FCtrl | Frame Control Octet |
| FCnt | Frame Counter, 2 octets long |
| FOpts | Frame Options used to transport MAC commands, 15 octets long |
Note: Information provided on this page is derived from LoRaWAN Specification V1.0 released on Jan.2015 by LoRa™ Alliance. Refer latest specifications published by LoRa Alliance ( https://www.lora-alliance.org ).
Conclusion
The LoRa physical layer forms the foundation of the LoRa protocol stack, providing essential modulation techniques and channel properties that define its performance. By optimizing parameters such as spreading factors and bandwidth, users can fine-tune their LoRa based applications for improved efficiency and coverage. Mastery of the LoRa physical layer is critical for successful deployment of IoT solutions using LoRa technology.