Decision on RF Protocol for Two Way Star Network, Incorporating Contention Based Medium Access

Thread Starter


Joined Mar 5, 2020
Hi all,

I am in the process of brainstorming a plan to implement a radio protocol for a set of GPS modules. The scope of this project and reasons for posting are outline below.

The Scenario:
The GPS system is configured in a "base-rover" or "master-slave" set up. There is a single GNSS receiver, that is in a fixed location, which acts as a base station. This station (the master) broadcasts "correctional data" to all "rovers". This correctional data allows rovers to calculate their position to an high level of accuracy. In a general application, the base does not require to hear any response from a rover unit, and this system can be configured in a one-way manner. My application requires rovers to reply to the rover at certain times, and hence I must make modifications to my current implementation to allow this.

Data Transmission:
The base sends 4 messages, 3 are transmitted once per second, the other once every 10 seconds. These messages range from 6 - 130 bytes including headers & CRC. They are fed into a serial buffer and sent over the air by UHF.

The rovers will be required to transmit a response upon the occurrence of a specific event (they cross a line), with packet size no more than 15 bytes. The reception of the rover data is not time critical, and the percentage of rovers that will want to transmit at any given time will be less than 5% of the total number of rovers (approx 200-300), that may be active. The rover will only have to transmit its data once during the course of this scenario.

Current Implementation:
Currently, my implementation supports only one base and one rover, in a point-to-point configuration. I am using HOPERF HM-TRP radio modules with a variant of SiK open-source firmware. I wish to expand to a multi-rover configuration as detailed above, and these radios will not be sufficient, so a change is required.

What I need:
I am looking to implement a two-way star network (1 base, N rovers), as well as some method of contention based medium access. Considering the lack of time sensitivity with the reception of message, I had considered Slotted ALOHA or similar. Similarly a range of 300+ m and a data rate of 50+kbps would be desired

I have considered two protocols, one on 2.4GHz (ZigBee) and one on 433MHz (LoRa), these protocols allow more flexibilities in terms of configuring star and mesh topologies. Each have their own respective advantages in terms of range and bit rate.

If anyone has any suggestions to point me in the right direction for an appropriately flexible protocol, to allow me to implement a contention based MAC such as slotted aloha, it would be greatly appreciated!