LoLRa Killed the Radio Chip

LoRa, short for Long Range, is a low-power wide-area network technology designed for long-range communication between Internet of Things (IoT) devices. Developed by Semtech Corporation, LoRa operates on unlicensed frequency bands, allowing for easy deployment and scalability of IoT networks. It utilizes chirp spread spectrum modulation to achieve long-range communication with low power consumption.

One of the key features of LoRa is its ability to transmit data over several kilometers in urban environments and even longer distances in rural areas, making it ideal for applications that require connectivity over vast areas. This long-range capability makes LoRa suitable for various use cases, including smart cities, agriculture, asset tracking, environmental monitoring, and industrial automation.

In general, specialized radio chips are used to transmit radio frequency (RF) signals. But while these chips handle many low-level details like signal modulation, strictly speaking, they are not necessary. Generating RF signals is actually quite simple — even unavoidable. Flicking on a light switch, for example, will create an RF field.

Producing random RF signals is not of much use, however, so radio chips are used to transmit a meaningful message to support a specific application. This is especially true with LoRa. Since it is a commercial protocol it is not easy to roll your own solution. But, given the popularity of LoRa, much work has been done to reverse engineer the protocol and figure out how it works.

That knowledge has opened the door to those that want to build their own LoRa transmitter. YouTuber and engineer CNLohr recently decided to take on this challenge. Recognizing that any time an electrical potential passing through a conductor changes an RF field is created, CNLohr decided to use low-cost microcontrollers to rapidly switch an output pin, connected to an antenna, on and off. Using this technique, a very capable LoRa transmitter can be created without a radio chip or any other specialized components.

Of course this I/O pin switching must happen in a very specific way, otherwise it would just be noise. So, CNLohr developed firmware, called LoLRa, for the CH32V203 RISC-V microcontroller and the ESP32 to switch an output pin on and off in a very specific pattern at hundreds of megahertz. While these specific microcontrollers were targeted in this project, the code could be adapted to other platforms without too much trouble.

A number of tests were conducted to determine the transmission range of the system. It proved to be surprisingly effective, with signals being received at distances of a few hundred meters all the way up to 2.5 kilometers. The longest range transmissions were achieved with an ESP32-S2 transmitting from a drone.

As CNLohr noted, the signals produced using this technique are by no means limited to a small range of LoRa frequencies. There is plenty of spill over into other areas of the RF spectrum where transmissions are banned, so you probably do not want to build this project for yourself, as it could get you into some legal trouble. Adding a filter to your setup could help, but there are no guarantees. It is also notable that LoLRa can only transmit signals — reception is not possible. So for nearly all practical use cases, using a traditional radio chip is almost certainly the right choice. But, whether you intend to build the device or not, the project write-up is very instructive and worth a read if you want to learn a thing or two about RF transmissions and LoRa.