What’s ExpressLRS Gemini, How to Enable it?

Gemini is a groundbreaking mode in ExpressLRS (ELRS), enhancing reliability and stability beyond the standard ExpressLRS. In this article, we explore this technology from a beginner’s perspective, highlighting its benefits and downsides, how to enable it, and whether or not you should upgrade your hardware for this new feature.

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The Basics of Radio System

At the simplest form, radio systems consist of a transmitter and a receiver, each equipped with an antenna to send and receive data.

To mitigate the risk of lost data packets due to interference or poor antenna orientation (cross polarization), the concept of diversity was introduced. It allows you to place the two antennas at 90-degree to each other to increase the chance of successful data transfer. There are two types of diversity receivers: Antenna diversity and Receiver diversity.

Antenna diversity employs dual antennas, allowing the receiver to choose the one with the stronger signal.

Receiver diversity, also known as “true diversity”, takes antenna diversity a step further by incorporating two separate receive chains. It’s like having two receivers on the same board, enabling the receiver to receive the same signal from both antennas simultaneously.

Understanding Gemini Mode

Gemini mode in ExpressLRS goes beyond the traditional concept of diversity. It employs not only dual antennas on the receiver for receiving data, but also two separate transmit chains and antennas to send information simultaneously. Each transmit chain operates on different frequencies, typically with a difference of 40MHz.

ExpressLRS uses LoRa technology, which has a feature called “frequency hopping”. This capability allows it to switch to an different frequency or channel when interference is detected, preserving the clarity and accuracy of the transmitted and received data. However this might result in a small drop in communication, or even potentially trigger a short failsafe.

The Gemini approach ensures that if the signal from one antenna is blocked or interfered with, the other antenna, operating on a different frequency, can still transmit the necessary data to the drone, maintaining a stable connection.

Gemini is NOT for Increasing Range

Gemini increases the odds of your FPV signal packets getting through by retransmitting each packet twice on different frequencies. It doesn’t enhance range or penetration significantly but can improve link quality (LQ), especially when you’re in areas where some packets aren’t getting through.

For example, if your LQ is already 95% or higher, Gemini is likely unnecessary. But if you are in an environment where LQ struggles, Gemini could be helpful. Gemini can also help racers by reducing jitter and dropped packets due to interference.

Advantages of Gemini

Gemini mode sends the same data packet multiple times on different frequencies (sending these data packets in parallel instead of in series like D-mode), it ensures that at least one set of TX packets reaches the drone. It effectively doubles the probability of successful packet delivery, making the communication twice as reliable as traditional methods without the penalty of latency.

In fact, the benefits of Gemini extend beyond just strengthening connections; it also significantly improves link quality. Maintaining high and consistent link quality is crucial for ensuring smooth and precise control, especially in environments with interference, such as urban areas with many Wi-Fi signals or situations where multiple drones are operating simultaneously, like drone races.

Having two transmitting antennas and positioning them perpendicular to each other can mitigate issues related to cross polarization and reduce the impact of suboptimal antenna arrangements on the drone during motion, ensuring optimal signal reception in various orientations.

Considerations and Limitations

While Gemini offers numerous advantages, it does come with certain considerations. Since it requires two transmit chains to work simultaneously, it consequently uses double the power consumption. It will run down your radio battery faster, therefore you might need to use an external battery to power to module alone.

Additionally, extra thoughts on how to properly manage the extra antenna is vital to maximizing the benefits of Gemini mode, which we will touch on later in this post.

Compatible Hardware

Any “True Diversity” ExpressLRS receivers should support Gemini Mode, but you need to update the firmware to the latest version. And you will need a Gemini compatible transmitter module (one with two antennas).

While it is anticipated that most ExpressLRS hardware manufacturers will develop transmitters and receivers supporting Gemini mode in the future, BetaFPV is currently the only maker of such compatible hardware at the time of this writing: SuperG transmitting module, and SuperD / SuperP receivers.

How to Enable Gemini Mode

To activate Gemini mode, firstly, confirm that both your transmitter and receiver are compatible with Gemini mode and have been updated to the latest firmware (V3.3 or newer).

Within the ExpressLRS LUA script on your radio, navigate to “Antenna mode” and select the Gemini option to activate it.

You also need to enable it in the receiver by navigating to Other Devices, selecting your receiver, and opting for the Gemini option in the RX Mode.

Antenna Mode

If you don’t have a compatible receiver, the Antenna mode of the transmitter module should be set to either Ant1, Ant2 or Switch.

Ant1 and Ant2 allows users to use one of the two available antenna, the other antenna will be idle.

The option Switch uses both antennas and alternates between them for data transmission. It uses less power than Gemini, but still offers the advantage of having two transmitting antennas.

Optimizing Antenna Placement

It’s crucial to position the antennas in a manner that allows both antennas on the receiver to receive signals from both transmitters as much as possible. This means maintaining line of sight between the transmitter and receiver antennas at all times is paramount to take full advantage of the Gemini system. Transmitter antennas placed at 90 degrees to each other can offer extra polarization diversity.

Conclusion

The introduction of ExpressLRS Gemini is a revolutionary step in drone radio technology, promising enhanced reliability and performance in communication between drones and controllers.

Is upgrading to new hardware that are Gemini compatible worth it? For many, it might seem excessive. If your current setup satisfies your needs and you usually operate in areas with minimal interference, upgrading might not be necessary. However, the ability to have two antennas in different orientations could still be helpful in certain situations.

While the benefits are clear, particularly in high-interference environments, users need to consider the implications on power consumption and optimize their setups to extract maximum benefits from this new feature.