ELRS Redundant Diversity RX

Status: Active

Owner: @Nolan Haines @Ishman Mann

Altium: https://warg.365.altium.com/designs/C4975292-FC49-480C-8328-02A26C366B34#design

Purpose

Why, what, how

Why:

Need airside receiver that complements the ELRS Gemini Tx system created by Farris
Want better link quality at comp
- Comp takes place at an airport (a very high interference RF environment)
- Large radars, other drones, people leaving their controllers on etc creates a lot of interference
- This project combines ELRS Gemini with “true” antenna diversity (both described in detail below)
- this should allow us to (ideally) maintain a near 100% link quality at all times even with interference
  - I don’t have any backing up for this number besides the video below. The reality is is that we don’t actually know which system will provide the highest LQ. That is the point of this project, basically: make this and see how much it improves LQ, and compare it to the other options we have. Based on the information currently available, I expect this system to significantly improve LQ, but again this needs to be rigourously tested and options weighed.
  - ExpressLRS Gemini Tx see the end of this video to see Gemini alone (without true diversity) in action in an environment with less RF interference. LQ stays nearly at 100 the whole time.
  - adding diversity antennas should allow us to do this in our context.
Want to be able to mix and match frequencies on each Gemini signal
- ie. want to be able to mix 2.4GHz/900MHz, 900/900, down to even lower (433, 144), so we can choose the freq with least interference
- NOTE: this feature will not be present on the first version of this board, for now we just want to get 2.4/2.4 working properly. the ability to change frequency will come later.
CAN support (formerly also had USB support for flashing, but this has been removed due to weight/size concerns)
Basically we’re trying to “stack every buff in the game” as in we’re trying to implement every technology related to LQ currently available for ELRS into a single receiver board
A receiver with this exact setup hasnt been done yet (afaik) by anyone in the ELRS community so we’d sorta be first
- Jye did do a bit of testing for it though (Github for that linked below)

What:

Intended for Pegasus

Board with fairly minimal size and weight
- have a bit of leeway here since our drone is bigger than most ELRS platforms, so things like a CAN transceiver is doable.
This board will need custom firmware
Antennas will be attached to this board but place elsewhere on the drone. (highly likely on the leg, but away from the motors)
- need to do the math for losses on the cables for the antennas, whether its worth using higher quality/more expensive cables
This is a massive test/integration effort.
note: it is planned to make a second version of this board with basically the same schem, but a much less dense layout, as a groundside Tx. it is possible to turn the schematic of this PCB into a Tx without changing basically anything by writing different firmware. This will essentially be an alterntive to Farris’s Gemini Tx for test, comparison, whatever EFS wants to do with it

How:

ESP32 based system
Four LR1121 transceivers
one LDO powers everything; LR1121s are ultra low power
JST connectors
UART pin on the JST for debug and EFS stuff
u.fl connectors for the antennas

Diversity and Gemini Explanation

Antenna diversity is when multiple antennas are used to either transmit or receive data in a wireless communication system. In this context, it will be used on the receiving end. Two antennas will be used to receive the same information, on the same frequency. Doing this allows for each antenna to receive a different version of the same signal. It is less likely that each version of the signal will experience significant interference, which means that a diversity setup is more likely to receive data than one without. There are different types of antenna diversity; they are explained in detail later in this section.

Gemini is a new feature added to ELRS. The idea is to transmit the same information at slightly different (~40MHz apart) frequencies to optimize LQ. Farris’s explanation:

The ELRS Gemini TX is a custom PCB that acts as the transmitter controller for the antennas on the ground to communicate with the drone. It involves using 2 transmitter modules with a TX power of 27dBm (about 8km range) simultaneously transmitting the same information on slightly different frequencies. The goal is to reduce the risk of lost data due to poor connectivity with a single antenna - transmitting the same info twice essentially doubles the chances of the command information reaching the drone successfully.

The ELRS Redundant Diversity RX project will combine both Gemini and True Diversity. Each of the signals transmitted by the TX will be received by a pair of antennas configured for True Diversity. This will, in theory, significantly improve LQ as the benefits of both systems are combined.

The rest of this section will explain the different types of diversity, as well as Gemini.

Figure 2: The differences between Antenna Diversity, True Diversity and Gemini.

Basic: One signal is transmitted to one antenna. Least amount of immunity to interference.

Antenna Diversity: One signal is transmitted, but there are two antennas on the receiving end. One receiver is attached to two antennas, and the receiver switches between them (based on the Link Quality?). Better immunity to interference.

True Diversity: One signal is transmitted to two antennas each attached to their own receiver (2 receivers). Whichever receives a valid packet first is sent to the flight controller. (According to someone on the ELRS Discord) Very good signal immunity.

Gemini: “In Gemini Mode, a TX module simultaneously transmits a packet in two frequencies 40MHz apart for 2.4GHz and ~10MHz apart for 900MHz users. The packet separation used is half of the frequency domain selected and will vary a little. A true diversity Receiver is used to receive both packets simultaneously. Transmitting on 2 separate frequencies provides better interference avoidance and/or mitigation, in a similar way DVDA does by sending repeat packets sequentially on different frequencies. This means, the Receiver has an increased chance of receiving the packet. This results in a much higher and stable LQ.”

Gemini - ExpressLRS

Design

Board Level System Diagram

prob gonna remove this because the file is corrupted and I can’t edit the original diagram

also it’s duplicate info

Drone Level System Diagram

Gemini TX ↔︎ RX Diagram

Diagram made by Farris Matar.

NOTE that on the drone side of this diagram it should have a CAN line as well.

Component Selection

Microcontroller: ESP32-PICO-D4

ESP32-PICO-D4 | DigiKey Electronics

RF Transceiver: LR1121

LR1121IMLTRT | DigiKey Electronics

User manual: https://www.mouser.com/pdfDocs/UserManual_LR1121_v1_1.pdf

Datasheet: https://semtech.my.salesforce.com/sfc/p/#E0000000JelG/a/3n000000vByp/PdIx6QpZ_oZmdRVW2iW.ciZPBKY8nZLQpybO9K0Bo.s

LDO:

AZ1117IH-3.3TRG1 | DigiKey Electronics

The ESP pulls 500mA tops, the LR1121 pull 10mA tops, TCXOs pull 2.5mA tops. This LDO is rated for 1A so we should be more than fine with just the one.