# Long-distance Video Streaming and Telemetry via Raw WiFi Radio

This page shows how to set up a companion computer with a camera (Logitech C920 or RaspberryPi camera) such that the video stream is transferred from the UAV to a ground computer and displayed in QGroundControl. The mechanism also provide a bidirectional telemetry link (i.e. like SiK radio). This setup uses WiFi in unconnected (broadcast) mode and software from the Wifibroadcast project (opens new window).

Note

Before using Wifibroadcast check regulators allow this kind of WiFi use in your country.

# Wifibroadcast Overview

The Wifibroadcast project provides video and telemetry transport that use low-level WiFi packets to avoid the distance and latency limitations of the ordinary IEEE 802.11 stack.

The high level benefits of Wifibroadcast include:

1:1 map RTP to IEEE 802.11 packets for minimum latency (doesn't serialize to byte steam).
Smart FEC support (immediately yield packet to video decoder if FEC pipeline without gaps).
Bidirectional MAVLink telemetry (opens new window). You can use it for MAVLink up/down and video down link.
Automatic TX diversity (select TX card based on RX RSSI).
Stream encryption and authentication (libsodium (opens new window)).
Distributed operation. It can gather data from cards on different hosts. So you aren't limited to bandwidth of single USB bus.
Aggregation of MAVLink packets. Doesn't send WiFi packet for every MAVLink packet.
Enhanced OSD (opens new window) for Raspberry PI (consume 10% CPU on PI Zero).
Compatible with any screen resolution. Supports aspect correction for PAL to HD scaling.

Additional information is provided in the FAQ below.

# Hardware Setup

The hardware setup consists of the following parts:

On TX (UAV) side:

NanoPI NEO2 (opens new window) (and/or Raspberry Pi if using Pi camera).
Logitech camera C920 (opens new window) or Raspberry Pi camera (opens new window).
WiFi module ALPHA AWUS036ACH (opens new window).

On RX (ground station side):

Any computer with Linux (tested on Fedora 25 x86-64).
WiFi module ALPHA AWUS036ACH (opens new window). See wifibroadcast wiki > WiFi hardware (opens new window) for more information on supported modules.

If you don't need high-power cards, you can use any card with rtl8812au chipset.

# Hardware Modification

Alpha AWUS036ACH is a high power card that uses too much current while transmitting. If you power it from USB it will reset the port on most ARM boards. So it must be directly connected to 5V BEC in one of two ways:

Make a custom USB cable (cut +5V wire from USB plug and connect it to BEC (opens new window)
Cut a +5V wire on PCB near USB port and wire it to BEC (don't do this if doubt - use custom cable instead). Also I suggest to add 470uF low ESR capacitor (like ESC has) between power and ground to filter voltage spikes. Be aware of ground loop (opens new window) when using several ground wires.

# Software Setup

To setup the (Linux) development computer:

Install libpcap and libsodium development libs and install python2.7 + python-twisted packages.
Download wifibroadcast sources (opens new window).
See Setup HOWTO (opens new window) how to build debian, rpm or tar.gz package and configure it.

# UAV Setup

Setup camera to output RTP stream:

a. Logitech camera C920 camera:

gst-launch-1.0 uvch264src device=/dev/video0 initial-bitrate=4000000 average-bitrate=4000000 iframe-period=3000 name=src auto-start=true \
         src.vidsrc ! queue ! video/x-h264,width=1280,height=720,framerate=30/1 ! h264parse ! rtph264pay ! udpsink host=localhost port=5602

b. RaspberryPi camera:

raspivid --nopreview --awb auto -ih -t 0 -w 1280 -h 720 -fps 49 -b 4000000 -g 147 -pf high -o - | gst-launch-1.0 fdsrc ! h264parse !  rtph264pay !  udpsink host=127.0.0.1 port=5602

Configure WFB for drone as described in Setup HOWTO (opens new window)
Configure autopilot (px4 stack) to output telemetry stream at 1500kbps (other UART speeds doesn't match well to NEO2 frequency dividers). Setup mavlink-router (opens new window) to route MAVLink packets to/from WFB:
```
[UdpEndpoint wifibroadcast]
Mode = Normal
Address = 127.0.0.1
Port = 14550
```

# Ground Station Setup

Run QGroundControl or use the following command to decode video:

gst-launch-1.0 udpsrc port=5600 caps='application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)H264' \
          ! rtph264depay ! avdec_h264 ! clockoverlay valignment=bottom ! autovideosink fps-update-interval=1000 sync=false

Configure WFB for ground station as described in Setup HOWTO (opens new window).

# Tuning Radio Settings

With default settings WFB use radio channel 165 (5825 MHz), width 20MHz, MCS #1 (QPSK 1/2) with long GI. This provides ~7 mbit/s of effective speed (i.e. usable speed after FEC and packet encoding) for both directions in sum, because WiFi is half-duplex. So it is suitable for video down stream 720p@49fps (4 mbit/s) + two full-speed telemetry streams (uplink and downlink). If you need a higher bandwidth you can use other MCS index (for example 2 or greater) and/or 40MHz channel.

# Antennas and Diversity

For simple cases you can use omnidirectional antennas with linear (that bundled with wifi cards) or circular leaf (circularly polarized Coverleaf Antenna (opens new window)) polarization. If you want to setup long distance link you can use multiple wifi adapters with directional and omnidirectional antennas. TX/RX diversity for multiple adapters supported out of box (just add multiple NICs to /etc/default/wifibroadcast). If your WiFi adapter has two antennas (like Alfa AWU036ACH) TX diversity is implemented via STBC (opens new window). Cards with 4 ports (like Alfa AWUS1900) are currently not supported for TX diversity (only RX is supported).

# FAQ

Q: What is a difference from original wifibroadcast?

A: The original version of wifibroadcast used a byte-stream as input and split it to packets of fixed size (1024 by default). With this scheme if radio packets were lost (and this was not corrected by FEC) the result was random/unexpected holes in the stream. This is especially bad if the data protocol is not resistant to such random erasures.

The new version was rewritten to use UDP as data source and pack one source UDP packet into one radio packet. Radio packets now have variable size that depends on payload size. This significantly reduces a video latency.

Q: What type of data can be transmitted using wifibroadcast?

A: Any UDP with packet size <= 1466. For example x264 inside RTP or MAVLink.

Q: What are transmission guarantees?

A: Wifibrodcast use FEC (forward error correction) which can recover 4 lost packets from 12 packets block with default settings. You can tune it (both TX and RX simultaneously!) to fit your needs.

:::caution Don't use band that the RC TX operates on! Or setup RTL properly to avoid model loss. :::

Q: Is only Raspberry PI supported?

A: Wifibroadcast is not tied to any GPU - it operates with UDP packets. But to get RTP stream you need a video encoder (with encode raw data from camera to x264 stream). In my case RPI is only used for video encoding (because RPI Zero is too slow to do anything else) and all other tasks (including wifibroadcast) are done by other board (NanoPI NEO2).

# Theory

Wifibroadcast puts the WiFi cards into monitor mode. This mode allows to send and receive arbitrary packets without association and waiting for ACK packets. Analysis of Injection Capabilities and Media Access of IEEE 802.11 Hardware in Monitor Mode (opens new window) 802.11 timings (opens new window)

# What ARM Boards are recommended for the UAV?

Board	Pros	Cons
Raspberry Pi Zero (opens new window)	- Huge community - Camera support - HW video encoder/decoder with OMX API.	- Hard to buy outside US (shipping costs >> its price) - Slow CPU - Only one USB bus - 512MB SDRAM
Odroid C0 (opens new window)	- Fast CPU - EMMC - 1GB SDRAM	- Very sensitive to radio interference - Doesn't supported by mainline kernel - High cost - HW video encoder is broken - Bad PCB quality (too thin, ground pins without thermal relief (opens new window))
NanoPI NEO2 (opens new window)	- ARM 64-bit CPU - Very cheap - Supported by mainline kernel - 3 independent USB busses - 1Gbps Ethernet port - 3 UARTs - Very small form-factor - Resistant to radio interference	- Small community - 512MB SDRAM - No camera interface

This article chose to use Pi Zero as camera board (encode video) and NEO2 as main UAV board (wifibroadcast, MAVLink telemetry, etc.)

# TODO

Make prebuilt images. Pull requests are welcome.
Do a flight test with different cards/antennas.

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