Wifibroadcast

This is transmitter and receiver of UDP packets using raw WiFi radio inspired by https://befinitiv.wordpress.com/wifibroadcast-analog-like-transmission-of-live-video-data/ . The main difference is how the data are encapsulated into ieee80211 frames. The original wifibroadcast accepts stream of bytes and split them into packets without attention to x264 stream elements. This can emit up to 100ms latencies. In my case wifibroadcast accepts UDP stream (for example x264 encapsulated into RTP packets). This provides low latency streaming.

Other features:

• Bidirectional mavlink telemetry. 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 using libsodium.
• Distributed operation. It can gather data from cards on different hosts. So you don't limited to bandwidth of single USB bus.
• Aggreagation of mavlink packets. Don't send wifi packet for every mavlink packet.
• Enhanced OSD https://github.com/svpcom/wifibroadcast_osd for raspberry pi (consume 10% CPU on PI Zero)

Theory:

Wifibroadcast puts the wifi cards into monitor mode. This mode allows to send and receive arbitrary packets without association. This way a true unidirectional connection is established which mimics the advantageous properties of an analog link. Those are:

• The transmitter sends its data regardless of any associated receivers. Thus there is no risk of sudden video stall due to the loss of association

• The receiver receives video as long as it is in range of the transmitter. If it gets slowly out of range the video quality degrades but does not stall.

• The traditional scheme “single broadcaster – multiple receivers” works out of the box. If bystanders want to watch the video stream with their devices they just have to “switch to the right channel”

• Wifibroadcast allows you to use several low cost receivers in parallel and combine their data to increase probability of correct data reception. This so-called software diversity allows you to use identical receivers to improve relieability as well as complementary receivers (think of one receiver with an omnidirectional antenna covering 360° and several directional antennas for high distance all working in parallel)

• Wifibroadcast uses Forward Error Correction to achieve a high reliability at low bandwidth requirements. It is able to repair lost or corrupted packets at the receiver.

Sample usage chain:

Camera -> gstreamer --[RTP stream (UDP)]--> wfb_tx --//--[ RADIO ]--//--> wfb_rx --[RTP stream (UDP)]--> gstreamer --> Display

For encode logitech c920 camera:

gst-launch-1.0 uvch264src device=/dev/video0 initial-bitrate=6000000 average-bitrate=6000000 iframe-period=1000 name=src auto-start=true \
               src.vidsrc ! queue ! video/x-h264,width=1920,height=1080,framerate=30/1 ! h264parse ! rtph264pay ! udpsink host=localhost port=5600

To decode:

 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

HOWTO build:

For development (inline build)

make

For binary distribution RHEL or Fedora

make rpm

For binary distribution Debian or Ubuntu

make deb

For binary distribution (tar.gz)

make bdist

You need to generate encryption keys for gs(ground station) and drone:

wfb_keygen

Leave them inplace for development build or copy to /etc for binary install. Put drone.key to drone and gs.key to gs.

Supported WiFi hardware:

My primary hardware targets are:

1. Realtek RTL8812au. 802.11ac capable. Easy to buy. Requires external patched driver! System was tested with ALPHA AWUS036ACH on both sides in 5GHz mode.
2. Ralink RT28xx family. Cheap, but doesn't produced anymore. System was tested with ALPHA AWUS051NH v2 as TX and array of RT5572 OEM cards as RX in 5GHz mode.

To maximize output power and/or increase bandwidth (in case of one-way transmitting) you need to apply kernel patches from patches directory. See https://github.com/svpcom/wifibroadcast/wiki/Kernel-patches for details.

Wifibroadcast + PX4 HOWTO:

https://dev.px4.io/en/qgc/video_streaming_wifi_broadcast.html

Wiki:

See https://github.com/svpcom/wifibroadcast/wiki for additional info