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Views: 504 Author: Site Editor Publish Time: 2019-07-26 Origin: Site
A video transmitter (VTX) is a device that is connected to your camera and transmits the video signal from your drone back down to you on the ground over the airwaves in real time. It is one of the most important pieces of equipment for drone FPV flying. The range of the wireless video link is limited by a number of factors. Among these factors signal interference is the most vital one. In this article we will let you make clear about the signal interference and how to resolve it.
The following is the content list:
1.What is FPV
2.What cause the signals interference
3.How to resolve or avoid the interference problem
4.Advanced Digital Wireless Video Links
5.Conclusion
Drones are operated by First Person View (FPV). In an FPV system
the video image captured from an on board camera and then transmitted
to a personal video monitor on the ground via a radio wireless link.
Picture on the right shows a typical set of video monitor
screen on GCS embedded receiver.
At present advanced FPV Systems with high-power and high sensitivity video transmitters and receivers could offer ranges of tens of km. More information regarding to long range FPV video transmitter, please check the website: www.hk-ifly.com. We will no longer introduce them here.
In the environment where your drone flying if there are other sources of radio transmission which will interfere with the main signal. When the interfering signals occur has same or close to your frequency band with your drone wireless video link, it will act as inband noise. This will reduce the signal to noise ratio, result in a noisy video image and limit range of the link.
1.A typical interferer can be the WiFi hotspot in the area, tv tower, high voltage power line, GPS and Fly controller on board, a nearby mobile phone or another drone’s video transmitters.
2.If the source of interference is much more powerful than the the wireless link band, it is called a blocker. The blocking signal can penetrate insufficient front-end channel filtering, and reduce the dynamics of the Low Noise Amplifier (LNA). A simplified diagram of a receiver signal chain is shown as below.
Typical high power blockers can be radars, broadcast towers or military radios.
Signal interference could be the result of a number of things, i can only suggest a few and hopefully one of those rings a bell.
For small power interferer, such as hotspot, mobile phone, GPS or other drone wireless link, we can do as follows:
1.The problem can be minimised by selecting a channel as far away in frequency from the interferer as possible and moving the video receiver and antenna.
2. Try your best to decrease the quantity of radio equipment on board. You can try to that use FPV system that combines Video and bi-directional digital serial data in one wireless channel which can save bandwidth and frequency resource, and reduce possible interference with other wireless devices.
For powerful interferer, such as radars, broadcast towers or military radios.
The technical measures is to ensure good front-end channel filtering of the video receiver, and use a directive ground antenna to minimise interference from other directions.
Directional antennas with a narrow beam and high directional gain will increase the receiving signal strength from drones. The antenna can even be equipped with a tracker that automatically directs your antenna correctly facing the drone (the strong signal comes out the flat face not the tips or bottom - therefore if your rx antenna is properly facing the drone, the signals will be very strong).
Over the past few years, rapid technology development has created low latency and long range HD video links for drones. The digital radio transmission system needs to keep a stable link between drone with GCS under rapidly deteriorating radio conditions. 
A wireless high frame rate HD video link need to encode each frame to nearly same size so that there is no additional latency in wireless channel caused by large I frame, and hence an advanced modulation is needed. This puts a strict requirement on the signal to noise ratio, so a various of MIMO antenna concept is applied together with coded orthogonal frequency division multiplexing (COFDM). COFDM is very effective for communication over channels with frequency selective fading (different frequency components of the signal experience different fading). With a traditional single wide band carrier, frequency selective fading is complex to handle. COFDM mitigates the problem by converting the high speed serial data into parallel low bandwidth sub-carriers. Some sub-carriers are reserved as pilot carriers (used for channel estimation/equalisation and to combat magnitude and phase errors in the receiver) and some are left unused to act as guard bands. The reservation of sub-carriers to guard bands helps to reduce the out of band radiation, and thus reduce the requirements on transmitter front-end filters. Advanced COFDM FPV modules adapt CABAC entropy encoding to achieve high compression rate. Hence we can get high video quality at very low bit-rate.
Wireless video for FPV drone piloting is still fast developing technology, and we will see compact and strong anti-interference HD FPV systems emerge in the near future.
