Drones have become increasingly popular in recent years as professional tools, entertainment and air sports competitions. Unmanned aerial vehicles (UAVS) are the generic term for unmanned aerial vehicles (UAVS). They include many types of unmanned remotely controlled aircraft, including fixed-wing aircraft, helicopters and multi-rotor aircraft.
Professional drones are becoming more widely used, aerial photography during sporting events does not have to rely on expensive full-size helicopters, and estate agents often use drones to record. Drones can also spot missing people and can monitor habitats at risk of pollution. Power companies are using drones to inspect high-voltage lines, avoiding costly blackouts and dangerous manual climbs. Even conservative industries like rail companies are considering using drones to check track conditions in areas with restricted access. There are also delivery companies planning to deliver small packages by drone
1.UAV operation technology
Drones can be piloted in two different ways; One is to visually observe the drone's line of sight, and the other is through a first-person perspective (FPV). In the FPV system, video images from an onboard camera are transmitted via radio to a personal video display on the ground in the form of a screen or video goggles.
2.video transmission wireless technology
Wi-Fi can be used to transmit signals over fairly short distances. Wi-Fi signals can range from 300 meters to 2,000 meters, depending on the device and conditions. Transmission range can vary due to a number of factors:
Transmitter power, the larger the antenna, the farther the signal radiation, the smaller the attenuation;
Antennas, arranged in ascending order of power Whip(or wire), Chip, PCB or external (via U.F.L or RPSMA connector);
Frequency is used, usually the lower the frequency, the further the signal can travel.
The environment, surrounding trees, buildings, direct line of sight, atmospheric conditions, etc. can negatively affect Wi-Fi signal range.
Frequency band, 5GHz Wifi network is preferred, which has less interference in urban areas. Other frequency band features are as follows:
2.1 Less than 1GHz band
Common solutions come from those who fly FPVS (first person views) using simple analog cameras connected to 900 MHz. Using a 1W 900MHz transmitter with alfalfa leaf antenna (a common antenna type) and an 18dB gain patch antenna pointed at your aircraft, a site line of over 5 miles can be easily obtained. It depends on the area one wants to operate in and the availability of frequency bands to use such applications.
2.2 3G/4G band
You can use the 3G/4G dongle that comes with the drone for wireless transmission at high data rates. The solution can be used based on 3G/4G network availability in the operating area.
2.3 Customize the solution.
Integrated RF transceivers are widely used not only in Software Defined radio (SDR)1 architectures in cellular telephone base stations, such as Multi-service Distributed Access Systems (MDAS) and small cells, but also for wireless high-definition video transmission in industrial, commercial and small cells. Military applications such as unmanned aerial vehicles (UAVs). You can use the RF transceiver family AD9361/AD9363 and manufacture suitable hardware based on their spectrum availability, as these transceivers have bandwidth up to 6GHz. A suitable baseband-side FPGA can be used for digital processing.
3. Wireless video transmission challenges
The range of wireless video links is limited by a number of factors. Path loss itself weakens the signal as distance increases, and obstructions in the line of sight produce additional attenuation. There are some uncertain challenges of wireless link in natural environment, and effective solutions need to be given. The following two aspects are the main problems:
3.1 interference
Other wireless transmission sources in natural environments may interfere with drone video transmission signals. If the jamming signal occurs in the same frequency band as the wireless video link, it will act as in-band noise. This will reduce the signal-to-noise ratio, resulting in noisy video images and limited link range. A typical source of interference might be the video transmitter of another drone in the area, a nearby WiFi hotspot, or a cell phone. Problems can be minimized by choosing a channel with a frequency as far away from the source of interference as possible or by moving the video receiver and antenna. If the interference source is strong but outside the frequency band of the wireless link, it is called a blocker. Blocking signals can penetrate inadequate front-end channel filtering and reduce the dynamics of a low noise amplifier (LNA).
3.2 Reflection induced multipath fading
Even with a strong, noiseless signal, wireless links can suddenly go down, especially in cluttered or urban environments. This may be due to reflection propagation paths cancelling out direct propagation paths. Cancellation occurs due to phase shifts associated with different propagation delays. This occurs at specific points in the receiving space and simply moves the antenna by less than one wavelength to disappear. In addition to signal cancellation,multipath propagation also causes symbol delay extension. Symbols from different paths arrive at different times, resulting in bit error if the delay is large.
4. Overcome challenges
4.1 RF Frequency Switching
The 2.4GHz frequency is widely used for Wi-Fi, Bluetooth, and IoT short distance communications, making it increasingly crowded. Its use for wireless video transmission and control signals increases the chance of signal interference and instability. This creates undesirable and often dangerous conditions for drones. Using frequency switching to maintain a clean frequency will make data and control connections more reliable. When the transmitter senses a crowded frequency, it automatically switches to another band. For example, two drones operating nearby using the frequency would interfere with each other's communications. Automatically switching LO frequencies and re-selecting bands will help maintain a stable wireless link. Adaptive selection of carrier frequency or channel during power-on is one of the excellent features of high-end UAVs.
4.2 Frequency Hopping
Fast frequency hopping, widely used in electronic countermeasures (ECM), also helps avoid interference. Usually if we want frequency hopping, the PLL needs to be relocked at the end of the program. This involves writing to the frequency register and, after VCO calibration time and PLL locking time, making the jump frequency interval close to tens of microseconds.
Figure 3. Schematic diagram of frequency hopping scheme
4.3 OFDM modulation at the PHY layer
Orthogonal frequency division multiplexing (OFDM) is a form of signal modulation that divides a high data rate modulated stream into a number of slowly modulated narrow band near-range ion carriers. This makes it less sensitive to selective frequency fading. Disadvantages are high peak-to-average power ratio and sensitivity to carrier migration and drift. OFDM is widely used in PHY layer of broadband wireless communication.
4.4 5G and WIFI technology
Wireless video for FPV UAVs is still an immature technology and we will see compact and low cost HD FPV systems in the near future. The key to cost reduction is to improve the integration of the system on chip and the resulting high yield. A paradigm shift occurs when an entirely new radio, camera, or display concept appears. The next generation of cellular and WiFi technology, called 5G, will utilize dynamic beamforming to increase system gain and keep interference low. Together with more complex MIMO, this will further improve performance and transmission bandwidth. These concepts are likely to be applied to future FPV systems as the technology matures. This results in higher performance, greater range, higher image quality, and better reliability. It will enable drones to deal with more of the challenges we face today, as well as challenges we haven't yet thought of.
