Drones have become increasi₽∑ngly popular in recent yea≤∞×rs as professional to&§ols, entertainment and air sports compe₩¥titions. Unmanned aerial vehicles ¶¶'¶(UAVS) are the generic te♠"♥rm for unmanned aerial vehicles (UAV•☆αS). They include many types of unman →λned remotely controlled aircraft, incl↕π uding fixed-wing aircraft, helicopters ✔♣and multi-rotor aircraft.
Professional drones are b≤≥ ecoming more widely u$♠sed, aerial photography during sporting∑←← events does not have to rely on exp'±§"ensive full-size helico÷& ↔pters, and estate agents of"₩Ωten use drones to recor€₹d. Drones can also spot missing pe₩≤γople and can monitor habitats at r↔€£isk of pollution. Power companies are♦ using drones to inspect high-voltage γ★¥lines, avoiding costly blackout≤s and dangerous manual climbs.✘ ∞® Even conservative indust ©ries like rail companies a☆₩¥re considering using ↕Ωdrones to check track conditions in£σ areas with restricted access×™. There are also delivery companies pla←¶®♦nning to deliver sma↓™•"ll packages by drone
1.UAV operation technology
Drones can be piloted in two differen☆γ€t ways; One is to visually obs≈♠erve the drone's line of sight, γ₽≤and the other is through a f☆φirst-person perspective (FPV₹≈). In the FPV system, &☆₹≥nbsp; video images from an onboard camera ar↑>¶±e transmitted via radio to a ÷®∏personal video display on the ground i↓€n the form of a screen or video g→oggles.
2.video transmission ₩wireless technology
Wi-Fi can be used to transmitε↕π signals over fairly short distances. W≠∞i-Fi signals can range from 300 meters ₩∏to 2,000 meters, depending on the devic≥Ωe and conditions. Transmissioφ£"Ωn range can vary due to a number of ↕ε £factors:
Transmitter power, the larger the ↑π♠antenna, the farther ≥↓the signal radiation, t±λ↕he smaller the attenua↔↑tion;
Antennas, arranged in ascending oπ≠rder of power Whip(or wire), Ch±∞♠λip, PCB or external (vδ®ia U.F.L or RPSMA connector);
Frequency is used, usualελ>♥ly the lower the frequency, π∑the further the signal ca¶α↑n travel.
The environment, surro↕αunding trees, buildings, direct line o"σ≤≥f sight, atmospheric co≤♥π€nditions, etc. can neg€≈✘atively affect Wi-Fi signal ran✔ge.
Frequency band, 5GHz Wifi network is p'¶referred, which has less in¶®terference in urban a±♣Ωreas. Other frequency band feature≤× ∏s are as follows:
2.1 Less than 1GHz band
Common solutions come from th¶↕£ose who fly FPVS (firs≥δt person views) using simple♣&εγ analog cameras connected t♠∏o 900 MHz. Using a 1W 900MH"★¶•z transmitter with alfalfa le☆'σ<af a€σ≠♦ntenna (a common antenna type) an&♥≥&d an 18dB gain patch€Ω antenna pointed at your aircraft, a s¶ite line of over 5 miles c★↕₽↕an be easily obtained. It depends on th★¥ e area one wants to operate i₩≤↕n and the availability of frequency ban£×ds to use such applic₽₹"ations.
2.2 3G/4G band
You can use the 3G/4G dongle that coΩ↕★mes with the drone for wireles$λ↑₽s transmission at high data rates. ♥The solution can be used based on""→→ 3G/4G network availa♥♣bility in the operating area.
2.3 Customize the solution.
Integrated RF transceiver≠£s are widely used not only in Softw↔≠≥are Defined radio (SDR)1 archit↕ ≠ectures in cellular ÷£telephone base statio↓$♦ns, such as Multi-service D±✘γ®istributed Access Systems (MDAS) ✘€and small cells, but<≥ also for wireless high-definitio ≤n video transmission in industri₩∏™±al, commercial and small cell∏¶≤s. Military applications such as unm₹≈∏✘anned aerial vehicles (UAVs). λ×δYou can use the RF trans αceiver family AD9361/AD9363 a¥β₽nd manufacture suitable hardware bπ€¥ased on their spectrum availability, as$× these transceivers have'₹× bandwidth up to 6GHz. A su≈±↑φitable baseband-side FPGA can b"★αe used for digital pr₽♠>ocessing.
3. Wireless video transmission c뱕>hallenges
The range of wireless video lin≥¶∑εks is limited by a number of fact♣δ¥σors. Path loss itself weakens the sign©∑™♥al as distance increases, and obst↓☆λructions in the line of sight produce•∑ additional attenuation. There <•↑are some uncertain challen♠±✘ges of wireless link in nδ®atural environment, and effectiv™®e solutions need to be ¶↔×±given. The following two aspects↔★↓ are the main ×₹; problems:
3.1 interference
Other wireless transmis←"sion sources in natural↕© environments may interfere ✘πwith drone video transm£'₩±ission signals. If the jamm∑γ<φing signal occurs in th↓εe same frequency band as the wλ₹ireless video link, it will act as✘♠≈™ in-band noise. This will reduce♥£♦₽ the signal-to-noise ratio, resuδσlting in noisy video images and limi♥×ted link range. A typic¶δ✘al source of interfere☆∏£nce might be the video trans" ♠mitter of another drone in tσ≈he area, a nearby WiFi hotspot, or a cγ→ell phone. Problems can φbe minimized by choosing a chεannel with a frequency as faδ×r away from the source of interfere ≈"nce as possible or by moving the vi&®λdeo receiver and antenna. If tΩβ≥ he interference source is str♣α∞ong but outside the frequency Ωε±band of the wireless link, ↑©it is called a blocker. Block≈↔σing signals can pene ✘trate inadequate front-end c→™hannel filtering and reduce th♣↑≠γe dynamics of a low noise ampliελδ fier (LNA).
3.2 Reflection induced multipath ≤↑₽fading
Even with a strong, noisele&&ss signal, wireless links c★≈an suddenly go down, especially in cl↑∞uttered or urban environments. ₩&©σThis may be due to reflection propa¥"•&gation paths cancelling out direct×←¥↕ propagation paths. Cancellation &∞occurs due to phase shif≥γts associated with different pr∏ αopagation delays. This occurs at ¶ specific points in the r↔≥αφeceiving space and simply move$φs the antenna by les↓βs than one wavelength to d ←isappear. In addition to si>αgnal cancellation,multipath p €ropagation also causes≥✔÷ symbol delay extension. Symbolε↕s from different paths ₽≠€♠arrive at different times±☆β, resulting in bit error ≤>if the delay is large.
4. Overcome challenges
4.1 RF Frequency Switc≤••hing
The 2.4GHz frequency is widely®♣ used for Wi-Fi, BluetoothΩ", and IoT short distance commu÷>→nications, making it increasingly crowdαδed. Its use for wire☆€←<less video transmiss≠™δ÷ion and control signals increase≈♥s the chance of signal interfe ♥♠rence and instability. ThisΩ•δ creates undesirable and often d±λ↓angerous conditions for dron≠ ™es. Using frequency switching to main'™tain a clean frequency will make dat©←a and control connections more reliabl♥'e. When the transmittλ÷er senses a crowded frequency, it a∏€utomatically switches to anot₽¥her band. For example©>π, two drones operating nearby usingα₩ the frequency would§Ω interfere with each o≠ther's communications. Au₽€βtomatically switching LO frequencies <₹↑☆and re-selecting bands will help ☆↓εmaintain a stable wireless link. Ad&∞™≈aptive selection of carrier frequα"βency or channel during pow♥ Ω er-on is one of the excλ♠γ→ellent features of high-ε₹end UAVs.
4.2 Frequency Hopping
Fast frequency hopping, widely use§≠©d in electronic countermeas≠β™♣ures (ECM), also helps avoid interfere↑↓nce. Usually if we want®₽₽ frequency hopping, t$®✔he PLL needs to be relocked at the e"εnd of the program. This invol®"←ves writing to the frequency register a∑φφnd, after VCO calibration time a£≠<nd PLL locking time, making the jump✘©♣' frequency interval close to ♣ ↕tens of microsecond<→s.
Figure 3. Schematic diagraα εm of frequency hopping scheme
4.3 OFDM modulation at the PHY layer
4.4 5G and WIFI technology
Wireless video for FPV UAV→✔¥s 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 impro∏σ✔ve the integration of the★δα∞ system on chip and the resulti≥ ♠ng high yield. A paradigm sh↑₹γift occurs when an entirely new radi'∏"$o, camera, or display concept appea→₩rs. The next generationΩ₩ of cellular and WiF÷&i technology, called ©×∏₩5G, will utilize dynamic b₽ eamforming to increase system gain and≥σ®≠ keep interference low. Togethπ♣er with more complex MIMO, this will fu™₩∏rther improve performance a♠☆nd transmission bandwidth. These conce∞≠↓pts are likely to be applie↔ d to future FPV systems as the techno&$÷logy matures. This results i>φn higher performance, g ™reater range, higher image quality, an$&d better reliability. It will enable drones to deal with mo§>re of the challenges we'φ♠→ face today, as well απφas challenges we haven't yet thoug↔Ωht of.
