What is the image transmission technology of UAV? Why doesn't WiFi have such a strong signal?

If the flight control is the brain of the drone, then the image transmission system is the "eye" of the drone, and we overlook the beautiful world from the perspective of God through the drone.

The UAV image transmission system adopts appropriate video compression technology, signal processing technology, channel coding technology and modulation and demodulation technology to transmit the video shot by the camera carried by the UAV to the remote receiver in real time in a wireless way.

If UAV image transmission systems are classified according to equipment types, they can usually be divided into two categories: analog image transmission and digital image transmission. Because the video quality and stability of digital image transmission are far better than that of analog image transmission system, digital image transmission is usually used in industrial applications.

UAV image transmission mainly uses 1.2G, 2.4G and 5.8G, and 2.4G and WiFi belong to the same frequency band. 1.2G is the specified frequency band. At present, there is no open amateur frequency band of 1.2G in China, which is only used legally by radio enthusiasts who have obtained qualification certificates. The 5.8G frequency band has been divided into open amateur frequency bands. There are fewer devices working at 5.8G and less interference. The high-frequency antenna can be miniaturized. However, the higher the frequency, the higher the cost of electronic components, the higher the accuracy of the antenna, and the easier it is to generate heat. The magnetizer near the transmitter is more sensitive than the low frequency, and it is more difficult to do high power than the low frequency.

At present, the mainstream technologies of UAV image transmission are OFDM, WiFi and so on. OFDM (Orthogonal Frequency Division Multiplexing) is a kind of multi-carrier modulation, which is more suitable for high-speed data transmission, and can also send a large amount of data in narrow-band bandwidth, and can resist frequency selective fading or narrow-band interference. However, OFDM also has some disadvantages, such as carrier frequency offset, sensitivity to phase noise and carrier frequency offset, and high peak-to-average ratio. WiFi image transmission is a cost-effective UAV image transmission technology. However, WiFi has many technical limitations, and many manufacturers directly use solutions to build it. No matter what the chip design format is, it cannot be modified. WiFi image transmission interference management strategy is not real-time and the signal utilization rate is relatively low.

The UAV image transmission system consists of four parts: remote server, airplane, storefront relay and mobile phone video control. For example, there are two high-power WiFi modules, which are embedded in UAV flying base and ground relay respectively.

The high-power WiFi module used by UAV has a transmission power of +28dBm and a transmission distance of 2 kilometers. The high-power WiFi module can not only transmit the video of the aerial camera in real time, but also transmit the control signal from the ground mobile terminal such as mobile phone.

Why can't the wireless router's WiFi signal travel so far? In the case of the same frequency, the UAV can transmit images over a long distance, while the wireless router's WiFi signal is so far away but there is no signal, largely because the wireless router, mobile phone and other mobile terminals have insufficient power. According to the national regulations, the transmission power of wireless routers should not exceed 100mW(20dBm), while the antenna gains are generally 3dBi and 5dBi. Some products use 6dBi and 7dBi gain antennas to highlight the wall penetration ability, and the signal strength improvement of antenna gain is still very limited, so it is good for wireless routers to cover 200m barrier-free.

In addition, low-power mobile phones and computers in daily life have also caused great limitations, leading to the phenomenon that the WiFi signal is obviously good, but it is still unable to access the Internet or the network quality is poor.

It's like two people entering the mountain at the same time. After a distance, the loud voice can be heard by the small voice, and the small voice can respond to the loud voice. The big voice can't hear anything, and naturally there will be no response.

Actually, mobile phones and so on. To access the Internet through WiFi, they need to go through the process of three-way handshake before they can truly establish a connection to the Internet. If the transmitting power of the WiFi signal terminal is very high, and the mobile phone exceeds the maximum distance that it can respond to the signal, it will cause the situation that the WiFi signal is very strong and can receive the information, but the information can't be sent out anyway.

In addition, the UAV is flying in an open outdoor place for image transmission, while the router's WiFi is generally used in a complex environment with many obstacles. Therefore, it is good that the WiFi signal of the router at home can cover 10 meter.

WiFi is one of the wireless technologies to realize UAV image transmission.

So why is the WiFi signal transmitted by drone maps stronger than ordinary WiFi devices?

1. Transmitting power of WiFi module (SKW77) for UAV image transmission:

2. Transmission power of ordinary WiFi module (taking MTK WiFi module as an example, the others are basically the same):

The maximum transmission power of UAV's WiFi module is 28dBm (about 640mW), while that of ordinary WiFi module is generally 20dBm, 100mW (18dBm in the legend), which is quite different.

This is the reason why the UAV image transmission WiFi signal is stronger than the ordinary WiFi module.

As we all know, drones must be used outdoors, while ordinary WiFi devices, such as mobile phones and AP, are generally used indoors. Different usage scenarios use different frequencies and powers, which is the biggest difference between them.

When the UAV is used outdoors, it requires long transmission distance, obstacle avoidance and penetration. , so it is generally used in the 2.4GHz band, because the 5.8GHz band has poor penetration because of its high frequency. And the power will be higher than the indoor WiFi module. What is the maximum transmission distance of outdoor WiFi module?

Let's look at the data of UBNT's bridge products, one of the best outdoor bridge products. The bridge of UBNT adopts TDMA technology based on WiFi protocol, with high transmission power and special antenna, and the effective transmission distance can reach 15km.

For ordinary indoor WiFi devices, the distance between devices is relatively close, usually several hundred meters. Moreover, considering the radiation to human body, the maximum power of general wireless AP/ router is 20dBm( 100mW). Some wireless routers have enhanced their functions, but the actual increase is the transmission power.

That is to say, the WiFi signal is not always weak, but will adopt different signal strengths according to the usage scenarios and distance requirements.

Image transmission is essentially a TDD information distribution and transmission system, just like WiFi.

In the application of UAV, image transmission is mainly used to send the real-time data collected by the camera to the user terminal.

Image transmission can work at 400M and 800M, of course, the vast majority of civil drones work in the 2.4G engineering medical frequency band. In the frequency band of 2400-2083.5MHz, the flight control of UAV uses frequency hopping signals similar to WiFi. This kind of frequency hopping signal disperses the RF energy, and then increases the anti-jamming ability, and the bandwidth is about 18M. The image transmission system occupies about ten megabytes of bandwidth. If you want to transmit high-quality images at high speed and stability, then image acquisition, coding and compression all need great technical strength. At present, DJI does its own flight control and map transmission, and its strength is quite good.

Because UAV is also an integrated system, it has limited mass, limited power and limited space area. The power of image transmission must be limited.

The power of WiFi you mentioned is not as high as that of the picture transmission system of the drone. This should be an illusion, because our country's Ministry of Industry and Information Technology stipulates that in the engineering medical frequency band of 2.4GHz, basically the maximum transmission power can only reach 0.5 watt.

The reason why there is the phenomenon you mentioned is roughly because the density of WiFi transmission content is not as good as that of UAV image transmission system.

This question is too general to be answered accurately.

Because such questions can't be understood in a few words, but the more you answer or answer in a technical detail, the more you can understand.

Because there are many ways and means to realize the technology, the technology is deep, and it can't be studied and understood in just a few books. Therefore, if the answer is not accurate, I am afraid of misleading children, and I will only answer according to my personal practical experience.

First of all, the maps carried by drones are divided into toy level, airplane model level and industrial level. The technologies used at different levels are also very different.

Here is just a brief talk about the common graphic transmission technology of industrial drones.

There are many technologies used in UAV image transmission, and the commonly used cofdm technology, namely code division orthogonal frequency division multiplexing, is an open technology.

WIFI is originally a short-range communication, and there is no signal far away, which is determined by application scenarios and technical standards.

Actually, to put it bluntly, radio transmission is electromagnetic wave. Everyone knows that the longer the wavelength, the less likely it is to be disturbed. Just choose the right channel, mainly because there are fewer people flying and fewer interference sources. You let 10 people and drones fly in the same frequency band as stable as possible.