Since one of the key features of Dajiang’s new Mavic product, the “7 km picture transmission†feature has been announced, in addition to its surprise, it also ignited people’s interest in the long-distance image transmission function of drones. Around this topic, we recently discussed ( link ) some of the implementation methods for drones in long-distance maps .
However, as one of the key technical barriers for consumer drones and a subset of the long-distance wireless transmission field, the long-distance wireless image transmission function has always been a key point in the technological development of the UAV industry, and it is also a popular geek friend. With a keen interest in a technical topic, people hope that from the point of view of technology realization, we can use the formula and principle to conduct a more in-depth and detailed discussion on the topic of image transmission. To this end, we have the honour to hire an industry tycoon today. He will set out from the technical details and introduce us to the core issues of the graphic transmission system design , 2 design red lines, 4 basic design parameters, and 8 design priorities. , as well as a sound design proposal.
 Guest introduction
Zhang Hao is a chief engineer of Beijing Linghe Electrical and Mechanical Co., Ltd., a researcher at the China Aerospace 9th Institute and an expert in electrical instrumentation. He has participated in the development of multiple types of aerospace monitoring and communications equipment. Is also a 20 years old model aircraft enthusiasts.
This open class was conducted in the form of a speech. The following is the speech of Zhang Jian:
 1 core issue, 2 design red linesWe can find out from the name of "picture transmission" that this is not a professional definition. Probably from some senior model aircraft developed from the mouth to play at home, and it only exists in the field of consumer drones. Professional aerospace vehicles do not have independent video image transmission equipment, and real-time video is part of the transmission over a wireless data link or a channel in it. It has also been included in aerospace telemetry systems.
The UAV transmits telemetry signals (including video images and various sensing information) . The terrestrial telemetry receiving system usually includes an automatic tracking antenna array and a synchronous multi-channel receiver. While receiving the data, calculates the position of the UAV, speed, heading and other information in real time. This is the most traditional one of the most ancient telemetry modes. In the absence of GPS, in addition to optical measurement can only rely on radio telemetry data, this model is a passive radar.
With the development of satellite navigation, drones mostly use GPS and satellite data links, but direct telemetry still cannot be replaced. For decades, as a large aerospace country, the level of domestic telemetry and remote control has been the world's first echelon. Picture transmission equipment has also been fully developed as part of its communication system. Picture transmission of medium-range unmanned aerial vehicles over 150 km is common, but prices of over 1 million are relatively expensive.
So for consumer drones, there are enough things to learn from. I think the core issue of consumer drones is only the "cost problem . " Do not have to doubt how fast it can be communicated, because wireless communication technology has developed to today, no one will doubt the return of 1080P Mars signal. What we really want to discuss is the question of price. At present, the price of 1080P graphics products on the market is basically within 1,700 US dollars, and the hardware cost control within 700 US dollars has become the first red line of consumer drone graphic transmission design .
Of course, there is also a second red line, that is, regulations. The highest legal documents China Radio Management is "The People's Republic of China Radio Regulations" for the legislature of the State Council and Central Military Commission, performed by the radio regulatory agencies at all levels of regulation. If the user wishes to apply for a separate license for the image transmission, he will need to obtain the “Radio Transmission Equipment Type Approval Certificate†first, which is based on the provisions of the “Radio Frequency Division Regulations†in the relevant technical specifications of the radio transmission equipment. Obtaining a professional radio license is not inoperable, but there is no way to promote it in the area of ​​consumer drones.
For professional aerospace vehicles, the time-frequency spectrum monitoring and control division has left a special band, and consumer drones can honestly give way to (ITU Radiocommunication Sector, the International Telecommunication Union Radiocommunication Bureau) of the ISM ITU-R Frequency band (Industrial Scientific Medical, Industrial Scientific Medical Band) . 13.56Mhz, 27.12Mhz, 40.68MHz, 433Mhz, 915Mhz, 2.4Ghz, and 5.8GHz are all within 1W without license emission. Wherein, the 433MHz band and it is often difficult to satisfy high-definition image transmission bandwidth requirements, frequency bands 915Mhz GSM half has been occupied, and L-band bandwidth is not rich, S 2.4GHz band will become distant obtained 1080P The first choice, but 4K or higher resolution graphic designers find it difficult to find the S band bandwidth cheaper, C band 5.8G can be made wider. However, at the same transmit power and receiver sensitivity compared with 5.8G 2.4G communication distance of only 41.4%, and it is more sensitive to water vapor attenuation, then the actual communication distance less than 30%, both advantages and disadvantages.
Note to editors: The L, S, and C bands mentioned here are usually classified as follows:
The first two red line identifies two basic parameters:
The first parameter: 1W of transmit power (+30dBm)
Second parameter: 2.4G or 5.8G ISM band
Maybe I'm sure many engineers will be disappointed 1W transmit power, then we will make them then I regain confidence, to see the future in the end be able to realize how far communication distance. Continue to set two parameters:
The third parameter: -105dBm receiver sensitivity
The fourth parameter: 3dBi transmit and receive antennas (It is difficult to find a worse antenna than this one)
According to the free space radio wave propagation law:
P Transmit power - P Receiver sensitivity + G Transmit antenna gain + G Receive antenna gain = 32.44 + 20lg (f frequency) + 20lg (D distance)
30dbm-(-105dbm)+3dbi+3dbi=32.44+20lg(2400Mhz)+20lg(D)
Solution D = 111.6Km Remove 3dB Rain attenuation and 3dB margin The actual communication distance is approximately 50km. This result will not let you restore confidence in 1W launch power. After all, we only used the most common 3dBi antenna. Everyone must have noticed the -105dBm receiver sensitivity. Yes, this is the key to success. Many low-cost digital maps are developed using WIFI devices or suites. A fundamental problem of unsatisfactory distance is the receiver sensitivity.
Take 802.11g/b OFDM mode 54Mb/s rate as an example, its receiving sensitivity is only -68dBm, consider the chip manufacturer raises receiving sensitivity to -70dBm. At +25dBm (316mW) transmit power, there are:
25-(-70)+3+3=32.44+20lg(2400)+20lg(D)
Solution D = 1.1Km, so the actual communication distance is about 500 meters. Therefore, many old RF engineers are admiring the principle of winning receiver sensitivity. Increase the transmission power of the costs are often too high, four times the transmission power can be doubled from this account between 1W and 4W also be able to count clear.
 8 design priorities
Next discuss eight design priorities:
The first point, video codec
At present, the encoding and decoding schemes adopted by 1080P image transmission products are basically hardware solutions, and the real-time performance of hardware solutions will have certain advantages. Unlike other areas of image transmission, image transmission UAV demanding for delay, is especially FPV flight. For aerial, if the image transmission delay is too large, head in hand it is easy to overshoot based video control PTZ process. Most of the 1080P graphics products on the market use the TI DaVinci and Hass programs. TI has not yet introduced hard-solved H.265 products, only H.264 DSPs, but it is more versatile . The Hays H.265 solution has obvious cost advantages and is faster to develop.
For the current codec algorithm , the 1080P 30FPS signal is compressed to a 6Mb/s stream, and the effect is already satisfactory for broadcast applications. Without handshake wireless communication (similar to UDP), error events will occur, long distances error rate will be very high, but the real factor is the deadly stream of critical information is lost. Such as frame synchronization data, once lost will result in the loss of the entire frame. It must be redundant processing of critical information in order to apply to a single radio channel. While the receiver baseband demodulation algorithm clock extraction, bit sync, word sync, frame synchronization must be designed for a high error rate, or the small error will produce catastrophic loss of image.
The picture above shows the video coding standard (less H.265)
Above is TI Da Vinci
Pictured above is Hass chips
The second point, low noise amplifier (LNA)
The noise factor of the first-stage LNA is a decisive link in the overall receiver design. To achieve better than -105dBm reception, the LNA must be faced with the impact. Old RF engineers will be more trusted discrete narrow band LNA FET, seemingly simple, but in reality complex, often online simulation vector or a month scrutiny is on the ADS. Although both ends have been matched to 50 ohm integrated LNA chip is also very easy to get noise figure (than as HMC376, ADL5523) of less than 0.9, but works with those old radio engineer than the affirmation or lose at the starting line.
The above figures are ADS simulation and vector network simulation, respectively
The figure above shows the overall composition of a RF baseband to the front end
The third point, local oscillator (LO)
For a long time, the phase noise of the local oscillator is not actually a problem for RF engineers. This is actually a cost issue, as long as you don't cherish $20 in frequency, you can get a phase noise of -212dBC/Hz (like the LMX2531). If your local oscillator budget is only $6, choosing a common PLL (such as the AD4360's -167dBC/Hz) will affect the overall signal-to-noise ratio. Receiving the final decision and the sensitivity magnifications of the RF signal is not, but overall noise figure and a baseband demodulation threshold of the receiver (SNR).
FIG 1080P pass current products on the market are invariably used ADI small base station plan AD9361 / AD9371, although the price expensive side, but compared DAC + AD8349 and AD8347 + ADC solution was more easily processed.
There is a good news that will cheer you up. According to Henry Ho of RichWave, they are integrating the RTC6763 several USD integrated RF chip with the Hass H.265 platform. The RTC6763 has a receiving sensitivity of -86dBm in a QPSK modulation mode of 4Mbit/s. With a 1W transmit power in the S-band and a 3dbi antenna at both ends, the RTC6763 has a communication capability of more than 7km.
The picture above shows ADI's small-cell solution
The picture above shows the solution of RichWave, because TI does not have DSP of H265, and the following DSP is actually the code of H264.
The fourth point, the antenna
The low-gain 3dBi antenna is used to calculate the communication distance. Actual transmission and reception can use 6dBi antenna, even more than 10dBi focusing antenna or array antenna on the ground. Every 6 dB increase, the communication distance doubles. If both transmitting and receiving are changed from 3dbi to 6dbi, it is to double the communication distance.
Within certain limits, don't miss the opportunity to get a farther communication distance from the antenna. The choice of pattern and polarization is the most problematic place in use. Many people use polarisation for transmission, and horizontal polarisation for reception, which compromises the communication distance. The ground receiving antenna should be as consistent as possible with the polarization of the transmitting antenna.
The illustration shows different directions of polarization
If you do not consider the height of vertical climb, the launch antenna on the plane is more suitable for the use of vertically polarized omnidirectional antennas. The main lobe angle can be very small, and the side lobe can be relied on at close range, so that the horizontal communication distance can be ensured to the maximum .
The figure shows the direction of the GP. The left side is the omnidirectional in the horizontal plane. The right side is the vertical plane. The gain is very low directly below.
If there is a need for vertical climb, a circularly polarized antenna with weak directionality can be considered. For GPS, a right-hand polarized antenna is selected .
The gain of the antenna reflects the ability to concentrate electromagnetic energy, and the higher the gain, the stronger the directionality. The higher the angle of the main lobe of the antenna radiation gain is smaller, high-gain antenna to get some experimental communication distance still not as good as low-gain antenna, because the directional problem. For example: high-gain directional antenna is often only a few degrees to the angle of ten several times.
The illustration shows a directional antenna pattern
There is no real sense of the three-dimensional omnidirectional antenna. An omnidirectional antenna means that its pattern has omnidirectional characteristics in a certain plane . For example, with a vertically polarized GP antenna, it is just omnidirectional on the horizontal plane, as shown in the figure below.
The fifth point, baseband (modulation and demodulation)
Many engineers choose high-order modulations as soon as they arrive, such as the 16QAM constellation and the 8PSK constellation. It is true that 1080P video requires transmission speed, but in fact it is sufficient for 3M QPSK of 6M stream, and lower-order modulation has a lower demodulation threshold to achieve better reception sensitivity.
Commonly used modulation methods
If you can further improve the compression ratio, BPSK is not a bad idea. 4G and WIFI with different transmission 1080P FIG not need such a high degree of speed, but requires higher receiver sensitivity, then get farther communication distance.
I found that a lot of people have a misconception: they think according to Shannon theorem, the faster the transmission speed, the lower the reception sensitivity. This is a conceptual error (it really does not have a dime relationship with the fragrant theorem). Let 's take a look at how Shannon says:
To be precise, the fragrant theorem tells us that the higher the modulation order (higher C/B), the higher the SNR requirement and the lower the receiving sensitivity . For example, the signal-to-noise ratio S/N=3 for the QPSK C/B=2 demodulation, C/B=1 for the BPSK, and the signal-to-noise ratio S/N=1 required for demodulation. If C (the data rate required to transmit video) does not change, as long as the RF bandwidth B is increased, then the C/B is reduced and the SNR requirement is not so high, that is, the low-order modulation is used. He reflects the ability of the band to transport information. Just like the train skin and cargo, the absolute quantity has nothing to do with the fragrant theorem.
The real effect of RF bandwidth is that its width determines the input thermal noise power, which is called the inherent noise floor, KTBRF = KTBRF (Hz), where K is Boltzmann's constant temperature, T is 290K room temperature, and BRF is RF bandwidth. Calculate the enough 3M bandwidth as described above to see how much hot input noise is mixed in the received signal.
KTBRF(log) = 10log(1.38110^-23W/Hz/K290KX3.84MHz1000mW/W) = -110dBm
Seeing this number, you will understand why the receiver sensitivity (third parameter) is set to -105 dBm. In fact, I just let the received signal be several dB stronger than the noise. I have less than 4 times the signal to noise ratio ( close to 4).
For the multi-machine communication problem of 1080P image transmission, taking into account the limited bandwidth of the ISM band, there are not so many frequency division channels available for frequency division (FDM) and frequency hopping (FHSS) of radio frequency, and time division (TD) and code division (CD) are also Not applicable, so baseband OFDM split orthogonal frequency division channels may be the most promising choice.
In fact, OFDM is not a modulation mode. It is only a frequency division method. When we listen to FM radio, people's stations and traffic stations are far apart at different frequencies, which is FD. If two stations are close to each other, overlapping and not interfering with each other is OFDM.
Schematic, horizontal axis is frequency
Baseband demodulation algorithm will play a decisive role in the anti-jamming problem, we can not use the picture transmission process to prevent the neighboring Pharaoh to launch on the same frequency, so find the appropriate baseband algorithm will occupy more than 70% of the entire picture transmission design Workload.
Sixth, line-of-sight communication distance
The line-of-sight communication distance in free space cannot be tested by ground pull, because the earth is round and its visible distance is as follows:
In the formula, h1 and h2 are the height of the transmitting and receiving antenna respectively, and K is the meteorological factor. Assess the LOS communication distance to fully understand the influence of the Fresnel zone.
In long-distance situations, the aircraft only has enough height to reach the theoretical communication distance.
Seventh point, amplitude modulation caused by vibration or rotation
Many aircraft have self-rotation or self-oscillation. These periodic vibrations are transmitted to the antenna to form amplitude modulation. Usually, they are relatively slight, but they also reduce the signal-to-noise ratio and can even be directly observed through the eye diagram. The demodulation algorithm tries to avoid this effect as much as possible.
This situation may not be easy to encounter, but please do not forget the amplitude modulation brought by the antenna vibration.
The eighth point, multipath issues
Multipath problems in air-to-ground communications can lead to symbol-bit crosstalk. If a symbol bit representing a 1 or 0 delay arrives, it will overlap with the symbol bit following it, causing bit errors.
WIFI usually cannot solve the multi-path problem of large delay. In order to pursue the transmission speed and cost, the protocol only considers the propagation delay within a small space. This is one of the reasons why WIFI is not suitable for long-distance communication. The baseband signal of the transmitter can use a unit impulse cosine-scattering filter to achieve waveform shaping. The “smearing†of the time domain waveform is attenuated quickly, and inter-symbol interference can be reduced. Another benefit of the raised cosine filter is the software bandwidth limitation. The SAW filter can be omitted in front of the power amplifier (PA).
If you want to do a good image transmission product, you can't stay in the concept of fiddling, and you can't rely on integrated chips and existing communication protocols. Be sure to start with RF fundamentals and each dB must compete. Lost in the slightest difference.
In the end, Zhang Qian emphasized that every dB must be an attitude to engage in radio frequency. The insertion loss of any cable, the insertion loss of any connector, the return loss of any one antenna, and the accuracy of the microstrip line on any PCB can be fatal.
The picture above shows the radio frequency part of the transmission and reception of picture transmission developed by Zhang Jian.
 question Time The following are some of the questions of the open class group friend about this lecture, and Zhang Hao's answer.
Q1: You mentioned just now that “For the multi-machine communication problem of 1080P image transmission, considering the limited bandwidth of the ISM band, there are not so many frequency division channels (FDM) and frequency hopping (FHSS) of the radio frequency. TD) and code division (CD) are also not applicable, so OFDM-based division of orthogonal frequency division channels at baseband may be the most promising option. What is the reason why time division and code division are not applicable in this passage?
A: Time and score are exchanged at the expense of speed and bandwidth. It is naturally easy to understand at the time. A staircase can only have one person at a time, and two people will line up. For code division, on the one hand, the bandwidth requirement after the spread spectrum is higher. On the other hand, the spread spectrum coding itself uses a string of code instead of a symbol, which also sacrifices the speed. The code division itself solves the multiple access problem and can also bring about code gain and anti-interference, but the channel does not need much for picture transmission.
Question 2: You just mentioned that “WIFI cannot usually solve the multi-path problem of large delays. The protocol only considers the propagation delay within a small space in order to pursue the transmission speed and cost. This is also because WIFI is not suitable for long-distance communication. One of the reasons for this.†So, what are the characteristics of an agreement for long-range wireless communications?
A: The first is the problem of high BER. Because the data link layer, the Internet layer, etc. of WIFI are originally designed to run the TCP transport layer. Picture transfer protocols do not handshake, do not do retransmission mechanisms, but need redundancy of key information, ordinary information will not bring mosaics, and picture transmission does not require WIFI and 4G so fast, then multi-channel, so low cost, So many devices are used in the same space. So the content of many existing agreements is redundant.
Question 3: Why is COFDM not preferred?
A: In fact, COFDM and OFDM are the same thing, and Coded's OFDM is the first choice.