What is Fourier Transform in layman's terms
(unmanned surface ship, UPS for short) is an unmanned surface ship. Mainly used to perform dangerous and unsuitable tasks for manned ships. Once equipped with advanced control systems, sensor systems, communication systems and weapons systems, it can carry out a variety of military missions in war and out of war, such as: B. Reconnaissance, search, detection and demining, search and rescue, navigation and hydrogeographic surveys. Anti-submarine combat, specialty combat and patrol, combat against pirates, counter-terrorism attacks, etc. In recent years, various countries have paid great attention to research on unmanned boats. Among them, the United States and Israel have always been leaders in the development and deployment of unmanned surface vehicles.
Status of unmanned boats
The US Navy has been investigating unmanned surface vehicles since the 1990s. In 2007 the US Navy published the "Navy Unmanned Surface Vessel Master Plan", which assigned 7 tasks to the unmanned surface vehicle. At the same time, it also defined the type of ship, size, and standard of the unmanned surface vehicle, which was marked the United States' unmanned surface vehicle. The surface vehicle entered the formal development phase. Typical unmanned boats developed by the USA are "Spartan Scout", "Ghost Guard" and "Piranha". A few days ago, the US submarine technology company in 2013 exhibited a new type of unmanned surface vehicle CAT for the surveillance and reconnaissance of inland rivers and port waters at the French exhibition for military and police equipment. Surveyo, CAT. The Surveyo unmanned surface vehicle uses a type of catamaran with remote control and autonomous navigation modes. It is also equipped with a small remote control submersible, such as the company's "Observer" model, which is equipped with a high resolution camera and is optionally available equipped with video sound for cloudy water. Satisfied. The land based command station can use a WiFi connection to visualize and control all of the embedded software on the boat in real time. The first-generation unmanned vehicle "Protector" developed by Israel is considered to be the most sophisticated unmanned surface vehicle today. The boat has started mass production and is armed with troops. The Singapore Navy is its first overseas user. In February 2014, Israel Aerospace Industries Corporation launched a new generation of unmanned surface vehicles called "KATANA". The unmanned vehicle includes automatic navigation and collision avoidance systems and is equipped with electro-optical cameras and infrared cameras, line of sight and non-line of sight, radar and weapon systems. It can perform a variety of tasks in a wide range of protecting exclusive economic zones and maritime borders. It can provide real-time intelligence imagery by identifying, tracking and classifying near and far targets and launching attacks on targets according to instructions, can be automatic or human operated. Remote control.
In comparison to foreign unmanned boat research, domestic technology is also catching up. The research institute represented by Shenyang Automation Institute has surpassed foreign counterparts in some areas such as automatic control, image recognition and navigation, etc. Major domestic research institutes include Harbin Engineering University, Shenyang Institute of Automation, Dalian Maritime University, Shanghai Maritime University, Jiangsu University of Science and Technology, National University of Defense Technology and China Aerospace Science and Industry Corporation It has always been the field of unmanned boats. A data connection with high bandwidth, long distance, low latency and low power consumption is very important for unmanned boats. In 2015, a certain type of unmanned boat from the Shenyang Automation Institute used Anykey-AV8 to implement a wireless data connection for high bandwidth industrial Ethernet long distance calls.
In recent years, private companies in China have successively entered the field of unmanned military model boats and civil unmanned boats. At the international high-tech exchange conference in Shenzhen in 2015, an institution demonstrated a new type of unmanned boat. The boat can be equipped with radar, sonar, and cameras to gather environmental information from nearby. It can also be equipped with a portable weapon system as needed and can be used Satellite and Anykey-AV8's wireless communication technology for private networks performs remote control, cruises on the sea surface, searches and rescues, and conducts submarine missions at sea by.
During the 2008 Olympic Sailing Games in Qingdao, the "Tianxiang No. 1", jointly developed by the China Aviation Science and Industry Corporation and the Atmospheric Observation Technology Center of the China Meteorological Administration, performed a series of tasks to measure environmental parameters such as wind direction and speed, water temperature and Air temperature in the specified range. [This boat is the first unmanned surface boat used for engineering practice independently developed by our country. Its emergence is of great importance for the detection of aquatic areas and the prediction of marine disasters in our country.
Unmanned boat communication
Early communication relied mainly on relatively backward communication methods such as digital radio and WiFi. The communication station has a low transmission rate and a poor confidentiality, does not have the characteristics of anti-interference, anti-multipath fading, mirror radiation, etc., and has defects such as a high bit error rate and a short distance. The WiFi rate is high, but the communication distance is short, which is practical.
Surface wireless communication features
The communication properties of radio waves on the water surface are very different from those on land. Communication has its own environmental characteristics: firstly, the sea obstacles on the site are less obscured, and the direct result of this is that the radio waves travel a large distance. When radio waves propagate in the ocean, the diffraction loss is therefore less than on land. At the same time, the propagation distance increases, which increases the reflection of radio waves, and when electromagnetic waves propagate on the water surface, the reflection coefficient is relatively large at a small viewing angle, so that the influence of reflected waves is greater than on land. When there are more obstacles, the radio waves will be blocked. Therefore, it is often believed that the main obstacle is isolated and the corresponding one on the sea is mainly caused by the single-edged tip diffraction caused by the waves caused by bad weather or when there is island separation between the ship and the ship The unit arc diffraction, the earth is a sphere, so no plane on the ground and the water surface, so the reflection of the electric wave by the water surface is actually the reflection of the electric wave on the smooth ball. It is also known as the mirror reflection effect of radio waves.
In addition, there is a certain atmospheric absorption damping, cloud damping and rain weakening in the environment with high humidity and high fog on the water surface. These attenuations can be compensated for by increasing the power redundancy in the system design. The most fundamental challenge for unmanned wireless boat communication is the multipath interference of radio waves. Since the antenna height of the unmanned boat is generally not high, it is close to the surface of the water and the transmitted radio waves pass the waves, islands and ship obstacles. The strength of the received signal is combined by the superposition of the direct waves and the reflected waves. Multipath effects can lead to signal fading. The electrical length of each path changes over time, so that the phase relationship between the component fields arriving at the receiving point also changes over time. The random interference of these component fields forms the fading of the entire received field. The phase relationship between the components is different for different frequencies. Therefore, their interference effects also vary with frequency. This property is known as frequency selectivity. In broadband signal transmission, the frequency selectivity can be obvious and form an intermodulation. Correspondingly, because different paths have different time delays, the simultaneously transmitted signals before and after the receiving point are scattered because they follow different paths while the narrow pulse signals overlap back and forth. This overlap makes the data unsolvable so that conventional radios with a carrier can hardly function normally in a reusable environment.
Anykey-AV8's core technology is Orthogonal Frequency Division Multiplexing (ofdm / cofdm). Orthogonal Frequency Division Multiplexing channel. Orthogonal signals can be separated by using related technologies on the receiving side, which can reduce mutual interference (ISI) between subchannels. The signal bandwidth on each subchannel is smaller than the relevant bandwidth of the channel, so each subchannel can be viewed as shallow fading, which can avoid cross-talk between symbols, and since the bandwidth of each subchannel is only a small fraction of the original channel bandwidth channel balancing becomes relatively easy.
OFDM technology is the basis of the industrial specifications of the HPA Alliance (HomePlug Powerline Alliance). It uses discontinuous multi-tone technology to combine a large number of signals at different frequencies called carrier waves into a single signal to complete the signal transmission. Because this technology can transmit signals under interference conditions, it is widely used in transmission media that are prone to external interference or have poor resistance to external interference. The data stream during transmission is divided into several parts, the radio channel is divided into N channels and the transmitted data stream is divided into N data pieces, which are each transmitted on the N channels. Although the overall channel is not flat and has frequency selectivity, each channel transmits on its own channel, which breaks the flatness. This is not only advantageous for increasing the cycle time of symbols in the transmission process, but can also reduce the interference between symbols. The most important point is that adding a guard interval can maximally eliminate the interference between symbols.
OFDM is a special multi-carrier modulation technology. The transmitted information is transmitted on several subchannels by serial-parallel conversion, in contrast to conventional modulation, in which only one frequency can be transmitted at a time.Signal, OFDM can transmit several signals at the same time on orthogonal frequencies, whereby the channel bandwidth can be used to the full.
In the OFDM system, the size of each transmission symbol rate is between ten bits / s and ten bits / s, and a serial-to-parallel conversion must be performed to convert the input to serial bit, the stream is converted into OFDM symbols that are transmitted can be.
Since the modulation mode can be changed and adaptively adjusted by itself, the modulation mode of each subcarrier can be changed. The parallel conversion is also different. The reverse process is carried out on the receiving side and the data transmitted by each subcarrier is converted back to the original serial data.
When an OFDM symbol is transmitted in a multipath radio channel, the frequency-selective fading results in certain groups of subcarriers suffering considerable attenuation, which leads to bit errors. These are located in the frequency response of the channel. The zero point an causes the information transmitted on the neighboring subcarriers to be destroyed, which leads to a series of bit errors in every signal. Compared to the case where a number of errors occur continuously, most forward error correcting codes work more effectively when the errors are evenly distributed. Therefore, to improve system performance, most systems use data encryption as part of the serial to parallel conversion work.
In OFDM, modulation and demodulation are essential. There are several sub-carriers during the modulation process and we perform differential phase key modulation on the signals on the sub-carriers, which is called PSK modulation for short. The principle of modulation and demodulation is as follows:
From technical analysis, it can be seen that OFDM's FFT maps the information of each symbol N times, which is expanded in the time domain, which corresponds to the effect of time diversity. As the symbol interval becomes larger, the receiving end takes into account the processing of N samples together in the acquisition and treats it as one symbol. The symbol interval is larger than the time delay and there is no multipath influence. Multipath only occurs when the time delay is longer than the symbol interval.
Due to the addition of the Fourier transform of the CP, the ordinary convolution is converted into a circular convolution, so that the end data caused by multipaths circulate forward as long as the multipath If the time delay does not exceed the CP length, no data information is lost. (The meaning of the next symbol is not lost.) That is, there is no interference between symbols.
Since Anykey-AV8 uses orthogonal frequency division multiplexing technology and error control technology, the number of carriers is up to 1024. The easy understanding is that 1024 ordinary digital radio stations transmit time data and received data at the same time, its anti-multipath effect is very significant. It is very well suited for use in environments with severe multipath interference at sea.
Communication method of the unmanned boat
Many domestic ships have adopted industrial Ethernet or TCP / IP technology. TCP / IP is the abbreviation for Transmission Control Protocol / Internet Protocol. The Chinese translation is called Transmission Control Protocol / Internet Protocol, also known as Network Communication Protocol Foundations of the Internet. Agreement. It consists of the IP protocol of the network layer and the TCP protocol of the transport layer. TCP / IP defines how electronic devices connect to the Internet and how data is transferred between them. The protocol uses a 4 tier hierarchy structure and each tier invokes the protocol provided by the next tier to meet its own needs. In layman's terms, TCP is responsible for detecting transmission problems, sending signals when there is a problem, and requesting retransmission until all data has been safely and correctly transmitted to the destination. And IP gives an address for every networked device on the Internet. As industrial Ethernet technology is mature, has a full checking mechanism, strong anti-interference capability and broad technical versatility, it can be connected to various cloud platforms via routing gateways. Almost different bus protocols can be converted to Ethernet using converters. B. CAN bus (Controller Area Network), RS232 / RS485, high-resolution video signals etc. can be converted to Ethernet without any problems. This promotes the integration of various devices on board. Ethernet is used for communication between the integrated information processing system on board, the integrated monitoring system of the ground station and the autonomous navigation control system. Use Ethernet to send control commands and real-time status information of the unmanned boat to realize remote access and remote control of the unmanned boat. The ground station integrated monitoring system and on-board information processing system are used as the information center of the unmanned boat bottom and board monitoring system, respectively, and are connected to the ground station electronic map terminal, the ground station video monitoring system and the on-board electronic map terminal, communication between the autonomous navigation control system and others Monitoring devices also use the Ethernet TCP protocol. The systems that use Ethernet communication are:
(1) The integrated monitoring system of the ground station and the integrated processing system for onboard information carry out the transmission of control commands and real-time movement status information of the unmanned boat.
(2) Integrated information monitoring system and autonomous navigation control system on board. The communication content includes trajectory, control commands and real-time movement status of the unmanned boat.
(3) The unmanned integrated information system for boat information and other monitoring and navigation devices on the boat, the communication content includes electronic map information, radar information and video surveillance information, and the communication is mainly image information.
(4) The ground station's integrated monitoring system and other monitoring and navigation equipment for the ground station. The communication content includes electronic map information, radar information, video surveillance information, mainly image information. Including GPS / Beidou, Inertial Navigation System, Millimeter Wave Radar, Sonar, Autopilot, Lidar, Video, etc. Others use the serial protocol, and finally, serial communication via RSS232 to Ethernet, which is built into the Ethernet communication system, is easy and quick. On the one hand, it supplies real-time position and orientation data for the information-integrated processing of the unmanned boat. On the other hand, it controls the command. It is transmitted to execution devices such as the main engine propulsion system, and at the same time the real-time state of motion of the unmanned boat detected by the equipment is fed back to the information-integrated processing system. The data transmission radio is mainly based on the RS232 / 485 interface. In terms of interface and communication protocol, it has not adapted to the communication data link requirements of modern unmanned boats.
The boat unmanned information network and control system consists of three main parts that use serial communication:
(1) Integrated processing system for unmanned boat information and integrated navigation devices for attitude and azimuth. The integrated navigation device for attitude and azimuth follows the NMEA0183 protocol which is based on the RS232 serial communication standard and can be implemented point-to-point, point-to-multipoint data transmission. Therefore, wired serial communication is used.
(2) Integrated unmanned boat information processing system and host propulsion system. The host drive system is controlled by a one-chip microcomputer so that a wired serial port is used to communicate with the on-board information processing system.
(3) Integrated processing system for unmanned boat information and integrated monitoring system for ground stations. Serial communication is not the main communication, mainly used for some devices without an ethernet communication interface, this data conversion connect to the main communication system for ethernet communication.
Anykey-AV8 is an IP core, a standard transparent Ethernet transmission interface. It can easily be docked to the unmanned boat control platform. Due to the additional real-time video transmission technology compared to the previous transmission of data and control instructions, the current unmanned boat communication places higher demands on the bandwidth. Take, for example, high-resolution three-channel video with a resolution of 1080P and H. 264 encoding. Each channel requires a rate of at least 6 Mbit / s, and the three channels require a total of 18 Mbit / s.With control commands and data, the required bandwidth is 20 Mbit / s.Conventional digital radios cannot achieve this bandwidth. Anykey-AV8 offers an available to meet demand, Anykey AV8-MSC base station equipment provides data bandwidth of up to 100Mbps downlink and 50Mbps uplink and supports point-to-multipoint applications with a delay of < 10 ms that can meet point-to-multi-point unmanned boat network data transmission.
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