Design of UAV wireless communication system

The increasing demand for communication has led to a surge in the development and deployment of wireless communication technology in unmanned aerial vehicles (UAVs). As a result, there has been extensive research conducted on UAV wireless communication systems over the past five years. This article provides an overview of the advancements made in three key areas of research: transmission of UAV wireless communication systems, wireless communication networks, and design optimization of UAV wireless communication systems. The article summarizes the existing literature on these topics and highlights the main findings and challenges faced by researchers. In addition, it explores the principles and implementation approaches of three types of UAV wireless communication systems, including a UAV wireless communication system, a UAV wireless transmission system based on the AD9361+ZYNQ7045 platform, and a UAV communication system based on wireless data transmission module. Overall, the research in UAV wireless communication systems has shown great potential for future applications in various fields such as aerial photography, disaster monitoring, and search and rescue operations. By improving the efficiency and reliability of wireless communication between UAVs, the technology can enhance their performance and increase their usefulness in a range of industries.


Introduction
UAV is a non-carrier aircraft operated by electric remote-control equipment and programmed control devices.As a typical means of air communication, UAV communication can provide users with high reliability, high flexibility, easy deployment and communication services.It is regarded as a supplement to future air network solutions and ground communication.In the past, research of UAV technology usually focused on navigation, control and autonomy.In contrast, the communication problem of UAV was either ignored or regarded as part of the control and autonomy problem [1].Since entering the 21st century, with the development of information technology, people's demand for communication has been increasing.Wireless communication technology of UAV has been widely concerned and studied.
Through literature reading and analysis, the research on three aspects.
(1) About the transmission of UAV wireless communication system.Zeng proposed a based-on radio frequency agility transceiver AD9361.The actual test results of the hardware platform show that the design can achieve reliable data transmission between UAV and ground station, and optimize the cost, volume, weight and power consumption of UAV [2].The UAV wireless communication system designed by [3] adopts advanced Orthogonal Frequency Division Multiplexing of channel coding alldigital modulation and demodulation technology and MPEG2/MPEG4 digital compression coding technology, which has the technical characteristics of multi-carrier and strong anti-multipath capability.Zheng proposed a method to realize the communication guarantee of the integration of space and earth [4].
(2) About the wireless communication network of UAV.Higashiura developed a sensor network system using UAVs can move freely in the air [5].Liu developed a communication system with a wireless data transmission module as the transceiver.The difference between these two parts is mainly related to the operational details of the remote control and the Radio Technical Commission for Aeronautics (RTCA) format, in which the GPS information and data format are designed to transmit these two types of information to correctly locate and control the steering gear.Compared with transmitter and receiver, the system can break through the limit of radio communication distance and effectively eliminate the interference of external electromagnetic fields on radio signals [6].Guo studied the path planning of UAV [7].
(3) About the design optimization of UAV wireless communication system.Lu studied interference suppression and resource allocation in UAV.In the existing case, since multiple GNs transmitting.there is interference at the receiver.In order to reduce the interference at the receiver, The minimum uplink is maximized by optimizing UAVs [8].Lu studied resource optimization in the UAV system [9].

A UAV wireless communication transmission system based on ARM+FPGA platform
The UAV wireless communication system adopts advanced orthogonal frequency division multiplexing with channel coding (OFDM), all-digital modulation and demodulation technology and MPEG2/MPEG4 digital compression coding technology.Its technical characteristics including multicarrier, strong anti-multipath ability, "non-line-of-sight", "diffraction" transmission characteristics and good penetration ability, which can meet the UAV wireless communication task.The system is based on the high-performance ARM+FPGA platform to realize the integrated business hardware module.The software adopts a self-developed operating system platform with a modular design and can be upgraded site [3].
The block diagram in an OFDM transmitter and receiver are represented as Figure 1 and Figure 2.  The symbol period of OFDM signal be T, when the minimum interval between the frequencies of N subcarriers is 1/T.The OFDM transmitter block diagram includes a source encoder, interleaver, modulator, inverse Fast Fourier Transform (IFFT), cyclic prefix insertion, and digital-to-analog converter (DAC).The source encoder compresses the data, and the interleaver rearranges the data to reduce the impact of interference.The modulator maps the interleaved data onto a constellation, and the IFFT converts the modulated data from the frequency domain to the time domain.The cyclic prefix is added to the time-domain signal to combat inter-symbol interference, and the DAC converts the signal to analog for transmission.
The OFDM receiver block diagram includes an analog-to-digital converter (ADC), cyclic prefix removal, Fast Fourier Transform (FFT), demodulator, deinterleaver, and source decoder.The ADC converts the received signal from analog to digital.The cyclic prefix is removed from the received signal, and the FFT converts the signal from the time domain to the frequency domain.The demodulator maps the received signal onto the constellation, and the deinterleaver rearranges the data.Finally, the source decoder decompresses the data to retrieve the original information.

A UAV wireless communication transmission system based on AD9361+ZYNQ7045 platform
UAV wireless communication transmission system comprises ground measurement and control equipment and airborne communication equipment.Both ground measurement and control equipment and airborne communication equipment have complete wireless communication transceiver function.Both ground measurement and control equipment and airborne communication equipment include radio frequency.The radio frequency part mainly uses AD9361 to complete up and down conversion, digital-to-analog conversion.The baseband part mainly uses ZYNQ7045 FPGA to complete modulation and demodulation, channel coding and decoding, and uplink and downlink framing and de-frame functions.The system aims to realize the mutual communication between the ground TT&C equipment and the UAV, that is, on the one hand, the ground TT&C equipment can send control instructions to the airborne equipment, and on the other hand, the airborne equipment can telemetry and transmit the collected information to the ground [10].
The system uses 16-bit CRC data to realize the verification process, and CRC16 can be generated.A description block diagram of convolutional coding used by the system is represented as Figure 3.

Designing a UAV communication system with wireless data transmission module
For improving the flying distance of the UAV, a communication system using wireless data transfer module as a carrier is designed.The system has two parts: the lower module and the upper module.The lower module mainly processes the remote information and the RTCA (Radio Technical Commission for Aeronautics) format differential GPS information.A data format was proposed to transmit the two kinds of information simultaneously.The upper computer module obtains these information to obtain high-precision GPS data and control the steering gear.The flight test shows that compared with the traditional transmission using a radio transmitter and receiver, the system can not only break the limit of radio communication distance but also effectively eliminate the interference of external electromagnetic fields on radio signals [11].An overview of the differential GPS used by the system is represented as Figure 4.

Implementation of a wireless communication transmission system for UAV based on ARM+FPGA platform
The UAV is equipped with video capture equipment, wireless transceiver, battery, etc. Fix the wireless transceiver and battery at the bottom of the UAV, and use -3 feeder to vertically install the transmission antenna at the tail of the UAV.The signal transmitted by the UAV wireless transceiver is received through the ground wireless receiving platform, which can clearly display the data collected by the UAV on the display screen and transmit the signal to other display/storage devices through the off-platform interface.At the same time, the receiving platform can embed a network transmission module to transmit the signal to the central station [3].
Addr1 --Addr8 is the address bus, D0-D15 is the data bus, nGCS5 is the bank chip selection of ARM, nWE is the write enable signal, and EINT0 is the interrupt signal.The module diagram of the main controller unit is represented as Figure 5.The FPGA chip of the main controller unit mainly completes baseband signal modulation and demodulation, digital signal up-conversion and down-conversion and some interface circuits.The data memory is high-speed and large-capacity static RAM.The program memory is a large-capacity FLASH ROM, and the parameters to be saved after the transceiver is shut down are placed in the SRAM with a lithium battery.In addition, it is also equipped with audio and RF A/D, D/A chips.This architecture is more general, and components are easy to complete.The embedded real-time operating system is adopted for software preparation.Each function is a relatively independent task module.The operating system manages communication between tasks, providing a standard, open and interoperable software platform.

Implementation of a UAV wireless communication transmission system based on AD9361+ZYNQ 7045 platform
Although the existing multiple platforms provide complete wireless communication solutions, UAV communication frequency points may be distributed in L-band, S-band, C-band and other non-fixed frequency points, which cannot meet the requirements.The AD9361 support software can configure the RF working frequency range from 70MHz to 6GHz, so the AD9361 chip is preferred for the RF part.The factors of modularization, scalability and product cost were fully considered.Finally, the AD9361 chip of ADI was selected to complete the relevant functions of the RF part of the system.At the same time, the ZYNQ7045 of Xilinx was used to complete the functional development of the baseband part.The ADI SDR platform provides a relatively detailed reference design scheme in the scheme demonstration stage.Various parameters in the RF RFIC AD9361 chip can be completely customized through software, with a signal output bandwidth of up to 56M, and the output frequency can be configured as 70MHz to 6GHz.The FPGA part has a variety of Xilinx ZC706 boards with large programmable logic resources.The SDR communication link's transmission distance, quality, stability and real-time meet this demand, so the ADI SDR hardware platform is finally selected.
The RF, ADC, DAC, digital filter, analog filter and digital module necessary for the transceiver are integrated in a single chip.Powerful programmable ability enables the chip to be used in all current mobile communication standards.The chip integrates 12-bit high-speed digital-to-analog conversion and analog-to-digital conversion RF 2x2 transceiver.Support configurable working frequency band 70MHz to 6.0GHz, covering L-band, S-band and some C-band.The supported tunable channel bandwidth is 200KHz to 56MHz.Internal Aux ADC and Aux DAC can be used for real-time monitoring and manual control of signal gain.Ground and onboard equipment adopt the same implementation architecture, and the transmission and reception links are symmetrical.It should be noted that the antenna is not shown in Figure 3.The standard gain horn antenna is used in the actual test, with the operating range of 1.60GHz to 6.95GHz and the gain of 15d B [10].Quantitative indicators of the system design are represented as Table .1

Designing the UAV communication system with wireless data transfer module
The ground part mainly comprises data transmission and ground monitoring systems.The main control part is AT91SAM7SE (512) 2 of ATMEI company.Its main function is to collect the control information of the remote controller and the DGPS information of the base station, which are interrupted and received and sent to the upper computer module.The data transmission system comprises Xtend-PKG data transmission radio and wireless routing from Digi, mainly used to send data to the upper computer module and receive the aircraft status information sent by the upper computer.The ground monitoring system is mainly composed of a monitoring computer.The airborne electronic warehouse mainly comprises a flight control computer, sensor system, servo system, and data transmission system.The core of flight control is AT91SAMTSE (512) of ATMEI.Designed quantitative indicators are: the RTCAOBS designed by the system contains the satellite observation information of the base station.The set transmission frequency is 1Hz, the amount of information data per frame is adjustable (up to 255 bytes), and the maximum number of bytes is [14O+(92 * N)]/8, where N is the maximum number of satellite records, which can be any value between 4 and 12, and the default value is 12. RTCAREF contains the location information of the base station.The set transmission frequency is 0.1Hz, and the amount of information data per frame is 24bytes.

Conclusions
UAV can be a robot that uses the braking system, automatic control and information technology to keep the aircraft on the scheduled route for operation.Through its strong practicality, it has been widely used in more and more fields.After sorting and analyzing, the research on the wireless communication system of UAV in the past five years is divided into three aspects, the transmission of the wireless communication system of UAV, the wireless communication network of UAV and the optimization design about wireless communication system of UAV.In later parts, the principle and implementation of a wireless communication transfer system in a UAV, a wireless communication transmission system in UAV about AD9361+ZYNQ7045 platform and a UAV communication system about wireless data transmission module are introduced respectively.One kind of UAV wireless communication transmission system is based on the high-performance ARM+FPGA platform, which can realize integrated business integration hardware module, a UAV wireless communication transmission system about AD9361 and ZYNQ7045 uses AD9361 and ZYNQ7045 FPGA to realize the design scheme of radio frequency part and baseband part, respectively.The wireless communication transmission system in UAV has strong scalability and a wide range, enough to adapt to the scene application of future UAV formation squadrons.
As an aerial robot, UAV can be used for military investigation, surveillance, military use, agricultural plant protection, electric patrol inspection, police law enforcement, geological exploration, environmental monitoring, forest protection, and aerial photography, and its applicable fields are rapidly expanding.With technology improvement, wireless communication transmission technology, network technology and resource management technology in UAV still need continuous innovation and development.

Figure 2 .
Figure 2. OFDM receiver block diagram.The symbol period of OFDM signal be T, when the minimum interval between the frequencies of N subcarriers is 1/T.The OFDM transmitter block diagram includes a source encoder, interleaver, modulator, inverse Fast Fourier Transform (IFFT), cyclic prefix insertion, and digital-to-analog

Figure 3 .
Figure 3.A description block diagram of convolutional coding used by the system.

Figure 4 .
Figure 4. Overview of differential GPS Differential GPS (DGPS) is a technique used to improve the accuracy of Global Positioning System (GPS) positioning.It involves the use of a reference station located at a known position that transmits correction signals to GPS receivers in the vicinity.The GPS receiver compares the signals received from the reference station to its own GPS signals, and calculates the difference in time and position.This difference is used to correct the position calculated by the GPS receiver, resulting in improved accuracy.DGPS is used in various applications, including navigation, surveying, and mapping, where accurate positioning is essential.

Figure 5 .
Figure 5. the module diagram of the main controller unit.The FPGA chip of the main controller unit mainly completes baseband signal modulation and demodulation, digital signal up-conversion and down-conversion and some interface circuits.The data memory is high-speed and large-capacity static RAM.The program memory is a large-capacity FLASH ROM, and the parameters to be saved after the transceiver is shut down are placed in the SRAM with a lithium battery.In addition, it is also equipped with audio and RF A/D, D/A chips.This architecture is more general, and components are easy to complete.The embedded real-time operating system is adopted for software preparation.Each function is a relatively independent task module.The operating system manages communication between tasks, providing a standard, open and interoperable software platform.
The sensor system comprises Inertial Measurement Unit (IMU), Electronic Compass (COMPASS) and NovAtel's high-precision GPS receiver 0EM4-G2.The servo system is composed of five steering engines, namely, roll, pitch, yaw, total torque and throttle, which are used to control five different aircraft attitudes.The data transmission system is composed of CM-X270 from CompuLab and XT9O-SI-NA data transmission radio from Digi.Its main function is to send the real-time status information of the aircraft to the ground monitoring platform and receive the data sent by the ground data transmission radio.The main functions of the flight control computer are: (1) processing the original data information of the sensor system, performing the Kalman filtering, and obtaining accurate state estimates.(2) Process the PCM+DGPS data received by the data transmission station.(3) Process control command and planning command uploaded by the ground monitoring platform, perform cascade PID control operation, and output the control information to control 5 steering engines respectively to complete the corresponding autonomous flight action[11].The AT91SAM7SE (512) of ATMEI company used in the main control part is shown in Figure6.

Figure 6 .
Figure 6. the AT91SAM7SE (512) of ATMEI company used in the main control part.

Table 1 .
. The main quantitative indicators.