I. Product
1. Integrated service VSAT satellite communication system
1.1. Summary
Integrated service VSAT satellite communication system, composed of satellite, earth station and network control center, is capable of supporting such services as voice, data and fax as well as working in the frequency bands of L, C and Ku. The earth station of VSAT system mainly consists of antenna, radio frequency (RF) equipment, channel equipment and terminal equipment.
1.2 Antenna
Antennae of VSAT earth station include such series of antennae as antenna for satellite communication in motion, antenna for satellite communication in stop and portable antenna.
1.2.1 Antenna for vehicle satellite communication in motion
Adopting the design of suspension isolating three-axis stability, the antenna, with the functions of automatic satellite acquisition and tracking, is capable of storing parameters of several satellites and being set in several work modes, including automatic tracking, semi-automatic tracking and manual tracking. The main technical indexes are as follows:
a) Working frequency band: Ku;
b) Antenna aperture: 0.6m, 1m and 1.2m;
c) Antenna gain:
1) 0.6m: receiving 35.6dBi and transmitting 36.6dBi;
2) 1m: receiving 40.0dBi and transmitting 41.2dBi;
d) Polarization mode: linear polarization (adjust automatically) ;
e) Tracking range: orientation: 360 without limit, pitching: 15-75;
f) Initial turn-on time: ≤4min;
g) Reacquisition time within 3 minutes after target loss: ≤1s;
h) Feed interface: WR-75.
1.2.2 Antenna for vehicle satellite communication antenna in stop
With the functions of automatic satellite acquisition and tracking, the antenna is capable of storing parameters of several satellites and being set in such work modes as automatic tracking, semi-automatic tracking and manual tracking. The main technical indexes are as follows:
a) Working frequency band: Ku;
b) Antenna aperture: 1.2m, 1.8m and 2.4m;
c) Antenna gain:
1) 1.2m: receiving 42.1dBi and transmitting 43.2dBi;
2) 1.8m: receiving 45.6dBi and transmitting 46.7dBi;
3) 2.4m: receiving 48.1dBi and transmitting 49.2dBi;
d) Polarization mode: linear polarization (adjust automatically) ;
e) Tracking range: orientation: ±180°, pitching: 10°～85°;
f) Initial turn-on time: ≤10min;
g) Feed interface: WR-75.
1.2.3 Portable antenna
Adopting an assembled structure and auxiliary manual satellite acquisition method with electronic compass and LCD display, portable antenna is characterized by small volume, light weight, portablility as well as quick and convenient satellite acquisition enabled by auxiliary satellite acquisition means. The main technical indexes are as follows:
a) Working frequency band：Ku;
b) Antenna gain:
1) 0.5m: receiving 34.5dBi and transmitting 35.6dBi;
2) 0.9m: receiving 39.6dBi and transmitting 40.7dBi;
c) Polarization mode: linear polarization;
d) Tracking range: orientation: 360 without limit, pitching: 10°～85°;
e) Initial turn-on time: ≤7min;
f) Feed interface: WR-75.
1.3 RF equipment
RF equipment includes such equipment as RF transceiver, L, C and Ku series converters, and is used for frequency conversion, reception and transmission of carrier signals, with the main technical indexes of the equipment as follows:
1.3.1 70~L up/down converter
a) Reference input: 10MHz;
b) Input frequency: up converter:70±20MHz，down converter: 950～1450MHz;
c) Output frequency: up converter: 950～1450MHz，down converter: 70±20MHz;
d) Frequency step: 125kHz;
e) Power level adjustment range: 30dB to 1dB in steps.
1.3.2 L～C up/down converter
a) Reference input: 10MHz;
b) Input frequency: up converter: 950～1525MHz，down converter: 3625～4200MHz;
c) Output frequency: up converter: 5850～6425MHz，down converter: 3625～4200MHz;
d) Gain: up converter: 10dB, down converter: 20dB.
1.3.3 L～Ku up/down converter
e) Reference input: 10MHz;
f) Input frequency: up converter: 950～1450MHz, down converter: 12.25～12.75GHz;
g) Output frequency: up converter: 14～14.5GHz, down converter: 950～1450MHz;
h) Gain: up converter: 10dB, down converter: 20dB.
1.3.4 Low-noise amplifier
a) Working frequency band: 12.25～12.75GHz;
b) Noise factor: 1.2dB;
c) Spurious-wave output: ≤－60dBc;
d) Power supply: 15～24V DC;
e) Input-terminal connector: WR-75;
f) Output-terminal connector: N-K.
1.4 Channel equipment
Base-band and interface processing equipment of VSAT system below medium frequency provides such services as voice and data, with the main technical indexes as follows:
Technology system: MBE-convolutional code- DQPSK-FDMA-SCPC;
Service category: voice, data, fax and web visit;
Maximum service rate: 9.6kbps~2048kbps;
Terminal interface type: 2-wire subscriber line, RS232, RS422 and LAN.
2. Station type equipment supported
2.1 Portable station
Used for providing scattered subscribers with digital voice and data communication services, portable station is characterized by strong anti-interference capability, high intelligent level, quick and convenient turn-on as well as simple operation. It is capable of automatically collecting local longitude and latitude information as well as determining antenna pointing conveniently and accurately with the function of displaying intensity of satellite signals; it can also connect with public telephone network, public data network and INTERNET conveniently through the central earth station of the system.
Working frequency band: Ku;
Antenna aperture: 0.5m and 0.9m;
Service type: voice and data;
Maximum service rate: 9.6kbps (0.5m antenna) and 1024kbps (0.9 m antenna);
Channel no.: 1 channel;
Stored dimension: 500 mm×400 mm×250 mm;
Total weight: 18kg.
2.2 Cabinet-type station
Composed of 2 cabinets and convenient for handling or carrying with vehicles, cabinet-type station can be established in a quick manner. Three-channel communication capacity enables voice, data and image service communication. It can connect with ground cable network. TV conference can be held through the imbedded video CODEC of the equipment together with the video terminal.
Working frequency band: Ku;
Antenna aperture: 0.9m;
Service type: voice and data;
Maximum service rate: 1024kbps;
Channel no.: 1 channel of broadband and 2 channels of narrowband.
2.2 Earth station of satellite communication in motion
As an important station type of the satellite communication system, earth station of satellite communication in motion can carry out communication with any central station and fixed station within the network as well as fixed vehicle station through the centralized network control management of the central station. The network structure is a network structure under centralized control and is capable of transmitting voice, data and fax services.
Working frequency band: Ku;
Antenna aperture: 0.6m;
Service type: voice, data, fax and web;
Maximum service rate: 512kbps (0.6m antenna);
Service channel: 1 channel.
2.3 Vehicle-borne station of satellite communication in stop
The vehicle-borne station adopts a series of international hi-techs and the equipment is featured by small size, digitalization and intellectualization, with all work done in automation. With excellent performance, desirable reliability and simple operation, the vehicle-borne station currently is the first at home and reaches the international advanced level. It can transmit such integrated services as telephone, fax, data and image.
Working frequency band: Ku;
Antenna aperture: 1.8m;
Service type: voice, data, fax and network services;
Maximum service rate: 2048kbps;
Service channel: 2 channels (standard configuration，extensible).
2.4 Network-controlled central station
As the control and management core of the satellite communication system, the central station is capable of carrying out monitoring and network management over various communication earth stations of the whole network. It can transmit such services as voice, data, telex, telegram and fax as well as carry out telephone conference.
Working frequency band: C and Ku;
Business type: voice, data, fax and web;
Maximum service rate: 64kbps;
Service channel: 4 channels (standard configuration, extensible);
Function of exiting/accessing network: supported;
Function of system management: supported.
II. Solution
1. Networking design scheme of satellite system
1.1 System components
The system includes several station types, including network control center, vehicle-borne station of satellite communication in stop, vehicle-borne station of satellite communication in motion and portable station. Various service communications are carried out under the uniform management of the network control center. The system supports voice, data and web visit. Among them, the large vehicle-borne station and network control center also support the function of ground telephone network access.
System component diagram
1.2 Network topological structure and resource allocation method
Service communication: mesh network;
Network control: star network;
Resource allocation method: combination of DAMA and PAMA.
1.3 Services carried out
Voice: establish link by traditional call dialing and data dialing;
Data transmission: provide interface conversion equipment to realize IP data transmission;
Video conference: realize access to video terminal.
1.4 Access scheme of various station types
The system authorizes allocation access to various station types and only allows legal subscribers to access network. Various subscribers have power to manage passable data rate, passable objects, etc.
2 Optimized design scheme of LNA simulation in Ku band
2.1 Introduction
Low-noise amplifier (LNA) is located at the front position of all transceiver systems and plays a significant role in the system. Its performance directly influences that of the whole machine. The main function of LNA is to provide the final stage with sufficiently high gain under the condition of overcoming noise, i.e. eliminating noise as much as possible while providing gain. Circuit design by using micro-wave CAD simulation software can avoid complicated theoretical calculation, thus greatly improving accuracy and efficiency of the design.
2.2 Design principle
The LNA design is a compromise among the following features, including optimized gain, low noise factor, matching of input and output ports, high linearity and low power consumption. The simple structure of LNA will make us mistakenly think that the design is very simple but the compromise of the design is very complicated.
The main performance indexes of LNA are work frequency, noise factor, output gain, gain flatness and voltage standing wave ratio, among which noise factor and output gain are the most important indexes.
Noise factor: Noise factor is defined as the ratio between the input signal-to-noise ratio and output signal-to-noise ratio of the amplifier. The noise factor depends on the minimum noise factor of the selected amplifier tube, equivalent noise resistance of the amplifier tube, source reflection coefficient of the output terminal, etc. When the input terminal of the amplifier is completely matched, the amplifier has the minimum noise factor output.
Output gain: Output gain of the amplifier is defined as the ratio between the output power and input power of the amplifier. Improvement of LNA gain is greatly conducive to reducing the noise factor of the whole machine, but excessively high LNA gain will affect the dynamic range of the whole receiver. Therefore, generally speaking, LNA gain should be determined with consideration given to noise factor of the whole machine and dynamic range of the receiver of the system, etc.
Dynamic range of the amplifier: The dynamic range of the whole receiver should be taken into consideration in the LNA design so as to avoid non-linear distortion to the final stage of the receiver. Generally, the relatively higher one of the input third-order intercept point of the low-noise amplifier should be chosen, at least 30dB higher than the maximum input signal so as to avoid non-linear distortion in the time of signal input.
2.3 Design procedure
First of all, appropriate microwave field effect transistor should be chosen to maintain the noise factor of the whole machine less than 1.3dB. Thus the noise factor of the amplifier in the input stage of LNA will be low. For the purpose of obtaining gain above 20dB, LNA must adopt series connection of multi-stage amplifiers. Upon analysis and calculation, three-stage amplifier in series connection should be adopted, with the first two stages choosing NE3210S01 (the noise factor is only 0.34dB) of NEC company and the third stage choosing ATF36077 of HP company.
Then the circuit diagram of microwave simulation software design is used to open Serenade8.7 microwave simulation software to generate schematic diagram of the first-stage amplifier circuit. Optimization and simulation are carried out with objective of minimizing the noise factor before series connection of the second stage amplifier circuit. Coupling is conducted in the center through coupling resistance to carry out inter-stage circuit matching before continuing analysis and optimization. The objective of inter-stage matching circuit is to match the input impedance of the final-stage amplifier with that of the first-stage output impedance, which may produce relatively great gain. In addition, there should be sufficiently low noise in the second stage. Upon completion of this process, independently generate the schematic diagram of the third-stage amplifier circuit to conduct matching optimization of the output circuit. The task of output matching circuit is to match the complex output impedance of field effect transistor to 50 Ohm of load real impedance and maximize the output gain of the amplifier. At last, connect the three-stage amplifier circuit in series.
2.4 Conclusion
Currently, CAD method is generally adopted for simulation during the design of low-noise amplifier of our company so as to avoid complicated calculation of matching circuit and stability problem during the microwave circuit design process through software optimization.
3 Design scheme of automatic control system of vehicle satellite communication antenna
3.1 Introduction
Satellite communication is featured by long communication distance, wide coverage, flexible networking, high quality channel and the correlation between communication and cost. Therefore, it has been popularized and developed rapidly in communication in recent years. Among various satellite earth stations, the vehicle-borne station is characterized by high flexibility as well as quick and convenient erection and turn-on. Therefore, it boasts a relatively broad application prospect. Since antenna is one of the major equipment of the station, it is of great significance in researching and developing automatic antenna control system of large aperture.
3.2 System design
3.2.1 Closed-loop control system
(1) Automatic control and automatic control system
Automatic control is to make the machine have certain performance by using the process of control device automatically controlling the machine. The controlled machine is called control object and the control device applied is called controller.
Controller and control object consist of automatic control system. The system input acts on the controller to make the control object have predetermined characteristic or predetermined output, called control input; the system has another kind of input acting on the control object, disturbing or damaging the predetermined characteristic or predetermined output of the system, called disturbance input. The system output is controlled amount of automatic control system. For example, automatic speed control system (speed adjustment system), the speed given voltage adjusted by potentionmeter is control input of the speed adjustment system; the load torque acting on the automatic shaft is the disturbance input of the speed adjustment system; rotation speed of the electric motor is the controlled amount of the speed adjustment system and the output of the system.
(2) Open-loop control system and closed-loop control system
Open-loop control system is the simplest automatic control system. For the control input of a system, there is a corresponding output of the system which, however, cannot have a determined value under the act of disturbance input. For example, for the open-loop speed adjustment system, under constant speed given voltage, the system output (rotation speed of the electric motor) will change with the change of such disturbance input as load torque and power supply voltage fluctuation, the control precision being low. An important reason for the low control precision of open-loop control system is the change of system output having no effect to the control input of system.
The control precision of the system will increase in case of feedback of a part of system output by feedback elements to the input terminal of the system for comparison with the control input of the system, and control of the system output with the difference (adjustment error) after comparison. The system composed of controller, control object and feedback device is called closed-loop control system or (negative) feedback control system. In the general speed adjustment system, tachometer generator measuring the rotation speed of the electric motor is a speed feedback device.
We call open loop of the system in case of feedback breakage of closed-loop control system. Pay attention to another situation in which there is no breakage of system feedback but saturation occurs to some link of the system (output of the link does not change with the input change), the electric motor carries out open-loop operation with constant acceleration. And the closed-loop operation of the system does not restore until the power supply of the electric motor is less than the flow limit value.
(3) Function of negative feedback
Since the closed-loop system adopts negative feedback, the adjustment error of the system is reduced and the control precision of the system is improved; since the adopted negative feedback reduces the influence of the change of internal element parameters (non-linear, drifting) to the system output, the system is not sensitive to the change of internal parameters.
Because of energy store elements in the internal system, large overshot, even oscillation will occur to the system output while reducing adjustment error of the system. The task of the controller is to ensure the stability, certain stability margin and relatively desirable dynamic performance of the system while ensuring control precision of the system.
3.2.2 System components
The control system is mainly composed of the following two parts:
(1) Driver part: motor driver and motor
(2) Controller part: control circuit, beacon receiver, power supply and polarization motor driver
3.2.3 System design
The design adopts closed-loop control, specifically divided into three closed-loop subsystems.
(1) Motor servo subsystem
Motor servo subsystem refers to the feedback loop consisting of servo driver and motor, which controls motor movement.
The design adopts S/CD system driver of Kollmorgen and GODLINE XT system motor. S/CD series driver is featured by advanced servo control function, advanced technical indexes, advanced movement control, flexible and diversified I/O configuration as well as advanced communication function. Various types of products can be flexibly chosen in accordance with requirements of different systems, and quickly applied to different antenna control systems under the condition that other 2 subsystems remain unchanged, with relatively strong universality and substitutability. S/CD series driver has 7 kinds of operating modes, and the design adopts analog speed control mode.
(2) Motor control subsystem
Motor control subsystem refers to driver and control circuit consisting of feedback circuit and the control circuit sending servo motor operation directive. After the motor is driven by the driver, angular displacement accumulative amount of the servo motor returning from the incremental encoder contained with the servo motor is sent back to control circuit upon initial processing by the driver and becomes absolute displacement of the servo motor upon processing by control circuit before being directly stored into NVRAM via DMA mode so as to avoid data loss in power down situation.
The motor servo subsystem adopts analog speed mode, variable control voltage of ±10V, balanced D/A output and an additional buffer amplifier so that the motor overload damage caused by mechanical jamming is avoided.
(3) Automatic tracking control subsystem
Automatic tracking subsystem is mainly feedback circuit composed of control circuit and beacon receiver.
Automatic tracking means that the beacon signal received by the antenna from satellite drives tracking system to enable automatic satellite acquisition of the antenna. The design adopts step tracking mode, i.e. at certain interval, let the antenna conduct step rotation at a slight angle within the orientation or pitching range, with beacon receiver calculating and receiving power level strength, control circuit determining the increase and decrease of power level. In case of increase of the receiving power level, the antenna will be controlled to rotate at a slight angle along the former orientation; in case of decrease of the receiving power level, the antenna will be controlled to rotate reversely. Pitching direction or azimuth direction occurs alternatively in turn, thus enabling gradual satellite acquisition of the antenna beam.
3.3 Conclusion
Currently, the automatic antenna control system has been extensively applied to the vehicle antennae of our company, with desirable application effect as well as quick, accurate and convenient satellite acquisition.
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