Laser-based wireless voice communication system
Electromagnetic signal as a wireless communications carrier has a long history, is still widely used in short wave, microwave, millimeter-wave wireless communications. However, there are fatal defects in them: the confidentiality of poor, low-capacity, restricted resources, such as band. Light as a fiber-optic communications carrier with fiber optics as the transmission medium. As the band of light is very rich in resources, it is a huge communication capacity has become a mainstay of modern communication. However, including the Optical Fiber Communication Network, cable and other communications infrastructure is a pre-planned, fixed, and will not reach the fiber optic cable or fiber optic cable to reach the geographical inconvenience, can not be fiber-optic communications. As early as age 70 in the twentieth century, people began laser communication technology, but because the more successful of optical fiber communications, free space laser communication can not be given adequate attention. In recent years, due to the needs of mobile communications and microwave communications bandwidth limitations, free-space optical communications has made great progress. Lucent Technologies United States of using 1.55μm-band fiber amplifier semiconductor lasers increases (EDFA) as a light source to launch, and the use of wavelength division multiplexing structure, the space capacity to achieve 10Gbps optical communication. Japan, Europe and other countries also reported a number of space laser communication devices. China University of Electronic Science and Technology of carbon dioxide laser (10.6μm wavelength, cavity-type), the realization of fixed-point duplex four-wire three-way telephone communications of the atmosphere (technical achievements No. 88210414); Sun Yat-Sen University Institute of Laser and Spectroscopy the use of audio or digital signal AM laser system to achieve the work of the atmospheric transmission (technology No. 89209283). However, because of their low capacity, in the structure of communication systems, there is no communication with other facilities (including fiber-optic communications, microwave communications) interface, it is a small practical value. To address the above issues, the CAS Shanghai Institute of Optics and Fine Mechanics reports a wireless laser communications client machine to achieve the communication facilities and other interface (technology No. 00217069.8), but because of the high-end equipment, has not been widely used . In this paper, laser-based wireless voice communications system designed to provide a cheap and easy to carry, the same plane with the laser signal transmitting and receiving devices, and laser receiver with automatic tracking devices duplex laser communication equipment . The device launcher modulation laser signal issued not only in the free-space transmission, but also the direct use of optical fiber transmission as a vector to overcome the adverse weather conditions in the case of defects not be able to communicate; the device signal transmission capacity can be directly related to fiber-optic communications, microwave communications network and network, and the flexibility to adapt to the use of a variety of occasions.
1 the overall program design
Laser wireless communication system is mainly fired by the laser device, laser and optical receiver telescope composed of three parts (as shown in Figure 1). Its working principle is: the transmitter, through the coaxial cable with high-frequency transmitter code-phase converter; Fiber Adapter Fiber Media Converter with transmitters connected to; code converter with Media Converter with the standard selector switch connected to, and then by the signal processing module shaping, amplification, clock extraction, such as dealing with the importation of laser-driven laser driver modulation components of the laser beam generated by laser directional antenna to the space launch. Collected by the optical receiver antenna then the modulation signal into the laser detector, the input signal into a signal processing module, followed by selector switch into the system after: Laser Driver to connect all the way through fiber-optic fiber-optic communication line connecting adapter; another way converter with the code phase, and then coaxial cable access to electric transmission line. The system is designed for voice laser wireless communications system shown in Figure 2 by the various parts.
2 major hardware design
2.1 The choice of laser devices
Space laser communication wavelength select the main consideration: to avoid the effects of solar radiation, reducing the beam launch angle and reduce the size of the antenna transceiver, light transmittance in the atmosphere, as well as the reality of the device or the feasibility of the expected, including the device performance the expected price. From the laser point of view of wireless communications, the transmission rate of the atmosphere is an important factor. Less than 300nm in the UV band, the transmittance of the atmosphere has dropped drastically. It is clear that ultraviolet light is not conducive to air communications. Visible-band laser, for example, the second harmonic generation YAG laser, is not conducive to avoid sunlight noise caused by background radiation. Have used laser-band 830 ~ 860nm, 980 ~ 1060nm and 1550 ~ 1600nm, the atmosphere is a good window.
2.2 light-emitting and receiving antenna
As a result of the function of the optical antenna is to transmit the optical signal to be effectively made to each other and from the other side to receive the signal light and efficient, therefore, optical antenna is designed to meet the premise of the overall design to ensure that the communications system in the setting distance and atmospheric attenuation can work, select a reasonable telescope launched far-field divergence angle of the receiving telescope field of view of the receiver and the other parameters of optical system. Were to be introduced below.
(1) design considerations
Optical properties of the main requirements: high optical quality (λ/20RMS); low block rate; high optical transmittance (T ≥ 0.92); low scattered light. In addition, small thermal expansion coefficient, high mechanical strength, light weight, long service life.
Optical design considerations: In order to meet the space communications requirements of the antenna, I choose Cassegrain antenna. Include: the primary parabolic reflector; hyperbolic sub-reflector; focusing mirror, so that the structure of imaging in the external antenna.
(2) Performance Analysis
Light electric field strength to meet the assumption of Gaussian amplitude distribution, that is,
Which, ω is the waist size of light, R, said radius of curvature.
Similar to the use of Fresnel's law of radiation field, as well as the definition of antenna gain, the observation point (r, θ) Department of the antenna gain values:
Sub-block rate mirror, antenna pointing error, as well as optical antenna effect of the truss has a greater impact on antenna gain. In addition, the gain of the receiving antenna to detect a greater impact on the way there.
In the optical design of space communications in order to satisfy the requirements of the antenna, optical transmitter antenna system in Figure 3 (a) as shown by the semiconductor laser light on the road and set up the launch of its composition mirrors. Optical receiver antenna system in Figure 3 (b) below, mainly by the correction mirror, mirror 2 mirror correction glue mirror, primary mirror, filter, condenser and filter glue condenser lens, the composition of detectors. Among them, the detector used SI-PIN GT101 photodiode completed composite optical signal is converted to the corresponding electrical signal. The device in the reverse-bias conditions, when the illumination, the semiconductor to absorb light, or from the depletion layer in the depletion layer produced a diffusion length e-hole pair, and finally separated by the electric field. When the carrier drift through the depletion layer, formed in the external circuit current, in order to achieve photovoltaic conversion.
2.3 The main circuit design
Selection of an integrated circuit power regulator.
Pre-processing circuit includes pre-amplifier and power amplifier in two parts. Modulated by the conversion of electrical signals within the usually weak, subject to the former pre-amplifier circuit output voltage amplification. Therefore, the quality pre-processor, marks the system as a whole to a large extent the level of sound quality. That is, pre-processor and power amplifier for the system of choice is very important. This system as a NE5532 chip preamplifier, LA4101 as a power amplifier. At the same time, in order to obtain good results, reducing the interference in the signal before the pre-amplifier input, design a high-pass filter.
Modulation circuit of the light source modulation method of a number of species, but from the light source and the relationship between the modulator can be divided into: light source within the modulation, external modulation source. The system uses a light source within the modulation.
Power amplifier circuit of the photodetector due to weak signal, subject to power amplifier voltage signal amplification, to generate sufficient output power yet really to promote the speaker pronunciation. More types of amplifiers, the system uses a power amplifier IC LA4101.
The launch of the above-mentioned circuit design shown in Figure 4, the receiving circuit shown in Figure 5.
3 experimental prototype and conclusions
Design thinking in the above-mentioned guidance, completed a 5km wireless atmospheric laser communication equipment of the trial. Communication 0.855μm wavelength of light using a single-mode quantum-well semiconductor laser, with 200μm core diameter for the fiber-optic coupling, a fiber optical power to 200mW. Optical Antenna Tx / Rx aperture telescope to pass φ110mm, laser far-field angle of launch 1.5mrad, receiver field of view for 3mrad.
128/256/512/1024/2048kbps rate of the system is compatible with AMI and HDB3 code two interface functions. The system has been opened to the outdoor test. Its equipment are located in approximately 5km away from high-rise between the two (non-line-of-sight requirements of shelter), 360h for as long as the opening Test, which has experienced heavy fog, heavy rain, light rain, sunny weather and so on. The experimental results show that, in addition to extremely low visibility in heavy fog, the communication system can work, good call quality. The system has the following advantages:
(1) has a duplex function spaces;
(2) can be achieved for more than a single multi-channel communications;
(3) detection through an optical telescope and receiving some part of the launch between the situation of being;
(4) simple structure, easy to carry;
(5) with the fiber-optic communication interface, the application scope, in particular, apply to the two other side of the river, between high mountains, between the high-rise building wireless communications;
Performance of the system, transmission rate of 155Mbps, the bit error rate less than 10-8, head-up communication from not less than 5km.
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