Variations on the theme of space laser communications. Laser communication with aliens

This chapter discusses laser communication network technology, as well as its advantages, such as cost-effectiveness; low operating costs; high throughput and quality of digital communications, as well as rapid deployment and change of network configuration.

Laser devices can transmit any network stream that is delivered to them using optical fiber or copper cable in the forward and reverse directions. The transmitter converts electrical signals into modulated laser radiation in the infrared range with a wavelength of 820 nm and a power of up to 40 mW. Laser communication uses the atmosphere as a propagation medium. The laser beam then hits a receiver that has maximum sensitivity within the wavelength range of the radiation. The receiver converts laser radiation into signals from the electrical or optical interface used. This is how communication is carried out using laser systems.

The optical range has many characteristic features and, due to its short wavelength, makes it possible to achieve high radiation directivity, significantly reduce the size of antenna systems, form extremely narrow laser beams and obtain a high concentration of electromagnetic radiation in space.

When transmitting information by modulated electromagnetic waves, it is necessary that the modulation frequency be 10...100 times less than the carrier frequency. In addition, modulation frequencies occupy a certain frequency band, and its width is determined by the amount of information transmitted per unit time. For example, the transmission of telegraph text requires a frequency band of 10 Hz, and for television images a frequency band of 107 Hz and a carrier frequency of at least 108 Hz are required. The radio range occupies the frequency band 104…108 Hz and is fully mastered. The information capacity of the communication channel in the microwave range (109..1012 Hz) is higher, but due to the characteristics of the propagation of microwave radiation in the atmosphere, communication between microwave stations is possible only at a line-of-sight distance. In the optical range, only the visible region occupies the frequency band from 41014 to 1015 Hz. Using a laser beam, it is theoretically possible to transmit 1015/107 = 108 television channels, which is several orders of magnitude higher than modern needs, or 1013 telephone conversations. Thus, one of the advantages of optical communication lines is the ability to transmit large amounts of information due to the ultra-wide frequency band. Mastering the optical range: creating laser light sources, sensitive semiconductor optical radiation receivers and developing low-loss fiber LEDs opens up new opportunities for creating communication systems.

The optical range opens up the possibility of creating information and control systems with characteristics that are fundamentally unattainable in the radio range. To date, a variety of ground, aviation and space systems have been developed. optical communications, laser ranging, laser systems for aerospace monitoring of the natural environment, aerial reconnaissance systems, collision avoidance systems for moving objects, laser systems for docking spacecraft, laser guidance and laser weapon control systems.

The potential capabilities of laser information systems, as well as optical methods of information transmission and processing in general, are very great. In many problems, the maximum achievable characteristics are limited only by quantum effects. However, in reality, the potential capabilities of the optical range cannot always be effectively realized in practice. There are many reasons for this.

The performance characteristics of real laser systems are greatly influenced by inevitable fluctuations in laser radiation sources and random changes in parameters information processes, the effects of various interferences, the probabilistic nature of the photo detection operation. Many Information Systems optical range are built using an open (most often atmospheric) channel. For laser radiation, the atmospheric channel is a channel with a randomly inhomogeneous propagation medium. The effects of absorption of optical radiation by atmospheric gases, molecular and aerosol scattering, distortions of the spatio-temporal structure and disruption of the coherence of laser radiation - all this has a noticeable impact on the energy potential, principles of information signal processing and range of action created systems. All of the listed features show that the analysis of laser information systems and the assessment of their potential and actually achievable characteristics cannot be carried out without a probabilistic study of the structure of information signals and interference.

To date, numerous results have been accumulated on the probabilistic analysis of various laser systems. However, most of these results seem to be very disparate, they are not based on a unified approach, and they are quite difficult to use in practical problems. The need for additional detailed studies of the probabilistic structure of signals, interference and, in general, information processes in radio optics is associated with the need to improve mathematical models, solve problems of optimizing the structure of signals and systems, and develop new promising algorithms for transmitting, receiving, converting and processing information in optical information systems.

Laser communication is an alternative to radio, cable and fiber optic communications. Laser systems make it possible to create a communication channel between two buildings located at a distance of up to 1.2 km from each other, and transmit telephone traffic (speed from 2 to 34 Mbit/s), data (speed up to 155 Mbit/s) or their combination. Unlike wireless radio systems, laser communication systems provide high noise immunity and transmission secrecy, since unauthorized access to information can only be obtained directly from the transceiver.

A company that uses laser communications to create a main (backup) short-range communication channel will not only avoid the need to lay new wire communications, but also the need to obtain permission to use the radio frequency. Moreover, don't high level the cost of organizing a high-performance communication channel, as well as the short time it takes to put it into operation, will ensure a quick return on investment. Thus, a wide range of capabilities and undoubted advantages of laser equipment make its use the best solution to the problem of organizing a reliable communication channel between two buildings.

Active research into microwaves began in the mid-20th century. American physicist Charles Townes decided to increase the intensity of the microwave beam. Having excited the ammonia molecules to high energy levels through heat or electrical stimulation, the scientist then passed a weak microwave beam through them. The result was powerful amplifier microwave radiation, which Townes called a “maser” in 1953. In 1958, Townes and Arthur Schawlow made next step: Instead of using microwaves, they tried to amplify visible light. Based on these experiments, Maiman created the first laser in 1960.

The creation of the laser made it possible to solve a wide range of problems that contributed to significant developments in science and technology. Which made it possible at the end of the 20th and beginning of the 21st centuries to obtain such developments as: fiber-optic communication lines, medical lasers, laser processing of materials (heat treatment, welding, cutting, engraving, etc.), laser guidance and target designation, laser printers, barcode readers and much more. All these inventions have greatly simplified the life of an ordinary person and allowed the development of new technical solutions.

This article will answer the following questions:

1) What is wireless laser communication? How was it accomplished?

2) What are the conditions of use laser communication in space?

3) What equipment is needed to implement laser communication?

Definition of wireless laser communication, methods of its implementation.

Wireless laser communication is a type of optical communication that uses electromagnetic waves in the optical range (light) transmitted through the atmosphere or vacuum.

Laser communication between two objects is carried out only through a point-to-point connection. The technology is based on data transmission using modulated radiation in the infrared part of the spectrum through the atmosphere. The transmitter is a powerful semiconductor laser diode. The information enters the transceiver module, in which it is encoded with various noise-resistant codes, modulated by an optical laser emitter and focused optical system transmitter into a narrow collimated laser beam and transmitted into the atmosphere.

At the receiving end, the optical system focuses the optical signal onto a highly sensitive photodiode (or avalanche photodiode), which converts the optical beam into an electrical signal. Moreover, the higher the frequency (up to 1.5 GHz), the greater the volume of transmitted information. The signal is then demodulated and converted into output interface signals.

The wavelength in most implemented systems varies between 700-950 nm or 1550 nm, depending on the application laser diode.

From the above it follows that the key instrument elements for laser communication are a semiconductor laser diode and a highly sensitive photodiode (avalanche photodiode). Let's look at the principle of their operation in a little more detail.

Laser diode is a semiconductor laser built on the basis of a diode. His work is based on the occurrence of population inversion in p-n areas transition upon injection of charge carriers. An example of a modern laser diode is provided in Figure 1.

Avalanche photodiodes are highly sensitive semiconductor devices that convert light into an electrical signal due to the photoelectric effect. They can be considered as photodetectors that provide internal amplification through the avalanche multiplication effect. From a functional point of view, they are solid-state analogs of photomultipliers. Avalanche photodiodes have greater sensitivity compared to other semiconductor photodetectors, which allows them to be used for recording low light powers (≲ 1 nW). An example of a modern avalanche photodiode is provided in Figure 2.

Conditions for using laser communications in space.

One of the promising areas for the development of space communication systems are systems based on transmitting information via a laser channel, since these systems can provide greater throughput with less power consumption, overall dimensions and weight of transceiver equipment than those used in this moment radio communication systems.

Potentially, space laser communication systems can provide exceptional high speed information flow – from 10-100 Mbit/s to 1-10 Gbit/s and higher.

However, there are a number technical problems that need to be solved to implement laser communication channels between the spacecraft (SC) and the Earth:

necessary high accuracy guidance and mutual tracking at distances from half a thousand to tens of thousands of kilometers and when carriers move at cosmic speeds.
The principles of receiving and transmitting information via a laser channel are becoming significantly more complicated.
Optical-electronic equipment is becoming more complex: precision optics, precision mechanics, semiconductor and fiber lasers, highly sensitive receivers.

Experiments on the implementation of space laser communications

Both Russia and the United States of America are conducting experiments on the implementation of laser communication systems for transmitting large amounts of information.

RF Laser Communication System (SLS)

In 2013, the first Russian experiment was carried out to transmit information using laser systems from Earth to the Russian segment of the International Space Station (RS ISS) and back.

The SLS space experiment was carried out with the aim of testing and demonstrating Russian technology and equipment for receiving and transmitting information via a space laser communication line.

The objectives of the experiment are:

testing, under conditions of space flight on the ISS RS, the main technological and design solutions incorporated into the standard equipment of the intersatellite laser information transmission system;
development of technology for receiving and transmitting information using a laser communication line;
study of the possibility and operating conditions of laser communication lines “on board the spacecraft - ground point» under different atmospheric conditions.

The experiment is planned to be carried out in two stages.

At the first stage, the reception-transmission system is worked out information flows via the lines “on board the RS ISS–Earth” (3, 125, 622 Mbit/s) and “Earth–on board the RS ISS” (3 Mbit/s).

At the second stage, it is planned to develop a high-precision guidance system and an information transmission system along the line “on board the ISS RS – relay satellite.”

The laser communication system at the first stage of the SLS experiment includes two main subsystems:

on-board laser communication terminal (BTLS), installed on the Russian segment of the International Space Station (Figure 3);
ground laser terminal (GLT) installed at the Arkhyz optical observation station in the North Caucasus (Figure 4).

Objects of study at stage 1 of FE:

on-board laser communication terminal equipment (BTLN);
ground laser communication terminal (GLT) equipment;
atmospheric radiation propagation channel.

Figure 4. Ground laser terminal: astro pavilion with optical-mechanical unit and alignment telescope

Laser communication system (LCS) - stage 2.

The second stage of the experiment will be carried out after the successful completion of the first stage and the readiness of a specialized spacecraft of the “Luch” type on the GEO with an on-board terminal of the inter-satellite laser information transmission system. Unfortunately, information about whether the second stage was carried out or not could not be found in open sources. Perhaps the results of the experiment were classified, or the second stage was never carried out. The information transfer scheme is shown in Figure 5.

Project OPALS USA

Almost simultaneously, the American space agency NASA begins deploying the OPALS (Optical Payload for Lasercomm Science) laser system.

“OPALS represents the first experimental site for the development of laser space communications technologies, and the International Space Station will serve as a test site for OPALS,” said Michael Kokorowski, OPALS project manager and a member of NASA's Jet Propulsion Laboratory (JPL). Jet Propulsion Laboratory, JPL) - "Future laser communications systems that will be developed based on OPALS technologies will be able to exchange large volumes of information, eliminating the bottleneck that in some cases is holding back scientific research and commercial enterprises."

The OPALS system is a sealed container containing electronics connected via an optical cable to a laser transmitting and receiving device (Figure 6). This device includes a laser collimator and a tracking camera mounted on a moving platform. The OPALS installation will be sent to the ISS aboard the Dragon spacecraft, which will launch into space in December this year. Once delivered, the container and transmitter will be installed outside the station and the 90-day program will begin. field tests systems.

Operating principle of OPALS:

From Earth, specialists from the Optical Communications Telescope Laboratory will send a beam of laser light towards the space station, which will act as a beacon. The equipment of the OPALS system, having caught this signal, using special drives, will aim its transmitter at a ground-based telescope, which will serve as a receiver, and transmit a response signal. If there is no interference in the path of laser light beams communication channel will be installed and the transmission of video and telemetric information will begin, which for the first time will last about 100 seconds.

European Data Relay System abbreviated EDRS.

The European Data Relay System (EDRS) is a project planned by the European Space Agency to create a constellation of modern geostationary satellites that will transfer information between satellites, spacecraft, unmanned aerial vehicles (UAVs) and ground stations, providing faster than traditional methods data transfer speed, even in conditions of natural and man-made disasters.

EDRS will use new technology laser communication Laser Communication Terminal (LCT). The laser terminal will allow transmitting information at a speed of 1.8 Gbit/s. LCT technology will enable EDRS satellites to transmit and receive about 50 terabytes of data per day in almost real time.

The first EDRS communications satellite is scheduled to launch into geostationary orbit in early 2016 from the Baikonur Cosmodrome on a Russian Proton launch vehicle. Once in geosynchronous orbit over Europe, the satellite will conduct laser communications links between the four unmanned Sentinel-1 and Sentinel-2 satellites operating as part of the Copernicus Earth observation space program. aircraft, as well as ground stations in Europe, Africa, Latin America, the Middle East and the northeast coast of the United States.

A second, similar satellite will be launched in 2017, and the launch of a third satellite is planned for 2020. Together, these three satellites will be able to cover the entire planet with laser communications.

Prospects for the development of laser communications in space.

Advantages of laser communication compared to radio communication:

transmission of information over long distances
high transfer speed
compactness and lightness of data transmission equipment
energy efficiency

Disadvantages of laser communication:

the need for precise pointing of receiving and transmitting devices
atmospheric problems (cloudiness, dust, etc.)

Laser communication makes it possible to transmit data over much greater distances relative to radio communication, the transmission speed due to the high concentration of energy and much more high frequency carrier (by orders of magnitude) is also higher. Energy efficiency, low weight and compactness are also several times or orders of magnitude better. Difficulties in the form of the need for precise guidance of receiving and transmitting devices can be solved with modern technical means. In addition, ground-based receiving devices can be located in areas of the Earth where the number of cloudy days is minimal.

In addition to the problems presented above, there is another problem - the divergence and attenuation of the laser beam when passing through the atmosphere. The problem is especially aggravated when the beam passes through layers with different densities. When passing through the interface between media, a light beam, including a laser beam, experiences particularly strong refractions, scattering and attenuation. In this case, we can observe a kind of light spot resulting precisely from passing such an interface between the media. There are several such boundaries in the Earth's atmosphere - at an altitude of about 2 km (active weather atmospheric layer), at an altitude of approximately 10 km, and at an altitude of approximately 80-100 km, i.e. already at the boundary of space. The heights of the layers are given for mid-latitudes in the summer. For other latitudes and other seasons, the heights and the very number of interfaces between the media may differ greatly from those described.

Thus, when entering the Earth’s atmosphere, a laser beam, which had previously calmly traveled millions of kilometers without any losses (except perhaps a slight defocusing), loses the lion’s share of its power within some unfortunate tens of kilometers. However, we can turn this fact, bad at first glance, to our advantage. Since this fact allows us to do without any serious aiming of the beam at the receiver. Because as such a receiver, or rather a primary receiver, we can use precisely these very boundaries between layers and media. We can point the telescope at the resulting spot of light and read information from it. Of course, this will significantly increase the amount of interference and reduce the data transfer rate. And it will make it completely impossible during the daytime. But this will make it possible to reduce the cost of the spacecraft by saving on the guidance system. This is especially true for satellites in non-stationary orbits, as well as for spacecraft for deep space research.

At the moment, if we consider the connection “Earth - spacecraft and spacecraft - Earth”, optimal solution is the synergy of laser and radio communications. It is quite convenient and promising to transmit data from the spacecraft to the Earth using laser communications, and from the Earth to the spacecraft using radio communications. This is due to the fact that the laser receiving module is a rather bulky system (most often a telescope), which captures laser radiation and converts it into electrical signals, which are then amplified using known methods and converted into useful information. Such a system is not easy to install on a spacecraft, since most often the requirements are compactness and low weight. At the same time, the laser signal transmitter is small in size and weight compared to antennas for transmitting radio signals.

Black Overlord January 4, 2015 at 05:04

Variations on the theme of space laser communications

Cosmonautics

One of the current topics in commercial astronautics, and not only that, is the topic of laser communications. Its benefits are known, tests have been carried out and have been successful or very successful. If anyone doesn’t know the pros and cons, I’ll briefly outline them.

Laser communications make it possible to transmit data over much greater distances compared to radio communications; the transmission speed, due to the high energy concentration and a much higher carrier frequency (by orders of magnitude), is also higher. Energy efficiency, low weight and compactness are also several times or orders of magnitude better. As well as the cost - in principle, an ordinary Chinese laser pointer with a power of around 1 W or higher may well be suitable for laser communication in space, which I intend to prove below.

Of the minuses, we can mention, first of all, the need for much more accurate guidance of the receiving and transmitting modules relative to radio communications. Well, there are well-known atmospheric problems with clouds and dust. In fact, all these problems can be easily solved if you approach them head-on.

First of all, let's look at how the receiving module works. It is a specialized (not always) telescope that captures laser radiation and turns it into electrical signals, which are then amplified using known methods and converted into useful information. Communication, naturally, as everywhere else now, must be digital and, accordingly, full-duplex. But should it be laser in both directions? Absolutely not necessary! Why this is so will become clear to us, we only need to consider how the receiving and transmitting devices for laser communication differ, and how the requirements for the weight and size parameters of communication devices on orbital spacecraft (or deep space spacecraft) and ground-based complexes differ.

As mentioned earlier, the receiving complex is a telescope. With lenses and (or) reflectors, a system for attaching them and pointing the telescope. And this means a heavy and bulky design - which is completely unacceptable for a spacecraft. Because for a spacecraft, any device must be as light and compact as possible. Which is quite typical for an LI transmitter - everyone has probably already seen modern PP lasers the size and weight of a fountain pen. Well, it’s true that power supply for a real, non-toy laser will weigh more, but for radio digital communication systems it will weigh even more due to its much lower energy efficiency.

What follows from all this? This means that there is absolutely no need to transmit data in both directions using a laser; it is enough to transmit it only from the satellite in an optical channel, and to the satellite (SC) in a radio channel, as before. Of course, this means that you will still have to use directional parabolic antenna for taking, which is not good for the weight of the spacecraft. But it should be taken into account that the antenna for reception, like the receiver itself, will still weigh several times less than the one for transmission. Because we can make the power of a ground-based transmitter orders of magnitude more powerful than on a spacecraft, which means we don’t need a large antenna. In some cases, a directional antenna will not be needed at all.

That. we have a reduction in the weight of the spacecraft almost by several times, as well as in energy consumption. Which is a direct path to the possibility of universally using microsatellites for communications, space exploration and other needs, which means a sharp reduction in the cost of space. But that's not all.

First, let's consider a way to solve the problem of pointing a laser beam from a satellite to a ground-based receiver. At first glance, the problem is serious, and in some cases, completely unsolvable (if the satellite is not at a geostationary station). But the question is: is it necessary to point the beam at the receiver?

Eat known issue- this is the divergence and attenuation of the laser beam when passing through the atmosphere. The problem is especially aggravated when the beam passes through layers with different densities. When passing through the interface between media, a beam of light, incl. and the laser beam experiences particularly strong refractions, scattering and attenuation. In this case, we can observe a kind of light spot resulting precisely from passing such an interface between the media. There are several such boundaries in the Earth's atmosphere - at an altitude of about 2 km (active weather atmospheric layer), at an altitude of approximately 10 km, and at an altitude of approximately 80-100 km, i.e. already at the boundary of space. The heights of the layers are given for mid-latitudes in the summer. For other latitudes and other seasons, the heights and the very number of interfaces between the media may differ greatly from those described.

That. When entering the Earth’s atmosphere, a laser beam, which had previously calmly traveled millions of kilometers without any losses (except perhaps a slight defocusing), loses the lion’s share of its power within some unfortunate tens of kilometers. However, we can very well turn this seemingly bad fact to our advantage. For this fact allows us to do without any serious aiming of the beam at the receiver. For as such a receiver, or rather a primary receiver, we can use the Earth’s atmosphere itself, or rather these very boundaries between layers and environments. We can simply point the telescope at the resulting light spot and read information from it. Of course, this will significantly increase the amount of interference and reduce the data transfer speed. And it will make it completely impossible in the daytime for obvious reasons - the sun! But how much we can reduce the cost of a satellite by saving on the guidance system! This is especially true for satellites in non-stationary orbits, as well as for spacecraft for deep space research. In addition, considering that lasers, even with such a low-quality, not narrow frequency band as Chinese lasers, can quite realistically be filtered out from interference using light filters or narrow-frequency photodetectors.

No less relevant could be the use of laser communication not for space, but for terrestrial long-distance communication in a manner similar to tropospheric communication. This refers to the transmission of data by laser also using atmospheric scattering at the interfaces of atmospheric layers from one point on the Earth’s surface to another. The range of such communication can reach hundreds and thousands of kilometers, and even more when using the relay principle.

Tags: laser communication, space

The advantages of a laser channel over a radio channel are that, firstly, it does not create radio interference; secondly, it is more confidential; thirdly, it can be used under conditions of exposure to high levels of electromagnetic radiation.

The schematic diagram of the transmitter is shown in Fig. 1. The transmitter consists of a command encoder made on an ATtiny2313 microcontroller (DD1), an output block on BC847V transistors (VT1, VT2) and an RS-232 interface, which, in turn, consists of a DB9-F connector (for cable) (XP1) and level converter - on MAX3232 (DD3).

The microcontroller reset circuit consists of elements DD2 (CD4011B), R2, C7. The output unit is an electronic switch made on transistor VT1, in the collector circuit of which a laser pointer is connected through a current limiter on transistor VT2. The transmitter is powered by a constant stabilized voltage of 9 - 12 V. Microcircuits DD1, DD2, DD3 are powered by a voltage of 5V, which is determined by the 78L05 stabilizer (DA1).

The DD1 controller is programmed in the BASCOM environment, which allows it to send commands from personal computer(PC) via RS-232 interface, from the Bascom terminal, using the “echo” function.

The microcontroller has clock frequency 4 MHz from internal oscillator. Packs of pulses with a frequency of about 1.3 KHz from the OS0A (PB2) output are supplied to the output block. The number of pulses in a packet is determined by the number of the command received from the PC.
To enter a command, you need to press any key on the PC keyboard, then when the words “Write command” and “Enter No. 1...8” appear, enter a number from 1 to 8 and press the “Enter” key.

The program for the transmitter microcontroller “TXlaser” consists of a main loop (DO...LOOP) and two interrupt processing subroutines: for reception (Urxc) and for timer 0 overflow (Timer0).

To obtain an output frequency of 1.3 KHz, the timer is configured with a frequency division factor (Prescale) = 1024. In addition, counting starts from the lower value Z = 253 (at a high level on PB2) and reaches 255. A timer overflow interrupt occurs when the processing of which switches the output of PB2, and the timer is again set to the value Z = 253. Thus, a signal with a frequency of 1.3 KHz appears at the output of PB2 (see Fig. 2). In the same subroutine, the number of pulses on PB2 is compared with the specified one, and if they are equal, the timer stops.

In the reception interrupt processing subroutine, the number of pulses that need to be transmitted is set (1 – 8). If this number is greater than 8, the message “ERROR” is displayed in the terminal.

During the operation of the subroutine, the PD6 pin is present low level(HL1 LED is off) and the timer is stopped.
In the main loop, pin PD6 is high, and the HL1 LED is turned on.
Text of the "TXlaser" program:

$regfile = "attiny2313a.dat"
$crystal = 1000000
$hwstack = 40
$swstack = 16
$framesize = 32

Config Pind.0 = Input "UART - RxD
Config Portd.1 = Output "UART - TxD
Config Portd.6 = Output "LED HL1
Config Portb.2 = Output "output OC0A

"timer configuration 0-division factor=1024:
Config Timer0 = Timer, Prescale = 1024
Stop Timer0 "stop the timer

Dim N As Byte "variable definition"
Dim N0 As Byte

Const Z = 253 "lower limit of the timer count for output frequency = 1.3 KHz
Timer0 = Z

On Urxc Rxd "receive interrupt processing subroutine
On Timer0 Pulse "overflow interrupt routine"

Enable Urxc
Enable Timer0

Do "main loop
Set Portd.6 "turn on the HL1 LED
Loop

Rxd: "receive interrupt processing subroutine
Stop Timer0
M1:
Print "Write commad"
Input "Enter No. 1...8:" , N0 "command input
If N0 > 8 Then "limit the number of commands
Print "Error"
Goto M1
End If
N0 = N0 * 2
N0 = N0 - 1 "set value of the number of pulses in a packet
Toggle Portb.2
Start Timer0 "start the timer
Return

Pulse: "overflow interrupt processing routine"
Stop Timer0
Toggle Portb.2
Reset Portd.6 "turn off the LED
Timer0 = Z
N = N + 1 "increment in the number of pulses
If N = N0 Then "if the number of pulses = specified
N=0
N0 = 0
Waitms 500 "delay 0.5s
Else
Start Timer0 "otherwise, continue counting
End If
Return
End "end program

The transmitter is made on a printed circuit board measuring 46x62 mm (see Fig. 3). All elements, except the microcontroller, are SMD type. The ATtiny2313 microcontroller is used in a DIP package. It is recommended to place it in the panel for DIP chips TRS (SCS) - 20 in order to be able to “painlessly” reprogram it.

Printed circuit board The transmitter TXD.PCB is located in the “FILE PCAD” folder.
The schematic diagram of the laser channel receiver is shown in Fig. 4. At the input of the first amplifier DA3.1 (LM358N), a low-pass filter formed by elements CE3, R8, R9 and having a cutoff frequency of 1 KHz attenuates background noise 50 -100 KHz from lighting fixtures. Amplifiers DA3.2 and DA4.2 amplify and increase the duration of received pulses of the useful signal. The comparator on DA4.1 generates an output signal (one), which is supplied through the inverters of the CD4011D (DD2) chip - DD2.1, DD2. The signal synchronously arrives at the contacts of the microcontroller ATtiny2313 (DD1) – T0 (PB4) and PB3. Thus, Timer0, operating in the external pulse counting mode, and Timer1, measuring the time of this counting, are launched synchronously. Controller DD1, performing the function of a decoder, displays received commands 1...8 by setting log.1 on the PORTB pins, respectively PB0...PB7, while the arrival of a subsequent command resets the previous one. When the command “8” arrives at PB7, log.1 appears, which, with the help of electronic key on transistor VT1, turns on relay K1.

The receiver is powered constant voltage 9 -12V. The analog and digital parts are powered by 5V voltages, which are determined by stabilizers of type 78L05 DA5 and DA2.

In the RXlaser program, Timer0 is configured as a counter of external pulses, and Timer1 as a timer that counts the period of passage of the maximum possible number of pulses (command 8).

In the main cycle (DO...LOOP), Timer1 is turned on when the first command pulse is received (K=0), the condition for enabling the inclusion of timer Z=1 is reset.
In the interrupt processing subroutine, when the Timer1 count coincides with the value of the maximum possible count, the command number is read and set in PORTB. The condition for enabling inclusion of Timer1 is also set - Z=0.
Text of the RXlaser program:

$regfile = "attiny2313a.dat"
$crystal = 4000000
$hwstack = 40
$swstack = 16
$framesize = 32

Ddrb = 255 "PORTB - all outputs
Portb = 0
Ddrd = 0 "PORTD-input
Portd = 255" pull-up PORTD
Config Timer0 = Counter , Prescale = 1 , Edge = Falling "as pulse counter
Config Timer1 = Timer, Prescale = 1024, Clear Timer = 1" as timer
Stop Timer1
Timer1 = 0
Counter0 = 0

"variable definition:
Dim X As Byte
Dim Comm As Byte
Dim Z As Bit
Dim K As Bit

X =80
Compare1a = X "number of pulses in the match register
Z=0

On Compare1a Pulse "interrupt routine by coincidence

Enable Interrupts
Enable Compare1a

Do "main loop
If Z = 0 Then "first condition for turning on the timer
K = Portd.3
If K = 0 Then "second condition for turning on the timer
Start Timer1
Z=1
End If
End If
Loop

Pulse: "subroutine interrupt processing by coincidence
Stop Timer1
Comm = Counter0 "reading from the external pulse counter
Comm = Comm - 1 "definition of the bit number in the port
Portb = 0 "port zeroing
Set Portb.comm "set the bit corresponding to the command number
Z=0
Counter0 = 0
Timer1 = 0
Return
End "end program

The programs "TXlaser" and "RXlaser" are located in the Lazer_prog folder.

The receiver is located on a board measuring 46x62 mm (see Fig. 5). All components are SMD type, with the exception of the microcontroller, which must be placed in a panel for DIP chips of type TRS(SCS) - 20.

Setting up the receiver comes down to setting the end-to-end transmission coefficient and the response threshold of the comparator. To solve the first problem, you need to connect an oscilloscope to pin 7 of DA4.2 and by selecting the value of R18, set the end-to-end transmission coefficient at which the maximum amplitude of noise emissions observed on the screen will not exceed 100 mV. Then the oscilloscope switches to pin 1 of DA4.1 and selecting a resistor (R21) sets the zero level of the comparator. By turning on the transmitter and directing the laser beam to the photodiode, you need to make sure that rectangular pulses appear at the output of the comparator.
The receiver circuit board RXD.PCB is also located in the FILE PCAD folder.

It is possible to increase the noise immunity of the laser channel by modulating the signal with a subcarrier frequency of 30 – 36 KHz. Modulation of pulse trains occurs in the transmitter, while the receiver contains a bandpass filter and an amplitude detector.

The diagram of such a transmitter (transmitter 2) is shown in Fig. 6. Unlike transmitter 1 discussed above, transmitter 2 has a subcarrier generator tuned to a frequency of 30 KHz and assembled on slots DD2.1, DD2.4.. The generator provides modulation of bursts of positive pulses.

The laser channel receiver with a subcarrier frequency (receiver 2) is assembled on domestic microcircuit K1056UP1 (DA1). The receiver circuit is shown in Fig. 7. To isolate command pulses, an amplitude detector with a low-pass filter and a pulse normalizer, assembled on logic elements DD3.1, DD3.2, a diode assembly DA3 and C9, R24, are connected to the output of the DA1 10 microcircuit. Otherwise, the circuit of receiver 2 coincides with the circuit of receiver 1.

Optical communication is carried out by transmitting information using electromagnetic waves optical range. An example of optical communication is the transmission of messages used in the past using fires or semaphore alphabet. In the 60s of the 20th century, lasers were created and it became possible to build broadband optical communication systems that transmit not only telephone, but also television and computer signals.
Optical communication systems are divided into open, where the signal is transmitted in the atmosphere or space, and closed, that is, using light guides . Below, only open atmospheric communication lines are considered.
An optical atmospheric communication system between two points consists of two paired transceiver devices located within line of sight at both ends of the line and directed towards each other. The transmitter contains a laser generator and a modulator of its optical radiation by the transmitted signal. The modulated laser beam is collimated by the optical system and directed towards the receiver. In the receiver, the radiation is focused onto a photodetector, where it is detected and the transmitted information is isolated. Since the laser beam is transmitted between communication points in the atmosphere, its distribution is highly dependent on weather conditions, the presence of smoke, dust and other air pollutants. In addition, turbulent phenomena are observed in the atmosphere, which lead to fluctuations in the refractive index of the medium, beam oscillations and distortions of the received signal. However, despite mentioned problems, atmospheric laser communication turned out to be quite reliable at distances of several kilometers and is especially promising for solving the problem of the latest problem. The propagation of laser radiation in the atmosphere is accompanied by a number of phenomena of linear and nonlinear interaction of light with the medium. Moreover, none of these phenomena manifests itself separately. Purely Qualitatively, these phenomena can be divided into three main groups: absorption and scattering by air gas molecules, attenuation by aerosols (dust, rain, snow, fog) and radiation fluctuations due to atmospheric turbulence. The main limiters of the ALS range are thick snow and thick fog, for which. aerosol attenuation is maximum. The propagation of the laser beam is also strongly influenced by atmospheric turbulence, that is, random spatiotemporal changes in the refractive index caused by the movement of air, fluctuations in its temperature and density. Therefore, light waves propagating in the atmosphere experience not only absorption, but also. fluctuations in transmitted power.
Atmospheric turbulence leads to distortions of the wave front and, consequently, to oscillations and broadening of the laser beam and redistribution of energy in its cross section. In the plane of the receiving antenna, this manifests itself in a chaotic alternation of dark and bright spots with a frequency from fractions of a hertz to several kilohertz. In this case, signal fading sometimes occurs (the term is borrowed from radio communications) and the connection becomes unstable. Fading is most pronounced in clear sunny weather, especially in the hot summer months, during the hours of sunrise and sunset, and in strong winds. ALS systems can be used not only in the “last mile” of communication channels, but also as inserts in fiber-optic lines on some difficult areas; for communication in mountainous conditions, at airports, between individual buildings of one organization (government bodies, shopping centers, industrial enterprises, university campuses, hospital complexes, construction sites, etc.); when creating spatially dispersed local computer networks; when organizing communications between switching centers and base stations cellular networks; for quickly laying a line with limited installation time. Therefore in Lately the interest of domestic producers in this new and promising sector is growing

The functional diagram of a laser communication system is very simple:

· the processing unit receives signals from various standard devices(telephone, fax, digital PBX, local computer network) and converts them into a form acceptable for transmission by a laser modem;

· the converted signal is transmitted by the electro-optical unit in the form of infrared radiation;

· on the receiving side, the light collected by the optical system falls on a photodetector, where it is converted back into electrical signals;

· The amplified and processed electrical signal is sent to a signal processing unit, where it is restored to its original form.

Transmission and reception are carried out by each of the paired modems simultaneously and independently of each other. Laser modems are installed in such a way that the optical axes of the transceivers coincide. The main difficulty is adjusting the direction of the optical axes of the transceivers. The divergence angle of the transmitter beam ranges from several arc minutes to 0.5° for different models, and the adjustment accuracy must correspond to these values.

After installing the transceiver units, you need to connect them to cable networks in both buildings. There are many models of devices with a wide variety of interfaces, however, unlike suppliers of radio communication equipment, manufacturers of wireless optics systems adhere to the following general connection ideology: a laser communication line is an emulation of a piece of cable (two twisted pairs or two fibers of an optical cable). Connected via wireless optics local networks function as if they were connected by a dedicated cable. Some models of laser modems have combined interfaces to Ethernet networks and E1 streams. As a result, one atmospheric link can connect the LAN and telephone networks buildings without using a multiplexer.

This is what an installed atmospheric laser communication system looks like. System throughput is 100Mbit/sec at a distance of up to 3! kilometers. photo:

Some wireless remote bridges are used to transmit data infrared radiation laser Typically, such a device contains a traditional wired Ethernet bridge and a laser modem that provides physical communication. In other words, laser device only sends data bits, and the rest of the work is done by a regular bridge. Laser modems generate radiation with a wavelength of 820 nm, which cannot be detected without special instruments. Obviously, for laser bridges, the emitter and receiver must be located on a straight line visibility. The typical distance between bridges is slightly more than 1 km and is limited by laser power.
One of the main advantages of such systems is their high throughput. Second the advantage is sufficient noise immunity, since infrared radiation does not interact with radio waves. Like fiber optic systems, laser bridges provide a high level of security. To intercept information, it is necessary to place the appropriate device on the beam line, which, firstly, can be easily detected, and secondly, this is very difficult to implement, since such systems are installed on the roofs of high-rise buildings. The disadvantages of laser-based systems are the influence of weather conditions on the stability of communications. Heavy rain, snow or fog will cause significant beam scattering and signal weakening. The connection can also be affected by sunrise or sunset if the channel is oriented east to west.
Wireless bridges are used to permanently connect networks, as a backup link, or as a temporary solution. Many companies are involved in their production. Prices depending on bandwidth and communication distances range from 5 to 75 thousand dollars per channel. Expensive, but over time this decision can pay off.

2.5 Gbit/s over laser beam

fSONA Communications has introduced a new wireless optical communication system, SONAbeam 2500-M, which allows data transfer rates of about 2.5 Gbit/s. The system is based on four redundant transmitters operating at a wavelength of 1550 nm with a laser signal output power of 560 mW. On a five-kilometer test site in clear weather, the system operated at maximum speed and with virtually no errors.

Control questions

1. What technologies are used to create wireless networks?

2. List the main technologies of radio networks.

3. What is t access points(access point)?

4. Describe 802.11 technology. What is a directional and omnidirectional antenna?

5. What is roaming(roaming).?

6. List technologies alternative to the IEEE 802.11 standard;

7. Characterize the technology Bluetooth.

8. Characterize the technology HiperLAN.

9. What are optical networks?

10. What are microwave systems?

11. Describe the IEEE 802.16 (WiMAX) standard?