MMDS data transmission systems. Antennas and downconverters. Technologies for wireless information distribution MMDS

Technologies for wireless information distribution MMDS

Cellular television systems based on MMDS technology have received last years widespread as an alternative to classic cable networks. These include:

MMDS - microwave multipoint distribution system;

LMDS - local multipoint distribution service;

MVDS - multipoint video distribution system.

The MMDS system was developed back in 1983. The main advantage of MMDS is the absence of expensive wired infrastructure. MMDS technology has been most intensively implemented in rural areas, as well as in countries where cable networks are insufficiently developed. The MMDS system is a serious competitor to CATV and satellite TV both in cost, quality and functionality. The equipment is manufactured for the 2.5–2.7 GHz range.

The small radius of propagation of millimeter waves determined the use of MVDS technology in networks with low-power transmitters built on the cellular principle. The wide bandwidth combined with the cellular structure makes this technology very suitable for organizing interactive multimedia networks, including television, telephony, video conferencing, and high-speed Internet access. To increase the range or expand the coverage area of MMDS, repeaters are used.

At the receiving post, a structure of a small antenna with a signal converter is mounted in the line of sight from the transmitter. Signals from the output unit are supplied via a coaxial cable through a standard decoder to the subscriber's apartment. The signals are decoded if they have permission from the address coding system. In accordance with the Radio Regulations, the following frequency bands are allocated for radio systems such as MMDS, LMDS and MVDS:

2.1-2.7 GHz.

30.8-33.4 GHz.

27.5-29.5 GHz.

40.5-42.5 GHz.

42.5-43.5 GHz.

To date, dozens of MMDS systems have been implemented that provide Internet access, interactive television and other broadband services using wireless access technology. The ability to integrate an MMDS system with high-speed wireless digital data exchange makes it easy to solve the last mile problem.

Data requested by users is broadcast downstream on digital channels using QPSK, 16-, 32-, 64-128- or 256-QAM modulation. At the same time, depending on the channel width and the selected signal modulation scheme, one TV channel with a width of up to 8 MHz provides data transfer rates of up to 56 Mb/s, which is significantly higher than the widespread ADSL.

The service area radius of the MMDS system is determined by the height of the transmitting antenna, the power of the transmitter, the number of transmitted channels, losses in the antenna-feeder path and the gain of the transmitting and receiving antennas.

The main advantage of MMDS broadcasting networks over cable ones is that they require lower capital costs (at least four times with 100 distribution points within a radius of 20 km from the television center). Secondly, the MMDS system, compared to a cable network, is more compact and mobile, and does not require a large staff of employees to operate and repair the network.

The use of MMDS systems in multi-channel terrestrial television systems has a number of advantages compared to conventional terrestrial television broadcasting systems:

Possibility of transmitting up to 25 television programs, depending on the standard with an analog signal and 4-6 times more when modulated with digital signals of the MPEG-2 standard.

Television broadcasting is carried out at an environmentally friendly level, the total transmitter power is mainly 1-10 W. (For reference: the current TV broadcasting systems use transmitters with a power in the meter range of up to 50 kW, in the decimeter range - up to 10 kW).

Using a compact antenna with linear dimensions of 15-25 cm.

High signal quality due to the relatively low level of interference in the frequency ranges allocated for these systems (2.5-2.7 GHz).

Elimination of so-called “shadow zones” in cities with multi-storey buildings using repeaters.

Reduced operating costs due to the absence of long main and sub-main lines.

High resistance to various types of reconstruction and natural disasters (fire, earthquake, man-made emergencies).

Internet traffic is asymmetrical: the intensity of information in the forward channel is 10-20 times higher than the intensity of request transmission. Therefore, simpler modulation schemes are used in the return channel, allowing speeds of up to 25 Mbit/s to be achieved in the 8 MHz band.

The MMDS system equipment kit includes the following components (Figure 4.1):

Modulators;

Input receiving system;

Digital/analog transmitters (or one group transmitter for N channels);

Digital/analog channel combiner (RF mixer, combiner);

Waveguide and coaxial cable.

Antennas;

Broadband repeaters (if necessary);

Control system;

Automatic or manual reservation system.

Figure 4.1 - Television and radio broadcasting based on the MMDS system

Modulators designed to transfer a television signal to the high-frequency range (2.5 GHz). Currently, terrestrial television uses analog signals with amplitude modulation to transmit images, and frequency modulation is used to transmit audio signals.

Input receiving system designed to receive incoming audio, video and data signals at the base station.

In the practice of designing and installing MMDS systems, two options for constructing block diagrams are used:

single channel,

multichannel.

In the single-channel version, N-transmitting devices are used to transmit N-television programs, including a modulator and the transmitter itself, and the power of different transmitters is summed in the antenna (see Figure 4.2).

Figure 4.2 - MMDS structure for single-channel transmitters

In the multi-channel version, the transmitted N-TV programs first arrive at their modulators, then a group signal is formed from them, which modulates a broadband transmitter operating on a common antenna (see Figure 4.3).

Figure 4.3 - MMDS structure with a multi-channel transmitter

In the 2500..2700 MHz band, 31 analogue television channels of the NTSC standard (6 MHz channel band) and 24 channels of the PAL and SECAM standard (8 MHz band) can be placed.

In the single-channel version, all the power is emitted in a given channel, and in the multi-channel version, it is reduced by about 50 times with 8 channels, i.e. the power in each channel drops by approximately 2N times.

When transmitting TV signals through the system, the radio source for transmitting them to consumers is a digital headend. Video signals from satellite, local television stations or VCRs are encoded (MPEG encoders) and multiplexed into transport streams, including signals from 4 to 10 individual video servers.

It is advisable to use multi-channel or group transmitters in small cities and towns, where the radius of the coverage area does not exceed 6 km.

The wideband transmitter allows you to simultaneously amplify and broadcast 24 television channels. This range of transmitters includes models with output power from 50 to 1300 W.

With almost equal signal-to-noise ratios (52-55 dB), a broadband transmitter, due to the division of power by the number of channels, will have a smaller broadcast area than a single-channel transmitter. However, for small cities and towns this is a more cost-effective solution. The broadband transmitter can be used both at the head MMDS station and as a broadband repeater to increase the broadcast area and, accordingly, the number of subscribers.

Channel adder(RF mixer, combiner) works with adjacent or non-adjacent digital and analog applications without exciting cross-resonances, is a welded aluminum structure, characterized by high-quality connections, low attenuation. The mixer filters and sums the MMDS channels through a broadband directional filter into a common waveguide. It has lower input loss compared to a system using separate spectral filtering and summation.

1. Block diagram of subscriber installationMMDS.

MMDS (Multichannel Multipoint Distribution System) – a microwave, multipoint signal distribution system designed to deliver a certain range of services to consumers. The MMDS scheme includes a multi-channel transmission complex and individual (collective) subscriber installations. The subscriber installation includes:

Receiving antenna

Buck converter

Buck Converter Power Supply

Local cable trunk

Subscriber converter (Set Top)

MMDS signals are encrypted to prevent unauthorized access. Authorized access is controlled by the computer special program ACS.

2. What is cable attenuation? Attenuation value for cablesRG-59 andRG-6.

Cable attenuation– this is a decrease in signal level per unit cable length. Measured in decibels. The signal attenuation in a cable is different at different frequencies. A large attenuation value reduces the level of the useful signal. In practice, this results in a decrease in image quality. The higher the frequency of the signal, the greater its attenuation. The greater the linear length of the cable, the greater the signal attenuation. The difference between the attenuation values on one section of cable at different frequencies is called slope.

3. Antenna designMMDS, composition, purpose, method of fastening.

Antenna is a system designed to receive or emit electromagnetic waves. Receiving antenna converts radio wave energy into electric current energy.

The MMDS antenna is designed to receive and convert the signal.

The MMDS antenna consists of:

Reticles (reflector 18T or 25T)

Integrated converter (T4010)

Counter reflector (sub-reflector)

- “L”-shaped bracket with holes for fastening the grid and converter and a groove for adjustment

- “U” - shaped bolt with a fixing bracket for fastening to the mast

Fastening material (bolts, nuts, washers)

The antenna is installed on a mast or on a wall mount, depending on the reception conditions and the design features of the building. Wall mount– these are special brackets for installing the antenna on a flat vertical and horizontal surface. The design of the mount allows for adjustment in tilt and height for precise adjustment. The design uses locking washers to prevent the connection from loosening when wind occurs.

There are some differences in the method of mounting the T4010 antennas from the T2127. For the T2127 antenna, additional brackets are provided for mounting the down converter on the mast.

4. Antenna typesMMDS, their differences.

Two types of antennas are used to receive MMDS signals.

Old type antenna It is structurally different in that the down converter and the amplifier are made in different blocks. They differ in the designation and method of fastening. Irradiator – T2127, converter – QL1047. The signal is first converted (converted), then it enters the amplifier unit via a cable. Converted and amplified signal transmitted via cable line to the consumer.

New type antenna consists of an integrated converter in which an amplifier is mounted. Structurally designed in one compact body. Designated as T4010. Signal conversion and amplification takes place in one unit and immediately reaches the consumer via cable line.

5. How to choose the right signal receiving pointMMDS.

For the right choice reception points, it is necessary to take measurements using a device (television signal level meter) and find a place where the level of the useful signal will be maximum and the signal-to-noise ratio will be the best. For collective receiving systems, the receiving point is selected where the signal spectrum (frequency response) will be the most uniform (linear), the signal-to-noise ratio will be the best. Increasing the level of the useful signal is achieved by carefully aligning the antenna in the direction of the transmitting center.

6. What is horizontal polarization of a receiving antenna.

Radio wave

V or horizontal N

Horizontal antenna polarization– this is the position of the antenna in space, for best regards horizontal component of electromagnetic radiation (television radio signal).

7. What is vertical polarization of a receiving antenna.

Radio wave is a combination of electric and magnetic fields interconnected. Radio waves can propagate in free space, cables, waveguides, etc. Radio waves are characterized by length, amplitude, and polarization.

Radio waves can be plane-polarized or circularly polarized. The type of wave is determined by the position of the electric field vector. According to their position to the horizon, they distinguish between vertical V or horizontal N polarization. To increase the amount of transmitted information, electromagnetic waves are polarized in different planes.

Vertical polarization of the antenna– this is the position of the antenna in space for the best reception of the vertical component of electromagnetic radiation (television radio signal).

8. What range do the antennas operate in?MMDS.

The MMDS antenna is designed to receive a signal in the range of 2.1 GHz...2.7 GHz (international standard), amplify and convert it into a special one. range (HB – hotel band) 295 MHz…487 MHz. We use the range 2.2 GHz...2.5 GHz.

9. What is the difference between 18T and 25T antennas?

The reflector (grid) of the antenna has a spherical surface of a cellular structure made of metal. Antennas 18T and 25T differ in design. The 18T mesh has a reception area approximately half that of the 25T mesh. Therefore, the received signal level is approximately 8 dB/mV less at the same receiving point. The 25T mesh consists of two identical sectors (for ease of transportation) connected to each other by fasteners.

10. What is a converter. Purpose of a step-down converter.

Converter– converter of one type of signal to another. The converter used in the MMDS system converts (lowers the signal frequency) the range of 2.2 GHz...2.7 GHz into special ones. range (hyper range NV) 295 MHz...487 MHz. The integrated converter has a built-in amplifier to amplify the signal.

11. Types of converters used to receive MMDS.

MMDS uses 2 types of converters:

T 4010 (integrated), in which the amplifier and step-down converter are made in one housing

QL1047 with T2127 feed, converter and amplifier are made in different housings.

12. Nominal level at the converter output, limits of change.

The nominal (for normal and stable operation) output signal level should be 75 dB/mV. Range of changes from 50dB/m to 95dB/m. At 40 dB/m Set Top closes. The control signal value must be at least 65dB/m.

13. Types of cables used in the installation of individual and collective installations.

RF cable – flexible coaxial cable, consists of a copper or copper-plated steel conductor (central core) 1, dielectric insulation (in some cables impregnated with a water-repellent composition) 2, screen (foil, woven metal mesh) 3, protective sheath 4 made of material that prevents harmful influences sun and environment.

For the construction of collective installations, several types of cables are used: RG-11(sub-highway, inter-access constrictions), RG-6(cable for reducing risers, supplying a signal to the consumer), RG-59(sending a signal to the consumer). 565 and RG-11 cables are used in collective trunks and submains.

14. What is the frequency response of a cable, how do the levels of transmitted signals change depending on the distance.

Frequency response– dependence showing the change in cable parameters (amplitude, signal attenuation) depending on frequency. Expressed graphically.

U U – signal level (amplitude) (dB/mV)

ΔU – level change (dB)

f – signal frequency (MHz)

As the linear length of the cable increases, the signal level decreases.

15. What are connectors, their types and purposes.

Connector (from Englishconnect- connect)– connector, connector. Designed for switching (connecting) various components in a cable television system (splitters, taps, amplifiers, cables). They differ in type and functionality. There are corresponding connectors for each brand of cable.

F-11 for RG-11 cable (connection of RG-11 cable with splitters, taps, etc.)

F-6 for RG-6 cable (connection of RG-6 cable with splitters, taps, etc.)

F-59 for RG-59 cable (connection of RG-59 cable with splitters, taps, etc.)

F-6 BEL- M for connecting an RG-6 cable to a consumer (TV, VCR)

F-59 BEL- M for connecting the RG-59 cable to the consumer (TV, VCR)

F-6 BEL- F for switching connectors type F-6 BEL-M

F-59 BEL- F for switching connectors type F-59 BEL-M

F-81 for connecting two cables with connectors at the ends (type F-59, F-6, F-11)

There are options for connectors with rubber rings to seal the connection. Used for outdoor installation to prevent moisture from entering the joint.

16. How the power supply of the converter is organized.

The converter is powered using a power supply unit (PSU) converting 220V alternating current at 20...24V DC through a power “injection” (power insert), which supplies voltage to the converter and prevents voltage from being supplied to the consumer (Set Top).

17. Is it possible to connect two or more subscribers to one antenna, how to organize power supply.

It is possible to connect several subscribers to one MMDS antenna (usually no more than 3…4). Using amplifiers, you can increase the number of subscribers connected to one antenna (collective network). When connecting several subscribers to one antenna, it is necessary to properly organize the power supply for the converter. Below are 2 options for organizing power supply for the converter. The option of using one power supply unit (PSU) is ideal, but unlikely in practice. The power supply is installed before the signal is branched; the signal can be divided either by a coupler (DC does not pass power) or a splitter (DSU passes power). The option of using several power supplies is often found in practice: one antenna, several consumers. (If one power supply is turned off, the other will power the antenna). The signal is divided (branched) using a splitter (DSU) to decouple several power supplies. If you use taps (DC), then the power supply may fail (for example, from a short circuit)), since DC does not pass power.

Purpose of the SET TOP device, their types, differences.

« SetTop" (decoder)– a multifunctional device that converts all frequencies of the operating range to a frequency of 85.25 MHz (4th meter channel), allows you to decode encoded channels, control sound volume, program “favorite” channels, implement “parental” control, implements the “bought” function – look”, is programmed for various packages without user intervention.

Set Top types:

5 inputs) allows you to mix the input signal from two sources, has a low-frequency output (RCA, video out, audio out connectors) for easy switching of consumer equipment (VCR, AV receiver, stereo system, etc.);

5input) a simplified version for working with one input signal source, does not have a low-frequency output;

All “Set Top” models implement the functions of “parental” control (setting a password for viewing selected channels), programming and scrolling through “favorite” channels, and the “bought - watch” function.

19. The main symptoms of a SET TOP malfunction, reflected on the display screen.

If a malfunction occurs, the SET TOP display displays information in the form of symbols, the interpretation of which is given in the table.

20. Tell us about the purpose of the remote controlR.C.-700.

To measure the level of a television signal, the following instruments are used:

IT 06 (digital display of the level of each channel separately)

IT 07 (graphical and digital display of signal level over the entire frequency spectrum)

PROMAX 8 (graphic and digital display of signal level over the entire frequency spectrum, sound)

Window Lite (graphic and digital display of signal level over the entire frequency spectrum, sound)

PTO (analog-digital indication, very convenient in the MMDS system).

28. What is an attenuator, its purpose.

Attenuator (from English.attenuate- weaken)– a device designed to reduce or change the amplitude of electrical signals or the power of electromagnetic oscillations. The attenuator is used to adjust the signal level (mainly when setting up amplifiers). Made from materials with high losses, or in the form of resistance. It is indicated on the device body as “FAM” with a digital indication of the level by which the signal will decrease (dB). FAM-3, FAM-6, FAM-8, FAM-10, FAM-12, FAM-16, FAM-20 are used. If the device or amplifier has a built-in attenuator, then it can be continuously adjusted. The regulation range is indicated on the device body or in the passport.

29. What is an equalizer, its purpose.

Equalizer (from English.equalize– call)– a frequency dependent device (bandpass filter) that allows you to isolate and regulate specific area signal spectrum. Designed to adjust the signal level (mainly when setting up amplifiers). Equalizers can be made as a separate device, or built-in (into the amplifier), continuously adjustable. The range of adjustments is marked on the case or in the passport. Specifically, in our case, the slope of the frequency response is adjusted by the amount indicated on the device body. The equalizer is designated “ILSEM” with a digital indication of the frequency response slope level (dB).

30. Effect of cable damage (cuts, crushing, etc.).

Damage (squeezing) of the cable leads to a change in the wave impedance and, as a result, leads to a loss of quality (the line is not fully matched). Violation (cuts) of the screen leads to interference with the cable of over-the-air channels and, as a result, deterioration of the image, double picture, loss of sound, and color distortion.

31. The influence of signal-to-noise ratio on image quality. Nominal signal-to-noise ratio for high-quality reception.

Quantitative indicators:

45 dB/mV excellent

43 dB/mV good

40 dB/mV satisfactory

37 dB/mV bad (decoder does not open, works unstable)

Noise interference greatly affects picture and sound quality. Color loss, ripples, distorted sound.

32. What is affected by the level of the signal supplied to the amplifier input. What happens when it is exceeded nominal signal level and when it decreases more than established standards.

The signal level supplied to the amplifier input affects the image quality of the entire collective network (all consumers). With a significant increase in the nominal level at the amplifier input, the television image becomes distorted in the form of small horizontal stripes, and the decoder does not decode the channels. When the nominal signal level at the amplifier input is significantly reduced, the television signal becomes distorted in the form of “ripples,” unstable images, loss of color, and sound distortion.

Multichannel Multipoint Distribution System - in English abbreviation MMDS(M ultichannel M ultipoint D istribution S ystem) is a terrestrial television broadcasting system, similar to cable television, but without cable.
MMDS system operates at very high frequencies (2.5 - 2.6 GHz), this ensures high quality (no ghosting, high clarity, high noise immunity), large coverage radius and a large number of transmitted television programs.
The frequency range width is 2686-2500 = 186 MHz. This band can accommodate up to 24 analogue television channels of the Russian standard D (SECAM, 8 MHz) or up to 31 channels of the European standard B (PAL, 6.5 MHz). For Western countries this is not much, so MMDS systems are built where creating a cable network is impossible or impractical. Currently, MMDS systems are used mainly for organizing multi-channel commercial television broadcasting, as well as other broadband services using wireless access technology, including interactive ones.
In many cases, this method of distributing television and radio programs has undeniable advantages over the long-known and widely used ones - via cable networks and via satellite relays. So, in particular, receiving antennas can be significantly smaller than satellite ones, since the power of the MMDS signal is much greater than the signal from the satellite.

The MMDS antenna can be installed anywhere under the only condition - the need for direct visibility from the installation site of the MMDS receiving antenna to the telecentre. The MMDS antenna, receiving a signal from the television center, converts (lowers) its frequency to a lower one, usually in the V UHF range. The antenna gain, or rather the converter gain (a converter is a device located in the center of the antenna), is enough on average to connect 1-4 TVs.
Advantages of MMDS:
Excellent broadcast quality.
There is no need to purchase an expensive antenna.
There is no need to purchase multiple antennas.
Ability to watch a large number of channels.
Programs for viewers of all ages.
Disadvantages of MMDS:
The signal propagation area is limited by the line of sight of the repeater.
The need for state licensing to use the frequency range.
The total number of broadcast television channels cannot exceed 24.
The power of MMDS transmitters is low - from fractions of a watt to several watts per channel, while conventional amplitude modulation is used - the same as in terrestrial television. Therefore, it is difficult to obtain a higher quality television signal in MMDS than in a traditional cable network. When transmitting digital channels and computer data, problems associated with multipath reception arise. Therefore, cable modem systems for MMDS are subject to more stringent requirements than the same equipment for wired cable networks. Accordingly, equipment with the same functionality for MMDS is more expensive than “cable” equipment.
Even leaves on trees contribute very significant attenuation. Re-reflection from buildings in the near zone from the transmitting center may also have some influence.
MMDS antenna
Interactive MMDS subscribers have a transceiver unit installed instead of a receiving antenna with a converter - a subscriber transceiver. Internet access is provided by a system of wireless cable modems.
Multichannel MMDS transmitters are the most popular solution for relatively small cities (radius up to 50 km). One broadband transmitter is installed at the transmitting station. To excite it, a set of television modulators is used that generate a signal in the MV range (approximately 200-400 MHz). The modulator signals are added by a conventional combiner (adder) and fed to the input of the transmitter, in which the group signal is converted into the 2.5 - 2.7 GHz range and amplified in power.
To receive digital programs, each MMDS subscriber must have a serial cable digital terminal installed. MMDS receiving antennas vary in gain: 21 dB, 24 dB, 27 dB.
MMDS subscriber downconverter
(Downconverter and power supply sold separately)

Designed for high-quality reception of digital and analogue television broadcasting signals and data in a wireless MMDS network in a difficult electromagnetic environment. Peculiarities:
has a low noise figure, and the high signal level at the device output makes it possible to serve both collective networks and individual subscribers;
compact, highly reliable design in a sealed housing;
low receiver noise figure;
wide dynamic range;
high suppression of out-of-band signals;
the ability to use external reflectors with different Kus;
support for digital broadcasting with QAM256 modulation;
the ability to service both collective networks and individual subscribers;
various frequency versions.

Earlier we mentioned such cellular television systems as MMDS (Multichannel Microwave Distribution System), LMDS() or MVDS (). Now we will analyze each system in detail and determine the pros and cons of each of them.

Multichannel Multipoint Distribution System - in English abbreviation MMDS (Multichannel Multipoint Distribution System) is a terrestrial television broadcasting system, an analogue of cable television, but without cable, in some way similar to a satellite television broadcasting system - only the relay satellite in this case is, as it were, located on the ground. In many cases, this method of distributing television and radio programs has undeniable advantages over the long-known and widely used ones - via cable networks and via satellite relays. So, in particular, receiving antennas can be significantly smaller than satellite ones, because the power MMDS- there is much more signal than the signal from the satellite. The frequency range width is 2686-2500 = 186 MHz. This band can accommodate up to 24 analogue television channels of the Russian standard D (SECAM, 8 MHz) or up to 31 channels of the European standard B (PAL, 6.5 MHz). For Western countries this is not much, so the systems MMDS are built, as a rule, where creating a cable network is impossible or impractical.

Benefits of MMDS:

· Inexpensive subscription service

· Minimum (compared to cable television networks) quantity technical specialists

· Ease of connection for the end user due to the absence of connection to the cable infrastructure

· Low cost of owning channels and maintaining channel-forming equipment compared to the cost of owning and maintaining the functionality of similar cable infrastructure

Disadvantages of MMDS:

· The total number of broadcast television channels cannot exceed 24

· Rapid deployment of the system in a specific area due to the ease of installation of both the basic transmission equipment and the repeater network

· Ability to use the network MMDS as a repeater for both state and local cable TV channels

Rice. 3 - Block diagram of MMDS

LMDS(Local Multipoint Distribution System) is a broadband point-to-multipoint wireless telecommunications system that operates in the frequency range above 20 GHz (the specific range varies by country and local band licensing). The LMDS system is designed for one- or two-way transmission of voice, data, Internet traffic and video. LMDS can be translated as Local Multipoint Distribution System.

At its core, LMDS technology is a cellular information transmission system for fixed subscribers based on a millimeter wave radio channel. The basis of its organization copies the principle of organizing a network in a mobile cellular communication. To cover a certain area (usually a city), a network of overlapping cells is deployed, with a base station installed in the center of each cell. One such station in the LMDS system allows you to cover an area with a radius of several kilometers and connect several thousand subscriber stations. At the same time, the stations themselves in the LMDS system are connected with each other by high-speed terrestrial communication channels or radio channels.

Advantages of LMDS:

A wireless system that does not require the installation of expensive cable communication lines.

Possibility of network deployment in a short period of time

If necessary, the system can be quickly dismantled and installed in another location.

Compared to wired communication channels of similar transmission speeds, the deployment of an LMDS subscriber terminal and the subscription fee for the channel are lower.

In Russia, the LMDS system has not yet become widespread.

MVDS (Multipoint Video Distribution System) is a broadband point-to-multipoint wireless telecommunications system, the main purpose of which is video transmission (including TV programs). Today, in the MVDS system, Internet, voice over IP and other types of services can be added to the video signal using an IP encapsulator. Therefore, the differences between LMDS and MVDS systems are gradually disappearing, although initially the first of them was intended for broadband transmission of mainly data, and the second - only video. MVDS can be translated as "multipoint video distribution system". At its core, MVDS is a cellular information transmission system for fixed subscribers based on a millimeter wave radio channel. According to the principle of its organization, MVDS copies the principle of network organization in mobile cellular communications. To cover a certain area (usually a city), a network of overlapping cells is deployed, in the center of each of which a base station (BS) is installed. One BS allows you to cover an area in the form of a circle (in reality it is a polygon) with a radius of several kilometers and connect several thousand subscriber stations (AU). The BSs themselves are connected to each other by high-speed terrestrial communication channels or radio channels.

The most attractive quality of MVDS systems is the width of the range provided - 2 GHz. Another feature of waves in this range is the straightness of their propagation. They are not able to bend around even small obstacles, but on the contrary, they are reflected from them with virtually no distortion. Practice has shown that at a frequency of 40 GHz, signals that have undergone 4-fold reflection are satisfactorily received. This property can be used in the design of high-frequency signal distribution systems. MVDS systems can use both analog and digital methods of information transmission, as well as various modulation systems. However, for the purposes of building multimedia networks, the development of purely digital systems is relevant. There are 2 types of systems: cable and satellite.

The “cable” type of system uses QAM modulation and a channel width of 8 MHz, and the “satellite” type uses QPSK modulation and a channel width of 36-40 MHz.

The satellite version of MVDS allowed transmission of up to 30 TV channels standard quality and provided signal reception on a 25-centimeter horn antenna within a radius of 10 km, and cable - up to 100 channels, but at a distance of up to 4.5 km, subject to reception on a 60-centimeter antenna. The MVDS multimedia network is built on the basis of the headend. When forming information flows, a wide variety of sources can be used - the Internet, terrestrial, cable and satellite television channels, and various local sources of information.

Rice. 5 – Block diagram of the MVDS system.

(2.5-2.686 GHz.)

Multichannel Multipoint Distribution System - in English abbreviation MMDS (Multichannel Multipoint Distribution System) is a terrestrial television broadcasting system, an analogue of cable television, but without cable, in some way similar to a satellite television broadcasting system - only the relay satellite in this case is, as it were, located on the ground.

In many cases, this method of distributing television and radio programs has undeniable advantages over the long-known and widely used ones - via cable networks and via satellite relays. So, in particular, receiving antennas can be significantly smaller than satellite ones, because the power MMDS- there is much more signal than the signal from the satellite.

Advantages MMDS:

Excellent broadcast quality
No need to purchase an expensive antenna
No need to purchase multiple antennas
Ability to watch a large number of channels
Programs for viewers of all ages

frequency range

Its width is 2686-2500 = 186 MHz. This band can accommodate up to 24 analogue television channels of the Russian standard D (SECAM, 8 MHz) or up to 31 channels of the European standard B (PAL, 6.5 MHz). For Western countries this is not much, so MMDS systems are built, as a rule, where creating a cable network is impossible or impractical. For Russian TV viewers, such a number of channels today is even excessive.

IN MMDS no return channel is expected. However, the system can be upgraded to bidirectional. The return channel signal is located in the MDS frequency band (2.15-2.16 GHz), WCS (2.30-2.35 GHz) or within the range itself MMDS(2.5-2.686 GHz). In the first case, it is necessary to obtain separate permits for an additional frequency range; in the second, the possible number of television channels, which is already small, is significantly reduced. The equipment for the return channel is an almost parallel system, the cost of which is commensurate with the cost of the transmitting station itself MMDS.

Transmitter power MMDS is small - from fractions of a Watt to several Watts for each channel, while conventional amplitude modulation is used - the same as in terrestrial television. So get in MMDS A better TV signal than a traditional cable network is difficult. When transmitting digital channels and computer data, problems associated with multipath reception arise. Therefore, cable modem systems for MMDS more stringent requirements are imposed than for the same equipment for wired cable networks. Accordingly, equipment with the same functionality for MMDS more expensive than cable.

MMDS in Russia

First MMDS-station began broadcasting in Moscow at the end of 1991. It was broadcast in analog format on 8 foreign channels, the target audience was very wealthy citizens (mainly foreigners living in Russia). And the technology began to truly become “widely known in narrow circles” in the second half of the 90s, when domestic companies began operating, providing services both for the sale of relevant equipment, and for carrying out design work, installation, etc., about MMDS information appeared in specialized publications, on the Internet, with equipment, features of construction and operation MMDS-systems could be seen at exhibitions and conferences.

On February 16, 1999, the Government of the Russian Federation issued Resolution No. 179 “On approval of the Regulations on holding a competition for the right to use radio frequencies for the purpose of distributing television programs using systems MMDS, LMDS and MVDS". Thus it was formed the legislative framework and clarity was brought to the licensing procedures.

At the moment there are more than 300 MMDS broadcast centers in Russia. Is it a lot or a little? For comparison, during the “golden” time for MMDS, there were more than 250 operators in the United States, and the number of subscribers was estimated at several million (although, as of October 2004, it was 100 thousand people, or 0.1% of the total number of subscribers with access to multi-program television). That is, we can say that MMDS “didn’t take off,” or more precisely, it turned out to be not as in demand as expected.

The equipment operates within a radius of several tens of kilometers (depending on the height of the antenna) and allows you to cover entire areas and regions. This method of solving the “last mile” problem is one of the most convenient for small cities and suburbs with low-rise and heterogeneous buildings, where laying a cable network is ineffective.

In Russia, systems have already been deployed and are successfully operating in more than 30 cities MMDS. The range of systems is determined by many factors, but the main requirement is the presence of line of sight between the transmitting and receiving antennas. Even leaves on trees contribute very significant attenuation. Only re-reflection from buildings in the near zone from the transmitting center can have some influence.

Systems MMDS work in Yekaterinburg, Yakutsk, Dudinka (TAO), Kemerovo, Anzhero-Sudzhensk, Novokuznetsk, Syzran, Lesnoy (Sverdlovsk region), Severodvinsk, Usinsk (Komi Republic), Volgograd, Tyumen, Moscow, Perm, Slavyansk-on-Kuban (Krasnodar Territory), Livny (Oryol region), Novosibirsk, Plesetsk and other cities.

Advantages and disadvantages

Advantages:

Inexpensive subscription service
Minimum (compared to cable television networks) number of technical specialists
Ease of connection for the end user due to the absence of connection to the cable infrastructure
Low cost of ownership of channels and maintenance of channel-forming equipment compared to the cost of ownership and maintenance of similar cable infrastructure

The signal propagation area is limited by the line of sight of the repeater. The need for state licensing to use the frequency range

Flaws:

The total number of broadcast television channels cannot exceed 24
Rapid deployment of the system in a specific area due to the ease of installation of both the basic transmission equipment and the repeater network
Possibility to use the network MMDS as a repeater for both state and local cable TV channels

Small channel capacity

Of course, in the past it seemed that 24 channels was more than enough (and this is an ideal case; in reality, licenses were issued for 12-16 channels). However, both cable and satellite operators could offer a noticeably larger number of television programs, which naturally attracts potential customers more strongly. Of course, one can object - after all MMDS allows the use of QAM modulation, and in one frequency channel the useful speed (bitrate) can reach almost 45 Mbit/s. True, for this, subscribers had to purchase decoding devices, and such a measure, naturally, did not evoke positive emotions in them - we all know well the level of purchasing power of the population, especially in the provinces.

A whole range of difficulties in ensuring broadcast quality caused by over-the-air transmission. Since the television signal is absolutely identical in its parameters to the standard over-the-air signal (amplitude modulation is used), and power levels are relatively low (from hundredths of a watt to several tens of watts), it is necessary to achieve quality at least comparable to signals in cable lines, it is very difficult. Of course, the transition to digital broadcasting theoretically improves the quality of broadcasting, but the problem remains associated with multipath signal reception, and it is known that the digital signal format for MMDS is no different from the DVB-C standard, that is, it has minimal noise immunity.

Broadcasting is carried out in the microwave range; it is necessary to ensure direct visibility on the radio link between the receiving and transmitting antennas. But this parameter also depends on the terrain, the presence of buildings and structures; even ordinary tree foliage interferes with signal reception in the gigahertz range. In this regard, the height of the transmitting antenna is determined not only from the required distance to the radio horizon line, but also from the calculation of the maximum availability of its service for subscribers. But then another problem arises: if in large cities there already exist television towers of sufficient height, or high-rise buildings on which, under certain conditions, a transmitting antenna can be placed (especially since a large number of potential subscribers are concentrated within the city limits - for example, the radius of Moscow, in which is home to over 10 million people, is approximately 36-40 km). For rural areas, small regional towns, with a low population density over a large area, where antenna masts of the highest possible height are required, they most likely do not exist, and building such ones is difficult, time-consuming and often unprofitable. Plus, to be able to receive signals over a long distance, channel transmitters with a power of 100 W are needed, and this is completely different money.

Price

Low deployment cost MMDS- this is one of the myths. No, of course, in the case of using a multi-channel transmitter, “ordinary” analog modulators from cable headends, a relatively low mounting height of the transmitting antenna, which makes it possible not to use an expensive waveguide. But in any case, you will have to think about buying another, backup transmitter, measuring equipment operating in MMDS-range. And if the project involves the use of powerful channel transmitters, a high mounting height of the transmitting antenna, and an automatic backup system, then the cost of the transmitting equipment alone can reach several hundred thousand “conventional units”. And this money will be needed immediately, because MMDS-the system cannot be built gradually, unlike cable networks.

The level of consumer properties is also limited by the fact that a large number of failure options MMDS-equipment leads to the complete absence of TV broadcasting and/or data transmission services for all subscribers. Such reasons may be failure of the transmitting feeder, antenna, combiner, group transmitter, or destruction of the antenna mast. For obvious reasons, this is an integral part of the MMDS concept itself. The only option for 100% redundancy is the construction of a duplicate transmission station, which, of course, is unrealistic from the point of view of financial feasibility.

There are certain difficulties from the point of view of general accessibility of the service provided (in relation to broadcasting television programs). The natural need of any commercial operator is to make a profit from the services provided to subscribers. In the case of CATV systems, control over unauthorized connections can be exercised even in such a simple way as periodically going around houses in order to search for “unauthorized people,” turning them off and then applying sanctions.

In the case of broadcasting, everything is not so obvious. Not only is it sometimes simply impossible to establish whether such a “pirate” has operating equipment, but also getting physical access to it (that is, the equipment) is very difficult, if not impossible. It follows from this that in order to combat unauthorized users, it is necessary to purchase, install and maintain conditional access equipment. And these are also additional financial expenses that must be compensated somehow.

Features related to the development of the Russian communication services market. Wired cable networks began to develop in the early 90s, and, naturally, most actively in cities where the most solvent population is concentrated. Of course, in the late 90s, when MMDS came to Russia, the most “delicious” client was already taken. In the provinces, the level of solvency of the population was noticeably lower, and there was no habit of paying “for television,” and wealthy residents who could afford access to multi-program television already had kits for receiving satellite TV.

And finally, there are licensing issues. The time required to obtain a license sometimes completely eliminated such an advantageous feature MMDS, as rapid system deployment. In principle, companies selling equipment offered services to speed up the procedure, but, of course, for separate and quite considerable money. Moreover, the operators were aware that this process would repeat again in a few years, when the license would need to be renewed. Which, of course, did not inspire optimism among potential and existing operators.

MMDS antenna

Subscribers have an interactive MMDS Instead of a receiving antenna with a converter, a transceiver unit is installed - a subscriber transceiver. Internet access is provided by a system of wireless cable modems, for example, from Vyyo.

Multichannel transmitters MMDS- the most popular solution for relatively small cities (radius up to 10 km). One broadband transmitter is installed at the transmitting station. To excite it, a set of television modulators is used that generate a signal in the MV range (approximately 200-400 MHz). The modulator signals are added by a conventional combiner (adder) and fed to the input of the transmitter, in which the group signal is converted into the 2.5 - 2.7 GHz range and amplified in power.

To receive digital programs for each subscriber MMDS a serial cable digital terminal costing $100 or more must be installed.

MMDS receiving antennas vary in gain: 21 dB, 24 dB, 27 dB.

Story

The well-known frequency range 2500-2700 MHz began to be used in the United States in the mid-60s of the last century, from the moment the US Federal Communications Commission (FCC) allowed its use for the so-called “educational fixed television broadcasting” services (ITFS - Instructional Television Fixed Service). Twenty-eight of the thirty-one channels (each 6 MHz wide) could use educational institutions and non-profit organizations. The remaining 3 channels were given to OFS (Operational Fixed Service) services - commercial organizations could use them for their own needs.

In 1969, a number of telecommunications operators approached the FCC with a proposal to authorize the 2150-1162 MHz band (then unused) for commercial use. In 1974, the FCC announced that it would begin issuing licenses to willing local telecommunications operators to broadcast on these single-channel frequencies (6 MHz band). The innovation was called MDS - Multipoint Distribution System; the first transmitter began operating in 1975, broadcasting the signal of the HBO cable channel. This year can rightfully be considered the year of birth MMDS.

MDS quickly gained popularity as a way to deliver television “pictures”, of course, on a paid basis, first to hotel complexes, and later to individuals. But over time, amid the boom in wired cable TV, MDS's prospects began to look bleak. Cable TV appeared in many cities, and a consumer paying $20 to $30 a month for one MDS channel could get 12 to 24 channels on cable for the same money. MDS operators did not want to give up and began to look for opportunities to increase the number of broadcast channels.

In 1979, they again asked the FCC to reconsider the terms of use of the frequency spectrum owned by the ITFS services. After some delay (perhaps hesitation in anticipation of “small” cash contributions?) in 1983, operators received the right to a block of eight adjacent channels, subject to the condition that simultaneous broadcasts would be carried out on only 4 of them. From that moment on, the abbreviation appeared MMDS. In the same year, permission was given, albeit with some reservations, to use the remaining ITFS channels.

So from now on MMDS operators had the technical ability to broadcast 33 TV channels, and around this it was possible to start building a business. For traditional cable companies, which at the time were struggling to produce up to 30 channels, this was a serious threat.

A solution was found in a price war, which was expressed in inflating the cost of television programs for operators MMDS. This is explained by the fact that the content was produced either by the owners of the cable networks themselves, or its authors were simply afraid of trouble from their largest consumer at that time. In this regard, the US Department of Justice initiated an investigation (naturally, at the instigation of the MMDS Wireless Cable Association (WCA), which had been formed by that time), then developed into a Senate investigation. As they say, “after long and protracted battles,” the WCA’s claims were satisfied. In October 1992, the Cable Television Competition and Consumer Protection Act came into force, which required, in particular, equal pricing policies for content producers in relation to all providers.

From this moment the golden years began MMDS. Subscriber base growth increased sharply, from late 1992 to 1995, driven solely by IPOs or debt issuances by market operator companies MMDS grew by more than a billion dollars.

This fact is indicative. In 1995, the FCC, overwhelmed by the number of applications for MMDS licenses, decided to launch an auction. The country's territory was divided into 51 “main” and 487 “main” regions (the type of region was determined not by geographical, but by commercial indicators). The auction was considered closed if no one could offer a higher price than someone else had already bid. After 181 rounds of bidding, the auction was declared closed in March 1996. 493 licenses were issued, the treasury received 216 million dollars - very significant money for owning a license for “village” television. So, in the mid-90s, the market prospects for MMDS seemed very compelling. The FCC approached equipment manufacturers with a proposal to develop systems using digital waveforms, which made it possible to broadcast more television programs, as well as provide new services, such as data transmission.

But at this moment happy life and ended. The high-speed data transfer market began to actively develop, and the main players, considering MMDS not ready for use in this context, they switched to the development of other technologies. On the other hand, cable networks were actively developing, which could now offer subscribers several times more channels compared to wireless networks. And to the main market MMDS- operators (broadcasting television signals to suburbs and rural areas), a new player has emerged - a direct satellite broadcast system - offering a larger number of TV channels with at least comparable quality. Number of subscribers MMDS-services reaching better times million values began to fall. Under these conditions, operators, who by that time had spent hundreds of millions of dollars of borrowed funds to create coverage areas, were forced to either resort to bankruptcy protection or restructure their business. Actually, this is how the “television” period ended MMDS in USA.

After the FCC approved the use in September 1998 MMDS- networks for two-way data transmission, such then-whales of the telecom market as MCI WorldCom and Sprint showed interest in them. For relatively little money, these companies bought licenses from the owners for the right to operate in the corresponding frequency range, and from now on MMDS began to be positioned as a fixed communication service in the context of data transmission. In what capacity does he remain in the USA to this day?