• The Physical Layer transforms data into bits that are sent across the physical media.
• The Data Link layer determines access to the network media in terms of frames. Its Media Access Control (MAC) sub layer is responsible for physical addressing.
• The Network Layer routes data through a large network.
• The Transport Layer provides end-to-end, reliable connections, often in terms of segments [Book02].
• The Session Layer allows users to establish connections using intelligently chosen names in packets.
• The Presentation Layer negotiates data exchange formats, also in terms of packets.
• Finally, the Application Layer provides the interface between the user's application and the network through messages.
OSI Model
• OSI model is just a guideline for protocol design, not the actual protocols
• Not all layers are always used
o Internet uses only five layers
• Some layers may be combined together
o Top three layers are normally combined into one layer .
Why a OSI model?
• Idea was originally to get a message across different networks.
• By layering, each layer performs a separate function. Makes changes and modifications easier. Change of lower layers does not affect higher layers as long as their interfaces are the same.
• Higher layers deal more with end-to-end communications, user services and applications .
• Lowest three layers deal primarily with the details of data transmission in networks.
• Each layer offers certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented.
The Open Systems Interconnect (OSI), established in 1984 by the ISO (International Standards Organization), divides network functions into seven layers: Physical, Data Link, Network, Transport, Session, Presentation and Application Protocol.
It ascertains that the RF equipment would cater to the requirements of the network topology and satellite modems in use. The link Budget estimates the ground station and satellite EIRP required. Equivalent isotropically radiated power (EIRP) is the power transmitted from a transmitting object. Satellite ERP can be defined as the sum of output power from the satellite’s amplifier, satellite antenna gain and losses. Calculations of signal levels through the system (from origination earth station to satellite to receiving earth station) to ensure the quality of service should normally be done prior to the establishment of a satellite link. This calculation of the link budget highlights the various aspects. Apart from the known losses due to various cables and inter-connecting devices, it is advisable to keep sufficient link margin for various extraneous noise which may affect the performance. It is also a safeguard to meet eventualities of signal attenuation due to rain/snow. As mentioned earlier a satellite provides two resources, bandwidth and amplification power. In most VSAT networks, the limiting resource in satellite trans-ponder is power rather than bandwidth .
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VSAT technology
Under this, a central network monitoring system allocates a unique code to each of the VSATs enabling multiple VSATs to transmit simultaneously and share a common frequency band. The data signal is combined with a high bit rate code signal which is independent of the data. Reception at the end of the link is accomplished by mixing the incoming composite data/code signal with a locally generated and correctly synchronized replica of the code . Since this network requires that the central network management system co-ordinates code management and clock synchronization of all remote VSATs, star topology is, by default, the best one. Although this is best applicable for very large networks with low data requirements, there are practical restrictions in the use of spread spectrum. It is employed mainly for interference rejection or for security reasons in military systems .
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VSAT technology
The network uses a pool of satellite channels, which are available for use by any station in that network. On demand, a pair of available channels is assigned so that a call can be established. Once the call is completed, the channels are returned to the pool for an assignment to another call. Since the satellite resource is used only in pro-portion to the active circuits and their holding times, this is ideally suited for voice traffic and data traffic in batch mode.
DAMA offers point-to-point voice, fax, and data requirements and supports video-conferencing. The ability to use on-board signal processing and multiple spot beams will enable future satellites to reuse the frequencies many times more than today’s’ system. In general, channel allocation may be static or dynamic, with the latter becoming. DE – 5 increasingly popular. DAMA systems allow the number of channels at any time be less than the number of potential users. Satellite connections are established and dropped only when traffic demands them .
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VSAT technology
DAMA offers point-to-point voice, fax, and data requirements and supports video-conferencing. The ability to use on-board signal processing and multiple spot beams will enable future satellites to reuse the frequencies many times more than today’s’ system. In general, channel allocation may be static or dynamic, with the latter becoming. DE – 5 increasingly popular. DAMA systems allow the number of channels at any time be less than the number of potential users. Satellite connections are established and dropped only when traffic demands them .
It implies that the VSATs are pre-allocated a designated frequency. Equivalent of the terrestrial leased line solutions, PAMA solutions use the satellite resources constantly. Consequently, there is no call-up delay what makes them most suited for interactive data applications or high traffic volumes. As such, PAMA connects high data traffic sites within an organization .
SCPC (Single Channel Per Carrier) refers to the usage of a single satellite carrier for carrying a single channel of user traffic. The frequency is allocated on a pre-assigned basis in case of SCPC VSAT which is also synonymously known as PAMA VSAT.
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VSAT technology
SCPC (Single Channel Per Carrier) refers to the usage of a single satellite carrier for carrying a single channel of user traffic. The frequency is allocated on a pre-assigned basis in case of SCPC VSAT which is also synonymously known as PAMA VSAT.
It is the oldest and still one of the most common methods for channel allocation. In this scheme, the available satellite channel bandwidth is broken into frequency bands for different earth stations. This means that guard bands are needed to provide separation between the bands. Also, the earth stations must be carefully power-controlled to prevent the microwave power spilling into the bands for the other channels. Here, all VSATs share the satellite resource on the frequency domain only. Typically implemented in a mesh or single satellite hop topology, FDMA has the following variants:
Labels: VSAT technologyWith TDMA networks, numerous remote sites communicate with one central hub – a design that is similar to packet-switched networks. As a leased-line equivalent, SCPC can deliver dedicated bandwidth of up to 2 Mbit/s. Remote sites in a TDMA network compete with one another for access to the central hub, restricting the maximum band-.4 – DE width in most cases to 19.2 kbit/s. Almost all international VSAT services in Asia-Pacific are based on SCPC. Most domestic offerings are based on TDMA, although some domestic operators offer point-to-point SCPC links as well. Here, we will discuss briefly TDMA, pre-assigned or demand-assigned FDMA, CDMA and other accessing techniques featuring merits and demerits of these schemes.
From the above figure the followings are noted:Switch - routing control between host & modulator/demodulator. Basically on the packets header
Modulator- modulate the outbound carriers (TDM)
Bank of Demodulators- receive inbound carriers, extract data packets.
Radio Frequency Terminal (RFT) - transmits subsystem (up converters, high power amplifier), receive subsystem (low noise amplifier, down converter)
Network Control Center (NCC) - controls and monitors hub and IDUs
Primary Power Subsystem
In a TDMA network, all VSATs share satellite resource on a time-slot basis. Remote VSATs use TDMA channels or in routes for communicating with the hub. There could be several in routes associated with one out route. Several VSATs share one in route hence sharing the bandwidth. Typical in routes operate at 64 or 128 Kbit/s. Generally systems with star topology use a TDMA transmission technique. Most common configuration of TDMA star network use one single high performance hub, many low cost VSAT terminals, centralized management, lower price, optimized use of satellite capacity .
Critical to all TDMA schemes is the function of clock synchronization what is performed by the TDMA hub or master earth station. The VSATs may also access the in route on a fixed assigned TDMA mode, wherein each VSAT is allocated a specific time slot or slots.
Broadcast Networks are used to transmit data, audio and video files to a number of users. The data is broadcast from central site for end user sites. This is a one-way system. The central site broadcasts the data. The remote site only receives data. The channel can provide an uplink speed of 36 to 40 depends on transponder’s BW .
• The main objective of 'Receive only VSAT' services is sharing and spread of information. Thus it is of benefit to society as a whole rather than few users as in the case of normal VSAT.
• The applications of this technology are mainly in areas of information, social services, education and medicine. The broadband multicast capability can provide innovative applications like Tele-Medicine, online newspapers, market rates and Tele-education. It can ensure that these applications are accessible to even the remotest parts of the country.
• This technology is predicted to play a crucial role in facilitating the government efforts to bridge the Digital Divide. Thus the success of Receive Only VSAT is in the favour of the government.
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VSAT technology
• The main objective of 'Receive only VSAT' services is sharing and spread of information. Thus it is of benefit to society as a whole rather than few users as in the case of normal VSAT.
• The applications of this technology are mainly in areas of information, social services, education and medicine. The broadband multicast capability can provide innovative applications like Tele-Medicine, online newspapers, market rates and Tele-education. It can ensure that these applications are accessible to even the remotest parts of the country.
• This technology is predicted to play a crucial role in facilitating the government efforts to bridge the Digital Divide. Thus the success of Receive Only VSAT is in the favour of the government.
i. The size of a VSAT antenna varies. The feedhorne directs the transmitted power towards the antenna dish or collects the received power from it.
ii. It consists of an array of microwave passive components. Antenna size is used to describe the ability of the antenna to amplify the signal strength.
iii. The Radio Frequency Terminal (RFT) is mounted on the antenna frame and interconnected to the feed-horn (outdoor electronics) includes Low Noise Amplifiers (LNA) and down-converters for amplification and down conversion of the received signal respectively .
iv. LNAs are designed to minimize the noise added to the signal during this first stage of the converter as the noise performance of this stage determines the overall noise performance of the converter unit. The noise temperature is the parameter used to describe the performance of an LNA.
v. Up- converters and High Powered Amplifiers (HPA) are also part of the RFT and are used for up converting and amplifying the signal before transmitting to the feed-horn. The Up/Down converters convert frequencies between intermediate frequency (IF level 70 MHz) and radio frequency.
vi. Extended C band, the down converter receives the signal at 4.500 to 4.800 GHz and the up converter converts it to 6.725 to 7.025 GHz. The HPA ratings for VSATs range between 1 to 40 watts.
vii. The Outdoor Unit (ODU) is connected through a low-loss coaxial cable to the indoor unit (IDU). The typical limit of an (Inter Facility Link) IFL cable is about 500 feet. The IDU consists of modulators that superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for up conversion, amplification and transmission.
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VSAT technology
ii. It consists of an array of microwave passive components. Antenna size is used to describe the ability of the antenna to amplify the signal strength.
iii. The Radio Frequency Terminal (RFT) is mounted on the antenna frame and interconnected to the feed-horn (outdoor electronics) includes Low Noise Amplifiers (LNA) and down-converters for amplification and down conversion of the received signal respectively .
iv. LNAs are designed to minimize the noise added to the signal during this first stage of the converter as the noise performance of this stage determines the overall noise performance of the converter unit. The noise temperature is the parameter used to describe the performance of an LNA.
v. Up- converters and High Powered Amplifiers (HPA) are also part of the RFT and are used for up converting and amplifying the signal before transmitting to the feed-horn. The Up/Down converters convert frequencies between intermediate frequency (IF level 70 MHz) and radio frequency.
vi. Extended C band, the down converter receives the signal at 4.500 to 4.800 GHz and the up converter converts it to 6.725 to 7.025 GHz. The HPA ratings for VSATs range between 1 to 40 watts.
vii. The Outdoor Unit (ODU) is connected through a low-loss coaxial cable to the indoor unit (IDU). The typical limit of an (Inter Facility Link) IFL cable is about 500 feet. The IDU consists of modulators that superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for up conversion, amplification and transmission.
Star
The hub station controls and monitors can communicate with a large number of dispersed VSATs. Generally, the Data Terminal Equipment and 3 hub antenna is in the range of 6-13m in diameter. Since all VSATs communicate with the central hub station only, this network is more suitable for centralized data applications .
Mesh
A group of VSATs communicate directly with any other VSAT in the network without going through a central hub. A hub station in a mesh network performs only the monitoring and control functions. These networks are more suitable for telephony applications.
i. Interactive real time application
• Point of Sale/retail/Banking (beg. ATM)
• Corporate data
ii. Telephony
• Rural: individual subscribers
• Corporate Telephony
iii. Intranet, Internet and IP infrastructure
• Multimedia delivery (ex. video streaming)
• Interactive distance learning/ training .
iv. Direct-to-home
• Broadband Internet access for consumers and businesses
VSAT is a term widely used in the satellite industry to describe an earth station that is installed on the ground to receive communications from a satellite or to communicate with other ground stations by transmitting to and receiving from satellite spacecraft. The ground station may be used only for reception, but is typically capable of both receiving and transmitting. Major components of a VSAT are generally grouped in two categories, ODU (Outdoor Unit) and IDU (Indoor Unit) .
Labels: VSAT technologyo computer communications
o reservation systems
o database enquiries
o billing systems
o file transfers
o electronic mail
o video conferencing
o point of sale transactions
o credit checks and credit card verification
o stock control and management
• Antenna base construction.
• Antenna installation.
• Antenna pointing.
• Cross pole test.
• Equipment configuration.
• BER test.
• Router configuration.
• Data port connects.
• End-to-End data flow test.
• Link test by different way.
• Ready for service
i. Rapid, reliable satellite transmission of data, voice and video and an ability to allocate resources (bandwidth and amplification power) to different users over the coverage region as needed.
ii. VSAT industry is offering fixed network solutions that can provide a full suite of services at reasonable price. e.g.: a toll quality voice channel via VSAT is available between 3-15 cents/minute today.
iii. Easy to provide point-to-multipoint (broadcast), multipoint-to-point (data collection), point-to-point communications and broadband multimedia services .
iv. VSATs are serviced not only in cases where the land areas are difficult to install, say in the case of remote locations, water areas, and large volumes of air space.
v. An ability to have direct access to users and user premises .
vi. VSAT systems can also provide a variety of services including broadband communication systems satellite-based video, audio, Internet and data distribution networks .
VSAT (Very Small Aperture Terminal) is a satellite-based communications service that offers businesses and government agencies flexible and reliable communications solutions, both nationally and internationally, on land and at sea and represents a cost effective solution for users seeking an independent communications network connecting a large number of geographically dispersed sites .
VSAT’s are small, software-based earth stations (generally 0.9 - 4.5 meters), which are used for transmission of data, voice, or video via satellite. It can be operated without additional manpower or technology. VSAT services are delivered using C or KU band GEO satellites .
Satellite communication provides services:
i. International Telephony
• using Public Switched Telephone Network (PSTN)
• Intermediate Data Rate (IDR)
• Time Division Multiple Access (TDMA)
ii. Broadcasting
• TV Uplink
• Television Receive Only (TVRO)
• Digital Satellite News Gathering (DSNG)
iii. VSAT ( Very Small Aperture Terminal)
• Personal Earth Station (PES-TDMA)
• Telephony Earth Station (TES-TDMA)
• Domestic IDR/Single Channel Per Carrier (SCPC)
• VSAT Dial away
• VSAT Sky Star Advantage
• VSAT Faraway
For satellite communications the following three types of frequencies are used which are IEEE standards .
C Band : Range between 4 GHz to 8 GHz
Ku Band : Range between 12 GHz to 18 GHz
Ka Band : Range between 20 GHz to 30 GHz
From the above figure the followings are noted:
HPA – High Power Amplifier
LNA - Low Noise Amplifier (Earth station equipment that amplifies the transmit RF signal)
CPE – Customer Premises Equipment (e.g. Telephone, PABX, Ethernet Hub, Host Server etc)
Satellite Communication is a technology of data transmission whether one-way data broadcasting or two-way interactive using radio frequency as a medium.
It consists of-
i. Space Segment or Satellite ( eg. Measat, Intelsat and Inmarsat)
ii. Ground Segment or earth station which includes Antenna, Outdoor Unit, Inter Facility Link, Indoor Unit and Customer Premises Equipment.
Satellite Communication using VSAT (Very Small Aperture Terminal) since the science fiction on radio transmission through space using geo-synchronous earth satellite, provider has progressed significantly in the field of satellite communications. The early earth stations were large and expensive. The reason for the size and complexity of the early stations was not related to inadequate performance. In fact, the antennas had very high efficiency and the noise temperatures of their receivers were low. However, the satellites at that time had a relatively poor performance providing considerably low RF (radio frequency) power per transponder and a rather high noise temperature for the on-board receivers. Additionally, satellites were then considered suitable only for very long distance communication. Gradually, satellite communications have appeared as regional systems requiring smaller coverage on the earth’s surface enabling higher gain antennas. Subsequently, increase in transponder out-put power, introduction of systems having several spot beams, development of field-effect transistor amplifier for low noise receivers as well as its availability as power amplifier have changed the satellite communication scenario. Once it was possible to envisage an all solid-state transmit and receive earth station even with a rather low power output, low price, large quantity, VSAT-based earth station design could be conceived.
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VSAT technology
Wavelength can be calculated as
c = n λ = f λ
So, Wavelength, λ = V/ f
Where,
λ = Carrier wavelength
f = Carrier frequency
c = Velocity of light ( 3 × 108 m/s)
In case of wavelength, when wavelength decreases then the speed of electromagnetic wave propagation decreases and loss increases. Wavelength inversely proportional to rainy season .
The signal-to-noise ratio which is the ratio of the power in a signal to the power contained in the noise that is present at a particular point in the transmission. Typically, this ratio is measured at a receiver, because it is at this point that an attempt is made to process the signal and recover the data. For convenience, this ratio is often reported in decibels:
SNR dB= 10 log 10signal powernoise power
This expression the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean a high-quality signal and low number of required intermediate repeaters .
The signal-to-noise ratio is important in the transmission of digital data because it sets the upper bound on the achievable data rate. Shannon’s result is that the maximum channel capacity, in bits per second, obeys the equation
C= B log2(1+SNR)
Where C is the capacity of the channel in bits per second and B is the bandwidth of the channel in hertz .
Bit Error Rate. The rate at which errors in transmission occur, normally related closely to the Signal to Noise Ratio (SNR). BER of 10-9, or one bit error for every billion bits, is a typical minimum system requirement .
Labels: Microwave antennaA waveguide is a hollow metal tube, either a cylinder or a rectangle in cross section that acts as a pipe for transmission of electromagnetic waves of the microwave range over short distance (1 to 10 cms) . A typical waveguide of rectangular cross section. Since the waveguide is of solid metal, any electromagnetic wave put into its cavity can not escape and travels through the guide to the other end. As a result, waveguide offers a very large bandwidth (2 GHz to 110 GHz) .
The RF power for acceleration of protons inside the accelerating structure, supplied from high-power klystrons, is taken up to the desired ports by means of wave-guide line. The factors of primary importance for a wave-guide system are: power handling capacity, insertion loss, impedance uniformity, band width, physical dimensions/tolerances, economic considerations and self strength. The wave-guides are made from aluminium alloy 6061 plates which are heliarc welded at four corners .
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Microwave antenna
The RF power for acceleration of protons inside the accelerating structure, supplied from high-power klystrons, is taken up to the desired ports by means of wave-guide line. The factors of primary importance for a wave-guide system are: power handling capacity, insertion loss, impedance uniformity, band width, physical dimensions/tolerances, economic considerations and self strength. The wave-guides are made from aluminium alloy 6061 plates which are heliarc welded at four corners .
There will always be some loss of signal strength through the cables and connectors used to connect to the antenna.
This loss is directly proportional to the length of the cable and generally inversely proportional to the diameter of the cable .Additional loss occurs for each connector used and must be considered in planning.Cable vendor can provide a chart indicating the loss for various types and lengths of cable. Table A-1 on page A-4 is an example of this kind of chart.
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Microwave antenna
Consider a dish antenna pointed at an isotropic antenna transmitting some distance away. We know that the isotropic antenna radiates uniformly in all directions, so it is a simple matter of spherical geometry to calculate how much of that power should be arriving at the dish over its whole aperture . Now to measure how much power is being received from the dish (at the electrical connection to the feed) – never greater than is arriving at the aperture. The ratio of power received to power arriving is the aperture efficiency.
The total antenna efficiency accounts for the following losses:
(1) Reflection because of mismatch between the feeding transmission line and the antenna and
(2) Antenna conductor and dielectric losses.
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Microwave antenna
The total antenna efficiency accounts for the following losses:
(1) Reflection because of mismatch between the feeding transmission line and the antenna and
(2) Antenna conductor and dielectric losses.
Antenna gain is essential for microwave communication---since it helps both transmitting and receiving, it is doubly valuable.
Antenna gain is an indicator of how well an antenna focuses RF energy in a preferred direction. Antenna gain is expressed in dBi (the ratio of the power radiated by the antenna in a specific direction to the power radiated in that direction by an isotropic antenna fed by the same transmitter). Antenna manufacturers normally specify the antenna gain for each antenna they manufacture .
The relationship between antenna gain and effective area is
G = 4πAe / λ 2 = 4π f 2Ae / c2
Where
G = Antenna Gain
Ae = Effective area
λ = Carrier wavelength
f = Carrier frequency
c = Speed of light ( 3 × 108 m/s)
The hypothetical isotropic antenna is a point source that radiates equally in all directions. Any real antenna will radiate more energy in some directions than in others. Since it cannot create energy, the total power radiated is the same as an isotropic antenna driven from the same transmitter: in some direction it radiates more energy than an isotropic antenna, so in others it must radiate less energy. The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. Usually we are only interested in the maximum gain-----the direction in which the antenna is radiating most of the power .
An antenna with a large aperture has more gain than smaller one; just as the captures more energy from a passing radio wave, it also radiates more energy in that direction. Gain may be calculated as
GdBi = 10 log10( η 4π/ λ 2 A)
With reference to an isotropic radiator; η is the efficiency of the antenna.
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Microwave antenna
Antenna gain is an indicator of how well an antenna focuses RF energy in a preferred direction. Antenna gain is expressed in dBi (the ratio of the power radiated by the antenna in a specific direction to the power radiated in that direction by an isotropic antenna fed by the same transmitter). Antenna manufacturers normally specify the antenna gain for each antenna they manufacture .
The relationship between antenna gain and effective area is
G = 4πAe / λ 2 = 4π f 2Ae / c2
Where
G = Antenna Gain
Ae = Effective area
λ = Carrier wavelength
f = Carrier frequency
c = Speed of light ( 3 × 108 m/s)
The hypothetical isotropic antenna is a point source that radiates equally in all directions. Any real antenna will radiate more energy in some directions than in others. Since it cannot create energy, the total power radiated is the same as an isotropic antenna driven from the same transmitter: in some direction it radiates more energy than an isotropic antenna, so in others it must radiate less energy. The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. Usually we are only interested in the maximum gain-----the direction in which the antenna is radiating most of the power .
An antenna with a large aperture has more gain than smaller one; just as the captures more energy from a passing radio wave, it also radiates more energy in that direction. Gain may be calculated as
GdBi = 10 log10( η 4π/ λ 2 A)
With reference to an isotropic radiator; η is the efficiency of the antenna.
A signal degrades as it moves through space. The longer the path, the more loss it experiences. This free-space path loss is a factor in calculating the link viability . Free-space path loss is easily calculated for miles or kilometers using one of the following formulas:
Lp = (96.6 + 20 log10 F) + (20 log10 D)
where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in miles
or
Lp = (92.4 + 20 log10 F) + (20 log10 D)
where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in kilometers
The sensitivity of a receiver is its ability to receive quick signal. This sensitivity may be defined in several ways.
First, it may be started in terms of the signal field strength of a signal that will produce a desired demodulated output level under a certain modulation level. The sensitivity is usually started in terms of the voltage developed by the antenna across the receiver antenna terminals in microvolts. This level ranges from a few microvolts to a few hundred microvolts for typical receiver .
Another way of stating the sensitivity is to state the antenna terminal signal voltage required to produce a specified signal- to- noise ratio. In the case of receiver for digital signals, the sensitivity is usually stated as the input signal level required to produce a desired bit- error rate which is related to signal- to- noise ratio.
Receiver sensitivity is typically specified in units of microvolts for a 12 dB SINAD (signal to noise and distortion). The amount of RF power required by the receiver to faithfully represent the actual signal transmitted is arbitrarily set at the 12 dB SINAD, which translates into -114 dBm for a typical receiver.
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Microwave antenna
The major issues with microwave link engineering are rain fade, multi path and interference. An RF engineer begins a design by doing a link budget analysis. A given radio system has a system gain that depends on the design on the radio and the modulation used. The gains from the antenna at each end are added to this gain. Larger antennas provide higher gain. The free space loss of the radio signal as it travels over the air is then subtracted from the system: the longer the link the higher the loss. These calculations result in a “fade margin” for the link. Anything that affects the radio signal within this margin will be overcome by the radio; if the margin is exceeded, then the link could go down. The next step, then, is to analyze impediments that could potentially affect the radio signal . With good understanding of the potential affects on the signal, the RF engineer can design links with availability and performance equal to or better than a wire line link. A link budget is a rough calculation of all known elements of the link to determine if the signal will have the proper strength when it reaches the other end of the link. To make this calculation, the following information should be available:
- Frequency of the link
- Free space path loss
- Power of the transmitter
- Antenna gain
- Total length of transmission cable and loss per unit length at the specified frequency
- Number of connectors used
- Loss of each connector at the specified frequency
- Path length
The amount of extra RF power radiated to overcome this phenomenon is referred to as fade margin. The exact amount of fade margin required depends on the desired reliability of the link, but a good rule-of-thumb is 20dB to 30dB.
Fade Margin = SG + AG - LC – LP
Where
SG = System gain (depend on modem)
AG = Antenna gain
LC = Cable loss
LP = Path loss
These parameters are also responsible for gain loss.
System Gain = Radiated Power of radio Equipment - (Receiver Sensitivity)
When planning for paths longer than seven miles, the curvature of the earth might become a factor in path planning and require that the antenna be located higher off the ground . The additional antenna height needed can be calculated using the following formula:
H = D2/8
Where,
H = Height of earth bulge (in feet)
D = Distance between antennas (in miles)
The minimum antenna height at each end of the link for paths longer than seven miles (for smooth terrain without obstructions) is the height of the First Fresnel Zone plus the additional height required to clear the earth bulge . The formula would be:
H = 43.3 √ (D/4F ) + D2/8
Where,
H = Height of the antenna (in feet)
D = Distance between antennas (in miles)
F = Frequency in GHz
The characteristics of a radio signal cause it to occupy a broad cross-section of space, called the Fresnel Zone, between the antennas. From figure shows the area occupied by the strongest radio signal, called the First Fresnel Zone, which surrounds the direct line between the antennas
Because of the shape of the First Fresnel Zone, what appears to be a clear line-of-sight path may not be. As long as 80 percent of the First Fresnel Zone is clear of obstructions, the link behaves essentially the same as a clear free-space path .
H = Height of the First Fresnel Zone (in feet)
D = Distance between the antennas (in miles)
F = Frequency in GHz
In the first Fresnel zone and all the odd numbered Fresnel zones, deflected signals are generally in phase with or the center lobe signal. In the second Fresnel zone, and all even-numbered Fresnel zones, deflected signals are up to 180 out of phase with the center lobe signal . Signals deflected from the second Fresnel zone can cause Inter Symbol Interference (ISI) which can result in great losses of the center lobe signal. To avoid this problem, must place the antenna at a height that is out of range from F2 deflections. (An antenna can be set too high as well as too low.) Where deflection and diffraction from ground-based objects cause interference, even a small relocation of the antenna often produces a substantial improvement.
The minimum antenna height at each end of the link for paths longer than seven miles (for smooth terrain without obstructions) is the height of the First Fresnel Zone plus the additional height required to clear the earth bulge . The formula would be:
H = 43.3 √ (D/4F ) + D2/8
Where,
H = Height of the antenna (in feet)
D = Distance between antennas (in miles)
F = Frequency in GHz
When planning antenna placement, it might be necessary to build a free-standing tower for the antenna. Regulations and limitations define the height and location of these towers with respect to airports, runways, and airplane approach paths. These regulations are controlled by the FAA. In some circumstances, the tower installations must be approved by the FAA, registered with the FCC, or both. To ensure compliance, review the current FCC regulations regarding antenna structure .
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Microwave antenna
In general, the best location for antenna placement is always the closest to the transmitter. The type of antenna selected also has an effect on placement as discussed earlier. Since the transmitted signal is typically vertically polarized, it is important to orient the receiver antennas in a vertical position. The best place is usually on an adjacent wall, near the vicinity of the transmitter, using the frequency length formula to determine the proper distance between the antennas. When a number of multiple systems are used in a given location it is a common practice to use antenna combiners to eliminate the antenna Farm. Antenna amplifier/combiners are a convenience item that allows a single pair of antennas to feed multiple receivers
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Microwave antenna
The orientation of the antenna will change the orientation of the signal. The transmitting and receiving antennas should be both polarized either horizontally or vertically. Adjacent antennas on different frequencies can be cross-polarized to help reduce interference between the two, if operating license permits this.
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Microwave antenna
Dipole antenna
Other antenna types include the “dipole”, where a section of wire, one-half the wavelength, is positioned either horizontally or vertically in the air to transmit signals. Dipoles emit their signals in more of a two dimensional semi-circular or “doughnut” pattern, the key being both the transmitter and receiver’s antennas must be aligned the same (horizontally or vertically). Dipoles do not require a ground-plane are considered “bi-directional,” in that their signals travel in two opposite directions, depending on how the antenna is oriented .
Yagi antenna
The more focused (uni-directional) type of antenna is called a “Yagi.” A Yagi antenna is basically a standard one-half wavelength antenna, but with additional “elements” placed in front of it to focus the energy for transmission in one direction. The “reflector” and “director” elements are just similar-sized resonators spaced appropriately to increase the strength and narrow the direction of the signal prior to transmission. Again, the key to successfully using Yagi antennas is the correct orientation and alignment of the transmitting/receiving antennas.
Sectoral antenna
The requirement to serve a number of small areas from a single base station has resulted in the development of the multiple beam technique, for which the sectoral antenna is ideal .
Parabolic Reflector antenna
This antenna consists of a parabolic metal surface (dish) with a feed antenna in front. The feed antenna consists of a directive antenna such as a dipole and reflector, log-periodic dipole array or horn antenna. This antenna is capable of producing extremely high gains, usually in the 20 - 30 dBi range
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Microwave antenna
Other antenna types include the “dipole”, where a section of wire, one-half the wavelength, is positioned either horizontally or vertically in the air to transmit signals. Dipoles emit their signals in more of a two dimensional semi-circular or “doughnut” pattern, the key being both the transmitter and receiver’s antennas must be aligned the same (horizontally or vertically). Dipoles do not require a ground-plane are considered “bi-directional,” in that their signals travel in two opposite directions, depending on how the antenna is oriented .
Yagi antenna
The more focused (uni-directional) type of antenna is called a “Yagi.” A Yagi antenna is basically a standard one-half wavelength antenna, but with additional “elements” placed in front of it to focus the energy for transmission in one direction. The “reflector” and “director” elements are just similar-sized resonators spaced appropriately to increase the strength and narrow the direction of the signal prior to transmission. Again, the key to successfully using Yagi antennas is the correct orientation and alignment of the transmitting/receiving antennas.
Sectoral antenna
The requirement to serve a number of small areas from a single base station has resulted in the development of the multiple beam technique, for which the sectoral antenna is ideal .
Parabolic Reflector antenna
This antenna consists of a parabolic metal surface (dish) with a feed antenna in front. The feed antenna consists of a directive antenna such as a dipole and reflector, log-periodic dipole array or horn antenna. This antenna is capable of producing extremely high gains, usually in the 20 - 30 dBi range
Antennas are mainly two types---
- Omni directional antenna
- Directional antenna
Omni directional Antenna
The omni directional antenna radiates or receives equally well in all directions. It is also called the "non-directional" antenna because it does not favor any particular direction.
Omni antennas usually resemble vertical rods but can come in other shapes as well. Some have horizontal rods of the same length placed at their base to increase their performance/distance. These are called “ground planes”.
The key factor to note is that for receivers all four signals (or signals from any direction, for that matter) are received equally well. For transmitters, the radiated signal has the same strength in all directions. This pattern is useful for broadcasting a signal to all points of the compass (as when calling "CQ"), or when listening for signals from all points.
The omni directional antenna radiates or receives equally well in all directions. It is also called the "non-directional" antenna because it does not favor any particular direction.
Omni antennas usually resemble vertical rods but can come in other shapes as well. Some have horizontal rods of the same length placed at their base to increase their performance/distance. These are called “ground planes”.
The key factor to note is that for receivers all four signals (or signals from any direction, for that matter) are received equally well. For transmitters, the radiated signal has the same strength in all directions. This pattern is useful for broadcasting a signal to all points of the compass (as when calling "CQ"), or when listening for signals from all points.
Directional Antennas
Gain and directivity are intimately related in antennas. The directivity of an antenna is a statement of how the RF energy is focused in one or two directions. Because the amount of RF energy remains the same, but is distributed over less area, the apparent signal strength is higher. This apparent increase in signal strength is the antenna gain. The gain is measured in decibels over either a dipole (dBd) or a theoretical construct called an isotropic radiator (dBi) . The isotropic radiator is a spherical signal source that radiates equally well in all directions. One way to view the omni directional pattern is that it is a slice taken horizontally through the three dimensional sphere
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Microwave antenna
"That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves". An antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and free shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions .
Antennas focus the radio signal in a specific direction and in a narrow beam. The increase in the signal power (compared to an omni directional antenna) when it is focused in the desired direction is called gain.
Antennas are tuned to operate on a specific group of frequencies. Other specific attributes such as beam-width and gain are also fixed by the manufacturer. Antennas should be selected and placed according to the site and the application .
In general, the larger the antenna, the higher the gain and the larger the mast required. It is best to use the smallest antenna that will provide sufficient protection from interference and enough signal at the far end of the link to provide good reception even with fading.
Other considerations include antenna beam-width, front-to-side ratios, front-to-back ratios, and cross-polarization rejection. Where interference from other licensees on the same channel or adjacent channels is an issue, narrow beam-widths, high front-to-back and front-to-side ratios, and high cross-polarization rejection are likely to be required. Even when other licensees are not an issue, if using a network deployment using the “cell” approach, all these considerations is still important to reduce interference between own adjacent installations .
Co-channel interference results when another RF link is using the same channel frequency. Adjacent-channel interference results when another RF link is using an adjacent channel frequency. In selecting a site, a spectrum analyzer can be used to determine if any strong signals are present at the site and, if they are, to determine how close they are to the desired frequency. The further away from the proposed frequency, the less likely they are to cause a problem. Antenna placement and polarization, as well as the use of high-gain, low-side lobe antennas, are the most effective method of reducing this type of interference.
When the entire control cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. Otherwise, each control cable requires one surge arrestor within two feet of the building entrance, and another surge arrestor within 10 feet of the transverter
Labels: Microwave antennaBecause the coaxial line carries a DC current to supply power to the transverter, gas-discharge surge arrestors are required. Do not use quarter-wave stub or solid-state type surge arrestors .
When the entire coaxial cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. However, local electrical codes require that the conduit be grounded where it enters the building.
When steel conduit is not used to encase the cable, each cable requires one surge arrestor within 2 feet of the building entrance, and another surge arrestor within 10 feet of the transverter .
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Microwave antenna
When the entire coaxial cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. However, local electrical codes require that the conduit be grounded where it enters the building.
When steel conduit is not used to encase the cable, each cable requires one surge arrestor within 2 feet of the building entrance, and another surge arrestor within 10 feet of the transverter .
Connection of two points by non-visible electromagnetic waves is known as radio link
Types
- Extremely Low Frequency(ELF)
- Very Low Frequency(VLF)
- Low Frequency(LF)
- Medium Frequency (MF)
- High Frequency (HF)
- Very High Frequency (VHF)
- Ultra High Frequency (UHF)
- Super High Frequency (SHF)
Electromagnetic waves at the frequency range of about 2 to 40 GHz are referred to as microwave . Microwave radio operates in unlicensed bands are 2.4 GHz and 5.7 GHz and are licensed band it could operate like 6GHz, 7 GHz, 8GHz, 10GHz, 11GHz and 13GHz, 15GHz, 18GHz and 23GHz, 38GHz frequency bands. At these frequencies, highly directional beams are possible and microwave is quite suitable for point-to-point transmission. Concentrating all the energy into a small beam using a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. It’s a type of unbounded network transmission medium. Microwave is mainly used for satellite communications .
A microwave system includes an antenna, radio, multiplexes, waveguide (hollow metal conductor connecting the RF equipment to the antenna) and feed cables. Based on capacity and radio equipment, antenna size, tower heights and terrain elevation will play a major role in how it will planned and construct the system. These four factors also will dictate system reliability, multi-path fading, fade margin calculations, freshnel zone clearance, interference analysis, system diversity and long-distance specifications.
Labels:
Microwave antenna
A microwave system includes an antenna, radio, multiplexes, waveguide (hollow metal conductor connecting the RF equipment to the antenna) and feed cables. Based on capacity and radio equipment, antenna size, tower heights and terrain elevation will play a major role in how it will planned and construct the system. These four factors also will dictate system reliability, multi-path fading, fade margin calculations, freshnel zone clearance, interference analysis, system diversity and long-distance specifications.
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