VSAT Glossary
A
Antenna Noise Temperature
A receiving antenna collects noise from radiating bodies falling within its radiation pattern. For an on-ground receiving antenna, this includes contributions from the sky and from the surroundings (the earth). The combined affect of these noise sources is modelled by an equivalent noise temperature for the antenna, which varies with elevation angle and antenna size.
Attenuation
The measure of the weakening of a signal, intentional or unintentional, that occurs as it travels through a device or transmission medium (e.g. radio waves through the atmosphere, an electrical signal through a cable…). Attenuation is usually measured in decibels.
Auto-Tracking
Refers to a type of satellite dish being coupled to a servo-mechanism which guarantees the permanent correct pointing of the antenna. Such mechanisms are necessary in case the satellite is not at a fixed pozsition in the sky (inclined orbit satellites) or in case the terminal itself is moving (terminals that are embarked on boats…).
Azimuth
The pointing direction of an antenna measured in the local horizontal plane in a clockwise direction from North. It is the horizontal co-ordinate that is used to align a satellite antenna. See also Elevation.
B
Bandwidth
Refers to the radio capacity which is needed for transmitting a signal, or to the characteristics of a given emitting or receiving equipment. It is Measured in “cycles per second” or Hertz. For instance, a voice transmission by telephone requires a bandwidth of about 13,000 cycles per second (13KHz) and a TV channel occupies a bandwidth between 3 and 6,000,000 cycles per second (3 - 6 MHz). On the equipment side, a satellite transponder is featured by its central frequency and its bandwidth, which is most of the time either 36 or 72MHz.
Beacon
A particular signal transmitted by a satellite that is used for monitoring telemetry data, tracking the satellite, or conducting propagation experiments.
Beam
A unidirectional flow of radio waves concentrated in a particular direction. A term commonly used to refer to an antenna’s radiation pattern by analogy with a light beam. It is most often used to describe the radiation pattern of satellite antennas. The intersection of a satellite beam with the earth’s surface is referred to as the (beam’s) footprint.
Beamwidth
A measure of the ability of an antenna to focus signal energy towards a particular direction in space (e.g. towards the satellite for a ground-based transmitting antenna), or to collect signal energy from a particular direction in space (e.g. from the satellite for a ground-based receiving antenna). The beamwidth is measured in a plane containing the direction of maximum signal strength. It is usually expressed as the angular separation between the two directions in which the signal strength is reduced to one-half of the maximum value (the -3 db half-power points).
Bit Error Rate (BER)
An overall measure of the quality of a received digital bit stream. It is the ratio of the number of information bits that are received in error to the total number of bits received, averaged over a period of time.
BPSK
Binary Phase Shift Keying. A method of modulating a carrier wave used for digital transmissions by changing its Phase characteristic between two states, one representing a “zero”, and the other representing a “1″.
BUC
A BUC (Block Up-Converter) is a small device mounted on emitting antennas, that includes both a frequency converter and an amplifier. Basically, it takes the L-band input from the satellite Modem and transmits it upstream to the satellite on Ka, Ku, or C band. BUC’s are rated according to their output power. A low power Ka band BUC can transmit with as little as 1 watt, while a high power C-band BUC can transmit with as much as 200 watts.
C
Carrier
A pure continuous radio emission at a fixed frequency, without modulation and without interruption. A carrier can be seen as the radio “tube” that is used for routing the content (the signal). When non-modulated (for tests purpose), a carrier is also referred to as a “blank carrier”. Several types of modulation can be applied to the carrier (see Modulation).
C/N - Carrier-to-Noise-Ratio
A measure of the quality of a modulated carrier at the receiver input. It is the ratio of the power of the carrier to the power of the noise introduced in the transmission medium, measured within a specified bandwidth (usually the modulated carrier’s bandwidth). It is usually in decibels. The higher the ratio, the better quality of the received carrier.
Channel
A band of radio frequencies assigned for a particular purpose, usually for the establishment of one complete communication link. This term is sometimes used interchangeably with Transponder, but in general the channel bandwidth is less than the transponder bandwidth.
CIR
“Commited Information Rate” The amount of bandwidth which is guaranteed permanently to the terminal, whatever the traffic in the network is. Comes together with “MIR” to feature a satellite data Service.
Clarke Belt
A region in space, 22,237 miles (35,785 km) from the surface of the earth, where geostationary satellites are placed. First proposed by Arthur C. Clarke in Wireless World magazine in 1945. Satellites placed in these orbits, although traveling around the earth at thousands of miles an hour, appear to be stationary when viewed from a point on the earth, since the earth is rotating upon its axis at the same angular rate that the satellite is traveling around the earth.
Clear Sky
A term describing the weather conditions encountered at the terrestrial end of a satellite communication link. It is used to describe the condition where the attenuation of radio waves caused by precipitation (rain, snow, sleet, dew, etc.) is lowest (i.e. cloud-free sky and good visibility).
D
Decibel (dB)
The standard unit used to express the ratio of two levels. It is typically used to express either a gain or loss in power between input and output devices. A Decibel is one-tenth of a Bel, a unit created by Bell Telephone Laboratories to describe the loss of signal in telephone lines. A Decibel is a “dimensionless” unit, like “per cent”.
Delay
The time it takes for a signal to go from the transmitter through the satellite to a receiver. The delay for LEO satellites is generally very small, but for GEO satellites this can exceed one-half of a second.
Downlink (or Download, or Forward Channel)
The part of a satellite communications link that includes signal (re-) transmission from the satellite to the ground and its reception on the ground.
DVB
DVB Digital Video Broadcasting, a coherent set of European standards for transmission and reception of digital television signals via satellite, cable or terrestrial means, developed under the auspices of the Digital Video Broadcasting project and formalised by the European Telecommunication Standards Institute (ETSI). Although European, the standards have been adopted in many countries worldwide. There are many standards within the DVB family, including specifications for satellite (DVB-S), cable (DVB-C) and terrestrial (DVB-T) transmission and reception. The DVB-RCS (”Return Channel on Satellite”) normalizes the transportation of the Upstream in VSATs networks.
E
EIRP
Effective Isotropic Radiated Power - This term describes the strength of a signal leaving an antenna. EIRP is usually the sum of transmitted power and antenna gain, minus any transmission line losses. Expressed in dBW.
Elevation
The angle a satellite is above the horizon as measured in degrees from zero at the horizon to 90 degrees if the satellite is directly overhead. See Azimuth.
Equatorial Orbit
An orbit the plane of which is parallel to the earth’s equator. See also Inclination and Clarke Belt. The all Geostationary satellites (”GEO”) are located in such orbit.
F
Feed
The feed system of an antenna, looking like an Hornet. It routes and concentrates the radio wave to and from the focus of the dish.
Footprint
The footprint is the coverage area of the satellite, where the satellite is in line of sight. Groundstations located within the footprint are able to use the satellites services. At the edge of the footprint, where the elevation angle is very low (< 15°), the transmitting conditions can be rather bad due to the thickness of the atmospheric coats. It is then necessary to use larger antennas and more powerful BUCs.
Forward Error Correction (FEC)
An error correcting algorithm that adds unique codes to the digital stream at the emitting source so errors can be detected and corrected at the receiving subsystem. An example of this is QPSK.
Frequency
The number of times that an alternating current goes through its complete cycle in one second of time. One cycle per second is also referred to as one hertz; 1000 cycles per second, one kilohertz; 1,000,000 cycles per second, one megahertz: and 1,000,000,000 cycles per second, one gigahertz.
Frequency Bands
L-Band - The frequency range from 0.5 to 1.5 GHz. Also used to refer to the 950 to 1,450 MHz sub-Band used for mobile communications.
C-Band - This is the band between 4 and 8 GHz with the 4 and 6 GHz band being used for satellite communications. C-Band transmissions are less susceptible to heavy weather conditions than higher satellite Bands (Ku or Ka-Band).
Ku-Band - The frequency range from 10.9 to 17 GHz. This Band is widely used for Digital TV Broadcast and VSAT Applications.
Ka-Band - The frequency range from 18 to 31 GHz. Due to its susceptibility to rain and clouds, it is used together with Spot beams, the power of which is concentrated on small footprints.
Frequency Reuse
A technique for utilising a specified range of frequencies more than once within the same satellite system so that the total capacity of the system is increased without increasing its allocated bandwidth.
G
Gain (Antenna)
A measure of the amplifying or focussing power of an antenna when transmitting to, or receiving from, a particular direction in space. The gain of an antenna is the ratio of the power radiated (or received) by the antenna in a given direction to the power radiated (or received) by an isotropic antenna fed with the same power. The gain is usually expressed in dBi.
Geostationary Orbit
The orbit of a geosynchronous satellite whose orbit lies in the plane of the earth’s equator.
Geosynchronous
An object orbiting the earth at the earth’s rotational speed and with the same direction of rotation. The object will appear at the same position in the sky at a particular time each day, but will not appear stationary if not orbiting in the equatorial plane.
Ground Segment
The ground segment consists of all the earth stations that are operating within a particular satellite system or network. These can be connected to the end-user’s equipment directly or via a terrestrial network.
H
Hertz (Hz)
The name given to the basic measure of radio frequency characteristics. An electromagnetic wave completes a full oscillation from its positive to its negative pole and back again in what is known as a cycle. A single Hertz is thus equal to one cycle per second.
HPA
High Power Amplifier. A device that accepts a relatively weak input signal and boosts it to a power level that is suitable for transmission over an earth-space link.
I
IDU (Indoor Unit)
The IDU the part of a satellite station which is placed indoor, close to the working station or the router that it is connected to. Basically, the IDU is a satellite modem. It is connected to the ODU (Outdoor Unit).
Inclination
The angle between the orbital plane of a satellite and the equatorial plane of the earth.
Inclined Orbit
An orbit that approximates the geostationary orbit but whose plane is tilted slightly with respect to the Equatorial plane, with the consequence that the satellite appears to move about its nominal position in a daily “figure-of-eight” motion when viewed from the ground. Satellites are often allowed to drift into an inclined orbit near the end of their nominal lifetime in order to conserve fuel on-board the satellite, which would otherwise be used to correct this natural drift caused by the gravitational pull of the Sun and the Moon.
Isotropic Antenna
An isotropic antenna is an ideal antenna that radiates power with unit gain uniformly in all directions and is often used as a reference for antenna gains in wireless systems. There is no actual physical isotropic antenna. The radiation pattern for the isotropic antenna is a sphere with the antenna at its center. Antenna gains are often specified in dBi, or decibels over isotropic. This is the power in the strongest direction relative to the power that would be transmitted by an isotropic antenna emitting the same total power.
L
Link Budget
A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It takes into account the attenuation of the transmitted signal due to propagation, as well as the loss, or gain, due to the antenna. Random attenuations such as fading are not taken into account in link budget calculations with the assumption that fading will be handled with diversity techniques.
Low Noise Amplifier (LNA)
A preamplifier usually mounted on a receiving dish antenna designed to amplify weak signals by contributing the least amount of thermal noise to the received signal.
Low Noise Block Downconverter (LNB)
A combination of a Low Noise Amplifier and a frequency Downconverter built into one device attached to the feed.
M
Margin
The difference in decibels between the C/N ratio achieved at the receiver input under clear sky conditions to the minimum C/N ratio required for just acceptable transmission quality. Also referred to as the “Rain Margin”.
MIR
“Maximum Information Rate”. Refers to the Maximum Bandwith that can be received by a terminal according to the service it has subscribed to. Depending on the state of the traffic at any moment, a terminal is made available a bandwidth which is between its CIR and its MIR.
MCPC - Multiple Channel Per Carrier
Refers to the multiplexing of many digital channels into a common digital bit stream, which is then used to modulate a single carrier that conveys all of the services (the channels) to the end users. The term MCPC is frequency used in the context of DVB systems, where the composite digital signal is referred to as a Transport Stream.
Modulation
The process of manipulating either the Amplitude, the Frequency or the Phase of a carrier in relation to an incoming video, voice or data signal. Basically, the emitter and the receiver are permanently tuned (syntonized) on the central frequency of the carrier. Then the modulation consists in a very small alteration of the carrier by the emitter: the signal wave, which is weak in comparison to the carrier wave, is “added” to it. At the receiving side, a “substractor” device separates this signal wave from the carrier.
Modulator
A device which modulates a carrier by adding the signal wave on it.
Multiple Access
Refers to the technologies which allow many users to access a common and shared ressource at any moment. Within the area of radio communications, the main existing technologies are…
FDMA Frequency Division Multiple Access: new incoming users are placed on different carriers or sub-carriers
TDMA Time Division Multiple Access: new incoming users are placed on the same carrier, but on different time-slots. Therefore, each one benefits the bandwidth by short period of times.
FTDMA: a mix between FDMA & TDMA
CDMA: Code Division Multiple Access: all users are accessing the bandwidth at any moment, but by using individual coding schemes; being assumed that most of the codes are “orthogonal”, the paired communications will not interfere one another.
N
Noise
Any unwanted energy that is always present to some extent within any signal.
Noise Temperature
A mathematical convenience for predicting the influence of noise in a communications system. It is a measure of the noise power generated by a practical device, expressed as the equivalent temperature of a resistor which, when placed at the input of a perfect noise-free device, generates the same amount of output noise. The noise temperature is usually expressed in kelvin or dBK.
O
ODU (Outdoor Unit)
The ODU is the part of a satellite station which is placed outdoor, and which is mounted either on a wall or on the ground. It includes the Antenna on which the Feed, the LNB, and the BUC are mounted, and the metal mount. 2 coaxial cables connect the LNB and the BUC to the IDU.
P
Phase Shift Keying (PSK)
A modulation technique that involves the Phase of the carrier, by shifting it of a certain angle (π/2 in Binary PSK, π/4 in Quadratic PSK, π/8 in 8-PSK, etc.). Using BPSK, two states (i.e. 1 bit) can be coded per cycle (i.e. per Hertz). Using QPSK, 4 states, i.e. 2 bits are coded. And so on… The more bits per cycle are encoded, the more susceptible the signal will be to interference and fading, and ultimately, the higher the need for emitting power. In satellite communications, an optimum is reached today by using QPSK modulation.
Polar Mount
An antenna mechanism that permits steering in both elevation and azimuth through rotation about a single axis. Satellite earth stations utilize a modified polar mount geometry that incorporates a declination offset.
Polarization
A technique used by the antenna designers to direct energy in the horizontal or vertical planes, or in a circular manner. This allows to route 2 data streams on a single wave by using 2 orthogonal polarization plans (See also frequency reuse).
Horizontal Polarisation. Type of linear polarisation where the electric field is approximately aligned with the local horizontal plane at an on-ground transmission or reception point.
Vertical Polarisation. Type of linear polarisation where the electric field is approximately aligned with the local vertical plane at an on-ground transmission or reception point.
X-Polarisation. A more precise definition of horizontal linear polarisation. X-polarisation is defined with respect to a particular direction from the satellite towards the earth, allowing precise calculation of the polarisation alignment angle for any given geographic location.
Y-Polarisation. A more precise definition of vertical linear polarisation (see X-Polarisation).
Q
QPSK - Quadrature Phase Shift Keying
See Phase Shift Keying (PSK) and FEC.
R
Random Access
The “Random Access” Mode refers to a particular (and most simple) way to access and share the radio ressource : any client-terminal can send a data packet at any moment (or at the beginning of any time slot, in case of slotted schemes). It opposes to “Reservation” or “Connection” modes, in which the client terminal must negotiate its access to the capacity (the time slots or the full carrier) before sending any data. The advantage of Random Access Modes is that there is no time lost in negotiating the capacity. The disadvantage is that when there are many terminals, the risk of access conflicts (collisions) becomes very high, as 2 or more terminals may try to send datas at the same time. In such cases, each terminal concerned by the colllision has to repeat its transmission, which becomes very unefficient when the network is overloaded. The advantage of Reservation Modes is that there are no such risks of collisions. The disadvantage is that there are significant delays invested in the upfront bandwidth negotiation.
S
SCPC - Single Channel Per Carrier
Opposes to MCPC. In SCPC systems, each communication signal is individually modulated onto its own carrier which is used to convey that signal to the end user. A number of similar carriers share a common satellite transponder and use a unique portion of its bandwidth. Each carrier supports a single communication channel only, hence the name of “Single Channel Per Carrier”.
Signal to Noise Ratio (S/N)
The ratio of the signal power and noise power. The better the signal to noise ratio, the better the quality of the signal.
Spread Spectrum
A radio transmission scheme in which the transmission of a signal uses a much wider bandwidth and power than would normally be required. Spread spectrum scheme also involves the use of narrower signals, that are frequency hopped through various parts of the total available bandwidth. The use of both techniques produce very low levels of interference between users, that are corrected using classical error correction techniques.
System Noise Temperature
The equivalent noise temperature of a complete receiving system, taking into account contributions from the antenna, the receiver and the transmission line that interconnects them, referred to the receiver input.
T
Transponder
The term “satellite transponder” refers to a transmitter-receiver subsystem on-board the satellite that uses a single high power amplification chain and processes a particular range of frequencies (the “transponder bandwidth”). There are many transponders on a typical satellite (between 20 and 70), the bandwith of each being either 36 or 72 MHz. Such a Transponder may support one or more communication channels.
U
Uplink (or Upload, or Return Channel)
The part of a satellite communications link that involves signal transmission from the ground and reception on-board the satellite.
V
VSAT Terminals
VSAT stands for “Very Small Aperture Terminal”. It refers to receive/transmit terminals installed at dispersed sites connecting to a central hub via satellite and using small diameter antenna dishes (0.6 to 3.8 meter). Basically, VSAT antennas are antennas that can be moved very easily, due to their size and to the way they are mounted. The VSAT terminals include an Indoor Unit (IDU) and an Outdoor Unit (ODU), interconnected by 2 coaxial cables.
