Satellite Communication Systems

Satellite Communication Systems

• Satellites helped to overcome the difficulties associated with long-range communication.

• Communication satellites are radio relay stations which offers the advantage of direct line-of-sight to almost 98% of the earth’s surface.

• Satellites communication systems are profitable than land based system if it allowed to stay in place for a reasonable number of years.

• The greatest strides in the development of satellites communication systems took place between 1965 and 1979, when COMSAT (Communication Satellite Corporation) launched four satellites within 6 years.

Satellite Communication Systems• If a transmitting station cannot communicate with one or

more receiving stations because of line-of-sight restrictions, satellites can be used to overcome this constraint.

• The transmitting station sends the information to the satellite which in turn retransmits it to the receiving stations, hence a communication satellite is essentially a repeater.

Kepler’s Laws and Newton’s Laws

• The fundamental properties of orbits are summarized by Kepler’s three laws of planetary motion which are applicable to satellite orbits also1. The orbit of each planet (satellite) is an ellipse with the sun (earth) at one focus2. The line joining the sun (earth) to a planet (satellite) sweeps out equal areas in equal times.3. The square of the period of revolution is proportional to the cube of the semi-major axis

Semimajor Axis

Kepler’s Laws and Newton’s Laws

• The fundamental laws of physics upon which the theory of orbital mechanics is based on Newton’s law of gravitation and Newton’s second law of motion.

• The law of gravitation states that the gravitational force of attraction between two bodies varies at the product of their masses M and m and inversely as the square of the distance r between them and is directed along a line connecting their centers.

• The second law of motion states that the acceleration of a body is proportional to the force acting on it and inversely proportional to the mass M.

Orbit Patterns

• All satellites are placed in an orbit in space.

• An orbit is a circular/elliptical path occupied by the satellitemoving in a direction parallel the surface of the earth.

• This velocity around the earth has a forward velocity sufficient to create an outward thrust (centrifugal force) equal to earth’s gravitational pull on the satellite.

• The satellite therefore stays in its orbit without falling back to earth’s surface.

• There are three common orbital patterns; the polar orbit, the inclined elliptical orbit and the equatorial geosynchronous orbit.

Orbital Patterns

• Polar Orbit: - An orbit which passes over the poles.

• Equitorial Orbit: - An orbit in the plane of the equator.

• Elliptical Orbit: - An orbit elliptical in shape.

Polar Orbit Equitorial Orbit Inclined Orbit with angle i

Orbital Patterns

Eclipse focal points

• Characteristics of satellite orbit include height, speed, angle of elevation and angle of inclination.

• When a satellite is in an elliptical orbit, the center of the earth is one of the focal points.

Typically two points are of great interest, the highest point referred to as the apogee and the lowest point referred to as the perigree.

Geosynchronous Satellites• Equitorial Geosynchronous orbits are also called the Clarke

Orbit.• Communication Satellites placed on the Geosynchronous

orbit are called Geosynchronous satellites.

• The following facts apply to all satellite orbits

1. The plane of the orbit must pass through the center of the planet being orbited.

2. The time to complete one orbit depends on the mass of the satellite, its velocity and the final orbital altitude.

Geosynchronous Satellites

• This means that the satellite must move in the same direction as the earth’s rotation.

• The only orbit that meets these requirements is the one that is directly over the equator, moving in a west-to-eastdirection at an altitude 35,838 kM (appr 36,000 kM) above sea level with a forward velocity of 10,998.4 kM/h (11,000 kM/h) to complete one orbit within 24 hrs .

• Communication satellites are to placed in such orbits (geostationary) so that it appears to be stationary over a selected location on the earth’s surface.


• The satellite placed in this orbit revolves in exact synchronism with the earth’s rotation and appears fixed or stationary w.r.t the earth.


• Since the satellites appear to be stationary in space special antennas for tracking are required.

• The antennas can be simply pointed at the satellite.

• Hence such orbits are called geostationary orbits

• Approximately 42% of earth’s surface can be seen or accessed from such a satellite, hence users within this area can utilize satellite for communication.

• This arrangement makes continuous communication possible.


• At the synchronous orbit the angular velocity ω of the satellite equals the velocity of a point on the earth.

• The geosynchronous orbit is placed at geo synchronous altitude.

Synchronous orbits

• Let M and m be the mass of the earth and that of the satellite respectively.

• The geosynchronous altitude is calculated as follows.

• Similarly let RE and R be the radius of earth and that of the satellite from the center of the earth respectively.


• This centrifugal force equals the gravitational force between the satellite and earth given by GmM/R2 - (2).

• If ω is angular velocity of the circular motion of the satellite in radian per second then the centrifugal force of the satellite would mRω2 – (1).

• Then

• At the earth’s surface the gravitational force is mg where gis the gravitational acceleration due to gravity.

2

2

R

GmMmR =ω• Equating the above

eqns.

mgR

GmM

E

=2


• Therefore 3

1

22

23

1

2

2

4

=

=f

gRgRR EE

πω

• Where f = 2π/ω is the revolutions per second.

• Putting f = 1 revolution per day and RE=6370kM and g=9.9m/s, R = 42,208 kM.

• Subtracting RE from R we get the distance of geosynchronous orbit from the surface of the earth equals 35,838kM.


• The angle subtended by the at this distance is 17.3˚.

• The highest latitude at which the satellite is visible at this distance is 81.3˚.

• For an earth station to communicate with the satellite requires a minimum elevation angle of atleast 5˚.

Satellite location

Illumination = footprint

Geosynchronous Satellites• This reduces the highest altitude to 76˚.

• Corresponding to this lattitude of coverage of the satellite is around 42% of earth’s surface.

• Polar and inclined orbits may be synchronous, but it need not be stationary.

• However the polar regions cannot be seen by the equatorial satellite.

• To cover the polar regions, therefore a satellite with an elliptical and inclined orbit is needed.

• A geostationary satellite is always synchronous.

• Geostationary satellites also has equatorial and circular orbit.


Satellites in the geostationary earth orbit (GEO) e nable1. Facsimiles,2. Video conferencing, 3. Internet, 4. Long distance fixed phone service, 5. Television and 6. Broad band communications.

• Hence all geosynchronous satellites need not be geostationary.

• Satellites placed in elliptical and inclined orbits whose period of revolution is equal to the earth’s siderial period of rotation (23 hrs, 56 min and 4.1 sec) is considered synchronous.

Medium Earth Orbit (MEO) Satellites

• The Medium Earth Orbits (MEO) extends roughly 10,000km to 20,000kM.

• Similar to the LEO satellites the MEO satellites also provide continuous coverage by a constellation of satellites rather than a single satellite.

• Although more satellites are required as compared to GEO the MEO satellites are economic, since the cost per satellite is less.

• The other advantages are that the mass is less and the launch costs are less.

• These are designed for shorter mission life time as compared to GEO.

Medium Earth Orbit (MEO) Satellites

• The GPS system developed by the US Dept of Defense is constellation of 24 satellites with 6 orbital planes at an altitude of 20,182kM.

• These are 12 hour orbit has the special property that the ground trace exactly closes upon itself without overlap.

Satellites in the medium earth orbit (MEO) enable1. Mobile cell phones,2. Fixed phones, 3. Other personal communications.

Low Earth Orbit (LEO) Satellites

• The first experimental satellites were low earth orbit satellites.

• Low earth orbits altitudes typically vary between 750 and 1500 kM.

• The potential advantages of LEO compared to GEO include reduced satellite power requirements and antenna size, smaller propagation delay, the availability of Doppler effect for position determination, the availability to develop the system incrementally and lower overall system cost.

• LEO satellites are used for world wide data messaging and Global position determination.


• LEO satellites provide continuous coverage by a constellation of satellites rather than a single satellite.

• The STARSYS constellation consists of 24 satellites at an altitude of 1300kM in orbits inclined at 60°.

• Polar orbits are a special case of low earth, circular orbits having an inclination of 90°.

• In LEO altitude range a polar orbiting constellation can provide global coverage with fewest number of satellites.

• Motorola’s IRIDIUM constellation consists of 77 satellites in the polar orbit at an altitude of 765kM with 11 satellites in each of 7 orbital planes.

• This network is designed to provide Mobile Satellite Service for handheld personal telephones.


Satellites in the Low earth orbit (LEO) are used fo r1. Hand held mobile phones,2. Fax, 3. Paging4. Tracking of ships and trucks5. Fixed ordinary phones6. Broad band multimedia7. Monitoring of remote industrial areas.8. Surveillance.

Advantages and Disadvantages of Satellite Communication

• Communication through satellite has several advantages and disadvantages.

• Until the advent of communication satellites, long distance communication could be done by using cascaded radio relays, very low frequency radio (below 30kHz) and high frequency or short wave radio (3-30MHz).

• Cascaded radio relays were limited to overland spans.• Very low frequency and short wave radio were suitable

only for specialized applications.


Advantages • Satellite communication offers high capacity

transmission over long distances over land or water.• It has the inherent ability to transmit simultaneously from

one point (location) to many points on the earth (location of the earth stations) within its coverage area. (this is known as point-to-multipoint relay), where as terrestrial relays are point-to-point.

• Satellites systems can be installed rapidly and communications can be established within a short time.

• It can be removed from one location and reinstalled elsewhere, relatively quickly as compared to earth stations.


Advantages• Mobile communication can be easily achieved by

satellites.• It provides unique flexibility in interconnecting mobile

vehicles. (Terrestrial networks may interconnect mobile vehicles by cellular radios).

• Satellites costs are independent of distance, where as terrestrial network costs escalates with distance.

• The quality of transmitted signal is independent of the the locations of earth stations as long as the station locations are within the coverage area.


Advantages • The quality of transmitted signal is independent of the the

locations of earth stations as long as the station locations are within the coverage area.

Disadvantages.• When the satellite is in position the communication path

between the terrestrial transmitter and the receiver is approximately 75000 km long.

• This will introduce a delay of ¼ second between the transmission and reception of a signal.

• Thus between the talks there is gap of ½ second which can be annoying.

• Repairing costs of satellites are high and expensive.

Satellite Frequency Allocations• Most communication

satellites operate in the microwave frequency region.

• The most widely used band is the C band.

• However this band is getting overcrowded.

• Most new satellite systems use the Ku band.

• For given antenna size the gain is higher for the Ku band, this improves reliability and antenna cost and size.

Earth Station

• The earth station or ground station is the terrestrial base of the system.

• The earth station can communicates (transmit and receive signals) with the satellite.

• An earth station consists of an antenna subsystem, a power amplifier subsystem, a low-noise receiver subsystem and a ground-communication equipment subsystem.

Earth Station• The performance of an earth station is specified by its

equivalent-isotropic-radiated-power (e.i.r.p) and its gain-to-noise ratio (G/T).

• The e.i.r.p is the product of the power output of the high power amplifier at the transmitting antenna and its gain .

• G/T is the ratio of the gain of the receiving antenna to the system noise temperature.

• The antenna gain is proportional to the square of its diameter and is dependent on the efficiency of the feed/reflector system.

• The system noise temperature is a measure of 1. the noise of the receiver, 2. losses between the antenna feed system and the receiver and 3. the antenna noise.

• The size of the antenna varies from 0.5m to 30m depending upon the type of earth station and the power capacity.

Inside the Satellite• There are basically five sections within the satellite, each

totally dependent on the other four.

A. Electronic section, called the transponder.

D. Station keeping section, made up of the Control and information section and the Rocket thruster section.

B. Antenna systems.

C. Power Packs.

Transponder

• Fig shows the basic operation of a communication satellite.

• An earth station transmits information to the satellite.

Inside the Satellite

• The satellite contains a receiver which picks up the transmitted signal, amplifies it and translates it to another frequency.

Transponder

• This new frequency is amplified and retransmitted to other receiving stations back on earth.


• The original signal being transmitted from the earth station to the satellite is known as up link.

Transponder

• The retransmitted signal from the satellite to the receiving station is called the down link.

• Usually the down link frequency is lower than the uplink frequency, typical uplink frequency is 6 GHz and a common downlink frequency is 4GHz.

• The transmitter-receiver combination in the satellite is known as a transponder.

• The basic function of a transponder is amplification and frequency translation.


• The reason for frequency translation is that the transponder cannot transmit and receive the same frequency.

Transponder

• The strength of the received signal (up link) is weak as compared the that of the transmitted signal (down link).

• If the frequency of the transmitted signal and the received signal are the same the receiver will be overload the high power transmitted signal and block out the weaker up linlsignal.

• By using widely spaced transmit and receive frequencies interference is minimized and separation is achieved.

• It is not economical to have one signal transponder in communication satellites, hence typical satellites will 12, 24 or even more transponders.


• Each transponder represents an individual communication channel.

Transponder

• Various multiplexing schemes are used so that each channel may carry multiple information transmissions.

Linkages• Regardless of the orbits used for communication

services, all satellite links have some elements in establishing end-to-end communication.

• The link in general form will have both transmit and receive facilities at both ends.

• The overall requirements for the satellite links can be divided into two parts.

• The first deals with the satellite radio-frequency (RF) link, which establishes a communication link between the transmitter and a receiver using the satellite as a repeater.

• The performance capacity of the satellite radio link is measured in terms of its overall available carrier-to-noise ratio (C/N).

Linkages

• This is the ratio of the carrier power to the noise power measured in a Bandwidth (B).

• The value of C/N depends on a number of factors, which in turn depends on the available power and bandwidth.

• The second requirement deals with link between earth terminals and user equipment.

• This ratio is a direct measure of the channel carrying capacity of the satellite link.

• The user equipment consists of mainly voice, data, video communication devices which are either simplex or duplex .

Linkages

• The quality of this baseband links is characterised by transmission rates, error rates, signal-to-noise ratio etc.

• A communication satellite operates as a distant line-of-sight microwave repeater providing communication services among multiple earth stations in various geographic locations.

• Some relevant definitions related to the RF link are given below.

Radio Frequency Satellite Link

• A channel is a one way link from the transmitting earth station through the satellite to the receiving earth station.

Linkages

• A circuit is a full-duplex link between two earth stations.

• A half-circuit is two way link between an earth station and the satellite only.

• The channel-carrying capacity of a satellite RF link is directly related to the overall available C/N ratio.

• The second component in the RF link is the down link.

• The first component in the RF link is the up link.

• The figure of merit or the performance factor of this link is called the up link carrier-to-noise ratio (C/N)U.

Linkages

Linkages

• Similar to the up link the down link performance factor is called the down link carrier-to-noise ratio (C/N)D .

• Both the values of both (C/N)U and (C/N)D ratios depend on the power of the transmitter , the transmitting and receiving antenna gains and the receiving system noise temperature .

• The third component of the RF link is the satellite electronics system, which by itself produces undesirable noise signals.

• The carrier-to-noise ratio this system is expressed as (C/N)t.

LinkagesUp Link .

• All of the ground equipment along with the transmission path and the receiving antenna at the satellite are included in the up-link system.

• A smaller receiver antenna has low gain and needs a larger beam width angle.

• To conserve space and weight on board the satellite, the receiving antenna is made much smaller than the transmitting antenna.

• This can be compensated by increasing the size of the ground station transmitting antenna as well as raising the ground station transmitted power.

• Typical diameter of ground station transmitter antenna is 32.5 ft with a transmitter power of 1 to 5kW.

LinkagesDown Link .

• Similar to the up link system, all of the ground equipment along with the receiving path and the transmitting antenna at the satellite are included in the down-link system.

• Fig shows the foot print patterns of a multi-beam satellite.

• The down-link is described in terms of a satellite transmitter output power, down link antenna gain and beam width and the ground area the transmitted signal will service (called the footprint ).

• The shape of the foot print is controlled by the design shape of the parabolic reflector.

Linkages.Down Link .

Linkages

• At the altitude of the Clarke’s orbit, one satellite could have a foot print of 42.2% of the earth’s surface.

Cross Link .

• The beam width from the satellite for such a coverage would only be 17.174˚.

• This would not allow a global coverage.

• A minimum of three satellites placed at 120˚ apart in the Clarke’s orbit would cover all the earth’s surface (98%) except for the polar caps.

Linkages

• This makes it possible for one earth station to transmit to another earth station at the opposite side of the globe by sending the data to its “in view ” satellite.

Cross Link .

A B

Multiple Access Techniques

• Multiple access techniques allow interconnection among large number or earth station terminals simultaneously via a single satellite transponder.

• Using multiple access techniques one earth station can communicate with all other earth stations using the same satellite.

• Multiple access techniques uses the concept of multiplexing used in various communication systems.

• Time Division Multiple Access (TDMA) as well as Frequency Division Multiple Access (FDMA) schemes are widely used.


• The Random Access Multiple scheme is mostly used in packet communication systems.

• With the development of spread spectrum techniques in communication systems the Code Division Multiple Access (CDMA) is also becoming popular in satellite communication.

• In addition to the above multiple access techniques, the access of earth stations to the satellite may be based on the fixed or demand basis.

• In the fixed case the access allocated to each earth station is fixed in advance, therefore it is termed fixed access (FA) or pre-assigned access (PA)

• In case of FA, a station has periodic access to the satellite isindependent of its actual need.


• In the demand access (DA) the resource is allocated as needed in response to the changing traffic conditions.

• The demand access techniques are more efficient as compared to the fixed access techniques.

Time Division Multiple Access (TDMA)

• Conceptually TDMA is more complex than time division multiplexing.

• Fig. indicates the principle of TDMA.

• Many earth stations in the satellite communication network use a single carrier for transmission via the satellite transponder on a time division basis.


Time Division Multiple Access (TDMA)

• These earth stations transmit traffic bursts in a periodic time frame which is termed TDMA frame.

• This time duration of the order of 125µSec, there is guard time between the transmitting times of the individual bursts, in order to ensure the bursts does not overlap.

• The satellite transponder receives one burst at a time, amplifies it and retransmits the signal back to the earth.

Multiple Access TechniquesTime Division Multiple Access (TDMA)

• TDMA would be called fixed assignment TDMA (FA-TDMA) if the frame time is divided into time slots of fixed duration, with the slots equally divided among the stations.

• In case the transmission time allocations are based on instantaneous load demands, the TDMA would be called demand assignment TDMA (DA-TDMA).

• Every receiving earth station that is served by the transponder may receive the entire burst stream and can extract the burst meant for it.

Multiple Access TechniquesTime Division Multiple Access (TDMA)

• In a TDMA network each earth station periodically transmits one or more bursts to the satellite and the TDMA frame structure consists of such bursts.

• There are two reference bursts (RB1 and RB2) meant for reliability.

TDMA Frame Structure

• Fig shows a typical TDMA frame structure.


• Out of these two reference bursts one serves as primary reference burst (PRB) and the corresponding earth station is termed primary reference station.

• Similarly the other reference burst serves as the secondary reference burst (SRB) and the corresponding earth station would be secondary reference sation (SRS).

• The secondary reference burst is mainly used in the event of failure of the primary reference burst so as to provide undisrupted service for the TDMA network.



• The switchover from the primary to the secondary in such eventualities is automatic.

• These two reference bursts carry no information, but serve to synchronize and identify the frame of signal bursts.

• These two reference bursts provide the timing references for all the earth stations accessing a particular satellite transponder.


• Each TDMA frame has only one reference burst (either PRB1 or PRB2) is transmitted and it allows a satisfactory interleaving of bursts within the TDMA frame.


• The traffic burst signals which form the main parts of the TDMA frame carries the information from the earth stations.

• The corresponding earth stations are termed traffic stations.

• There may be one or more slots in one frame.


• Different traffic burst occupy fixed slot positions in the TDMA frame according to the burst time plan.

• Traffic burst may be long or short depending upon the amount of information to be transmitted.


• Thus by referencing the timing of the occurrence of primary reference burst, traffic bursts carrying information can be located in the TDMA frame.

• When the signal bursts from the transponder are received traffic stations detect the primary reference burst and then locate and extract the burst corresponding to it by precisely controlling time of reception.


• Similarly during transmitting to the transponder the traffic station (earth stations) precisely control the transmit timing of their bursts according to the allocated slot positions in theTDMA frame so that no overlapping occurs among the bursts.


• The time interval between different traffic bursts from different traffic stations to the satellite transponder so as to prevent overlap is known as guard time.

TDMA Frame Efficiency


• The TDMA frame efficiency (η) depends on the ratio of the time devoted for transmission of information in the frame to the total frame length is thus defined as

FT

t∑−=1η where ∑ t

represents the sum of all the guard times and other preambles including the reference bursts.


• The sum of all the guard times and other preambles including the reference bursts Σt is the time overhead required for the transmission of information.

• If these N sequential frames are combined in an orderly manner, the resulting combination reduces the total overhead considerably, thus increasing the TDMA frame efficiency.

TDMA Superframe

• This orderly arrangement of TDMA frame is called TDMA Superframe.

• If a number of frames (N frames) are send independently each frame would require the same amount of time overhead.


• To identify the frames in a superframe a frame identification number (marker) is required.

TDMA Superframe

• Normally the idenfication number of frame 1 serves as the superframe reference marker.

• The process is repeated in the same fashion for every N frames and increases the TDMA frame efficiency.

Demand Assigned Multiple Access Techniques

• Most users in a communication system do not communicate continuously.

• Hence the access allocation to the transponder need not have a fixed pattern and it will advantageous to dynamically assign the channel according to traffic requirements.

• The fixed assignment works best when all the earth stations are operating at high capacity.

• When the traffic from the earth station is low and intermittent,dynamic acess allocation improves efficiency by reducing waste of time slots.


DA-TDMA• Figure shows the allocation

for a FA TDMA scheme.• A,B,C and D are users.• Each time frame has four

slots assigned sequentially to the users.

• Each user has three slots.• The last slot of user A , the

second slot of B and the first slot of C are all inactive.

• With fixed assignment the inactive slots with no data transmission, are wasted.


• The demand assignment scheme allows the possibilty of reallocation of the empty slots due to the lightly loaded earth stations.

DA TDMA

• Hence stations can be allocated with more slots per time frame than the others.

• DA TDMA can also allow variable duration slots for the earth stations.

• Therefore the DA TDMA frame structure is different from the TDMA frame structure.


DA TDMA

• Figure indicates a typical DA TDMA frame structure.


• A super frame consists of N frames where N represents the number of stations transmitting from the frame.

DA TDMA

• The idenfication number of frame 1 serves as the superframe reference marker, therefore earth station 1 serves as the reference station.

• Each traffic station can be addressed once per frame, eg station 1 is addressed in frame 1, station 2 in frame 2 etc.

• Each station also transmits a superframe short burst (SSB) once per frame.

• The reference station (frame 1) sends the capacity assignment messages to other stations via capacity assignment channel.


• The SSB is normally at the end of frame and also carries the capacity request channel, by which the traffic station communicates with the reference station and request additional capacity.

DA TDMA

• The station continues to send this signal once per superframe until it receives an acknowledgment from the reference station.

• The reference reconfigures the frame and send the information of the reconfigured frame to all stations affected via the capacity assignment channel.

• When the traffic at a station exceeds its allocated capacity in the frame, it sends a capacity request message to the reference station via the capacity request channel.


• Thus the most important function of the DA-TDMA is the ability of the reference station to quickly search the frame to find the available capacity and then assign it to the traffic station that request it.

DA TDMA

• Call signalling messages are transferred between originating and the destination station on this channel.

• The common signalling channel is employed between traffic stations to implement distributed signal processing and call assignments.


• It is the earliest of all multiple access techniques used in satellite communication.

Frequency Division Multiple Access (FDMA)

• Each of the earth stations within the satellite footprint transmits one or more carriers at different center frequencies.

• Each carrier is assigned a frequency band with a small guard band to avoid overlapping between adjacent carriers.


• The satellite transponder receives all the carriers within its bandwidth.

Frequency Division Multiple Access (FDMA)

• The transponder translates the frequencies, amplifies the signals and retransmits them back to the earth.

• Two FDMA techniques are in use.1. Multichannel per carrier where the earth station multiplexes several channels into one carrier using frequency division multiplexing before transmission to the transponder.2. Single channel per carrier where each channel modulates a separate carrier before transmission.


• With the DA FDMA method the carrier frequencies in the satellite transponder is not fixed or pre-assigned for any earth station

DA FDMA

• The transponder and the earth station are paired on demand .

• Neither end of a channel is permanently associated with any fixed carrier frequency.

• A pool of available carrier frequencies is known to the satellite transponder.

• Each carrier frequency within the satellite transponder bandwidth becomes a part of this pool.


• The carrier frequencies may be assigned to any channel between the earth station and the satellite as required.

DA FDMA

• The first fully demand assigned single channel per carrier SCPC system was the SPADE system, developed for use by the INTELSAT in late 1960s.

Satellite Communication Systems

Documents

Transcript of Satellite Communication Systems