LAAS system consists of LAAS ground station/processing unit/power supply (one shelter on airport property), 4 reference receivers/antennas, one VHF data link antenna. LAAS uses a similar approach to WAAS except all of the equipment is installed at the airport and the corrections and integrity information is transmitted to the aircraft via a local VHF transmitter. LAAS accuracy is typically 2-3 times better than WAAS and the system is designed to support landing operations is zero visibility conditions, commonly referred to at Category-III operations. LAAS provides a more cost beneficial alternative to legacy landing systems because a single facility can provide service to all runways at an airport and the precise positioning service is available 360 degrees coverage out to 23 miles to enable three dimensional arrivals, closely space parallel runway operations, and NextGen super density operations.
Transponders are microwave repeaters carried by communications satellites. Transparent transponders can handle any signal whose format can fit in the transponder bandwidth. No signal processing occurs other than that of heterodyning (frequency changing) the uplink frequency bands to those of the downlinks. Such a satellite communications system is referred to as a bent-pipe system. Connectivity among earth stations is reduced when multiple narrow beams are used. Hence, the evolution proceeded from the transparent transponder to transponders that can perform signal switching and format processing.
Breakeven Distance: As the cost of Satellite Circuit is independent of distance on the Earth between the two ends, whilst the cost of a terrestrial circuit is approximately directly proportional to that distance, the concept of a &quot;breakeven&quot; distance where the costs are equal has been used to determine where services should be routed via satellite. This breakeven distance varies according to the size of the route, growth rate, and any special networking requirements.
1 for N Diversity: Where there is negligible likelihood of route failure, there is no need for route diversity protection and the type of protection used is known as &quot;1 for N&quot;. In point to point radio systems it is (typically 7 : 1) throughout the world. If a worker section down a route fails, the traffic is switched to a stand-by section. After repair of the worker, traffic is returned to it after a suitable period of time. This period of time is that necessary for a stability test, to check that the fault has been genuinely cleared. Traffic loss due to section failure can typically be reduced by several hundred times by the use of &quot;1-for-N&quot; protection.
AJAL ASC Chap2 revIew
MODULE 2Satellite Access Methods AJAL.A.J Assistant Professor –Dept of ECE, Federal Institute of Science And Technology (FISAT) TM MAIL: firstname.lastname@example.org
The Earth is divided up into the northern hemisphere and the southern hemisphere: Northernhemisphere Southernhemisphere Equator
The Earth is tilted on an axisNorth poleSouth pole
The Earth is kept in orbit by the force of… Gravity …and by the fact that is is moving at a high velocity
Digital Signaling Example - PCM (Coder-Decoder)
Reasons for Choosing Data and Signal Combinations • Digital data, digital signal – Equipment for encoding is less expensive than digital- to-analog equipment • Analog data, digital signal – Conversion permits use of modern digital transmission, computational resources and switching equipment • Digital data, analog signal – Transmission media will only propagate analog signals – Examples include optical fiber and POTS (3 kHz bandwidth limited) • Analog data, analog signal – Analog data easily converted to an analog signal via some form of modulation (AM, FM, etc.)
Unguided Media• Transmission and reception are achieved by means of an antenna (rcvr + xmtr)• Configurations for wireless transmission – Directional (infers gain) – Omnidirectional – Polarization (vertical, horizontal, circular)
A Simplified Wireless Communications System – Unguided Media Antenna Information to be Coding Modulator Transmitter transmitted(Voice/Data) Carrier Antenna Information received Decoding Demodulator Receiver(Voice/Data) Carrier
Modulation Terms adding data to a radio frequency signalBaseband – modulation techniques that do not use asinusoidal carrier but encodes information directly as theamplitude, width of position of a pulse. PAM – pulseamplitude modulation PWM – pulse width modulationBandpass – modulation techniques that encodeinformation as the amplitude, frequency or phase of asinusoidal carrier. FSK – frequency shift keying, PSK –phase shift keying, AM, FM
Communication frequencies• Microwave band terminology – L band 800 MHz - 2 GHz – S band 2-3 GHz – C band 3-6 GHz – X band 7-9 GHz – Ku band 10-17 GHz – Ka band 18-22 GHz
• Satellite up links and down links can operate in different frequency bands: Band Up-Link Down-link ISSUES (Ghz) (Ghz) C 4 6 Interference with ground links. Ku 11 14 Attenuation due to rain Ka 20 30 High Equipment cost• The up-link is a highly directional, point to point link• The down-link can have a footprint providing coverage for a substantial area "spot beam“.
Early satellite communications• Used C band in the range 3.7-4.2 GHz• Could interfere with terrestrial communications• Beamwidth is narrower with higher frequencies
More recent communications• Greater use made of Ku band• Use is now being made of Ka band
Rain fade• Above 10 GHz rain and other disturbances can have a severe effect on reception• This can be countered by using larger receiver dishes so moderate rain will have less effect• In severe rainstorms reception can be lost• In some countries sandstorms can also be a problem
Characteristics of some Frequencies• Microwave frequency range – 1 GHz to 40 GHz – Directional beams possible (small) – Suitable for point-to-point transmission – Used for satellite communications• VHF/UHF Radio frequency range – 30 MHz to 1 GHz (no atmospheric propagation, LOS) – Suitable for omnidirectional applications• Infrared frequency range – Roughly 3x1011 to 2x1014 Hz – Useful in local point-to-point multipoint applications within confined areas
Terrestrial Microwave• Description of common microwave antenna – Parabolic "dish", 3 m in diameter – Fixed rigidly which focuses a narrow beam – Achieves a line-of-sight (LOS) transmission path to the receiving antenna – Located at substantial heights above ground level• Applications – Long haul telecommunications service (many repeaters) – Short point-to-point links between buildings
Satellite Microwave• Description of communication satellite – Microwave relay station – Used to link two or more ground-based microwave transmitter/receivers – Receives transmissions on one frequency band (uplink), amplifies or repeats the signal and transmits it on another frequency (downlink)• Applications – Television distribution (e.g., Direct TV) – Long-distance telephone transmission – Private business networks
Broadcast Radio• Description of broadcast radio antennas – Omnidirectional (HF-vertical polarization, VHF/UHF- horizontal polarization) – Antennas not required to be dish-shaped – Antennas need not be rigidly mounted to a precise alignment• Applications – Broadcast radio • VHF and part of the UHF band; 30 MHz to 1GHz • Covers FM radio and UHF and VHF television • Below 30 MHz transmission (AM radio) is subjected to propagation effects so not reliable for point-to-point communications (MUF or max usable freq)
Network Architectures and Protocols Systematic Signaling Steps for Information Exchange Open Systems Interconnections (OSI) Transmission Control Protocol (TCP) Internet Protocol (IP) Internet Protocol Version 4 (IPv4) Internet Protocol Version 6 (IPv6) – essentially larger MAC addressing space for the influx of IP based devices Mobile IP
Ad Hoc Network (peer to peer)Versus an infrastructure network (centralized) with its AP(Access Points) which is your WiFi/Hotspot/typical wireless network normally used to access the Internet.
Multiplexing• Capacity of transmission medium usually exceeds capacity required for transmission of a single signal• Multiplexing - carrying multiple signals on a single medium – More efficient use of transmission medium
Reasons for Widespread Use of Multiplexing• Cost per kbps of transmission facility declines with an increase in the data rate (economy of scale)• Effective cost of transmission and receiving equipment declines with increased data rate (cost per bit)• Most individual data communication devices with their associated applications require relatively modest data rate support
Multiplexing Techniques• Frequency-division multiplexing (FDM) – Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal – Requires guard bands• Time-division multiplexing (TDM) – Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal – Requires accurate clock• Code-division multiple access(CDMA) – Use of orthogonal codes to separate users who are all using the same band of frequencies
TDMA Frame Illustration for Multiple Users User 1 Time 1 Time 2User 2 … … … Time nUser n Mobile Stations Base Station
CDMA (Code Division Multiple Access) Frequency User 1 User 2 . .. User n TimeCode
Transmitted and Received Signals in a CDMA System Information bits Code attransmitting endTransmitted signal Received signal Code at receiving end Decoded signal at the receiver 42
OFDM (Orthogonal Frequency Division Multiplexing) Frequency Conventional multicarrier modulation used in FDMA FrequencyOrthogonal multicarrier modulation used in OFDM (normally a single user)
Satellite Microwave Transmission• a microwave relay station in space• can relay signals over long distances• geostationary satellites – remain above the equator at a height of 22,300 miles (geosynchronous orbit) – travel around the earth in exactly the time the earth takes to rotate
Satellite Transmission Links• earth stations communicate by sending signals to the satellite on an uplink• the satellite then repeats those signals on a downlink• the broadcast nature of the downlink makes it attractive for services such as the distribution of television programming
Satellite Transmission Process satellite transponder dish dish 22,300 milesuplink station downlink station
WORKING • The receiver only knows that the satellite is neither approaching or departing • So the ship must be on a line perpendicular to the orbit of the satellite • However, farther from the orbit, the frequency transition is less • A calculation will tell the receiver how far, but not which side Universiteit Utrecht
Satellite Transmission Applications• television distribution – a network provides programming from a central location – direct broadcast satellite (DBS)• long-distance telephone transmission – high-usage international trunks• private business networks
Principal Satellite Transmission Bands• C band: 4(downlink) - 6(uplink) GHz – the first to be designated• Ku band: 12(downlink) -14(uplink) GHz – rain interference is the major problem• Ka band: 19(downlink) - 29(uplink) GHz – equipment needed to use the band is still very expensive
Satellite-Related Terms• Earth Stations – antenna systems on or near earth• Uplink – transmission from an earth station to a satellite• Downlink – transmission from a satellite to an earth station• Transponder – electronics in the satellite that convert uplink signals to downlink signals
Ways to Categorize Communications Satellites• Coverage area – Global, regional, national• Service type – Fixed service satellite (FSS) – Broadcast service satellite (BSS) – Mobile service satellite (MSS)• General usage – Commercial, military, amateur, experimental
Classification of Satellite Orbits• Circular or elliptical orbit – Circular with center at earth’s center – Elliptical with one foci at earth’s center• Orbit around earth in different planes – Equatorial orbit above earth’s equator – Polar orbit passes over both poles – Other orbits referred to as inclined orbits• Altitude of satellites – Geostationary orbit (GEO) – Medium earth orbit (MEO) – Low earth orbit (LEO)
Geometry Terms• Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite• Minimum elevation angle• Coverage angle - the measure of the portion of the earths surface visible to the satellite
Minimum Elevation Angle• Reasons affecting minimum elevation angle of earth station’s antenna (>0o) – Buildings, trees, and other terrestrial objects block the line of sight – Atmospheric attenuation is greater at low elevation angles – Electrical noise generated by the earths heat near its surface adversely affects reception
NGSO - Non Geostationary Orbits Orbit should avoid Van Allen radiation belts: • Region of charged particles that can cause damage to satellite • Occur at ~2000-4000 km and ~13000-25000 km
GEO Orbit• Advantages of the the GEO orbit – No problem with frequency changes – Tracking of the satellite is simplified – High coverage area• Disadvantages of the GEO orbit – Weak signal after traveling over 35,000 km – Polar regions are poorly served – Signal sending delay is substantialGEO : Geosynchronous equatorial orbit
LEO - Low Earth Orbits• Circular or inclined orbit with < 1400 km altitude – Satellite travels across sky from horizon to horizon in 5 - 15 minutes => needs handoff – Earth stations must track satellite or have Omni directional antennas – Large constellation of satellites is needed for continuous communication (66 satellites needed to cover earth) – Requires complex architecture – Requires tracking at ground
LEO Satellite Characteristics• Circular/slightly elliptical orbit under 2000 km• Orbit period ranges from 1.5 to 2 hours• Diameter of coverage is about 8000 km• Round-trip signal propagation delay less than 20 ms• Maximum satellite visible time up to 20 min• System must cope with large Doppler shifts• Atmospheric drag results in orbital deteriorationLEO : Low earth orbit
LEO Categories• Little LEOs – Frequencies below 1 GHz – 5MHz of bandwidth – Data rates up to 10 kbps – Aimed at paging, tracking, and low-rate messaging• Big LEOs – Frequencies above 1 GHz – Support data rates up to a few megabits per sec – Offer same services as little LEOs in addition to voice and positioning services
MEO Satellite Characteristics• Circular orbit at an altitude in the range of 5000 to 12,000 km• Orbit period of 6 hours• Diameter of coverage is 10,000 to 15,000 km• Round trip signal propagation delay less than 50 ms• Maximum satellite visible time is a few hoursMEO : Medium Earth Orbit
HEO - Highly Elliptical Orbits• HEOs (i = 63.4°) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere)• Depending on selected orbit (e.g. Molniya, Tundra, etc.) two or three satellites are sufficient for continuous time coverage of the service area.• All traffic must be periodically transferred from the “setting” satellite to the “rising” satellite (Satellite Handover)
Satellite SystemsGEO GEO (22,300 mi., equatorial) high bandwidth, power,M EO latency MEOLEO high bandwidth, power, latency LEO (400 mi.) low power, latency more satellites small footprint V-SAT (Very Small Aperture Terminal) private WAN
GPS Satellite Constellation • Global Positioning System • Operated by USAF • 28 satellites • 6 orbital planes at a height of 20,200 km • Positioned so a minimum of 5 satellites are visible at all times • Receiver measures distance to satellite USAF - United States Air Force
Frequency Bands Available for Satellite Communications
Satellite Link Performance Factors• Distance between earth station antenna and satellite antenna• For downlink, terrestrial distance between earth station antenna and “aim point” of satellite – Displayed as a satellite footprint (Figure 9.6)• Atmospheric attenuation – Affected by oxygen, water, angle of elevation, and higher frequencies
Satellite Communications Alternating vertical and horizontal polarisation is widely used on satellite communications This reduces interference between programs on the same frequency band transmitted from adjacent satellites (One uses vertical, the next horizontal, and so on) Allows for reduced angular separation between the satellites.
Capacity Allocation Strategies• Frequency division multiple access (FDMA)• Time division multiple access (TDMA)• Code division multiple access (CDMA)
Frequency-Division Multiplexing• Alternative uses of channels in point-to-point configuration – 1200 voice-frequency (VF) voice channels – One 50-Mbps data stream – 16 channels of 1.544 Mbps each – 400 channels of 64 kbps each – 600 channels of 40 kbps each – One analog video signal – Six to nine digital video signals
Frequency-Division Multiple Access• Factors which limit the number of subchannels provided within a satellite channel via FDMA – Thermal noise – Intermodulation noise – Crosstalk
Forms of FDMA• Fixed-assignment multiple access (FAMA) – The assignment of capacity is distributed in a fixed manner among multiple stations – Demand may fluctuate – Results in the significant underuse of capacity• Demand-assignment multiple access (DAMA) – Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations
FAMA-FDMA• FAMA – logical links between stations are preassigned• FAMA – multiple stations access the satellite by using different frequency bands• Uses considerable bandwidth
DAMA-FDMA• Single channel per carrier (SCPC) – bandwidth divided into individual VF channels – Attractive for remote areas with few user stations near each site – Suffers from inefficiency of fixed assignment• DAMA – set of subchannels in a channel is treated as a pool of available links – For full-duplex between two earth stations, a pair of subchannels is dynamically assigned on demand – Demand assignment performed in a distributed fashion by earth station using CSC
Reasons for Increasing Use of TDM Techniques• Cost of digital components continues to drop• Advantages of digital components – Use of error correction• Increased efficiency of TDM – Lack of intermodulation noise
FAMA-TDMA Operation• Transmission in the form of repetitive sequence of frames – Each frame is divided into a number of time slots – Each slot is dedicated to a particular transmitter• Earth stations take turns using uplink channel – Sends data in assigned time slot• Satellite repeats incoming transmissions – Broadcast to all stations• Stations must know which slot to use for transmission and which to use for reception
Satellite Signals► Used to transmit signals and data over long distances Weather forecasting Television broadcasting Internet communication Global Positioning Systems
Communication Satellite►A Communication Satellite can be looked upon as a large microwave repeater► It contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.
Types of Satellite Orbits► Based on the inclination, i, over the equatorial plane: Equatorial Orbits above Earth’s equator (i=0°) Polar Orbits pass over both poles (i=90°) Other orbits called inclined orbits (0°<i<90°)► Based on Eccentricity Circular with centre at the earth’s centre Elliptical with one foci at earth’s centre
Intelsat► INTELSAT is the original "Inter-governmental Satellite organization". It once owned and operated most of the Worlds satellites used for international communications, and still maintains a substantial fleet of satellites.► INTELSAT is moving towards "privatization", with increasing competition from commercial operators (e.g. Panamsat, Loral Skynet, etc.).► INTELSAT Timeline:► Interim organization formed in 1964 by 11 countries► Permanent structure formed in 1973► Commercial "spin-off", New Skies Satellites in 1998► Full "privatization" by April 2001► INTELSAT has 143 members.
Advantages of Satellite Communication Can reach over large geographical area Flexible (if transparent transponders) Easy to install new circuits Circuit costs independent of distance Broadcast possibilities Temporary applications (restoration) Niche applications Mobile applications (especially "fill-in") Terrestrial network "by-pass" Provision of service to remote or underdeveloped areas User has control over own network 1-for-N multipoint standby possibilities
Disadvantages of Satellite Communication Large up front capital costs (space segment and launch) Terrestrial break even distance expanding (now approx. size of Europe) Interference and propagation delay Congestion of frequencies and orbits
When to use Satellites When the unique features of satellite communications make it attractive When the costs are lower than terrestrial routing When it is the only solution Examples: • Communications to ships and aircraft (especially safety communications) • TV services - contribution links, direct to cable head, direct to home • Data services - private networks • Overload traffic • Delaying terrestrial investments • 1 for N diversity • Special events