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Bandwidth Optimization in Satellite Communication
INTRODUCTION
Satellite communications systems exist because earth is a sphere.
Radio waves travel in straight lines at the microwave frequencies used for wideband communications.
Satellites are important in: voice communications, video & radio transmission, navigation (GPS),remote sensing (maps, weather satellites) etc.
They cover large areas.
Inherent broadcast.
Inherent capability of by-passing the whole terrestrial system.
HOW DO SATELLITES WORK?
Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.
The two stations can use a satellite as a relay station for their communication.
One Earth Station transmits the signals to the satellite. Up link frequency is the frequency at which Ground Station is communicating with Satellite.
The satellite Transponder converts the signal and sends it down to the second earth station. This frequency is called a Downlink.
ADVANTAGES OF SATELLITE COMMUNICATIONS
LARGE COVERAGE HIGH QUALITY HIGH RELIABILITY HIGH CAPACITY FLEXIBILITY SPEED OF INSTALLATION EMERGENCY COMMUNICATION
APPLICATIONS OF SATELLITE COMMUNICATION
TELEPHONESATELLITE TELEVISIONFIXED SERVICE SATELLITEDIRECT BROADCAST SATELLITEMOBILE SATELLITE TECHNOLOGIESSATELLITE RADIOSATELLITE INTERNET ACCESSMILITARY USES
Ku Band
The Ku band is a portion of the electromagnetic spectrum that
ranges from 10.95-14.5 GHz
More flexibility
For the End users Ku band is generally cheaper and enables smaller
antennas
The satellite operator's Earth Station antenna do require more
accurate position control when operating at Ku band than compared
to C band.
C Band
Range : 4 – 8 GHz
At frequencies higher than 10 GHz in heavy rain fall areas, a
noticeable degradation occurs
The C-band perform better in comparison with Ku band under
adverse weather conditions
The Ku band satellites typically require considerably more power to
transmit than the C-band satellites.
•Available bandwidth is limited and insufficient to meet
demand
• Existing capacity is usually running at maximum capacity– As a result it is often unusable– Universal flat lining during working hours
•The cost of bandwidth is extremely high•Expanding bandwidth capacity is limited due to finances, supply, technology
THE BANDWIDTH CHALLENGE
optimizing the traffic
advanced modulation techniques to reduce the bandwidth allocated to a given service
HOW TO REDUCE BANDWIDTH
Who Benefits from BANDWIDTH OPTIMIZATION SOLUTIONS
SATELLITE SERVICE PROVIDERS
MARITIME INDUSTRIES
OIL and GAS COMPANIES
CONSTRUCTION and MINING INDUSTRY
MILITARY and GOVERNMENT OPERATIONS
DISASTER RECOVERY , EMERGENCY AID
BROADCASTING COMPANIES
BLOCK DIAGRAM
MODULATION TECHNIQUES
Modulation is the process by which information is conveyed by
means of an electromagnetic wave.
The power and bandwidth necessary for the transmission of a signal with a given level of quality depends on the method of modulation.
1. QPSK
2. 8-PSK
QPSK v/s 8PSK
QPSK occupies 1/2 of the bandwidth of BPSK whereas
8PSK uses 1/3rd of the bandwidth that BPSK for a given bit
rate
With a 8PSK capable satellite receiver you can
demodulate QPSK as well as 8PSK
8PSK makes better use of bandwidth than QPSK
8PSK is not as phase-tolerant as QPSK and has a slightly
longer acquisition time
UP/DOWN CONVERTER
Up converter accepts IF signal in the 70±18 MHz band Convert to an RF signal in 5.925-6.425 GHz band
Down converter accepts RF signal in 3.7-4.2 GHz band
convert to an IF signal in 70±18 MHz band
Same transponder is used for transmitter and receiver channels
HIGH POWER AMPLIFIER
Obtain Necessary EIRP (Equivalent Isotropic Radiated Power) from an earth station
Three types: - klystron power amplifier(KPA)- traveling wave tube amplifier(TWTA)- solid state power amplifier(SSPA)
For large power of the order of few kilowatts, traveling wave tube amplifiers (TWTAs) or Klystron are used
Klystrons amplifiers are used in ONGC
Klystrons have narrow instantaneous bandwidth around 40MHz tunable over 500MHz range
TWTAs have wide bandwidth typically around 500MHz
LOW NOISE AMPLIFIER
Amplify very weak signals
Located very close to the detection device
Placed at the front-end of a radio receiver circuit
The effect of noise from subsequent stages of the receive chain is reduced
Low NF (like 1db)
Large enough gain (like 20db) Large enough intermodulation and compression point (IP3 and P1dB)
The gain of the LNA That is used in satellite earth station, ONGC is 60db
Modems Modems currently in use at ONGC :
- DMD15 Universal Satellite Modem- DMD20 Universal Satellite Modem
DMD15 Universal Satellite Modem
Main Features:
•BPSK and QPSK modulation.
•9.6 Kbps to 8.448 Mbps in 1 bps steps.
•Configuration, monitor and control features are fully user-programmable.
•Excellent spurious performance.
•Fully-compliant with IESS 308/309.•Industry standard I/O interfaces.
•Customize for closed network applications.
•50-90,100-180 MHz IF in 1 Hz steps.
DMD20 Universal Satellite ModemHighlights:•BPSK/QPSK/OQPSK/8-PSK/8-QAM/16-QAM Operation
• 2.4 Kbps to 20 Mbps in 1 bps Steps
• FEC - Viterbi, Reed-Solomon, Sequential, Trellis, Turbo Product Code, Low Density Parity Check Code
• Configuration, Monitor and Control Features Fully User-Programmable
• Excellent Spurious Performance
• Fully Compliant with IESS 308/309/310/314/315
•Industry-standard Universal Interface Module
•50 to 90 MHz and 100 to 180 MHz IF, and 950 to 2050 MHz L-Band in 1 Hz Steps
• Standard Features Include: Reed-Solomon,Asynchronous Overhead, Satellite Control Channel and Automatic Uplink Power Control
The required occupied bandwidth is
B = k ( Rb / m )(1/ r )
Where,
Rb = information bit rate
m = number of bits per symbol
r = code rate
K = bandwidth expansion factor
used
to minimize intersymbol
interference
Link Design, Link Budget and Power
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Link Power Budget
Transmission:HPA PowerTransmission Losses (cables & connectors)Antenna Gain
EIRPTx
Antenna Pointing LossFree Space LossAtmospheric Loss (gaseous, clouds, rain)Rx Antenna Pointing Loss
Rx
Reception:Antenna gainReception Losses (cables & connectors)Noise Temperature Contribution
Pr
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Link BudgetsThe transmission formula can be written in dB as:
The calculation of received signal based on transmitted power and all losses and gains involved until the receiver is called “Link Power Budget”, or “Link Budget”.
The received power Pr is commonly referred to as “Carrier Power”, C.
rrotherrapolaptar LGLLLLLLEIRPP
27
Why calculate Link Budgets?System performance tied to operation
thresholds.Operation thresholds Cmin tell the
minimum power that should be received at the demodulator in order for communications to work properly.
Operation thresholds depend on:Modulation scheme being used.Desired communication quality.Coding gain.Additional overheads.Channel Bandwidth.Thermal Noise power.
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Simple Link Power BudgetParameter Value Totals Units Parameter Value Totals Units Frequency 11.75 GHz Transmitter Receive Antenna Transmitter Power 40.00 dBm Radome Loss 0.50 dB Modulation Loss 3.00 dB Diameter 1.5 m Transmission Line Loss 0.75 dB Aperture Efficiency 0.6 none Transmitted Power 36.25 dBm Gain 43.10 dBi Polarization Loss 0.20 dB Transmit Antenna Effective RX Ant. Gain 42.40 dB Diameter 0.5 m Aperture Efficiency 0.55 none Received Power -98.54 dBm Transmit Antenna Gain 33.18 dBi Slant Path Summary Satellite Altitude 35,786 km Transmitted Power 36.25 dBm Elevation Angle 14.5 degrees Transmit Anntenna Gain 33.18 dBi Slant Range 41,602 km EIRP 69.43 dBmi Free-space Path Loss 206.22 dB Path Loss 210.37 dB Gaseous Loss 0.65 dB Effective RX Antenna Gain 42.4 dBi Rain Loss (allocated) 3.50 dB Received Power -98.54 dBm Path Loss 210.37 dB
BANDWIDTH OPTIMIZATION
Transponder bandwidth is usually the most expensive resource in a satellite communication link. For maximum efficiency, a satellite link should be engineered to balance bandwidth and power.
Available bandwidth can be optimized by using one of the following techniques:
• Using higher modulation• lower order FEC technique• Increased Antenna size
Our project is based on using higher order modulation techniques for efficient utilization of available bandwidth.
FORWARD ERROR CORRECTION In communication forward error
correction(FEC) a system of error control for data transmission, whereby the sender adds systematically generated redundant data to its messages, also known as an error-correcting code (ECC).
The carefully designed redundancy allows the receiver to detect and correct a limited number of errors occurring anywhere in the message without the need to ask the sender for additional data. FEC gives the receiver an ability to correct errors without needing a reverse channel to request retransmission of data.
Benefits of Forward Error Correction (FEC)
Reduce bandwidth by 50%.Increase data throughput by a factor of 2.Reduce antenna size by 30%.Reduce transmitter power by a factor of 2.Provide 3dB more link margin.
if we have bandwidth to spare, then use a lower order modulation or a
higher rate FEC (like 1/2 or 2/3) to spread the signal out.
If we have power to spare then use a higher order modulation and/or
lower rate FEC (like 3/4 or 7/8).
Ideally use all of both the available bandwidth and power simultaneously
to obtain the highest user information rate.
Bandwidth-Power Trade-Off
RESULTS
The following results were derived from these calculations:
Bandwidth requirement has been reduced by a considerable amount when 8PSK is used as compared to QPSK.
Different FEC rates used also has an effect on the bandwidth requirement of the transmission and receiving link.
By using a proper combination of modulation technique and FEC rate we can achieve efficient utilization of bandwidth.
Allocated Bandwidth Bandwidth, Allocated Bandwidth or Occupied Bandwidth is the frequency space required by a carrier on a transponder.
E.g. : a duplex E1 (2.048 Mbps) circuit with 8-PSK modulation, FEC rate 3/4 and 1.4 spacing requires:
Bandwidth = data rate/(no. of bits per symbol * FEC)* frequency spacing * 2 [for duplex circuit]
B = 2.048 / (3 * 0.75) * 1.4 * 2 = 2.548 MHz
For a 36 MHz transponder, 2.548 MHz corresponds to 7.078% bandwidth utilization.
Power Equivalent Bandwidth Power Equivalent Bandwidth (PEB) is the transponder power used by a carrier, represented as bandwidth equivalent. PEB calculation example:
• Transponder EIRP = 37 dBW • Output Backoff (OBO) = 4 dB • Available EIRP = 37 – 4 = 33 dBW = 10^3.3= 1995.26 Watts • Transponder Bandwidth = 36 MHz • Power Available / MHz = 1955.26 / 36 = 55.424 W • If a carrier uses 24 dBW, then
PEB = Power used by your carrier/transponder saturated power PEB = 10^2.4/ 55.424 = 4.532 MHz
This corresponds to 12.59% of available transponder power.
CONCLUSION
In the design of a communication system, the choice of
modulation is of fundamental importance and always involves
tradeoffs between power and bandwidth.
In the past, frequency spectrum was relatively plentiful but
the power available on a satellite was limited. Today, the
equation has been reversed. Spectrum is now scarce.
More spectrum efficient forms of digital modulation such as
8PSK and 16QAM are becoming more attractive, even though
the power requirements are higher.
Coupled with powerful coding methods such as concatenated
Reed Solomon/Viterbi coding, these methods offer the
prospect of enhanced spectral efficiency with virtually error-
free digital signal transmission.
Thank You
