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Fundamentals of Satellite Communications
Lawrence N. GoellerOASD/PA&E
12 February 2004
Outline• Satellites and Communications Frequencies
• Amplifiers
• Antennas and Antenna Gain
• Signal to Noise Ratio (SNR): The Link Budget Equation
• Carrier Waves, Modulation, and Bandwidth
• Systems: Narrowband, Wideband, and Protected
• Summary
Geostationary Satellites• Communications satellites are like very
tall relay towers– Most communications satellites use
Geosynchronous Equatorial Orbits– Key concept: GEO sats appear to
stand motionless in the sky• Satellites are easy to find, track• No handoffs required• Fixed full-period coverage areas
– Very valuable orbital slots• Orbit must be circular• Orbit Must be equatorial• Only one altitude (for 24 hours)
– GEO is high; 1/10th of way to moon• Speed of light c = 3 ×108 m/sec• Round trip signal time: ~0.25 sec
RE, Radiusof Earth
~6300 km
About 6 RE~36,000 km
RelayTower
Satcom Transmission Frequencies• Satellite communications can only
take place at certain frequencies– Below few hundred MHz, will
not penetrate ionosphere– Above few ten’s GHz, will not
penetrate rain or atmosphere
• So carrier waves in these bands used; band names from radar world– Lower range: “UHF”, L, S-band– Middle: C-band, X-band– High: Ku, Ka-band, “EHF”– Special case: optical (laser)
• Key comm. satellite design factor: carrier frequency band– Assigned commercial or gov’t.
Highest frequencies absorbed by atmosphere
Low frequencies reflected by ionosphere
High frequencies absorbed by rain
100 MHz 100 GHz10 GHz1 GHz
Iono-sphere
rainatmosphere
Satcom Spectrum Chart
Com’l.K
MILSTAR*AEHF Uplink
2 GHz
ACTSUplink1 GHz
DSCSDownlink500 MHz
GBSUplink1 GHz
ACTSDownlink
1 GHz
MILSTAR, AEHF, GBS
Downlink1 GHz
Commercial SATCOM Services
Government / Military SATCOM Services
VHF UHF L S X K V
VHF EHF
UFOMUOS175 MHz
SGLSWeatherNASA150 MHz
MilitaryUHF Band
GovernmentS-Band Military
X-Band
1GHz 2GHz 4GHz 8GHz 12GHz 18GHz 27GHz 40GHz 75GHz
DSCSUplink
500 MHz
GovernmentExploratory Ka-
Band
Military EHF
INMARSAT, ODYSSEY, IRIDIUM, GLOBALSTAR, MSAT, ELLIPSO
ODYSSEY, INMARSAT GLOBALSTAR, ELLIPSO
INTELSAT, INMARSAT
INTELSAT,VSATS
IRIDIUM,ODYSSEY
TELEDESIC, IRIDIUM,ODYSSEY, SPACEWAY
(PCS Services)
Com’l.L
Com’l.S
Com’l.C
Com’l.Ku
Com’l.Ka
3GHz
30GHz
UHF
KaC Ku
SHF
Com’l.UHF
300 MHz
LEOSTAR
Comm: Transporting BitsCommunications satellite
as a relay• Digital Communications is about moving bits from one place to another– Communications satellites are not
sources or destinations– Other satellites often are
• Physics: Communications via radio waves must use bandwidth and power– “Comm” means bits per second
(bps), with an associated error rate– Power, in watts, is generated by
transmitter and collected by receiver– Bandwidth, in hertz (Hz), is the
range of frequencies this power uses– Both Power & Bandwidth required
UplinkDownlink
Shannon’s Law
⎟⎠⎞
⎜⎝⎛ +⋅=
NSWRb 1log2
Data rate (bps)
Bandwidth (Hz)
Signal power to noise power ratio
(watts / watts)
Transmitters and HPAsHigh Power Amplifiers
Signalin
Transmit antenna
Receive antenna
Signal &Noise out1 2 3 4 Etc.
High Power Amplifier (HPA)LNA
• Satellites and terminals employ multiple stages of amplifiers– 9 to 12 orders of magnitude of amplification common in GEO satellites– Last stage of amplifier chain is called High Power Amplifier (HPA)
• Several types of high-freq amplifiers, with names like Klystron, Magnetron– Very popular one: Traveling Wave Tube Amplifier (TWTA)– TWTA’s widely used in satellites; good efficiency, high power, reliable
• Up to 100’s watts in satellites, 1000’s of watts in large terminals– Solid-state amplifiers (SSA’s) also; rugged, but less power, efficiency– HPA’s generally: The higher the frequency, the harder to do
Antennas and Antenna Gain• Antennas radiate and collect radio waves
• They do not radiate equally in all directions– Except for mythical “isotropic”
antennas, useful as a reference– Antennas can be high-gain (highly
directional) or low-gain
• Gain pattern is like a contour map– Lines represent constant power density
(watts per square meter)– If only one line, it is usually at the half-
power contour
• Transmit gain GT expresses concentration– PT × GT = EIRP (Equivalent Isotropic
Radiated Power)
Dipole Antenna Gain:Pattern symmetric about antenna axis
Isotropic gain pattern
High gainpattern
Commercial spot beam coverage of Europe
Diffraction and Beam Width
Large D, High Freq Large D, Low Freq
Small D, High Freq Small D, Low Freq
large θ
Mediumθ
Mediumθ
Small θ
TDλθ ⋅≈ 70 θ
DT
• In geometry, parallel lines remain parallel forever
• In physics, sharp-edged wave trains diverge past the Rayleigh range – Diffraction; all waves do this– Beam stabilizes at angle θ
c = fλ= freq. × wavelength
= 3 ×108 m/sec
Rule of thumb: GT = (4π/λ2)AT, A = areaθ in degrees; λ and D in same units
Receive Gain and PointingAll transmitted power that does not hit receive antenna is “lost” to free space
Surface area of sphere with radius R:Asphere = 4πR2
R
Transmitter
ReceiverFS
rec
sphere
rec
LG
AA
=Surface area of receive antenna: Arec
Grec = receive antenna gain= gain it would have if transmitting
LFS defined as “free space loss”Receive gain matters for pointing accuracy:
TransmitterReceivers
The higher the frequency, or the larger the receive antenna’s diameter, the more
accurately it must pointθ
θ OK
Not OK
Other Loss Terms
17
1 10 3.
Atmi
R10i
R25i
1001 fi
1 10 1001 10 3
0.01
0.1
1
10
100
86532 4 20 80605030 40
Frequency (GHz)
Atte
nuat
ion
(dB
/km
)Rain (25 mm/hr)Rain (10 mm/hr)
Atmosphere
0.001
Rain and Atmospheric Losses• Power that is transmitted but not received is “lost”– Free Space loss, LFS
• Other types of losses too– Absorption or scattering
by rain, air, ionosphere– Absorption by trees,
buildings, etc.– Mis-pointing of either
antenna– Losses inside terminals
• Collectively, Lother = Lo
H20
O2
Signal and Noise• Physics again: detectors fundamentally
have to detect energy– Energy per second is power (in watts)– S and N are both powers– Ratio of S to N is what matters
• There is always internal noise in receiver– Amplifiers themselves generate it!
Noise power formula: N = kTW– N = noise power (watts), T = temp, k =
Boltzmanns’s constant, W = bandwidth– Amps are poor blackbodies! So have to
define Teff, or “effective noise temp.”
• Also external noise: sky, sun, rain, Earth (if looking down); all affect Teff
– Man-made: nearby users, jamming
Received signal power: Threshold detector can be
100% accurate
Noise power in receiver
S+N: Threshold Detectioncan generate errors
200K amp 75K amp
“Room” T: 293K
Teff
Receivers and LNAsLow Noise Amplifiers
Receive antenna
Transmit antenna
Low Noise Amplifier (LNA)
SignalSin
(Watts)
Sout+Nout(Watts)
Consider each stage has Gain G, Noise N
1 2 3 4
G1(Sin+N1)
G2G1(Sin + N1) + G2(N2)
Etc.
High Power Amplifier (HPA)
G3G2G1(Sin + N1) + G3G2(N2) + G3(N3)
• The noise generated in the first amplifier is amplified by every successive stage– Noise from other stages amplified by successively smaller number of stages– So it is worth paying extra to minimize the noise in the first stage
• Appropriately called “Low Noise Amplifiers” or LNAs• Common LNA Teff: 200K for X-band, 500K for Ka-band
The Link Budget EquationThe Link Budget Equation calculates the signal to noise ratio at the receiver.
WTkLLGGP
NS
effoFS
RTT
⋅⋅⋅
⋅⋅⋅
=1 S: Received signal power, watts
N: Noise power in receiver, wattsPT: Transmitted power, wattsGT: Antenna gain of transmitterGR: Antenna gain of recieverLFS: Free space lossLo: Other loss termsK: Boltzmann’s constantTeff: Effective Noise TemperatureW: Bandwidth consideredEb: Signal energy per bitNo: Noise power per hertzRb: Data rate in bits per second
We can write: S = Rb × EbAnd also: No = N / W (watts per hertz)
WkTLLGGP
WNRE
NS
effoFS
RTT
o
bb
⋅⋅⋅⋅
=⋅⋅
=
Expressing in terms of data rate Rb:
The link budget equation
( )effoFS
obRTTb kTLL
NEGGPR⋅⋅⋅⋅⋅
=−1/
Important: Rb is associated with a specific bit error rate (BER)
Carrier Waves and Modulation• Carriers transmit power from one place
to another– Necessary but not sufficient for
communications– Carriers do not carry “information”
in the comm sense of this word
• Carriers do not carry “information” in the communications sense
– You have to “jiggle” (modulate) them; info is in the jiggles
– Carriers have three key properties• Amplitude, Frequency, and
Phase– Any of these can be modulated
• And combinations
Unmodulated Carrier
10 0
Amplitude Modulation
Frequency Modulation
Phase Modulation
Modulation and BandwidthBandwidth Physics:
Unmodulated carrier: 0 0 0 0
Time t Am
plitu
de
Sine wave: Monochromatic, so bandwidth = 0
Symbolin
Bitsout
0 0 0 1 0 12 1 03 1 1
0 1 2 3 00011011
modem
Frequency f
0 1 1 0 1 Modulated carrierhas bandwidth
Higher symbol rate, more bandwidth
0 1 1 0 1 0 1 0 0 1
Transmitted symbols
“Bandwidth efficient modulation”; more bps/Hz. But also
more power required!
0 1 2 3 0 2 3 1 0 2
Am
plitu
de BW depends on symbol rate, but not bits/symbol!
Channel Encoding
⎟⎠⎞
⎜⎝⎛ +⋅=
NSWRb 1log2
Shannon’s Law
Channel encoding lets you balance these two quantities
• Due to noise, some errors are inevitable– Can we use computers to detect
and even correct received errors before more processing? Yes.
• Channel Encoding:– Add extra bits to message in clever
ways to help receiver ID errors– Allows less power in noisy
environments– But requires more bps (thus BW)
• Channel encoding allows you to trade bandwidth for power
– Specifically, Γ vs Eb/No
Coding rates expressed by “r”“r = 1/2” = “rate one-half coding” means 1 user bit for 2 sent bits(very robust, not BW efficient)Other common rates: r = 3/4, 7/8Uncoded would be r = 1
Spectral efficiency Γ (gamma):Γ = user’s bits per second / Hertz(Γ includes modulation & coding)
What Modems Do• Modems do the following:
– MO-DEMs modulate and demodulate a carrier wave with a baseband signal• Modify A, F, φ, combo’s, with
specified number of bits per symbol • Or: Convert bps to hertz and back, at a BER
– That is:• You give modem an Eb/No
• It returns a Bit Error Rate– Modem specs often state a coder/decoder
algorithm; this is a separate function, but is usually included in the same box today
• Transponded vs Processing satellites– Former satellites do not demodulate the
signals on board, merely amplify them– Latter have modems; adds weight, $$
Comtech EF Data 8650 (Used with DSCS satellites)
From the spec sheet:
16QAM w/ R/SBER 3/4 7/8
Viterbi Decoder, QPSKBER 1/2 3/4 7/8
Implied: numbers in tables are Eb/No (dB)
Not shown: Γ
10-4 7.9 9.310-5 8.1 9.610-6 8.4 9.810-7 8.6 10.010-8 8.8 10.310-9 9.0 10.5
10-3 4.2 5.2 6.4 10-6 6.1 7.5 8.610-8 7.2 8.8 9.9
Viterbi w/ R/S, QPSKBER 1/2 3/4 10-6 4.1 5.610-8 4.4 6.010-10 5.0 6.3
Narrowband Satellites• Low gain antennas transmit power in
many directions– So limited power collected
• Affecting data rate– So OK to use low end of frequency
window, where there is less BW– Interference limits system users– But accurate pointing not required
• “Comm on the move” (COTM)• “Omni-directional,” nearly
• Ideal length of dipole antenna (for radiating or collecting power):– Quarter wavelength (λ/4)– > 3 meters, ionosphere reflection– < 10 cm, not enough surface area– λ/4 = 25 cm at 300 MHz (UHF)– UFO, MUOS; 5/25 kHz, COTM
UHF Follow-on (UFO) Spacecraft 225-400 MHz band
Narrowband Satcom associated with voice, low data rate; also COTM,
rain/foliage penetration, simple and inexpensive terminals
Wideband Satellites
DSCS III Wideband Milsatcom system
• High gain antennas transmit, collect lots of power– Supports high capacity– But high bandwidth also needed– So higher bands with more total
frequency used• X-band: 500 MHz• Ka-band: 1000 MHz
• Terminals can reuse frequencies because the beams are so narrow– Have to stop and point: no COTM– Exception: Navy, some aircraft
• Satellites: (DSCS, GBS, WGS): Tradeoff between coverage and capacity
1° beam:24 Mbps
3° beam:1.5 Mbps
SamePower
(120W)
Wideband gain patterns for GBS
Protected SystemsMilstar II
44 GHz up (BW: 2GHz),20 GHz down (BW: 1 GHz)
Low Capacity, Good Protection
• Protection can be tied to any data rate– But usually a tradeoff with capacity
• Protection has come to mean three things:– Jammer resistance (Antijam, or AJ)
• Jamming is essentially adding noise, thus decreasing S/N
• Countermeasures include small beams, “nulling,” special waveforms
– Performance in nuclear environment• High altitude nucs disrupt ionosphere;
requires anti-scintillation (AS)• Essentially, intermittent loss of signal
– Stealth: low probability of interception (LPI) or detection (LPD)
• Use narrow beams, waveform approaches, short bursts User UserJammer
NuclearEffects
Ionosphere
Summary• Communications is a complex field;
Satcom even more so!– Many pieces, but each manageable
• Key concept: Need both power and bandwidth for communications– Power: Link Budget Equation– Bandwidth: Modulation schemes– Coding can balance the two
• Key concept: Satellite frequencies are well-suited to different missions– Consider capacity, rain, foliage,
COTM, protection, cost
Some Common Satcom Tradeoffs
• Capacity vs Coverage area
• Capacity vs. Mobility
• Capacity vs. Protection
• Link Availability vs. Frequency
• Protection vs. Frequency
Antenna Arrays• Waves undergo a phenomenon called
interference– Not noticeable for different signals– But for same signals (with perhaps
different relative phase, amplitude), patterns can be formed
• Used in many fields– Radio, cell towers (dipole antennas)– Helical antennas– High gain too: Phased Arrays
• Expensive, thermal management issues– Satellites: many beams from one aperture– Terminals: Rapid beam steering Airborne Wideband
Terminal (AWT)
Cell tower phased array
“Phased Array” of dipoles(view from top)
Narrowband/Wideband SynergyAs of 2004
Wideband
• Frequencies: C, X, Ku, Ka-bands– Rain effects as freq. increases
• Data rates: 128 kbps – Gbps
• High-gain antennas– Must point antenna at satellite– Limited coverage areas
• Uses– Trunked voice; VTCs– High rate data (imagery, video)
Narrowband
• Frequencies: “UHF,” L-Band– Unaffected by rain, atm.
• Data rates: 75 bps – 64 kbps
• Low-gain antennas– Comm on the move– Decent reception
• Uses– Single voice channel– Low rate data (text, data links)