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)