ECE 5233 Satellite Communications Prepared by:

Dr. Ivica Kostanic

Lecture 10: Satellite link design(Section 4.4)

Spring 2014

Florida Institute of technologies

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Example of noise temperature calculationsHeterodyningDownlink link budgetExample link budget in C-band

Outline

Important note: Slides present summary of the results. Detailed derivations are given in notes.

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LNA – Low Noise Amplifier – first signal amplifying stage

BPF1 – band filter, selection of the band of operation

Mixer – down conversion to IF stage BPF2 – channel filter, section of the

appropriate satellite channel IF amplifier – main signal amplification

on RX chain Demod – demodulation of the signal BB – base band processing

Single conversion receiver

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Block diagram of SC super heterodyne RX

snT

T

fmfrf

IF

mfrf

f

frf

m

rf

farf

n

G

rffmfifn

TkTBG

GGGGT

GGGT

GGT

GT

TT

kBGGGGGP

S

T

211

2

1

1

12

Noise power at the output of the IF stageNote 1: LNA noise temperature is the most dominant term

Note 2. band selection filter insertion losses need to be small to minimize noise in mixer

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Single conversion receiver –Ex 4.3.1

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Suppose we have 4GHz receiver with following gains and temperatures: Ta = 25K, Trf = 50K, Tif = 1000K, Tm = 500K, Grf = 23dB, Gif = 30dB . Calculate system noise temperature assuming no losses in the mixer. Recalculate the system noise temperature assuming 10dB mixer losses. How can the system temperature be minimized in the 10dB mixer lose case.

Answers:

a) Ts = 82.5K

b) Ts = 127.5K

c) Increase LNA gain

Note: spreadsheet allows for insertion losses and noise temperatures of the filtering components.

Antenna Ta 25 K

LNA Trf 50 KGrf 23 dB 199.53

Band selection filter Gf1 0 dB 1.00Tf1 0 K

Mixer Gm -10 dB 0.10Tm 500 K

IF stageChannel selection filter Gf2 0 dB 1.00

Tf2 0 K

IF amplifier Gif 30 dB 1000.00Tif 1000 K

Total gain Gt 43 dB 19952.623Ts (system noise temperature) Ts 127.62 K

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RX with antenna feed losses (Ex 4.3.2)

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RX front endAntenna Ta 25 K

Antenna feed Ga -2 dB 0.63Tp 300 KTe 175.47 KTa' 126.49

LNA Trf 50 KGrf 50 dB 100000.00 linear

Band selection filter Gf1 0 dB 1.00 linearTf1 0 K

Mixer Gm 0 dB 1.00 linearTm 500 K

IF stageChannel selection filter Gf2 0 dB 1.00 linear

Tf2 0 K

IF amplifier Gif 30 dB 1000.00 linearTif 1000 K

Total gain Gt 80 dB 100000000.00 linearTs (system noise temperature) Ts 176.50 K

The system in Ex. 4.4.2 has an LNA with a 50dB gain. A section of lossy wave-guide with attenuation of 2dB is inserted between antenna and LNA. Find the new noise temperature if the physical temperature of the waveguide is 300K.

Answers:

a) Ts = 176.5 K

Note: To increase sensitivity of the front end, the receiver may be cryogenically-cooled. For example using liquid Nitrogen, the physical temperature may be brought down to 77K.

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Super-heterodyne receiver

Heterodyning – translation of signal from RF to IF frequency Reason – to facilitate filtering and demodulation May be single conversion or double conversion Opposite to heterodyning – direct conversion receivers

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Front end filter (band selection)

XIF filter

(Channel selection)

DemodulationLNA

~

IFA

Mixing stage

RF Stage IF Stage Baseband

Messagesignal

Block diagram of a SH RX

Note: double conversion RX has two IF stages

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Downlink link budget

Downlink – satellite to ground Downlink link budget – usually limiting budget in fixed satcomm Link budget components

o Satellite TX (PA power, backoff, antenna, pointing)o Path losses (FSPL, additional losses due to elements, fade margin)o Ground RX (antenna, G/T ratio, system noise figure)

Link budget – evaluates various system design trade-offso Date rate / capacityo Reliabilityo Component size and cost

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Components of the link budget table

1. Transmitter sideo TX componentso Principle result: EiRP

2. Receiver sideo RX componentso Principle results

RxSensitivity System temperature RSLA

3. Propagation modelingo Free space losso Environmental losseso Other system losses

4. Link reliability and margin calculations

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Elements of the radio link

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Satellite transmission – link budget components Transponder output power Transponder backoff

o Dependent on Peak to Average Power Ratio (PAPR) of the modulated signal

o Backoff ensures that transponder PA works in linear region of power amplification

Satellite antenna gain at the edge of coverage region

o Usually the coverage region is defined through 3dB beamwidth – gain to the edge is 3dB lower than the maximum gain of the antenna

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antotT LBGP EiRP

PAPR for a given modulation scheme is usually specified through CCDF curves

EiRP – effective isotropic radiated power [dBm]

PT – transponder power [dBm]

Gt – transmit antenna gain on axis [dB]

Bo – backoff [dB]

Lant – edge of beam loss of antenna gain [dB]

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Propagation losses

Components of propagation losseso Free Space Path Loss (path length)o Atmospheric losseso Design margin (misalignment, severe

weather, drift in component parameters) Propagation losses - calculated for worst case

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Note: All losses are frequency dependent.

Earth station

Satellite

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Earth station – link budget components

Antenna gaino Function of efficiency and size

System noise temperatureo Determined from the Rx front end

Receiver sensitivityo Minimum required RX power for proper

link operationo Determined from noise power and

required S/N ratio Required S/N ratio

o Determined from the modulation typeo Ultimately driven by bandwidth efficiency

requiremento Obtained from receiver operating curves

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dBIM1210log10

12

/

/

BR

BR

NSNS

Signal to noise ratio requirement

R – information rate

B – bandwidth of the channel

IM – implementation margin

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Downlink link budget example

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Typical downlink LB spreadsheet

Note: Link budget as shown here has four distinct sections

1. Transmitter

2. Receiver

3. Propagation

4. Budget results

Transponder TXPt 20 W 43.01 dBm Antenna efficiency 0.65Bo -5 dB Antenna size 0.07 m^2Lant -3 dB Antenna dimension 0.30 mGt 20 dB

EiRP 55.01 dBm

Propagation lossesPath length 36000 kmf 4000 MHz lambda 0.075 mFSPL -195.61 dB

Atmospheric losses -2 dBDesign margin -3 dBTotal environmental losses -200.61 dB

Earth stationAntenna gain 49.7 dB Antenna efficiency 0.7System noise temperature 75 K 18.7506 dBK Antenna size 59.68 m^2G/T 30.95 dB/K Antenna dimension 8.72 mNoise bandwidth 27 MHzC/N required 10 dB

Rx Sensitivity -95.537 dBm

Budget resultsMax al lowed path loss 200.25 dBBudgeted path loss 200.61 dBExcess path loss -0.36 dB

Microsoft Excel Worksheet