ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 10: Satellite link design...

ECE 5233 Satellite Communications

Prepared by:

Dr. Ivica Kostanic

Lecture 10: Satellite link design

(Section 4.4)

Spring 2014

Florida Institute of technologies

Example of noise temperature calculations

Heterodyning

Downlink link budget

Example link budget in C-band

Outline

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


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

Block diagram of SC super heterodyne RX

snT

T

fmfrf

IF

mfrf

f

frf

m

rf

farf

n

G

rffmfifn

TkTBG

GGGG

T

GGG

T

GG

T

G

TTT

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


Single conversion receiver –Ex 4.3.1

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


RX with antenna feed losses (Ex 4.3.2)

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.


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

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


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-offs

o Date rate / capacity

o Reliability

o Component size and cost


Components of the link budget table

1. Transmitter side

o TX components

o Principle result: EiRP

2. Receiver side

o RX components

o Principle results

RxSensitivity

System temperature

RSLA

3. Propagation modeling

o Free space loss

o Environmental losses

o Other system losses

4. Link reliability and margin calculations

Elements of the radio link


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

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]


Propagation losses

Components of propagation losses

o Free Space Path Loss (path length)

o Atmospheric losses

o Design margin (misalignment, severe weather, drift in component parameters)

Propagation losses - calculated for worst case

Note: All losses are frequency dependent.

Earth station

Satellite


Earth station – link budget components

Antenna gain

o Function of efficiency and size

System noise temperature

o Determined from the Rx front end

Receiver sensitivity

o Minimum required RX power for proper link operation

o Determined from noise power and required S/N ratio

Required S/N ratio

o Determined from the modulation type

o Ultimately driven by bandwidth efficiency requirement

o Obtained from receiver operating curves

dBIM1210log10

12

/

/

BR

BR

N

SN

S

Signal to noise ratio requirement

R – information rate

B – bandwidth of the channel

IM – implementation margin


Downlink link budget example

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 effi ciency 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 effi ciency 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 allowed path loss 200.25 dBBudgeted path loss 200.61 dBExcess path loss -0.36 dB

Microsoft Excel Worksheet

ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 10: Satellite link design...

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