unit-3 scr satellite link design
DESCRIPTION
satellite link designTRANSCRIPT
1
Satellite Communication Systems
By- Kanchan Bakade
Reference book:Satellite communication- Dennis Roddy
Satellite Communication- Pratt and Bostian
Transmission Losses Losses occur along the way, some of which are
constant. Losses for clear weather conditions = losses
which don’t vary significantly with time and losses which are calculated statistically
Free-space loss Antenna misalignment losses Fixed atmospheric and ionospheric losses Losses which are weather related which
fluctuate with time , allowed for by introducing fade margins into the transmission equation.
Transmission Losses – Free-space loss
Free-space loss = power loss that comes from the spreading of the signal in space
Most significant type of transmission loss
24
log10][][][
r
GEIRPP RR
24
log10][
r
FSL
effR FAP
4
2R
eff
GA
22
2 444
RGEIRP
G
R
EIRPP R
RR
224 m
W
R
PGF tt
Basic form of link equation:
where and
Transmission Losses – Feeder losses
Basic form of link equation only accounts for free-space loss
Other types of losses need to be accounted for Feeder losses: Losses that occur between the
receive antenna and the receiver proper Eg: Losses in the connecting waveguides, filters, and
couplers Receiver feeder losses = [RFL] dB RFL values are added to FSL Similarly losses occur at transmitting antenna too
when connected to HPA o/p – needed
Transmission losses – Antenna misalignment losses
Ideal situation: earth station and satellite antenna aligned for maximum gain
Two possible sources of off-axis loss: satellite and earth station
Off-axis loss at ES is antenna pointing loss ( only few tenths of a decibel)
Misalignment of the polarization direction - [AML] dB include both pointing and polarization losses.
The AML for uplink and downlink must be taken into account seperately.
Figure: ref. 1
Transmission Losses – Fixed atmospheric and ionospheric losses
Atmospheric gases result in losses by absorption
atmospheric absorption loss = [AA] dB Polarization loss (dB); PL= 20 log(cosθ) θ = angle of mismatch
Basic Transmission Theory
Calculation of the power received by an earth station from a satellite transmitter.
Had two approach: The use of flux density The link equation.
Transmitting source is an isotropic radiator which uniformly radiates a total power in all directions.
The flux density crossing the surface of a sphere with radius R is given by
22 W/m
4 R
PF
t
2
2 W/m
4 R
PF
t
Antenna Gain Antenna gain is G(θ) is defined as the ratio of power per unit
solid angle radiated in a direction θ to the average power radiated per unit solid angle.
We need directive antennas to get power to go in wanted direction.
Gain of antenna at θ = 0º (bore sight) get increase in power in a given direction compared to isotropic antenna.
4/
)()(
0P
PG
• P() is variation of power with angle.
• G() is gain at the direction .
• P0 is total power transmitted.
• sphere = 4solid radians
EIRP
For a transmitter with o/p power P watts driving a lossless antenna with gain G , the flux density in the direction of the antenna bore sight at a distance R meters is
PtGt=Effective Isotropic Radiated Power (EIRP) Note that EIRP may vary as a function of
direction because of changes in the antenna gain vs. angle
22 W/m
4 R
GPF tt
Received Power The power available to a receive antenna of area Ar m2 we
get:24
x R
AGPAFP ett
rr
Real antennas have effective flux collecting areas LESS THAN the physical aperture areaDefine Effective Aperture Area Ae: x e rAA Where Ar is the actual (physical) aperture area. = aperture efficiency; losses between the incident wavefront and the antenna o/p port.
Includes illumination efficiency, aperture taper efficiency of the antenna, and other losses due to spill,blockage, phase due to spill, blockage, phase errors, Diffraction effects, polarization and mismatch losses.
It is range 50 to 75% for parabolic reflector and lower for small antennas and Higher for large cassegrain antennas. Horn antenna efficiency is close to 90%.
Back to Received Power… Antennas have (maximum) gain G related to the
effective aperture area as follows:
where λ is the wavelength (m) at the frequency of operation.
The power available to a receive antenna of effective area Ar = Ae m2 is:
24 x
R
AGPAFP ett
rr
2
4
e
r
AG
Inverting…
4
2r
e
GA
2
4
eA
Gain
Back to Received Power…
2
4
RGGPP rttr
Friis Transmission Formula
• The inverse of the term at the right referred to as “Path Loss”, also known as “Free Space Loss” (Lp):
24
R
Lp
Therefore…
p
rttr L
GGPP
Signal TransmissionLink-Power Budget Formula
The decibel equation for the received power is: [PR] = [EIRP] + [GR] - [LOSSES] dBWWhere:
[PR] = received power in dBW [EIRP] = equivalent isotropic radiated power in
dBW [GR] = receiver antenna gain in dB [LOSSES] = total link loss in dB
dBW = 10 log10(P/(1 W)), where P is an arbitrary power in watts, is a unit for the measurement of the strength of a signal relative to one watt.
More complete formulation
rotherpolrataap
rttr LLLLLLL
GGPP
Demonstrated formula assumes idealized case. Free Space Loss (Lp) represents spherical spreading
only. Other effects need to be accounted for in the
transmission equation: La = Losses due to attenuation in atmosphere Lta = Losses associated with transmitting antenna Lra = Losses associates with receiving antenna Lpol = Losses due to polarization mismatch Lother = (any other known loss - as much detail as available) Lr = additional Losses at receiver (after receiving antenna)
Signal TransmissionLink-Power Budget Formula Variables
Link-Power Budget Formula for the received power [PR]: [PR] = [EIRP] + [GR] - [LOSSES]
The equivalent isotropic radiated power [EIRP] is: [EIRP] = [Pt] + [Gt] dBW, where: [Pt] is the transmit power in dBW and [Gt] is the
transmitting antenna gain in dB. [GR] is the receiver antenna gain in dB. Losses for clear sky conditions are: [LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL],
where: [FSL] = free-space spreading loss in dB = PT/PR (in watts) [RFL] = receiver feeder loss in dB [AML] = antenna misalignment loss in dB [AA] = atmospheric absorption loss in dB [PL] = polarisation mismatch loss in dB
The major source of loss in any ground-satellite link is the free-space spreading loss.
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
Translating to dBs
The transmission formula can be written in dB as:
This form of the equation is easily handled as a spreadsheet (additions and subtractions!!)
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
Link Power Budget
Transmission:+ HPA Power- Transmission Losses (cables & connectors)+ Antenna Gain
EIRPTx
- Antenna Pointing Loss- Free Space Loss- Atmospheric Loss (gaseous, clouds, rain)- Rx Antenna Pointing Loss
Rx
Reception:+ Antenna gain- Reception Losses (cables & connectors)+ Noise Temperature Contribution
Pr
Now all factors are accounted for as additions and subtractions
Link Budget parameters
Transmitter power at the antenna Antenna gain compared to isotropic radiator EIRP Free space path loss System noise temperature Figure of merit for receiving system Carrier to thermal noise ratio Carrier to noise density ratio Carrier to noise ratio
Easy Steps to a Good Link Power Budget
First, draw a sketch of the link path Doesn’t have to be artistic quality Helps you find the stuff you might forget
Next, think carefully about the system of interest Include all significant effects in the link power budget Note and justify which common effects are insignificant here
Roll-up large sections of the link power budget Ie.: TXd power, TX ant. gain, Path loss, RX ant. gain, RX losses Show all components for these calculations in the detailed budget Use the rolled-up results in build a link overview
Comment the link budget Always, always, always use units on parameters (dBi, W, Hz ...) Describe any unusual elements (eg. loss caused by H20 on radome)
Simple Link Power Budget
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.
Closing the Link We need to calculate the Link Budget in order
to verify if we are “closing the link”.Pr >= Cmin Link Closed
Pr < Cmin Link not closed
Usually, we obtain the “Link Margin”, which tells how tight we are in closing the link:
Margin = Pr – Cmin
Equivalently:Margin > 0 Link ClosedMargin < 0 Link not closed
System Noise
Random thermal motion of electron in the resistive and active devices in the receiver and also lossy components of antennas
Available noise power: Thermal noise: Flat frequency spectrum Noise power spectral density: BN ≈ 1.12 B-3dB
NNN BkTP
NN
N kTB
PN 0
System Noise – Antenna Noise
Two types of antenna noise:1. Noise originating from antenna losses 2. Sky noise = microwave radiation present
throughout the universe• Figure: Equivalent noise temperature of the sky as seen from earth station antenna
• Equivalent noise temperature of the earth as seen from the satellite antenna is about 290 K
Figure: ref. 1
System Noise – Amplifiers Single stage amplifier:
Amplifiers in cascade:
)(,0 eantout TTGkN
...21
3
1
21
GG
T
G
TTTT ee
eantS
Figures: ref. 1
)(,0,0 eant
outin TTk
G
NN
For amplifiers in cascade, it’s important to have a low noise, high gain amplifier for the first stage.
System Noise Nout= FGkTo
F = noise factor, To = Room temperature =290 K
Also Nout= Gk(Te+To)
Noise figure NF = 10logF
0)1( TFTe
1
0
1
0 )1()1(
G
TFL
G
TLTT antS
System Noise Power - 1
Performance of system is determined by C/N ratio.
Most systems require C/N > 10 dB. (Remember, in dBs: C - N > 10 dB)
Hence usually: C > N + 10 dB We need to know the noise temperature of our
receiver so that we can calculate N, the noise power (N = Pn).
Tn (noise temperature) is in Kelvins (symbol K):
2739
5320 FTKT 2730 CTKT
System Noise Power - 2 System noise is caused by thermal noise
sources External to RX system
Transmitted noise on link Scene noise observed by antenna
Internal to RX system The power available from thermal noise is:
where k = Boltzmann’s constant = 1.38x10-23 J/K(-228.6 dBW/HzK),
Ts is the effective system noise temperature, andB is the effective system bandwidth
(dBW) BkTN s
Noise Spectral Density
N = K.T.B N/B = N0 is the noise spectral density (density of noise power per hertz):
N0 = noise spectral density is constant up to 300GHz.
All bodies with Tp >0K radiate microwave energy.
(dBW/Hz) 0 ss kT
B
BkT
B
NN
System Noise Temperature
1) System noise power is proportional tosystem noise temperature
2) Noise from different sources is uncorrelated (AWGN)
Therefore, we can Add up noise powers from different contributions Work with noise temperature directly
So:
But, we must: Calculate the effective noise temperature of each
contribution Reference these noise temperatures to the same
location
Additive White Gaussian Noise (AWGN)
RXlinelossLNAantennadtransmittes TTTTTT
Typical Receiver
Noise Model
Noise is added and then multiplied by the gain of the device (which is now assumed to be noiseless since the noise was already added prior to the device)
Equivalent Noise Model of Receiver
Equivalent model: Equivalent noise Ts is added and then multiplied by the equivalent gain of the device, GRFGmGIF
(noiseless).
Calculating System Noise Temperature - 1
Receiver noise comes from several sources. We need a method which reduces several
sources to a single equivalent noise source at the receiver input.
Using model in Fig. 4.5.a gives:
End)-(Front
(Mixer)
(IF)
inRFRFmIF
mmIF
IFIFn
TTkBGGG
BkTGG
BkTGP
Calculating System Noise Temperature - 2
Divide by GIFGmGRFkB:
If we replace the model in Fig. 4.5.a by that in Fig. 4.5b
RFm
IF
RF
minRFRFmIFn GG
T
G
TTTkBGGGP
BkTGGGP sRFmIFn
Calculating System Noise Temperature - 3
Equate Eqns :
Since C is invariably small, N must be minimized.
How can we make N as small as possible?
RFm
IF
RF
minRF GG
T
G
TTTT S
For a noiseless lossy device
Tno = Tp (1 - Gl) Where Gl is linear gain of attenuating device
Tno is noise temperature at the output
Tp is the physical temperature of the device Therefore, for an attenuation of A dB,
Gl =10A/10
TinGain
Gl
Noiseless lossy device
+
Pn
Noise Source Tno
Reducing Noise Power
Make B as small as possible – just enough bandwidth to accept all of the signal power (C ).
Make TS as small as possible Lowest TRF
Lowest Tin (How?) High GRF
If we have a good low noise amplifier (LNA), i.e., low TRF, high GRF, then rest of receiver does not matter that much.
inRFRFm
IF
RF
minRF TT
GG
T
G
TTTT
S
Reducing Noise Power Discussion on Tin
Earth Stations: Antennas looking at space which appears cold and produces little thermal noise power (about 50K).
Satellites: antennas beaming towards earth (about 300 K): Making the LNA noise temperature much less
gives diminishing returns. Improvements aim reduction of size and weight.
Antenna Noise Temperature
Contributes for Tin Natural Sources (sky noise):
Cosmic noise (star and inter-stellar matter), decreases with frequency, (negligible above 1GHz). Certain parts of the sky have punctual “hot sources” (hot sky).
Sun (T 12000 f-0.75 K): point earth-station antennas away from it.
Moon (black body radiator): 200 to 300K if pointed directly to it.
Earth (satellite) Propagation medium (e.g. rain, oxygen, water vapor):
noise reduced as elevation angle increases. Man-made sources:
Vehicles, industrial machinery Other terrestrial and satellite systems operating
at the same frequency of interest.
Antenna Noise Temperature Useful approximation for Earth Station
antenna temperature on clear sky (no rain): Earth Station Antenna - Noise Temperature
15
20
25
30
35
40
45
50
0 10 20 30 40 50 60 70 80 90 100
Elevation Angle (degrees)
Ta
(K)
NOISE FIGURE AND NOISE TEMPERATURE
Noise figure : specify the noise generated within a device.
The noise temperature T=T0(NF-1)Where NF is a linear ratio, not in dB. T is the reference temperature(290K)
out
in
NS
NSNF
/
/
G/T ratio for Earth Stations When link equation rewritten in terms of
(C/N) at the earth station, we had
Thus Gr / Ts used to specify the quality of a receiving earth station.
Increasing G/T, increases the received C/N ratio.
Satellite terminals has negative G/T which is below 0 dB/K. i.e Gr < Ts
s
r
n
rt
ns
rtt
T
G
RkB
GP
RBkT
GGP
N
C22
44
System Figure of Merit
G/Ts: RX antenna gain/system temperature Also called the System Figure of Merit, G/Ts
Easily describes the sensitivity of a receive system Must be used with caution:
Some (most) vendors measure G/Ts under ideal conditions only
G/Ts degrades for most systems when rain loss increases This is caused by the increase in the sky noise component This is in addition to the loss of received power flux density
Power Budget Example - 14.1.1 Satellite at 40,000 km (range)
Transmits 2WAntenna gain Gt = 17 dB (global beam)
Calculate: a. Flux density on earth’s surface b. Power received by antenna with effective aperture of 10m2
c. Gain of receiving antenna. d. Received C/N assuming Ts =152 K, and Bw =500 MHz
a. Using Eqn. 4.3: (Gt = 17 dB = 50)
2215-
2722
dBW/m 143W/m10 x 4.97
) x(4x104
50 x 2
44
R
GP
R
EIRPF tt
(Solving in dB…)
dBW/m2 1431521120
114
dB[meter] )10x4(log x 2 R
dBW 20173)(27
102
F
dB
GtPtEIRP
Power Budget Example - 1b. Received Power
dBWW 13310 x 4.97 P
10 x )(4.97x10 A x FP14-
r
-15r
(Solving in dB…)
dBW 133
10)143(
r
r
P
AFP
c. Gain given Ae = 10 m2 and Frequency = 11GHz ( eqn. 4.7)
dBA
G er 3.52
0273.010 x 4π4
2
Power Budget Example - 1b. System Noise Temperature
dBNC
NCNC
dBWW
dBW
BTKor
KTB
dBdBdB
2.13/
)79.119(133/
13310 x 4.97 PC
97.119
99.8682.21 6.228
10 x 500 x 152x 01 x 38.1PN
14-r
623n
C/N carrier to noise ratio
rotherpolrataap
rttr LLLLLLL
GGPP
BKT
P
N
C
s
r
RFm
IF
RF
minRF GG
T
G
TTTT S
2D
G
24
R
LpFLa
Carrier to Noise Ratios
C/N: carrier/noise power in RX BW (dB) Allows simple calculation of margin if: Receiver bandwidth is known Required C/N is known for desired signal type
C/No:carrier/noise p.s.d. (dbHz) Allows simple calculation of allowable RX
bandwidth if required C/N is known for desired signal type
Critical for calculations involving carrier recovery loop performance calculations
Carrier-to-Noise Ratio
][][][][][ NBkLOSSEST
GEIRP
N
C
N
R
P
P
N
C
otherRR LR
GEIRPP1
4
2
NNN BkTP
otherNsys
R
LFSLkBT
GEIRP
N
C 111
NN
BN
C
BN
C
N
C
00
Definition of C/N ratio
Received power with all losses taken into account
Noise power
C/N ratio in product form
C/N ratio in dB form
Definition of carrier-to-noise density ratio
Power Budget Example - 2Generic DBS-TV:
Received PowerTransponder output power , 160 W 22.0 dBWAntenna beam on-axis gain 34.3 dBPath loss at 12 GHz, 38,500 km path -205.7 dBReceiving antenna gain, on axis 33.5 dBEdge of beam -3.0 dBMiscellaneous losses -0.8 dBReceived power, C -119.7 dBW
Power Budget Example - 2Noise powerBoltzmann’s constant, k -228.6
dBW/K/HzSystem noise temperature, clear air, 143 K 21.6
dBKReceiver noise bandwidth, 20MHz 73.0
dBHzNoise power, N -134.0 dBW C/N in clear air 14.3 dBLink margin over 8.6 dB threshold 5.7
dBLink availability throughout US Better than
99.7 %
55
Thank you
Carrier-to-Noise Ratio – Uplink
Uplink:
Earth station EIRP, satellite receiver feeder losses, satellite receiver G/T,
Frequency dependent calculations calculated for the uplink frequency
][][][][0
kLOSSEST
GEIRP
N
CUUU
U
subscript “U” stands for uplink
Traveling wave tube amplifiers (TWTAs)
Widely used in transponders to provide the final output power required to the transmit antenna
Provides amplification over a very wide bandwidth
Nonlinear transfer characteristic Input power of the TWTA needs to be carefully
controlled to minimize distortion Low input powers: input-output relationship is linear
Higher input powers: output power saturates
Figure: ref. 1
Uplink – Saturation Flux Density
In the uplink, the TWTA will be at the receiving end
Received signal from earth will be input to the TWTA
Saturation flux density ( Fs ): flux density required at the satellite’s receiving antenna to produce saturation of the TWTA
Using the saturation flux density, one can calculate the required EIRP at the earth station to produce saturation of the TWTA at the satellite
RFLLOSSESAFEIRP USUS 0
4
log102
0A
Effective area of an isotropic antenna subscript “S” stands for saturation
Uplink – Input Back-off
A number of simultaneous carriers present in the TWTA requires back-off of the operating point to reduce intermodulation distortion
It’s the input back-off because the received signal will be input to the TWTA
Earth station EIRP has to be reduced by this specified back-off
iUSU BOEIRPEIRP ][][][ iBO = input back-off
Uplink – Earth Station HPA Earth station high power amplifier is at the transmitting end
of the uplink Supplies the radiated power plus the transmitter feeder
losses
Earth station HPA transfer characteristic can also be nonlinear, requiring output back-off
Output back-off since HPA’s output is the transmitted signal
An HPA with a high saturation point has larger physical size and higher power consumption, but penalty for this is not as large since it’s on the earth.
][][][][ TFLGEIRPP THPA TFL = transmitter feeder losses
HPAsatHPAHPA BOPP ][][][ ,
Carrier-to-Noise Ratio – Dowlink
Downlink:
Satellite EIRP, earth station receiver feeder losses, earth station receiver G/T
Frequency dependent calculations calculated for the downlink frequency
][][][][0
kLOSSEST
GEIRP
N
CDDD
D
subscript “D” stands for downlink
Downlink – Output Back-off, Satellite TWTA Output
Output back-off:
Satellite TWTA Output: TWTA supplies radiated power and transmit feeder
losses, the saturates o/p of TWTA is given by
ODSD BOEIRPEIRP ][][][
dBBOBO Oi 5][][
DTDTWTA TFLGEIRPP ][][][][
The output of the TWTA is being transmitted, so it’s the output back-off
A rule of thumb:
OSTWTATWTA BOPP ][][][
Figure: ref. 1
Effects of Rain So far, calculations have been made for
clear-sky conditions Rainfall is a significant cause of fading in
the C band and especially in the Ku band Rainfall causes attenuation by scattering
and absorption of the radio waves Rain attenuation for horizontal polarization
is greater than for vertical polarization
Rain Attenuation Rain attenuation data is usually
available as curves or tables Tables gives percentage of time
over a year that the attenuation exceeds the dB values
Figure: ref. 1Figure: ref. 4
Radomes are truncated spherical shells composed of panels to protect the earth station antenna from the environment.
Radome transmission loss: ordinary insertion loss, scattering loss
Layer of water caused by rain introduces attenuation by absorption and reflection
Rain-fade margins – Uplink and Downlink
Uplink (satellite is receiving): Increase in noise due to rain usually not a major factor
since satellite antenna is pointed toward a “hot” earth With uplink power control, power output from the earth
station may be increased to compensate for fading Downlink (earth station is receiving): No power control since user doesn’t have control of
satellite EIRP A = rain attenuation caused by absorption Equivalent noise temperature for the rain:
ATT arain
11 rainCSsky TTT
Combined Uplink and Downlink
overall C/N ratio is less than the lower of the uplink and downlink C/N ratios
111
DUoverall N
C
N
C
N
C
Lets denote noise power per unit bandwidth by PNU and the average carrier at the same point by PRU . Therefore,
The carrier power at the end of the space link is PR (i.e received carrier power for the downlink). It is equal to ƴ (system power gain ) times the carrier power input at the satellite.
At the end-of-link, noise is ƴ PNU + PND
When not counting ƴ PNU contribution, then The combined C/No is given by
NU
RU
UO P
P
N
C
NDNU
R
DO PP
P
N
C
ND
R
DO P
P
N
C
11
1
)(
DOUO
O
D
O
U
O
R
ND
RU
NUO
RUR
R
ND
R
NU
R
NDNU
R
NO
N
C
N
CN
C
C
N
C
N
P
P
P
P
C
N
then
termiforPPsubstitute
P
P
P
P
P
PP
P
P
C
N
Intermodulation Noise Occurs whenever multiple carriers pass through a
device with nonlinear characteristics, such as TWTAs1111
IMDUoverall N
C
N
C
N
C
N
C
C/N ratio for intermodulation noise is a function of the number of carriers and their modulation characteristics, and the amplitude and phase characteristics of the high-power amplifier.
Figure: ref. 5
To reduce intermodulation noise, we can operate the traveling wave tube in a back off condition
Increasing back off decreases uplink and downlink C/N ratios
There is an optimum operating point that gives maximum overall C/N ratio as a function of back-off
System Design Example Ku-band geostationary satellite with bent pipe
transponders to distribute digital TV signals from an earth station to many receiving stations
Bent pipe transponder: transponder that amplifies the received signal and retransmits it at a different frequency
Figure: ref. 2
Need a minimum overall C/N ratio of about 9.5 dB in the TV receiver
Table of specifications
Figure: ref. 2
Ku-Band Uplink Design
Uplink Noise Power Budget
k = Boltzmann’s constant -228.6 dBW/K/Hz
T_n= 500 K 27.0 dBK
B = 43.2 MHz 76.4 dBHz
P_n= transponder noise power
-125.2 dBW
NNN BTkP ][][][][ LOSSESGEIRPP RR
Uplink Power Budget
P_t = Earth station transmitter power
P_t dBW
G_t = Earth station antenna gain
55.7 dB
G_r = Satellite antenna gain 31.0 dB
FSL = Free-space loss -207.2 dB
L_ant = Earth station on 2 dB contour
-2.0 dB
Other losses -1.0 dB
P_r = Received power at transponder
P_t - 123.5 dB
Minimum required receive power:
[P_r] = [C/N] + [P_n]
= 30 + -125.3 = -95.2 dBW
dBD
Gt 7.55*68.0log102
dBR
FSL 2.2074
log102
[P_r] = [P_t] – 123.5 dB = -95.2 dBW
=> [P_t] = 28.3 dBW => P_t = 675 W
NRN
R PPP
P
N
C
Ku-Band Downlink Design
Downlink Noise Power Budget
k = Boltzmann’s constant -228.6 dBW/K/Hz
T_n= 30 + 110 K = 140 21.5 dBK
B = 43.2 MHz 76.4 dBHz
P_n= transponder noise power
-130.7 dBW
Downlink Power Budget
P_t = Satellite station transmitter power
18.0 dBW
G_t = Satellite station antenna gain
31.0 dB
G_r = Earth station antenna gain G_r dB
FSL = Free-space loss -205.4 dB
L_ant= Earth station on 3 dB contour
-3.0 dB
Other losses -0.8 dB
P_r = Received power at earth station
G_r - 160.2 dB
dBN
C
N
C
N
C
N
C
N
C
DDUoverallD
2.176.52111
5017
overalloverall N
CdB
N
C
100030
UU N
CdB
N
C
Minimum required receive power:
[P_r] = [C/N]+[P_n] = 17.2 + -130.7 = -113.5 dBW
OSTWTATWTA BOPP ][][][ P_t, sat = 80 W => [P_t, sat] = 19 dBW
Output back-off = 1 dB
[P_t] = 19 – 1 = 18 dBW dBR
FSL 4.2054
log102
[P_r] = [G_r] – 160.2 = -113.5 dBW => [G_r] = 46.7 dB
=> G_r = 46,774
mDD
GR 14.2774,46*65.02
NRN
R PPP
P
N
C
Satellite Communication Link Design Procedure
1. Determine the frequency band in which the system must operate. Comparative designs may be required to help make the selection.
2. Determine the communications parameters of the satellite. Estimate any values that are not known.
3. Determine the parameters of the transmitting and receiving earth stations.
4. Start at the transmitting earth station. Establish an uplink budget and a transponder noise power budget to find uplink C/N in the transponder.
5. Find the output power of the transponder based on transponder gain or output back-off.
6. Establish a downlink power and noise budget for the receiving earth station. Calculate downlink C/N and overall C/N for a station at the edge of the coverage zone (worst case).
7. Calculate S/N or BER in the baseband channel. Find the link margins. 8. Evaluate the result and compare with the specification requirements.
Change parameters of the system as required to obtain acceptable overall C/N or S/N or BER values. This may require several trial designs.
9. Determine the propagation conditions under which the link must operate. Calculate outage times for the uplinks and downlinks.
10. Redesign the system by changing some parameters if the link margins are inadequate. Check that all parameters are reasonable, and that the design can be implemented within the expected budget.
The above can be found in ref. 2
Summary Transmission losses include free-space loss, feeder
losses, antenna misalignment losses and fixed atmospheric and ionospheric losses
To reduce system noise for amplifiers in cascade, have a low noise, high gain amplifier in the first stage
C/N ratio gives error probability and capacity Multiple carriers present means back-off must be
accounted for Rain attenuation can be overcome with uplink power
control, increasing the antenna diameter, or using an amplifier with higher gain and lower noise
C/N ratios add as reciprocals Space link calculations are an iterative process since
it’s hard to get it all right on the first try