Link Budget Tutorial Xx

Link Budget Tutorial

CS 401

Full-Duplex Communications

VHF MonopoleReceiving

VHF YagiTransmitting(Command)

UHF DipoleTransmitting

UHF YagiReceiving

Using UHF for spacecrafttransmission and VHFfor command. Most commonamateur radio modes.

Basic HardwareCubesat Ground

Station

Transmitting Dipole Yagi

Transmitting Antenna Gain

0dB 10.2dB

Receiving Antenna

Monopole Yagi

Receiving Antenna Gain

0dB 14.15dB

Transceiver Helio-100 TS-9000

RF output Power

.5-1W 50W

Link Budget Overview• Gains – losses• Done for both sides

– Satellite to ground station (transmit mode)– Ground station to satellite (command mode)

• Gains – Transmitter power (RF output)– Antenna

• Losses– Line loss from cable– Connector losses– Free space loss– Atmospheric loss– Ionospheric loss– System noise loss

Decibel units

• Logarithmic unit• Ratio of power/intensity to a reference value

• P1 and P0 must measure same type and have the same units

• Solving for P1

1(log10 10 P

0101 10 PPdBL

• dB’s are easier to add and subtract• 500mW = ? dBm• dBm = 10log10(500/1)• =10log10(500)• ~27dBm• 500mW = ? dBW• =10log10(0.5/1)• =0dBW (don’t show –dBW)

• dB’s are easier to add and subtract• 500mW ~= 27dBm • Examples:• 10mW = 10dB• 100mW = 20dB• 1000mW = 30dB

dB units for RF links

• dBm or dBmW – power relative to 1mWReferenced to a 50ohm load• dBc – noise or peak power relative to carrier

power• dBi – forward gain relative to a theoretical

isotropic antenna. Gain for a ½ wave dipole is 2dBi

• dBd – forward gain relative to a ½ wave dipole so gain for a ½ wave dipole is 0 dBd

Elevation Angle• Angle between ground

station and spacecraft relative to earth

• When spacecraft is directly overhead, elevation angle is 90

• When spacecraft is on horizon, elevation angle is 0

• 0 degrees difficult to communicate, requires full line of sight. Trees, buildings, etc. may obstruct view.

Specs required for link budget

Cubesat Ground Station

Transmitter power 500mW 50W

Antenna Gain 0dB 10.2dB

Cable length (estimate) 19cm (max) 25m

Frequency 434MHz 144MHz

Bandwidth 15000Hz 15000Hz

Modulation/BER

Altitude 800km 800km

Other specs are estimated

Link elements

• XMTR power in Watts – RF output power• XMTR power in dBW – convert Watts to dBW• XMTR system losses in dB

-cable loss – function of frequency and length -connectors -filters -antenna mismatch – see next slide

Antenna Mismatch• Based on VSWR ratio which will be > 1• Calculated for both GS and Spacecraft separatelyLet TX = transmitter output power, V = VSWRPL – power lossPT – power transmittedPLdB – Power loss due to mismatchThen the following are used to compute PLdB

PL = TX*((V-1)2/(V+1)2)PT = TX-PLPLdB = -10*log10(PT/TX)

Antenna Mismatch Examples

PL = TX*((V-1)2/(V+1)2)PT = TX-PLPLdB = -10*log10(PT/TX)

Let TX = 50W and V = 1.5PL = 50((1.5-1)^2/(1.5+1)^2) =

2WPT = 48WPLdB = -10*log(48/50) =

0.18dB

Let TX = 1W and V = 2PL = 0.11WPT = 0.89WPLdB = -10*log(0.89/1) = 0.51 dB

Link Values so farCubesat ->Ground Station

Ground Station-> Cubesat

XMTR Power (W) 1W 50W

XMTR Power (dBm) 0 17

XMTR System Losses (dB)1

XMTR Frequency (MHz) 434 144

XMTR Antenna Gain (dBi) 0 9.35

1 - estimates

Antenna Gain – take gain of antenna and add 2dBi. Because we have no attitudecontrol, we are subtracting 3dB to account for pointing losses.Cubesat has gain of 0dB + 2dBi - 3 dB = 0dBiGS uplink antenna has gain of 10.2dB + 2.15dBi = 12.35dB – 3dB = 9.35dBi

EIRPEffective (or equivalent)isotropically radiatedpower (EIRP)

EIRP (dBm) = Pt(dBm)– Lc(dB) + Ga (dBi),

wherePt – transmitted power (dBm)Lc – Line/cable losses (dB)Ga – Antenna Gain (dBi)

Cubesat ->Ground Station

XMTR Power (W)

1W 50W

XMTR Power (dBm)

XMTR System Losses (dB)1

XMTR Frequency (MHz)

434 144

XMTR Antenna Gain (dBi)

XMTR EIRP(dBm)

-0.5 24.4

Slant Range

d – elevation angle

S -slant rangeRe – radius of the earth, 6376.136kmh – altitude of Cubesat, e.g. 800km

r = h + Re

Earth surface

Satellite orbit

S = Re[{r^2/Re^2 – cos^2(d)}^1/2-sin(d)]

Slant Range Calculations

)sin(Re*))cos*Re)(Re( 222 hS

S - slant rangeRe – radius of the earthh– mean orbital altitudeΘ – elevation angle

Use whichever formula is easiest to calculate in Excel:

)sin())(cosRe/)(Re(Re* 222 hS

Slant Range Computations• Use Excel and compute

the slant ranges for an altitude of 325km at the elevations given on the chart.

• Use the following constants:

Re – radius of the earth = 6378.14

h – mean orbit altitude = 325km

Slant ranges for mean orbit altitudeof 800km

Elevation Slant Range

(deg) (km)

5 2784

10 2367

20 1769

30 1395

40 1159

50 1006

60 907.2

70 845.1

80 810.9

90 800

Link Budget so far…Cubesat ->Ground Station

XMTR System Losses (dB)1 0.5 2

XMTR EIRP(dBm) -0.5 24.4

Slant Range (at 5deg elevation)

2784 2784

The slant range is the same no matter which direction – from space to ground orground to space.

Source: http://www.vetechnet.com/lesson-1.htm

Path Loss (Free Space Loss)• Path loss is a function of distance

and wavelength.• Recall that distance, i.e. the slant

range is a function of elevation• RF often refers to wavelengths

instead of frequency• Note: Cubesat transmits on UHF but

ground station transmits on VHF• Make sure that wavelength and

distance are in the same units• UHF (amateur bands)

– Frequency: 420-450MHz– Wavelength: 70cm

• VHF (amateur bands)– Frequency: 144-148MHz– Wavelength: 2m

[log*20 10 d

PathLoss

See Diallo link budget for alternative formula.

Using formula above for UHF and elevation angle of 5 degrees:

D= 2783.9 km (slant range at 5 degrees)λ = 70x10-5 (km)

Path loss = 153.97dB

Adding Path Loss Cubesat ->Ground Station

Slant Range (at 5deg elevation)

2784 2784

Path Loss (5deg elevation) 153.97 144.85

Antenna Polarization• Linear polarization –

confines the electrical field vector to a given plane along the direction of propagation

• Circular polarization describes an electrical field that is circular over time. See URL:

• http://upload.wikimedia.org/wikipedia/commons/8/81/Circular.Polarization.Circularly.Polarized.Light_Right.Handed.Animation.305x190.255Colors.gif

Linear polarization

VerticalHorizontal

Circular polarization

Source: http://www.ccrs.nrcan.gc.ca/glossary/index_e.php?id=3089

More Polarization Examples

Circularly polarizedRight-Hand CP

Linearly polarized(Vertical)

Polarization Mismatches

• Antennas transmit and receive in exactly the same way.

• A vertically polarized antenna will not communicate with a horizontally polarized antenna

Polarization Loss Factors• Given two linearly polarized

antennas rotated from each other by angle φ

• Polarization Loss Factor (PLF) = cos2φ

• If they have the same polarization, PLF = 0

• If one is vertically polarized and the other is horizontally polarized, then φ=90 and they will not communicate.

• Given a circularly polarized antenna and a linearly polarized antenna

• The LP antenna will pick up the in-phase component of the CP wave.

• So polarization mismatch will be 0.5 or -3dB no matter what angle the LP antenna is rotated to.

• Since using CP antennas (Yagis) for GS and LP (dipole and monopole) antennas on the Cubesat, PLF = 3dB

Adding Polarization Matching LossCubesat ->Ground Station

Slant Range (km) (at 5deg elevation)

2784 2784

Path Loss (5deg elevation) (dB)

153.97 144.85

Polarization Matching Loss (dB)

Atmospheric Losses• Power loss due to

absorption, refraction and scattering.

• Lower the elevation, longer the distance in the troposphere

• Major cause of signal attentuation – rain and fog

• Use a look-up table • Source: “Radiowave

Propagation in Satellite Communications” Louis J. Ippolito

ElevationAngle (deg)

Atmospheric Loss (dB)

0 10.2

10 1.1

30 0.4

45 0.3

Adding Atmospheric LossesCubesat ->Ground Station

Slant Range (km) (at 5deg elevation)

2784 2784

Path Loss (5deg elevation) (dB)

153.97 144.85

Polarization Matching Loss (dB)

Atmospheric Losses (dB) (5 deg elev)

2.1 2.1

Isotropic Signal at Received Antenna

• Basically gain – losses• EIRP – Losses which are

the sum of:– Path Loss– Polarization Matching

Losses – Atmospheric Losses

Adding to Link Budget…Cubesat ->Ground Station

Slant Range (at 5° deg elev) 2784 2784

Path Loss (5° elev) 153.97 144.85

Polarization Matching Loss 3dB 3dB

Atmospheric Losses (5° elev) 2.1 2.1

Isotropic Sig at Rcvr Ant (dBW) (5° elev) -159.57 -121.45

Receiver Component

• Received signal at GS (transmit from Cubesat)

• Received signal on Cubesat (command from GS)

• Elements– Receive Antenna Gain– Receive Noise

Temperature– Receive G/T– Receive C/No– Bandwidth– Receive Eb/No– Required Receive Eb/No– Link Margin

Receive Antenna Gain

• Receiving at Cubesat• Recall antenna gain =

• Receiving at GS• Antenna gain = 14.15dB• Convert to dBi• =14.15dB + 2.15 dBi =

16.3dBi• Subtract 3dB for

pointing loss• =16.3-3 = 13.3dBi

Cubesat ->Ground Station

Path Loss (5° elev) 153.97 144.85

Receive Antenna Gain (dBi) 13.3 0

Receive Noise Temperature• Somewhat complicated• Factors include:

– Antenna Temperature/Sky Temperature

– System Line (physical) Temperature

– Noise temperature of amplifiers

– Computed feedline coefficent

– More• Will use estimates

• 377K – Receive Noise Temperature at GS

• 293K – Receive Noise Temperature at Spacecraft

Cubesat ->Ground StationDownlink

Ground Station-> CubesatUplink

Path Loss (5° elev) 153.97 144.85

Receive Noise Temperature (K) 377 293

Receive G/T

• Receive G/T - Real measure of the receiver’s performance

• Is the “figure of merit”• Receive G/T • = Receive gain in dBi -

10 log10 ( receive noise temperature T ).

• Example• Let RG=receive antenna

gain in dBI, e.g. 13.3• Let RT = receive noise

temperature, e.g. 377• Receive G/T • =RG – 10log10(RT)

• =13.3 – 10log10(377)

• =-12.5

Cubesat ->Ground StationDownlink

Ground Station-> CubesatUplink

Path Loss (5° elev) 153.97 144.85

Receive G/T (dB/K) -12.5 -24.7

Receive C/No (dB-Hz)• Carrier to Receiver Noise Density• C/No = Signal at Rcvr Ant + Received G/T –

Boltzmans Constant(dBW/K/Hz)• Boltzman’s constant = -228.6 dBW/K/Hz• Downlink Example:Iso. Signal at Rcvr Ant (at 5deg elevation) = -159.57Received G/T = -12.5C/No = -159.57 + (-12.5) – (-228.6) = 56.5dB-Hz

Bandwidth

• Need to indicate bandwidth in Hz• Bandwidth – difference between upper and

lower frequency for a range of frequencies• CW < 100Hz• Bandwidth – 15000Hz – frequency range for

Cubesat and GS receivers

Cubesat -> GSDownlink

GS-> CubesatUplink

Path Loss (5° elev) 153.97 144.85

Receive C/N0(dB-Hz) (5° elev) 56.5 82.5

Bandwidth(Hz) 15000 15000

• Eb/N0 (the energy per bit to noise power spectral density ratio) is an important parameter in digital communication or data transmission.

• normalized signal-to-noise ratio(SNR) measure, also known as the "SNR per bit".

• No is the thermal noise density = kT , k – Boltman’s constant in Joules/Kelvin and T is in Kelvin.

• Measure of the signal-to-noise ratio for a digital communications system

• Receive Eb/N0 = C/N0 – 10log10(Bandwidth)• Required Eb/N0 is a function of the modulation scheme and

desired bit error rate (BER)

Modulation/Demodulation Method: CUBE -Sat 2005 July 16

NOTE: Select Here: Choice Made: Result:

UPLINK:

Modulation, Coding & BER Option: 10 GMSK Eb/No:

Command Link Threshold

Option: Modulation Type: Coding: Bit Error Rate Spec: Required Eb/No (dB): 10.6

1 AFSK/FM None 1.00E-04 21.0 dB

2 AFSK/FM None 1.00E-05 23.2

3 G3RUH FSK None 1.00E-04 16.7

4 G3RUH FSK None 1.00E-05 18.0

5 Non-Coherent FSK None 1.00E-04 13.4

6 Non-Coherent FSK None 1.00E-05 13.8

7 Coherent FSK None 1.00E-04 10.5

8 Coherent FSK None 1.00E-05 11.9

9 GMSK None 1.00E-04 8.4

10 GMSK None 1.00E-05 9.6

11 BPSK None 1.00E-05 9.6

12 BPSK None 1.00E-06 10.5

13 QPSK None 1.00E-05 9.6

14 QPSK None 1.00E-06 10.5

15 BPSK Convolutional R=1/2, K=7 1.00E-06 4.8

16 BPSK Conv. R=1/2,K=7 & R.S. (255,223) 1.00E-06 2.5

17 BPSK Conv. R=1/6,K=15 & R.S. (255,223) 1.00E-07 0.8

18 User Defined None 1.00E-05 9.6

Operator Estimate of Implementation Loss

NOTE: Implementation Loss Estimate: 1.0 dB

Required Eb/N0

• From AMSAT chart, required Eb/N0 is 9.6dB + an implementation loss of 1.0dB = 10.6dB

• Receive Eb/N0 (for elevation of 5deg)

C/N0 – 10log10(Bandwidth)

Downlink: 56.5 – 10log10(15000)Uplink: ?

GS-> CubesatUplink

Path Loss (5° elev) 153.97 144.85

Receive Eb/N0 (dB) 14.74 40.74

Required Eb/N0(dB) 10.6 10.6

Link Margin (dB) 4.14 30.14

GS-> CubesatUplink

Path Loss (5° elev) 153.97 144.85

Receive Eb/N0 (dB) 14.74 40.74

Required Eb/N0(dB) 10.6 10.6

Link Margin (dB) 4.14 30.14

Your assignment• Complete Diallo’s link budget using Excel • Add 5, 20, 30, 45, 60 degree elevation angles for uplink• Add 10 degree elevation angle for downlink• Add formulas to compute slant ranges• Add all needed values/formulas for added elevation angles• Change the mean orbital altitude to 325km• Write a 1-page description of link budget in your own

words emphasizing the key elements. Indicate assumptions: RF power, orbital altitude, antenna gain, etc.

• Link budget due today• Paper due next Wednesday

Link Budget Tutorial Xx

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