satellite communications a part 3

Autumn2004 © University of Surrey SatComms A - part 3 - B G Evans 3.1

Satellite Communications APart 3

Link planning / budgetting-Professor Barry G Evans-

EEM.scmA


Link budget & system planning


Mobile System


Performance

• (i) QoS – b.e.r.– 10-4 if speech– 10-6 – 10-8 data (extra coding)

• (ii) Availability– 95%– Channel conditions


Basic Transmission


Carrier Transmission Budget-Antenna Gain-

The antenna gain is defined as the ratio of the power per unit solid angle received/radiated by the antenna in a given direction to the power per unit solid angle received/radiated by an isotropic antenna supplied with the same power.


Basic Transmission


Antenna radiation pattern

Antenna radiation pattern = gain variations as a function of the angle relative to boresight


Transmitted power in a given direction


Predicted coverage areas for the HOTBIRD satellites

(a) Superbeam(b) Widebeam(courtesy of EUTELSAT)


Effective isotropically radiated power (EIRP)


Exercise (1) - Carrier Transmission Budget

• Given– Power fed to antenna: PT = 10W– Antenna gain (at boresight): GTmax = 40dB– Distance: R = 36000km (earth to geostationary satellite

• Calculate– Transmitter EIRP in dB(W)– Flux density at receiver in dB(W/m2)


Down Path


GEO - Geometry


Earth station from the geostationary orbit

• Satellite– Height h above the equator

– Sub-satellite point, longitude ΦS

• Earth station– Latitude E, longitude ΦE

– Relative longitude satellite = (ΦE – ΦS) = ΦES


Exercise (2) – Carrier Transmission Budget

• Given– Uplink frequency = 14GHz– Eart station

• Power fed to the antenna: PT=100W• Antenna diameter: D=4 (efficiency =0.6)• Location: Bercenay (France)

Latitude = 48º13’07”N Longitude = 03º53’13”E

– Satellite• Receiving antenna gain at boresight: GRmax=40dB• Location: 7ºE (EUTELSAT 1-F2)

• Calculate– EIRP of earth station– Free space loss– Received power


Noise in an Earth Station

– Noise comes from:• Ta= picked up by antenna from outside ( =effective noise)• Tf= lossy feeder• TLNA, TIPA= amplifiers in receiver chain• TD/C= down converter

– Refer all noise to a reference plane into the LNA

kB

Pa

DEMODBASEBAND

QoS(BER)

LNA IPA

rf if

G/T Ref

Lo

DOWN CONVC/NOD

Ta Tf

Ts

TLNA TIPA

TD/L


Noise in a Payload

G/T Ref

Cu

D/C

C/Nou

• Noise comes from:– Antenna received noise –earth + galaxy– Feeder lossy noise (nb.290K)– Equipment noise –amps / D/C etc. added in same way as for earth

station.

CD

eirps


Noise Characterisation (1)


Noise Characterisation (2)


Noise contribution of an attenuator


B/K10Log(Ts)dL)(GaTG

L)dB - (G referenceat antenna ofGain

10log(g)GdB

α10log(l),LdB

...g x g

T

g

T T )Tf-(1 TaTs

a

IPALNA

CD

LNA

IPALNA

l1

D/CLNA IPA

Ref

Ta Tf TLNA TIPA TD/C

LD/C

LdB

xTal

1

)Tfl

(1

1

IPALNA

D/Cxgg

T

LNA

IPAg

TTLNA

GLNA GIPA

Earth-station system G/Tand noise temp.


Earth station antenna noise temperatureExamples (clear sky conditions)


Exercise (3) - Noise Contribution Budget

• Operating frequency = 12 GHz• LNA: TLNA = 150K, GLNA = 50dB• MIXER: TMX = 850K, GMX = -10dB• IF AMP: TIF = 400K, GIF = 30dB

• Calculate– Receiver effective input noise temperature TR

– Receiver noise figure


Exercise (4) - G/T of C-band earth station

• Dish=15m, n=70%• Ta=30K• Tf=290K• Loss f=0.5dB• TLNA=35K• GLNA=30dB• FIPA=3dB• GIPA=20dB• TD/C=1000K• Loss D/C=-10dB

• Calculate the earth station G/T– What are the advantages of trading off dish size and LNA

temp.?

D/CLNA IPA

Feeder if


Propagation-Effects to be considered-

• Radio noise• Ionospheric effects

– Absorption– Total electron content effects (group delay, refraction,

polarisation rotation)– Scintillation

• Tropospheric effects– Attenuation by rain– Depolarisation– Refraction effects

• Shadowing and multipath effects


• Any ATTENUATION process which involves energy absorption is associated with THERMAL NOISE GENERATION from the medium

• Absorption by atmospheric gases is frequency dependent, hence clear sky noise temperature exhibits similar variations with frequency

Clear Sky Noise Temperature


• See CCIR Rep.719 for a detailed description of practical techniques of calculation for LAG. The following curve displays AAG(E) versus frequency; E is the elevation angle.

Attenuation by atmospheric gases


Noise temperature of the sun


Ionospheric effects


Attenuation due to rain, etc.

• Mist

• Clouds

• Snow

• Ice


References for calculation methodology

• Course notes or chapter 8 of the book

• ITU-R PN 618-3 splant path rain induced attenuation and depolarisation and scintillatin

(available from lending libraries or ITU, Geneva)


Attenuation due to precipitation and cloudsRelevant techniques described in CCIR (see rep.563, 564, 721, 723)


Contours of RAINFALL RATE

R₀․₀₁ (mm/h) exceeded for 0.01% OF AN AVERAGE YEAR:

Maps of rainfall contours (1/3)


with circular polarization use the arithmetic mean of attenuation with horizontal and vertical polarization

Nomogram for determination of specific attenuation


Comments:30/20 GHz systems face a problem, especially in tropical regions where rainfall rate is very high during small percentage of time.Performance objective must be achieved when rain occurs. The link will probably be over dimensioned during most of the time (margin).

Typical values of rain attenuation


DEPOLARISATION

• Rain and ice cause this due to shape of particles– Need to know shape and orientation of particles– Linear and circular POLN different– Circular POLN is worst case– Can form a model linking depolarisation (XPD)

and attenuation


XPD STATISTICS


Raininduced XPD circular polarisation(for 1% worsth month)

CO-POLAR ATTENUATIONdB


Other tropospheric effects

• Snow– Dry snow –ok (little effect)

– Wet snow –as bad as rain

– Problem if snow builds up on antenna

• Atmospheric absorption– Gas and particle absorption (worse at low

elevation angles)


Noise Contribution Budget-Satellite Antenna Noise Temperature-(1)


Noise Contribution Budget-Satellite Antenna Noise Temperature-(2)


Influence of Rain


Noise


Exercise (5.a) - Carrier Transmission Budget

• Ta=50K, =0.9, Tf=290k• Pointing loss = 0.7dB• Atmospheric loss = 0.3dB• Rain loss = 3dB for 99% lime• Rain temp = 275K

• Calculate the G/T of the earth station under worst weather conditions

• Calculate the down link C/No• Calculate the down link C/N if the link bandwidth is 100KHz• Complete the link budget sheet


Exercise (5.b) – Link budget sheet

• Link budget sheet – Downlink

Satellite EIRP dBW

Pointing loss dB

Atmospheric loss dB

Rain loss (99%) dB

Free space loss dB

Gain E/S dB

Downlink carrier dBW

E/S noise temp. dB-K

E/S G/T dB/K

Boltzmann constant -228.6 dBW/Hz/K

Downlink noise density (NOD)

dBW/Hz

C/NOD dB-Hz


DOWNLINK


UPLINK


Up Path


Exercise (6) – Up-link

• Ku-band uplink 14GHz

• Dish size=5m, =0.65

• Distance to satellite=38,000km

• Uplink atmospheric loss=0.3dB

• Uplink pointing loss=0.7dB

• Uplink rain loss=3dB

• Tx e/s w.g. feed loss=3dB

• Satellite Rx antenna gain=26dBi (from Tx e/s)

• Satellite Tx antenna gain=25dBi (at boresight)

• Rx earth station AR=-2dB

• Satellite Tpdr gain=120dB

• The satellite transponder has a single carrier saturation condition PFDi=-76dBW/m2, eirp sut=50dBW

• The transponder’s is operated at 8dB input and 5dB output back off

Calculate1. The uplink HPA rating if this has to operate at 6dB back off

2. The uplink carrier at the satellite Cu

3. The downlink carrier eirp from the satellite

SAT

120dBCu

eirps


• The order of any intermodulation product is defined as ‘(n+m) where the IMP’s are:

where n, m= 1,2,3,…

• When the center frequency of the amplifier is large compared to its bandwidth, odd-order intermodulation products are the only ones falling within the useful frequency band

• Intermodulation product power decreases with the order of the product. So only third and fifth order intermodulation products are concerned

Characteristics of intermodulation products

)21( fmfn


Link Performance-Intermodulation noise

Intermodulation products may appear at:- the output of the transmitting earth station non linear power amplifier- the output of the satellite repeaterThese intermodulation products can interfere with the desired carriers, and hence be considered as noise called “intermodulation noise”.With modulated carriers, the intermodulation noise is distributed over the entire frequency band.

Example: Intermodulation noise spectrum for a typical TWT with 10 carriers. (6 central carriers modulated by a multiplex of 24 telephone channels, two 64 channels carriers and two 132 channels carriers)

Intermodulation products can be considered as filtered white noise with constant spectral density (No)IM


Intermodulation

• (nf1 mf2), order = (n+m)

• IMP’s vary (order-1)dB/dB with carrier.

– E.g. 3rd – 2dB/dB 5th – 4dB/dB

• Payloads –linearisers to reduce IMP’s

• Ku-band transponders– TV– 3rd order important

• L/S-band transponder –1000’s small mobile carriers. 5th and 7th important


Total Link Operation(link from earth station to earth station)

1 – LINEAR OPERATION: PT‹(PT)max’ (NO)IM = O

Repeater power gain GS is constant. Satellite transmitter output power is shared between:- amplified carriers- amplified input noise

2 – SATURATION REGION OPERATION:

Available power from satellite repeater is limited.

Output power is shared between:- amplified carriers- amplified input noise- intermodulation products

Power gain value depends on operating point.


Total Link Budget-Non linear operation-


Interference


Interference Management (1)

• Between Satellite and Terrestrial systems –limit PFD’s satellites and terrestrial Tx’s

• Radio Reg’s –Appendix S7• 1st stage

– Calculate coordination contour– Calculate all Tx’s inside contour

• 2nd stage– If needed– Detailed calc’s using all parameters– Site shielding– Energy dispersal etc.


EARTH STATION: MADLEYSAT: INTELSAT 5 LONGITUDE 3415RX FREQUENCY: 4.18 GHZ

- CO-ORDINATION CONTOUR- MODE 1 CONTOUR

Radio regulations(Appendix S7)


Interference


Interference Management (2)

• Between satellite networks• Radio Reg’s –Appendix S8• Analyse noise increase ΔT6% (otherwise go to second

detailed stage)

S1 S2

I

C

E1 E2


Total Link Budget-Non Linear Operation-

Total noise at receiver input = uplink retransmitted noise + intermodulation noise + downlink noise:

1 – Assuming that all incoming carriers at repeater input have SAME POWER(as with controlled uplink power FDMA for instance):

(C/N0)T-1 = (C/N0)U

-1 + (C/N0)D-1 + (C/N0)IM

-1+ (C/I0)U/D-1

2 – If incoming carriers at repeater input DO NOT HAVE SAME POWER:

There is a CARRIER SUPPRESSION EFFECT: large power carriers tend to suppress small power carriers.

Generally speaking:

- for SMALL POWER carriers:(C/N0)T is smaller than in the case of equal power carriers.

- for LARGE POWER carriers:(C/N0)T is larger than in the case of equal power carriers.


Exercise (7) – Link Performance

• Given– Uplink carrier power-to-noise power spectral density: (C/No)U=85dB(Hz)

– Downlink carrier power-to-noise power spectral density: (C/No)D=83dB(Hz)

– Carrier power-to-intermodulation noise power spectral density: (C/No)IM=87dB(Hz)

– Uplink carrier power-to-interference power spectral density: (C/No)I,U=90dB(Hz)

– Downlink carrier power-to-interference power spectral density: (C/No)I,D=90dB(Hz)

– Noise equivalent bandwidth of earth station receiver: BN=5MHz

• Calculate– Overall link carrier power-to-noise power spectral density: (C/No)T

– Overall link carrier power-to-noise power ratio: (C/N)T

SATUplink interference

Downlink interference

Uplink path Downlink path


Total Link Budget-Non linear operation with interference


Figure 6.11 Gains and losses in power of signals being relayed by satellite. The signal falls to about one hundred billion billionth of its strength (10-34) on each of its 25,000-mile journeys through space. This great loss is balanced by the grains of the antennas and amplifier


UplinkEIRP

C/NouSatellite

C/IM oC/IU

C/ID

C/ND

Sat. Link Performance

C/NTOT

Requirements for QoS

C/No.Req

Margin =C/NTOT – C/NoReq

If Not 2-3 dB

Increase EIRPAnd Repeat

Setting out Link budgets


Types of Objectives

• SIGNAL QUALITY OBJECTIVES:

In terms of thresholds which must not be exceeded for more than a given percentage of time.

• SYSTEM AVAILABILITY OBJECTIVES:

Asys = (required time – down time) / required time

Required time = period of time during which the user requires the link to be in condition to perform a required functionDown time = cumulative time of link interruption within the required time

Interruption is a period in which there is a complete or partial loss of signal, excessive noise, or a discontinuity or severe distortion of the signal.

• PROPAGATION TIME:

The overall link propagation time should not overstep a maximum value depending on the user’s requirement.


System Availability

SYSTEM AVAILABILITY ASYS implies that the quality objectives be met during a given percentage of time (typically between 99 and 99.9%).

This requires the link C/N0 ratio to be larger or equal to a given value for the considered percentage of time.

C/N0 varies according to:- propagation effects (mainly influence of rain)- implementation losses (mainly antenna depointing or equipment failure)

ASYS = ATX Asat Alink ARX

where:ATX = transmitting earth station availabilityAsat = satellite availabilityAlink = link availabilityARX = receiving earth station availability


RAIN INDUCED attenuation and depolarization can reduce the C/N0 value, and cause link outage.

A LARGER MARGIN value leads to a HIGHER LINK AVAILABILITY, as C/N0 will understep the required value during a shorter time interval.

Link Availability


System cost increases rapidly with system availability:

CUSTOMER SHOULD NOT ASK FOR TIGHT SPECIFICATIONS, UNLESS STRICTLY NEEDED.

Cost of System Availability


PERFORMANCE: - BIT ERROR RATE- BANDWIDTH

Digital Transmission Techniques-System Model-


Time Division Multiplexing

Digital signals are organized in bursts by means of buffers where bits are stored and then read at a higher clock rate.

Bursts are transmitted sequentially within time slots according to a time frame structure.


Time Division Multiplex Standards

(CCITT,Rec. G702, G732, G733):


Two types of encryption techniques:- stream cipher: each bit of the plain text is combined bit per bit with the keystream,- block ciphering: the plain text is modified block per block.

Data Encryption

DATA ENCRYPTION entails two aspects:- confidentiality: avoid access to message by an unauthorized party.- authentication: protection against someone changing the message content.


Scrambling

• At receiver side CLOCK TIMING for bit detection is extracted from DATA SYMBOL TRANSITIONS. Long data streams of 0’s and 1’s can result in the loss of data synchronization.DIGITAL DATA CSRAMBLING at transmitter side provides a data symbol transition probability close to 0.5. At receiver side DESCRAMBLING is performed to restore original data.

• SCRAMBLING also removes any periodic pattern in the baseband pulse train. Hence it CANCELS any DISCRETE LINE COMPONENT in the modulated RF spectrum. This offers better protection against overstepping the permissible level of radiated power flux density: scrambling is an ENERGY DISPERSAL technique.


Channel encoding


Decoding gain


Digital Speech Interpolation (DSI)

• A talker has an activity factor which is less than 1. By INSERTING BITS from another channel INTO PAUSES of a given channel, DSI compresses a number m of voice channels into a SMALLER number n of satellite channels.


Performance objectives

• Quality objectives for telephony (CCIR Rec.522)DIGITAL TRANSMISSION

– The BIT ERROR RATE (BER) should not exceed:a) 10-6, 10-minute mean value, for more than 20% of any monthb) 10-4, 1-minute mean value, for more than 0.3% of any monthc) c) 10-3, 1-second mean value, for more than 0.01% of any year

• Quality objectives for data transfer (CCIR Rec.614)– for 64 kbit/s channels as part of ISDN:

The BIT ERROR RATE (BER) should NOT EXCEED:a) 10-7, for more than 10% of any monthb) 10-6, for more than 2% of any monthc) 10-3, for more than 0.03% of any month

• No standard yet for bit rates in excess of 64 kbit/s. Likely 10-9 to 10-12.


Noise performance

• Problems– Additive white Gaussian noise– Co-channel interference

• Frequency re-use

– Adjacent channel interference• From other signals

– Intersymbol interference• Due to band limiting

– Phase noise• From carrier recovery


Which modulation?

• Power limited satellite use B/QPSK• Mobiles need OQPSK/MSK to avoid non linear amp problems• Bandwidth limited satellite –16 QAM etc.

PSK

FSKASK

BER

Eb/Nominimum power from satellite


Digital transmission techniquesM-PSK modulation


Carrier-to-noise power ratio at demodulator input

The noise equivalent bandwidth BN of their receiver is assumed to be matched to the modulated carrier bandwidth B


BPSK QPSK

• QPSK is half bandwidth of BPSK• PSD in QPSK is 3dB higher

Rs= Rb/2Rs= Rb

1

0

DECN

ERROR

Prob(Error) = BER

NoEberfcBER

AWGNSIGNAL

2

1

+/2

Vn=AWGN

VR

VR

-/2

2 x errorsBw=1/2 BPSKResult same

11

10 00

01

ERROR

NoEberfcBER

2

1

ERROR


Theoretical Bit Error Probability (BEP)


Bit Error Probability (BEP)

Useful values


Theoretical Bit Error Probability(BEP)


Carrier & Bit Error Probability (cont’d)


Demodulation of digital signalsCONCLUSION


M-PSK modulation Power spectral density


Matched filtering


Bit Rates and Bandwidth

• Example

– What is Rb, Rc and Rs?– If filtering is Rc, =0.5, what is the bandwidth?

PCMcoder

=1/2FEC coder

QPSKMODRb Rc Rs


Spectral efficiency


Exercise (8) – Digital Transmission Techniques

• Given– Type of modulation: coherent QPSK (spectral efficiency r = 1.5 bits/s.Hz)

– Received information bit rate: Rb = 36 Mbit/s

– Required BER = 10-5

• Calculate required value of C/No and bandwidth:– 1. Without coding– 2. With coding (code rate = 3/4)– Could you use a smaller code rate (for instance 2/3 or 1/2)?

satellite transponder bandwidth B=36MHz

Free space loss L=200dB

G/T=14dB(K-1)receiver bandwidth BIF

PT=10W

GT=30dBEIRP=40dB(W)

C/No = EIRP x 1/L x G/T x 1/k


Power/Bandwidthtrade off and coding

• Power and bandwidth limitations are complimentary

• A transponder has finite bandwidth BT and hence traffic limit = BT/BC (BC = channel spacing)

Bandwidth limit case

• A transponder has fixed power so can only support ‘n’ channels at a given QoS (ber)

– Traffic limit (PTPD-Pc)=10log(n) (Pc=power per channel)

Power limit case

• Coding allows trade-off between bandwidth and power to optimise throughput

cnc No

Eb

No

EbPower

1RbB


Exercise (9) – Overall Link Budget

• The attached link budget has been calculated for a 9.6 kb/s email service for a VSAT into a hub. The modulation used is BPSK and the coding gives a coding gain of 4dB. The desired quality of service is a BER of 10-6.

• The hub available has the following parameters:– Antenna diameter = 4m– Efficiency = 65%– Feeder loss = 0.5dB– Skynoise = 50K– LNA temp = 75K– Rain temp = 275K

• Fill in the missing components of the budget.


Exercise (9) cont’d


Exercise A – Earth Station

• An INTELSAT-A (4/6 GHz) earth-station is required to transmit two IDR carriers (eirp = 65 dBW/carrier). The specification of the station is:– G/T >35 dB/K at 5° elevation– 2 carrier IDR, intermodulation eirp is not to exceed 10dBW/4kHz.

• A list of major available equipment is shown in Table next page.• The earth-station waveguide feed loss is 0.5dB on receive and 3dB on

transmit. All components beyond the LNA can be neglected for noise calculation purposes and all antennas have a 70% efficiency. The output back-offs of HPAs can be taken as 10dB for multicarrier operation and the IMP eirp is given by;

• E12 = 2E1 + E2 – 2(GTX – LTX) + D - SF• Where:

– (GTX – LTX) is the effective atenna gain at the HPA flange.

– D = (-2.PSAT – 28 + 2 x BOo)dBW2.– SF (Spreading Factor) = 20 dBW/4kHz.

• Calculate the minimum cost earth-station configuration to meet the specification.


Exercise A – Earth Station (cont.)

• List of equipmentEquipment Cost (£k)

Antenna

13.0 m

15.5 m

18.0 m

Noise Temp

30 K

25 K

20 K

160

200

300

HPA

125 W

400 W

700 W

50

60

80

LNA

33 K

55 K

80 K

30

6

3


Exercise B – Payload

• Figure A shows a 20/30 GHz payload with specification– G/T 16.5 dB/K– eirp 50 dBW– C/I3 20 dB

• The input power at the antenna receive terminals is –111 dBW. There is an input feeder loss of 1dB at a temperature of 75K.

i. Determine the noise specification for the payload.ii. 20 GHz LNAs of gain 10, 13 and 20 dB with noise figure of 3, 4 and 5 dB

respectively are available. Determine the LNA configuration to be used.iii. Estimate whether the noise specification is achievable.iv. Describe how you would check the linearity specification.

satellite communications a part 3

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