Link Budget in Satellite Communications

Slide Number 1Rev -, July 2001

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

Volume 5



Vol 5: Link Analysis

Section 1

Getting Started




What is a Link Budget?Part 1

5.1: Getting Started



Part 1: What is a Link Budget

5.1.1.1: Definition

Vol 5: Link Analysis, Sec 1: Getting Started

DefinitionLink analysis, or budgeting, is the process of calculating the carrier power levels to be transmitted from the Earth Station and the satellite in order to provide the required overall carrier- or signal-to-noise ratio at the receive end of the link.

The resultant carrier power levels will depend on the type of service, the coding rate, the Earth Station locations, and the satellite’s characteristics.

A Link Budget is an engineering model that follows the path of a radio signal from source to destination. It ties together the signal level, interference components, and noise contributions of both the uplink and downlink side of a satellite link.

The terms “Link Budget” and “Link Analysis” will here be used interchangeably.



Link Analysis GoalsLink analysis provides an opportunity to:

• Calculate the transmit power levels required from both the satellite and the Earth Station.

• Balance satellite power and bandwidth utilization, thus reducing space segment costs.

• Allocate sufficient link margin to provide an acceptable propagation availability.

• Account for interference, path losses, and other variables.

• Determine hardware requirements.

• Optimize system design in terms of trade-offs between Earth-segment and space-segment costs.

Part 1: What is a Link Budget

5.1.1.2: Goals





Minimum Required InformationPart 2




Sec 1: Getting Started

5.1.2: Minimum Required Information


PreambleLink budgeting is done when there is a specific need to implement a customer’s service on a given satellite.

Engineers tasked with link analysis can be faced with very vague customer requirements, or very specific ones.

In either case, part of the process should be the completion of a questionnaire designed to determine the minimum information required to get started.

The required information can be divided into three categories:

The engineer must interface with the satellite provider’s traffic planning group for the satellite-specific information.

The customer’s performance criteria. The geographic location of the proposed Earth Stations. Satellite-specific information.



PreambleDifferent types of services carry different performance criteria.

The primary division between types of service lies between analog and digital.

A secondary division to consider is whether the service is to carry voice, data, or video.

In all cases, the customer must also specify an availability figure.

Depending on the service type, the customer may specify other performance requirements. These additional parameters are usually encompassed by the primary ones considered here.

Ultimately, each link is unique and must be designed on a case-by-case basis.

Part 2: Minimum Required Information

5.1.2.1: Desired System Performance




GeneralAnalog voice and television services are usually specified in terms of the baseband signal-to-noise (S/N) ratio necessary for some final, target quality of service.

The satellite service provider will turn this S/N requirement into a carrier-to-noise (C/N) or carrier-to-noise density (C/No) value.

The resultant C/N value is the performance threshold value at which the required S/N threshold is met.

However, note that the final performance threshold to be selected does not necessarily occur at the same carrier to noise density ratio as other threshold effects. It is defined simply as the value required to provide the quality and service availability needed for that link.

Sub 1: Desired System Performance

5.1.2.1.1: Analog Systems

Vol 5: Link Analysis, Sec 1: Getting Started, Part 2: Minimum Required Information



5.1.2.1.1.1: Analog VoiceQualityThree broad, voice quality categories are recognized to exist:

Orderwire: Intelligible but noisy, and possibly unnatural sounding. The speaker may not be recognized even if known to the listener.This service quality is provided at low cost and is used only for infrequent maintenance activities.

Field: Offers a low signal to noise ratio, and probably reduced audio bandwidth, but the speaker is easily recognized.

It is used for some private line or military services. Toll: This is a high quality service offering low noise and low

distortion.It is used for standard domestic and international telephone circuits.









5.1.2.1.1.1: Analog VoicePerformance Threshold vs. Analog TechniqueThe following are examples of several analog voice transmission schemes, their relative qualities, and the C/No ratios required to carry them.

Modulation Technique Performance Threshold Quality(C/No in dB-Hz)

Single Channel Per Carrier 53 Toll(SCPC-FM)

49 Field/Orderwire

Frequency Division Multiplex Varies* TollFM (FDM-FM)

SCPC Amplitude Companded 45 FieldSingle Sideband (ACSSB)

* Depends on the number of circuits and the RF bandwidth






5.1.2.1.1.2: Analog VideoBecause of its extensive satellite capacity requirement, analog video service is being phased out in favor of digital video.

Analog video performance thresholds are specified by the signal-to-noise ratio of the baseband signal.

The signal-to-noise ratio required at the receiving Earth Station depends on the use to be made of the signal.

The following table lists the video signal-to-noise ratios typically used in system designs:

Service S/N ratio (db)Direct to home 40 - 45Remote reception 45 - 48Cable head ends 48 - 50Network feeds 52 - 54



GeneralThe principal quality indicator in all digital systems is the baseband bit error ratio (BER).

BER is a unitless measure, being the ratio of errored bits to total bits being received from a communication medium.

BER performance is related closely with received Eb/No and, consequently, with link C/N.

Since BER captures only error magnitude and tells nothing about error character, other quality measurements are sometimes called for.

Chief among these is the concept of error free seconds (EFS), which can give some indication of the frequency of error occurrence: bursting or dribbling.


5.1.2.1.2: Digital Systems




5.1.2.1.2.1: Digital VoiceThe following are examples of several digital voice encoding schemes, their relative qualities, and the bit rates required to transmit them.




Encoding Technique Bit Rate (kbps) Threshold BER Quality

PCM 64 10-5 Toll

ADPCM 32 10-5 Toll

Delta Modulation 64 10-3 Near Toll

Other Proprietary 8 to 16 10-4 Good FieldSystems Quality

Linear Predictive 2.4 to 9.6 10-3 or 10-4 Field / TollCoding Quality

Digital Multiplex 32 per Voice 10-5 TollADPCM Channel



5.1.2.1.2.2: Digital VideoFor digital video, subjective evaluations are required to determine a bit rate that provides adequate quality for the application.

This bit rate may vary from one manufacturer’s hardware to another’s.

Typical bit rates are in the 3 Mbps to 10 Mbps range, with 4-6 Mbps most common.

Based on the use of Viterbi FEC and concatenated Reed-Solomon coding, the typical system will require an Eb/No of about 5 to 7 dB, depending on FEC rate.

Once the bit rate and performance threshold have been determined, the system design is the same as for any high speed digital link, whether SCPC or single carrier per transponder.






5.1.2.1.2.3: DataFrom the perspective of the satellite link, data is a series of digital 1’s an 0’s regardless of the protocol or framing method employed.

All that is necessary is to establish a threshold BER for the service and then design a link that will meet the EbNo requirements for that threshold.

A performance threshold of 1 error in 10 million bits, or 1 X 10-7, is typical in satellite communications when FEC is used. If concatenated Reed-Solomon coding is also used, the BER may be in the range of 10-9 to 10-11.

Satellite modems use forward error correction to achieve this level of performance while minimizing the satellite power required per carrier.






IntroductionThe availability of a service is specified as the percentage of the time that information can be sent from end to end with a quality that meets or exceeds the established performance threshold.

This is usually specified as a percentage of a year, although percentage of the worst month can also be used.

Typical values are in the range of 99% to 99.95% per year.

After the service availability has been determined, the allowable outage time must be allocated among the following effects:

Atmospheric Propagation Effects Hardware Failures Sun Transit Occurrences


5.1.2.1.3: System Availability


Prime Power Failures Maintenance Activities Non-Satellite System Outages



5.1.2.1.3.1: Service AvailabilityThe availability specification has a direct effect upon two major design parameters:1. The propagation availability determines the satellite link

margin.2. The hardware availability determines the level of equipment

redundancy and the sparing and maintenance philosophies.These two availability specifications must be jointly defined so that each has an appropriate effect on the overall service availability.Propagation effects depend primarily upon the transmission frequency, local weather conditions, and antenna elevation angle. The contribution to outages is small at C-Band, but quite significant at Ku-Band in high rain-rate areas.






5.1.2.1.3.1: Service AvailabilityThe availability of the hardware is given by:




MTBF

MTBF + MTTRAVAILABILITY =

In the formula, MTBF, the Mean Time Between Failure, is a function of the MTBF of each component—available from manufacturing data or from experience with given units—and the level of redundancy employed in the design.

MTTR, the Mean Time To Restore, is a function of:• Site staffing• Remote monitor, alarm and control facilities• Travel time to the site if not staffed• Spare unit availability (on-site or shipping time)• Time to repair after arrival on site

EQ. 5.1.2.1.3.1 Availability



5.1.2.1.3.2: Outage AllocationsSun TransitReceiver outages due to high noise levels from the sun are known in advance as to their times of occurrence and duration.

These outages may be included in the calculation of availability, or with the user's agreement they could be excluded, since the effects are totally predictable.

The duration of the outage is a function of the antenna diameter, the frequency band, and the amount of link margin allocated.

The amount of degradation is also a function of the Earth Station G/T, or the normal receive system noise temperature. Low noise systems suffer more degradation than high noise systems.






5.1.2.1.3.2: Outage Allocations

C-Band Typical Sun Transit Degradations on the Worst Day




ANTENNA PEAK MINUTES OF DEGRADATION > 3 dBDIAMETER (M) DEGRADATION (dB) SINGLE EVENT ANNUAL TOTAL

16 26 4 1912 25 5 307.3 23 7 504.5 20 10 1003.7 19 12 1442.4 15 16 2641.8 13 19 350

S SY T = 7 0 KWhere



5.1.2.1.3.2: Outage Allocations

Ku-Band Typical Sun Transit Degradations on the Worst Day




ANTENNA PEAK MINUTES OF DEGRADATION > 3 dBDIAMETER (M) DEGRADATION (dB) SINGLE EVENT ANNUAL TOTAL

5.6 17 4 144.5 17 4 153.7 16 5 202.4 14 6 361.8 12 7 47

S SY T = 1 0 0 KWhere

The effect is less noticeable in noisier systems.



5.1.2.1.3.2: Outage AllocationsPrime Power FailureEquipment outages due to failure of an Earth Station’s prime power source are difficult to predict with certainty.

In urban centers, the power is usually quite reliable, and a specific outage allocation may not be required.

When a station is located on a user's premises, the user's equipment may also incur an outage when the power fails. Again in this case, since the user’s equipment is also non-functional during the outage, a specific allocation may not be required.

If the user’s availability requirements are high, user equipment may be protected by a UPS. As a general rule of thumb, if the user’s equipment is protected by UPS, the satellite equipment should be as well.






5.1.2.1.3.2: Outage AllocationsMaintenance ActivitiesSome outage time due to routine scheduled maintenance activity is inevitable.

If maintenance can be carried out during off-peak hours then the effect is minimal and an outage allocation for maintenace may not be necessary.System upgrades and expansion are usually not considered part of the outage allocation. These types of changes are planned in advance and can be scheduled to have minimal impact on the service.

Alternatively, they may be implemented without disruption to the service if redundant equipment is available.






5.1.2.1.3.2: Outage AllocationsNon-Satellite System OutagesThe end user may experience outages due to failure or disruptive activity involving other components of the network.

This is not of direct concern to the satellite system designer, however it is important to consider the possible impact on the design.

One of the most common causes of outages are failures of the backhauls connecting the end user to the satellite Earth Station.

It would be inappropriate to design a highly reliable satellite system if the backhauls were not of comparable availability.

Typically, no allocations are made explicitly for outages due to factors outside of the satellite system unless specific factors can be foreseen.






5.1.2.1.3.2: Outage AllocationsSummaryFor almost all services, except those of extremely high availability, outages are considered to come from propagation effects and hardware failures only.

The following is a list of typical services and their associated end-to-end availability.




FREQ AVAIL PROP HARDSERVICE REDUNDANT BAND OBJ. AVAIL AVAIL

VOICE TO REM YES C 99.40 99.95 99.45VOICE TO REM NO C 98.00 99.95 98.00PT-PT DATA YES Ku 99.90 99.95 99.95PT-PT DATA NO Ku 99.75 99.90 99.85VSAT DATA NO Ku 99.80 99.90 99.90FMTV (UPLINK) or DVC YES C or Ku 99.96 99.97 99.99



5.1.2.1.3.2: Outage AllocationsITU-R Recommendations on Performance and AvailabilityITU-R recommendation 614, Bit error ratio in ISDN:

< 10-7 for 10% of any month

< 10-6 for 2% of any month

< 10-3 for 0.03% of any month

ITU-R recommendation 579 - PCM telephony or ISDN unavailability objectives:

0.2% of a year for equipment

0.2% of any month for propagation






5.1.2.1.3.2: Outage AllocationsIntelsat’s Business Service Performance and Availability




CLEAR SKY THRESHOLD PROPAGATIONSERVICE TYPE PERFORMANCE PERFORMANCE OUTAGE

BASIC IBS BER < 10-8 BER = 10-6 0.04% of the year(C-BAND UPLINK)

BASIC IBS BER < 10-8 BER = 10-6 1% of the year(Ku-BAND UPLINK)

SUPER IBS BER < 10-8 BER = 10-6 0.04% of the year(Ku-BAND UPLINK)



Earth Station LocationIn addition to availability and performance criteria, the designer of a satellite link must know where the Earth Stations will be located.

This is, first, because the geographic location of the Earth Station dictates where in the satellite’s EIRP and SFD footprint the service will operate.

This has a direct effect on the resultant power levels that must be transmitted to establish a link which meets the required performance criteria.

Secondly, geographic location has an effect on link margins, since different sites lie in different rainfall zones.

Finally, if geographic location results in low elevation angles for Earth Station antennas, receive system noise will increase. This too must be accounted for when link budgeting.


5.1.2.2: Earth Station Location




5.1.2.3.1: What Satellite, Transponder, Frequency?The designer of a satellite link will interface internally with sales and traffic planning departments to determine which satellite, transponder, and frequency to use for a proposed link.

The choice of satellite and transponder is usually dictated by the type of service, since satellite traffic planning often allocates whole transponders, and sometimes whole satellites, to specific types of service.

There is more flexibility in the final choice of frequencies. As the design of the link coalesces and final choices are made concerning bandwidth and power requirements, a suitable frequency location must found on the satellite.

Alternately, if there is sufficient flexibility in other link criteria, the link might be specifically designed to fit a given frequency slot on the satellite.


5.1.2.3: Satellite Characteristics




5.1.2.3.2: Satellite EIRP, SFD and G/TThe transponder chosen to carry the service will exhibit unique EIRP, SFD and receiver G/T values.

These values will ultimately affect the power level that must be transmitted on the uplink to achieve the desired downlink power level.

Because these values may differ from transponder to transponder, if an existing link is moved to a different transponder, the link may need to be re-engineered.






5.1.2.3.3: The Input and Output Backoff RelationshipTypical satellite transponders are fixed gain devices: to get more power out, more power must be put in.

Over the transponder’s linear operating range there is a fixed relationship between the input, or drive, power, and the output power. This relationship changes over non-linear portions of transponder response.




INPUTO

UTP

UT

Transponders are carefully tested over their operating range and a transfer curve is produced. This curve graphically details the relationship between transponder input and output across the whole operating range. Figure 5.1.2.3. TWTA AM/AM Transfer Curve



5.1.2.3.3: The Input and Output Backoff RelationshipTransfer curves are traditionally expressed in power relative to the transponder’s maximum output power.

Since operating points will always be less than maximum, the expression backed off is used, giving rise the terms Input Backoff (IBO) and Output Backoff (OBO).

This transfer curve is alternately called the AM/AM Transfer Curve and the IBO/OBO Curve.

As with other transponder characteristics, this relationship differs somewhat from one transponder to the next. For this reason as well, if a satellite link is to be moved to a different transponder it may need to be re-engineered.






POWERAMPLIFIER

RECEIVER

Sub 3: Satellite Characteristics

5.1.2.3.4: Satellite Non-Linearity and Intermodulation Effects


5.1.2.3.4.1: Satellite Transfer Characteristics

Figure 5.1.2.3.4.1a: Transponder Functional Diagram

For the purpose of this discussion, the transponder can be simplified to a receive portion containing mostly passive devices and a power amplification portion.

The receiver portion of the satellite is usually specified to be very linear. The signal impairments that occur in the satellite are therefore caused primarily by the transmit power amplifiers.



POWERAMPLIFIERS

INPUTFILTERS

OUTPUTFILTERS

RECEIVER





Channelization

Figure 5.1.2.3.4.1b: Transponder Functional Diagram



The use of a single carrier in each RF channel is well suited to analog or digital television transmission and for occasional, very wideband data applications.

However, numerous applications involve small, low power carriers.

In that case it is not feasible to use a separate satellite channel for every carrier.

Some satellite channels will always be operated in a Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) mode.

For the moment, let’s focus on FDMA.







Whenever multiple carriers are used in a satellite transponder, special design considerations must be followed in order to minimize intermodulation products.

A tradeoff must be made between the satellite RF output power available and the level of intermodulation noise or distortion created.

However, FDMA systems have the advantage of allowing multiple access to the satellite by small Earth Stations, thereby enabling systems with very light traffic requirements to economically use the satellite.

The reduction in cost at these Earth Stations more than compensates for the reduced power efficiency in the satellite power amplifier.











INPUT POWER, dB RELATIVE TO SINGLECARRIER SATURATING POWER

100-10-20-30-20

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MULTIPLECARRIERS

Figure 5.1.2.3.4.1c: AM/AM Transfer Curves







Figure 5.1.2.3.4.1d: AM/PM Transfer Curves, TWT

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AM / AM AM / PHASE SHIFT

AM/AM

PHASE SHIFT







Figure 5.1.2.3.4.1e: AM/PM Transfer Curves, SSPA

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INPUT POWER, dB RELATIVE TOSINGLE CARRIER MAXIMUM POWER

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Figure 5.1.2.3.4.1f: AM/PM Transfer Curves, Linearized TWTA

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INPUT POWER, dB RELATIVE TOSINGLE CARRIER MAXIMUM POWER

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Figure 5.1.2.3.4.1g: Intermodulation Generation

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RELATIVE FREQUENCY50 60 70 80 904030 10025



Intermodulation-Free AssignmentsFor a small number of carriers assigned in a sufficiently wide band, it is possible to define a third order intermodulation-free assignment.

The basic requirement is to pick the carrier spacings so that no two spacings are alike.





1 2 4 5 8RELATIVE CARRIER SPACINGS

Figure 5.1.2.3.4.1h: An Intermodulation-Free Assignment



INPUT POWER, dB RELATIVE TO SINGLECARRIER SATURATING POWER Figure 5.1.2.3.4.1i: Total Intermodulation Power

010203040

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TOTAL INTERMODPOWER

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The intermodulation noise power shown on the previous slide is calculated based on the AM/AM and AM/PM data for the specific transponder amplifier.

An accurate intermodulation noise spectrum can be obtained by making calculations of this nature using specific input carrier information.

However, for a large number of modulated carriers randomly assigned across the bandwidth of an FDMA channel, the intermodulation noise power will be approximately uniformly spread across the bandwidth occupied by the modulated carriers.

For design purposes, this interference can be treated as a noise power density in the same way that thermal noise is included in designs.







Power density is calculated by dividing the total transponder output intermodulation power by the occupied RF channel bandwidth.




INPUT BACKOFFIBO (dB)

OUTPUT BACKOFFOBO (dB)

SATURATED CARRIERTO INTERMOD NOISE

DENSITY (dB-Hz)14 8.1 102.013 7.2 100.512 6.5 99.011 5.8 97.510 5.1 96.09 4.5 94.58 3.9 93.07 3.4 91.56 2.9 90.05 2.5 89.04 2.3 88.03 2.1 86.5




The following table shows typical data for a linearized TWTA. Note that the intermodulation power density is lower than for the non-linearized TWTA for the same OBO.




INPUT BACKOFFIBO (dB)

OUTPUT BACKOFFOBO (dB)

SATURATED CARRIERTO INTERMOD NOISE

DENSITY (dB-Hz)10 7.1 109.49 6.3 105.48 5.5 102.67 4.8 99.36 4.1 96.65 3.5 94.34 2.9 92.33 2.4 90.5




MULTICARRIER INPUT POWER, dB RELATIVETO SINGLE CARRIER SATURATING POWER

18 0246810121416

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TYPICAL SATURATED OUTPUT POWER TOINTERMODULATION NOISE DENSITY

FOR AN FDMA RF CHANNEL





Figure 5.1.2.3.4.1j: Total Intermodulation Power



The output from a computer program designed to calculate intermodulation power densities can be an invaluable design tool.

Here, the output from Telesat’s program, IMSHI, is used for demonstrative purposes.

Two TV CarriersThe first example shows the calculated intermodulation for two TV carriers sharing a 54 MHz RF channel.



5.1.2.3.4.2: Computer-Generated Intermodulation Examples








Frequency (MHz)20 30 40 50 60 70 80 90 100 110 120

-22.0-21.0-20.0-19.0-18.0-17.0-16.0-15.0-14.0-13.0-12.0-11.0-10.0-9.0-8.0-7.0-6.0-5.0-4.0

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Figure 5.1.2.3.4.2a Intermod Products of Two Unmodulated Carriers







Figure 5.1.2.3.4.2b Intermod Products of Two Modulated Carriers20 30 40 50 60 70 80 90 100 110 120

-127.0-125.0-123.0-121.0-119.0-117.0-115.0-113.0-111.0-109.0-107.0-105.0-103.0-101.0-99.0-97.0-95.0-93.0

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Frequency (MHz)35 40 45 50 55 60 65 70 75 80 85 90

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Figure 5.1.2.3.4.2c Intermod Products of Mixed Unmodulated Carriers







30 35 40 45 50 55 60 65 70 75 80 85 90-105.5-105.0-104.5-104.0-103.5-103.0-102.5-102.0-101.5-101.0-100.5-100.0-99.5-99.0-98.5-98.0-97.5-97.0-96.5

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Figure 5.1.2.3.4.2d Intermod Products of Mixed Modulated Carriers







-20 -15 -10 -5 0 5 10 15 20 25 30 35-57.5

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Figure 5.1.2.3.4.2e Intermod Products of 15 Unmodulated SCPC Carriers







-20 -15 -10 -5 0 5 10 15 20 25 30 35-122.5-120.5

-118.5-116.5

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-110.5-108.5-106.5

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Figure 5.1.2.3.4.2f Intermod Power of 15 Modulated SCPC Carriers





5.1.2.3.4.3: FDMA Satellite Channel Design Tradeoffs


UPLINK C/No

DOWNLINK C/No

C/Io

TOTAL C/No

CARRIERTO NOISEPOWERDENSITYRATIO

(dB-Hz)

0MULTICARRIER INPUT POWER, dB RELATIVETO SINGLE CARRIER SATURATING POWER

-10-20

Figure 5.1.2.3.4.3a: Carrier to Noise Power Density







0MULTICARRIER INPUT POWER, dB RELATIVETO SINGLE CARRIER SATURATING POWER

RFCHANNELCAPACITY

-10-20

Figure 5.1.2.3.4.3b: RF Channel Capacity



The optimum operating point occurs where channel capacity is maximum.

However, other considerations such as Earth Station HPA sizes, may move the optimum, in terms of lowest overall cost, toward lower uplink powers and slightly lower channel capacity.

It can be seen that the capacity is not very sensitive to variations in the power level. A typical operating point for an FDMA RF TWTA channel might be:

IBO = 9 db

OBO = 4.5 db

These backoff levels are defined relative to single carrier saturation.








What is to ComePart 3




Sec 1: Getting Started

5.1.3: What is to Come


What is to ComeIn Section 5.3 of this volume, you will be given a chance to design links and calculate link budgets.

These exercises will begin with the simplest point-to-point link and will progress to more difficult examples.

At first, you will use calculators to make calculations. Later, the ________________ program used by Arabsat for link budgeting will be introduced.

With this program, exercises of somewhat greater difficulty will be conducted.

Engineering

Link Budget in Satellite Communications