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A HUGHES ELECTRONICS COMPASun Transit Outages for Ku-Band

Typical Sun Transit Degradations on the Worst Day

Because of the very large losses on the downlink from the satellite, the signals must be processed by a power amplifier before re-transmission. Because the power and mass a vailable onboard the satellite is limited, the amplifiers must be very efficient. The amplifier chosen is usually a travelling wave tube (TWT), although solid state amplifiers are becoming more popular for use at C-Band.To keep the efficiency high, the amplifier is operated at or near its output power saturation point.

The TWT amplifier exhibits a nonlinear response resulting in AM/PM conversion (amplitude variations at the TWT input cause phase variations on the signals at the TWT output) and intermodulation products when operated with multiple carriers. In addition, strong carriers will suppress the weak carriers when they share the same amplifier.

For a 3.7 m antenna, sun transit events occur on typically 2 or 3 consecutivedays twice a year. A total of 25 minutes outage time per year is equal to anavailability of about 99.995%. This factor is often insignificant compared toother events.


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A HUGHES ELECTRONICS COMPAChannelized Satellite Transmitter

In order to minimize the intermodulation and suppressing effects, thesatellite is channelized, which allows many of the channels to operatewith only a single carrier, thereby maximizing the power available fromthe TWTA. Because of the AM/PM conversion effects, the TWTA worksbest with constant envelope carriers such as those using frequencymodulation (television or FDM/FM voice channels).

RECEIVER

TWTAMPLIFIERS

OUTPUT

FILTERS

INPUT

FILTERS


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A HUGHES ELECTRONICS COMPA

Frequency Division MultipleAccess Channels

The use of a single carrier in each RF channel is well suited to TVtransmission and for high capacity data services, but for applicationsinvolving many earth stations, small low-power carriers are required. It is notfeasible to use a separate satellite channel for every carrier; therefore somechannels must be operated in a frequency division multiple access (FDMA)mode.

Whenever multiple carriers are used in a satellite transponder, special design

considerations must be followed to allow for the intermodulation productsthat are generated both within the RF channel and into adjacent RF channels.A tradeoff must be made between the satellite RF output power available andthe level of intermodulation noise or distortion created.

FDMA systems have the advantage of allowing multiple access to the satelliteby small earth stations, thereby enabling systems with very light trafficrequirements to use the satellite economically. The reduction in cost at theearth station more than compensates for the reduced power efficiency in thesatellite TWTA. These tradeoffs will be covered in detail later in the link designsections.


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A HUGHES ELECTRONICS COMPAFDMA Channel Characteristics

Advantages

Allows a variety of different traffic types to share the transponder efficiently (wide range of powers and bandwidths are allowed for thecarriers)

Low space segment cost for light traffic systems

Small low-cost earth stations can be used

Each station using the RF channel is independent

Circuits can easily be added to the each earth station and stations can easily be added to the RF channel

Major Disadvantage

Reduction of RF channel capacity due to the production of multicarrier intermodulation noise and due to the output power backoffrequired on the RF channel.


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Satellite TWTA Characteristics -AM/AM

The following curve shows the typical AM/AM response for a C-Bandsatellite TWT amplifier. Note that the saturated output power for multiplecarrier operation is less than for single carrier operation, and that the gainis reduced as the input power approaches saturation.

0

-10

-20

-30 -20 -10 0 10

MULTIPLE

CARRIERS

SINGLE

CARRIER

INPUT POWER, dB RELATIVE TO SINGLE

CARRIER SATURATING POWER

OUTPUTP

OW

ER,dBRELA

TIVETOSINGLE

CAR

RIERSAT

URATEDOUTPUT


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Satellite TWTA Characteristics -AM/PM

The following figure shows the typical phase shift of a C-Band satelliteTWT amplifier. This shows that variations in input power result in phasemodulation of the amplifiers output signals.

0

-10

-20

-30 -20 -10 0 10

PHASE SHIFT

AM/AM



OUTPUTPOW

ER,

d

BRELATIVE

TOSATURATION

20

30

50

0

10

40

PHASESHIFT(DEGREE)


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A HUGHES ELECTRONICS COMPAIntermodulation Generation

This figure illustrates the third order intermodulation products generated bythree carriers. For a large number of carriers, products like these aregenerated by every possible two- and three-carrier combination.

0

-10

-20

25 40 50

A+C-B B+C-A

RELATIVE FREQUENCY

OUTPUTdBRELATIVET

OSATURATION

30 60 70 80 90 100

-30

2C-B 2C-A2B-A2B-C2A-B2A-C

A+B-C

A B C


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A HUGHES ELECTRONICS COMPAIntermodulation Generation

It can be shown1 that the reduction in amplitude of the fifth and higher-order products, with an increasein number of carriers, more than compensates for the increase in the number of products, so that themulticarrier performance of a TWTA is essentially determined by the third order distortion even up tooutput power saturation.

1 REFERENCE

R. J. Westcott, Investigation of Multiple FM/FDM Carriers Through a TWT Operating Near to Saturation,Proc. IEE, Vol 114, No. 6, 1967.


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Intermodulation FreeAssignments

For a small number of carriers assigned in a sufficiently wide band, it is possible to define a third-order intermodulation-free assignment. The basicrequirement is to pick the carrier spacings so that no two spacings are alike.

For example, if each carrier requires one unit of bandwidth, and the first two carriers are spaced one unit apart, the third must be two units away from thesecond (three units from the first), the fourth carrier must be four units from the third carrier, the f ifth five units from the fourth carrier, etc.

In the above example, six carriers require 21 units of bandwidth rather than the 6 units that world be used if the carriers were assigned consecutively. Thismethod requires a great deal of bandwidth and becomes impractical for more than 8 or 10 carriers.

1 2 4 5 8

RELATIVE CARRIER SPACINGS


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A HUGHES ELECTRONICS COMPASatellite TWTA Intermodulation

This figure shows the typical multicarrier intermodulation noise output poweras a function of the total TWT input:

-50 -40 -30 -20 -10

TOTAL INTERMOD

POWER

MULTICARRIER

AM/AM



OU

TPUTPOW

ER,dBRELA

TIVE

TO

SING

LECARR

IERSATURATEDOU

TPUT

-30

0

-40

0

-20

-50


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FDMA Satellite ChannelIntermodulation

For a large number of modulated carriers randomly assignedacross the bandwidth of an FDMA channel, the intermodulationnoise power will be approximately uniformly spread across thebandwidth occupied by the modulated carriers.

For design purposes then, this interference can be treated as anoise power density in the same way that thermal noise isincluded in designs. The power density is calculated by dividingthe total TWTA output intermodulation power by the occupied RFchannel bandwidth.


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Saturated Carrier

to Intermod Noise

Density (dB-Hz)

102

100.5

99

97.5

96

94.593

91.5

90

89

88

86.5

Input Backoff

IPBO (dB)

14

13

12

11

10

98

7

6

5

4

3

Output Backoff

OPBO (dB)

8.1

7.2

6.5

5.8

5.1

4.53.9

3.4

2.9

2.5

2.3

2.1

The following table shows a typical C-Band transponderintermodulation performance:


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The following curve displays the information from the previous table in agraphical format.

-18

TYPICAL SATURATED OUTPUT POWER

TO INTERMODULATION NOISE DENSITY

FOR AN FDMA RF CHANNEL

MULTICARRIER INPUT POWER, dB RELATIVE TO SINGLE


SATURATED

OUTPUTPOW

ER

TO

NOISEDENSI T

Y(dB-Hz)

95

90

105

100

85

80

-16 -14 -12 -10 -8 -6 -4 -2 0


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Earth Station UplinkIntermodulation Noise

Similar to the satellite amplifier, the earth station amplifiers are not linear,and they generate intermodulation products when operated in amulticarrier mode.

The effect here is potentially much worse, because the earth stationamplifier output is often not restricted by a channel filter.

Since the cost of transmit power is lower on the earth, these amplifiersare operated with larger output backoffs than those on the satellite inorder to reduce the level of the intermodulation products.

Typical multicarrier mode values of output backoffs are 7 dB fromsaturation for a TWTA and 3 dB below the 1 dB GCP for a SSPA. Theseoutput backoffs will yield a two-carrier C/I ratio of about 25 dB. This is areasonable compromise between the cost of the amplifier and the level ofintermodulation products generated.


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Earth Station UplinkIntermodulation Noise

An allocation for this uplink interference must be made for any systemusing multiple carriers in the earth station amplifiers. Even if a particularlink uses only single carriers in the amplifiers, other earth stationssharing the RF channel in an FDMA system may cause uplinkintermodulation noise to fall on the carriers.

Therefore it is recommended to make an allocation for this interferenceinto all carriers in FDMA RF channel.

INTELSAT also provides a number of specifications for the permitted levelof out-of-band intermodulation products transmitted from an HPA. TheseEIRP densities range from 12 dBW/4 kHz at Ku-Band to 23.4 dBW/4 kHz at

C-Band depending upon the satellite in use.

These allocations are normally not required for single carrier RF channelssuch as television systems.


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Cross-Polarized RF ChannelInterference

The bandwidth available for satellite use at a specific orbital location is a limited resource. Most current satellites increase the available bandwidth throughfrequency reuse by means of orthogonally polarized RF channels.

Most satellites use linear polarization with half of the channels transmitting in the vertical polarization, and the other half in the horizontal polarization. Othersatellites may use left and right circular polarizations. In general, the satellite re-transmits a signal on the opposite polarization to that on which it was received.

The atmosphere can affect the orthogonal relationship of the two polarizations resulting in some cross talk between RF channels. The depolarization is causedby non-spherical water droplets, and the level depends on the rainfall rate, the droplet size, the raindrop orientation, and the frequency of transmission.

Circularly polarized signals are somewhat more sensitive to atmospheric depolarization, but the effects are negligible at frequencies below 10 GHz. C-Bandsystems can therefore ignore the effects of the atmosphere on polarization isolation.


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Cross-Polarized RF ChannelInterference

The antennas used on the satellite and at the earth stations should beable to meet cross-polarization isolation specifications of about 35 dB.

A typical cross-polarization isolation of better than 30 dB can usually bemaintained on an uplink or downlink including the effects of both theearth station and the satellite. Care must be taken to maintain proper

alignment of the earth station feeds to meet this level of performance.

The RF channel frequencies of the two polarizations are often offset bysome amount in order to minimize the effects of interference from highlevel signals into the opposite polarization (at C-Band, this is usually halfa transponder, or 20 MHz).

The spectral densities of the different types of traffic must be included inany interference analysis and in general it is preferable to assign similartypes of carriers on RF channels of opposite polarization.


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C-Band Satellite Frequency andPolarization Plan

1A

3720

5945

2A

3760

5985

3A

3800

6025

4A

3840

6064

5A

3880

6105

6A

3920

6145

7A

3960

6185

8A

4000

6225

9A

4040

6265

10A

4080

6305

11A

4120

6345

12A

4160

6385

Uplink Frequencies, Vertical Polarization

Downlink Frequencies, Horizontal Polarization

A

POL

1B

3740

5965

2B

3780

6005

3B

3820

6045

4B

3860

6085

5B

3900

6125

6B

3940

6165

7B

3980

6205

8B

4020

6245

9B

4060

6285

10B

4100

6325

11B

4140

6365

12B

4180

6405

Uplink Frequencies, Horizontal Polarization

Downlink Frequencies, Vertical Polarization

B

POL

G-8344 P 08/15/97


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Cross-Polarized InterferenceCalculation

The following pages illustrate the procedure for estimating the levels ofinterference between cross-polarized RF channels.

The examples are as follows:

Uplink interference from an FDMA RF channel Uplink interference from a TDMA RF channel

Uplink C/I for a TV carrier due to FDMA or TDMA on thecross-polarized RF channel


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Cross-Polarized InterferenceExamples

Uplink Interference from FDMA

Assume that cross-polarized interference occurs on the uplink from an FDMA RF channel on a C-Band satellite. If the FDMA channel is fullyloaded, the input backoff will be about 9 dB, and we will assume that the uplink carrier energy is uniformly spread over the 36 MHz bandwidth.

Based on a net uplink cross-polarization isolation of 30 dB, the ratio of saturating input power to cross-polarized interference noise powerdensity in the oppositely polarized RF channel is:

CSAT/IO = 0 - {-9 - 30 -10 LOG (36 x 106)} dB-Hz

= 9 + 30 + 75.6 dB-Hz

= 114.6 dB-Hz


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Cross-Polarized InterferenceExamples

Uplink Interference from TDMA

As another example, consider the uplink cross-polarized interference from awideband digital signal such as a TDMA carrier.

Assume the TDMA carrier operates the RF channel at saturation and that its

energy is uniformly spread over 30 MHz of bandwidth.

If the net uplink cross-polarization isolation is 30 dB, then as in the previousexample the ratio of saturating input power to cross-polarized interferencenoise power density in the oppositely polarized RF channel is:

CSAT/IO = 0 - {0 - 30 -10 Log (30 x 106)} dB-Hz

= 30 + 74.8 dB-Hz

= 104.8 dB-Hz


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Interference from FM TV intoFDMA

In most cases the interference noise power can be treated in the same way as thermal noise and added on a power basis with other noise and interferenceterms. Interference from FM modulated TV carriers into narrowband carriers, such as SCPC voice or data carriers, is a special case.

Because of the time varying nature of the FM TV signal, much of the RF carrier energy can be instantaneously concentrated in a very narrow bandwidth.This results in interference power densities that greatly exceed the RMS values derived by conventional analysis.

A number of measurements have been carried out on the effects of FM TV carrier interference into QPSK carriers by measuring the degradation in bit errorrate and calculating the equivalent interference noise power density that would produce the same degradation. By doing measurements at variousfrequency offsets from the TV carrier, an equivalent interference noise power density spectrum can be estimated for the TV carrier.


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Interference from FM TV intoFDMA Channel

The following graph summarizes measurements made for a typical C-Band modulated TV carrier with a peak deviation of about 9 MHz and hasbeen scaled for use with Ku-Band systems having a peak deviation ofabout 8 MHz. Note that the power density is shown per Hz relative to thetotal TV carrier power.

These envelopes can be used to estimate the interference effects of TVcarriers into narrowband carriers. Several MHz near the TV centerfrequency (more than just the energy dispersion bandwidth) mustnormally be avoided due to the high interference power density.

G S


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Equivalent TV InterferencePower Spectrum

POWER IN

100 kHz

RELATIVETO TOTAL

CARRIER

POWER

(dB)

Frequency Relative to Carrier Center (MHz)

0

Ku-BAND

-3.6 f -55.8 dB/Hz

C-BAND-2.7 f -53.8 dB/Hz

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Terrestrial Microwave SystemInterference

Fixed satellite systems currently operate in the following frequency bands:

North America

Uplink 5.925 - 6.425 GHz

Downlink 3.7 - 4.2 GHz

Uplink 14.0 - 14.5 GHzDownlink 10.95 - 11.2 GHz

11.45 - 11.7 GHz

11.7 - 12.2 GHz

Worldwide

5.845 - 6.425 GHz

3.62 - 4.2 GHz

14.0 - 14.5 GHz10.95 - 11.2 GHz

11.45 - 11.7 GHz

12.25 - 12.75 GHZ

In North America, the Ku-Band frequencies are dedicated to satellite

transmissions, and there is no mutual interference problem with terrestrialsystems. The C-Band frequencies are shared with terrestrial microwavesystems and therefore all earth station or microwave system installationsmust be made with appropriate allowance for interference as follows:

6 GHz earth stations into terrestrial stations

4 GHz terrestrial stations into earth stations


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Earth Station FrequencyCoordination

Required for 6/4 GHz systems

Ensures that the operation of the earth station and the terrestrial system are within specifications

Usually required by a government licensing authority

Coordination must be carried out with terrestrial and satellite system users within an appropriately defined

coordination zone

This coordination could involve users in other countries

Note that this is different from the international coordination of satellites, which is carried out through the International

Telecommunications Union (ITU).

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STEP 1

In North America, due to the very large number of terrestrial stations, the initialanalysis is carried out by computer. The program is administered by the FrequencyCoordination System Association, and it contains:

A master database of licensed terrestrial and satellite stations and theircharacteristics

Software for the calculation of interference levels Mutually agreed interference objectives (for earth stations this is typically a

desired C/I ratio)

This procedure is very conservative because it assumes:

A flat earth within the coordination zone No natural or man-made objects are in the path

The two systems are co-polarized

Guaranteed antenna pattern envelopes are used

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If the program predicts no interference, then the site is considered to be

interference free. If there are some interferences predicted, then proceed to...

STEP 2

Draw path profiles on topographical contour maps to assess theeffect of blockages due to hills, etc.

Carry out a calculation of the attenuation provided by the objects inthe path. This is usually done with the aid of computer programs.

This may clear the site of interference effects. If not, proceed to...

STEP 3

Carry out field measurements with a transportable antenna. This willassess protection provided by buildings and the fine structure of theantenna sidelobes.

If this is not successful in achieving the C/I objective, then shielding is thenext alternative.

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Site Shielding

Interfering signals may sometimes be blocked by the use of site shielding using:

Wire mesh screen

Earth berm

Solid wall built of concrete or steel

Shielding will yield the following typical signal attenuations:

Buildings 6 - 20 dB

Wire mesh screen 6 - 20 dB

Wall or earth berm 10 - 20 dB

If these techniques do not result in the C/I objective being met, the only alternativeleft is to relocate the earth station.

Basics of Link Engineering

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