Frequency Reuse Schemes in Satellite Communications

Slide Number 1Rev -, July 2001

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



Vol 3: Satellite Communication Principles

3.2. Frequency Reuse Schemes

Satellite design engineers use a variety of polarization schemes and beam patterns, and to a lesser extent, satellite co-location configurations, in order to increase the capacity of satellite transmission channels.

This permits the reuse of satellite transponder frequencies, which is a finite commodity in C- and Ku-Band satellite networks.

It is therefore an imperative design objective to implement frequency reuse schemes during the frequency planning phase, given the current congestion of geostationary satellites.

Introduction



Global C-Band FSS Satellite Frequency Assignments

3.4 3.6 3.7 4.2 4.5 4.8

U S D om sats , A s iaS a t1 , P A S -1 ~4 , In te lsa t V& V II, A n ik E , S o lid a rid ad

Insat 2A, 2B, 2C, 2D

Gorizont

JCSat 3

In te lsa t V I, A p s ta r 1 , 1 A & 2 R , A s iaS a t 2 & 3 ,E xp ress

Raduga

Palapa C1&C2, Thaicom 3, PAS-7

3.95 GHz

3.65 GHz

Frequency Assignments


Sec 2: Frequency Reuse Schemes

3.2.1. Frequency Assignments

Figure 3.2.1a. Global C-Band FSS Satellite Frequency Assignments Image Courtesy of Telesat Canada



Figure 3.2.1b. Global Ku-Band FSS & BSS Satellite Frequency Assignments

Global Ku-Band FSS & BSS Satellite Frequency Assignments 10.7 10.95 11.2 11.45 11.7 11.95 12.2 12.5 12.75

P A S -7 P A S -7 U .S FS SS A T E LLIT E S

U .S B S SS A T E LLIT E S

A S T R A1C A S T R A IE A S T R A 1F

H O TB IR D

E U T E LS A T 2

H O TB IR D

E U R O P E A N B S SS A T E LLIT E S /H O T B IR D

IN T E L-S A T

IN T E L-S A T

P A S -1B S A T

M E A -S A T 1

IN T E L-S A T

M E A -S A T 1 IN T E LS A T K A S IA /P A C IFIC FS S

S A T E LLIT E S

P A LA P A C 2 T H A I-C O M 2

A S IA -S A T 2

A S T R A1D

A S T R A1A

A S T R A1B

A S T R A1G

E U T E LS A T 2

E U T E LS A T 2

Image Courtesy of Telesat Canada






Frequency Assignments



Figure 3.2.1c. North America’s Fixed Satellite Service (FSS) Ku-Band Frequency Plan




Orthogonal PolarizationLinear Polarization

The signals transmitted by satellites such as EUTELSAT, Astra, PAS 4, Arabsat 1 D, INTELSAT (Ku band only) and AsiaSat are sent by an antenna aboard the satellite that positions the microwave energy in either a relatively vertical (straight up and down) or horizontal (lying flat) polarization.

In linear cross polarization schemes, half of the transponders beam their signals to Earth in a vertically polarized mode; the other half horizontally polarize their down links.

Although the two sets of frequencies overlap, they are 90 degree out of phase, and interference between them will be minimal.


3.2.2. By Orthogonal PolarizationSec 2: Frequency Reuse Schemes



Orthogonal PolarizationFor best reception of these signals, the pick up probe at the back of the waveguide must be oriented in the same plane, horizontal or vertical, as that of the desired satellite transponder.

To successfully receive and decode these signals on Earth, the Earth Station must be outfitted with a properly polarized feedhorn to select the vertically or horizontally polarized signals as desired.

In some installations, the feedhorn has the capability of receiving the vertical and horizontal transponder signals simultaneously, and routing them into separate LNAs for delivery to two or more satellite receivers.



3.2.2. By Orthogonal Polarization



Orthogonal Polarization



Figure 3.2.2a. Linear Polarization Satellites


Coverage footprint of a satellite with horizontal polarization.

The same coverage footprint with vertical polarization lines added in green.

The same frequencies may be used within the same footprint, provided they are separated by orthogonal (right-angled) polarization angle.




Circular PolarizationSome satellites, such as the INTELSAT (C-Band only), Arabsat 1 C, Gorizont and Express spacecraft, use an alternate polarization format known as circular polarization. For the best possible reception of circularly polarized satellite transmissions, a feedhorn that has been specifically constructed to receive these signals is necessary.

Instead of beaming the microwave energy along a linear plane, whether vertical or horizontal, circular polarization is transmitted in a helical rotating pattern, with right hand circular rotating in a clockwise direction as along the direction of transmissions, and left hand circular signals rotating in a counterclockwise direction.

Although standard linear feedhorns can still pick up any circular polarized signal, half of the available signal power will be lost.






Circular Polarization




As is the case with horizontal and vertical linear polarization, there is tremendous isolation between right- and left-hand circular polarization. This again allows two satellites, or one, to illuminate the same location on Earth with the same frequencies, without interference between signals.

Figure 3.2.2b. Circular Polarization Satellites




Exploiting Multiple Beams



Through the employment of narrow, focused spot beams, a frequency planner can reuse transponder frequencies and thereby increase the total bandwidth capacity of the satellite.

Figure 3.2.3. Exploiting Multiple Beams

3.2.3. By Exploiting Multiple BeamsIm

age Courtesy of Telesat Canada



There is a frequency reuse scheme being studied that would re-assign FSS and BSS Ku-Band spectrum to a satellite constellation referred to as Virtual Geosynchronous Orbit (VGSO).

The Virtual Geo constellation, though operating at non-geostationary altitudes, will appear virtually geostationary to users within the system’s coverage area.

VGSO systems use a specific pattern of inclined, elliptical medium earth orbits, which appear to “hang” over their service areas.

VGSO systems always have a minimum angular separation from the geostationary satellite arc of at least 40 degrees therefore there will be virtually no RF interference to and from geostationary satellites.

Virtual Geosynchronous Orbit



3.2.4. By Virtual Geosynchronous Orbit



This contrasts with geostationary orbits that rely on a complex pattern of satellite co-ordination and exclusion zones in order to avoid interference with each other.

VGSO systems can accommodate many non-geostationary satellite systems without complex administration and enforcement procedures.

A VGSO system consisting of 15 satellites can provide global coverage.







Some of the key features of VGSO satellite systems are:• Good satellite visibility

• Low signal propagation delays (compared to geostationary satellites)

• Limited satellite handoffs

• Distribution of capacity between the Northern and Southern Hemispheres

• No RF interference with geostationary satellites using the same Ku-Band FSS and BSS spectrum







Paired Carrier Multiple Access (PCMA) amounts to an Earth Station-local frequency reuse scheme.

In a typical full-duplex FDMA point-to-point link, two carrier frequencies are required, one for each transmit direction.

Employing PCMA, however, a full duplex link between two points can be established with one carrier frequency.

This is accomplished by having each transmitting location retain in memory a digital image of the transmitted carrier.

Each transmitting site, after appropriate delays, will receive, on the same frequency, both its own transmitted carrier and that of the other site.



Paired Carrier Multiple Access

3.2.5. By Paired Carrier Multiple Access (PCMA)



Each site now subtracts the stored image of its own transmission from the combined RF received carriers.

In theory, this would leave only the carrier that was sent by the other site, which could then be passed to the demodulator for processing.

In practice, however, the unwanted carrier cannot be fully subtracted. The remaining, undesired signal energy is treated as another source of C/Io, adding approximately 0.5 dB of noise to the system.

When this noise is properly accounted for in link budgeting, demodulators have no difficulty in demodulating the signal.



Paired Carrier Multiple Access

3.2.5. By Paired Carrier Multiple Access (PCMA)

Frequency Reuse Schemes in Satellite Communications

Engineering

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