Lecture 1, General Principles

Presentation / Lecture 1, General Principles

Date Submitted: 06 June 2001

Written by RPC Telecommunications Ltd.. Website: http://www.rpctelecom.com

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This is the first in a series of general satcom tutorial lectures submitted by RPC Telecommunications.

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 Printable Version
-Section 1
The pioneers of satellite communications
-Section 2
Historical Perspective
-Section 3
INTELSAT & EUTELSAT
-Section 4
Advantages & Disadvantages of satellite communications.
-Section 5
Frequency bands & the Satellite Link.
Frequency Bands Allocated to the FSS
Frequency bands are allocated to different services at World Radio-communication Conferences (WRCs). Allocations are set out in Article S5 of the ITU Radio Regulations.

It is important to note that (with a few exceptions) bands are generally allocated to more than one radio service

  • Sharing
  • Interference
  • -> CONSTRAINTS

Bands have traditionally been divided into "commercial" and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilise "government" bands.

The Satellite Link
Radio Links:

Antenna Gain: 

g = (Power density on-axis at given distance) / (power density from isotropic source at same distance)

g = 4π.a/λ2

Antenna Aperture:

a = η.π.r2

Power density from transmit antenna at receive aperture is:

pden = (pt.gt)/(4π.d2)

Power collected by receive antenna:

pr = ((pt.gt)/(4π.d2)).ar.α = pt.gt.gr(λ/(4π.d))2

Path transmission loss ("free space loss"):

lp = ((4π.d)/λ)2

Thermal noise at output of receive antenna:

n = k.T.B

Carrier-to-noise ratio:

pr/nr = (pt.gt.gr.α)/(lp.k.T.B)

Hence "Link Budget Equation" expressed in dB:

C/N = Pt+Gt-Lp-La+(G/T)|r-k-10.log(B)

Note: |r means evaluated at r.

When calculating the link budget, we must take account of additional factors:

  • Interferences from other radio systems
  • Intermodulation noise
  • Additional loss, interference and noise due to propagation events (e.g. rain, rain-scatter, noise degradation, ducting, etc.)

Figure of merit (G/T) is a useful way of categorising the performance of a receiving system. 

In a constant RF field, different receiving systems (with the same receive bandwidth) will yield a C/N proportional to the system G/T.

EIRP is the usual way of combining transmit power and gain:

EIRP = Pt+Gt

So, the link budget equation is normally expressed as:

C/N = EIRP - Lp - La - Lm + G/T - 10log(B) + 228.6

The satellite link:

Uplink and downlink are analysed seperately as individual radio links:

Overall Link Quality:

C/N = C/(Nup+Ndown+Iup+Idown+Nimod)

(c/ntot)-1=(c/nup)-1+(c/ndown)-1+(c/iup)-1+...

 
 

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