1. ECE 5233 Satellite Communications
Prepared by:
Dr. Ivica Kostanic
Lecture 8: Satellite link design
(Section 4.1, 4.2)
Spring 2014

2. Outline Objectives of link design Elements of satellite link
Free space path loss equation
Signal to noise ratio and link capacity
Examples
Important note: Slides present summary of the results. Detailed derivations are given in notes.

3. Objective of a link analysis
Link analysis determines properties of satellite equipment (antennas, amplifiers, data rate, etc.)
Two links need to be planned
Uplink – from ground to satellite
Downlink – from satellite to ground
Two way communication – 4 links (two way maritime communications)
One way communication – 2 links (example – TV broadcast)
Two links are not at the same frequency
Two links may or may not be in the same band
Fixed / broadcast satellite services – usually same band
Mobile satellite services may use different bands
In some systems satellite links may be combined with terrestrial returns
One way communication
Two way communication

4. Elements of a satellite link
Transmit power
TX antenna gain
Path losses
Free space
TX/RX antenna losses
Environmental losses
RX antenna gain
RX properties
Noise temperature
Sensitivity (S/N and ROC)
Design margins required to guarantee certain reliability
Note: satellite signals are usually very weak – requires careful link budget planning

5. Free space path loss – transmit side
Free Space Path Losses (FSPL) due to dispersion of EM wave energy
Antenna used to focus the energy of the wave in the direction of the receiver
Note: antenna gain is usually quoted in the direction of radiation maximum. For other direction need to use the actual radiation pattern
Power flux in the direction of maximum radiation

6. Free space path loss – receive side
Received power
Using
One obtains
Effective antenna gain (effective aperture)
FSPL equation
hA – aperture efficiency of the antenna (50-90%)

7. Free Space Path Loss (FSPL)
Equation for FSPL (linear)
R = distance between TX and RX
l = wavelength of the RF wave
Equation for FSPL (logarithmic) – Friis’ equations
Notes:
FSPL grow 20dB/dec as a function of distance
FSPL grows 20dB/dec as a function of frequency
FSPL curves are straight lines in log-log coordinate system
For Geo-Stationary satellites – loss may be above 200dB!
FSPL curves 1-32GHz range

8. Additional losses Link equation AL – additional losses
Misalignment of the antennas
Atmospheric losses
Radome losses
Component mismatch losses
The additional losses are taken into account through appropriate design margins
Typical design margin 5-10dB
Component accuracy
Operating frequency
Required reliability
Link equation
AL – additional losses

9. Shannon capacity formula
Shannon capacity formula – establishes fundamental limits on communication
In the case of AWGN satellite channel
Minimum energy per bit normalized to noise power density that is required for a given spectrum utilization
C – capacity of the channel in bits/sec
B – bandwidth of the channel in Hz
S/N – signal to noise ratio (linear)
Note: g is the fundamental measure of spectrum utilization. Ultimate goal of every wireless communication system is to provide largest g for a given set of constraints.
Define g = R/B - bandwidth utilization in bps/Hz, where R is the information rate in bps.

10. Bandwidth utilization vs. power trade-off
Bandwidth utilization increases with an increase of available power
In power limited regions small increase of power produce significant increase in bandwidth utilization
In bandwidth limited region large power increase is required for increase in bandwidth utilization
For systems that are in bandwidth limited region – capacity is increased through frequency reuse
By combining power and reuse methods, contemporary systems reach spectrum utilization of 3-7bps/Hz
Note: most of contemporary satellite systems are bandwidth limited – lot of efforts invested in means for spectrum reuse

11. Examples
Example A satellite at a distance of 40000km from a point on Earth surface radiates power of 10W into antenna gain of 17dB. Find the flux density on the Earth surface and the power received using antenna with effective aperture of 10 square meters. Answers: Flux density: 2.49e-14 W/m2 Received power: -126dBW (-96dBm)
Example The satellite in Example operates at a frequency of 11GHz (Ku band). The gain of the receiving antenna is 52.3dB. Find the received power. Answer: Received power: -126dBW