ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 8: Satellite link design...

ECE 5233 Satellite Communications

Prepared by:

Dr. Ivica Kostanic

Lecture 8: Satellite link design

(Section 4.1, 4.2)

Spring 2014

Florida Institute of technologies

Objectives of link design

Elements of satellite link

Free space path loss equation

Signal to noise ratio and link capacity

Examples

Outline

Important note: Slides present summary of the results. Detailed derivations are given in notes.


Objective of a link analysis

Link analysis determines properties of satellite equipment (antennas, amplifiers, data rate, etc.)

Two links need to be planned

o Uplink – from ground to satellite

o 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

o Fixed / broadcast satellite services – usually same band

o Mobile satellite services may use different bands

In some systems satellite links may be combined with terrestrial returns

One way communication

Two way communication


Elements of a satellite link

Transmit power

TX antenna gain

Path losses

o Free space

o TX/RX antenna losses

o Environmental losses

RX antenna gain

RX properties

o Noise temperature

o Sensitivity (S/N and ROC)

Design margins required to guarantee certain reliability

Note: satellite signals are usually very weak – requires careful link budget planning


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

24 R

GPW TT

Power flux in the direction of maximum radiation


Free space path loss – receive side

Effective antenna gain (effective aperture)

AA Ae hA – aperture efficiency of the antenna (50-90%)

Received power

eTT

eR AR

GPAWP

24

Using

Re GA 4

2

2/4 R

GGPP RTT

R

One obtains

FSPL equation

2/4 RFSPL


Free Space Path Loss (FSPL)

Equation for FSPL (linear)

2/4 RFSPL

R = distance between TX and RX

l = wavelength of the RF wave

Equation for FSPL (logarithmic) – Friis’ equations

GHzlog20mileslog205.96 fdFSPL

GHzlog20kmlog2044.92 fdFSPL

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


Additional losses

Additional losses

o Misalignment of the antennas

o Atmospheric losses

o Radome losses

o Component mismatch losses

The additional losses are taken into account through appropriate design margins

Typical design margin 5-10dB

o Component accuracy

o Operating frequency

o Required reliability

ALFSPLEiRP RR GP

Link equation

AL – additional losses


Shannon capacity formula

Shannon capacity formula – establishes fundamental limits on communication

In the case of AWGN satellite channel

N

SBC 1log2

C – capacity of the channel in bits/sec

B – bandwidth of the channel in Hz

S/N – signal to noise ratio (linear)

Define g = R/B - bandwidth utilization in bps/Hz, where R is the information rate in bps.

02

02

1log

1log

N

E

BN

RE

B

C

b

b

Minimum energy per bit normalized to noise power density that is required for a given spectrum utilization

12min

00

N

E

N

E bb

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.


0 2 4 6 8 10 12 14 16 18 200

1

2

3

4

5

6

7

EbNo ~ Power (linear ratio)

Spe

ctra

l eff

icie

ncy

[bps

/Hz]

Bandwidth limited

Power limited

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

12min

00

N

E

N

E bb


Examples

Example 4.2.1. 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 4.2.2. The satellite in Example 4.2.1 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

ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 8: Satellite link design...

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