ΑΣΥΡΜΑΤΕΣ-ΔΟΡΥΦΟΡΙΚΕΣΕΠΙΚΟΙΝΩΝΙΕΣ

Διδάσκων

Λούβρος Σπυρίδων

Επίκουρος Καθηγητής - ΤΕΣΥΔ

ΔΙΑΛΕΞΗ 1

ΕΙΣΑΓΩΓΗ ΣΤΗΝ ΑΣΥΡΜΑΤΗ ΕΠΙΚΟΙΝΩΝΙΑ

Radio communication

VoiceVideoData

VoiceVideoData

• Broadcast radio• Television• Mobile phones• Bluetooth• Wireless LAN• Radio links• etc

Carriergenerator

Modulator PowerAmplifier

Transmitter (Tx)

PreAmplifier

De-modulator

Localoscillator

Receiver (Rx)Modulated carrier

Radio path

Equipment properties• Carrier frequency• Bandwidth• Modulation type• Transmitter output power• Receiver sensitivity• Antenna gain/directivity• Antenna mounting position• Diversity

Path properties• Information density

• Coverage area• Interference

• Obstacles• Climate

• Atmospheric properties

Give

Radio communication

Hop (or Link)

Radio pathTerminal Terminal

Digital bit stream in each direction e.g. …100110101…Capacity in Mbit/s

Radio links

Radio pathAntennaRadio•Transmitter•Receiver

AntennaRadio•Transmitter•Receiver

Two different frequencies, f and f’,used as carriers. Together forming one duplex channel.

f

f’Bandwidth/Capacity

Bandwidth/Capacity

Radio links

Am

plitu

deS

igna

l stre

ngth

, pow

er[d

Bm

]

Frequency, the number of oscillations per second [Hz]

t

Frequency and power

Frequency and power are chosen to give optimum performancesfor the intended radio communication use

The numbers of oscillations per second (the frequency), measured in Hertz [Hz].

By changing the frequency different things are achieved:• Steady large changes will give different propagation properties• Steady small changes will give separate channels• Fast small changes are used to carry the information (modulation)

Frequency

• In radio communication the signal strength is usually measured in Watt or in the logarithmic unit dBm

• The reference for dBm is 1mW 0dBm = 1mW

• Negative dBm values power below 1mWPositive dBm values power above 1mW

Power

Radio link typical transmitter power: 20dBm=0.1WRadio link typical receiver threshold: -80dBm=0.00000000001WGSM 900 cell phone transmitter power: 2W (33dBm)GSM 900 radio base station transmitter power: 20W (43dBm)Broadcast transmitter power: 60 000W (78dBm)

Examples of different carrier power levels

Power

Basic free space loss

d1 d2

• The energy within a certain area will be less as the distance increases.• Frequency dependency,

“Low” frequency → low loss over distance“High” frequency → high loss over distance

Basic free space loss, link budget

Transmitteroutput power

Input power to

the receiver

Receiver threshold level

Distance [km]

PowerLevel[dBm]

7 GHz

Basic free space loss23 GHz38 GHz

0 km n km

Basic free space loss

Generally frequency licenses shall be applied for from national administrations

“High” frequencyEasier to get licenseShort rangeUrban use in general

“Low” frequencyLong rangeGenerally used in rural areas

Modulation techniques, examples

Low modulation order: + long hops, fairly uncomplicated techniqueHigh modulation order: + high traffic capacity per bandwidth

C-QPSK (4QAM)

4 symbols2 bit/symbol

Phase

128QAM

128 symbols7 bit/symbol

Phase and amplitude

Antenna

Parabolic 0.6 m

Parabolic 0.2 m

Parabolic antennas • gives high directivity

(well focused beam).• are referred to

by the diameter.• in diameters from 0.2 to 3.7m

Low radio frequency Large antennaLong path length Large antenna

A “small” antenna gives: Less windload, less visibility and lower cost for antenna and installation

A “large” antenna gives: Higher gain, thereby longer hopand/or higher transmissionquality

Antenna

Antenna gain

Transmitteroutput power

Input power to

the receiver

Receiver threshold level

Distance [km]

PowerLevel[dBm] Antenna

Gain[dBi]

0.3m antenna

0.6m antenna

1.2m antenna

0 km n km

Antenna polarization

Different polarization can be used to reduce interference between neighboring paths when using the same or nearby frequencies

V (Default)

H

Point to multipoint antenna

Hub: 0.2m 90º sector antenna

AT: Point to point antenna

Line of sightRadio optical line of sight

Geometrical line of sight

• Heights of masts must be designed to give free line of sight and a sufficiently large ground clearance.

• Due to atmospheric properties the radio beam is normallybent slightly downwards

Ground clearance

BER = Ratio of bits not possible to detect correctlyBER 10-3 = One bit out of 1000 bits corrupted

BER 10-6 = One bit out of 1000 000 bits corrupted

BER 10-9 = One bit out of 1000 000 000 bits corrupted

Two bits out of 8 corrupted

Sent bitstream

Detected bitstream

P

D

…0 0 0 1 1 0 1 0… …0 0 0 0 1 1 1 0…

Transmission quality

Terminal Terminal

Transmitter Receiver

Receiver

Switch Switch

1+1 protection

Terminal Terminal

Transmitter Receiver

Receiver

Switch Switch

1+1 protection

Quality & Availability targets

• All links are designed to meet a certain transmission Quality & Availability.• Internationally accepted recommendations from:

- ITU-T (media independent)- ITU-R (radio-link specific)

• Quality = the ratio of errors in the bit stream giving short term cuts.• Availability = long term cuts• Limiting factors for radio links

- Distance, - Rain- Multipath fading- Hardware errors

• It is the role of the Planner to design individual links and network to meet the Q&A targets.