ΑΣΥΡΜΑΤΕΣ-ΔΟΡΥΦΟΡΙΚΕΣ_διάλεξη 1η
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Transcript of ΑΣΥΡΜΑΤΕΣ-ΔΟΡΥΦΟΡΙΚΕΣ_διάλεξη 1η
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.