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ECE 4710: Lecture #36 1
Chapter 8
Chapter 8 : Wired and Wireless Communication Systems Telephone Fiber Optic DSL Satellite Digital & Analog TV Cellular Telephone Personal Communication Systems (PCS) Link Budget Analysis and System Design
ECE 4710: Lecture #36 2
Link Budget Analysis
BER baseband performance determined by received signal to noise ratio (S/N)
How do we predict the received signal and noise power? Link Budget Analysis
Predict received signal power at input to Rx» Depends on Tx output power, channel attenuation (path loss), antenna
gains (wireless), etc.
Predict received noise power at input to Rx» Depends on frequency, antenna field of view, temperature, etc.
Predict signal + noise power at detector or MF input in Rx» Depends on Rx gain, noise characteristics, etc.
» S/N (or Eb/No) at detector or MF input determines BER of digital system
ECE 4710: Lecture #36 3
Signal Power @ Rx
Signal power at Rx input is a critical parameter in the design of any communication system
For a given Tx power how do we predict the received signal power?
Basic communication system (Tx + Channel + Rx)
Receiver
ECE 4710: Lecture #36 4
Signal Power @ Rx
Free Space Transmission Channel Wireless Communication System Atmosphere (usually) or Outer Space
Power gain of channel Gain? Book includes Tx and Rx antennas as part of
channel» Not standard perspective but is OK
All channels attenuate the Tx signal and are therefore lossy do NOT have gain (signal amplification)
Wired Channel cable attenuation Wireless Channel free space path loss
ECE 4710: Lecture #36 5
Signal Power @ Rx
Power gain of channel
PRx is almost always much smaller than PTx
Example: Cell phone tower PTx W while PRx nW
gainpower space free
antennaRx ofgain power
antennaTx ofgain power
output) antennaRx (at Rx intopower signal
antennaTx intopower signal
where
FS
AR
AT
Rx
Tx
ARFSATTx
Rx
G
G
G
P
P
GGGP
P
ECE 4710: Lecture #36 6
densitypower same with theantenna isotropican ofdensity power radiation radiation maximum ofdirection in the antenna actual ofdensity power radiation AG
Antennas
Antenna Power Gain (GA) NOT actual amplification of signal Power gain relative to “isotropic” antenna Isotropic antenna
» “Iso” = same» Theoretical non-realizable antenna that radiates equal (same)
power in all directions (spherical expansion)» Useful reference to compare performance of practical antennas
Power gain
ECE 4710: Lecture #36 7
Antennas
Practical antennas Purpose is to radiate power in specific direction(s)
towards Rx
Focus PTx in given direction greater focus larger “gain”
Larger antenna size (relative to ) greater ability to focus energy in specific direction larger gain
Antenna is effectively a transducer which takes a time-varying voltage from a circuit and launches a time-varying EM wave in free space» Only time-varying EM waves can effectively propagate large
distances
ECE 4710: Lecture #36 8
Power Density
Radiated EM wave characterized by power density Power density = power per unit area (W / m2) Power density of isotropic antenna at distance d
EIRP = Effective Isotropic Radiated Power Equal power at any given distance isotropic
Power decays 1 / d 2 as surface area of sphere expands» Point source of EM energy
Best case free space path gain (loss) is
EIRPTx Pd
P
d
24 radius with sphere of area surface
powerTx
24/1 d
ECE 4710: Lecture #36 9
Power Density
For real antenna the radiated power density is larger than PEIRP for direction of max radiation antenna gain
FCC specifies EM radiation safety regulations in terms of electric field intensity E (V / m) instead of power density (W / m2)
Conversion :
Free-space wave impedance = 377
377density power
2
E
12
7
0
0
1085.8104
antenna real ofdensity power radiated4
2 d
PGPG Tx
ATEIRPAT
ECE 4710: Lecture #36 10
Signal Power @ Rx
Radiated power density of real antenna :
Rx antenna at distance d will intercept / capture some of the Tx power density
The amount of power captured at Rx is directly related to Rx antenna size or effective area (Ae) Larger area = more power captured from Tx density
24
d
PG Tx
AT
where4
22 mAA
d
PGP ee
TxATRx
ECE 4710: Lecture #36 11
Signal Power @ Rx
Rx antenna gain is related to effective area by
Thus the signal power at output of Rx antenna is
Link Formula Friis Transmission Formula
2
22
22 4
44
4 d
GGPGd
PGA
d
PGP ARATTxARTx
ATeTx
ATRx
4or
4 2
2AR
ee
AR
GA
AG
ECE 4710: Lecture #36 12
Antenna is a reciprocal element Gain is the same whether it is transmitting or receiving
GA is linear quantity that is unitless relative measure between to powers
In decibels GA (dB) = 10 log (GA)
Antenna GA & Ae
ECE 4710: Lecture #36 13
dBm 43.6or nW .734 W107.43
30004
)157.0(26320 92
2
RxP
Signal Power @ Rx
Example 1: A PCS cell phone tower transmits at a frequency of 1.9 GHz, has a Tx power of 20 W, and an antenna gain 18 dB. Determine the Rx signal power (in dBm) of a mobile phone at a distance of 3 km assuming the Rx antenna has a gain of 3 dB and has a LOS link to Tx.
2
2
4
d
GGPP ARATTx
Rx
210 & 6310 10/310/18 ARAT GG
m 157.0109.1
1039
8
f
c
ECE 4710: Lecture #36 14
W p 68.5 W 105.681061.1300,224
)075.0(352185,35200
1223
2
RxP
Signal Power @ Rx
Example 2: A DirecTV satellite is in geosynchronous orbit above the earth at an altitude of 22,300 miles. The satellite transmits at a frequency of 4 GHz, has a Tx power of 200 W, and uses a dish antenna with a 3 m radius. Determine the Rx signal power at a home Rx that also uses a dish antenna with 0.3 m ( 1 ft ) radius.
dB) 45.5 (~ 185,35/(0.075))3(7 22 ATG
2/7dish satellite 4-8 Table AGA
m 075.0/ fc
2rA
dB) 25.5 (~ 9.351/(0.075))3.0(7 22 ARG
ECE 4710: Lecture #36 15
Free Space Loss
Recall that book defines
where GFS is free space power gain
Using
Then
where LFS is the free space loss :
ARFSATTx
Rx GGGP
P
4 then
4 2
2
2
2
ARFSATARAT
Tx
RxARATTxRx GGG
d
GGP
P
d
GGPP
1
4 2
2
FSFS Ld
G
2
24
d
LFS
ECE 4710: Lecture #36 16
Free Space Loss
Free space loss :
in dB :
Best case loss Free space free of all matter and particles (vacuum) Earth’s atmosphere can cause additional loss due to
attenuation of EM wave by atmospheric molecules (O2, H20, etc.)
» Only significant for f > 2 GHz and distances > 100 km
Many links are not Line of Sight (LOS) obstructed (OBS)
22 /4 dLFS
dd
LFS
4log20
4log10)dB(
2
ECE 4710: Lecture #36 17
Free Space Loss
For obstructed conditions (mobile radio) then useful model is
n is path loss exponent n = 2 free space or LOS with no atmospheric
attenuation n > 2 for OBS conditions Mobile radio path loss models use n = 2 – 5 and n in the
range of 2.8 - 3.5 is typical
d
nd
Ln
FS
4log10
4log10)dB(
ECE 4710: Lecture #36 18
Link Formula
Link formula in simplest form predicts the best case received signal power
Many factors can cause Rx signal power to be lower than simple form of link formula, e.g. Obstructed link
» Building, trees, hills, earth curvature, etc.
Atmospheric attenuation» f > 2 GHz and/or large separation distances
Antenna misalignment (gain is less than max value)
4 2
2
d
GGPP ARATTx
Rx