l04-propagation lecture 2
TRANSCRIPT
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Basics o f Smal l Scale
Fading : Towards choice
o f PHY
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Basic Quest ions
Desert Metro Street Indoor
What will happen if the transmitter
- changes transmit power ?
- changes frequency ?- operates at higher speed ?
What will happen if
the receiver moves?
What will happen if we conduct
this experiment in different types
of environments?
Channel effects
Effect of mobility
Transmit power, data rate,
signal bandwidth, frequency
tradeoff
Tx
Rx
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Review of basic concepts
Channel Impulse response
Power delay profile Inter Symbol Interference
Coherence bandwidth
Coherence time
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Channel Impu lse Response
)(tx
Channel
)(ty
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Power delay Prof i le
ReceivedSigna
lLevel(dBm)
-105
-100
-95
-90
-90
0 50 100 150 200 250 300 350 400 450
Excess Delay (ns)
RMS Delay Spread () = 46.4 ns
Mean Excess delay () = 45 ns
Maximum Excess delay < 10 dB = 110 ns
Noise threshold
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Examp le (Power delay p ro f i le)
-30 dB
-20 dB
-10 dB
0 dB
0 1 2 5
Pr()
(s)
s 38.4
]11.01.001.0[
)0)(01.0()2)(1.0()1)(1.0()5)(1(_
2
2222_
2 07.21]11.01.001.0[
)0)(01.0()2)(1.0()1)(1.0()5)(1( s
s
37.1)38.4(07.21 2
1.37 s
4.38 s
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RMS Delay Spread : Typ ical values
10ns 50ns 150ns 1s 2s 5s 10s 25s500ns
Office building 1
San Francisco
Manhattan
Suburban
Office building 2 SFO
Delay spread is a good measure of Multipath
3m 15m 45m 150m 300m 600m 3Km 7.5Km
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Inter Symbo l In terference
-30 dB
-20 dB
-10 dB
0 dB
0 1 2 5
Pr()
(s)
1.37 s
4.38 s
0 1 2 5 (s)
Symbol time
4.38
Symbol time > 10* --- No equalization required
Symbol time < 10* --- Equalization will be required todeal with ISI
In the above example, symbol time should be more than 14s to avoid ISI.
This means that link speed must be less than 70Kbps (approx)
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Coherence Bandw idth
)(tx
Time domain view
High correlation of amplitudebetween two different freq.
components
Range of freq over
which response is flat
Bc
delay spread
)( fX
Freq. domain view
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RMS delay spread and coherence b/w
RMS delay spread and
coherence b/w (Bc) are
inversely proportional
1
cB
.501cB For 0.9 correlation
.5
1
cBFor 0.5 correlation
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Time dispersive nature of channel
RMS delay spread () Coherence b/w (Bc)
Time domain view Freq domain view
Delay spread and coherence bandwidth are parameters which
describe the time dispersive nature of the channel.
channel 1
channel 2
channel 3
Signal
Cha
nnel
Symbol Time (Ts) Signal bandwidth (Bs)
signal 1
signal 2
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Revis i t Examp le (Power delay p ro f i le)
-30 dB
-20 dB
-10 dB
0 dB
0 1 2 5
Pr()
(s)
s 38.4_
2
_2 07.21 s
s
37.1
1.37 s
4.38 s
kHzBcoherence c 146
.5
1)%50(
Signal bandwidth for Analog Cellular = 30 KHz
Signal bandwidth for GSM = 200 KHz
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Doppler Shif t
cosvfv
Doppler shift
Example
- Carrier frequency fc= 1850 MHz (i.e. = 16.2 cm)
- Vehicle speed v= 60 mph = 26.82 m/s
- If the vehicle is moving directly towards the transmitter
- If the vehicle is moving perpendicular to the angle of arrival of the
transmitted signal
Hzf 165162.0
82.26
0f
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Coherence Time
Time domain view Frequency domain view
Coherence Time: Time interval
over which channel impulse
responses are highly
correlated
Tc
signal bandwidth
symbol time
fc+fdfc-fd
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Doppler sp read and coherence t ime
Doppler spread and
coherence time (Tc) are
inversely proportional mc
f
T1
m
cf
T 423.0 Rule of thumb
mc fT 16
9
For 0.5 correlation
fmis the max doppler shift
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Time varying nature of channel
Coherence Time (TC) Doppler spread (BD)
Symbol Time (TS) Signal bandwidth (BS)
Time domain view Freq domain view
Doppler spread and coherence time are parameters which
describe the time varying nature of the channel.
channel 1
channel 2
channel 3
Signal
Cha
nnel
signal 1
signal 2
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Small scale fading
Multi path time delay
Doppler spread
Flat fading BC
BS
Frequency selective fadingB
C
BS
TC
TSSlow fading
Fast fading TC
TS
fading
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PHY Layer Design Choices ?
Required Data Rates
Determines channel : frequency selective or flat fading; fastor slow fading
Required QoS at the PHY: bit-error-rate (BER),
packet-error-rate (PER), Frame-error-rate (FER)
May be determined by application needs (higher layers)
Affected by Interference and Noise levels
PHY layer choices include selection of
Modulation/Demodulation
Techniques to mitigate fading: diversity, equalization, OFDM,
MIMO
Techniques to mitigate interference (if necessary)
Error correction Coding