Channel issues Accuracy of Models Large-scale: Path loss Medium-scale: Shadowing Small-scale: Multipath fading

1. Small Scale Fading:
By
AJAL JOSE

2. Radio Block Diagram
 In today's class:
 How does the signal propagate? What are the prominent
effects? --- Small scale fading
Coding Modulation Antenna
DemodulationDecoding Antenna

3. Main story
 Communication over a flat fading channel
has poor performance due to significant
probability that channel is in deep fading.
 Reliability is increased by provide more
signal paths that fade independently.
 Diversity can be provided across time,
frequency and space.
 Name of the game is how to expoited the
added diversity in an efficient manner.

4. Antenna Diversity
Receive Transmit Both

5. Current strategy :
Transmit More when Channel is Good

6. Small-scale Multipath Propagation
The mobile radio channel
as a function of time and space.
Illustration of Doppler effect.
d
X Y
Δl
d
υ

7. Channel issues

8. Accuracy of Models
• The accuracy of the models depends on their
purpose:
– detailed models are needed for detailed coverage and
capacity analysis
– for rough capacity and range calculation needed in
radio interface design, simple and easy-to-use
models are sufficient
• It is important to analyze the sensitivity of the
result with respect to the propagation model
used

9. Accuracy of Modeling

10. Small-Scale Multipath Propagation
• The three most important effects
– Rapid changes in signal strength over a small travel distance or
time interval
– Random frequency modulation due to varying Doppler shifts on
different multipath signals
– Time dispersion caused by multipath propagation delays
• Factors influencing small-scale fading
– Multipath propagation: reflection objects and scatters
– Speed of the mobile: Doppler shifts
– Speed of surrounding objects
– Transmission bandwidth of the signal
• The received signal will be distorted if the transmission bandwidth is
greater than the bandwidth of the multipath channel.
• Coherent bandwidth: bandwidth of the multipath channel.

11. Fading Channel
Large-scale: Path loss
Medium-scale: Shadowing
Small-scale: Multipath fading
d
Pr/Pt

12. 12
From Signals to Packets
Analog Signal
“Digital” Signal
Bit Stream 0 0 1 0 1 1 1 0 0 0 1
Packets
0100010101011100101010101011101110000001111010101110101010101101011010111001
Header/Body Header/Body Header/Body
ReceiverSender
Packet
Transmission

13. Materials
• Attenuation values for different materials
Material Loss (dB) Frequency
Concrete block 13-20 1.3 GHz
Plywood (3/4”) 2 9.6 GHz
Plywood (2 sheets) 4 9.6 GHz
Plywood (2 sheets) 6 28.8 GHz
Aluminum siding 20.4 815 MHz
Sheetrock (3/4”) 2 9.6 GHz
Sheetrock (3/4”) 5 57.6 GHz
Turn corner in corridor 10-15 1.3 GHz

14. Can I see
Whats there taking place actually in
the channel

15. NORMAL SHADOWING
Tx Rx0 Rx
d0
d

16. Small Scale Fading
• Causes of small scale fading
– Multipaths
• Coherence distance, time, and Doppler
spread
– Coherence distance: the signal strength experiences change
from largest to smallest traveling the distance
– Coherence time: the time a moving Rx travels the coherence
distance
– Doppler spread: coherence time expressed in time domain
• Coherence bandwidth and delay spread
– Coherence bandwidth: the frequency band causing signal
changes from largest to smallest
– Delay spread: coherence bandwidth expressed in time domain

17. Small scale fading
 Rapid fluctuations of the signal
over short period of time
 Invalidates Large-scale path loss
 Occurs due to multi-path waves
 Two or more waves (e.g:
reflected/diffracted/scattered waves)
 Such waves differ in amplitude and
phase
 Can combine constructively or
destructively resulting in rapid signal
strength fluctuation over small
distances
Example of Multipath
Phase difference between
original and reflected wave

18. Small Scale Fading
• The signal variation over a short
period of time or a short distance

19. Large Scale Fading
• Large scale fading is due to the shadowing
effect of large size objects (buildings,
mountains)
• It is the mean signal strength (or power)
vs. (large) distance between Tx and Rx
• Path loss and path gain
– The ratio of total transmitted power over the received
power
– The path gain is 1/path loss

21. ECE6331 Spring 2009
Multipathradio propagation in urban areasMultipathradio propagation in urban areas

22. Small Scale Fading:
Different types of transmitted signals undergo different
types of fading depending upon the relation between the
Signal Parameters: Bandwidth, Symbol Period
&
Channel Parameters: RMS Delay Spread,
Doppler Spread
 In any mobile radio channel a wave can be dispersed
either in Time or in Frequency.
 These time and frequency dispersion mechanisms lead
to four possible distinct effects which depend on the
nature of transmitted signal, the channel and the velocity.

23. Fading Channel
Large-scale Fading Small-scale Fading
Path Loss Shadowing Effect Multipath
Delay Spread
Doppler
Spread
Flat
Fading
Frequency
Selective
Fading
Fast
Fading
Slow
Fading
Signal BW << Channel BW
Symbol period >> Delay spread
Signal BW > Channel BW
Symbol period < Delay spread
High Doppler spread
Symbol period > Coherence Time
Signal variation < Channel variation
Low Doppler spread
Symbol period << Coherence Time
Signal variation >>Channel variation
Mobile SpeedPropagation
Environment

24. 2424
Types of Small Scale FadingTypes of Small Scale Fading
Multipath time delayMultipath time delay
Doppler SpreadDoppler SpreadDoppler SpreadDoppler Spread
Flat fadingFlat fading Frequency
Selective
Fading
Frequency
Selective
Fading
FastFast
FadingFading
FastFast
FadingFading
Slow fadingSlow fadingSlow fadingSlow fading
Two types of fading are independent of each other.Two types of fading are independent of each other.

25. Types of Small-scale Fading
Small-scale Fading
(Based on Multipath Tİme Delay Spread)
Flat Fading
1. BW Signal < BW of Channel
2. Delay Spread < Symbol Period
Frequency Selective Fading
1. BW Signal > Bw of Channel
2. Delay Spread > Symbol Period
Small-scale Fading
(Based on Doppler Spread)
Fast Fading
1. High Doppler Spread
2. Coherence Time < Symbol Period
3. Channel variations faster than baseband
signal variations
Slow Fading
1. Low Doppler Spread
2. Coherence Time > Symbol Period
3. Channel variations smaller than baseband
signal variations

26. Small scale fading
Multi path time delay
Doppler spread
Flat fading
BC
BS
Frequency selective fading
BC
BS
TC
TSSlow fading
Fast fading
TC
TS
Small scale fading

27. Simulating Doppler/Small-scale fadingSimulating Doppler/Small-scale fading

28. Doppler Shift Geomerty

30. Doppler ShiftDoppler Shift

31. • Doppler Shift
– A mobile moves at a constant velocity v, along a path segment having
length d between points X and Y.
– Path length difference
– Phase change
– Doppler shift
θθ coscos tvdl ∆==∆
θ
λ
π
λ
π
φ cos
22 tvl ∆
=
∆
=∆
θ
λ
φ
π
cos
2
1 v
t
fd =
∆
∆
⋅=

32. Simulating Doppler fadingSimulating Doppler fading

33. Doppler spectrumDoppler spectrum

34. Doppler Spectrum
If one transmits a sinusoid, …
what are the frequency components in the received signal?
• Power density spectrum versus received frequency
• Probability density of Doppler shift versus received
frequency
• The Doppler spectrum has a characteristic U-shape.
• Note the similarity with sampling a randomly-phased
sinusoid
• No components fall outside interval [fc- fD, fc+ fD]
• Components of + fD or -fD appear relatively often
• Fades are not entirely “memory-less”

35. How do systems handle Doppler
Spreads?•Analog
•Carrier frequency is low enough to avoid problems
•GSM
• Channel bit rate well above Doppler spread
• TDMA during each bit / burst transmission the channel is fairly
constant.
• Receiver training/updating during each transmission burst
• Feedback frequency correction
•DECT
•Intended to pedestrian use:
•only small Doppler spreads are to be anticipated for
•Original DECT concept did not standardize an equalizer
•IS95
•Downlink: Pilot signal for synchronization and channel estimation
•Uplink: Continuous tracking of each signal

36. How to handle fast multipath fading?
Analog
•User must speak slowly
GSM
•Error correction and interleaving to avoid
burst errors
•Error detection and speech decoding
•Fade margins in cell planning
DECT
•Diversity reception at base station
IS95
•Wideband transmission averages channel
behaviour
This avoids burst errors and deep fades

37. How do systems handle delay spreads?
Analog
• Narrowband transmission
GSM
• Adaptive channel equalization
• Channel estimation training sequence
DECT
• Use the handset only in small cells with small delay spreads
• Diversity and channel selection can help a little bit
“pick a channel where late reflections are in a fade”
IS95
• Rake receiver separately recovers signals over paths with excessive delays
Digital Audio Broacasting
• OFDM multi-carrier modulation
The radio channel is split into many narrowband (ISI-free) subchannels

38. Flat Fading
 Occurs when the amplitude of the received
signal changes with time
 For example according to Rayleigh Distribution
 Occurs when symbol period of the
transmitted signal is much larger than the
Delay Spread of the channel
 Bandwidth of the applied signal is narrow.
 May cause deep fades.
 Increase the transmit power to combat this situation.

39. Flat Fading
h(t,τ)
s(t) r(t)
0 TS 0 τ 0 TS+τ
τ << TS
Occurs when:
BS << BC
and
TS >> στ
BC: Coherence bandwidth
BS: Signal bandwidth
TS: Symbol period
στ: Delay Spread

40. Frequency Selective Fading
 Occurs when channel multipath delay spread
is greater than the symbol period.
 Symbols face time dispersion
 Channel induces Intersymbol Interference (ISI)
 Bandwidth of the signal s(t) is wider than the
channel impulse response.

41. Frequency Selective Fading
h(t,τ)
s(t) r(t)
0 TS 0 τ 0 TS+τ
τ >> TS
TS
Causes distortion of the received baseband signal
Causes Inter-Symbol Interference (ISI)
Occurs when:
BS > BC
and
TS < στ
As a rule of thumb: TS < στ

42. Frequency Selective Fading
• If the channel possesses a constant-gain and linear phase
response over a bandwidth that is smaller than the bandwidth of
transmitted signal, then the channel creates frequency selective
fading. signal spectrum
channel response
received signal spectrum
f
f
f
)( fS
CB

43. Fast Fading
 Due to Doppler Spread
 Rate of change of the channel characteristics
is larger than the
Rate of change of the transmitted signal
 The channel changes during a symbol period.
 The channel changes because of receiver motion.
 Coherence time of the channel is smaller than the symbol
period of the transmitter signal
Occurs when:
BS < BD
and
TS > TC
BS: Bandwidth of the signal
BD: Doppler Spread
TS: Symbol Period
TC: Coherence Bandwidth

44. Slow Fading
 Due to Doppler Spread
 Rate of change of the channel characteristics
is much smaller than the
Rate of change of the transmitted signal
Occurs when:
BS >> BD
and
TS << TC
BS: Bandwidth of the signal
BD: Doppler Spread
TS: Symbol Period
TC: Coherence Bandwidth

45. 4545
Multipath terms associated with fadingMultipath terms associated with fading
TTss = Symbol period or reciprocal bandwidth= Symbol period or reciprocal bandwidth
BBss = Bandwidth of transmitted signal= Bandwidth of transmitted signal
BBcc = coherence bandwidth of channel= coherence bandwidth of channel
Note :Note :
BBcc= 1/50= 1/50σσττ wherewhere σσττ is rms delay spreadis rms delay spread

46. Different Types of Fading
Transmitted Symbol Period
Symbol Period of
Transmitting Signal
TS
TS
TC
στ
Flat Slow
Fading
Flat Fast
Fading
Frequency Selective
Slow Fading
Frequency Selective
Fast Fading
With Respect To SYMBOL PERIOD

47. Different Types of Fading
Transmitted Baseband Signal Bandwidth
BS
BD
Flat Fast
Fading
Frequency Selective
Slow Fading
Frequency Selective
Fast Fading
BS
Transmitted
Baseband
Signal Bandwidth
Flat Slow
Fading
BC
With Respect To BASEBAND SIGNAL BANDWIDTH

48. Small-scale Multipath Measurements

49. Small-scale Multipath Measurements
• Three methods of wideband
channel sounding techniques
1.Direct RF Pulse System
2.Spread Spectrum Sliding Correlator
Channel Sounding
3.Frequency Domain Channel Sounding

50. Small-scale Multipath Measurements)
• Direct RF Pulse System
★Determine the power delay profile of any
channel by using pulse signal with pulse width
τbb
. The main problem with this system is that it
is subject to interference and noise.
• Another disadvantage is that the phases of
the individual multipath components are not
received.

51. Small-scale Multipath Measurements
Direct RF channel impulse response measurement system
Pulse Generator
fc
Tx
RF link
τbb
τREP
τ
x(τ)
bbτ
2
BPF Digital storage
Oscilloscope
Resolution = Pulse Width
BW =
Rx
detector

52. Small-scale Multipath Measurements
• Spread Spectrum Sliding Correlator Channel
Sounding
★The advantage of a spread spectrum system is that, while
the probing signal may be wideband, it is possible to detect the
transmitted signal using a narrow band receiver, thus
improving the dynamic range of the system as compared to the
direct RF pulse system.
★The transmitter chip clock is run at a slightly faster rate than
the receiver chip clock. This implementation is called a sliding
correlator.
★A disadvantage of the spread spectrum system is that
measurements are not made in real time, but they are
compiled as the PN codes slide past one another.

53. Small-scale Multipath Measurements
Spread spectrum channel impulse response measurement system.
Pulse Sequence
Generator
fc
Tx chip clock
Rc
= α[Hz] = 1/Tc
Tx
BPF detector
Digital storage
Oscilloscope
BW = 2Rc
Rc = β[Hz]
Pulse Sequence
Generator
Correlation Bandwidth
BW 2(α-β)≒
Wideband Filter Narrowband Filter
Rx
@ fc
BPF

54. Small-scale Multipath Measurements
• Frequency Domain Channel Sounding
★Measure the frequency response of the
channel first then convert it to time response.
★It is useful only for very close measurements
(indoor channel sounding).
★It is a non-real time measurement.

55. Small-scale Multipath Measurements
Frequency domain channel impulse response measurement system.
)(
)(
)()(21
wX
wY
wHwS =∝
Vector Network Analyzer
Swept Frequency Oscillator
S-parameter test set
Inverse
DFT Processor
h(t)=FT-1
[H(w)]
Tx
Rx
X(w) Y(w)
Port 1 Port 2

56. Extra slides

57. Multipath Components
Component 2
Component 1
Component N
Radio Signals Arriving from different directions to receiver
Receiver may be stationary or mobile.

71. Trace Collection Setup
Receiver
Transmitter
Receiver movement trajectory
Origin 2o feet
If you measure signal at the receiver, what do you expect to see?

72. Measured Signal
Signal Strength
-80
-70
-60
-50
-40
-30
-20
-10
0
Time
dBm
Channel 4 Avg. Signal Strength
+20 feet +20 feet +20 feet +20 feet +20 feet +20 feet +20 feetOrigin
Long range path lossSmall scale fading

73. Fundamental Design considerations
Data signal x(t) Recovered
data signal
power spectrum
Noise, interference
ratio must be
above some threshold
for correct reception
I
C
Channel
Attenuation Distortion

74. Radio Propagation: Fading and multipath
Tx
Rx
Fading: rapid fluctuation of the amplitude of a radio signal over a
short period of time or travel distance
• Fading
• Varying doppler shifts on different multipath signals
• Time dispersion (causing inter symbol interference)
Effects of multipath

75. Review of basic concepts
 Fourier Transform
 Channel Impulse response
 Power delay profile
 Inter Symbol Interference
 Coherence bandwidth
 Coherence time

76. Understanding the effect of attenuation
Parabolic
Yagi
Patch
Omni
24 dBi
14 dBi
8 dBi
0 dBi
20 dBm
cable
loss

77. Questions?Questions?