This chapter discusses small-scale fading and multipath propagation effects in mobile radio channels. It explains that multipath waves traveling along paths of different lengths interfere at the receiver, causing rapid fluctuations in signal strength over short distances. The key points are:
1) Small-scale fading is caused by multipath interference and depends on factors like surrounding objects, signal bandwidth, and mobile speed.
2) Multipath propagation can be modeled using the time-varying impulse response of the channel.
3) Important parameters used to characterize fading include coherence bandwidth, Doppler spread, coherence time, delay spread, and Ricean/Rayleigh distributions.
2. Introduction
Reflection, Diffraction and Scattering of waves
Most cellular systems operate in urban area,
where no direct line-of-site path between Rx
and Tx
Presence of high rise buildings causes severe
diffraction loss
Due to multiple reflections, em waves travel
along different paths of varying lengths
3. Propagation models focused on predicting
average received signal strength at a given
distance from transmitter
Models predicting mean signal strength for an
arbitrary T-R separation distance are large
scale models
Models that characterize rapid fluctuations of
received signal strength over very short
distances or short time intervals are small
scale models
6. Small-Scale Fading and
Multipath
Rapid fluctuations of the amplitude,
phase or multipath delays of a radio
signal over a short period of time or
travel distance is known as small-scale
fading
Large- scale path loss effects may be
ignored
7. Fading is caused by interference
between two or more versions of the
transmitted signal which arrive at the
receiver at slightly different times
Multipath waves combine at the
receiver antenna to give a resultant
signal which can widely vary in
amplitude and phase
8. This depends on the distribution of the
intensity and relative propagation time
of the waves and the bandwidth of the
transmitted signal
Multipath in the radio channel creates
small-scale fading effects
9. Three most important fading
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 (echoes) caused by
multipath propagation delays
10. In Built-up urban areas, fading occurs
because there is no single line-of- sight
path MS and BTS antennas
Mobile antennas are well below the
height of surrounding structures
Even if LOS exists, there are reflections
from ground and surrounding structures
11. Multipath signals add vectorially at
receiver antenna and cause the signal
received by the mobile to distort or fade
Even when the mobile is stationary,
fading may occur because of moving
objects in radio channel
12. Due to constructive and destructive effects of
multipath waves, receiver moving at high
speed can pass through several fades in a
small period of time
More serious case is that of a deep fade in
received signal
Due to relative motion between mobile and
base station ,Doppler shift in frequency takes
place
13. Factors influencing Small-Scale
fading
Multipath propagation: Presence of
reflecting objects and scatterers in the
channel creates constantly changing
environment
Random phase and amplitudes of
different multipath signals induce small-
scale fading and or distortion
14. Speed of the mobile: Relative motion
results in random frequency modulation
due to different Doppler shifts on each
multipath component
Speed of surrounding objects: If
objects in radio channel are in motion, a
time varying Doppler shift is induced
15. If surrounding objects move at a
greater rate than the mobile, this effect
dominates the small-scale fading
Motion of surrounding objects may be
ignored otherwise and only speed of
the mobile needs to be considered
Coherence time defines staticness of
the channel
16. Transmission bandwidth of the signal: If
the transmitted radio signal bandwidth is
greater than bandwidth of multipath channel
,received signal will be distorted but small-
scale fading will be insignificant
In the other case, signal will not be distorted
in time but signal will change rapidly
Coherence bandwidth is a measure of max.
freq. difference for which signals are strongly
correlated in amplitude
18. Difference in path lengths traveled by
the wave from source S to mobile at
points X and Y is ∆l=d cosθ =v∆t cos θ
Phase change in received signal due to
difference in path lengths is
∆Φ=(2π∆l)/λ = (2π v∆t cos θ) /λ
Doppler shift is given by
fd =(1/ 2π)*(∆Ф/∆t)= (v cos θ) /λ
19. Example 5.1
Solved the problem with all the
students participating in the process
20. Impulse Response Model of a
Multipath Channel
The small-scale variations of a mobile radio
signal can be directly related to the impulse
response of the mobile radio channel
Impulse response is a wideband channel
characterization and contains all information
necessary to simulate or analyze any type of
radio transmission through the channel
21. Mobile radio channel may be modeled
as a linear filter with time varying
impulse response
Time variation is due to receiver motion
in space
Filtering nature is caused by summation
of amplitudes and delays of multiple
arriving waves at any instant of time
22. Consider the case where time variation
is due strictly to receiver motion in
space
Receiver moves along the ground at
some constant velocity v
For a fixed position d, channel between
T and R can be modeled as a linear
time invariant system
24. Due to different multipath waves which
have propagation delays which vary
over different spatial locations of the
receiver, impulse response of the
channel should be a function of the
position of the receiver
Channel impulse response can be
expressed as h (d, t).
26. y(vt,t) =∫x(τ)h( vt,t-τ)d τ
Since v is constant, y(vt,t) is just function of
t.
Mobile radio channel can be modeled as a
linear time varying channel ,where the
channel changes with time and distance
Since v may be assumed constant over a
short time interval ,we may let x(t) represent
transmitted bandpass waveform
27. Impulse response h (t,τ)completely
characterizes the channel and is a
function of both t and τ
Variable t represents time variations
due to motion , variable τ represents
channel multipath delay for a fixed
value of t
28. If multipath channel is assumed to be
bandlimited channel, h(t,τ) may be
represented by a complex baseband impulse
response hb(t,τ)
Received signal in Multipath channel consists
of a series of attenuated ,time delayed ,phase
shifted replicas of the transmitted signal, the
baseband impulse response of a multipath
channel can be expressed by
29. hb(t,τ)=Σai(t,τ)exp[j(2πfcτ,i(t)+φi(t,τ))]§
(τ-τi(t))
Ai and Ti are real amplitudes and
excess delays respectively of the ith
multipath component at time t
Phase term represents the phase shift
due to free space propagation of ith
multipath component
33. Relationship between
bandwidth and received power
In actual wireless communication
systems, the impulse response of a
multipath channel is measured in the
field using channel sounding techniques
Illustrate how the small scale fading
behaves quite differently for two signals
with different bandwidths in identical
multipath channel
34. Received local ensemble average
power of wideband and narrowband
signals are equivalent
Pulse is wideband signal and CW signal
is a narrowband signal
Received power is computed for both
the cases
35. Small Scale Multipath
measurements
Three techniques
Direct pulse measurements, spread
spectrum sliding correlator
measurement and swept frequency
measurements
37. Allows engineers to determine rapidly the
power delay profile of any channel
The system transmits a repetitive pulse of
width Tbb and uses a receiver with a wide
bandpass filter.
BW=2/ Tbb Hz
Signal is amplified ,detected with an envelope
detector and displayed and stored on high
speed oscilloscope
38. This gives immediate measurement of
the square of the channel impulse
response convolved with the probing
pulse
If oscilloscope is set on average mode,
this system can provide a local average
power delay profile
40. Spread Spectrum Sliding
Correlator Channel Sounding
A carrier is spread over a large
bandwidth by mixing it with a binary
pseudo-noise sequence having a chip
duration Tc and chip rate Rc equal to
1/Tc
The power spectrum envelope is given
by [sin π(f-fc)Tc] / π(f-fc)Tc] 2
41. Null-to-null RF bandwidth is BW=2Rc
Spread spectrum signal is received,
filtered and despread using a PN
sequence generator identical to that
used at the transmitter
Two PN sequences are identical but chip
clock rate is higher at Tx than that at
Rx
42. Mixing the chip sequences in this fashion
implements a sliding correlator
When faster chip clock catches up with PN
code of slower chip clock, two will be virtually
identically aligned , giving maximal
correlation
When the two sequences are not maximally
correlated ,mixing the incoming signal with
unsynchronized receiver chip sequence will
spread the signal into the bandwidth at least
as large as receiver’s PN sequence.
43. Narrowband filter following correlator can
reject almost all of the incoming power
Processing gain= 2Rc/Rbb= 2Tbb/Tc
Tbb is period of baseband signal
When incoming signal is correlated with
received sequence, the signal is despread
,envelope detected and displayed on an
oscilloscope
44. Different multipath signals have
different time delays, they will
maximally correlate with receiver PN
sequence at different times
After envelope detection, channel
impulse response convolved with the
pulse shape of a single chip is displayed
on the oscilloscope
45. Time resolution ∆τ of multipath
components using spread spectrum
system with sliding correlator is
2Tc=2/Rc
The system can resolve two multipath
components as long as they are equal
to or greater than two chip durations
apart(2Tc seconds)
46. The sliding correlation process gives
equivalent time measurements that are
updated every time the two sequences
are maximally correlated
Time between maximal correlations ∆T
is given by Tc =γ l /Rc
Tc = chip period (seconds)
Rc=chip rate (Hz)
47. γ =slide factor and l=sequence length
in chips
Slide factor is defined as ratio between
transmitter chip clock rate and
difference between transmitter and
receiver chip clock rates
γ =α/(α-β) alpha is Tx chip clock rate
and beta is Rx chip clock rate
48. Since incoming spread spectrum signal
is mixed with a receiver PN sequence
that is slower than transmitted
sequence, the signal is essentially
down-converted to a low-frequency
narrowband signal.
Processing gain is realized using a
narrowband filter.
49. The equivalent time measurements
refer to the relative times of multipath
components as they are displayed on
the oscilloscope
Observed time scale on oscilloscope is
related to actual propagation time scale
by Actual propagation time=observed
time/γ
51. Vector network analyzer controls a
synthesized frequency sweeper
Sweeper scans a particular frequency
band centered on the carrier by
stepping through discrete frequencies
Number and spacings of frequency
steps impact the time resolution of
impulse measurements
52. For each frequency step, the S
parameter test set transmits a known
signal at port 1 and monitors the
received signal level at port 2.
These signal levels allow the analyzer to
determine the complex response which
is frequency domain representation of
channel impulse response
53. Time Dispersion Parameters
Parameters which grossly quantify
multipath channel
Excess delay, rms delay spread and
excess delay spread are determined
from a power delay profile(Graph of
received signal power V/s excess delay)
54. Time dispersive properties of wideband
multipath channel are most commonly
quantified by their excess delay( τ bar)
and rms delay spread στ.
Mean access delay is Σ[P(τk) τk ]/ P(τk)
Rms delay spread is sqrt(tau square
bar-tau bar square)
55. These delays are measured relative to
first detectable signal arriving at
receiver at τ0 =0
Typical values are of the order of
microseconds in outdoor mobile and
nanoseconds in indoor radio channels
58. Maximum excess delay (X dB) of the power
delay profile is defined to be the time delay
during which multipath energy falls to X dB
below the maximum.
In practice, values of rms delay spread, mean
excess delay and excess delay spread depend
on the choice of noise threshold
60. Coherence Bandwidth
It is a statistical measure of the range of
frequencies over which channel can be
considered flat.
Flat channel passes all spectral components
with approximately equal gain and linear
phase
It is the range of frequencies over which two
frequency components have a strong
potential for amplitude correlation
61. Two sinusoids with frequency
separation greater than Bc are affected
quite differently by the channel.
If coherence bandwidth is defined as
the bandwidth over which the
frequency correlation function is above
0.9, Bc =1/(50στ) approximately
62. Doppler Spread and
Coherence Time
These parameters describe time varying
describe the time varying nature of channel in
small-scale region
Doppler spread BD is a measure of spectral
broadening caused by time rate of change of
mobile radio channel
Defined as range of frequencies over which
received Doppler spectrum is non-zero
63. Doppler spectrum is fc-fd to fc+fd
fd is function relative velocity of mobile
and angle theta between direction of
mobile and arrival of scattered waves
If baseband signal bandwidth is much
greater than BD the effects of Doppler
spread are negligible. This is slow
fading channel
64. Coherence Time
Inversely proportional to Doppler
Spread
Tc=1/Bd
Coherence time is statistical measure of
the time duration over which channel
impulse response is essentially
invariant
65. Coherence time is the time duration over
which two received signals have a strong
potential for amplitude correlation.
Tc=0.423/fm where fm =maximum Doppler
shift =v/λ
Two signals arriving with a time separation
greater than Tc are affected differently by the
channel.
70. Rayleigh and Ricean
Distributions
Rayleigh distribution is commonly used
to describe statistical time varying
nature of the received envelope of a flat
fading signal or the envelope of an
individual multipath component
Mean value of Rayleigh distribution is
given by 1.2533 σ
71. Ricean Fading Distribution
When there is a dominant stationary
signal component present, such as LOS
propagation path, the small-scale fading
envelope distribution is Ricean.
In such a situation, random multipath
components arriving at different angles
are superimposed on a stationary
dominant signal.
72. At the output of the envelope detector, this
has effect of adding a dc component to the
random multipath
The effect of a dominant signal arriving with
many weaker multipath signals give rise to
the Ricean distribution.
As dominant signal becomes weaker, the
composite signal envelope is Rayleigh
type.The Ricean distribution degenerates into
a Rayleigh distribution when the dominant
component fades away.