Fading in wireless communication.ppt

Presentation on different types of fading in wireless communication

1. Fading in wireless communication
By
Dr. Pravinkumar Patil

2. WirelessResearch Challenges
•Wireless communicationstechnology isgrowingextensively in all
segments.
•Next generation wireless networks are expected to provide reliable
Multimedia services (such as web browsing, music and video streaming,
video conference etc.) at very high rate of data transmission.
•This ever increasing demand leads to new research challenges in wireless
technology such as
-High data rates
-Reliable transmission(QoS)
-Multimedia service support in multi user environment.
-Efficient spectral utilization.
-Power efficient solution.
•Metrics
-Throughput
-BER
-PER
-Power consumption.

3. Wireless propagation and fading
•As the wireless mobile device moves through the environment, the field
strength varies due to :
–Free space path loss: In free space, received power attenuates like 1/d
2
.
–Large scale fading: With reflections, diffraction and scattering received
power attenuate even more rapidly with distance.
–Small scale fading: Rapid fluctuations in received power due to
multipath propagation (constructive and destructive interference of the
transmitted waves)log(distance)
S
i
g
n
a
l

L
e
v
e
l
(
d
B
)
path loss
long term fading
short term fading

4. Doppler
•Changes in the received carrier frequency due to the relative motion
of the mobile to the base station
•f= f
d = (v/l) cos(q)
–for f=900 MHz, v = 70 MPH (112 km/h)
–f
D-max= v/l= 93.3 Hzq D=v. t

5. Small-scale fading
•Small-scale fading (or simply fading) is used to describe the rapid fluctuation
of the amplitude of a radio signal over a short period of time or travel distance.
•Fading is caused by interference between two or more versions of the
transmitted signal which arrive at the receiver at slightly different times.
•These waves, called multipath waves, combine at the receiver antenna to give
a resultant signal which can vary widely in amplitude and phase.
•Multipath in the radio channel creates small-scale fading effects.

6. Types of small-scale Fading

7. Flat fading
–Most common type of fading described in the literature
–The spectral characteristics of the transmitted signal are preserved at the
receiver
–The most common amplitude distribution is the Rayleigh distribution.
–To summarize, a signal undergoes flat if
and

8. Frequency selective fading
•If the delay spread of the channel is larger than the symbol interval, time
dispersion of the transmitted symbols within the channel
•Leads to inter-symbol interference (ISI).
•Leads to a frequency selective channel.
–Viewed in the frequency domain, certain frequency components in the
received signal spectrum have greater gains than others.
–To summarized, a signal undergoes frequency selective fading if
And

9. Fast fading
–The channel impulse response changes rapidly within the symbol
duration. That is,
B
s<B
Dand T
s>T
c
–This causes frequency dispersion (also called time selective fading) due to
Doppler Spreading, which leads to signal distortion.
Slow fading
–The channel impulse response changes at a rate much slower than the
transmitted baseband signal
T
s<<T
cand B
s>>B
D

10. THANK YOU!