Concept of Diversity & Fading (wireless communication) Advance Communication System

1. Advance Communication Systems
Topic: Concept of Fading and diversity.
School Of Electrical Engineering, MIT Academy Of Engineering.
Instructor: Prof. Dr.Debashis Adhikari & Prof. Mandar Nalavade
- Omkar Rane BETA17 0120160172 Block-A B.Tech E&TC

2. Content
1)Introduction to fading.
2)Types of fading.
3)Introduction to diversity.
4)Classification to diversity.
5)Diversity Techniques.
6)References

3. Introduction to fading
 In conventional wireline communication systems, there is single signal propagation path between the transmitter and
the receiver, which is constrained by the propagation medium such as coaxial cable or twisted pair.
 However in wireless systems the signal can reach receiver via direct, reflected and scattered paths.
 As a result at receiver there is a superposition of multiple copies of the transmitted signal. These signal copies
experience different attenuations, delays and phase shifts arising from varied propagation distances and properties of
scattering media. Hence at the wireless receiver there is interference of signals received from these multiple
propagation paths, which is termed ‘Multipath Interference’.
 The multipath interference, in turn, results in an amplification or attenuation of the net received signal power observed
at the receiver ,and this variation in the received signal power observed at the receiver, and this variation in the
received signal strength arising from the multipath propagation phenomenon is termed ‘multipath fading or simply
fading’.
 Strong destructive interference at the receiver is referred to as deep fade, and such a condition may result in temporary
failure of communication due to severe drop in SNR at receiver.
Wireless propagation environment.

4. Types of fading
Fading
Large Scale
Fading
Path Loss
Shadowing
Small Scale
Fading
Fast Fading Slow Fading
Multipath
Fading
Frequency
Selective
Fading
Flat Fading

5. 1. Large Scale Fading:
This refers to the attenuation of signal power due to obstacles between the transmitter and receiver. It also covers the attenuation
and fluctuations of signal when the signal is transmitted over a long distance (usually in kilometers).
 Path Loss:
• It refers to the attenuation when a signal is transmitted over large distances.
• Wireless signal as they spread and propagate through medium, As the distance increase energy per unit area decreases.
• This is a fundamental loss that is independent of the type of transmitter and medium.
• we can minimize its effects by increasing the capture area/dimension of the receiver.
 Shadowing:
• This refers to the loss in signal power due to the obstructions in the path of propagation.
• One that is most effective way to reduce signal loss is to have a Line-Of-Sight(LOS) propagation.
• Shadowing losses also depend on the frequency of the E.M wave. As we know, EM waves can penetrate through various
surfaces but at the cost of loss in power i.e. signal attenuation.
• The losses depend on the type of the surface and frequency of the signal. Generally, the penetration power of a signal is
inversely proportional to the frequency of the signal.

6. 2. Small Scale Fading:
This refers to the fluctuations in signal strength and phase over short distance and small duration of time. It is also called
Rayleigh Fading. It affects all wireless communications and is a necessary to increase efficiency and decrease error.
(Based on doppler spread leads to frequency dispersion)
 Fast Fading:
• It occurs mainly due to reflections from surfaces and movement of transmitter or receiver.
• High doppler spread is observed in the fast fading with Doppler bandwidth comparable to or greater than the bandwidth
of the signal and the channel variations are as fast or faster than the signal variations.
• creates Inter Symbol Interference (ISI). One way to remove ISI is adaptive equalization.
 High Doppler spread.
 Coherence time < symbol period.
 Bandwidth of channel varies faster than bandwidth of signal.
 Signal Variation < Channel variation
 Slow Fading:
• It occurs mainly due to shadowing where large buildings or geographical structures obstruct the LOS.
• Low doppler spread is observed in Slow Fading with the doppler bandwidth being smaller compared to the bandwidth of
the signal and the channel variations are slow relative to the signal variations.
• It results in reduction of SNR which can be overcome using error correction techniques and receiver diversity techniques.
 Lower doppler spread
 Coherence time>symbol period
 Bandwidth of channel slower than the bandwidth of signal.
 Signal Variation > Channel variation

7. Multipath Fading: (based on time delay spread leads to time dispersion)
It occurs when a signal reaches the receiver from various path i.e. when multipath propagation takes place. Multipath fading can
affect all ranges of frequencies starting from low frequency to microwave and beyond. It affects both the amplitude and the
phase of the signal causing phase distortions and ISI. Multipath fading can affect signal transmission in two ways:
(Based upon multipath time delay spread)
 Flat Fading:
• In flat fading, all frequency components get affected almost equally. Flat multipath fading causes the amplitude to fluctuate
over a period of time.
• Flat fading, where the bandwidth of the signal is less than the coherence bandwidth of the channel or the delay spread is less
than the symbol period. There is no ISI(Inter symbol interference)
• Coherence bandwidth: It is a range of frequencies over which the channel can be considered "flat“.
 Bandwidth of signal < Coherence Bandwidth of channel.
 Delay Spread < Symbol Spread.
 Frequency Selective Fading:
• Selective Fading or Selective Frequency Fading refers to multipath fading when the selected frequency component of the
signal is affected.
• It means selected frequencies will have increased error and attenuation as compared to other frequency components of the
same signal.
• This can be overcome by techniques such as OFDM which spreads the data across the frequency components of the signal to
reduce data loss. Inter Symbol interference(ISI) is present.
 Bandwidth of signal >Bandwidth of channel.
 Delay Spread > Symbol Spread.

8. Frequency Selective ChannelFlat Fading Channel
Wireless
channel
Transmitte
d signal
Received
Signal
𝑌 𝑓 = 𝑋 𝑓 × 𝐻(𝑓)
𝑦 𝑡 = 𝑥 𝑡 ∗ ℎ(𝑡)
Ref:Mod-01 Lec-10 Coherence Bandwidth of the Wireless Channel

9. Small Scale Fading
(Based upon multipath time delay spread)
Flat Fading
1) Bandwidth of signal < Bandwidth of
channel.
2)Delay spread <Symbol period
Frequency Fading
1) Bandwidth of signal > Bandwidth of
channel.
2)Delay spread > Symbol period
Small Scale Fading
(Based upon Doppler spread)
Fast Fading
1) High doppler spread.
2)Coherence time <Symbol period
3)Channel variation are faster than
baseband signal variation.
Slow Fading
1) Low doppler spread.
2)Coherence time > Symbol period.
3) Channel variation are slower than
baseband signal variations.
Summary of Small Scale Fading

10.  Diversity is a powerful communication technique that provides wireless link improvements at relatively low cost.
 Diversity exploits the random nature of radio propagation by finding independent signal path for
communication.These independent paths are highly uncorrelated.
 It is by far the best tool available to combat the effect of multipath fading in wireless channel and thereby ensure
reliable communication.
 Diversity is based on the simple fact that independent wireless channels experience randomly independent levels of
fading. Hence, the probability that multiple independent wireless channels are simultaneously in a deep fade is
drastically lower compared to that multiple independent wireless channels simultaneously deep fade is drastically
lower compared to that of a single channel fading.
 Once can therefore significantly improve the reliability of symbol detection by transmitting multiple version of the
same information signal over set of independent signal over set of independent fading channels.
 If one radio path undergoes a deep fade another independent path may have a strong signal.
Introduction to diversity
Single link wireless Wireless system with diversity

11. Classification to diversity
Macro diversity:
 provides a method to mitigate the effects of shadowing , as in case of Large scale fading.
 Large scale fading is caused by shadowing due to the presence of fixed obstacles in the radio path.
 Long term fading can be mitigated by macroscopic diversity(apply on separated antenna sites) like the diversity using two base
stations
Micro diversity:
 provides a method to mitigate the effects of multi-path fading as in case of small scale fading.
 Small scale fades are characterized by deep and rapid amplitude fluctuations which occur as the mobiles moves over distances
of just a few wavelengths.
 These fades are caused by multiple reflections from surroundings in the vicinity of the mobile.
 Short term fading can be mitigated by the diversity using multiple antennas on the base station or mobile unit.
Classification to diversity

12. Diversity techniques
1)Space Diversity:
Using antennas spaced enough (at Tx or Rx). Use more
than one antenna to receive the signal.
2) Polarization Diversity:
 Using antennas with different polarizations.
Polarization diversity uses antennas of different
polarizations i.e. horizontal and vertical.
 The antennas take advantage of the multipath
propagation characteristics to receive separate
uncorrelated signals SNR is improved by up to 12 dB
even in line-of-sight channels.

13. 4)Time Diversity:
 Using time slots separated in time more than the channel
coherence time.
 In time diversity, the signals representing the same
information are sent over the same channel at different times.
 Time diversity repeatedly transmits information at time
spacing that exceeds the coherence time of the channel.
 Multiple repetition of the signal will be received with
independent fading conditions, thereby providing for
diversity.
3)Frequency Diversity:
 Using frequency channels separated in frequency more than
the channel coherence bandwidth.
 In Frequency Diversity, the same information signal is
transmitted and received simultaneously on two or more
independent fading carrier frequencies.
 Rationale behind this technique is that frequencies separated
by more than the coherence bandwidth of the channel will be
uncorrelated and will thus not experience the same fades.
 The probability of simultaneous fading will be the product of
the individual fading probabilities. The frequency diversity is
use to reduce frequency selective fading.

14. [1] https://www.everythingrf.com/community/what-are-the-different-types-of-fading ,definitions
[2] https://www.slideshare.net/s4h4rr/diversity-techniques-for-wireless-communication-42991111 ,diagrams
[3] Principle of modern wireless communication (Theory & practice) – By Prof. Aditya K. Jagannathan , definitions
[4] https://www.everythingrf.com/community/what-are-the-different-types-of-fading ,definitions & diagram
[5] Wireless Communications “Principles and Practice” by Theodore S. Rappaport. (2nd edition).
[6] J.T. Wang et al, “Design of Space-Time-Frequency Transmitter Diversity Scheme for TDS-OFDM System”.
IEEE Transactions on Consumer Electronics,Vol. 51, No. 3, AUGUST 2005 .
[7] Effective Fading Reduction Techniques in Wireless Communication System 1Ekwe O. A., 2Abioye, A. E.,
3Oluwe, M. O., 4Okoro, K.C. 1,2,3,4,Department of Electrical /Electronics Engineering, Michael Okpara
University of Agriculture, Umudike, Abia State, Nigeria .
[8] Mod-01 Lec-10 Coherence Bandwidth of the Wireless Channel, Advanced 3G and 4G Wireless Mobile
Communications by Prof. Aditya K. Jagannathan, Department of Electronics & Communication Engineering, IIT
Kanpur.
[9] Notes and PPT given by sir.
References