4.3 types of diversity

1. TYPES
OF
DIVERSITY

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3. Types of Diversity
• How to obtain the diversity in the signals ?
• Diversity are classified based on the fading methods.
•Small scale fading
• Microscopic diversity techniques are used.
•Large scale fading
• Macroscopic diversity techniques are used.

4. Micro diversity Techniques.
• There are 5 methods available.
• Spatial diversity
• Antenna elements separated by space
• Temporal diversity
• Transmission of signals at different time
• Frequency diversity
• Transmission of signals at different frequencies
• Angular diversity
• Multiple antennas with different antenna patterns
• Polarization diversity
• Multiple antenna with different polarizations

5. 1. Spatial diversity
• Signals are Transmitted / Received by more than one
antenna and the best signal is selected for processing.
• Oldest and simplest form of diversity
• Large correlation between the signals are undesirable, as
it decreases the effectiveness of diversity.
• Important factor is to establish the relation between the
antenna spacing (Different for Tr side and Different for the
Rx side)
Transmitter Side
Receiver side 

6. Envelope Correlation Coefficient as a
function of antenna separation

7. 2.Temporal Diversity
• As the wireless propagation channels are time variant, signals that
are received at different times are uncorrelated.
• Temporal diversity can be realized in 3 different ways.
• Repetition coding
• Automatic repeat request
• Combination of interleaving and coding

8. • Repetition coding:
• The signal is repeated several times, where the repeat
intervals are sufficient for decorrelation of signals.
• This method is highly bandwidth inefficient
• Automatic Repeat ReQuest – (ARQ):
• Rx sends a message to transmitter to indicate weather it
received the data with sufficient quality.
• If not then transmission is repeated again.
• Efficiency is better than repetition coding.
• Retransmission occurs only at certain cases.
• Combination of interleaving and coding:
• The signals are interleaved and coded. Instead of
sending the actual data those code words are sent.
• The transmitted codeword can be reconstructed .

9. 3. Frequency Diversity
• Frequency diversity is implemented by
transmitting information on more than one
carrier frequency.
• It does not mean that same messages are
transmitted in two different frequencies.
• When this is done then the signals undergoes different
fading levels.
• Instead, information is spread over a large
bandwidth, so the small parts of the information
are conveyed by different frequency components.
• Then the receiver averages over the different
frequencies to recover the original information.

10. Frequency Spreading

11. • The spreading is achieved by different methods:
• Compressing the information in time.(TDMA)
• Code division multiple access.(CDMA)
• Orthogonal frequency division multiplexing(OFDM)
• Frequency hopping in conjunction with coding.
Demerits:
- It requires large band width.
- More number of receivers are required.
- High cost.

12. 4. Angular Diversity
• It enhances the decorrelation of signals at closely spaced
antennas.
• Different antenna patterns can be achieved very easily.
• This effect is due to Mutual Coupling.

13. Mutual Coupling
• Place 2 identical
antennas close to each
other
• Here antenna B acts as a
reflector for antenna A.
• Antenna A acts as a
reflector for Antenna B.
• Hence the pattern of
both the antennas are
skewed as shown in
figure.
• Of Course Antennas with
different radiation
pattern can also be used

14. • Mutual Coupling can also be increased by locating
the antenna at different parts of the casing.
• The various possibilities are shown in the figure.

15. 5. Polarization Diversity
• Signals are differentiated with the horizontally and
vertically polarizations.
• Reflection and the diffraction depends upon the
polarizations.
• The fading levels at each polarization are independent.
Thus the diversity is achieved.

16. Macro Diversity
• Macro Diversity:
• To reduce the large scale fading these macro diversity is
used.
• Large scale fading are generally caused by shadowing.
• If there is an hill between the BS and the MS then
simply increasing the Tx antenna and Rx antenna
doesn’t make any difference.
• Thus a intermediate BS (BS2) is placed in between the
BS and MS so that the hill does not lie in between the BS
and BS2.
• Receive – Amplify – Retransmit.
Merits:
• Distance between BS and MS can be increased.
Demerits:
• It requires large bandwidth.
• frequency repeaters causes delay dispersion.