The document discusses diversity techniques in wireless communication. It introduces different types of diversity including frequency diversity and time diversity. Frequency diversity involves transmitting the same information over multiple carrier frequencies separated by more than the coherence bandwidth. Time diversity involves repeated transmission of information with time spacing exceeding the channel coherence time. The document provides examples of how techniques like frequency division multiplexing and rake receivers implement frequency and time diversity respectively.

1. PRESENTED BY:-
DiwakER PaNT
mE (EcE REgulaR)
October 9, 2012
Diversity Techniques By Diwaker Pant 1

2.  Introduction.
 Need of Diversity.
 Classification of Diversity.
 Diversity Techniques.
 Frequency diversity.
 Time Diversity.
 Related Paper.
 Conclusion.
 References.
October 9, 2012 Diversity Techniques By Diwaker Pant 2

3.  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.
October 9, 2012 Diversity Techniques By Diwaker Pant 3

4.  If one radio path undergoes a deep fade another independent
path may have a strong signal.
October 9, 2012 Diversity Techniques By Diwaker Pant 4

5. Two criteria are necessary to obtain a high degree
of improvement from a diversity system are :
 First, the fading in individual branches should have
low cross-correlation or highly uncorrelated.
 Second, the mean power available from each branch
should be almost equal.
October 9, 2012 Diversity Techniques By Diwaker Pant 5

6.  M acro diversity: provides a method to mitigate
the effects of shadowing , as in case of Large
scale fading.
 M icro diversity: provides a method to mitigate
the effects of multi-path fading as in case of small
scale fading.
October 9, 2012 Diversity Techniques By Diwaker Pant 6

7.  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
October 9, 2012 Diversity Techniques By Diwaker Pant 7

8.  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.
October 9, 2012 Diversity Techniques By Diwaker Pant 8

9. Space Diversity:
Using antennas spaced enough (at Tx or Rx).
Polarization Diversity:
Using antennas with different polarizations.
Frequency Diversity:
Using frequency channels separated in frequency more than
the channel coherence bandwidth.
Time Diversity:
Using time slots separated in time more than the channel
coherence time.
October 9, 2012 Diversity Techniques By Diwaker Pant 9

10.  Use more than one antenna to receive the signal.
October 9, 2012 Diversity Techniques By Diwaker Pant 10

11.  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.
October 9, 2012 Diversity Techniques By Diwaker Pant 11

12.  Frequency diversity is implemented by transmitting
same information on more than one carrier
frequency.
 Our aim is to make these carrier frequency
uncorrelated to each other, so that they will not
experience the same fades.
 To make them least correlated, these carrier
frequencies are separated by more than the coherence
bandwidth of the channel.
October 9, 2012 Diversity Techniques By Diwaker Pant 12

13.  Theoretically if the channels are uncorrelated , then the
probability of simultaneous fading will be the product of the
individual fading probabilities.
 Frequency diversity is often employed in microwave line –of-
sight links.
 These links uses Frequency division multiplexing mode(FDM).
October 9, 2012 Diversity Techniques By Diwaker Pant 13

14.  In Practice 1:N protection switching is used as
shown below.
Idle(Backup Frequency)
1
Normal Frequency
2
.
.
Transmitter
.
.
Normal Frequency
N
Receiver
October 9, 2012 Diversity Techniques By Diwaker Pant 14

15.  Protection switching is provided by a radio licensee.
 In this case one frequency is nominally idle but is
available on a stand by basis to provide frequency
diversity switching for any one of the other N
carrier.
 When diversity is needed , the appropriate traffic is
simply switched to backup frequency.
October 9, 2012 Diversity Techniques By Diwaker Pant 15

16.  FH-SS is a special case of frequency diversity.
 New OFDM modulation and access techniques
exploit frequency diversity.
 This can be achieved by providing simultaneous
modulation signals with error control coding across
a large bandwidth.
 If a particular frequency undergoes a fade , the
composite signal will still be demodulated.
October 9, 2012 Diversity Techniques By Diwaker Pant 16

17. October 9, 2012 Diversity Techniques By Diwaker Pant 17

18.  Time diversity repeatedly transmits information at
the time spacing that exceeds the coherence time of
the channel.
 Multiple repetitions of the signals will be received
with independent fading conditions, thereby
providing diversity.
 Our modern implementation of time diversity
involves the use of RAKE receiver for spread
spectrum CDMA.
October 9, 2012 Diversity Techniques By Diwaker Pant 18

19.  The rake receiver is so named because it reminds the function of a
garden rake, each finger collecting symbol energy similarly to how tines
on a rake collect leaves.
RAK Receiver
E
October 9, 2012 Diversity Techniques By Diwaker Pant 19

20.  A rake receiver is a radio receiver designed to counter the
effects of multipath fading, It does this by using several "sub-
receivers" called fing e rs.
 The rake receiver was patented in the US in 1956 by “ Price
and Green” .
 Each correlator detects a time-shifted version of the original
transmission, and each finger correlates to a portion of the
signal, which is delayed by at least one chip in time(1/Rc)
from the other fingers.
 This will result in higher SNR (Eb/N0) in a multipath
environment than in a "clean" environment.
October 9, 2012 Diversity Techniques By Diwaker Pant 20

21.  Multipath component appears like uncorrelated noise at a
CDMA receiver and equalization is not required.
 The outputs of each correlator are weighted to provide better
estimate of the transmitted signal than is provided by a single
component.
 The weighting coefficients are based on the power or the
SNR from each correlator output.
o If the power or SNR is small out of a particular correlator, it
will be assigned a small weighting factor or vice versa.
October 9, 2012 Diversity Techniques By Diwaker Pant 21

22. Interleaving is used to obtain time diversity in the digital
communication system.
Interleaving is a way to arrange data in a non-contiguous
way to increase performance.
It is typical for many speech coders to produce several
important bits in succession.
The function of interleaver is to spread these bits out in time
so that if there is a deep fade or error burst, the important bits
from a block of source data are not corrupted at the same time.
October 9, 2012 Diversity Techniques By Diwaker Pant 22

23. At the receiver , de-interleaver is used.
Convolutional interleaver can be used in place of block
interleaver and ideally suited for use with convolutional
codes.
October 9, 2012 Diversity Techniques By Diwaker Pant 23

24.  Jin-Tao Wang, Zhi-Xing Yang, Chang-Yong Pan,
Jian Song and Lin Yang, “ Design of Space-Time-
Frequency Transmitter Diversity Scheme for TDS-
OFDM System” , IEEE Transactions on Consumer
Electronics, Vol. 51, No. 3, AUGUST 2005 .
October 9, 2012 Diversity Techniques By Diwaker Pant 24

25.  Abstract
o To improve the performance of the Time Domain Synchronous -
Orthogonal Frequency Division Multiplexing (TDS-OFDM)
system, an efficient space-time-frequency coding scheme is
proposed. The block coding is employed in both time and
frequency dimensions simultaneously and proved to be a quasi-
orthogonal design. The performance improvement of TDS-OFDM
system with this method is theoretically analyzed and computer
simulated, showing its superior performance in the presence of both
time and frequency selective environments. Comparing to the pure
space-time or space-frequency coding scheme with straightforward
implementations, the proposed implementation scheme reduces the
computational load by about 70% in TDS-OFDM system.
October 9, 2012 Diversity Techniques By Diwaker Pant 25

26.  Recently, transmitter diversity has been extensively studied to
combat the detrimental effects in wireless fading channels
because of its relative simplicity in implementation.
 One attractive approach to the transmitter diversity is the
space-time block coding scheme in which a very simple
maximum-likelihood (ML) decoding algorithm is used at the
receiver.
 Space-time block codes (STBCs) from orthogonal designs
can achieve full diversity.
 The main characteristic of these codes is the orthogonality.
October 9, 2012 Diversity Techniques By Diwaker Pant 26

27.  A design of space-time-frequency transmitter diversity
scheme is proposed in this paper and its effectiveness is
demonstrated using the Time Domain Synchronous - OFDM
(TDS-OFDM) system as an example.
Fig. (A) Diagram of the space-time-frequency diversity method for TDS-OFDM
system
October 9, 2012 Diversity Techniques By Diwaker Pant 27

28.  The BER performance of the introduced STFC-
OFDM method for TDS-OFDM system was further
verified by computer simulations.
 The guard interval was set to 1/9 of the length of the
frame body.
 The simulation system used rate 2/3 Convolutional
code and QPSK modulation on each subcarrier.
October 9, 2012 Diversity Techniques By Diwaker Pant 28

29.  channel A is a six-ray model from the Brazil digital television test
report, channel model B is constructed by replacing the sixth ray with
a 0dB echo, representing a channel with severe frequency selective
fading.
October 9, 2012 Diversity Techniques By Diwaker Pant 29

30. The BER performance comparison among the STC-OFDM, SFC-
OFDM and STFC-OFDM systems under channel A without Doppler
shift.
October 9, 2012 Diversity Techniques By Diwaker Pant 30

31. The BER performance comparison among the STC-OFDM, SFC-
OFDM and STFC-OFDM systems under channel B without
Doppler shift.
October 9, 2012 Diversity Techniques By Diwaker Pant 31

32.  To improve the transmission performance of TDS-OFDM
system developed for digital television broadcasting, an
efficient STFC-OFDM transmitter diversity scheme is
introduced and the performance improvement is both
theoretically analyzed and computer simulated.
 STFC-OFDM scheme is proved to be essentially a quasi-
orthogonal design. The block coding is employed in both time
and frequency dimensions simultaneously. Compared with the
conventional STC-OFDM and SFC-OFDM systems, this
scheme performs better in both time and frequency selective
channels because the channel condition assumption is much
more close to the reality.
October 9, 2012 Diversity Techniques By Diwaker Pant 32

33.  Diversity is an extremely powerful technique for improving
the quality of communication systems and it is easy to achieve
gains equivalent to power savings in excess of 10 dB.
 These gains are achieved at the expense of extra hardware,
particularly in terms of extra antennas, bandwidth and
receivers, which must be balanced against the benefits.
 The key requirements for achieving the maximum benefit are
that the multiple branches of the system should encounter
substantially equal mean powers and near-zero cross-
correlation of the fading signals.
October 9, 2012 Diversity Techniques By Diwaker Pant 33

34.  Wireless Communications “ Principles and Practice” by
Theodore S. Rappaport. (2nd edition).
 http://en.wikipedia.org/wiki/Rakereceiver.
 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 .
October 9, 2012 Diversity Techniques By Diwaker Pant 34

35. October 9, 2012 Diversity Techniques By Diwaker Pant 35

36. Diversity Techniques By Diwaker Pant October 9, 2012 36