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PRESENTED BY:-
                                       DiwakER PaNT
                                       mE (EcE REgulaR)


October 9, 2012
                  Diversity Techniques By Diwaker Pant    1
 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
 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
    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
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
    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
    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
    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
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
    Use more than one antenna to receive the signal.




October 9, 2012                                  Diversity Techniques By Diwaker Pant   10
    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
    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
    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
    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
    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
    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
October 9, 2012   Diversity Techniques By Diwaker Pant   17
    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
 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
    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
    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
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
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
     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
    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
    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
    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
    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
 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
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
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
    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
    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
   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
October 9, 2012   Diversity Techniques By Diwaker Pant   35
Diversity Techniques By Diwaker Pant   October 9, 2012   36

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Diversity Techniques in mobile communications

  • 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