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Diversity Diversity Presented by ENG Ahmed Hamza Supervisor

VIEWS: 19 PAGES: 29

									  Diversity

Presented by ENG.: Ahmed Hamza
Supervisor: Dr. Mohab Mangoud
Outline
Introduction.
What is diversity? Why?
Types of diversity
   Space diversity.
   Polarization diversity.
   Frequency diversity.
   Time diversity.
Receiver diversity system model & combining techniques.
Types of combining techniques.
Comparative Performance of Diversity Combining
Techniques
    Introduction
         In a fading environment the received signal power varies
         randomly over distance or time due to shadowing and/or
         multipath fading.




Transmitter

                                                          Receiver
     CON.


Transmitter




              Receiver
What is diversity? Why?
 Diversity – Transmitting and processing multiple
 copies of the same signal, is an effective way of
 mitigating fading
 Diversity-combining uses the fact that independent
 signal paths have a low probability of experiencing
 deep fades simultaneously.
       Prob(all channels are faded) < Prob(one channel is in a fade)

 Microdiversity: mitigates the effect of multipath
 fading.
 Macrodiversity: mitigates the effects of shadowing
 from buildings and objects.
   Type of diversity
   How to create and take advantage of independent paths?


                          Diversity



Space         Frequency               Polarization   Time
Space diversity
 Definition
   Uses multiple transmit or receive antennas separated
   in distance.
 Advantages
   Reduce fading (at receiver) without increase
   transmitted signal power or bandwidth.
   Increase SNR.
 Disadvantages
   The transmit power must be divided among multiple
   antennas.
   the received SNR is the same as single transmit
   antenna was used.
Polarization diversity
 Definition
   Using two transmit or receive antennas with
   different polarization (e.g. vertically and
   horizontally polarized waves).
 Disadvantages
   Uses at most two branches corresponding to two
   types of polarization.
   The transmit or received power is divided between two
   differently polarized antennas.
 Frequency diversity
Definition:
   Transmit the narrowband signal over multiple carrier frequencies
   Frequency separation must be greater than the coherence
   bandwidth of the channel

           coherence bandwidth = 1/(2πτrms) and it represents the frequency
           separation of uncorrelated signals
           For τrms = 10 µs, coh. BW = 16 kHz

Pros and Cons:
   Fewer antenna elements and corresponding circuitry.
   Requires twice the bandwidth.
   Multiple mixers for the different carriers.
   requires additional power to send the signal over multiple
   frequency bands.
Time diversity
Definition:
  transmitting the same signal at different times, with time
  separation >= 1/fdoppler
  Can also be realized using interleaving


Advantages:
  not require increased transmit power.
Disadvantage:
  decrease the data rate.
Receiver diversity system model & combining


 How to take advantage of the independent diversity paths to
 improve the communications link?

 The out of the combiner is just a weighted sum of the
 different fading paths or branches
Receiver diversity system model & combining

 The signal output from the combiner equals the original
 transmitted signal s(t) multiplied by a random complex
 amplitude term.
            Sout   (t)=    S(t)



 the distribution of SNR is a function of:
         the number of diversity paths.
        the fading distribution on each path.
        combining technique
Definitions:

   ‫ע‬s :the SNR per symbol.
   Ps :The symbol error probability
   Pout: the probability that ‫ע‬s falls below a given value
   corresponding to the maximum allowable Ps




Diversity gain: SNR gain given by extracting the same
information from two independently fading channels.
M : diversity order of the system
Types of combining techniques

                        Combining




Selection   Threshold               Maximal ratio   Equal gain
Selection Combining
 Definition:
      the combiner outputs the signal on the branch with
      the highest SNR
 Advantages:
      requires just one receiver
      co-phasing of multiple branches is not required.
 Disadvantage:
      only one branch is used at a time.
Performance




the average SNR gain increases with M, but not linearly.
Performance
Threshold Combining
 Definition:
       Once a branch is chosen, as long as the SNR on that
       branch remains above the desired threshold
       called switch and stay combining (SSC)
 Advantages:
       avoids the need for a dedicated receiver on each branch
       co-phasing of multiple branches is not required.
 Disadvantage:
       only one branch is used at a time.
   Performance




SSC with an optimized threshold has the same outage probability as
SC.
Maximal Ratio Combining
 Definition:
      Measure the signal amplitude and phase on
      each antenna.
      the output is a weighted sum of all branches.
      the signals are cophased
                               r1       r2             rM


                              G1       G2              GM


                                    Co-Phase and Add

                                             rsum
  Performance
                                                     r1       r2             rM

Consider M diversity branches
   Co-phase the signal from all branches             G1      G2              GM
   Individually weight and then add all the
   signals to provide optimal SNR                         Co-Phase and Add
   Resulting signal amplitude at the detector will
                                                                   rsum
   be given by:             M
                  rM  = ∑ r G       i   i
                            i = 1




     Assuming the same noise power N on all branches, the
     noise power at the output of the combiner is:
                              M
                       N T = ∑ Gi2 N
                             i =1

     SNR at the detector: γM = rM2 / NT
     Find the Gi such that γM is maximized
Optimum Combiner Weight
Performance Improvements
with MRC
Advantage & Disadvantage
 Advantage:
     Multi_branch is used at a time.
     Good performance .
 Disadvantage:
     requires knowledge of the time-varying SNR on
     each branch.
     co-phasing of multiple branches is required.
Equal Gain Combining
 Definition:
       co-phases the signals on each branch and then
       combines them with equal weighting.
       Same as MRC but with Gi = 1

 Performance:
      Instantaneous SNR after combining



       The performance gain with EGC is very close to
       that of MRC
Comparative Performance of Diversity Combining
                  Techniques
Comparative Performance of Diversity Combining
                  Techniques




SC                                MRC
Comparative Performance of Diversity Combining
                  Techniques

								
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