In rate is 128 Kbps should be 128 bps

In th?s part of the paper we would look at the rake reciever perfomance
analysis and discuss it. especially  in this
chapter we go in details in every stage of our model to show how this model
work,

 

1)
Modulation and spreading; After we generate a local signal as explained in
previous chapter, we apply the out put to block of modulation and spreading,
the result of modulation and spreading is shown Figure

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Fig.6 System Model

Fig.
7 System Model Results

 

In
figure 7 the first graph shows the modulated signal, second one shows the output
of PN code, and the final one shows modulated signal after spreading, if the
chip rate in WCDMA is 3.84 Mcps then the sample time should be 2.6µsec, but
this value makes the simulation very small , so that it is multiplied as in all
our parameters by 1000 ,hence it becomes 2.6e-3 sec .While the bit rate is 128
Kbps should be 128 bps , this signal has the processing gain 15dB (10log
3.84e6/128e3) , from the interference present in the CDMA system .This effect
is termed ‘processing gain’ and is a fundamental aspect of all CDMA systems. It
is important to understand that spreading/despreading by itself does not
provide any signal improvement for wireless applications. Indeed, the
processing gain comes at the price of an increased transmission bandwidth.

 

2)
Multipath Fading Channel and Noise; As mentioned in earlier chapters, the
Multipath is implemented by delay blocks and combined together; the result of
this process is shown in Figure (8) The first graph shows the output from the
first path, the second one shows the second path and the third one shows the
third path. The fourth graph shows the combined of all these three paths before
adding together, the output looks as one signal because all the three paths are
delayed with the same delay sample as shown below. The fifth one after adding
and the last one indicates the signal before Multipath channel The last two
figure show the combination of three paths in case of different delay sample
before adding together and after adding together respectively.

 

                              

Fig. 8 Multipath
Fading Channel Results

 

 

The
effect of adding noise to the signal is shown in Figure (9) The first figure
shows the signal before fading channel and the second one is after fading
channel, and the last one after adding noise The signal power needs to be
typically a few decibels above the noise, the required power density is
designated as Es/No ,where Es is the energy or power density and No is the
noise power density .In this model Es/No = 10 dB

 

 

Fig. 9 Signal With
Added Noise

 

3)
Rake Receiver; As we mentioned before , the RAKE receiver contains five stages,
these stages are (1)bank of correlators ,(2) selector , (3)tracking loop ,
(4)alignment ,despreading and combiner , and (5)demodulation .here we discuss
the result of each stage below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I)                   
Bank of
Correlators

   The
result is shown in Figure(10)

 

 

Fig. 10 Bank Of Correlators Results

 

II)                 
Selector

        In this
stage we select the strongest signal from correlator, a test of this block is
shown in Figure (11)

      Fig 11
A Test Of  Block

 

As shown above
when we apply random input ,the selector selects the highest one and goes to
next one and so on ,it determines the number of highest input not the value,
now we need to organize these outputs from the highest number of input to the
lowest one, this process is shown in Figure (12)

 

Fig 12 
Selection Process

 

III)               
Tracking Loop

        Once the
complete received signal, we try to synchronize it. Within each correlation
receiver, we have to track the fast-changing phase and amplitude values
originating from the fading process .this tracking process has to be very fast
.the result is shown in Figure (13) The first graph illustrates the PN code
while the other one shows the code generated by the tracking loop. As we see
the local code synchronizes With PN code

 

Fig. 13 The Tracking Loop Process

 

IV)               
Alignment,
despreading and combiner

        In this
stage all outputs signals are aligned as in step seen earlier, then it is
multiplied by the outputs of the tracking loops such that all these signal have
the same phase as shown in   

 

Fig. 14 After Spreading

 

 

Fig. 15 After Despreading

 

Figure (15) On the figure (15), the last diagram shows
the combined signal while the first ,2nd ,3rd and 4th diagram show the output
after despreading (modulated signal)

 

 

 

 

 

 

 

 

 

 

V)                 
Demodulation

        The
received signal after demodulation is illustrated in second diagram in Figure
(16) below where the transmitted signal is the above one.

 

 

Fig. 16 Transmitted and Demodulated Signal

 

     IV. ANALYSIS AND COMMENTS OF RESULTS

 

 From the results
obtained after running the program and for several cases to see the influence
of the change on these result due to an increase in the number of paths and
increase the number of selectors, as shown in Figures (17-1, 17-2, 17-3, and 17-4)
which show the rate of error change in each case. When we have two paths and
three selectors, the error range is about .0009 and reduces gradually when the
number of selectors increases to four, to become .0005. The error then drops to
.00029 when paths increase to three paths and selectors to three. The error
drops to .00025 when number of selectors becomes four, so an increase in the
number of paths and figures in the rake receiver leads to improvement in the
bit error rate. Then the rake receiver is able to overcome the problem of
fading channel due to multipath, though the present multipath was quite a
handicap to the proper reception of CDMA, it has now become an advantage for a
better performance because the more paths that exit, the better the reception
of RAKE receiver and the lesser the error rate incurred. We note that from these
results that the error ratios were constant. As the number of bits increases,
the error ratio starts to rise such that its ratio is constant As the error
ratio is in the shape of a straight line, this indicates that the receiver
deals with the signal according to various conditions in a good manner. The
error ratio drops only if the number of paths or the number of selectors
increases.

 

Fig.17-1 Result of two path, three selectors

 

 

Fig. 17-2

 

 

Fig.17-3 Result of three path.three selectors

 

 

 

 

 

 

 

 

Fig. 17-4 Result of three path, four selectors

 

Effect of noise There are many ways in which digital
communication systems might be compared. One of the most important comparisons
is based on how efficiency the system can utilize the available signal energy
to transmit information. A useful measure of this efficiency is the energy per
bit (or per symbol). Since all systems have noise in them, the energy
utilization is defined as Es/No, where Es is the energy per symbol and No is
the one sided noise spectral density To employ the energy utilization in
comparing our model, we simulate this model in four cases as discuss before, it
is necessary to relate (Es/No) to the bit error, to show how the change in
Es/No will effect on bit error rate, however, the change in fingers and paths.
This result shown below;

 

 

Fig. 18 log(BER) vs. Es/No

 

The Figure (18) shows clearly the effect of bit error
rate, whenever the Es/No, for each case in the study. From it we can note the
extent of the influence of the multipath. The more paths there are, the less
error rate we have. In the case of three paths and three selectors, the error
rate drops more than the case where these are two paths and four selectors,
despite the increase in the number of selectors. In the second case the signal
did not improve while it improved when the paths increase from two to three. In
the case of four selectors and two paths, and four selectors and three paths,
bit error dropped in the second case at a greater rate There is a great
proximity between three selectors, three paths and four selectors, three paths
because the influence of multipath has been nullified in this case and only the
effect of the selector exits, which improves the signal slightly To compare this
study with previous one, and to a conventional receiver. Figure(19) below shows
previous study compare between RAKE and conventional receiver. In this diagram
the red color shows RAKE receiver, where the green one shows a conventional
receiver. The RAKE receiver will gives about 2dB of gain when compare to the
conventional receiver.

 

 

Fig. 19

 

It is clearly seen that a RAKE receiver performed much
better than conventional receiver.

 

                                   V.
CONCLUSION

 

The block of fading channel in simulink program does
not give acceptable results because it operates only in the baseband
frequencies, so we applied some delays multiplied by factors that lead to
Rayleigh fading. Other limitations we have never changed the code of any blocks
in simulink, hence have not been able to control the results of our model. We
have only changed the properties of the blocks to get as good results as
possible In future models, either the blocks codes are to be altered in order
to have better control or to device our own code without using simulink
program. Simulation was carried out to study the performance of RAKE receiver
in a multipath channel. It was noticed that, the greater the number of paths,
the better the signal received. Also the more fingers (selectors) we have, the
better the received signal. Then AWGN channel model is fallowed to our model,
to provide proper channel noise. The result shows that increasing of Es/No,
decrease the bit error rate. These results as discussed before are limited by
simulink program blocks. But we have adjusted them to obtain the best possible
results.

 

 

 

 

Appendix A

 

Model of RAKE receiver

Four selectors

 

Fig.    
Model Of A Rake Reciever With Four Selectors

 

 

Referrences

 

(1)Rodger E. Ziemer & Rodger L. Peterson “DIGITAL
COMMUNICATION AND SPREAD SPECTRUM SYSTEMS” Macmillan Publishing Company, Inc,
1985

 

(2) Peterson, Ziemer, Borth INTRODUCTION TO SPREAD
SPECTRUM COMMUNICATION Prentice-Hall, 1995

 

(3) B.Sklar DIGITAL COMMUNICATION FUNDAMENTAL AND
APPLICATION Prentice-Hall, 1988

 

(4)Harri Holma and Antti Toskala “WCDMA FOR UMTS” John
Wiley & Sons, Ltd, 2000

 

(5)George R. Cooper & Clare D. McGillem “MODERN
COMMUNICATIONS AND SPREAD SPECTRUM” McGraw-Hill, Inc. 1986

 

(6) ECPE4654: DSP Implementation of Communication
System RAKE RECIEVER IN ASYNCHRONOUS CDMA SYSTEM Project Report 10th May 2001

 

(7) http://kilyos.ee.bilkent.edu.tr/~signal/BCSP/szabo.pdf

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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