Simulation of digital communication system under various fading channels

As part of my final year project, i have to design a digital
communication system in simulink using any modulation scheme. the
modulation scheme that i chose is BPSK.... i have to plot the BER of
the system against SNR and compare the acheived results to the
theoretical results in order to proove that my system is correct. i
also have to use the AWGN channel, the multipath rayleih fading
channel and the rician fading channel. i already know that in order to
simulate the rayleigh fading channel it has to be connected to the
AWGN channel but the BER values acheived are very high compared to the
theoretical. I'm not sure if my system is correct or not but it seems
to run smoothely... below is a general description of the way the
system is connected


Bernoulli random number generator >>connected to>> BPSK modulator
>>connected to>>multipath rayleigh fading channel >>connected to>>
AWGN channel>>connected to>> BPSK demodulator


to calculate the BER the error rate block is used with two inputs one
from the transmitter and the other is from the receiver.



please advise me on how to improve the system so that the correct
results can be acheived

<saleh.lootah@gmail.com> wrote in message 
news:1177439410.146901.50520@c18g2000prb.googlegroups.com...
> As part of my final year project, i have to design a digital
> communication system in simulink using any modulation scheme. the
> modulation scheme that i chose is BPSK.... i have to plot the BER of
> the system against SNR and compare the acheived results to the
> theoretical results in order to proove that my system is correct. i
> also have to use the AWGN channel, the multipath rayleih fading
> channel and the rician fading channel. i already know that in order to
> simulate the rayleigh fading channel it has to be connected to the
> AWGN channel but the BER values acheived are very high compared to the
> theoretical. I'm not sure if my system is correct or not but it seems
> to run smoothely... below is a general description of the way the
> system is connected
>
>
> Bernoulli random number generator >>connected to>> BPSK modulator
>>>connected to>>multipath rayleigh fading channel >>connected to>>
> AWGN channel>>connected to>> BPSK demodulator
>
>
> to calculate the BER the error rate block is used with two inputs one
> from the transmitter and the other is from the receiver.
>
>
>
> please advise me on how to improve the system so that the correct
> results can be acheived
>

Hi Saleh,

You may not be accounting for the delays in your system in the Tx-to-Rx 
path.

You probably should read the Error Rate Calculation block doc reference page 
for more information on this:

http://www.mathworks.com/access/helpdesk/help/toolbox/commblks/ref/errorratecalculation.html

If that's not the issue, then try starting with a simple model (e.g. 
everything except for the channel impairment blocks) and get that simple 
model working first with the Error Rate Calc block.  Then start adding more 
blocks one at a time and adjust the delays as you go.

Hope this helps,
Steve

P.S. This section in the doc link above discusses this in more detail:

Delays
The Receive delay and Computation delay parameters implement two different 
types of delays for this block. One is useful when part of your model causes 
a lag in the received data, and the other is useful when you want to ignore 
the transient behavior of both input signals:

  a.. The Receive delay parameter is the number of samples by which the 
received data lags behind the transmitted data. This parameter tells the 
block which samples "correspond" to each other and should be compared. The 
receive delay persists throughout the simulation.

  b.. The Computation delay parameter tells the block to ignore the 
specified number of samples at the beginning of the comparison.


If you do not know the receive delay in your model, you can use the Align 
Signals block, which automatically compensates for the delay. If you use the 
Align Signals block, you should set the Receive delay in the Error Rate 
Calculation block to 0.

Alternatively, you can use the Find Delay block to find the value of the 
delay, and then set the Receive delay parameter in the Error Rate 
Calculation block to that value.