B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

12.12 MATLAB Exercises 727
Figure 12.25
Comparison of
the in channel
gain for 32
subcarriers.
1.8
1.6
1.4
Subcannel gain Hl11J
2 .-----,------r-----,----.--------.-----.-----,
(J i)
I)
)
(i)
)
(i)
(i)
1.2
0
)
()
()
(i)
0.8 O
0.6
(i) (J (i)
(i)
)
( l Ii)
0.4
0.2
I) ()
(D ) Ii)
5
10
15 20
Subcarrier label
25
30
35
ylabel ('Imaginary' );
title ('(b) Subchannel 10 output ');axis ('square' );
subplot (223) ;plot ( z_data (15,l:800) ,'.');
% subchannel 15 output
xlabel ('Real') ;ylabel ('Imaginary ');
title (' (c) Subchannel 15 output ');axis ('square');
subplot (224) ;plot (z_data (:,l:800) , 'b. '); % mixed subchannel output
xlabel ('Real') ;ylabel ('Imaginary ');
title ('(d) Mixed OFDM output') ;axis ('square' );
% Plot the average OFDM SER versus SER under "ideal channel"
% By Disabling 5 poor subcarriers, average SER can be reduced .
figure (4) ;
figc=semilogy ( SNRa ,Q, 'k-' ,SNR , mean ( SEReq) , 'b- o' , ...
SNR,mean ([SEReq(l:14, :) ;SEReq(20:32, :)]), 'b- s');
set ( figc , 'LineWidth ' ,2);
legend ('Ideal channel ', 'Using all subcarriers ', 'Di sabling 5 poor
subcarriers ')
title ('Average OFDM SER' );
axis ([l 30 1.e-4 1] ) ;hold off;
xlabel ('SNR (dB) ') ;ylabel ('Symbol Error Rate (SER) ');
First, we display the subchannel gain H[n] in Fig. 12.25. We can clearly see that, among the 32
subchannels, the 5 near the center have the lowest gains and hence the lowest SNR. We therefore expect
them to exhibit the worst performance. By fixing the average channel SNR at 30 dB, we can take a quick
peek at the equalizer outputs of the different subcarrier equalizers. In particular, we select subchannels 1,
I 0, and 15 because they represent the moderate, good, and poor channels, respectively. Scatter plots of the
output samples (Fig. 12.26a-c) clearly demonstrate the quality contrast among them. If we do not make
any distinction among subchannels, we can see from Fig. 12.26d that the overall OFDM performance is
dominated mainly by the poor subchannels.
We can also look at the SER of all 32 individual subcarriers in Fig. 12.27. We see very clearly that
the 5 worst channels are responsible for the 5 worst SER performances. Naturally if we average the SER