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

722 DIGITAL COMMUNICATIONS UNDER LINEARLY DISTORTIVE CHANNELS
COMPUTER EXERCISE 12.2: DECISION FEEDBACK EQUALIZATION
In this exercise, we use the main MATLAB program, Exl 2_2 . m, to generate the same kind of data as
in the last exercise. The main difference is that we adopt a slightly different two-ray multipath channel
h(t) = g(t) - 0.83g(t - 3T /8)
in which the ISI is much more severe. At the receiver, instead of using linear equalizers, we will implement
and test the decision feedback equalizer (DFE) as described in Sec. 12.6. For simplicity, we will implement
only a DFE feedback filter, without using the FFW filter.
% Matlab Program <Ex12_2 .m>
% This Matlab exercise <Ex12_2 .m> performs simulation of
% decision feedback equalization under QAM- 16 baseband transmission
% a multipath channel with AWGN .
% Correct carrier and synchronization is assumed .
% Root-raised cosine pulse of rolloff factor = 0.5 is used
% Matched filter is applied at the receiver front end .
% The program estimates the symbol error rate (SER) at different Eb/N
clear;clf;
L=l00000;
% Total data symbols in experiment is 1 mi llion
% To display the pulse shape , we oversample the signal
% by factor of f_ovsamp=B
f_ovsamp=B;
% Oversampling factor vs data rate
delay_rc=4;
% Generating root-raised cosine pulseshape (rolloff factor = 0.5)
prcos=rcosflt ( [ 1 ], 1, f_ovsamp ,
prcos=prcos (l:end- f_ovsamp+ l) ;
prcos=prcos/norm (prcos);
pcmatch=prcos (end : -1:1) ;
'sqrt', 0.5, delay_rc ); % RRC pulse
% remove O's
% normalize
% MF
% Generating random signal data for polar signaling
s_data=4* round (rand (L,1) ) +2* round (rand (L, l) ) -3+...
+j *(4 * round (rand (L,1))+2*round (rand(L,l) )-3) ;
% upsample to match the 'oversampling rate ' (normalize by 1/T) .
% It is f_ovsamp /T (T=l is the symbol duration)
s_up=upsample ( s_data, f_ovsamp) ;
% Identify the decision delays due to pulse shaping
% and matched filters
delayrc=2*delay_rc* f_ovsamp ;
% Generate polar signaling of different pulse-shaping
xrcos=conv ( s_up ,prcos);
[c_num ,c_den] = cheby2 (12,20, (1+ 0.5) /8) ;
% The next commented line finds frequency response
%[H ,fnlz]=freqz ( c_num , c_den , 512,8) ;
% The lowpass filter is the Tx filter before signal is sent to channel
xchout=filter ( c_num ,c_den ,xrcos) ;
% We can now plot the power spectral densities of the two signals
% xrcos and xchout
% This shows the filtering effect of the Tx filter before
% transmission in terms of the signal power spectral densities