How to plot frequency response after down sampling

2018 11 28
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Hi There,
I have an FIR filter expressed in this form:
Ha = 0 -11*exp(-j*w) + 0*exp(-j*2*w) + 74*exp(-j*3*w)+ 168*exp(-j*4*w)+ 74*exp(-j*5*w)+ 0*exp(-j*6*w)-11*exp(-j*7*w)+ 0*exp(-j*8*w);
And I know how to plot the frequency reponse (x axis is frequency, and y axis is magnitude) of this filter. For example, like this:
w=-2*pi:2*pi/400:0;
amp_max = max(abs(Ha));
amp_normalize = abs(Ha)/amp_max;
dB=mag2db(amp_normalize);%normliazed to zero db
fs=15*2*2; %sampling freq.
%plot magnitude response
plot(fs/2*w/pi,dB);
Now after this filter, one additional step is to down sample by factor of 2. That is, to discard alternate samples.
My question is: how to plot again the frequency reponse after down-sampling? Example code or pointer to example code is highly appreciated.
Thanks.

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