about spectrum sensing and bandpass filter

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Talha Rahman
Talha Rahman 2011-8-22
Hi there, i am doing simulation for energy detector and i am having strange problems (as it looks to me..). whenever i modulate my 5Khz carrier signal with my BPSK data....i transmit it to the channel awgn ....and when i receive it at the receiver i apply bandpass filter that results into zero detection as the energy of signal is below the threshold.so i get Probability of detection equal to zero. whereas without Bandpass filter i get my signal detected with some probability of detection. Why is that so? and also can one share the threshold calculation equation....gaminv or chi2inv....
Thanks
Talha

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Walter Roberson
Walter Roberson 2011-8-22
What are your bandpass filter settings? Pass between what and what, and roll off how quickly?
How are you doing the modulation? If you are inserting more than one bit per cycle of the carrier, then the data you are passing would be at much higher frequency than the carrier signal (especially if you are using triangular or square wave modulation as those call for infinite frequencies.)
I only have a very small amount of experience with filtering, but it seems to me that you do not want bandpass: that instead you want to filter out the 5 KHz and lower, leaving only the higher frequency data that you had modulated the carrier with.
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Talha Rahman
Talha Rahman 2011-8-22
Fs = 48000; % Sampling Frequency
Fstop1 = 1200; % First Stopband Frequency
Fpass1 = 2000; % First Passband Frequency
Fpass2 = 8000; % Second Passband Frequency
Fstop2 = 8800; % Second Stopband Frequency
Dstop1 = 0.001; % First Stopband Attenuation
Dpass = 0.057501127785; % Passband Ripple
Dstop2 = 0.0001; % Second Stopband Attenuation
These above are my bandpass settings. i created my BP filter using sptool ....modulation is done by simply multiplying row vector of 1s and -1s with sine (2pi*f* [1 length(data)]) where f is 5khz (i.e. carrier frequency). i just want to pass a band that includes 5khz frequency ...... m not considering any data rates in my simulation but still how can i generate high frequencies.....and when i observe my signal using periodogram function i get the band (as mention in BP specs above with 5khz in the center)... now?

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