Post by maran on Apr 11, 2011 17:52:12 GMT -5
Hi guys, Im designing an OFDM system, with OFDM symbol timing of 66.7 us (15 kHz) and 20 MHz bandwidth.
So I use the normal process in matlab, after IFFT I added a cyclic prefix, I have built my entire system and the back to back works perfectly fine.
The problem is, I dont know how to fix my symbol timing as 66.7 us and I dont know my sampling rate right out of IFFT modulator cause its a default function of matlab.
The problem is, Im upsampling after adding cyclic prefix to 20 MHz, im doing this by taking the signal and multiplying it, just multiplying it with a square pulse with frequency at 20 MHz.
This is how I do it in matlab:
say X is the output after adding cyclic prefix.
Y=X*square(2*pi*fc*t) where fc is 20 MHz and it is sampled with sampling frequency,fs of 4*fc.
When I look at the spectrum, its a sinc wave with first crossing at 0 Hz on left and 20 MHz at the right side. Which is totally correct and exactly what I want.
So I want to filter the rest of the harmonics after the 20 MHz crossing, the problem occurs when I use a low pass filter with same (fs) and when I look at the spectrum after filtering, the first crossing disappeared even after I compensated the delay, its like some wave overlapping at the first crossing. And also the delay, I could only compensate for certain vectors, some of them are still not right after the filtering.
I thought may be its because of the sampling frequency offset cause I dont have the default sampling frequency of IFFT, please help me, I really dont have any idea of solving this problem, Im stuck here and cant move further and my block diagram is right compare to all other references!
I even thought windowing will solve, but then references only use it for PAPR reduction which is not in my concern at the moment. Please help.
Also guide me on setting the symbol period.
Thank you very much!
So I use the normal process in matlab, after IFFT I added a cyclic prefix, I have built my entire system and the back to back works perfectly fine.
The problem is, I dont know how to fix my symbol timing as 66.7 us and I dont know my sampling rate right out of IFFT modulator cause its a default function of matlab.
The problem is, Im upsampling after adding cyclic prefix to 20 MHz, im doing this by taking the signal and multiplying it, just multiplying it with a square pulse with frequency at 20 MHz.
This is how I do it in matlab:
say X is the output after adding cyclic prefix.
Y=X*square(2*pi*fc*t) where fc is 20 MHz and it is sampled with sampling frequency,fs of 4*fc.
When I look at the spectrum, its a sinc wave with first crossing at 0 Hz on left and 20 MHz at the right side. Which is totally correct and exactly what I want.
So I want to filter the rest of the harmonics after the 20 MHz crossing, the problem occurs when I use a low pass filter with same (fs) and when I look at the spectrum after filtering, the first crossing disappeared even after I compensated the delay, its like some wave overlapping at the first crossing. And also the delay, I could only compensate for certain vectors, some of them are still not right after the filtering.
I thought may be its because of the sampling frequency offset cause I dont have the default sampling frequency of IFFT, please help me, I really dont have any idea of solving this problem, Im stuck here and cant move further and my block diagram is right compare to all other references!
I even thought windowing will solve, but then references only use it for PAPR reduction which is not in my concern at the moment. Please help.
Also guide me on setting the symbol period.
Thank you very much!