Does the MATLAB's ifft()/fft() do any spectral shaping ?

Greetings,

I generate an OFDM signal using :

Qam = qammod(randi([0 3],Nfft,1),M);
OFDMsym = ifft(Qam,Nfft);

OFDMsym_CP = [OFDMsym(end-Ncp+1:end); OFDMsym];

This is then transmitted using SDR (USRP2+RFX2400). When I view the spectrum it looks like the following:

http://i.imgur.com/WYTgw.png

It appears that there is some spectral shaping happening at the ends of the bands. The bandwidth I am using to transmit is well within the bandwidth limit offered by the hardware so it was suggested from the support mailing list that MATLAB was doing some spectral shaping. When I turn off say, 50-75 subcarriers on opposite sides of my OFDM band, I see a normal 'OFDM-like' spectrum:

http://i.imgur.com/jVXqu.png

How can I fix this? In these sub carriers I see my QAM constellation points drawn towards the origin. For ex. :

http://i.imgur.com/wxxuy.png

Thanks in advance. Best Regards, Sanat

Answer 1

[Walter Roberson]

I would be pretty sure that fft() and ifft() do not do any spectral shaping. I do not have any information about qammod() though.

Answer 2

[Rick Rosson]

Walter is correct. FFT does not do any shaping. It is an implementation of the DFT, plain and simple. MATLAB does provide additional functions that do provide windowing and shaping. Please search the doc for PSD, pwelch, and windowing for more info.