Comments on: Transmit pulse shaping filter – rectangular and sinc (Nyquist)

By: Krishna Pillai

Krishna Pillai — Sat, 20 Jun 2009 04:12:03 +0000

@joel: Thanks for noticing and reporting it. For some reason, my Octave version cleanly handles that case and outputs the value 1. Hence I missed it. I corrected it, anyways.

By: joel

joel — Sun, 14 Jun 2009 01:33:04 +0000

Dear Krishna,

there is a small error in your script, the sinc function is undefined there for t=0. This value shall be set to 1.

Regards

By: Krishna Pillai

Krishna Pillai — Sun, 31 May 2009 15:18:22 +0000

@kiran: You may use the command rcosflt() in Matlab
http://www.mathworks.com/access/helpdesk/help/toolbox/comm/ref/rcosflt.html

By: kiran cp

kiran cp — Mon, 25 May 2009 11:24:29 +0000

hi friends. i want to design low pass multi stage raised cosine filter. how to design multi stage filter with raised cosine response. filter specification are:

sampling frequency = 15.36 MHz
rolloff factor = 0.22
cuttoff frequency = 3.84 MHz
passband frequency = 4.5 MHz

please tell me how to design.

By: Krishna Pillai

Krishna Pillai — Tue, 12 May 2009 00:09:23 +0000

@Jamil: To model inter carrier interference (ICI) in an OFDM system, you may introduce frequency offset. The code in the post http://www.dsplog.com/2008/03/03/frequency-offset-estimation-using-80211a-short-preamble/ will hopefully provide you a reference for how to introduce carrier frequency offset.

Removing the effect of intercarrier interference after taking FFT is difficult (unless the effect of ICI is small). Else you may estimate carrier frequency offset in time domain and remove the effect of that in time domain itself prior to taking FFT.

Hope this helps.

By: Jamil

Jamil — Tue, 05 May 2009 11:30:30 +0000

i have a question .plz ans me
how can i implement ICI and eliminate that in our OFDM system.

By: Krishna Pillai

Krishna Pillai — Sat, 04 Apr 2009 11:35:33 +0000

@Claire: Well both are oversampling, but there is no direct comparison between both the approaches. However, there is a nice time-frequency duality to understand.

With the 64pt FFT vs 128pt FFT case, its oversampling in frequency domain. If for example, we use only even subcarriers in a 128pt, then in time domain the first 64 samples at the o/p of 128pt iFFT will be same as the last 64 samples.

The other case, where we insert zeros between samples, is over sampling in time domain. If for example, we insert a zero between every samples and observe the spectrum, we can see that the same spectrum is repeated twice.