Comments on: Understanding an OFDM transmission http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/ Signal Processing for Communication Fri, 10 Feb 2012 01:03:15 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: Krishna Sankar http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-101382 Krishna Sankar Fri, 10 Feb 2012 00:54:09 +0000 http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-101382 @Michael: Well, the sampling frequency is a notional in matlab. So, if its 20Hz it means we can 'see' frequencies from [-10Hz to +10Hz); and if it is 20MHz the range is from [-10MHz to +10MHz). Helps? @Michael: Well, the sampling frequency is a notional in matlab. So, if its 20Hz it means we can ‘see’ frequencies from [-10Hz to +10Hz); and if it is 20MHz the range is from [-10MHz to +10MHz).

Helps?

]]> By: Michael http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-100600 Michael Mon, 06 Feb 2012 15:06:59 +0000 http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-100600 Hi Krishna, i have a question about the pwelch() function ,in ur code,like this: ************************************************************* fsMHz = 20; [Pxx,W] = pwelch(st,[],[],4096,20); plot([-2048:2047]*fsMHz/4096,10*log10(fftshift(Pxx))); xlabel('frequency, MHz') ylabel('power spectral density') title('Transmit spectrum OFDM (based on 802.11a)'); ************************************************************ the fifth parameter '20',i know it is the rate of sampling.But,our rate is 20 000 000,not that 20. so ,is there something i ignore or some logical in that expression,why do you put '20 ' in that place instead of 20000000? Hi Krishna,
i have a question about the pwelch() function ,in ur code,like this:
*************************************************************
fsMHz = 20;
[Pxx,W] = pwelch(st,[],[],4096,20);
plot([-2048:2047]*fsMHz/4096,10*log10(fftshift(Pxx)));
xlabel(‘frequency, MHz’)
ylabel(‘power spectral density’)
title(‘Transmit spectrum OFDM (based on 802.11a)’);
************************************************************
the fifth parameter ’20′,i know it is the rate of sampling.But,our rate is 20 000 000,not that 20.

so ,is there something i ignore or some logical in that expression,why do you put ’20 ‘ in that place instead of 20000000?

]]> By: Krishna Sankar http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-97771 Krishna Sankar Mon, 23 Jan 2012 00:39:08 +0000 http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-97771 @Jo: Well, unless you are doing some filtering, it should be close to rectangular... @Jo: Well, unless you are doing some filtering, it should be close to rectangular…

]]> By: Krishna Sankar http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-97764 Krishna Sankar Mon, 23 Jan 2012 00:29:33 +0000 http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-97764 @khalil: The concept of time between samples is notional in Matlab. For eg, if we assume that a) the time between the samples is 1/20MHz b) number of samples in a symbol is 64, then the symbol duration 64/20 = 3.2us @khalil: The concept of time between samples is notional in Matlab. For eg, if we assume that
a) the time between the samples is 1/20MHz
b) number of samples in a symbol is 64,
then the symbol duration 64/20 = 3.2us

]]> By: Jo http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-96908 Jo Wed, 18 Jan 2012 21:22:08 +0000 http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-96908 Hi Krishna, Any thoughts on reason for the OFDM spectrum not to be rectangular? I am simulating real, unipolar ofdm signal, but the psd estimate gives me half of raised-cosine filter like shape. I am intrigued. Please can you email me back if you do reply. Many Thanks. Hi Krishna,

Any thoughts on reason for the OFDM spectrum not to be rectangular? I am simulating real, unipolar ofdm signal, but the psd estimate gives me half of raised-cosine filter like shape. I am intrigued. Please can you email me back if you do reply. Many Thanks.

]]> By: khalil http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-96281 khalil Mon, 16 Jan 2012 17:01:50 +0000 http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-96281 Me too i don't find the symbol duration or exactly bit duration. Each symbol contains many bits that are transmitted with a bit duration. I don't find it in the code matlab Me too i don’t find the symbol duration or exactly bit duration.
Each symbol contains many bits that are transmitted with a bit duration.
I don’t find it in the code matlab

]]> By: MFS http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-95953 MFS Sun, 15 Jan 2012 08:28:52 +0000 http://www.dsplog.com/2008/02/03/understanding-an-ofdm-transmission/#comment-95953 @Krishna: thanks, received @Krishna: thanks, received

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