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Why Padding Zeros before OFDM IFFT?

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davyzhu

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ifft ofdm

Hi all,

In many OFDM system(like 802.11a and DVB-T), they always padding zeros to data before go to the IFFT module. Why use padding zeros?

Best regards,
Davy
 

ofdm ifft

this is related to window size and quality of IFFT
 

ifft zero padding

It is important as well when you deal with PAPR ....
 

ofdm zero padding before ifft

Hi,

I am a newbie on this subject.
But what is PAPR?

Regards,
Davy
 

why we use ifft algorithm in ofdm

PAPR=Peak to Average Power Ratio. It is a serious problem in OFDM system because normal front-end circuit cannot afford large PAPR.
 

zero padding ofdm

hello
for FFT or IFFT all program(i.e. Matlab)make priodic function to calculte.
because of above the input and out put matrixes have same dimention. but we know in convolution N*M has N+M-1. for addopte this we padding zero.
above is the very short reply by my knowlege.
we hope these are is usefull.
 

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    davyzhu

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zero padding ifft

hi,
The zeros are padded to make bin size of IFFT an integer no. For example, if the sampling frequency is 2048 khz and i want to keep the bins 1 khz apart, then i should do 2048 point fft.
 

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    davyzhu

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ofdm+ifft

Zero padding is performed on both sides of the OFDM symbol and acts like a guard band (not cyclic prefix) as for example in 802.11 you have 48 data subcarrier and 3 pilot carrier which makes 52 subcarrier while you perform FFT or IFFT at 64, so you you 'add zeros' at the front and at the end of the signal.
 
ifft+ofdm

in order to FFT/IFFT algorithms be fast and efficient ,the size of FFT must be an integer which is a power of 2 .If we have less FFT points, we pad the remaining points so that the total number of FFT points become a power of 2.
 

fft ifft 　パディング

can somebody explaine more about zero pad

appreciated.
 

Re: fft ifft 　パディング

Aya2002 said:
can somebody explaine more about zero pad

appreciated.
Click to expand...

Hi Aya2002

we could simply use zero padded pre/postfix, and before the DFT at the receiver copy the beginning and end of frame as prefix and postfix. This creates the same effect of cyclic extension with the advantage of reducing transmit power and causing less ISI.

regards
 

Hi I am new to OFDM, i will like to know how I can use matlab to design a simple OFDM signal,
 

besongsamuel said:
Hi I am new to OFDM, i will like to know how I can use matlab to design a simple OFDM signal,
Click to expand...



firstly: you must understand the basic principle of operation of the OFDM then you can use matlab to simulate it.
 

I'll be honest here and say that making programs for OFDM in Matlab helped me understand the theory of OFDM even more. Maybe I'm a special case :)

Of course, you need to learn about OFDM in theory first!
 

There is something I am confused with. I read the paper on cyclic prefixing and zero padding and in this paper, the signal is padded after IFFT at the transmitter. But in OFDM system designs, the zero padding is done before IFFT. What is the difference???
 

besongsamuel said:
There is something I am confused with. I read the paper on cyclic prefixing and zero padding and in this paper, the signal is padded after IFFT at the transmitter. But in OFDM system designs, the zero padding is done before IFFT. What is the difference???
Click to expand...

send to me the paper to see that
 

besongsamuel said:
There is something I am confused with. I read the paper on cyclic prefixing and zero padding and in this paper, the signal is padded after IFFT at the transmitter. But in OFDM system designs, the zero padding is done before IFFT. What is the difference???
Click to expand...


That doesn't seem right.
 

Could you please look into my answers about the zero padding in the page

(What are pilot carriers for ?)
 

there are 2 main concept in OFDM, Synchronization issue and channel estimation calcualtion, for which different methods are used.

Some algorithm use zeropadding for synchronization like time and frequency. some use pilot and some use some sequence called preamble.

If pilots are used , some are used for synchronization and some are used for channel estimation.

Zero padding mainly used for time and frequency synchronization.

zero padding before IFFT is done then it is just to make system robust. if after IFFt is done to get the size of FFT an integer which is a power of 2.

instead of Cyclic prefix , zero padding also dome to reduce the power consumption.

Added after 2 minutes:

Its all depends on which way u want to synchronize ur receiver and how u want to estimate ur channel.....the algorithm there will decide the design of OFDM singal....
 

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