FPGA Central

Hi all,
I have just started reading about the OFDM systems .I have read in
some paper that zero padding is advantageous over Cyclic prefix
approach.
In most of the systems i have found Cyclic prefix there not zero
padding.
ONe more thing ,in a particular system 64 pt IFFT/FFT was taken .in
which 52 were carriers(48 data + 4 pilots) rest were zeros.
and 16 CP.
Is there any advantage of using 12 zeros?
I could not find any reference for the same if any paper is there
please send me the one.
All i could make out is that it may reduce the PAPR.

With regards
pria

> One more thing ,in a particular system 64 pt IFFT/FFT was taken .in
> which 52 were carriers(48 data + 4 pilots) rest were zeros.
> and 16 CP.

This is from the IEEE 802.11 Standard.

On Jun 17, 8:44*pm, Porterboy 76 <porterbo...@gmail.com> wrote:
> > One more thing ,in a particular system 64 pt IFFT/FFT was taken .in
> > which 52 were carriers(48 data + 4 pilots) rest were zeros.
> > and 16 CP.
>
> This is from the IEEE 802.11 Standard.

ya but i want to know whats the advantage of padding zeros .

>On Jun 17, 8:44=A0pm, Porterboy 76 <porterbo...@gmail.com> wrote:
>> > One more thing ,in a particular system 64 pt IFFT/FFT was taken .in
>> > which 52 were carriers(48 data + 4 pilots) rest were zeros.
>> > and 16 CP.
>>
>> This is from the IEEE 802.11 Standard.
>
>ya but i want to know whats the advantage of padding zeros .
>

Hi,
Zero padding increases the sampling rate. In other words the resolution o
the signal increases...
Where as CP provides resistance against inter symbol interference..(ISI)

Good Luck..

On Jun 18, 11:43*am, "san3885" <santosh.3...@gmail.com> wrote:
> >On Jun 17, 8:44=A0pm, Porterboy 76 <porterbo...@gmail.com> wrote:
> >> > One more thing ,in a particular system 64 pt IFFT/FFT was taken .in
> >> > which 52 were carriers(48 data + 4 pilots) rest were zeros.
> >> > and 16 CP.
>
> >> This is from the IEEE 802.11 Standard.
>
> >ya but i want to know whats the advantage of padding zeros .
>
> Hi,
> Zero padding increases the sampling rate. In other words the resolution of
> the signal increases...
> Where as CP provides resistance against inter symbol interference..(ISI)
>
> Good Luck..

hi
you sure about it.Interpolation increases sampling freq , thats
true,but here we aren't doing the same we are appending zeros at the
end.
Will it really increase the resolution?

>On Jun 18, 11:43=A0am, "san3885" <santosh.3...@gmail.com> wrote:
>> >On Jun 17, 8:44=3DA0pm, Porterboy 76 <porterbo...@gmail.com> wrote:
>> >> > One more thing ,in a particular system 64 pt IFFT/FFT was take
.in
>> >> > which 52 were carriers(48 data + 4 pilots) rest were zeros.
>> >> > and 16 CP.
>>
>> >> This is from the IEEE 802.11 Standard.
>>
>> >ya but i want to know whats the advantage of padding zeros .
>>
>> Hi,
>> Zero padding increases the sampling rate. In other words the resolutio
o=
>f
>> the signal increases...
>> Where as CP provides resistance against inter symbo
interference..(ISI)
>>
>> Good Luck..
>
>hi
>you sure about it.Interpolation increases sampling freq , thats
>true,but here we aren't doing the same we are appending zeros at the
>end.
>Will it really increase the resolution?
>
>
No. Resolution increases only if you pad zeros in the middle (in frequenc
domain).
Are you padding zeros to both the ends or to any one ?
If you add to both the ends it will serve as guard band as well...
Also, i observe in your example above..

Data + Pilots = 52 freq bins
52 is not a power of 2, hence you cant perform IFFT/FFT operation on it.
Otherwise its a mess to perform N-point DFT/IDFT in hardware.
I think this is one of the main reason for zero pad.
May be someone can throw more light on this...

On Jun 17, 10:41*am, priya <kaushik.priya...@gmail.com> wrote:
> Hi all,
> I have just started reading about the OFDM systems .I have read in
> some paper that zero padding is advantageous over Cyclic prefix
> approach.
> In most of the systems i have found Cyclic prefix there not zero
> padding.

True. The padding is in time domain.

> ONe more thing ,in a particular system 64 pt IFFT/FFT was taken .in
> which 52 were carriers(48 data + 4 pilots) rest were zeros.
> and 16 CP.
> Is there any advantage of using 12 zeros?

This padding is different from the previous one as this padding is in
frequency domain. If you do not pad zeros, what else could you pad?

> I could not find any reference for the same if any paper is there
> please send me the one.
> All i could make out is that it may reduce the PAPR.
>
> With regards
> pria

On 6/17/2009 7:41 AM, priya wrote:
> Hi all,
> I have just started reading about the OFDM systems .I have read in
> some paper that zero padding is advantageous over Cyclic prefix
> approach.
> In most of the systems i have found Cyclic prefix there not zero
> padding.

There are some disadvantages to zero-padding instead of using a cyclic
prefix. It's been discussed here before, so maybe search around.

> ONe more thing ,in a particular system 64 pt IFFT/FFT was taken .in
> which 52 were carriers(48 data + 4 pilots) rest were zeros.
> and 16 CP.
> Is there any advantage of using 12 zeros?

You need a guard region to prevent aliasing around the edge subcarriers.
In other words, to use the edge subcarriers one would need a brickwall
anti-alias filter, and those don't exist.

> I could not find any reference for the same if any paper is there
> please send me the one.
> All i could make out is that it may reduce the PAPR.
>
> With regards
> pria