On Sep 10, 8:37�pm, phil <phi...@gmail.com> wrote:
> Thanks for the replies. Yes, I agree. We need to be protective from
> adjacent bands.
>
> But what if the following? Suppose each band is 20MHz but the total
> OFDM signal takes a bandwidth (counting the null tones and all others)
> of only 16MHz, while the 5MHz on the boundaries (2MHz each side) are
> kept unused for interference avoidance between adjacent bands. I think
> we can use those null tones inside the OFDM modulation.
>
> The bandwidth efficiency would be 16/20, around 80%. Of course, the
> numbers here are only for illustration purpose and the actual
> efficiency depends on actual implementation and design.
>
> Do you think there is any serious problem with the above? Please share
> your criticism/comment. Thanks.
>
> On Sep 9, 10:42�pm, Krishna <krishna.pil...@gmail.com> wrote:
>
> > On Sep 10, 2:22�am, phil <phi...@gmail.com> wrote:> Hi there,
>
> > > My question is regarding the null tones, which are usually placed on
> > > the two boundaries of the total 64 subcarriers. My understanding of
> > > using null tones is that, we can ease the filter implementation
> > > complexity/cost in the following DAC. I heard that doing so, we may
> > > skip oversampling (or an oversampling factor of 1). Please correct me
> > > if I am wrong.
can anybody please help to comment here?
thanks
>
> > [kp] Well, in typical designs even though there are null tones, there
> > will be an oversampling factor of 2 (or even 3). The objective of
> > oversampling is to remove the aliases created outside the desired
> > band.
>
> > > But, can I choose not to force those boundary tones to be null tones
> > > and instead transmit data on those tones? My current understanding is
> > > that, I need to do oversampling (say factor 2) in order to make the
> > > filter implementation affordable.
>
> > > My ultimate question is, if I am willing to do oversampling instead of
> > > forcing null tones on the boundaries, is it a workable OFDM
> > > modulation? Do I need to do anything else if I do not use any null
> > > tones?
>
> > [kp] It might create problems when the available spectrum is divided
> > in multiple channels.
> > The right most subcarrier of your channel might interfere with left
> > most subcarrier from a similar
> > transmission happening on the adjacent band.
>
> > > Please help. Thanks
>
> > Hope this helps.
> > Krishna
> > ~blogs @http://www.dsplog.com
Reply by phil●September 11, 20082008-09-11
Thanks for the replies. Yes, I agree. We need to be protective from
adjacent bands.
But what if the following? Suppose each band is 20MHz but the total
OFDM signal takes a bandwidth (counting the null tones and all others)
of only 16MHz, while the 5MHz on the boundaries (2MHz each side) are
kept unused for interference avoidance between adjacent bands. I think
we can use those null tones inside the OFDM modulation.
The bandwidth efficiency would be 16/20, around 80%. Of course, the
numbers here are only for illustration purpose and the actual
efficiency depends on actual implementation and design.
Do you think there is any serious problem with the above? Please share
your criticism/comment. Thanks.
On Sep 9, 10:42�pm, Krishna <krishna.pil...@gmail.com> wrote:
> On Sep 10, 2:22�am, phil <phi...@gmail.com> wrote:> Hi there,
>
> > My question is regarding the null tones, which are usually placed on
> > the two boundaries of the total 64 subcarriers. My understanding of
> > using null tones is that, we can ease the filter implementation
> > complexity/cost in the following DAC. I heard that doing so, we may
> > skip oversampling (or an oversampling factor of 1). Please correct me
> > if I am wrong.
>
> [kp] Well, in typical designs even though there are null tones, there
> will be an oversampling factor of 2 (or even 3). The objective of
> oversampling is to remove the aliases created outside the desired
> band.
>
> > But, can I choose not to force those boundary tones to be null tones
> > and instead transmit data on those tones? My current understanding is
> > that, I need to do oversampling (say factor 2) in order to make the
> > filter implementation affordable.
>
> > My ultimate question is, if I am willing to do oversampling instead of
> > forcing null tones on the boundaries, is it a workable OFDM
> > modulation? Do I need to do anything else if I do not use any null
> > tones?
>
> [kp] It might create problems when the available spectrum is divided
> in multiple channels.
> The right most subcarrier of your channel might interfere with left
> most subcarrier from a similar
> transmission happening on the adjacent band.
>
> > Please help. Thanks
>
> Hope this helps.
> Krishna
> ~blogs @http://www.dsplog.com
Reply by Krishna●September 10, 20082008-09-10
On Sep 10, 2:22�am, phil <phi...@gmail.com> wrote:
> Hi there,
>
> My question is regarding the null tones, which are usually placed on
> the two boundaries of the total 64 subcarriers. My understanding of
> using null tones is that, we can ease the filter implementation
> complexity/cost in the following DAC. I heard that doing so, we may
> skip oversampling (or an oversampling factor of 1). Please correct me
> if I am wrong.
[kp] Well, in typical designs even though there are null tones, there
will be an oversampling factor of 2 (or even 3). The objective of
oversampling is to remove the aliases created outside the desired
band.
> But, can I choose not to force those boundary tones to be null tones
> and instead transmit data on those tones? My current understanding is
> that, I need to do oversampling (say factor 2) in order to make the
> filter implementation affordable.
>
> My ultimate question is, if I am willing to do oversampling instead of
> forcing null tones on the boundaries, is it a workable OFDM
> modulation? Do I need to do anything else if I do not use any null
> tones?
>
[kp] It might create problems when the available spectrum is divided
in multiple channels.
The right most subcarrier of your channel might interfere with left
most subcarrier from a similar
transmission happening on the adjacent band.
Reply by Vladimir Vassilevsky●September 9, 20082008-09-09
Null tones provide the guard band. If you place the data into the guard
band, then you will spill it into the adjacent channel (and/or pick the
spillage from the adjacent channel).
VLV
phil wrote:
> Hi there,
>
> I have a question on OFDM. As we know, most OFDM systems nowadays do
> not use all the available tones (subcarriers) for data modulation.
> Let us say we have 64 tones in total and we might end up with 48 data
> tones, 4 pilot tones, 3 DC tones and 9 null tones, which is kind of an
> typical setup.
>
> My question is regarding the null tones, which are usually placed on
> the two boundaries of the total 64 subcarriers. My understanding of
> using null tones is that, we can ease the filter implementation
> complexity/cost in the following DAC. I heard that doing so, we may
> skip oversampling (or an oversampling factor of 1). Please correct me
> if I am wrong.
>
> But, can I choose not to force those boundary tones to be null tones
> and instead transmit data on those tones? My current understanding is
> that, I need to do oversampling (say factor 2) in order to make the
> filter implementation affordable.
>
> My ultimate question is, if I am willing to do oversampling instead of
> forcing null tones on the boundaries, is it a workable OFDM
> modulation? Do I need to do anything else if I do not use any null
> tones?
>
> Please help. Thanks
Reply by phil●September 9, 20082008-09-09
Hi there,
I have a question on OFDM. As we know, most OFDM systems nowadays do
not use all the available tones (subcarriers) for data modulation.
Let us say we have 64 tones in total and we might end up with 48 data
tones, 4 pilot tones, 3 DC tones and 9 null tones, which is kind of an
typical setup.
My question is regarding the null tones, which are usually placed on
the two boundaries of the total 64 subcarriers. My understanding of
using null tones is that, we can ease the filter implementation
complexity/cost in the following DAC. I heard that doing so, we may
skip oversampling (or an oversampling factor of 1). Please correct me
if I am wrong.
But, can I choose not to force those boundary tones to be null tones
and instead transmit data on those tones? My current understanding is
that, I need to do oversampling (say factor 2) in order to make the
filter implementation affordable.
My ultimate question is, if I am willing to do oversampling instead of
forcing null tones on the boundaries, is it a workable OFDM
modulation? Do I need to do anything else if I do not use any null
tones?
Please help. Thanks