LTE, FFT, OFDM

On Sun, 29 Nov 2015 19:33:51 +0000 (UTC), spope33@speedymail.org
(Steve Pope) wrote:

>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>
>>On Sun, 29 Nov 2015 19:02:01 +0000 (UTC), spope33@speedymail.org
>
>>>the cyclic prefix is there for a reason, and
>>>it is only after a certain amount of receiver processing --
>>>that uses the entire signal format, including the cyclic prefix,
>>>to compensate for impairments -- that the signal can then be time-sliced, 
>>>and becomes orthogonal.  
>
>>The nice thing about the CP is that you don't really have to do any
>>processing with it in the receiver.   You can just throw it away.
>>It's done it's job just by being there when the signal went through
>>the channel.
>
>>You just need synchronization to get the subcarrier orthogonality
>>back, and there are a variety of ways to do that.  Some common
>>synchronization methods use the CP, but it isn't necessary to do so.
>>Regardless, once synchronization is achieved, which can be done with
>>or without the CP, the only time-slicing is to take the synchronized
>>samples from the symbol and put them in the FFT (or iFFT, whichever
>>doesn't match the modulator). 
>
>>The question hasn't been about synchronization, so I think it would be
>>counterproductive to muddy up an answer with that when it isn't
>>relevant to the question.    Assuming perfect synchronization is a
>>very typical thing in comm when talking about other parts of the
>>system.   That's very appropriate here.
>
>It's certainly appropriate -- in fact necessary, in a tutorial sense --
>to talk about toy OFDM systems in which the cyclic prefix and its 
>processing can be either left out entirely, or ignored.

The CP length or presence or absence or the processing thereof has no
affect on effective subcarrier width in an OFDM system, so I would
disagree with that statement in the context of answering the OPs
questions in that area so far.  There are lots of parts and pieces to
OFDM systems that have not been touched on here, appropriately so
because they're not germaine to the question.

Not sure what a "toy OFDM system" is, either.

>However, it is better to make clear this is what you're doing 
>from the outset.  Otherwise you find yourself arguing against true
>statements... 

It is counterproductive to go back to the Big Bang to answer every
question, although I do know people who seem to think otherwise.   An
answer, even a true one, that produces more confusion than clarity is
not helpful in most cases.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

>The CP length or presence or absence or the processing thereof has no
>affect on effective subcarrier width in an OFDM system, so I would
>disagree with that statement in the context of answering the OPs
>questions in that area so far.  There are lots of parts and pieces to
>OFDM systems that have not been touched on here, appropriately so
>because they're not germaine to the question.

Well, the question was whether the subcarrier width is the same
as the subcarrier spacing; so the possible differences between these 
two was thereby "touched upon".

A readily-explainable reason why the two will have different 
values is germane to this question, even if the question was not
(in your opinion) gemane to the remainder of the discussion.


Steve

On Mon, 30 Nov 2015 03:27:36 +0000 (UTC), spope33@speedymail.org
(Steve Pope) wrote:

>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>
>>The CP length or presence or absence or the processing thereof has no
>>affect on effective subcarrier width in an OFDM system, so I would
>>disagree with that statement in the context of answering the OPs
>>questions in that area so far.  There are lots of parts and pieces to
>>OFDM systems that have not been touched on here, appropriately so
>>because they're not germaine to the question.
>
>Well, the question was whether the subcarrier width is the same
>as the subcarrier spacing; so the possible differences between these 
>two was thereby "touched upon".

And if the idea is to understand the workings of an OFDM system, which
I think was the case, an answer in that context is, IMHO, better than
one outside of it.

>A readily-explainable reason why the two will have different 
>values is germane to this question, even if the question was not
>(in your opinion) gemane to the remainder of the discussion.

Which has zero to do with the CP unless one is interested in splitting
the finest of hairs.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

>(Steve Pope) wrote:


>>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

>>> There are lots of parts and pieces to
>>> OFDM systems that have not been touched on here, appropriately so
>>> because they're not germaine to the question.

>>Well, the question was whether the subcarrier width is the same
>>as the subcarrier spacing; so the possible differences between these 
>>two was thereby "touched upon".

>And if the idea is to understand the workings of an OFDM system, which
>I think was the case, an answer in that context is, IMHO, better than
>one outside of it.

I think the disconnect here is that you believe my reply to
the OP's question, which was all of eight sentences, somehow
derailed the preferred (by who?) context of the discussion, rather 
than contributing to the discussion.

I think this belief has gotta be baseless.

Carry on...


S.

On Mon, 30 Nov 2015 08:51:15 +0000 (UTC), spope33@speedymail.org
(Steve Pope) wrote:

>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>
>>(Steve Pope) wrote:
>
>
>>>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>
>>>> There are lots of parts and pieces to
>>>> OFDM systems that have not been touched on here, appropriately so
>>>> because they're not germaine to the question.
>
>>>Well, the question was whether the subcarrier width is the same
>>>as the subcarrier spacing; so the possible differences between these 
>>>two was thereby "touched upon".
>
>>And if the idea is to understand the workings of an OFDM system, which
>>I think was the case, an answer in that context is, IMHO, better than
>>one outside of it.
>
>I think the disconnect here is that you believe my reply to
>the OP's question, which was all of eight sentences, somehow
>derailed the preferred (by who?) context of the discussion, rather 
>than contributing to the discussion.
>
>I think this belief has gotta be baseless.
>
>Carry on...

If a young primary-school student asks "Is 2+2 always 4?",  I think
the proper answer is "Yes, it is."     In my view your answer is akin
to replacing that with, "No, if the answer is in binary 2+2 is 100."
The answer is technically correct, but not appropriate.

OFDM is often tricky even for experienced people to get their heads
around when they first start to learn it, so I think answers that
confuse rather than clarify are unhelpful.   I think your answer falls
in that category.

It's not baseless.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

>On Mon, 30 Nov 2015 08:51:15 +0000 (UTC), spope33@speedymail.org

>>I think the disconnect here is that you believe my reply to
>>the OP's question, which was all of eight sentences, somehow
>>derailed the preferred (by who?) context of the discussion, rather 
>>than contributing to the discussion.

>>I think this belief has gotta be baseless.

>If a young primary-school student asks "Is 2+2 always 4?",  I think
>the proper answer is "Yes, it is."     In my view your answer is akin
>to replacing that with, "No, if the answer is in binary 2+2 is 100."
>The answer is technically correct, but not appropriate.
>
>OFDM is often tricky even for experienced people to get their heads
>around when they first start to learn it, so I think answers that
>confuse rather than clarify are unhelpful.   I think your answer falls
>in that category.
>
>It's not baseless.

And I think that's totally whack.  I have not confused anyone.

Steve

On Tue, 1 Dec 2015 00:26:28 +0000 (UTC), spope33@speedymail.org (Steve
Pope) wrote:

>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>
>>On Mon, 30 Nov 2015 08:51:15 +0000 (UTC), spope33@speedymail.org
>
>>>I think the disconnect here is that you believe my reply to
>>>the OP's question, which was all of eight sentences, somehow
>>>derailed the preferred (by who?) context of the discussion, rather 
>>>than contributing to the discussion.
>
>>>I think this belief has gotta be baseless.
>
>>If a young primary-school student asks "Is 2+2 always 4?",  I think
>>the proper answer is "Yes, it is."     In my view your answer is akin
>>to replacing that with, "No, if the answer is in binary 2+2 is 100."
>>The answer is technically correct, but not appropriate.
>>
>>OFDM is often tricky even for experienced people to get their heads
>>around when they first start to learn it, so I think answers that
>>confuse rather than clarify are unhelpful.   I think your answer falls
>>in that category.
>>
>>It's not baseless.
>
>And I think that's totally whack.  I have not confused anyone.

How do you know?  Given that the full audience is unknown, I don't
think you have any way of knowing.  ;)


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

eric.jacobsen@ieee.org (Eric Jacobsen) writes:

> On Mon, 30 Nov 2015 03:27:36 +0000 (UTC), spope33@speedymail.org
> (Steve Pope) wrote:
>
>>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>>
>>>The CP length or presence or absence or the processing thereof has no
>>>affect on effective subcarrier width in an OFDM system, so I would
>>>disagree with that statement in the context of answering the OPs
>>>questions in that area so far.  There are lots of parts and pieces to
>>>OFDM systems that have not been touched on here, appropriately so
>>>because they're not germaine to the question.
>>
>>Well, the question was whether the subcarrier width is the same
>>as the subcarrier spacing; so the possible differences between these 
>>two was thereby "touched upon".
>
> And if the idea is to understand the workings of an OFDM system, which
> I think was the case, an answer in that context is, IMHO, better than
> one outside of it.

Eric / Steve,

I don't know what you mean by "inside/outside" but this question
has perked my interest in the topic. Why IS the carrier bandwidth
different than the carrier spacing?

I am woefully ignorant on the subject, but isn't the answer independent
of OFDM? Isn't it that the FFT can viewed as a set of bandpass filters
(a filter bank), and that each of these filters are not Fs / N wide but
something less due to the mechanics of the FFT? 

By the way, I've watched you guys argue for days (weeks?) now. I wish
you would both acknowledge each other's maturity and capabilities, and
spend all this energy for positive things. I, for one, respect you both
immensely.
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com

>>4) Considering the used sub-carriers of 1200, the used bandwidth is
turns
>>out to be - 1200*1500 = 18 MHz. But in any case 2048 sub-carriers are
>>present (though not used). Hence the bandwidth is really - 2048*1500 =
30
>>MHz.
>
>Those "guard band" samples are zero, so no energy comes out at those
>frequencies other than the sidelobe leakage from the populated
>subcarriers.   The system is designed so that by the edge of the
>channel that energy is low enough, and/or can be suppressed by an IF
>filter, so that it is not an issue for the adjacent channels.

Dear Eric, Steve, Kaz,

I have couple of questions based on the response above.

1. The guard band is 20-18 = 2 MHz or 30-18 = 12 MHz?

2. Also, if guard band samples are zero, I assume that in frequency
domain, we still end up using 30 MHz and those unused frequencies are not
available for usage from a spectrum perspective.

Thanks a lot ...

---------------------------------------
Posted through http://www.DSPRelated.com

On Tue, 01 Dec 2015 12:09:14 -0600, "Sharan123" <99077@DSPRelated>
wrote:

>>>4) Considering the used sub-carriers of 1200, the used bandwidth is
>turns
>>>out to be - 1200*1500 = 18 MHz. But in any case 2048 sub-carriers are
>>>present (though not used). Hence the bandwidth is really - 2048*1500 =
>30
>>>MHz.
>>
>>Those "guard band" samples are zero, so no energy comes out at those
>>frequencies other than the sidelobe leakage from the populated
>>subcarriers.   The system is designed so that by the edge of the
>>channel that energy is low enough, and/or can be suppressed by an IF
>>filter, so that it is not an issue for the adjacent channels.
>
>Dear Eric, Steve, Kaz,
>
>I have couple of questions based on the response above.
>
>1. The guard band is 20-18 = 2 MHz or 30-18 = 12 MHz?

The 20MHz channels are adjacent to each other, so the guard band if
from the occupied part of the spectrum to the next adjacent channel,
or 20-18, split between the two sides (since there can be an adjacent
channel on each side), so 1MHz on each side. 

It is therefore imperative in the system that the additional 5MHz on
each side has significant filtering/suppression so that no energy
intrudes into the other channel.   This isn't difficult if those
subcarriers are zeros in the transmitter, and even easier if there is
an IF filter that helps out.   Having the extra transmit bandwidth
zeroed makes the design of the IF filter a bit easier.

>2. Also, if guard band samples are zero, I assume that in frequency
>domain, we still end up using 30 MHz and those unused frequencies are not
>available for usage from a spectrum perspective.

They adjacent channels are available for other transmitters in the
system to use, so they cannot be used by the original transmitter.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com