On Dec 13, 11:30�am, Randy Yates <ya...@ieee.org> wrote:
> Eric Jacobsen <eric.jacob...@ieee.org> writes:
> > On Fri, 12 Dec 2008 11:26:57 -0800 (PST), Mark1 <makol...@yahoo.com>
> > wrote:
>
> >> �Whether 2k taps would be needed in an SC
> >>> system in a similar environment is arguable...8-VSB has essentially
> >>> the same BW and works in the same environments just fine with a
> >>> traditional EQ.
>
> >>There are people that would argue that point..
>
> >>I have no personal knowledge one way or the other yet, but I have seen
> >>this point sometimes hotly debated. �And will probably hear more come
> >>Feb 18...
>
> >>Mark
>
> Mark, Eric, Vladimir, et al., thanks for the illumination. I may
> come back to this in a few days.
>
> PS: Mark, see this series of posts in which we discussed (or perhaps
> we just cussed!) this very issue:
>
> http://groups.google.com/group/comp.dsp/browse_thread/thread/ceac6381...
> --
> % �Randy Yates � � � � � � � � �% "So now it's getting late,
> %% Fuquay-Varina, NC � � � � � �% � �and those who hesitate
> %%% 919-577-9882 � � � � � � � �% � �got no one..."
> %%%% <ya...@ieee.org> � � � � � % 'Waterfall', *Face The Music*, ELOhttp://www.digitalsignallabs.com- Hide quoted text -
>
> - Show quoted text -
thanks Randy..
Eric there are mnay anecdotal reports of people with strong signals as
indicated by signal strength indications and or simply the fact that
they are line of site to the Tx etc but still have erratic reception
of 8VSB. One case I read about, the guy actually had a spectrum
analyzer hooked up and can see the multipath/selective fading change
as planes fly by and corresponding to the picture break up..
I would guess that most cases of reception problems with 8VSB in
places where analog is/was possible, are due to multipath and not weak
signal. But I have no idea if OFDM would do any better. Also the EQs
in the 8VSB demods are getting better with each generation. It is
going to be interesting.
Mark
Reply by Randy Yates●December 13, 20082008-12-13
Eric Jacobsen <eric.jacobsen@ieee.org> writes:
> On Fri, 12 Dec 2008 11:26:57 -0800 (PST), Mark1 <makolber@yahoo.com>
> wrote:
>
>>
>> �Whether 2k taps would be needed in an SC
>>> system in a similar environment is arguable...8-VSB has essentially
>>> the same BW and works in the same environments just fine with a
>>> traditional EQ.
>>
>>There are people that would argue that point..
>>
>>I have no personal knowledge one way or the other yet, but I have seen
>>this point sometimes hotly debated. And will probably hear more come
>>Feb 18...
>>
>>Mark
Reply by Eric Jacobsen●December 12, 20082008-12-12
On Fri, 12 Dec 2008 11:26:57 -0800 (PST), Mark1 <makolber@yahoo.com>
wrote:
>
> �Whether 2k taps would be needed in an SC
>> system in a similar environment is arguable...8-VSB has essentially
>> the same BW and works in the same environments just fine with a
>> traditional EQ.
>
>There are people that would argue that point..
>
>I have no personal knowledge one way or the other yet, but I have seen
>this point sometimes hotly debated. And will probably hear more come
>Feb 18...
>
>Mark
It'll be impossible for nearly everyone to tell whether failures are
due to signal strength or equalization issues. And since no vendor
will tell you how their EQ is constructed, there'll be no way to tell
whether it's an implementation issue or a genuine system limitation.
So, yeah, the debates will rage. There's nothing to be done about
it, either.
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
Reply by Mark1●December 12, 20082008-12-12
�Whether 2k taps would be needed in an SC
> system in a similar environment is arguable...8-VSB has essentially
> the same BW and works in the same environments just fine with a
> traditional EQ.
There are people that would argue that point..
I have no personal knowledge one way or the other yet, but I have seen
this point sometimes hotly debated. And will probably hear more come
Feb 18...
Mark
Reply by Eric Jacobsen●December 12, 20082008-12-12
On Fri, 12 Dec 2008 09:05:51 -0600, Vladimir Vassilevsky
<antispam_bogus@hotmail.com> wrote:
>
>
>Randy Yates wrote:
>> Vladimir Vassilevsky <antispam_bogus@hotmail.com> writes:
>>
>>>[...]
>>>OFDM is a wideband modulation which allows simple suboptimal
>>>receivers.
>>
>>
>> Suboptimal in what sense?
>
>In the sense that OFDM is not approaching the channel capacity limit,
>being wasteful both in power and in bandwidth. However they are buying
>the implementation simplicity for that.
>
>
>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com
Yup. OFDM suffers from nasty PAPR and lots of overhead due to the
cyclic prefix and the pilot tones that are usually necessary to make
it work. It's also generally more sensitive to phase noise.
But what you get for that is something that works really well in nasty
multipath, even nasty dynamic multipath. So it's all just the usual
tradeoff compromise management.
Consider that a system like DVB-T, take the 2k subcarrier case, has
what essentially boils down to a 2k-coefficient EQ. Building a 2k
coefficient EQ for a single-carrier system is a pretty formidable
task, and it'd probably be tough to make it adapt as quickly as the
taps in an OFDM system. Whether 2k taps would be needed in an SC
system in a similar environment is arguable...8-VSB has essentially
the same BW and works in the same environments just fine with a
traditional EQ.
It is often necessary to explain to people that, despite the hype,
OFDM is neither more spectrally efficient nor higher capacity than
single-carrier systems. OFDM is very useful in certain
circumstances, but it is often applied where a single-carrier system
might work better. In most applications where OFDM systems can be
found there are comparable SC systems that do the same or similar jobs
with similar performance.
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
Reply by Mark1●December 12, 20082008-12-12
> Randy Yates wrote:
> >>BTW, it takes a lot of bandwidth to be multipath resistant: ~tens of
> >>MHz for outdoors and ~hundreds of MHz for indoors.
>
Hi Randy,
while comparing the pros and cons of OFDM to other systems..here are
two other factors to consider:
1) resistance to impulse and burst noise
2) PAR or peak to average ratio of the Tx signal in the transmitter
and the impact this has on the hardware design re to linearity and
power consumption..
these are two of the downsides to OFDM as far as I know..
Mark
Reply by Eric Jacobsen●December 12, 20082008-12-12
On Fri, 12 Dec 2008 05:49:55 -0800 (PST), Tom <tom.derham@gmail.com>
wrote:
>On Dec 12, 9:37�pm, Randy Yates <ya...@ieee.org> wrote:
>> Tom <tom.der...@gmail.com> writes:
>> > [...]
>> > OFDM (with cyclic prefix) allows one-tap equalization - in other words
>> > the channel equalization process requires only one complex multiply
>> > per subcarrier.
>>
>> Why? This seems to be presuming some relationship between total
>> bandwidth and number of subcarriers. Otherwise, consider the
>> scenario in which a single subcarrier's bandwidth is so great
>> that a fade is not flat even within that subcarrier.
>
>Remember that detection at the receiver is done over finite size
>blocks of samples corresponding to one OFDM symbol length.
>The response of the DFT at an evaluated frequency is a sinc function
>where nulls lie in all the other subcarriers. So there is no energy
>spread from one subcarrier into the others (orthogonality).
>There is no ability to resolve other frequencies between two
>subcarriers - due to the finite length of the DFT input. So a single
>subcarrier is "flat fading" by definition.
>In fact, in order to get the very low coherence bandwidth that you
>describe (i.e. where the actual channel is frequency selective between
>the frequencies corresponding to adjacent subcarriers), it would
>require the channel length (maximum multipath delay) to be longer than
>the OFDM symbol length! This is not allowed to happen since the guard
>interval length (which is normally much shorter than the OFDM symbol
>length) must be at least as long as the channel length.
>
>-T
And that does have an effect on dictating the width of the
subcarriers. Generally as the range gets longer and the channel
delay spread (PDP) gets longer, the subcarriers have to get narrower.
So one does have to make the subcarriers narrower as the delay spread
gets longer in order to maintain flat fading in each subcarrier. So
Randy's question was pretty good, actually, and I think the answers
have been a little misleading in a way.
It *is* possible to have a subcarrier wider than the channel nulls,
and then you'll need more EQ taps per subcarrier since the fading per
subcarrier is no longer flat. That does mean that the whole idea of
keeping the delay spread contained within the cyclic prefix has been
violated, but it can be done (and some systems are actually designed
that way).
For examples:
UWB - very short range, very wide subcarriers (MHz).
802.11 - LAN range, medium-width subcarriers (360kHz)
DVB-T - Long range, narrow subcarrier (~1-2kHz)
The symbol rate per subcarrier just has to be low enough to cover the
channel delay spread in order to use a single coefficient EQ. Just
like a single-carrier system.
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
Reply by Vladimir Vassilevsky●December 12, 20082008-12-12
Randy Yates wrote:
>>BTW, it takes a lot of bandwidth to be multipath resistant: ~tens of
>>MHz for outdoors and ~hundreds of MHz for indoors.
>
> Is that the sort of bandwidth that the new standards based on OFDM
> use? For example, DVT-B and 802.16a?
While ago, when working on my thesis, I was surprised by that fact, too.
Many systems advertised as multipath resistant evidently don't have
enough bandwidth for that.
As for the modulations, my preference is the frequency hopped scheme. It
is simple, robust and efficient in the implementation. The bandwidth can
be made as wide as required.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Vladimir Vassilevsky●December 12, 20082008-12-12
Randy Yates wrote:
> Vladimir Vassilevsky <antispam_bogus@hotmail.com> writes:
>
>>[...]
>>OFDM is a wideband modulation which allows simple suboptimal
>>receivers.
>
>
> Suboptimal in what sense?
In the sense that OFDM is not approaching the channel capacity limit,
being wasteful both in power and in bandwidth. However they are buying
the implementation simplicity for that.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by julius●December 12, 20082008-12-12
On Dec 12, 8:19 am, Randy Yates <ya...@ieee.org> wrote:
>
> Cyclic convolution? Orthogonalizes the operation? What do these mean?
> Is cyclic convolution the same as circular convolution?
Cyclic convolution = circular convolution.
Recall that the DFT is the correct transform for periodic (circular,
cyclic)
signals, so you learned that if you were to compute a linear, non-
cyclic
convolution using DFTs or FFTs, then you have to do this overlap-add.
Now you have to do it backwards, in a way!
> PDP? Last I knew, that was the prefix for a line of Digital Equipment
> Corporation computers.