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.comYup. 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
Multipath Question and OFDM
Started by ●December 11, 2008
Reply by ●December 12, 20082008-12-12
Reply by ●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 ●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... > >MarkIt'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 ●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... >> >>MarkMark, 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/ceac6381c6fe422e/a72338294a3a88ab?hl=en&q=8vsb+ofdm&lnk=ol& -- % Randy Yates % "So now it's getting late, %% Fuquay-Varina, NC % and those who hesitate %%% 919-577-9882 % got no one..." %%%% <yates@ieee.org> % 'Waterfall', *Face The Music*, ELO http://www.digitalsignallabs.com
Reply by ●December 13, 20082008-12-13
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
