What is the best modulation scheme?

"Eric Jacobsen" <eric.jacobsen@ieee.org> wrote in message
news:esu4m1pv81dunalatgulhoms0spjf17h5n@4ax.com...
>
> The symbol rate is typically equal to the 3dB bandwidth of the signal
> for QAM.  So in 1MHz bandwidth you can have a 1MHz symbol rate, but
> you need some extra on either side for the skirts, and how much more
> is needed depends on the type of filtering that is being employed.
>
> As Mark mentioned, it's not too hard to get a 5.36MHz symbol rate in a
> 6Mhz channel with the rest of it used as guardband for the skirts and
> margin for adjacent channel interference.
>
> You have to do exactly the same thing for OFDM.   You can't just pile
> the subcarriers up to the edges of the band, because spectral regrowth
> from the amplifier (and other impairments) still mean that you need
> some guard band, and it winds up being about the same as for a single
> carrier system.
>
> Not only that, but in OFDM you have to slow the symbol rate further
> than you might think in order to accomodate the cyclic prefix or
> cyclic extension.   The CP/CE provide lots of goodness for multipath
> handling, but are dead weight as far as information transmission is
> concerned.  For this reason sometimes when somebody waves the "OFDM is
> more spectrally efficient" card, I wave the "no it's not, it's LESS
> spectrally efficient" card just to make the comparison.   In most
> cases when it gets down to the real engineering there is little or no
> difference in spectral efficiency between OFDM and single carrier
> systems in my experience.
>
> Eric Jacobsen
> Minister of Algorithms, Intel Corp.
> My opinions may not be Intel's opinions.
> http://www.ericjacobsen.org

Hi Eric,

Thanks for the answer, I realized I was wrong after I had a look at the
pre-adsl modem protocols (e.g. v.34)... Anyway, I agree with you 100%.

Hi Eric,

Thank you for some of the opinions about the OFDM trade-offs study.
Well, the bill will be too costly to a poor student :-)

Regards,
Lindah


> Lindah,
>
> To me it comes down to equalizer complexity.   For OFDM systems the
> equalizer is easily bounded, for a QAM system, especially at a very
> high symbol rate, the equalizer complexity can quickly get out of hand
> if the channel delay spread is very long at all.
>
> It's not difficult to make a first-order tradeoff analysis.  For the
> OFDM system add a cyclic prefix to the symbols that's _at minimum_ the
> length of the longest expected channel delay spread, and see how much
> it'll hurt to maybe use 1.5x or 2x that length to accomodate filter
> effects and other stuff that gets in the way.   Tradeoff subcarrier
> spacing against the coherence bandwidth and the symbol rates that you
> can process (keeping the CP to 1/3 or less, I like a lot less, of the
> symbol is good for efficiency) and see what the system starts to look
> like.   This gives an idea of how big/fast/ugly the FFT, channel
> estimator, etc., are going to be.
>
> Figure out the data rates that you can expect with such a system, and
> then see what it takes to do the same with a single-carrier.
> Whatever the symbol rate turns out to be, the EQ then needs to be
> around the number of taps as T goes into the delay spread.  It gets
> ugly in a hurry for high-symbol-rate systems.   This is why so many
> wideband systems these days are OFDM if the channel delay spread is
> very long.
>
> You've probably already done some or all of this, so ignore me as
> suitable.  But to me that's pretty much the basis of the real
> tradeoffs between OFDM and single carrier.   OFDM does, however, let
> you do all kinds of other cool things like adaptive bit loading, power
> loading, OFDMA, etc., that just isn't practical with single carrier
> systems.
>
> Hope that helps a bit or two.   Where should I send the bill?   ;)
>
>
> Eric Jacobsen
> Minister of Algorithms, Intel Corp.
> My opinions may not be Intel's opinions.
> http://www.ericjacobsen.org

On 1 Nov 2005 05:38:48 -0800, lindah74uk@yahoo.co.uk wrote:

>Hi Eric,
>
>Thank you for some of the opinions about the OFDM trade-offs study.
>Well, the bill will be too costly to a poor student :-)
>
>Regards,
>Lindah

:)   No problem on the bill.   I'll just start an account for you...  ;)


Just kidding, you're most welcome to the advice.


Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org