Modulation capacity -- 2MSK

I'm thinking about the following:

We know that BPSK has a mutual-information capacity around 0 dB
(some say, 0.2 dB by a different argument, but it's close), and
that this is ... most would calculate, 1.2 dB off from the channel
capacity.

Does 2-MSK have the same capacity?  Or is it somehow compromised
by whatever small factor?

(Yes, lame question, I should be able to convince myself of the
answer one way or another.)

Any insights?


Steve

On Tue, 1 May 2012 04:51:59 +0000 (UTC), spope33@speedymail.org (Steve
Pope) wrote:

>I'm thinking about the following:
>
>We know that BPSK has a mutual-information capacity around 0 dB
>(some say, 0.2 dB by a different argument, but it's close), and
>that this is ... most would calculate, 1.2 dB off from the channel
>capacity.
>
>Does 2-MSK have the same capacity?  Or is it somehow compromised
>by whatever small factor?
>
>(Yes, lame question, I should be able to convince myself of the
>answer one way or another.)
>
>Any insights?
>
>
>Steve

Since you're down in the details of the differences between the
various interpretations of "capacity", it might be tough to get either
enough cognizant people to chime in or agree if they do.

Some of the practical capacity computations, like Ungerboeck's BICO
capacity, depend only on the received signal constellation shape.
From that standpoint 2-MSK and BPSK should be indistinguishable.

Since the signal spectra differ, I wouldn't necessarily bet they'll be
the same for all interpretations or definitions of capacity, but I
agree that if there's a difference it should be pretty small.

That probably doesn't help much, but thought I'd throw it out anyway.


Eric Jacobsen
Anchor Hill Communications
www.anchorhill.com