Re: TCP-Group Digest V95 #97

[brandon@rrd.bbc.co.uk (BrandonButterworth)]

> From: brian@nothing.ucsd.edu (Brian Kantor)
> Subject: 56 kbit/s on 2 meters?
>
> >Don't you also get a lot of multi-path problems at 56kb?
>
> Yes, and the higher in frequency you go, the worse it gets.
>
> Pretty soon, the multipath isn't just blurring the bits, it's shifting
> them, and you lose.  The only solution appears to be spread-spectrum.

Or as we are doing for Digital Audio Broadcasting at 210MHz use
COFDM. The idea is that you have many carriers but the symbol period
of each is low enough to allow for the maximum multi-path delay.

It also has some redundancy so that some carriers can get lost
at any instant.

For the multichannel radio system there are about 1500 carriers
each QPSK at 1msec symbol period (approx from memory).

For digital TV it will be about 8000 carriers in 8MHz using 64QAM
giving about 500Mbit/sec.

Between the two there will be some good cheap receiver chips coming
out. Generation needs DSP for low data rates (like the Telebit
PEP modems) or FFT chips for higher rates.

We could scale this down to fit whatever channel we want (less carriers
on lower bands)

Brandon

 **************************************************************************
 *         Brandon Butterworth, BBC Research & Development Department     *
 *                Kingswood Warren, Tadworth, Surrey, KT20 6NP            *
 * email: brandon@rd.bbc.co.uk         ampr: G1OZZ@GB3KP                  *
 * phone: 0737 836592 (+44 737 836592) fax:  0737 832336 (+44 737 832336) *
 **************************************************************************