[opendtv] Re: MIMO

Al Limberg wrote:

> Vertical polarised antennas were preferred in the early
> days because they were not as good pigeon roosts.  So
> roofs kept cleaner.

But I think now, in the UK and in Italy, vertical and horizontal are
used to help in frequency planning. Especially because mountainous
regions require so many translators. I guess you can reuse the frequency
channel somewhat sooner if you change the polarization.

In some parts of Italy, you're expected to use vertical on some channels
and horizontal for others. Here's an example:

http://www.dgtvi.it/dgtvi/copertura.aspx?TRS_ID=1550000&PROVID=38&CITYID
=4779&CS=1

However, this MIMO idea the BBC is attempting to use for doubling its
channel capacity is a lot more complicated than just assuming you can
double the capacity by sending one stream in v and a totally separate
stream in h. This space-time code stuff is still fairly new, but it
seems from what I've read that if the transmitter does not have
knowledge of the channel properties, which it wouldn't have in a one-way
broadcast, you would need more than just two transmit and two receive
antennas to hope to double the channel capacity. And not only that, but
it helps to be in a Rayleigh channel to get the capacity gains. As the
channel goes Ricean and Gaussian, the bit error rates at the high
capacity settings start to rise, which means the high capacity setting
becomes less robust.

In short, if you really want "multipath to be your friend," as the
saying goes, then lack of multipath works against you.

Bert
 
 
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