Re: frequency band

On Sat, 13 May 2006 14:27:57 GMT, "DAB sounds worse than FM"
<dab.is@xxxxxxxxxxxxxx> wrote:

Don Pearce wrote:
On Sat, 13 May 2006 13:45:04 GMT, "DAB sounds worse than FM"
<dab.is@xxxxxxxxxxxxxx> wrote:


No, the noise drops.


How can the noise drop? We're talking about Gaussian noise.


No we aren't.


http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem

"In information theory, the Shannon-Hartley theorem is an application of the
noisy channel coding theorem to the archetypal case of a continuous-time
analog communications channel subject to Gaussian noise."


Sure - that is the only way to define the theory. The other spacial
channels tend to be sufficiently incoherent that the noise has
Gaussian characteristics, so the theory still holds. It isn't thermal
in origin, though, so it is amenable to reduction by the same
mechanism that boosts the wanted spacial channel.


The noise includes interference from other channels,
cells, whatever. That is what sets the noise floor in a MIMO system.
It reduces when you can direct the power where it is needed.


You improve the signal level and reduce the
interference level simultaneously - that is where the huge
improvement comes from.


No, because Shannon's Capacity Theorem assumes there is no
interference -- just signal received with no interference, add
noise, demodulate. So MIMO beats the case where there is no
interference.

The reason why MIMO beats Shannon's Capacity is because you send
*different* data on the different streams, and MIMO allows you to
recover the data. Each individual data stream must obey Shannon's
Capacity Theorem, but as a whole MIMO beats the Capacity Theorem.

No. Certainly in the iBurst system, you send a different signal, but
not different data. Each antenna sounds its channel on the uplink
training burst and builds a matrix of all the multipaths it can find.
It then inverts that matrix for the downlink. This happens on each
antenna. Although the signals from each antenna are thus different, by
the time they reach the wanted remote receiver, they have cohered into
a single, in phase signal. The same thing is happening simultaneously
for the other remotes being served on that channel. The result of all
this is that what the remote receives is a single, particularly clean
channel, while the "interference" from the other spatial channels is
randomly averaged and thus reduced.

So as far as the receiver is concerned it does not receive different
data on each MIMO channel, it simply sums to the same data.


I'm not going to comment on how this iBurst system works, but all the
space-time codes I can remember reading about transmit different data from
different antennas.

Also, just because this iBurst system uses MIMO, i.e. multiple transmit and
receive antennas, does not mean that it's anywhere near optimal. The
equation I quoted before is the optimal case, and which shows that it is
possible to beat Shannon's Capacity by using MIMO.

It is very close to optimal. And of course although the signal on the
various antennas is different, the data can not be. The system only
works by cohering the various multipaths into a single data stream.

d

--
Pearce Consulting
http://www.pearce.uk.com
.

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