[opendtv] Re: Is LTE in Broadcast's Future?
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 2 Mar 2011 16:56:08 -0600
Craig Birkmaier wrote:
> A migration to LTE broadcasting would provide improved spectral efficiency,
> a much wider range of potential services and the ability to leverage the
> same infastructure as that being used for broadband. Sounds like a win-win
> to me.
Both WCDMA and LTE have migrated to (a) 64-QAM, (b) MIMO, (c) 20 MHz channel
widths. Tell me where you get the idea that LTE gets better spectral
efficiency. Better than what? Better than the earlier iterations of WCDMA? You
mean, a 20 MHz LTE MIMO channel has more capacity than a 5 MHz WCDMA channel
that doesn't use MIMO? Quelle surprise!
WCDMA can keep jacking up spectral efficiency to the point that the noise,
created by the other users (or the other program streams?) in the frequency
channel, becomes too high for the receiver to sift through. This is no
different from increasing the spectral efficiency of an OFDM-based scheme, e.g.
by making the constellation as dense as possible and the FEC as weak as
possible, and still achieve reliable reception. In both cases, the limiting
factor is SNR. Given the same receiver performance, thermal noise etc., where
would anyone get the idea that one scheme is better than the other?
The two are entirely comparable in this regard.
Another aspect of hype is where you say one thing, then you say another, and
then you hope that the listener puts 2 and 2 together to obtain 6. To wit, yes,
MIMO can give you amazing b/s/Hz figures, BUT it can only do so as long as the
different propagation paths are highly uncorrelated. The figures drop as the
propagation path goes from rayleigh to ricean, and heaven forfend, to gaussian.
So if you can get, say, 30 b/s/Hz in an urban canyon, does this mean that you
should count on 30 b/s/Hz at 30-60 miles from the transmitter, over open fields?
Another little tid-bit. If you read the test results in the literature, you
will find that 1 X m on n X 1 variants of MIMO show a spectral efficiency of
somewhat more than 3 b/s/Hz, with SNR of 15 dB, is quite respectable. This is
what we achieve now with 8T-VSB. These special cases of MIMO are where you have
either one transmit antenna and multiple receive antennas, or the other way
around.
In a rayleigh channel, where the spectral efficiency of MIMO is roughly x * the
spectral efficiency of a single path, and x = the lesser of m or n, the results
scale up from that 3.something number. For example, if you use a 16 X 16 MIMO
scheme, the best results appear to be somewhat higher than 40 b/s/Hz for 15 dB
of SNR. Maybe 42 b/s/Hz or so. (And 3 b/s/Hz * 16 paths = 48 b/s/Hz aggregate.)
But again, this result applies to rayleigh channels that support 16 highly
uncorrelated paths. The number drops to around 30 b/s/Hz at distances of a few
miles, and of course much less than that as one propagation path begins to
dominate over the others.
Don't just succumb to hype, Craig.
Bert
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