[opendtv] Re: Latest S/N test
- From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 25 Jan 2005 17:22:04 -0800
Good post Bert, you have an impressive grasp of DTV receiver processing
issues. RF interference and other related distortion issues also greatly
impact channel S/N performance and tend to be overlooked. The attached URL
is for one of a series of articles written on the subject by the highly
regarded engineer, Charles Rhodes. I believe you will find this and his
other relates articles of interest.
http://www.tvtechnology.com/features/digital_tv/f_DTV_interference.shtml
----- Original Message -----
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
Sent: Tuesday, January 25, 2005 3:21 PM
Subject: [opendtv] Re: Latest S/N test
> Bob Miller wrote:
>
>> I am waiting for more info. They did say that
>> the data rates were as close as possible to
>> 19.3 Mbps for all modulations tested.
>
> Okay, so I'll assume either an 8 MHz or a 6 MHz
> RF channel. And I'll use the same percentage of
> the band in each case, to provide for credible
> guard bands.
>
> 6 MHz (5.38 MHz used), at 19.3 Mb/s
> Shannon limit 10.42 dB S/N
>
> 8 MHz (7.17 MHz used), at 19.3 Mb/s
> Shannon limit 7.37 dB S/N
>
> There's also the propagation channels to think
> about. If the channel is Gaussian, you will
> approach the Shannon limit more easily. If the
> RF channel is degraded by multipath fading,
> you will require progressively more S/N for
> solid reception, moving away from the Shannon
> limit.
>
> When robust schemes are achieved through pilots
> or other modulation tricks, the S/N required for
> reception in degraded channels will suffer
> least compared with Gaussian channel
> performance, but under benign conditions you'll
> pay a price in higher S/N required.
>
> If robustness is achieved through clever signal
> processing, then you can more easily approach
> the Shannon limit in benign conditions, but the
> robustness will depend entirely on your cunning
> SP routines. So you tend to pay a price in
> degraded channels.
>
> This is another example of the no free lunch
> hypothesis.
>
> Also, to understand the numbers and to get a
> good idea about where you are wrt the Shannon
> limit, it helps to use (S+I)/N rather than
> S/(N+I), where I is the sum of interfering
> echoes of the main signal. Gives much less
> impressive numbers unless you know what to
> look for.
>
> What's impressive is to see (S+I)/N in
> Rayleigh channels that approach or even beat
> the S/N in a Gaussian channel. Even more
> impressive when these two numbers are close to
> the Shannon limit.
>
> In principle, (S+I)/N in a fading channel can
> beat S/N of a Gaussian channel because
> knowledge of the fading mechanism can give the
> receiver information to better untwist the
> distorted symbols. Whereas in a Gaussian
> channel, the distortion is random, which means
> unpredictable.
>
> Bert
>
>
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