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From: bando on 18 Jun 2010 18:52
OP here... let me refer to one instance where I was told by VLK that I was
incorrect. Is alpha is not an important parameter? Alpha does dictate the
occupied bandwidth in the channel. This is very important, though perhaps
importance is a subjective valuation (adjacent channel interference,
spectral efficiency etc). I can not see holding the opinion that alpha is
important is totally wrong.

I am fine with being wrong, in fact I expect it when I'm asking a question
in a post. That's why I'm looking for advice, to beome more right. I
apologize for my dismissal of the terse reply, I do appreciate even minimal
free help. However, I was hoping for more. Specifically, I expected a
more verbose explanation as to why I was wrong.

Thanks everyone who has replied. I think from Dilip's last reply I have
the answer I sought.

Appreciatively,
Bando


From: bando on 18 Jun 2010 19:35
OP again... I feel the need to clarify. I did not assume that varying
pulse shape (to something other than RRC pulse shaping) would be immaterial
to the value of equivalent noise bandwidth. I was, in the question posed,
intending to refer only to RRC pulse shaping.

Furthermore, I admit that I clearly did not understand the difference
between equivalent noise bandwidth, and the actual noise bandwidth. In the
receive filter, the actual bandwidth over which noise is received is larger
than the equivalent noise bandwidth (except for alpha = 0). The total
noise power received in that band however is only Rs times the noise power
spectral density because of the shape of the filter. Hopefully I have now
stated this correctly (though if I'm wrong I'd love to hear it!).

The reason I wanted to know what noise bandwidth to use, and maybe I should
have explained my reason for asking the question in first place, is that I
am maintaining a link budget that accounts for many impairments not modeled
in the digital comm simulation (atmospheric loss, gain variation etc.). In
this link budget I need to compute the "ideal Eb/N0" that should used to
feed the simulation.

The question I had to ask myself was whether to use Rs or Rs(1+alpha) to
compute N = kTB and obtain the noise power for computing C/N. Now I
clearly understand the right answer was to use Rs (for noise equivalent
bandwidth when dealing with RRC pusle shaped signals).

I knew this had to be the case since otherwise increasing alpha to 1 would
have reduced my ideal Eb/N0 by 3dB. From modeling this could not be so.
Now I not only know Rs is the correct equivalent noise bandiwidth, but I
know why. My boss had put together the link budget with the (1+alpha)
factor included for computation of noise power. Fixing this problem
improves our C/N by 20%!

Again, thanks.

~Bando
From: cl7teckie on 6 Jul 2010 07:27
Following up on this thread, is a common confusion I noticed among cable
operator. RRC is extensively used in DVB. When one measures the SNR after
equalizer and the Nyquist filter, the is always the question on equivalent
noise bandwidth. Often, these operator assumes SNR is exactly equal to CNR,
as the equivalent noise bandwidth is equal to that of Rs in RRC.

What I found is that for RRC, while the signal power is like a S-curve
around Rs (after eq and filter), a white noise is not exposed to the RRC on
the transmit side. Thus the equvalent noise bandwidth, while is very close
to Rs, it is NOT equal to Rs. What I found is that the noise bandwidth is
slightly larger than Rs. The SNR thus has a factor of 10*log(alpha/4) lower
than that of CNR before the receiver.

Any comment or disagreement are welcome.
From: Mark on 6 Jul 2010 22:04
On Jul 6, 7:27 am, "cl7teckie" <paul(a)n_o_s_p_a_m.pixelmetrix.com>
wrote:
> Following up on this thread, is a common confusion I noticed among cable
> operator. RRC is extensively used in DVB. When one measures the SNR after
> equalizer and the Nyquist filter, the is always the question on equivalent
> noise bandwidth. Often, these operator assumes SNR is exactly equal to CNR,
> as the equivalent noise bandwidth is equal to that of Rs in RRC.
>
> What I found is that for RRC, while the signal power is like a S-curve
> around Rs (after eq and filter), a white noise is not exposed to the RRC on
> the transmit side. Thus the equvalent noise bandwidth, while is very close
> to Rs, it is NOT equal to Rs. What I found is that the noise bandwidth is
> slightly larger than Rs. The SNR thus has a factor of 10*log(alpha/4) lower
> than that of CNR before the receiver.
>
> Any comment or disagreement are welcome.

yes it is often assumed that noise BW of the receiver is equal to the
symbol rate

Note the -3 dB points of an RRC (square root raised cosine) Rx filter
are equal to the symbol rate BW so as ALPHA approaches 1 the above
assumption gets closer and closer to true...

Mark
From: dvsarwate on 6 Jul 2010 22:46
On Jul 6, 6:27 am, "cl7teckie" <paul(a)n_o_s_p_a_m.pixelmetrix.com>
wrote:

> Any comment or disagreement are welcome.

OK, I disagree.

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