noise in a WLAN system

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Does any one know how one can simulate the noise in a WLAN system? I
used the thermal noise equation (N=KTB) but using the noise factor of
10dB results in a very low noise and consequently in a too high SNR and
simulation results comparing to the measurements are not acceptable.

On 10 Oct 2005 07:58:35 -0700, (E-Mail Removed) wrote:

>Does any one know how one can simulate the noise in a WLAN system?

Simulate as in calculate or on the bench?

For the bench test, any 802.11 source of spread spectrum transmission
is considered noise if it's not correlated with the desired receive
signal. I use an old Teletronics frequency hopping client radio as a
noise source. The frequency hopping give it a nice flat frequency
response as seen on the spectrum analyzer. I sometimes wish that it
worked somewhat out of band but for the price, I can't expect much
more.

For calculations, it's just the usual Boltzmans Constant noise floor
times bandwidth thermal noise calculation.

>I used the thermal noise equation (N=KTB) but using the noise factor of
>10dB results in a very low noise and consequently in a too high SNR and
>simulation results comparing to the measurements are not acceptable.

I can't tell where you devined the 10dB noise figure (not noise
factor), what type of modulation you're using, what you're working
with or even what you're trying to accomplish. See:
http://www.maxim-ic.com/appnotes.cfm...te_number/2875
for a reasonable explanation of the calculations and techniques
involved.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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Thank you for the information.
I meant Noise Figure of 10dB. I read in different articles that NF of
5-10 dB is considered for systems based on IEEE 802.11 WLAN.

The problem is that if only thermal noise will be considered as the
noise source in the system the noise power density will be around
10^(-20) and as a result SNR at locations even far from AP will be very
high!

For example if we calculate the received power[dB] as transmit power
minus path loss minus noise power, considering the 802.11a parameters
(carrier frequency at 5 GHz and transmit power of 100 mW) with path
loss exponent of 3 at distance of 20m from the AP the SNR will be
around 20dB! Compared to reality this is too high!


Jeff Liebermann wrote:
> On 10 Oct 2005 07:58:35 -0700, (E-Mail Removed) wrote:
>
> >Does any one know how one can simulate the noise in a WLAN system?
>
> Simulate as in calculate or on the bench?
>
> For the bench test, any 802.11 source of spread spectrum transmission
> is considered noise if it's not correlated with the desired receive
> signal. I use an old Teletronics frequency hopping client radio as a
> noise source. The frequency hopping give it a nice flat frequency
> response as seen on the spectrum analyzer. I sometimes wish that it
> worked somewhat out of band but for the price, I can't expect much
> more.
>
> For calculations, it's just the usual Boltzmans Constant noise floor
> times bandwidth thermal noise calculation.
>
> >I used the thermal noise equation (N=KTB) but using the noise factor of
> >10dB results in a very low noise and consequently in a too high SNR and
> >simulation results comparing to the measurements are not acceptable.
>
> I can't tell where you devined the 10dB noise figure (not noise
> factor), what type of modulation you're using, what you're working
> with or even what you're trying to accomplish. See:
> http://www.maxim-ic.com/appnotes.cfm...te_number/2875
> for a reasonable explanation of the calculations and techniques
> involved.
>
> --
> Jeff Liebermann (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

On 10 Oct 2005 14:13:41 -0700, (E-Mail Removed) wrote:

>X-No-Archive

Why?

>I meant Noise Figure of 10dB. I read in different articles that NF of
>5-10 dB is considered for systems based on IEEE 802.11 WLAN.

Read again. 10dB is a tolerable number for the *SYSTEM* noise figure
which includes everything between the antenna and the demodulator. The
actual minimum acceptable noise figure is heavily dependent on the
type of modulation, data speed, and desired BER (bit error rate). My
guess(tm) is that the 10dB figure is for a BPSK 1-2 Mbit 802.11
system. This doesn't apply to an ODFM 802.11a system.

>The problem is that if only thermal noise will be considered as the
>noise source in the system the noise power density will be around
>10^(-20) and as a result SNR at locations even far from AP will be very
>high!

Well, I can't check your calculations without seeing them first. I'm
also not going to try and reproduce your calculations without the
slightest clue as to which system parameters you're using, and which
assumptions you're making. However, I do have a guess. The usual
mistake is to forget to include the 10dB minimum (FCC required) spread
spectrum processing gain.

>For example if we calculate the received power[dB] as transmit power
>minus path loss minus noise power,

You forgot the antenna gains and coax losses.

You can subtract the noise power but getting a value of noise power is
a bit tricky as it's affected by the noise figure of the receiver
front end. If you're using the occupied bandwidth of the spread
spectrum signal as your noise power bandwidth, you're doing it wrong.
Think digital filter and demodulator bandwidth.

I suggest you switch to using the receiver sensitivity at a reference
BER, which will yield a corresponding S/N ratio at the demodulator
depending on modulation type and speed. Model the system from the
transmitter to the receiver input seperately from the receiver input
to the demodulator output.

>considering the 802.11a parameters
>(carrier frequency at 5 GHz and transmit power of 100 mW) with path
>loss exponent of 3 at distance of 20m from the AP

5GHz or 5.7GHz?

Path loss is measured in dB and is easily calculated.
http://www.microindustrie.com/fcc/path.htm
I'm too lazy to work the numbers.

Path loss exponent is used to determine the variation in path loss
caused by various edge obstructions and atmospherics and should not
enter into your equation unless you're looking for a worst case
analysis or doing interior reflective ray tracing. The value is also
empirically derived somewhat of a guess. At this point, I have no
clue as to why you introduced loss exponent.

>the SNR will be
>around 20dB! Compared to reality this is too high!

The system noise figure and the minimum demodulated S/N ratio are not
the same number. The S/N ratio is more commonly related to the
modulation type and the BER.

I have a feeling this is your homework project. I don't mind helping
but most schools have better resources available. If you would kindly
disclose:
1. What you're trying to accomplish?
2. What you have to work with?
I'll try to be more helpful.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558