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Design >> RF Design >> How close is kTB to actual received noise
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Message started by aaron_do on Jul 17^th, 2008, 11:45pm

Title: How close is kTB to actual received noise
Post by aaron_do on Jul 17^th, 2008, 11:45pm

Hi all,

just wondering, a received signal is usually received with AWGN equal to kTB. What can cause it to increase or even decrease? I'm not really talking about interference, just noise (blackbody radiation).

thanks,
aaron

Title: Re: How close is kTB to actual received noise
Post by didac on Jul 18^th, 2008, 12:03pm

Hi,
If I remember correctly an antenna will only seen an equivalent to black body radiation if it's inside a perfect anechoic chamber since it will achive thermal equilibrium with the environment, in reality KTB it's KT_aB (T_a it's the antenna equivalent noise temperature). T=290ºK it's obtained through statistical measurements in earth environment in the more general case(or for a more realistic estimation) it's necessary to know the noise sources that "see" the antenna and apply:
T_a=n_radT_b+(1-n_rad)T_p
where :n_rad is the antenna efficiency
T_b:equivalent brightness temperature seen by the main beam(secondary lobes pick up noise also but usually are neglected for this calculation)
T_p:it's the physical temperature of the antenna(it takes into account the ohmic component of the antenna).
So in reality if an antenna picks up through it's main lobe a noisy source you can see a higher level of thermal floor(or viceversa).
Hope it helps,
PD:in which band are you working?, maybe it could be a problem of fast fading on the received signal so the SNR will degrade(or improve) but not caused by noise, i.e it's not a true AWGN channel, it's a frequency selective-time varying one and the attenuation it's not fixed.

Title: Re: How close is kTB to actual received noise
Post by aaron_do on Jul 19^th, 2008, 12:41am

Hi didac,

i'm working in the 2.4 GHz ISM band. thanks for the info. Seems like i need to read up on antennas...

Aaron

Title: Re: How close is kTB to actual received noise
Post by didac on Jul 20^th, 2008, 7:30am

Hi,
In classical antenna book you will find the definition of T_a and how to calculate T_b(it's a ratio of two 2-D integrals...) but the general concept it's more likely to be found in radiocommunications texts. In this band I think that it's more dangerous fading than variation of the noise(10-20 dB's of variation of the incoming signal it's not at all strange from frame to frame), of course it depends on system specific details.
Hope it helps,
PD:as a reference for antenna theory I think that Antenna Theory:Analysis and Design by Constantine A.Balanis it's a good book .
For general radiocommunications I think that Wireless Communications:Principles and Practice by Theodore S. Rappaport it's a good reference(although I think that it doesn't cover MIMO...) .