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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> SNR and Multipath Fading https://designers-guide.org/forum/YaBB.pl?num=1273043957 Message started by aaron_do on May 5th, 2010, 12:19am |
Title: SNR and Multipath Fading Post by aaron_do on May 5th, 2010, 12:19am Hi all, why do we need higher SNR when multipath fading is considered than in a normal AWGN channel? My understanding is that multipath fading is equivalent to delay spread. What is the connection between delay spread and white noise? thanks, Aaron |
Title: Re: SNR and Multipath Fading Post by pancho_hideboo on May 5th, 2010, 12:28am Consider function and purpose of "Rake Receiver". Answer is self-evident. |
Title: Re: SNR and Multipath Fading Post by aaron_do on May 5th, 2010, 1:24am Thanks. I only know roughly what a Rake receiver is. Anyway I think i'm starting to see where my misunderstanding is. I think SNR is a very basic definition of the ratio of the received signal power to the noise power. However, the received signal power includes components which may add constructively or destructively resulting in a drop in the SNR. Does that sound right? If so, then I mistakenly believed that SNR already takes into account the vector addition of different components of the received signal. If I'm wrong then I guess I need to read up more about the Rake receiver. thanks, Aaron |
Title: Re: SNR and Multipath Fading Post by pancho_hideboo on May 5th, 2010, 2:49am aaron_do wrote on May 5th, 2010, 12:19am:
Consider digital communication. aaron_do wrote on May 5th, 2010, 1:24am:
http://www.designers-guide.org/Forum/YaBB.pl?num=1266622237/1#1 http://edocs.soco.agilent.com/display/ads2009/Equalized+16-QAM+with+Multipath+and+Phase+Noise http://edocs.soco.agilent.com/display/ads2009/Interactive+Controls+and+Displays+for+ADS+Ptolemy+Simulation Even in analog communication, multipath causes linear distortion. |
Title: Re: SNR and Multipath Fading Post by aaron_do on May 5th, 2010, 7:35am Quote:
I understand that the linear distortion causes ISI. Can such ISI be overcome by an improvement in the SNR? thanks, Aaron |
Title: Re: SNR and Multipath Fading Post by pancho_hideboo on May 5th, 2010, 8:04am aaron_do wrote on May 5th, 2010, 7:35am:
Again study "Eb/N0", "Es/N0", "S/N" and "C/N" in digital communication. ISI is improved by equalization. |
Title: Re: SNR and Multipath Fading Post by aaron_do on May 5th, 2010, 8:45am Hi pancho_hideboo, Quote:
that's really what I'm trying to understand actually. I was looking up a bit about Rayleigh multipath channel model, and apparently the channel can be modeled as a vector addition of two signals with different attenuation, phase shift, and time shift. So ignoring the time shift for now, the actual power which we receive is ultimately attenuated due to the possible phase difference of the two signals (in the Rayleigh model). My understanding is that the SNR does not include the effect of this attenuation. Therefore, when comparing the Rayleigh model to the AWGN model, a higher SNR is required for the same BER. Am I on the right track? BTW the main reason I'm asking is I'm trying to figure out the SNR requirement for the IEEE 802.15.4 standard. Several papers have quoted an SNR requirement of about 0 dB, but I don't think they took multipath fading into account. The IEEE 802.15.4 standard says the SNR required is about 5-6 dB while one or two papers which study the effect of multipath fading for the 802.15.4 standard show that the required SNR is much higher (around 14 dB). So are they all talking about the same definition of SNR? I'm inclined to believe that works which include multipath fading... thanks, Aaron |
Title: Re: SNR and Multipath Fading Post by pancho_hideboo on May 5th, 2010, 8:55am Model of multipath fading is no more than FIR filter with time varying coefficients. aaron_do wrote on May 5th, 2010, 8:45am:
Again I use Eb/N0 not S/N for studying quality of communication system. In simple multipath fading model, there are main path and delayed path. You can define C/N simply for main path with delayed path as noise. |
Title: Re: SNR and Multipath Fading Post by aaron_do on May 5th, 2010, 7:37pm Hi pancho_hideboo, I have looked into the concepts you mentioned as well as matched filter detection, but the mathematics is quite involved, so its not easy to get a deep understanding. Anyway I've included a diagram of how I guess multipath affects the SNR requirement in a receiver. Does it look right? Basically |N| is the allowable noise power and |S| is the signal power. The cross hatched region represents the region where the PER would fall below the required value (1% in this case). These are constellation diagrams (I know I haven't labelled the axes). thanks, Aaron |
Title: Re: SNR and Multipath Fading Post by RFICDUDE on May 6th, 2010, 7:43pm The simple models just assume flat fading over the channel BW, so the effect is that the signal power is time varying. But of course the thermal noise of the receiver does not fade, so you need a higher SNR to insure reliable reception when flat fading occurs. Frequency selective fading is much more difficult to analyze. Rake receivers are a possible solution, but not all standards use them. I'm not sure if 802.15 receivers typically use RAKE receivers or not. I think one of the advantages of OFDM is that fading is frequency selective over the whole channel, but the fading looks flat over any given subcarrier (subcarrier ISI is not a problem). I'm no expert here, just offering my miss informed opinion (guess). |
Title: Re: SNR and Multipath Fading Post by aaron_do on May 6th, 2010, 7:57pm thanks for the response. I think 802.15.4 was designed so that Rake receivers would be unnecessary. |
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