sept. 27, 2006
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Sept. 27, 2006. Lab #3 Due Today Tie it all together: Bandwidth, bandlimiting, information capacity example EM Spectrum Noise. 802.11 Bandwidth. 802.11b and g use 2.4GHz band They have 14 channels with 5MHz spacing - PowerPoint PPT PresentationTRANSCRIPT
Sept. 27, 2006Sept. 27, 2006
Lab #3 Due TodayLab #3 Due Today
Tie it all together: Bandwidth, bandlimiting, Tie it all together: Bandwidth, bandlimiting, information capacity exampleinformation capacity example
EM SpectrumEM Spectrum
NoiseNoise
802.11 Bandwidth802.11 Bandwidth
802.11b and g use 2.4GHz band802.11b and g use 2.4GHz band
They have 14 channels with 5MHz They have 14 channels with 5MHz spacingspacing– Bandlimiting – at +/-11MHz, signal must be Bandlimiting – at +/-11MHz, signal must be
30dB down. At +/- 22MHz, signal must be 30dB down. At +/- 22MHz, signal must be 50dB down.50dB down.
– Lots of overlap between channels, requires Lots of overlap between channels, requires good network designgood network design
802.11 WLAN Example802.11 WLAN Example
Dumb design: co-locate 3 APs using Dumb design: co-locate 3 APs using 802.11 channels 1, 2, 3802.11 channels 1, 2, 3
Peak power for each channel at center Peak power for each channel at center frequency (ffrequency (fcc) of channel) of channel
Power drops moving away from fPower drops moving away from fcc but is but is
not 0 when next channel is reachednot 0 when next channel is reached
802.11 Example – Issues With 802.11 Example – Issues With WLANWLAN
Channels overlapChannels overlap
SNR is lowSNR is low
If co-located, even channels farther apart If co-located, even channels farther apart in frequency domain will overlapin frequency domain will overlap
Noise from other sources will decrease Noise from other sources will decrease SNR even more (microwave ovens, SNR even more (microwave ovens, bluetooth, cordless phones)bluetooth, cordless phones)
What we can doWhat we can do
Move APs apart to reduce interferenceMove APs apart to reduce interference
Channels adjacent in space should not be Channels adjacent in space should not be adjacent in frequencyadjacent in frequency– If using 3 APs, use channels 1,6,11If using 3 APs, use channels 1,6,11– If using 3 APs, use channels 1,4,8,11If using 3 APs, use channels 1,4,8,11
Noise will go down, SNR will go up, Noise will go down, SNR will go up, information capacity will go upinformation capacity will go up
Electromagnetic SpectrumElectromagnetic Spectrum
range of all frequencies of electromagnetic range of all frequencies of electromagnetic radiation broken into subrangesradiation broken into subranges
EM Spectrum - physical characteristic EM Spectrum - physical characteristic Spectrum allocation - humans dividing Spectrum allocation - humans dividing spectrum into different uses and spectrum into different uses and designating who can do whatdesignating who can do what
EM Spectrum DivisionsEM Spectrum Divisions
www.ntia.doc.gov/osmhome/allochrt.pdfwww.ntia.doc.gov/osmhome/allochrt.pdf
Frequency AllocationsFrequency Allocations
Allocations in Canada by Industry Canada, and Allocations in Canada by Industry Canada, and in US by FCCin US by FCCHistory of EM Spectrum Allocation:History of EM Spectrum Allocation:– No allocations at birth of wireless communicationNo allocations at birth of wireless communication– 1912 first wireless communication control in US – 1912 first wireless communication control in US –
easy to get bandwidth easy to get bandwidth – 1927, Federal Radio Commission given power to 1927, Federal Radio Commission given power to
grant or deny radio accessgrant or deny radio access– Power to grant or deny access passed to Federal Power to grant or deny access passed to Federal
Communication Commission in 1934Communication Commission in 1934– Granted spectrum based on “Best Public Interest”Granted spectrum based on “Best Public Interest”– In 1994, started to auction off frequencies in 1994 In 1994, started to auction off frequencies in 1994
Frequency Spectrum AuctionsFrequency Spectrum Auctions
Highest bidder obtains license granting Highest bidder obtains license granting holder right to emit EM at certain holder right to emit EM at certain frequencies in a certain areafrequencies in a certain area– License generally granted for 8 yearsLicense generally granted for 8 years
Supposedly done to create “free market” Supposedly done to create “free market” for EM spectrum.for EM spectrum.
Fetches billions of dollars for FCC which is Fetches billions of dollars for FCC which is then put into general coffersthen put into general coffers
Frequency Spectrum Auction Frequency Spectrum Auction CriticismsCriticisms
How can FCC auction off something that is part How can FCC auction off something that is part of natureof nature
How can market price be established before How can market price be established before there is a marketthere is a market
Unlicensed bands are what has driven WiFi Unlicensed bands are what has driven WiFi growth, unlicense more (or all) bands to drive growth, unlicense more (or all) bands to drive more R&Dmore R&D
New technologies could make EM spectrum for New technologies could make EM spectrum for communication a “black box” (see next slide)communication a “black box” (see next slide)
EM Spectrum as a Black BoxEM Spectrum as a Black Box
Why do bands of spectrum have to be allocated Why do bands of spectrum have to be allocated for certain things?for certain things?If you have data to transmit, why can’t we just If you have data to transmit, why can’t we just transmit it without regard for which frequency it transmit it without regard for which frequency it will actually go out onwill actually go out onTherefore eliminate idea of allocationTherefore eliminate idea of allocationDon’t eliminate regulatory commissions – would Don’t eliminate regulatory commissions – would still need “traffic cops”still need “traffic cops”Would bring down barriers to entry, and create Would bring down barriers to entry, and create true “free market”true “free market”
EM Spectrum as a Black Box (2)EM Spectrum as a Black Box (2)
Basic technology exists to do this (frequency Basic technology exists to do this (frequency hopping/spread spectrum)hopping/spread spectrum)Still would require a large advancement in Still would require a large advancement in technologytechnologyIdeal: Enormous world-wide wireless network Ideal: Enormous world-wide wireless network using all of RF band. using all of RF band. – Don’t need to know which part of band you use, just Don’t need to know which part of band you use, just
need to know how to send/receive dataneed to know how to send/receive data
Not there yet, radio commissions unlikely to Not there yet, radio commissions unlikely to budge on current state until technologies can budge on current state until technologies can support thissupport this
NoiseNoise
NoiseNoise
Arch-enemy to signalArch-enemy to signal
Distorts analog signals, causes errors in Distorts analog signals, causes errors in digital signalsdigital signals
Limits information capacityLimits information capacity– I = 3.32 * BW * log(1 + SNR)I = 3.32 * BW * log(1 + SNR)– If noise = 0, SNR→∞, log(1+SNR)→∞, I→∞If noise = 0, SNR→∞, log(1+SNR)→∞, I→∞– Data rate only limited by total amount of data Data rate only limited by total amount of data
you have if no noiseyou have if no noise
Uncorrelated NoiseUncorrelated Noise
Noise from system not signalNoise from system not signal– Is present even if signal is notIs present even if signal is not
Sources External to system:Sources External to system:– Atmosphere: electrical disturbancesAtmosphere: electrical disturbances– outer space: cosmic radiationouter space: cosmic radiation– man-made: electronics, motors, lights etc.man-made: electronics, motors, lights etc.
Internal Uncorrelated NoiseInternal Uncorrelated Noise
Source of noise is internal to systemSource of noise is internal to system
Thermal noiseThermal noise– thermal movement of electrons. thermal movement of electrons. – Is constant across all frequencies. Is constant across all frequencies. – Can be expressed mathematically:Can be expressed mathematically:
Noise (in watts) = KTBNoise (in watts) = KTBK=Boltzmann’s Constant = 1.38x10K=Boltzmann’s Constant = 1.38x10-23-23 J/k J/kT = temperature in KelvinsT = temperature in KelvinsB = Bandwidth in hertzB = Bandwidth in hertz
Internal Uncorrelated Noise (2)Internal Uncorrelated Noise (2)
Shot NoiseShot Noise– Due to random movement of electron flow in Due to random movement of electron flow in
currentscurrents– electrons take different paths, sometimes electrons take different paths, sometimes
faster, sometimes slower. This random flow faster, sometimes slower. This random flow causes shot noise. causes shot noise.
Correlated NoiseCorrelated NoiseNoise related to or generated by signalNoise related to or generated by signalDistortionDistortion– Sine wave is amplified too much to give a square Sine wave is amplified too much to give a square
wavewave– Harmonic DistortionHarmonic Distortion
Signal to Noise Ratio (SNR)Signal to Noise Ratio (SNR)
Measure of signal power with respect to Measure of signal power with respect to noise powernoise power– Expressed as ratio, or in dBExpressed as ratio, or in dB
Most fundamental of communication Most fundamental of communication system measurementssystem measurements– Gives first order estimate of how well system Gives first order estimate of how well system
will performwill perform
Example: Signal strength is 1 watt, noise Example: Signal strength is 1 watt, noise strength is .002 watts. What is SNR?strength is .002 watts. What is SNR?
Noise Factor and FigureNoise Factor and Figure
Measurements of how much a system Measurements of how much a system distorts or changes the signal to noise distorts or changes the signal to noise ratioratio– Usually measures how much extra noise the Usually measures how much extra noise the
system addedsystem added
Noise factor = SNRNoise factor = SNR(in)(in)/SNR/SNR(out)(out)
Noise Figure is just the Noise Factor Noise Figure is just the Noise Factor expressed in dBexpressed in dB– Noise Figure = 10 log (SNRNoise Figure = 10 log (SNR(in)(in)/SNR/SNR(out)(out)))
ExampleExample
A nonideal amplifier has the following A nonideal amplifier has the following parameters:parameters:– Input signal power = 2x10Input signal power = 2x10-10-10 W W – Input noise power = 2x10Input noise power = 2x10-18-18 W W– Power Gain = 1,000,000Power Gain = 1,000,000– Internal Noise = 6x10Internal Noise = 6x10-12-12 W W
Determine Input SNR, Output SNR, noise Determine Input SNR, Output SNR, noise factor, and noise figurefactor, and noise figure