Sept. 27, 2006

1. Sept. 27, 2006 • Lab #3 Due Today • Tie it all together: Bandwidth, bandlimiting, information capacity example • EM Spectrum • Noise

2. 802.11 Bandwidth • 802.11b and g use 2.4GHz band • They have 14 channels with 5MHz spacing • Bandlimiting – at +/-11MHz, signal must be 30dB down. At +/- 22MHz, signal must be 50dB down. • Lots of overlap between channels, requires good network design

3. 802.11 WLAN Example • Dumb design: co-locate 3 APs using 802.11 channels 1, 2, 3 • Peak power for each channel at center frequency (fc) of channel • Power drops moving away from fc but is not 0 when next channel is reached

5. 802.11 Example – Issues With WLAN • Channels overlap • SNR is low • If co-located, even channels farther apart in frequency domain will overlap • Noise from other sources will decrease SNR even more (microwave ovens, bluetooth, cordless phones)

6. What we can do • Move APs apart to reduce interference • Channels adjacent in space should not be adjacent in frequency • If using 3 APs, use channels 1,6,11 • If using 3 APs, use channels 1,4,8,11 • Noise will go down, SNR will go up, information capacity will go up

7. Electromagnetic Spectrum • range of all frequencies of electromagnetic radiation broken into subranges • EM Spectrum - physical characteristic Spectrum allocation - humans dividing spectrum into different uses and designating who can do what

8. EM Spectrum Divisions • www.ntia.doc.gov/osmhome/allochrt.pdf

9. Frequency Allocations • Allocations in Canada by Industry Canada, and in US by FCC • History of EM Spectrum Allocation: • No allocations at birth of wireless communication • 1912 first wireless communication control in US – easy to get bandwidth • 1927, Federal Radio Commission given power to grant or deny radio access • Power to grant or deny access passed to Federal Communication Commission in 1934 • Granted spectrum based on “Best Public Interest” • In 1994, started to auction off frequencies in 1994

10. Frequency Spectrum Auctions • Highest bidder obtains license granting holder right to emit EM at certain frequencies in a certain area • License generally granted for 8 years • Supposedly done to create “free market” for EM spectrum. • Fetches billions of dollars for FCC which is then put into general coffers

11. Frequency Spectrum Auction Criticisms • How can FCC auction off something that is part of nature • How can market price be established before there is a market • Unlicensed bands are what has driven WiFi growth, unlicense more (or all) bands to drive more R&D • New technologies could make EM spectrum for communication a “black box” (see next slide)

12. EM Spectrum as a Black Box • Why do bands of spectrum have to be allocated for certain things? • If you have data to transmit, why can’t we just transmit it without regard for which frequency it will actually go out on • Therefore eliminate idea of allocation • Don’t eliminate regulatory commissions – would still need “traffic cops” • Would bring down barriers to entry, and create true “free market”

13. EM Spectrum as a Black Box (2) • Basic technology exists to do this (frequency hopping/spread spectrum) • Still would require a large advancement in technology • Ideal: Enormous world-wide wireless network using all of RF band. • Don’t need to know which part of band you use, just need to know how to send/receive data • Not there yet, radio commissions unlikely to budge on current state until technologies can support this

14. Noise

15. Noise • Arch-enemy to signal • Distorts analog signals, causes errors in digital signals • Limits information capacity • I = 3.32 * BW * log(1 + SNR) • If noise = 0, SNR→∞, log(1+SNR)→∞, I→∞ • Data rate only limited by total amount of data you have if no noise

16. Uncorrelated Noise • Noise from system not signal • Is present even if signal is not • Sources External to system: • Atmosphere: electrical disturbances • outer space: cosmic radiation • man-made: electronics, motors, lights etc.

17. Internal Uncorrelated Noise • Source of noise is internal to system • Thermal noise • thermal movement of electrons. • Is constant across all frequencies. • Can be expressed mathematically:Noise (in watts) = KTBK=Boltzmann’s Constant = 1.38x10-23 J/kT = temperature in KelvinsB = Bandwidth in hertz

18. Internal Uncorrelated Noise (2) • Shot Noise • Due to random movement of electron flow in currents • electrons take different paths, sometimes faster, sometimes slower. This random flow causes shot noise.

19. Correlated Noise • Noise related to or generated by signal • Distortion • Sine wave is amplified too much to give a square wave • Harmonic Distortion

20. Signal to Noise Ratio (SNR) • Measure of signal power with respect to noise power • Expressed as ratio, or in dB • Most fundamental of communication system measurements • Gives first order estimate of how well system will perform • Example: Signal strength is 1 watt, noise strength is .002 watts. What is SNR?

21. Noise Factor and Figure • Measurements of how much a system distorts or changes the signal to noise ratio • Usually measures how much extra noise the system added • Noise factor = SNR(in)/SNR(out) • Noise Figure is just the Noise Factor expressed in dB • Noise Figure = 10 log (SNR(in)/SNR(out))

22. Example • A nonideal amplifier has the following parameters: • Input signal power = 2x10-10 W • Input noise power = 2x10-18 W • Power Gain = 1,000,000 • Internal Noise = 6x10-12 W • Determine Input SNR, Output SNR, noise factor, and noise figure