Week 2. Topics in Wireless Systems EE584-F 03 9/9/2003. Copyright 2003 Stevens Institute of Technology - All rights reserved

Transcription

1 Week Topics in Wireless Systems 43

2 0 th Generation Wireless Systems Mobile Telephone Service Few, high-power, long-range basestations -> No sharing of spectrum -> few users -> expensive 44

3 Cellular Systems st Generation 45

4 Frequency Re-use Covering the MTS service area with cells: s of miles A few miles

5 Full Duplex Communications in Cellular North American AMPS frequencies: MHz 849 MHz 869 MHz 894 MHz 30 khz Cellular uplink Cellular downlink 47

6 Cellular Hand-off Providing coverage as mobile moves between cell site coverage areas Extent of site B coverage Extent of site B coverage (6) Look for Site B on Channel K () your signal is getting weak, prepare for hand -off Cell Site B (4) Channel #K (3) handoff Mobile X MTSO () I need to handoff Mobile X (5) Proceed Cell Site A 48

7 Channel dispersion multipath fading, dispersion Multipath reflections create time dispersion of the received signal Movement of the receiver, transmitter or objects in the environment create changes in the multipath environment 49

8 Characterizing the RF Fading Environment Line-of-sight plus multipath Multipath with no line-of-sight 50

9 Effects of Multipath Reflecting surface Path length d Wavelength λ TX RX Path length d Conditions for complete, destructive interference between path and path : A = A = +.5 ( ) d d k λ 5

10 Rayleigh Fading.0 0 db t Probability that signal level is below abscissa db -0 db Signal level relative to average, db -30 db -40 db Average interval between fades 5

11 Dealing with the RF Environment Consider a representative fading profile. Assume that a transmission block is lost if any part of it is in fade: t Fade depth Minimum usable signal (e.g.) Transmission blocks P(success)=/8=67% 53

12 Dealing with the RF Environment: Understand the channel characteristics Consider a representative fading profile. Assume that a transmission block is lost if any part of it is in fade: t Fade depth Minimum usable signal (e.g.) Transmission blocks P(success)=/8=67% P(success)=30/39=77% 54

13 Dealing with the RF Environment: Interleaving Consider a representative fading profile. Assume that a transmission block is lost if any part of it is in fade: t Fade depth Minimum usable signal (e.g.) Transmission blocks P(success)=/8=67% P(success)=30/39=77% P(success)=36/39=9% 55

14 Dealing with the RF Environment: Diversity Consider a two representative fading profiles measured at two antennas. Assume that a transmission block is lost if any part of it is in fade at both: t Fade depth Minimum usable signal (e.g.) Transmission blocks P(success)=6/8=89% P(success)=38/39=97% P(success)=39/39=00% For description of diversity experiments, see and 56

15 Multiple Access Techniques Commonplace multiple access techniques: frequency In Frequency Frequency Division Multiple Access (FDMA) In time Time Division Multiple Access (TDMA) time 57

16 TDMA nd Generation IS-54/IS-36: 40 ms frame Base User 4 User 5 User

17 CDMA nd Generation Consider a two channel frequency division system: sin(w t) TX RX sin(w t) sin(w t) TX RX sin(w t) Fundamentally, what allows RX to receive TX while rejecting TX? For ω ω, sin( ωt)sin( ω tdt ) = 0 59

18 CDMA nd Generation What is magical about sinusoids? Consider some arbitrary functions: f (t) TX RX f (t) f (t) TX RX f (t) Constraint on f, f : f() t f() tdt = 0 60

19 CDMA Spreading and Despreading TX RX TX RX 6

20 CDMA Spreading and Despreading TX RX TX RX Spreading factor ~ (RF Bandwidth)/(Baseband bandwidth) 6

21 General Receiver Considerations Tunability Gain ~0dB Filter Demodulator Baseband Interface Output signal Selectivity Sensitivity 63

22 Radiation from an Antenna z y Er (, θφ, ) = θ F (, ) θ θφ + φfφ( θφ, ) e π r j λ r θ r V H Er (, θφ, ) Hr (, θφ, ) H H φ = θ Eθ 377Ω Eφ = 377Ω S = E H * φ x antenna 64

23 Radiation Pattern U( φ) φ

24 Polarization y z y z Horizontal Vertical x x y z Circular (Left-Hand) x 66

25 The Friis Equation P r = GG t r λ ( 4π R) P t G t G r TX R RX P t P r 67

26 EIRP G t = R G r TX P t P r RX G t R G r TX P t P r RX 68

27 Free Space Propagation 0 0 R L R 50 0 log( L( d, ) ) d

28 Realistic Path Loss L R n 0 0 Environment Free space n 0 log( L( d, ) ) 0 log( L( d, 3.5) ) 50 Urban Shadowed urban log( L( d,, 500, 4) ) 00 R d L R R> d 4 R d

29 The Earliest Radio-location services NNNNNNN AAAAAAA Right Center Left Morse A Tone Morse N Note: Right and left may be reversed! 7

30 Geolocation Services TX TX t t =t + RX t 3 =t + TX 3 7

31 Geolocation Services TX TX RX TX 3 73

32 Representative Wireless Communications Systems Satellite GEO-Geosynchronous orbit: 4000 mi Broadcast TV, Long distance communications PSK/TDMA FM/FDMA GHz 4 GHz Multiple satellites Low gain receive antennas Encrypted links Large spreading factor for A/J 4 GHz 6 GHz Ku Band LEO-Low Earth orbit: mi High path loss C Band High gain antennas High power uplink Open or minimally protected communications PSK Spread-spectrum GPS Geolocation L Band.8,.575 GHz 74

33 Representative Wireless Communications Systems AMPS Cellular MacroCell spacing ~5-0 miles MHz uplink MHz downlink Analog FM 30 khz channels 5 khz deviation Analog signaling A and B competing carriers 75

34 Representative Wireless Communications Systems -G PCS MacroCell spacing ~5-0 miles at 850 MHz, 3-5 miles at 900 MHz 850: IS-95 CDMA.5 MHz channels IS-36 QPSK TDMA 30 khz channels 900: IS-95 CDMA.5 MHz channels IS-36 QPSK TDMA 30 khz channels GSM GMSK TDMA 00 khz channels Digital signaling A and B competing carriers on 850 MHz A,B,C,D,E,F competing carriers on 900 MHz MHz MHz uplink MHz, downlink 76

35 Representative Wireless Communications Systems Terrestrial Microwave mi LOS 4-0+ GHz Analog SSB FDMA Digital QPSK, 6QAM, 64QAM TDM: DS-DS3 Multichannel Voice, Data traffic Generally not encrypted 77

36 Representative Wireless Communications Systems Tactical Military Point-to-point net operation MHz Analog FM 6 kb/s FSK Encrypted communications FH/DS ECCM 78

37 Representative Wireless Communications Systems 80. WLAN 80.b GHz TDD overlapping channels Mc/s DS-SS, CCK/PSK 80.a GHz, GHz 0 non-overlapping TDD channels 50 kbaud OFDM (5 carriers) BPSK, QPSK, 6QAM, 64QAM on each carrier 79