CS5530 Mobile/Wireless Systems Key Wireless Physical Layer Concepts

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1 Mbile/Wireless Systems Key Wireless Physical Layer Cncepts Yanyan Zhuang Department f Cmputer Science UC. Clrad Springs

2 Outline Electrmagnetic spectrum Reflectin, diffractin and scattering f signals Multipath, Dppler shift Digital mdulatin and multiplexing Nise 2

3 Electrmagnetic Spectrum Wireless cmmunicatin 100 khz t 60 GHz Higher frequency: nly g in a straight line, can t g far Image: 3

4 Antenna Transmitter cnverts electrical energy t electrmagnetic waves Receiver cnverts electrmagnetic waves t electrical energy Same antenna used fr transmissin and receptin Signal f same frequency cause interference At receiver side 4 Ref. CN5E, NT@UW

5 Reflectin, diffractin and scattering 5

6 Reflectin, diffractin and scattering Reflectin Surface large relative t the wavelength f signal Diffractin Edge f impenetrable bdy is large relative t the wavelength f signal Scattering Obstacle size in rder f wavelength (lamp pst) LOS Diffracted and scattered signals are nt significant Nn-LOS Diffractin and scattering are primary means f receptin 6

7 Multipath Prpagatin Image: research/prpagatin/physicalradi-channel-mdels Each prpagatin path travels frm transmitting antenna t receiving antenna while interacting with physical bjects in the envirnment Signals bunce ff bjects and take multiple paths 7

8 Dppler Shift If transmitter r receiver is mbile, the frequency f received signal changes Mving twards each ther: higher frequency Mving away frm each ther: lwer frequency 8

9 Dppler Shift (cnt.) Frequency difference = speed (m/sec) / wavelength (m) Example: 2.4 GHz: wavelength = speed f light / frequency = 3*10^8/2.4*10^9 = m 120 km/h (75 m/h) = 120*10^3/3600 = 33.3 m/s Frequency difference = 33.3/0.125 = 267 Hz 9

10 Dppler Shift (cnt.) Frequency difference = speed (m/sec) / wavelength (m) Example: 2.4 GHz: wavelength = speed f light / frequency = 3*10^8/2.4*10^9 = m 120 km/h (75 m/h) = 120*10^3/3600 = 33.3 m/s Frequency difference = 33.3/0.125 = 267 Hz Why imprtant? Mbile envirnment: walking, driving Example: WiMax is nly designed fr speed lwer than 60 km/h (37.5 m/h) 10

11 Digital Mdulatin and Multiplexing Digital Mdulatin Prcess f cnverting between bits and signals that represent them Regulate amplitude, phase, r frequency f a signal t cnvey bits } ASK, PSK, FSK Multiplexing Use a single medium t carry several signals 11

12 Digital Mdulatin and Multiplexing Digital Mdulatin ASK (Amplitude Shift Keying) } Tw different amplitudes: 0/1 FSK (Frequency Shift Keying) } Tw different frequencies PSK (Phase Shift Keying) } Wave is shifted 0 r 180 degrees Only ne f frequency / phase can be mdulated at a time: they are related Amplitude and phase can be mdulated in cmbinatin 12

13 Digital Mdulatin and Multiplexing Digital Mdulatin PSK (Phase Shift Keying) } Wave is shifted 0 r 180 degrees: BPSK } Wave is shifted 0/90/180/270 degrees: QPSK QAM (Quadrature Amplitude Mdulatin) } Amplitude and phase are mdulated in cmbinatin (a) QPSK. (b) 16 QAM. (c) 64 QAM. 13

14 Digital Mdulatin and Multiplexing Multiplexing FDM (Frequency Divisin Multiplexing) } Divides spectrum int frequency bands, with each user having exclusive pssessin f sme band t send their signal 14

15 Digital Mdulatin and Multiplexing Multiplexing FDM (Frequency Divisin Multiplexing) TDM (Time Divisin Multiplexing) } Users take turns (rund-rbin), each ne peridically getting entire bandwidth fr a little burst f time 15

16 Digital Mdulatin and Multiplexing Multiplexing FDM (Frequency Divisin Multiplexing) TDM (Time Divisin Multiplexing) CDM (Cde Divisin Multiplexing) } A signal is spread ut ver a wider frequency band } Mre tlerant f interference } Allw multiple signals t share the same frequency band CDMA (Cde Divisin Multiple Access) 16

receiver: training signal befre transmissin Develped initially fr military (prevent

17 Frequency Hpping Spread Spectrum (FHSS) Transmitting signals by rapidly switching amng many frequency channels Using a pseudrandm sequence knwn t nly transmitter and receiver: training signal befre transmissin Develped initially fr military (prevent jamming and cllisin) Spreads pwer ver a wide spectrum (spread spectrum) Hedy Lamarr 17

18 Direct Sequence Spread Spectrum (DSSS) Each bit is represented by multiple bits using a spreading cde r chipping cde Transmitters XOR the chipping cde with data t be transmitted bit chipping cde: lnger chipping cde, mre secure FHSS and DSSS are tw methds f CDMA 18

Applicatins FHSS in 802.11 A typical FHSS WLAN will subdivide the bandwidth int 79 nn-verlapping channels, each 1MHz wide 802.

19 Applicatins FHSS in A typical FHSS WLAN will subdivide the bandwidth int 79 nn-verlapping channels, each 1MHz wide standard defines 78 different hpping patterns The patterns allw fr 26 netwrks t be c-lcated and still perate simultaneusly 19

20 Nise and Different Surces Nise has 3 different surces Thermal nise } Prprtinal t abslute temperature } Temperature measured frm abslute zer in kelvins Spurius emissins } Car ignitin and electrnic devices } Mre nise in urban areas Receiver nise } Amplifier adds nise } Nise generated befre the amplifier als gets amplified 20

21 Summary Electrmagnetic Spectrum Reflectin, diffractin and scattering f signals Multipath, Dppler shift Digital mdulatin and multiplexing Nise 21 Ref. CN5E, NT@UW

Fundamentals. Antennas Mobility Management. Frequencies Modulation. Mobile Communication Fundamentals 1

Fundamentals. Antennas Mobility Management. Frequencies Modulation. Mobile Communication Fundamentals 1 Mbile Cmmunicatins Fundamentals Frequencies Mdulatin Antennas Mbility Management Mbile Cmmunicatin Fundamentals 1 Frequency Spectrum fr Cmmunicatin Frequencies, Examples: Audi TV VLF LF HF VHF UHF IR UV