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  1. Physical layer basics • transmit bit stream from sender to receiver • what does sender need to do? • what is transmitted into the air? • what happens in the air? • what does receiver need to do?

  2. modulation channel coding source coding At sender analog signal bit stream • source coding/compression • reduce redundancy • channel coding • add redundancy (why?) • modulation • from bit stream to wave form • from analog form (e.g., music) to another form

  3. Modulation in wireless communication • translate digital data to analog signal (baseband) • shifts center frequency of baseband signal up to the radio carrier • example carrier frequency • 802.11b/g: 2.4 GHz, 802.11a (5 GHz), GSM: 1.9 GHz • why? • Antenna size: on the order of signal’s wavelength • More bandwidth available at higher carrier frequency • Medium characteristics (path loss, shadowing, reflection, scattering, diffraction) depend on signal’s wavelength

  4. analog baseband signal digital data modulation modulation 101101001 radio carrier Modulation at sender

  5. Modulation schemes • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK)

  6. Amplitude Shift Keying (ASK) • Pros: simple • Cons: susceptible to noise • Example: optical system, infra-red 1 0 1 t

  7. Frequency Shift Keying (FSK) • Pros: less susceptible to noise • Cons: requires larger bandwidth 1 0 1 t 1 0 1

  8. Phase Shift Keying (PSK) • Pros: • Less susceptible to noise • Bandwidth efficient • Cons: • Receiver must synchronize in frequency and phase w/ transmitter t

  9. Q I 1 0 Q 11 10 I • QPSK (Quadrature Phase Shift Keying): • 2 bits coded as one symbol • needs less bandwidth compared to BPSK • symbol determines shift of sine wave • Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK 00 01 A t 01 11 10 00 Variant of phase shift keying • BPSK (Binary Phase Shift Keying): • bit value 0: sine wave • bit value 1: inverted sine wave • very simple PSK • low spectral efficiency • robust, used in satellite systems

  10. Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ,but different amplitude a. 0000 and 1000 have same amplitude but different phase Used in Modem Q 0010 0001 0011 0000 φ I a 1000 Quadrature Amplitude Modulation (QAM) • combines amplitude and phase shift keying • It is possible to code n bits using one symbol • 2n discrete levels • bit error rate increases with n

  11. What is transmitted in air? • radio wave (baseband modulated w/ carrier radio) • high-frequency, short wavelength • wave length * frequency = speed of light  3x108m/s • e.g., 802.11b, wavelength 0.1m

  12. Frequency range (bandwidth) • need a wide spectrum • e.g., 802.11b bandwidth 20 MHz • w/o noise: Nyquist’s result • w/ noise (e.g., thermal noise, background radiation) • Shannon’s channel capacity theorem: the maximum number of bits that can be transmitted per second by a physical channel is: • W: frequency range • S/N: signal noise ratio

  13. Frequencies for communication twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency

  14. Frequency regulation • ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)

  15. What happens in the air? • path loss: attenuation due to distance • fading (frequency dependent) • shadowing • reflection at large obstacles • refraction depending on the density of a medium • scattering at small obstacles • diffraction at edges refraction shadowing reflection scattering diffraction

  16. Received Signal Power (dB) path loss shadow fading Rayleigh fading log (distance) Typical picture

  17. Real world example

  18. Detour: dB and Power conversion • dB • Denote the difference between two power levels • (P2/P1)[dB] = 10 * log10 (P2/P1) • P2/P1 = 10^(A/10) • Example: P2 = 100 P1 • dBm and dBW • Denote the power level relative to 1 mW or 1 W • P[dBm] = 10*log10(P/1mW) • P[dB] = 10*log10(P/1W) • Example: P = 0.001 mW, P = 100 W

  19. Path loss • determine average received power • proportional to 1/dn • d: distance from sender to receiver • vacuum: n=2 • in practice: n larger than 2

  20. Path loss models • Free space model • Pr(d): receiver power, Pt: transmitter power • d: distance from transmitter to receiver • Gt: transmitter antenna gain • Gr: receiver antenna gain • lambda: wave length, L: constant • Two-ray ground reflection model • ht: height of transmitter, hr: height of receiver

  21. Log-normal/shadow fading • long-term variation or fading Gaussian r.v. Average received power

  22. Multipath fading • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction LOS pulses multipath pulses LOS: Line Of Sight signal at sender signal at receiver

  23. When transmitter and/or receiver moves? • Short-term fading • quick changes in the power received • Doppler shift long term fading power t short term fading

  24. signal power interference spread signal power spread interference detection at receiver f f Spread spectrum technology • spread a narrow band signal into a broad band signal using a special code • Resilient to narrow band interference • Side effects: • coexistence of several signals without dynamic coordination • tap-proof

  25. Effects of spreading and interference dP/df dP/df user signal broadband interference narrowband interference i) ii) f f sender dP/df dP/df dP/df iii) iv) v) f f f receiver

  26. tb user data 0 1 XOR tc chipping sequence 0 1 1 0 1 0 1 0 1 1 0 1 0 1 = resulting signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0 tb: bit period tc: chip period DSSS (Direct Sequence Spread Spectrum) • XOR the signal with pseudo-random number (chipping sequence) • generate a signal with a wider range of frequency: spread spectrum • spread factor: tb/tc • military applications use spread factor up to 10,000

  27. spread spectrum signal transmit signal user data X modulator chipping sequence radio carrier transmitter correlator lowpass filtered signal sampled sums products received signal data demodulator X integrator decision radio carrier chipping sequence receiver DSSS System

  28. FHSS (Frequency Hopping Spread Spectrum) • Discrete changes of carrier frequency • sequence of frequency changes determined via pseudo random number sequence • Two versions • Fast Hopping: several frequencies per user bit • Slow Hopping: several user bits per frequency • Advantages • frequency selective fading and interference limited to short period • simple implementation • uses only small portion of spectrum at any time

  29. tb user data 0 1 0 1 1 t f td f3 slow hopping (3 bits/hop) f2 f1 t td f f3 fast hopping (3 hops/bit) f2 f1 t tb: bit period td: dwell time FHSS: Example

  30. Comparison between slow hopping and fast hopping • Slow hopping • Pros: cheaper • Cons: less immune to narrowband interference • Fast hopping • Pros: more immune to narrowband interference • Cons: tight synchronization  increased complexity

  31. FHSS (Frequency Hopping Spread Spectrum) spread transmit signal narrowband signal user data modulator modulator frequency synthesizer hopping sequence transmitter narrowband signal received signal data demodulator demodulator hopping sequence frequency synthesizer receiver

  32. demodulation At receiver analog signal bit stream source decoding channel decoding

  33. analog baseband signal digital data demodulation synchronization decision 101101001 radio carrier Demodulation at receiver

  34. Signal interference and noise ratio • Signal (S) • Noise (N) • Includes thermal noise and background radiation • Often modeled as additive white Gaussian noise • Interference (I) • Signals from other transmitting sources • SINR = S/(N+I) (sometimes also denoted as SNR)