Wireless Channels Y. Richard Yang 01/12/2011

Outline Recap Characteristic of wireless channels

Recap: Wireless and Mobile Computing Driven by technology and infrastructure wireless communication technology global infrastructure device miniaturization and capabilities software development platforms Challenges: wireless channel: unreliable, open access mobility portability changing environment heterogeneity

Recap: Overview of Wireless Transmissions source coding bit stream channel coding analog signal sender modulation receiver bit stream source decoding channel decoding demodulation

ideal periodical digital signal Fourier Transform: Every Signal Can be Decomposed as a Collection of Harmonics Time domain Frequency domain 1 1 t t ideal periodical digital signal decomposition Two representations: time domain; frequency domain Knowing one can recover the other

Try spectrum1.m and spectrum2.m Examples Try spectrum1.m and spectrum2.m

Recap: Modulation Objective Basic schemes encode digital data into analog signals at the right frequency range Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): Frequency Shift Keying (FSK): Phase Shift Keying (PSK): 1 t

Example Suppose fc = 1 GHz (fc1 = 1 GHz, fc0 = 900 GHz for FSK) Bit rate is 1 Mbps Encode one bit at a time Bit seq: 1 0 0 1 0 Q: How does the wave look like for each scheme? 1 t t

Phase Shift Keying: BPSK BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK Properties robust, used e.g. in satellite systems Q I 1 Q: What is the spectrum usage of BPSK?

Spectral Density of BPSK Spectral Density = bit rate ------------------- width of spectrum used b fc : freq. of carrier Rb =Bb = 1/Tb b fc

Phase Shift Keying: QPSK 11 10 00 01 Q I A t QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK

Phase Shift Keying: Comparison fc: carrier freq. Rb: freq. of data 10dB = 10; 20dB =100 BPSK A QPSK t 11 10 00 01

Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation it is possible to code n bits using one symbol 2n discrete levels 0000 0001 0011 1000 Q I 0010 φ a 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 Q: why would any one use BPSK, but the highest QAM?

Antennas and Signal Propagation

Antennas: Isotropic Radiator Isotropic radiator: a single point equal radiation in all directions (three dimensional) only a theoretical reference antenna Radiation pattern: measurement of radiation around an antenna z y z ideal isotropic radiator y x x Q: how does power level decrease as a function of d, the distance from the transmitter to the receiver?

Free-Space Isotropic Signal Propagation In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver) Suppose transmitted signal is x, received signal y = h x, where h is proportional to 1/d² Pr: received power Pt: transmitted power Gr, Gt: receiver and transmitter antenna gain  (=c/f): wave length Sometime we write path loss in log scale: Lp = 10 log(Pt) – 10log(Pr)

Free Space Signal Propagation 1 t at distance d ?

Real Antennas Q: Assume frequency 1 Ghz,  = ? Real antennas are not isotropic radiators Some simple antennas: quarter wave /4 on car roofs or half wave dipole /2  size of antenna proportional to wavelength for better transmission/receiving /4 /2 Q: Assume frequency 1 Ghz,  = ?

Dipole: Radiation Pattern of a Dipole http://www.tpub.com/content/neets/14182/index.htm http://en.wikipedia.org/wiki/Dipole_antenna

Why Not Digital Signal (revisited) Not good for spectrum usage/sharing The wavelength can be extremely large to build portal devices e.g., T = 1 us -> f=1/T = 1MHz -> wavelength = 3x108/106 = 300m

Figure for Thought: Real Measurements

Signal Propagation Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles scattering at small obstacles diffraction at edges diffraction reflection refraction scattering shadow fading

Signal Propagation: Scenarios Details of signal propagation are very complicated We want to understand the key characteristics that are important to our objective

i.e. reduces to ¼ of signal 10 log(1/4) = -6.02 Shadowing Signal strength loss after passing through obstacles Some sample numbers i.e. reduces to ¼ of signal 10 log(1/4) = -6.02

Multipath Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Multipath Can Reduce Signal Strength Example: reflection from the ground: received power decreases proportional to 1/d4 instead of 1/d² due to the destructive interference between the direct signal and the signal reflected from the ground ground For detail, see page 9: http://www.eecs.berkeley.edu/~dtse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter2.pdf

Multipath Fading Due to constructive and destructive interference of multiple transmitted waves, signal strength may vary widely as a function of receiver position

Multipath Fading: A Simple Two-path Example receiver - Wavelength is about 0.3 m for 1 GHz cellular

More detail see page 16 Eqn. (2.13): Multipath Fading with Mobility: A Simple Two-path Example r(t) = r0 + v t, assume transmitter sends out signal cos(2 fc t) r0 More detail see page 16 Eqn. (2.13): http://www.eecs.berkeley.edu/~dtse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter2.pdf

Received Waveform v = 65 miles/h, fc = 1 GHz: 10 ms deep fade v = 65 miles/h, fc = 1 GHz: fc v/c = 109 * 30 / 3x108 = 100 Hz Why is fast multipath fading bad?

Small-Scale Fading

Multipath Can Spread Delay signal at sender LOS pulse Time dispersion: signal is dispersed over time multipath pulses signal at receiver LOS: Line Of Sight

RMS: root-mean-square Delay Spread RMS: root-mean-square

Multipath Can Cause ISI dispersed signal can cause interference between “neighbor” symbols, Inter Symbol Interference (ISI) Assume 300 meters delay spread, the arrival time difference is 300/3x108 = 1 ms if symbol rate > 1 Ms/sec, we will have serious ISI In practice, fractional ISI can already substantially increase loss rate signal at sender LOS pulse multipath pulses signal at receiver LOS: Line Of Sight

Summary: Wireless Channels Channel characteristics change over location, time, and frequency Received Signal Large-scale fading Power power (dB) path loss log (distance) time small-scale fading frequency