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This 22 page Study Guide was uploaded by akilbabu on Wednesday February 24, 2016. The Study Guide belongs to ee 5368 at University of Texas at Arlington taught by peter wang in Spring 2016. Since its upload, it has received 44 views. For similar materials see wireless ommunications systems in Electrical Engineering at University of Texas at Arlington.
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Date Created: 02/24/16
Wireless Fundamentals Peter Wang January 16,2011 1 © Wirelessfundamentals_04022010 OUTLINE • Wireless channels • Far-field average power • Long-term fading channel • Short-term fading channel • Signal transmission through a system • Nonlinear system • Linear system • Linear time invariant • Linear time variant • Fading mitigation techniques • Six ways to mitigate fading • Diversity combining methods 2 © Wirelessfundamentals_04022010 Wireless channels • Wireless -> Mobility -> Channel complexity • The radiating electromagnetic field is reflected, diffracted, and scattered by various obstacles • The received signal has a spatially and temporally varying energy pattern (fading) due to the combined interference • Three phenomena together effects the received power • Far-field average power (i.e., the area-mean power) • Long-term fading due to the shadowing • Short-term fading due to multipath and mobility • The statistically varying received signal power can be 2 X /10 modeled as: P R 10 g(d) PTG GT R 2ismultipath fading g(d)is pathloss 3 © Wirelessfundamentals_04022010sshadow fading Long-term & short-term fading regions Scatterer Building Semi-transmission Reflector Diffrector Ground Short-term fading region Long-term fading region P 2 10 X /10g(d) P G G R T T R 4 © Wirelessfundamentals_04022010 Long-term fading • Local mean power P 10 X /10 g(d) P G G R_localmean T T R n • Path Loss g(d) k(d) d • For Free-space propagation n = 2 h t 1 d2 • For two ray propagation n=4 r r • Long-term fading (i.e., Lognormal shadowing) • Slowly varying fading (~20 wavelength in distance) • Lognormal distribution 5 © Wirelessfundamentals_04022010 Short-term fading due to multipath (1/2) • Flat Fading (e.g., Rayleigh or Ricean distribution) • Frequency-Selected Fading Short-term fading due to multipath (2/2) • Flat Fading Example Transmitted SignalReceived Signal 1 ( f)c T m Coherence Bandwidth Delay Spread • Frequency Selective Fading Example Transmitted Signal Received Signal 7 © Wirelessfundamentals_04022010 Short-term fading due to mobility • Slow Fading • Fast Fading velocity velocity 1 ( t) c B d Coherence Time Doppler Spread 8 © Wirelessfundamentals_04022010 Example of short-term fading channel Power density 5 4 3 2 1 Frequency (Hz) 1 us 2 us 3 us 4 us 5 -80 -20 0 20 40 6 80 Hz us Hz 0 Time delay (us) Fast channel varying (~0.5 wavelength in distance) 9 © Wirelessfundamentals_04022010 Fading channel measurement 10 © Wirelessfundamentals_04022010 Types of received power variations “Wireless Communication”, Andreas F. Molisch, IEEE Press, John Wiley 11 © Wirelessfundamentals_04022010 Example of received power measurement 12 © Wirelessfundamentals_04022010 Summary of propagation channels Propagation effects Rayleigh /Ricean distribution Large-scale/Long-term fading Small-scale/Short-term fading Log-normal distribution Pathloss Shadowing variation (~20 ) Suzuki distribution Multipath delay spread Doppler spread Flat fading Frequency Slow fading Fast fading selective fading 13 © Wirelessfundamentals_04022010 Signal transmission through a system (1/3) • Nonlinear Systems • A memoryless nonlinear case: y(t) a a g(t) a g (t) a g (t) 0 1 2 k • If g(t) bandwidth is B(Hz), then g (t) bandwidth is kB(Hz) • If a signal is transmitted over a nonlinear channel (system), the nonlinearity – Distorts the signal – Causes interference with other signals in the channel because of its spectral dispersion (spreading) 14 © Wirelessfundamentals_04022010 Signal transmission through a system (2/3) • Linear Systems • Distortionless System • The input and the output have identical waveshapes within a multiplicative constant. A delay output that retains the input waveform is also considered distortionless d y(t) k x(t t ) d LTI system h t j2 f d hr Y(f ) k X(f ) e • For distortionless transmission condition r H(f ) H(f ) k H(f ) (f ) 2 f t d (f ) 2 t k h d df h d f h(f) is linear with respect to f (f ) h 15 © Wirelessfundamentals_04022010 Signal transmission through a system (3/3) • Linear Distortion Systems • Signal distortion caused by non-ideal channel characteristics of magnitude distortion, phase distortion, or both • Linear channel distortion (time dispersion) introduce the intersymbol interference (ISI) • Linear time-invariant distortion caused by multipath effect • Linear time-variant distortion caused by mobility + multipath d ht d1 d2 hr r 16 © Wirelessfundamentals_04022010 Fading mitigation techniques (1/2) • Various techniques to overcome the combined effects of fading, noise, and signal interference •Interleaving: reduce the effect of fast fading and possible bursts and noise •OFDM: transmit sequences of digital signals in parallel reducing their BW requirements. Frequency-selective-fading becomes flat- fading for each signal BW reducing ISI •Channel coding: detect or correct digital-signal errors 17 © Wirelessfundamentals_04022010 Fading mitigation techniques (2/2) • Another three techniques to mitigate the effect of fading on signal • Equalizer: Overcome ISI (used in GSM system) • Diversity techniques: Overcome short-term fading • Space, Frequency, Time, Angle, Polarization, Multipath diversities • RAKE receiver: overcome multipath fading (used in CDMA system) 18 © Wirelessfundamentals_04022010 Diversity Combining Methods • Receive diversity (Linear combining) • Selection combining (simple implementation) • Equal-gain combining • Maximum-ratio combining (optimum combining) • Transmit diversity • Space-time block code • MIMO reception • Trade off between diversity and multiplexing schemes • Diversity scheme improves SINR • Multiplexing scheme improves transmission bit rate 19 © Wirelessfundamentals_04022010 Summary • Wireless channels • Far-field average power (A general equation) • Long-term fading channel (Large-scale fading, Log-normal shadowing) • Short-term fading channel (Caused by Delay spread & Doppler pread) • Signal transmission through a system • Nonlinear system (Spectral Dispersion) • Linear system (Time Dispersion) • Time invariant (Due to multipath) • Time variant (Due to multipath & mobility) • Fading mitigation techniques • Six methods to mitigate fading • Diversity combining methods (e.g., Maximum Ratio Combining, Space- Time Block Code) 20 © Wirelessfundamentals_04022010 Reference Reference website: http://www-ee.uta.edu/Online/PWang/ee5368/index.htm) Reference books: (1) “Mobile Wireless Communications”, by M. Schwartz. (2) “Modern Digital & Analog Communication Systems”, by B. P. Lathi and Zhi Ding. (3) “Wireless Communications”, by A. F. Molisch. 21 © Wirelessfundamentals_04022010 Cellular System Evolutions LTE GSM IS-95 IS-136 AMPS Analog or Digital Digital Digital Digital Analog Digital System Multiple OFDMA/ FD/TDMA FD/CDMA FD/TDMA FDMA Access SC-FDMA Method FDD or TDD FDD FDD FDD FDD FDD Mode TDD BW/Radio Flexible 200 KHz 1.25 MHz 30 KHz 30 KHz Channel (Up to 40 MHz) Voice Flexible 8 Users (~20 3 Users 1 Users annels/Radio Users) Channel Modulation PSK, QAM MSK PSK /4 DQPSK FM Seme Hard or SoftHard Hard Soft Hard Hard Handoff 22 © Wirelessfundamentals_04022010
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