Lecture7 - E4703 Wireless Communications Slide Set 7...

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E4703 Wireless Communications Slide Set 7
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Outline Summary of last lecture. Introduction to Diversity. Receiver diversity: ¾ Selection combining. ¾ Threshold combining. ¾ Maximum-ratio combining. ¾ Equal-gain combining. Transmit diversity. ¾ Transmitter CSI ¾ No Transmitter CSI: the Alamouti scheme
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Summary of Last Lecture Derived exact and approximate error probabilities for various constellations in AWGN (no fading): ¾ For BPSK and QPSK, exact and approximate. ¾ For M-PSK, only approximate. ¾ For M-PAM, exact. ¾ For M-QAM, exact and approximate. Synthesize above approximations into the general formula where α M and β M depend on the modulation. Derived exact error probability also for DPSK. For slowly fading channels, relevant performance metric is the outage probability. ) ( ) ( s M M s e Q P γβ αγ
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For fast fading channels, relevant performance metric is the average error probability. Derived exact expressions for BPSK and DPSK in Rayleigh fading and approximate expression for other modulations. Critical observation: while without fading P e decays exponentially with γ s , the average error probability decays only reciprocally with γ s . Outage and average error probabilities can be combined to account for both slow fading (shadowing) and fast fading (multipath). Doppler causes an irreducible error floor in differential modulations, which improves with growing data rates. Delay spread causes ISI and, in turn, an irreducible error floor with both coherent and differential modulations. This floor degrades with growing data rates.
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Diversity Fading, both shadow and multipath, is a major impairment in wireless communication A chief technique to mitigate fading is diversity: send/receive the data over several routes undergoing independent fading. Even if one (or several) are in bad fading conditions, some of the others may be in good conditions. The more routes, the better chances. Example: receive antenna diversity (picture above ). Two levels of diversity: ¾ Macroscopic diversity. Designed to combat shadow fading. ¾ Microscopic diversity. Designed to combat multipath fading.
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The main difference between macro and micro diversity is in the scale. For antenna diversity, micro requires antenna spacings on the order of wavelength (inches) while macro requires spacings of tens of meters. Macro antenna diversity is thus implemented using antennas at different base stations, while micro is implemented using co-located antennas at the base and/or terminal. In this lecture we focus on micro diversity only (against multipath fading). We will visit macro diversity towards end of the course. Main forms of micro diversity: ¾ Space diversity.* ¾ Polarization diversity.* ¾ Angle diversity.* ¾ Frequency diversity.
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This note was uploaded on 08/05/2008 for the course ELEN E4702 taught by Professor Lazano during the Summer '08 term at Columbia.

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Lecture7 - E4703 Wireless Communications Slide Set 7...

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