Lecture8 - E4703 Wireless Communications Slide Set 8...

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E4703 Wireless Communications Slide Set 8
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Outline Summary of last lecture. Introduction to coding. Coding for AWGN channels. ¾ A baby example: single-bit parity check. ¾ Linear block codes • Hard decision • Soft decision. ¾ Convolutional codes • Trellis diagrams • The Viterbi algorithm • Distance properties Coding & interleaving for fading channels.
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Summary of Last Lecture Diversity mitigates fading by providing multiple (independent) routes for the signal. Two levels: ¾ Macroscopic diversity. Designed to combat shadow fading. ¾ Microscopic diversity. Designed to combat multipath fading. Variety of forms, several of them relying on the use of multiple transmit or receive antennas. Focused on microscopic antenna diversity to illustrate basic notions: ¾ Receive diversity. ¾ Transmit diversity. Receive diversity based on co-phasing, weighting and combining signals from M receive branches.
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A particularly simple version is to select the best branch. Benefit of receive diversity is a more favorable SNR distribution, including an increase in average SNR (the array gain). This improves the outage probability, average error probability, etc. The superior form of weighting and combining is MRC. Transmit diversity based on pre-phasing, weighting and launching signals from M transmit antennas. If transmitter has CSI, it can achieve exact same performance as MRC. If not, for 2 antennas Alamouti can still provide order 2 diversity but without the array gain.
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Coding Coding is essential for reliable communication and it is therefore a central ingredient of any digital wireless system. It allows reducing error probability (as long as the rate is below capacity) at the expense of complexity and latency. The coding gain is the reduction in SNR required for a given error probability. Powerful coding schemes are featured in modern cellular standards. Very broad and deep topic. We will only scratch the surface and cover some basic notions. Begin by looking at codes for AWGN. Then we’ll see how to modify this codes in the face of fading, which might cause long strings of errors.
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Baby Example: Parity-Check Single-bit parity check may the most basic code that exists. Add a bit (called parity bit) to every block of k bits such that the total number of 1 s is even. We can then detect the occurrence of certain errors in the block transmission. Say we have the block 10011101. We add a 1 for parity and transmit 100111011. Suppose that the 5 th bit is flipped in error so we receive 100101011. This has an odd number of 1s and thus we know an error has occurred. We will detect an odd number of errors but not an even
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Lecture8 - E4703 Wireless Communications Slide Set 8...

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