Lecture9 - E4703 Wireless Communications Slide Set 9...

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E4703 Wireless Communications Slide Set 9
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Announcement Only half lecture today. Afterwards, coffee/tea drinkers are invited to come to the SIPA coffee-shop (118 th th floor). My treat !
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Outline Summary of last lecture. Spread Spectrum ¾ Direct-Sequence ¾ Frequency Hopping Multiuser channels Multiple Access: FDMA, TDMA, CDMA and SDMA. Random Access: ¾ Pure Aloha ¾ Slotted Aloha ¾ CSMA
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Summary of Last Lecture Coding essential for reliable communication. Central ingredient of any digital wireless system. It allows reducing error probability (at rates below capacity) at the expense of complexity and latency. Studied ( n , k ) binary linear block codes, which map blocks of k bits into codewords of n bits. Systematic if the first k digits are the original information bits while the last ( n - k ) are parity (redundancy) bits. The Hamming distance between 2 codewords is the number of digits in which they differ. The minimum distance between any two codewords, d min , is a key parameter.
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In hard-decision decoding, the demodulator maps every received bit onto 1 or 0 (as if the transmission was uncoded) and then feeds this string of 1s and 0s to the decoder. Up to d min -1 errors can be detected and can be corrected. In hard decoding, the demodulator discards useful information: how far each received symbol lied from the hard decision that was made. With soft decoding, this information is passed to the decoder. Convolutional codes are generated by passing the information bits through a linear finite-state shift register consisting of K stages (constraint length) with k bits per stage. A codeword length n is produced for every k input bits. Conveniently represented as a trellis. ML decoding (tracking the most likely paths through the trellis) drastically simplified by the Viterbi algorithm. ⎣ ⎦ 2 / ) 1 ( min = d t
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With hard decoding, can correct up to errors, with d free the minimum Hamming distance between 2 codewords. State of the art in coding embodied by turbo codes and other iteratively decodable codes, which perform very close to capacity in AWGN. Interleaving essential complement of coding in fading channels. = 2 1 free d t
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Spread Spectrum SS is a digital communication technique that increases the bandwidth of the transmit signal to values much larger that is needed to transmit the information. Although SS would appear to waste a precious commodity (bandwidth), it also enables overlaying multiple users which ultimately may make the bandwidth usage efficient. This is the basis of CDMA systems. Furthermore, there are other advantages that motivate the use of SS, such as eavesdropping and jamming resistance. In fact, SS techniques were widely used by the military (e.g. GPS) before entering the commercial arena.
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An added advantage of SS is that, by virtue of the bandwidth expansion, it provides frequency diversity. Two types of SS:
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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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Lecture9 - E4703 Wireless Communications Slide Set 9...

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