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1 S.72-227 Digital Communication Systems Advanced Modulation and Random Access Techniques

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Timo O. Korhonen, HUT Communication Laboratory 2 Agenda today u Review of code error detection and correction capability u ARQ-techniques –Stop-and-wait –go-back-N –Selective-repeat u ARQ throughput efficiency u Selective-repeat in polar signaling AWGN channel u Trellis coded modulation (TCM) –set-partitioning –subset selection u Dynamic medium access t Delay-bandwidth product t Throughput –ALOHA –Slotted ALOHA

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Timo O. Korhonen, HUT Communication Laboratory 11 Trellis coded modulation (TCM) [2-3] u In TCM modulation and coding are treated as –a combined entity to –maximize total effective Euclidean distance between –mapped code paths in decoder state trellis. u This is realized by –Setting the number of points in constellation larger than required by the modulation format. Extra used to give space for redundancy required by error control –Convolutional coding used to introduce dependency between constellation points such that only certain constellation patterns (sequences) allowed –Soft-decision decoding is used at the receiver to get additional sensitivity gain of order of 2-3 dB

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Timo O. Korhonen, HUT Communication Laboratory 21 Poisson frequency distribution[1]

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Timo O. Korhonen, HUT Communication Laboratory 22 Poisson distribution - example [1] u For Poisson distribution we have and therefore average number of errors is and the probability frequency distribution function is u Cumulative distribution yields then required probability as u Checking with Mathematica yields:

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Timo O. Korhonen, HUT Communication Laboratory 23 ALOHA’s throughput u The probability that the transmitted packet will not overlap with a another packet is the prob. that no packet is transmitted within the vulnerable period u Assuming that the offered mean traffic is 2G in 2X seconds (=vulnerable period) (X :time to transmit the frame) results that the probability of making k transmission within the vulnerable period is u Channel throughput S equals (offered traffic G)x(probability of successful transmission) or X=L/R L: frame length (bits)

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Timo O. Korhonen, HUT Communication Laboratory 24 Slotted-ALOHA (slot size = frame length) ALOHA S-ALOHA u In the Slotted ALOHA-system transmission is allowed at yielding for throughput u Note that ALOHA yields maximum throughput for G = 1/2, that means offering in average one frame within the vulnerable period (if more is offered, collision prob. increases) u However, if the offered traffic G is very small, actually almost all offered traffic goes through because then offered traffic throughput

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Timo O. Korhonen, HUT Communication Laboratory 25 References u [1] A.B. Carlson: Communication Systems (4th ed) u [2] Haykin S: Communication Systems (3th ed) u [3] J. G. Proakis: Digital Communications (4th ed) u [4] Stallings W: Data and Computer Communications (7th ed) u [5] M. Duck, R. Read: Data Communications and Computer Networks (2th ed) u [6] G. Ungerboeck: “Trellis-coded Modulation with Redundant Signal Sets, Parts I and II, IEEE Communications Magazine, vol. 25, pp. 5-21, Feb. 1987 u [7] A. Leon-Garcia, I. Widjaja: Communication Networks (2th ed)

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