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S.72-227 Digital Communication Systems

S.72-227 Digital Communication Systems. Advanced Modulation and Random Access Techniques. Agenda today. Review of code error detection and correction capability ARQ-techniques Stop-and-wait go-back- N Selective-repeat ARQ throughput efficiency

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S.72-227 Digital Communication Systems

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

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

  3. Trellis coded modulation (TCM) [2-3] • 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. • 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

  4. Poisson frequency distribution[1]

  5. Poisson distribution - example [1] • For Poisson distribution we haveand therefore average number of errors isand the probability frequency distribution function is • Cumulative distribution yields then required probability as • Checking with Mathematica yields:

  6. ALOHA’s throughput • 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 • 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 • Channel throughput S equals (offered traffic G)x(probability of successful transmission) or • X=L/R • L: frame length (bits)

  7. S-ALOHA ALOHA Slotted-ALOHA (slot size = frame length) • In the Slotted ALOHA-system transmission is allowed atyielding for throughput • Note that ALOHA yields maximumthroughput for G = 1/2, that means offering in average one frame within the vulnerable period (if more is offered, collision prob. increases) • However, if the offered traffic G is very small, actually almost all offered traffic goes through because then throughput offered traffic

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

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