C03-Media_Access

C03-Media_Access - Mobile Communications Chapter 3 : Media...

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3.1 Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2009 Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha, reservation schemes Collision avoidance, MACA Polling CDMA, SAMA Comparison
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3.2 Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2009 Motivation Can we apply media access methods from fixed networks? Example CSMA/CD C arrier S ense M ultiple A ccess with C ollision D etection send as soon as the medium is free, listen into the medium if a collision occurs (legacy method in IEEE 802.3) Problems in wireless networks signal strength decreases proportional to the square of the distance the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot “hear” the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is “hidden”
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3.3 Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2009 Motivation - hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B B A C
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3.4 Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2009 Motivation - near and far terminals Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A’s signal C cannot receive A If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! A B C
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3.5 Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2009 Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time The multiplexing schemes presented in chapter 2 are now used to control medium access!
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3.6 Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2009 FDD/FDMA - general scheme, example GSM f t 124 1 124 1 20 MHz 200 kHz 890.2 MHz 935.2 MHz 915 MHz 960 MHz
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3.7 Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2009 TDD/TDMA - general scheme, example DECT 1 2 3 11 12 1 2 3 11 12 t downlink uplink 417 μs
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3.8
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C03-Media_Access - Mobile Communications Chapter 3 : Media...

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