mac - 2004 Matt Welsh Harvard University 1 CS263: Wireless...

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Unformatted text preview: 2004 Matt Welsh Harvard University 1 CS263: Wireless Communications and Sensor Networks Matt Welsh Lecture 4: Medium Access Control October 5, 2004 2004 Matt Welsh Harvard University 2 Today's Lecture Medium Access Control Schemes: FDMA TDMA CDMA Examples from cell phone technology: AMPS, GSM Packet networks: ALOHA and Slotted ALOHA Capture effect and hidden terminal problem Use of power control Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 2004 Matt Welsh Harvard University 3 Medium Access Control (MAC) Many mobile devices must share limited spectrum e.g., 802.11b networks in the US operate in the frequency range 2.4022.479 GHz Most GSM networks in the US operate at 1850-1910 MHz (uplink) and 1930-1990 MHz (downlink) This is not a lot of spectrum!!! So ... how do we carve up the spectrum to give multiple users access to it?? Three basic approaches: Divide the spectrum up by frequency Divide the spectrum up by time Divide the spectrum up by code (e.g., pattern of usage more later!) 2004 Matt Welsh Harvard University 4 Duplexing The first kind of multiple access is duplexing : Allowing simultaneous transmit and receive to a single user Frequency Division Duplexing (FDD) Each user assigned two channels Channel = range of frequencies for a single user to transmit or receive One channel used for transmission, another used for reception Time Division Duplexing (TDD) Uses only a single channel for both transmission and reception But, communication divided into time slots One time slot used for transmission, another used for reception tx rx c c 1 c transmission reception time time tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx rx tx 2004 Matt Welsh Harvard University 5 FDD vs TDD Why use FDD or TDD? FDD requires each user to be allocated multiple channels Less efficient use of spectrum FDD requires slightly more complex electronics ($$$) TDD induces some latency between transmit and receive cycles Generally operates on level of milliseconds, so not generally noticeable With TDD, propagation delays may limit the distance of a user to a station e.g., In GSM, user must be < 35 km from the cell tower for timing to work out 2004 Matt Welsh Harvard University 6 Frequency Division Multiple Access How to allocate spectrum to multiple mobile users? Frequency Division Multiple Access (FDMA) Give each user her own channel (or pair of channels if using FDD) Can be combined with either FDD or TDD If channel is unused by that user, it sits idle Channel is effectively wasted Must design system to avoid crosstalk across frequencies Increases cost of base must use bandpass filters on each channel time User 1 User 2 User 3 User 4 2004 Matt Welsh Harvard University 7 Example: AMPS First US analog cellular system, early 80's While call in progress, phone occupies one...
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mac - 2004 Matt Welsh Harvard University 1 CS263: Wireless...

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