© 2004 Matt Welsh – Harvard University1CS263: Wireless Communicationsand Sensor NetworksMatt WelshLecture 4: Medium Access ControlOctober 5, 2004© 2004 Matt Welsh – Harvard University2Today's LectureMedium Access Control Schemes:●FDMA●TDMA ●CDMA●Examples from cell phone technology: AMPS, GSMPacket networks: ALOHA and Slotted ALOHACapture effect and hidden terminal problem●Use of power controlCarrier Sense Multiple Access with Collision Detection (CSMA/CD)© 2004 Matt Welsh – Harvard University3Medium Access Control (MAC)Many mobile devices must share limited spectrum●e.g., 802.11b networks in the US operate in the frequency range 2.402—2.479 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 University4DuplexingThe first kind of multiple access is duplexing:●Allowing simultaneous transmit and receive to a single userFrequency 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 receptionTime 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 receptiontx rxc0c1c0transmissionreceptiontimetimetx rx tx rx tx rx tx rxtx rxtx rx tx rx tx rx tx rxtx rxtx rx tx rx tx rx tx rx tx© 2004 Matt Welsh – Harvard University5FDD vs TDDWhy use FDD or TDD?FDD requires each user to be allocated multiple channels●Less efficient use of spectrumFDD requires slightly more complex electronics ($$$)TDD induces some latency between transmit and receive cycles●Generally operates on level of milliseconds, so not generally noticeableWith 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 University6Frequency Division Multiple AccessHow 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 TDDIf 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 channeltimeUser 1User 2User 3User 4© 2004 Matt Welsh – Harvard University7Example: AMPSFirst US analog cellular system, early 80'sWhile call in progress, phone occupies onechannel in each of the uplink and downlink bands●Base-to-mobile spectrum 869-894 MHz●Mobile-to-base spectrum 824-849 MHzEach channel is 30 kHz. So, how many simultaneous users?Bandwidth = (894-869) = 25 MHz●However, each operator allocated only half of this12.5 MHz / 30 kHz per channel = 416 channels●However, 21 data channels used for control messages●Leaving 395 simultaneous channels available for voiceCan you hear me now?NO!© 2004 Matt Welsh – Harvard University8Time Division Multiple Access (TDMA)Divide radio channel into time slots●In each slot, a given user is allowed to either transmit or receiveTDMA typically uses a cyclic frame structure●Each user allocated one time slot out of each frame●Max number of users therefore bounded by number of slots in a frameEach frame often has preamble/postamble bits●Used for control, synchronization, etc.●All nodes sharing a frame must be synchronized so they know when their slot begins!Slots within a frame protected by guard bits●Extra (unused) bits used to prevent two users from overlapping in timeFrame 1 Frame 2 Frame 3Frame 4time© 2004 Matt Welsh – Harvard University9Example: GSMGlobal System for Mobile communications●First commercial launch, mid-1991 in EuropeDigital cellular system, combination of FDMA and TDMAForward and reverse links each 25 MHz●Broken into multiple channels of 200 kHz each == 125 channels total●Each channel broken into 8 time slots of 0.577 ms each...RX Channel 125RX Channel 1Frame...TX Channel 125TX Channel 1© 2004 Matt Welsh – Harvard University10GSM CapacityGSM data transmission occurs at ~ 270 kbps●Each user therefore gets 270 / 8 = 33.75 kbps of “bandwidth”●Framing overhead limits this to 24.7 kbps per user●Slots are only 0.577 ms long!How many simultaneous users can GSM support?●125 channels x 8 slots/channel = 1000 users●However, each of the 1000 “physical channels” (FDMA channel plus TDMA time slot) can be assigned to different “logical channels” dynamically●Example: voice data, control and signalling traffic, GPRS data, etc.General Packet Radio Service (GPRS)●Allows GSM time slots to carry digital data (21.4 kbps per slot)●If one user took over all 8 timeslots, could get 172.2 kbps through●Most phones support 4 downstream channels + 1 upstream channel●85.6 kbps downstream, 21.4 kbps up●EDGE (Enhanced Data GSM Environment) pushes each slot to 48 kbps© 2004 Matt Welsh – Harvard University11Code Division Multiple AccessRecall Direct Sequence Spread Spectrum (DSSS)●Original signal is modulated with a spreading code (pseudorandom bits)●This effectively spreads the signal out over a wider bandwidth.CDMA = Use different spreading codes for each user●All users share same carrier frequency and can transmit simultaneously●Use codewords (spreading sequences) that are orthogonal●That is, avoid overlap between different spreading sequencesIssue: The more users, the higher the noise floor●All other users' signals appear as noise to a particular receiver●Adding users makes it more difficult to differentiate individual usersRelated problem: Weaker signals may be “drowned out”●CDMA systems generally incorporate transmission power control●Base may tell mobile unit to increase or decrease transmit power to equalize.© 2004 Matt Welsh – Harvard University12Example: IS-95CDMA standard used in North America (e.g., Sprint PCS)Single 1.228 MHz (!!!) band shared by 64 code “channels”●Mobile-to-base channel slightly different ...●One channel allocated for control data●8 channels