Prof.R.K.NADESH;SITE;VIT MEDIUM ACCESS CONTROL Wireless channel is a shared medium MAC coordinates transmission between users sharing the spectrum Goals: prevent collisions while maximizing throughput and minimizing delay Why Special MAC needed in Wireless Domain? Why not CSMA/CD?

Prof.R.K.NADESH;SITE;VIT HIDDEN & EXPOSED TERMINAL

Prof.R.K.NADESH;SITE;VIT NEAR AND FAR TERMINALS

Prof.R.K.NADESH;SITE;VIT MAC SCHEMES SDMA FDMA TDMA CDMA

Prof.R.K.NADESH;SITE;VIT Space Division Multiple Access Allocating Separate Space Eg.BS-MS User SDMA is always used in combinations Basis of SDMA – Cells,Sectorized Antennas which constitute the infrastructure implementing SDM.

Prof.R.K.NADESH;SITE;VIT Frequency Division Multiple Access Allocating Frequecies to Transmission Channel FIXED(Radion Station) /Dynamic (Demand Driven) FDMA+TDMA Hopping Pattern FDM-Simultaneous access to network Duplex Channel ;MS-BS;BS-MS

Prof.R.K.NADESH;SITE;VIT Contd.. Two Different Directions;Separated by two different frequencies(FDD) UPLINK & 915 MHZ DOWNLINK – to 960 MHZ 124 Channels

Prof.R.K.NADESH;SITE;VIT Contd…

Prof.R.K.NADESH;SITE;VIT Time Division Multiple Access Allocates certain time slot for communication Same frequency Synchronization must be done (s-r) Fixed and Dynamic -(WLC) Combined FDMA->Greater Flexibility & Transmission Capacity

Prof.R.K.NADESH;SITE;VIT Fixed TDMA Allocating Time Slots in a fixed pattern (Fixed Bandwidth ;BS-MS)

Prof.R.K.NADESH;SITE;VIT Contd.. Different slots for U/D Link in same frequency (TDD) Multiple Access & Duplex Channel(BS- MS) 1O ms the pattern is repeated. Each slot has the time duration of 417µs 64bit Duplex is inefficient for bursty data Too Static and too inflexible for data communication

Prof.R.K.NADESH;SITE;VIT Classical Aloha Scheme from University of HAWAII If TDM is applied without controlling access Each station can access medium at any time(Random Access Scheme-No Control)

Prof.R.K.NADESH;SITE;VIT Slotted Aloha Refinement by introducing Time Slots All Senders- Synchronized Transmission starts when each timeslot begins Still Access is not coordinated Suited for light loads(18-36%)

Prof.R.K.NADESH;SITE;VIT Carrier Sense Multiple Access Sensing the carrier and accessing the medium if it is idle Decreases the probability of Collision Hidden Terminals not detected Versions Non-Persistent CSMA Sense carrier & Starts Sending if it is idle otherwise repeat sensing after some time P-Persistent CSMA Sense the medium but only transmit with a probability of p.

Prof.R.K.NADESH;SITE;VIT Demand Assigned Multiple Access (Reservation Aloha) Reservation period followed by transmission period Eg.Satellite Systems ( Station on earth-Satellite ) Timeslot alloted successfully, no collision Satellite collects all requests & sends back reservation list indicating access rights EXPLICIT RESERVATION

Prof.R.K.NADESH;SITE;VIT PACKET RESERVATION MULTIPLE ACCESS(PRMA) DAMA with implicit reservation Implicit Reservation Certain slots form a frame Frame repeated in time BS broadcasts status of each slot All MS will know free & reserved slot

Prof.R.K.NADESH;SITE;VIT Each station has certain bandwidth and fixed delay RESERVATION TDMA Each station has certain bandwidth and fixed delay

Prof.R.K.NADESH;SITE;VIT Multiple Access with Collision Avoidance Solves Hidden and Terminal Problem

Prof.R.K.NADESH;SITE;VIT MACA-Examples

Prof.R.K.NADESH;SITE;VIT BASESTATION FOR COORDINATION POLLING  ONE STATION TO BE HEARED BY ALL STATIONS One Master with several slaves Round Robin & Randomly Accessed Polling

Prof.R.K.NADESH;SITE;VIT Inhibit Sense Multiple Access Current state of the medium is signaled via a “busy tone” the base station signals on the downlink (base station to terminals) if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals collisions and successful transmissionsvia the busy tone and acknowledgements, respectively

Prof.R.K.NADESH;SITE;VIT Code Division Multiple Access- (CDMA) All terminals send on the same frequency probably at same time and can use the whole bandwidth of the transmission channel Uses codes to separate users and enable access to shared medium without interference

Prof.R.K.NADESH;SITE;VIT CDMA Orthogonal Codes 2 vectors are called orthogonal if the inner product is Zero (2,5,0) & (0,0,17) 2*0+5*0+0*17=0 (3,-2,4) &(-2,3,3) =0

Prof.R.K.NADESH;SITE;VIT BASIC FUNCTION OF CDMA Two senders A & B want to send data CDMA assigns 2 unique orthogonal code Key A k = B k = A d =>1; B d =>0 Assumptions 0 as -1 & 1 as 1

Prof.R.K.NADESH;SITE;VIT CDMA-Spreading Signal Sender Spreading the Signal (Multiplication of data bits) A s =A d *A k = 1* (-1,1,-1,-1,1,1) =(-1,1,-1,-1,1,1) Bs= Bd*Bk = -1 * (1, 1, -1, 1, -1, 1) = (-1,-1,1,-1,1,-1)

Prof.R.K.NADESH;SITE;VIT CDMA Both Signals are time,frequency C= A S +B S (-2,0,0,-2,2,0) Receiver wants A Code :C*AK=(-2,0,0,-2,2,0)*(-1,1,-1,-1,1,1) = =6 >0 ;Receiver detects as binary 1

Prof.R.K.NADESH;SITE;VIT CDMA Receiver wants B Code C*Bk=(-2,0,0,-2,2,0)*(1, 1, -1, 1, -1, 1) = = -6 <0 detects as ZERO

Prof.R.K.NADESH;SITE;VIT CDMA On Signal - I

Prof.R.K.NADESH;SITE;VIT CDMA On Signal - II

Prof.R.K.NADESH;SITE;VIT CDMA On Signal - III

Prof.R.K.NADESH;SITE;VIT CDMA On Signal - IV

Prof.R.K.NADESH;SITE;VIT CDMA On Signal - V

Prof.R.K.NADESH;SITE;VIT COMPARISON