Prof.R.K.NADESH;SITE;VIT MEDIUM ACCESS CONTROL Wireless channel is a shared medium MAC coordinates transmission between users sharing the spectrum Goals:

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?


HIDDEN & EXPOSED TERMINAL


NEAR AND FAR TERMINALS


MAC SCHEMES

• SDMA

• FDMA

• TDMA

• CDMA


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.


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


Contd..

• Two Different Directions;Separated by two different frequencies(FDD)

• UPLINK-890.2 & 915 MHZ

• DOWNLINK – 935.2 to 960 MHZ

• 124 Channels


Contd…


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


Fixed TDMA

• Allocating Time Slots in a fixed pattern

(Fixed Bandwidth ;BS-MS)


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


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)


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%)


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.


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


PACKET RESERVATION MULTIPLE ACCESS(PRMA)PACKET RESERVATION MULTIPLE ACCESS(PRMA)DAMA with implicit reservationDAMA 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


RESERVATION TDMAEach station has certain bandwidth and fixed delayEach station has certain bandwidth and fixed delay


Multiple Access with Collision AvoidanceMultiple Access with Collision Avoidance

• Solves Hidden and Terminal Problem


MACA-Examples


BASESTATION FOR COORDINATION

• POLLING

ONE STATION TO BE HEARED BY ALL STATIONS

• One Master with several slaves

• Round Robin & Randomly Accessed Polling


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


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


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)

-6-6+12=0


BASIC FUNCTION OF CDMA

• Two senders A & B want to send data

• CDMA assigns 2 unique orthogonal code

• Key Ak = 010011 Bk=110101

• Ad=>1; Bd=>0

• Assumptions

0 as -1 & 1 as 1


CDMA-Spreading Signal

• Sender Spreading the Signal (Multiplication of data bits)

As =Ad*Ak

= 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)


CDMA

• Both Signals are transmitted@same time,frequency

C= AS+BS

(-2,0,0,-2,2,0)Receiver wants A Code

:C*AK=(-2,0,0,-2,2,0)*(-1,1,-1,-1,1,1) = 2+0+0+2+2+0

=6 >0 ;Receiver detects as binary 1


CDMA

• Receiver wants B Code

C*Bk=(-2,0,0,-2,2,0)*(1, 1, -1, 1, -1, 1)

= -2+0+0-2-2+0

= -6

<0 detects as ZERO


CDMA On Signal - I


CDMA On Signal - II


CDMA On Signal - III


CDMA On Signal - IV


CDMA On Signal - V


COMPARISON

Documents

Prof.R.K.NADESH;SITE;VIT MEDIUM ACCESS CONTROL Wireless channel is a shared medium MAC coordinates transmission between users sharing the spectrum Goals: