1.2 MAC

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1. Introduction1.2 Medium Access Control

Prof. JP Hubaux

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Modulation and demodulation (reminder)

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter

1.2 MAC

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Motivation

Can we apply media access methods from fixed networks?

Example of CSMA/CDCarrier Sense Multiple Access with Collision Detectionsend as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)

Problems in wireless networkssignal strength decreases proportional to the square of the distance or even morethe sender would apply CS and CD, but the collisions happen at the receiverit might be the case that a sender cannot “hear” the collision, i.e., CD does not workfurthermore, CS might not work if, e.g., a terminal is “hidden”

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Hidden terminalsA 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 terminalsB sends to A, C wants to send to another terminal (not A or B)C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is not necessaryC is “exposed” to B

Hidden and exposed terminals

BA C

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Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out A’s signalC 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!

Motivation - near and far terminals

A B C

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Access methods SDMA/FDMA/TDMA/CDMA

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 receiverpermanent (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

CDMA (Code Division Multiple Access)assign an appropriate code to each transmission channel

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Frequency multiplex

Separation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:

no dynamic coordination necessaryworks also for analog signals

Disadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces

k2 k3 k4 k5 k6k1

f

t

c

8

f

t

c

k2 k3 k4 k5 k6k1

Time multiplex

A channel gets the whole spectrum for a certain amount of time

Advantages:only one carrier in themedium at any timethroughput high even for many users

Disadvantages:precise synchronization necessary

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f

Time and frequency multiplex

Combination of both methodsA channel gets a certain frequency band for a certain amount of

timeExample: GSM Advantages:

better protection against tappingprotection against frequency selective interference

But: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

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Code multiplex

Each channel has a unique code

All channels use the same spectrum at the same time

Advantages:bandwidth efficientno coordination and synchronization necessarygood protection against interference and tapping

Disadvantages:lower user data ratesmore complex signal regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

t

c

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Some medium access control mechanisms for wireless

TDMA CDMAFDMASDMA

FDD

Fixed Aloha ReservationsDAMA

MultipleAccess withCollisionAvoidance

Polling

Pure

CSMA

• Used inGSM Slotted

Non-persistent p-persistent CSMA/CA• Copes with hiddenand exposed terminal

• RTS/CTS• Used in 802.11

(optional)

MACAW MACA-BI FAMA

CARMA

• Used in 802.11(mandatory)

• Used in 802.11(optional)

CSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFDD: Frequency Division Duplex

CSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFDD: Frequency Division Duplex

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FDMA/FDD – example: GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHzdownlink

uplink

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TDMA/TDD – example: DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

DECT: Digital Enhanced Cordless TelecommunicationsTDD: Time Division Duplex

DECT: Digital Enhanced Cordless TelecommunicationsTDD: Time Division Duplex

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Mechanismrandom, distributed (no central arbiter), time-multiplexSlotted Aloha additionally uses time-slots, sending must always start at slot boundaries

Aloha

Slotted Aloha

Aloha/slotted aloha

sender A

sender B

sender C

collision

sender A

sender B

sender C

collision

t

t

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Performance of Aloha (1/3)

First transmissionRetransmission(if necessary)

t0 t0+Xt0-X

Vulnerable period

t0+X+2tprop

Time-out

t0+X+2tprop+B

Backoff period

• tprop : maximum one-way propagation time betwwen 2 stations• Information about the outcome of the transmission is obtained after the

reaction time 2 tprop• B: backoff time

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Performance of Aloha (2/3)S: new packets S: throughput of the system

{G

[ ]

: total load: arrival rate of new packets

Assumption: Poisson distribution of the aggregate arrival process, with an average number of arrivals of 2G arrivals/2X seconds

transmissions in 2 seconds

GS

P k X ( )

[ ] [ ]( )

2

02

2

2, 0,1, 2,...

!Throughput S: total arrival rate G times the prob. of a successful transmission:

no collision 0 transmissions in 2 seconds

2 =

0! =

Peakvalue at 0.5 :

kG

G

G

Ge k

k

S GP GP X

GG e

Ge

G

−

−

−

= =

= =

= 1 0.1842S e= ≈

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Performance of Aloha (3/3)

2

2

Average number of transmission attempts/packet:

attempts per packet

Average number of unsuccessful attempts per packet:

= 1 1

The first transmission requires seconds,

and each subs

G

G

prop

G eS

G eSX t

ε

=

− = −

+

[ ]

[ ]

2

2

equent retransmission requires 2

Thus the average packet transmission time is approx:( 1)( 2 )

expressed relatively to X:

/ 1 ( 1)(1 2 )

where i

prop

Galoha prop prop

Galoha

prop

t X B

E T X t e X t B

BE T X a e a Xta X

+ +

= + + − + +

= + + − + +

= s the normalized one-way propagation delay

Computation of the average packet transmission time

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Performance of Slotted Aloha First transmission

Retransmission(if necessary)

t0=kX (k+1)X

Vulnerableperiod

t0+X+2tprop

Time-out

t0+X+2tprop+B

Backoff period

[ ]

- 1Peakvalue at 1 : 0.368

Average packet delay:

/ 1 ( 1)(1 2 )

G

Gslotaloha

S Ge

G S e

BE T X a e a X

=

= = ≈

= + + − + +

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Carrier Sense Multiple Access (CSMA)

Goal: reduce the wastage of bandwidth due to packet collisionsPrinciple: sensing the channel before transmitting (never transmit when the channel is busy)Many variants:

Collision detection (CSMA/CD) or collision avoidance(CSMA/CA)Persistency (in sensing and transmitting)

Station A beginstransmissionat t=0

A

Station A capturesthe channelat t=tprop A

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1-Persistent CSMA

Stations having a packet to send sense the channel continuously, waiting until the channel becomes idle.As soon as the channel is sensed idle, they transmit their packet.If more than one station is waiting, a collision occurs.Stations involved in a collision perform a the backoff algorithm to schedule a future time for resensing the channel Optional backoff algorithm may be used in addition for fairness

Consequence : The channel is highly used (greedy algorithm).

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Non-Persistent CSMA

Attempts to reduce the incidence of collisionsStations with a packet to transmit sense the channelIf the channel is busy, the station immediately runs the back-off algorithm and reschedules a future sensing timeIf the channel is idle, then the station transmits

Consequence : channel may be free even though some users have packets to transmit.

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p-Persistent CSMA

Combines elements of the above two schemesStations with a packet to transmit sense the channelIf it is busy, they persist with sensing until the channel becomes idleIf it is idle:

With probability p, the station transmits its packetWith probability 1-p, the station waits for a random time and senses again

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Throughput expression

( )[ ] ( )

( ) ( ) ( ) ( )

[ ] ( )

( )( ) ( )

( )

aeaGeS

eaGGeS

aeeaeeaGS

eaGeaGeaGGaGGGS

GeS

GeS

aG

aG

aG

aG

aGaG

aGaG

aGaG

aG

G

G

+−=

++=

+−+−+

=

++−−+++++

=

=

=

−

−

−

−

+−−

+−−

+−−

+−

−

−

1

21

111

11212/11

1

1

1

21

2Pure ALOHA

Slotted ALOHA

Unslotted1-persistent CSMA

Slotted1-persistent CSMA

Unslottednonpersistent CSMA

Slottednonpersistent CSMA

Protocol Throughput

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Throughput plot

Normalized propagation delay is a =0 .01

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CSMA/CD (reminder)

Operating principleCheck whether the channel is idle before transmittingListen while transmitting, stop transmission when collisionIf collision, one of the 3 schemes above (1-persistent, non-persistent or p-persistent)

CS: Carrier Sense (Is someone already talking ?)MA: Multiple Access (I hear what you hear !)CD: Collision Detection (We are both talking !!)

Three states for the channel : contention, transmission, idle

Station

Repeater Terminator

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Why CSMA/CD is unfit for WLANs

Collision Detection requires simultaneous transmission and reception operations (which a radio transceiver is usually unable to do) detecting a collision is difficultCarrier Sensing may be suitable to reduce interference at sender, but Collision Avoidance is needed at receiverCSMA/CD does not address the hidden terminalproblem

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CSMA/CA

Is described in the section devoted to IEEE 802-11

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DAMA - Demand Assigned Multiple Access

Channel efficiency only 18% for Aloha, 36% for Slotted AlohaReservation can increase efficiency to 80%

a sender reserves a future time-slotsending within this reserved time-slot is possible without collisionreservation also causes higher delaystypical scheme for satellite links

Examples for reservation algorithms:Explicit Reservation (Reservation-ALOHA)Implicit Reservation (PRMA)Reservation-TDMA

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DAMA / Explicit Reservation

Explicit Reservation (Reservation Aloha):two modes:

ALOHA mode for reservation:competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible)

it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time

Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t

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DAMA / Packet reservation (PRMA)Implicit reservation

based on slotted Alohaa certain number of slots form a frame, frames are repeatedstations compete for empty slots according to the slotted aloha principleonce a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to sendcompetition for a slot starts again as soon as the slot was empty in the last frame

frame1

frame2

frame3

frame4

frame5

1 2 3 4 5 6 7 8 time-slot

collision at reservation attempts

A C D A B A F

A C A B A

A B A F

A B A F D

A C E E B A F Dt

ACDABA-F

ACDABA-F

AC-ABAF-

A---BAFD

ACEEBAFD

reservation

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DAMA / Reservation-TDMA

Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slotsevery station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k).other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic)

N mini-slots N * k data-slots

reservationsfor data-slots

other stations can use free data-slotsbased on a round-robin scheme

e.g. N=6, k=2

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MACA - collision avoidance

MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance

Designed especially for packet radio networks (Phil Karn, 1990)Principle:

RTS (request to send): a sender request the right to send from areceiver with a short RTS packet before it sends a data packetCTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

Signaling packets containsender addressreceiver addresspacket size

Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

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MACA avoids the problem of hidden terminalsA and C want to send to BA sends RTS firstC waits after receiving CTS from B

MACA avoids the problem of exposed terminalsB wants to send to A, C to another terminalnow C does not have to wait for it cannot receive CTS from A

MACA principle

A B C

RTS

CTSCTS

A B C

RTS

CTS

RTS

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MACA example

A B

C

D

E

A B

C

D

E

A B

C

D

E

RTS CTS

DATA

: blocked from Tx

1 2

3

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MACA variant: application in IEEE802.11

idle

wait for the right to send

wait for ACK

sender receiver

packet ready to send; RTS

time-out; RTS

CTS; data

ACK

RxBusy

idle

wait fordata

RTS; RxBusy

RTS; CTS

data; ACK

time-out ∨Data with errors;NAK

ACK: positive acknowledgementNAK: negative acknowledgement

RxBusy: receiver busy

time-out ∨NAK;RTS

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Polling mechanisms

If one terminal can be heard by all others, this “central” terminal (e.g., base station) can poll all other terminals according to acertain scheme

all schemes known from fixed networks can be used (typical mainframe - terminal scenario)

Example: Randomly Addressed Pollingbase station signals readiness to all mobile terminalsterminals ready to send can now transmit a random number withoutcollision with the help of CDMA or FDMA (the random number can be seen as a dynamic address)the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminalthis cycle starts again after polling all terminals of the list

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ISMA (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 stopsthe base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach)mechanism used, e.g., for CDPD (Cellular Digital Packet Data)Similar approach was proposedfor Packet Radio Networks(Kleinrock + Tobagi, 1975)

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CDMA (Code Division Multiple Access)Principles

all terminals send on the same frequency and can use the whole bandwidth of the transmission channel each sender has a unique codeThe sender XORs the signal with this codethe receiver can “tune” into this signal if it knows the code of the sendertuning is done via a correlation function

Disadvantages:higher complexity of the receiver (receiver cannot just listen into the medium and start receiving if there is a signal)all signals should have approximately the same strength at the receiver

Advantages: all terminals can use the same frequency, no planning neededhuge code space (e.g., 232) compared to frequency spaceinterferences (e.g. white noise) is not codedmore robust to eavesdropping and jamming (military applications…)forward error correction and encryption can be easily integrated

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DSSS (Direct Sequence Spread Spectrum) (1/2)

XOR of the signal with pseudo-random number (chipping sequence)many chips per bit (e.g., 128) result in higher bandwidth of the signal

Advantagesreduces frequency selective fadingin cellular networks

base stations can use the same frequency rangeseveral base stations can detect and recover the signalsoft handover

Disadvantagesprecise power control necessarycomplexity of the receiver

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period

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DSSS (Direct Sequence Spread Spectrum) (2/2)

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator

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CDMA: principle (very simplified)

Ak

X AsAd

Bk

X BsBd

As + Bs

Ak

X

Bk

X

C+D

C+D

Ad

Bd

C+D: Correlation and DecisionC+D: Correlation and Decision

Spreading Despreading

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CDMA: example

Sender A sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1)sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

Sender Bsends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1)sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space interference neglected (noise etc.)As + Bs = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive signal from sender Aapply key Ak bitwise (inner product)

Ae = (-2, 0, 0, -2, +2, 0) • Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6result greater than 0, therefore, original bit was „1“

receiving BBe = (-2, 0, 0, -2, +2, 0) • Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“

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Spreading of signal A

data Ad

signal As

key sequence Ak

1 0 1

10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Real systems use much longer keys resulting in a larger distancebetween single code words in code space.

Ad+Ak

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Spreading of signal B

signal As

As + Bs

1 0 0

00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1

11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

data Bd

signal Bs

key sequence Bk

Bd+Bk

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Despreading of signal A

Ak

(As + Bs) * Ak

correlatoroutput

decisionoutput

As + Bs

0 1 0

1 0 1data Ad

Note: the received signal is invertedNote: the received signal is inverted

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Despreading of signal B

correlatoroutput

decisionoutput

Bk

(As + Bs) * Bk

As + Bs

0 1 1

1 0 0data Bd

Note: the received signal is invertedNote: the received signal is inverted

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decisionoutput

Despreading with a wrong key

(1) (1) ?

wrong key K

correlatoroutput

(As + Bs) * K

As + Bs

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Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time

Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha

SAMA - Spread Aloha Multiple Access

1sender A0sender B

01

t

narrowband

send for a shorter periodwith higher power

spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)

Problem: find a chipping sequence with good characteristics

11

collision

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Comparison SDMA/TDMA/FDMA/CDMAApproach SDMA TDMA FDMA CDMAIdea segment space into

cells/sectorssegment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km²

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

In practice, several access methods are used in combinationExample :SDMA/TDMA/FDMA for GSM and IS-54

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References

J. Schiller: Mobile Communications, Addison-Wesley, 2000Leon-Garcia & Widjaja: Communication Networks, McGrawHill,

2000