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