The document discusses different media access methods for wireless networks, including their advantages and disadvantages. It begins by explaining why CSMA/CD is not suitable for wireless due to signal strength decreasing with distance and the possibility of collisions occurring at the receiver instead of the sender. It then discusses four main media access methods - SDMA, FDMA, TDMA, and CDMA. For each method, it provides a brief overview and examples of technologies that use each method, as well as their key features, advantages, and disadvantages. Hidden and exposed terminals problems that can occur in wireless networks are also explained.

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2. Motivation forspecialized
MAC
 C a n we apply media access methods from fixednetworks?
Example CSMA/CD
Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium if a collision occurs
(legacy method in IEEE 802.3)
Problems in wirelessnetworks
signal strength decreases proportional to the square of the distance
 the sender would apply CS and CD,but the collisions happen at thereceiver
 i t might be the case that a sender cannot “hear” the collision, i.e., CD does not
work
furthermore, CS might not work if, e.g.,a terminal is“hidden”
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3. CSMA/CD fails in wireless N/w because CSMA/CD is not really interested
in collisions at the sender , but rather in those at the receiver.
 T h e signal should reach the receiver without collisions. But sender isthe one
detecting collisions
 This is not a problem using wire, asmore or less the same signal
strength can be assumed all over the wire
 T h e strength of a signal in wireless N/w decreases proportionally tothe square
of the distance to the sender
 T h e senderstart sending but a collision happens at the receiver due to a
second sender.
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4. Motivation: Hidden and exposed terminals
• consider the scenario with three mobile phones as shown in Figure.
• The transmission range of A reaches B, but not C(the detection range
does not reach C either). The transmission range of C reaches B, but
not A.
• Finally, the transmission range of B reaches A and C, i.e A cannot
detect C and vice versa.
•
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6. HiddenTerminals:
A sends to B, C cannot receiveA
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 ,and vice versa.
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7. Exposed Terminals:
B sends toA, C wants to send to another terminal (notA or B)
outsiderange
C senses the carrier and detects that carrier is busy
C postpones its transmission until it detects the medium as being
idle again
butA is outside the radio range of C, therefore waiting is not
necessary
C is “exposed” to B
Hidden terminals cause collisions, where asExposed terminals
causes unnecessary delay
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9. Access methods SDMA/FDMA/TDMA/CDMA
 SDMA (Space Division MultipleAccess)
segment space into sectors, use directedantennas
 cell structure
 FDMA (Frequency Division MultipleAccess)
assign a certain frequency to a transmission channel between a sender and a
receiver
permanent (e.g.,radio broadcast), slow hopping (e.g.,GSM), fast hopping(FHSS,
Frequency Hopping Spread Spectrum)
 TDMA (Time Division MultipleAccess)
assign the fixed sending frequency to a transmission channel between a sender
and a receiver for a certain amount of time
 CDMA (Code Division MultipleAccess)
 Same bandwidth is occupied by all the users however they are all assigned separate code
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10. SDMA
 SDMA is used for allocating a separated space to users in wirelessnetworks.
 A typical application involves assigning an optimal base station to a mobile
phone user
 T h e mobile phone may receive several base stations with differentquality.
 A MAC algorithm could now decide which base station is best, taking into
account with frequencies (FDM), time slots(TDM) or code(CDM) are still
available(depending on technology)
Typically SDMA is never used in isolation but always in combination with one
or more other schemes
 T h e basis for the SDMA algorithm is formed by cell and sectorized antennas
which constitute the infrastructure implementing SDM
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11. FDMA ( Frequency Division Multiple Access)
 FDMA assigns individual channels to individualusers
 Eac h user is allocated a unique freq., band or channel
These channels are assigned on demand to users who request service
During the period of the call, no other user can share thesamechannel
 T h e FDMA channel carries only one phone circuit at a time.
 I f an FDMA channel is not in use, then it sits idle and can not be usedby other
users.
 FDMA requires tight RF filtering to minimize adjacentchannel
interference
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12.  Uplink frequency:824-849MHZ
 Downlink frequency:869-894MHZ
 8 3 2 channels spaced 30KHZ apart(3users/channel)
 48.6 kbpsbitrate
 Used inAMPS(Analog cellular telephone systems)
 Used FDD(Frequency DivisionDuplexing)
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13. Main features
• Continuous transmission
• Narrow bandwidth
• Low overhead
• Simple hardware at mobile unit and BS : (1) no digital
processing needed (2) ease of framing and synchronization.
• FDMA can be used with both analogue and digitalsignal.
• FDMA requires high-performing filters in the radio hardware, in
contrast toTDMA andCDMA.
• FDMA is not vulnerable to the timing problems thatTDMAhas.
• Due to the frequency filtering, FDMA is not sensitive to nearfar
problem.
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14. Advantages
• I f channel is not in use, it sits idle
• Channel bandwidth is relatively narrow(30kHz)
• Simple algorithmically,and from a hardwarestandpoint
• Fairly efficient when the number of stations is small andthetraffic is
uniformly constant
• Capacity increase can be obtained by reducing the
information bit rate and using efficient digital code
• No need for network timing
• No restriction regarding the type of baseband or type of
modulation
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15. Disadvantages
• The presence of guardbands
• Requires right RF filtering to minimize adjacent channel interference
• Maximum bit rate per channel is fixed
• Small inhibiting flexibility in bit rate capability
• Does not differ significantly from analog system
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FDD/FDMA - general scheme, example GSM
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz

17. TDMA (Time Division Multiple Access)
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• TDMA (Time Division MultipleAccess) System divide the ratio
spectrum into time slots.
• In each slot only one user is allowed to either transmit or receive
• Each user occupies a cyclically repeating time slot
transmission for any user is non continuous
• Listening to different frequencies at the same time is quite difficult

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Uplink frequency:824-849MHZ
Downlink frequency:869-894MHZ
 832 channels spaced 30KHZ apart(3users/channel)
48.6 kbpsbitrate
Used in DigitalAMPS(Advanced Mobile Phone systems)
UsedTDD(Time Division Duplexing)

19. MAIN FEATURES
Shares single carrier frequency with multipleusers.
Non-continuous transmission.This results in low battery
consumption since the subscriber transmitter can be turned OFF
when not in use.
Slots can be assigned on demand in dynamicTDMA.
TDMA uses different time slots forTx and Rx,thus
duplexers are not required.
 Global Systems for Mobile communications (GSM) uses the DMA
technique
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20. Advantages
 I t carry data rates of 64 kbps to 120 Mbps .
 I t provides the user with extended battery life and talk time.
 I t is the most cost effective technology to convert an
analogue system to digital.
TDMA technology separates users according to time, it
ensures that there will be no interference
TDMA allows the operator to do services like fax, voice band data,
and SMS as well as bandwidth-intensive application such as
multimedia and videoconferencing.
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21. Disadvantages
 Each user has a predefined time slot.When moving from one
cell to other,if all the time slots in this cell are full the user might
be disconnected.
 I t is subjected to multipath distortion.A signal coming from atower
to a handset might come from any one of several directions. It might
have bounced off several different buildings before arriving.
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22. TDD/TDMA - general scheme, example DECT
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11 12 1 2 3 11 12
t
downlink uplink
417 µs
1 2 3

23. CDMA
 CDMA (Code Division MultipleAccess)
There is no restriction on time and frequency in thisscheme.
 Parallel communication without collision and whole bandwidth canbe used
Users are separated by code not by time slot and frequencyslot
 A l l terminals send on the same frequency probably at the sametimeand can
use the whole bandwidth of the transmission channel
Eac h sender has a unique random number, the sender XORsthesignal
with this random number
 T h e receiver can “tune” into this signal if it knows the pseudorandom
number, tuning is done via a correlation function
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25. Advantages
Potentially larger capacity (more users can communicate
simultaneously) If users don’t use the medium all the time (e.g.,they
are just reading email), CDMA will allowmuch more users to
communicate simultaneously.
CDMA will use the resource (the radio spectrum)more
efficiently.
Provides larger spread spectrum, thus more robustagainst noise
bursts and multipath frequency selective fading
 GSM bandwidth = 200kHz
 IS-95 bandwidth = 1.25 MHz
W-CDMA (3G) bandwidth = 10MHz
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26.  The transition from one BS to another (handoff) is not abrupt,as
inTDMA,and provides better quality No absolute limit on the
number of users
Easy addition of moreusers
Impossible for hackers to decipher the code sent
Better signal quality
Huge code space (e.g.232) compared to frequencyspace
Interferences (e.g.white noise) is notcoded
forward error correction and encryption can beeasily
integrated
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27. Disadvantages:
 A s the number of users increases, the overallquality of service
decreases
Self-jamming
 Near- Far- problem arises
Higher complexity of a receiver(receiver cannot just listen into
the medium and start receiving if there is a signal)
 A l l signals should have the same strengthat areceiver
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28. CDMA in theory
SenderA
 s e n d s Ad= 1,keyAk= 010011 (assign:“0”=-1,“1”=+1)
 s e n d i n g signalAs=Ad*Ak= (-1,+1,-1,-1,+1,+1)
Sender B
 s e n d s Bd= 0, keyBk= 110101 (assign: “0”= -1, “1”= +1)
 s e n d i n g signal Bs= Bd* Bk= (-1, -1, +1, -1, +1,-1)
Both signals superimpose in space
 interference neglected(noiseetc.)
 A s + Bs= (-2,0,0,-2,+2,0)
Receiver wants to receive signal from senderA
 a p p l y keyAkbitwise (innerproduct)
 Ae= (-2,0,0,-2,+2,0) Ak= 2 + 0 + 0 + 2 + 2 + 0 = 6
 result greater than 0, therefore, original bit was “1”
 r e c e i v i n g B
 Be= (-2,0,0,-2,+2,0)  Bk= -2 + 0 + 0 - 2 - 2 + 0 = -6,i.e.“0”
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29. CDMA on signal level I
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data A
signal A
key A
key
sequence A
data  key
Real systems use much longer keys resulting in a larger distance
between single code words in code space.
1 0 1
0 1 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1
1 0 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0
Ad
Ak
As

30. CDMA on signal level II
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signal A
data B
signal B
As + Bs
key B
key
sequence B
data  key
1 0 0
0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1
1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1
Bd
Bk
Bs
As

31. CDMA on signal level III
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Ak
(As + Bs)
* Ak
As + Bs
data A
1 0 1
1 0 1
integrator
output
comparator
output
Ad

32. CDMA on signal level IV
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Bk
(As + Bs)
* Bk
As + Bs
data B
1 0 0
1 0 0
integrator
output
comparator
output
Bd

33. CDMA on signal level V
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wrong
key K
(As + Bs)
* K
As + Bs
(0) (0) ?
integrator
output
comparator
output

34. SAMA - Spread Aloha Multiple Access
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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
1
0
sender A
sender B
0
1
narrow
band
send for a
shorter period
with higher power
spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)
t
Problem: find a chipping sequence with good characteristics
1
1
collision

35. Comparison SDMA/TDMA/FDMA/CDMA
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Approach SDMA TDMA FDMA CDMA
Idea segment space into
cells/sectors
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
segment the
frequency band into
disjoint sub-bands
spread the spectrum
using orthogonal codes
Terminals only one terminal can
be active in one
cell/one sector
all terminals are
active for short
periods of time on
the same frequency
every terminal has its
own frequency,
uninterrupted
all terminals can be active
at the same place at the
same moment,
uninterrupted
Signal
separation
cell structure, directed
antennas
synchronization in
the time domain
filtering in the
frequency domain
code plus special
receivers
Advantages very simple, increases
capacity per km²
established, fully
digital, flexible
simple, established,
robust
flexible, less frequency
planning needed, soft
handover
Dis-
advantages
inflexible, antennas
typically fixed
guard space
needed (multipath
propagation),
synchronization
difficult
inflexible,
frequencies are a
scarce resource
complex receivers, needs
more complicated power
control for senders
Comment only in combination
with TDMA, FDMA or
CDMA useful
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA