Mobile Communication

IT2402 MOBILE COMMUNICATION
UNIT – I
Dr.A.Kathirvel, Professor and Head, Dept of IT
Anand Institute of Higher Technology, Chennai

Unit - I
WIRELESS COMMUNICATION
Cellular systems- Frequency Management and Channel
Assignment- types of handoff and their characteristics,
dropped call rates & their evaluation -MAC – SDMA – FDMA –
TDMA – CDMA – Cellular Wireless Networks

Cellular Systems-Cellular Concepts
 The cellular concept was a major breakthrough in solving the
problem of spectral congestion and user capacity. It offered
very high capacity in a limited spectrum allocation without
any major technological changes.
 The cellular concept has the following system level ideas
 Replacing a single, high power transmitter with many low power
transmitters, each providing coverage to only a small area.
 Neighboring cells are assigned different groups of channels in order to
minimize interference.
 The same set of channels is then reused at different geographical
locations.

Cellular Concepts
When designing a cellular mobile communication system,
it is important to provide good coverage and services in a
high user-density area.
Reuse can be done once the total interference from all
users in the cells using the same frequency (co-channel
cell) for transmission suffers from sufficient attenuation.
Factors need to be considered include:
Geographical separation (path loss)
Shadowing effect

Cell Footprint
The actual radio coverage of a cell is known as
the cell footprint.
Irregular cell structure and irregular placing of the
transmitter may be acceptable in the initial system
design. However as traffic grows, where new cells
and channels need to be added, it may lead to
inability to reuse frequencies because of co-
channel interference.
For systematic cell planning, a regular shape is
assumed for the footprint.

Cell Footprint
Coverage contour should be circular. However it is
impractical because it provides ambiguous areas
with either multiple or no coverage.
Due to economic reasons, the hexagon has been
chosen due to its maximum area coverage.
Hence, a conventional cellular layout is often
defined by a uniform grid of regular hexagons.

Frequency reuse
A cellular system which has a total of S duplex
channels.
S channels are divided among N cells, with
each cell uses unique and disjoint channels.
If each cell is allocated a group of k channels,
then
 S = k N .

Terminology
Cluster size : The N cells which collectively use
the complete set of available frequency is
called the cluster size.
Co-channel cell : The set of cells using the
same set of frequencies as the target cell.
Interference tier : A set of co-channel cells at
the same distance from the reference cell is
called an interference tier. The set of closest
co-channel cells is call the first tier. There is
always 6 co-channel cells in the first tier.

Co-ordinates for hexagonal cellular geometry
With these co-
ordinates, an array
of cells can be laid
out so that the
center of every cell
falls on a point
specified by a pair
of integer co-
ordinates.

Co-ordinates for hexagonal cellular geometry

Designing a cellular system
N=19
(i=3, j=2)

The cluster size must satisfy: N = i2 + ij + j2
where i, j are non-negative integers.

Can also verify that
 where Q is the co-channel reuse ratio

Handover / Handoff
Occurs as a mobile moves into a different cell
during an existing call, or when going from
one cellular system into another.
It must be user transparent, successful and not
too frequent.
Not only involves identifying a new BS, but also
requires that the voice and control signals be
allocated to channels associated with the new BS.

Handover / Handoff
Once a particular signal level Pmin is specified as the
minimum usable signal for acceptable voice quality at the
BS receiver, a slightly stronger signal level PHO is used as
a threshold at which a handover is made.

Handover / Handoff
 =handoff threshold -
 Minimum acceptable
 signal to maintain the call
  too small:
Insufficient time
to complete handoff
before call is lost
More call losses
  too large:
Too many handoffs
Burden for MSC

Dwell Time
The time over which a user remains within
one cell is called the dwell time.
The statistics of the dwell time are important
for the practical design of handover
algorithms.
The statistics of the dwell time vary greatly,
depending on the speed of the user and the
type of radio coverage.

Handover indicator
Each BS constantly monitors the signal strengths of all of
its reverse voice channels to determine the relative
location of each mobile user with respect to the BS. This
information is forwarded to the MSC who makes
decisions regarding handover.
Mobile assisted handover (MAHO) : The mobile station
measures the received power from surrounding BSs and
continually reports the results of these measurements to
the serving BS.

Prioritizing Handover
Dropped call is considered a more serious event than call
blocking. Channel assignment schemes therefore must
give priority to handover requests.
A fraction of the total available channels in a cell is
reserved only for handover requests. However, this
reduces the total carried traffic. Dynamic allocation can
improve this.
Queuing of handover requests is another method to
decrease the probability of forced termination of a call
due to a lack of available channel. The time span over
which a handover is usually required leaves room for
queuing handover request.

Practical handover
High speed users and low speed users have
vastly different dwell times which might cause
a high number of handover requests for high
speed users. This will result in interference
and traffic management problem.

Practical handover
The Umbrella Cell
approach will help to
solve this problems.
High speed users are
serviced by large
(macro) cells, while low
speed users are handled
by small (micro) cells.

Practical handover
 A hard handover does “break before
make”, ie. The old channel connection
is broken before the new allocated
channel connection is setup. This
obviously can cause call dropping.
 In soft handover, we do “make before
break”, ie. The new channel connection
is established before the old channel
connection is released. This is realized
in CDMA where also BS diversity is
used to improve boundary condition.

Interference and System Capacity
 In a given coverage area, there are several cells that use
the same set of frequencies. These cells are called co-
channel cells. The interference between signals from
these cells is called co-channel interference.
 If all cells are approximately of the same size and the
path loss exponent is the same throughout the
coverage area, the transmit power of each BS is almost
equal. We can show that worse case signal to co-
channel interference is independent of the transmitted
power. It becomes a function of the cell radius R, and
the distance to the nearest co-channel cell D’.

Received power at a distance d from the
transmitting antenna is approximated by
Useful signal at the cell boundary is the weakest,
given by Pr (R). Interference signal from the co-
channel cell is given to be Pr (D′) .

D’ is normally
approximated by
the base station
separation
between the two
cells D, unless
when accuracy is
needed. Hence

For the forward link, a very general case,
 where Di is the distance of the ith interfering
cell from the mobile, i0 is the total number of
co-channel cells exist.

If only first tier co-channel cells are considered,
then i0 = 6.
 Unless otherwise stated, normally assuming
Di ≈ D for all i.

Outage probability
The probability that a mobile station does not receive a
usable signal.
For GSM, this is 12 dB and for AMPS, this is 18 dB. If
there is 6 co-channel cells, then
Exercise : please verify this
For n=4, a minimum cluster size of N=7 is needed to meet the
SIR requirements for AMPS.
For n=4, a minimum cluster size of N=4 is required to meet the
SIR requirements for GSM

Outage probability
Approximation
in distance has
been made on
the 2nd tier
onwards.

Outage probability
More accurate SIR can be
obtained by computing the
actual distance.
Our computation of outage
only based on path loss. For
more accurate modeling,
shadowing and fast fading
need to be taken into
consideration. This will not be
covered in this course.

Coverage Problems
Revision:
Recall that the mean measured value,
Measurement shows that at any value of d, the path loss PL(d)
at a particular location is random and distributed log-normally
(normal in dB) about this mean value.
 Pr (d)dB = Pr (d)dB + Xσ
 where Xσ is a zero-mean Gaussian distributed random variable
(in dB) with standard deviation σ(in dB).

Boundary coverage
 There will be a proportion of locations at distance R (cell
radius) where a terminal would experience a received signal
above a threshold γ. (γ is usually the receiver sensitivity)
 where Q(x) is the standard normal distribution.

Cell coverage
 Proportion of locations within the area defined by the cell
radius R, receiving a signal above the threshold γ.

Cell coverage
 Solution can be found using the graph provided. (n : path
loss exponent)

Cell coverage
Example: if n=4, σ=8 dB, and if the boundary is to have
75% coverage (75% of the time the signal is to exceed the
threshold at the boundary), then the area coverage is
equal to 94%.
If n=2, σ=8 dB, and if the boundary is to have 75%
coverage, then the area coverage is equal to 91%.
An operator needs to meet certain coverage criteria. This
is typically the “90% rule” – 90% of a given geographical
area must be covered for 90% of the time.

Cell coverage
 The mean signal level at any distance is determined by path
loss and the variance is determined by the resulting fading
distribution (log-normal shadowing, Rayleigh fading,
Nakagami-m, etc). In this course, we will deal with log-normal
shadowing only.
 The proportion of locations covered at a given distance (cell
boundary, for example) from BS can be found directly from
the resultant signal pdf/cdf.
 The proportion of locations covered within a circular region
defined by a radius R (the cell area, for example) can be found
by integrating the resultant cdf over the cell area.

Cell coverage --Cellular Traffic
The basic consideration in the design of a
cellular system is the sizing of the system.
Sizing has two components to be considered.
Coverage area
Traffic handling capability
After the system is sized, channels are
assigned to cells using the assignment
schemes mentioned before.

Cell coverage --Terminology in traffic theory
Trunking : exploits the statistical characteristics of the users
calling behaviour. Any efficient communication system relies
on trunking to accommodate a large number of users with a
limited number of channels.
Grade of service (GoS) : A user is allocated a channel on a per
call basis. GoS is a measure of the ability of a user to access a
trunked system during the busiest hour. It is typically given
as the likelihood that a call is blocked (also known as
blocking probability mentioned before).
Trunking theory : is used to determine the number of
channels required to service a certain offered traffic at a
specific GoS.
Call holding time (H) : the average duration of a call.
Request rate (λ) : average number of call requests per unit
time.

Cell coverage --Traffic flow or intensity A
Measured in Erlang, which is defined as the call minute per
minute.
Total offered traffic for such a system is given as
A = λ ⋅H
Exercise : There are 3000 calls per hour in a cell, each
lasting an average of 1.76 min. Offered traffic A =
(3000/60)(1.76) = 88 Erlangs

Cell coverage
If the offered traffic exceeds the maximum possible
carried traffic, blocking occurs. There are two different
strategies to be used.
Blocked calls cleared
Blocked calls delayed
Trunking efficiency : is defined as the carried traffic
intensity in Erlangs per channel, which is a value between
zero and one. It is a function of the number of channels
per cell and the specific GoS parameters.
Call arrival process: it is widely accepted that calls have a
Poisson arrival.

Channel Assignment Strategies
Channel allocation schemes can affect the
performance of the system.
Fixed Channel Allocation (FCA) :
Channels are divided in sets.
A set of channels is permanently allocated to each cell in
the network. Same set of channels must be assigned to
cells separated by a certain distance to reduce co-channel
interference.
Any call attempt within the cell can only be served by the
unused channels in that particular cell. The service is
blocked if all channels have used up.

Channel Assignment Strategies (FCA)
Most easiest to implement but least flexibility.
An modification to this is ‘borrowing scheme’. Cell
(acceptor cell) that has used all its nominal channels can
borrow free channels from its neighboring cell (donor cell)
to accommodate new calls.
Borrowing can be done in a few ways: borrowing from the
adjacent cell which has largest number of free channels,
select the first free channel found, etc.
To be available for borrowing, the channel must not
interfere with existing calls. The borrowed channel should
be returned once the channel becomes free.

Channel Assignment Strategies (DCA)
 Dynamic Channel Allocation (DCA) :
 Voice channels are not allocated to any cell permanently. All channels
are kept in a central pool and are assigned dynamically to new calls as
they arrive in the system.
 Each time a call request is made, the serving BS requests a channel from
the MSC. It then allocates a channel to the requested cell following an
algorithm that takes into acount the likelihood of future blocking within
the cell, the reuse distance of the channel and other cost functions ⇒
increase in complexity
 Centralized DCA scheme involves a single controller selecting a channel
for each cell. Distributed DCA scheme involves a number of controllers
scattered across the network.
 For a new call, a free channel from central pool is selected based on
either the co-channel distance, signal strength or signal to noise
interference ratio.

Channel Assignment Strategies
Flexible channel assignment
Divide the total number of channels into two groups, one of
which is used for fixed allocation to the cells, while the other is
kept as a central poor to be shared by all users.
Mix the advantages the FCA and DCA, available schemes are
scheduled and predictive.
Channels need to be assigned to users to accommodate
new calls
handovers
 with the objective of increasing capacity and minimizing
probability of a blocked call.

System Expansion Techniques
As demand for wireless services increases, the number of
channels assigned to a cell eventually becomes
insufficient to support the required number of users.
More channels must therefore be made available per
unit area.
This can be accomplished by dividing each initial cell area into a
number of smaller cells, a technique known as cell-splitting.
It can also be accomplished by having more channels per cell, i.e.
by having a smaller reuse factor. However, to have a smaller
reuse factor, the co-channel interference must be reduced. This
can be done by using antenna sectorization.

System Expansion Techniques--Cell
splitting
Cell splitting increases the number of BSs in order to
increase capacity. There will be a corresponding
reduction in antenna height and transmitter power.
Cell splitting accommodates a modular growth capability.
This in turn leads to capacity increase essentially via a
system re-scaling of the cellular geometry without any
changes in frequency planning.
Small cells lead to more cells/area which in turn leads to
increased traffic capacity.

splitting

splitting
 For new cells to be smaller in size, the transmit power
must be reduced. If n=4, then with a reduction of cell
radius by a factor of 2, the transmit power should be
reduced by a factor of 24 (why?)
 In theory, cell splitting could be repeated indefinitely.
 In practice it is limited
 By the cost of base stations
 Handover (fast and low speed traffic)
 Not all cells are split at the same time : practical problems of
BS sites, such as co-channel interference exist
 Innovative channel assignment schemes must be developed to
address this problem for practical systems.

System Expansion Techniques --
Sectorization
Keep the cell radius but decrease the D/R ratio.
In order to do this, we must reduce the
relative interference without increasing the
transmit power.
 Sectorization relies on antenna placement
and directivity to reduce co-channel
interference. Beams are kept within either a
60° or a 120° sector.

Sectorization

Sectorization
If we partition a cell into three 120° sectors, the
number of co-channel cells are reduced from 6 to 2 in
the first tier.
Using six sectors of 60°, we have only one co-channel
cell in the first tier.
Each sector is limited to only using 1/3 or 1/6 of the
available channels. We therefore have a decrease in
trunking efficiency and an increase in the number of
required antennas.
But how can the increase in system capacity be achieved?

System Expansion Techniques -Micro cells
Micro cells can be introduced to alleviate
capacity problems caused by “hotspots”.
By clever channel assignment, the reuse factor
is unchanged. As for cell splitting, there will
occur interference problems when macro and
micro cells must co-exist.

System Expansion Techniques -Micro cells

61
Modulation and demodulation
synchronization
decision
digital
dataanalog
demodulation
radio
carrier
analog
baseband
signal
101101001 radio receiver
digital
modulation
digital
data analog
modulation
radio
carrier
analog
baseband
signal
101101001 radio transmitter

62
About CSMA/CD
 Can we borrow media access methods from fixed networks?
 Example of 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 (original method in IEEE 802.3)
 Problems in wireless networks
 a radio can usually not transmit and receive at the same time
 signal strength decreases proportionally to the square of the distance or even
more
 the sender would apply CS and CD, but the collisions happen at the receiver
 it 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”

63
 Hidden terminals
A 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 terminals
B sends to A, C wants to send to another terminal (not A or B)
C has to wait, CS signals a medium in use
but A is outside the radio range of C, therefore waiting is not
necessary
C is “exposed” to B
Hidden and exposed terminals
BA C

64
 Terminals A and B send, C receives
 signal strength decreases (at least) proportionally to the square of the
distance
 the signal of terminal B therefore drowns out A’s signal, C 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

65
SDMA/TDMA/FDMA/CDMA
 SDMA (Space Division Multiple Access)
 segment space into sectors, use directed antennas
 cell structure
 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
 FDMA (Frequency Division Multiple Access)
 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)
 CDMA (Code Division Multiple Access)
 assign an appropriate code to each transmission channel (DSSS, Direct
Sequency Spread Spectrum)
 frequency hopping over separate channels (FHSS, Frequency Hopping
Spread Spectrum)

66
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha Reservations
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden
and exposed terminal
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS: Frequency-Hopping Spread Spectrum
DSSS: Direct Sequence Spread Spectrum
CSMA: Carrier Sense Multiple Access
CA: Collision Avoidance
DAMA: Demand-Assigned Multiple Access
MACA-BI: MACA by invitation
FAMA: Floor Acquisition Multiple Access
CARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth • Used in UMTS

67
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth

68
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 the
medium at any time
Disadvantages:
 precise
synchronization
required

69
Frequency multiplex
 Separation of the whole spectrum into smaller frequency bands.
 A channel gets a certain band of the spectrum for the whole
time.
Advantages:
 looser coordination
 works also for analog signals
Disadvantages:
 wastage of bandwidth
if the traffic is
distributed unevenly
 inflexible
 guard spaces
k2 k3 k4 k5 k6k1
f
t
c

70
f
Time and frequency multiplex
 Combination of both methods.
 A channel gets a certain frequency band for a certain amount
of time.
Example: GSM
Advantages:
 more flexibility
 But: precise coordination
required
t
c
k2 k3 k4 k5 k6k1

71
Code multiplex
 Each channel has a unique code
 All channels use the same spectrum at the
same time
 Advantages:
bandwidth efficient
good protection against interference
and eavesdropping
 Disadvantage:
more complex signal regeneration
 Implemented using spread spectrum
technology
k2 k3 k4 k5 k6k1
f
t
c

72
TDMA/TDD – example: DECT
1 2 3 11 12 1 2 3 11 12
t
downlink uplink
417 µs
DECT: Digital Enhanced Cordless Telecommunications
TDD: Time Division Duplex

73
FDMA/FDD – example: GSM
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
downlink
uplink
FDD: Frequency Division Duplex

74
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed

75
 Mechanism
 random, distributed (no central arbiter), time-multiplex
 Slotted 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

76
Performance of Aloha (1/4)
First transmission
Retransmission
(if necessary)
t0 t0+Xt0-X
Vulnerable period
t0+X+2tprop
Time-out
t0+X+2tprop+B
Backoff period B
 tprop : maximum one-way propagation time between 2 stations
 Information about the outcome of the transmission is obtained after the
reaction time 2 tprop
 B: backoff time

77
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
Pr transmissions in 2 second
G
S
k X 
 
   
 
2
0
2
2
2
s , 0,1,2,...
!
Throughput S: total arrival rate G times the prob. of a successful transmission:
.Pr no collision .Pr 0 transmissions in 2 seconds
2
=
0!
=
Peakvalue at 0
k
G
G
G
G
e k
k
S G G X
G
G e
Ge
G



 
 
 1.5 : 0.184
2
S
e
 

78
1
Detail of computation of throughput of previous slide:
Define:
: Transmission by a given station
: Absence of transmission by other station
Throughput:
1.Pr( , ) .Pr( , )
.Pr( | ).Pr( )
.Pr( )
N
i
T
A any
T A N T A
N A T T
N T





2
. (0,2 )
. (0,2 )
. G
Poisson G
G Poisson G
G e



79
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 e
S
G e
S
X t


  

 
 
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
G
aloha prop prop
G
aloha
prop
t X B
E T X t e X t B
BE T X a e a
X
t
a
X
 
     
     
 s the normalized one-way propagation delay
Computation of the average packet transmission time

80
Performance of Slotted Aloha
First transmission
Retransmission
(if necessary)
t0=kX (k+1)X
Vulnerable
period
t0+X+2tprop
Time-out
t0+X+2tprop+B
Backoff period
 
-
1Peakvalue at 1 : 0.368
Average packet delay:
/ 1 ( 1)(1 2 )
G
G
slotaloha
S Ge
G S
e
BE T X a e a
X

  
     

81
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed

82
Carrier Sense Multiple Access(CSMA)
 Goal: reduce the wastage of bandwidth due to packet collisions
 Principle: 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 begins
transmission
at t=0
A
Station A captures
the channel
at t=tprop
A

83
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).

84
Non-Persistent CSMA
 Attempts to reduce the incidence of collisions
 Stations with a packet to transmit sense the channel
 If the channel is busy, the station immediately runs the back-
off algorithm and reschedules a future sensing time
 If the channel is idle, then the station transmits
Consequence : channel may be free even though some
users have packets to transmit.

85
p-Persistent CSMA
Combines elements of the above two schemes
Stations with a packet to transmit sense the
channel
If it is busy, they persist with sensing until the
channel becomes idle
If it is idle:
With probability p, the station transmits its packet
With probability 1-p, the station waits for a random
time and senses again

86
Throughput expression
    
       
   
    
 
ae
aGe
S
eaG
Ge
S
aeea
eeaG
S
eaGeaG
eaGGaGGG
S
GeS
GeS
aG
aG
aG
aG
aGaG
aGaG
aGaG
aG
G
G






















1
21
11
1
1121
2/11
1
1
1
21
2
Pure ALOHA
Slotted ALOHA
Unslotted
1-persistent CSMA
Slotted
1-persistent CSMA
Unslotted
nonpersistent CSMA
Slotted
nonpersistent CSMA
Protocol Throughput

87
Throughput plot
Normalized propagation delay is a =0 .01

88
CSMA/CD (reminder)
• Operating principle
 Check whether the channel is idle before transmitting
 Listen while transmitting, stop transmission when collision
 If 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

89
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 difficult
 Carrier Sensing may be suitable to reduce interference at
sender, but Collision Avoidance is needed at receiver
 CSMA/CD does not address the hidden terminal problem

90
CSMA/CA
Is described in the module B devoted to IEEE
802.11

91
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed

92
Demand Assigned Multiple
Access(DAMA)
 Channel efficiency only 18% for Aloha, 36% for Slotted Aloha
 Reservation can increase efficiency to 80%
a sender reserves a future time-slot
sending within this reserved time-slot is possible without
collision
reservation also causes higher delays
typical scheme for satellite links
 Examples for reservation algorithms:
Explicit Reservation (Reservation-ALOHA)
Implicit Reservation (PRMA)
Reservation-TDMA

93
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

94
DAMA / Packet reservation (PRMA)
Implicit reservation
 based on slotted Aloha
 a certain number of slots form a frame, frames are repeated
 stations compete for empty slots according to the slotted aloha principle
 once 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 send
 competition 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 D
t
ACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservation

95
DAMA / Reservation-TDMA
Reservation Time Division Multiple Access
 every frame consists of N mini-slots and x data-slots
 every 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
reservations
for data-slots
other stations can use free data-slots
based on a round-robin scheme
e.g. N=6, k=2

96
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed

97
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 a receiver
with a short RTS packet before it sends a data packet
CTS (clear to send): the receiver grants the right to send as soon as it is
ready to receive
 Signaling packets contain
 sender address
 receiver address
 packet size
 Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed
Foundation Wireless MAC)

98
 MACA mitigates the problem of hidden terminals
A and C want to
send to B
A sends RTS first
C waits after receiving
CTS from B
 The hidden terminal problem might still arise, especially in
case of mobility of the nodes
MACA principle
A B C
RTS
CTSCTS

99
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

100
MACA variant: application in 802.11
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 for
data
RTS; RxBusy
RTS;
CTS
data;
ACK
time-out 
Data with errors;
NAK
ACK: positive acknowledgement
NAK: negative acknowledgement RxBusy: receiver busy
time-out 
NAK;
RTS

101
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed

102
Spread Spectrum principle
c(t) c(t)
 ( ) f
 j f( )
r f( )
~
( ) f
t f( )
Synchronization
n
Pseudo-random
code
FilterDecoderCoder
s f( )
f
s f( )
sS
Bs
( )s f power density spectrum of the original signal
sS power density of the original signal
sB bandwidth of the original signal
( )j f power density spectrum of the jamming signal

103
c(t) c(t)
 ( ) f
 j f( )
r f( )
~
( ) f
t f( )
Synchronization
Pseudo-random
code
FilterDecoderCoder
s f( )
f
t f( )
s s
t
t
S B
S
B

Bt

104
c(t) c(t)
 ( ) f
 j f( )
r f( )
~
( ) f
t f( )
Synchronization
Pseudo-random
code
FilterDecoderCoder
s f( )
jS
f
 j f( )
Bj

105
c(t) c(t)
 ( ) f
 j f( )
r f( )
~
( ) f
t f( )
Synchronization
Pseudo-random
code
FilterDecoderCoder
s f( )
Bj
tS
jS
f
r f( )
Bt

106
c(t) c(t)
 ( ) f
 j f( )
r f( )
~
( ) f
t f( )
Synchronization
Pseudo-random
code
FilterDecoderCoder
s f( )
sS
f
~
( ) f
j
j
t
B
S
B
Bt
Bs
 
signal s s s s t
jjamming j j s
j s
ProcessingPt signal gain
Pjamming original
P S B S B B
BP S B B
S B
B
  
Processing gain: Increase in received
signal power thanks to spreading

107
c(t) c(t)
 ( ) f
 j f( )
r f( )
~
( ) f
t f( )
Synchronization
Pseudo-random
code
FilterDecoderCoder
s f( )
sS
f
 ( ) f
j
j
t
B
S
B
Bs

108
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed

109
Frequency Hopping Spread Spectrum
(FHSS) (1/2)
 Signal broadcast over seemingly random series of frequencies
 Receiver hops between frequencies in sync with transmitter
 Eavesdroppers hear unintelligible blips
 Jamming on one frequency affects only a few bits
 Rate of hopping versus Symbol rate
 Fast Frequency Hopping: One bit transmitted in multiple hops.
 Slow Frequency Hopping: Multiple bits are transmitted in a hopping
period
 Example: Bluetooth (79 channels, 1600 hops/s)

110
Frequency Hopping Spread Spectrum
(FHSS) (2/2)
tb
tc
Fast Frequency Hopping:b ct ttb : duration of one bit
tc : duration of one chip
Chip: name of the sample period in spread-spectrum jargon

111
TDMA CDMAFDMASDMA
DAMA
Multiple
Access with
Collision
Avoidance
Polling
Pure
CSMA
• Used in
GSM Slotted
• RTS/CTS
• Used in 802.11
(optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11
(mandatory)
• Used in 802.11
(optional)
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth • Used in UMTS

112
Direct Sequence Spread Spectrum
(DSSS) (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
 Advantages
 reduces frequency selective
fading
 in cellular networks
neighboring base stations can use
the same frequency range
neighboring base stations can
detect and recover the signal
enables soft handover
 Disadvantages
 precise power control necessary
 complexity of the receiver
user data
chipping
sequence
resulting
signal
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 period
tc: chip period

113
Direct Sequence Spread Spectrum
(DSSS) (2/2)
X
user data
chipping
sequence
modulator
radio
carrier
spread
spectrum
signal
transmit
signal
transmitter
demodulator
received
signal
radio
carrier
X
chipping
sequence
lowpass
filtered
signal
receiver
integrator
products
decision
data
sampled
sums
correlator

114
Categories of spreading (chipping)
sequences
 Spreading Sequence Categories
– Pseudo-random Noise (PN) sequences
– Orthogonal codes
 For FHSS systems
– PN sequences most common
 For DSSS beside multiple access
– PN sequences most common
 For DSSS CDMA systems
– PN sequences
– Orthogonal codes

115
Orthogonal Codes
Orthogonal codes
All pairwise cross correlations are zero
Fixed- and variable-length codes used in CDMA
systems
For CDMA application, each mobile user uses one
sequence in the set as a spreading code
Provides zero cross correlation among all users
Types
Walsh codes
Variable-Length Orthogonal codes

116
Walsh Codes
1
1 1
H
1 0
 
  
 
1 1
1 1
H H
H
H H
k k
k
k k
 
 
 
   
 
1
1 1
H
1 0
 
  
 
 Set of Walsh codes of length n consists of the n rows of an n
x n Hadamard matrix:
 Sylvester's construction:
 Every row is orthogonal to every other row and to the logical
not of every other row
 Requires tight synchronization
 Cross correlation between different shifts of Walsh sequences
is not zero
2
1 1 1 1
1 0 1 0
H
1 1 0 0
1 0 0 1
 
 
 
 
 
 
...

117
Typical Multiple Spreading
Approach
 Spread data rate by an orthogonal code (channelization
code)
 Provides mutual orthogonality among all users in the same cell
 Further spread result by a PN sequence (scrambling
code)
 Provides mutual randomness (low cross correlation) between
users in different cells

118
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 code
 The sender XORs the signal with this code
 the receiver can “tune” into this signal if it knows the code of the sender
 tuning 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 needed
 huge code space (e.g., 232) compared to frequency space
 more robust to eavesdropping and jamming (military applications…)
 forward error correction and encryption can be easily integrated

119
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 Decision
Spreading Despreading

120
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 B
 sends 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 A
 apply key Ak bitwise (inner product)
Ae = (-2, 0, 0, -2, +2, 0)  Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6
result greater than 0, therefore, original bit was „1“
 receiving B
Be = (-2, 0, 0, -2, +2, 0)  Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“

121
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 distance
between single code words in code space.
Ad+Ak
1
-1

122
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
1
-1
1
-1
2
-2
0

123
Despreading of signal A
Ak
(As + Bs)
* Ak
correlator
output
decision
output
As + Bs
0 1 0
1 0 1data Ad
Note: the received signal is inverted
2
-2
0
1
-1
0
2
-2
0

124
Despreading of signal B
correlator
output
decision
output
Bk
(As + Bs)
* Bk
As + Bs
0 1 1
1 0 0data Bd
Note: the received signal is inverted
2
-2
0
1
-1
0
2
-2
0

125
Despreading with a wrong key
decision
output (1) (1) ?
wrong
key K
correlator
output
(As + Bs)
* K
As + Bs
2
-2
0
1
-1
0
2
-2
0

126
Comparison SDMA/TDMA/FDMA/CDMA
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 used in all cellular
systems
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
higher complexity
In practice, several access methods are used in combination
Example: SDMA/TDMA/FDMA for GSM

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