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CellularCellular SystemsSystems

Broadcast channelBroadcast channel

Centralized broadcast channelDistributed broadcast channel

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CentralizedCentralized broadcast broadcast channelchannel

� Fixed access point (Cellular systems, WLAN, WMAN …)

Base Station

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Wired network

� Mobile-access point

connection

Centralized broadcast channelCentralized broadcast channel

� Cellular coverage:The territory coverage is obtained by Base Stations–BS (or Access Points) that provide radio access (Radio Access Network, RAN) to Mobile Stations–MS within a service area called CELL

BaseStation

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Station

MobileStation

Cell

Distributed broadcast channelDistributed broadcast channel

� Ad-hoc wireless networks (mesh networks, sensor networks)

� mobile - mobile

connections

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WiredWired--Wireless networks:Wireless networks:Main differencesMain differences

� Shared transmission medium

� Multiple access mechanisms

� Radio resource reuse

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CentralSwitch

cable

Radiochannel

WiredWired--Wireless networks:Wireless networks:Main differencesMain differences

� Radio channel� Variable channel characteristics� Advanced modulation and coding schemes

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TDM (TDM (TimeTime DivisionDivision Multiplexing)Multiplexing)

� Each source/sender can use only a single time slot every N� Hence we define a frame structure, where the frame is

constituted by N consecutive time slots� If we give a number to each time slot, each source/sender is

associated to a time-slot number, and it can transmit only insidesuch slot

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1 2 3 4 5 1 2 3 4 53 4 5 1

frame frame

... ...

slot

Centralized broadcast channelCentralized broadcast channel

� The Base Station is vital to enforce synchronization among mobile terminals

� Its transmissions are used to synchronizeall transmissions (e.g., sending a signal to say when the frame starts)

TDMA Burst

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Starting timesignal

2τ Propagation timeτ = d/vd : distancev : light speed in the medium (here, the air)

TDMA Burst

Centralized broadcast Centralized broadcast channelchannel� Guard time: )2(max i

igT τ=

TDMA Burst = T

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Obviously: the guard time is dominated by the farthestnode from the BS

TDMA Burst = T

Guard Time = 2τ

τ2+=

+=

T

T

TimeGuardBurstTDMA

BurstTDMAEfficiency

Centralized broadcast Centralized broadcast channelchannel� Timing Advance:

� If each node knows the propagation delay towards the BS, it can anticipate its transmission!

Propagation delay τ τ τ τ must

2) Delayestimation 4) Next transmissions

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� Propagation delay τ τ τ τ mustbe estimated (it can be time-varying)

� Estimation error is still possible: time guards are reduced, but they are not null!

� Technique used in GSM

1) First transmission

3) BS sendsthe estimateddelay to the MS

Cellular (Mobile) SystemsCellular (Mobile) Systems

BS

MS

uplink

downlink

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MS

cell

MS = Mobile StationBS = Base Station

Uplink = from the MS to the BSDownlink = from the BS to the MS

Radio AccessRadio Access

� The radio access problem is related to the way in which the users in the cell share radio resources

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� downlink: � multiplexing is used

� uplink: � multiple accessis used

Radio AccessRadio Access

� 1st generation systems: � TACS (Europe)

AMPS (US)� FDM/FDMA

(downlink/uplink)

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� 3rd generation:UMTS CDM/CDMA

� 4th generationLTEOFDMA/SC-FDMA(Single Carrier – FDMA)

� 2nd generation:GSM (Europe) D-AMPS (US)� multi-carrier TDM/TDMA

Frequency reuseFrequency reuse

� Available frequencies are notsufficient for all users

� Solution: we reuse the same frequency in different cells (spatial reuse)

� Spatial reuse causes co-channel interference

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� Spatial reuse causes co-channel interference

� Spatial reuse is made possible if cells are sufficiently far apart so that interference can be small/tolerable (in order to guarantee a good quality of the transmitted signal)

Spatial reuseSpatial reuse

� Interferenceis therefore a fundamental, intrinsic feature of cellular systems

� Usually we assume that system quality is good when the ratio between the signal

21

31

2

1

13

2

13

1

322

12

13

1

3

1

2

12

3

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the ratio between the signal power and the interference power, named SIR (Signal-to-Interference Ratio) is higher than a predefined threshold, SIRmin

211

23

12

311

2

1

3

1

32

Cluster dimensioningCluster dimensioning� All available frequencies are divided into K groups� We assign a group to each cell in order to maximize the

distance between 2 cells that use the same group of frequencies

� Frequency reuse efficiency = 1/K� Possible K values: K=1,3,4,7,9,12,13, …

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3215

67

42

31

12

734

3215

4

15

643

15

67

4

3

4

215

67 2

132

12

12

31

23

112

1321

3

13

221

21

31

3

3

1

213

23

K = 7 K = 3

Cluster dimensioningCluster dimensioning

� If we know/if we set the SIRmin value tolerated by the system, then we can estimate the maximal efficiency of the system, i.e., the minimum K value that can be used

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minimum K value that can be used

� Received power:η−⋅⋅= dGPP tr

Cluster Cluster dimensioningdimensioning

d

D

d

d2

d3

d

� Hip.: same antennas (G) and same tx power (Pt)

∑

∑

−

−

=−

−

=

=⋅⋅

⋅⋅=

6

6

1i it

t

d

dGP

dGPSIR

η

η

η

η

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dr d1

d4

d5 d6

∑ =−

= 6

1i id η

� Worst case: d = r

� Approximation: di = Dη

η

η −

−

−

=≅RD

rSIR

1

6

1

6

Cluster Cluster dimensioningdimensioning� The SIR depends exclusivelyon the reuse

ratio R=D/r (and on η) but not on the absolute transmission power or on the cell dimension

� If we fix SIRmin we can compute Rmin

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� Then, if Rmin is known, we can obtain K since we can observe that:

� and therefore:3

2RK =

( )3

6 /2

min

ηSIRK =

ExerciseExercise� Let us dimension a cluster for a cellular system

that tolerates SIRmin = 18 dB, considering the case where the path-loss exponent ηηηη is equal to 3.9

( ) ( )99.6

1.6366 9.3/2/2

=⋅==ηSIR

K

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99.633min ===K

SummarySummary� dB

� Logarithmic scale

� If we use absolute powers

10log10dB PP =

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10/

10

10 dBP

dB

P =

SummarySummary� The product in linear scale corresponds to a

sumusing dB

� The ratio corresponds to a differencein dB

dBdB PGPG +→⋅

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dBdB

dBdB

APAP −→/

SummarySummary

� Notable values

dB

dB

54.99

98

→→

dB

dB

77.43

32

→→

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dB

dB

dB

dB

301000

20100

1010

54.99

→→

→→

dB

dB

dB

77.76

75

633224

→→

=+→⋅=