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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
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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
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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
→→
=+→⋅=