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Cellular Wireless Networks
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Examples of wireless networks
Cellular telephony
Satellite networks
Metropolitan-area data networks
Local-area networksInfostations (mobile hosts traveling through fixed network)
Ad hoc networks (mobile nodes dynamically forming a temporary
network without the use of any existing network infrastructure)
Paging networks
Other networks: Personal area networks, sensor networks, home
networks, smart dust, ubiquitous computing environments, ambientintelligence buildings, etc
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1.
2.
3. 4.
5.
6.
7.
8.
( anywhere. any time)
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senderr
transmission
detection
interference
short term fading
long term
fading
t
power
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With path loss, shadowfading, and Rayleigh fading
With path loss
log (distance)
Received
Signal
Power
(dB)
With path loss andshadow fading
dB With Respect
to Average Value
0 0.5
0.5P
1.5
-30
-20
-10
10
0
1t, in seconds
0 10 3020
x, in wavelength
Received
Signal
Power
(dB)
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Cellular Network Organization Use multiple low-power transmitters (100 W or less)
Areas divided into cells
Each served by its own antenna
Served by base station consisting of transmitter, receiver, and
control unit
Certain channels (e.g. bands of frequencies) allocated to each
cell
Cells set up such that antennas of all neighbors are
approximately equidistant (hexagonal pattern)
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(base station)
K (mobile
station)
MSC
MTSO
(Mobile Switching Center
obile Telephone Switching
Office)
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TDMA 3
FDMA 3.
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Direct Sequence CDMA (DS-CDMA)
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Frequency Hopping CDMA
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Frequency Reuse
Adjacent cells assigned different frequencies to avoid interference
Objective is to reuse frequency in nearby cells
10-50 channels (TDM, FDM or CDMA) assigned per cell
Transmission power controlled to limit power at that frequency
escaping to adjacent cells
Cellular systems are interference-limited, not noise limited.
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The issue is to determine how many cells must intervene
between two cells using the same frequency
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2, 1/3
.
GSM, FDMA
- 200Hz . -
TDMA 8 ,
.
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Adjacent channel interference can be controlled with transmit and receive filters
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Case c0=1, c1=1, c2=c3==0
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Approaches to Cope with Increasing Capacity
Adding new channels Channel borrowing channels are taken from
adjacent cells by congested cells
Cell splitting cells in areas of high usage can be
split into smaller cells
Cell sectoring cells are divided using directionalantennas into a number of sectors, each with their
own set of channels
Microcells antennas move to buildings, lamp
posts.Very Small Cells, possibly an antenna in
every room
Cell splitting
Directional
antennas
Cell sectoring
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120 sectoring reduces interference from co-channel cells.
Out of the 6 co-channel cells in the first tier, only 2 interfere with the
center cell.
If omni-directional antennas were used at each base station, all 6 co-
channel cells would interfere with the center cell.
5
5
5
5
5
5
7
6
14
2
3
SECTORIZATION
Omnidirectional Sectorized Directive
90
270
180
150
120
30
300240
210 330
60
0
90
270
180
150
120
30
300240
210 330
60
0
90
270
180
150
120
30
300240
210 330
60
0
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Cellular System Overview
Base Station (BS) includes an antenna, a controller, and a number of receivers
Mobile telecommunications switching office (MTSO) connects calls betweenmobile units. Also called MSC (Mobile Switching Center). Responsible for handoffs
Two types of channels available between mobile unit and BS
Control channels used to exchange information having to do with setting upand maintaining calls (out-band or in-band through stealing bits)
Traffic channels carry voice or data connection between users
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Handoffs ( handovers)
BASESTATION
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Usually relative signal strength with hysteresis and
threshold is used. Also, prediction techniques.
short term fading
long term
fading
t
power
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More efficient to work with cell history than with mobile history.
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Power Control
It is desirable to include dynamic power control ina cellular system Received power must be sufficiently above the background noise
for effective communication
Desirable to minimize power in the transmitted signal from the
mobile, in order to reduce cochannel interference, alleviate healthconcerns, save battery power
In SS systems using CDMA, its desirable to equalize the received
power level from all mobile units at the BS (the so called near-far
problem)
Open-loop power control (depends solely on mobile unit; not as accurateas closed-loop, but can react quicker to fluctuations in signal strength)
Closed-loop power control ( BS makes power adjustment decision and
communicates to mobile on control channel)
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Estimate channel requirements per cell
Littles law:
P = mean rate of calls attempted per unit time
h = mean holding time per successful call
A = average number of calls present
hA P!
Better to use M/M/m and M/M/m/m formulas
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HLR may become a bottleneck
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Mobile Wireless TDMA Design Considerations
Number of logical channels (number of time slots in TDMA frame): 8
Maximum cell radius (R): 35 km
Frequency: region around 900 MHz
Maximum vehicle speed (Vm): 250 km/hr
Bandwidth: Not to exceed 200 kHz (25 kHz per channel)
Slot duration: 0.577ms (GSM).Each slot contains 156bits.
Bit rate 33.8Kbits/sec for each of the 8 channels
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GSM Network Architecture