lecture1 cellular wireless networks 2004

8/9/2019 Lecture1 Cellular Wireless Networks 2004

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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

lecture1 cellular wireless networks 2004

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