Lecture 9: Wireless Networks Anders Västberg 08-790 44 55 vastberg@kth.se

Lecture 9:Wireless Networks

Anders Västberg

08-790 44 55

vastberg@kth.se

Wireless Networks

• Provide fixed network access to a large number of stationary or mobile users.

• Examples:– Radio and television broadcasting systems– Mobile telephone systems (PSTN access)– Mobile broadband systems (Internet access)

Taxonomy of Wireless Networks

Wireless Networks

Fixed NetworksMobile Access

NetworksAd Hoc Networks

Cellular NetworksRandom Acess

NetworksMultihop Wireless

InternetsSensor Networks

[Kumar et al., 2004]

Duplex Communication

• Downlink: Communication from the BS to the MT.

• Upplink: Communication from the MT to the BS.

• Multiplexed by – Frequency Division Duplex (FDD)– Time Division Duplex (TDD)

Cellular System Overview

[Stallings, 2005]

Radio Access Ports (RAP)

• Provides Wireless Connections to Mobile Terminal (MT)

• Service area

• Coverage area

• Area availability

• Population availability

Capacity

• System capacity– Maximum number of users that can be served– Given a certain quality level

• Blocking probability

Range/Interference

• Few RAP, Low user densities =>Range limited systems

• Many RAP and user terminals =>Interference limited systems

Reuse distance

0 500 1000 1500 2000 2500 3000

140

130

120

110

100

90

80

D

R R

tt

dBW

Capacity

• Reuse distance

• Capacity only dependent on the ration between resuse distance and cell radius

• The system is scalable

Cell GeometryTesselation

[Stallings, 2005]

Frequency Reuse

Fixed Channel Allocation

K

C

KR

D3

,...3,2,1,0,22 jijijiK

}21,19,16,13,12,9,7,4,3,1{K

Capacity (channels/cell)

Normalized resuse distance

Cluster size

cA A

Area Capacity (channels/a.u.)

Worst Case Downlink Scenario

[Ahlin et al., 2006]

Worst Case Uplink Scenario

[Ahlin et al., 2006]

Capacity increase

• Adding new channels– Increase C

• New modulation and coding schemes– Decrease t

• Smaller Cells– Decrease Ac

– Cell splitting

• Divide cells into sectors– Use directional antennas

Directional Antennas

2/1

2/2

)(

hh

hh

A

S

Ideal sector antenna

If main lobe is

Nh

2 the Interference power is reduced by a factor of N