Lecture 11: Cellular Networks

• Introduction

• Principle of wireless networks

• The principle of frequency reuse

• Cellular system overview

Ben Slimane

slimane@kth.se

Cellular Networks

• The purpose of wireless networks is to provide wireless access to the fixed network (PSTN)

Cellular Networks

• Multiple low-power transmitters (100 W or less) are used

• The service area is divided into cells– Each cell is served by its own antenna– Each base station consists of a transmitter, a

receiver, and control unit– Base station placed in the middle or at the border

of the cell – Each base station is allocated a certain frequency

band (frequency allocation)

Cellular Geometries

Cellular Geometries

Rd 3

• The most common model used for wireless networks is uniform hexagonal shape areas– A base station with omni-directional antenna is placed in

the middle of the cell

Cellular Geometries

• Cells are classified based on their sizes• Macrocells with radius of 1km or more (wide

area)– Hexagonal shape cells

• Microcells with radius of 100m or more (cities)– Hexagonal shape cells– Manhattan (city) type cell structure

• Picocells with radius in the meters (indoor)– Shape depends on the room

Design of Wireless Networks

• The design is done in two steps– Area coverage planning

– Channel (Frequency) allocation

• Outage area• Coverage area

Frequency Reuse

• An efficient way of managing the radio spectrum is by reusing the same frequency, within the service area, as often as possible

• This frequency reuse is possible thanks to the propagation properties of radio waves

Frequency Reuse

• We form a cluster of cells– Divide the total number of channels (frequencies) between

the cells of the cluster.

– All the channels within the cluster are orthogonal

• No interference between cells of the same cluster

• We repeat the cluster over the service area• The distance between the clusters is called the reuse

distance D• The design reduces to finding D!

Frequency Reuse

NRD

N

JIJIJIN

3

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

...4,3,2,1,),(22

• For hexagonal cells, the number of cells in the cluster is given by

Frequency Reuse Pattern

• Frequency reuse pattern for N=3

Frequency Reuse Patterns

• Frequency reuse pattern for N=7

Capacity of Wireless Networks

• The capacity of a wireless network is measured as the average of simultaneous radio links supported by the systems

η=C/N, users/cell

• The area capacity is defined as

η=C/(NxAcell), users/unit area

• Acell is the cell area

Approaches of Increasing Capacity

• Adding new channels• Frequency borrowing – frequencies

are taken from adjacent cells by congested cells

• Cell splitting – cells in areas of high usage can be split into smaller cells

• Directional antennas – cells are divided into a number of wedge-shaped sectors, each with their own set of channels

• Microcells – antennas move to buildings, hills, and lamp posts

Cellular System Overview

Cellular Systems Terms

• Base Station (BS) – includes an antenna, a controller, and a number of transceivers

• Mobile telecommunications switching office (MTSO) – connects calls between mobile units

• Two types of channels available between mobile unit and BS– Control channels – used to exchange information having to

do with setting up and maintaining calls– Traffic channels – carry voice or data connection between

users

Steps in an MTSO Controlled Call between

Mobile Users

• Mobile unit initialization

• Mobile-originated call

• Paging

• Call accepted

• Ongoing call

• Handoff

Examples of Mobil Cellular Calls

Examples of Mobile Cellular Calls

Examples of Mobile Cellular Calls

Additional Functions in an MTSO Controlled Call

• Call blocking

• Call termination

• Call drop

• Calls to/from fixed and remote mobile subscriber

Mobile Radio Propagation Effects

• Signal strength– Must be strong enough between base station and mobile

unit to maintain signal quality at the receiver

– Must not be so strong as to create too much cochannel interference with channels in another cell using the same frequency band

• Fading– Signal propagation effects may disrupt the signal and cause

errors

Radio Resource Allocation problem

i

j

To each active terminal assign - Base station - Channel (“Frequency”) - Transmitter powersuch that Link Quality & power constraints are satisfied for as many terminals as possible

Handover Performance Metrics

• Cell blocking probability – probability of a new call being blocked

• Call dropping probability – probability that a call is terminated due to a handover

• Call completion probability – probability that an admitted call is not dropped before it terminates

• Probability of unsuccessful handover – probability that a handover is executed while the reception conditions are inadequate

Handover Performance Metrics

• Handoff blocking probability – probability that a handoff cannot be successfully completed

• Handoff probability – probability that a handoff occurs before call termination

• Rate of handoff – number of handoffs per unit time• Interruption duration – duration of time during a

handoff in which a mobile is not connected to either base station

• Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur

Handover Strategies Used to Determine Instant of Handover

• Relative signal strength

• Relative signal strength with threshold

• Relative signal strength with hysteresis

• Relative signal strength with hysteresis and threshold

• Prediction techniques

Handover decision

Transmitter Power Control

• Why transmitter power control?– Reduce terminal power consumption– Reduce interference within the cellular system and

improve quality– Efficient handling of mobility– In SS systems using CDMA, it’s desirable to

equalize the received power level from all mobile units at the BS

• Reduce near-far problem

Types of Power Control

• Open-loop power control– Depends solely on mobile unit– No feedback from BS– Not as accurate as closed-loop, but can react quicker to

fluctuations in signal strength

• Closed-loop power control– Adjusts signal strength in reverse channel based on metric

of performance– BS makes power adjustment decision and communicates to

mobile on control channel

Traffic Engineering

• In cellular systems, the number of active users (calls) is random.

• Ideally, available channels would equal number of subscribers active at any time– Not possible in practice

• For N channels per cell and L active subscribers per cell we have

• L < N non-blocking system• L > N blocking system

Performance Questions

• Blocking Probability – Probability that a call request is blocked?

• System capacity for a given blocking probability?

• What is the average delay?

• System capacity for a certain average delay?

Traffic Intensity

• In cellular systems, calls are Poisson distributed with calls/s

• The traffic load of the system is

is the number of calls per seconds• h is the average call duration in seconds• A = average number of calls arriving during average

holding period (in Erlangs)

hA

Factors that Determine the Nature of the Traffic Model

• Manner in which blocked calls are handled– Lost calls delayed (LCD) – blocked calls put in a queue

awaiting a free channel

– Blocked calls rejected and dropped• Lost calls cleared (LCC) – user waits before another attempt

• Lost calls held (LCH) – user repeatedly attempts calling

• Number of traffic sources– Whether number of users is assumed to be finite or infinite