CELLULAR COMMUNICATIONS

Cellular Basics

Spectrum Reuse

Earlier systems: single central transmitter

Cover wide area Single channel per user 25kHz for sufficient audio quality and

guard interval 40 users in 1MHz, 400 users for 100MHz Modern systems have millions of

subscribers

Spectrum Reuse

Several transmitters, each having only certain coverage area

Cell==coverage area

Reuse same spectrum in many transmitters

Cells

Cells

Often shown as hexagonal shapes In reality, very irregular boundaries Signal strength decreases

gradually=>no exact cell edges Some cell areas may overlap Allocate different spectrum to adjacent

cells Can overlap without causing interference

Cells

Clusters

Cells with different spectrum grouped together as cluster

Often clusters of size 7

Cluster: set of different frequencies used in group of cells

Cluster is repeated by linear shifti steps along one directionj steps in the other direction

How many different frequencies does a cluster contain?

Theoretical Network Planning

Honeycomb (hexagonal) cell structure

Reuse Distance

Distance between cell centers = × Cell Radius

Reuse distance

distance between the centers of two co-channel cells

u2 2

R = i + j + 2ij3

3 Rcosp

whereR is Cell RadiusRu is Reuse Distance

and cos(p /3) = 1/2

3

Cluster Radius

Radius of a cluster

cu

2 2

R = R3

= i + j + ij

3 R

Cluster Size C: number of channels needed for (i,j) grid

is proportional to surface area of cluster

Surface area of one hexagonal cell is

R2S =

3 3

2 R

Surface area of a (hexagonal) cluster of C cells is

uR R

2u

S = CS = 3 3

2R3{ }

Combining these two expressions gives uR = R 3C

Possible Cluster Sizes

We have seen

uR = R 3C

and also

u2 2

R = i + j + ij 3 R

Thus:

C = i + j + ij2 2

with integer i and j .

C = 1 i = 1, j = 0 } Cluster size for CDMA netC = 3 i = 1, j = 1C = 4 i = 2, j = 0C = 7 i = 2, j = 1 } Usual cluster sizes for TDMAC = 9 i = 3, j = 0 } cellular telephone netsC = 12 i = 2, j = 2

·

Cluster size C = i2+ ij + j2 = 1, 3, 4, 7, 9, ...

· Cellular Telephony Chose C to ensure acceptable link quality at cell boundary

Typical Cluster Sizes

Reuse distance 2 – reuse pattern

One frequency can be (re)used in all cells of the same color

Reuse distance 3 – reuse pattern

Design Objectives for Cluster Size

•High spectrum efficiency

many users per cell

Small cluster size gives much bandwidth per cell

• High performance

Little interference

Large cluster sizes

The effect of decreasing cell size•Increased user capacity

•Increased number of handovers per call

•Increased complexity in locating the subscriber

•Lower power consumption in mobile terminal:· Longer talk time,· Safer operation

•Different propagation environment, shorter delay spreads•Different cell layout,

· lower path loss exponent, more interference· cells follow street pattern· more difficult to predict and plan· more flexible, self-organizing system needed (cf. DECT vs. GSM)

Cells

Macrocells 10km, sparsely populated area

Microcells 1km, densely populated area

Picocell 200m, particular buildings, streets

Reuse distance 3 – reuse pattern

Fixed and Dynamic assignment Fixed frequency assignment: permanent

certain frequencies are assigned to a certain cell

problem: different traffic load in different cells Dynamic frequency assignment:

temporary base station chooses frequencies depending on

the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference

measurements

21

Increasing Capacity

Add new channels Dynamic channel allocation – frequencies

can be 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 into a number of wedge-shaped sectors, each with their own set of channels (typical: 3)

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

Cell sectorization

Use directional antennas

Collocate cell antenna at the cell edges

Reduce cost

Handoff/Handover

Maintain call while moving

Basic Network Architecture

Basic Architecture

Base Station Controller (BSC) Control each base station Manage hand-off of a call from one base

station to other Mobile Switching Center(MSC)

Manages setup and tear down of calls to and from mobile subscribers

Home Location Register (HLR) HLR subscriber database including location

Network

Base Transceiver Station (BTS) Antenna Tower Radio

transceivers Power Supply Link to BSC (land

lines or microwave)

Setting up calls/registration

Make a call originated from mobile handset Allocate resources (channel)

Receive a call Locate the current cell

After the telephone is switched on Contact base station Register to use a network

Registration

Authenticate (e.g. for billing) Authentication Center (AuC)

Store my location HLR for “home” subscribers VLR for “visiting”/roaming subscribers

Mobile communicates with the network to update status/location

Network keeps last known location

Receiving a calls

Network should send a notification to a mobile

Network send to the area where mobile is located

Mobile listen to a “paging” channel Examine each message on the paging

channel and compares number with his own

Respond if match

Paging channel

Always listening to the paging channel drains the battery

Divide paging channel into 10 subgroups according to a last digit of mobile phone number

Mobile has to listen only 1/10 of time Longer call setup time

Random Access Channel(RACH) Respond to call /paging channel

message Initiate a call “Access” message Request a channel/slot/resources for

further communications Slotted ALOHA

Handover(EU)/Handoff(US)

Mobile monitor signal strength Network knows about availability of

channels Mobile monitors strength of signal from

current and adjacent cells and sends this information to network

When signal drops below certain level, network reserved new channel at adjacent cell

Mobile switch channel, network shuts down old channel

33

Handoff Region

BSi

Signal strength due to BSj

E

X1

Signal strength due to BSi

BSjX3 X4 X2X5 Xth

MS

Pmin

Pi(x) Pj(x)

• By looking at the variation of signal strength from either base station it is possible to decide on the optimum area where handoff can take place.

Types of Handoffs Hard handoff

A hard handoff is a “break before make” connection. MS is linked to no more than one BS at any given

time. Hard handoff is primarily used in FDMA and TDMA.

Soft handoff It isn't a “break before make” transition. The call can be carried on both cells simultaneously. Soft handoff is used in CDMA.

Handoff Decisions

Decision-making process of handoff may be centralized or decentralized

Three different kinds of handoff decisions Network-Controlled Handoff Mobile-Assisted Handoff Mobile-Controlled Handoff

Umbrella Cells

Operation Support Systems

Network Management Systems Service Delivery Service Fulfillment, including the

Network Inventory, Activation and Provisioning

Service Assurance Customer Care Billing