CELLULAR COMMUNICATIONSCellular 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

Cell Footprint

R2S = 3 3

2 R

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

Co-ordinates for hexagonal cellular geometry

With these co-ordinates, an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates.

Cosine Rule

Reuse Distance

Distance between cell centers = × Cell Radius

Reuse distancedistance between the centers of two co-channel cells

u2 2R = i + j + 2ij

3 3 Rcos

p

whereR is Cell RadiusRu is Reuse Distance

and cos(p /3) = 1/2

3

Cluster Radius

Radius of a cluster

cuR = R3

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

2uS = CS = 3 3

2R3{ }

Combining these two expressions gives uR = R 3C

Possible Cluster Sizes

We have seenuR = R 3C

and also

u2 2R = 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

Cells Macrocells

10km, sparsely populated area

Microcells 1km, densely populated area

Picocell 200m, particular buildings, streets

Umbrella Cells

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

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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 cell of the subscriber

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

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Handoff Region

BSi

Signal strength due to BSj

E

X1

Signal strength due to BSi

BSjX3 X4 X2X5 Xth

MSPmin

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

Operation Support Systems Network Management Systems Service Delivery Service Fulfillment, including the

Network Inventory, Activation and Provisioning

Service Assurance Customer Care Billing

GSM

Groupe Speciale Mobile/Global System for Mobile

GSM Air Interface TDMA with FDD

200Khz channels with 200KHz guard bands GSM 900 has 124 carriers GMSK modulation, 270kbps per carrier Up to 8 users, 24.8kbps per user FEC reduces to 13kbps per user for voice

Physical Channel RF carrier divided into 8 slots, numbered

0..7 Timeslots carrying data

At most 8 traffic channels Control messages

At least 1 control channels More control (logical) channels Packed into RF carrier

Single Burst/Slot

Frame Structure

Trafficchannels

(TCH)

Signalingchannel

TCH/F: Full-rate Traffic ChannelTCH/H: Half-rate Traffic Channel

FCCH: Frequency correctionSCH: SynchronizationBCCH: Broadcast control

PCH: PagingAGCH: Access grantRACH: Random access

SDCCH: Stand-alone dedicated control

SACCH: Slow associated controlFACCH: Fast associated control

Two-way

Base-to-mobile

Two-way

Logical Channel List

BCH

CCCH

DCCH

Broadcast Control Channels

Common Control Channels

Dedicated Control Channels

Channel Coding

International Mobile Station Equipment Identity (IMEI)

Type Approval Code (TAC): 6 decimal places, centrally assigned.

Final Assembly Code (FAC): 6 decimal places, assigned by the manufacturer.

Serial Number (SNR): 6 decimal places, assigned by the manufacturer.

Spare (SP): 1 decimal place.

International Mobile Subscriber Identity ( IMSI)

Mobile Country Code (MCC): 3 decimal places, internationally standardized.

Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within the country.

Mobile Subscriber Identification Number (MSIN): Maximum 10 decimal places, identification number of the subscriber in the home mobile network.

Mobile Subscriber ISDN Number ( MSISDN):

Country Code (CC) : Up to 3 decimal places.

National Destination Code (NDC): Typically 2-3 decimal places.

Subscriber Number (SN): Maximum 10 decimal places.

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What is a location area (LA)? A powered-on mobile is informed of an incoming

call by a paging message sent over the PAGCH channel of a cell

One extreme is to page every cell in the network for each call - a waste of radio bandwidth

Other extreme is to have a mobile send location updates at the cell level. Paging cut to 1 cell, but large number of location updating messages.

Hence, in GSM, cells are grouped into Location Areas – updates sent only when LA is changed; paging message sent to all cells in last known LA

SMS SMS allowed

Two way communications of the text messages Maximum character length of 160 characters

This can change though depending on the operator or the character set used

Character sets supported are ASCII + additional European characters Unicode

First Text Was sent in December 1992, to a Vodafone device

Sent by Neil Papworth, saying “Merry Christmas” Standard

Defined by ETSI and is known as “GSM 03.40”

SMS

SMS Continued The success is SMS was never planned for! It was only ever intended as the Pager replacement,

with limited use This will explain some of the design decisions made

SMS GSM

At a defined time interval in GSM all devices will listen to a transmission. This is when a Digital Control Channel (DCCH) packet of

information is being sent across the network. These DCCH packets are used to transfer essential information

into the devices. Information like a call is in coming Paging signals from the Base stations, to work out if a handover

is needed One of these packet formats is called SMS point to point

messaging, Paging, access control channel (SPACH) This message type can be used to carry a text message.

Advantage of this method is a text message can still be delivered during a phone conversation.

SMS

SMS Packet format All data is transferred in a single DCCH SPACH

packet

SCA Service Centre Address

MR Message Reference PID Protocol Identifier

PDU Type Protocol Data Unit Type

DA Destination Address DCS Data Coding Scheme

VP Validity Period UDL User Data Length UD User Data

GPRS: General Packet Radio Service

GSM data CSD: circuit switched data Max 14kbps Similar to voice call Inefficient usage of spectrum

GPRS packet-based service Upgrade of infrastructure GGSN is a gateway to outside world SGSN is a gateway within the network

GPRS architecture

GPRS handset classes Class A

Class A terminals have 2 transceivers which allow them to send / receive data and voice at thesame time. This class of device takes full advantage of GPRS and GSM. You can be taking a call and receiving data all at the same time.

Class B

Class B devices can send / receive data or voice but not both at the same time. Generally if you are using GPRS and you receive a voice call you will get an option to answer the call or carry on.

Class CThis device only allows one means of connectivity. An example would be a GPRS data card in a laptop.

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Packet switched Upgrades the modulation scheme

From GMSK to 8-PSK Maximum speed ~59 Kb/sec per time slot, ~473.6 Kb/sec for all 8 time

slots Variable data rate – depending on the channel conditions

Defines several different classes of service and mobile terminals

Enhanced Data GSM Environment (EDGE)

EDGE enabled data mobile

Page 68

Practically achievable data rates

Theoretical rates are constrained by mobile power and processing capabilities

Most mobiles support less than the maximum allowed by standardPractically achievable

data rates

Page 69

Migration:1. High speed circuits

switched data (HSCSD)

2. Packet switched data (GPRS,EDGE)

3. Integrated packet services – possibly under different access scheme (UMTS)

GSM Migration Towards 3G

G SM 2+9.6 Kb/sec

H S C S D64 K b/sec

G P R S114 Kb /sec

E D G E384 K b/sec

U M TS2M b/sec

1999 1Q2000

2Q2000

3Q2001

4Q2002

T im eline

D ata R ate

H S C S D - H igh S peed C ircu it S witched D ataG P R S - G ene ra l P acke t R adio S ystemED G E - Enhanced D ata G SM Environm entU M T S - U niversa l M o bile Te lephone Se rvice

70

HSDPAHigh Speed Downlink Packet Access Standardized in 3GPP Release 5 Improves System Capacity and User Data Rates in the Downlink

Direction to 10Mbps

Adaptive Modulation and Coding (AMC) Replaces Fast Power Control :

User farer from Base Station utilizes a coding and modulation that requires lower Bit Energy to Interference Ratio, leading to a lower throughput

Replaces Variable Spreading Factor :Use of more robust coding and fast Hybrid Automatic Repeat Request (HARQ, retransmit occurs only between MS and BS)