Gsm,gprs,cdma,3 g

3G Basic Understanding

Vishal Sharma

Aims of Course• To attain a general understanding of UMTS systems

– GSM Evolution Towards UMTS

– 3g Standards

– Code Division Multiple Access Technology

– UMTS Network Elements and Architecture

– UMTS Air Interface

Introductory Session

Locator Slide• 1st and 2nd Generation Cellular Systems Overview• 3rd Generation Drivers and Standards• CDMA Mobile Technology Overview• UMTS Architecture Overview• UMTS Air Interface

Locator Slide

Cellular Generations• People talk about mobile

technology in terms of generations:

– 1st Generation or 1G– 2nd Generation or 2G– 2.5G– 3rd Generation or 3G

• But what do these mean?

time

Data rate

Progress of data rates with time and generation

1978 1992 2000 2001

1st and 2nd Generation Cellular Systems Overview

1st Generation• 1976+, though really the technology of

the 1980’s

• Analogue modulation

• Frequency Division Multiple Access

• Voice traffic only

• No inter-network roaming possible

• Insecure air interface


1st Generation Standards• AMPS (Analogue Advanced Mobile Phone System)

– North American Standard in cellular band (800MHz)

• TACS (Total Access Communications System)– UK originated Standard based on AMPS in 900MHz band

• NMT (Nordic Mobile Telephony System)– Scandinavian Standard in 450MHz and 900MHz bands

• C-450– German Standard in 450MHz band

• JTACS (Japanese Total Access Communications System)– Japanese Standard in 900MHz band


1st Generation Planning• Macrocellular

– High sites for coverage driven planning– Antennas above roof height

• Frequency planning required– For networks with more cells than frequencies these

must be planned

• Large cell size – Order 30km

• Handover– Mobile only ever connected to a single cell


2nd Generation• 1990’s

• 1st system to use Digital modulation

• Voice and low rate circuit switched data

• Same technology allows international

roaming

• Secure air interface


GSM• First networks in 1992• European developed standard, but

with worldwide subscriber base• Different frequency bands

– GSM450, GSM900, GSM1800, GSM1900

• Largest 2nd Generation subscriber base

• Frequency/Time Division Multiple Access

GSM phones from 1999/2000


GSM Planning• Macrocells and microcells

– Capacity driven planning

• Frequency planning required• Optional parameters requiring

planning– Hierarchical Cell Structures– Frequency Hopping– Discontinuous Transmission– Power Control

• Simple subscriber/traffic analysis– Capacity limited by number of TRX’s

• Handover - yes


D-AMPS/PDC• TDMA (D-AMPS)

– North American TDMA/FDMA based standard based upon AMPS

– Predominantly used in North and South America

– ANSI-41 Core Network– Planning Similar to GSM

• PDC– Japanese TDMA/FDMA based standard– Predominantly used in Asia– Planning Similar to GSM

TDMA and PDC phones from 1999/2000


cdmaOne• First networks in 1996• Derived from Qualcomm IS-95 air

interface• Largely American subscriber base with

some Asian networks• Code Division Multiple Access

– The closest 2nd generation standard to many of the 3rd generation standards

• ANSI-41 core network• Chip rate of 1.2288Mcps


cdmaOne Planning

• Macrocells and microcells• Single Frequency

– multiple frequencies for hotspots

• Soft Handover (multiple connections between mobile and network)

• Code Planning• Capacity Interference Limited

1 Connection

2 Connections

3 Connections


Worldwide Mobile Communications

0100200300400500600700

1991

1993

1995

1997

1999

2001

Second Generation -D-AMPS

Second Generation -PDC

Second Generation -GSM

Second Generation -cdmaOne

First Generation -Analogue

Millio

n

Su

bscrib

ers

Year Source:Wideband CDMA for 3rd Generation Mobile Communications, Artech House, 1998


Worldwide Mobile Subscribers

0

500

1000

1500

2000

1995 2000 2005 2010

European UnionCountries

North America

Asia Pacific

Rest of World

Millio

n

Su

bscrib

ers

Year Source:Third Generation Mobile Communications, Artech House, 2000


2.5G• Digital modulation

• Voice and intermediate rate circuit/packet

switched data

• Same technology roaming

• Secure air interface

• Based upon existing dominant standards

such as GSM and cdmaOne


GPRS• General Packet Radio Service• Enhancement to the GSM standard• Utlilises

– Multiple Timeslots– Packet Switching

• Packet Switched Data typically to rates of 56kbps


IS-95B• Enhancement to cdmaOne standard

• Utilises– High rate coding scheme– Combined code channels– Packet switching

• Packet Switched Data to rates of 114kbpsQualcomm PDQ Smartphone



Locator Slide

IMT-2000• International Mobile Telecommunications 2000 is a program focussed on

providing a single global standard for mobile communications

• Development started in 1985 as FPLMTS – Future Public Land Mobile Telecommunications System

• Proposed by the ITU (International Telecommunications Union)

3rd Generation Drivers and Standards

Who does IMT-2000 serve?

• Integrating all the following users– fixed– cellular– cordless– professional mobile radio– paging– satellite– specialised (aeroplane, etc)


Aspects of IMT-2000 Networks


The Road to 3G

HSCSD


•HDR – High Data Rate

ftp.tiaonline.org/uwc136

www.cdg.orgwww.3gpp.org

What are the IMT-2000 goals?• Data Rates

– Local area - 2 Mbps

• In office, stationary– Limited mobility - 384 kbps

• Urban pedestrian– Full mobility - 144 kbps

• Rural in car

• High spectrum efficiency compared to existing systems

• High flexibility to introduce new services


IMT-2000 Spectrum

1800 20501900 1950 20001850 2100 2150 2200

ITU

Allocation

Europe

Japan

Korea

USA

1885 1980 20102025 2110 2170 2200

1920 1980 20102025 2110 2170 2200

1920 1980 2110 2170

2110 21701920 1980

1850 1910 1930 1990 2110 2200

MSS MSS

IMT-2000

Land Mobile

IMT-2000

Land Mobile UL

IMT-2000

Land Mobile UL

IMT-2000

Land Mobile

IMT-2000

Land Mobile DL

IMT-2000

Land Mobile DL

UMTS

Paired UL

UMTS

Paired DLMSS MSSUMTS

Unpaired

UMTS

Unpaired

IMT-2000

Land Mobile

PCS

UL

PCS

DLReserved

1900

DECTGSM 1800

1880


IMT-2000 Spectrum

1800 20501900 1950 20001850 2100 2150 2200

ITU

(WARC-92)

Europe

Japan

Korea

USA

1885 1980 20102025 2110 2170 2200

1920 1980 20102025 2110 2170 2200

1920 1980 2110 2170

2110 21701920 1980

1850 1910 1930 1990 2110 2200

MSS MSS

IMT-2000

Land Mobile

IMT-2000

Land Mobile UL

IMT-2000

Land Mobile UL

IMT-2000

Land Mobile

IMT-2000

Land Mobile DL

IMT-2000

Land Mobile DL

UMTS

Paired UL

UMTS

Paired DL

UMTS

SAT

UMTS

SAT

UMTS

Unpaired

UMTS

Unpaired

IMT-2000

Land Mobile

PCS

UL

PCS

DLReserved

1900

DECTGSM 1800

1880


3rd Generation Cellular2002+•Digital modulation

•Voice and high rate data

•Multi technology roaming

•Secure air interface

•Standards

• UMTS FDD (CDMA based)

• UMTS TDD (CDMA based)

• cdma2000 (CDMA based)

• EDGE (TDMA based)


UMTS FDD• UMTS Frequency Division Duplexing Mode

• Built onto enhanced GSM core network

• Utilises:– QPSK modulation (Quadrature phase shift keying)– Multiple channel coding and bearer rates– Variable spreading factors and multi-code transmission– CDMA– FDD

• Data up to rates of 2Mbps


UMTS TDD• UMTS Time Division Duplexing Mode

• Built onto enhanced GSM core network

• Utilises:– QPSK modulation– Multiple channel coding and bearer rates– CDMA– TDD

• Data up to rates of 2Mbps

• Will happen after UMTS FDD


cdma2000• Built onto ANSI - 41 core network

• Utilises:– QPSK modulation– Multiple channel coding and bearer rates– CDMA– FDD– Multiple carriers on the downlink

• allows compatibility with cdmaOne– Synchronous operation

• Data up to rates of 2Mbps (typically less)


EDGE• Enhanced Data for GSM Evolution

– Sometimes called E-GPRS (Enhanced GPRS)

• Enhancement to the GSM and TDMA standards

• Utlilises:– 8PSK Modulation– Possible 1.6 MHz carrier under IS-136– 8 Channel Coding Schemes– Multiple Timeslots (similar frame structure to GSM)– TDMA

• Data rates up to 384kbps (typically less)


4th Generation...



Locator Slide

Multiple Access Explained • Imagine you are in a cocktail party…

• Now imagine you are trying to talk to somebody

• If you are trying to listen to somebody you need to be able to pick out their speech from everybody else’s speech.

• Everybody is using the same medium to talk - the air in the room

CDMA Mobile Technology Overview

Terminology Explanation• This is Multiple Access

– Many conversations/channels share the same medium

• There are a number of different Multiple Access (MA) strategies you can try:

– Time Division Multiple Access (TDMA)– Frequency Division Multiple Access (FDMA)– Code Division Multiple Access (CDMA)


TDMA at the Party• We divide time into a number of timeslots

• Everybody takes turns to speak within a timeslot

• Once everybody has spoken we go back to the start of the list and begin again - this is a frame

• This ensures that two conversations/channels don’t get confused.

• Conversation/Channel separation is provided in time.

• Bit of a problem if people speak too late or too early…– We may need guard periods between timeslots when nobody speaks

• People need to know when to speak…– We need signalling to tell people their timeslot


TDMA

freq

uen

cy

time

User 1 User 1

Timeslot Period Frame Period

Idealised TDMA (with no guard periods)

Available Frequency Band


FDMA at the cocktail party• We divide the available frequency band into a number of frequency channels of the

same channel bandwidth

• People speak continuously at different frequencies/pitches, and use earpieces to filter out frequencies they’re not interested in.

• Again this ensures that two conversations don’t get confused.

• Conversation/Channel separation provided in frequency.

• Bit of a problem as the filters aren’t perfect…– We may need guard bands between channels where nobody speaks

• People need to know the frequency of the conversation…– We need signalling to tell people their frequency channel


FDMA at the cocktail party• FDMA is difficult to illustrate in terms of speech as everyone’s voice is at

a similar frequency.

• FDMA is the oldest form of multiple access technique.

• You have used FDM when you tune into a radio channel.

• Radio channels advertise their frequency. E.g. 95.1 MHz Capital.


FDMA

freq

uen

cy

time

User 1

Frame Period (we may still need frames/timeslots for signalling)

Channel Bandwidth

Idealised FDMA (with no guard bands)


FDMA and TDMA compared• FDMA can be used for digital or analogue systems

• TDMA is realistically a digital technology

• TDMA reduces distortion interference effects (inter-modulation) by allowing only one user access to the system at a time.

• Digital signals are easier to buffer and re-process and therefore cope better with a hostile radio channel.


FDMA/TDMA

• Of course we could also be clever and use a combination of TDMA and FDMA…like in GSM

• This is commonly referred to as simply TDMA


CDMA Spreading•Essentially Spreading involves changing the symbol rate on the air interface

Identical codes

Tx Bit Stream

P

f

Code Chip Stream

Spreading

P

f

Channel

Air Interface Chip Stream

P

f

Code Chip Stream

Despreading

P

f

Rx Bit Stream

P

f


Spreading and Despreading

Rx Bit Stream


Tx Bit Stream1

-1

Code Chip Stream

XSpreading

Code Chip StreamXDespreading


Spreading and Dispreading with code Y


Tx Bit Stream1

-1

Code Chip Stream

XSpreading

XDespreadingCode Chip Stream Y

Rx Bit Stream


Spreading in noise

• The gain due to Despreading of the signal over wideband noise is the Processing Gain

Signal

P

f

Spreading Code

Tx SignalP

f

Rx Signal (= Tx Signal + Noise)

fP

Channel

Wideband Noise/Interference

P

f

Spreading Code Signal

P

f


Spreading in noise (time domain)

• Here, the message is recovered with a SNR of -6 dB. The spreading code is at a rate 8 times greater than the data.


Run exe

CDMA in Cellular• Cellular systems have multipath

propagation with variable delay

• Channels from the same transmitter are no longer perfectly orthogonal

• i.e Channelisation codes are no longer perfectly synchronised

• Downlink Channels on the same cell will interfere with each other

• An ‘orthogonality factor’ (0.6 in urban macrocells typically)

– The orthogonality factor gives the percentage of interference that is rejected


A Channelised TransmitterChannel 1 Bit Stream

Channel 2 Bit Stream

Channel 3 Bit Stream

Pulse Shaping and Modulation

c1

c2

c3

s1

In a Base Station, channels are first spread and channelised using the channelisation codes, then combined and finally scrambled together.

Each base station will be allocated one of 512 primary scrambling codes.


Types of Code• Summarising:

– Channelisation Codes•Are used to separate channels from a single cell or terminal

– Scrambling Codes •Are used to separate cells and terminals from each other rather than purely channels

• Different base stations will use the same spreading codes with separation being provided by the use of different scrambling codes.

S1

S2

S3

C1 C2 C3

C1 C2 C3

C1 C2 C3


Pilot Channels• Pilot channels are used in the cell selection process (i.e. “best

server” means “strongest pilot”)• Pilots contain no baseband information - no ‘bits’.• The pilot is spread by the all 1’s channelisation code.

– Effectively the pilot is the scrambling code

• The required pilot channel SNR is referred to as Ec/Io (EcIo)• Pilots allow channel estimation to be carried out.• The result of channel estimation is used to programme the Rake

receiver.


Soft Handover• Soft Handover is where more then one cell is in communication with a

terminal• The cells in communication with the terminal are known as an active set• The best serving cell is known as the primary cell - and maintains the

primary channel• Other channels are known as handover channels• The gain associated with soft handover is known as the macrodiversity

gain– This occurs due to the uncorrelated nature of fast fading between cells and

the variation in slow fading between cells– Note that slow fading is not entirely uncorrelated for different cells


Hard Handover (e.g.GSM)

Handover Hysteresis

Margin

RX_Level

Direction of Travel

Cell A Cell B

Distance

In hard handover the mobile is only ever instantaneously connected to a single cell


Soft Handover• During soft handover more than one cell

is in communication with the mobile.

MS


Soft Handover (e.g. in cdmaOne)Active set = 1 = 2 = 1

Pilot Ec/Io

T_ADDT_DROP

Cell A Cell A and Cell B Cell B

Direction of Travel

Add Time Delay Drop Time Delay

In soft handover the mobile may be instantaneously connected to more than one cell

Distance


More CDMA at the Cocktail Party - Cell Breathing

• The higher the noise at a party the louder you have to speak • You get to a point where you can’t shout louder and can’t have a conversation

where you are standing• The further away you are to the listener the louder you have to speak• If it is noisy only people standing close together can have a conversation• As it gets noisy the area that can be covered by a conversation decreases• Conversely the quieter it is then the area covered by a conversation can be

larger• This is called Cell Breathing and occurs in mobile CDMA networks


Cell Breathing• An increase in traffic results in an increase in interference• Mobiles at the extremities of cells may be pushed out of the cells effective

coverage area due to decreased Eb/No

• This effect may occur over the course of 24 hours due to changes in traffic demand over peak hours

6am Noon 9pm


Cell Breathing :- “good” or “bad” ?

.

• Cell Breathing is integral to WCDMA cellular radio systems.

• Its disadvantage is that it leads to the creation of gaps in the network coverage.

• Its advantage is that it maximises capacity when it is demanded.

• The amount of cell breathing can be controlled by limiting the Noise Rise in the admission algorithm.

• It cannot, however be eliminated.


Cell Breathing

.

Very rough rule of thumb.

Area shrinkage (%) =

Coverage with 3 dB Noise Rise

Coverage with 10 dB Noise Rise

Unloaded Coverage

5.17101100

NR


More CDMA at the Cocktail Party - Power Control

• If somebody is shouting louder than they need, it increases the overall noise

• This is inefficient, as it reduces the number of people who can have conversations

• We need to speak as quietly as possible to maximise the number of simultaneous conversations.

• This is called Power Control in mobile networks

• In CDMA networks it is very important that this power control is efficient

– We use fast power control with a much quicker feedback loop than in TDMA networks



Locator Slide

User Equipment

UMTS Terrestrial

Radio Access Network

Core Network

UU IU

UE UTRAN CN

UMTS Architecture Overview

UMTS High Level ArchitectureNew

Major Network Elements in UMTS

UU IU

UE UTRAN CN

CU

IUb

IUr

USIM

ME

Node B

Node B

Node B

Node B

RNC

RNC

MSC/VLR

SGSN GGSN

GMSC

HLR

PLMN, PSTN, ISDN

Internet, X25

Packet Network

Mobile Equipment

UMTS SIM

Radio Network

Controller

Radio Network

Controller

Serving GSN

Gateway GSN

Gateway MSC

Mobile Switching

Centre

Home Location Register

Iu-ps

Iu-cs

IUb


General UE Architecture

UU

UE

CU

USIM

ME

Mobile Equipment

UMTS SIM

UTRANTerminal

Equipment


Elements of the UE• Mobile Equipment

– The radio terminal used for radio communication over the Uu interface

• UMTS Subscriber Identity Module– The smartcard that holds the subscriber

identity, authentication and encryption keys etc

• Additionally one can define a Terminal Equipment item that connects to the UE– This carries the application specific user

interface


General Core Network Architecture

IU

CN

MSC/VLR

SGSN GGSN

GMSC

HLR

Serving GSN

Gateway GSN

Gateway MSC

Mobile Switching

Centre

Home Location Register

Other SGSN

Other MSC

UTRAN

UTRAN

External Circuit

Switched Networks

External Packet Switched Networks

Iu-cs

Iu-ps

Gs

GnGn

Gr Gc

DD

Gi

FF


Functions of the CN• Switching

• Service Provision

• Transmission of user traffic between UTRAN(s) and/or fixed network

• Mobility Management

• Operations, Administration and Maintenance


General UTRAN Architecture

UU IU

UE

UTRAN

IUb

IUr

Node B

Node B

Node B

Node B

RNC

RNC

Radio Network Controller

Radio Network Controller

Iu-ps

Iu-cs

IUb

CN (MSC)

CN (SGSN)


UTRAN• UTRAN is the UMTS Terrestrial Radio Access

Network

• The functions of UTRAN are:

– Provision of Radio Coverage

– Security and privacy

– Handover

– Radio resource management and control


Elements of UTRAN• Radio Network Controller

– Owns and controls radio resources in its domain (BSC in GSM)– Service Access point for all services that UTRAN provides for the CN

• Node B– Acts as the radio basestation (BTS in GSM)

– Converts the data flow between the Iub and Uu interfaces


Radio Network Subsystem (RNS)• A Radio Network Subsystem consists

of:– A single RNC– One or more Node B’s– Cells belonging to Node B’s

RNC

Node B

Cell

Cell

Cell

Node B

Cell

Cell

Cell

Iur

Iu

Uu


Radio Network Controller (RNC)

• Responsible for the use and integrity of the radio resources

• Responsible for the handover decisions that require signalling to the UE

• Provides a combining/splitting function to support macrodiversity between different Node Bs

RNC

Node B

Cell

Cell

Cell

Node B

Cell

Cell

Cell

Iur

Iu

Uu


Node B

• Logical node responsible for radio transmission / reception in one or more cells to/from the UE

• Dual mode Node B can support FDD and TDD mode

RNC

Node B

Cell

Cell

Cell

Node B

Cell

Cell

Cell

Iur

Iu

Uu


Cell

• A cell is an area of radio coverage serviced by one or more carriers RNC

Node B

Cell

Cell

Cell

Node B

Cell

Cell

Cell

Iur

Iu

Uu


Major Interfaces in UMTS• There are four major new interfaces

defined in UMTS – Iu

•The interface between UTRAN and the CN

– Iur

•The Interface between different RNCs

– Iub

•The interface between the Node B and the RNC

– Uu

•The air interface

RNC

Node-B

RNC

UE

CN

Uu

Iu

Iub

Iur


Iub

• The Iub is the interface between the RNC and the Node-B

• The Node B effectively performs a relay function between the Iub and the Uu

• Thus the Iub needs to carry:– Layer 2+ signalling between the UE and the UTRAN– Signalling directly to the Node B

• To control radio resource allocation• General control of the Node-B• O&M Functionality


Iur

• The Iur is the interface between two RNCs

• Thus the Iur needs to support:– Basic Inter RNC Mobility– Dedicated Channel Traffic– Common Channel Traffic– Global Resource Management


Iu

• The Iu is the interface between the Core Network and the UTRAN

• There are two instances of the Iu:

– The Iu-ps connecting UTRAN to the Packet

Switched Network

– The Iu-cs connecting UTRAN to the Circuit

Switched Network


Handover in UMTS• There are three basic types of handover

– Intra frequency handovers• Handovers between 2 UMTS codes at the same

frequency• These can be soft handovers

– Inter frequency handovers• Handovers between 2 UMTS carriers at different

frequencies• These are hard handovers

– Inter system handovers• Handovers between UMTS and GSM carriers• These are hard handovers


Handover Sets in UMTS• Active Set

– Cells forming a soft handover connection to the mobile

• Candidate Set

• Neighbour Set– Those cells which are continuously monitored

but do not yet qualify for the Active Set


Macrodiversity between Node B’s• If an active set consists of two

connections to cells parented to different Node Bs then the combining of the two channels occurs at the RNC

• This is known as a soft handover

• This doubles the transmission ‘cost’ of the call

RNC

Node B

Cell

Cell

Cell

Node B

Cell

Cell

Cell

Iur

Iu

Uu


Macrodiversity between Cells on the Same Node B

• If an active set consists of two connections to cells parented to the same Node B

– combining of the two channels occurs at the Node B

• This is known as a softer handover• This has no transmission

implication • But does have capacity

implications, if cells are collocated.

RNC

Node B

Cell

Cell

Cell

Node B

Cell

Cell

Cell

Iur

Iu

Uu


= 2Cell A and Cell C

= 2Cell A and Cell B

Handover Decisions in UMTS

Pilot Ec/Io

Direction of Travel

Window_DROP

Drop Time Delay

Window_ADD

Add Time Delay Replace Time Delay

Window_REPLACE

Active set = 1Cell A

A Active

B Active

C Active



Locator Slide

Contents and Session Aims• This session aims to explain the

protocols and operation of the air interface

– To give an overview of the UMTS specific operation of the air interface

UMTS Air Interface

•Overview of the Air Interface

Role of the Air Interface• To provide a number of bearer or physical channels

– supports data transfer over the radio path.

• To provide control channels

– to manage the cell

• To provide a number of traffic channels

– at an acceptable error performance and at various rates

• To provide signalling channels

– for call set up, etc.

UMTS Air Interface

Role of the Air Interface• In providing all of this we must also:

• Ensure an efficient use of the available spectrum

• Minimise interference to other cells and services

• Minimise the use of power, particularly from the mobile

• Provide synchronisation

UMTS Air Interface

UMTS FDD Air Interface Overview

Parameter ValueMultiple Access Scheme Direct Sequence CDMADuplexing Method FDDChip Rate 3.84 McpsCarrier Spacing 5 MHzCarrier Spacing Raster 200 kHzFrame Length 10 msSlots per Frame 15Inter-cell Synchronisation NoneSpreading factor Variable (4-512)User Data Rate 8->384 kbps

UMTS Air Interface

Multiple Access Scheme• UMTS FDD mode makes use of CDMA

• In the case of UMTS this is commonly referred to as Wideband CDMA - WCDMA

• However there are elements of FDMA and TDMA in UMTS

– Common channels for paging (TDMA)

– Packet access share codes between UEs (TDMA)

– Multiple carriers are used per operator (FDMA)

UMTS Air Interface

Duplexing Method

• UMTS FDD mode makes use of Frequency Division Duplexing

– The UL and DL Channels are carried on separate carriers

• In the case of UMTS in Europe:– The Uplink band is between 1.92 and

1.98GHz– The Downlink band is between 2.11

and 2.17GHz – The Uplink/Downlink Separation is

190MHz1.8GHz

2.05GHz

1.9GHz

1.95GHz

2GHz

1.85GHz

2.1GHz

2.15GHz

2.2GHz

190MHz2.17GHz

2.11GHz

1.98GHz

1.92GHz

UL DL

UMTS Air Interface

Chip Rate• The chiprate used in UMTS FDD mode is 3.84Mcps

• This leads to a carrier bandwidth of approximately 5MHz

• This chip rate was chosen because it:– Could be generated simply from existing GSM clock rates– Provided a similar bandwidth to cdma2000 to allow shared use of filters

etc in UEs

UMTS Air Interface

Carrier Spacing and Carrier Spacing Raster

• The nominal carrier spacing for UMTS is 5MHz

• This was chosen to comply with the American market, where spectrum has been awarded in 5MHz blocks

• It is possible to move the centre frequency of the carrier on a 200kHz raster

5MHz

200kHz

UMTS Air Interface

Adjacent Channel Interference• Adjacent channel interference may have a significant impact on UMTS

capacity

• Required attenuation (by standards)– adjacent carrier 33dB– 2nd adjacent carrier 43dB

UMTS Air Interface

Inter Cell Synchronisation• Cells in a UMTS network are not synchronised in time with each

other.

• This is a significant difference from cdmaOne which is synchronised.

• This removes the need for tight synchronisation between the base stations

• There is no need for GPS receivers at sites

– This makes implementation of picocells and their integration with the network simpler as satellite Line of Sight is not required

UMTS Air Interface

Spreading Factor and User Data Rates

• UMTS has been designed to provide flexibility to allow the user to use multiple services, some of which we cannot foresee at the moment

• Rather than having a fixed bit rate and spreading factor, each of the channels on the user interface has a range of bit rates that can be used

• This makes the channels more complicated than for GSM…but certainly more flexible

UMTS Air Interface

Power Control• Power control commands are either Power Up or

Power Down.• Step size is usually 1 dB.• It is intended to compensate for fast and slow

fading.• Fast fading results from multi-path propagation

resulting in signal strength gradients of up to approximately 1 dB per centimetre in space.

• Mobile speeds faster than approximately 15 m/s may cause problems with power control.

UMTS Air Interface

Power Control – Power Rise• If the mobile reacts to Power

Control commands, it is usual for the average power to increase.

• This increases the level of interference experienced by neighbouring cells

• The difference between the average power level on a fading and non-fading channel is known as the Power Rise.

-5

0

5

10

15

20

25

Mobile Tx Pwr Average Non-fading

•Power Rise

Power Control – Soft Handover• A mobile near the edge of a cell will be causing almost as

much Noise Rise on the neighbouring cell as it is on the serving cell.

•Serving Cell•Neighbouring Cell

Power Control – Soft Handover• Establishing a second connection from the neighbouring cell

provides advantages on the uplink.• Tx power on the uplink is reduced by:• Macro-diversity: Eb/N0 estimate is passed to RNC that

selects best connection.• Lower incidence of power up commands results in lower

Power Rise.

RNC

Power Control – Soft(er) Handover• If both active cells are on the same site, the handover is

called “softer” handover.• In this case, the benefits are greater as the two signals are

combined using a maximal combiner.• A maximal combiner is also used in the mobile.

Thank You

Date post:	06-May-2015
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