gsm,gprs,cdma,3 g
TRANSCRIPT
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 and 2nd Generation Cellular Systems Overview
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 and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
GPRS• General Packet Radio Service• Enhancement to the GSM standard• Utlilises
– Multiple Timeslots– Packet Switching
• Packet Switched Data typically to rates of 56kbps
1st and 2nd Generation Cellular Systems Overview
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
1st and 2nd Generation Cellular Systems Overview
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
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)
3rd Generation Drivers and Standards
Aspects of IMT-2000 Networks
3rd Generation Drivers and Standards
The Road to 3G
HSCSD
3rd Generation Drivers and Standards
•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
3rd Generation Drivers and Standards
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
3rd Generation Drivers and Standards
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 Drivers and Standards
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)
3rd Generation Drivers and Standards
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
3rd Generation Drivers and Standards
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
3rd Generation Drivers and Standards
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)
3rd Generation Drivers and Standards
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)
3rd Generation Drivers and Standards
4th Generation...
3rd Generation Drivers and Standards
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
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)
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
TDMA
freq
uen
cy
time
User 1 User 1
Timeslot Period Frame Period
Idealised TDMA (with no guard periods)
Available Frequency Band
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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.
CDMA Mobile Technology Overview
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)
CDMA Mobile Technology Overview
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.
CDMA Mobile Technology Overview
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 Mobile Technology Overview
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
CDMA Mobile Technology Overview
Spreading and Despreading
Rx Bit Stream
Air Interface Chip Stream
Tx Bit Stream1
-1
Code Chip Stream
XSpreading
Code Chip StreamXDespreading
CDMA Mobile Technology Overview
Spreading and Dispreading with code Y
Air Interface Chip Stream
Tx Bit Stream1
-1
Code Chip Stream
XSpreading
XDespreadingCode Chip Stream Y
Rx Bit Stream
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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.
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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.
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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.
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
Soft Handover• During soft handover more than one cell
is in communication with the mobile.
MS
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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.
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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
CDMA Mobile Technology Overview
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
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
UMTS Architecture Overview
General UE Architecture
UU
UE
CU
USIM
ME
Mobile Equipment
UMTS SIM
UTRANTerminal
Equipment
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
Functions of the CN• Switching
• Service Provision
• Transmission of user traffic between UTRAN(s) and/or fixed network
• Mobility Management
• Operations, Administration and Maintenance
UMTS Architecture Overview
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)
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
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
UMTS Architecture Overview
= 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
UMTS Architecture Overview
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
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