10_UTRAN-RRM_ws11

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UTRAN Radio Resource Management

Introduction

Handover Control

Soft/Softer Handover

Inter Frequency Handover

Power Control

Closed Loop Power Control Open Loop Power Control

Interference Management

Load Control

Call Admission Control

Congestion Control

Packet Data Transmission

Packet Data Control

Dynamic Scheduling

BTS 1

BTS 3

BTS 2UE

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UMTS Networks 2Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011

References

H. Holma, A. Toskala (Ed.), WCDMA for UMTS, Wiley, 5th edition, Wiley, 2010

Walke, Althoff, Seidenberg: UMTS Ein Kurs. J. Schlembach Fachverlag, 2002

H. Kaaranen, et.al., UMTS Networks: Architecture, Mobility and Services,Wiley, 2001. (see chapter 4)

A. Viterbi: CDMA: Principles of Spread Spectrum Communications, AddisonWesley, 1995.

J. Laiho, A. Wacker, T. Novosad (ed.): Radio Network Planning andOptimisation for UMTS, Wiley, 2001

T. Ojanper, R. Prasad, Wideband CDMA for Third Generation MobileCommunication, Artech House, 1998.

R. Prasad, W. Mohr, W. Konhuser, Third Generation Mobile Communications

Systems, Artech House, March 2000.

3GPP standards:

TS 25.214: Physical Layer Procedures (esp. power control)

TR 25.922: Radio Resource Management Strategies

TR 25.942: RF System Scenarios

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Efficient use of limited radio resources (spectrum, power, code space)

Minimizing interference Flexibility regarding services (Quality of Service, user behaviour)

Simple algorithms requiring small signalling overhead only

Stability and overload protection

Self adaptive in varying environments

Allow interoperability in multi-vendor environments

Radio Resource Managementalgorithms control theefficient use of resourceswith respect tointerdependent objectives:

cell coverage cell capacity quality of service

RRM High-Level Requirements

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RRM Components

Radio Resource Management

MediumAccessControl

Packet DataControl

LoadControl

HandoverControl

PowerControl

Core Network/ other RNCs

Physical layer

typically inRNC

typically inNodeB

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Handover Control: Basics

General: mechanism of changing a cell or base station during a call or session

Handover in UMTS:

UE may have active radio links to more than one Node B

Mobile-assisted & network-based handover in UMTS:

UE reports measurements to UTRAN if reporting criteria (which are set bythe UTRAN) are met

UTRAN then decides to dynamically add or delete radio links dependingon the measurement results

Types of Handover:

Soft/Softer Handover (dedicated channels)

Hard Handover (shared channels)

Inter Frequency (Hard) Handover

Inter System Handover (e.g. UMTS-GSM)

Cell selection/re-selection (inactive or idle)

All handover types require heavy support from the UMTS network infrastructure!

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Macro Diversity & Soft Handover (Wrap-Up)

Downlink: combining in the mobile stationUplink: combining in the base station and/or radio network controller

NodeB 1NodeB 2

UE

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Soft/ Softer Handover

In soft/softer handover the UE maintains active radio links to more than oneNode B

Combination of the signals from multiple active radio links is necessary

Soft Handover

The mobile is connected to (at least) two cells belonging to different NodeBs

In uplink, the signals are combined in the RNC,e.g. by means ofselection combining using CRC

Softer Handover

The mobile is connected to two sectors within one NodeB More efficient combining in the uplink is possible like

maximum ratio combining (MRC) in the NodeB instead of RNC

Note:

In uplink no additional signal is transmitted, while in downlink each new linkcauses interference to other users, therefore:

Uplink: HO general increase performance

Downlink: Trade-off

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Soft and Softer Handover in Practice

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Soft Handover Control

Measurement quantity, e.g.EC/I0 on CPICH

Relative thresholds add &drop for adding & dropping

Preservation time Tlink toavoid ping-pong effects

Event triggered measurementreporting to decreasesignalling load

MeasurementQuantity

Link to 1 Link to 1 & 2 Link to 2 time

CPICH 1

CPICH 2

drop

NodeB 1 NodeB 2

add Tlink

UE

soft handoverarea

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Soft Handover Simulation Results

Soft handover significantly improves the performance, but

0%

5%

10%

15%

20%

25%

5 15 25 35 45 55

Offered Traffic [Erlang per site]

Ou

tage

Pro

ba

bility

(Bloc

king

an

dDropp

ing

)

1 link

max 2 SHO links

max 4 SHO links

max 6 SHO links

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Soft Handover Simulation Results II

the overhead due to simultaneous connections becomes higher!

0

0,5

1

1,5

2

1 2 4 6

Max . Active Se t S ize

M

ean

Num

berofActive

Links

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Handover f1

f2 always neededbetween layers

Hierarchical cell structure (HCS)

Macro Micro Macro

f1

f2

f1

Handover f1

f2 neededsometimes at hot spot

Hot-spot

f1

f2f1 f1

Hot spot

Inter-Frequency Handover

Hard handover

Inter-frequency measurements of target cell needed in both scenarios

Mobile-assisted handover (MAHO)

slotted (compressed) mode for inter-frequency measurements to find suitabletarget cell

also supports GSM system measurements Database assisted handover (DAHO)

no measurements performed on other frequencies or systems

use cell mapping information stored in data base to identify the target cell

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Power Control: Basics

Controls the setting of the transmit power in order to:

Keep the QoS within the required limits, e.g. data rate, delay and BER

Minimise interference, i.e. the overall power consumption

Power control handles:

Path Loss (Near-Far-Problem), Shadowing (Log-Normal-Fading) andFast Fading (Rayleigh-, Ricean-Fading)

Environment (delay spread, UE speed, ) which implies differentperformance of the de-interleaver and decoder

Uplink: per mobile

Downlink: per physical channel

Three types of power control: Inner loop power control Outer loop power control (SIR-target adjustment)

Open loop power control (power allocation)

Downlink power overload control to protect amplifier Gain Clipping (GC) Aggregated Overload Control (AOC)

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Near-Far Problem: Spreading sequences are not orthogonal

(multi-user interference) Near mobile dominate Signal to interference ratio is lower for far

mobiles and performance degrades

The problem can be resolved throughdynamic power control to equalize allreceived power levels

AND/OR

By means of joint multi-user detection

Near-Far Problem Power Control

NodeB

UE 1

UE 2

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Closed Loop Power Control

Closed loop power control is used on channels, which are established in bothdirections, such as DCH

There are two parts Inner Loop Power Control (ILPC): receiver generates up/ down

commands to incrementally adjust the senders transmit power

Outer Loop Power Control (OLPC): readjusts the target settings of theILPC to cope with different fading performance

NodeB

UE

control command: Up/Down

Example: Uplink Closed Loop Power Control

Inner Loop(1500 Hz)

SIR > SIRtarget ?

Outer Loop( 100Hz)

target adjustmentBLERtarget

RNC

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Impact of Power Control

-8

-6

-4

-2

0

2

4

6

0 0.2 0.4 0.6 0.8 1

power/fading[dB]

time [sec]

2

3

4

5

6

7

8

0 0.2 0.4 0.6 0.8 1

Eb

/N0

[dB]

speed = 3 km/h

Example: UMTS Closed Loop Power Control in the slow fading channel

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Power Control Performance

5

5.5

6

6.5

7

7.5

0 20 40 60 80 100 120

Velocity [km/h]

RequiredULSIR

[dB]

PedA

VehA

SIR requirement strongly depends on the environment (due to differentfast fading conditions Jakes models)

outer loop power control needed to adapt SIR

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Open Loop Power Control

Open loop power control is used on channels that cannot apply closedloop power control, e.g. RACH, FACH

The transmitter power is determined on the basis of a path loss estimatefrom the received power measure of the opposite direction

To avoid excessive interference, probes with incremental power stepsuntil a response is obtained: power ramping

NodeB

UE

Open Loop Power Control on RACH

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CDMA Overload

CDMA systems tend tobecome unstable More traffic increases the

interference More interference requires

higher power More power increases the

interference Methods are required to limit

the system load Restrict the access to the

system Overcome overload

situations

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Interference in CDMA Networks

Frequency reuse factor is one

CDMA is subject to high multiple access interference

Soft capacity: CDMA capacity (e.g. number of users) determined by theinterference is soft

Handling of interference is the main challenge in designing CDMA networks

Interference Problem

Inter-Symbolinterferenz (ISI) Delayed components from thesame user signal interfere due tomultipath propagation

Multiple Acces Interference MAI Different user signals interferedependent on the access scheme

Intra-Cell Interference Interference caused by the users

belonging to same cell

Inter-Cell Interferenz Interference caused by the usersbelonging to neighbor cells.

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Cell Breathing

CDMA systems: cell size depends on the actual loading

Additional traffic will cause more interference

If the interference becomes too strong, users at the cell edge can nomore communicate with the basestation

CDMA interference management

Restriction of the users access necessary

Cell breathing makes network planning difficult

Example: cell brething with increasing traffic

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Cell Breathing (contd.)

coveragelow load

coveragemedium load

coveragehigh load

Coverage depending on load: load causes interference, which reduces thearea where a SIR sufficient for communication can be provided

yellow area: connection may drop or blocked

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Coverage vs. Capacity

Capacity depends on:

QoS of the users (data rate, error performance (bit-error-rate))

User behaviour (activity)

Interference (out of cell)

Number of carriers/ sectors

Coverage (service area) depends on:

Interference (intra- & inter-cell) + noise

Pathloss (propagation conditions)

QoS of the users (data rate, error performance (bit-error-rate))

Thus, trade-off between capacity and coverage

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Coverage vs. Capacity

Downlink limits capacity while uplink limits coverage Downlink depends more on the load (users share total transmit BS power)

0 20 40 60 80 100 120 140 160 180 2000

0.5

1

1.5

2

2.5

3

3.5

Downlink

Uplink

Erlangs (2% GOS)

M

aximum

cellradius(km)

13kbps circuit switched service capacity versus maximum cell radius

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Example of Coverage and Best Server Map

Application: RF engineering (cell layout)

Legend: violet indicates high signal level, yellowindicates low level

coveragemap

Application: HO decision

Legend: color indicates cell with bestCPICH in area

bestse

rvermap

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Load Control: Basics

Main objective:

Avoid overload situations by controlling system load

Monitor and controls radio resources of users Call Admission Control (CAC)

Admit or deny new users, new radio access bearers or new radiolinks

Avoid overload situations, e.g. by means ofblocking the request

Decisions are based on interference and resource measurements Congestion Control (ConC)

Monitor, detect and handle overload situations with the alreadyconnected users

Bring the system back to a stable state, e.g. by means ofdropping an existing call

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Resource Consumption

Service/BLER-dependentresource consumption

Uplink example: Service I: Voice

Rb = 12.2kbps, Eb/Nt= 5dBI = 0.99%

Service II: Data

Rb = 144kbps, Eb/Nt= 3.1dBII = 7.11%

In downlink there is additionaldependency on the locationof the user Cell center low

consumption Cell edge high

consumption

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Admission/ Congestion Control

Basic algorithm Admission control is triggered

when load thr_CAC New users are blocked Existing users are not

affected as long as load

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