UTRAN Radio Resource Management...Cellular Communication Networks Andreas Mitschele-Thiel, Jens...

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

Cellular Communication Networks 2 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015

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 Management algorithms control the efficient use of resources with respect to interdependent objectives: – cell coverage – cell capacity – quality of service

RRM – High-Level Requirements


RRM – Components

Radio Resource Management

Medium Access Control

Packet Data

Control

Load

Control

Handover

Control

Power Control

Core Network/ other RNCs

Physical layer

typically in RNC

typically in NodeB


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 by the UTRAN) are met

UTRAN then decides to dynamically add or delete radio links depending on 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!


Macro Diversity & Soft Handover (Wrap-Up)

Downlink: combining in the mobile station

Uplink: combining in the base station and/or radio network controller

NodeB 1 NodeB 2

UE


Soft/ Softer Handover

In soft/softer handover the UE maintains active radio links to more than one Node 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 of selection 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 link causes interference to other users, therefore:

Uplink: HO general increase performance

Downlink: Trade-off


Soft and Softer Handover in Practice


Soft Handover Control

• Measurement quantity, e.g.

EC/I0 on CPICH

• Relative thresholds dadd &

ddrop for adding & dropping

• Preservation time Tlink to

avoid “ping-pong” effects

• Event triggered measurement

reporting to decrease

signalling load

Measurement

Quantity

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

CPICH 1

CPICH 2

ddrop

NodeB 1 NodeB 2

dadd Tlink

UE

soft handover

area


Soft Handover – Simulation Results

Soft handover significantly improves the performance, but …

0%

5%

10%

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20%

25%

5 15 25 35 45 55

Offered Traffic [Erlang per site]

Ou

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e P

rob

ab

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y

(Blo

ckin

g a

nd

Dro

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)

1 link

max 2 SHO links

max 4 SHO links

max 6 SHO links


Soft Handover – Simulation Results (contd.)

… the overhead due to simultaneous connections becomes higher!

0

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1 2 4 6

Max. Active Set Size

Mean

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mb

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of

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Handover f1 f2 always needed

between layers

Hierarchical cell structure (HCS)

Macro Micro Macro

f1

f2

f1

Handover f1 f2 needed

sometimes at hot spot

Hot-spot

f1

f2

f1 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 suitable target 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


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) and Fast Fading (Rayleigh-, Ricean-Fading)

Environment (delay spread, UE speed, …) which implies different performance 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)


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 through

dynamic power control to equalize all

received power levels

AND/OR

By means of joint multi-user detection

Near-Far Problem – Power Control (Wrap-Up)

NodeB

UE 1

UE 2


Closed Loop Power Control

Closed loop power control is used on channels, which are established in both directions, 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 the ILPC 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 adjustment

BLERtarget

RNC


Impact of Power Control

-8

-6

-4

-2

0

2

4

6

0 0.2 0.4 0.6 0.8 1

pow

er/

fadin

g [

dB]

time [sec]

2

3

4

5

6

7

8

0 0.2 0.4 0.6 0.8 1

Eb/N

0 [

dB]

speed = 3 km/h

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


Power Control Performance

5

5.5

6

6.5

7

7.5

0 20 40 60 80 100 120

Velocity [km/h]

Req

uir

ed

UL

SIR

[d

B]

PedA

VehA

SIR requirement strongly depends on the environment (due to different fast fading conditions – Jakes models)

outer loop power control needed to adapt target SIR


Open Loop Power Control

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

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

To avoid excessive interference, probes with incremental power steps until a response is obtained: “power ramping”

NodeB

UE

Open Loop Power Control on RACH


CDMA Overload

• CDMA systems tend to

become 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


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 no more 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


Cell Breathing (contd.)

coverage low load

coverage medium load

coverage high load

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

shadowed area: connection maybe lost


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 the interference is soft

Handling of interference is the main challenge in designing CDMA networks

Interference Problem

Inter-Symbol Interference (ISI) Delayed components from the same user signal interfere due to multipath propagation

Multiple Acces Interference – MAI Different user signals interfere dependent on the access scheme

Intra-Cell Interference Interference caused by the users belonging to same cell

Inter-Cell Interference Interference caused by the users belonging to neighbor cells.


Interference in CDMA Networks (contd.)

Uplink

Multi access channel: access of the same medium but with different channel conditions

Intra-cell interference: sum of unsynchronized signals

Inter-cell interference: sum of signals not assigned to BS

Downlink

Broadcast channel: signals are summed up and then broadcast

Intra-cell interference: sum signal (synchronized) with same channel condition

Inter-cell interference: signals from BS not assigned


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


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)

Maxim

um

cell

radiu

s (

km

)

13kbps circuit switched service capacity versus maximum cell radius


Example of Coverage and Best Server Map

Application: RF engineering (cell layout)

Legend: violet indicates high signal level, yellow indicates low level

co

ve

rag

e m

ap

Application: HO decision

Legend: color indicates cell with best CPICH in area

be

st

se

rve

r m

ap


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 radio links

Avoid overload situations, e.g. by means of blocking the request

Decisions are based on interference and resource measurements

Congestion Control (ConC)

Monitor, detect and handle overload situations with the already connected users

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


Resource Consumption

• Service/BLER-dependent

resource consumption

• Uplink example:

– Service I: Voice

Rb = 12.2kbps, Eb/Nt = 5dB

aI = 0.99%

– Service II: Data

Rb = 144kbps, Eb/Nt = 3.1dB

aII = 7.11%

• In downlink there is additional

dependency on the location

of the user

– Cell center ® low

consumption

– Cell edge ® high

consumption

a


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 < thr_ConC

• Congestion Control is

triggered when

load thr_ConC

– Reduce consumption of one

or several users

– Simple action: drop the user

– Repeat until

load < thr_ConC


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50%

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thr_CAC = 50%thr_CAC = 75%thr_CAC = 90%

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20%

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Dro

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Tradeoff between blocking and dropping

Example: 64k per user, urban

Call Admission Control: Simulation Results I


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90%

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Cell

Lo

ad

ing


Cell load depending on CAC threshold

Example: 64k per user, urban

Call Admission Control: Simulation Results II


Packet Data Control: Channel Switching

Flexibility of packet services

Asymmetrical data rates

Very low to very high data rates

Control information/user information

Efficient transmission making good use of CDMA characteristics

Dedicated channel (DCH)

Minimise transmission power by closed-loop power control

Independence between uplink and downlink capacity

Common channel

Random access in the uplink (RACH)

Dynamic scheduling in the downlink (FACH)

Adaptive channel usage depending on traffic characteristics

Infrequent or short packets Common channel (Cell_FACH)

Frequent or large packets Dedicated channel (Cell_DCH)

No packet transmission UE “stand by” modus (URA_PCH)


CELL_DCH CELL_FACH CELL_DCH

Page Download Time Reading Time

DCH Active Time “Chatty Applications”

Channel Switching – Example

Example: Web service

Chatty apps.: keep alive message, stock tickers, etc. (e.g. 100 bytes every 15 sec)

Second stage: when no activity in CELL_FACH then switch to URA_PCH


Power Control:

– Balances user received quality (BLER, SIR)

– Users at cell center get less share of BTS

transmit power assigned than at cell edge

– Occurrence of power overload

Rate Adaptation:

– Transmit power is proportional to data rate

– Users at cell edge get lower data rate assigned

than at cell center

– Reduces also power overload

On DCH combination of power control and

rate adaptation

– Rate assignment at begin of a transmission

based on load and user location

– Rate adaptation when ongoing transmission

according to power consumption and overload

– Based on RRC-signaling (time horizon:

100msec … 10sec)

NodeB

UE 1

UE 2

low data rate

area

high data rate

area

Power Control vs. Rate Adaptation


Rate Adaptation Performance

Rate adaptation significantly improves the RRM performance on DCH.

UMTS_urban, 50 kByte

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Cell Throughput [kBit/sec]

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384k

64k

adaptive

UMTS_urban, 50 kByte

0

1

2

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6

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8

200 300 400 500 600

Cell Throughput [kBit/sec]

Mean

Dela

y [

sec]

384k

64k

adaptive


Dynamic Scheduling

• “Statistical multiplexing” of data

packets from different data

flows on one shared medium,

e.g. on DSCH or HSDPA

• Scheduling with time-horizon of

2msec … 1sec

• Optimised usage of radio

resources

• Exploitation of the short-term

variations on the radio

channels (opportunistic

scheduling)

• Can provide certain degree of

QoS

NodeB

Sample Flow

Flow #1

Flow #3

UE 3

UE 2

UE 1

Flow #2


References

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

H. Kaaranen, et.al., “UMTS Networks: Architecture, Mobility and Services”,

2nd edition, Wiley, 2001.

A. Viterbi: “CDMA: Principles of Spread Spectrum Communications”, Addison Wesley, 1995.

J. Laiho, A. Wacker, T. Novosad (ed.): “Radio Network Planning and Optimisation for UMTS“, 2nd edition, Wiley, 2005

T. Ojanperä, R. Prasad, “Wideband CDMA for Third Generation Mobile

Communication”, Artech House, 1998.

R. Prasad, W. Mohr, W. Konhäuser, “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“

UTRAN Radio Resource Management...Cellular Communication Networks Andreas Mitschele-Thiel, Jens...

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