05 CT82355EN01GLA1 Radio Resource Management

CT82355EN01GLA1 ©2014 Nokia Solutions and Networks. All rights reserved.

Radio Resource Management

LTE Air Interface Course

2 CT82355EN01GLA1 ©2014 Nokia Solutions and Networks. All rights reserved.

Nokia Solutions and Networks Academy

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At the end of this module, you will be able to:

• List the main tasks of RRM in LTE

• Describe the functionalities of the eNB related to RRM

• Underline the radio admission control in LTE

• Explain the scheduling functionality

• Distinguish the RRM functionalities related with the control of the radio

link quality (i.e. adaptive modulation and coding, modulation and

transport block size, outer link quality control, power control)

• Describe the handover process in LTE

• Introduce the discontinuous transmission functionality

•

Module Objectives



RRM in eNodeB

Radio Admission Control

Scheduling

Link Quality Control

– Adaptive Modulation and Coding

– Modulation and Transport Block Size

– Outer Link Quality Control

– Power Control

Handover Control

Discontinuous Transmission (DTX)


Scope of RRM

Scope of RRM:

• Management and optimized utilization of the (scarce) radio resources:

• Provision for each service/bearer/user an adequate QoS (if applicable)

• Increasing the overall radio network capacity and optimizing quality

• RRM is located in eNodeB

• See next slide

LTE-UE

Evolved Node B

(eNB)

X2

LTE-Uu

eNB


Evolved Node B (eNB)

Inter-cell RRM: HO, load balancing between cells

Radio Bearer Control: setup , modifications and

release of Radio Resources

Connection Mgt. Control: UE State Management,

MME-UE Connection

Radio Admission Control

eNode B Meas. collection and evaluation

Dynamic Resource Allocation (Scheduler)

eNB Functions

IP Header Compression/ de-compression

Access Layer Security: ciphering and integrity

protection on the radio interface

MME Selection at Attach of the UE

User Data Routing to the S-GW/ P-GW

Transmission of Paging Msg coming from MME

Transmission of Broadcast Info (e.g. System info,

MBMS)

• Only network element defined as part of

eUTRAN

• Replaces the old Node B / RNC

combination from 3G.

• Provides all radio management functions

• To enable efficient inter-cell radio

management for cells not attached to the

same eNB, there is a inter-eNB interface

X2 specified. It will allow to coordinate

inter-eNB handovers without direct

involvement of EPC during this process.



RRM in eNodeB

Radio Admision Control

Scheduling





– Power Control

Handover Control



Radio Admission Control ( RAC)

Objective: To admit or to reject the requests for establishment of Radio Bearers (RB) on a cell basis

• Based on number of RRC connections and number of active users per cell

– Non QoS aware

• Operator configures both max. number of established RRC connections and max. number of active users per cell by O&M threshold

– RRC connection is established when the SRBs have been admitted and successfully configured

– UE is considered as active when Radio bearer is established

– Upper bound for maximum number of supported connections depends on the BB configuration of eNB

• RL T: All RRC connection setup request are admitted by default to avoid RAC complexity

• Improvement in RL10: Possibility of giving priority to the HO cases based on the HO cause

At reception of the HO request message the RAC decides in an ‘all-or-nothing’ manner on the

admission / rejection of the resources used by the UE in the source cell (prior to HO). 'All-or-nothing'

manner means that either both SRB AND (logical) DRB are admitted or the UE is rejected. RLT all

SRB are admitted



RRM in eNodeB


Scheduling





– Power Control

Handover Control



LTE vs. R99 Scheduling

NodeB Rel. 99 eNodeB LTE

Fast pipe is shared among UEs Dedicated pipe for every UE

Channel quality

time/frequency/space


LTE vs. R99 Scheduling, cont.

Deployment of shared channels offers the possibility for scheduling. In this way

information on varying channel conditions can be exploited to increase the overall

throughput.

Fast scheduling in time (1 ms) and frequency domain reduces latency and improves

peak rate. Adaptive Modulation and Coding leads to higher data rates and optimizes

spectral efficiency Hybrid ARQ leads to higher efficiency in transmission and error

correction. A scheduler deploys mechanisms to determine which user(s) is(are)

served in a given transmission time interval.

Dynamic assignment of radio resources to the UE is done by taking into account

channel conditions and prioritization for the UE with the better channel conditions.

Benefit is the maximization of the Node B throughput, high peak date rates for the

UE and an efficient usage of the radio resources.

Furthermore OFDMA / MIMO allows scheduling decisions on the basis of three

dimensions: time, frequency and space.


-10

10

30

50

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

S f

r

20 dB

10 cm

1MHz

Fast Fading in Time and Frequency Domain


Fast Fading in Time and Frequency Domain, cont.

As shown in the figure the channel's decorrelation in frequency and time offers the

possibility to exploit the varying conditions.

Scheduling resource is the time-frequency grid. In detail, the basic scheduled

resource consists of a 1 ms (sub-frame, TTI) and 12 subcarriers, 180 kHz. The

efficiency of the scheduling strongly depends on the deployed algorithm.

Additionally the performance depends on the UE speed. Furthermore the gain of the

scheduling may be higher the higher the number of scheduled UE's.

The scheduling functionality is provided by the MAC layer.


Scheduler Types

A variety of scheduling strategies is available. The scheduling strategy is based on a certain metric.

Examples are:

- Round-Robin

No quality indication is taken into consideration. The resources are mainly shared in an equal manner.

- Max C/I.

The UE with the best channel conditions gets the highest priority. The cell throughput is maximized. Starvation of UEs with channels of low quality may be a disadvantage.

- Proportional Fair.

This algorithm defines priorities based on the quality and the averaged scheduled rate.

- QoS

Different strategies exist to get QoS related information integrated.

E.g. Depending on the priority of the service and/or the UE, RT/NRT service type. a scheduling weight can be introduced.

Combinations of the different types can also be applied.


Downlink Scheduler Algorithm

Evaluation of available resources (PRBs/RBGs)

for dynamic allocation on PDSCH

Resource allocation and scheduling

for common channels

DL scheduling of UEs:

Scheduling of UEs/bearers to PRBs/RBGs

Start

End

Pre-Scheduling:

Select UEs eligible for scheduling

-> Determination of Candidate Set 1

Time domain scheduling

of UEs according to simple criteria

-> Determination of Candidate Set 2

Start

End

Frequency domain scheduling

of UEs/bearers

-> PRB/RBG allocation to UEs/bearers


Downlink Scheduler Algorithm, cont. • Determine which PRBs are available (free) and can be allocated to Ues

• Allocate PRBs needed for common channels like SIB, paging, and random access procedure (RAP)

• Final allocation of UEs (bearers) onto PRB. Considering only the PRBs available after the previous steps

– Pre-Scheduling: All UEs with data available for transmission based on the buffer fill levels

– Time Domain Scheduling: Parameter MAX-#_UE_DL decides how many UEs are allocated in the TTI being scheduled

– Frequency Domain Scheduling for Candidate Set 2 UEs: Resource allocation in Frequency Domain including number and location of allocated PRBs

The scheduling is performed on cell basis. The two main functions are to decide which UE(s) shall be scheduled, the number of resources and the MCS to be applied.

Furthermore the scheduler needs to be QoS aware. There is priority given to random access responses, control data, HARQ retransmissions.

The channel quality may be taken in consideration.

In RL10 the DL exploits CQI reports to decide on frequency and time resources. In UL the scheduling decisions in RL10 are not based on quality but a random frequency allocation is deployed.


Uplink Scheduler Channel Unaware

Example of allocation in frequency domain:

• Full Allocation: All available PRBs are

assigned to the UEs scheduled per TTI

• Fractional: Not all PRBs are assigned,

still the hoping function handles the

unassigned PRB as if they were

allocated to keep the equal distribution

per TTI

a) b)


Uplink Scheduler Channel Unaware, cont.

Time domain:

• Evaluation of the #PRBs that will be assigned to UEs

• Available number of PRBs per user

– Multiple of 2, 3 or 5

• Max. # of UEs which can be scheduled per TTI time frame is restricted by an O&M parameter. RL T and RL10 limit the number to a max. of 10 UEs per TTI

Frequency Domain:

• Uses a random function to assure equal distribution of PRBs over the available frequency range ( random frequency hopping)



RRM in eNodeB


Scheduling





– Power Control

Handover Control



Link Adaptation by AMC (UL/DL)

• Motivation of link adaptation: Modify the signal transmitted to and by a particular user

according to the signal quality variation to improve the system capacity and coverage

reliability.

• If SINR is good then higher MCS can be used -> more bits per byte -> more

throughput.

• If SINR is bad then lower MCS should be use ( more robust)

• Flexi Multimode BTS performs the link adaptation for DL and UL on a TTI basis

• The selection of the modulation and the channel coding rate is based:

• Downlink data channel: CQI report from UE

• Downlink common channel (PDCCH): signaling payload, CQI report

• Uplink: BLER measurements in Flexi LTE BTS

The effective Eb/No and hence the spectral efficiency depend on BLER. However there are QoS

requirements which also have to be considered. Taking both into account leads to a target

BLER.

AMC is in use in order to tune BLER so that the target value is reached. Therefore when channel

conditions change modulation and/or coding modifications might be needed.


PDSCH – AMC Algorithm

START

Retrieve Default MCS

Dynamic AMC

active?

HARQ

retransmission?

Determine avaraged CQI

value for allocated PRBs

Use the same MCS as for

initial transmission

Determine MCS

Use Default MCS

END


PDSCH – AMC Algorithm, cont.

An initial MCS is provided by O&M (parameter INI_MCS_DL) and is set as default MCS. If DL AMC is not

activated (O&M parameter ENABLE_AMC_DL) the algorithm always uses this default MCS.

If DL AMC is activated HARQ retransmissions are handled differently from initial transmissions ( For HARQ

retransmission the same MCS has to be used as for the initial transmission).

An MCS based on CQI reporting from UE shall be determined for the PRBs assigned to the UE as indicated

by the downlink scheduler. So the mechanism has UE scope with a frequency of several TTIs based on

configurable CQI measurement intervals.

In RL T the algorithm is based on overall signaling payload (control data volume) for all users and does not

depend on the actual radio condition.

The adaptation is done on cell-basis and per TTI. In RL10 the code rate is selected for PDCCH resources

(QPSK only) based on CQI reports. These CQI reports indicate the ‘CCE (Control Channel Elements)

aggregation level’ and hence the coding rate.

The usage of PDCCH resources are based on channel condition and in addition on the availability of PDCCH

resources into account. The feature may be enabled by O&M.

For AMC of the PUSCH a UE specific slow link adaptation ( 10-100ms) is applied. The decisions are based

on BLER measurements.

AMC works independently of UL scheduler and UL power control. Interactions to UL PC and scheduler are

result driven, i.e. to keep signaling load on eNodeB internal interfaces low, MCS is reported at the start of

data transfer and only when there are changes of MCS. In case of long link adaptation updates and to avoid

low and high BLER situations, the link adaptation can act based on adjustable target BLER values:

• “Emergency Downgrade” if BLER goes above a MAX BLER threshold (poor radio conditions);

• “Fast Upgrade” if BLER goes below of a MIN BLER threshold (excellent radio conditions).



RRM in eNodeB


Scheduling





– Power Control

Handover Control



Modulation and TB Size

• Modulation and Coding Scheme (MCS) – 3GPP TS 36.211 specifies QPSK, 16QAM and 64QAM for the Physical

Downlink/Uplink Shared Channel

– Affects the amount of resources that will be used for user data

• Transport Block Set (TBS) – Number of user data bits transmitted to single user during one TTI (1ms)

– The TB occupies two resource blocks in time domain

3GPP TS 36.213 specifies tables to:

• link the MCS Index -> Modulation Order (modulation type) and TBS Index

• link the TBS Index -> Transport Block Size (TBS) for a specific number of Physical Resource Blocks (PRB)

• The exact mapping is shown in the next slides


Modulation and TB Size DL MCSs

MCS ITBS MCS_index Mod order

0-QPSK 0 0 2

1-QPSK 1 1 2

2-QPSK 2 2 2

3-QPSK 3 3 2

4-QPSK 4 4 2

5-QPSK 5 5 2

6-QPSK 6 6 2

7-QPSK 7 7 2

8-QPSK 8 8 2

9-QPSK 9 9 2

10-16QAM 9 10 4

11-16QAM 10 11 4

12-16QAM 11 12 4

13-16QAM 12 13 4

14-16QAM 13 14 4

15-16QAM 14 15 4

16-16QAM 15 16 4

17-64QAM 15 17 6

18-64QAM 16 18 6

19-64QAM 17 19 6

20-64QAM 18 20 6

21-64QAM 19 21 6

22-64QAM 20 22 6

23-64QAM 21 23 6

24-64QAM 22 24 6

25-64QAM 23 25 6

26-64QAM 24 26 6

27-64QAM 25 27 6

28-64QAM 26 28 6


Modulation and TB Size, cont.

From TS 36.213 (DL example shown here)

MCS index -> from 0 to 28 -> it is decided by the scheduler which should translate a

specific CQI in an MCS index

Modulation Order -> indicates the modulation type (QPSK, …) by indicating the

number of bits per symbol

QPSK = 2

16QAM = 4

64QAM = 6

ITBS = TBS index

The TBS Index is mapped to a specific TBS size for a specific #RBs

Uses a different table


Modulation and TB Size

Example of mapping (DL-SCH) ITBS to a TBS size for a specific number of RBs (1..12 in this

example)



RRM in eNodeB


Scheduling





– Power Control

Handover Control



Outer Link Quality Control (OLQC)

Feature: CQI Adaptation ( DL)

• Only used in DL

• Used for CQI measurement error compensation

– CQI estimation error of the UE

– CQI quantization error or

– CQI reporting error

• It adds a CQI offset to the CQI reports provided by UE. The corrected CQI report is

provided to the DL Link adaptation for further processing

• CQI offset derived from ACK/NACK feedback

Due to inaccuracies in the CQI reporting the performance may be downgraded. In

order to compensate these effects CQI adaptation is applied. The algorithm is based

on the comparison of the observed (averaged) ACK/NACK ratio with the target BLER.

A CQI offset can be derived which is added to the actual reported CQI values and so

forms the basis for the AMC algorithm.



RRM in eNodeB


Scheduling





– Power Control

Handover Control



Power Control – Principle

Very low

Low

High

Very high


Power Control – Principle, cont.

The transmission power is adapted in order to achieve the desired QoS (BLER/BER).

This adaptation is necessary since the propagation channel is subject to several conditions, which generally

vary in space and/or time, e.g. path loss, log normal fading, short term fading , UE speed

• location (outdoor, indoor, in-car) etc.

Downlink power control determines the energy per resource element (EPRE). The term resource element

energy denotes the energy prior to CP insertion. The term resource element energy also denotes the average

energy taken over all constellation points for the modulation scheme applied.

Uplink power control determines the average power over a DFT-SOFDM symbol in which the physical channel

is transmitted. Compared with UTRAN the UL power control is slower. The PUSCH and the PUCCH are

subject to a combined open and closed loop power control algorithm, i.e. to control the transmission power for

UL channels a combination of an open (input: pathloss, sysinfo and signaling) and a closed loop (TPC) method

is used.

A cell wide overload indicator (OI) and a High Interference Indicator (HII) to control UL interference are

exchanged over X2. An indication is given which PRBs an eNodeB scheduler allocates to cell edge UEs and

hence will be most sensitive to inter-cell interference.

Power control - already being applied in 2nd and 3rd generation networks - has high potential for improvement

of the performance of mobile networks.

Main benefits are:

It can bring down the interference in up- and downlink and hence enhances the capacity of the networks.

Additionally it helps to keep down the uplink-power consumption, thereby increasing the stand-by time for the

UE.

Furthermore, from the EMC (Electro Magnetic Compatibility) point of view it can improve the situation

considerably


DL Power Assignment Example

t

f

P

0 1 2 3 4 5 6

The eNodeB determines the downlink transmit energy per resource element (EPRE).

Downlink cell-specific reference-signal (RS) EPRE is constant across the downlink system bandwidth and

constant across all subframes until different cell-specific RS power information is received. The downlink RS

EPRE is given by the parameter Reference-signal-power provided by higher layers.

In cases 16QAM, 64QAM, spatial multiplex TRI>1 or multi-user MIMO the DL power is given by rA and rB


UL Power Control

1) Initial TX power level

2) SINR measurment

3) Setting new power offset4) TX power level

adjustment with the new

offset

Procedure for Slow UL Power control

UE controls the Tx power to keep the transmitted power spectral density (PSD) constant independent

of the allocated transmit bandwidth (#PRBs)

If no feedback from eNodeB ( in the PDCCH UL PC command) the UE performs open loop PC based

on path loss measurements

If feedback from eNodeB the UE corrects the PSD when receiving PC commands from eNodeB ( in the

PDCCH UL PC command)

PC commands ( up and down) based on UL quality and signal level measurements

Applied separately for PUSCH, PUCCH

Scope of UL PC is UE level ( performed separately for each UL in a cell)



RRM in eNodeB


Scheduling





– Power Control

Handover Control



Handover Types

E-UMTS micro cells

Intra-frequency HO

(intra eNB)

intra-frequency HO

(inter eNB, inter MME)

1a

interfrequency HO

other RAT

E-UMTS macro cell

intersystem HO

triggered by e.g.

- coverage of E-UMTS

- service

- load

1b

3

2

intersystem HO

triggered by other RAT


Handover Types, cont.

Handovers (HO) can be grouped into

INTRA-SYSTEM: EUTRAN EUTRAN (1):

Intra-frequency -(example 1a):

Inter-frequency -(example 1b):

INTER-SYSTEM / INTER-RAT:

EUTRAN UTRAN, GERAN .. (example 2)

GERAN, UTRAN … EUTRAN HO’s. (example 3),

Not discussed further, since this HO type is triggered by GERAN.


Handover Principles -> Lossless

– Packets are forwarded from the source to the target

-> Network-controlled

– Target cell is selected by the network, not by the UE

– Handover control in E-UTRAN (not in packet core)

-> UE-assisted

– Measurements are made and reported by the UE to the network

-> Late path switch

– Only once the handover is successful, the packet core is involved

Handover Algorithm

A Handover will be initiated by a measurement report, which is sent via the Radio Resource Control (RRC)

protocol. Upon the reception of this measurement report, the handover algorithm will decide whether a handover

should take place.

In response to the handover decision, the handover execution will be carried out using the corresponding

procedures. After the handover execution, the handover algorithm will be informed, whether the handover was

successful or not.

The Handover procedure is composed of a number of single functions:

• Measurements

• Filtering of measurements

• Reporting of measurement results

• Hard handover algorithm

• Execution of handover


Handover Procedure

= GTP signaling

S-GW + P-GW

MME

Source eNB

Target eNB

MME MME MME

= Data in radio

= Signaling in radio

= GTP tunnel

= S1 signaling

= X2 signaling

Before

handover

Handover

preparation Radio handover

Late path

switching

S-GW + P-

GW

S-GW + P-GW S-GW + P-GW

X2


Handover Preparation

UE Source Target MME GW

1. Measurement control

2. Measurement report

3. HO decision

4. HO request

5. Admission control

6. HO request ack.


Handover Preparation, cont.

1. The source eNB configures the UE measurement procedures with MEASUREMENT CONTROL

2. UE is triggered to send MEASUREMENT REPORT to the source eNB. It can be event triggered or

periodic

3. Source eNB makes handover decision based on UE report + load and service information

4. The source eNB issues a HANDOVER REQUEST to the target eNB

5. Target eNB performs admission control

6. Target eNB sends the HANDOVER REQUEST ACKNOWLEDGE to the source eNB

When the UE is in LTE_ACTIVE state, mobility handling takes place via network controlled handovers with

UE assistance. UE assistance here simply means that the UE does measurements and reports them to the

eNB to assist in the handover decision. Currently it is planed that neighbor cell measurements are based on

the UE’s cell detection capabilities rather than on a network supplied neighbor cell list.

When the source (current serving) eNB decides to start a handover of an UE to a neighbor cell in a new

(target) eNB it will contact this target eNB. This is done via the X2-AP message HANDOVER REQUEST. The

message will contain the target cell for the UE, the current serving MME and SAE GW. It is task of the target

eNB to allocate virtual capacity in the target cell via its admission control function.

If this is done the target eNB returns part of the handover message for the UE within the X2-AP message

HANDOVER REQUEST ACKNOWLEDGE. In this message also a data forwarding tunnel (TEID from target

eNB) is indicated. It allows the source eNB to forward still buffered or still arriving downlink packets to the

target eNB.


Handover Execution


7. HO command

8. Status transfer

Forward packets to target

Buffer packets from source

9. Synchronization

10. UL allocation and timing advance

11. Handover confirm


Handover Execution, cont.

7. Source eNB generates the HANDOVER COMMAND towards UE, Source eNB starts

forwarding packets to target eNB

8. Source eNB sends status information to target eNB

9. UE performs the final synchronization to target eNB and accesses the cell via RACH

procedure DL pre-synchronization is obtained during cell identification and measurements

10. Target eNB gives the uplink allocation and timing advance information

11. UE sends HANDOVER CONFIRM to target eNB, Target eNB can begin to send data to UE

The source eNB can now give the HANDOVER COMMAND (RRC) to the UE. The command

contains the configuration for the UE in the new cell and possibly already an UL/DL resource

allocation. The UE will detach from the old cell and synchronize itself to the new cell. In the

mean time the source eNB can start downlink packet forwarding via X2 interface.

Once synchronization between UE and the new cell is achieved, the UE confirms the handover

with RRC message HANDOVER CONFIRM. This will trigger a HANDOVER COMPLETE

message of S1-AP to be sent to the MME. It simply informs the MME that now a new eNB is

responsible for the UE. Thus this message will contain the IP addresses and TEIDs of the target

eNB for the S1 tunnels.


Handover Completion


12. Path switch request

13. User plane update request

14. Switch downlink path

15. User plane update response

16. Path switch request ack.

17. Release resources

18. Release resources


Handover Completion, cont.

12. Target eNB sends a PATH SWITCH message to MME to inform that the UE has changed

cell

13. MME sends a USER PLANE UPDATE REQUEST message to Serving Gateway

14. Serving Gateway switches the downlink data path to the target side

15. Serving Gateway sends a USER PLANE UPDATE RESPONSE message to MME

16. MME confirms the PATH SWITCH message with the PATH SWITCH ACK message

17. By sending RELEASE RESOURCE the target eNB informs success of handover to source

eNB and triggers the release of resources

18. Upon reception of the RELEASE RESOURCE message, the source eNB can release radio

and C-plane related resources associated to the UE context

The MME’s task is to send this information via GTP-C UPDATE BEARER REQUEST to the SAE

GW. This will switch the traffic path now completely from SAE GW to target eNB.

When the path is switched, the old eNB will get the S1-AP message RELEASE RESOURCE

which will clear down all allocated resources for the UE that is already in the new eNB.



RRM in eNodeB


Scheduling





– Power Control

Handover Control



Discontinuous Transmission in DL (DTX)

Sleep periods needed for mobiles in RRC Connected Mode to save UE battery power

Basic idea → UE is not monitoring PDCCH in some specified subframes and it sleeps:

→ ON period to be defined (UE active and monitoring PDCCH)

→ OFF period (UE sleeping not monitoring PDCCH)

The ON/OFF periods should be set-up in such way to maintain the QoS (latency) of the application

→ Example web browsing: ON period = 1 ms (1 subframe)

OFF period 100 ms (100 subframes)

DTX is configured via higher layer parameters

05 CT82355EN01GLA1 Radio Resource Management

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