S-72_3260_lecture_8.3260_2012_l8

Lecture 8 – Radio Resource

Management in LTE System S-72.3260 Radio Resource Management Methods Course

Fall 2012

Topics

• Short introduction to LTE and its architecture

• RRM in LTE – Scheduling

– Link Adaptation

– Power Control

– Handover

– Inter-Cell Interference Coordination (ICIC) & Enhanced ICIC

– Load Balancing

– MIMO

– Multi-RAT

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S-72.3260 Radio Resource Management Methods

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References

This lecture material has been reproduced solely from following references:

(1) “Architecture and protocol support for radio resource management (RRM)”, Gábor Fodor et al, 2009 (Chapter 4 of the book: Long Term Evolution edited by Borko Furht and Syed A. Ahson)

(2) LTE for UMTS, edited by Harri Holma and Antti Toskala, 2009

(3) LTE, The UMTS Long Term Evolution, Second Edition, Edited by Stefania Sesia, Issam Toufik and Matthew Baker, 2011

(4) “MIMO and COMP in LTE-Advanced”, NTT DoCoMo Technical Journal, Vol.12, No. 2

(5) “Introducing LTE-Advanced”, Application Note, Agilent Technologies

(6) 3GPP TS 25.913. Requirements for evolved UTRA (E-UTRA) and evolved UTRAN (E-UTRAN)

(7) 3GPP TS 36.211. E-UTRA physical channels and modulation

(8) 3GPP TS 36.213. E-UTRA physical layer procedures

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Introduction

• We discuss RRM in Long Term Evolution or LTE system

• RRM covers all functions related to assignmnet and the sharing of radio resources among the users in a wireless communication system

• Radio link conditions change in wireless system and there is a need to adapt the transmission and reception parameters to the actual link conditions

• So far you have seen that the type of multiple access technology (whether FDMA or TDMA or CDMA,...) used plays an important role in required resource control or sharing or assignments.

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Why LTE?

The set targets for the LTE when it was initiated:

• Significantly increased peak data rate e.g. 100 Mbps (downlink) and 50

Mbps (uplink)

• Increase "cell edge bitrate" whilst maintaining same site locations as

deployed today

• Significantly improved spectrum efficiency ( e.g. 2-4 x Release 6)

• Possibility for a Radio-access network latency below 10 ms

• Significantly reduced C-plane latency to less than 100 ms

• Scalable bandwidth: 5, 10, 20 and possibly 15 MHz

• Support for inter-working with existing 3G systems and non-3GPP

specified systems

• And many more… Please see 3GPP TR 25.913 V9.0.0 for more details

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Evolution of wireless standards 1990-2012

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Evolution of wireless standards

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Courtesy of ”HSPA to LTE-Advanced”, RYSAVY Research, 3GPP America, September 2009

Evolution of UMTS specifications

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LTE Architecture

• The radio interface (multiple access) in LTE system is based on OFDM technology. -> Many RRM functions here depends on OFDM

• LTE system promises on high data rates, low latency and high spectrum efficiency: – Partially achieved because radio resource control functions are

designed close to the radio interface, which makes instantaneous radio link quality information readily available

• Let’s take a look at LTE Radio Access Network (RAN) Architecture briefly

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LTE Architecture

• Two nodes:

– Basestations or eNodeB (eNB)

– Serving Gateway(S-GW) in user plane and Mobility Management Entity (MME) in

the control plane.

• Both S-GW and MME belong to core network, called as Evolved Packet Core

(EPC)

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LTE Architecture

• S-GW: performs generic packet processing such as packet filtering and classification -> similar to router functions

• MME: maintains UE (user equipment) context such as established bearer, security context, locations of the UE; Nonaccess startum (NAS) signaling protocol

• Radio resources are solely owned and controlled by eNodeB

• There are two planes, user plane and control plane between UE and the network

• In the control plane in eNB side: – Radio Link Control / Medium Access Control (RLC/MAC): Short time-scale radio

resource control toward the UE, e.g., signaling of assigned resources and tranport formats

– Packet Data Convergence Protocol (PDCP): header compression & ciphering

– Radio Resource Control (RRC): To execute the longer time-scale resource control toward the UE,e.g., QoS-based radio bearer establishment, handover control,...

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LTE Architecture

• Services are provided to the UE in terms of evolved packet system

(EPS) bearers.

• End-to-end EPS can be divided into two:

– Radio bearer between the UE and eNB -> determines the QoS treatment on the

radio interface

– Access bearer between eNB and S-GW-> determines the QoS that the packets

get on the transport network

• The primary goal of RRM is to control the use of radio resources so

that:

– QoS requirements are met

– Overall usage of radio resources on the system level are minimized

=> To meet the service requirements at the lowest possible of system costs

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Radio Resources in LTE

• The radio interface of LTE is based on the OFDM technology. Radio

resources appear as one common shared channel.

• Let’s take a look at resource grid of the uplink and downlink shared

channels

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Radio Resources in LTE • The schedular located in eNB controls and assigns of time-frequency

blocks to UEs in an orthogonal manner

• The smallest unit in the resource grid is called ”Resource Element”

(RE) -> it corresponds to one subcarrier during one symbol duration.

• 7 symbols (with the normal CP) makes one slot with the length of 0.5

ms and 12 subcarriers in one slot is called ”Resource Block” (RB)

– Slot can include 6 symbols in case of extended Cyclic Prefix (CP)

• 2 consecutive time slots make a subframe

• 10 subframes create a frame

#0 #1 #2 #3 #19#18

One radio frame, Tf = 307200Ts = 10 ms

One slot, Tslot = 15360Ts = 0.5 ms

One subframe

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OFDM and SC-FDMA

• In downlink of LTE system, the modulation scheme is OFDM

• In uplink because of acheiving better peak-to-average power ratio,

Single-Carrier FDMA (SC-FDMA) is used

• It is besically a discrete Fourier Transform- (DFT-) precoded

modulation

Figure: Courtesy of

Wikipedia: Single-Carrier

FDMA

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• Because of SC-FDMA in UL, the allocation of RBs per UE has to be on consecutive RBs in frequency.

• LTE supports various MIMO schemes from transmit diversity to spatial multiplexing => therefore the virtual space of radio resources extended another dimension corresponding to the antenna port, in addition to time and frequency.

– This means that a time-frequency resource grid is available per antenna port.

– For LTE system (Rel. 8 & 9):

• Downlink supports up to 4 transmit antenna but uplink supports no multi-stream transmission except multiuser MIMO

– For LTE-Advanced (LTE Rel. 10)

• Downlink supports up to 8 transmit antenna and uplink supports up to 4 multi-stream transmission

• The RRM in LTE can be formulated as the solution for optimal allocation of time, frequency and antenna port to UEs with the required QoS and minimum use of radio resources

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• In LTE, Link Adaptation (LA) selects the transport format, i.e., modulation and coding scheme (MCS) and allocates power to assigned resoutces

• Scheduler chooses the time-frequency based on CSI for a UE, then LA selects MCS and allocates the power to the selected time-frequency resources.

• Antennas and their corresponding precoding matrices are selected separately from time-frequency assignments mechanism.

• We discuss following topics (in terms of RRM not limited to these): – Dynamic packet assignment – scheduling

– Link adaptation and power allocation

– Handover control

– Intercell interference coordination (ICIC) & eICIC

– Load balancing

– MIMO configuration control

– Multi-RAT

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Dynamic Packet Assignment - Scheduling

• eNB controls the assignment of resources on shared channels: PUSCH (physical uplink shared channel) in uplink and PDSCH (physical downlink shared channel) in downlink.

• Both DL and UL schedulers are located in eNB. The reason to place also the UL scheduler in eNB is to maintain the intracell orthogonality.

– Essentially both UL and DL schedulers function similarly except for some available channel states information (CSI) and buffer status information

• The scheduler assigns RBs in pairs and to signal which RBs are assigned to a particular UE, PDCCH (physical downlink control channel) is used.

• Procedure: UE recognize its identity on the PDCCH (blind decoding) => Find its control information, decodes it and identifies the DL RBs that carry data for that UE (on PDSCH), also UL RBs on PUSCH that have been granted to send UL data

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Dynamic Packet Assignment - Scheduling

• PDCCH is carried in the first one to three OFDM symbols in each subframe.

• Sometimes in order to limit the control signaling overhead associated with the dynamic signaling of the RB allocation to the UE, semipersistent scheduling is applied:

– It allows to allocate resources ahead in time, typically in a periodic manner. It can be useful for applications which produce predictable amounts of data perioidically (e.g., VoIP)

• In DL, the scheduler can assign freely any RBs for the same UE rather than in UL this does not hold

– Only consecutive RBs can be assigned to a UE to maintain the single carrier property

– In UL the possibility of utilizing frequency selective scheduling is limited

• The scheduler selects UEs and RBs based on: Channel quality and QoS requirements of the radio bearers along with traffics in Tx buffers

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Channel Quality Information

• In DL, eNB should get the channel quality

reports from the UE in order to make

channel-dependent scheduling.

• In LTE, Channel Quality Indicator (CQI) is

for this purpose

• In DL:

– Reference Signals (RS) are sent in each RB so

that UE can measure the channel quality.

– In each RB=12X7 REs (in case of normal CP), 4

REs are dedicated for RS (in single antenna

scenario) -> RSs are also used for channel

estimation

– CQI are sent either on PUCCH (if UE has no UL

allocation) or on the PUSCH (if UE has a valid UL

allocation).

Cell-specific reference signal (CRS)

arrangement for normal CP and one

antenna port; Courtesy from 3GPP

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Channel Quality Information & Scheduler

• In UL:

– RSs are sent similar to DL, eNB perform the measurements

– Channel quality can be measured only on RBs that UE is actually transmitting ->

because typically UE doesn’t use the whole BW -> But RBs must be allocated

upon choosing the best ones in the full BW, therefore,

• For measuring the channel quality on all RBs, channel Sounding Reference Signal (SRS)

is transmitted from the UE. The eNB instructs UE to send SRS for one symbol duration

within a subframe occupying the entire BW.

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Link Adaptation (LA) • After choosing RBs by scheduler for allocation to a UE, the MCS and

power has to be specified -> Link Adaptation (LA) is in charge of this.

• MCS selection for both UL and DL is done by eNB

– In DL, the selection done based on CQI report from the UE; It is signalled on

PDCCH-> UE decodes its data received on PDSCH according to MCS depecified

on the PDCCH.

– In UL, based on the measured link quality at the eNB and the buffer status report

from the UE-> eNB decodes the data from UE based on MCS assigned to it

CQI Values - reproduced

from 3GPP

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Power Control • Power allocation depends on selected MCS which is based on a target

SINR-> The ultimate goal of power control is to set the transmit power levels such that a target SINR is achieved.

• In DL, eNB is in charge and basically there is no standard as such for the power control mechanism

– Simplest form: eNB distribute the power uniformly over the RBs

– More optimum way: eNB uses water-filling power allocation -> higher power is assigned to subcarriers whose fading and interference are in favorable conditions

• In UL, eNB sends the power control commands to UE -> Standardized – The UE transmit power equation is as follows:

min(Pmax, )

Pmax is the maximum power allowed; P0 is a UE specific parameter; α is a cell specific path loss compensation factor; PL is the downlink path loss caculated in the UE based on reference power; ΔTF is a MCS-specific parameter; f() is a function signaled via RRC; ΔTPC is the actual transmit power command signaled in each scheduling assignment; M is the number of RBs assigned to the UE measurements;

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Handover

• Handover maintains the radio link coverage of an UE when it moves from the coverage of one cell to another.

• UE is always connected to the cell with the best average path gain.

• In LTE like in GSM, only hard handovers exist, i.e., UE is connected only to one cell at a time -> no soft handover like in WCDMA. Because

– LTE is not as sensitive as WCDMA to intracell interference

– No needs to maintain diversity with soft handover in LTE as there are other means such as MIMO.

– Link adaptation and channel-dependent scheduling functions are fast in LTE

• Handover must be fast to combat the rapid change of link quality, and to create the best user experience, i.e., no hiccup in the call or downloading/streaming data

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Handover sequence

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Handover Failure Recovery

• LTE provides an efficient recovery mechanism for handover failures

• If UE loses the radio link, it re-selects to a suitable cell and initiates a connection re-establishment

• If the UE context is available at the selected eNB, the UE context can be recovered. The time for re-etablishment is very good which creates still a good user experience

• If the context does not exist, UE re-establishes the connectivity and goes from idle to active state and gets the context. This takes longer time than the previous one.

• If eNB wants to reduce the probablity of failure for the handover, it can prepare multiple target eNBs and later after handover to the best eNB is done, cancels the other eNBs.

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ICIC • ICIC: Inter-Cell Interference Coordination -> mechanisms to

manage radio resources in order to keep the intercell interference under control.

• It is a multi-cell RRM approach which takes into account resources and loads situation in multiple cells

• In reuse-1 system the collision might happen: UEs in neighboring cells (uplink) may cause interference to eNB (below Fig) or eNBs may cause interference

to served UE.

• Reuse-1: all RBs

should be used

by each cell.

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ICIC

• To avoid or reduce the collision between two cells: avoid scheduling

some of the RBs in the some of the cells -> Reuse-n (n>1)/fractional

reuse or by coordinating the allocation of RBs in neighboring cells

• LTE is reuse-1 system, therefore reuse-n or fractional reuse has not

been adopted -> because not to underutilize the radio resources

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ICIC

• Coordination of RB allocation between the cells can be done either in time or in frequency. LTE release 8, rely on frequency domain sharing between cells and adjustment of tramsmit power

– Because time domain coordination on the scheduling time scale (1ms) is hard to achieve due to, e.g., delay and generated signaling load on the interface between eNBs (X2 interface).

• ICIC methods in LTE rel 8 can be categorized as: – Reacive schemes: Based on the measurement from past, monitoring the performance

and if there is too high interference, appropriate actions to be taken place, e.g., adjusting tranmit power or packet scheduling

– Proactive schemes: eNB informs its neighboring eNBs how it plans to schedule its users in the future; Neighboring cells take this info into account. The proactive schemes are handled via signaling through the X2 interface between eNBs.

• In Rel 8, ICIC is primarily for improving the performance of shared data channels (PDSCH & PUSCH) and there is no explicit ICIC techniques for common channels such as PDCCH and PUCCH

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ICIC

• In DL: – Proactive DL ICIC is facilitaed by standardized Relative Narrowband Transmit Power

(RNTP)

– RNTP is an indicator per RB signaled to neighboring eNBs, indicating the max anticipated DL transmit power level per RB. So, neigboring cells know about power level, and different power power patterns can be used in those cells to improve the overall SINR conditions for UEs.

• In UL: – Proactive ICIC technique is based on High Interference Indicator (HII)

• The fundamental idea in here is that the serving cell informs its neighboring eNBs at which RBs it intends to schedule high interference users in the future. The neighboring eNBs should aim at scheduling low interference UEs at those RBs to avoid scheduling of cell-edge users at the same RBs between two neighboring cells.

– There is also reactive ICIC based on Overload Indicator (OI).

• eNB measures the uplink interference +noise power, creates OI reports, signals over the X2 to neighboring cells.

• OI is based on the interference from other cells and not from the serving cell

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eICIC in LTE-Advanced

• ICIC techniques in LTE system (Rel 8 & 9) can be summarized as frequency-domain scheduling, power setting, increasing robustness (e.g., by beamforming or interference cancellation for DATA channels)

• The main motivation for Enhanced-ICIC (eICIC) is to mitigate the interference in CONTROL channels:

– It’s essentially time-domain-based ICIC in Release 10 of 3GPP specifications-> LTE-A

– The overall objective is to mute certaion subframes of one layer of cells so that the interference becomes less in the other layer

– These muted subframes are called Almost Blank Subframes (ABS):

• Subframes with reduced downlink transmission power and/or activity.

• It is ALMOST blank because it must contain Reference Signals, Synchronization Signals, Paging Channels and Broadcast channels due to backward compatibility issues for Realease 8/9 (LTE) UEs. Although these are transmitted but with much less energy than normal subframes, to reduce the interference.

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eICIC in LTE-Advanced

• Essentially in heteregenous network where co-channel

deployments can be categorized into two scenrios as macro-pico

and macro-femto.

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Load Balancing (LB)

• The task of LB is to balance the the traffic load over multiple inter-frequency and inter-RAT (Radio Access Technology) cells.

• With LB, loads are distributed over multiple frequency and RAT layers so that radio resources remain highly utilized and the QoS is maintained with the minimum probability for any hiccups (call drps etc).

• LB algorithms may result in handover or cell re-selection decisions because of distributing the traffic from highly loaded cells to under-utilized cells.

• The inter-RAT was purposely designed to be independent of the handover in order that it can be triggered at any time by a requesting eNB even when there is no mobility event.

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MIMO Configuraion Control

• Radio resources in LTE can be managed in three domains: time, frequency and antenna port.

• There are basically three ways to get advantages of MIMO: Spatial Multiplexing, Diversity Schemes and Beamforming (They have been addressed in MIMO Lecture)

• Just to recap: – Spatial multiplexing: Different data streams are sent from different antennas; Can

be employed when the channel rank is greater than 1 and SINR is high enough -> because, transmit power has to be shared between streams sent from the different antennas, and the SINR per antenna will be lower than the original one.

– Diversity schemes: Some kind of transformation on the data prior to transmission. The transformation is usually called as ”precoding”. Diversity schemes are used when the SINR is poor. -> To improve the SINR by exploiting diversity gains.

• Example: Alamouti codes

• In LTE precoding based on SFBC (Space-Frequency Block Codes) is used (not STBC, i.e., not in time domain).

– Beamforming: a single symbol is multiplied by different weight factors and transmitted on different antenna elements

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MIMO Configuraion Control • Spatial Multiplexing: The precoding involves a set of precoder matrices called as a

codebook -> depending on channel feedback the appropriate precoder can be used:

{W(1), W(2),...,W(K)}

Y=W.x

– In LTE specification different codebooks have been defined

– The precoding can be further combined with cyclic delay diversity (CDD):

Y= D(k)W(i)x where

k is the frequency domain index of the resource element on which the transmission

is mapped and δ is the delay shift.

– The idea of CDD is to increase phase shift

on antenna port so that it possibly becomes

match to the actual channel and results

in an increased SINR

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MIMO – Transmit Diversity &

Beamforming • Diversity Scheme: SFBC

• Beamforming: Different phase adjustment by multiplying one

symbol with different weight factors and sent on different antenna

elements -> Signal can be steered in specific direction:

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*

1

*

2

21

xx

xxY

x

w

w

w

w

4

3

2

1

Y

MIMO Techniques in LTE and LTE-A

• LTE System (Rel. 8-9) – Up to 4 streams in downlink and 1 stream in UL

– Closed-Loop SU-MIMO and transmit diversity

– Open-Loop SU-MIMO and transmit diversity

– Cyclic delay diversity (CDD)

– Adaptive Beamforming

– MU-MIMO (on uplink)

• LTE-Advanced (Rel. 10) – Backward compatible to LTE

– High-dimensional SU-MIMO (up to 8 × 8)

– Enhanced MU-MIMO

– Coordinated Multipoint (CoMP)

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MIMO Transmission in LTE (SU-MIMO) • LTE Downlink: The SU-MIMO scheme is applied to the physical downlink

shared channel (PDSCH); Support both closed-loop (codebook based) and

open-loop operation;

– Open-loop: when reliable PMI feedback is unavailable at the eNB.

– Closed-loop: UE needs to feedback the channel quality indicator (CQI), precoding matrix

indicator (PMI), and rank indicator (RI)

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PMI and RI

• Precoding Matrix Indicator (PMI): Indicates the preferred codebook

element:

• Rank indicator (RI) : It indicates the number of supported layers

– For open-loop transmit diversity: RI = 1

– For open-loop spatial multiplexing: RI > 1

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RRM in Multi-RAT Networks

• General principle for inter-RAT, that is, intra-3GPP cellular radio technologies: UE is connected only to one Radio Access Technology (RAT).

– When UE is using a service that can be best served by LTE system, all other services used by this UE should use also use LTE

• Triggering on inter-RAT measurements and handover decisions are made by the RAN that serves the UE at that moment

• The target RAN gives guidance to UE on how to make the radio access:

– Other information such as redio resource configuration, target cell information, all the required identities come from target RAN

• For handover in this case, target RAN:

– Trigger inter-RAT measurements

– Make the comparison between different radio access technologies

– Make a handover decision and command the UE

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RRM in Multi-RAT Networks

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