Radio Resource Management - Connection Mobility Control eNodeB Source Handover StrategiesRadio Resource Management (RRM) is an E-UTRAN Node B (eNodeB) application level function that ensures the efficient use of available radio resources. RRM manages the assignment, re-assignment and release of radio resources taking into account single and multi-cell aspects.

Connection Mobility Control (CMC) is a sub-function of RRM. It is required in the idle as well as connected mode. In idle mode there will be cell selection and reselection. The connected mode will involve Handover procedures triggered on the basis of the outcome of CMC Algorithms.

This paper primarily focuses on the connected mode of User Equipment (UE) at the Source eNodeB. We discuss a strategy for CMC, including a determination of Handover and the criteria for Target cell determination for Handover.

White Paper

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About the AuthorMukesh Kumar Das

Mukesh Kumar Das is working as an IT Analyst with Tata Consultancy Services (TCS) in the Telecom Next-Gen R&D group for the past one year. He is involved in Initiatives and Proof of Concept( PoC )for LTE technology. He has used his experience and understanding of CMC to write this paper.

Late Saugata Mukherjee was working as a Consultant with TCS in a Large R&D Telecom account and with the Telecom Next-Gen R&D group. He had significant experience in wireless access and core technologies.

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Table of Contents

Introduction 4

– Radio Resource Management (RRM) Overview 4

– eNodeB Architecture 4

- Radio Admission Control (RAC) 5

- Radio Bearer Control (RBC) 5

- Connection Mobility Control (CMC) 5

- Dynamic Allocation of resources to UE’s in both uplink downlink (DRA) 5

– Inter-Cell Interference co-ordination (ICIC) 5

– Load Balancing (LB) 5

Connected Mode CMC - Key Inputs 6

– UE Power Measurement Configuration 6

UE Power and Quality Measurement Report 6

eNodeB Physical Layer Measurements 7

Neighbor & Network Loads 7

Phases of Handover Preparation 7

CMC Strategies in Phases of Handover 7

Phase I: Handover Required Determinations 7

Phase II - Handover Preparation 9

Phase III: Handover Target Determination 10

- Phase IIIa - X2 Handover 10

- Phase IIIb - S1 Handover or Release Flowchart 11

Conclusion 12

Acknowledgement 12

References 12

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IntroductionRadio Resource Management (RRM) Overview

Radio Resource Management (RRM) is an eNodeB application level function that ensures the efficient use of available radio resources. RRM manages the assignment, re-assignment and release of radio resources with consideration of single and multi-cell aspects.

There are various functions for Radio Resource Management. Selected key functions are described in the following sections.

eNodeB Architecture

eNodeB is the hardware that is connected to a mobile phone network that communicates directly with mobile handsets (User Equipment), like a base transceiver station (BTS) in GSM networks.

The radio link specific protocols, including radio link control (RLC) and medium access control (MAC) protocols are terminated in the eNodeB. The packet data convergence protocol (PDCP), which is responsible for header compression and ciphering, is also located in the eNodeB. In the control plane, the eNodeB uses the radio resource control (RRC) protocol for application level Radio Resource Control.

The primary goal of RRM is to control the use of radio resources in the system. This must be accomplished while the Quality of Service (QoS ) requirements of the individual radio bearers are met and the overall used radio resources on the system level are minimized. The objective of RRM is to satisfy the service requirements at the smallest possible cost for the system, ensuring optimized use of spectrum.

Long Term Evolution (LTE) RRM includes a variety of algorithms that provide services,such as, power control, allocation of resources, mobility control, and QoS management, to ensure the best use of available radio resources.

Figure 1 depicts the eNodeB (eNodeB) architecture.

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Figure 1: eNodeB Architecture

Radio Admission Control (RAC)

Radio Admission Control (RAC) admits or rejects establishment requests for new radio bearers.

The goal of RAC is to ensure high radio resource utilization by accepting radio bearer requests as long as radio resources are available. This simultaneously ensures proper QoS for in-progress sessions - by rejecting radio bearer requests when they cannot be accommodated.

Radio Bearer Control (RBC)

Radio Bearer Control (RBC) involves the establishment, maintenance and release of radio bearers.RBC is also concerned with the maintenance of radio bearers of in-progress sessions at the change of the radio resource situation, for example, due to mobility.

RBC is involved in the release of radio resources associated with radio bearers including at session termination and handover.

Connection Mobility Control (CMC)

Connection mobility control (CMC), overlooks the management of radio resources in connection with idle or connected mode mobility.

In this paper we are concerned with UE in connected mode that is, Handover.

Handover decisions may be based on UE and eNodeB measurements. In addition, Handover decisions may take other inputs, including neighbor cell load, traffic distribution, transport and hardware resources and operator defined policies, into account.

Dynamic Allocation of resources to UE’s in both uplink and downlink (DRA)

Dynamic Resource Allocation (DRA) or Packet Scheduling (PS) allocates and de-allocates resources including buffer and processing resources and resource blocks) to user and control plane packets.

Inter-Cell Interference co-ordination (ICIC)

Inter-Cell Interference Coordination manages radio resource blocks to keep inter-cell interference under control based on feedback from multiple cells.

Load Balancing (LB)

Load balancing (LB) handles uneven distribution of the traffic load over multiple cells. The purpose of LB is to influence the load distribution in such a manner that radio resources remain highly utilized and the QoS of in-progress sessions are maintained, and call dropping probabilities are kept sufficiently small.

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Application RRM as a service function on the eNodeB can be considered in the following logical realization form:

Figure 2: Application RRM Logical View

UE Power Measurement Configuration

Uplink power control is a key radio resource management function. It is typically used to maximize the power of the desired received signals while limiting the generated interference.

Measurement commands are used by Evolved – Universal Terrestrial Radio Access Network (E-UTRAN) in order to direct the UE to start measurements, modify measurements or stop measurements.

UE Power and Quality Measurement Report

Measurements of the following parameters are done at the UE and reported to eNodeB:Reference Signal Received Power (RSRP)Reference Signal Received Quality (RSRQ)

For the current paper it is considered that the setting for periodic reporting for strongest cells is set by the eNodeB.

Connected Mode CMC - Key Inputs

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Figure 2: Application RRM Logical View

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eNodeB Physical Layer Measurements

Measurements are reported by Physical Layer for Intra-Frequency, Inter-Frequency Intra-Radio Access Technology (RAT) cases to the RRM. These include parameters such as DL RS TX Power.

Neighbor & Network Loads

Neighboring eNodeB Cell Loads are reported over the X2 interface. Serving MME loads are reported over the S1 interfaces.

When a UE is handed over to another Cell it needs to follow a set of sub-steps or phases. The eNodeB that is currently serving the UE is called Source and the eNodeB that will be handed over is referred to as the Target.

Prior to the current situation, where UE which is active from Cell A (Source) and notifies Cell B (Target) that a handover is required, Cell A’s CMC (also termed as the Source eNodeB) has the following responsibilities:

Handover Required DeterminationHandover PreparationHandover Target Determination

The next section describes a sample strategy for each of the above preparation phases in CMC on the Source eNodeB.

Phase I: Handover Required Determinations

This strategy is a part of the CMC function and will trigger a Handover if required based on UE’s Reference Signal Received Power (RSRP)(Reference Signal Received Quality )(RSRQ)Measurements and Thresholds configured for the Serving Cell.

Phases of Handover Preparation

CMC Strategies In Phases of Handover

Figure 3: LTE Intra-RAT Handover

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Figure 4 describes a sample approach for determining Handover.

Figure 4: Handover Required Determination approach

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Phase II - Handover Preparation

The LTE network has several connection paths for the Target cell. This function maps the path to be followed. In addition to this, it initially determines whether Network assistance is required.

Dynamic Candidate List is considered a key consideration parameter.

Figure 5: Handover Preparation Flowchart

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Phase III: Handover Target Determination

This strategy is a part of CMC function that selects the target cell. If we consider a scenario where a Handover to Cell C is also possible, this phase will determine that Cell B needs to be chosen as Target.

When the Handover occurs between Cell A and Cell B through a direct link it is termed as a X2 handover. If Core Network assistance is needed, it is called a S1 Handover.

This strategy considers that the flexibility of keeping or releasing the bearers in case Handover is deemed mandatory (possibly due to system aspects).

The following figures explain an approach for each of the Target Determination Variants.

Phase IIIa - X2 Handover

Figure 6: X2 Handover Execution Flowchart

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Phase IIIb - S1 Handover or Release Flowchart

Figure 7: S1 Handover Execution or Release

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References[1] 3GPP TS 36.101 UE Radio Transmission and Reception http://www.3gpp.org .[2] 3GPP TS 36.331 RRC Protocol Specification http://www.3gpp.org .[3] 3GPP TS 36.300 http://www.3gpp.org[4] 3GPP TS 36.322. E-UTRA radio link control (RLC) protocol specification. ftp://ftp.3gpp.org/Specs/archive/36_series/36.322/[5] 3GPP TS 36.321. E-UTRA medium access control (MAC) protocol specification. ftp://ftp.3gpp.org/Specs/archive/36_series/36.321/[6] 3GPP TS 25.913. Requirements for evolved UTRA (E-UTRA) and evolved UTRAN (E-UTRAN). ftp://ftp.3gpp.org/Specs/archive/25_series/25.913/[7] http://lte-epc.blogspot.in/2012/03/rrm-functions.html

Conclusion

Acknowledgement

Connection Mobility Control (CMC) is vital for eNodeB operation, session continuity and keeping network performance optimized while users are on the move.

Several studies have shown that users have a significantly high probability (even 50%-60%) of being on cell edges at an instant . Without correct CMC implementation, we could experience call drops, ping pongs and so on. The result is a negative impact on the performance of the eNodeB and the network. Furthermore, effective utilization of Spectrum is hindered.

We consider an optimized CMC approach that can be adapted to engineering parameters, because accommodating field results is necessary to ensure that system resources are effectively utilized.

I (Mukesh Kumar Das) would like to express my gratitude to all those who made it a possibility to complete this white paper. I would also like to thank my colleagues from the Next-Gen R&D Group for all their help and support during this activity.

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other applicable laws, and could result in criminal or civil penalties. Copyright © 2012 Tata Consultancy Services Limited TCS

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