16 Radio Resource Management aspects

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

16.0 General

The purpose of radio resource management (RRM) is to ensure the efficient use the available radio resources and to provide mechanisms that enable E-UTRAN to meet radio resource related requirements identified in clause 10 of TR 25.913 [2]. In particular, RRM in E-UTRAN provides means to manage (e.g. assign, re-assign and release) radio resources taking into account single and multi-cell aspects.

16.1 RRM functions

16.1.1 Radio Bearer Control (RBC)

The establishment, maintenance and release of Radio Bearers involve the configuration of radio resources associated with them. When setting up a radio bearer for a service, radio bearer control (RBC) takes into account the overall resource situation in E-UTRAN, the QoS requirements of in-progress sessions and the QoS requirement for the new service. RBC is also concerned with the maintenance of radio bearers of in-progress sessions at the change of the radio resource situation due to mobility or other reasons. RBC is involved in the release of radio resources associated with radio bearers at session termination, handover or at other occasions.

RBC is located in the eNB.

16.1.2 Radio Admission Control (RAC)

The task of radio admission control (RAC) is to admit or reject the establishment requests for new radio bearers. In order to do this, RAC takes into account the overall resource situation in E-UTRAN, the QoS requirements, the priority levels and the provided QoS of in-progress sessions and the QoS requirement of the new radio bearer request. The goal of RAC is to ensure high radio resource utilization (by accepting radio bearer requests as long as radio resources available) and at the same time to ensure proper QoS for in-progress sessions (by rejecting radio bearer requests when they cannot be accommodated).

RAC is located in the eNB.

16.1.3 Connection Mobility Control (CMC)

Connection mobility control (CMC) is concerned with the management of radio resources in connection with idle or connected mode mobility. In idle mode, the cell reselection algorithms are controlled by setting of parameters (thresholds and hysteresis values) that define the best cell and/or determine when the UE should select a new cell. Also, E-UTRAN broadcasts parameters that configure the UE measurement and reporting procedures. In connected mode, the mobility of radio connections has to be supported. Handover decisions may be based on UE and eNB measurements. In addition, handover decisions may take other inputs, such as neighbour cell load, traffic distribution, transport and hardware resources and Operator defined policies into account.

CMC is located in the eNB.

16.1.4 Dynamic Resource Allocation (DRA) – Packet Scheduling (PS)

The task of dynamic resource allocation (DRA) or packet scheduling (PS) is to allocate and de-allocate resources (including buffer and processing resources and resource blocks (i.e. chunks)) to user and control plane packets. DRA involves several sub-tasks, including the selection of radio bearers whose packets are to be scheduled and managing the necessary resources (e.g. the power levels or the specific resource blocks used). PS typically takes into account the QoS requirements associated with the radio bearers, the channel quality information for UEs, buffer status, interference situation, etc. DRA may also take into account restrictions or preferences on some of the available resource blocks or resource block sets due to inter-cell interference coordination considerations.

DRA is located in the eNB.

16.1.5 Inter-cell Interference Coordination (ICIC)

16.1.5.0 General

Inter-cell interference coordination has the task to manage radio resources such that inter-cell interference is kept under control. ICIC mechanism includes a frequency domain component and time domain component. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells. The preferred ICIC method may be different in the uplink and downlink.

The frequency domain ICIC manages radio resource, notably the radio resource blocks, such that multiple cells coordinate use of frequency domain resources.

In TDD, intended UL-DL configuration may be exchanged through backhaul signalling, and frequency domain ICIC information may be exchanged per subframe set, such that multiple cells may coordinate the usage of frequency domain resources in the subframe sets.

For the time domain ICIC, subframe utilization across different cells are coordinated in time through backhaul signalling or OAM configuration of so called Almost Blank Subframe patterns. The Almost Blank Subframes (ABSs) in an aggressor cell are used to protect resources in subframes in the victim cell receiving strong inter-cell interference. Almost blank subframes are subframes with reduced transmit power (including no transmission) on some physical channels and/or reduced activity. The eNB ensures backwards compatibility towards UEs by transmitting necessary control channels and physical signals as well as System Information. Patterns based on ABSs are signalled to the UE to restrict the UE measurement to specific subframes, called measurement resource restrictions. There are different patterns depending on the type of measured cell (serving or neighbour cell) and measurement type (e.g. RRM, RLM). MBSFN subframes can be used for time domain ICIC when they are also included in ABS patterns. The eNB cannot configure MBSFN subframes, as specified in TS 36.211 [4], as ABSs when these MBSFN subframes are used for other usages (e.g., MBMS, LCS).

Extending the coverage of a cell by means of connecting a UE to cell that is weaker than the strongest detected cell is referred to as cell range extension (CRE). With time domain ICIC, a CRE UE may continue to be served by a victim cell (i.e., the weaker cell) even while under strong interference from aggressor cells (i.e., the stronger cell).

A UE under strong interference from aggressor cells may need to mitigate interference from the aggressor cells on some physical channels and signals in order to receive data from serving cell or to detect the weak cells or to perform measurements on the weak cells.

The network may provide SIB1 to the UE in the CRE region by a dedicated RRC signalling to assist UE system information acquisition.

ICIC is located in the eNB.

16.1.5.1 UE configurations for time domain ICIC

For the UE to measure "protected" resources of the serving cell and/or neighbour cells, RRM/RLM/CSI measurement resource restriction is signalled to the UE. There are three kinds of measurement resource restriction patterns that may be configured for the UE.

– Pattern 1: A single RRM/RLM measurement resource restriction for the PCell.

– Pattern 2: A single RRM measurement resource restriction for indicated list of neighbour cells operating in the same carrier frequency as the PCell.

– Pattern 3: Resource restriction for CSI measurement of the PCell. If configured, two subframe subsets are configured per UE. The UE reports CSI for each configured subframe subset.

For pattern 3, it is up to the network to choose the two subframe subsets but typically the two subframe subsets are chosen with the expectation that CSI measurements using the two configured subframe subsets are subject to different levels of interference (e.g., one subframe subset indicates ABSs while the second subframe subset indicates non-ABSs). For periodic CSI reports, linkage of each CSI report to a configured subset of subframe is defined in TS 36.331 [16]. For aperiodic CSI reports, the UE reports CSI based on the subframe subset containing the CSI reference resource.

In RRC_CONNECTED, the RRM/RLM/CSI measurement resource restrictions are configured by dedicated RRC signalling.

The network may configure the UE with CRS assistance information of the aggressor cells in order to aid the UE to mitigate the interference from CRS of the aggressor cells.

16.1.5.2 OAM requirements for time domain ICIC

16.1.5.2.1 Configuration for CSG cell

When the time-domain inter-cell interference coordination is used for non-members UE in close proximity of a CSG cell, OAM configures a CSG cell not to use a time domain resource set (i.e. a set of subframes), so that a non-member UE in close proximity of the CSG cell can be still served by another cell. OAM also configures a cell neighbour to a CSG cell with the protected time domain resource set not used by the CSG cell, so that the neighbour cell knows which time domain resource can be used for a non-member UE in close proximity of the CSG cell.

16.1.5.2.2 Configuration for interfering non-CSG cell

When the time-domain inter-cell interference coordination is used to mitigate interference between two cells using X2 signalling of ABS patterns from an interfering eNB to an interfered eNB, the following OAM requirements are applied.

– OAM may configure association between eNBs to use the time-domain inter-cell interference coordination.

– For the deployment scenarios where common subset for ABS patterns from multiple interfering cells is desirable, OAM configuration ensures that a ‘common subset’ exists between the ABS patterns of those interfering cells.

NOTE 1: The possibility of whether the common ABS pattern from multiple eNBs is desirable or not depends on the deployment cases of the time domain solution of inter-cell interference coordination.

NOTE 2: It is up to eNB implementation how a receiving eNB derives the ‘usable ABS subset’ from the ABS patterns coming from multiple neighbour eNBs.

16.1.6 Load Balancing (LB)

Load balancing has the task to handle uneven distribution of the traffic load over multiple cells. The purpose of LB is thus to influence the load distribution in such a manner that radio resources remain highly utilized and the QoS of in-progress sessions are maintained to the extent possible and call dropping probabilities are kept sufficiently small. LB algorithms may result in hand-over or cell reselection decisions with the purpose of redistribute traffic from highly loaded cells to underutilized cells.

LB is located in the eNB.

16.1.7 Inter-RAT Radio Resource Management

Inter-RAT RRM is primarily concerned with the management of radio resources in connection with inter-RAT mobility, notably inter-RAT handover. At inter-RAT handover, the handover decision may take into account the involved RATs resource situation as well as UE capabilities and Operator policies. The importance of Inter-RAT RRM may depend on the specific scenario in which E-UTRAN is deployed. Inter-RAT RRM may also include functionality for inter-RAT load balancing for idle and connected mode UEs.

16.1.8 Subscriber Profile ID for RAT/Frequency Priority

The RRM strategy in E-UTRAN may be based on user specific information.

The Subscriber Profile ID for RAT/Frequency Priority (SPID) parameter and the Additional RRM Policy Index (ARPI) received by the eNB via the S1 interface or the X2 interface are indices referring to user information (e.g. mobility profile, service usage profile). The information is UE specific and applies to all its Radio Bearers.

NOTE: The Additional RRM Policy Index (ARPI) may be applied for specific RRM strategies independently or in combination with the Subscriber Profile ID for RAT/Frequency Priority (SPID).

TS 23.401 [17] specifies that a target eNB may receive an ARPI from the MME while receiving an ARPI from the source eNB, in which case information received from the MME shall prevail.

Both indices are mapped by the eNB to locally defined configuration in order to apply specific RRM strategies (e.g. to define RRC_IDLE mode priorities and control inter-RAT/inter frequency handover in RRC_CONNECTED mode).

16.1.9 Inter-eNB CoMP

The task of inter-eNB CoMP is to coordinate multiple eNBs in order that the coverage of high data rates and the cell-edge throughput are improved, and also the system throughput is increased. The coordination of multiple eNBs is achieved by signalling between eNBs of hypothetical resource allocation information, CoMP hypotheses, associated with benefit metrics. Each of the signalled CoMP hypotheses is concerned with a cell belonging to either the receiving eNB, the sending eNB or their neighbour. The benefit metric associated with the CoMP hypotheses quantifies the benefit assuming that the CoMP hypotheses are applied. The receiving eNB of the CoMP hypotheses and the benefit metrics may take them into account for RRM.

RSRP measurement reports and CSI reports may be exploited for inter-eNB CoMP. For example, the RSRP measurement reports and CSI reports can be used to determine and/or validate CoMP hypotheses and benefit metrics.

The enhanced RNTP may be used in inter-eNB CoMP to exchange information between eNBs concerning the adopted power allocation.

Inter-eNB CoMP is located in the eNB.

16.1.10 Cell on/off and cell discovery

The eNB using cell on/off may adaptively turn the downlink transmission of a cell on and off. A cell whose downlink transmission is turned off may be configured as a deactivated SCell for a UE. A cell performing on/off may transmit only periodic discovery signals and UEs may be configured to measure the discovery signals for RRM. Cell on/off may be performed for the purpose of e.g. inter-cell interference coordination and avoidance, load balancing, and energy saving, etc. The criteria used for cell on/off may be e.g. traffic load increase/decrease, UE arrival/departure (i.e. UE-cell association), and packet arrival/completion.

A UE performs RRM measurement and may discover a cell or transmission point of a cell based on discovery signals when the UE is configured with discovery-signal-based measurements.

16.1.11 Resource reservation

E-UTRAN may reserve resources in uplink and downlink on a NB-IoT non-anchor carrier to avoid resource overlap e.g. with NR when NB-IoT is deployed within an NR carrier. The resource reservation signalled to the UE is carrier specific and is for use in unicast transmission in connected mode.

For BL UEs or UEs in enhanced coverage, E-UTRAN may reserve resources in uplink and downlink to avoid resource overlap e.g. with NR when it is deployed within an NR carrier. The resource reservation signalled to the UE is cell specific and is for use in unicast transmission in connected mode.

16.2 RRM architecture

16.2.1 Centralised Handling of certain RRM Functions

Void.

16.2.2 De-Centralised RRM

16.2.2.1 UE History Information

The source eNB collects and stores the UE History Information for as long as the UE stays in one of its cells.

When information needs to be discarded because the list is full, such information will be discarded in order of its position in the list, starting with the oldest cell record. If the list is full, and the UE history information from the UE is available, the UE history information from the UE should also be discarded.

The resulting information is then used in subsequent handover preparations by means of the Handover Preparation procedures over the S1 and X2 interfaces, which provide the target eNB with a list of previously visited cells and associated (per-cell) information elements. The Handover Preparation procedures also trigger the target eNB to start collection and storage of UE history Information and thus to propagate the collected information.

16.2.3 Void

16.3 UE assistance information for RRM, and UE power optimisations and UE overheating

Except for NB-IoT UEs, in order to optimise the user experience and (for instance) to assist the eNB in configuring connected mode parameters and connection release handling, the UE may be configured to send assistance information to the eNB comprising:

– UE preference for power optimised configuration (1 bit):

– When this bit is sent by the UE, the UE shall set this in accordance with its preference for a configuration that is primarily optimised for power saving (e.g. a long value for the long DRX cycle or RRC connection release) or not;

– The details regarding how the UE sets the indicator are left to UE implementation.

– UE bandwidth preference on maximum PDSCH/PUSCH bandwidth:

– When this information is sent by the UE that supports CE mode, the UE shall set this in accordance with its preference on maximum PDSCH/PUSCH bandwidth to assist the eNB for a reconfiguration of the CE mode for the UE in RRC_CONNECTED state;

– The details regarding how the UE sets the bandwidth preference are left to UE implementation.

– UE indication on detected overheating:

– When this information is send by the UE, the UE shall set this information to inform the eNB about UE internal overheating caused by configurations concerning carrier aggregation/dual connectivity, MIMO transmissions, and/or modulation schemes being concurrently configured. The eNB may mitigate the indicated overheating by downgrading the UE configuration. Details regarding how the eNB mitigates the overheating are left to implementation (e.g. the eNB may choose to mitigate overheating by downgrading E-UTRA configuration and/or NR in case of EN-DC taking into account the assistance information provided by the UE). If the eNB does not provide any mitigation, the UE may need to mitigate the indicated overheating based on UE implementation.

– The details regarding how the UE detects the internal overheating are left to UE implementation.

In 5GS and, if configured, in EPS, a NB-IoT UE or BL UE may send assistance information to the eNB to assist the eNB in connection release handling.

The network response to the UE assistance information is left to network implementation. The eNB ensures that an appropriate QoS level is provided irrespective of received power preference indication or the bandwidth preference.