11 Scheduling and Rate Control

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

11.0 General

In order to utilise the SCH resources efficiently, a scheduling function is used in MAC. In this clause, an overview of the scheduler is given in terms of scheduler operation, signalling of scheduler decisions, and measurements to support scheduler operation.

For NB-IoT, the Basic Scheduler Operation in 11.1, the uplink buffer status reports part in 11.3 and the DL channel quality reporting in 11.7 are applicable, the UE-AMBR part in 11.4 is applicable only for UE which is enabled to use S1-U data transfer or User Plane CIoT EPS optimisation or for UE which is enabled to use NG-U data transfer or User Plane CIoT 5GS Optimisation, and all other subclauses of clause 11 are not applicable.

11.1 Basic Scheduler Operation

MAC in eNB includes dynamic resource schedulers that allocate physical layer resources for the DL-SCH, UL-SCH and SL-SCH transport channels. Different schedulers operate for the DL-SCH, UL-SCH and SL-SCH.

The scheduler should take account of the traffic volume and the QoS requirements of each UE and associated radio bearers, when sharing resources between UEs. Only "per UE" grants are used to grant the right to transmit on the UL-SCH and SL-SCH (i.e. there are no "per UE per RB" grants).

Schedulers may assign resources taking account the radio conditions at the UE identified through measurements made at the eNB and/or reported by the UE.

Radio resource allocations can be valid for one or multiple TTIs.

Resource assignment consists of physical resource blocks (PRB) and MCS. Allocations for time periods longer than one TTI might also require additional information (allocation time, allocation repetition factor…).

When CA is configured, a UE may be scheduled over multiple serving cells simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling with the Carrier Indicator Field (CIF) allows the PDCCH of a serving cell to schedule resources on another serving cell but with the following restrictions:

– Cross-carrier scheduling does not apply to PCell i.e. PCell is always scheduled via its PDCCH;

– When the PDCCH of an SCell is configured except for an LAA SCell, cross-carrier scheduling for uplink transmission and downlink transmission does not apply to this SCell i.e. it is always scheduled for uplink transmission and downlink transmission via its PDCCH;

– When the PDCCH of an LAA SCell is configured:

– If cross-carrier scheduling applies only to uplink transmission, it is scheduled for downlink transmission via its PDCCH and for uplink transmission via the PDCCH of one other serving cell;

– If self-scheduling applies to both uplink transmission and downlink transmission, it is always scheduled for uplink transmission and downlink transmission via its PDCCH.

– When the PDCCH of an SCell is not configured, cross-carrier scheduling for uplink transmission and downlink transmission applies and this SCell is always scheduled for uplink transmission and downlink transmission via the PDCCH of one other serving cell.

A linking between UL and DL allows identifying the serving cell for which the DL assignment or UL grant applies when the CIF is not present:

– DL assignment received on PCell corresponds to downlink transmission on PCell;

– For DC, DL assignment received on PSCell corresponds to downlink transmission on PSCell;

– UL grant received on PCell corresponds to uplink transmission on PCell, except for the UL grant in Random Access Response from PCell in response to a random access preamble on SCell of MCG for which case the UL grant is for the SCell where the preamble is sent;

– For DC, UL grant received on PSCell corresponds to uplink transmission on PSCell, except for the UL grant in Random Access Response from PSCell in response to a random access preamble on SCell of SCG for which case the UL grant is for the SCell where the preamble is sent.

– DL assignment received on SCell_n corresponds to downlink transmission on SCell_n;

– UL grant received on SCell_n corresponds to uplink transmission on SCell_n. If SCell_n is not configured for uplink usage by the UE, the grant is ignored by the UE.

When DC is configured, cross-carrier scheduling can only be used across serving cells within the same CG. Within a CG, neither PCell of MCG nor PSCell of SCG can be cross-carrier scheduled.

When SPT is configured, cross-carrier scheduling can be used, but is limited to serving cells within the same PUCCH group. In this case, both the scheduling cell and the scheduled cell shall be configured with SPT.

For BL UEs or UEs in enhanced coverage, when multi-TB scheduling is configured, a single MPDCCH can indicate scheduling of multiple downlink transmissions, where each transmission corresponds to one HARQ process

11.1.1 Downlink Scheduling

In the downlink, E-UTRAN can dynamically allocate resources (PRBs and MCS) to UEs at each TTI via the C-RNTI on PDCCH(s). A UE always monitors the PDCCH(s) in order to find possible allocation when its downlink reception is enabled (activity governed by DRX when configured). When CA is configured, the same C-RNTI applies to all serving cells.

In addition, E-UTRAN can allocate semi-persistent downlink resources for the first HARQ transmissions to UEs:

– RRC defines the periodicity of the semi-persistent downlink grant;

– PDCCH indicates whether the downlink grant is a semi-persistent one i.e. whether it can be implicitly reused in the following TTIs according to the periodicity defined by RRC.

When required, retransmissions are explicitly signalled via the PDCCH(s). In the TTIs where the UE has semi-persistent downlink resource, if the UE cannot find its C-RNTI on the PDCCH(s), a downlink transmission according to the semi-persistent allocation that the UE has been assigned in the TTI is assumed. Otherwise, in the sub-TTIs where the UE has semi-persistent downlink resource, if the UE finds its C-RNTI on the PDCCH(s), the PDCCH allocation overrides the semi-persistent allocation for that TTI and the UE does not decode the semi-persistent resources.

Semi-persistent downlink resources can be configured per serving cell with the restriction that multiple DL SPS configurations per serving cell are not supported. SPS configurations can be active simultaneously for different cells. PDCCH allocations made on a given serving cell can only override the semi-persistent allocation for that serving cell.

For NB-IoT:

– Scheduling information for downlink data is transmitted on the downlink physical control channel NPDCCH. The scheduled downlink data is transmitted on the shared data channel NPDSCH;

– Only cross-subframe scheduling is supported, cross-carrier scheduling is not supported. The transmission duration in number of sub-frames for the NPDCCH and the NPDSCH is variable;

– The transmission duration in number of sub-frames is semi-static for the NPDCCH and is indicated for the NPDSCH as part of the scheduling information transmitted on the NPDCCH;

– The start time of the NPDSCH relative to the NPDCCH is signaled as part of the scheduling message;

– When multi-TB scheduling is configured, a single NPDCCH can indicate scheduling of multiple downlink transmissions, where each transmission corresponds to one HARQ process.

11.1.2 Uplink Scheduling

In the uplink, E-UTRAN can dynamically allocate resources (PRBs and MCS) to UEs at each TTI via the C-RNTI on PDCCH(s). A UE always monitors the PDCCH(s) in order to find possible allocation for uplink transmission when its downlink reception is enabled (activity governed by DRX when configured). When CA is configured, the same C-RNTI applies to all serving cells.

In addition, E-UTRAN can allocate a semi-persistent uplink resource or autonomous uplink resource for the first HARQ transmissions and potentially retransmissions to UEs:

– RRC defines the periodicity of the semi-persistent uplink grant or the bitmap of the autonomous uplink grant;

– PDCCH indicates whether the uplink grant is a semi-persistent one or an autonomous uplink one i.e. whether it can be implicitly reused in the following TTIs according to the periodicity or the bitmap defined by RRC.

In the TTIs where the UE has semi-persistent uplink resource or autonomous uplink resource, if the UE cannot find its C-RNTI on the PDCCH(s), an uplink transmission according to the semi-persistent allocation or autonomous uplink allocation that the UE has been assigned in the TTI can be made. The network performs decoding of the pre-defined PRBs according to the pre-defined MCS. Otherwise, in theTTIs where the UE has semi-persistent uplink resource or autonomous uplink resource, if the UE finds its C-RNTI on the PDCCH(s), the PDCCH allocation overrides the persistent allocation or autonomous uplink allocation for that TTI and the UE’s transmission follows the PDCCH allocation, not the semi-persistent allocation or autonomous uplink. Retransmissions are either implicitly allocated in which case the UE uses the semi-persistent uplink allocation or autonomous uplink allocation, or explicitly allocated via PDCCH(s) in which case the UE does not follow the semi-persistent allocation or autonomous uplink allocation. The UE is not allowed to use autonomous uplink resource for retransmission of dynamically scheduled transmission.

NOTE: there is no blind decoding in uplink and when the UE does not have enough data to fill the allocated resource, padding is used.

When the UE is provided with valid uplink grants in several serving cells in one TTI, the order in which the grants are processed during logical channel prioritisation and whether joint or serial processing is applied are left up to UE implementation, while adhering to transmission restrictions of a logical channel via LAA SCells.

Similar to the downlink, semi-persistent uplink resources can be configured. Multiple UL SPS configurations are supported per Serving Cell. On one Serving Cell, multiple such configurations can be active simultaneously only for the same TTI length. SPS configurations can also be active simultaneously for different cells. PDCCH allocations made on a given serving cell can only override the semi-persistent allocation for that serving cell.

When UL skipping is configured, the UE will not transmit a MAC PDU with only padding BSR and padding if no data is available for transmission in the UE buffer. When UL Skippping and an SPS interval shorter than 10ms is configured, a retransmission is prioritised over a new transmission on semi-persistent uplink resources if no dynamic grant is allocated for that subframe.

For a UE capable of V2X communication, multiple semi-persistent configurations can be configured in uplink, regardless of the specific services the UE is operating. The uplink resources for each semi-persistent configuration can only be configured for the PCell. When DC is configured, the uplink resources for each semi-persistent configuration can only be configured for the PCell or PSCell.

Autonomous uplink allocation can be configured for LAA SCell(s). The UE will not transmit on autonomous uplink resources if no data is available for transmission.

For BL UEs or UEs in enhanced coverage, when multi-TB scheduling is configured, a single MPDCCH can indicate scheduling of multiple uplink transmissions, where each transmission corresponds to one HARQ process.

For BL UEs or UEs in enhanced coverage, E-UTRAN can allocate preconfigured uplink resources to be used in RRC_IDLE for transmission using PUR, see clause 7.3d.

For NB-IoT:

– Scheduling information for uplink data is transmitted on the downlink physical control channel NPDCCH. The scheduled uplink data is transmitted on the shared data channel NPUSCH;

– The transmission duration in number of sub-frames for the NPUSCH is variable;

– The transmission duration in number of sub-frames is semi-static for the NPDCCH and is indicated for the NPUSCH as part of the scheduling information transmitted on the NPDCCH;

– The start time of the NPUSCH relative to the NPDCCH is signaled as part of the scheduling message;

– E-UTRAN can allocate semi-persistent uplink resource for sending a BSR acting as a Scheduling Request;

– When multi-TB scheduling is configured, a single NPDCCH can indicate scheduling of multiple uplink transmissions, where each transmission corresponds to one HARQ process;

– E-UTRAN can allocate preconfigured uplink resources to be used in RRC_IDLE for transmission using PUR, see clause 7.3d.

11.2 Activation/Deactivation Mechanism

To enable reasonable UE battery consumption when CA is configured, an activation/deactivation mechanism of SCells is supported (i.e. activation/deactivation does not apply to PCell). When an SCell is deactivated, the UE does not need to receive the corresponding PDCCH or PDSCH, cannot transmit in the corresponding uplink, nor is it required to perform CQI measurements. Conversely, when an SCell is active, the UE shall receive PDSCH and PDCCH (if the UE is configured to monitor PDCCH from this SCell), and is expected to be able to perform CQI measurements. To enable faster CQI reporting, a temporary CQI reporting period (called short CQI period) can be supported during SCell activation period. E-UTRAN ensures that while PUCCH SCell is deactivated, SCells of secondary PUCCH group should not be activated or dormant. E-UTRAN ensures that SCells mapped to PUCCH SCell are deactivated before the PUCCH SCell is changed or removed.

To enable faster transition to activated state, a dormant state for SCells (i.e. not PCell or PSCell) is supported. When an SCell is in dormant state, the UE does not need to receive the corresponding PDCCH or PDSCH, cannot transmit in the corresponding uplink, but is required to perform CQI measurements. A PUCCH SCell cannot be in dormant state.

The activation/deactivation mechanism is based on the combination of a MAC control element and deactivation timers. The MAC control element carries a bitmap for the activation and deactivation of SCells: a bit set to 1 denotes activation of the corresponding SCell, while a bit set to 0 denotes deactivation. With the bitmap, SCells can be activated and deactivated individually, and a single activation/deactivation command can activate/deactivate a subset of the SCells. One deactivation timer is maintained per SCell but one common value is configured per UE by RRC.

The state transitions to and from dormant Scell state use MAC control elements.

At reconfiguration without mobility control information:

– SCells added to the set of serving cells are initially "deactivated", "dormant" or "activated";

– SCells which remain in the set of serving cells (either unchanged or reconfigured) do not change their activation status ("activated", "deactivated" or "dormant").

At reconfiguration with mobility control information (i.e. handover) or connection resume from RRC_INACTIVE:

– SCells are "deactivated", "dormant" or "activated".

In DC, the serving cells of the MCG other than the PCell can only be activated/deactivated by the MAC Control Element received on MCG, and the serving cells of the SCG other than PSCell can only be activated/ deactivated by the MAC Control Element received on SCG. The MAC entity applies the bitmap for the associated cells of either MCG or SCG. PSCell in SCG is always activated like the PCell (i.e. deactivation timer is not applied to PSCell). With the exception of PUCCH SCell, one deactivation timer is maintained per SCell but one common value is configured per CG by RRC.

11.3 Measurements to Support Scheduler Operation

Measurement reports are required to enable the scheduler to operate in both uplink and downlink. These include transport volume and measurements of a UEs radio environment.

Uplink buffer status reports (BSR) are needed to provide support for QoS-aware packet scheduling. In E-UTRAN uplink buffer status reports refer to the data that is buffered in for a group of logical channel (LCG) in the UE. Four LCGs and two formats are used for reporting in uplink:

– A short format for which only one BSR (of one LCG) is reported;

– A long format for which all four BSRs (of all four LCGs) are reported.

Uplink buffer status reports are transmitted using MAC signalling.

In DC, LCG is configured per CG.

In DC, BSR configuration, triggering and reporting are independently performed per CG. For split bearers, the PDCP data is considered in BSR in the CG(s) configured by RRC.For LWA bearers in the UL, the bearers configured to use WLAN only do not trigger BSR. For bearers configured to use WLAN and LTE, only the data that may be sent over LTE (i.e., excluding UL data already sent or decided to be sent over WLAN) is considered for BSR.

11.4 Rate Control of GBR, MBR and UE-AMBR

11.4.1 Downlink

The eNB guarantees the downlink GBR associated with a GBR bearer, enforces the downlink MBR associated with a GBR bearer and enforces the downlink AMBR associated with a group of Non-GBR bearers.

11.4.2 Uplink

The UE has an uplink rate control function which manages the sharing of uplink resources between radio bearers. RRC controls the uplink rate control function by giving each bearer a priority and a prioritised bit rate (PBR). The values signalled may not be related to the ones signalled via S1 to the eNB.

The uplink rate control function ensures that the UE serves its radio bearer(s) in the following sequence:

1. All the radio bearer(s) in decreasing priority order up to their PBR;

2. All the radio bearer(s) in decreasing priority order for the remaining resources assigned by the grant.

NOTE1: In case the PBRs are all set to zero, the first step is skipped and the radio bearer(s) are served in strict priority order: the UE maximises the transmission of higher priority data.

NOTE2: By limiting the total grant to the UE, the eNB can ensure that the UE-AMBR plus the sum of MBRs is not exceeded.

NOTE3: Provided the higher layers are responsive to congestion indications, the eNB can enforce the MBR of an uplink radio bearer by triggering congestion indications towards higher layers and by shaping the data rate towards the S1 interface.

If more than one radio bearer has the same priority, the UE shall serve these radio bearers equally.

11.4.3 UE-AMBR for Dual Connectivity

In DC, the MeNB ensures that the UE-AMBR is not exceeded by:

1) limiting the resources it allocates to the UE in MCG; and

2) indicating to the SeNB a limit so that the SeNB can also in turn guarantee that this limit is not exceeded.

For split bearers the SeNB ignores the indicated downlink UE-AMBR. If the SeNB is not configured to serve the uplink for split bearers, the SeNB ignores the indicated uplink UE-AMBR.

11.5 CQI reporting for Scheduling

The time and frequency resources used by the UE to report CQI are under the control of the eNB. CQI reporting can be either periodic or aperiodic. A UE can be configured to have both periodic and aperiodic reporting at the same time. In case both periodic and aperiodic reporting occurs in the same subframe for a particular CG, only the aperiodic report is transmitted in that subframe.

For efficient support of localized, distributed and MIMO transmissions, E-UTRA supports three types of CQI reporting:

– Wideband type: providing channel quality information of entire system bandwidth of the cell;

– Multi-band type: providing channel quality information of some subset(s) of system bandwidth of the cell;

– MIMO type: open loop or closed loop operation (with or without PMI feedback).

Periodic CQI reporting is defined by the following characteristics:

– When the UE is allocated PUSCH resources in a subframe where a periodic CQI report is configured to be sent, the periodic CQI report is transmitted together with uplink data on the PUSCH. Otherwise, the periodic CQI reports are sent on the PUCCH.

Aperiodic CQI reporting is defined by the following characteristics:

– The report is scheduled by the eNB via the PDCCH;

– Transmitted together with uplink data on PUSCH.

When a CQI report is transmitted together with uplink data on PUSCH, it is multiplexed with the transport block by L1 (i.e. the CQI report is not part of the uplink the transport block).

The eNB configures a set of sizes and formats of the reports. Size and format of the report depends on whether it is transmitted over PUCCH or PUSCH and whether it is a periodic or aperiodic CQI report.

11.6 Explicit Congestion Notification

The eNB and the UE support of the Explicit Congestion Notification (ECN) is specified in clause 5 of IETF RFC 3168 [35] (i.e., the normative part of IETF RFC 3168 [35] that applies to the end-to-end flow of IP packets), and below. ECN is beneficial especially for latency sensitive interactived applications such as chat and gaming as well as for real-time voice and video, this because loss as a congestion signal is avoided, losses that would otherwise necessitate retransmission of packets with additional application delay as a result.

The eNB should set the Congestion Experienced (CE) codepoint (’11’) in PDCP SDUs in the downlink direction to indicate downlink (radio) congestion if those PDCP SDUs have one of the two ECN-Capable Transport (ECT) codepoints set. The eNB should set the Congestion Experienced (CE) codepoint (’11’) in PDCP SDUs in the uplink direction to indicate uplink (radio) congestion if those PDCP SDUs have one of the two ECN-Capable Transport (ECT) codepoints set. ECN marking should be per the recommendations in IETF RFC 7567 [73].

11.7 DL channel quality reporting

The DL channel quality report is only applicable to BL UEs, UEs in enhanced coverage and NB-IoT UEs.

The DL channel quality report in RRC_IDLE is defined by the following characteristics:

– The reporting is configured by eNB via system information;

– For NB-IOT UEs:

– The report is related to the DL carrier used for the initial random access procedure;

– The report is carried in the RRC message during the random access procedure.

The DL channel quality report in RRC_CONNECTED is defined by the following characteristics:

– The reporting is triggered by the eNB via a MAC Control Element;

– For NB-IOT UEs, the report is related to the configured DL carrier used in unicast transmission;

– The report is carried in a MAC Control Element.