LTE2664 functional description - LTE2664: Load-based PUCCH Region

3.5 LTE2664: Load-based PUCCH Region

3.5.2 LTE2664 functional description

Functional overview

The LTE2664: Load-based PUCCH Region feature adjusts the range of physical resource blocks (PRB) allocated to the physical uplink control channel (PUCCH). The adjustment is made proportionally to the number of UEs connecting to a cell.

Cooperation with the LTE1130: Dynamic PUCCH Allocation feature enables the operator to expand or compress the PUCCH region according to the actual cell’s load. Thanks to the live adjustment of scheduling request (SR) and channel state information (CSI) periodicities, defined with the LTE1130: Dynamic PUCCH Allocation feature, the adaptation of PRBs assigned to PUCCH is swift and efficient; that is, adequate to the number of UEs connected to the cell.

g

Note: The PUCCH region may be released once there are PUCCH resources in it that are no longer occupied. To avoid undesirable bandwidth fragmentation, the system stops assigning new calls to the PUCCH area at the risk of being soon converted to PUSCH.

Figure 2 PUCCH resource allocation in LTE2664

Contribution of the LTE1130: Dynamic PUCCH Allocation feature

The physical uplink control channel (PUCCH) is used to transmit the uplink control information (UCI), which includes:

• Scheduling requests (SR)

• HARQ ACK/NACK in response to DL transmissions in the physical downlink shared channel (PDSCH)

• Channel state information (CSI), which includes:

– Channel quality indicators (CQI)

– MIMO-related feedback such as the rank indicators (RI) or the precoding matrix indicators (PMI)

The LTE1130: Dynamic PUCCH Allocation feature simplifies the process of PUCCH configuration by introducing:

• Dynamic switching between channel state indicator (CSI) periodicities: 40 ms and 80 ms

• Dynamic switching between UL scheduling request (SR) periodicities: 10 ms, 20 ms, and 40 ms

Switching between SR periodicities depends on cell load, which is related to number of RRC Connected UEs in the cell. Switching between CSI periodicities depends on a cell load and configuration of MIMO, carrier aggregation and enhanced inter-cell interference coordination (eICIC).

The adaptation of the CSI or SR periodicity is done only for UEs for which

reconfiguration needs to be applied. To avoid generating extra signaling, new CSI or SR periodicity is given to every UE experiencing one of the following events:

• RRC-idle to RRC-connected state transition

• UE entering the cell via handover

• Secondary cell (SCell) configuration or reconfiguration

Enhancements introduced by LTE2664: Load-based PUCCH Region feature In addition to the shift in CSI and SR periodicities managed by the LTE1130: Dynamic PUCCH Allocation feature, the LTE2664: Load-based PUCCH Region feature allows operator to expand or contract PUCCH region according to the minimal configuration of resources allocated to PUCCH (2 PRBs assigned) . The eNB responds in advance to the rising number of UEs connecting to the cell by expanding the PUCCH area. A

configurable safety margin prevents the PUCCH resources from running out and helps to avoid the radio resource control (RRC) blocking.

When the PUCCH expansion process is triggered, eNB calculates new parameter values for the next PUCCH expansion step. Parameters that define PUCCH area size and PRACH offset (APUCCH_FDD:assignedNCqiRb,

APUCCH_FDD:assignedPrachFreqOff and

APUCCH_FDD:assignedN1PucchAn) are modified. UEs get the new parameter values through a SIB broadcast (SIB#2 in the image) which contains the new values for nRB-CQI, prach-FreqOffset, and n1PUCCH-AN.

Figure 3 PUCCH expansion procedure

SIB#2

nRB-CQI Prach-FreqOffset

N1PUCCH-AN

APUCCH_FDD/TDD

assignedPrachFreqOff AssignedN1PucchAn

assignedNCqiRB

If the PUCCH expansion process is triggered because the systemInfoValueTag is above the value set in LNBTS:sibUpdateRateThreshold, then new parameter values are calculated for maximum possible PUCCH configuration. A SIB update (SIB#2 in the image) containing the new information is broadcasted.

Figure 4 PUCCH expansion procedure (systemInfoValueTag >

LNBTS: sibUpdateThreshold)

SIB#1 SIB#2

SIB#1

systemInfoValueTag 28

SIB#2

nRB-CQI

Prach-FreqOffset n1PUCCH-AN

LNBTS

sibUpdateRateThreshold 25

When a cell’s load decreases, spare physical uplink control channel (PUCCH) capacity is acquired. To reconfigure the area assigned to PUCCH, a system information block (SIB) sent by the eNB must be modified. Introducing SIB modifications requires a sufficient amount of time. Due to restrictions, they cannot occur too often and cannot be delayed too much either to avoid the RRC blocking. That is why a PUCCH area downgrade has to be carried out with an appropriate time margin in regard to the previous PUCCH modification. After the established interval has passed, some of the PUCCH-allocated physical resource blocks (PRB) exceeding the threshold are reassigned by the eNB to be used by the physical uplink shared channel (PUSCH).

The operator is enabled to define the amount of supported UEs. This includes the maximum number of UEs calculated considering parameters:

• maxNumActUe

• aPucchMinNumRrcNoDrb

• aPucchMinNumEmergencySessions

• aPucchAddAUeRrHo

• aPucchAddAUeTcHo

Setting correctly parameters for minimum number of UEs hosted by the cell allows to reach minimal configuration containing only 2 or 4 PRBs.

• aPucchMinNumRrc

• aPucchMinNumRrcNoDrb

• aPucchMinNumEmergencySessions

• aPucchAddAUeRrHo

• aPucchAddAUeTcHo

Expansion or compression of PUCCH resources is followed by shift of PRBs used by physical random access channel (PRACH). Location of these resources is corresponding to the changes of the area occupied by PUCCH.

g

Note: When LTE944: PUSH Masking feature is active PRACH resources are steadily allocated due to the PUSCH masking. PRACH resources are allocated according to the LTE1130: Dynamic PUCCH allocation feature calculated largest PUCCH BW

occupation.

Reconfiguration of PUCCH resources

Reconfiguration of PUCCH is a complex process that might be divided into three distinctive phases:

1. Preparation phase when the UE in the cell is prepared for the process to trigger.

Changes in the system information are introduced and notifications in the eNB software are prepared.

2. Communication phase when notifications are sent to UEs, informing that in the next broadcast control channel (BCCH) modification window new SIBs will be available.

3. Broadcasting phase when the new SIBs are transmitted.

Resources reconfiguration event is triggered when UE's reosurces must be relocated.

Each UE subbmited to reconfiguration recieves a RRCConnectionReconfiguration message from eNB followed by a feedback RRCConnectionReconfigurationComplete message from the UE.

Extending the PUCCH region is a fast process, devoid of risk of dropping the UEs. The default value for the parameter countdownPucchExp defining the extent of time that need to pass between following expansions is set for 10 minutes. In contrast the

compression is carried out with much more attention. The default value for the parameter countdownPucchCompr is set to 60 minutes. To avoid unnecessary repositioning,the number of UEs requiring reconfiguration must be below 50.

Borderline PUCCH reconfiguration instances

There is possibility that after triggering PUCCH reconfiguration, cell's load may

substantially change which requires appropriate LTE2664: Load-based PUCCH Region feature response.

• When expansion of PUCCH region is triggered and the number of SIB modification per interval is above sibUpdateRateThresholdthreshold, PUCCH area is expanded to the maximal configurable value.

• When compression of PUCCH region is triggered and the number of SIB

modification is above sibUpdateRateThreshold threshold, the compression is aborted and postponed to the next countdown expiration.

Data carried in PUCCH