Section: 5.4.6 Power Headroom Reporting
Content: The Power Headroom reporting procedure is used to provide the serving eNB with information about the difference between the nominal UE maximum transmit power and the estimated power for UL-SCH transmission or SRS transmission per activated Serving Cell and also with information about the difference between the nominal UE maximum power and the estimated power for UL-SCH and PUCCH/SPUCCH transmission on SpCell and PUCCH SCell. The reporting period, delay and mapping of Power Headroom are defined in TS 36.133[ Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for support of radio resource management ]  [9] and TS 38.133[ NR; Requirements for support of radio resource management ]  [19]. RRC controls Power Headroom reporting by configuring the two timers periodicPHR-Timer and prohibitPHR-Timer, and by signalling dl-PathlossChange which sets the change in measured downlink pathloss and the required power backoff due to power management (as allowed by P-MPRc, see TS 36.101[ Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception ]  [10] and TS 38.101[ None ] -3 [21]) to trigger a PHR, as specified in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [8]. A Power Headroom Report (PHR) shall be triggered if any of the following events occur: - prohibitPHR-Timer expires or has expired and the path loss has changed more than dl-PathlossChange dB for at least one activated Serving Cell of any MAC entity which is used as a pathloss reference since the last transmission of a PHR in this MAC entity when the MAC entity has UL resources for new transmission; - periodicPHR-Timer expires; - upon configuration or reconfiguration of the power headroom reporting functionality by upper layers, as specified in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [8], which is not used to disable the function; - activation of an SCell of any MAC entity with configured uplink; - addition of the PSCell (i.e. PSCell is newly added or PSCell is changed); - prohibitPHR-Timer expires or has expired, when the MAC entity has UL resources for new transmission, and the following is true in this TTI for any of the activated Serving Cells of any MAC entity with configured uplink: - there are UL resources allocated for transmission or there is a PUCCH/SPUCCH transmission on this cell, and the required power backoff due to power management (as allowed by P-MPRc, see TS 36.101[ Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception ]  [10] and TS 38.101[ None ] -3 [21]) for this cell has changed more than dl-PathlossChange dB since the last transmission of a PHR when the MAC entity had UL resources allocated for transmission or PUCCH/SPUCCH transmission on this cell. NOTE 1: The MAC entity should avoid triggering a PHR when the required power backoff due to power management decreases only temporarily (e.g. for up to a few tens of milliseconds) and it should avoid reflecting such temporary decrease in the values of PCMAX,c/PH when a PHR is triggered by other triggering conditions. NOTE 2: If UL HARQ operation is autonomous for the HARQ entity and if the PHR is already included in a MAC PDU for transmission by this HARQ entity, but not yet transmitted by lower layers, it is up to UE implementation how to handle the PHR content. If the MAC entity has UL resources allocated for new transmission for this TTI the MAC entity shall: - if it is the first UL resource allocated for a new transmission since the last MAC reset, start periodicPHR-Timer; - if the Power Headroom reporting procedure determines that at least one PHR has been triggered and not cancelled, and; - if the allocated UL resources can accommodate the MAC control element for PHR which the MAC entity is configured to transmit, plus its subheader, as a result of logical channel prioritization: - if extendedPHR is configured: - for each activated Serving Cell with configured uplink: - obtain the value of the Type 1 or Type 3 power headroom; - if the MAC entity has UL resources allocated for transmission on this Serving Cell for this TTI: - obtain the value for the corresponding PCMAX,c field from the physical layer; - if simultaneousPUCCH-PUSCH is configured or a serving cell operating according to Frame Structure Type 3 with uplink is configured and activated: - obtain the value of the Type 2 power headroom for the PCell; - obtain the value for the corresponding PCMAX,c field from the physical layer (see clause 5.1.1.2 of TS 36.213[ Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures ]  [2]); - instruct the Multiplexing and Assembly procedure to generate and transmit an Extended PHR MAC control element for extendedPHR as defined in clause 6.1.3.6a based on the values reported by the physical layer; - else if extendedPHR2 is configured: - for each activated Serving Cell with configured uplink: - obtain the value of the Type 1 or Type 3 power headroom; - if the MAC entity has UL resources allocated for transmission on this Serving Cell for this TTI: - obtain the value for the corresponding PCMAX,c field from the physical layer; - if a PUCCH SCell is configured and activated: - obtain the value of the Type 2 power headroom for the PCell and PUCCH SCell; - obtain the values for the corresponding PCMAX,c fields from the physical layer (see clause 5.1.1.2 ofTS 36.213[ Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures ]  [2]); - else: - if simultaneousPUCCH-PUSCH is configured for the PCell or a serving cell operating according to Frame Structure Type 3 with uplink is configured and activated: - obtain the value of the Type 2 power headroom for the PCell; - obtain the value for the corresponding PCMAX,c field from the physical layer (see clause 5.1.1.2 of TS 36.213[ Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures ]  [2]); - instruct the Multiplexing and Assembly procedure to generate and transmit an Extended PHR MAC control element for extendedPHR2 according to configured ServCellIndex and the PUCCH(s) for the MAC entity as defined in clause 6.1.3.6a based on the values reported by the physical layer; - else if dualConnectivityPHR is configured: - for each activated Serving Cell with configured uplink associated with any MAC entity: - obtain the value of the Type 1 or Type 3 power headroom; - if this MAC entity has UL resources allocated for transmission on this Serving Cell for this TTI or if the other MAC entity has UL resources allocated for transmission on this Serving Cell for this TTI and phr-ModeOtherCG is set to real by upper layers: - obtain the value for the corresponding PCMAX,c field from the physical layer; - if simultaneousPUCCH-PUSCH is configured or a serving cell operating according to Frame Structure Type 3 with uplink is configured and activated: - obtain the value of the Type 2 power headroom for the SpCell; - obtain the value for the corresponding PCMAX,c field for the SpCell from the physical layer (see clause 5.1.1.2 of TS 36.213[ Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures ]  [2]); - if the other MAC entity is E-UTRA MAC entity: - obtain the value of the Type 2 power headroom for the SpCell of the other MAC entity. - if phr-ModeOtherCG is set to real by upper layers: - obtain the value for the corresponding PCMAX,c field for the SpCell of the other MAC entity from the physical layer (see clause 5.1.1.2 of TS 36.213[ Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures ]  [2]); - instruct the Multiplexing and Assembly procedure to generate and transmit a Dual Connectivity PHR MAC control element as defined in clause 6.1.3.6b based on the values reported by the physical layer; - else: - obtain the value of the Type 1 power headroom from the physical layer; - instruct the Multiplexing and Assembly procedure to generate and transmit a PHR MAC control element as defined in clause 6.1.3.6 based on the value reported by the physical layer; - start or restart periodicPHR-Timer; - start or restart prohibitPHR-Timer; - cancel all triggered PHR(s).