4 RF power control

3GPP45.008GSM/EDGE Radio subsystem link controlTS

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

4.1 Overall process

RF power control is employed to minimize the transmit power required by MS or BSS whilst maintaining the quality of the radio links. By minimizing the transmit power levels, interference to co‑channel users is reduced.

4.2 MS implementation

4.2.1 General

RF power control shall be implemented in the MS.

In A/Gb mode, the power control level to be employed by the MS on each uplink channel, except PDCH, is indicated by means of the power control information sent either in the layer 1 header of each SACCH message block (see 3GPP TS 44.004) on the corresponding downlink channel, or in a dedicated signalling block (see 3GPP TS 44.018). Power control for PDCH is defined in subclause 10.2.

Similarly in Iu mode in MAC-Dedicated state and MAC-DTM state, the power control level to be employed by the MS on each uplink channel, is indicated by means of the power control information sent either in the layer 1 header of each SACCH message block (see 3GPP TS 44.004) on the corresponding downlink channel, or in a dedicated signalling block. Power control for MAC-Shared state is defined in subclause 10.2.

The MS shall employ the most recently commanded power control level appropriate to each channel for all transmitted bursts on either a TCH (including handover access burst), FACCH, SACCH, PDTCH or SDCCH. For FLO in Iu mode the MS shall employ the most recently commanded power control level appropriate to each DBPSCH for all transmitted bursts (including radio packets, handover access burst and SACCH).

The MS shall confirm the power control level that it is currently employing in the SACCH L1 header on each uplink channel. The indicated value shall be the power control level actually used by the mobile for the last burst of the previous SACCH period.

When on an E-TCH, the MS shall, if so indicated by the BSS in the SACCH L1 header (see 3GPP TS 44.004) or Assignment command (see 3GPP TS 44.018), use FPC (fast power control). The MS shall employ the most recently commanded fast power control level on each uplink E-TCH channel. The power control level to be employed by the MS is indicated by means of the power control information sent via E-IACCH once every FPC reporting period (see subclause 4.7). If FPC is in use, the MS shall report, in the SACCH L1 header, the power control level used at the end of the normal power control reporting period.

When on an E-TCH using 8 PSK for the uplink, the MS shall use the E-IACCH in the uplink for fast measurement reporting.

In A/Gb mode, when assigned a TCH or O-TCH, the MS shall configure the channel in enhanced power control (EPC) mode if so commanded by BSS in the channel assignment (see 3GPP TS 44.018). On such a channel, EPC may be used for uplink power control and/or downlink power control.

Similarly in Iu mode, when assigned a DBPSCH, the MS shall configure the channel in enhanced power control (EPC) mode if so commanded by BSS in the channel assignment. On such a channel, EPC may be used for uplink power control and/or downlink power control.

The enhanced power control (EPC) is part of the GERAN Feature Package 2 (see 3GPP TS 24.008).

When on a channel in EPC mode,

– the MS shall use the EPCCH in the uplink for EPC measurement reporting (see subclause 8.4.1b).

– the MS shall, depending on what is signalled in the L1 header of the downlink SACCH (see 3GPP TS 44.004) and during channel assignment (see 3GPP TS 44.018), obey either the EPC Uplink Power Control Command (sent on the EPCCH in the downlink) or the Ordered MS Power Level (sent in the L1 header of the downlink SACCH).

– If the signalling indicates that EPC shall be used in the uplink, the MS shall employ the most recently commanded EPC power control level, as indicated by the EPC Uplink Power Control Command sent on the corresponding EPCCH in the downlink. The EPC Uplink Power Control Command is sent once every EPC reporting period (see subclause 8.4.1b). The MS shall ignore the Ordered MS Power Level sent in the SACCH L1 header in the downlink.

– If the signalling indicates that normal power control shall be used in the uplink, the MS shall ignore the EPC Uplink Power Control Command and use normal power control.

– the MS shall confirm, in the SACCH L1 header on the uplink, the RF power control level used at the last burst of the previous SACCH period, as specified for normal power control.

NOTE: The term "normal power control" is used in this specification only for clarification and is otherwise only referred to as "power control".

In case of a multislot configuration, each bi‑directional channel shall be power controlled individually by the corresponding SACCH, E-IACCH or EPCCH, whichever is applicable. Power control information on downlink unidirectional channels shall be neglected.

4.2.2 (EC-)RACH

4.2.2.1 Random access procedure when EC operation is not enabled

When accessing a cell on the RACH (random access) and before receiving the first power command during a communication on a DCCH or TCH (after an IMMEDIATE ASSIGNMENT), on DCS 1800 and PCS 1900 frequency bands the MS shall use the power level defined by the MS_TXPWR_MAX_CCH parameter broadcast on the BCCH of the cell. On all other bands the MS shall limit the power level to LB_MS_TXPWR_MAX_CCH + Band_offset, if LB_MS_TXPWR_MAX_CCH parameter is broadcast, otherwise the power level is limited according to the MS_TXPWR_MAX_CCH parameter. Band_offset equals 0 dB for GSM 850, ER-GSM 900 and GSM 900, -2 dB for GSM 700 and -6 dB for GSM 400. As an exception, on the DCS 1800 band the class 3 DCS 1 800 capable MS shall use the power level defined by MS_TXPWR_MAX_CCH plus the value POWER_OFFSET also broadcast on the BCCH of the cell.

In addition, if the network indicates support for MS power reduction by broadcasting parameter INIT_PWR_RED (see 3GPP TS 44.018) and if the latest RLA-value, RLA_C or RLA_P (see section 6.1) for the measured signal strength from the BTS the MS is accessing is -48 dBm or higher immediately before the access attempt, the MS power shall not exceed

PRED = min{(MS_TXPWR_MAX_CCH, (LB_MS_TXPWR_MAX_CCH + Band_offset), (P5‑INIT_PWR_RED)} for GSM 400, GSM 700, T-GSM 810, GSM 850, ER-GSM 900 and GSM 900 and

PRED = min{ MS_TXPWR_MAX_CCH, (P0+2‑INIT_PWR_RED)} for DCS 1800 and PCS 1900,

where P5 and P0 are the power control levels for respective band in 3GPP TS 45.005.

The power reduction only applies for the first transmission of the access burst on the RACH. If the initial transmission fails due to no response from the network, the MS shall not apply power reduction in remaining transmissions. The power reduction also applies for DCCH or TCH (after an IMMEDIATE ASSIGNMENT) under the same received signal strength conditions until the ordered power control level in the SACCH L1 header differs from MS_TXPWR_MAX_CCH or LB_MS_TXPWR_MAX_CCH + Band_offset, whichever is applicable or a L3 message with a valid power control command is received.
If INIT_PWR_RED is not broadcast, no power reduction shall apply.

If a power control level defined in 3GPP TS 45.005 is received but the level is not supported by the MS, the MS shall use the supported output power which is closest to the output power indicated by the received power control level.

4.2.2.2 Random access procedure when EC operation is enabled

When accessing a cell on the RACH or EC-RACH when EC operation has been enabled the MS shall use an open-loop power control in determining the power level for the first transmission.

The calculated output power, P, to be employed by the MS for the first transmission of the access burst (including any blind physical layer transmissions) on (EC-)RACH after uplink coverage class selection (see subclause 6.10.3) shall be:

P = min(MSPWR, MS_TXPWR_MAX_CCH, L+TARGET_RX_PWR)

where

TARGET_RX_PWR = BT_Threshold_UL + BT_Threshold_UL_Margin

L = BSPWR – RLA_EC (see subclause 6.9)

BT_Threshold_UL indicates the BS_RX_PWR below which blind physical layer transmissions are used on EC-RACH (broadcast in EC SI 2, see 3GPP TS 44.018 [17])

BT_Threshold_UL_Margin indicates the power margin, in dB, above BT_Threshold_UL (see 3GPP TS 44.018 [17]) that is used to indicate the targeted received power at the BTS.

BSPWR is the output power of the BTS used on FCCH and EC-SCH (broadcast in EC SI 2 (see 3GPP TS 44.018 [17]))

MS_TXPWR_MAX_CCH is the maximum allowed output power on the (EC-)RACH, broadcast in EC SI 2 (see 3GPP TS 44.018 [17]) and

MSPWR is the nominal maximum output power of the MS.

All power values are expressed in dBm, except L and BT_Threshold_UL_Margin which are expressed in dB.

If the first transmission of the access burst (including any blind physical layer transmissions) fails (i.e., no response is received from the network), the calculated output power for retransmissions shall be:

P = min(MSPWR, MS_TXPWR_MAX_CCH).

The MS may round the calculated output power P to the nearest nominal output power value (see 3GPP TS 45.005). The output power actually transmitted by the MS shall fulfil the absolute accuracy as specified in 3GPP TS 45.005 [24]. In addition, the transmitted power shall be a monotonic function of the calculated output power and any change of 2 dB in the calculated value shall correspond to a change of 2 1.5 dB in the transmitted value.

If a calculated output power is not supported by the MS, the MS shall use the supported output power which is closest to the calculated output power.

An MS transmitting blind physical layer transmissions shall use the same output power on all blind physical layer transmissions of the access burst.

4.3 MS power control range

The range over which a MS shall be capable of varying its RF output power shall be from its maximum output down to its minimum, in steps of nominally 2 dB.

3GPP TS 45.005 gives a detailed definition of the RF power level step size and tolerances.

In A/Gb mode, the fast power control scheme for E-TCH and the enhanced power control scheme for TCH and O-TCH are based on differential control to adjust the employed RF power level. Similarly in Iu mode, the fast power control scheme for E-TCH and the enhanced power control scheme for DBPSCH are also based on differential control to adjust the employed RF power level. The possible DL power control commands are listed in the following table.

Codeword	Power control command
0	Not used
1	Increase output power by four power control levels
2	Increase output power by three power control levels
3	Increase output power by two power control levels
4	Increase output power by one power control level
5	No output power level change
6	Decrease output power by one power control level
7	Decrease output power by two power control levels

If a power control command is received but the requested output power is not supported by the MS, the MS shall use the supported output power which is closest to the requested output power.

4.4 BSS implementation

In A/Gb mode, RF power control, including fast power control for E-TCH and enhanced power control for TCH and O-TCH, may optionally be implemented in the BSS.

Similarly in Iu mode, RF power control, including fast power control for E-TCH and enhanced power control for DBPSCH, may optionally be implemented in the BSS.

4.4.1 VAMOS subchannel power control for BSS in downlink

For a TCH in VAMOS mode in downlink, the BSS may optionally implement VAMOS subchannel specific power control. A BSS supporting VAMOS shall support AQPSK modulation with at least one value of (see 3GPP TS 45.004).

4.5 BSS power control range

The range over which the BSS shall be capable of reducing its RF output power from its maximum level shall be nominally 30 dB, in 15 steps of nominally 2 dB.

3GPP TS 45.005 gives a detailed definition of the RF power level step size and tolerances.

4.6 Strategy

The RF power control strategy employed by the network determines the ordered power level that is signalled to the MS, and the power level that is employed by the BSS.

The power level to be employed in each case will be based on the measurement results reported by the MS/BTS and various parameters set for each cell. The exact strategies will be determined by the network operator. A detailed example of a basic algorithm appears in annex A.

4.7 Timing

4.7.1 Normal Power Control

Upon receipt of a command from an SACCH to change its power level on the corresponding uplink channel, the MS shall change to the new level at a rate of one nominal 2 dB power control step every 60 ms (13 TDMA frames), i.e. a range change of 15 steps should take about 900 ms. The change shall commence at the first TDMA frame belonging to the next reporting period (as specified in subclause 8.4). The MS shall change the power one nominal 2 dB step at a time, at a rate of one step every 60 ms following the initial change, irrespective of whether actual transmission takes place or not.

In case of channel change, except for multislot configuration change, the commanded power control level shall be applied on each new channel immediately. The multislot configuration change message does not command the MS to use new power control levels. For those time slots not used by the MS before the multislot configuration change procedure, the MS shall use the power control level used on the main channel before the multislot configuration change.

4.7.2 Fast Power Control

Switching between the normal power control mechanism and FPC shall be done if FPC is enabled or disabled via signalling in the SACCH L1 header. The respective power control mechanism to be used shall then be active as from the first TDMA frame belonging to the next reporting period (see subclause 8.4). The initial power control level to be used by the MS immediately after switching shall, in both cases, be the level last commanded by the normal power control mechanism.

The basic timing cycle for the fast power control mechanism is the FPC reporting period of length 4 TDMA frames, which is mapped into the 26-multiframe according to the following figure.

DL measurements made during RP(n) shall be reported to the network during the next occurance of RP((n+2) mod 6). Power control commands received from the network during RP(n) are effectuated on the corresponding UL channel during the next occurance of RP((n+1) mod 6).

4.7.3 Enhanced Power Control

When in enhanced power control (EPC) mode, the MS shall for uplink power control obey either the EPC Uplink Power Control Commands or the Ordered MS Power Level. This is controlled by signalling via the SACCH L1 header in the downlink (see 3GPP TS 44.004) and during channel assignment (see 3GPP TS 44.018). The type of power control commands to be obeyed by the MS during one SACCH period is determined by what is signalled in the L1 header during the previous SACCH period and, before any SACCH block has been correctly decoded, by what is signalled during channel assignment.

NOTE: This signalling via the SACCH L1 header and during channel assignment only controls the uplink power control mechanism. In A/Gb mode, EPC measurement procedures shall always be followed by the MS when on a TCH or O-TCH in EPC mode. Similarly in Iu mode, EPC measurement procedures shall always be followed by the MS when on a DBPSCH in EPC mode.

When the MS is ordered to obey the Ordered MS Power Level, the timing according to subclause 4.7.1 applies.

When the MS is ordered to obey the EPC Uplink Power Control Command, it shall, upon receipt of an EPC Uplink Power Control Command on an EPCCH in the downlink, change to the new power level on the corresponding uplink channel at the first TDMA frame belonging to the next EPC reporting period (as specified in subclause 8.4.1b).

4.8 Dedicated channels used for a voice group call or voice broadcast

The network shall not allocate the uplink of the channel used for a voice group call to more than one MS. If marked busy, no other MS shall transmit on the channel. This marking is indicated by the network, as defined in 3GPP TS 43.068 and 3GPP TS 44.018. Any MS allocated the uplink of a channel used for a voice group call shall only transmit if the uplink is marked busy, and shall stop using the uplink if it happens to become marked free. An MS not allocated the uplink may perform a random access procedure on the uplink to gain access to talk, only if the uplink is marked as free.

On a channel used during a voice group call, the uplink power control shall only apply to the MS currently allocated that uplink, and the MS power control level ordered by the network shall be ignored by all other MSs listening to the downlink.

When performing a random access on a cell to gain access to the uplink of a channel used for a voice group call, until receiving the first dedicated power command from the network, the MS shall use the last received power level command as defined by the MS_TXPWR_MAX_CCH parameter broadcast on the BCCH of the cell, or if MS_TXPWR_MAX_CCH corresponds to a power control level not supported by the MS as defined by its power class in 3GPP TS 45.005, the MS shall act as though the closest supported power control level had been broadcast.

RF downlink power control will normally not be applied on channels used for a voice group call or voice broadcast.