LTE Power Control
Purpose of Power Control
+ Compensate path loss , including shadow fading and multiple path fading
+ Reduce interference on the edge cell
-LTE systems use the Orthogonal Frequency Division Multiple Access (OFDMA) technique on the downlink and the Single Carrier Frequency Division Multiple Access (SC-FDMA) technique on the uplink. With these techniques, the subcarriers of UEs in a cell are orthogonal. Power control compensates for path loss and shadow fading and counteracts interference between cells. In LTE systems, power control is performed on eNodeBs and UEs.
DL Power Distribution
- Downlink power control is achieved through fixed power assignment or dynamic power control.
+ Fixed power assignment
Fixed power assignment is applicable to the cell-specific reference signal, synchronization signal, PBCH, PCFICH, and the PDCCH and PDSCH that carry common information of the cell. Users configure fixed power based on channel quality. The configured power must meet the requirements for the downlink coverage of the cell.
+ Dynamic power control
Dynamic power control is applicable to the PHICH and the PDCCH and PDSCH that carry dedicated information sent to UEs. Dynamic power control lowers interference, expands cell capacity, and increases coverage while meeting users' QoS requirements. However, these channels can also support fix power assignment, and in fact, this is our recommendation because the AMC function can also meet the requirement of QoS
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Cell Specific RS Power Setting
EPRE: Energy Per Resource Element
The RS power setting is based on EPRE
RS power is the reference power for the other channel
- The cell-specific reference signal is transmitted in all downlink subframes.
- The signal serves as a basis for downlink channel estimation, which is used for data demodulation.
The power for the cell-specific reference signal is set through the ReferenceSignalPwr parameter, which indicates the Energy Per Resource Element (EPRE) of the cell-specific reference signal.
PB indicator the power ratio between type B symbol and type A symbol, which is specified by 3GPP protocol
RS Power ＝ Total power per channel(dbm) – 10lg(total subcarrier)+10lg(Pb + 1)
Pb determine the RS occupation of total power, Pb=1 indicate the RS power is 9.4% of total power
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Synchronization Signal Power Setting
The synchronization signal is used for cell search and system synchronization. There are two types of synchronization signals, the Primary Synchronization Channel (P-SCH) and the Secondary Synchronization Channel (S-SCH).
The offset of the power for the P-SCH and S-SCH against the power for the cell-specific reference signal is set through the SchPwr parameter.
PowerSCH ＝ ReferenceSignalPwr +SchPwr
PBCH/PCFICH Power Setting
On the PBCH, broadcast messages are sent in each frame. The messages carry the basic system information of the cell, such as the cell bandwidth, antenna configuration, and frame number.
The offset of the power for the PBCH against the power for the cell-specific reference signal is set through the PbchPwr parameter.
The PCFICH carries the number of OFDM symbols used for PDCCH transmission in a subframe. The PCFICH is always mapped to the first OFDM symbol of each subframe.
The power for the PCFICH is set through the PcfichPwr parameter, which indicates an offset of the power for the PCFICH against the power for the cell-specific reference signal.
PowerPBCH ＝ReferenceSignalPwr + PbchPwr
PowerPCFICH ＝ ReferenceSignalPwr + PcfichPwr
PDCCH/PDSCH Power Setting
In the following two status, the power for PDCCH and PDSCH power is fixed setting
When PDCCH carry scheduling information of common control information (RACH response /paging/SIBs indicator )
When PDSCH carry the common info (RACH response/SIB/paging message)
Dynamic Power Allocation - PHICH
The PHICH occupies very few resources, which means decreasing its transmit power cannot significantly reduce power consumption. In addition, the PHICH carries the ACK/NACK messages for the uplink data, which requires high accuracy. Decreasing the transmit power of the PHICH may reduce accuracy and uplink data rate. Therefore, the dynamic power allocation is not recommended in commercial network.
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Dynamic Power Control - PDCCH
When PDCCH carry the following dedicate info, power control should be performed to ensure the receive reliability
Uplink scheduling information (DCI format 0)
Downlink scheduling information (DCI format 1/1A/1B/2/2A)
PUSCH/PUCCH TPC commands (DCI format 3/3A)
PDSCH Power Presentation
The presentation of PDCCH power
Regarding power control for the PDSCH, the OFDM symbols on one slot can be classified into two types. Above table shows the OFDM symbol indexes within a slot where the ratio of the EPRE to the EPRE of RS is denoted by ρA or ρB.
Power control for the PDSCH determines the EPREs of different OFDM symbols using ρA and ρB. ρA determines the power offset against the power for the RS when there is no reference signal on the PDSCH, and ρB determines the power offset against the power for the cell-specific reference signal when there is a reference signal on the PDSCH.
PPDSCH_A ＝ ρA + ReferenceSignalPwr
PPDSCH_B ＝ ρB + ReferenceSignalPwr