Telecom knowledge and experience sharing: UMTS Power Control

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Power Control Overview
R99 Power Control
HSDPA Power Control

Power Control Overview

Power Control

CDMA is not a new technology.

Power control is a key technology of CDMA system.

Power control is the key path for launching the large scale CDMA commercial network.

UMTS is a typical self-interference system, thus the chief principle is that any potential surplus transmitted power for service must be controlled.

Why Power Control?

All UMTS users occupy the same frequency spectrum at the same time! Frequency and time are not used as discriminators.

UMTS operates by using codes to discriminate between users.

UMTS interference comes mainly from nearby users.

Each user is a small voice in a roaring crowd -- but with a uniquely recoverable code.

To achieve acceptable service quality, the transmit power of all users must be tightly controlled so that their signals reach the base station with the same signal strength and the absolute minimum power level demanded to avoid the Near-Far Effect.

Near-Far Effect

Eb/No & Power Control

Fast Power Control

Purpose of Power Control

Category of Power control

Open Loop

Measure the channel interference condition and adjust the initial transmitted power

Close Loop－Inner Loop

Measure the SIR (Signaling to Interference Ratio), compare with the target SIR value, and then send power control instruction to UE.

The frequency of WCDMA inner loop power control is 1500Hz.

ØIf measured SIR>target SIR, decrease the UE transmitted power.

ØIf measured SIR

Close Loop－Outer Loop

Measure the BLER (Block Error Rate), and adjust the target SIR.

The frequency of WCDMA outer loop power control is 10~100Hz.

ØIf measured BLER>target BLER, decrease the target SIR value.

ØIf measured BLER

Open Loop Power Control

Unbalanced for UL/DL signal, not accurate, only used in initial stage

Open loop power control

Close Loop – Inner Loop Power Control

Close Loop – Outer Loop Power Control

Closed Loop Power Control

R99 Power Control

Physical Random Access Channel

Uplink open loop power control of PRACH

Preamble_Initial_Power = Primary CPICH DL TX power – CPICH_RSCP +UL interference + Constant Value

Primary CPICH DL TX power is the transmit power of the main pilot channel (SIB5/6).
UL interference is the uplink interference, which is measured and obtained by NodeB and updated in real time in SIB7.
Constant Value is a value related with the cell environment. It is a value depends on the service rate and quality carried by PRACH.

If the value of Preamble_Initial_Power exceeds the allowed maximum power, UE sets the preamble transmit power as the allowed maximum power.

Transmit power of PRACH message part

Power Ramp Step is the power offset between two continuous preambles.
Pp_m is the power offset between the control channel and the last preamble of the message part.

Uplink Dedicated Physical Channel

Uplink open loop power control of dedicated channel

DPCCH_Initial_power = DPCCH_Power_offset - CPICH_RSCP

The value of DPCCH_Power_offset is determined by DPCCH open loop power control method.
The value of CPICH_RSCP is the CPICH channel code power obtained by UE through measurement.

Eb/No is the quality factor of the DPCCH PILOT domain.

NT+IT is the uplink interference, which is obtained by NodeB through measurement and updated in real time in SIB7.
PG is the spectrum spread gain, 256.
CPICH_TX_Power is the transmit power of the P-CPICH.

DPCCH & DPDCH power gain factor

Downlink common channel initial power

The transmit power of P-CPICH depends on the proportion of maximum transmit power of a cell.
The values of P-CCPCH, P-SCH, S-SCH, AICH, and PICH depend on the offset to P-CPICH.
The transmit power of the data domain of S-CCPCH depends on the PCH transmit power and the maximum value of the maximum transmit power of FACH. The transmit power of the TFCI domain and Pilot domain of S-CCPCH are indicated respectively by the offsets (PO1 and PO3) as opposed to the transmit power of the data domain.

Downlink Dedicated Physical Channel

Downlink open loop power control of dedicated channel

PG is the service processing gain.

Ptx,CPICH is transmission power of the CPICH (dBm).

Ec-cpich/No is CPICH Ec/N0(dB) reported by the UE.

αmin is the lower limit of the downlink orthogonal factor.

αmax is the upper limit of the downlink orthogonal factor.

k is the coefficient factor. Its fixed value is 0.01.

L represents path loss. L is obtained from the measurement result reported by the UE. If L cannot be obtained, its value is 130dB.

k1 and k2 are scenario parameters.

Ptx,total is the total transmit power of a cell before a subscriber accesses the cell.

β=10^((Eb/N0)/10), where Eb/No is the Eb/No of the sub-service configured corresponding to the current rate of the access service.

PowerOffset is different for different situation.

Downlink open loop power control of dedicated channel

On the DPCH, the bits of TFCI, TPC and PILOT are also multiplexed besides the data bits because the information carried by these bits is important. Therefore, the needed power is also a little higher than that of the data domain. The power value depends on the offset as opposed to the power of the data domain and is indicated with PO1, PO2 and PO3 respectively.

Uplink inner loop power control

At the receiving end, the SIR measurement (SIR=Eb/No) is done for each received radio link. The measurement result is compared with the target SIR (SIRtarget) required by the service.
If SIR ≥ SIRtarget, control information is returned to the sender with a transmit power command whose bit value is 0.
If SIR < SIRtarget, a TPC command whose bit value being 1 is returned through the downlink control channel to the sender.
The sender judges whether to increase or decrease the transmit power depending on the received TPC command and specified power control algorithm.

Uplink inner loop power control algorithms

Algorithm 1: the transmit power of sender can be adjusted in every timeslot. Each timeslot, the receiver judges, whether to increase or decrease the transmit power of the sender depending on the received TPC command.

Suppose the TPCs of all radio link sets are 1, then TPC_cmd=1 (increase transmit power). Suppose one TPC coming from any radio link set is 0, then TPC_cmd=-1 (decrease transmit power).

Algorithm 2: the transmit power of sender is adjusted once every five timeslots.

Transmit power is not adjusted in the first four timeslots (TPC_cmd=0). When the TPC command of the 5th timeslot is received, TPC_cmd=1 if all five received TPC commands are 1， TPC_cmd=-1 if all five received TPC commands are 0， TPC_cmd=0 in other cases.

Algorithm 1 is to perform inner loop power control at each timeslot, while algorithm 2 is to perform inner loop power control only once every five timeslots. That is, the frequency is higher to perform inner loop power control in algorithm 1, When the environment of mobile communication is quite unfavourable and the channel fades very quickly, algorithm 1 helps the transmit power to converge fast to meet the service quality requirement.
With algorithm 2, the inner loop power control is performed every five timeslots, that is, the frequency is lower to perform inner loop power control in algorithm 2. So algorithm 2 is applicable when the environment of mobile environment is quite favourable and the channel fades slowly or hardly fade.

Downlink Inner Loop Power Control

Algorithm 1: UE sends a TPC command at each timeslot. The UTRAN adjusts the transmit power at each timeslot according to the TPC command.
Algorithm 2: UE sends a TPC command for three timeslots. The UTRAN adjusts the transmit power once every three timeslots according to the TPC command.
Algorithm 1 is for fast channel fading and Algorithm 2 for slow channel fading.

Uplink Outer Loop Power Control

The initial SIRTarget value is determined upon service access, and produce the decision command by the quality information that is obtained from the measurement report. If adjustment is necessary, SIRTarget is adjusted slowly and is used to notify NodeB. NodeB compares the SIR in the dedicated measurement report with the latest SIRTarget and makes the single link SIR approach to SIRTarget through inner loop power control. In this way, the service quality will not fluctuate drastically in a changing radio environment.

Uplink Outer Loop Power Control algorithms

Principle for increase: When the tolerance BLER period (BLERAccpPeriod) has not expired yet, but the number error blocks has already exceeded the error transport block number threshold (ErrorThresh), now increase SIRTarget.
Principle for decrease: When the error block tolerance counter is no less than the tolerance BLER period (BLERAccpPeriod).

Decrease SIRTarget if now the received number of error blocks is less than the error transport block number threshold (ErrorThresh).
Keep the SIRTarget same if now the received number of error blocks equals to the error transport block number threshold (ErrorThresh).

Downlink Outer Loop Power Control

The downlink outer loop power control is realized in the UE. RNC provides BLERtarget to UE.

Control Part Gain Factor

HSDPA Power Control

Total Power Allocation of HSDPA

Static allocation by RNC

Count beforehand the average data throughput in a related area, and estimate the number of HS-PDSCHs to be configured and needed power.
Configure the percentage of power occupied by HSDPA: HspaPwrRatio in OMCR.
If the resource has to be reallocated due to changes in the average data throughput in this area, make the configuration in OMCR again and notify Node B.

Dynamic allocation by RNC

Initial HS-PDSCH and HS-SCCH total power (HspaPwrRatio) are configured in OMCR according to the number of physical HS-PDSCH+HS-SCCH channels configured for the cell.
HspaPwrRatio is adjusted dynamically along with the power occupation ratio by non-HSDPA and HS-DSCH users.
HspaPwrRatio is dynamically adjusted when the HSDPA resource congestion occurs.
The overload control module triggers HspaPwrRatio to decrease in the event of overload.
When there is no HS-DSCH user, HspaPwrRatio can only be decreased along with the power change of non-HS.

Dynamic Power Adjustment for HSDPA and DPCH

Free allocation of Node B

Free power allocation is determined by algorithm of Node B based on available power, service priority and QoS. RNC should have the allowed available power of HSDPA configured as 100%.

HS-DPCCH ACK Power Offset for Single Radio Link or Intra-Node B Handover