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Huawei Confidential. All Rights Reserved
WCDMA Power Control Principle
ISSUE 1.0
2 Internal Use
Chapter 1 Power OverviewChapter 1 Power Overview
Chapter 2 Power Control AlgorithmChapter 2 Power Control Algorithm
3 Internal Use
Purpose of power controlPurpose of power control
Purpose of power controlPurpose of power control
Uplink channelUplink channel: To overcome the near-far effect.
Downlink channelDownlink channel : Overcome fast fading and the interferences of adjacent cells.
Power control must be used in CDMA system to ensure every user transmits
at minimum power, thus maximizing network capacity
4 Internal Use
Power control classificationPower control classification
Power control classification:
Open loop Power controlOpen loop Power control
Closed loop Power controlClosed loop Power control
Uplink inner power control
Downlink inner-power control
Uplink outer power control
Downlink outer power control
The purpose of inner loop power control in WCDMA system is to maintain a certain
signal-to-interference ratio of transmission signal power when the signals reach the
receiving end.
However, for different multi-path environments, even if the mean signal-to-
interference ratio is kept above a certain threshold, the communication quality
requirement (BER) can not always be satisfied .
5 Internal Use
The Relationship between Transmitted Power and The Relationship between Transmitted Power and Received Power after Power Control Methods IntroducedReceived Power after Power Control Methods Introduced
0 200 400 600 800-20
-15
-10
-5
0
5
10
15
20
Time (ms)
Rel
ati
ve
po
wer
(d
B)
Channel
Transmitted power
Received power
6 Internal Use
Benefit of Power ControlBenefit of Power Control
Benefit of power control
Overcome uplink near-far problem
Ensure good communication quality in uplink and downlink by adjust
transmission power
Overcome the influences of unfavorable factors such as fast fading, slow fading
on radio channels
Decrease network interference, increase the capacity and quality of network
In other word, the purpose of power control is to ensure the QoS with minimum
power in the CDMA system.
7 Internal Use
Power control methods adopted for various physical channelsPower control methods adopted for various physical channels
Power control methods adopted for various physical channels "X" – can be applied, "–" – not applied
Physical channel
Open loop power control
Inner loop power control
Outer loop power Control
No power control process, power is specified by upper
layers.
DPDCH - X X -
DPCCH X X X -
PCCPCH - - - X
SCCPCH - - - X
PRACH X - - -
AICH - - - X
PICH - - - X
8 Internal Use
Chapter 1 Power OverviewChapter 1 Power Overview
Chapter 2 Power Control AlgorithmChapter 2 Power Control Algorithm
9 Internal Use
1.Open loop power control1.Open loop power control
2.Inner-loop power control2.Inner-loop power control
3.Outer loop power control3.Outer loop power control
Chapter 2 Power Control AlgorithmChapter 2 Power Control Algorithm
10 Internal Use
Open Loop Power Control OverviewOpen Loop Power Control Overview
Purpose
UE estimates the power loss of signals on the propagation path by
measuring the downlink channel signals, then calculate the transmission
power of the uplink channel.
Mainly used for initial connection setup.
Under the FDD mode, fast fading of the uplink channel is unrelated to fast
fading of the downlink channel.
11 Internal Use
Open Loop Power Control OverviewOpen Loop Power Control Overview
Disadvantage of open loop power control
This power control method is rather vague
Application scenarios of open loop power control
In the range of a cell, signal fading caused by fast fading is usually more
serious than that caused by propagation loss. Therefore, open loop
power control is applied only at the beginning of connection setup,
generally in setting the initial power value.
12 Internal Use
Open Loop Power Control of PRACHOpen Loop Power Control of PRACH
Random access procedure of PRACHRandom access procedure of PRACH : - UE transmit a preamble using the selected uplink access slot, signature, and preamble
transmission power. - Then ,UTARN response by sending AI if the preamble is received. - Next, UE transmit the message part if the AI is received. - But, if UE does not receive the AI from UTRAN in τp-p period, a next preamble will be
transmitted.
AICH accessslots RX at UE
PRACH accessslots TX at UE
One access slot
p-a
p-mp-p
Pre-amble
Pre-amble
Message part
Acq.Ind.
13 Internal Use
Open Loop Power Control of PRACHOpen Loop Power Control of PRACH
The initial value of PRACH power is set through open loop power control
Preamble_Initial_Power = PCPICH DL TX power Preamble_Initial_Power = PCPICH DL TX power -- CPICH_RSCP + UL CPICH_RSCP + UL
Interference + Constant ValueInterference + Constant Value
ParametersParameters
PCPICH DL TX power, UL interference and Constant are given in system information.
The value of CPICH_RSCP is measured by UE
PCPICH DL TX power is very closed to the downlink coverage ability,
which is already given in cell setup.
UL interference can be measured by NodeB, then it will be reported to RNC.
Constant Value is the threshold of preamble message. This value has to be
analysed very carefully.
14 Internal Use
Open loop power control of PRACHOpen loop power control of PRACH
NO. Parameter Parameter meaning
1 Power Offset Pp-m The power offset of the last access preamble and message control part. This value plus the access preamble power is the power of the control part
2 Constant Value This parameter is the correction constant used for the UE to estimate the initial transmission power of PRACH according to the open loop power
3 PRACH Power Ramp Step This parameter is the ramp step of the preamble power when the UE has not received the capture indication from NodeB
4 Preamble Retrans Max This parameter is the permitted maximum preamble repeat times of the UE within a preamble ramp cycle
Power Ramp Step
Pp-m
10ms/20ms
Preable_Initial_power
15 Internal Use
Open loop power control of PRACHOpen loop power control of PRACH
Different Constant Values for different stage of WCDMA network
lifecycle. Take the beginning stage for example:
Constant Value can be greater (-16dB or -15dB) so that the preamble
message can be received easier by UTRAN
The power ramp step could be greater so that the possibility which the
preamble message can be received correctly will be higher
With the development of network, the number of users increased
very fast. On this stage, the Constant value could be less 1dB.
16 Internal Use
Open loop power control of PRACHOpen loop power control of PRACH Open loop power control of PRACHOpen loop power control of PRACH
Application scenariosApplication scenarios
1. CCCH : RRC Connection Request
Open loop power control of PRACH
5. Downlink Synchronisation
UE Node BServing RNS
Serving RNC
DCH - FP
Allocate RNTISelect L1 and L2parameters
RRC RRC
NBAP NBAP
3. Radio Link Setup Response
NBAP NBAP
2. Radio Link Setup Request
RRC RRC
7. CCCH : RRC Connection Set up
Start RX description
Start TX description
4. ALCAP Iub Data Transport Bearer Setup
RRC RRC
9. DCCH : RRC Connection Setup Complete
6. Uplink Synchronisation
NBAP NBAP
8. Radio Link Restore Indication
DCH - FP
DCH - FP
DCH - FP
17 Internal Use
Open loop power control of DL DPCCHOpen loop power control of DL DPCCH
The DL DPCCH open loop power control can be calculated by the
following formula:
P =P = (( Ec/Io)Ec/Io)ReqReq - CPICH_Ec/Io + PCPICH - CPICH_Ec/Io + PCPICH
ParametersParameters
(Ec/Io)req is the required Ec/Io, which should be satisfied for UE to be able to
received the message from dedicated channel correctly
CPICH_Ec/Io is measured by UE, then it is given to UTRAN by RACH
PCPICH is the transmission power of CPICH
Comments
Similar to UL, the (Ec/Io)Req value should be considered very carefully
Because there is no power ramp in the initial DL DPCCH, the initial power should
be satisfied with the requirements. Therefore, this value can be greater than the
one from simulation to ensure the success ratio
18 Internal Use
Open loop power control of DL Open loop power control of DL DPCCHDPCCH Open loop power control of DL Open loop power control of DL DPCCHDPCCH
Application scenariosApplication scenarios
1. CCCH : RRC Connection Request
Open loop power control of DPCCH
5. Downlink Synchronisation
UE Node BServing RNS
Serving RNC
DCH - FP
Allocate RNTISelect L1 and L2 parameters
RRC RRC
NBAP NBAP
3. Radio Link Setup Response
NBAP NBAP
2. Radio Link Setup Request
RRC RRC
7. CCCH : RRC Connection Set up
Start RX description
Start TX description
4. ALCAP Iub Data Transport Bearer Setup
RRC RRC
9. DCCH : RRC Connection Setup Complete
6. Uplink Synchronisation
NBAP NBAP
8. Radio Link Restore Indication
DCH - FP
DCH - FP
DCH - FP
19 Internal Use
Open loop power control of UL DPCCHOpen loop power control of UL DPCCH
The UL DPCCH open loop power control can be calculated by the following formula:
DPCCH_Initial_powerDPCCH_Initial_power == PCPICH DL TX power - PCPICH DL TX power - CPICH_RSCPCPICH_RSCP
+ UL interference + + UL interference + DPCCH_SIRtargetDPCCH_SIRtarget
ParameterParameter PCPICH DL TX power is the transmission power of CPICH CPICH_RSCP can be measured by UE UL interference can be measured by NodeB
Comments The DPCCH_SIRtarget value should be considered very carefully.
It reflects the lowest requirement for decoding the DPCCH in a certain multiple path environment
20 Internal Use
Open loop power control of UL Open loop power control of UL DPCCHDPCCH Open loop power control of UL Open loop power control of UL DPCCHDPCCH
Application scenariosApplication scenarios
1. CCCH : RRC Connection Request
Open loop power control of DPCCH
5. Downlink Synchronisation
UE Node BServing RNS
Serving RNC
DCH - FP
Allocate RNTISelect L1 and L2parameters
RRC RRC
NBAP NBAP
3. Radio Link Setup Response
NBAP NBAP
2. Radio Link Setup Request
RRC RRC
7. CCCH : RRC Connection Set up
Start RX description
Start TX description
4. ALCAP Iub Data Transport Bearer Setup
RRC RRC
9. DCCH : RRC Connection Setup Complete
6. Uplink Synchronisation
NBAP NBAP
8. Radio Link Restore Indication
DCH - FP
DCH - FP
DCH - FP
21 Internal Use
1.Open loop power control1.Open loop power control
2.Inner-loop power control2.Inner-loop power control
3.Outer loop power control3.Outer loop power control
Chapter 2 Power Control AlgorithmChapter 2 Power Control Algorithm
22 Internal Use
CClose loop power controllose loop power control
Weakness of open loop power control
Open loop power control can decided the initial power, but it is not accurate
For WCDMA-FDD system, the uplink fading is not related to the downlink
one because of the big frequency interval of them
Therefore, the path loss and interference estimated by downlink can not reflect
the one in uplink completely. But, the close loop power control can solve this
problem
The advantages of close loop power control
Can control the transmission power of uplink and downlink very fast, and
decrease interference in system.
Maintains a higher quality of service
Why the close loop power control is neededWhy the close loop power control is needed
23 Internal Use
IInner-loop power controlnner-loop power control
The receivers calculate the SIR by estimating the power strength
and the current interference. Then, compare this one to SIRtarget
If SIRSIRmeasured <measured < SIRtarget SIRtarget, send TPC = TPC = 11 to inform receivers increase
its transmission power
If SIRSIRmeasured >measured > SIRtarget SIRtarget, send TPC = 0TPC = 0 to inform receivers decrease
transmission power
The receiver that receives the TPC will adjust the transmission power by
algorithms. The inner loop power control can convergence the
estimated SIR to SIR target
The principle for Inner-loop power controlThe principle for Inner-loop power control
24 Internal Use
IInner-loop power controlnner-loop power control
In 3GPP protocol, two algorithms are adopted in the inner-loop
power control of uplink DPCCH
PCA1 : PCA1 : uplink power control step is tpc=△tpc=△ 11dB or dB or 22dBdB
PCA2 : PCA2 : uplink power control step is tpc=△tpc=△ 11dBdB
The power control adjustment range in DPCCH is
△△ DPCCH= tpc×TPC_cmd△DPCCH= tpc×TPC_cmd△
TPC_cmd is achieved by different algorithm
The power offset shows the difference of transmission power of UL
DPCCH and UL DPDCH
The adjustment range of DPDCH is the same as the DPCCH.
The power offset is decided by the signaling from higher layer
IInner-loop power control Algorithmnner-loop power control Algorithm
25 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
NodeB compares SIRSIRmeasuredmeasured with the preset target signal-to-interference ratio
(SIRtargetSIRtarget).
NodeB
UETransmit TPC
Inner-loop
set SIRtar
1500Hz 1500Hz
Each UE has own loop Each UE has own loop
26 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
2
2
d
c
DPDCH/DPCCH structureDPDCH/DPCCH structure
The power ratio of DPCCH to DPDCH is
Pilot N pilot bits
TPC NTPC bits
DataNdata bits
Slot #0 Slot #1 Slot #i Slot #14
Tslot = 2560 chips, 10 bits
1 radio frame: T f = 10 ms
DPDCH
DPCCHFBI
N FBI bitsTFCI
N TFCI bits
Tslot = 2560 chips, N data = 10*2 k bits (k=0..6)
27 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
The uplink DPCCH SIR should be estimated by different serving cells.
In each time slot, the TPC can be generated by the following rules:
No soft handover
If SIR estimation > SIR target,
then TPC = 0 to decrease the transmission power
If SIR estimation < SIR target,
then TPC = 1 to increase the transmission power
Soft handover
In one time slot, UE received several TPC, then combine then.
Comments
In the last situation, the PCA decides how TPC_cmd are combined.
The PCA has two methods. UTRAN decides which one is used.
TPCTPC
28 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
UE can adjust the UL DPCCH transmission power with △△ tpctpc step
according to the received TPC_cmdTPC_cmd
The step △△ tpctpc can be 1dB or 2dB, which is decided by UTRAN
If the TPC_cmd = 1 , the UL DPCCH and UL DPDCH transmission
power should be increased △ tpc
If the TPC_cmd = -1 , the UL DPCCH and UL DPDCH transmission
power should be decreased △ tpc
If the TPC_cmd = 0 , the UL DPCCH and UL DPDCH transmission
power should be decreased △ tpc
29 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
UE in Non - Soft handover caseUE in Non - Soft handover case
UE receives only one TPC
If TPC = 0, TPC_cmd = -1
If TPC = 1, TPC_cmd = 1
PCA1PCA1
30 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
UE in Soft handoverUE in Soft handover
UE receives several TPCs in one time slot and combine them to get one TPC_cmd
by the following two steps:
First, combine the TPCs from one RLS
Then, combine the TPCs from different RLS
Comments
The TPC from RLSi is Wi
PCA1PCA1
31 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
Wi can be achieved by the following rules
If the TPC = 0, Wi=0
If the TPC = 1, Wi = 1
Assume UE has N RLSs , N TPC can be obtained after
combination, W1 、 W2…WN. The combination method for these
N TPCs from N RLSes can be described as following formula
TPC_cmd = γ (W1, W2, … WN)
γ function should satisfied:
If one Wi = 0, TPC_cmd = -1
If all Wi are 1 , TPC_cmd = 1
PCA1PCA1
32 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
UE in non-soft handover modeUE in non-soft handover mode
Only one TPC is received in one time slot. The power control can be done once by
each 5 time slots. Each frame is divided 3 groups of 5 time slots. In the first 4 time
slots, the TPC_cmds are 0, which means the power does not change. In the 5th
time slot, the TPC_cmd can be achieved by the following rules:
If all TPC are 0, the TPC_cmd is -1 and the transmission will decrease 1dB;
If all TPC are 1, the TPC_cmd is 1 and the transmission will increase 1dB;
Otherwise, TPC_cmd=0.
TPC ( RX) TPC_cmd
0000 0 0000 -1
1111 1 0000 1
else 0000 0
PCA2PCA2
33 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
UE in soft handoverUE in soft handover
TPC_cmd can be achieved by the following two steps
First, combine the TPC from the same RLS
N TPCi (i = 1,2......N) obtained from N RLSs in each time slot
The N TPC_cmds from different RLS can be achieved by the above mentioned rules.
Assume each final TPC_cmd from N RLS are TPC_tempi ( i = 1,2......N )
TPC_cmd in the fifth time slot can be obtain by the following rules :
TPC_cmd (5th slot) = γ(TPC_temp1, TPC_temp2, …, TPC_tempN)
where γ is defined as follows:
or
otherwise, TPC_cmd = 0.
PCA2PCA2
N
ii cmdTPCtempTPC
N 1
1_,5.0_1
N
ii cmdTPCtempTPC
N 1
1_,5.0_1
34 Internal Use
Uplink-inner loop power controlUplink-inner loop power control
The control frequency
PCA1, the power control frequency is 1500Hz
PCA2, the power control frequency is 300Hz
Application scenarios
When UE is moving with high speed (80Km/h), the fast inner-loop
power control can not catch up with the fast fading, but will produce
negative gain. In this situation, PCA2 is preferred.
Comparison between PCA1 and PCA2Comparison between PCA1 and PCA2
35 Internal Use
Downlink Inner-loop power controlDownlink Inner-loop power control
NodeB
Transmit TPC
Measure SIR and compare
Inner-loop
1500Hz
36 Internal Use
Downlink inner-loop power controlDownlink inner-loop power control
Inner-loop power control of downlink DPCCH include two types: - Inner-loop power control in compressed mode, - Inner-loop power control in non-compressed mode.
Timeslot structure of Downlink DPCH :
- PO3 defines the power offset of the TFCI bit in the downlink DPCCH to DPDCH.
- PO2 defines the power offset of the TPC bit in the downlink DPCCH to DPDCH.
- PO1 defines the power offset of the Pilot bit in the downlink DPCCH to DPDCH.
- The values of PO1 、 PO2 and PO3 are defined by RNC.
37 Internal Use
Downlink inner-loop power controlDownlink inner-loop power control
First, UE should estimate the downlink DPDCH/DPCCH power and the
current SIR
Then, UE can generate TPC by comparing SIRestimated to SIRtarget
If SIRestimated > SIRtarget ; TPC = 0 (decrease power)
If SIRestimated < SIRtarget ; TPC = 1 (increase power)
The step of DL inner-loop power control could be 0.5, 1, 1.5 or 2dB
38 Internal Use
Downlink inner-loop power controlDownlink inner-loop power control
When UE is not in soft handover TPC generated by UE is transmitted in TPC domain of UL channel
When UE in soft handover, two power control modes can be used, which is
decided by DPC_mode: DPC_MODE = 0 , UE will transmit TPC in every slot
DPC_MODE = 1 , UE will transmit the same TPC in every three time slot
When the downlink channel is out of synchronization, UE will transmit
TPC=1 because UE can not measure the downlink SIR
Upon reception of TPC, UTRAN will adjust the downlink power of
DPCCH/DPDCH. But the transmission power can not higher than
Maximum_DL_Power and not less than Minimum_DL_Power neither.
Power control in different statePower control in different state
39 Internal Use
1.Open loop power control1.Open loop power control
2.Inner-loop power control2.Inner-loop power control
3.Outer loop power control3.Outer loop power control
Chapter 2 Power Control AlgorithmChapter 2 Power Control Algorithm
40 Internal Use
OOuter-loop power controluter-loop power control
The limitation of inner loop power control
The purpose of inner loop power control of the WCDMA system is to
maintain a certain signal-to-interference ratio of transmission signal
power when the signals reach the receiving end.
The character of outer-loop power control
The Qos which NAS provide to CN is BLER, not SIR
The relationship between inner-loop power control and outer-loop
power control
SIR target should be satisfied to be able to decoding receive signal correctly.
But different radio path radio environment has different requirement on SIR
Therefore, the outer-loop power control can adjust the SIR to get a stable
BLER in the changeable radio environment
41 Internal Use
Uplink outer loop power controlUplink outer loop power control
NodeB UE
Transmit TPC
Measure and compare SIR
Inner-loop
Set SIRtarget
get the good quality service data get the good quality service data
Out loop
RNC
Measure received data and
compare BLER
Set BLERtarget
10-100Hz
42 Internal Use
NodeB
set SIRtar
Transmit TPC
Measure and compare SIR
Measure and compare BLER
Outer loop
Inner loop L1
L3
10-100Hz1500Hz
Downlink outer loop power controlDownlink outer loop power control
43 Internal Use
Outer loop power controlOuter loop power control
SIR target SIR target adjustment step adjustment step
etBLERt
etBLERtBLERmeastepSIRAdjustSoefficientSIRAdjustcSIRtar
arg
arg**
44 Internal Use