Power Control (PC)
# In CDMA-based 3G systems all users can share a
common frequency
–
interference control
–
keep the transmission powers at a minimum level
–
ensure adequate signal quality and level at the
receiving end.
Open-loop PC
–
is responsible for setting the initial UL and
DL transmission powers when a UE is accessing the network.
Slow PC
–
is applied on the DL common channels.
Inner-loop PC
(also called fast closed-loop power control)
–
adjusts the transmission powers dynamically on
a 1500 Hz basis.
Outer-loop PC
estimates the received quality and adjusts the
target SIR (Signal to Interference Ratio) for the fast closed-loop PC so that
the required quality is provided.
Open Loop Power Control:
-Open loop power control is used on channels that cannot
apply closed loop power control, e.g. RACH, FACH-The transmitter power
is determined on the basis of a path loss estimate from the received power
measure of the opposite direction-To avoid excessive
interference, probes
with incremental power steps until a response is obtained: “power ramping”
Uplink Open Loop Power Control
–
The UL open-loop PC function is located both in
the UE and in the UTRAN.
Based on the calculation of the open-loop PC,
the terminal sets the initial power for the first Physical Random Access
Channel (PRACH) preamble and for the UL Dedicated Physical Control Channel
(DPCCH) before starting the inner-loop PC.
Downlink Open Loop Power Control
This function is located in both the UTRAN and the UE
In the Downlink, the open-loop PC is used to set
the initial power of the downlink channels based on the DL measurement reports
from the UE.
Closed Loop Power Control
- Closed loop power
control is used on channels which are established in both directions, such as
DCH
-The receiver generates
transmit power commands (TPC) based on the estimated received quality; the TPC are sent back to the
transmitter in the opposite direction
-The transmit power is adjusted according to the
received TPC
Inner Loop Power Control
-The inner-loop PC relies on the feedback
information at Layer I
-This allows the UE/Node B to adjust its
transmitted power based on the received SIR level at the Node B/UE for
compensating the fading of the radio channel.
-The inner-loop PC function in UMTS is used for
the dedicated channels in both the UL and DL directions and for the Common
Packet Channel (CPCH) in UL.
-In
W-CDMA fast PC with a frequency of 1.5 kHz is supported
Outer Loop Power Control
-The aim of the outer-loop PC algorithm is to
maintain the quality of the communication at the level defined by the quality
requirements of the bearer service in question by producing adequate target SIR
for the inner-loop PC.
-Done for each DCH belonging to the same RRC connection.
-The frequency of outer- loop PC ranges
typically from 10 to 100 Hz.
