Third Generation Network (3G), UMTS
9.1 The WCDMA Concept
9.1.4 Power Control
The signal variations (caused by fast fading) decrease with a respective increase in sig- nal bandwidth. This phenomenon is due to the loss of coherence in different multipath components. This is the same effect as the frequency diversity that can be used when the system bandwidth becomes larger than the channel coherence (correlation) band- width. The power regulation is somewhat simplified when the spread bandwidth is much larger than the correlation bandwidth. This is applicable to a typical WCDMA system radio environment. Nonetheless, power control is instrumental for WCDMA operation and used basically to:
• estimate minimum transmit power (target Signal-to-Interference Ratio) for accept- able radio link performance and
• maintain the level of Signal-to-Interference Ratio during the call.
Power control is implemented at two levels of control speed, relatively slow (open and outer) and rather fast (closed loop). Altogether, three power control mechanisms are deployed in 3G WCDMA:
• Open-loop power control
• Outer-loop power control
• Closed-loop power control.
9.1.4.1 Open-Loop Power Control
The open-loop power control mechanism is applied in both downlink and uplink com- munications. In relation to the uplink, it provides a coarse initial power setting of the mobile station for the uplink channel. The Frequency Division Duplex (FDD) WCDMA system cannot use the signal level estimation of the downlink pilot channel received by a mobile, as, for instance, is possible in GSM. The duplex separation in WCDMA is too large to the extent that fading in the uplink and downlink become uncorrelated.
The open-loop power control function is located both in the mobile station and in the network (base station and radio network controller). In the uplink direction, it sets the initial power for the random-access channel and for the uplink traffic channel. In the downlink, the open-loop power control sets the power for the downlink channels. The power levels are based on the mobile station measurements reported to the base station in terms of frame reliability indicators (FRI) related, in turn, to block error rate (BLER).
For each level of BLER of FRI there is a respective set point for Signal-to-Interference Ratio (SIR) or output power level.
9.1.4.2 Outer-Loop Power Control
The radio link performance is impacted by mobile speed, slow shadowing and multipath profile. This means that the target SIR may need to be adjusted over time as the mobile environment changes. The changes in SIR target are controlled by outer-loop power control, as shown in Figure 9.10.
Outer-loop power control function resides in the Radio Network Controller (RNC).
The RNC monitors the received quality at the mobile station and the base station. If there is a difference in the BLER and the targeted BLER with current SIR, the RNC may change the respective SIR target for either the MS or BS. An additional reason for placement of outer power control in the RNC is a soft handover in WCDMA. The outer control is always performed after soft handover combining in RNC.
Closed-Loop Power Control Closed-loop power control keeps the received signal level of the uplink signal from every mobile at the same specified level in order to minimize the interference level within the cell. Figure 9.11 shows an example of uplink closed-loop
RNC NodeB
outer-loop power control:
target SIR adjustment
Fast power control: compare SIR and target SIR, send power up/down command Figure 9.10 Outer-loop power control.
Third Generation Network (3G), UMTS 131
power control. Mobile stations UE 1 and UE 2 operate at the same carrier frequency, separable at the base station only by their respective spreading codes thus producing a noise like interference to each other. Apparently, the path loss for UE 2 is significantly higher than for UE 1. In the absence of a power control mechanism, the uplink signal from UE 1 can easily block reception of the signal from UE 1. This blocking is known as thenear-far problemin CDMA systems. The optimum strategy to ensure minimum interference and maximum possible capacity is to continuously equalize the received power toPminof all mobile stations, see Figure 9.11.
With the closed-loop power control in the uplink, the base station performs frequent estimates of the received Signal-to-Interference Ratio (SIR) and compares it to a target SIR. If the measured SIR is higher than the target SIR, the base station will command the mobile station to lower the power or increase the power in opposite case. The measure- ment cycle is executed at a rate of 1.5 kHz for each mobile station and hence operates faster than the speed of fast Rayleigh fading in car or the pedestrian environment. The power control can be implemented in steps of 1 or 2 dB.
Figure 9.12 illustrates how uplink closed-loop power control should work on a fad- ing channel at low speed. Based on measured SIR, the base station sends a closed-loop power control command to the mobile station to reduce/increase power proportional to the inverse of the SIR. In a nearly perfect situation, the received signal in the base station is essentially non-fading, as shown in the upper section of Figure 9.12.
The positive effect of increased transmit power for mitigating the fading at the cell edge or in a deep fade may produce a negative effect of increased inter-cell interfer- ence [5]. This means that the maximum output power of the mobile should also be optimized in order to minimize inter-cell interference. The same closed-loop power con- trol technique is also used on the downlink. In addition to the task of keeping reasonable link performance around the SIR target, the base station tends to keep the overall trans- mit power to a minimum in order to create some marginal reserve in power of the RF carrier to ensure a trade-off between serving the mobile stations at the cell edge and reducing inter-cell interference.
UE 1
UE 2 NB
P1 → Pmin
P2 → Pmin
Figure 9.11 Closed-loop power control.
Transmit Power Received
Power
Time Time Time
Figure 9.12 Closed-loop power control compensates for a fading channel.