Power Control in UMTS

In this post I will look at different power control mechanisms that are present within UMTS. Those new to this topic will find it informative. For those familiar with the technology, it is hoped that this post will bring greater insight that are often not mentioned in the standards. Standards tell us what is to be done, not how (implementation issue) or why (design issue which happens during study, analysis and writing of standards).

It is a known fact that power control is important in any system, particularly in an age of global warming in which everyone is trying to achieve a lean carbon footprint. When it comes to mobile phones, the idea is to extend battery life by using the minimum possible power while maintaining reliable communications. From the point of any cellular network, proper power control helps in keeping interference at a manageable level while improving capacity and the overall service to the mobile subscriber.

UMTS, unlike GSM, has a greater need to combat the near-far problem. A UE close to the Node-B transmitting at the same power as another at the cell edge, will potentially block out the latter. To maintain reliable links to all UEs, the received power at the Node-B should be about the same. This means that propagation path loss between theUE and the Node-B should be taken into account. In an ideal environment, this alone is sufficient. But real environments are rarely ideal. Channel conditions vary, in the short term and in the long term. Recognizing all these, we can relate easily to the three main power control mechanism in UMTS:

1. Open loop power control: this relates directly to the path loss. As the name suggests, this control has no feedback. It simply sets the initial power at which the UE should transmit. This initial settings happens via RRC signalling. This control is in the UE and the RNC.
2. Outer loop power control: this relates to long term variations of the channel. A target SIR is specified. If the received SIR is less than this target, transmit power needs to be increased. Otherwise, it needs to be decreased. In practice, DL target quality is in terms of transport channel block error ratio (BLER). The BLER can be related to a target SIR. If the received SIR is less than the target, BLER is likely to be not met. Alternatively, if the BLER is more than the target, transmit power has to be increased. This control is in the UE and the RNC. This is also known as slow closed loop power control. It happens at the rate of 10-100 Hz.
3. Inner loop power control: this is also known as fast closed loop power control. It happens at a rate of 1500 Hz to combat fast fading. This control is with the UE and the Node-B. While outer loop control is set at RRC level and executed at Layer 1, fast power control happens at Layer 1 in order to meet the BLER target set by outer loop control. The effect of this control is that even in a fading channel, the received power is maintained constant so as to achieve the BLER target. This is represented in Figure 1 [2].

Figure 1: UMTS Fast Power Control Combating Fast Fading

Fast power control is important in keeping interference to a minimum and improving capacity. Without it, transmit power would have higher to meet quality targets. The gain from this control is as much as 5.8 dB at the receiver for pedestrian speeds for 8kbps speech with 10ms interleaving and antenna diversity. The gain is less at the transmitter and for higher speeds [2].

The problem with fast power control are the spikes in power when deep fades are encountered. This may be necessary for the connection but it also introduces interference to neighbouring cells where the UEs may not necessarily be experiencing adverse channel conditions. Recognizing this fact, the rate of fast power control can be adjusted to suit the need. For example, for non-real time services, a higher BLER can be tolerated. As a result, it is permissible to be in a fade and lose packets, leaving it to RLC to retransmit. So although 1500 Hz is the maximum rate, both UL and DL allow for lower rates by which it is meant that TPC bits do not change from slot to slot. For DL power control, DPC_MODE controls this behaviour enabling the use of same TPC for 3 slots. For UL power control, ‘Power Control Algorithm’ tells the UE how TPC bits are processed. For the slower rate, the UE considers TPC bits from 5 slots before changing its power [TS 25.214].

Anyone familiar with the operations of transport channels and their multiplexing on a CCTrCH will realize the difficulty of meeting BLER target. The reason is each transport channel can have its own quality target based on the Q0S of the service it carries. How can we then meet diverse BLER targets of transport channels mapped to the same physical channel? I am not aware of any solutions to this problem but my belief is that in practice only one BLER target is used. In other words, the target is used as an indication of the RL quality and not the QoS of the Radio Access Bearer. Service QoS is implemented differently in terms of bandwidth, level of error protection (CRC), channel coding (convolutional vs turbo) and spreading gain (spreading factor).

It must be mentioned at this point that BLER target cannot sometimes be met. For example, if the Node-B is already transmitting at its highest possible power, there is no way it can respond positively to a TPC “UP” command. Decisions have to be made by Admission Control in the SRNC. Possibly, some calls have to be dropped. Possibly, data rates have to be reduced to meet the target BLER. It has been shown that dynamic bearer switching in bad channel conditions improves BLER performance [4]. For example, in bad channels when meeting BLER is proving to be difficult, the service is switched from 384 kbps (10 ms TTI, SF 8, 12 TBs) to 128 kbps (20 ms, SF 16, 4 TBs). This is far better than dropping the call.

Differentiation of power control happens at a finer level too. In the UL, DPCCH and DPDCH operates at different power levels and these can vary with the TFC. Every TFC has its own gain factors, βc and βd that adjust the transmit power. These gain factors are set independent of fast power control. For PRACH, the preamble and the message parts can operate at different power levels. If E-DCH is used, the power levels of E-DPCCH and E-DPDCH can be different and are in relation to DPCCH and DPDCH powers. In the DL, DPCCH and DPDCH are time multiplexed and each can operate at a different power. In addition, different fields of DL DPCCH can operate at different power levels. Different DL channels can operate at different power levels. If compressed mode is enabled, further dynamics are involved. The step sizes (in dB) for power adjustment can also be varied at the same rate as outer loop power control. Specific rules apply for F-DPCH. Power control in HSDPA is done very differently from R99 channels. HSDPA and E-DCH power control will be a separate post. Likewise, power control during SHO will be a separate post.

In conclusion, power control is extremely important in UMTS. The design contains a lot of flexibility to allow power control at different levels.

References/Further Reading:

1. Fredrik Gunnarsson and Fredrik Gustafsson, Control theory aspects of power control in UMTS, Control Engineering Practice, Volume 11, Issue 10, Pages 1113-1125, October 2003.
2. Jaana Laiho, Introduction to RRM/PC, (Powerpoint presentation), Nokia1999.
3. Bo Bernhardsson, Power Control in WCDMA–Background, Dept. of Automatic Control, Lund Institute of Technology.
4. Wolfgang Karner, Philipp Svoboda, Markus Rupp, A UMTS DL DCH Error Model Based on Measurements in Live Networks, Institut f¨ur Nachrichtentechnik und Hochfrequenztechnik, Technische Universit¨at Wien, Austria.

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