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Archive for December, 2007

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 UMTS

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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GPRS Dual Transfer Mode

They say necessity is the mother of invention. So when GPRS saw the upcoming competition from UMTS it had to invent new methods to keep itself alive longer in the cellular world. The greater truth is that everyone knew that UMTS will not replace GSM/GPRS overnight. Rather, these two access standards will co-exist for a long time to come before 3G completely replaces 2.5G. More than competition, it was completion and cooperation.

It was correctly envisaged that this coexistence will happen for the following reasons:

  1. Going by demand, deployment of 3G cells will happen in phases whereby urban areas could be covered by 3G while rural areas continue on 2.5G. This means that movement from urban to rural will trigger an inter-RAT handover.
  2. 3G may be deployed at the level of microcells while at the macrocell level 2.5G may be used.
  3. Operators have invested considerably in 2.5G and the move to 3G will at best be a transition.
  4. Because WCDMA at 2100 MHz generally lacks the indoor coverage that’s possible with GSM 900 MHz, this leads to another use case for inter-RAT handover.
  5. It will take time for users to switch consumption patterns, to move from voice-centric to data-centric.
  6. It will take time for users to give up their old handsets in favour of 3G-enabled handsets.

Whatever be the case, one of the keys problems for GPRS is that CS and PS traffic cannot happen simultaneously. How then can we meet user expectations when an inter-RAT handover from UMTS to GPRS will necessarily mean dropping the packet connection?

GPRS standards provides for three classes of MS:

  • Class A: supports simultaneous attach, simultaneous activation, simultaneous monitor, simultaneous invocation and simultaneous traffic. A CS call in the middle of PS call will not disrupt the latter. This class of a mobile may even require two transceivers because CS and PS could be on different frequencies. RF duplexers may be needed along with multiple call processing units. There is no coordination between the two service domains. As such, such a mobile is quite complex and rarely implemented. The cost of such a mobile is also likely to be high and with possibly low battery life.
  • Class B: supports simultaneous attach, simultaneous activation and simultaneous monitor. Service invocation and traffic are mutually exclusive. This means that a CS call in the midst of a PS call will suspend the latter.
  • Class C: the two domains are mutually exclusive even for attach. If MS is used for the CS domain (possibly chosen by the user), PS domain becomes unavailable.

Thus, although the class A mobile will suit inter-RAT handover from UTRAN, it is prohibitively complex. To overcome this problem, designers have standardized the Class A Dual Transfer Mode (DTM) MS which can be seen as a subset of a Class A MS. This work started in November 1999. Today it is fairly stable and we are beginning to see DTM mobiles in the market [1]. Its characteristics are the following:

  1. Simultaneous traffic on both domains is possible and network coordinates this. Thus, the domains are not independent as a true Class A MS. Changes in the network are necessary to enable this coordination. In particular, the BSS maintains a mapping of the IMSI and TLLI. This enables the BSS to “page” a mobile (using PACCH) in Packet Transfer Mode for an incoming CS call. Thus, even without a Gs interface, such paging coordination is possible. This is a new network mode known as NMO-II bis.
  2. While it is true that a CS call can be added to an MS in Packet Transfer Mode, this happens in three stages. TBF is released. A dedicated connection is initiated. Finally, the MS transits to DTM for re-establishing the data connection. The state transitions are depicted in Figure 1.
  3. Resources assignments towards DTM state are made using one of three messages: DTM Assignment Command (generally used when CS resources have to be reallocated), Packet Assignment Command (when only PS resources have to be added while keeping CS resources the same), Main DCCH Assignment (for single timeslot packet signalling on main DCCH).
  4. CS and PS are carried on the same frequency. The use of timeslots is also restricted. Both calls could be shared on a single slot. In this case, both are half-rate channels (TCH/H + PDTCH/H). To clarify, although an RLC/MAC block is sent in four bursts, at half rate it is physically sent over 8 TDMA frames. If multislots are used, the slots have to be contiguous (TCH + multiple PDTCH). Although multislot approach gives higher data rates, in the event of a handover, it will be difficult to find just as many empty slots in the target cell.
  5. To simplify signalling procedures, main DCCH can be used for packet procedures. Thus, an MS is dedicated mode can be paged for a packet call using the main DCCH. Likewise, the MS can request to perform Cell Update or Routing Area Update during a CS call using the main DCCH.
  6. Use of main DCCH for PS signalling is on SAPI 0. For the purpose of demultiplexing, a new protocol discriminator (PD) has been defined – GPRS Transparent Transport Protocol (GTTP). All messages with this PD are passed transparently by the BSS to the SGSN. This reduced the congestion caused by GPRS signalling that happens on the border of RA/LA.
  7. The use of main DCCH for PS domain signalling may affect the signalling of an active CS call. As an alternative, new allocations using DTM procedures can be used. These may reallocate resources of the ongoing CS call.
  8. Certain things can be coordinated between the two domains – measurements, timing advance, power control.
  9. An MS in DTM, can be commanded by the BSS to make a handover. MS will not perform any cell reselection and NC mode does not matter.
  10. An MS supporting a certain multislot class will support all lower classes as well.
  11. While a non-DTM MS will support dynamic allocation and fixed allocation, a DTM mobile can have exclusive allocation in the uplink as an extra.
  12. It will be clear from Figure 1, that a dedicated connection cannot be released while maintaining the TBF. Thus, PS call will be temporarily suspended before it is again established. However, there is an enhanced CS release procedure by which the MS may be sent SI/PSI to ease re-entry into Packet Transfer Mode. PSI 14 has been defined for this purpose which may received in DTM state. Packet CS Release Indication (on PACCH), Packet SI Status, Packet PSI Status and Packet Serving Cell Data (PSCD) are also used in this regard.

Figure 1: RR Operating Modes and State Transitions

DTM States

From my search on the Internet, I discovered that one common class implemented by mobile phone vendors is the DTM Multislot Class 11. The technical specification of Nokia N95 states a speed of DL/UL 177.6/118.4 kbps. Given that Multislot Class 11 represents 4 Rx, 3 Tx and 5 active slots per frame, it is not clear how these data rates have been obtained.

Overall, DTM is a significant feature for a GPRS phone. Its has great value within a dual-mode phone. Users may notice slower data rates following a handover from UTRAN, but at least they will not lose their data connection; nor will they get into a habit of terminating their voice calls to get their important data through. Eventually, DTM will not be required as we move to an all-IP core and the inevitable death of CS domain. In the meantime while we have it, we need to ask one question: How often do we download/upload data while on a voice call?

References:

  1. List of S60 devices supporting GPRS Class A (Dual Transfer Mode), Nokia Forum, 27 Sept 2007.
  2. Mark Pecen and Andrew Howell, Simultaneous Voice and Data Operation for GPRS/EDGE: Class A Dual Transfer Mode, IEEE Personal Communications, April 2001.

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