WO2005125040A2

WO2005125040A2 - A method of controlling uplink power level

Info

Publication number: WO2005125040A2
Application number: PCT/GB2005/002200
Authority: WO
Inventors: Volker Breuer; Thomas Chapman; Burghard Unteregger
Original assignee: Siemens Aktiengesellschaft
Priority date: 2004-06-16
Filing date: 2005-06-07
Publication date: 2005-12-29
Also published as: GB0413421D0; GB2415324B; WO2005125040A3; GB2415324A

Abstract

A method of controlling uplink power level in a terminal (1) of a communication system, wherein the terminal transmits on a first uplink channel type (8) to a first base station (2) and transmits in soft handover on a second uplink channel type (9) to the first base station and to a second or subsequent base station (3, 4) comprises receiving power control commands at the terminal on a downlink dedicated channel (5, 6, 7) from each base station. The uplink power level for the first and second uplink channel type (8, 9) is adapted, wherein the downlink dedicated channel (7) from the first base station provides power control commands for the first uplink channel type; and wherein the downlink dedicated channel (5, 6) from the second and subsequent base station (3, 4) provides power control commands for the second uplink channel type.

Description

A METHOD OF CONTROLLING UPLINK POWER LEVEL

This invention relates to a method of controlling uplink power level in a terminal of a communication system. Although, the examples are described with respect to a HSDPA, the invention is applicable to any communication system in which there is a signalling channel which transmits to only one base station in soft handover and another channel which transmits to all base stations in an active set. Existing 3GPP FDD mobile phone systems are set up such that a terminal which receives a reduce power instruction from any one base station during soft handover will reduce its uplink transmit power and ignore any request for an increase in power from other base stations. This works perfectly well when all of the base stations are communicating on the same channel types as the terminal is only interested in communicating with the base station with the optimum uplink conditions. However, the introduction of cells which support high speed downlink packet access (HSDPA) means that there could be a request for a power increase for the high speed channel at the same time as another base station is demanding a power decrease for its dedicated channel. Following the convention of always reducing power if there is at least one reduce power request, the base station which has sent data on its high speed channel will fail to receive an acknowledgement from the terminal and so resend data unnecessarily. In accordance with the present invention, a method of controlling uplink power level in a terminal of a communication system, wherein the terminal transmits on a first uplink channel type to a first base station; and transmits in soft handover on a second uplink channel type to the first base station and to a second or subsequent base station; comprises receiving power control commands at the terminal on a downlink dedicated channel from each base station and adapting the uplink power level for the first and second uplink channel type; wherein the downlink dedicated channel from the first base station provides power control commands for the first uplink channel type; and wherein the downlink dedicated channel from the second and subsequent base station provides power control commands for the second uplink channel type. The present invention overcomes the problems of the prior art by separating the power control for the two channel types, to ensure that the first base station can receive its acknowledgments, even if it does not have the best link to the terminal of all of the base stations in an active set. Preferably, a standard offset between the second uplink channel type power level and the first uplink channel type power level is pre-set. This gives a reference point for the expected difference in power. Preferably, a maximum offset is set. This ensures that one power level cannot increase significantly with respect to the other without taking some other action, e.g. initiating a handover. Preferably, the first base station ceases to transmit power control commands for the second uplink channel type when an increase in offset is detected. This is generally caused by another base station having a better link to the terminal and the terminal interpreting transmit power control (TPC) bits from the first base station as applying to the first downlink channel only Preferably, the terminal resets the offset and applies power control commands for both the second and first channel types, if the first base station is the only base station in an active set. If no other base station is active, it cannot have a better link, so the first base station needs to send power control commands on both channel types. Preferably, the power level of the first uplink channel type of the first base station is determined by the first base station and the power level of the second uplink channel type of the first base station is determined by the base station in the active set requiring the lowest power level from the terminal. Preferably, the terminal reduces the transmit power for both the first and second uplink channel types, if the detected offset is equal to the standard offset and the first base station transmits a "down" power control command. Preferably, the terminal increases the transmit power for both first and second uplink channel types, if the detected offset is equal to the maximum offset and all of the base stations transmit an "up" power control command. Preferably, a limit for the maximum increase in first uplink channel type power level is set by the base station in the active set with the best uplink conditions. If the first uplink channel type power level could increase by any amount, then it could create unacceptable interference in the operation of the base station having the best uplink conditions. Preferably, a handover is initialised when a maximum first uplink channel type power level or a predetermined difference between the first and second uplink channel type power levels is reached. This helps to minimise unnecessary interference by initiating a handover, rather than increasing power continuously if the path loss for a particular terminal to base station link becomes too great, e.g. if the terminal moves out of range. Generally, the power level of the first uplink channel type is increased and the power level of the second uplink channel type is decreased. This is because it is likely that another base station could have a better second uplink channel type connection to the terminal, but since no other base station uses the first uplink channel type, then it may need alteration. Preferably, the node B operates an outer power control loop, if an acknowledgement (ACK) detection error occurs. The first uplink channel type may be any type of signalling channel that is transmitted to only one base station when in soft handover, but preferably, the first uplink channel type is an uplink signalling channel associated with a downlink high speed channel. Typically, the second uplink channel type is a dedicated channel. An example of a method of controlling uplink power level in a terminal of a communication system in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates typical uplink and downlink connections for a terminal in soft handover. Figure 1 shows a typical arrangement for a terminal 1 in soft handover, communicating with a number of base stations 2, 3, 4. The following example is described with respect to UMTS. For UMTS, a terminal is know as user equipment, UE and a base station is a cell or node B. Each cell with a radio link to the UE forms part of an active set (AS). In soft handover (SHO), the UE has dedicated physical channels (DPCH) on radio links to more than one cell. The DPCH consists of a signalling part (DPCCH) and a data part (DPDCH). The signalling part carries control information exchanged between the cell and UE. In addition to the dedicated channel, if the UE 1 and a cell 2 support high speed downlink packet access (HSDPA), then there may be a connection on the High Speed Downlink Shared Channel (HS-DSCH) 10. This HS-DSCH and the associated signalling channels are transmitted and received from only one cell 2, cell C in Fig.1 for this example. The signalling channels in downlink are the High Speed Shared Control Channel (HS-SCCH) and in uplink the High Speed Dedicated Physical Control Channel (HS-DPCCH). The dedicated physical channel is under fast power control. That means, that the cells provide transmit power control information on DPCCH for the uplink channels and the UE provides the information for the downlink channels. Fast power control is used to adapt the transmit power to the current interference situation providing a certain signal interference ratio (SIR), the target SIR, at the UE 1 and at an antenna of the cell respectively. TPC information is sent in downlinks 5, 6, 7 from all cells 2, 3, 4 in the active set in SHO, so the UE interprets incoming TPC bits from the active set node Bs in such a manner that it uses the lowest signalled transmit power, i.e. if any of the active set node Bs signal "power down", then the UE reduces it's power. The transmit power for the HS-DPCCH 8 is bound to UL DPCCH transmit power on the uplink dedicated physical channel (UL-DPCH) 9 by a certain offset (Beta). This Beta is configured or reconfigured by higher layer signalling. If the UE receives a data block correctly on the HS-DSCH 10, it signals this with an Acknowledgement (ACK) to the cell. If the cell correctly receives this ACK, it will send new data in the next transmission, otherwise it will send a retransmission. For example, assuming that cell A 4 is the cell with the best uplink condition, then the transmit power for the UL DPCCH, DPDCH and HS-DPCCH are selected according to the power control of cell A. Consequently, cell C 2, the cell providing the HS-DSCH 10 and receiving the HS-DPCCH 8, which is not the cell with the best uplink conditions, may not receive the HS-DPCCH correctly because it was not sent with enough power. As the HS-DPCCH carries the acknowledgement for correctly received data on HS-DSCH and channel quality information, any information that is not received or is received erroneously degrades the data throughput to this particular UE 1 and so degrades throughput and increases interference in the cell. Although, it is possible to reconfigure the Beta factor via higher layer signalling, this higher layer signalling is performed by the Radio Network Controller (RNC) and as a result the beta factor cannot follow a fast varying UL radio channel situation, so the performance is poor in such situations. In addition, the problem of lost or degraded information may also occur when users move quickly from a serving HSDPA NodeB towards another NodeB in the active set. On average, both the DPCH and HSDPCCH powers are reduced as, in an unmodified system, the levels can only be adjusted together, even though the HS-DPCCH transmission level needs to be increased due to increasing pathloss. The RNC is not aware of the diverging path loss conditions for the two channels, as it is only related to fast inner loop power control.. There are also other impacts of the reconfiguration of the beta factor which makes this a less desirable option. Another possible solution to the problem of the cell not receiving the HS- DPCCH correctly, is to use HS-DSCH handover to move the HSDPA radio link to the best DPCH cell, i.e. in our example, from cell C to cell A, but there are drawbacks with this solution too. Firstly, the HSDPA handover is normally based on the reception quality on the downlink, the information about which is provided by the UE 1 , but the downlink channel quality tends to differ from uplink channel quality, because they are sent on different frequencies. Secondly, HSDPA handover takes some time because the complete HSDPA context is held in one NodeB only, so it is undesirable to change too often. Finally, every HSPDA handover, without careful planning leads to packet loss in the queue buffers in the transmitting station, which means it is not an attractive option to overcome fast varying physical channel conditions. The fundamental problem is that the power for the HS-DPCCH 8 is tied to the power of the UL DPCCH 9, which in turn results from the power requirements for the best uplink in the active set. The solution of the present invention is to make the transmit power on the HS-DPCCH 8 depend upon fast transmit power control signals only from the cell that carries HSDPA channels. There are a number of possible features which can be included in the method of the present invention, which are described below. They need not all be included in one implementation, but can be selected according to their relative importance in a particular system of operation. If the cell 2 to which the HS-DPCCH is sent requests a power up, then the UE 1 increases the transmit power for the HS-DPCCH 8 with respect to DPCCH, while the transmit power on the UL DPCCH 9 stays as it is, or is lowered as a result of the power control commands of other cells 3, 4. There needs to be a means to distinguish whether the power commands from the HSDPA serving NodeB apply to both channels or only to the HS-DPCCH. Otherwise, power up commands will be sent continuously when the DPCH is not increased and the HS- DPCCH will hit the maximum power. The distinction is made in the terminal 1 that if it receives different commands from the HSDPA NodeB and other NodeBs in the set, then the terminal will follow the command of the HSDPA serving NodeB 2 for the HS-DPCCH and the commands of the other NodeBs 3, 4 for the DPCH. The serving HSDPA NodeB 2 is aware that its commands only apply to the HS-DPCCH 8 because it detects that the offset between the two channels has increased. Any further commands from the serving HSDPA NodeB 2 are only transmitted with respect to the HS-DPCCH 8. If all other cells 3, 4 are released from the active set, then the UE 1 returns the HS-DPCCH offset to the nominal, or standard, level and then sends power control commands by applying TPC bits to both the DPCH and HS-DPCCH. When the offset between HS-DPCCH and DPCH is reduced to the nominal level as a result of improved propagation conditions to the HSDPA serving NodeB 2 and the terminal detects that the offset between the channels is the nominal offset, then the UE 1 applies TPC "down" bits to both channels. Similarly, if the HS-DPCCH offset reaches its maximum limit, and is detected by the nodeB, then the UE applies power "up" commands to both channels. The UE 1 can set its transmit power to the maximum allowed offset limit for the HS-DPCCH 8 with respect to DPCCH, if the cell sends a retransmission of a data block after the UE has sent an ACK for this data block. The node B may detect a sudden change in power offset, which indicates that an ACK detection error has occurred and then the node B will operate an outer power control loop for the HS-DPCCH.

Increasing and decreasing the transmit power can be by means of fixed or variable step size and the limits for increase and decrease and for the step size can either be fixed or configurable by higher layers. The maximum allowed increase in HS-DPCCH power, as derived and provided via higher layer signalling by the RNC, is generally determined by the NodeB in the active set having the best uplink conditions because this NodeB will suffer most from increased interference as a result of the increased HS- DPCCH. The limit can also take into account power amplifier (PA) backoff thresholds, so that changes in PA backoff are not caused by the HS-DPCCH power control. In addition, corresponding threshold levels may be set on the UE HS-DPCCH transmission power, which are used to trigger reports, so that a handover can be pre- initialised, if one is deemed necessary. Examples of suitable thresholds are reaching maximum HS-DPCCH power level or reaching a certain DPCH HS-DPCCH power difference. Although it may not provide the best downlink values, such a strategy could be used to lower the uplink interference in the system and continue the HSDPA session at a lower throughput. Any of these methods of triggering reports may need to include some hysteresis to avoid ping-pong effects, which can also occur in case of HSDPA only NodeBs. The present invention is relevant for all communications where a terminal receives simultaneous services (in these examples, the simultaneous services are HSDPA and DPCH) which are partly controlled by different NodeBs, or only a subset (one at the minimum) of NodeBs.

Claims

1. A method of controlling uplink power level in a terminal of a communication system, wherein the terminal transmits on a first uplink channel type to a first base station; and transmits in soft handover on a second uplink channel type to the first base station and to a second or subsequent base station; the method comprising receiving power control commands at the terminal on a downlink dedicated channel from each base station and adapting the uplink power level for the first and second uplink channel type; wherein the downlink dedicated channel from the first base station provides power control commands for the first uplink channel type; and wherein the downlink dedicated channel from the second and subsequent base station provides power control commands for the second uplink channel type.

2. A method according to claim 1 , wherein a standard offset between the second uplink channel type power level and the first uplink channel type power level is pre-set.

3. A method according to claim 1 or claim 2, wherein a maximum offset is set.

4. A method according to claim 2 or claim 3, wherein the first base station ceases to transmit power control commands for the second uplink channel type when an increase in offset is detected.

5. A method according to claim 2 or claim 3, wherein the terminal resets the offset and applies power control commands for both the second and first channel types, if the first base station is the only base station in an active set.

6. A method according to any preceding claim, wherein the power level of the first uplink channel type of the first base station is determined by the first base station and the power level of the second uplink channel type of the first base station is determined by the base station in the active set requiring the lowest power level from the terminal.

7. A method according to at least claim 2, wherein the terminal reduces the transmit power for both the first and second uplink channel types, if the detected offset is equal to the standard offset and the first base station transmits a "down" power control command.

8. A method according to at least claim 3, wherein the terminal increases the transmit power for both first and second uplink channel types, if the detected offset is equal to the maximum offset and all of the base stations transmit and "up" power control command.

9. A method according to any preceding claim, wherein a limit for the maximum increase in first uplink channel type power level is set by the base station in the active set with the best uplink conditions.

10. A method according to any preceding claim, wherein a handover is initialised when a maximum first uplink channel type power level or a predetermined difference between the first and second uplink channel type power levels is reached.

11. A method according to any preceding claim, wherein the power level of the first uplink channel type is increased and the power level of the second uplink channel type is decreased.

12. A method according to any preceding claim, wherein the node B operates an outer power control loop, if an acknowledgment (ACK) detection error occurs.

13. A method according to any preceding claim, wherein the first uplink channel type is an uplink signalling channel associated with a downlink high speed channel.

14. A method according to any preceding claim, wherein the second uplink channel type is a dedicated channel.