WO2006006895A1

WO2006006895A1 - Method and arrangement in a radio communication system

Info

Publication number: WO2006006895A1
Application number: PCT/SE2004/001117
Authority: WO
Inventors: Mats-Ola FORSLÖW; Niklas Denkert
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2004-07-08
Filing date: 2004-07-08
Publication date: 2006-01-19
Anticipated expiration: 2007-01-08

Abstract

The present invention relates to a method and apparatus for adjusting a transmit power control process in a mobile radio communication system (SYS1), wherein transmit power (204) is controlled by issuing power control commands (101, 102) indicating whether the transmit power should be increased or decreased in order to adjust towards a transmission quality target (203a-d), and wherein a first transmit power adjustment step (205a, 205b, 205c) is applied in predefined circumstances. Sequences of issued transmit power control commands (101, 102) are registered (301). The first transmit power adjustment step (205a, 205b, 205c) is adjusted (302) according to a predetermined rule based on transmit power control command sequences (101, 102) following previous instances of applying said first transmit power adjustment step (205a, 205b, 205c).

Description

METHOD AND ARRANGEMENT IN A RADIO COMMUNICATION SYSTEM

TECHNICAL FIELD OF THE INVENTION

This invention relates to the control of power levels of transmitted signals in radio communication systems. More in particular, the invention relates to a method for adjusting a transmit power control process in a mobile radio communication system, an apparatus for implementing this method as well as a mobile station, a radio communication network and a radio base station including such an apparatus.

DESCRIPTION OF RELATED ART

Good transmit power control methods can be important to radio communication systems having many simultaneous transmitters because such methods reduce the mutual interference of such transmitters. For example, transmit power control is necessary to obtain high system capacity in interference limited communication systems, e.g., those that use code division multiple access (CDMA) . Depending upon the system characteristics, power control in such systems can be important for the uplink (i.e., for transmissions from a remote terminal/mobile station to the network), the downlink, (i.e., for transmissions from the network to the remote terminal/mobile station) or both.

In a Universal Mobile Telecommunication System (UMTS) specified by the 3^rd Generation Partnership Project (3GPP) , power control is applied both in the uplink and downlink directions. Power control in each direction is based on both an inner loop power control algorithm (alternatively referred to as a fast power control loop) and an outer loop power control algorithm

(alternatively referred to as a slow power control loop) .

In the uplink direction, the inner loop power control is performed by the network (radio base station or Node B) frequently estimating the received Signal-to-Interference (SIR) and comparing it to a target SIR. If the estimated SIR is lower than the target SIR, the radio base station orders the mobile station to increase its transmit power. If the estimated SIR is higher than the target SIR, the base station instead orders the mobile station to decrease its transmit power. Estimating the received SIR and ordering the mobile station to increase/decrease its transmit power is performed 1500 times per second (once for each time slot) . The orders to increase/decrease the transmit power are referred to as Transmit Power Control (TPC) commands.

The outer loop power control algorithm in the uplink direction is performed to adjust the target SIR in the base station. Thus, the quality, usually defined as a certain target bit error rate (BER) or block error rate (BLER) , of the radio link is regularly evaluated and compared to a required quality of service. If the quality is below the required quality of service (QoS) , the target SIR is increased. If the quality is found to be above the required quality of service, the target SIR is decreased. Typically the evaluation of the quality is based on an average over several frames (each frame consists of 15 time slots) in order to provide reliable estimates and hence the outer loop power control is a much slower process than the inner loop power control. The required quality of service would typically be different for different services.

Power control in the downlink direction is performed essentially as in the uplink direction, but the roles are then of course reversed, i.e. it is the mobile station that evaluates the signal from the base station and issues TPC-commands to the base station ordering it to increase/decrease its transmit power.

In view of the outer loop power control being a rather slow process, it is not well suited to track fast changing requirements on the SIR which occurs in specific circumstances, e.g. in connection with compressed mode, in a UMTS system. In order to better cope with the fast changes in required target SIR/transmit power during compressed mode, the 3GPP specifications provide for target SIR offsets and transmit power adjustment steps supplementing the basic inner loop and outer loop power control mechanisms during compressed mode.

US 6,564,067 discloses a method for setting a transmission quality target value for power control. The method involves applying an offset to said transmission quality target in order to compensate for the effects of a compressed mode whereby transmission is interrupted during transmission gaps. The offset includes components compensating for the effects of both a transmission rate increase as well as the transmission gaps themselves.

International patent application published as WO 03/076964 discloses a system and method for speed indication wherein a speed indication signal is determined from a sequence of Transmit Power Control (TPC) commands sent from a mobile station to an access point of a wireless communication network.

SUMMARY OF THE INVENTION

The problem addressed by the present invention is improving the power control process of mobile radio communication systems.

The problem is solved by a method according to claim 1 and an apparatus according to claim 12.

Α general advantage of the invention is that it enables improvements of the power control process in radio communication systems.

A further advantage of the invention, is that it enables improved power control in compressed mode. Yet another advantage of the invention, is that it affords increased robustness against inappropriate selection of transmit power adjustment step values.

The invention will now be described in more detail with reference to exemplary embodiments thereof and also with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of an example mobile communication system in which the present invention may be advantageously employed.

Fig. 2 is a diagram illustrating an example scenario of target SIR and transmit power in compressed mode.

Fig. 3 is a flow diagram illustrating a basic method for adjusting a transmit power control process according to the invention.

Fig. 4 is a schematic view illustrating hardware structure relevant to a first exemplary embodiment of the invention

Fig. 5A-5B are flow diagrams illustrating a method according to the first exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 illustrates a non-limiting example of a communication system SYSl in which the present invention may be employed. The exemplary communication system SYSl illustrated in Fig. 1 is a Universal Mobile Telecommunication System (UMTS) . The communication system SYSl includes a core network CNl, a UMTS Terrestrial Radio Access Network (UTRAN) RANl and User Equipment (UE) , alternatively referred to as mobile stations (MS) .

The core network CNl includes a Mobile services Switching Center

(MSC) node MSCl that provides circuit-switched services and a General Packet Radio Service (GPRS) node SGSNl, sometimes referred to as a Serving GPRS Support node (SGSN) , which is tailored to provide packet-switched type services.

Each of the core network nodes MSCl and SGSNl connects to the the radio access network RANl over a radio access network interface referred to as the Iu interface. The radio access network RANl includes one or more radio network controllers

(RNCs) . For sake of simplicity, the radio access network RANl of

Fig. 1 is shown with only one radio network controller node

RNCl. Each radio network controller is connected to and controls a plurality of radio base stations (RBSs) . For example, and again for sake of simplicity, Fig. 1 only illustrates a first radio base station node RBSl and a second radio base station node RBS2 connected to the radio network controller node RNCl. The interface between the radio network controller RNCl and the base stations RBSl and RBS2 is referred to as the Iub interface.

Mobile stations, such as mobile station MSl shown in Fig. 1, communicate with one or more radio base stations RBS1-RBS2 over a radio or air interface referred to as the Uu interface.

An operation and maintenance center OMCl provides operation and maintenance of the radio network controller RNCl and the radio base stations RBS1-RBS2 via interfaces denoted Mur and Mub respectively.

All of the above mentioned interfaces Uu, Iu, Iur, Mur and Mub are illustrated by dashed lines in Fig. 1

In the UMTS system SYSl of Fig. 1, power control is applied both in the uplink and the downlink direction. Power control in each direction is based on both an inner loop power control algorithm (alternatively referred to as a fast power control loop) and an outer loop power control algorithm (alternatively referred to as a slow power control loop) .

As an example, assuming a radio link has been established between the first radio base station RBSl and the mobile station MSl of Fig. 1, the power control process in the uplink direction would be performed as follows.

Inner loop power control would be performed by the first radio base station RBSl frequently (for each time slot) estimating the received Signal-to-Interference ratio (SIR) and comparing it to a target SIR. Based on the result of said comparison, the first radio base station RBSl issues transmit power control (TPC) commands 101 to the mobile station MSl ordering it to increase or decrease its transmit power depending on whether the estimated SIR was below or above the target SIR.

Outer loop power control would be performed by the radio network controller RNCl to adjust the uplink target SIR. Thus, the quality, usually defined as a certain target bit error rate

(BER) or block error rate (BLER) , of the radio link is regularly evaluated and compared to a required quality of service (QoS) .

Depending on the outcome of said comparison, the target SIR would be increased or decreased. The radio network controller

RNCl informs the first radio base station RBSl of any changes in the uplink target SIR.

Power control in the downlink direction would be performed essentially as in the uplink direction, but the roles are then of course reversed, i.e. it is the mobile station MSl that evaluates the signal from the first radio base station RBSl and issues TPC-commands (inner loop power control) to the first radio base station RBSl ordering it to increase/decrease its transmit power and it is also the mobile station MSl that evaluates the quality and updates the downlink SIR target (outer loop power control) if need be.

In order to enable a mobile station, such as mobile station MSl, to perform measurements on other frequencies and other modes and radio access technologies supported by the mobile station, the 3GPP specifications provide for so called compressed mode. Compressed mode basically involves stopping the downlink transmission for a defined period of time (a transmission gap) , thus making receiver circuitry in the mobile station available for performing said measurements. Simultaneous uplink and downlink compressed mode may be applied e.g. when measuring on frequencies close to the uplink frequency.

In order to create transmission gaps, the data transmission is compressed in the time domain. The 3GPP specifications provide the following mechanisms for this purpose:

• reducing the spreading factor;

• puncturing;

• higher layer scheduling.

All three methods are applicable in the downlink, while puncturing is not used in the uplink.

Operation in compressed mode has a significant effect on the power control processes. The increased bit rate during compressed frames when reducing the spreading factor as well as the weaker channel coding in a transmission time interval (TTI) when applying puncturing, necessitate increased target SIR.

Also, the breaks in the closed loop power control caused by transmission gaps in compressed frames, causes significant performance degradation which needs to be compensated for during the compressed frames and frames following a compressed frames

(recovery frames) by another increase of the target SIR during these frames.

In order to better cope with the fast changes in required target SIR/transmit power during compressed mode, the basic inner loop and outer loop power control mechanisms are supplemented with target SIR offsets and transmit power adjustment steps as illustrated by the example scenario of Fig. 2, illustrating compressed mode utilizing spreading factor reduction. Fig. 2 includes two diagrams wherein the horizontal axis of both diagrams represent time, the vertical axis of the lower diagram represents transmit power 204 (illustrated by the solid line) and the vertical axis of the upper diagram represents target SIR 203a-d (illustrated by the dashed line) .

Time is organized into a sequence of 10 ms frames 202a-202d, each frame consisting of 15 time slots 201. Frame 202b is a compressed frame, i.e. a frame including a transmission gap, frame 202c is a recovery frame following the compressed frame 202b while frames 202a and 202d are normal frames preceding and following the compressed frame 202b and recovery frame 202c respectively.

In frame 202a, the SIR of the received signal matches the target SIR 203a and hence the TPC command sequence during frame 202a consists of alternating orders to increase and decrease the transmit power causing the transmit power 204 to vary accordingly.

Since frame 202b is a compressed frame, the target SIR and transmit power needs to be adjusted to compensate for the effects of the increased bit rate and the transmission gap in the frame. Therefore, in the beginning of the compressed frame 202b, the target SIR is increased to a new target SIR 203b by adding corresponding target SIR offsets (denoted ΔSIR_Pi_LOτ and DeltaSIR for the uplink in the 3GPP specifications) to the basic target SIR determined by the slow power control loop. Further, in conjunction with applying said variation of the SIR target an associated transmit power adjustment step 205a is applied to the transmit power 204. Except for during the transmission gap 206, the inner loop power control process issues transmit power control commands for each time slot 201 of the compressed frame 202b in order to adjust the SIR of the received signal towards the target SIR 203b. In the recovery frame 202c, the target SIR and transmit power needs to be adjusted again in order to compensate for the effects of the reduced bit rate in the recovery frame 202c as compared to the preceding compressed frame 202b and the transmission gap 206 in the preceding compressed frame 202b. Therefore, in the beginning of the recovery frame 202c, the target SIR offsets added for the compressed mode frame 202b are removed and instead another target SIR offset (denoted DeltaSIRafter in the 3GPP specifications) is applied, the net effect being a reduction of the target SIR 203c for the recovery frame 202c as compared to the target SIR 203b of the compressed frame 202b. Further, in conjunction with applying said variation to the target SIR in the beginning of the recovery frame 202b, an associated transmit power adjustment step 205b is applied to the transmit power 204. The inner loop power control process issues transmit power control commands for each time slot 201 of the recovery frame 202c in order to adjust the SIR of the received signal towards the target SIR 203c applicable in the recovery frame 202c.

In the frame 202d following the recovery frame 202c, yet another adjustment of the target SIR and transmit power is necessary. Therefore, in the beginning of frame 202d, the target SIR offset added in the recovery frame 202c is removed causing a reduction of the target SIR 203d for the frame 202d as compared to the target SIR 203c of the recovery frame 202c. An associated transmit power adjustment step 205c is applied in conjunction with applying said variation to the target SIR in the beginning of the frame 202d.

Ideally, when applying a transmit power adjustment step in conjunction with a target SIR variation, the transmit power adjustment step and target SIR variation should be well matched so that immediately after applying said transmit power adjustment step and target SIR variation, the transmit power would be just right for providing a received SIR corresponding to the SIR target. The inventors of the present invention have recognized that this is far from always the case as illustrated by the example scenario of Fig. 2.

In Fig. 2, the transmit power 204 increases for all time slots transmitted in the compressed frame 202b following application of the transmit power adjustment step 205a, indicating that the transmit power adjustment step 205a was insufficient to meet the target SIR value 203b of the compressed frame 202b. In the recovery frame 202c the transmit power 204 is decreased for 5-6 time slots following application of the transmit power adjustment step 205b before leveling out, indicating that the transmit power adjustment step 205b was not sufficient to immediately reach the new target SIR value 203c of the recovery frame 202c, but still enough so as to meet the target SIR value 203c after half a frame. Finally, in the frame 202d following the recovery frame 202c, the transmit power 204 immediately exhibits an alternating pattern of transmit power increase/decrease following transmit power adjustment step 205c, which indicates that the transmit power adjustment step 205c was just right for meeting the target SIR value 203d of- the frame 202d following the recovery frame 202c.

Please note that the effects of fading could for an individual sequence of frames cause similar TPC-command sequences as illustrated in Fig. 2 even if the transmit power adjustment steps and associated target SIR variations are well matched. Thus, in order to assess whether a transmit power adjustment step matches an associated target SIR variation, it is necessary to consider several instances of applying said transmit power adjustment step and target SIR variation in order to eliminate the effects of fading on such an assessment.

The present invention addresses the situation elaborated above, wherein transmit power adjustment steps associated with and applied in conjunction with transmission quality target variations are not always well matched with the corresponding _ _

transmission quality target variation, and hence the present invention provides improvements of the power control process of mobile radio communication systems

Fig. 3 illustrates a basic method according to the invention for adjusting a transmit power control process in a mobile radio communication system, such as SYSl of Fig. 1, wherein transmit power is controlled by issuing power control commands indicating whether the transmit power should be increased or decreased in order to adjust towards a transmission quality target, e.g. a target SIR 203a-203d, and wherein a first transmit power adjustment step is applied in predefined circumstances.

At step 301, sequences of issued transmit power control commands are registered.

At step 302, the first transmit power adjustment step is adjusted according to a predetermined rule based on transmit power control command sequences following previous instances of applying said first transmit power adjustment step.

Thus the basic method of the invention enables, in the context of the example scenario of Fig. 2, the transmit power adjustment step 205a to be adjusted based on transmit power control command sequences following previous instances of applying the transmit power adjustment step 205a, i.e. TPC-sequences in previous compressed frames 202b. This method may also be applied to the other transmit power adjustment steps 205b (based on TPC- sequences in previous recovery frames 202c) and 205c (based on TPC-sequences frames 202d following recovery frames) .

A first exemplary embodiment of the invention for use in the context of the radio communication system SYSl of Fig. 1 is illustrated in Fig. 4 together with Figs. 5A-5B.

Fig. 4 illustrates schematically exemplary hardware elements of relevance for the invention in the first radio base station RBSl, the operation and maintenance center OMCT and mobile stations such as the mobile station MSl.

The first radio base station RBSl includes receiver circuitry 401 for receiving and decoding radio signals transmitted by mobile stations such as mobile station MSl, transmitter circuitry 402 for encoding and transmitting radio signals to mobile stations, a processor 403 and a memory unit 404. The processor 403 receives TPC-commands 102 relating to downlink power control via the receiving circuitry 401. Based on the received TPC-commands, the processor 403 adjusts the transmit power of the transmitter circuitry 402. The processor 403 further estimates the SIR of received signals, compares the estimated SIR to the applicable target SIR value and issues TPC- commands 101 relating to uplink power control to mobile stations via the transmitter circuitry 402.

The mobile station MSl includes receiver circuitry 411 for receiving and decoding radio signals transmitted by radio base stations such as the first radio base station RBSl, transmitter circuitry 412 for encoding and transmitting radio signals to radio base stations, a processor 413 and a memory unit 414. The processor 413 receives TPC-commands 101 relating to uplink power control via the receiving circuitry 411. Based on the received TPC-commands, the processor 413 adjusts the transmit power of the transmitter circuitry 412. The processor 413 further estimates the SIR of received signals, compares the estimated SIR to the applicable target SIR value and issues TPC-commands 102 relating to downlink power control to radio base stations via the transmitter circuitry 402.

The operation and maintenance center OMCl includes a processor 423 and a memory unit 424.

In the first exemplary embodiment of the invention, the power adjustment steps associated with each of the target SIR variations in compressed frames, recovery frames and frames following an recovery frame in both the uplink and downlink directions are adjusted using the invention.

The first radio base station RBSl registers TPC-command sequences relating to each of said frame categories in both uplink and downlink direction. For power control in the uplink direction, the first radio base station RBSl registers TPC- commands issued to mobile stations such as mobile station MSl.

For power control in the downlink direction, the first radio base station RBSl instead registers TPC-commands received from mobile stations.

In this exemplary embodiment, the first radio base station RBSl maintains a FRAME_CNT variable and a TPC_ACK variable in memory 404 for each different transmit power adjustment step being monitored. There are three different transmit power adjustment steps (205a, 205b and 205c in Fig. 2) monitored in each direction, i.e. a total of 6 different transmit power adjustment steps.

TPC-command registration are organized in recording periods of e.g. 15 minutes. In the beginning of each recording period, all FRAME__CNT and TPC_ACK variables are initialized to zero.

Each time a certain transmit power adjustment step is applied during a recording period, the processing steps illustrated in Fig. 5A are performed by the processor 403 in the first radio base station RBSl.

At step 501, the FRAME_CNT variable associated with this particular transmit power adjustment step is incremented by one. At step 502, each TPC-command is checked to determine whether it was an order to increase or decrease the transmit power. If an increase in transmit power is ordered (an alternative INCREASE at step 502), the TPC_ACK variable associated with this particular power adjustment step is incremented by one at step 503. If a decrease of the transmit power is ordered (an alternative DECREASE at step 502), the TPC_ACK variable is decremented by one at step 504. If there are more TPC-commands to process for the frame (an alternative YES at step 505) , processing returns to step 502 where the next TPC-command is considered. Otherwise (an alternative NO at step 505) , processing of this frame is completed.

After each recording period, the data registered during the recording period is transferred to the operation and maintenance center OMCl which performs an analysis on the registered data to determine whether the power adjustment steps should be adjusted.

A new recording period may immediately be initiated following the end of the previous recording period.

Fig. 5B illustrates processing performed by the processor 423 in the operation and maintenance center OMCl to evaluate the data registered during a recording period. The same processing is performed for each set of FRAME_CNT and TPC_ACK variable associated with a specific transmit power adjustment step.

At step 510, the FRAME_CNT variable is compared to a first threshold SAMPLE_THRESHOLD (e.g. in the order of 10 000) to check whether enough samples of TPC-command sequences have been obtained to form a reliable basis for adjusting the associated transmit power adjustment step. If to few TPC-command sequences have been registered during the recording period (an alternative NO at step 510), processing stops without adjusting the transmit power adjustment step. If enough samples of TPC-command sequences have been obtained (an alternative YES at step 510) , processing continues at step 511 where a value TPC_AVG is calculated by dividing the TPC_ACK value with FRAME_CNT. TPC_AVG reflects the average number of increase/decrease TPC-commands in each frame following application of the transmit power adjustment step. A positive TPC_AVG value indicates that there have been more TPC-commands ordering an increase of transmit power than TPC-commands ordering a decrease of transmit power, while a negative TPC_AVG value indicates the reverse proportions of increase/decrease TPC-commands.

At step 512, the TPC_AVG value is compared to a second threshold INCREASEJTHRESHOLD. If the TPC_AVG value is above the second threshold INCREASEJTHRESHOLD (an alternative YES at step 512), the transmit power adjustment step is increased DELTA_ADJUSTMENT dB at step 513. Otherwise (an alternative NO at step 512), the TPC_AVG is compared to a third threshold DECREASEJTHRESHOLD at step 514. If the TPC_AVG value is below the third threshold DECREASEJTHRESHOLD (an alternative YES at step 512), the transmit power adjustment step is decreased DELTA_ADJUSTMENT dB at step 515.

The parameter DELTA_ADJUSTMENT may preferably be set equal to the size of the steps by which the transmit power is adjusted in response to the TPC-commands (e.g. typically 1 dB) . The

INCREASEJTHRESHOLD and DECREASEJTHRESHOLD could preferably be selected so that the transmit power adjustment step is adjusted only if the TPC_AVG value indicates that the average net effect of TPC-commands per frame is an increase/decrease of transmit power by more than one step, i.e. the INCREASEJIHRESHOLD and

DECREASEJTHRESHOLD could be set equal to +1 and -1 respectively.

In order to gain some extra margin (especially in embodiments basing adjustment decisions on fewer samples, e.g. due to short measurement periods) , the INCREASE_THRESHOLD and

DECREASEJTHRESHOLD could be selected as larger than +1 and less than -1, e.g. as +/- 2.

If the transmit power adjustment step was adjusted, i.e. either step 513 or 515 of Fig. 5B was executed, the operation and maintenance center OMCl communicates said adjustment to either the first radio base station RBSl or the mobile station MSl at step 516. If the transmit power adjustment step relates to the downlink power control process, the first radio base station RBSl is informed directly of the adjustment by means of a message transmitted over the Mub interface. If the transmit power adjustment step relates to the uplink power control process, the operation and maintenance center OMCl communicates the adjustment over the Mur interface to the radio network controller RNCl which informs the mobile station MSl of the adjustment in a RRC-message (e.g. Radio Link Reconfiguration) transmitted over the Iub interface.

In the exemplary first embodiment discussed above, the processor 403 and memory 404 of the first radio base station RBSl together functions as registering means for registering sequences of issued transmit power commands, while the processor 423 of the operation and maintenance center OMCl acts as adjusting means for adjusting each of the transmit power adjustment steps discussed above.

Apart from the exemplary first embodiment of the invention disclosed above, there are several ways of providing rearrangements, modifications and substitutions of the first embodiment resulting in additional embodiments' of the invention.

Instead of having the operation and maintenance center OMCl perform the processing of Fig. 5B to decide whether a transmit power adjustment step should be adjusted, the corresponding processing could be performed in either the first radio base station RBSl or the radio network controller RNCl.

Different recording period lengths may of course be defined. Thus the recording period could be both considerable longer (e.g. several hours of even longer periods) or shorter than the 15 minutes used in the first exemplary embodiment. By selecting an appropriate recording period length it is possible, if desired, to adapt to e.g. variations of the interference situation during a day.

Instead of gathering and evaluating data for all radio connections served by a base station during a recording period, data could be gathered and evaluated on a per radio connection basis. Such a solution would require faster adaptation based on less TPC-command samples (e.g. in the order of 10-100 samples), but would be able to adapt to conditions specific to an individual radio connection. When gathering and evaluating data on a per radio connection basis, it is preferable to have the radio base stations perform data gathering and evaluation for transmit power adjustment steps applicable in uplink power control while having the mobile stations perform the corresponding tasks for transmit power adjustment steps applicable in downlink power control.

Usually a transmit power adjustment step is associated with and applied in conjunction with a variation of the transmission quality target (e.g. target SIR) as illustrated by the example scenario of Fig. 2. However, it is also possible to apply a transmit power adjustment step without applying an associated variation of the transmission quality target. One example of such an embodiment would be in the context of compressed mode based on higher layer scheduling, where a transmit power adjustment step could be applied, without any corresponding variation of the target SIR, in order to provide an extra power margin for fading dips that might occur while there is a break in the inner loop power control process.

There are very many alternative ways for registering sequences of issued transmit power control commands that could be used in connection with the present invention. One alternative to the solution illustrated in the first exemplary embodiment, i.e. the use of the FRAME_CNT and TPC_ACK variables, would be to have a separate counter for each possible proportion of INCREASE and DECREASE TPC-commands in a frame which is incremented each time that specific proportion of INCREASE and DECREASE TPC-commands occurs in a frame. At the end of a recording period, it would then be possible to determine how frequent each possible proportion of INCREASE and DECREASE TPC-commands in a frame occurs. Yet another alternative for registering sequences of issued transmit power commands would be to count the number of INCREASE and DECREASE TPC-commands respectively for each different time slot position in a frame.

Different kinds of statistical analysis could be performed in order to determine whether a transmit power adjustment step should be adjusted. The available statistical analysis depends to some extent on how TPC-command sequences have been registered. In the first exemplary embodiment of the invention, a statistical analysis is performed by comparing an average value for the proportions of INCREASE/DECREASE TPC-commands to threshold values in order to determine if the transmit power adjustment step should remain the same, be increased or be decreased. In other embodiments, such statistical analysis could e.g. be based on the median value for the proportions of INCREASE/DECREASE TPC-commands.

Adjustment of a transmit power adjustment step could be implemented by means of an associated adjustment offset variable which defines how much the original transmit power adjustment step should be adjusted. Alternatively, the variable/parameter defining the transmit power adjustment step could be modified each time an adjustment is made.

Even though the invention in its first exemplary embodiment has been applied in the context of a UMTS radio communication system, the invention may of course be applied in other radio communication systems where transmit power is controlled by power control commands indicating whether the transmit power should be increased or decreased in order to adjust towards a transmission quality target and wherein a transmit power adjustment step is applied in predefined circumstances.

Claims

1. A method for adjusting a transmit power control process in a mobile radio communication system (SYSl) , wherein transmit power

(204) is controlled by issuing power control commands indicating whether the transmit power should be increased or decreased in order to adjust towards a transmission quality target (203a-d) , and wherein a first transmit power adjustment step (205a-205c) is applied in predefined circumstances, said method c h a r a c t e r i z e d in the steps of:

registering (301) sequences of issued transmit power control commands (101, 102);

adjusting (302) said first transmit power adjustment step (205a- 205c) according to a predetermined rule based on transmit power control command sequences following previous instances of applying said first transmit power adjustment step.

2. A method according to claim 1, wherein time is organized into frames (202a-202d) and wherein said first transmit power adjustment step is associated with a compressed mode of operation in which transmission gaps (206) are included in some of the frames.

3. A method according to claim 2, wherein said first transmit power adjustment step is applied in conjunction with one of:

the beginning of a compressed frame (202b) including a transmission gap;

the beginning of a recovery frame (202c) following a compressed frame including a transmission gap;

the beginning of a frame (202d) following a recovery frame.

4. A method according to any one of claims 1-3, wherein according to said predetermined rule, if a statistical analysis of the registered transmit power control command sequences following previous instances of applying said first transmit power adjustment step indicates that it would be desirable to increase said first transmit power adjustment step, said first transmit power adjustment step is increased.

5. A method according to any one of claims 1-4, wherein according to said predetermined rule, if a statistical analysis of the registered transmit power control command sequences following previous instances of applying said first transmit power adjustment step indicates that it would be desirable to decrease said first transmit power adjustment step, said first transmit power adjustment step is decreased.

6. A method according to any one of claims 1-5, wherein the first transmit power adjustment step (205a-205c) is associated with and applied in conjunction with a first variation of the transmission quality target (203a-203d) by addition or removal of one or more quality target offsets.

7. A method according to any one of claims 1-6, wherein the transmit power control process is applied in the uplink direction between a radio communication network and a first mobile station.

8. A method according to claim 7, wherein said registering and adjusting steps are performed by the first mobile station.

9. A method according to claim 7, wherein said registering and adjusting steps are performed by the radio communication network and the method further comprising the step of the radio communication network informing the mobile station of any adjustments to said first transmit power adjustment step.

10. A method according to any one of claims 1-6, wherein the transmit power control process is applied in the downlink direction between a radio communication network and a first mobile station.

11. A method according to claim 10, wherein said registering and adjusting steps are performed by the radio communication network.

12. An apparatus for adjusting a transmit power control process in a mobile radio communication system, wherein transmit power (204) is controlled by issuing power control commands (101, 102) indicating whether the transmit power should be increased or decreased in order to adjust towards a transmission quality target (203a-d) , and wherein a first transmit power adjustment step (205a, 205b, 205c) is applied in predefined circumstances, said apparatus c h a r a c t e r ! z e d by:

registering means (403, 404) for registering sequences of issued transmit power control commands (101, 102);

adjusting means (423) for adjusting said first transmit power adjustment step (205a, 205b, 205c) according to a predetermined rule based on transmit power control command sequences following previous instances of applying said first transmit power adjustment step (205a, 205b, 205c) .

13. An apparatus according to claim 12, wherein time is organized into frames (202a-202d) and wherein said first transmit power adjustment step (205a, 205b, 205c) is associated with a compressed mode of operation in which transmission gaps (206) are included in some of the frames.

14. An apparatus according to claim 13, wherein said first transmit power adjustment step (205a, 205b, 205c) is applied in conjunction with one of:

the beginning of a compressed frame (202b) including a transmission gap (206) ;

the beginning of a recovery frame (202c) following a compressed frame (202b) including a transmission gap (206) ; the beginning of a frame (202d) following a recovery frame (202c) .

15. An apparatus according to any one of claims 12-14, wherein according to said predetermined rule, if a statistical analysis of the registered transmit power control command sequences following previous instances of applying said first transmit power adjustment step indicates that it would be desirable to increase said first transmit power adjustment step, said first transmit power adjustment step is increased.

16. An apparatus according to any one of claims 12-15, wherein according to said predetermined rule, if a statistical analysis of the registered transmit power control command sequences following previous instances of applying said first transmit power adjustment step indicates that it would be desirable to decrease said first transmit power adjustment step, said first transmit power adjustment step is decreased.

17. An apparatus according to any one of claims 12-16, wherein the first transmit power adjustment step is associated with and applied in conjunction with a first variation of the transmission quality target by addition or removal of one or more quality target offsets.

18. An apparatus according to any one of claims 12-17, wherein the transmit power control process is applied in the uplink direction between a radio communication network (RANl) and a first mobile station (MSl) .

19. An apparatus according to claim 18, wherein said registering and adjusting means are included in the radio communication network and the apparatus further comprises means in the radio communication network for informing the mobile station of any adjustments to said first transmit power adjustment step.

20. An apparatus according to any one of claims 12-17, wherein the transmit power control process is applied in the downlink direction between a radio communication network and a first mobile station.

21. A mobile station including an apparatus according to any one of claims 12-18.

22. A radio communication network including an apparatus according to any one of claims 12-20.

23. A radio base station including an apparatus according to any one of claims 12-20.