WO2008115700A1

WO2008115700A1 - Method for switching between power control modes

Info

Publication number: WO2008115700A1
Application number: PCT/US2008/055986
Authority: WO
Inventors: Gabriel Nagy; Walter Featherstone
Original assignee: Motorola, Inc.; Leyra, Luisa, A.
Priority date: 2007-03-21
Filing date: 2008-03-06
Publication date: 2008-09-25
Also published as: GB2447889A; GB2447889B; GB0705397D0

Abstract

A power control apparatus for a cellular communication system comprises a transmitter (109) transmitting to a user equipment (103) at a transmit power level, TPL. A receiver (107) receives power control commands representing a requested transmit power change of the TPL to achieve a quality level. A transmit power controller (111) adjusts the TPL in response to the power control commands. A quality processor (129) determines a quality parameter for the downlink communication in response to measurement reports and a mode processor (131) switches the power control apparatus between a default power control mode and a bias power control mode of operation in response to the quality parameter. A bias processor (133) biases the TPL when in the bias power control mode.

Description

METHOD FOR SWITCHING BETWEEN POWER CONTROL MODES

Field of the invention

The invention relates to power control in a cellular communication system and in particular, but not exclusively, to downlink power control in a 3^rd Generation Partnership Project (3GPP) communication systems such as the Universal Mobile Telecommunication System (UMTS) .

Background of the Invention

In cellular communication systems it is imperative to manage the radio links between the base stations such that the resource used by a given communication link is as low as possible. Thus, it is important to minimise the interference caused by the communication to or from a mobile station, and consequently it is important to use the lowest possible transmit power. As the required transmit power depends on the instantaneous propagation conditions, it is necessary to dynamically control transmit powers to closely match the conditions. For this purpose, the base stations and mobile stations operate power control loops, where the receiving end reports information on the receive quality back to the transmitting end, which in response adjusts its transmit power .

Reducing the transmit power is particularly important in Code Division Multiple Access (CDMA) systems where users share the available bandwidth and therefore minimising the interference level is critical in maximising the overall system capacity.

In 3^rd Generation Partnership Project (3GPP) communication systems such as the Universal Mobile Telecommunication System (UMTS) or Wideband CDMA, both an inner power control loop and an outer power control loop are implemented. Inner loop power control operates as follows. The receiving entity of a radio link measures the received signal to noise ratio (SIR) , and compares it to a locally stored target SIR. A command is sent back to the transmitter to increase transmitted power if the measured SIR is less than the target. Conversely, if the measured SIR is greater than the target, a command is sent to the transmitter to decrease the transmitted power. The target SIR is set by a feature called outer loop power control. Its function is to maintain the Frame Error Rate (FER) or BLock Error Rate (BLER) of the radio link at or below a given value or threshold. The FER/BLER of the received signal is measured by one of a number of known techniques, and the SIR target is adjusted to try to ensure that the FER/BLER is at or below the given value .

For downlink power control, the mobile stations control the power control loop by determining whether to transmit power up or power down messages thereby controlling the downlink transmit power. The mobile stations operate the power control such that the resulting FER/BLER is maintained close to a reference FER/BLER. This FER/BLER may be provided to mobile stations from the network. As the downlink power control loop is controlled by mobile stations, the network and network operator relies on the mobile stations only requesting the minimum power level necessary to meet the reference value. If the mobile stations request more power than is strictly necessary, excessive interference will be experienced by all users in the system which could lead to an overload situation, coverage gaps, reduced capacity and/or a reduced Quality of Service (QoS) being experienced by at least some mobile stations.

In order to ensure that user equipments operate in accordance with the technical specifications, systems such as UMTS require mobile stations to undergo a conformance test which includes a test of the downlink power control loop performance. However, such tests are performed under tightly controlled lab environments which are not always representative of the mobile station' s behaviour in live networks.

Indeed experiments have shown that power control loop functionality of many mobile stations tend to operate such that the experienced downlink quality exceeds the quality requirement imposed by the network. Thus, it has been found that a significant proportion of mobile stations in a live cellular communication system tend to request unnecessarily high power levels. While this may improve the perceived communication quality for the individual mobile station requesting the excessive power, it results in a degraded performance of the communication system as a whole. Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, reduced interference, compatibility with existing standards, low complexity, facilitated operation, and/or improved performance would be advantageous. In particular, a practical and/or efficient system for the network partially controlling the downlink power control operation of user equipments would be advantageous.

Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a power control apparatus for a fixed network of a cellular communication system, the apparatus comprising: transmitting means for transmitting a downlink communication to a user equipment at a transmit power level; receiving means for receiving power control commands from the user equipment, the power control commands representing a requested transmit power change of the transmit power level to achieve a quality level for the downlink communication; means for adjusting the transmit power level in response to the power control commands; quality means for determining a quality parameter for the downlink communication in response to measurement reports received from the user equipment; mode means for switching the power control apparatus between a default power control mode of operation and a bias power control mode of operation in response to the quality parameter; and biasing means for biasing the transmit power level when the power control apparatus is in the bias power control mode.

The invention may provide improved performance in a cellular communication system. Improved downlink power control performance may be achieved in many embodiments leading to reduced interference, increased capacity and/or improved quality of service. In particular, the invention may allow the downlink power control to be partially controlled from the network side. The invention may allow an operator of the cellular communication system to identify and compensate for user equipments requesting a higher than necessary downlink transmit power. The approach may allow compensation to be performed while maintaining a constant quality target for the power control loop. The approach may provide improved backwards compatibility and may specifically be compatible with 3GPP communication systems such as UMTS, and may typically be introduced to such system without requiring changes to the technical specifications.

The power control apparatus implement may implement a nested power control loop comprising an inner and outer power control loop. The quality level may be a quality target for an outer power control loop. The power control apparatus may be distributed between different network elements of a cellular communication system. For example, the transmitting means may be comprised in a base station whereas the quality means and mode means may be located in a base station controller/ radio network controller. The fixed network may comprise the elements of the cellular communication system which are not user equipments .

According to another aspect of the invention there is provided a cellular communication system comprising: transmitting means for transmitting a downlink communication to a user equipment at a transmit power level; receiving means for receiving power control commands from the user equipment, the power control commands representing a requested transmit power change of the transmit power level to achieve a quality level for the downlink communication; means for adjusting the transmit power level in response to the power control commands; quality means for determining a quality parameter for the downlink communication in response to measurement reports received from the user equipment; mode means for switching the power control apparatus between a default power control mode of operation and a bias power control mode of operation in response to the quality parameter; and biasing means for biasing the transmit power level when the power control apparatus is in the bias power control mode.

According to another aspect of the invention there is provided a method of power control for a fixed network of a cellular communication system, the method comprising: transmitting a downlink communication to a user equipment at a transmit power level; receiving power control commands from the user equipment, the power control commands representing a requested transmit power change of the transmit power level to achieve a quality level for the downlink communication; adjusting the transmit power level in response to the power control commands; determining a quality parameter for the downlink communication in response to measurement reports received from the user equipment; switching between a default power control mode of operation and a bias power control mode of operation in response to the quality parameter; and biasing the transmit power level when in the bias power control mode.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment (s) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 is an illustration of an example of a cellular communication system in accordance with some embodiments of the invention;

FIG. 2 illustrates in example of a method of power control in accordance with some embodiments of the invention; and

FIG. 3 illustrates in example of a method of power control in accordance with some embodiments of the invention .

Detailed Description of Some Embodiments of the Invention The following description focuses on an embodiment of the invention applicable to a CDMA cellular communication system and in particular to the downlink of a UMTS cellular communication system. However, it will be appreciated that the invention is not limited to this application but may be applied to many other cellular communication systems.

FIG. 1 illustrates a block diagram of a base station 101, a user equipment 103 (UE) and a base station controller 105 of a cellular communication system. The UE 103 may typically be a remote station, a subscriber unit, a mobile station, a communication terminal, a personal digital assistant, a laptop computer, an embedded communication processor or any communication element communicating with a base station over the air interface. In the specific example, the cellular communication system is a UMTS system and thus the base station may correspond to a UMTS Node B and the base station controller to a Radio Network Controller (RNC) .

For clarity and brevity only the elements of the cellular communication system required for describing the current embodiment are shown in FIG. 1. It will be appreciated that the cellular communication system and base station comprise additional functionality required or desired for the operation and management of a cellular communication system.

In the embodiment of FIG. 1, the base station 101 is supporting a communication service of the UE 103 and accordingly transmits downlink communications to the UE 103. The base station 101 comprises a receiver 107 which receives transmissions from the UE 103. As well known to the person skilled in the art, the receiver may receive user data, control messages, data measurements etc from the UE 103. The received data may be forwarded to other network elements such as the RNC 105 to which the base station 101 is coupled. In addition, the base station 101 may use received data for the internal operation and control of the base station 101. For example, received data may be used for transmit power control as will be described below.

The base station 101 comprises a transmitter 109 which is operable to transmit data over the air interface to the UE 103 in accordance with the UMTS technical specifications. In particular, the transmitter 109 transmits user data, control messages and broadcast information to the UE 103. The transmitter 109 of the base station transmits the user data to the UE 103 at a transmit power level that may be varied to suit the current conditions.

The transmitter 109 is coupled to a power controller 111 which is operable to control the transmit power of the transmitter 109. The power controller 111 is coupled to the receiver 107 which receives power control commands from the UE 103 and forwards them to the power controller 111 which then adjusts the transmit power level in response thereto. The power control commands may be power up commands resulting in the downlink transmit power being increased or may be power down commands resulting in the downlink transmit power being decreased. The UE 103 comprises a receiver 113 for receiving signals from the base station 101 and a transmitter 115 for transmitting messages to the base station 101. The UE 103 generates the power control commands for the downlink transmit power control in the following way.

The receiver 113 is coupled to a SIR estimator 117 which generates a SIR estimate for the signal received at the UE 103. In particular, the SIR estimator 117 generates the SIR estimate in response to the characteristics of the messages received by the receiver 113 from the base station 101 as is well known in the art.

The SIR estimator 117 is coupled to an inner power controller 119 which compares the SIR estimate to a SIR reference value. If the SIR estimate is lower than the reference value the inner power controller 119 generates a power up command and if the SIR estimate is higher than the reference value the inner power controller 119 generates a power down command. The inner power controller 119 is coupled to the transmitter 115 which transmits the power commands to the base station 101. Thus, the transmit power of the base station 101 is controlled to achieve a SIR at the UE 103 corresponding to the SIR reference value.

The inner power controller 117 is furthermore coupled to an outer power controller 121, which generates a reference value for the inner power controller 121. In particular, the outer power controller 121 generates the SIR reference value and feeds it to the inner power controller 119. Thus, the inner power controller 119 controls the transmit power to preferably result in a SIR at the UE 103 equal to the SIR reference value generated by the outer power controller 121.

The outer power controller 121 is further coupled to a BLER estimator 123 which determines a BLER estimate for the signal received at the UE 103. In the described embodiment, the BLER estimator 123 is coupled to the receiver 113 and determines a BLER estimate based on the received messages as is well known in the art. For example, each block may comprise a check sum and if a check sum check is successful, the block is determined to be received without errors, and if the check sum fails, a block error is deemed to have occurred.

The outer power controller 121 further receives a signal quality target from a target reference 125. In particular, the outer power controller 121 receives a target BLER and compares the BLER estimate with this target. If the BLER estimate is lower than the BLER target value, the outer power controller 121 increases the SIR reference value and if the BLER estimate is higher than the BLER target value, the outer power controller 121 decreases the SIR reference value. Thus the outer power controller 121 controls the SIR reference value to result in a desired BLER experienced by the UE.

Thus, in the example of FIG. 1, a nested power control loop comprising an inner and outer power control loop is used to control the downlink transmit power level of the base station 101 such that the downlink communications achieve a given quality level. In the example, the quality measure is a BLER measure but it will be appreciated that other quality measures may be used. Also, it will be appreciated that other power control loop structures can be used and specifically that a nonnested power control loop comprising only a single loop driving the transmit power to achieve a given reference target can be used. For example, the combined operation of the SIR estimator 117, the BLER estimator 123 and the inner and outer power controllers 119, 121 may be considered to operate as a single power control loop controller 127 driving the downlink transmit power level towards the reference BLER target.

In the described embodiment, the signal quality target is determined in the fixed network and communicated to the UE 103 over the air interface. Thus, in the example, the target reference 125 simply receives a BLER target from the receiver 113. The BLER target is typically determined at the RNC 105 based on predetermined quality of service characteristics associated with the communication service. The target value is for a UMTS system transmitted at the initialisation of the communication service and cannot subsequently be modified.

From the description above it is clear that the UE 103 has control over both the inner and outer power control loops and therefore the power level transmitted by the base station 101. A problem arising from this scenario is that the fixed network/ network operator is reliant on the UE 103 only requesting the minimum power level necessary to meet the quality target that the network has specified (the target BLER) . However, experiments have demonstrated that many UEs frequently request a higher transmit power than necessary to achieve the target. Accordingly, many communication services are supported by excessive downlink transmit powers resulting in quality levels for the individual communication which are higher than specified. Such, situations may for example arise from design margins for the UEs being higher than strictly necessary for many scenarios not specifically tested by UMTS conformance tests. However, this approach leads to excessive interference and thus a quality degradation for other users in the system and may lead to reduced performance and capacity of the system.

Furthermore, in UMTS, such problems cannot be addressed by dynamically modifying the BLER threshold as the specifications only provide for this to be set during communication initialisation. Furthermore, although UMTS includes a possible transmit power limit this still allows the transmit power to be higher than necessary (even if below the limit) thereby increasing interference .

In the system of FIG. 1, the RNC 105 comprises functionality that allows some control of the downlink transmit power control loop from the network side.

Specifically, the RNC 105 comprises a quality processor 129 which is coupled to the receiver 107 of the base station 103. It will be appreciated that the coupling is a logical coupling representing any coupling that allows information to be exchanged between the receiver 107 and the quality processor 129. Typically, the coupling will be implemented by communication of data over the Iub interface between the base station 101 and RNC 105. The quality processor 129 is operable to determine a quality parameter for the downlink communication based on measurement reports received from the UE 103. In the specific example, the quality parameter is a parameter indicating time since the last report that a BLER parameter has exceeded a threshold. However, it will be appreciated that in other embodiments other quality measures may be used and that the quality parameter used by the quality processor 129 may be different than the quality measure used to control the power control loop.

In some embodiments, the UE 103 may regularly and continuously transmit measurement reports to the RNC 105 which can be used to determine the quality parameter. However, in the example of FIG. 1, the network transmits a reporting command to the UE 103 which requests that a specific measurement report is transmitted to the base station 101 (and from there forwarded to the quality processor 129) if a quality measurement performed by the user equipment meets a quality criterion. It will be appreciated that any suitable criterion can be used. In the specific example, the criterion is a low quality criterion indicating that the quality of the downlink communication has dropped below a given level.

Thus, in the example, event based reporting is used to provide information to the quality processor 127 about the actual quality achieved for the downlink communication. In the example, the criterion is set to be a BLER falling below a given threshold and thus the quality processor 127 will receive a measurement report whenever the actual BLER falls below a given value. The trigger value for the measurement report may be set to correspond to the required BLER level for the communication; potentially offset by a suitable design margin .

The 3GPP specifications provide a mechanism for the network requesting downlink BLER information from the UE. It can specifically request event based reporting of the transport channel (TrCH) quality when the amount of bad TrCH Cyclic Redundancy Codes (CRCs) during a predefined sliding window exceeds a predefined number. Each failed CRC is an indication of a block error. Specifically a pre-defined measurement report of type 5A as specified in 3GPP Technical Specifications TS 25.331 can be used. The 3GPP reporting mechanism allows downlink BLER information to be provided on a per TrCH basis thereby allowing the RNC 105 to perform quality metric control of individual TrCHs if there are multiple connections (TrCHs) active between the network and the UE 103.

The quality processor 129 is coupled to a mode processor 131 which is operable to switch the operation of the power control loop between a default power control mode of operation and a bias power control mode of operation in response to the quality parameter. When in the default mode, the power control loop operates normally with the network having no dynamic control of the transmit power level. Thus, the downlink transmit power is controlled by the UE 103 (except for the limit values imposed by the network) . In the bias mode, the power control loop is still active and specifically the operation of the UE 103 is unchanged. Thus, in the bias mode, the UE 103 continues to transmit power up and power down commands as determined by the inner power controller 119. However, in addition, the network side biases the transmit power level such that this deviates from the transmit power that would result purely from the received power control commands .

It will be appreciated that depending on the embodiment/ scenario, the bias may be a positive bias resulting in increased transmit power levels or may be a negative power resulting in decreased transmit power levels. The following description will focus on an example where the bias is negative, i.e. the RNC 105 has detected that the actual BLER achieved for the downlink communication is too low and that the transmit power level should accordingly be reduced to reduce interference.

Thus, if the mode processor 131 detects that the quality parameter is indicative of a current communication quality higher than a first reference quality the operation is switched to a bias mode wherein the transmit power level for the downlink communication is biased towards a reduced transmit power level. In the specific example, the quality parameter from the quality processor may for example be an indication of the time that has passed since the last measurement report indicating BLERs above a given level was received. If this duration exceeds a given predetermined threshold, the mode processor 131 switches the power control loop to the bias mode .

The mode processor 131 may then continue in the bias mode until the quality parameter indicates that the quality of the downlink has fallen below a given second reference level. For example, as the transmit power level is reduced, the BLERs may increase and if this results in a measurement report being generated from the UE 103, the quality parameter will change to reflect that a measurement report has recently been received. This may cause the mode processor 131 to change back to the default mode where the transmit power level is fully controlled by the UE 103.

The mode processor 131 is coupled to a bias processor 133 which is further coupled to the power controller 111 of the base station 101. It will be appreciated that the coupling is a logical coupling representing any coupling that allows information to be exchanged between the quality processor 129 and the power controller 111. Typically, the coupling will be implemented by a communication of data over the Iub interface between the base station 101 and RNC 105.

When the mode processor 131 switches to the bias mode, the bias processor 133 proceeds to introduce the bias to the transmit power. In the specific example, this is achieved by transmitting a bias command to the power controller 111 in response to which the power controller 111 introduces the required bias. In the example the bias command message is a UMTS NBAP DL POWER CONTROL message.

Thus, the described system provides an efficient, reliable and high performance means of monitoring the power control loop operation of UEs and to automatically take corrective action if the UEs control the loops to result in a quality level which is too high thereby resulting in excessive interference. The described system is compatible with the technical specifications of e.g. UMTS and requires no standardisation change. The system also allows a gradual correction of the power control operation and will not affect UEs requesting the appropriate transmit powers.

It will be appreciated that in different embodiments different criteria can be used for switching between the different modes.

Thus, in the specific example, the mode processor 131 may switch the power control loop to the bias mode if no measurement reports indicative of a high BLER have been received in a given time interval and may switch back to the default mode if a measurement report is received when in the bias mode. Alternatively or additionally, quality measurements reported to the Radio Link Control layers may be used to switch between the different modes.

It will be appreciated that any suitable way of biasing the transmit power in the bias mode may be used without detracting from the invention.

For example, the bias may be introduced by weighting at least some power control commands. In the extreme case, all power commands may be ignored (i.e. each command being weighted by a zero weight) allowing the transmit power to be fully set by the bias processor 133. As another example, the relative weighting of power up and power down commands may be changed. For example, the weight of power up commands may be reduced to only correspond to a tenth of a power down command such that ten power up commands are required to compensate for one power down command. As another example, the downlink transmit power may be directly biased. E.g. when the power controller 111 receives a bias command it can introduces a power offset e.g. in the form of a sudden power step. For example, it may instantly reduce the transmit power by, say, 1 dB . As the power control loop is still operating, this power reduction will be compensated for by the loop resulting in the transmit power increasing back to the original transmit power (as this is controlled by the UE 103) . However, the temporary power reduction will reduce the average transmit power and therefore the average interference in the system.

Also, in order to further reduce the average power and to compensate for the restoring effect of the power control loop, the power offset may be repeatedly introduced. For example, the bias command from the bias processor 133 may specify a power step size and a repetition time. In response, the power controller 111 can repeatedly introduce a power offset of the specified step size and with the specified repetition rate. It will be appreciated that by specifying the power step size and the repetition rate, the transmit power reduction during the bias mode can be controlled by the bias processor 133. The repetition rate may for example be set to reflect the response time of the power control loop.

In some embodiments, the power offset may be dynamically changed to compensate for the restoring effect of the power control loops. For example, a linearly increasing power offset with a given rate of change may be introduced to result in an average transmit power below the level requested by the UE 103.

In some embodiments, the bias processor 133 may be arranged to increase the power bias with time. For example, initially when the power control loop enters the bias mode, a relatively modest bias can be introduced. If this does not result in measurement reports being generated within a given time interval, i.e. if the communication quality is still too high despite the bias being introduced, the bias may be increased. This approach may be iterated until a measurement report is received indicating that the communication quality has dropped to result in a BLER which is too high.

It will be appreciated that the introduced bias level may be a predetermined bias or may be determined dynamically depending on the current conditions in the system.

For example, the bias processor 133 may determine the bias in response to a load characteristic of the base station 101. Thus, the amount of bias can be made dependant on the serving cell (s) load situation. The power reduction can thereby be pursued more aggressively for UEs which are served by overloaded cells and thus where interference is more critical. This would allow the power level to be brought down quickly in order to match the desired quality target. This approach would thereby manage the trade-off between the objective to reduce the amount of extra power transmitted unnecessarily (in doing so saving resources and reducing interference) , against the probability of a temporary increase of errors due to e.g. a sudden change in the path loss/interference level s .

As another example, the bias may be determined in response to a mobility characteristic of the UE 101. Thus, the power reduction can be pursued more aggressively for stationary UEs than for fast moving UEs. This may typically be possible as the probability of fast variations in the propagation conditions is lower for slower moving mobiles than for faster moving mobiles thereby reducing the probability that a propagation degradation will occur during the bias mode.

As another example, the amount of bias may be determined in response to measurement reports received from the UE 101. For example, if a BLER measurement report is received indicating that the communication link is relatively close to the target BLER a relatively small amount of bias may be introduced whereas if a measurement report is received indicating that the BLER is close to zero a high level of bias may be introduced.

In some embodiments, the downlink communication may comprise a plurality of parallel downlink transmissions with shared power control commands. For example, UMTS allows a number of TrCHs to support a single communication service. Furthermore, the TrCHs may use the same power control loop to control the transmit power for all transmissions. Specifically, the power control loop may be operated for a specific TrCH and the transmit power for the other TrCHs may be set with a fixed offset relative to the transmit power of this TrCH. In such cases, the RNC 105 may monitor the quality on each individual TrCH and may specifically set up event based measurement reporting of BLER as previously described. The mode processor 131 may then detect that one of the TrCHs is operating with a quality level above the required and may accordingly enter the bias mode. In the example, the bias processor 133 may bias the transmit power for this TrCH by modifying the transmit power for the downlink transmissions of this TrCH relative to a transmit power of the transmissions of the other TrCHs and specifically relative to the TrCH for which the power control loop is operated.

In some embodiments, the RNC 105 may be arranged to bias the transmit power level for a time interval following a switch from the bias mode of operation to the default mode of operation where this bias is an opposite bias of the bias of the first bias means.

For example, when exciting a bias mode wherein the transmit power has been reduced, the RNC 105 may transmit a message to the power controller 111 resulting in the transmit power being increased for a given time interval. This may result in the quality of the communication being rapidly increased following a detecting that it has fallen too low in the bias mode.

In some embodiments, the measurement reports used to determine the quality parameter by the quality processor 129 may be retransmission requests of a retransmission request scheme used for the downlink communication.

For example, for a communication service, each retransmission request may be taken as an indication of a block error. The quality processor 129 may thus determine the BLER in a sliding window and use this to determine whether to enter the bias mode. Specifically, if the number of retransmission requests within the sliding window exceeds a threshold, the RNC 105 may enter the power control into the bias mode.

The system of FIG. 1 has primarily been described with reference to reducing the transmitted power level during the bias mode. However, it will be appreciated that the approach could be applied in a more general manner and in particular is suitable for embodiments wherein the average transmitted power level is biased towards a higher transmit power level.

FIG. 2 illustrates in example of a method of power control in accordance with some embodiments of the invention .

The method initiates in step 201 wherein a downlink communication is transmitted to a user equipment at a transmit power level.

Step 201 is followed by step 203 wherein power control commands are received from the user equipment. The power control commands represent a requested transmit power change of the transmit power level to achieve a quality level for the downlink communication.

Step 203 is followed by step 205 wherein the transmit power level is adjusted in response to the power control commands . Step 205 is followed by step 207 wherein the transmit power level is biased if the power control is in a bias power control mode and is not biased if the power control is not in the bias power control mode.

Step 207 is followed by step 209 wherein a quality parameter for the downlink communication is determined in response to measurement reports received from the user equipment.

Step 209 is followed by step 211 wherein the quality parameter is evaluated and the power control is switched between a default power control mode of operation and a bias power control mode of operation in response to the quality parameter.

FIG. 3 illustrates a more detailed example of a method of power control in accordance with some embodiments of the invention. The method may be operated by the system of FIG. 1.

In the example, the variables PWR_STEP_DOWN and PWR_STEP_DOWN_TIME control the power decrease rate by a fixed amount of dB in a given number of radio frames, imposed by the base station on the fast closed power control loop, using the last reported average downlink transmit power as reference power. This process of fixed rate power reduction continues until receiving a downlink quality event report from the UE indicating that the quality fell below the desired level (e.g. a measurement report indicating a high BLER) . The variable PWR_STEP_DOWN_INIT_TIME controls the penalty delay before the forced power reduction is resumed after receiving a downlink quality event report from the UE provided no new downlink quality event reports arrive. Each new downlink quality event report from the UE restarts the penalty timer and ensures that forced power reduction remains deactivated, allowing the fast closed loop power control full control of the transmit power levels .

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

Claims

1. A power control apparatus for a fixed network of a cellular communication system, the apparatus comprising transmitting means for transmitting a downlink communication to a user equipment at a transmit power level; receiving means for receiving power control commands from the user equipment, the power control commands representing a requested transmit power change of the transmit power level to achieve a quality level for the downlink communication; means for adjusting the transmit power level in response to the power control commands; quality means for determining a quality parameter for the downlink communication in response to measurement reports received from the user equipment; mode means for switching the power control apparatus between a default power control mode of operation and a bias power control mode of operation in response to the quality parameter; and biasing means for biasing the transmit power level when the power control apparatus is in the bias power control mode.

2. The power control apparatus of claim 1 wherein the mode means is arranged to switch the power control apparatus from the default mode to the bias mode if the quality parameter is indicative of a communication quality higher than a first reference quality and from the bias mode to the default mode if the quality parameter is indicative of a communication quality lower than a second reference quality, and the bias means is arranged to bias the transmit power level towards a reduced transmit power level.

3. The power control apparatus of claim 1 further comprising means for transmitting a reporting command to the user equipment, the reporting command requesting a measurement report to be transmitted to the receiving means if a quality measurement performed by the user equipment meets a quality criterion.

4. The power control apparatus of claim 3 wherein the mode means is arranged to switch the power control apparatus from the bias mode to the default mode in response to receiving the measurement report.

5. The power control apparatus of claim 3 wherein the bias means is arranged to increase a bias of the transmit power level if the measurement report is not received within a time interval.

6. The power control apparatus of claim 1 wherein the bias means is arranged to bias the transmit power level by introducing a power offset to the transmit power level .

7. The power control apparatus of claim 1 further comprising second bias means arranged to bias the transmit power level for a time interval following a switch from the bias mode of operation to the default mode of operation; the bias of the second bias means being an opposite bias of the bias of the first bias means .

8. The power control apparatus of claim 1 wherein the bias means is arranged to determine a bias level in response to one of the group of: a load characteristic of a base station supporting the downlink communication, and a mobility characteristic of the user equipment.

9. The power control apparatus of claim 1 wherein the downlink communication comprises a plurality of parallel downlink transmissions with shared power control commands, and the bias means is arranged to bias the transmit power level by modifying a transmit power of one downlink transmission relative to a transmit power of another downlink transmission.

10. A method of power control for a fixed network of a cellular communication system, the method comprising transmitting a downlink communication to a user equipment at a transmit power level; receiving power control commands from the user equipment, the power control commands representing a requested transmit power change of the transmit power level to achieve a quality level for the downlink communication; adjusting the transmit power level in response to the power control commands; determining a quality parameter for the downlink communication in response to measurement reports received from the user equipment; switching between a default power control mode of operation and a bias power control mode of operation in response to the quality parameter; and biasing the transmit power level when in the bias power control mode.