GB2413922A

GB2413922A - Method and Apparatus for controlling a transmit power of a radio transmitter.

Info

Publication number: GB2413922A
Application number: GB0410209A
Authority: GB
Inventors: Jonathan Wallington; David L Bhatoolaul
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2004-05-07
Filing date: 2004-05-07
Publication date: 2005-11-09
Anticipated expiration: 2024-05-07
Also published as: GB2413922B; GB0410209D0

Abstract

A communication service is supported between a base station (101) and a user equipment (UE) (103) of a cellular communication system (100). The communication service utilises a power control and a data retransmission mechanism to reduce the resulting number of errors. The base station comprises a transmitter (107), the transmit power of which is controlled by an inner power control loop (109, 115, 117). The inner power control loop (109, 115, 117) receives a reference value from an outer power controller (119) which sets the reference value in response to a signal quality target. The signal quality target is associated with a retransmission probability as an increased signal quality target reduces the retransmission probability. The signal quality target is generated by the target performance controller (125) which is operable to modify the signal quality target dynamically during the communication service. The invention allows a flexible and dynamic balancing of different error management mechanisms resulting in improved performance of the communication system.

Description

24 1 3922

METHOD AND APPARATUS FOR CONTROLLING A TRANSMIT POWER

OF A RADIO TRANSMITTER

Field of the invention

The invention relates to a method and apparatus for controlling a transmit power of a radio transmitter and in particular to controlling a transmit power In a cellular communication system.

Background of the Invention

In a cellular communication system a geographical region is divided into a number of cells each of which is served by a base station. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated.

As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. As the mobile station moves towards a base station, it enters a region of overlapping coverage of two base stations and within this overlap region it changes to be supported by the new base station. As the mobile station moves further into the new cell, it continues to be supported by the new base station. This is known as a handover or handoff of a mobile station between cells.

A typical cellular communication system extends coverage over typically an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile CE12576EP F'nalVerson PWdoc station to a base station is known as uplink, and communication from a base station to a mobile station is known as downlink.

The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc. Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. One carrier is used for a pilot signal which further contains broadcast information. This carrier is used by mobile stations for measuring of the signal level of transmissions from different base stations, and the obtained information is used for determining a suitable serving cell during initial access or handovers. Further description of the GSM TDMA communication system can be found in 'The GSM System for Mobile Communications' by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

CE12576EP FlnalVersion.PWdoc To further enhance the services and performance of the GSM communication system, a number of enhancements and additions have been introduced to the GSM communication system over the years.

One such enhancement is the General Packet Radio System (GPRS), which is a system developed for enabling packet data based communication in a GSM communication system. Thus, the GPRS system is compatible with the GSM (voice) system and provides a number of additional services including provision of packet data communication, which augments and complements the circuit switched communication of a traditional communication system.

Furthermore, the packet based data communication may also support packet based speech services. The GPRS system has been standardized as an add-on to an existing GSM communication system, and can be introduced to an existing GSM commumcation system by introducing new network elements.

Specifically, a number of Serving GPRS Support Nodes (SGSN) and Gateway GPRS Support Nodes (GGSN) may be introduced to provide a packet based fixed network communication.

Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) wherein user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de-spread the received signal thereby regenerating the original signal. Each base station has a code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed.

CE12576EP F'nalVerson PWdoc Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

In a UMTS CDMA communication system, the communication network comprises a core network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user equipment over a radio link being part of the air interface. The wireless user equipment may be a mobile station, a communication terminal, a personal digital assistant, a laptop computer, an embedded communication processor or any communication element communicating over the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the Node Bs and the communication over the air interface.

Common to all types of cellular communication systems is that it is imperative to manage the radio links between the base stations and the mobile 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 it's transmit power.

CE12576EP FloalVersion PWdoc In WCDMA, 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 (interference) 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 known 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 or 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 or BLER is at or below the given value.

In recent years, the flexibility and variety of communication services supported by a cellular communication system has increased significantly. For example, 3rd generation systems, such as UMTS, along with enhancements to 2n generation systems, such as GPRS, have introduced a number of services known as Acknowledge Mode (AM) services. For AM services, the receiving end provides feedback to the transmitting end indicating whether data has been successfully received or not. Specifically, data may be sent in distinct blocks and for each block a positive acknowledge message (ACK) indicating that the block was received successfully or a negative acknowledge message (NACK) indicating that the block was not received successfully may be sent. If a NACK is received, the transmitting end retransmits one or more of the data blocks. Thus, the AM services use retransmissions to compensate for transmission errors.

It will be appreciated that the power control functionality and retransmission functionality are heavily interrelated and mutually affect each other. Both mechanisms are concerned with transmission error management and control CE12576EP F'nalVerson.PWdoc and are aimed at ensuring reliable communication without excessive resource use and interference. For example, for decreased transmission power, the signal to noise ratio at the receiver deteriorates resulting in an increased number of receive errors. The retransmission mechanism of the AM services accordingly compensates by generating an increased number of retransmissions. Conversely, if the transmit power is increased for given propagation conditions, fewer errors occur and fewer retransmissions are required.

As the transmit power is controlled by the power control loop, it is important that this is operated optimally. Setting the target too low will result in too many transmission errors leading to an excessive increase in retransmissions resulting in congestion, delays and increased resource consumption. Setting it too high will result in excessive transmit powers resulting in increased interference and a resulting loss of capacity for the cell.

More specifically for a UMTS AM communication service, the choice of BLER target employed by a Radio Bearer (RB) is a compromise between the instantaneous transmit power usage, the link quality and the retransmission performance. A higher BLER target means less power is needed for the transmission but at the cost of more errors, whereas a lower BLER target means fewer errors but at the cost of a higher power requirement. The price of higher errors and the subsequent retransmissions is a reduction in goodput = throughput (ABLER), increased delays, an increased probability of data being discarded due to e.g. buffer overflow, an increased probability of a dropped call and an increase in the total transmitted energy. The price of a higher instantaneous power requirement is increased power consumption and increased interference to other users. This will further cause the transmit power for these users to be increased. This non-linear feedback effect leads to increased overall interference and transmit power levels.

CE12576EP F'nalVersion PW.doc Conventionally, the BLER target is set to a predetermined value dependent on the desired service characteristics. Thus, the predetermined BLER target is selected to provide a reasonable trade off between all the above described factors. However, this results in an inflexible and suboptimal control of the transmit power resulting in increased interference and reduced capacity of the communication system as a whole. It furthermore does not exploit and utilise the interaction between the power control mechanism and retransmission mechanism thereby leading to suboptimal performance.

Hence, an improved system for controlling transmit powers would be advantageous and in particular a system allowing increased flexibility, improved performance, reduced interference, reduced delay and/or an improved resource utilization 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 an apparatus for controlling a transmit power of a radio transmitter of a cellular communication system, the radio transmitter supporting a communication service using a retransmission scheme; the apparatus comprising: power control means for controlling the transmit power in response to a signal quality target associated with a retransmission probability; and means for modifying the signal quality target dynamically during the communication service.

CE12576EP FlnalVersion PWdoc The inventors of the current invention have realised that power control of a transmit power is closely interrelated with retransmission performance for a communication service using a retransmission scheme and that a significantly improved performance and operation of a cellular communication system can be achieved by modifying a signal quality target for a power control means dynamically during a communication service. In particular, the invention may provide for an improved flexibility of error management allowing the trade off between increased transmit power and increased retransmissions to be dynamically optimised during a communication service. This may provide improved control over the total required transmit energy for achieving a given goodput. The invention may thus provide reduced interference, reduced delay, increased capacity, improved quality of service, increased flexibility and/or improved resource usage.

The invention may be applied to the uplink and/or the downlink of a cellular communication system. The signal quality target is associated with the retransmission probability since a change of the signal quality target will result in a changed retransmission probability for given transmission conditions. In particular, an increased signal quality target typically results in a reduced probability of retransmissions as fewer errors are received.

According to a preferred feature of the invention, the signal quality target is a target error rate. Preferably, the target error rate is a block error rate target.

The block error rate target may for example be a frame erasure rate. In particular, for a UMTS cellular communication system, the block error rate target may be a BLER or a PER as defined in the Technical Specifications TS 25.215 version 5.5.0 section 5.1.6.

An error rate is a particularly suitable signal quality target providing efficient performance as it has a particularly strong and direct association with the CE, 2576EP.F'naiVersion.PW ooc retransmission probability. It furthermore provides for compatibility with existing communication systems such as UMTS.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a resource characteristic.

The resource characteristic may e.g. be a previous, current, future, measured, calculated and/or estimated resource characteristic. The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a characteristic of the transmit power.

The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole. In particular, the signal quality target may be reduced for increasing transmit powers thereby resulting in an increasing amount of error management being handled by retransmissions. For example, if the transmit power is above a given threshold, the signal quality target may be reduced until the transmit power falls below the threshold. This may prevent excessive transmit powers causing power amplifier overload and/or excessive interference.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a transmit power associated with at least one other communication service.

CE12576EP F'nalVerson PWdoc The transmit power may be a transmit power associated with a single other communication service but is typically associated with a group of communication services. In particular, the transmit power of all communication services of a given cell may be used.

The transmit power of the other communication services may e.g. be a previous, current, future, measured, calculated and/or estimated transmit power. The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a quality of service characteristic of the communication service.

The quality of service characteristic may e.g. be a previous, current, future, measured, calculated and/or estimated quality of service characteristic. The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole.

The quality of service characteristic may for example be a delay, an error rate, or a data rate of the communication service. The feature may provide an improved service to a user and may be used to provide a guaranteed quality of service for the communication service.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a functionality limitation of a communication element associated with the communication service. The functionality limitation may be a memory limitation.

CE12576EP FinaiVerson PW.doc The communication element is preferably a receiver receiving the communication service, a network element of a network of the cellular communication system and/or a user equipment of the cellular communication system.

The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole.

The functionality limitation is preferably a functionality limitation affecting the power control, transmit power and/or retransmission functionality performance. For example, the functionality may be a maximum transmit power capability of the transmitter or a memory limitation of a buffer. The signal quality target is preferably controlled to avoid the functionality limitation being exceeded thereby reducing the impact of the functionality limitation.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a cell loading.

Preferably, the cell loading is of the serving cell associated with the communication service but alternatively or additionally, the cell loading may be a cell loading of another cell such as a neighbour cell.

The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole. In particular, the signal quality target may be controlled to increase the emphasis on retransmissions for high loading levels where additional transmit powers may be particularly undesirable.

CE12576EP F'nalVerson PWooc According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a retransmission characteristic.

The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole. In particular, a delay or retransmission probability or rate may be used to control the signal quality target. For example, if the retransmission probability or frequency becomes too high, the signal quality target may be increased to reduce the retransmission probability on account of increased transmission power.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a communication traffic characteristic.

The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole. The communication traffic characteristic is preferably a characteristic of the communication service such as a traffic volume associated with the communication service.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a priority of the communication service.

The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole. In particular, the signal quality target may be controlled to allow high priority services to be communicated with low error probabilities and few retransmissions whereas CE12576EP F'nalVerson PWdoc low priority services are allowed to rely on retransmissions for successful communication. Hence, a higher reliability and lower call drop probability and delay of a priority service may be achieved.

According to a preferred feature of the invention, the cellular communication system is a CDMA cellular communication system.

In particular, the cellular communication system may be a 3r generation cellular communication system such as a UMTS communication system. Thus, the invention may provide for improved performance in a CDMA cellular communication system.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a transmit power associated with a spreading code supporting the communication service.

The spreading code may in particular be an OVSF (Orthogonal Variable Spreading Factor) code of a UMTS cellular communication system and the signal quality target may be modified such that the transmit power of an individual OVSF code does not increase beyond a given threshold. The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a total transmit power associated with a plurality of spreading codes supporting communication services.

The spreading code may in particular be an OVSF code of a UMTS cellular communication system and the signal quality target may be modified such CE12576EP.F'nalVerson PWdoc 1' that the transmit power of the plurality of OVSF codes does not increase beyond a given threshold. The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole. In particular, it may reduce a maximum interference level or combined transmit power of for example a base station.

According to a preferred feature of the invention, the means for modifying is operable to modify the signal quality target in response to a spreading code resource usage.

The spreading code may in particular be an OVSF code of a UMTS cellular communication system. The spreading code resource usage may e.g. be a previous, current, future, measured, calculated and/or estimated resource usage. The feature may provide a suitable parameter for controlling the signal quality target resulting in improved performance of the communication service, the serving cell and/or the communication system as a whole. For example, the feature may allow improved flexibility in managing resource usage in response to the availability and use of OVSF codes.

According to a preferred feature of the invention, the communication service is supported by a shared channel.

The shared channel is a channel that is or may be shared between a plurality of communication services and in particular between a plurality of users or mobile stations. Retransmissions for shared channel services may affect other communication services than the service requesting the retransmission and the optimum control of retransmissions is thus of particular importance for shared channels.

Preferably, the communication service is a packet data service.

CE12576EP F'nalVerson PWdoc According to a preferred feature of the invention, the power control means controls an inner loop power control loop in response to a reference value generated by an outer loop power control loop, the signal quality target being a reference value for the outer loop power control loop.

This provides for a particularly advantageous implementation and in particular provides compatibility with 3r generation cellular communication systems such as UMTS.

According to a preferred feature of the invention, the apparatus further comprises means for modifying a service parameter of the communication service.

For example, a data rate, delay or other quality of service parameter may be changed. The feature allows a further flexibility in resource and interference management and trade off and in particular provides for increased control in association with the control of the signal quality target. In particular, small variations of the current conditions may be taken into account by modifying the signal quality target whereas larger variations may be taken into account by modifying a service parameter. Modification of a service parameter may for example be performed by switching between a range of predefined service types having given quality of service parameters.

According to a second aspect of the invention, there is provided a cellular communication system comprising an apparatus as described above.

According to a third aspect of the invention, there is provided a method for controlling a transmit power of a radio transmitter of a cellular communication system, the radio transmitter supporting a communication service using a retransmission scheme; the method comprising the steps of CE12576EP F'nalVerson PWdoc controlling the transmit power in response to a signal quality target associated with a retransmission probability; and modifying the signal quality target dynamically during the communication service.

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

An embodiment of the invention will be described, by way of example only, with reference to the drawing, in which FIG. l illustrates a block diagram of a base station and a user equipment (UK) of a cellular communication system.

Detailed Description of a Preferred Embodiment of the Invention The following description focuses on an embodiment of the invention applicableto 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. The invention is applicable to the uplink as well as the downlink.

FIG. 1 illustrates a block diagram of a base station lot and a user equipment 103 (UK) of a cellular communication system lOO. The UE may typically be a subscriber unit, a mobile station, a communication terminal, a personal digital CE12576EP F'nalVerson PWdoc assistant, a laptop computer, an embedded communication processor or any communication element communicating over the air interface.

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 for the UE 103. In the specific example, the communication service is a data packet service wherein data packets are transmitted from the base station 101 to the UE 103. The data packet communication service supports an application of the UK, such as for example a file download, a voice communication or Internet browsing.

The communication service is an Acknowledged Mode (AM) service using retransmissions to reduce the error rate. In particular, the communication service uses an Automatic Repeat request (ARQ) scheme where each data packet is positively or negatively acknowledged as is well known in the art.

The cellular communication system stores the transmitted data packets in a suitable buffer and if a negative acknowledge message is received from the UE 103, the corresponding data packet is retransmitted. If a positive acknowledgement is received, the corresponding data packet is discarded from the buffer thereby freeing up memory.

It will be appreciated that most of the functionality of the retransmission functionality may be implemented in other network elements than the base station 101 and in particular may be implemented in a Radio Network Controller (RNC) coupled to the base station 101. For example, the data buffer may be implemented in the base station 101 or an RNC (not shown).

CE12576EP F'nalVerson.PWdoc The base station 101 comprises a receiver 105 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 (not shown) 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 furthermore comprises a transmitter 107 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 transmits the data packets, control messages and broadcast information to the UE 103. The transmitter 107 of the base station transmits the data packets to the UE 103 at a transmit power level that may be varied to suit the current conditions.

The transmitter 107 is coupled to a power controller 109 which is operable to control the transmit power of the transmitter 107. The power controller is coupled to the receiver and receives power control commands from the UE 103.

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 111 for receiving signals from the base station 101 and a transmitter 113 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.

CE12576EP FinalVersion PW.doc The receiver 111 is coupled to a SIR estimator 115 which generates a SIR estimate for the signal received at the UE 103. In particular, the SIR estimator 115 generates the SIR estimate in response to the characteristics of the messages received by the receiver 111 from the base station 101 as is well known in the art.

The SIR estimator 115 is coupled to an inner power controller 117 which compares the SIR estimate to a SIR reference value. If the SIR estimate is lower than the reference value the inner power controller 117 generates a power up command and if the SIR estimate is higher than the reference value the inner power controller 117 generates a power down command. The inner power controller 117 is coupled to the transmitter 113 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 119, which generates a reference value for the inner power controller 117. In particular, the outer power controller 119 generates the SIR reference value and feeds it to the inner power controller 117. Thus, the inner power controller 117 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 119.

The outer power controller 119 is further coupled to a BLER estimator 121 which determines a BLER estimate for the signal received at the UE 103. In the described embodiment, the BLER estimator 121 is coupled to the receiver 111 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 CE12576EP F'nalVerson PWooc without errors, and if the check sum fails, a block error is deemed to have occurred.

The outer power controller 119 further receives a signal quality target from a target reference 123. In particular, the outer power controller 119 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 119 increases the SIR reference value and if the BLER estimate is higher than the BLER target value, the outer power controller 119 decreases the SIR reference value. Thus the outer power controller 119 controls the SIR reference value to result in a desired BLER experienced by the UK.

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 123 simply receives a BLER target from the receiver 111. In the described embodiment, the base station 101 comprises a target performance controller 125 which generates a suitable signal quality target. The target performance controller 125is coupled to the transmitter 107 of the base station 101 which transmits the signal quality target to the UE 103. It will be appreciated that in other embodiments, the target performance controller 125 may be located elsewhere, and that it in particular may be fully or partially implemented in an RNC (not shown) coupled to the base station 101.

It will be appreciated that in the cellular communication system of FIG. 1, the error performance is controlled by a double power control loop mechanism. In addition, the communication service uses retransmissions to perform error recovery. Hence, two different mechanisms are used to control the error performance and to achieve the desired overall link quality. The inventors have realised that these mechanisms have very different performance and in particular impact differently on other aspects and characteristics of the CE12576EP.F'nalVerswn PWooc communication performance than the error performance. For example, retransmissions introduce additional delays, additional memory requirements for buffering of data packets and additional congestion whereas increased transmit powers result in increased instantaneous interference and increased power consumption.

The inventors have realised that the performance of the communication system may be improved by dynamically adjusting the relative significance of the different mechanisms depending on the specific conditions in the cellular communication systems. For example, in some situations, it may be preferable to have lower transmit powers with a higher rate of retransmissions whereas it in other situations may be preferable to have higher transmit powers with the resulting lower rate of retransmissions.

The inventors have furthermore realised that the balance between the transmit power control and the retransmission mechanisms may be adjusted by modifying the signal quality target used by the power control means. In particular, the weighting between the two mechanisms may be achieved by dynamically modifying the target BLER target value. Thus, the target performance controller 125 is in the described embodiment capable of modifying a signal quality target, in the form of the BLER target value, dynamically during the communication service. The modified BLER target value is communicated to the UE 103 which changes the value stored in the target reference 123 when a new value is received.

The dynamic variation of the signal quality target provides an improved optimization and allows the different error management mechanisms to be adjusted with respect to each other. In particular, the embodiment may provide an increased flexibility in the error management which may result in reduced interference and improved performance not only for the communication service but for the communication system as a whole.

CE12576EP.F'nal\/erswn PWdoc The embodiment for example allows the total energy required for a transmission of a given number to be optimised. Specifically, by reducing the transmit power, a decreased energy consumption for transmission of a data packet is decreased. However, this increases the probability of retransmissions, each of which results in increased energy consumption. Thus, the total energy consumption for transmission of a data packet in a retransmission scheme has a minimum corresponding to a given transmit power setting. This transmit power depends on the specific conditions and the target performance controller 125 may therefore control the signal quality target in response to characteristics that allow the transmit power to approach the optimal value for the current conditions.

It will be appreciated that many different characteristics, parameters and algorithms may be used to control the signal quality target. In the following, a number of possible characteristics and parameters in response to which the signal quality target may be controlled will be presented. It will be appreciated that other characteristics may additionally or alternatively be used and that the described characteristics and parameters may be used individually or in any suitable combination. It will furthermore be appreciated that the characteristics and parameters may be determined, generated, estimated, measured and/or received in any suitable way and at any suitable functional, physical or logical location.

The signal quality target generated by the target performance controller 125 may be modified in response to a resource characteristic. In one such embodiment, a resource distribution for the base station 101 supporting the communication service is determined. The resource distribution may then be used to adjust the signal quality target.

CE12576EP F'nalVersion PW.doc As a specific example, a dedicated channel may be used for transmissions of data packets whereas a shared channel is used for retransmissions. In this case, the current resource usage of the shared channel and the dedicated channel may be determined. If the resource usage of the dedicated channel is low and the resource usage of the shared channel is high, the signal quality target is set to a high value resulting in reduced shared channel resource usage in exchange for an increased dedicated channel resource usage.

Conversely, if the resource usage of the shared channel is low and the resource usage of the dedicated channel is high, the signal quality target is set to a low 1 0 value.

As another example suitable for a CDMA communication system, the target performance controller 125 may modify the signal quality target in response to a spreading code resource usage. For example, the signal quality target may be increased as the number of unused OVSF codes of a base station of a UMTS communication system is reduced.

For instance, if many retransmissions occur, the communication service may require additional OVSF codes for the retransmissions or may switch to an OVSF code having a lower spreading factor in order to increase the reliability (e.g. by switching to a stronger forward error correcting code using more redundancy). This may be an advantageous strategy if the cell is interference limited but has sufficient available codes. However, if the interference levels are low, but most OVSF codes are allocated, this is highly undesirable and a preferred approach is to increase the signal quality target in order to reduce the number of retransmissions at the expense of an increased interference level. Thus, the embodiment will allow a dynamic optimization to the current interference and OVSF resource use conditions.

CE12576EP F'nalVerson PWdoc In some embodiments the target performance controller 125 may modify the signal quality target in response to a characteristic of the transmit power which is being controlled.

Preferably, the transmit power level is taken into account when determining the signal quality target. Typically, it will be advantageous to maintain the transmit power as close to a nominal power as possible. For example, if the transmit power moves towards high values, the resulting interference increases and the margin for transmit power variations reduces. Therefore, the signal quality target is preferably controlled such that the transmit power is biased towards nominal transmit power levels. Thus, the signal quality target may be reduced for increasing transmit power values. The transmit power may specifically be a transmit power for a spreading code (e.g. an OVSF code) supporting the communication service.

As a specific example suitable for the uplink scenario, the power amplifier of a UE typically has a limited dynamic range. Therefore, the signal quality target used as a reference value for the outer power control loop in the UE is preferably reduced if the transmit power level increases beyond a given threshold. This will bias the operation of the UE towards power levels within the dynamic range and thus may reduce the probability of clipping and/or non- linearity effects.

In some embodiments the target performance controller 125 may modify the signal quality target in response to a transmit power associated with at least one other communication service.

For example, the total transmit power for all communication services sharing a power amplifier of a base station may be determined. The power amplifier typically has a preferred operating range wherein it exhibits linearity suitable for amplification of multiple signals. If the total transmit power is outside this CE12576EP F'nalVerson PWdoc operating range, non-linearity effects result in cross modulation thereby degrading the performance. Consequently, the signal quality target is preferably controlled to bias the transmit power towards the optimal transmit power level.

In a specific embodiment suitable for a UMTS communication system, the target performance controller 125 may modify the signal quality target in response to a total transmit power associated with a plurality of spreading codes, such as OVSF codes, that support other communication services. For example, a number of OVSF codes may be supported by a single power amplifier and the total transmit power for these OVSF codes may be used in controlling the signal quality target.

In some embodiments, the target performance controller 125 may modify the signal quality target in response to a quality of service characteristic of the communication service.

For example, it is well known that retransmissions introduce a significant delay to a communication service. Thus, if the number of retransmissions increases, the delay may increase substantially. This may be acceptable if the delay requirement of the communication service is sufficiently large. However, if the delay approaches the maximum delay guaranteed for the service, the signal quality target is preferably increased to result in fewer retransmissions and thus in a reduction of the delay at the expense of an increase in the instantaneous interference.

In some embodiments, the target performance controller 125 may modify the signal quality target in response to a functionality limitation of a communication element associated with the communication service.

CE12576EP FinalVersion PWdoc In particular, the retransmission scheme requires that data packets are stored until a positive acknowledgement is received from the UK. This requires a suitable buffer having sufficient memory capacity to store the data packets until they can be discarded. However, if the conditions result in too many transmission errors, the requirement for temporary data packet storage may exceed the memory capacity of the buffer. Therefore, if the buffer load level increases beyond a given level, the signal quality target is preferably increased to reduce the requirement for retransmissions. Preferably, when the buffer level drops below a lower level, the signal quality target may be reduced thereby allowing an increase of the number of retransmissions in order to reduce the average transmit power.

The buffer may be a dedicated buffer for the communication service or may be a shared buffer for a number of retransmission communication services.

As another example, the functionality limitation may be a maximum transmit power and the signal quality target may be modified to bias the transmit power towards lower levels as described above.

The functionality limitation may be a functionality limitation of the base station or of another network element of the cellular communication system, such as the RNC to which the base station is connected. For example, the data packet buffer may be implemented in the RNC or in the base station and the signal quality target may be modified in response to this buffer level.

The functionality limitation may also be a functionality limitation of the UK.

This may be particularly appropriate for uplink applications wherein the data packet buffer and transmitter are typically implemented in the UK.

In some embodiments, the target performance controller 125 may modify the signal quality target in response to a cell loading.

CE12576EP.F'naiVersion PWdoc For example, in CDMA communication systems, high cell loadings result in high levels of interference. These high levels of interference cause the transmit powers to be increased, which again increases interference. This interference feedback may in extreme cases result in a runaway transmit power increase that overloads the cell resulting in many dropped calls. Accordingly, for high cell loadings, the signal quality target is preferably reduced thereby causing an increased number of retransmissions at a lower instantaneous transmit power. This will be particularly advantageous in highly loaded cells where propagation conditions may change rapidly.

Specifically, the transmit power may be sufficient for the majority of data packets to be received correctly. The data packets which are not received correctly will typically have been transmitted during disadvantageous propagation and interference conditions. However, by the time these data packets are retransmitted, the interference and propagation conditions may have improved such that successful communication is possible at the lower data rate. Thus, the mechanism allows the communication to be supported by a lower average power when the cell loading is critically high thereby increasing the capacity of the communication system and increasing the reliability of the communication service. The increased reliance on retransmissions increase the delay of the communication service and therefore the signal quality target is preferably increased at lower cell loadings when the instantaneous transmit power is less critical. Thus, the performance is dynamically modified to suit the current conditions.

In some embodiments the target performance controller 125 may modify the signal quality target in response to a retransmission characteristic.

For example, the signal quality target may be modified in response to for example a current delay of the retransmissions or a frequency or rate of CE12576EP F'nalVerson PWdoc retransmissions. As a specific example, if more than a certain percentage of the data packets are retransmitted, the signal quality target may be reduced in order to reduce the probability of retransmissions. In other embodiments, a retransmission delay may alternatively or additionally be controlled more directly for example by determining the duration in which the oldest data packet has been stored in the buffer and increasing the signal quality target for an increasing duration. Thus, retransmission characteristics may directly be measured and used to bias the error management towards a nominal balance wherein the error management relies suitably on both the power control mechanism and the retransmission mechanism.

In some embodiments, the target performance controller 125 may modify the signal quality target in response to a communication traffic characteristic. For example, the signal quality target may be modified in response to a traffic volume or a burstyness of the communication. For example, for high volume, bursty data services, an increased reliance on retransmissions may be preferred. This will have a smoothing effect on the resource usage as the retransmissions will cause the data communication to be spread over an increasing time interval.

In some embodiments, the target performance controller 125 may modify the signal quality target in response to a priority of the communication service. In particular, the signal quality target may be set higher for a priority service than for a non-priority service. This will result in the high priority service to be communicated with a reduced delay and increased reliability but with a higher interference impact on other communications than for the non-priority communication service.

It will be appreciated that in a preferred embodiment, a complex algorithm for controlling the signal quality target taking into account a plurality of the described parameters may be implemented.

CE12576EP.F'nalVerson PWdoc In some embodiments, the cellular communication system may additionally comprise means for modifying a service parameter of the communication service as well as the signal quality target. For example, the data rate, delay requirement or reliability of the communication service may be changed. For example, for a UNITS communication system Dynamic Channel Configuration Control (DCCC) allows switching between data rates or between Radio Resource Control (RRC) states in response to a cell loading. Hence, depending on the conditions, the target performance controller 125 may change the signal quality target without changing a service parameter or a service parameter may be changed typically in addition to a change in the signal quality target.

In some embodiments, the communication service is supported by a shared channel. For example, a plurality of bursty communication services may share the same channel by transmitting using the same spreading code. In the downlink, the data packets may be time multiplexed unto the same spreading code, and it will be appreciated that retransmissions also using the shared channel reduce the capacity of the shared channel. However, increasing the transmit power to reduce the number of retransmissions increases interference to other users. Thus, the signal quality target may preferably be modified depending on the load of the shared channel and the interference conditions in the cell. Specifically, if the cell is interference limited and the shared channel is not heavily loaded, the signal quality target is reduced to achieve a low transmit power with many retransmissions. However, if the interference levels are low and the shared channel is heavily loaded, the signal quality target is increased to reduce the number of retransmissions.

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units of the cellular communication system. However, it will be apparent that any suitable distribution of functionality between different functional units may be CE12576EP.F'nalVerson PWdoc used without detracting *om the invention. 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. However, preferably, the invention is 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 1 5 processors.

Although the present invention has been described in connection with the preferred embodiment, 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. 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. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

CE12576EP.FnalVerson PWdoc

Claims

1. An apparatus for controlling a transmit power of a radio transmitter of a cellular communication system, the radio transmitter supporting a communication service using a retransmission scheme; the apparatus comprising: power control means for controlling the transmit power in response to a signal quality target associated with a retransmission probability; and means for modifying the signal quality target dynamically during the commumcatlon service.

2. An apparatus as claimed in claim 1 wherein the signal quality target is a target error rate.

3. An apparatus as claimed in claim 2 wherein the target error rate is a block error rate target.

4. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a resource characteristic.

5. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a characteristic of the transmit power.

6. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a transmit power associated with at least one other communication service.

CE12576EP F'nalVerson PWdoc

7. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a quality of service characteristic of the communication service.

8. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a functionality limitation of a communication element associated with the commumcatlon service.

9. An apparatus as claimed in claim 8 wherein the communication element is a receiver receiving the communication service.

10. An apparatus as claimed in claim 8 wherein the communication element is a network element of a network of the cellular communication system.

11. An apparatus as claimed in claim 8 wherein the communication element is a user equipment of the cellular communication system.

12. An apparatus as claimed in any previous claim 8 to 11 wherein the functionality limitation is a memory limitation.

13. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a cell loading.

14. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a retransmission characteristic.

CE12576EP.F'naiVersion PWdoc

15. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a communication traffic characteristic.

16. An apparatus as claimed in any previous claim wherein the means for modifying is operable to modify the signal quality target in response to a priority of the communication service.

17. An apparatus as claimed in any previous claim wherein the cellular communication system is a CDMA cellular communication system.

18. An apparatus as claimed in claim 17 wherein the means for modifying is operable to modify the signal quality target in response to a transmit power associated with a spreading code supporting the communication service.

19. An apparatus as claimed in any previous claim 17 or 18 wherein the means for modifying is operable to modify the signal quality target in response to a total transmit power associated with a plurality of spreading codes supporting communication services.

20. An apparatus as claimed in any previous claim 17 to 19 wherein the means for modifying is operable to modify the signal quality target in response to a spreading code resource usage.

21. An apparatus as claimed in any previous claim wherein the communication service is supported by a shared channel.

22. An apparatus as claimed in any previous claim wherein the communication service is a packet data service.

CE12576EP.FlnalVersion PWdoc

23. An apparatus as claimed in any previous claim wherein the power control means controls an inner loop power control loop in response to a reference value generated by an outer loop power control loop, the signal quality target being a reference value for the outer loop power control loop.

24. An apparatus as claimed in any previous claim wherein the apparatus further comprises means for modifying a service parameter of the communication service.

25. A cellular communication system comprising an apparatus according to any of the previous claims.

26. A method for controlling a transmit power of a radio transmitter of a cellular communication system, the radio transmitter supporting a communication service using a retransmission scheme; the method comprising the steps of controlling the transmit power in response to a signal quality target associated with a retransmission probability; and modifying the signal quality target dynamically during the communication service.

27. A computer program enabling the carrying out of a method according to claim 26.

28. A record carrier comprising a computer program as claimed in claim 27.

CE12576EP F'nalVerson PWdoc