GB2421150A - Power control during soft handover - Google Patents

Abstract

A cellular communication system (100) comprises an inner power control for controlling transmission powers of a user equipment (101). A plurality of base stations (103, 105, 107) receive a first signal from the user equipment 101 in a soft handover communication channel. A serving base station (103) of the plurality of base stations (103, 105, 107) receives a second signal from the user equipment (101) in a non-soft handover communication channel. The cellular communication system comprises an error processor (303) for determining a quality characteristic of a radio link (113) between the user equipment (101) and the base station (103). A comparator (211) and a target processor (30) then determine a target parameter for the inner power control loop in response to the quality characteristic. The non-soft handover communication channel may specifically be an HSDPA (High Speed Downlink Packet Access) channel of a 3<rd> Generation cellular communication system.

Description

A CELLULAR COMMUNICATION SYSTEM AND A METHOD OF POWER

CONTROL THEREFOR

Field of the invention

The invention relates to power control in a cellular communication system and in particular, but not exclusively, to power control in a 3rd generation cellular communication system.

Background of the Invention

In a cellular communication system, a geographical region is divided into a number of cells served by base stations. 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 from the base station of the cell within which the mobile station is situated.

A typical cellular communication system extends coverage over an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as the uplink, and communication from a base station to a mobile station is known as the 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 * * S.. S.. * *.S * S S S * * * *SS S * S * S S S * S S * * S *S S * . . S * ** S ** S * * I Se I with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Internet or 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. 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. 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.

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) technology. Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) techniques employ this CDMA technology. In CDMA systems, user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency and * * S.. **S S *SS * * * S S S S S.. S S S S S S S S S S S * S 55 S * S S S S 55 5 *S S S S S *5 S in the same time intervals. In TDD, additional user separation is achieved by assigning different time slots to different users in a similar way to TDMA. However, in contrast to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in WCIDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

In a 3rd generation cellular 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. The RAN is operable to support wireless user equipment over a radio link of the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC5) which control the base stations and the communication over the air interface.

The RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate base stations. It further provides the interface between the RAN and the core network. An RNC and associated base stations are collectively known as a Radio Network Subsystem (RNS).

* S *SS *SS S SI.

* S S * * S * S.. S * S S * S S * S S S S S *S S * S * * S SI * S. S * S IS S 3rd generation cellular communication systems have been specified to provide a large number of different services including efficient packet data services. For example, downlink packet data services are supported within the 3rd Generation Partnership Project (3GPP) release 5 Technical Specifications in the form of the High Speed Downlink Packet Access (HSDPA) service.

In accordance with the 3GPP specifications, the HSDPA service may be used in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode.

In HSDPA, transmission code resources are shared amongst users according to their traffic needs. The base station (also known as the Node-B for UMTS) is responsible for allocating and distributing the HSDPA resources amongst the individual calls. In a UMTS system that supports HSDPA, some of the code allocation is performed by the RNC whereas other code allocation, or more specifically, scheduling is performed by the base station. Specifically, the RNC allocates a set of resources to each base station, which the base station can use exclusively for high speed packet services. The RNC furthermore controls the flow of data to and from the base stations. However, the base station is responsible for scheduling HS-DSCH transmissions to the mobile stations that are attached to it, for operating a retransmission scheme on the HS-DSCH channels, for controlling the coding and modulation for HS- DSCH transmissions to the mobile stations and for transmitting data packets to the mobile stations.

* S *SS *SS S 55.

* S S S S S S S.. S * S S V S S * S S S V S *S S * S I I S IS * II S S S S S* S HSDPA seeks to provide packet access techniques with a relatively low resource usage and with low latency.

Specifically, HSDPA uses a number of techniques in order to reduce the resource required to communicate data and to increase the capacity of the communication system. These techniques include Adaptive Coding and Modulation (AMC), retransmission with soft combining and fast scheduling performed at the base station.

Although 3rd Generation cellular communication systems support soft handovers wherein transmissions between a mobile station and a plurality of base stations are combined for improved performance, HSDPA communications are designed to involve only a single cell. Accordingly, HSDPA relies on only a single radio link and soft handover of HSDPA signals is not supported. Thus, in an HSDPA enabled cellular communication system some communication channels may support soft handover whereas other communication channels (such as HSDPA channels) do not.

It is important to manage radio links between base stations and communication units such that the resource used by a given communication link is as low as possible. It is therefore 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 ioops, where the receiving a a a.. as. a a,.

S S S S S S

a. S S S S S S S * . S a a.

* a. a * *i * p * * *9 a end reports information on the receive quality back to the transmitting end, which in response adjusts the transmit power.

Specifically, in WCDMA, the uplink power control operates by the base station calculating the signal to interference ratio (SIR), comparing this to a desired uplink SIR threshold and transmitting a power down control signal to the mobile station if the SIR is above the threshold. If the SIR is not above the threshold, the base station transmits a power up control signal.

If a mobile station is in an active soft handover, it may receive transmit power commands from the active base stations. The mobile station will increase the transmit power only if all of the active base stations transmit a power up command. Thus, if one or more of the base stations transmit a power down command, this is an indication that at least one of the base stations of the active set receives the uplink transmission at a sufficiently high quality. In soft handover the transmit power of the mobile station is accordingly selected to ensure that at least one soft handover link is of sufficient quality.

In order to avoid a high complexity, an individual power control loop is not implemented for each physical channel in a cellular communication system. Specifically, for a UMTS cellular communication system, the transmit power commands are determined by the base stations measuring the SIR of the uplink control channel known as the Dedicated Physical Control CHannel (DPCCH). The DPCCH may be operated in soft handover and thus the transmit power of the DPCCH will be * * SI. ISa * S..

* S * d. S SI S S. I I a * e * * I a S S *5 5 55 5 5s controlled to ensure reliable communication to at least one of the base stations of the active set but not necessarily to all of the base stations.

When a mobile station is involved in an HSDPA service, a number of control messages are transmitted from the mobile station to the single base station supporting the HSDPA service. For example, the mobile station may transmit retransmission acknowledge messages (Hybrid ARQ ACK/NACK messages) and indications of the quality of the communication channel (CQI - Channel Quality Indicators).

These messages are transmitted on a continuous HSDPA uplink control channel known as the HS-DPCCH (High Speed - Dedicated Physical Control CHannel). The 3GPP Technical Specifications do not allow for a separate power control loop being implemented for this channel. Rather the Technical Specifications prescribe that the HS-DPCCH may use a transmit power which is given as a constant power offset relative to the DPCCH.

However, when the DPCCH is in a soft handover state, this approach frequently results in the HS-DPCCH not being receivable at the base station supporting the HSDPA service.

This is due to the propagation conditions from the mobile station to the soft handover base stations typically varying such that the radio links to other base stations of the soft handover are dominant. In such cases, the power control loop for the DPCCH is effectively controlled by the channel characteristics of the dominant link(s). Thus, the transmit power of the mobile station will not ensure that the DPCCH can be received by the HS-DSCH serving base station (only * * ** *** a *.s * * I S I S S S.. S * S * * S S S * S S S S *S S * S * S S ** S 55 S S S S *S 5 that it can be received by one of the base stations in the soft handover).

Furthermore, unless the power offset between the HS-DPCCH and the DPCCH is sufficiently large, the HS-DPCHH cannot be received by the serving base station either. However, in this case the HS-DPCCH cannot be received by the RAN at all as the HS-DPCCH cannot be in a soft handover state. Setting the power offset sufficiently high to ensure that the HS- DPCCH can be received in all situations results in the transmit power being excessive for most of the time thereby causing increased interference and reduced capacity.

A solution which has been proposed to address this problem is for the RNC to signal to the mobile station to use a different power offset. Furthermore, the transmissions on the HS-DPCCH may be repeated and the RNC may also signal different repetition rates to the mobile station. Providing that the power offsets and repetition factors are set to be sufficiently large for the different conditions, the probability of the HS-DPCCH being received correctly at the serving base station may be improved while the transmit powers may be reduced. However, as the propagation conditions vary significantly, the required power offsets will typically be very high as they must allow for the worst case conditions for each scenario. Thus, although the approach may allow for a further refinement of the power offset it will typically lead to excessive transmit powers and/or lost HS-DPCCH transmissions. For example, if a power offset is selected to provide for acceptable performance for the HS-DPCCH in situations where the DPCCH is in a soft handover with another link being dominant, this will result * * S.. *S* * *$S * S S * S S * *S. S S * * S * * * a * * * * S. S * S * S S SS S SS * S S S *S S in an unnecessarily high transmit power if the link to the serving base station is dominant.

Erroneous reception of HS-DPCCH may significantly degrade the performance and efficiency of conveying services on HSDPA. For example, retransmission acknowledgements/ non- acknowledgements (ACK/NACK) are transmitted on the HS-DPCCH and data errors may therefore affect the retransmission scheme resulting in reduced efficiency and increased resource consumption. Furthermore, Channel Quality Indications (CQI) used by HSDPA schedulers at the base station are also transmitted on the HS-DPCCH and errors in the CQI5 may result in an inefficient scheduling. This may reduce capacity and degrade the quality of service.

Hence, an improved system for power control in a cellular communication system would be advantageous and in particular a system allowing increased flexibility, improved resource usage, reduced interference, increased battery life, increased communication reliability and/or increased performance of the communication system 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 cellular communication system comprising: means * * S.. **e S *SS * S * * * . S *.* * * S * * S * S S * S S S. * * S * S S *S S 55 * S S * ** S for operating a power control loop operable to control transmission powers of a user equipment; means for receiving a first signal from the user equipment in a soft handover communication channel supported by a plurality of base stations; means for receiving a second signal from the user equipment in a non-soft handover communication channel supported by a first base station of the plurality of base stations; means for determining a quality characteristic of a radio link between the user equipment and the first base station; and means for setting a target parameter for the power control loop in response to the quality characteristic.

The invention may allow improved performance and may specifically in some embodiments allow for an improved power control performance and in particular for an improved setting of the transmit power of the user equipment for the non-soft handover communication channel. Furthermore, a low complexity implementation may be achieved in many embodiments as the target parameter may be set centrally and distributed to each base station of the plurality of base stations. Thus, each individual base station may generate transmit power commands for the user equipment without exchanging information with other base stations.

The invention may allow improved performance while complying with the standards of many cellular communication systems.

For example, no additional signalling between the user equipment and the base stations is required.

The invention may increase the reliability of the non-soft handover communication channel and may in particular reduce * S *SS *** * S..

S.. S a S * S * * * . S S * S 55 5 * S S S S S* * 55 * S S 55 5 the error rate of the second signal. Alternatively or additionally, the invention may reduce the transmit power of the user equipment and may reduce the interference caused by the user equipment resulting in an improved quality of service and/or increased capacity of the cellular communication system as a whole.

According to an optional feature of the invention, the means for setting the target parameter comprises an outer power control loop setting the target parameter; and the power control loop comprises a number of inner power control loops operable to use the target parameter as a reference value.

The outer power control loop may set the target parameter for the number of inner power control loops in response to a signal of a non-soft handover communication channel. The feature may for example provide improved performance, practical implementation and/or commonality with defined technical specifications for many cellular communication systems.

According to an optional feature of the invention, the number of inner power control loops is a plurality of inner power control loops. The invention may allow for an efficient control of a plurality of inner power control loops by e.g. a centralised outer power control loop operated in response to a quality characteristic of a signal in a non- soft handover communication channel.

According to an optional feature of the invention, each of the plurality of base stations comprises at least one of the plurality of inner power control loops.

* * *** *** a *** * S * * * a a ** S S * * * * a * a S * S S S. * S S S S *S S 55 * * * S ** S This feature may allow a practical implementation and/or may improve performance. In particular, it may allow base stations to operate an inner power control loop based on received signals from the user equipment and without requiring communication between base stations. Indeed in some embodiments the target parameter is the only information shared by the inner power control loops. The feature may further allow compatibility with existing standards.

According to an optional feature of the invention, the outer power control loop has an associated time constant more than five times higher than an associated time constant of the number of inner power control loops. This may facilitate operation and may provide an arrangement of the power control loops which allows the inner power control loop to operate with time constants allowing it to track fast propagation channel variations, whereas the time constant of the outer power control loop is sufficiently high to allow for time delays and bandwidth restrictions associated with communications between the outer power control loop and the inner power control loops.

According to an optional feature of the invention, the target parameter is a signal to noise target. This is a suitable parameter which may provide accurate control of the inner power control loops and thus a high performance of the transmit power regulation. The signal to noise target may for example be a Signal to Noise Ratio (SNR) target, a Signal to Interference Ratio (SIR) target or a Signal to Noise and Interference Ratio (SNIR) target * * *.. *** a *** a.. * * * . a * * * . a. * * .. . * a * * * ** * ..

* a. * a. * According to an optional feature of the invention, the cellular communication system further comprises a base station controller which comprises the means for setting the target parameter. This may allow for an effective distribution of functionality allowing for a centralised determination of the target parameter using available information and low complexity distribution of the target parameter.

According to an optional feature of the invention, the base station controller is a Radio Network Controller (RNC). This may allow facilitated implementation and high performance.

According to an optional feature of the invention, the quality characteristic is an error rate of the second signal. This may provide for a practical implementation and/or high performance. The error rate may for example be a BLock Error Rate (BLER) of the second signal.

According to an optional feature of the invention, the means for setting the target parameter comprises means for comparing the quality characteristic with a target quality characteristic. This provides for a simple but highly efficient implementation. For example, the means for setting the target parameter may compare a BLER of the second signal with a maximum acceptable BLER and if the measured BLER exceeds this value, the target parameter may be modified to increase the transmit power of the user equipment.

According to an optional feature of the invention, the means for setting the target parameter is further operable to set * , *.* *.* S *SS * S * * * S S S.. * S * * * S S * S * * S 5 *I * * S * P 5 ** * *S * S ** S the target parameter in response to a quality characteristic of the first signal.

This may provide improved performance. For example, the power control loop may be operated in response to characteristics of the soft handover signals as long as the radio link between the user equipment and the first base station is of sufficient quality to allow the non-soft handover signals of the non-soft handover communication channels to be received.

According to an optional feature of the invention, the cellular communication system is a 3 Generation cellular communication system. The invention may provide improved performance in a 3 Generation cellular communication system, such as a UMTS cellular communication system. In particular, the invention may improve performance of many 3rd Generation cellular communication systems while complying

with the Technical Specifications defined by the 3

Generation Partnership project.

According to an optional feature of the invention, the second signal is a High Speed Downlink Packet Access (HSDPA) signal. In particular, the nonsoft handover communication channel may be a High Speed-Dedicated Physical Control CHannel (HS-DPCCH) channel, the soft handover communication channel may be a Dedicated Physical Control CHannel (DPCCH) channel and the first base station may be operable to transmit an HSDPA signal to the user equipment on a High Speed-Dedicated Shared CHannel (HS- DSCH).

* * ..* *** * S..

* . S * * . S S.. S S S S * * * * S S S S * S. S S S S * S *S S ** S S S S *S S The invention may provide improved performance and may in particular provide power control improving performance in a 3rd Generation cellular communication system supporting HSDPA services. The power control may be modified to improve performance while complying with the specifications for the cellular communication system and in particular for HSDPA services.

Specifically, in some embodiments, the invention may substantially improve reliability of signals transmitted on the HS-DPCCH while the DPCCH is operating in a soft handover state. Additionally or alternatively, the transmit power of the HS-DPCCH may be substantially reduced to match the current requirements rather than using a simple power offset based on a worst case assumption.

According to a second aspect of the invention, there is provided a method of power control in a cellular communication system comprising: operating a power control loop operable to control transmission powers of a user equipment; receiving a first signal from the user equipment in a soft handover communication channel supported by a plurality of base stations; receiving a second signal from the user equipment in a non-soft handover communication channel supported by a first base station of the plurality of base stations; determining a quality characteristic of a radio link between the user equipment and the first base station; and setting a target parameter for the power control loop in response to the quality characteristic.

* * *** *** * *** * S S S S * * S.. S * * S S S * * . . S * 5 ** S * * * * S e* . ** * S 5 * S. S 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 a cellular communication system incorporating some embodiments of the invention; FIG. 2 illustrates an example of an inner power control of a base station; FIG. 3 illustrates an example of an outer power control of an RNC; and FIG. 4 illustrates another example of an outer power control of an RNC Detailed Description of Some Embodiments of the Invention The following description focuses on some embodiments of the invention applicable to a 3rd Generation cellular communication system and in particular to 3rd Generation cellular communication system supporting HSDPA services.

However, it will be appreciated that the invention is not limited to this application but may be applied to many other communication systems and services.

* * *** .*e * *..

* * * * * S S S.. * S S S S S S * S S S S I S. * * S S * * IS S *S * S I S 55 S FIG. 1 is an illustration of a UMTS cellular communication system 100 incorporating some embodiments of the invention.

In the example of FIG. 1, a user equipment 101 is supported by three base stations (node Bs) 103, 105, 107. The three base stations 103-107 are coupled to a Radio Network Controller (RNC) 109 which is coupled to a core network 111 as is typical for UMTS cellular communication systems. In the example of FIG. 1, the user equipment 101 is in an overlap area between three different cells supported by the three different base stations 103-107. It will be appreciated that although each cell of the current example is supported by a separate base station, individual base stations may in other examples support more than one cell.

The user equipment 101 may typically be a communication unit, a 3rd Generation User Equipment (UE), a subscriber unit, a mobile station, a communication terminal, a personal digital assistant, a laptop computer, an embedded communication processor or any physical, functional or logical communication element which is capable of communicating over the air interface of the cellular communication system.

In the current example, the user equipment 101 is communicating with a serving base station 103 through a first radio link 113 but is also communicating with two other base stations 105, 107 over other radio links 115, 117. specifically, the user equipment 101 is currently in a soft handover configuration with an active set comprising the three base stations 103- 107.

* * *** a.. S *** * a S * S S S a.. S S * S * S S S * S S S S. S * I * I * *S S S. a. . S ** * For clarity and brevity, FIG. 1 illustrates only aspects of the communication system required to describe exemplary embodiments of the invention. Similarly, only the functionality and features required to describe the embodiments will be described and it will be apparent to the person skilled in the art that the illustrated elements will be capable of performing other functions and provide features required or desired for the operation of a 3rd Generation cellular communication system as appropriate.

In the example of FIG. 1, the user equipment 101 is currently involved in an HSDPA call supported by a first of the base stations 103. Thus, the user equipment 101 is communicating with the first base station 103 using HSDPA communication channels. In particular, the first serving base station 103 is transmitting data to the user equipment 101 on an HS-DSCH (High Speed - Downlink Shared CHannel).

Similarly, an uplink HS-DPCCH (High Speed Dedicated Physical Control CHannel) has been setup to communicate control data from the userequipment 101 to the base station 103 as is known from conventional HSDPA systems. The HSDPA channels cannot be involved in soft handovers but are dedicated communication links between the user equipment 101 and the serving base station 103. This facilitates operation for HSDPA services, and for example allows that a fast and individual resource allocation for HSDPA services can be performed by the individual base station 103 in response to fluctuations of the radio link 113 between the base station 103 and the user equipment 101.

* a as. a.. a * . S * * S S a.. a S S S S S S * S * S S * a* S * . e * * S. S SS S * a *S S The HS-DPCCH is used to transmit various control messages including Hybrid ARQ ACK/NACK and CQI (Channel Quality Indicator) data. The Hybrid ARQ ACK/NACK data comprises acknowledge data used by the Hybrid ARQ retransmission scheme of the HSDPA service whereas the CQI commands are indicative of a quality of the radio link 113 between the serving base station 103 and the user equipment 101. The user equipment 101 measures the current receive quality of a pilot signal of the base station 103 and reports the result by transmitting the CQI commands. Thus, the CQI commands are indicative of the current radio propagation conditions from the base station 103 to the user equipment 101 and are used by the scheduling function of the base station 103 to schedule HSDPA data on the shared HS-DSCH to user equipment experiencing advantageous conditions. Such link adaptation scheduling may result in a substantially improved efficiency of the resource usage and may increase the capacity of the cellular communication system as a whole.

Furthermore, in the example of FIG. 1, a number of non-HSDPA communication channels are set up for the user equipment 101. Specifically, the user equipment 101 is supporting a DPCCH (Dedicated Physical Control CHannel) which is used to transmit various control data and commands from the user equipment 101 to the fixed network. The user equipment 101 also receives transmission from the base stations on non- HSDPA channels. For example, the user equipment 101 will receive a DPCCH and may additionally receive a DPDCH.

In the example, the non-HSDPA communication channels are in a soft handover state where the communication between the base stations 103-107 and the user equipment 101 utilise a * * *** *** a *4a * S S S S S 5. 5 5 S S S S S * S S S * S. S * * a * a. . .4 S - .a * plurality of propagation channels with the received signals of the plurality of propagation paths being combined by the receiving end (for example by selection combining). Thus, in the example of FIG. 1, the user equipment 101 is in a configuration wherein it is simultaneously supporting HSDPA channels which are not in a soft handover and non-HSDPA channels which are in a soft handover.

It is important to manage radio links between base stations and user equipments 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 transmission of signals from the communication unit to the base stations and therefore the lowest possible transmit power should be used by the user equipment 101 when transmitting to the base stations 103- 107. Accordingly, the base stations 103-107 and user equipment 101 operate power control loops to dynamically control transmit powers to closely match the varying conditions.

Specifically, cellular communication systems such as UMTS operate both an inner power control loop and an outer power control loop, Conventionally, the inner power control loop measures the received signal to noise ratio (SIR) of pilot symbols of the DPCCH, and compares it to a locally stored target SIR. If the measured SIR is less than the target SIR, the base station transmits a power up command and otherwise it transmits a power down command. If the user equipment 101 is in a soft handover, each of the base stations 103-107 in the soft handover transmits a transmit power command to the user equipment 101. Thus, in the example of FIG. 1, the user equipment 101 receives a transmit power command from each of I I III III S *.

* h S - U S S

S S S S S S

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I S S S S S 41 P Sm P the three base stations 103-107. Only if all of the received transmit power commands are power up commands will the user equipment 101 increase the transmit power. In other words, as long as one of the links 113-117 provides sufficient performance, the transmit power is not increased.

Accordingly, the transmit power is controlled such that the dominant link is of sufficient quality whereas other links of the soft handover may provide a poor quality signal.

However, in UMTS the same power control commands are used to control the transmit power of the HS-DPCCH channel which cannot be in a soft handover state. Thus, the transmit power control based on a soft handover state may result in the HS- DPCCH not being receivable by the base station 103. In order to compensate for this potential problem, the 3GPP R5 standards allow for a power offset to be specified between the transmit power of the DPCCH and the HS-DPCCH such that the HS-DPCCH is transmitted at a relatively higher power.

However, this results in an inaccurate setting of the transmit power as it will typically either be too high (if the HS-DPCCH experiences better conditions than assumed when setting the power offset) or too low (if the HS-DPCCH experiences worse conditions than assumed when setting the power offset).

In accordance with some embodiments of the current invention, the cellular communication system comprises functionality for determining a quality characteristic associated with a signal of the non-soft handover communication channel (e.g. the HS-DPCCH) and for setting a target parameter for the power control loop in response to the quality characteristic. In this way the operation of the S *sS SSS * S. S I b I I S S.) S S S I I I I $ I I I I 55 S $S I $ * IS S I S S. S power control loop may be controlled such that a given quality characteristic of the non soft handover channel is driven towards a desired value. In particular, controlling the target parameter in response to a quality characteristic of a signal which is not (or cannot be) in a soft handover state will allow the power control to reflect conditions of this link. Thus, in situations where the soft handover power control arrangement results in the power control being controlled by a dominant link to a different base station, the target parameter for the power control may be changed to ensure that the quality of non-soft handover communication to other base stations is of sufficient quality.

FIG. 2 illustrates an example of an inner power control of a base station. The base station may be the base station 103 of FIG. 1 and will be described with reference to this.

The base station 103 comprises a transmitter 201 which is connected to an antenna 203 and which is operable to transmit signals to the user equipment 101. The antenna 203 is also coupled to a receiver 205 (for example through a duplexer or a switch (not shown)). The receiver 203 is operable to receive signals transmitted over the air interface from the user equipment 101.

The receiver is coupled to a SIR processor 207 which is operable to determine a SIR indication based on a signal received from user equipment 101 and in particular based on a DPCCH signal transmitted from the user equipment 101. The person skilled in the art will be aware of many different techniques and algorithms for determining a SIR indication * * S.. *S. t *S* * * * S S * S S.. S S S S S * S * S S S S S 55 * * S S S S 55 S *5 S S * 55 0 and any suitable algorithm or technique may be used without detracting from the invention.

The receiver 205 is furthermore coupled to an RNC interface 209 which is operable to communicate with the RNC 109 over a UMTS lub interface. In particular, the RNC interface 209 transmits received data to the RNC 109 and may receive a target parameter for the inner power control from the RNC 109. In the specific example, the RNC interface receives a SIR (Signal to Interference Ratio) reference from the RNC 109.

The SIR processor 207 and the RNC interface 209 are coupled to a comparator 211 which compares the estimated SIR value received from the SIR processor 207 and the SIR reference from the RNC interface and generates an error signal. In a simple embodiment, the error signal may be a filtered difference between the SIR estimate and the SIR reference.

The comparator 211 is coupled to a transmit command processor 213 which determines an appropriate power control command. In particular, in some embodiments, the transmit command processor 213 may simply select a power up command if the error signal indicates that the SIR estimate is below the SIR reference and a power down command otherwise. The transmit command processor 213 is coupled to the transmitter 201 which transmits the selected transmit power command to the user equipment 101.

Each of the base stations 105, 107 of the example of FIG. 1 comprises an inner power control loop as the one described with reference to FIG. 2. Thus, when in soft handover, the * * *.. *** , S..

* . S S * S S eS. S * * * p * * * S S S * S 55 5 S S S S * *S S 55 S S * S 55 5 user equipment 101 receives transmit power commands from all three base stations 103-107.

In accordance with the UMTS Technical Specifications, the user equipment 101 proceeds to adjust the transmit powers of the soft handover channels (including the DPCCH) in response to the received transmit power commands. In particular, it proceeds to determine if any of the received transmit power commands are power down commands and if so it proceeds to decrease the transmit power. If all transmit power commands are power up commands, the user equipment 101 increases the transmit power.

Furthermore, the user equipment 101 proceeds to control the transmit power of non-soft handover channels, and in particular the HS-DPCCH channel, to match the fluctuations for the soft handover channels. specifically, the user equipment 101 may transmit the HS-DPCCH at a transmit power which is a fixed offset of the transmit power of the DPCCH (the offset may be zero).

However, in accordance with some embodiments of the invention, the cellular communication system further comprises means for setting the target parameter used by the inner control loop in response to a parameter which is only associated with the non-soft handover HSDPA channel. In particular, an outer power control loop may be used to modify the target parameter for the inner power control loop such that the transmit power of the user equipment 101 may be adjusted to provide acceptable quality on the single radio link between the user equipment 101 and the HSDPA * . *.. *** * *** * * * . S S S S.. * * S S * S S * S S S S S *5 S * S S S S *S * 55 S S S S *S * serving base station 103. The outer power control loop may be implemented in the RNC 111.

FIG. 3 illustrates an example of an outer power control of an RNC. The RNC may be the RNC 111 of FIG. 1 and will be described with reference to this.

The RNC 111 comprises a base station interface 301 which is coupled to the three base stations 103-107 through an Iub interface. The base station interface 301 transmits and receives data from the three base stations 103-107 and in particular receives the data transmitted from the user equipment 101.

In some embodiments, the base station interface 301 is coupled to an error processor 303 which receives data from the HS-DPCCH channel transmitted from the user equipment 101 to the HS_DSCH serving base station 103. The error processor determines an error rate of the HS-DPCCH communication from the user equipment 101. In particular, the error processor 303 may determine the BLock Error Rate (BLER) of the communication on the HS-DPCCH.

In other embodiments, the error processor may determine a quality characteristic for a communication channel which may be in a soft handover state but using only a signal of the radio link 113 between the user equipment 101 and the HS- DSCH serving base station 103. For example, the error processor 303 may receive the DPDCH data from the base station 103 and may determine the BLER estimate based on the signal prior to any combination.

* * St* *S* * *S* * a * * a S * a.. S * * S S * * * . . . . S SI S * S * * * *S a.* S * S S ** S Thus, the BLER estimate is determined for signals transmitted on the radio link 113 between the user equipment 101 and the HSDPA serving base station 103.

The error processor 303 is coupled to a BLER comparator 305 which is furthermore coupled to a BLER target processor 307.

The BLER target processor 307 determines a desired BLER value for the DPDCH (or e.g. the HS-DPCCH) channel. The BLER target processor 307 may in particular determine the desired BLER simply by providing a BLER target which has been received from an external source, such as an Operations and Management Center (OMC). Thus, the BLER comparator 305 compares the BLER estimate from the BLER processor to the BLER reference from the BLER target processor 307 and generates an error signal. Specifically, the BLER comparator 305 may simply determine the error signal as the filtered difference between the BLER estimate and the BLER reference.

The BLER comparator 305 is coupled to a SIR target processor 309 which sets a SIR target for the inner power control loops of the base stations 103-107 in response to the error signal from the BLER comparator 305. In a simple embodiment, the SIR target processor 309 may simply increase the SIR target value by a given increment when the error rate signal indicates that the BLER estimate is below the BLER target value and decrease it by a given increment otherwise.

The SIR target processor 309 is coupled to the base station interface 301 which transmits the SIR target to the base stations 103-107. The base stations 103-107 then proceed to use the revised SIR target as the reference value for the inner power control loops.

* * *** *** . * * * * S * * I.. * S S S * S * * S S S S S St S * S * S * *S S *S * S S P ** * The time constants of the outer and inner power control loops are typically different in order to provide suitable power control loop dynamics. The inner power control loop may for example be fast enough to deal with most fast fading whereas the outer power loop is only sufficiently fast to deal with slow fading. In most embodiments the outer loop time constant may be determined by the filter and may be more than five times higher than the time constant of the inner power control loops.

In accordance with the described example, each of the base stations 103107 may operate an inner power control and may transmit power control commands to the user equipment 101 in

accordance with the UMTS Technical Specifications.

Furthermore, the user equipment 101 may operate a conventional power control algorithm in accordance with the Technical Specifications of the UMTS cellular communication system. However, in contrast to conventional systems, a centralised operation may control the individual power control loops by setting a target parameter in response to a characteristic of a non-soft handover channel and specifically in response to a characteristic of an HSDPA associated channel. Thus, the inner power control loops may automatically be operated to ensure that the transmission of the non-soft handover channels is dynamically varied to ensure reliable reception without requiring a worst case excessive transmit power. Hence, interference may be reduced, battery life increased, reliability increased and performance of the communication system as a whole may be improved.

* * *.* *** * *** * S * * * . * I.. * * * * * . S * S S S S S *. * * S S S S *S S *S * S S S *S * More specifically, reliable and efficient HS-DPCCH communication may be achieved by modifying the operation of the uplink outer power control loop without requiring any modifications to inner power control loops and while maintaining compatibility with the Technical Specifications of UMTS.

The uplink outer loop power control algorithm may specifically modify a SIR target in response to differences between a desired BLER target and a measured BLER. For example, the RNC 111 may measure BLER on the uplink DPDCH radio link 113 that is associated with the same base station 103 from which the HS-DSCH is transmitted (i.e. at a stage before RNC combining is performed). The RNC 111 may then use this BLER measurement alone in order to modify the SIR target. The SIR target is then passed from the RNC 111 to all the base stations 103-107 in the active set for use by the inner power control loop as in a conventional system.

The outer loop power control algorithm will in a UMTS communication system typically send SIR target updates at intervals in the order of lOOms. Hence the power control system can respond to changes in the propagation conditions which occur on that timescale. In typical systems, the approach may control the user equipment to increase or decrease transmit powers as necessary in response to eg.

slow fading or changes in shadowing conditions.

By ensuring that the DPDCH is received with adequate BLER at the HSDPA serving base station from which HS-DSCH is transmitted, the technique may also ensure that, on average, * * *** .** * S..

**. . . . . S S S * * S S * S *0 * * S * S S ** S ** S S * * *S S the DPCCH is received at the desired levels and therefore that HS-DPCCH is received at the desired level.

It will be appreciated that in some embodiments, the SIR target will not be controlled only in response to the BLER target of a communication link between the user equipment and the (HS-DSCH serving) base station.

For example, FIG. 4 illustrates an example of an outer power control of an RNC which may be the RNC 111 of FIG. 1 and will be described with reference to this. The RNC 111 is similar to the RNC 111 example of FIG. 3 and the same functional modules are referred to using the same references and will not be described further.

Instead of the error processor 303 of FIG. 3, the RNC 111 of FIG. 4 comprises a soft handover error processor 401 which determines a BLER of the soft handover communication. The estimated BLER is compared to the target BLER determined by the BLER target processor 307 and the resulting error signal is used to modify the SIR target for the inner power control loops. Thus, the outer power control loop may control the SIR target such that a suitable soft handover performance is achieved.

However, in order to avoid that this approach may result in an unreliable HS-DPCCH communication due to another radio link 115, 117 being dominant, the RNC 111 comprises a link error processor 403. This link error processor 403 receives data from the HS-DPCCH or from the DPDCH received on the link 113 to the HSDPA base station 103 only (i.e. before combining) . In response, the link error processor 403 * * *** *** * *** * S * * S * S ** S S * * * * S * * S * S * ** S * a * * * S* * ** * S 5 * ** * determines the BLER of the radio link 113 to the HSDPA base station 103. As long as this BLER is acceptable, the outer power control loop operates based on the soft handover measurements thereby ensuring efficient power control of the soft handover communication. However, if the link error processor 403 detects that the BLER falls below a given threshold, it forwards a signal to the BLER target processor 307 causing a reduction in the BLER target which will have the effect of the SIR target being increased. Subsequently, if the BLER of the radio link 103 falls below a given threshold, the link error processor 403 causes the BLER target processor 307 to return the BLER target to its normal value. Thus, an efficient soft handover power control may be achieved while ensuring that transmit powers are sufficient to ensure reliable communications for non-soft handover channels.

Thus, in some embodiments, the target parameter for the inner power control loop may be updated both as a function of the non-soft handover signals and of the soft handover signals.

As a specific example, suitable for an HSDPA implementation, the SIR target for the inner power control may be updated as a function of both the BLER measured on the DPDCH received at the HS-DSCH serving base station and the BLER of the DPDCH measured following soft handover combining.

Specifically, the RNC may comprise a first and a second error threshold one related to the non-soft handover DPDCH and one related to the soft combined DPDCH. The error rate (typically the BLER) of the non-soft handover DPDCH signal * * *** S.. S *S* * S * * S S S *** S S S * S I S * S 5 0 5 S S. S * S 5 5 * ** * 55 * S S S 55 S is measured and compared to the first error threshold.

Similarly, the error rate (typically the BLER) of the soft handover (combined) DPDCH signal is measured and compared to the second error threshold. If one or both of the error rates exceed the corresponding threshold, the target parameter is increased and only if both measured error rates are below their corresponding target thresholds is the target parameter (SIR target) reduced.

Thus, in an embodiment of an HSDPA system, the BLER measured on the DPDCH received at the HS DSCH serving base station alone is compared to a first BLER target and the BLER measured on the output of the soft handover combiner is compared to a second BLER target. The first BLER target may be set higher than the second BLER target. This may allow the radio quality on the non-soft handover link to deteriorate (below what is acceptable to receive the DPDCH service), but not so much so that it would effect the ability to receive HS-DPCCH on the non-soft handover radio link.

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 * S *SS **S * *aS * S S S S * S S.. S S * S S S S * . S a S S ** S a a S * S S* * S. * . a S *S * 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 * * *** *** a *s* * S S * S S * SSS I S a * * a S * * a a * * a, S * S * * a *. S S * * S ** * 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 do 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. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

* * S.. 555 S 585 * S * S S S S S.. S S * S S S S S S S I S I S. S * S S * * SI I *5 S I 4 * *. S

Claims (18)

1. A cellular communication system comprising: means for operating a power control ioop operable to control transmission powers of a user equipment; means for receiving a first signal from the user equipment in a soft handover communication channel supported by a plurality of base stations; means for receiving a second signal from the user equipment in a non-soft handover communication channel supported by a first base station of the plurality of base stations; means for determining a quality characteristic of a radio link between the user equipment and the first base station; and means for setting a target parameter for the power control loop in response to the quality characteristic.

2. The cellular communication system claimed in claim 1 wherein the means for setting the target parameter comprises an outer power control loop setting the target parameter; and the power control loop comprises a number of inner power control loops operable to use the target parameter as a reference value.

3. The cellular communication system claimed in claim 2 wherein the number of inner power control loops is a plurality of inner power control loops.

* * S.. **S S *** * . * a * S.. S a a * * S * * S P 5. 5 * P 5 4 4 5 4* S I * am a 4. The cellular communication system claimed in claim 3 wherein each of the plurality of base stations comprises at least one of the plurality of inner power control loops.

5. The cellular communication system claimed in any of the previous claims 2 to 5 wherein the outer power control loop has an associated time constant more than five times higher than an associated time constant of the number of inner power control loops.

6. The cellular communication system claimed in any of the previous claims wherein the target parameter is a signal to noise target.

7. The cellular communication system claimed in any of the previous claims further comprising a base station controller and wherein the means for setting the target parameter is comprised in the base station controller.

8. The cellular communication system claimed in claim 7 wherein the base station controller is a Radio Network Controller (RNC).

9. The cellular communication system claimed in any of the previous claims wherein the quality characteristic is an error rate indication of the second signal.

10. The cellular communication system claimed in any previous claim wherein the means for setting the target parameter comprises means for comparing the quality characteristic with a target quality characteristic.

- . . (.

S S

SI S S S S S I

S S S S. S * . S 10 5 SO L S a. a 11. The cellular communication system claimed in any previous claim wherein the means for setting the target parameter is further operable to set the target parameter in response to a quality characteristic of the first signal.

12. The cellular communication system claimed in any previous claim wherein the cellular communication system is a 3rd Generation cellular communication system.

13. The cellular communication system claimed in claim 12 wherein the second signal is a High Speed Downlink Packet Access (HSDPA) signal.

14. The cellular communication system claimed in claim 13 wherein the nonsoft handover communication channel is a High Speed-Dedicated Physical Control CHannel (HS-DPCCH) channel.

15. The cellular communication system claimed in any of the claims 12 to 14 wherein the soft handover communication channel is a Dedicated Physical Data CHannel (DPDCH) channel.

16. The cellular communication system claimed in any of the claims 12 to 15 wherein the first base station is operable to transmit an HSDPA signal to the user equipment.

17. The cellular communication system claimed in any of the claims 12 to 16 wherein the first base station is operable to transmit the second signal to the user equipment on a High Speed-Downlink Shared CHannel (HSDSCH).

$ . II S S 4 $ e S IS S $ S S S 5 IS. $ S S * S $ .5 SI I $5 I I ** S 18. A method of power control in a cellular communication system comprising: operating a power control loop in order to control transmission powers of a user equipment receiving a first signal from the user equipment in a soft handover communication channel supported by a plurality of base stations; receiving a second signal from the user equipment in a non-soft handover communication channel supported by a first base station of the plurality of base stations; determining a quality characteristic of a radio link between the user equipment and the first base station; and setting a target parameter for the power control loop in response to the quality characteristic.

S

S *S S.. *5* *.. . : S. * S * S.S S 5 * * * S S S * ** * S * ** * * *. : Amendments to the claims have been filed as follows 1. A UMTS cellular communication system including High Speed Downhink Packet Access (HSDPA) service, comprising: means for operating a power control loop operable to control transmission powers of a user equipment; means for receiving first data from the user equipment in a non-HSDPA soft handover communication control channel supported by a plurality of base stations; means for receiving second data from the user equipment in an HSDPA non-soft handover communication control channel supported by a first base station of the plural'ity of base stations; means for determining a quality characteristic of a radio link between the user equipment and the first base station in response to both the first and second signals; S and means for setting a target parameter for the power control loop in response to the quality characteristic.

2. The cellular communication system claimed in claim 1 whereIn the means for setting the -arget parameter comprises an outer power control loop setting the target parameter; and the power control loop comprises a number of inner power control loops operable to use the target parameter as a reference value.

3. The cellular communication system claimed in claim 2 wherein the number of inner power control loops is a plurality of inner power control loops. 3,'

4. The cellular communication system claimed in claim 3 wherein each of the plurality of base stations comprises at least one of the plurality of inner power control loops.

5. The cellular communication system claimed in any of the previous claims 2 to 5 wherein the outer power control loop has an associated time constant more than five times higher than an associated time constant of the number of inner power control loops.

6. The cellular communication system claimed in any of the previous claims wherein the target parameter is a signal to noise target.

7. The cellular communication system claimed in any of the previous claims further comprising a base station controller and wherein the means for setting the target parameter is comprised in the base station controller.

8. The cellular communication system claimed in claim 7 wherein the base station controller is a Radio Network Controller (RNC).

9. The cellular communication system claimed in any of the previous claims wherein the quality characteristic is an error rate indication of the second data.

10. The cellular communication system claimed in any previous claim wherein the means for setting the target parameter comprises means for comparing the quality characteristic with a target quality characteristic. (10

11. The cellular communication system claimed in any previous claim wherein the means for setting the target parameter is further operable to set the target parameter in response to a quality characteristic of the first data.

12. The cellular communication system claimed in any previous claim wherein the cellular communication system is a 3rd Generation cellular communication system.

13. The cellular communication system claimed in claim 12 wherein the second data is from a Dedicated Physical Control Channel (DPCCH).

14. The cellular communication system claimed in claim 13 wherein the non-soft handover communication channel is a - - High Speed-Dedicated Physical Control CHannel (HS-DPCCH) channel.

15. The cellular communication system claimed in any of the claims 12 to 14 wherein the soft handover communication channel is a Dedicated Physical Data CHannel (DPDCH) channel.

16. The cellular communication system claimed in any of the claims 12 to 15 wherein the first base station is operable to transmit an HSDPA signal to the user equipment.

17. The cellular communication system claimed in any of the claims 12 to 16 wherein the first base station is operable to transmit the second data to the user equipment on a High Speed-Downlink Shared CHannel (HS-DSCH). (k(

18. A method of power control in a UMTS cellular communication system comprising: operating a power control loop in order to control transmission powers of a user equipment; receiving first data from the user equipment in a non- HSDPA soft handover communication control channel supported by a plurality of base stations; receiving second data from the user equipment in an HSDPA non-soft handover communication control channel supported by a first base station of the plurality of base stations; determining a quality characteristic of a radio link between the user equipment and the first base station in response to both the first and second signals; and setting a target parameter for the power control loop -- in response to the quality characteristic.