GB2391755A

GB2391755A - Apparatus and method for cell selection in mobile communcation system.

Info

Publication number: GB2391755A
Application number: GB0218642A
Authority: GB
Inventors: Eric Villier; Philip Jones
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2002-08-10
Filing date: 2002-08-10
Publication date: 2004-02-11
Anticipated expiration: 2022-08-10
Also published as: GB0218642D0; GB2391755B

Abstract

A system for cell selection in a cellular communication system (200) having a plurality of base stations (201,203) supporting user equipment (201). A radio characteristics processor (225) determines radio characteristics associated with a user equipment (201) and all base stations (201,203) of the neighbour list. Specifically the transmit power required for the signals of the user equipment (201) to be received at each of the base stations (201, 203) at a given signal to interference ratio is calculated. A cell selector (229) selects a serving cell using a selection criterion associated with uplink conditions in the communication system, and specifically the cell for which the calculated user equipment transmit power is the lowest. The invention is suitable for cellular communication systems such as GSM or UMTS.

Description

r 1 239 1755 . APPARATUS AND METHOD FOR CELL SELECTION IN CELLULAR

COMMUNICATION SYSTEM

5 Field of the invention

The invention relates to an apparatus and method for cell selection in cellular communication system and in particular a 3rd generation cellular communication system.

Background of the Invention

FIG. 1 illustrates the principle of a conventional cellular communication l 5 system 100 in accordance with prior art. A geographical region is divided

into a number of cells 101, 103, 105, 107 each of which is served by base station 109, 111, 113, 115. The base stations are interconnected by a fixed network which can communicate data between the base stations 101, 103, 105, 107. A mobile station is served via a radio communication link by the 20 base station of the cell within which the mobile station is situated. In the example if FIG. 1, mobile station 117 is served by base station 109 over radio link 119, mobile station 121 is served by base station 111 over radio link 123 and so on.

25 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. For example mobile station 125 is initially served by base station 113 over radio link 127. As it moves towards base station 115 it enters a region of overlapping coverage of the two base stations 111 and 113 and within this 30 overlap region it changes to be supported by base station 115 over radio link 129. As the mobile station 125 moves further into cell 107, it

( continues to be supported by base station 115. This is known as a handover or handoff of a mobile station between cells.

A typical cellular communication system extends coverage over typically 5 an entire country and comprises hundred 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 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 l 5 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 20 routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc. In 3rd Generation communication systems, the communication network comprises a core network and a Radio Access Network (RAN). The core 25 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.

30 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 known as Node Bs, as well as Radio Network Controllers (RNC) which control the Node Bs and the communication over the air interface.

S The frequency band allocated for a cellular communication system is typically severely limited, and therefore the resource must be effectively divided between mobile stations. A fundamental property of a cellular communication system is that the resource is divided geographically by the division into different cells. Thus a certain amount of resource (for 10 example a frequency band) may at a given time be allocated to a given cell thereby reducing the resource allocation to neighbouring cells. In order to optimise the capacity of a cellular communication system, it is important to minimise the impact of interference caused by or to other mobile stations. An important advantage of a cellular communication system is 15 that due to the radio signal attenuation with distance, the interference caused by communication within one cell is negligible in a cell sufficiently far removed, and therefore the resource can be reused in this cell. In addition, the resource is typically divided within one cell and between cells by division of the resource in the time domain, the frequency domain 20 and/or the code domain. Different communication systems use different principles for this division. The resource allocation may be static or dynamic dependent on the current load of the communication system, and typically a combination of static and dynamic resource allocation is used.

25 First generation analogue communication systems use a frequency division multiple access (FDMA) system, where the frequency domain is used for dividing the resource between cells. In these systems, the frequency band is divided into narrowband channels of typically 26 kHz bandwidth. A number of these channels are allocated to each base station 30 and upon call setup each mobile station will be allocated a specific narrowband channel for uplink communication and one for downlink communication.

!' Currently the most ubiquitous cellular communication system is the 2nd Generation system known as the Global System for Mobile communication (GSM). Similarly to analogue systems, the frequency band is divided into 5 relatively narrow channels of 200 kHz and each base station is allocated one or more of these frequency channels. However, in contrast to the analogue systems, each frequency channel is divided into eight separate time slots allowing up to eight mobile stations to use each frequency channel. This method of sharing the available resource is known as Time 10 Division Multiple Access (TDMA). 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.

l 5 Another principle of resource distribution is employed in the 2nd generation system known as IS96, as well as in 3r Generation systems such as the Universal Mobile Telecommunication System (UMTS). These systems divide the frequency into one or few wide band channels, which for UMTS has a bandwidth of 5 MHz. Typically, one wide band frequency 20 channel is used for uplink in all cells and a different wide band frequency channel is used for downlink. In this case, separation between cells is achieved through the use of spread spectrum techniques, where each cell is allocated a cell specific long user spreading code.

25 In these systems, a signal to be transmitted is multiplied by the spreading code, which has a chip rate typically much larger than the data rate of the signal. Consequently, a narrowband signal is spread over the wideband frequency channel. In the receiver, the received signal is multiplied by the same spreading code thereby causing the original narrowband signal to be 30 regenerated. However, signals from other cells having a different spreading code are not despread by the multiplication in the receiver, and remain wideband signals. The majority of the interference from these

signals can consequently be removed by filtering of the despread narrowband signal, which can then be received.

Separation between mobile stations of the same cell is also achieved by use 5 of spread spectrum techniques. The signal to be transmitted is multiplied by a shorter user specific code. Similarly, the receiver multiplies the received signal with the user specific code, thereby recovering the originally transmitted signal without Respreading signals from any of the other mobile stations. Thus, the interference from all other mobile lO stations, whether in the same or a different cell, can effectively be reduced by filtering.

A consequence of the spread spectrum techniques employed is that the amount of the interfering spread signals, which fall within the bandwidth l 5 of the narrowband signal cannot be removed by filtering, and will thus reduce the signal to interference ratio of the received signal.

Consequently, it is of the outmost importance that the interference between mobile stations is optimised in order to maximise the capacity of the system. The reduction of the interference from an unwanted mobile 20 station is equal to the ratio between the bandwidth of the spread signal and the narrowband despread signal, equivalent to the ratio between the chip rate and the symbol rate of the transmitted signal. This ratio is known as the processing gain. The technique is known as Code Division Multiple Access (CDMA), and further description of CDMA and

25 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.

Common for all types of cellular communication systems is that, it is 30 imperative to manage the radio links between the base stations such that the resource used by a given communication link is as low as possible.

Thus it is important to minimise the interference caused by the

communication to or from a mobile station and consequently it is important to use the lowest possible transmit power. As the required transmit power depends on the instantaneous propagation conditions, it is necessary to dynamically control transmit powers to closely match the 5 conditions. For this purpose the base stations and mobile stations operate power control loops, where the receiving end reports information on the receive quality bacl; to the transmitting end, which adjusts it's transmit power in response.

10 However, in order for the power controls to effectively minimise resource usage of the radio links, it is essential that the optimal base station is chosen as the serving cell. Thus a good cell selection procedure is essential to maximize the performance of a cellular communication system and in particular a CDMA cellular communication system. If the best cell is not 15 selected either the mobile or the base station, or indeed both of them, would transmit a power higher than the minimum required to meet a given target, hence creating more interference than necessary. Depending on which criterion the operator wants to optimize, there are several ways of determining which cell should be selected.

A cell selection criterion, which is typically used in analyzing the performance of a CDMA communication system, is based on minimum path loss. It is convenient for mathematical analysis purposes, because it is independent of the level of interference on either the uplink or the 25 downlink, and is symmetrical with respect to both links. However it does not take into consideration the load on either link, a parameter which may be important.

live other criteria are defined in the UMTS standards specifications ("UK

30 Procedures in Idle Mode and Procedures for Cell Reselection in Connected Mode", 3GPP TS25.30, both of them based on downlink load. The first consists in selecting the cell for which the measured carrier energy to

fir interference ratio Ec /lo is maximum, where EC is the energy per chip, measured by the mobile on the common pilot channel, and lo is the interference at the mobile. This last quantity lo depends on the cell the mobile is connected to when performing the measurements, since it 5 depends on the relative orthogonality of the intra-cell signals. The second criterion removes that dependency by selecting the cell for which the Signal to Interference Ratio (SIR) of the common pilot channel at the mobile is maximum.

10 Let Pa be the total power transmitted in cell a, Pin the power transmitted on the common pilot channel of cell a, go the channel gain between the mobile and cell n, In the orthogonality factor on cell a, No the power spectral density of the noise at the mobile's receiver and W the signal bandwidth. The two selection criteria are defined as max E/lo Select the cell n for which the quantity E, n / lo n iS maximised, where 20 En n a: Pings and lo n = cnPngn + P'g' + No ken max SIR Select the cell n for which the quantity Sn I I is maximised, where 25 Sn = Pin gn = received power at the mobile on the common pilot channel I = Pi go + No W = total interference power at the mobile (does not depend on a)

( Hence, as I is a constant independent of I, the cell selection process chooses the cell for which S., is maximum. If we assume that the power transmitted on the common pilot channel is the same in all cells, the max SIR criterion is equivalent to the minimum path loss criterion.

It is clear that this criterion amounts to selecting the cell for which the transmit power towards that mobile, required to meet a certain Eb / No target, will be minimum. In other words, it is a criterion which is biased towards the downlink load. If it is assumed that the capacity will be 10 downlink limited, it makes sense for each link to minimize the required transmit power at the base station, in order to support as many users as possible. However, the current cell selection criteria focus only on the downlink 15 interference ends seeks to minimise the downlink transmitted power.

Consequently, the criteria are highly inefficient in scenarios which are not limited by the downlink interference conditions. Consequently, the current cell selection criteria result in less than optimal resource utilisation in many scenarios, thereby significantly limiting the communication capacity 20 of the communication system in these circumstances. Consequently, an improved system for cell selection would be advantageous.

Summary of the Invention

The inventors of the current invention have realised that current cell selection criteria are sub-optimal by focussing exclusively on downlink conditions, and that improvements can be obtained by consideration of other conditions. Thus a system is provided seeking to alleviate and 30 mitigate one or more of the above mentioned disadvantages.

f Accordingly there is provided a method of cell selection for a cellular communication system having a plurality of base stations, each base station serving user equipment in a cell, the method comprising the steps of determining radio characteristics associated with a user equipment and 5 a plurality of base stations; selecting a base station out of the plurality of base stations as a serving base station by applying a selection criterion to the determined radio characteristics; and wherein the selection criterion comprises an evaluation of uplink conditions for each base station of the plurality of base stations derived from the determined radio 1 0 characteristics.

The invention provides a number of advantages including providing optimization of resource allocation and minimization of interference in the uplink direction, thereby increasing system capacity for uplink limited 15 systems or scenarios. The consideration of uplink characteristics provides an optimization of the cell selection in situations, where the communication system is not downlink limited. Consequently, the resource usage is optimised also for uplink limited scenarios thereby leading to an increased capacity of the communication system.

According to a first feature of the invention the selection criterion further comprises an evaluation of downlink conditions for each base station of the plurality of base stations determined from the determined radio characteristics. This allows for a balanced trade off between the 25 optimization of resource and interference in one direction relative to that of the other direction, and specifically allows an optimization of uplink conditions while ensuring that the downlink conditions do not deteriorate unacceptably. 30 According to a second feature of the invention, the method further comprises the step of transmitting, from at least a first of the plurality of base stations, an indication of whether the user equipment should perform

cell selection based on uplink conditions or downlink conditions and preferably all base stations of the plurality of base stations transmit equivalent indications of whether the user equipment should perform cell selection based on uplink conditions or downlink conditions. Thus, the cell 5 selection may be performed in the user equipment with the network determining whether an uplink or downlink criterion should be used. This allows the network operator to control the cell selection and in particular whether to optimise for the uplink or downlink direction.

10 According to a third feature of the invention the indication indicates that uplink conditions should be used for cell selection when the plurality of base stations are predominantly uplink interference limited and that downlink conditions should be used for cell selection when the plurality of base stations are predominantly downlink interference limited. Thus the 15 optimization is performed for the direction which is most critical and for which the current usage and/or interference is closest to maximum.

According to a fourth feature of the invention the uplink conditions for each base station is a transmit power of the user equipment and 20 preferably the selection criterion is to choose the base station resulting in a lowest estimated transmit power of the user equipment. By keeping the transmit power of the user equipment at a minimum, the interference caused by the user equipment is minimised and thus the capacity of the communication system increased.

According to a fifth feature of the invention the transmit power level is; determined as a monotonically increasing function of a total interference power received at each base station of the plurality of base stations. This provides a convenient and simple method for determining suitable radio 30 characteristics as the interference power can be broadcast from the base station and therefore easily be determined by the user equipment.

Further, the required transmit power will typically in most communication systems increase for increasing interference levels.

According to a sixth feature of the invention, the transmit power level is 5 determined as a monotonically increasing function of a propagation loss between the user equipment and each base station of the plurality of base stations. This provides a convenient and simple method for determining suitable radio characteristics as path loss can easily be determined from knowledge of transmitted power and the received signal level. The path 10 loss of the uplink can be determined as equivalent to the downlink path loss for most communication systems. Further, the required transmit power will typically in most communication systems increase for increasing path loss.

t 5 According to a seventh feature of the invention, the uplink conditions for each base station is an interference level at each base station caused by transmission from the user equipment and preferably the selection criterion is to choose the base station resulting in a lowest estimated interference level at a base station out of the plurality of base stations 20 caused by transmission from user equipment. This provides the advantage of optimization of the interference at a given base station which may specifically be in a critical situation.

According to a second aspect of the invention, there is provided an 25 apparatus for cell selection for a cellular communication system having a plurality of base stations, each base station serving user equipment in a cell, the apparatus comprising: means for determining radio characteristics associated with a user equipment and a plurality of base stations; means for selecting a base station out of the plurality of base 30 stations as a serving base station by applying a selection criterion to the determined radio characteristics; and wherein the selection criterion comprises an evaluation of uplink conditions for each base station of the

f plurality of base stations derived from the determined radio characteristics. 5 Brief Description of the Drawings

An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which lO FIG 1. is an illustration of a cellular communication system in accordance with the prior art;

FIG. 2 is an illustration of a communication system in accordance with a preferred embodiment of the invention; and FIG. 3 illustrates a flow chart for a method of cell selection in accordance with a preferred embodiment of the invention.

Detailed Description of a Preferred Embodiment of the Invention

In the following, a preferred embodiment of the invention is described with reference to a UMTS communication system. However the invention can 25 be applied to any suitable communication system including for example GSM. FIG. 2 is an illustration of a communication system 200 in accordance with a preferred embodiment of the invention.

In FIG. 2 two base stations 201, 203 of the communication system 200 are shown. In the preferred embodiment, the communication system is a

( UMTS communication system and consequently the base stations 201, 203 are UMTS base stations, known as Node B's. Each of the base stations 201, 203 are connected to the fixed network 205 for routing to other destinations, system management etc as is well known in the art. The 5 base stations 201, 203 transmit radio signals over the air interface from their respective antennas 207, 209. Specifically, the base stations 201, 203 transmit pilot signals having a substantially time invariant signal level.

The pilot signals transmit various data including broadcast data related to the characteristics of the base station and current operating conditions of 10 the base station andJor the communication system 200.

The communication system 200 comprises at least one user equipment 211. The user equipment 211 may be any entity communicating with a base station 201, 203 of the communication system over the air interface.

15 The user equipment 211 comprises an antenna 213, at which the signals from the base stations 201, 203 are received through radio links 215, 217 of the air interface. The antenna is connected to a duplexes 219, which separates the transmitted and received signals such that the same antenna 213 can be used both for receiving and transmitting. The 20 duplexer 219 is connected to a receiver 221, which receives the radio signals from the antenna 213, and as is well known in the art converts them into suitable signals for the user of the user equipment 211, i.e. typically a data signal or a speech signal for normal voice communication.

25 Further, the receiver 221 is connected to a measurement unit 223 which, together with the receiver 221, performs various measurements of the radio characteristics associated with the user equipment 211 and the base stations 201, 203. Different radio characteristics can be measured in different embodiments, but in the preferred embodiment radio 30 characteristics of the radio signals received by the user equipment 211 from each of the base stations 201' 203 are measured. In particular, the received signal levels of the pilot signals are measured.

The measurement unit 223 is connected to a radio characteristics processor 225 which derives a characteristic of the uplink conditions at least partly based on the measurements of the measurement unit 223. The 5 uplink conditions characteristic may also be determined by taking into account any other suitable and /or relevant parameter. For example the base stations 201, 203 may transmit broadcast information related to characteristics of the transmitted signal, and the radio characteristics processor 225 may receive this broadcast information from the receiver 10 directly or through the measurement unit 223. Specifically, the radio characteristics processor 225 may receive information of the transmit power level for the pilot signals transmitted by the base stations 201, 203.

The radio characteristics processor 225 may from this information and the received signal levels determine a path loss for each radio link 215, 217. In 15 addition, the radio characteristics processor 225 may receive broadcast information of the interference level at the receiver of the base stations 201, 203. Assuming the uplink path loss is equivalent to the downlink path loss, the radio characteristics processor 225 is able to calculate the transmit power required by the user equipment 211 to access each of the 20 base stations 201, 203 at a given signal to interference level.

The radio characteristics processor 225 is connected to a measurement controller 227, which is further connected to the receiver 221 and the measurement unit 223. The radio characteristics processor 225 transmits 25 signals to the measurement controller 227 over this connection. The signals comprise information as to which measurements are required, and in response the measurement controller 227 controls the operation of the receiver 221 and the measurement unit 223 to perform the required measurements. The radio characteristics processor 225 is also connected to a cell selector 229 which selects a serving base station from the base stations 201, 203 of

the communication system. The selection criterion includes an evaluation of the uplink conditions, and in the preferred embodiment the evaluation of the uplink conditions comprises derivation of the uplink conditions characteristic in the radio characteristics processor 225. Specifically, the 5 evaluation of the uplink conditions comprises evaluating the transmit power of the user equipment 211 required to meet a given signal to interference ratio for each of the base stations 201, 203, and selecting the serving cell as that of the base station 201,203 requiring the lowest transmit power.

The cell selector 229 is connected to a user equipment controller 230, which performs all the functions required for controlling the operation of the user equipment 211 in accordance with the requirements of the communication system. Specifically, the user equipment controller 230 l S controls the functional blocks of the user equipment 211 to enable the appropriate communication of user data and control data between the user equipment 211 and the serving base station, as is well known in the art. In accordance with an embodiment of the invention, the user equipment controller 230 further controls the user equipment 211 to access, remain 20 attached to or hand over to the serving base station chosen by the cell selector 229. Any suitable method or mechanism for associating the user equipment 211 with the selected cell can be used and in the preferred embodiment the standard mechanisms for the communication system arc used in accordance with the UMTS technical specifications.

The user equipment controller 230 is in FIG. 2 shown connected to the receiver 221 and transmitter 231 as required for the control of the operation of the user equipment 211. In addition, the user equipment controller 230 may be connected to any other functional block of the user 30 equipment 211 as required for proper operation of the user equipment 211.

f In the preferred embodiment, the user equipment 211 is able to use different selection criteria dependent on the operation conditions.

Specifically, the cell selector 229 is operable to select a serving cell in accordance with an uplink criterion or a downlink criterion. In this 5 embodiment, the fixed infrastructure comprises a cell selection controller 233 connected to the base stations 201, 203. The cell selection controller 233 can be implemented as part of and/or be connected to the base stations through the fixed network. The cell selection controller 201, 203 receives signals from the base stations 201, 203 related to the operatingconditions to in the communication system 200 and in particular related to the interference levels and blocking levels for different base stations. Based on this information, the cell selection controller 233 determines if the communication system in the appropriate region is downlink limited or uplink limited, i.e. if there is more capacity available in the uplink l5 direction than in the downlink direction. The cell selection controller 233 selects whether uplink or downlink selection criteria should be used by the user equipment in response to this determination. Specifically, the selection criteria is chosen to correspond to the direction having the least free capacity available, thereby optimising resource allocation for this 20 direction. The cell selection controller 233 communicates the information to the base stations 201, 203 which transmit it to the user equipment 211.

The user equipment 211 receives and decodes this information in the receiver 221 and forwards the information to the cell selector 229, which uses the selection criterion specified. In addition, the information is in the 25 preferred embodiment fed to the radio characteristics processor 225 to ensure the appropriate measurements are made and the appropriate radio characteristics are derived. In other embodiments, a combination of uplink and downlink criteria is used for the cell selection.

30 In general, the operation of the communication system of the preferred embodiment of FIG. 2 for cell selection comprises first determining radio characteristics associated with a user equipment and a plurality of base

( stations; and selecting a base station out of the plurality of base stations as a serving base station by applying a selection criterion to the determined radio characteristics, wherein the selection criterion comprises an evaluation of uplink conditions for each base station of the plurality of 5 base stations derived from the determined radio characteristics.

More specifically, FIG. 3 illustrates a flow chart for a method of cell selection in accordance with a preferred embodiment of the invention. The method will be described with specific reference to the communication l O system of FIG. 2.

In step 301, the user equipment 211 measures all the radio parameters required in the cell selection process for a given cell. The measurements are made by the measurement unit 223 together with the receiver 221 l5 under the control of the measurement controller 227 as previously described. Specifically, the receiver chooses a specific base station 201 and receives the pilot signal of that base station 201. A number of parameters are preferably measured, including the received signal level for the pilot signal and the error rate of the pilot signal. Step 301 further comprises 20 deriving information transmitted from the current base station 201 required for the cell selection process. In the preferred embodiment, this includes receiving the cell broadcast information transmitted as data on the pilot signal. This information preferably includes a number of parameters related to the operation characteristics of the base station 201, 25 such as for example the transmit power for the pilot signal and the current uplink interference level at the base station 201.

In step 303, one or more radio characteristics are determined, at least one of which is associated with the user equipment 211 and the current base 30 station 201. In the preferred embodiment, the radio characteristics processor 225 uses the measurements performed and information received to determine a characteristic, which is associatiated with the conditions

for communication in the uplink direction. Specifically, the radio characteristics processor 225 calculates the transmit power required by the user equipment 211 to be received at the current base station 201 at a given signal to interference ratio.

Specifically, let Pro be the transmit power level of the user equipment 211, lot the total interference power received at the base station 201,i. e. the total uplink interference level at the base station 201, and pi the path loss between the user equipment 211 and the base station 201. The transmit lO power is calculated to meet a given signal to interference ratio SIR target at the base station, i.e. the radio characteristic PTI is calculated from: MIX = 5/R

Ivt. pi lS and hence PT. = SIR ILL pi or Pr:= SIR+IUL+ pi when all values are measured in dB.

25 The path loss of the uplink connection can be determined in different ways as is known in the art, but in the preferred embodiment, it is assumed to be equivalent to the downlink path loss and therefore is calculated from Pi PTX BS PRx,tlE

( where PRX UE is the received signal level of the pilot tone as measured by the user equipment 211, and PTX BS is the transmit power of the pilot tone from the base station. Information of PTX BS is included in the broadcast 5 information from the base station.

In step 305, it is determined if the user equipment 211 has determined radio characteristics for all required cells. In communication systems such as UNITS, the base stations transmit neighbour lists of potential i lo candidates for serving cells. These neighbour lists are used to determine which cell to attach to or to handover to for an ongoing call. In the preferred embodiment, the neighbour list is used to determine which cells are included in the cell selection method. Specifically, step 305 investigates if all cells included in the neighbour list have had one or more 15 radio characteristics determined, and if not the process repeats step 301 and 303 for another base station 203.

If radio characteristic(s) have been determined for all cells of the neighbour list, the method continues at step 307. Since the steps for 20 determining one or more radio characteristics associated with a given cell is repeated for all cells of the neighbour list, radio characteristics associated with the user equipment and a plurality of base stations have been determined prior to step 307. Specifically, at least one radio characteristic associated with the user equipment 201 and each of the base 25 stations in the neighbour list has been determined. In the specific example of FIG. 2, two base stations 201, 203 are considered, and the method thus derives at least two radio characteristics associated with the user equipment 211 and the plurality of base stations 201, 203.

30 In the preferred embodiment, the radio characteristics processor 225 also determines characteristics associated with downlink conditions, and specifically it determines the radio characteristics associated with the

( downlink load in accordance with the UMTS standards specifications ("UK

Procedures in Idle Mode and Procedures for Cell Reselection in Connected Mode", 3GPP TS25.30. Thus the radio characteristics processor 225 provides characteristics suitable for selecting a serving cell based on 5 uplink conditions, downlink conditions or a combination of both.

In step 307, it is determined whether the cell selection should be based on uplink conditions or downlink conditions. In the preferred embodiment, the fixed infrastructure determines whether an appropriate region of the 10 communication system is in a state of being uplink limited or down link limited. In communication systems such as UMTS, the performance and general operating conditions of the communication system are continuously monitored in an Operations and Maintenance Center (OMC).

This central function receives information from network elements, 15 including the base stations, and based on this information, it can be determined if a region of the communication system is close to full capacity and/or in particular whether the uplink or downlink has the lowest available capacity.

20 Alternatively, the base stations may individually, or in small groups, determine if the uplink or downlink has the least available capacity. As shown in FIG. 2, a group of base stations may provide information to a cell selection controller 233, which based on this information selects whether optimization of the uplink or the downlink is preferable. In one specific 25 embodiment, the base stations provide information of the uplink interference level at the base station, as well as a measure of downlink interference levels measured and reported back by the user equipment in the area. In this case, the preferability of the uplink or downlink optimization is determined in response to whether the uplink or downlink 30 direction is closest to being interference limited.

( The cell selection controller 233 thus determines if optimization in the uplink or downlink direction is preferred in a given region comprising a plurality of base stations. It consequently instructs the relevant base stations to transmit information on the pilot signal indicating this 5 preference. The user equipment associated with the relevant base stations receive the information and forwards it to the cell selector 229. Thus step 307 is preferably executed in the user equipment by receiving control information from the fixed network (including base stations) and basing the cell selection on uplink or downlink conditions in response to the lo received information.

Hence, in the preferred embodiment the plurality of base stations transmit equivalent indications of whether the user equipment should perform cell selection based on uplink conditions or downlink conditions. This allows l 5 the optimization of the communication system in a region of the communication system, as it optimises the cell selection and thus resource allocation for the direction which is most critical, thereby leading to higher total capacity of the communication system.

20 If it is determined in step 307, that downlink based selection should be used, the method continues in step 309 by selecting a serving base station based on a criterion associated with the downlink conditions. The cell selection is based on the earlier derived radio characteristics, and specifically the cell selection is in the preferred embodiment based on one 25 of the criteria specified in the UMTS standards specifications 3GPP TS

25.304.

If it is determined in step 307, that uplink based selection should be used, the method continues in step 311 by selecting a serving base station in 30 accordance with a criterion comprising an evaluation of uplink conditions for each base station of the plurality of base stations derived from the determined radio characteristics. In the preferred embodiment, the

( selection criterion is based on the radio characteristics generated by the radio characteristics processor 225 and relating to the uplink conditions.

In particular, the selection criterion is such that the base station for which the transmit power required to achieve a given signal to interference ratio 5 at the base station is minimised. Thus, the cell selector 229 receives the calculated transmit power for each of the base stations of the neighbour list from the radio characteristics processor 225, and selects the base station for which the lowest value has been calculated. This will minimise the uplink interference caused by the user equipment thereby maximising lo the capacity of the communication system.

It is within the contemplation of the invention that any suitable criterion comprising an evaluation of the uplink conditions for each of a plurality of base stations can be used. As such, it is preferred but not essential, that 15 the transmit power level is determined as a monotonically increasing function of a total interference power received at each base station of the plurality of base stations, as the higher the interference level the higher the required transmit power is likely to be. Similarly the transmit power level is preferably determined as a monotonically increasing function of a 20 propagation loss received at each base station of the plurality of base stations. Following either step 309 or step 311, the method continues in step 313 by the user equipment attaching to the selected base station. Depending on 25 the operating state of the user equipment, this attachment may comprise initial access to the communication system, remaining on the base station or handover to the base station. The method of cell selection may be carried out at any convenient time and is preferably repeated continuously or at regular intervals. As such, the cell selection may be performed when 30 the user equipment is first switched on and the user equipment registers with the communication system. In this scenario, step 313 comprises accessing the communication system with an initial access message

transmitted to the selected cell. Further, the method may be carried out at regular intervals while the user equipment is idle. If the cell selection indicates that a new cell should be selected, the user equipment will, together with the network, perform the necessary operations to change the 5 attachment from the previous cell to the selected cell. The method is preferably also carried out repeatedly during an ongoing call. If the cell selection process determines that a new cell is selected, step 313 comprises handing over to the selected cell in accordance with the specifications of

the communication system. If the selected cell is the same as the current to serving cell step 313 comprises performing the operations required for remaining on the serving cell. In Borne embodiments, no operations are required for this.

In the above description, an embodiment of the invention was described

15 with specific reference to one user equipment and two base stations.

However, it will be apparent that a typical cellular communication system will comprise a large number of base stations and user equipment.

Preferably, the described cell selection method is used for all user equipment and all base stations, although in some embodiments only a 20 subset of user equipment may use the described method whereas other user equipment may use conventional methods of cell selection, and specifically cell selection methods wherein only downlink considerations are considered. This allows a communication system to exist wherein some enhanced, new and/or updated user equipment can use the described 25 method, whereas existing standard user equipment can use conventional cell selection techniques. This facilities roll out of the system as there is no requirement for replacing existing user equipment.

In some embodiments, the cell selection is not based exclusively on either 30 uplink or downlink conditions but on a combination of both. In one such embodiment, a penalty value is associated with each calculated uplink radio characteristic, and a penalty value is associated with each downlink

radio characteristic. A total penalty value is calculated for each base station by weighting and adding the uplink and downlink penalty values.

The cell having the lowest total penalty value is selected. The weighting between uplink and downlink may be dynamically changed in accordance 5 with the uplink and downlink conditions of the communication system, and specifically with the amount of available resource on the uplink and downlink. This provides a selection criterion wherein a balanced optimization of both uplink and downlink can be achieved. Further, the degree of optimization of one direction in preference to the other can be 10 dynamically adjusted in response to the criticality of each link for the current operating conditions.

In some embodiments, the uplink conditions for each base station used in the selection criterion is an interference level at each base station caused 15 by transmission from the user equipment and specifically, the cell selection criterion is to choose the base station resulting in a lowest estimated interference level at a base station out of the plurality of base stations caused by transmission from user equipment. In this embodiment, it is not the transmit power and consequently the total 20 interference from the user equipment which is minimised. Rather, the interference caused at a specific base station is minimised. In this way, it is possible to minimise the interference at a specific critical base station at the cost of a possible increase in the total interference caused by a user equipment. The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. However, preferably, the invention is implemented as computer software running on one or more data processors. The elements and components of an 30 embodiment of the invention may be located in the user equipment, the core network, the radio access network or any suitable physical or functional location. Indeed the functionality may be implemented in a

f 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 in the communication system.

5 The invention thus tends to provide a number of advantages including one or more of the following: It provides for an efficient cell selection system wherein uplink considerations may be optimized.

10. An increase in uplink capacity can be achieved.

Interference caused by user equipment can be reduced thereby improving error performance and/or reducing resource use.

It provides for trade off between unlink and downlink optimization in response to current operating conditions.

Claims

( Claims

1. A method of cell selection for a cellular communication system having a plurality of base stations, each base station serving user 5 equipment in a cell, the method comprising the steps of determining radio characteristics associated with a user equipment and a plurality of base stations; selecting a base station out of the plurality of base stations as a serving base station by applying a selection criterion to the determined 10 radio characteristics; and wherein the selection criterion comprises an evaluation of uplink conditions for each base station of the plurality of base stations derived from the determined radio characteristics.

2. A method of cell selection as claimed in claim 1 wherein the selection criterion further comprises an evaluation of downlink conditions for each base station of the plurality of base stations determined from the determined radio characteristics.

3. A method of cell selection as claimed in claim 2 further comprising the step of transmitting, from at least a first of the plurality of base stations, an indication of whether the user equipment should perform cell selection 25 based on uplink conditions or downlink conditions.

4. A method of cell selection as claimed in claim 3 wherein all base stations of the plurality of base stations transmit equivalent indications of whether the user equipment should perform cell selection based on uplink 30 conditions or downlink conditions.

f

5. A method of cell selection as claimed in claim 3 to 4 wherein the indication indicates that uplink conditions should be used for cell selection when the plurality of base stations are predominantly uplink interference limited and that downlink conditions should be used for cell selection 5 when the plurality of base stations are predominantly downlink interference limited

6. A method of cell selection as claimed in any previous claim wherein the uplink conditions for each base station is a transmit power of the user 10 equipment.

7. A method of cell selection as claimed in claim 6 wherein the selection criterion is to choose the base station resulting in a lowest estimated transmit power of the user equipment.

8. A method of cell selection as claimed in any of the previous claims or 7 wherein the transmit power level is determined as a monotonically increasing function of a total interference power received at each base station of the plurality of base stations.

9. A method of cell selection as claimed in any of the previous claims to 8 wherein the transmit power level is determined as a monotonically increasing function of a propagation loss between the user equipment and each base station of the plurality of base stations.

10. A method of cell selection as claimed in previous claim 9 as dependent on 8 wherein the transmit power level is determined as a monotonically increasing function of the total interference power multiplied by the propagation loss.

11. A method of cell selection as claimed in any of the previous claims 1 or 2 wherein the uplink conditions for each base station is an interference

level at each base station caused by transmission from the user equipment.

12. A method of cell selection as claimed in claim 11 wherein the 5 selection criterion is to choose the base station resulting in a lowest estimated interference level at a base station out of the plurality of base stations caused by transmission from user equipment.

13. An apparatus for cell selection in a cellular communication system 10 having a plurality of base stations, each base station serving user equipment in a cell, the apparatus comprising: means for determining radio characteristics associated with a user equipment and a plurality of base stations; means for selecting a base station out of the plurality of base 15 stations as a serving base station by applying a selection criterion to the determined radio characteristics; and wherein the selection criterion comprises an evaluation of uplink conditions for each base station of the plurality of base stations derived from the 20 determined radio characteristics.

14. A user equipment comprising the apparatus of claim 13.

15. A communication system comprising the apparatus of claim 13.