GB2392799A

GB2392799A - Base station receiver initialisation

Info

Publication number: GB2392799A
Application number: GB0217749A
Authority: GB
Inventors: Christopher Smart; Suresh Sharma
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2002-07-31
Filing date: 2002-07-31
Publication date: 2004-03-10
Anticipated expiration: 2022-07-31
Also published as: GB2392799B; GB0217749D0

Abstract

The invention relates to initialisation of a receiver in a base station (205) for a cellular communication system. A receiver (217) receives an access signal and derives synchronisation parameters there from. An identification processor (221) and a base station controller (225) derive an identification and transmit a communication request message comprising the identification to an RNC (229). In addition, a synchronisation processor (219) stores the determined synchronisation parameters. The RNC (229) configures a new communication link and transmits a configuration message to the base station (205). The base station (205) checks the identification and retrieves any stored synchronisation parameters corresponding to the identification. The retrieved synchronisation parameters are then used to initialise the receiver (217). The invention is particularly applicable to the UMTS communication system.

Description

A BASE STATION AND A METHOD OF RECEIVER INITIALISATION

THEREFOR

Field of the invention

The invention relates to a base station and a method of receiver initialization therefor and in particular to a base station and a method for l0 a cellular communication system.

Background of the Invention

15 FIG. 1 illustrates the principle of a conventional cellular communication 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, 20 105, 107. A mobile station is served via a radio communication link by the 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.

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 30 overlapping coverage of the two base stations 111 and 113 and within this 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 15 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. Currently the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile 25 communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. One carrier is used for a pilot signal which further 30 contains broadcast information. This carrier is used by mobile stations for measuring of the signal level of transmissions from different base stations, and the obtained information is used for determining a suitable serving

cell during initial access or hangovers. 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) wherein user separation is obtained by allocating lo different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de-spread the received signal thereby regenerating the original signal. Each base station has a 15 code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the

20 Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

25 When a mobile station initiates a call in UMTS or GSM, it transmits an initial access message to a selected base station. This access message is known as a RACH (Random Access Channel) message. In UMTS, the RACH access mechanism uses a slotted ALOHA protocol wherein the channel is divided into discrete time slots that can be used for accessing 30 the base station. The base station broadcasts timing information that the mobile station uses to synchronize to the time slots of the RACH channel.

The mobile station transmits the RACH message by choosing a RACH

time slot at random, and transmitting the RACH message in this time slot. In UMTS, an initial RACH message known as a preamble is transmitted 5 in the RACH time slot. If the preamble is successfully received at the base station, the base station transmits an acknowledgement back to the mobile station, which in return transmits a second longer message part consisting in a RACH information message. The RACH information message includes information related to the service requested, the lO destination address and further information required for setting up a link.

If the mobile station does not receive an acknowledgement from the base station within a given time, it retransmits the preamble in a new RACH time slot with an increased power level. The increased power level increases the probability of successful reception and a successful l 5 acknowledgement. The mobile station repeats the transmission of the preamble at increasing power levels until a positive acknowledgement is received. In order to increase the capacity of the RACH channel, the preamble 20 comprises a given signature in the form of a Walsh scrambling code used in the spreading process for the signal. This signature allows the base station receiver to differentiate between preambles of different mobile stations. Further, the signature acts as an identification of the mobile station transmitting the preamble, and is used in the acknowledgement 25 from the base station to identify which preamble is acknowledged. In UMTS 16 different signatures can be used for the preamble, and upon generation of the preamble the mobile station selects one of these pre-

defined signatures at random. If the mobile station within a given time interval detects an acknowledgement on the acquisition indication channel 30 (AICH) of a preamble received in the given RACH time slot and with the selected signature, it will determine that the preamble has been

s successfully received, and it will proceed to transmit the RACH information message.

When the base station receives the RACH information message, it 5 generates a data packet comprising the contained information and communicates it to a Radio Network Controller (RNC). The RNC is in charge of resource allocation for the air interface, and in response to the received information it proceeds to allocate communication resource to the originating mobile station. The RNC communicates the required 10 information back to the (or possibly another) base station, which in response proceeds to setup and configure the communication link with the mobile station.

Further description of the access mechanism for UMTS can be found in

15 Third Generation Partnership (3GPP) Technical Specification TS 25.214.

Following the resource allocation in the RNC, communication between the mobile station and the base station is initiated using the configured traffic communication link. Initial detection and demodulation of the 20 communication from the mobile station is hampered by errors and uncertainties in the precise timing and carrier frequency synchronization required for receiving the signal. This results in increased acquisition times and therefore a delay in establishing the communication link.

Additionally and/or alternatively the initial communication may have 25 increased error rate due to errors in the synchronization parameters.

Consequently an improved system for initialization of the communication link would be advantageous.

30 Summary of the Invention

Accordingly the Invention seeks to provide an improved system for initialization of a communication link.

Accordingly there is provided a base station for a cellular communication 5 system comprising: a receiver for receiving radio access signals; means for deriving at least one synchronisation parameter for at least a first of the radio access signals; means for deriving a first identification associated with the first radio access signal; memory means for storing the at least one synchronisation parameter and the first identification; means for 10 generating a message in response to receiving the first radio access signal, the message comprising the first identification; means for communicating the message to a network of the cellular communication system; means for receiving communication link configuration information from the network of the communication system; receiver initialising means for retrieving the 15 at least one synchronisation parameter from the memory means and initiating the receiver of the base station with the at least one synchronisation parameter if the communication link configuration information comprises an identification corresponding to the first identification. Thus the initialization of the communication link is improved by improving the receiver initialization. Determining if pre-determined synchronisation parameter(s) exist and if so using them in initialization of a receiver provides significantly improved synchronisation performance 25 resulting in faster acquisition times and reduced initial error rates.

Further, if no pre-determined synchronisation parameters are identified, the performance of the receiver is not degraded compared to normal operation. 30 Preferably, the at least one synchronisation parameter comprises a frequency estimate and/or a plurality of timing estimates each timing estimate being associated with a signal component of the radio access

signal and/or a timing estimate more accurate than the coarse timing estimate. This provides for improved synchronization performance of all synchronization parameters, which change sufficiently slowly to maintain correlation between the time of the radio access signal and the 5 initialization of the receiver.

According to one feature of the invention, the first indication comprises a coarse timing estimate. This has the advantage of using an identification signal which in many communication systems, such as UMTS, are anyway 10 generated and communicated to the network for other purposes. Hence, no modifications to the technical specifications or existing equipment are

required. According to a further feature of the invention, the receiver initializing 15 means is operable to determine that the communication link configuration information comprises an identification corresponding to the first identification, if it comprises a timing indication corresponding to the coarse timing estimate. Hence, this provides for a simple yet reliable system for determining the identification based on the coarse timing 20 estimate generated and communicated.

According to yet a further feature of the invention, the receiver initializing means is operable to determine that the communication link configuration information does not comprise an identification corresponding to the first 25 identification, if it comprises a timing indication corresponding to more than one coarse timing estimate stored in the base station. This improves the protection against using synchronization parameters not corresponding with the first radio access signal and thereby reduces the probability of initializing the receiver with wrong synchronization 30 parameters.

According to a different feature of the invention, the receiver initializing means is operable to initialise the receiver with a coarse timing estimate comprised in the communication link configuration information, if it is determined that the communication link configuration information does S not comprise an identification corresponding to the first identification.

This provides the advantage that even if no stored synchronisation parameter(s) have been established, the timing synchronisation can be initiated with a coarse timing estimate.

10 According to a further feature of the invention, the first identification comprises a time stamp related to the arrival time of the first radio access signal. Preferably, the receiver initializing means is further operable to initialise the receiver with the at least first synchronisation parameter only if the communication link configuration information comprises a time 15 stamp indication proceeding a current time by less than a given threshold.

This provides protection against using stored synchronisation parameter(s) , wherein the delay from determining the synchronisation parameter(s) to the receiver being initialized is so long, that the estimates are no longer valid. Hence, it reduces the risk of initializing the receiver 20 with unusable synchronisation parameters.

According to another feature of the invention, the radio access signal can be received at a plurality of sectors of the cell and the first identification comprises an identification of a first sector out of the plurality of sectors 25 from which the radio access signal was received. Preferably, the receiver initializing means is operable to initialise the receiver with the at least first synchronisation parameter, only if the communication link configuration information comprises a sector indication corresponding to the first sector. This prevents that a signal arrived in one sector can be 30 confused with a signal arriving in a different sector, and thus reduces the risk of initializing the receiver with synchronisation parameters derived from a different communication unit than the desired.

According to yet another feature of the invention, the means for communicating the message to a network of the cellular communication system is operable to communicate the message to a first network element 5 of the network and the means for receiving communication link configuration information from the network of the communication system is operable to receive the communication link configuration from a different network element of the communication system. This allows for a flexible and efficient communication in networks of communication 10 systems wherein the base station may be connected to a plurality of different network elements.

The cellular communication system is preferably a CDMA cellular communication system and in particular the CDMA cellular 15 communication system is preferably a UMTS cellular communication system. According to a second aspect of the invention, there is provided a method of receiver initialization for a base station for a cellular communication 20 system comprising the steps of receiving radio access signals; deriving at least one synchronization parameter for at least a first of the radio access signals; deriving a first identification associated with the radio access signal; storing the at least one synchronization parameter and the first identification; generating a message in response to receiving the first radio 25 access signal, the message comprising the first identification; communicating the message to a network of the cellular communication system; receiving communication link configuration information from the network of the communication system; determining if the communication link configuration information comprises an identification corresponding 30 to the first identification; and if so: retrieving the at least one synchronization parameter from the memory means; and initiating a

receiver of the base station with the at least one synchronization parameter. 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 10 FIG. 1 is an illustration of a cellular communication system in accordance with the prior art;

FIG. 2 is an illustration of a UMTS communication system comprising a base station in accordance with a preferred embodiment of the invention; FIG. 3 is an illustration of a flow chart for steps of a method for receiving an access message in accordance with elements of the preferred embodiment of the invention; and 20 FIG. 4 is an illustration of a flow chart for steps of a method for receiver initialization in accordance with elements of the preferred embodiment of the invention.

25 Detailed Description of a Preferred Embodiment of the Invention

A detailed description of a preferred embodiment of the invention is in the

following described with specific focus on a base station of a UMTS communication system. However, it will be clear that the invention is not 30 limited to this application but is equally applicable to other communication systems and suitable communication elements.

FIG. 2 is an illustration of a UMTS communication system 200 comprising a base station in accordance with a preferred embodiment of the invention.

FIG. 2 illustrates two communication units 201, 203 communicating with 5 a base station 205, in UMTS known as a Node B. over air interface links 207, 209. The communication units 201, 203 may typically be mobile stations, communication terminals, wireless devices, user equipment (UE), remote terminals, subscriber units etc. In the preferred embodiment, the communication units 201, 203 communicate with the base station 205 in lo accordance with the protocols and requirements as specified in the technical specifications of the UMTS communication system.

Specifically, the communication units 201, 203 initiate a call by transmitting an access message to the base station 205 on the RACH 15 access channel using a slotted ALOHA protocol i.e. wherein the access channel is separated into separate time slots. In particular, the communication units 201, 203 initially transmit a preamble comprising an identifying signature and upon receiving acknowledgement of this signature from the base station 205, the communication units 201, 203 20 transmit an information message, as previously described.

The base station 205 comprises an antenna 211 connected to a duplexer 213 which is further connected to a transmitter 215 and a receiver 217.

The antenna 211 receives and transmits radio signals over the 25 communication links 201, 203. The duplexer 213 isolates between the transmitter 215 and receiver 217 such that the same antenna can simultaneously be used for both receiving and transmitting.

The transmitter 215 is operable to transmit signals to the communication 30 units 201, 203 in accordance with the UMTS specifications. The

transmissions from the base station 205 to the communication units 201, 203 comprise traffic data, control data and broadcast data and are

communicated on logical channels, including traffic channels, broadcast channels, control channels and access channels.

Similarly, the receiver 217 is operable to receive signals transmitted from 5 the communication units 201, 203 in accordance with the UMTS specifications. The transmissions from the communication units 201, 203

comprise both traffic data and control data (including access and configuration data) and the uplink communication therefore also comprises logical channels including traffic channels, control channels and lO access channels.

Specifically the receiver 217 is operable to receive access messages transmitted from the subscriber units. In some embodiments, a single access message is received comprising information required for setting up 15 communication links. However, in the preferred embodiment of a UMTS communication system, the access message comprises two parts. Initially, a preamble is transmitted, and upon acknowledgement of the preamble a message part is transmitted. The message part comprises information related to e.g. the requested service, the destination address and the 20 identity of the communication unit 201, 203. The information of the message part is used in configuring the communication link.

The base station 205 further comprises a synchronization processor 219 for deriving at least one synchronization parameter for at least a first of 25 the radio access signals. The synchronization processor 219 is connected to the receiver 217 for determining the synchronization parameter(s). In order to successfully demodulate a received signal, local synchronization estimates must be generated as is well known in the art of receiver design.

Typically for digital receivers, such as RAKE receivers, estimates are 30 generated for the carrier phase, carrier frequency and relative timing of all significant signal components of the received multi path signal. The estimates may be explicitly generated or may be implicitly generated by

for example synchronizing a carrier phase by use of a phase locked loop.

The synchronisation processor 219 thus generates at least one of these synchronisation parameters and in the preferred embodiment the carrier frequency and relative timing of the significant signal components are 5 determined.

The synchronisation processor 219 is connected to an identification processor 221 which derives a first identification associated with the radio access signal. Any kind of identification can be used, and in a simple 10 embodiment the identification is simply generated as a running serial number. The synchronisation processor 219 is further connected to a memory 223 for storing the at least one synchronisation parameters and the associated 15 identification. Thus the synchronisation processor 219 receives the derived identification from the identification processor 221 and stores it together with the derived synchronisation parameters such that the identification can be used as an index to retrieve the stored synchronisation parameters.

20 The base station 205 further comprises a base station controller 225 which is connected to the synchronisation processor 219. The base station controller 225 controls the operation of the various entities of the base station 205 including the receiver 217, the transmitter 215 and the synchronisation processor 219. The base station controller 205 is 25 connected to most elements of the base station 205 including the synchronisation processor 219, the transmitter 215, the receiver 217 and the identification processor 221. Specifically the base station controller 205 is operable to generate a message in response to receiving the first radio access signal, the message comprising the first identification. When 30 the identification processor 221 generates an identification, it communicates the identification to the base station controller 225, which in response generates a message that includes the received identification.

The message further comprises configuration information which includes the information received from the communication unit in the message part of the access message. The message thus comprise information related to the communication request from the communication unit 201, the 5 destination address and other information required or desired for the network to set up a communication link for the communication unit 201.

The base station controller 225 is also connected to an interface 227 which interfaces the base station 205 to the remaining part of the fixed network.

lO In the preferred embodiment, the interface 227 is connected to a Radio Network Controller (RNC) 229. The interface 227 is responsible for receiving and transmitting all communication between the base station 205 and the network. This information includes traffic data, control data, monitoring data and configuration data. Specifically, the interface 227 is 15 operable to communicate the message generated by the base station controller 225 to a network of the cellular communication system, which in the preferred embodiment is achieved by communicating it to the RNC 229. 20 The interface 227 is further operable to receive communication link configuration information from the network of the communication system.

The configuration information relates to information for configuring a communication link with the communication units. In the preferred embodiment, the RNC 229 will upon receiving the message from the base 25 station 205 proceed to attempt to set up a communication link for the communication unit 201. This includes allocating a radio resource in the form of a traffic channel, and choosing a transmission scheme and other parameters that are necessary to meet the requirements of the service requested by the communication unit 201. When the communication link 30 has been configured in the RNC 229, the configuration information required by the base station 205 and/or the communication unit 201 is included in one or more configuration messages, which are communicated

to the base station 205. In addition, the RNC 229 may include the identification of the original message received from the base station 205.

When receiving the configuration messages from the RNC 229, the 5 interface 227 feeds the communication link configuration information to the base station controller 225. The base station controller 225 is connected to a receiver initializing processor 231, and upon receiving the communication link configuration information, the base station controller 225 controls the receiver initializing processor 231 to initiate the receiver lO 217 for reception of further signals from the communication unit 201. This initialization comprises analysing the received communication link configuration information to determine if it comprises any identification information. l 5 The receiver initializing processor is operable to retrieve the synchronisation parameters from the memory 233 and initiating the receiver 217 of the base station 205 with the at least one synchronisation parameters, if the communication link configuration information comprises an identification corresponding to the first identification. Thus, 20 if an identification is received in the communication link configuration information, the receiver initializing processor 231 accesses the memory to detect if it comprises an entry corresponding to the received identification.

If so, it retrieves the stored synchronisation parameters and initializes the receiver with the retrieved parameters. The initialization is preferably 25 achieved by initializing the appropriate variables of a firmware implemented synchronisation unit with the synchronisation parameters or by using the synchronisation parameters to pre-set the of initial values of a synchronisation feedback loop such as a phase locked loop. If the memory does not comprise an entry corresponding to the identification, or 30 if the communication link configuration information does not comprise any identification information, the receiver is initialized with default values.

The operation of the communication system of FIG. 2 will in the following be described in more detail with reference to FIG. 3 and 4. FIG. 3 is an illustration of a method for receiving an access message in accordance with elements of the preferred embodiment of the invention. In step 301, the receiver 217 receives a radio access signal from one of

the communication units 201, 203. The radio access signal is in this example an access signal message part that comprises information related to a communication request from the transmitting communication unit 201.

In step 303, the synchronisation processor 219 derives at least one synchronisation parameter for the radio access signal. As previously mentioned, the receiver 217, as part of the receiving process for the access message, estimates the carrier phase and carrier frequency of the 15 transmitted signal. In the preferred embodiment, these parameters are fed to the synchronisation processor 219. Thus the synchronisation processor 219 is in the preferred embodiment an integrated part of the receiver 217.

In addition to the carrier synchronisation parameters, the timing of the received access signal is determined. The typical propagation environment 20 of a UMTS communication system comprises a plurality of propagation paths between the communication unit and the base station. The multi path environment gives rise to a plurality of signal components of the transmitted signal being received with varying time offsets, depending on the length of each individual propagation path. In the preferred 25 embodiment, the receiver 217 further generates accurate timing estimates of the relative time offsets of each of these multi path signal components.

These time offsets are fed to the synchronisation processor 219 and the at least one synchronisation parameter thus comprises a plurality of timing estimates, each timing estimate being associated with a signal component 30 of the radio access signal.

In addition to the timing estimates for each of the signal components, the receiver generates a coarse timing estimate for the received signal. The coarse timing estimate is a single time estimate of the received signal from one communication unit and comprises no information related to how 5 many signal components are received or their relative time offsets. In the preferred embodiment, the coarse timing estimate is generated as a weighted average of the timing estimates of the individual signal components. lO Step 305 comprises storing the at least one synchronisation parameters and the associated identification. Thus, the determined synchronisation parameter(s) are stored by the synchronisation processor 219 in the memory 223 for later retrieval.

lS Step 307 comprises deriving a first identification associated with the radio access signal. In the preferred embodiment, the identification processor 221 determines the identification in response the synchronisation parameters being determined by the synchronisation processor 219.

Specifically, the identification is furthermore derived from the determined 20 synchronisation parameters. In the described embodiment, the identification comprises the coarse timing estimate and may specifically be identical to the coarse timing estimate. In this case, the identification processor 221 simply receives the determined coarse timing estimate from the synchronisation processor 219 and sets the identification to a 25 representation of this value.

In step 309, the base station controller 225 generates a message in response to receiving the first radio access signal, the message comprising the first identification. The base station controller 225 generates a 30 message in accordance with a protocol of the communication system for communication between the base station 205 and the network and in particular the RNC 229. In UMTS a specific message is specified to be

generated in response to receiving a message part of an access signal. The specific format of this message is specified in Third Generation Partnership (3GPP) Technical Specification TS 25.435. The specification

defines a number of parameters to be included wherein some of the 5 parameters are comprised in the message part of the access message. One parameter which is specified to be included is a coarse time estimate, and in the preferred embodiment the base station controller 225 therefore includes the coarse timing estimate generated by the identification processor in the message.

In step 311, the message is transmitted by the interface to a network of the cellular communication system and specifically to the RNC 229. The communication is performed using a communication protocol specified for communication between the base station 205 and the RNC 229.

When receiving the message from the base station 205, the RNC 229 proceeds to configure a new communication channel that can be used for the communication between the base station 205 and the communication unit 201. This includes allocating a communication resource in the form of 20 a CDMA code, as well as identifying and authenticating the communication unit. The configuration typically requires the RNC 229 to communicate and interact with other communication elements of the network. Once the configuration has completed in the RNC 229, a configuration message to be transmitted to the base station 205 is 25 generated. The configuration message comprises data and information related to the configured communication link or channel. As such, it comprises information required by the base station 205 and/or the communication unit 201 to setup and initialise the communication link.

30 Specifically the message comprises communication link configuration information that is used for initializing and configuring the base station 205 and the communication unit 201 to communicate using the configured

communication link. The communication link configuration information may in some situations but not necessarily always comprise an identification related to the access message instigating the configuration.

In UMTS, the coarse timing estimate originally received from the base 5 station 205 is included in the configuration message from the RNC 229. In conventional UMTS systems, the base station 205 configures and initiates the new communication link upon receiving the configuration message from the RNC 229 based solely on the information comprised in the configuration message. The initialization and configuration of the base lO station 205 and communication unit 201 is thus performed without any consideration of the information in or parameters of the previously received access messages. For this reason, the coarse timing estimate is passed back to the base station 205, such that it can be used as an initial estimate for the initialization of the base station 205 receiver 217.

FIG. 4 is an illustration of a flow chart for steps of a method for receiver initialization in accordance with elements of the preferred embodiment of the invention. The figure illustrates the operation of the base station 205 upon receiving a configuration message from the RNC 229.

In step 401, the interface 227 receives communication link configuration information from the network of the communication system. In the preferred embodiment, the information is received in a configuration message from the RNC 229, but in other embodiments it may be received 25 for example in a plurality of messages or as part of a continuous data stream. The communication link configuration information is extracted from the configuration message and fed to the base station controller 225.

The base station controller 225 forwards the information to the receiver initialization processor 231.

In step 403, the receiver initialization processor 231 determines if the communication link configuration information comprises an identification

corresponding to the first identification. In some embodiments, the identification is a dedicated identification used for the access message, such as a running serial number. In this case, the determination of whether the communication link configuration information comprises a S corresponding identification is preferably made in the base station controller 225 by simply comparing the identification received with prior identifications generated by the identification processor 221.

In the preferred embodiment, the first indication comprises a coarse 10 timing estimate and this coarse timing estimate is used as an identification. In this embodiment, the receiver initialization processor 231 is operable to determine that the communication link configuration information comprises an identification corresponding to the first identification, if it comprises a timing indication corresponding to the 15 coarse timing estimate. The receiver initialization processor 231 detects if the communication link configuration information comprises a coarse timing estimate. If not, it determines that the communication link configuration information does not comprise a corresponding identification and the method continues in step 406.

If a coarse timing estimate is found, it is extracted and the receiver initialization processor 231 searches the stored coarse timing estimates stored as identification in the memory 223 to determine if any stored values correspond to the extracted timing estimate. If no matches are 25 found, it is determined that the communication link configuration information does not comprise a corresponding identification and the method continues in step 405. If a match is found, it is determined that the communication link configuration information does comprise a corresponding identification, and the method continues in step 409. Any 30 suitable algorithm may be used to determine if a match exists. In a simple embodiment, a match is determined to exist if the timing estimates are sufficiently close, e.g. that the time difference is less than a threshold. In

other embodiments, an exact identity may be required or more advanced statistically derived algorithms may be used.

Using the coarse timing estimate as an identification has the significant 5 advantage that it does not require a separate identity to be generated and communicated between base station and network. Rather, a timing estimate is used which may be generated for other purposes. Specifically, in UMTS, the Technical Specifications require that a coarse timing

estimate is generated, that this may be communicated to the RNC and 10 further that it may be received back from the RNC in the configuration message. However, the communication protocol for UMTS does not allow for a specific identification to be included. Thus using the timing estimate as an identification provides the advantage that the described system can be implemented without requiring any modifications to the Technical 15 Specifications or to existing network equipment. Simple software

modifications to the base station are sufficient.

However, using a coarse timing estimate, rather than a unique identification, results in the possibility of an increased error rate in 20 determining a corresponding identification. Specifically, in some situations ambiguities may arise where more than one match exists in the memory for a given received coarse timing estimate. For this reason, the receiver initialization processor 231, in the preferred embodiment, is operable to determine that the communication link configuration information does not 25 comprise an identification corresponding to the first identification, if it comprises a timing indication corresponding to more than one coarse timing estimate stored in the base station 205. Thus if more than one match is found, the receiver initialization processor 231 cannot determine which is the right one, and therefore refrains from relying on the 30 identification. The method in this case continues in step 405. This reduces the probability of a wrong identification being used, and therefore reduces

the increased delay and error rate resulting from initializing the receiver 217 with erroneous synchronisation parameters.

If no corresponding identification has been found, the receiver 5 initialization processor 231 in step 405 proceeds to initialise the receiver 217 with default parameters. The default parameters may typically be typical or expected values for the synchronisation parameters.

Alternatively, the receiver 217 may be initialised without synchronisation parameters, which it then needs to derive from the ensuing lO communication signals. In the preferred embodiment, the receiver initialization processor 231 is operable to initialise the receiver with a coarse timing estimate comprised in the communication link configuration information, if it is determined that the communication link configuration information does not comprise an identification corresponding to the first 15 identification. Thus if the base station receives a coarse timing estimate in the configuration, which does not have a match in the memory, it will use the coarse timing estimate as an initial value for the timing synchronisation of the receiver 217. This improves the acquisition time and the initial error rate of the receiver when no synchronisation 20 parameters have been stored.

In step 407, the base station 205 proceeds to communicate with the communication unit 201 in accordance with the received configuration, and specifically the receiver 217 begins the demodulation and decoding 25 based on the initialization by the receiver initialization processor 231.

If the receiver initialization processor 231 has determined that the communication link configuration information comprises a corresponding identification, it proceeds in step 409 to retrieve the at least one 30 synchronisation parameters from the memory 223. The receiver initialization processor 231 then proceeds in step 411 with initiating the receiver 217 of the base station 205 with the at least one synchronisation

parameter. The receiver is thus initialised with synchronisation parameters for the specific communication unit 201 that have previously been generated and stored. Hence, the receiver 217is able to very quickly, and in some situations instantly, acquire synchronisation with the 5 received signal thereby resulting in much reduced acquisition delay and error rate for the initial communication.

Following the initialization with the retrieved synchronisation parameters, the method proceeds in step 407 with the base station 205 10 proceeding to communicate with the communication unit 201 in accordance with the received configuration, and specifically the receiver 217 begins the demodulation and decoding based on the initialization by the receiver initialization processor 231.

15 Thus, a significantly improved synchronisation performance is achieved wherein earlier derived accurate measurements can be used as initial synchronisation parameters for a newly configured communication link.

The method automatically detects if such pre-stored parameters are available, and the operation of the base station is only changed if that is 20 the case. Hence, the advantages of using pre-stored values are exploited when possible, without reducing the performance of the base station when no pre-stored parameters are available. Consequently, when possible, the system described provides much faster synchronisation thereby reducing the synchronisation delay and the initial error rate of the communication.

In the preferred embodiment, all synchronisation parameters which have rates of change so low that the synchronisation parameter of the access message is correlated to the value for the communication following configuration is stored. Thus the carrier frequency is stored, thereby 30 allowing a very fast synchronisation to the carrier frequency of the later communication. In addition, an accurate timing estimate is stored.

Preferably, the at least one synchronisation parameter comprises a

plurality of timing estimates each timing estimate being associated with a signal component of the radio access signal. Thus, for each of the received multi path signal components, a separate timing estimate is stored and retrieved upon receiving the configuration message. Following the 5 configuration, the receiver can consequently be initiated and the demodulation based on a full multi path channel estimate. This allows for instant demodulation of the received signal with low error rate.

Preferably, the receiver is a RAKE receiver wherein the finger management is initialised with one finger allocated and synchronized to lO each of the signal components.

In the preferred embodiment, the carrier phase is not pre-stored as it changes very fast, and therefore carrier phase estimates for the access message and the communication after the configuration tend to be l5 uncorrelated. However, in some embodiments timing may be kept so accurately that the receiver is initialised with a carrier phase derived from that estimated for the access message compensated by the phase drift caused by the carrier frequency offset over the time interval between the access message and the ensuing communication.

In one embodiment, the identification comprises a time stamp related to the arrival time of the first radio access signal. Specifically, the receiver initialization processor 231 is operable to initialise the receiver with the at least first synchronization parameter only if the communication link 25 configuration information comprises a time stamp indication proceeding a current time by less than a given threshold. In this embodiment, the time stamp of the received signal is compared to the current time, and if the time stamp indicates that the stored estimates are too old, the receiver is initialised with the default values rather than the pre-stored values.

30 Hence, the stored values are only used if there is a reasonable change that the stored values have sufficient correlation with the current parameters, thereby reducing the risk of initializing the receiver with outdated values.

In most cellular communication systems, the base station comprises directional antennas having angular offsets with respect to each other.

This results in a sectorisation of the cell wherein typically three different 5 sectors of the cell are served by different antennas and possibly different receivers and transmitters. Various form of cell sectorisation is well known in the art. Thus in some embodiments, the radio access signal can be received at a plurality of sectors of the cell, and the identification comprises an identification of a first sector out of the plurality of sectors 10 from which the radio access signal was received. Preferably, the receiver initialization processor 231 is operable to initialise the receiver with the at least first synchronization parameter only if the communication link configuration information comprises a sector indication corresponding to the first sector. Hence, only if the access message was received in the same 15 sector as the sector identified in the configuration message is the receiver initialized with the pre-stored values. This provides further protection against the receiver being initialized with the wrong parameters due to a false match.

20 In the described embodiment, the means for communicating the message to a network of the cellular communication system is operable to communicate the message to a first network element of the network and the means for receiving communication link configuration information from the network of the communication system is operable to receive the 25 communication link configuration from the same first network element of the communication system. Specifically, the message generated in response to the access signal is transmitted to the RNC 229 and the configuration message comprising the communication link configuration information is received from the same RNC 229.

However in other embodiments, the means for communicating the message to a network of the cellular communication system is operable to

communicate the message to a first network element of the network and the means for receiving communication link configuration information from the network of the communication system is operable to receive the communication link configuration from a different network element of the 5 communication system. Thus the first message comprising the identification may be transmitted to a first network element. This network element may pass information on to other network elements, and the configuration of the communication link may be performed at one or more of the network elements. Thus another element may generate the 10 communication link configuration information and this can be transmitted to the base station 205 through any suitable path, and thus return to the base station through another route. Since the base station is only reliant on the identification information to associate the communication link configuration information with pre-stored synchronization parameters, 15 there is no requirement that it arrives from any specific network element.

Hence, in embodiments where the base station may be connected to a plurality of different network element, this provides for a more flexible and therefore more efficient routing to be performed.

20 Although the above description has focussed on a UMTS communication

system, it will be apparent that the invention is equally applicable to many other communication systems.

For clarity and brevity, only the functionality of the communication 25 system, and specifically the base station, required for a description of a

preferred embodiment of the invention has been described, and it will be clear to the person skilled in the art that further well known features, algorithms and functionality will typically be part of a cellular communication system.

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 embodiment of the invention may be located in the core network, the radio access network or any suitable physical or functional location. Indeed the 5 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 in the network. Preferably the method is executed in a single Digital Signal Processor of a base station.

It will follow from the above, that the invention tends to provide a number of advantages, singly or in combination, including the following: Improved synchronization performance is achieved for a base station. 15. The acquisition delay is significantly reduced.

The initial error performance for the receiver is significantly increased. Improved performance can be achieved in a UMTS communication system without changes to the Technical Specifications or existing

20 equipment.

Claims

1. A base station for a cellular communication system comprising a receiver for receiving radio access signals; 5 means for deriving at least one synchronization parameter for at least a first of the radio access signals; means for deriving a first identification associated with the first radio access signal; memory means for storing the at least one synchronization 10 parameter and the first identification; means for generating a message in response to receiving the first radio access signal, the message comprising the first identification; means for communicating the message to a network of the cellular communication system; 15 means for receiving communication link configuration information from the network of the communication system; receiver initializing means for retrieving the at least one synchronization parameter from the memory means and initiating the receiver of the base station with the at least one synchronization 20 parameter if the communication link configuration information comprises an identification corresponding to the first identification.

2. A base station as claimed in claim 1 wherein the at least one synchronization parameter comprises a frequency estimate.

3. A base station as claimed in claim 1 wherein the at least one synchronization parameter comprises a plurality of timing estimates each timing estimate being associated with a signal component of the radio access signal.

4. A base station as claimed in any of the previous claims wherein the first indication comprises a coarse timing estimate.

5. A base station as claimed in claim 4 wherein the at least one synchronization parameter comprises a timing estimate more accurate than the coarse timing estimate.

6. A base station as claimed in any of the claims 4 to 5 wherein the receiver initializing means is operable to determine that the communication link configuration information comprises an identification corresponding to the first identification if it comprises a timing indication 10 corresponding to the coarse timing estimate.

7. A base station as claimed in any of the claims 4 to 6 wherein the receiver initializing means is operable to determine that the communication link configuration information does not comprise an 15 identification corresponding to the first identification if it comprises a timing indication corresponding to more than one coarse timing estimate stored in the base station.

8. A base station as claimed in any of the claims 4 to 7 wherein the 20 receiver initializing means is operable to initialise the receiver with a coarse timing estimate comprised in the communication link configuration information if it is determined that the communication link configuration information does not comprise an identification corresponding to the first identification.

9. A base station as claimed in any previous claim wherein the first identification comprises a time stamp related to the arrival time of the first radio access signal.

30

10. A base station as claimed in any previous claim wherein the receiver initializing means is operable to initialise the receiver with the at least first synchronization parameter only if the communication link

configuration information comprises a time stamp indication preceding a current time by less than a given threshold.

11. A base station as claimed in any previous claim wherein the radio 5 access signal can be received at a plurality of sectors of the cell and the first identification comprises an identification of a first sector out of the plurality of sectors from which the radio access signal was received.

12. A base station as claimed in claim 11 wherein the receiver 10 initializing means is operable to initialise the receiver with the at least first synchronization parameter only if the communication link configuration information comprises a sector indication corresponding to the first sector.

15

13. A base station as claimed in any previous claim wherein the means for communicating the message to a network of the cellular communication system is operable to communicate the message to a first network element of the network and the means for receiving communication link configuration information from the network of the 20 communication system is operable to receive the communication link configuration from a different network element of the communication system.

14. A base station as claimed in any previous claim wherein the means 25 for communicating the message to a network of the cellular communication system is operable to communicate the message to a first network element of the network and the means for receiving communication link configuration information from the network of the communication system is operable to receive the communication link 30 configuration from the same first network element of the communication system.

15. A CDMA cellular communication system comprising a base station as claimed in any of the claims 1 to 14.

16. A CDMA cellular communication system as claimed in claim 15 5 wherein the CDMA cellular communication system is a UMTS cellular communication system.

17. A method of receiver initialization for a base station for a cellular communication system comprising the steps of: 10 receiving radio access signals; deriving at least one synchronization parameter for at least a first of the radio access signals; deriving a first identification associated with the radio access signal; 15 storing the at least one synchronization parameter and the first identification; generating a message in response to receiving the first radio access signal, the message comprising the first identification; communicating the message to a network of the cellular 20 communication system; receiving communication link configuration information from the network of the communication system; determining if the communication link configuration information comprises an identification corresponding to the first identification; and if 25 so: retrieving the at least one synchronization parameter from the memory means; and initiating a receiver of the base station with the at least one synchronization parameter.