GB2404527A - Baseband processing of control signals in a multi sector base station - Google Patents

Abstract

A multi sector base station for a cellular communication system comprises a plurality of base band processing units 115-125 coupled to a plurality of transceivers 107-111 through a combiner 129. Each sector of the base station is assigned to one transceiver 107-111. The base station further comprises a segmenter 127, which segments the base band processing of one or more channel signals for a sector over at least two of the base band processing means 115-125. The segmenting may be by time or code multiplexing, or reduced gain processing. If one base band processing unit 115-125 fails, only a gradual degradation occurs as segments covered by other base band processing units 115-125 are still operable. Hence, improved reliability is achieved.

Description

A MULTI SECTOR BASE STATION AND METHOD OF OPERATION

THEREFOR

Field of the invention

The invention relates to a multi sector base station and a method of operation therefor and in particular to a multi sector base station for a cellular communication system, such as a cellular third generation (3G) cellular communication system.

Background of the Invention

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

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

The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc. Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) wherein user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de- spread the received signal thereby regenerating the original signal. Each base station has a code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed.

Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876 In a UMTS CDMA communication system, the communication network comprises a Core Network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user CE3091 6P equipment over a radio link being part of the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the Node Bs and the communication over the air interface.

The RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate Node Bs. It further provides the interface between the RAN and the CN. An RNC and associated Node Bs is known as a Radio Network System 1 0 (RNS).

In a typical 3G communication system, each RNC controls a number of base stations and the network component type deployed in the largest quantity is the base station or Node B. Accordingly, the combined cost of the base stations is a significant contribution to the total cost of deploying a communication system. Furthermore, many characteristics of the performance of a cellular communication system depend directly on the performance of the base stations. For example, the provided quality of service, reliability and capacity achieved in a cellular communication system depends significantly on the performance of the individual base stations. Accordingly, it is important to optimise the performance of base stations while maintaining low cost.

A performance parameter which is of critical importance for a base station of a cellular communication system is the reliability of the base station. In order to achieve high performance of the communication system as a whole, it is important that the base stations can provide the required communication capacity with only very few interruptions. Therefore base stations are designed to meet very strict requirements limiting the allowed full or partial outages for a base station. For example, a desired requirement for a base station may be that the downtime due to outage is less than an average of 1 minute per year.

CE3091 6P Clearly, this imposes a very strict design requirement on base stations and it is generally not possible to achieve such a level of reliability without introducing redundant functionality which can take replace a malfunctioning unit. Thus, base stations typically comprise additional circuitry which may prevent or mitigate outages caused by malfunctioning equipment.

A typical base station design comprises one or more base band processing means which are coupled to RF (radio frequency) means. For a transmitter, the base band processing means is typically operable to process received data streams and to format and encode them for transmission and the RF means is operable to up-convert and amplify the signals for transmissions. For a receiver, the RF means is operable to filter, down-convert and amplify the received signals and the base band processing means is typically operable to process the down converted signals to recover the comprised data.

In a typical base station architecture, the RF means are dedicated to one or more antennas of a specific sector and is used for receiving and transmitting all signals of these antennas. Thus, for a six sector base station, the RF means is typically implemented by six substantially identical RF units. In some cases, a single RF unit may support e.g. two sectors resulting in three RF units being required for a six sector base station. In order to meet the reliability of the base station, the architecture comprises one or more redundant RF units which can replace any of the other RF units in case of a malfunction.

In a base station architecture, the base band processing is often implemented in a similar way by a number of base band units being allocated to specific sectors with redundant units standing by to take over in case of a malfunction.

However, in some embodiments, the base band units are not allocated to specific sectors but may process channels corresponding to more than one CE30916P sector. Thus, the base band processing may be pooled over a plurality of base band units. This may allow for a dynamic redundancy wherein the channels processed by a base band unit which malfunctions may be re-allocated to other base band units. Accordingly a disruption may occur which may cause calls handled by the malfunctioning base band unit to be dropped. However, the sector remains operational and this is generally considered to be acceptable.

Nevertheless, the malfunctioning of a base band unit does result in a disruption to the performance of the base station and decreases the reliability of the base station and the communication system as a whole.

Accordingly an improved base station and method of operation therefor would be advantageous and in particular a multi sector base station allowing for increased flexibility, reduced cost, increased capacity of the communication system and/or increased reliability would be advantageous.

Summary of the Invention

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

According to a first aspect of the invention there is provided a multi sector base station for a cellular communication system comprising: a plurality of base band processing means; a plurality of transceiver means coupled to the base band processing means, each sector of the base station being assignable to a transceiver means of the plurality of transceiver means; means for segmenting the base band processing of at least one channel signal for a sector of the base station over at least two of the plurality of base band processing means.

CE3091 6P The inventors have realised that failure of some channel signals has a significantly increased impact on the reliability of a base station than other signals. The inventors have furthermore realised that improved reliability can be achieved by segmenting a channel signal for a sector of the base station over a plurality of base band processing means. Specifically, by segmenting a critical channel signal over a plurality of base band processing means a gradual degradation of the critical channel signal may be achieved if one base band processing means fails.

In particular, the inventors have realised that a channel signal which is common for many or all user equipment of a sector and/or cell of a cellular communication system may be segmented over a plurality of base band processing means.

An architecture of a base station in accordance with an embodiment of the invention may furthermore allow for increased flexibility in allocation of the resource of the base band processing means. Specifically a common pool of resource may be flexibly allocated while retaining a high flexibility. The flexibility may allow for efficient re-allocation and minimal disruption following a malfunction of a base band processing means.

The invention may allow for increased reliability of base stations and thus reduce outage and accordingly improve the performance of the communication system as a whole. The invention may furthermore allow a desired reliability figure to be achieved by less complex circuitry thereby allowing for a decreased cost of the base stations.

The transceiver means may comprise receiver and/or transmitter means and preferably comprise RF processing means. In addition, the transceiver means may comprise some base band processing functionality. The channel signal is CE30916P typically a signal associated with a physical or logical channel of the cellular communication system.

According to a feature of the invention, the at least one channel signal is at least one common channel signal. The common channel signal is common for a plurality of mobile stations or user terminals and is preferably common for all mobile stations of a sector or a cell of a base station. Segmenting the common channel signal may allow for a partial degradation wherein an acceptable albeit degraded level of service may still be achieved. This service level may possibly be achieved following a failure of a base band processing means and before reallocation of the channel signal to other base band processing means is achieved.

According to another feature of the invention, the at least one common channel signal is a common pilot channel signal. By segmenting a common pilot channel signal over a plurality of base band processing means, at least part of the pilot signal may be maintained even when a base band processing means fails. For example, if the pilot channel signal is time multiplexed between different base band processing means the pilot signal will be present for at least part of the total time even if a processing means fails. This may allow for some or all mobile stations to maintain the service thereby increasing reliability and reducing the performance and outage impact of a failing base band processing means. Segmentation of a common pilot channel signal is particularly suitable for the downlink and is thus preferably implemented by the transmitting elements of a base station.

According to another feature of the invention, the at least one common channel signal is a common random access signal. By segmenting a common random access channel signal over a plurality of base band processing means at least part of the random access signal may be maintained even when a base band processing means fails. For example, if the random access channel signal is CE3091 6P time multiplexed between different base band processing means, the random access channel will be operable for at least part of the total time even if a processing means fails. This may allow for some or all mobile stations to access the base station even if a base band processing means has failed thereby increasing reliability and reducing the performance and outage impact of a failing base band processing means. Segmentation of a common random access channel signal is particularly suitable for the uplink and is thus preferably implemented by the receiving elements of a base station.

According to another feature of the invention, the means for segmenting is operable to segment the base band processing in the time domain. This provides for a low complexity and easy to implement means of segmentation which results in high performance. Furthermore, segmentation in the time domain results in only a small degradation for some channel signals.

According to another feature of the invention, the segmentation is by time multiplexing of the at least one channel signal between the at least two base band processing means. Time multiplexing may be easily implemented and provides for a system which is easy to implement and which results in a system that may provide for low complexity management and control of the operation. Furthermore, time multiplexing may result in a particularly suitable segmentation for many channel signals. For example, a random access channel may advantageously be time multiplexed between base band processing means. If one base band processing means fails, the random access channel may not be available in some time intervals but typically mobile stations which are unsuccessful in accessing a base station retry at a later time which may be when the random access channel is available. Thus, only a small degradation results.

According to another feature of the invention, the means for segmenting is operable to segment the base band processing in the amplitude domain. This CE3091 6P provides for a low complexity and easy to implement means of segmentation which results in high performance. Furthermore, segmentation in the amplitude domain results in only a small degradation for some channel signals.

According to another feature of the invention, each base band processor means of the at least two base band processing means comprise reduced gain processing such that a combined gain of the at least two base band processing means corresponds to a desired gain. This allows for a low complexity and high performance way of segmenting a channel signal in the amplitude domain. For example, if a pilot channel signal is segmented over L base band processing means, each base band processing means may set a gain such as to generate a signal having an amplitude of A/L where A is the desired amplitude of the pilot channel signal. If a base band processing means fails the resulting amplitude will be reduced to (L- 1) A/L which depending on L will only result in a minor reduction of the amplitude of the pilot channel amplitude received by a mobile station.

According to another feature of the invention, the multi sector base station is a CDMA base station. The invention thus allow for an improved CDMA base station having increased reliability.

According to another feature of the invention, the means for segmenting is operable to segment the base band processing in the code domain. This provides for a low complexity and easy to implement means of segmentation which results in high performance. Furthermore, segmentation in the code domain results in only a small degradation for some channel signals.

According to another feature of the invention, the multi sector base station comprises means for detecting a fault of a base band processing means of the at least two base band processing means and the means for segmenting is CE3091 6P operable to segment the base band processing of the at least one channel signal over the remaining base band processing means of the at least two base band processing means. This allows for a failure of a base band processing means to readily be compensated for by modification of the segmentation. This allows for increased reliability and reduced impact of a failure. Additionally or alternatively it may allow for the reliability of the base station already having one failed base band processing means to be improved.

According to another feature of the invention, the means for segmenting is operable to segment the channel signal if it is a common channel signal and not to segment the channel signal if it is a user equipment dedicated channel signal.

The inventors have realised that the result of a failure of a base band processing means is different for different types of signal. Specifically, the inventors have realised that significantly increased reliability can be achieved by particularly segmenting common channel signals as the impact extends to a plurality of mobile stations. However, as the impact of a failure of a base band processing means on a user equipment dedicated channel typically only extends to that user equipment, the impact is significantly less. Hence, segmenting of channels depending on the type of channel allows for a simplified implementation and simplified control and management while achieving a desirable reliability.

A user equipment may typically be a wireless user equipment, mobile station, subscriber unit, communication terminal, personal digital assistant, laptop computer, embedded communication processor or any other communication element communicating over the air interface of the cellular communication system.

CE3091 6P According to another feature of the invention, the means for segmenting is operable to distribute user equipment dedicated channel signals of a single sector substantially equally over the plurality of base band processing means.

This allows for an improvement of the reliability of the base station and reduces the impact of a base band processing means failure.

According to another feature of the invention, the base band processing means are transmit base band processing means and the transceiver means are transmitter means. The invention may improve the transmit functionality of a base station and particularly may improve the reliability.

According to another feature of the invention, the transceiver means comprise means for combining a segment of the at least one channel signal from each of the second plurality of base band processing means to generate a combined channel signal. This allows for a suitable low complexity and high performance implementation.

According to another feature of the invention, the transceiver means comprise a plurality of intermediate transmit means coupled to a plurality of power amplifiers. This allows for a suitable low complexity and high performance implementation.

According to another feature of the invention, the base band processing means are receive base band processing means and the transceiver means are receiver means. The invention may improve the receive functionality of a base station and particularly may improve the reliability.

According to another feature of the invention, the base band processing means comprise both receive base band processing means and transmit base band processing means. Preferably, the invention may improve both the transmit CE3091 6P and receive functionality of a base station thereby increasing the reliability of the base station.

According to another feature of the invention, all of the plurality of base band processing means is coupled to a common data bus. This allows for a suitable low complexity and high performance implementation which particularly facilitates segmentation of the channel signal.

According to another feature of the invention, the at least two base band processing means comprise all of the plurality of base band processing means.

Preferably, the segmentation is across all the base band processing means whereby the reliability and impact of a failure of a base band processing means may be minimised.

According to second aspect of the invention, there is provided a method of operation for a multi sector base station for a cellular communication system, the multi-sector base station having a plurality of base band processing means and a plurality of transceiver means coupled to the base band processing means, each sector of the base station being assignable to a transceiver means of the plurality of transceiver means; the method comprising segmenting the base band processing of at least one channel signal for a sector of the base station over at least two of the plurality of base band processing means.

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

Brief Description of the Drawings

CE3091 6P An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 illustrates a block diagram of a transmit section of a multi sector base station in accordance with an embodiment of the invention; FIG. 2 illustrates a block diagram of a receive section of a multi sector base station in accordance with an embodiment of the invention; and FIG. 3 illustrates a block diagram of a transmit section of a multi sector UMTS base station in accordance with an embodiment of the invention.

Detailed Description of a Preferred Embodiment of the Invention The following description focuses on an embodiment of the invention applicable to a CDMA base station and in particular to a CDMA base station for a UMTS communication system. However, it will be appreciated that the invention is not limited to this application but may be applied to many other base stations.

FIG. 1 illustrates a transmit section of a multi sector base station in accordance with an embodiment of the invention.

In the illustrated embodiment, the base station 100 is a three sector base station comprising three antennas 101, 103, 105 with one for each sector. It will be appreciated that the invention is equally applicable to e.g. a base station using transmit diversity and having a plurality of antennas for each sector.

CE3091 6P Each of the antennas 101, 103, 105 is coupled to transceiver means which specifically are transmitter units 107, 109, 111. Each transmitter unit 107, 109, 111 is operable to perform the required Radio Frequency (RF) operations required for transmission of a base band signal in accordance with the specifications of the communication system. The transmitter units 107, 109, 111 typically comprise circuitry for up-converting, modulating and amplifying the base band signal as is well known to the person skilled in the art. In the shown embodiment, the transmitter units 107, 109, 111 further comprise a digital to analog converter and thus in the illustrated embodiment the transmitter units 107, 109, 111 perform operations on analog signals whereas the signal processing prior to the signals being fed to the transmitter units is digital.

In the shown embodiment, each sector is assigned to a single transmitter unit 107, 109, 111 and specifically each transmitter unit 107, 109, 111 is assigned to only one sector. In the preferred embodiment, the transmit section of the base station 100 further comprises one or more redundant transmitter units 113 which can be coupled to replace any of the other transmitter units 107, 109, 111 if a fault occurs in one of these.

The transmit section further comprises a plurality of base band processing means which in the specific embodiment corresponds to six base band processing units 115 - 125. The base band processing units 115 perform transmit base band processing of channel signals which are to be transmitted.

A channel signal will typically be received by the transmit section as one or more data streams corresponding to a logical or physical channel of the communication system. As will be appreciated by the person skilled in the art, the base band processing units 115 - 125 typically comprise functionality for e.g. coding, spreading and interleaving the data streams.

CE3091 6P In the preferred embodiment, the transmit section further comprises a segmenter 127 which is operable to segment at least one channel signal for a sector of the base station over at least two of the base band processing units - 125. The base band processing units 115 are coupled to a combiner 129 which is operable to combine the segments generated by the base band processing units 115 - 125 into a single common channel signal.

In the preferred embodiment, the outputs of the base band processing units may sum the signals for a particular antenna 115 - 125 and the combiner 129 may comprise a mesh network which is operable to send these sector signals to the corresponding transmitter units 107, 109, 111with the further combining function residing in the transmitter units (the combiner may be considered to comprise part of the functionality of the transmitter units 107, 109, 111).

In the preferred embodiment, the common channel signals are segmented by the segmenter 127 whereas the user equipment dedicated channels are not segmented but assigned to only one of the base band processing units 115 125. Specifically, all user equipment dedicated channels which are active for a given sector are distributed over the base band processing units 115 - 125 without being segmented. Thus each user equipment dedicated channel is allocated to a single base band processing unit 115 - 125. However, the combined group of user equipment dedicated channels of a sector are evenly distributed over the base band processing units 115 - 125. Accordingly, if a base band processing unit 115 - 125 fails, only approximately one sixth of the channels are affected by the failure.

However, all or most of common channel signals are in the described embodiment segmented by the segmenter 127 and preferably the segmentation is across all of the available base band processing units 115 - 125.

CE3091 6P A common channel signal that is advantageously segmented across a plurality of base band processing units 115 - 125 is a pilot channel signal. The pilot channel signal is used by all user equipment of the sector and neighbour sectors and cells to measure the signal strength of the base station. If the pilot channel signal is allocated to a single base band processing unit 115 - 125 as for the user equipment dedicated channels, a failure of this base band processing unit 115 - 125 results in a total loss of the pilot signals thereby impacting a large number of user equipment. Accordingly, the pilot channel signal is segmented over a plurality of base band processing units 115 - 125 such that the base band processing of the pilot channel signal is performed by more than one base band processing unit 115 - 125.

In the described embodiment, the segmentation is performed in the amplitude domain by each of the six base band processing units 115 processing a reduced amplitude pilot channel signal. Thus, the common pilot channel signal is preferably fed to all of the six base band processing units 115 - 125 which perform equivalent base band processing to generate equivalent output signals. However, each of the base bandprocessing units 115 - 125 generate an output signal having an amplitude of a sixth of the desired amplitude. In the described embodiment, the combiner 129 comprises an adder which adds the output signals of each of the base band processing unit 115 - 125. Thus a combined signal having the desired amplitude is generated.

It will be appreciated that in order for the common pilot channel signal to be processed in parallel for later summation, the processing of the base band processing units 115 - 125 must be such that this is feasible. Specifically, the base band processing units 115 - 125 preferably use linear and synchronized operations. For example, any spreading of the pilot channel signal performed by the base band processing units 115 is performed using the same spreading codes which are closely synchronised between the different base band processing units 115 - 125. This typically does not impose any CE3091 6P significantly stricter requirements as close synchronization is also required when multiple base band processing units process signals for the same sector.

Although additional processing is required by the parallel processing of the base band processing units 115 - 125 this is in the described embodiment limited to the common channel signals and therefore cause only a small overall increase in the required computational burden.

Furthermore, significantly increased flexibility, reliability and robustness is achieved by the described embodiment. For example, should one of the base band processing units 115 - 125 fail, this will typically cause an absence of the corresponding output signal (either as a direct consequence of the failure or due to protection circuitry). However, this will simply cause a slight reduction in the amplitude of the combined pilot channel signal which is transmitted.

Specifically, the signal strength of the common pilot channel signal will drop by 10 (6/5)= 0.8 dB which is acceptable in most situations and which will only have an effect on a small subset of user equipment located at the cell borders.

FIG. 2 illustrates a receive section of a multi sector base station in accordance with an embodiment of the invention.

In the illustrated embodiment, the base station 200 is a three sector base station comprising an antenna 201, 203, 205 for each sector. It will be appreciated that the invention is equally applicable to for example a base station using receive diversity and having a plurality of antennas for each sector.

Each of the antennas 201, 203, 205 is coupled to transceiver means which specifically are receiver units 207, 209, 211. Each receiver unit 207, 209, 211 is operable to perform the required Radio Frequency operations required for generating a base band signal from a radio signal received in accordance with CE3091 6P the specifications of the communication system. The receiver units 207, 209, 211 typically comprise circuitry for down-converting, filtering and amplifying the radio signal as is well known to the person skilled in the art. In the shown embodiment, the receiver units 207, 209, 211 further comprise an analog to digital converter, and in the illustrated embodiment the receiver units 207, 209, 211 perform operations on analog signals whereas the subsequent signal processing is of digital signals.

In the shown embodiment, each sector is assigned to a single receiver unit 207, 209, 211 and specifically each receiver unit 207, 209, 211 is assigned to only one sector. In the preferred embodiment, the receive section of the base station further comprises one or more redundant receiver units 213 which can be coupled to replace any of the other receiver units 207, 209, 211 if a fault occurs in one of these.

The receiver section further comprises a plurality of base band processing means which in the specific embodiment corresponds to six base band receive processing units 215 - 225. The base band receive processing units 215 - 225 perform receive base band processing of channel signals which are being received. As will be appreciated by the person skilled in the art, the base band receive processing units 215 - 225 typically comprise functionality for decoding, de-spreading and de-interleaving of data of the received signal.

In the preferred embodiment, the receive section further comprises a segmentation processor 227 which is operable to segment at least one channel signal for a sector of the base station over at least two of the base band receive processing units 215 - 225. The base band receive processing units 215 - 225 are coupled to a combiner 229 which is operable to combine the segments generated by the base band processing units 115 125 into a single common channel signal.

CE30916P As a specific example, a mesh network may be used to distribute each antenna signal to each base band receive processing unit 215 - 226 with each antenna signal comprising the sum of multiple channel signals. The segmentation may be performed by selectively enabling selected base band receive processing units 215 - 225 to process a channel signal. Thus the segmentation may in this embodiment be performed by the base band receive processing units 215 - 225.

In this embodiment, the segmentation processor 227 may thus be considered implemented by the group of base band receive processing units 215 - 225.

In the preferred embodiment, the common channel signals are segmented by the segmentation processor 227 whereas the user equipment dedicated channels are not segmented but assigned to one of the base band receive processing units 215 - 225. Specifically, all user equipment dedicated channels active in a given sector are distributed over the base band receive processing units 215 - 225 without being segmented. Thus each user equipment dedicated channel is allocated to a single segmentation processor 227 but the combined group of user equipment dedicated channels of a sector are evenly distributed over the base band receive processing units 215 - 225. Thus if a base band receive processing unit 215 - 225 fails, only approximately one sixth of the channels are affected by the failure.

However, all or most of the common channel signals are in the described embodiment segmented by the segmentation processor 227 and preferably the segmentation is across all of the base band receive processing units 215 225.

A common channel signal that is advantageously segmented across a plurality of base band receive processing units 215 - 225 is a random access channel signal. The random access channel signal is used by all user equipment of a sector to initially access the base station. If the random access channel signal is allocated to a single base band processing unit 115 - 125, a failure of this base band receive processing unit 215 225 results in a total loss of the CE30916P random access messages thereby impacting a large number of user equipment.

Accordingly, the random access channel signal is segmented over a plurality of base band receive processing units 215 - 225 such that the base band processing of the random access channel signal is performed by more than one base band receive processing unit 215 - 225.

In the described embodiment, the segmentation is performed in the time domain by each of the six base band receive processing units 215 - 225 taking turns in processing the random access channel for a short time interval. Thus, the random access channel signal is by the segmentation processor 227 alternately fed to one of the six base band receive processing units 215 - 225 such that at any given time instant, the random access channel is only processed by one of the base band receive processing units 215 - 225. Hence, in this embodiment, the random access channel is divided into suitably small time segments with each consecutive time segment being processed by a different base band receive processing unit 215 - 225.

In the described embodiment, the combiner 229 comprises a de-multiplexer which is operable to combine the time segment output signals of the base band receive processing units 215 - 225 into a single combined signal.

Thus, significantly increased flexibility, reliability and robustness is achieved by the described embodiment. During normal operation, the random access channel signal is continuously processed and received by alternating between the base band receive processing units 215 - 225. However, if one of the base band receive processing units 225 fails, the random access channel signal is only lost for the time segment in which the failed base band receive processing unit 225 should be processing the random access channel signal.

Consequently, only a sixth of the access messages will be lost in the described embodiment and thus a very gradual degradation is achieved.

CE3091 6P Furthermore, in communication systems such as UMTS, a user equipment which does not receive an acknowledgement in response to a transmitted access message transmits another access message after a delay. As only the time segments of the failed base band receive processing unit 225 are inoperative, it is likely that the second (or subsequent) access message is successfully received. Thus, it is unlikely that any user equipment access attempts fail as a direct consequence of the failure of the base band receive processing unit 225 and the impact of the fault is limited to a minor degradation of the capacity of the access channel.

In the preferred embodiment, the multi sector base station comprises both the transmit section illustrated in FIG. 1 and the receive section illustrated in FIG. 2.

In the preferred embodiment, the base station further comprises a fault controller for detecting that a fault has arisen in one of the base band processing means. Circuitry and methods for detecting faults of circuits and processors are well known in the field, and it is within the contemplation of the invention that any suitable approach can be used. If the fault controller detects the a fault has occurred in one of the base band processing means, it is furthermore operable to reconfigure the base station such that the base band processing of the channel signal is segmented over the remaining base band processing means.

Specifically, the fault controller may be coupled to the segmented 127 and/or segmentation processor 227 and may control the segmentation operation such that no segment is allocated for the failed unit. The fault controller may further be coupled to the combiner 129, 229 such that the combination can be modified to reflect the altered segmentation. Preferably, the fault controller is also coupled to the base band processing means such that parameters of the processing can be amended to reflect the changed segmentation.

CE3091 6P For example, for the transmit section of FIG. 1, a fault of one of the base band processing units 115 - 125 can be detected by a lack of output signal. For example, the fault controller may detect that base band processing unit 125 does not produce an output signal. Accordingly, base band processing unit 125 may be shut down. The fault controller will consequently amend the segmented 117 such that the channel signal is only fed to the remaining base band processing units 115 - 123. The fault controller will furthermore amend the gain of each base band processing units 115 - 123 from one sixth of the desired gain to one fifth of the desired gain. Accordingly, a very flexible and fast redistribution and compensation of the channel signal is achieved resulting in a minimization of the impact of the failure.

Similarly for the receive section of FIG. 2, a failure of one of the base band receive processing units 215 - 225 may be detected and notified to the fault controller. In response, the fault controller may control the segmentation processor 217 to change the segmentation such that only five segments are generated instead of six segments (for each cycle or time frame). Thus, the channel signal may be distributed over five base band receive processing units 215 - 223 rather than the original six base band receive processing units 215 - 225. The segmentation processor accordingly changes the time segment interval (and/or the repetition interval) such that a continuous random channel signal is achieved. In addition, the base band receive processing units 215 - 223 and the combiner 229 are controlled to operate according to the amended segmentation. Accordingly, a very flexible and fast redistribution and compensation of the channel signal is achieved resulting in a minimization of the impact of the failure.

In some embodiments, the base station is a CDMA base station and the segmentation is additionally or alternatively performed in the code domain.

For example, a random access channel may be provided which uses a plurality CE3091 6P of alternative spreading codes. In this case, the channel may be segmented by each of the base band processing means processing one of the plurality of spreading codes. If a base band processing means fails, only the access messages using the spreading code processed by that base band processing means will be lost and thus only a gradual degradation will occur.

As will be appreciated, the embodiments described specifically allow for reliable performance without requiring additional redundant base band processing means. Thus, not only may improved reliability be achieved but also reduced cost and complexity of the base station is possible.

In the following, a more detailed description of a preferred embodiment of a transmit section of a multi sector base station is provided. FIG. 3 illustrates a block diagram of the transmit section 300 of a six sector UMTS base station in accordance with an embodiment of the invention.

The transmit section comprises a segmentation controller 301 which receives data streams for transmission in logical and physical channels of the UMTS communication system. The segmentation controller 301 is coupled to a parallel data bus 303 to which is coupled a plurality of base band units 305- 315.

The base band units 305-315 are responsible for the baseband processing of individual signal channels. The base band unit processing resource is in the preferred embodiment pooled across all sectors of the base station such that any channel signal from any sector can be processed on any base band unit 305-315. This provides for a "soft fail" redundancy model where a failure of one base band unit 305-315 leads to a proportionate reduction in capacity rather than a sector shut-down. The user data for a given channel signal is encoded, spread and modulated in the base band unit 305315. At the output of the base band units 305-315, all base band modulated signals pertaining to each of the CE3091 6P six sectors are summed together. Hence each base band unit 305-315 has one output for each of the sectors (each output may comprise signals for multiple antennas). In addition, each base band unit 305-315 has an output for a redundant transmit unit.

An output of each of the base band units 305 315 is coupled to a transmit unit 317-327. In the shown embodiment, the base station comprises one transmit unit 317-327 for each sector. The main function of the transmit units 317- 327 is baseband and IF processing of the aggregate CDMA signal such as filtering and up-conversion. The aggregation, i.e. the second stage of summation of partially summed signals from each of the base band units, is performed at the transmit unit input. Hence, the transmit unit 317-327 represents a sector specific resource. Therefore, redundancy of the transmit units 317-327 is in the preferred embodiment achieved by including an additional transmit unit 329.

Each base band unit 305-315 connects to each transmit unit 317-327 including the redundant module. This fully meshed cross connect is realised by the base band bus 329. The base band bus 329 together with the digital interfaces on the base band units 305-315 and the transmit units 317-327 guarantee clock accurate time alignment of physical channels directed to the input of a particular transmit unit 317-327. This is required to maintain orthogonality of physical channels generated on different base band units but transmitted in the same sector.

The transmit units 317-327 are coupled to a transmit switch 331 which is further coupled to one power amplifier 333-343 for each sector. Each of the six power amplifiers 333-343 is connected to a sector antenna (not shown). The transmit switch 331 is operable to couple any transmit unit 317-327, including the redundant unit 329, to any power amplifier 333-343. Thus the transmit switch 331 allows for a flexible configuration which includes the possibility of replacing a faulty transmitter unit by a redundant unit 329.

CE3091 6P In the preferred embodiment, each base band unit is capable of supporting at least the following UMTS downlink channel types: À Downlink Dedicated Physical Channel (DPCH) À Primary Common Control Physical Channel (PCCPCH) À Secondary Common Control Physical Channel (SCCPCH) À Physical Downlink Shared Channel (PDSCH) À Page Indicator Channel (PICH) À Common Pilot Channel (CPICH) À Primary Synchronisation Channel (P-SCH) Secondary Synchronisation Channel (S-SCH) À Acquisition Indication Channel (AICH) In the uplink, the base band unit is capable of supporting at least the following 3GPP FDD channel types.

Uplink Dedicated Physical Data Channel (DPDCH) Uplink Dedicated Physical Control Channel (DPCCH) Physical Random Access Channel (PRACH) Physical Common Packet Channel (PCPCH) RACH and CPCH channels are shared between many users.

A typical requirement for a UMTS base station is that the downtime per year caused by outages must be less than one minute per year. A suitable outage definition is that the total outage for a base station is the total loss of the provisioned traffic capacity due to causes affecting the base station for more than 30 seconds and the partial outage of a base station is a loss of 10% or more of the provisioned traffic capacity for more than 30 seconds.

The downtime is calculated as follows: CE3091 6P Downtime = Time for Total Outages + (Time for Partial Outages x fraction of provisioned capacity loss).

The downtime is measured from the moment when the outage event occurs to the moment when the base station is ready to process calls at the capacity level.

To clarify the implications, the following example is provided. As a result of a base band unit failure, the base station may fail to support any of the physical channels, including the common ones, in both uplink and downlink. The user equipment previously served by this sector are returned to the idle mode upon expiry of their T313 counters (see section 8.5.6 "Radio Link Failure Criteria" in 3GPP Technical Specification TS 25. 331) and upon the expiry of the V301 counting number of retransmissions of the RRC CONNECTION REESTABLISHMENT REQUEST message. Thus, all circuit and packet calls in the sector will be dropped. After 30 see, the base station restores the common channels (i.e. starts transmitting CPICH, PCCPCH etc in the downlink and is ready to detect RACH in the uplink) as well as its capability to process the dedicated channels. The user equipments will have to re establish the RRC connections, and there will be an additional time associated with this procedure. This is clearly undesirable as the users will need to go through a number of procedures to bring them back to the connected mode from the idle mode.

As a different example, after a base band unit failure, the CPICH in one of the sectors disappears, but the dedicated channels in this sector continue to run because they are supported by a different base band unit. Even in this case, there is a strong likelihood that all the user equipments will disconnect from this sector and/or drop their calls. This is because of the following two reasons: (a) a good user equipment receiver design will use CPICH for channel estimation, so with no CPICH, a loss of sync may be declared causing a return CE3091 6P to the idle state and (b) the Ec/I measurements for handover are performed on the CPICH, so even if the call is not dropped, the network may eventually switch the user equipment to another cell.

Perhaps, a more serious issue is also related to the complete disappearance of the common pilot CPICH in the sector after the failure. During the next 30 seconds, some of the user equipments will connect to the neighboring cells and will be power-controlled by these. Close to the base station with the failed sector, such mobiles will be transmitting at maximum power. When the sector capacity and the CPICH are restored, the receiver of the base station will find itself overloaded by high receive levels, and may consequently be incapable of normal operation.

It is thus desirable to limit the number of dropped calls and to restore the normal operation of the calls in connected mode as quickly as possible. It is specifically desirable to maintain the CPICH level as much as possible in all sectors.

This is in the preferred embodiment achieved by segmenting the common channels (uplink and downlink) and distributing the segments across all the installed base band units. The segmentation can be carried out either in the amplitude domain, the code domain or the time domain For the dedicated channels, it is preferred that no segmentation is provided.

Instead, it is preferred that the dedicated channels in a given sector are distributed as evenly as possible across all the base band units. Hence, each dedicated channel is processed by a single base band unit only. Consider an example of 60 active radio links supported by 60 dedicated channels in a sector. The number of base band units is six, and preferably each base band unit has 60/6=10 dedicated channels in the sector. Of course, it is not always possible to divide the total number of dedicated channels in a sector equally CE3091 6P between the base band units. This will be exacerbated by the fact that softer handover legs and multi-code components for a given radio link will have to be allocated to the same base band unit. The approximately equal distribution will maximise the number of dedicated channels that are maintained immediately after a failure of any base band unit.

Segmentation in the amplitude domain is preferred for common channels such as PCCPCH, SCCPCH, CPICH, and PICH. This is equivalent to generating the same signal at each base band unit with the appropriate scaling, so that after the summation in the input adder in the transmit units the required output power is obtained. Thus for a given sector, each base band unit generates identical PCCPCH at 1/6 of the required power. These 6 physical channels are routed over the base band bus to the appropriate transmit unit, where they are added together.

The CPICH is handled in a similar way with the only difference being that there is no data carried on this channel and therefore no requirement for a data input to the base band units. A failure of one base band unit will result in a reduction of CPICH amplitude by 1/6. The CPICH coverage area accordingly shrinks, but the majority of the user equipments are still served by the sector. It may be necessary to adjust power of the maintained dedicated channels as this is referenced to the CPICH. Alternatively, the need for adjustment may simply be ignored for a given period. After this given period, the correct CPICH level is restored, i.e. each of the base band unit cards remaining in operation now generates 1/5 of the required CPICH power.

Before this happens, the user equipments at cell edge may switch to another cell, but because at least 5/6 of the CPICH level is maintained, the missed handover-like performance does not occur when the CPICH is back to full power operation.

CE3091 6P The approach requires extra processing resource in the base band units but the extra overhead is relatively small and will be acceptable in most applications.

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 and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term comprising does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

CE3091 6P

Claims (21)

1. A multi sector base station for a cellular communication system comprising: a plurality of base band processing means; a plurality of transceiver means coupled to the base band processing means, each sector of the base station being assignable to a transceiver means of the plurality of transceiver means; means for segmenting the base band processing of at least one channel signal for a sector of the base station over at least two of the plurality of base band processing means.

2. A multi sector base station as claimed in claim 1 wherein the at least one channel signal is at least one common channel signal.

3. A multi sector base station as claimed in claim 2 wherein the at least one common channel signal is a common pilot channel signal.

4. A multi sector base station as claimed in claim 2 wherein the at least one common channel signal is a common random access signal.

5. A multi sector base station as claimed in any previous claim wherein the means for segmenting is operable to segment the base band processing in the time domain.

6. A multi sector base station as claimed in claim 5 wherein the segmentation is by time multiplexing of the at least one channel signal between the at least two base band processing means.

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7. A multi sector base station as claimed in any previous claim wherein the means for segmenting is operable to segment the base band processing in the amplitude domain.

8. A multi sector base station as claimed in claim 7 wherein each base band processor means of at least two base band processing means comprise reduced gain processing such that a combined gain of the at least two base band processing means corresponds to a desired gain

9. A multi sector base station as claimed in any previous claim wherein the multi sector base station is a CDMA base station.

10. A multi sector base station as claimed in any previous claim wherein the means for segmenting is operable to segment the base band processing in the code domain.

11. A multi sector base station as claimed in any previous claim comprising means for detecting a fault of a base band processing means of the at least two base band processing means and wherein the means for segmenting is operable to segment the base band processing of the at least one channel signal over the remaining base band processing means of the at least two base band processing means.

12. A multi sector base station as claimed in any previous claim wherein the means for segmenting is operable to segment the channel signal if it is a common channel signal and not to segment the channel signal if it is a user equipment dedicated channel signal.

13. A multi sector base station as claimed in any previous claim wherein the means for segmenting is operable to distribute user equipment dedicated CE3091 6P channel signals of a single sector substantially equally over the plurality of base band processing means.

14. A multi sector base station as claimed in any previous claim wherein the base band processing means are transmit base band processing means and the transceiver means are transmitter means.

15. A multi sector base station as claimed in claim 14 wherein the transceiver means comprise means for combining a segment of the at least one channel signal from each of the second plurality of base band processing means to generate a combined channel signal.

16. A multi sector base station as claimed in claim 14 or 15 wherein the transceiver means comprise a plurality of intermediate transmit means coupled to a plurality of power amplifiers.

17. A multi sector base station as claimed in any previous claim 1 to 13 wherein the base band processing means are receive base band processing means and the transceiver means are receiver means.

18. A multi sector base station as claimed in any previous claim 1 to 13 wherein the base band processing means comprise both receive base band processing means and transmit base band processing means.

19. A multi sector base station as claimed in any previous claim wherein all of the plurality of base band processing means are coupled to a common data bus.

20. A multi sector base station as claimed in any previous claim wherein the at least two base band processing means comprise all of the plurality of base band processing means.

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21. A method of operation for a multi sector base station for a cellular communication system, the multi-sector base station having a plurality of base band processing means and a plurality of transceiver means coupled to the base band processing means, each sector of the base station being assignable to a transceiver means of the plurality of transceiver means; the method comprising segmenting the base band processing of at least one channel signal for a sector of the base station over at least two of the plurality of base band processing means.

CE3091 6P