GB2443236A

GB2443236A - Scrambling code assignment in a cellular communication system

Info

Publication number: GB2443236A
Application number: GB0621379A
Authority: GB
Inventors: Gerard Foster; Luis Lopes; David Padfield
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2006-10-27
Filing date: 2006-10-27
Publication date: 2008-04-30
Anticipated expiration: 2026-10-27
Also published as: WO2008057658A2; GB2443236B; WO2008057658A3; GB0621379D0

Abstract

A cellular communication system comprises a plurality of macrocells underlayed by a plurality of underlay microcells. A scrambling code unit (113) comprises a subset processor (201) which divides a set of scrambling codes assigned to the underlay cells into distinct subsets. A scramble code group processor (203) assigns a scrambling code group comprising a plurality of scrambling codes to underlay cells within a region. Each group is a unique group within the region and comprises one scrambling code from each of the subsets. A base station (109) for each of the underlay cells is arranged to transmit a pilot signal on each scrambling code of the group assigned to the underlay cell. Measurements by a remote station (111) may detect specific scrambling codes within each subset thereby allowing a detection of an underlay cell while using a reduced number of scrambling codes. Furthermore, a reduced neighbour list may be maintained.

Description

A CELLULAR COMMUNICATION SYSTEM

Field of the invention

The invention relates to identification of cells in a cellular communication system and in particular, but not exclusively, to identification of and handover to underlay cells in a Code Division Multiple Access (CDMA) cellular communication system.

Background of the Invention

A method which has been used to increase the capacity of cellular communication systems is the concept of hierarchical cells wherein a macrocell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macrocell. In this way, smaller cells, known as microcells or picocells (or even femtocells), are located within larger macrocells. The microcells and picocells have much smaller coverage thereby allowing a much closer reuse of resources. Frequently, the macrocells are used to provide coverage over a large area, and microcells and picocells are used to provide additional capacity in e.g. densely populated areas and hotspots.

Furthermore, picocells can also be used to provide coverage in specific locations such as within a residential home or office.

In order to efficiently exploit the additional resource, it is important that handover performance between the macrocell layer and the underlying layer is optimised. The process of handover can be separated into three phases. Firstly, identifying that a handover might be required, secondly, identifying a suitable handover candidate and finally, switching the mobile user from one base station to another.

Currently 3rd generation (3G) cellular communication systems based on Code Division Multiple Access (CDMA) technology (such as the Universal Mobile Telecommunication System (UMTS)) are being deployed. In these systems, each cell is allocated a unique scrambling code which is used to spread the air interface signals in order to provide cell separation. The User Equipments (UE5) of such systems receive a neighbour list identifying a number of scrambling codes for neighbour cells and the UEs apply the neighbour scrambling codes to the received signal using in order to measure receive levels. The signal level for each neighbour scrambling code is measured and reported back to the network. The network then uses these measurements to determine if a handover should be performed, and if so to which cell the UE should be handed over.

The current trend is moving towards introducing a large number of picocells to 3G systems. For example, it is envisaged that residential access points may be deployed having only a target coverage area of a single residential dwelling or house. A widespread introduction of such systems would result in a very large number of small underlay cells within a single macrocell.

However, underlaying a macrolayer of a 3G network with a picocell (or microcell) layer creates several issues for the CE1 6228EP management of handovers of tiEs between the different layers.

In particular, 3G communication systems are developed based on each cell having a relatively low number of neighbours and extending the current approach to scenarios wherein the tiE may need to consider large numbers of potential neighbour cells is not practical.

One problem of extending current approaches to scenarios where there are many underlaying picocells is how to uniquely and efficiently identify a picocell (or microcell) Specifically, it is not practically feasible to list every underlay cell as a potential neighbour of the macrocell as this would require very large neighbour lists. These large neighbour lists would e.g. result in the neighbour list exceeding the maximum allowable number of neighbours in the list, slow tiE measurement performance as a large number of measurements would need to be made. It would furthermore require significant operations and management resource in order to configure each macrocell with a large number of neighbours.

Hence, an improved cellular communication system would be advantageous and in particular a system allowing increased flexibility, improved suitability for large numbers of potential handover target/neighbour cells, improved suitability for underlay/overlay handovers, reduced neighbour lists, increased practicality, reduced measurement requirements and/or improved performance would be advantageous.

CE16228EP

Summary of the Invention

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

According to a first aspect of the invention there is provided a cellular communication system comprising a plurality of macrocells underlayed by a plurality of underlay cells, the system comprising: means for dividing a set of scrambling codes into distinct subsets; means for assigning a scrambling code group comprising a plurality of scrambling codes to underlay cells within a region, each scrambling code group being a unique group within the region and comprising one scrambling code from each of the subsets; and a base station for each of the underlay cells arranged to transmit a pilot signal on each scrambling code of the scrambling code group assigned to the underlay cell.

The invention may allow improved identification of underlay cells thereby improving e.g. handover performance. In particular, the invention may allow improved identification of an underlay target handover cell for a remote station currently served by a macro-cell. The invention may allow highly robust underlay cell identification while using a reduced amount of resources. In particular, the invention may require fewer scrambling codes while still allowing a given number of underlay cells to be identified.

The underlay cells may e.g. be microcells, picocells and/or femtocells. The distinct subsets do not share any scrambling CE1622BEP codes i.e. each of the scrambling codes is part of one and only one of the subsets. The cellular communication system may be a ODMA cellular communication system such as UNTS.

The remote station may for example be a User Equipment or a mobile communication unit, e.g. of a 3rd generation cellular communication system.

The pilot signals for the scrambling codes may be transmitted on the same frequency or may be transmitted on two or more different frequencies. Also, the pilot signals for the scrambling codes of the scrambling code groups may be transmitted on the same frequency as a macrocell scrambling code pilot signal or may be transmitted on a different frequency.

According to an optional feature of the invention, the cellular communication further comprises a network controller arranged to generate a neighbour list for a remote station in a macro-cell, the network controller comprising: sequential means for sequentially including different subsets of scrambling codes in the neighbour list, each neighbour list comprising one subset of the distinct subsets.

The feature may allow an efficient identification of an underlay cell from a large number of cells while requiring only a low number of cells in the neighbour list. Thus, the approach allows a limited neighbour list to be used and in particular reduces the number of scrambling codes that need to be included in the neighbour list in order to identify underlay cells.

CE1 6228EP According to an optional feature of the invention, the network controller furthermore comprises means for receiving measurement reports for scrambling codes of the neighbour list from the remote station; and the communication system further comprises identifying means for identifying a first underlay cell in response to measurement reports for scrambling codes of different subsets.

An efficient, reliable and/or low resource means of identifying an underlay cell may be achieved. Specifically, remote station measurements based on neighbour lists comprising only a limited number of neighbour scrambling codes can be used to identify an underlay cell among a large number of underlay cells (the number of underlay cells may e.g. substantially exceed the number of scrambling codes that can be included in a neighbour list).

According to an optional feature of the invention, the cellular communication system further comprises means for generating a relocation request for a handover of the remote station to the first underlay cell in response to the identification of the first underlay cell.

An improved handover performance can be achieved and/or less resource demanding handovers can be achieved. The means for generating the relocation request may be co-located with the identifying means and the relocation request may comprise an identifier for the identified underlay cell.

According to another aspect of the invention, there is provided a network element for a cellular communication system comprising a plurality of macrocells underlayed by a CE1 6228EP plurality of underlay cells, the network element comprising: means for dividing a set of scrambling codes into distinct subsets; means for assigning a scrambling code group comprising a plurality of scrambling codes to underlay cells within a region, each scrambling code group being a unique group within the region and comprising one scrambling code from each of the subsets.

According to another aspect of the invention, there is provided a method of operation in a cellular communication system comprising a plurality of macrocells underlayed by a plurality of underlay cells, the method comprising: dividing a set of scrambling codes into distinct subsets; assigning a scrambling code group comprising a plurality of scrambling codes to underlay cells within a region, each scrambling code group being a unique group within the region and comprising one scrambling code from each of the subsets; and a base station for each of the underlay cells transmitting a pilot signal on each scrambling code of the scrambling code group assigned to the underlay cell.

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

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which CE1 6228EP FIG. 1 illustrates an example of a cellular communication system in accordance with some embodiments of the invention; FIG. 2 illustrates an example of a scrambling code controller in accordance with some embodiments of the invention; FIG. 3 illustrates an example of a Radio Network Controller in accordance with some embodiments of the invention; FIG. 4 illustrates an example of a cellular communication system in accordance with some embodiments of the invention; FIG. 5 illustrates an example of a cellular communication system in accordance with some embodiments of the invention; FIG. 6 illustrates an example of a cellular communication system in accordance with some embodiments of the invention; and FIG. 7 illustrates an example of a method of operation for a cellular communication system in accordance with some embodiments of the invention.

Detailed Description of Some Embodiments of the Invention The following description focuses on embodiments of the invention applicable to a CDNA cellular communication system and in particular to a 3'' Generation Cellular communication system such as a UMTS System. However, it will be appreciated that the invention is not limited to this CE1 6228EP application but may be applied to many other communication systems.

FIG. 1 illustrates an example of a cellular communication system in accordance with some embodiments of the invention.

In the system, a macro-layer is formed by a macrocells supported by base stations. Furthermore, an underlay layer of picocells are supported by a large number of picocell base stations. Specifically, each picocell may have an intended coverage area of a single house or dwelling, and for a typical macrocell coverage area of 10 to 30 km there may be hundreds or even thousands of picocells each supported by an individual picocell base station.

In the system, the macro base stations each have a cell separation code in the form of a scrambling code which is unique within the given reuse area. Specifically the macro base stations have an assigned scrambling code which is unique within the reuse area such that a set of defined neighbours for each cell always have unique macro cell scrambling codes. Accordingly, the neighbour lists transmitted by the base stations comprise indications of macro-cells which all have different cell scrambling codes.

In contrast, the picocell base stations can use shared scrambling codes where the scrambling codes are shared between a plurality of picocell base stations within the reuse area and specifically within the area that is controlled by the macro RNC. Also, within a given macrocell picocells may share scrambling codes. By sharing a scrambling code between a plurality of picocell base stations, a much reduced number of scrambling codes are required by the system. Furthermore, by keeping the number GEl 6228EP of scrambling codes low, the number of scrambling codes that must be monitored by the remote station for handover determination can be reduced substantially thereby reducing the measurement time, power consumption and/or complexity of the remote station.

FIG. 1 illustrates a macro-base station 101 which supports a macrocell with a typical coverage area of 10-30 kilometres.

The macro base station 101 is coupled to a macro RNC 103 which is furthermore coupled to other macro base stations (not shown) . The macro RNC 103 is also coupled to a core network 105 which interfaces to other radio access networks and RNC5 (not shown) . In particular, the core network 105 is coupled to a pico-RNC 107. The pico-RNC 107 is coupled to a plurality of picocell base stations 109 (of which three are shown in FIG. 9) . Each of the picocell base stations 109 supports a picocell having a coverage area of typically 20 to 100 meters. The picocell base stations 109 implement the required functionality of a UMTS base station in order to support UMTS air interface communications within the picocell. However, in contrast to conventional UMTS base stations, the picocell base stations 109 can use shared scrambling codes assigned to the picocell base stations in accordance with rules that will be described later.

In the example, a remote station 111 is located within the macrocell of the macro base station 101 and is currently supported by the macro base station 101. However, if the remote station 111 is within the coverage area of a picocell it may perform a handover to the corresponding picocell base station 109.

GEl 6228EP In a typical scenario, the picocells are not overlapping but rather form a pattern of individual islands of coverage within the coverage area of the macrocell. Accordingly, when the remote station 111 detects the presence of a shared scrambling code it is likely to indicate that the remote station 111 has entered the coverage area of a picocell (e.g. the remote station has moved into a house covered by a single local picocell base station) . As the interference between different picocell base stations is low, this is a strong indication that the picocell base station can efficiently support the communication of the remote station 111.

In the communication system of FIG. 1, a plurality of shared scrambling codes are assigned to each picocell base station 109 in accordance with an assignment algorithm that allows a detected picocell base station to be effectively identified with a high probability. Specifically, the communication system comprises a scrambling code controller 113 which allocates scrambling codes to the individual picocell base stations 109 as will be described in the following.

In the system, a set of scrambling codes used by the picocells are divided into distinct subsets. A group of scrambling codes is assigned to each picocell such that each group has a unique set of scrambling codes and includes one scrambling code from each subset. Thus, although the scrambling codes may be shared between a plurality of picocells, the combination of scrambling codes in each group is unique.

CE1 6228EP Specifically, the pico-layer is allocated a set of scrambling codes which are not used in the macro layer.

Furthermore, this set of scrambling codes is divided into a number of subsets of equal or different numbers of scrambling codes such that none of the allocated scrambling codes appear in more than one subset. Each picocell is then allocated a tuple/group of scrambling codes with exactly one code being assigned from each subset and such that the resultant tuple is unique between all picocells within the reuse region. The reuse region may for example be the coverage area supported by an RNC and/or the algorithm may ensure that the tuples are unique between all picocells which share macro neighbours (e.g. within each macrocell) As a specific example, 32 scrambling codes may be assigned to the pico-layer. The pico-layer can e.g. divide these into 2 subsets of 16 scrambling codes, {xl, ... , x16} and {yl, ...

y16}. Each picocell underlying a given macrocell will be assigned a unique tuple of scrambling codes (xj,yj) such that no other picocell under the same macrocell has the same tuple. In this case 256 combinations of (x1,y3) exist so 256 picocells could be defined under a single macrocell.

In the system, the picocell base stations 109 transmit a pilot or beacon signal for each of the scrambling codes of the group assigned to the picocell. Furthermore, as will be described later, the remote stations are controlled to sequentially measure different scrambling codes and by comparing the detected scrambling codes to the unique groups of scrambling codes for each picocell, the identity of the detected picocell can be determined.

CE1 6228EP FIG. 2 illustrates the scrambling code controller 113 in more detail. For clarity, the scrambling code controller 113 is in FIG. 1 shown as a separate functional element coupled to the core network 105. However, it will be appreciated that in many embodiments, the scrambling code controller 113 will be part of a network element of the core network 105.

Specifically the scrambling code controller 113 can be part of a Mobile Switching Centre (MSC) or an Operations and Management Centre (OMC) of the core network 105.

The scrambling code controller 113 comprises a subset processor 201 which is provided with a set of scrambling codes that are allocated to the picocell layer. The subset processor 201 divides the set of scrambling codes into distinct subsets such that each scrambling code of the set is allocated to one and only one of the subsets.

The subset processor 201 is coupled to a scrambling code group processor 203 which generates unique (within the reuse region) groups of scrambling codes where each group comprises one scrambling code selected from each subset generated by the subset processor 201. Each of the unique groups is assigned to a picocell. Accordingly, for each picocell base station 109 the scrambling code group processor 209 generates a configuration message which is transmitted to the appropriate picocell base station 109 via a core network interface 205 that interfaces the scrambling code controller 113 to the core network 105.

The picocell base stations 109 receive the configuration messages and proceed to transmit a pilot signal on each of the scrambling codes of the assigned group.

CE1 6228EP Furthermore, the scrambling code controller 113 generates a message which contains all the unique groups of scrambling codes as well as the associated base station identifier. The message is transmitted to the macro RNC 103. The macro RNC 103 then configures the remote station 111 to make measurements of the picocell scrambling codes in one of the subsets at a time.

FIG. 3 illustrates an example of the macro RNC 103 in more detail. The RNC 103 comprises a core network interface 301 which interfaces the RNC 103 to the core network 105 and which specifically receives the message containing the association between scrambling code groups and picocell base station identifiers from the scrambling code controller 113.

The core network interface 301 is coupled to a neighbour list processor 303 which generates a neighbour list for the remote stations supported by the macrocell of the macro base station 101. The neighbour list processor 303 is arranged to include scrambling codes for the picocells in the neighbour list together with scrambling codes for neighbour macrocells. The neighbour list processor 303 at any given time only includes picocell scrambling codes from one of the distinct subsets determined by the spreading code controller 113. Thus, the macro RNC 103 configures the remote stations of the macrocell to monitor scrambling codes of one subset at a time. Furthermore, the neighbour list processor 303 is arranged to sequentially include different subsets of scrambling codes in the neighbour list while maintaining that each neighbour list comprises only one subset of the distinct subsets.

GEl 6228EP E.g. in the example where 32 scrambling codes were divided into two subsets, the remote station 111 is configured to measure the 16 scrambling codes of the first upset in a first time interval and to measure the 16 scrambling codes of the second subset in the following time interval.

If the remote station 111 detects a sufficiently strong receive signal level for one of the picocell scrambling codes, it generates a measurement report which indicates that the scrambling code has been detected. The measurement report is transmitted from the remote station 111 to the macro base station 101 which forwards it to the macro RNC 103. The remote station 111 can specifically measure the receive level for the scrambling codes and compare this to a predetermined threshold. If the receive level exceeds the predetermined threshold, the presence of the scrambling code is considered to have been detected.

It will be appreciated that although the description focuses on event triggered measurement reports, some or all of the measurement reports may be non-event triggered such as periodic measurement reports.

In the RNC 103, the measurement report is received by a measurement receiver 307 coupled to the base station interface 305. The measurement receiver 307 is coupled to an identification processor 309 which is operable to identify a first underlay cell in response to the measurement reports received from the remote station 111.

GEl 6228EP For example, the identification processor 309 may compare the detected picocell scrambling codes to the unique groups of scrambling codes in order to identify the detected picocells. E.g. in the previous example of two subsets of 32 scrambling codes, the remote station 111 may report that it has detected scrambling code x1 in the first time interval and Y2 in a second time interval. The identification processor 309 then determines which of the picocell base stations 109 has been assigned the scrambling code group containing (x1, Y2) and it then retrieves the base station identifier (e.g. the Base Station Identity Code, BSIC) stored for this group.

The identification processor 309 is coupled to a handover controller 311 which is provided with the identifier of the target picocell base station 109. The handover controller 311 is coupled to the base station interface 305 and to the core network interface 301. Upon receiving the identifier, the handover controller 311 proceeds to initiate a handover of the remote station 111 to the identified picocell base station 109.

In particular, the handover controller 311 generates a relocation request message which is transmitted to the identified picocell base station 109 by the core network interface 301. If the identified picocell base station 109 accepts the relocation request, it transmits a relocation acknowledgement message back to the RNC 103. In response, the handover controller 311 generates a handover command message which is transmitted to the remote station 111 by the base station 101. When the remote station 111 receives the handover command, it proceeds to handover to the CE1 6228EP identified picocell base station 109 using typical UMTS handover procedures.

The described approach allows an efficient detection and identification of underlay cells while maintaining short neighbour lists. Furthermore, the number of scrambling codes that must be monitored by the remote stations at any given time can be kept low even for large number of possible handover picocells.

In some embodiments, the identification processor 309 may be coupled to the neighbour list processor 303 which accordingly may select the scrambling codes to include in the neighbour list in response to the measurement reports that are received from the remote station(s) . Specifically, the subset included in the neighbour list may be changed when one of the scrambling codes from the current subset of the neighbour list is detected by a remote station.

Thus, when a remote station currently served by a macrocell reports a measurement for one of the scrambling codes of the subsets in its current neighbour list (indicating that this scrambling code has been detected), the macro RNC 103 records the reported scrambling code and configures a new neighbour list for the remote station using the next subset of scrambling codes. This process of reporting measurements and updating the neighbour list to a new subset of scrambling codes can continue until each of the subsets has elicited a measurement report from the remote station. The identification processor 309 accordingly records a sequence of detected scrambling codes resulting in a detected group GEl 6228EP of scrambling codes which uniquely (within the reuse region) identifies a local picocell. Accordingly, a picocell handover target has been identified.

If no scrambling code is detected within a given time interval for a given subset, the RNC 103 may abandon the current detection process and clear the record of the already detected scrambling codes. The neighbour list may in such a case maintain the current subset or revert to the original subset. This time out functionality may prevent that a spurious detection results from a slow detection process that allows a remote station to move between different picocells before the completion of the detection process.

In some embodiments, the measurement reports may be event triggered but in other embodiments some or all of the measurement reports may be non-event triggered measurement reports. For example, the measurement of the first subset may be event triggered whereas the following measurements may be e.g. period triggered. This may be advantageous as the first event triggered measurement report indicates that a potential handover target has been detected and the following measurement reports mainly provide a confirmation and identification of the detected pilot signal.

In some embodiments, the neighbour lists comprising the subsequent subsets used to identify the pilot signal may also include one or more scrambling codes from the detected group. This would allow the subsequent measurements to monitor the solution space. In particular, it would allow a CE1 6228EP simultaneous measurement of all the scrambling codes of the target picocell.

It will be appreciated that in some embodiments, the detection of a scrambling code simply comprises comparing the received signal level of a scrambling code to a predetermined threshold. However, alternatively or additionally, the received signal levels for different scrambling codes of a subset may be compared to each other.

For example, if a plurality of scrambling codes have receive levels exceeding the predetermined threshold, the detected scrambling code may be determined as the scrambling code having the highest receive level.

It will be appreciated that the transmit power used for different scrambling codes may be substantially identical but does not need to beso. For example, the first set of scrambling codes of the initial subset (i.e. when no scrambling codes have been detected) could be transmitted at lower power than subsequent subsets (when at least one scrambling code has been detected) . This will increase the probability that the subsequent scrambling codes will be detected following an initial detection of the presence of a picocell.

Each of the picocells may have assigned a pilot scrambling code which is different within a picocell reuse region. For example, a reuse pattern of scrambling codes may be applied to the picocells such that a remote station currently in a picocell can be provided with a neighbour list comprising an individual scrambling code for each of the neighbour picocells. This approach may allow efficient handover CE1 6228EP between picocells based on conventional tiNTS handover algorithms using unique neighbour scrambling codes. It will furthermore be appreciated that in some embodiments these picocell neighbour scrambling codes may be scrambling codes of the subsets used to form the unique groups. For example, a first subset of scrambling codes may be assigned to picocells such that neighboring picocells are always assigned different scrambling codes from the first subset.

It will be appreciated that the identification of the detected picocell from the measurements of the scrambling codes of the different subsets may be located at different physical and logical locations.

For example, FIG. 4 illustrates an example wherein the identification is performed in the RNC as described in the

previous examples.

In the example, six scrambling codes are allocated to picocells in a first macrocell 401 supported by the macro base station (103 -not shown in the example) and the macro RNC 103. The scrambling codes are split into two subsets {Al,A2,A3} and {B1,B2,B3} such that 9 different picocells can be identified uniquely using only three entries in the neighbour list.

In the example, the remote station 111 is in a first picocell 403 assigned the scrambling code group of (A3,B3) The remote station 111 is first configured to search for {A1,A2,A3} neighbours and it consequently reports A3. The neighbour list is then updated such that the remote station GEl 6228EP 111 is configured to search for {B1,B2,B3} and accordingly it reports B3.

In the example, the RNC 103 has information that the scrambling code group of (A3,B3) maps to the first picocell 403 and that the serving RNC of this picocell is the pico RNC 107. Accordingly, the RNC 103 generates a relocation request message which is transmitted to the pico RNC 107. In the example of FIG. 4 this transmission is via a first MSC 405 and a second MSC 407.

Thus, in this example the macro RNC is configured to perform the mapping between the detected scrambling code group and the picocell identity.

FIG. 5 illustrates a corresponding example wherein the identification of the picocell identity is performed in the first MSC 405. In this case, the RNC 103 transmits a message to the first MSC 405 which contains an indication of the detected scrambling code group. The message may furthermore contain an identity indication of the macrocell currently serving the remote station 111. The MSC 405 then performs the mapping to the picocell identity.

Thus, in this example, the macro RNC passes the source cell identity and the detected scrambling group to the first MSC 405 which is configured with the mapping between the scrambling code groups and the picocell identities. The MSC examines the scrambling code group in order to identify the target pico RNC to which to send a relocation request.

GEl 6228EP FIG. 6 illustrates a corresponding example wherein the identification of the picocell identity is performed in the pico RNC 107. For example, all picocells within a macrocell may in some embodiments be supported by a single pico RNC and therefore the macro RNC 103 may know the identity of the appropriate pico RNC even if the individual picocell identity is not known. In this example, the message containing an indication of the detected scrambling code group is transmitted from the macro RNC 103 to the pico RNC 107 wherein the picocell identity is identified.

It will be appreciated that although the above description has focussed on various embodiments wherein much or all of the functionality is implemented in an RNC, the functionality may be implemented partly or fully in other entities in other embodiments. In particular, the pilot detection and assignment functionality may be implemented in any entity which comprises all or part of the Radio Resource Control functionality for the air interface. For example, the functionality may be implemented in e.g. the access point itself, an access gateway or a UMA (Unlicensed Mobile Access) Network Controller (UNC) FIG. 7 illustrates a method of operation in a CDMA cellular communication system comprising a plurality of macrocells underlayed by a plurality of underlay cells.

The method starts in step 701 wherein a set of scrambling codes assigned to an underlay layer is divided into distinct subsets.

CE1 6228EP Step 701 is followed by step 703 wherein a scrambling code group comprising a plurality of scrambling codes is assigned to underlay cells within a region. Each group is a unique group within the region and comprises one scrambling code from each of the subsets.

Step 703 is followed by step 705 wherein a base station for each of the underlay cells transmits a pilot signal on each scrambling code of the group assigned to the underlay cell.

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors.

However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in CE16228EP a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply CE16228EP that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

GEl 6228EP

Claims

1. A cellular communication system comprising a plurality of macrocells underlayed by a plurality of underlay cells, the system comprising: means for dividing a set of scrambling codes into distinct subsets; means for assigning a scrambling code group comprising a plurality of scrambling codes to underlay cells within a region, each scrambling code group being a unique group within the region and comprising one scrambling code from each of the subsets; and a base station for each of the underlay cells arranged to transmit a pilot signal on each scrambling code of the scrambling code group assigned to the underlay cell.

2. The cellular communication system of claim 1 further comprising a network controller arranged to generate a neighbour list for a remote station in a macro-cell, the network controller comprising: sequential means for sequentially including different subsets of scrambling codes in the neighbour list, each neighbour list comprising one subset of the distinct subsets.

3. The cellular communication system of claim 2 wherein the network controller furthermore comprises means for receiving measurement reports for scrambling codes of the neighbour list from the remote station; and the communication system further comprises identifying means for CE1 6228EP identifying a first underlay cell in response to measurement reports for scrambling codes of different subsets.

4. The cellular communication system of claim 3 wherein the identifying means comprises first means for determining a detected group of scrambling codes for the remote station in response to the measurement reports, the detected group comprising a detected scrambling code from each subset; and second means for identifying the first underlay cell as an underlay cell being assigned a scrambling code group corresponding to the detected group.

5. The cellular communication system of claim 4 wherein the first means is arranged to select a scrambling code for the detected group from each subset as the scrambling code received with the highest signal level out of the scrambling codes of the subset.

6. The cellular communication system of claim 4 or 5 wherein the first and second means is located in a serving Radio Resource Control entity for a macrocell of the remote station.

7. The cellular communication system of any of the previous claims 4 or 5 wherein the first means is located in a serving Radio Resource Control entity for a macro cell of the remote station and the second means is located in a Mobile Switch Centre, MSC; and the serving RNC comprises means for transmitting a report message to the MSC comprising an indication of the detected group.

CE16228EP

8. The cellular communication system of any of the previous claims 4 or 5 wherein the first means is located in a serving Radio Resource Control entity for a macrocell of the remote station and the second means is located in a Radio Resource Control entity; and the serving RNC comprises means for transmitting a report message to the underlay RNC comprising an indication of the detected group.

9. The cellular communication system of any of the previous claims 7 or 8 wherein the report message furthermore comprises an identity indication for the macrocell.

10. The cellular communication system of any of the previous claims 4 to 9 wherein the sequential means is arranged to change the subset of the neighbour list in response to a measurement report indicating that a first scrambling code of a current subset of the neighbour list has been detected and to include the first scrambling code in the detected group.

11. The cellular communication system of claim 10 wherein the sequential means is arranged to clear the detected group if no measurement report indicating that a scrambling code is detected in a current subset of the neighbour list is received within a time interval.

12. The cellular communication system of claim 10 or 11 wherein the neighbour list is arranged to include a scrambling code of the detected group for a previous subset in the neighbour list.

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13. The cellular communication system of any of the claims to 12 wherein the base station is arranged to transmit a scrambling code of a subset of the neighbour list corresponding to an empty detected group at a different transmit power than scrambling codes of subsets of the neighbour list corresponding to a non-empty detected group.

14. The cellular communication system of any of the previous claims 3 to 11 further comprising means for generating a core network relocation request for a handover of the remote station to the first underlay cell in response to the identification of the first underlay cell.

15. The cellular communication system of any of the previous claims wherein the set of scrambling codes does not contain scrambling codes used for the macrocells.

16. The cellular communication system of any of the previous claims wherein each underlay cell has at least one assigned cell scrambling code different from scrambling codes of neighbour cells of the underlay cell; and the base station for each of the underlay cell is arranged to transmit a pilot signal using the cell scrambling code.

17. The cellular communication system of claim 15 wherein the assigned cell scrambling code is comprised in the group assigned to the underlay cell.

18. A network element for a cellular communication system comprising a plurality of macrocells underlayed by a plurality of underlay cells, the network element comprising: CE1622OEP means for dividing a set of scrambling codes into distinct subsets; means for assigning a scrambling code group comprising a plurality of scrambling codes to underlay cells within a region, each scrambling code group being a unique group within the region and comprising one scrambling code from each of the subsets.

19. A method of operation in a cellular communication system comprising a plurality of macrocells underlayed by a plurality of underlay cells, the method comprising: dividing a set of scrambling codes into distinct subsets; assigning a scrambling code group comprising a plurality of scrambling codes to underlay cells within a region, each scrambling code group being a unique group within the region and comprising one scrambling code from each of the subsets; and a base station for each of the underlay cells transmitting a pilot signal on each scrambling code of the scrambling code group assigned to the underlay cell.

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