Patent Application
20130295916
The present invention relates to a mobile telecommunications system, and in particular to the reduction of interference within a cell of the system, which may be caused by transmissions of neighbouring cells.
In third generation wireless mobile communications, and in particular to the Universal Mobile Telephone System (UMTS) and other systems employing CDMA, RF transmissions between a base station and user equipment are subject to two main coding operations. Firstly a spreading using channelization code, known as CDMA, is applied to separate individual transmission channels.
Secondly, a scrambling code is applied to transmissions in order to distinguish transmission operations in one cell from those of neighbouring cells. The requirements for scrambling and channelization codes are set out in Standard 3GPP TS 25.213 V9.1.0 (2009-12). It has however been found that there is a risk of interference with transmissions from neighbouring cells, which may be operating on the same carrier frequency, but employing different scrambling codes. This arises because, unlike CDMA codes, scrambling codes are non-orthogonal. The above 3GPP Standard specification does not provide any guidance on this problem.
US 2008/0039141 (Lucent) describes a process wherein base stations hold neighbour lists, including associated scrambling codes. If there is unacceptable interference with a neighbouring cell, then another scrambling code is assigned. This is assigned on a random basis from a set of available codes, excluding the neighbour list codes.
EP-A-2214434 (Alcatel) describes the generation of a “neighbour list” containing information of neighbouring femtocells, including primary scrambling code (PSC) and timing offset. For reduction of interference between cells, each cell upon deployment initializes with a PSC different from its neighbours. The PSCs may also have differing timing offsets, and timing offsets of neighbouring cells are determined, and the timing offset of the cell being installed is randomly selected from those unused.
WO 2008/093100 (Ubiquisys) relates to control of RF transmit power in femtocells for the purpose of reducing interference of neighbouring cells, wherein a carrier frequency and scrambling code from lists provided to it from a management system. The lists are shared with other base stations so that the carrier frequency and scrambling code are chosen to result in lowest interference to neighbor cells. In selection of scrambling code, if there are unused codes, then a scrambling code is chosen on a random basis from such unused codes. If however there is not an unused code, then the code is selected with the lowest code channel signal power, CPICH_RSCP.
Further improvements are desirable in reducing interference between neighbouring cells, in particular femtocells and other small cells, and in particular in the choice of Primary Scrambling codes, in a process that may be efficiently employed in an auto-configuration deployment.
The present invention provides, in a first aspect, a method for transmission in a mobile telecommunications system, comprising:
In a second aspect, the invention provides apparatus for a mobile telecommunications system, wherein the system includes a base station means for communicating with user equipment within a first cell region, and the apparatus comprising scanning means or listening means for monitoring transmissions of one or more neighbouring cells, means for determining scrambling codes of said transmissions and/or for determining timing offsets of said codes, with respect to the timing of said first cell, of said transmissions, and cross-correlation means for determining cross-correlation values between the determined codes and scrambling codes that may be employed in said first cell, and/or cross-correlation values as a function of timing offset between the determined codes and scrambling codes that may be employed, and means for selecting, in dependence on said cross-correlation values, a scrambling code and/or a timing offset for said first cell which has a reduced likelihood of interference with transmissions of neighbouring cells.
Thus, embodiments of the invention may operate by providing a revised scrambling code for the first cell in dependence on cross-correlation values of potential scrambling codes with scrambling codes and/or timing offsets of neighbouring cells. Alternatively, embodiments of the invention may operate by providing a timing offset to revise the timing of the scrambling code of the first cell in dependence on cross-correlations with scrambling codes and/or timing offsets of neighbouring cells. In a specific embodiment as described below, a revised scrambling code together with a revised timing offset for the scrambling code of the first cell are provided in dependence on cross-correlations with both scrambling codes and timing offsets of neighbouring cells.
In embodiments of the invention, assignment of Primary Scrambling Code (PSC) to CMDA-based (CDMA and W-CDMA) base stations and control of the relative timing offset is carried out such that mutual interference is reduced or eliminated. This procedure is applicable to any system based on spread spectrum-based systems such as WCDMA, CDMA, etc and includes systems which have been implemented or are proposals for future implementation.
Said apparatus may be located in the base station means, which may be a base station router (BSR) for use with small cells. Conceivably, the apparatus might be located elsewhere, for example in User Equipment (UE) under the control of the respective small cell. An embodiment is concerned with processing the downlink Primary Scrambling Code (PSC) that is determined by the UE in an initial cell search procedure. Conceivably, an embodiment may be concerned with processing Secondary Scrambling Codes.
The method is applicable particularly to small cells such as micro-, pico- and femtocells, where the problems of interference with neighbouring cells may be more marked than with very large macro cells due to the fact that small cells are deployed in larger numbers and also exist, in general, under the coverage of the macro cell. However the method is applicable to all scenarios with cells used in either embedded form (small cells within the coverage of macro cell), or adjacent cells (macro near macro cell, or small cells near another small cell). Small cells are likely to be deployed en masse (i.e. larger number than those used as macro), hence the issues encountered would be more apparent and severe on the performance of the network.
In an embodiment, transmissions of neighbouring cells are scanned or monitored and assessed in terms of timing relative to the subject cell, and in terms of scrambling codes used by these cells. In addition transmissions below a threshold strength may be disregarded as unlikely to cause interference. In one embodiment, in order to determine scrambling codes and timing offsets, the base station or BSR of the subject cell, which may be a new cell being installed, acts in a listening mode as a UE, using a “sniffer” function to perform the measurements and decoding of SIB messages of the other cells. In order to emulate the function of a UE (User Equipment), the base station includes all necessary information, files and codes. SIB (System Information Blocks) are broadcast to a UE from a base station (Node B, RNC) and from the Core Network, and are part of the Radio Resource Protocol (RRC) defined in Specification 3G TS 25.331. Hence the decoding of SIB of neighbouring cells may provide the BSR with the information it needs in terms of scrambling codes and timing offsets in use by neighbouring cells. The emulated UE may then undergo the specific synchronization process and scrambling code process, which is defined in Standard 3GPP TS 25.214, to determine the Primary Downlink Scrambling Code of a neighbouring cell. In an alternative embodiment, the BSR may include a scanning mechanism which acts to monitor all interfering transmissions for the BSR.
An embodiment provides one or more cross-correlation matrices that have been determined in an initial characterising step, and which contain information as to the cross-correlation between scrambling codes that may be used, and how the cross-correlation is affected by relative timing offsets between the codes. In use, the matrices may be accessed, once codes and timings of neighbouring cells have been determined, to provide scrambling codes and timing offsets for the subject cell. Whilst the initial characterising step may involve an extremely thorough analysis of all possible cross-correlations, the information present in the matrices may be a very much reduced subset of all this information, from the point of view of reducing cost and complexity. Thus for any particular code, a cross correlation matrix may contain a list of codes that will reduce interference to an acceptable level. The matrix may also contain optimum timing offsets for each of the listed codes that will reduce interference to a minimum level. However more complex arrangements may be envisaged. For example a matrix may be provided with rows representing relative timing offsets, and columns, one for each possible combination of code pairs, giving cross-correlation values.
An alternative to using pre-calculated matrices is to perform a real-time calculation of the best scrambling code and offset. In this embodiment, the base station, after scanning the scrambling codes and determining the offsets of detected neighbours, would perform the cross correlation of all scrambling codes in its database. It then selects the scrambling code and offset with the best (lowest) cross correlation. This gives the base station more flexibility in the decision making process, and negates the need for operators to ensure the selection matrices are up-to-date with allowed scrambling codes. Obviously the amount of computational resources required at the base station for this embodiment is higher.
An embodiment may additionally include adjusting the power levels of transmission and/or frequency of transmission in said first cell, in dependence on interference levels from neighbouring cells, in order further to reduce interference levels.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:
Embodiments of the invention address the issue of automatic downlink scrambling code configuration of small cell Base Station Routers (BSR).
Cells are usually classified by cell radius and maximum output power as defined in 3GPP (TS25.104). Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Microcells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors. Femtocells are cells designed for use primarily in residential or small business environments and connect to the service provider's network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells. For the purposes of the present invention, “small cells” are regarded as cells which fall into one of the above categories, other than the category of macro cells.
Base Station Routers (BSR) are commonly designed for use with femtocells for example within the home. The Router is of a small size, for example a single large circuit board, and besides having the functionality of a base station, additionally includes network functions of Radio Network Control and core network router functionalities, so that the router can be connected to an existing network, without requiring any modification of the network.
A known procedure for invoking the scrambling process (Standard 3GPP TS 25.214) is that User Equipment (UE) performs a synchronization operation on synchronization channels, identifying synchronization codes that are common to all cells. Having synchronised to the appropriate Base Station (BS), the UE then decodes the downlink Primary Scrambling Code (PSC), which is unique to that BS. This is done by a correlation operation, using the possible PSCs available. Once this is done, cell-specific information broadcast by the BS can then be read by the UE. Further secondary scrambling codes, related to the PSC, may be invoked, for further operations.
As stated in the above referenced standard Specification TS 25.213 “The scrambling codes are divided into 512 sets each of a primary scrambling code and 15 secondary scrambling codes. The primary scrambling codes consist of scrambling codes n=161 where i=0 . . . 511. The i:th set of secondary scrambling codes consists of scrambling codes 16*i+k, where k=1 . . . 15”. There is a one-to-one mapping between each primary scrambling code and 15 secondary scrambling codes in a set such that i:th primary scrambling code corresponds to i:th set of secondary scrambling codes.”
There is a risk of interference with transmissions from neighbouring cells, which may be on the same carrier frequency, while employing different scrambling codes. This arises because scrambling codes are non-orthogonal. Further this risk is particularly apparent when dealing with small cells such as femtocells, which generally employ the same carrier frequency as other neighbouring femtocells.
In this embodiment, scrambling code selection made by the small cells is based on minimizing interference from other adjacent cells (e.g. macro cells or femtocells). Since scrambling codes are non-orthogonal, the selection of code and adjusting a coordinated or relative time offset will allow great reduction of cross-talk between different cells.
One of the main aspects of small cells provisioning is the cell set up procedure, which requires the assignment of a primary scrambling code. In UMTS, a cell (or sector) is identified by the downlink scrambling codes. The 3GPP standard specifies a total of 218−1=262,143 scrambling codes, numbered 0 . . . 262,142 that can be generated. However not all the scrambling codes are used. The scrambling codes used by the UMTS system are divided into 512 sets each of a primary scrambling code and 15 secondary scrambling codes (total number of codes is 8192 codes).
In this embodiment, to exploit a low cross correlation between scrambling codes, the subject small cell is required to identify the scrambling codes being used by other neighbouring cells (macro cells and small cells) and then assign itself a code and an offset relative to the other cells. This will enable signal reception within the cell (often a RAKE receiver is used within the BS or BSR) to benefit from the lowest peak of the cross correlation between unwanted codes (i.e. used by other cells) and its own selected code. The subject small cell uses a listening mode upon powering up or during optimization time to identify the primary scrambling codes (PSC) used by other cells, to select a PSC for use within the subject cell, and to derive the required relative timing offset, such that interference is reduced.
In some cases, the subject small cell may be given a subset of the possible 512 primary scrambling codes to use. In this case, the small cell selects a code from this subset that will reduce interference. The codes are reused by the small cells based on reduction of interference as well, since the scrambling code selection of the small cells would be made to maximize orthogonality between other neighbouring small cells.
Referring to
Relative timing offset allows further reduction of interference. This can be seen by studying the results of the cross correlation between two codes, as shown in
The subject small cell is required to perform the following operations to derive information either in real time, or offline in a preliminary operation. Steps are as follows:
Having determined a scrambling code, the sniffer unit 14 then accesses a look up table, to determine appropriate scrambling codes and timing offsets for use by the subject cell. Exemplary look-up tables are shown in
Commonly small cells and macro cells operate using shared carrier (i.e. same carrier). Also, note that, in general, small cells use one carrier but differ by using different PSCs. The critical matter is that shared carrier means the only means to control interference is by using different PSCs and to control output power. However, a careful selection and coordination of the PSCs and their relative time offset has shown to offer an additional means to lower mutual interference. The sniffer can scan different carriers in case we want to switch the small cells carrier frequency to another one from the allowed set (usually operators have more than one carrier in their license), so we propose to have auto-configuration of carrier frequency for the small cells in order to add more dimensions to interference mitigation and control.
Further auto-configuration will also be used such as optimisation of the transmit power, in the sense that risk of interference with a neighbouring cell may be reduced by adjusting transmit power of each cell to ensure carefully control based on the indented coverage area.
The primary scrambling code allocation has an impact on level of interference received from other adjacent cells which manifests itself as a noise rise on the receiver side.
In use the present embodiment operates as follows:
For auto cell primary scrambling code allocation, planning, it is convenient to visualize the small cell (homes) as hexagonal cells. The location of the centre cell as well as the other cells may be established using, for example, location based services when an existing macro network exists.
In all cases above, a relative time offset is also computed in order to align the small cell receiver with the optimum point at which the lowest cross correlation occurs.
The above embodiment provides an efficient method for interference management for small cells while using the same carrier frequency as the macro cell and indeed when other small cells exist that are using the same carrier frequency.
An advantage of this embodiment is that the proposed method is low cost and easy for implementation. It utilises the characteristics of the codes used in order to yield the lowest possible mutual interference.
Referring now to a second embodiment, a real-time computation is provided where the small cell scans the air interface for the codes being used by other cell, and then it performs the computation based on real-time data to determine the best code and corresponding offset to use, this process is performed using the processor of the small cell. The process is as follows, referring to
Case c. is seen as a general case of b, but we exploit the fact that the small cells does not need to compute more than 511 cross correlations. That means that even in the case of detecting more than one code being used by other cells (i.e. there are more than other cell in the vicinity), then the small cells still computes the same data when it comes to cross correlation and determining the best code with the appropriate offset to use. Thus, the cross correlations of the discovered code is first computed, then the small cell performs optimization process in order to find the “best option” from the for example 508 remaining codes (assuming we discovered 4 codes) in terms of code and corresponding offset. The small cell computes 507 cross correlations and searches amongst them for one code that at a given offset yields the “best” cross correlation (best means lower cross correlation here). The best might not be the lowest possible when one cross correlation is performed, but is the code and offset that offers optimum interference reduction.
Therefore, referring to
Relative power of each discovered code is measured (30) by the UEs (when there is at least one under the small cell coverage) and the small cell. An average value is then computed.
Cross correlation is computed between code x relative average receive power (32) (this multiplication provides us with a measure on how bad the interference is from the cell using that code).
The “weighted” cross correlation is then used in a logic loop (34, 36) to find the code from the remaining set that provides a best fit. In the logic loop, an optimization process or algorithm is employed to assess weighted cross-correlations. Any suitable optimization algorithm (34) may be employed; such algorithms are widely available. In step (36), the results of the optimization are assessed. If further improvement appears necessary the optimization process is repeated, until such time as no further improvement appears possible. At that point the optimum or best fit is assessed from the optimisation results.
Timing offset may be taken into account in addition to differing codes, in which case complexity of computation increases. If we consider performing 511 cross correlations with accuracy of delay equals to half a chip, then each correlation requires 38400×2=76800 points to compute. This means we need to compute 76800×511 different multiplications and summation (this is how cross correlation is computed). So this is a large number of arithmetic operations to do. However, given that the scrambling codes are categorised into 64 different groups, we may do the computation based on performing 64−n cross correlations when n>1 and is the number of discovered codes. This way we reduce the computation by a factor of 8 without affecting the result.
Thus, an alternative to using pre-calculated matrices is to perform a real-time calculation of the best scrambling code and offset. In this case, the base station, after scanning the air interface to discover the scrambling codes being used by other cell and determining their offsets, performs the cross correlation of all scrambling codes in its database. It then selects the scrambling code and offset with the best (lowest) cross correlation. This gives the base station more flexibility in the decision making process, and eliminates the need for operators to continuously perform manual radio frequency cell planning (i.e. we make the network fully automated with the ability of parameters discovery and assignment). There is a need for allocating computational resources to perform this function, but the following two cases are addressed as follows:
The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10290605.4 | Nov 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/005536 | 11/3/2011 | WO | 00 | 7/16/2013 |
