Abstract:
The application relates to the selection of cells to form a cell coordination group for which interference management techniques such as COMP are to be applied. The cells to be selected to improve the performance of a UE in terms of throughput or delay are not necessarily the ones which are geographically the closest because in city environments shadowing effects of buildings exist. Thus, choosing the closest cells for coordinated interference management will not be a good solution in many cases. Hence, there is a need to develop another method and mechanism for more accurately generating a cell coordination group for interference management activities. This problem is solved by the application in that either the downlink interference toward the UE from the neighboring cells is estimated or the neighbor cells listen for the UE and then predict the downlink interference. Furthermore, the algorithm starts with an initial list of cells comprising the most likely handover candidates. Furthermore, cells are eliminated if the interconnection link to the serving base station is not fast enough for the interference reducing algorithms.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to networks, software and methods and, more particularly, to mechanisms and techniques for determining a cell coordination group for interference coordination techniques. 
       BACKGROUND 
       [0002]    An explosion of computing devices, stationary and portable, is currently underway. These computing devices include, but are not limited to, smartphones, laptops, tablets, phablets, etc. Demand for these devices is made more acute by social media&#39;s rapid evolution, which encourages a need to communicate and be in contact with others. This creates continual competition among various manufacturers to generate better and faster computing devices. 
         [0003]    However, an issue associated with this development that becomes more acute in dense urban locations is the poor cell coverage for small geographical pockets that are either in the shadow of a large building, or inside a large building, effects known in the field as “street canyon” and/or “building shadowing.” To address the poor or lack of coverage in these pockets, the modern mobile telecommunication networks have started to deploy small cells, in addition to the traditional macro cells. 
         [0004]    Such a heterogeneous telecommunication network  100  is schematically illustrated in  FIG. 1 , in which a macro cell  102  (only one macro cell is illustrated for simplicity) is located on a building  104  in a densely built up area  106 . This area includes streets  108  on which high buildings  110  are located. Due to these high-rise buildings, there might be areas  120  that have poor or no signal from macro cell  102 . Deploying another macro cell for these pockets of no or poor cell coverage is not efficient, both from a financial and a frequency bandwidth point of view. Thus, there is a trend to deploy a small cell  122  in or close to the area  120  having poor cell coverage. 
         [0005]    Regarding the macro and small cells, a small cell may operate in the 10 m to 2 km range while a macro cell may operate in the tens of kilometers. However, note that there is no one agreed upon definition in the industry for the small and macro cell ranges and the numbers provided herein are for exemplary purposes. 
         [0006]    However, by deploying many small cells to improve coverage, it leads to higher inter-cell interference levels as well as increased complexity in interference management features to achieve the best performance. 
         [0007]    One way to implement a heterogeneous network is to use a base station architecture based on remote radio equipment and radio equipment controllers. For example, as illustrated in  FIG. 2 , macro cell  102  includes a base station  202  that has the remote radio equipment  204  including radio frequency generation unit  206  and, possibly, antenna elements  208  located in one place while the radio equipment controller  210 , which typically includes baseband signal processing units, may be located in another place. The remote radio equipment  204  and radio equipment controller  210  are typically interconnected by fiber  212  carrying user plane information of in-phase and quadrature modulation data (digital baseband signals). Because of the fiber, the two locations can be far away from each other. 
         [0008]    By using this approach, a centralized radio access network (C-RAN)  220  with common baseband units  210  for multiple base stations  202 ′ to  202 ″ can be used for a large number of macro and small cells covering a larger geographical area. With the C-RAN  220 , inter-cell interference can be efficiently managed through interference mitigation features, such as Coordinated Multi-Point (CoMP) for reception and transmission. Note that the main idea behind CoMP is as follows: when user equipment (UE) is in the cell-edge region, it may be able to receive signals from multiple cell sites and the UE&#39;s transmission may be received at multiple cell sites regardless of the system load. Thus, if the signals transmitted from the multiple cell sites are coordinated, the downlink performance can be increased significantly. Further algorithms may be used for interference management when the macro and small cells use the same baseband units. 
         [0009]    Other interference coordination techniques may be used when macro and small cells use different baseband units (i.e., radio equipment controllers). In this case, there need to be a short interconnecting delay and high transport capacity on the X2 interface (e.g., the fiber  212 ) or other similar interface between the baseband units. 
         [0010]    However, a common problem with applying the existing interference coordination techniques to the various implementations of heterogeneous networks is the following. The existing interference techniques manage the multiple cells (macro and small) by using coordination, i.e., identification of the cells that are responsible for the highest amount of interference. In other words, for a given cell in the telecommunication network, the existing interference techniques need to determine and group those neighbor cells that generate the largest amount of interference. Thus, these techniques require (i) that the macro and small cells share the same radio equipment controller or (ii) there is a fast interconnect between radio equipment controllers when separate baseband units are deployed. 
         [0011]    However, there is a practical limit on the number of remote radio equipment that can be connected to the same radio equipment controller or a group of radio equipment controllers. Also, the delay between coordinated cells needs to be below a limit or otherwise the coordination cannot be efficient. For example, a signal copy arriving outside a cyclic prefix in Long Term Evolution (LTE) is treated as interference by the receiver. There can also be a limit regarding the number of connections between the radio equipment controller and the multiple cells due to hardware and/or software limitations (processing power, memory, interconnection interface speed etc.). 
         [0012]    A problem is thus to determine what remote radio equipment  204  (used for macro and small cell areas) that shares the same baseband unit  210  should belong to a same cell coordination group to benefit from CoMP and other interference management techniques. If the heterogeneous network has remote radio equipment  204  that do not share the baseband unit  210 , then, the problem is what baseband units should belong to the same cell coordination group to benefit the interference management. 
         [0013]    As now discussed, the existing methods for generating the cell coordination group have their own limitations. Such a method needs to identify the cells that generate a lot of interference to maximize the efficiency of the interference management. This identification is difficult in city environments due to e.g., street canyon and/or building shadowing effects as discussed with regard to  FIG. 1 . The geographically closest cell may not be the most interfering cell. To illustrate this concept,  FIG. 3  shows a heterogeneous network  300  having macro cells  302  and small cells  304 . Small cells  304  are connected by a line to the macro cell that generates the most downlink interference. In many cases, this is not the closest (in term of geographical distance) macro cell. Solid lines  310  are used when the most interfering macro cell also is the closest one and dash lines  312  otherwise. Dash lines  312  are dominating the picture, meaning that the majority of the small cells does not receive the strongest interference from the geographically closest macro cell. 
         [0014]    Thus, choosing the closest cells for coordinated interference management will not be a good solution in many cases. Hence, there is a need to develop another method and mechanism for more accurately generating a cell coordination group for interference management activities. 
       SUMMARY 
       [0015]    As the telecommunication networks become more complex by using more types of cells, there is the potential to coordinate groups of cells for reducing the inherent interference that appear between the cells. However, the existing coordination processes need to know the cell coordination group of a given cell in order to apply interference reducing methods. Present techniques for determining the cell coordination group fails, as noted above. Thus, there a need for a new cell coordination group generation technique. 
         [0016]    According to one embodiment, there is a method for forming a cell coordination group in a telecommunication network. The method includes a step of obtaining, for a serving cell, a neighbor cell list indicative of neighbor cells of the serving cell; a step of selecting a user equipment connected to the serving cell; a step of estimating downlink interference of the user equipment for the neighbor cells or a path loss of the user equipment for each neighbor cell; and a step of grouping the serving cell with selected neighboring cells to form a cell coordination group. The selected neighboring cells are selected based on the estimated downlink interference or the path loss. 
         [0017]    According to another embodiment, there is a node in a telecommunication network that includes a first module configured to obtain, for a serving cell, a neighbor cell list indicative of neighbor cells of the serving cell; a second module configured to select a user equipment connected to the serving cell; a third module configured to estimate downlink interference of the user equipment for the neighbor cells or a path loss of the user equipment for each neighbor cell; and a fourth module configured to group the serving cell with selected neighboring cells to form a cell coordination group. The selected neighboring cells are selected based on the estimated downlink interference or the path loss. 
         [0018]    Thus, it is an object to overcome some of the deficiencies discussed in the previous section and to provide a method and node for generating a cell coordination group that is more accurate. One or more of the embodiments discussed herein advantageously provides such a mechanism. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
           [0020]      FIG. 1  is a schematic diagram of a heterogeneous telecommunication network; 
           [0021]      FIG. 2  is a schematic diagram of a macro cell; 
           [0022]      FIG. 3  illustrates that maximum cell interference is not always related to geographical proximity in a heterogeneous telecommunication network; 
           [0023]      FIG. 4  illustrates a heterogeneous telecommunication network in which the base stations have baseband units connected by fiber to each other; 
           [0024]      FIG. 5  illustrates a method for forming a cell coordination group in a telecommunication network; 
           [0025]      FIG. 6  is a schematic diagram of a node and associated modules that execute various functions as noted in  FIG. 5 ; and 
           [0026]      FIG. 7  if a flowchart of a method for forming a cell coordination group in a telecommunication network. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a heterogeneous telecommunication network. However, the embodiments discussed herein are not limited to heterogeneous telecommunication networks, but they may be applied to other types of networks, for example, a traditional network that includes only macro cells or a network that includes only small cells. 
         [0028]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily all referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments or claims. 
         [0029]    According to an embodiment, there is a method for identifying and grouping of cells (depending on their configuration, identifying the remote radio units or radio equipment controllers) that need to cooperate for interference management purposes. The method may be executed in a network node, e.g., base station, radio network controller, radio equipment controller or other node. The grouping may be based on cell traffic statistics, path loss, signal strength, delay and transport capacity. 
         [0030]    In one embodiment, the method determines neighbor cells having interconnecting transport delay below a pre-determined limit and transport capacity above another predetermined limit to a serving cell. Then, various measurements for downlink interference and/or path loss for the neighbor cells are performed, and, based on these measurements, the serving cell and a selected number of neighbour cells are grouped together for interference management purposes. The neighbour cells with highest interference and/or path loss are selected, in order of level of interference/path loss, for the group until the maximum number of cells (due to hardware and/or software limitation) that can cooperate for coordination is reached. 
         [0031]    According to an embodiment illustrated in  FIG. 4 , a telecommunication network  400  includes multiple base stations  410 ,  420 ,  430 , and  440  (only four shown for simplicity) distributed over an area. Note that each base station is associated with a corresponding cell area, which in this case, are cell areas  410 A,  420 A,  430 A and  440 A. In the following, the term “base station” is sometimes interchangeably used with the term “cell” although they have different meanings. Each base station includes a radio frequency generation (RFG) unit  412 ,  422 ,  432  and  442  and a baseband unit  414 ,  424 ,  434  and  444 . The baseband units  414 ,  424 ,  434  and  444  are configured to communicate among them via corresponding cables  450 . Because each base station has its own baseband unit, a C-RAN is not practical. This is different from the embodiment illustrated in  FIG. 2 , in which same baseband units  210  are shared by base stations  204  to  204 ″. However, the methods discussed herein equally apply to the configurations shown in  FIGS. 2 and 4 . 
         [0032]    One method is illustrated in  FIG. 5  and includes a preparation step  500 . Considering that preparation step  500  is applied to base station  410 , i.e., the serving cell, it includes determining the most likely handover candidates based on the neighbour cell list and handover statistics (history) in the serving cell. This means that base station  410  stores (in a memory  416 ) or has access to a list  418  of the neighbours, e.g., base stations  420 ,  430  and  440 , and determines, based on prior data indicative of how many of the UEs from the serving cell were handover to the neighbour cells, the most likely handover candidates, e.g., base stations  420  and  430 . In one application, the handover statistics can be, for example, the number of handovers from the serving cell to each one of the neighbour cells in the neighbour cell list at a busy hour of the day. The cell with the highest count is most likely the cell that users will handover to. 
         [0033]    Note that one or more of the steps discussed next may be omitted and they do not have to be performed by each base station. In other words, it is possible that only base stations  410  and  440  implement these steps while base stations  420  and  430  do not. The base stations that are selected to implement the method of  FIG. 5  may be determined based on, for example, network operator&#39;s experience, historical data, number of UEs experiencing poor performance, etc. Also, the steps to be discussed next do not have to be performed in the order illustrated in  FIG. 5 . Variations of the order presented in  FIG. 5  may be implemented by those skilled in the art. 
         [0034]    Next, the serving cell  410  (in one application, more cells perform this step not only the serving cell) monitors in step  502  transport parameters between its baseband unit  414  and the neighbour cells&#39; baseband units  424 ,  434  and  444  along communication cables  450  to determine whether the transport capacity fulfils delay requirements, i.e., transmission delays below a first predetermined threshold and transport capacity above a second predetermined threshold between each base station and the serving cell. The first and second predetermined thresholds may depend on the configuration of the base station, the type of equipment used for controlling the radio equipment, the type of telecommunication network, etc. The monitoring step  502  for a network architecture as illustrated in  FIG. 4  (i.e., with radio equipment controllers for the various base stations geographically separated) is desired because delayed or low capacity communications between the baseband units prevent the interference reducing algorithms to be applicable. If a network architecture as illustrated in  FIG. 2  is used, the monitoring step may be performed once, at the beginning of the procedure, or this step may be skipped. 
         [0035]    In one optional step, the serving cell continuously determines the neighbour cells that fulfil delay requirement and have a transport capacity above a predetermined limit. If the serving cell determines that one or more of the neighbour cells fail to fulfil these requirements, those cells are removed from the candidate neighbour cells. If a dedicated transport link is used, this step can be incorporated in the preparation step  500 . 
         [0036]    In step  504 , a UE  460  is selected for further signal processing. UE  460  is physically located within serving cell  410 A. The selection is based on a detection of a service performance below a given threshold. For example, one of the service performance factors may be throughput or delay. Thus, if low throughput or high delay is determined comparative to the given threshold, that is indicative of the service performance. Those skilled in the art would recognize that other factors or a combination of factors may be used for determining the service performance. 
         [0037]    Once the UE  460  has been selected to have service performance below the given threshold, the method instructs in step  506 , each cell of the set of handover cell candidates  420 ,  430  and  440  from step  500 , to estimate/calculate/obtain its downlink interference associated with the selected UE  460 . For example, this step may be implemented such that the neighbor cells listen if they can “hear” the UE  460  and then predict downlink interference they generate based on power, traffic load and received power from UE  460 . Other methods for determining downlink interference are known, and some of them are disclosed, for example, in U.S. Pat. No. 6,137,991, the entire content of which is incorporated herein by reference. The downlink measurement is a good representation of the downlink interference from neighbors if the traffic load of the neighbor cells is known. 
         [0038]    The set of neighbor cells  420 ,  430  and  440  that are also handover cell candidates may be ordered to estimate, in an optional step  508 , path loss from the selected UE to each of the neighbor cells  420 ,  430  and  440 . Note that the downlink measurement from step  506  can be used for estimating path loss to a neighbor cell. Alternatively, uplink and/or downlink signal strength may be measured and path loss and/or downlink interference may be estimated based on the measured signal strength. This alternative may be preferred because estimating the signal strength in uplink is easy, e.g., just measure the received signal from the radio base station and this is a direct measure of the path loss toward the UE from the radio base station. More specifically, in one application, the UE may measure downlink signal strength of neighbors for handover purposes. These measurements may be then sent to the serving base station, from which path loss and downlink interference can be estimated using knowledge of the radio base station&#39;s used power and cell traffic information, etc. In another application, the serving and neighbor radio base stations may measure the uplink signal strength of the selected UE. The uplink signal strength is a direct measure of path loss. Downlink interference can be estimated from uplink signal strength using knowledge of the radio base station&#39;s used power and cell traffic information, etc. In other words, each of steps  506  and  508  may be modified to measure signal strength (uplink or downlink or both) and to estimate path loss and/or downlink interference. Thus, those skilled in the art would understand that steps  506  and  508  may be replaced by a step of measuring signal strength (uplink or downlink or both) followed, optionally, by a step of estimating the path loss, or the downlink interference or both. 
         [0039]    If there are limited measurement capabilities in the network  400 , the above noted estimations/calculations associated with steps  506  and  508  may be performed in cell priority order based on the handover statistics noted in step  500 , i.e., the cell with the highest probability of handover is calculated first and so on. Note that in one embodiment some or all of the estimations/calculations performed in steps  506  and/or  508  may be performed by the UE. Thus, the UE service performance threshold can be based on path loss and/or downlink performance. The decision whether to use only the path loss performance, only the downlink performance or a combination of the two may be made in step  510  or prior to preparation step  500 . If the decision is made in step  510 , it may be implemented as now discussed. In one application, it is determined whether the UE experiences poor uplink or downlink service performance. If poor uplink service performance is determined, then path loss or uplink signal strength is used for grouping. If poor downlink service performance is determined, then downlink interference is used for grouping. If the decision is made prior to step  500 , for example, based on historic data, the software supporting this method is programmed to not execute step  508  or to execute step  508  instead of step  506  or to execute both of them. In another application, the path loss may be used for grouping to apply a downlink interference management technique when a UE experiences bad downlink service performance. 
         [0040]    In step  512 , the cell coordination group is formed by including the serving cell and the neighbour cells with the highest interference and/or highest path loss (as estimated in steps  506  and  508 ). Because the number of members of the cell coordination group may be limited due to hardware and/or software features as discussed above, the number of neighbour cells that are part of the group is selected from the highest to lowest in terms of interference and/or path loss. 
         [0041]    In one application, the process advances to step  514 , where a decision is made whether to return to step  502  or to end the process. If the decision is made to return to step  502 , a consequence of the monitoring step is that the number of neighbor cells may increase or decrease. The method returns to step  502  until UE  460  has completed its session, leaves the serving cell  410  or its performance becomes better than the threshold. 
         [0042]    The entire method may be repeated for another UE for determining its cell coordination group. In one application, the UEs in the serving cell are addressed sequentially while in another application the UEs are addressed simultaneously. The approach depends on the amount of calculating capabilities present in the base stations and the node hosting the method of  FIG. 5 . In one application it is possible that some of the UEs present in the serving cell are addressed at the same time. Note that if interconnect transport characteristics are violated for a neighbour cell, the neighbour cell may be excluded from the cell coordination group. 
         [0043]    The above discussed methods may be implemented in software, hardware or a combination of the two in a node of the telecommunication network. They may be implemented in part or all of the nodes of the telecommunication network. A node may be considered any part or sub-part of the telecommunication network. They also may be implemented at least in telecommunication networks having base stations structured as illustrated in  FIGS. 2 and/or 4 . If the methods are implemented in software, one or more modules may be designed to accomplish the steps illustrated in  FIG. 5 . Thus,  FIG. 6  shows a node (belonging to a telecommunication network)  600  having modules  602  to  614  that execute in software, when run on the node, the functions noted in steps  500  to  512 . Modules  602  to  614  are labelled similar to the functions noted in steps  500  to  512  of  FIG. 5  for a simple identification of each function. Node  600  may also include a processor  620  that runs one or more of modules  602  to  614 , a data storage unit  622  (for storing, for example, the neighbour cell list) connected through a bus  623  to processor  620  and an input/output unit  624  that links the node to one or more base stations or other equipment of the telecommunication network. In one embodiment, node  600  is one of the base stations. Note that  FIG. 6  shows the modules not connected to each other as they may be software representations. However, the modules may be connected to each other, for example, in the order shown in  FIG. 5 . Processor  620  may be configured to implement each step of the method illustrated in  FIG. 5 . 
         [0044]    According to an embodiment illustrated in  FIG. 7 , a method for forming a cell coordination group in a telecommunication network is now discussed. The method includes a step  700  of obtaining, for a serving cell, a neighbor cell list indicative of neighbor cells of the serving cell, a step  702  of selecting a user equipment connected to the serving cell, wherein the selection of the user equipment is based on service performance provided by the serving cell to the user equipment, a step  704  of estimating downlink interference of the user equipment for the neighbor cells or a path loss of the user equipment for each neighbor cell, and a step  706  of grouping the serving cell with selected neighboring cells to form a cell coordination group, wherein the selected neighboring cells are selected based on the estimated downlink interference or the path loss. Note that it is possible to measure signal strength and then to estimate downlink interference and/or path loss based on the measured signal strength. 
         [0045]    The disclosed embodiments provide a telecommunication network node and method for grouping various cells in a cell coordination group that is used by the interference management unit of the network to reduce the interference between the cells. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
         [0046]    As also will be appreciated by one skilled in the art, the embodiments may be embodied in a wireless communication device, a telecommunication network, as a method or in a computer program product. Accordingly, the embodiments may take the form of an entirely hardware embodiment or an embodiment combining hardware and software aspects. Further, the embodiments may take the form of a computer program product stored on a computer-readable storage medium having computer-readable instructions embodied in the medium. Any suitable computer-readable medium may be utilized, including hard disks, CD-ROMs, digital versatile disc (DVD), optical storage devices, or magnetic storage devices such a floppy disk or magnetic tape. Other non-limiting examples of computer readable media include flash-type memories or other known memories. 
         [0047]    Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. The methods or flow charts provided in the present application may be implemented in a computer program, software or firmware tangibly embodied in a computer-readable storage medium for execution by a specifically programmed computer or processor.