Abstract:
The invention relates to a method for determining a set of replacement transmission parameters for a plurality of cells ( 108; 110; 112 ) of a digital cellular wireless telecommunication network ( 100 ) for transmissions, wherein the method comprises: determining (S 1 ) constraints; determining (S 2 ) a set of current transmission parameters, wherein the set of current transmission parameters comprises parameters currently used in the plurality of cells ( 108; 110; 112 ); evaluating (S 3 ) several candidate sets of replacement transmission parameters by considering the constraints, wherein each candidate set is adapted for replacing the set of current transmission parameters; simulating (S 4 ) network conditions for the plurality of cells ( 108; 110; 112 ) for each candidate set of the several candidate sets; comparing (S 5 ) the simulated network conditions; determining (S 6 ) a best set from the candidate sets by using the results of the comparison; setting (S 7 ) the best set as the set of replacement transmission parameters; using (S 8 ) the set of replacement transmission parameters for wireless telecommunication in the plurality of cells ( 108; 110; 112 ).

Description:
[0001]    The invention relates to the field of wireless digital telecommunication, more particularly to digital cellular wireless telecommunication networks. 
       BACKGROUND AND RELATED ART 
       [0002]    It is known in the state of the art to change data transmission parameters in one cell or in a plurality of cells in order to optimize mobile communication in the respective cells. Changing the data transmission parameters can cause interferences or decrease interferences. For example increasing the transmission power would increase the size of a cell and at the same time increase interferences caused in a neighboring cell. Other parameters such as handover parameters, antenna parameters or used frequencies for transmitting data have an impact on the quality of service of the telecommunication network and/or the energy consumption of the telecommunication network. 
         [0003]    WO 2009/083035 A1 describes a method of upgrading a wireless mobile communications network deployed on the field, comprising: capturing network events from the wireless mobile communications network; obtaining network simulation data from an automated network simulation planning tool; combining the captured network events and the network simulation data to derive diagnostic indicators adapted to evidence criticalities in a current network configuration; and modifying the current network configuration to overcome the criticalities. 
         [0004]    US 2010/298022 A1 describes a cellular radio communications network includes a plurality of radio cells. A target radio coverage is associated with each radio cell. 
         [0005]    Each operational radio cell provides an effective radio coverage defined by a transmission power value of said radio cell. A given transmission power value is applied to a particular set of radio cells. A radio cell is then selected. Thereafter, cellular information is obtained relating to a group of radio cells comprising the selected radio cell and neighboring cells. On the basis of the cellular information, the effective radio coverages of the cells of said group and the respective target radio coverages of the cells of said group are compared. If the effective radio coverage of at least one cell of the group of radio cells is less than its target radio coverage, respective new transmission power values are applied to radio cells of said group of cells. Certain steps are then repeated, as appropriate. 
       SUMMARY 
       [0006]    It is an object of the present invention to provide an improved method for determining a set of replacement transmission parameters, an improved network entity, and an improved computer program product according to the independent claims. Embodiments of the invention are given in the dependent claims. 
         [0007]    The invention relates to a method for determining a set of replacement transmission parameters for a plurality of cells of a digital cellular wireless telecommunication network for data transmissions in the digital cellular wireless telecommunication network. The method comprises determining constraints. Constraints may for example be unchangeable parameters, unusable frequencies or other constraints being not changeable by this method. For example this method is performed by a network entity and another network entity, for example a base station of another vendor, does not accept changes performed within this method. For example constraints may also be that certain frequencies can be used for data transmissions in the respective cells. For example the European Cognitive Radio Project QOSMOS defines frequencies that may be used under certain circumstances by digital cellular wireless telecommunication networks. 
         [0008]    Further a set of current transmission parameters is determined, wherein the set of current transmission parameters comprises parameters currently used in the plurality of cells for data transmissions. Transmission parameters may also be referred to as cell configuration parameters. Transmission parameters may for example comprise transmission times, transmission frequencies, transmission powers, handover related parameters, and/or antenna parameters. Handover related parameters may for example be offsets when handovers are performed or any other parameter related to a handover procedure of a mobile device from a first base station to a second base station. Antenna parameters may for example be a radiation direction of the antenna or an antenna tilt. 
         [0009]    Furthermore, several candidate sets of replacement transmission parameters are evaluated by considering the constraints. Each candidate set comprises transmission parameters that may be used for wireless telecommunication in the plurality of cells. Considering the constraints here means that it is considered that certain parameters may not be changed or may only be changed in a certain range because of the constraints. For example certain frequencies may not be used by the digital cellular wireless telecommunication network. In this case this is a constraint considered for evaluating the several candidate sets. Another constraint would be that a cell does not accept replacement parameters that are not determined on its own. Each candidate set is adapted for replacing the set of current transmission parameters. This means that each candidate set could be used for data transmissions in the plurality of cells. However, this does not mean that each candidate set would cause a better quality of service or more throughput in the plurality of cells than the current set of transmission parameters. 
         [0010]    Then, network conditions are simulated for the plurality of cells for each candidate set of the several candidate sets. Simulating here means that the candidate sets are not applied in the plurality of cells. It is only a simulation. In other words, when the simulation is performed, offline calculations are performed. Data transmissions in the plurality of cells are not affected by these simulations. Afterwards, the simulated network conditions are compared with each other and with the current network conditions. A best set is determined from the candidate sets by using the results of the comparison. For example for each candidate set a quality indicator is determined that may be a relative value or an absolute value. Then these values are compared and a best set is determined. For example the network conditions for candidate sets A, B and C are simulated and the current network conditions when using the current set of transmission parameters D are also known. For example it is determined that parameters A are better than all the other parameters. In this case the candidate set A would be determined as the best set. 
         [0011]    For determining the best set the various factors that are considered for determining the simulation results, such as the quality of service, the data throughput and/or the are taken into account with different or the same weighting factors. For example the quality of service is weighted very high as well as the data throughput. The energy consumption may be weighted not as high as the both previously mentioned indicators. The best candidate set is understood herein as the candidate set that optimizes the network conditions in the plurality of cells. Optimizing the network conditions comprises improving energy consumption, and/or quality of service, and/or data throughput, and/or interference conditions. It is possible that the best set is not explicitly numercially the best set (e.g. the one offering the best quality of service). 
         [0012]    The best set is then set as the set of replacement transmission parameters. The set of replacement transmission parameters is used for wireless telecommunication in the plurality of cells instead of the current transmission parameters. In other words the current transmission parameters have been replaced by the replacement transmission parameters. 
         [0013]    According to embodiments of the invention the constraints comprise fixed parameters of at least one cell of the plurality of cells, and/or radio frequencies that are allowed to be used for wireless telecommunication in the plurality of cells. Parameters of at least one cell of the plurality of cells can be fixed for example because an administrator of the network has set these parameters as fixed for a reason not known to the plurality of cells. Another possibility would be that a base station of the at least one cell of the plurality of cells does not cooperate with other base stations of the wireless telecommunication network and does not accept to modify parameters being determined by a method according to embodiments of the invention. 
         [0014]    Herein the term base station refers to a network entity which serves at least one cell of the digital cellular wireless telecommunication network. Several cells can be served by the same base station. A cell may also be referred to as a sector. 
         [0015]    Radio frequencies that are allowed to be used for wireless telecommunication can also be whole frequency bands that are allowed to be used for wireless telecommunication or sub-bands. In other words, the term radio frequencies as used herein refer to a range of frequencies. 
         [0016]    According to embodiments of the invention the allowed radio frequencies are Cognitive Radio frequencies. In the context of Cognitive Radio, one of the major challenges is to organize and to decide, which radio access entity (e.g. basestation) is using which part of the spectrum and with which power. This challenge is subject to strong interactions between the basestations, such as inter-cell interferences and interactions in the “coverage areas” of different nodes. 
         [0017]    As this issue is too complex for manual handling, powerful self-organizing networks (SON) techniques are required to solve this automatic self-organizing problem for Cognitive Radio. 
         [0018]    Thereby distributed SON functionalities are needed, because of the diversity of the cognitive system with many different “players” who all want to paticipate at potentially using a part of the available spectrum and a centralized control “of everything” is not possible. 
         [0019]    Furthermore, the distributed SON solution shall optimize the situation individually for each cell, as in the potentially very diverse cognitive radio scenario, it is not possible to have the same configurations for each cell; each cell needs to be optimized, while considering all the interactions and couplings with other cells and with other radio nodes. 
         [0020]    As a particular challenge, the SON entities and algorithms have to be “robust” against external “disturbances” and “robust” against “non-desired, strange, non-cooperating, . . . ” behaviour of other nodes within the whole cognitive system. For example, another node may not behave “correctly” and is causing much interference on a particular part of the spectrum or another node is non-cooperating and is doing whatever that other node “likes” to do. In such situations, the distributed cognitive SON system needs to optimize itself around that “disturbing node” and then optimize itself, its distributed cognitive SON participating nodes, in such a way, that they consider the disturbance as a kind of external constraints and optimize around the non-cooperating other radio node. 
         [0021]    This is a related challenge to mixing non-cooperating basestations from different vendors. For example when a vendor wants to sell small metro cells into an existing and non-cooperating macro-cell network of another vendor, then the small cells have to adapt and optimize themselves in the best possible way to the external situation imposed by the other vendors macro cells. Such a self-X feature would then allow the small metro cells to work well together with existing and non-cooperating macro basestations of the other vendor. 
         [0022]    The European cognitive radio project QoSMOS (http://www.ict-qosmos.eu) is addressing exactly this challenge, and has introduced an architecture with distinct cognitive radio entities, the “Spectrum Databases/Repositories etc”, the “Spectrum Manager” and the “Resource Manager”. 
         [0023]    For clarity, the naming and functionalities is recalled as used in the QoSMOS project and are related to typical nodes in cellular mobile networks: 
         [0024]    1) (Cognitive Radio) Spectrum Database(s), Repositories, Sensing Information, . . . : 
         [0025]    These are—possibly operator independent-entities—which provides information about the amount of spectrum, about the frequency bands, which are available in a particular area. This could be compared to (more or less) static network planning. This information could be considered as external constraints, which the SON entity for the Spectrum Manager has to obey. 
         [0026]    2) (Cognitive Manager) Spectrum Manager (“CM-SM”): 
         [0027]    This functionality coordinates and decides which concrete frequency resources and power level are allowed be used by a particular cell. It needs to consider the interactions between different cells. This is related to other semi-static system optimizations like e.g. semi-static load balancing. The functionality of this cognitive manager could be related to an Operation&amp;Maintainance Centre or to a powerful SON functionality which finds and sets the configuration parameters of a cell or basestation. 
         [0028]    3) (Cognitive Manger) Resource Manager (“CM-RM”): 
         [0029]    This functionality schedules dynamically the resources to the mobile users on a very short time scale. This is related to the resource scheduler of a cell 
         [0030]    Herein the “Spectrum Manager” may also be referred to as the network entity performing the various method steps. 
         [0031]    While for the QoSMOS project this invention solves the Spectrum and Power configuration, adaptation and optimization challenge, the same invented distributed SON solutions can equally be implemented in LTE basestation product where a related SON challenge with strongly interacting+coupled parameters needs to be solved, also for several other SON use cases and for Light Radio. Best is a generic solution, which can be used as a building block to be added or adapted to several SON use cases in different applications. 
         [0032]    In other words, cognitive radio could also be referred to as dynamic spectrum access. Dynamic spectrum access means that certain frequencies are allowed for certain times to be used. It is determined by measurements or by other information if these frequencies are allowed to be used. For example the frequencies are not allowed to be used if they are used by other systems or entities that are not associated with the digital cellular wireless telecommunication network. 
         [0033]    According to embodiments of the invention the constraints are retrieved from a database, and/or determined by performing measurements, and/or entered manually, and/or received from at least one base station of the plurality of cells. 
         [0034]    For example the constraints can be retrieved from a database, wherein the database is located inside the same network entity which also performs the method according to embodiments of the invention. The database may for example be stored on a storage medium such as a hard disc drive, a solid state drive, random access memory, read only memory and/or optical storage drives. For example the database defines base stations of the network which do not cooperate and do not accept parameters determined by the method according to embodiments of the invention. Another example would be that certain frequency ranges are comprised by the database which are allowed to be used by the digital cellular wireless telecommunication network. Another possibility is that the constraints are determined by performing measurements. This could also be referred to as sensing the spectrum. Sensing the spectrum means that it is measured, for example by the same network entity that also performs the other method steps, or another base station, which frequencies may be used for telecommunication in the network. For example some frequency ranges are defined as optionally being used by other systems. In this case it is measured if these frequency ranges are used by other systems. If the frequency ranges are not used by other systems they may be used for telecommunication in the telecommunication network. Another possibility would be to enter the constraints manually. This could be for example done by an administrator in an operation and maintenance center. The administrator could for example define certain frequency ranges that are not allowed to be used by the network or other parameters that are fixed or that may be varied only in a certain range. Another possibility is that the constraints are received from at least one base station of the plurality of cells. This means that the at least one base station sends the constraints to the network entity that performs the method according to embodiments of the invention. For example the base station sends a signal to the network entity that performs the method according to embodiments of the invention, wherein the signal defines certain parameters applied by the at least one base station as fixed. Another possibility would be that the at least one base station sends a signal to the network entity that performs the method according to embodiments of the invention, wherein this signal defines certain ranges for certain parameters which are allowed ranges. This means that the at least one base station would apply parameters inside these ranges. Parameters outside these ranges would not be applied by the at least one base station. 
         [0035]    According to embodiments of the invention the method is performed by a network entity, which is associated with a first cell. The plurality of cells comprises the first cell and cells being located in a neighboring region of the first cell. In other words, the method is performed for a center cell, which is referred herein as first cell, and a neighboring area of this center cell. The network entity performing the method according to embodiments of the invention could either be associated with a base station or be part of a base station. The neighboring area of the first cell comprises at least direct neighbors of the first cell. Preferably, even second or third neighbors are comprised by the neighboring area. The size of the neighboring area may be determined according to network conditions. Choosing a relatively big neighboring area of the first cell has the advantage that the parameters determined by the method according to embodiments of the invention are simulated for a large number of cells. This results in the fact that the replacement parameters are most likely advantageous for a large region of the network. Choosing a relatively small neighboring area would reduce computation effort. 
         [0036]    According to embodiments of the invention the neighboring area comprises direct neighbors of the first cell and neighbors of the direct neighbors. In other words the neighboring area comprises first neighbors of the first cell and second neighbors of the first cell. The second neighbors are the neighbors&#39; neighbors. 
         [0037]    According to embodiments of the invention the steps of evaluating, simulating, comparing, determining the best setting and using the best set are triggered by a periodic timer, and/or a random timer, and/or a trigger message, and/or a change of the current transmission parameters, and/or a change of the constraints, and/or a traffic load threshold. Using a periodic timer for triggering the steps is advantageous for example for defining time periods after which the method according to embodiments of the invention shall be performed. 
         [0038]    Using a random timer is advantageous for avoiding that first changes performed by a first network entity are changed again by a second network entity before they are changed back again to the previously set parameters by the first network entity. This could for example happen when a first network entity for example sets a parameter set A, then a second network entity sets parameter set B and then the first network entity again sets the parameter set A. By using the random timer it can be avoided that always the first network entity performs the method according to embodiments of the invention before the second network entity. By using the random timer for example a third network entity could perform the method according to embodiments of the invention in between the first and the second network entity. By this change such previously described ring changes can be avoided. 
         [0039]    A trigger message for triggering the method steps could for example be a message transmitted from at least one base station of the plurality of cells to the network entity performing the method according to embodiments of the invention. The trigger message can for example simply be a trigger to perform the method steps or another message indicating to perform the method steps. A change of the current transmission parameters can also trigger the method steps. For example the transmission parameters can be changed by another network entity, manually by an administrator or by another base station. In this case performing the method can be triggered when the change of the transmission parameters is detected. 
         [0040]    The same applies for the constraints. If a change of the constraints is detected, the whole network situation may be changed and a replacement set of parameters may be advantageous for data transmissions in the network. 
         [0041]    Also a traffic load threshold may be used for triggering the method steps. For example a traffic load threshold is defined and when the traffic load in at least one cell of the plurality of cells reaches the traffic load threshold the method steps are performed. This may be advantageous when replacement parameters could be better suited for handling the high traffic load. 
         [0042]    According to embodiments of the invention the method is interrupted or cancelled when it is indicated that the set of current transmission parameters shall be changed. This may for example happen when the method according to embodiments of the invention is performed by a second network entity and a replacement set of transmission parameters is determined by this second network entity. In this case the first network entity performing the method according to embodiments of the invention interrupts or cancels the method. This is advantageous before any method step performed by the first network entity is based on the previously used transmission parameters because these transmission parameters are changed the basis for the method steps is not valid anymore and interrupting or cancelling the method is performed preferably. The method may also be interrupted or cancelled when the set of current transmission parameters is changed for another reason, for example a base station changes the current transmission parameters based on its own computations or another set of current transmission parameters is set by an administrator. 
         [0043]    According to embodiments of the invention the current set of transmission parameters is received from base stations of the plurality of cells. The best set of transmission parameters is sent to each base station of the plurality of cells. In this case the steps of evaluating, simulating, comparing, determining the best set, setting and using the best set are performed at least a second time, only if it is determined that the constraints and/or the set of current transmission parameters have been changed significantly since having determined the constraints and/or the set of current transmission parameters most recently. For example a difference between the set of current transmission parameters and the best set is used for determining if the parameters have been changed significantly. For example the steps are only performed at least a second time if it is determined that the set of current transmission parameters differs more than a difference threshold from the best set of transmission parameters. The same applies analogously for the constraints. For example if the constraints differ only slightly the steps are not performed a second time because of this change. However, if the constraints have changed and the new constraints differ from the old constraints more than a constraints threshold it is determined that the steps are performed at least a second time. 
         [0044]    According to embodiments of the invention each set of transmission parameters comprises at least one of the following parameters: transmission times, transmission frequencies, transmission powers, handover related parameters, and/or antenna parameters. The parameters transmission times and transmission frequencies may for example be used for avoiding interferences. For example the same frequencies may be used in neighboring cells at different transmission times. The transmission powers define the size of the respective cells and are also related to interferences caused by one cell with another. Antenna parameters comprise parameters such as radiation direction and antenna tilt. Handover related parameters may be signal thresholds for determining when a handover shall be performed and/or times for how long a signal of a target cell of the handover procedure has to be stronger than a signal power threshold. 
         [0045]    According to embodiments of the invention the step of simulating is performed by dividing each cell of the plurality of cells into virtual sub areas and by simulating interference conditions and data transmission efficiency in the sub-areas. Using the sub-areas for simulating is advantageous for reducing computation effort and achieving good simulation results. 
         [0046]    The term “data transmission efficiency” may also be referred to as “resource efficiency”. 
         [0047]    According to embodiments of the invention the simulation is performed based on previously performed measurements. For example the network entity performing the method knows from previously performed measurements the network conditions when applying certain parameters. For example the network entity knows an equation of how the cell size is changed when the transmission power is changed by a certain value. 
         [0048]    According to embodiments of the invention the set of replacement transmission parameters is used for wireless telecommunication, only if the set of replacement transmission parameters has not been used for wireless telecommunication during a predetermined time period in the past, and/or the set of replacement transmission parameters lies inside a predetermined region for allowed transmission parameters, and/or the set of replacement transmission parameters does not downgrade network conditions in at least one cell of the plurality of cells more than a downgrade threshold. 
         [0049]    By using the predetermined time period in the past it is avoided that changes that have been set by a first network entity are not changed back by a second network entity and then changed back again by the first network entity and so on. For example it could be the case that a first network entity sets the parameter set A, then the second network entity sets the parameter set B and then the first network entity again sets the parameter set A. This can be avoided by using the predetermined time period. If the parameter set A has been set in this predetermined time period the parameter set A is not set again for avoiding the previously described ping-pong change. Further, it may be advantageous for determining a region for allowed transmission parameters. For example only some frequencies are allowed for wireless telecommunication because of law restrictions or restrictions set by the administrator. Furthermore, it is avoided that the network conditions in one certain cell of the plurality of cells is downgraded by more than a downgrade threshold because this would be too disadvantageous for users being located in this cell. 
         [0050]    In another aspect the invention relates to a network entity comprising means for determining constraints, means for determining a set of current transmission parameters, wherein the set of current transmission parameters comprises parameters currently used in the set of plurality of cells. Further, the network entity comprises means for evaluating several candidate sets of replacement transmission parameters, wherein each candidate set is adapted for replacing the set of current transmission parameters. Furthermore, the network entity comprises means for simulating network conditions for the plurality of cells for each candidate set of the several candidate sets, means for comparing the simulated network conditions, means for determining a best set from the candidate sets by using the comparison results, means for setting the best set as the set of replacement transmission parameters, and means for using the set of replacement transmission parameters for wireless telecommunication in the plurality of cells. The various means of the network entity may be implemented by a processor executing program instructions stored on a storage medium. The network entity is adapted for performing a method according to embodiments of the invention. 
         [0051]    In another aspect the invention relates to a computer program product comprising instructions that when being executed cause a network entity to perform a method according to embodiments of the invention. 
         [0052]    The following description explains embodiments of the invention in a more detailed way. 
         [0053]    1) Fully distributed CM-SM SON architecture:
       There is an individual “Cognitive Manager Spectrum Manager” (CM-SM) in (or for) each (e.g.) radio access node, e.g. for each basestation or for each cell.   This CM-RM decides on a “longer”, e.g. semi-static time scale, which part of the spectrum portfolio (which part(s) of the bandwidth part(s), which part(s) of the frequencies, which part(s) of the spectrum(s)) and which other relevant configuration parameters (i.e. transmission power) the “Cognitive Manager Resource Manager” (CM-RM) is allowed to used. The CM-RM then operates on a shorter time scale (e.g. dynamic) within the parts of the spectrum portfolio and within the configuration constraints set by the CM-SM.       
 
         [0056]    2) the SON Entity of the CM-SM Operates on a “Distributed Local Area”:
       Each CM-SM is optimizing a “local area”, this means it is optimizing the spectrum portfolio and the relevant parameters (such as i.e. transition power) for itself, and for other “neighbouring” CM-SMs within a “local area”.       
 
         [0058]    Due to the interactions and due to the interferences the spectrum- and power settings of neighbouring CM-SM entities are highly coupled, they cannot be individually optimized, and during the parameter finding process, the situation, setting, interactions with and from neighbouring entities have to be considered. The local area contains that group of CM-SMs which need (or should) be considered as there are directly interactions with the “centre” CM-SM. 
         [0059]    3) CM-SM SON Entity Optimization Procedure
       The SON entity/functionality of the “centre” CM-SM″ is evaluating possible candidate sets of parameter combinations out of the complete parameter space of all possible parameter combinations within the CM-SMs in the local area. The simplest algorithm would be to assess all options via brute force, but there are more intelligent and more runtime efficient search algorithms. Thereby each particular parameter set is predicted via a “sufficiently well suited” prediction model to access the expected performance of the system when this particular parameter set would be installed. The prediction of the future network performance is very tricky, requires innovative novel approaches, and this solution is part of another invention. It shall be noted again, that this is an offline assessment of possible candidate parameter sets, without actually installing (testing trying) these in the field. After having virtually assessed a/the large set of candidate combinations, the SON entity then selects the best suited one and these settings are installed within the local area.       
 
         [0061]    In this way, the optimal (predicted) parameter set is found and installed for each CM-SM within the local area. 
         [0062]    4) CM-SM signallings/message exchanges 
         [0063]    The CM-SMs exchange the following kind of information
       a) Information about their current configurations and settings, e.g. which part of the spectrum portfolio is
           assigned to use and parameter configurations such as e.g. transmission power.   
           b) Information about—e.g. averaged values—about currently experienced (average) “radio and load
           conditions”, such as e.g. about their traffic load and about how much interference is observed on   a particular part of the spectrum portfolio.   
           c) Commands (suggestions) from one CM-SM to another CM-SM to use a certain part of the spectrum
           portfolio, and to use a certain configuration parameters, such as e.g. a certain transmission power.   
           d) Optionally, direct trigger messages to initiate an action such as to start the local area evaluation+
           optimization procedure.   
           Thereby the invention should not be restricted to the actual message exchange path, i.e. the invention shall protect all options, whether the exchange is via direct messages between the CM-RM entities, whether the message exchange goes via the core network or whether maybe—e.g. via tunnelling techniques—the transport formats of underplaying legacy network.       
 
         [0074]    5) CM-SM Triggers, Timers, Delays, Interrupts
       The local optimization procedure of the SON entity of one CM-SM is or can be triggered by the following events:   a) Periodic timer, optionally with a short random time variation: Each SON entity of each CM-SM may “periodically” trigger itself to check the situation and to evaluate whether to run the local area optimization procedure.   b) When the CM-SM receives new status information, e.g. a changed configuration in its neighbourhood, or a new load or interference information, or new external constraints (e.g. spectrum database), then the SON entity of the CM-SM is triggering itself to evaluate the situation to access whether or not to run the complete local area optimization procedure.   c) External trigger, asking the CM-SM to assess and if needed to optimize the situation.       
 
         [0079]    An external entity (e.g. spectrum database) or the CM-RM have the possibility to ask the CM-SM to optimize its local area situation. The CM-RM may evaluate itself that there is the room for optimization (e.g. the CM-RM evaluates, that it may better be assigned more spectrum resources) and is then asking its CM-SM to make a re-evaluation of the situation—and thereby considering the new—e.g. high load—situation within the CM-RM.
       d) All trigger timings have an—e.g. small—random component in order to make it likely that different cells start and finish their optimization procedures at different times. (But when this case somewhen occurs, then see next point). Different kind of actions/events may have different timer delays in order to priories the order of which CM-SM runs (with a large probability) its SON optimization first.   e) Interrupts: If a CM-SM is currently running the optimization procedure, and exactly when executing the computations this CM-SM is at the same time receiving new external information, such as e.g. a changed spectrum portfolio in the neighbouring cell, then the already started optimization procedure is stopped, no spectrum or parameters are changed, and a new optimization procedure is scheduled to be re-started in the near future (with a small random delay component)       
 
         [0082]    Prior to running the actual optimization procedure with varying all the parameter combinations, the SON algorithm may optionally check or evaluate whether the situation has “sufficiently” changed since the last optimization procedure in order to decide whether actually running the optimization procedure is likely to improve the situation, or whether it may not be necessary because the result of the previous optimization run is still valid and thus this newly triggered optimization procedure may be skipped. 
         [0083]    6) Ensuring and Enforcing Stability and Convergence in these Distributed CM-SM SON Entities:
       These stability and convergence issues are a very critical aspect, and needs to be solved by intelligent and advanced techniques. Different SON entities optimize parameters of each other and of other cells, there are several—independent—SON entities who all try to set/configure on the same parameters, as the local areas of different SON entities strongly overlap. But this is not an issue arising from the tool of “local areas”. Even if each SON entity would only configure its own CM-SM, then still there are strong interactions between neighbouring CM-SMs (interferences, spectrum coordination issues, “CM-SM controlled areas” (e.g. cell areas) etc.   In these distributed CM-SM entities, there are i.e. two kind of stability effects which risk to occur:   1) Ping-Pongs and Ring-Ping-Pongs: The SON entity of one CM-SM is changing a parameter and then
           another CM-SM entity is changing it back to its original value.   
           2) Propagating wave of changes: One (first) SON entity of one (first) CM-SM is changing a (first) parameter.
           This change then induces a new SON decisions in a (second) SON entity of a (second) CM-SM with the   result that a (second) parameter is changed. This change of the (second) parameter then leads—via a third
               SON process to a change in a third parameter . . . and so on.   
               The following stability and robustness solutions are to be employed:   a) History lists, including information of the surrounding cells, in order to detect Ping-Pong and Ring-Ping-Pongs   b) Selection of a possibly suitable parameter combination in order to break the (Ring-) Ping-Pong loop.
               Temporarily storing a (large) set of acceptable parameter settings and   thereafter choosing a suitable combination out of the pre-stored ones in order to break the loop.   
               c) Considering the “change cost” for performing a particular type of parameter change.   d) Optionally including self learning functionalities to dynamically modify the parameter change costs
               for individual cells.   
               e) Preference to restrict any modifications to an as small local area as possible.   Optionally the sensitivity (damping) threshold can be made self-learningly, e.g. in such a way, that the threshold may slightly be increased if needed or if beneficial.   
               
 
         [0102]    7) Handling and Optimizing Around Non-Cooperating Nodes and Handling Erroneous or Disturbing Situations.
       Optimally, all nodes have a SON entity implemented, cooperate and work correctly. However, the situation can occur, that there are nodes which do not have a powerful SON functionality, or which do not cooperate such as not following orders, and it may also occur that a node shows erroneous or a particular fixed behaviour such as that node transmits on a certain frequency band but without any possibilities of the SON system to influence the behaviour of that other node. This non-cooperating behaviour could e.g. occur, when the SON supporting basestations are installed in an area where also non-SON supporting basestations of another vendor are present. In this case, the SON supporting basestations have to handle this situation and self-organizingly optimize themselves in the best possible way to the given situation imposed by the disturbing other vendors node.       
 
         [0104]    During the local area parameter optimizations, the SON algorithm in the (centre) cell searches and evaluates the available parameter space within the local area, which includes the option to modify configuration 
         [0105]    parameters in neighbouring cells. In the case, that another cell within the local area, e.g. another cell nearby which causes interference, does not support (for whatever reason) the SON features, then the (centre) cell considers the settings of the other non-cooperating cell as fixed constraints which cannot be modified. Then the parameter search algorithm analyses the possible parameter variations in those cells within the local area which support the SON feature. Thereby the SON algorithm “optimizes around” the non-cooperating node; the SON supporting cells self-optimize themselves to the best possible solution, using their SON-degrees-of-freedom, to adapt themselves to the given external situation of other cells. 
         [0106]    One typical application is that new cells are added to an existing network which does not support SON—such as e.g. an old or other vendors macro cell network. Then the SON-supporting small metro cells optimize all their own possible configuration parameters in such a way, that these are optimally set to cope with the external situation. As a result, the newly added cells adapt themselves automatically to ensure that they will work well together with the existing non-cooperating network. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0107]      FIG. 1  is a schematic view of a digital cellular wireless telecommunication network, 
           [0108]      FIG. 2  is a block diagram of a network entity and a base station being adapted for performing a method according to embodiments of the invention, 
           [0109]      FIG. 3  is a schematic view of a plurality of base stations and network entities being adapted for performing a method according to embodiments of the invention, 
           [0110]      FIG. 4  is a block diagram illustrating the message exchange between network entities according to embodiments of the invention, 
           [0111]      FIG. 5  is a block diagram illustrating the message exchange between a network entity and base stations according to embodiments of the invention, 
           [0112]      FIG. 6  is a schematic view of a digital cellular telecommunication network according to embodiments of the invention, 
           [0113]      FIG. 7  is a block diagram of a network entity according to embodiments of the invention, and 
           [0114]      FIG. 8  is a flow diagram of a method according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0115]      FIG. 1  is a schematic view of a digital cellular wireless telecommunication network  100  comprising a plurality of cells  108 ,  110  and  112 . The cells  108 - 112  may also be referred to as sectors. Cell  108  is served by base station  102 , cell  110  is served by base station  102 ′, and cell  112  is served by base station  102 ″. Although in  FIG. 1  each cell is served by one base station it may be the case that one base station serves several cells.  FIG. 1  further comprises smaller base stations (without reference sign) that serve cells which lie inside the cells  108 ,  110  and  112 . Each base station  102  uses resource blocks for digital telecommunication with mobile devices. Resource blocks may for example a frequency range that is used for a time period for telecommunication in the respective cell. In  FIG. 1  the resource blocks are schematically depicted as blocks below the respective base station  102 . For example base station  102  uses three out of six resource blocks. Base station  102  uses the first, the second and the sixth resource block. Base station  102 ′ uses all six resource blocks and base station  102 ″ uses four resource blocks, namely the first and the fourth, the fifth and the sixth. Using the same resource blocks neighboring cells may cause interferences for communication with mobile devices in the region where the two cells overlap. Further interferences may be caused by the smaller base stations that serve cells that lie inside cells  108 ,  110  and  112 . Parameters used for telecommunication in the respective cells are for example the resource blocks, and the transmission power. The transmission power may be used for varying the cell size as indicated by the arrows in  FIG. 1 . For example when the transmission power of base station  102  is increased the cell size of cell  108  is also increased and interferences between cell  108  and cells  110  and  112  may also become more significant. Another parameter that can be set for each base station  102  are the resource blocks. A resource block is a frequency range and time period that is used for transmitting data inside the cell. When the same frequency range is used at the same time in neighboring cells interferences may happen. By optimizing the parameters described above, interferences may be decreased and telecommunication in the network  100  is made more comfortable for users. 
         [0116]    For example the base stations  102 ,  102 ′ and  102 ″ are adapted to accept parameter changes that are determined by a method according to embodiments of the invention. However, the smaller base stations that serve the cells that lie inside the cells  108 - 112  may not be adapted to accept external changes. For example one or more of the smaller base stations use parameters that are set manually by an administrator. The parameters used by these smaller base stations are considered for the method according to embodiments of the invention as constraints such as other constraints of the network  100 . This is advantageous because the parameters of the base stations  102 ,  102 ′ and  102 ″ can be optimized by methods according to embodiments of the invention although the smaller base stations cannot be optimized in the same way. Other constraints for the method according to embodiments of the invention can be for example the fact that only certain frequency ranges are allowed to be used for data transmissions in the cells  108 - 112 . 
         [0117]      FIG. 2  is a block diagram of a network entity  200 , and a base station  202 . The network entity is adapted for performing a method according to embodiments of the invention. The network entity retrieves information from a spectrum database  210 , spectrum sensing  212 , and/or further information about network conditions  214 . The base station  202  retrieves information from other base stations  204 , information from mobile devices  206 , and/or information about interferences from other network devices or from a database. In the example of  FIG. 2 , the network entity  200  uses the information retrieved from the spectrum database  210 , spectrum sensing  212  and the further information  214  for determining constraints, which may for example be frequencies that are allowed to be used or parameters of other base stations that cannot be changed. Afterwards a set of current transmission parameters is determined by the network entity  200 . The set of current transmission parameters is determined by requesting it from base station  202 . Afterwards, several candidate sets of replacement transmission parameters are evaluated by network entity  200 . When performing the evaluation step network entity  200  considers the constraints. Preferably the several candidate sets are evaluated by varying the current transmission parameters. 
         [0118]    Then, the network entity  200  simulates network conditions for the plurality of cells for each candidate set of the several candidate sets. The simulation results are compared with each other and it is determined which set from the candidate sets is the best set. Then, the best set is transmitted to base station  202  and is used for data transmissions in the respective cell. 
         [0119]    When performing this method the network entity  200  considers the information  210 ,  212  and  214 . The information may be previously acquired or directly measured at the time of performing the method. For example the network entity  200  retrieves information from the spectrum database concerning frequency ranges that may be used for telecommunication. Then, further information is retrieved from spectrum sensing  212 . Spectrum sensing could for example mean that certain frequency ranges that are allowed to be used are sensed for data transmissions from other systems. If there are no data transmissions from other systems the respective frequency ranges can be used for wireless telecommunication by base station  202 . Base station  202  can retrieve information such as information about interferences, quality of service, energy consumption, and/or traffic load from other base stations and forwards this information  204  to network entity  200 . Then, network entity  200  can consider this information also for evaluating the candidate sets and determining the best set. Further, the base station  202  can use information  206  from mobile devices and forward this information  206  also to network entity  200 . This information  206  may for example be sensing information that has been sensed by the mobile devices. Also information about network conditions  208 , such as information about interferences, quality of service, energy consumption, and/or traffic load can be forwarded by base station  202  to network entity  200 . 
         [0120]      FIG. 3  is a block diagram of several base stations  202 ,  202 ′, and  202 ″ with respective network entities  200 ,  200 ′ and  200 ″. Each network entity  200  is associated with a base station  202 . The network entities  200  can exchange information with each other and retrieve further information from database  210  and/or from spectrum sensing  212 . 
         [0121]      FIG. 4  is a block diagram illustrating the message exchanged between network entities  200  and  200 ′. First, network entity  200  requests information from database  210 , from spectrum sensing  212  and/or further information about network conditions  214  by transmitting request message  400  to the database. The database may for example be located inside network entity  200  or in another network entity, for example a central network entity. The database may for example be stored in a storage medium. The information requested by network entity  200  is transmitted to network entity  200  from the database by transmitting message  402  as a response to the request message  400 . Optionally network entity  200  may also request information from the second network entity  200 ′ by transmitting request message  400 ′ to network entity  200 ′. Network entity  200 ′ then transmits response message  402 ′ that comprises the requested information to network entity  200 . Then, network entity  200  determines the candidate sets and simulates network conditions with these candidate sets and determines the best candidate set. It is important to be noted that network entity  200  only simulates the network conditions. At this time no parameters in the wireless telecommunication network have been changed. The best set of parameters is then determined and transmitted in message  404  to network entity  200 ′. Message  404  can be an install command that instructs network entity  200 ′ to apply the determined parameter set. Alternatively message  404  can only be a suggestion and network entity  200 ′ is not forced to apply the determined parameters. 
         [0122]      FIG. 5  is a block diagram illustrating message exchange between two network entities  200 ,  200 ′ and base station  202 . First, network entity  200  requests information about network conditions and/or currently used parameters from base station  202  by transmitting the request  500  to base station  202 . Base station  202  then answers this request by transmitting information  502 . Information  502  may for example be information about currently used parameters, constraints, and/or information about network conditions. Information about network conditions may for example be interference information, cell load information or information about handover parameters used by the cells served by base station  202 . 
         [0123]    Optionally the base station  202  may also transmit a request for resources  504  to the network entity  200 . The request for resources indicates how many and/or which resources are required by the base station for data transmissions in the respective cell. The request  504  can be considered by the network entity  200  when evaluating the candidate sets. It is also possible for the network entity  200  to ignore the request  504 . 
         [0124]    Network entity  200  may also exchange information in step  506  with another network entity  200 ′. The exchange of information may be advantageous for both network entities  200  and  200 ′ as both network entities need as much information as possible about the plurality of cells for which the replacement set of parameters shall be determined in order to determine the best possible set. Network entity  200  may also request information from spectrum sensing  212 , spectrum database  210  and/or further information about network conditions  214  by transmitting request message  400  to the database. The database may transmit a response message  402  comprising the requested information to network entity  200 . The information transmitted within the response message  402  may be previously acquired information or information acquired at that moment. In step  508  network entity  200  evaluates the candidate sets, simulates the network conditions for each candidate set and determines the best candidate set. Then, by sending message  509 , the replacement set of parameters is transmitted to base station  202  and optionally also to other base stations being not depicted in  FIG. 5 . The replacement set of parameters is then used in step  510  by the base station  202  for data transmissions in the cell served by base station  202 . 
         [0125]      FIG. 6  is a schematic view of a digital cellular wireless telecommunication network comprising a plurality of base stations  102 . Each base station  102  serves one cell, which may also be referred to as a sector. In each cell at least one smaller base station  600  is located. The smaller base station  600  can be considered by the method according to embodiments of the invention as constraints. The parameters of the smaller base station  600  may not be changed by the method according to embodiments of the invention. In other words, a network entity according to embodiments of the invention may determine a replacement set of transmission parameters for base stations  102 ,  102 ′ and  102 ″. However, the method may not determine replacement parameters for the small base station  600 . This is why these parameters are considered as constraints in the sense of the invention. This helps to optimize the parameters of base stations  102 ,  102 ′ and  102 ″ by considering the unchangeable parameters of the smaller base station  600 . 
         [0126]      FIG. 7  is a block diagram of a network entity  700 . The network entity  700  comprises a processor  702  and a storage medium  704 . The storage medium  704  comprises program instructions  705  that may be executed by a processor  702 . The network entity  700  further comprises an interface  706  which is adapted for communication with another network entity and/or a base station according to embodiments of the invention. Optionally the storage medium  704  may also comprise a database comprising previously acquired information that may be used for determining the candidate set according to embodiments of the invention. 
         [0127]    In operation, the processor  702  executes program instructions  705  in storage medium  704 . This causes the processor  702  to determine constraints. The constraints may for example be fixed parameters of at least one cell of the plurality of cells and/or radio frequencies that are allowed to be used for wireless telecommunication in the plurality of cells. Then, the processor  702  determines a set of current transmission parameters. These current transmission parameters are currently used in the plurality of cells by the base stations for wireless telecommunication. The processor  702  then evaluates several candidate sets of replacement transmission parameters, which consider the constraints. Then, the processor  702  simulates network conditions for the plurality of cells for each set of the several candidate sets. Simulating the network conditions means that the parameters are not set. Simulation may be performed by a simulation algorithm. The simulated network conditions are compared and a best set from the candidate sets is determined by the processor  700 . This best set is then set as the replacement set of transmission parameters and is transmitted via interface  706  to the base stations and optionally also to other network entities. 
         [0128]    The network entity  700  may be associated with only one base station, which means that the method is performed in a self-organized distributed manner. 
         [0129]      FIG. 8  is a flow diagram of a method according to embodiments of the invention. In step S 1  constraints set by the cellular wireless telecommunication network are determined. These may for example be fixed parameters set by an administrator or frequency ranges that are allowed to be used for wireless telecommunication. In step S 2  a set of current transmission parameters is determined. For example the set of current transmission parameters is transmitted to the network entity by a base station. Several candidate sets are evaluated in step S 3 . These candidate sets may be replacement transmission parameters and consider the constraints. In step S 4  network conditions are simulated for each candidate set. In step S 5  these network conditions are compared and in step S 6  a best set from the candidate sets is determined. In step S 7  the best set is set as the set of replacement transmission parameters, which is then used in step S 8  for wireless telecommunication in the plurality of cells. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           100  digital cellular wireless telecommunication network 
           102  base station 
           108  cell 
           110  cell 
           112  cell 
           200  network entity 
           202  base station 
           204  information from other base stations 
           206  information from mobile devices 
           208  interferences 
           210  spectrum database 
           212  spectrum sensing 
           214  further information about network conditions 
           400  request 
           402  response 
           404  install command 
           500  request 
           502  information 
           504  request for resources 
           506  information exchange 
           508  decision 
           509  replacement set of parameters 
           510  use replacement set of parameters 
           600  other base station 
           700  network entity 
           702  processor 
           704  storage medium 
           705  program instructions 
           706  interface