Abstract:
One example method is provided for dynamic allocation of air interface resources in a cellular network comprising at least three wireless cells located within a geographical proximity of each other, the method comprising determining, by a central managing entity, one or more classification rules for classifying each of said plurality of mobile devices according to the one or more classification rules; providing, by the central management entity, to a group of base stations associated with the at least three cells, information that comprises: information that relates to the determined one or more classification rules; information that relates to semi-static allocation of blocks of air interface resources adapted for use by one or more specific members of the group of base stations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims the benefit of priority under 35 U.S.C. §119 from Israeli Patent Application No. 224926 filed in the Israel Patent Office on Feb. 26, 2013, entitled “METHOD AND SYSTEM FOR DYNAMIC ALLOCATION OF RESOURCES IN A CELLULAR NETWORK,” the contents of which are hereby incorporated by reference in their entirety. 
       TECHNICAL FIELD 
       [0002]    The disclosure relates to a system and a method for allocating resources in wireless networks, and in particularly to allocating resources and mitigating interference in cellular mobile communication systems. 
       BACKGROUND 
       [0003]    The 3GPP Long Term Evolution (“LTE”) Specifications define two types of interference mitigation techniques: The first one being interference mitigation by interference reduction, and the second one is interference mitigation by inter cell interference coordination (ICIC). The 3GPP standard handles the two types of interference minimization differently. The first type, interference reduction, is used in conjunction with coverage and capacity optimization. The enablement of interference reduction are RF techniques such as antenna tilt, transmit power reduction, and handover mechanisms. 
         [0004]    The LTE Recommendation has defined an interface between base stations (referred to herein as “eNBs”) which enables the transfer of ICIC related indicators. This interface is referred to as X2. These ICIC function indicators are: Relative Narrowband Transmit Power Indicator (“RNTPI”), High Interference Indicator (“HII”), and Interference Overload Indicator (“OI”). 
         [0005]    The RNTPI indicator message is sent to neighbor eNBs. It contains 1 bit per each physical resource block (PRB) in the downlink transmission, indicating if the transmission power associated with that PRB will be greater than a pre-defined threshold. Thus, neighbor eNBs may anticipate which bands would suffer more severe interference and take the appropriate scheduling decisions immediately, rather than waiting to and relying on the UEs&#39; Channel Quality Information (“CQI”) reports. 
         [0006]    The HII indicator for uplink transmissions has a somewhat similar function as that which was described above in connection with the RNTPI message for downlink transmissions. There is one bit per each PRB, enabling the neighboring eNBs to assess whether they should expect high interference power in the near future. Reference Signal Received Power (“RSRP”) measurements which are reported as part of handover measurement reports, can identify cell edge UEs. In a similar way, this indicator can be used to identify the bands used in a frequency-partitioning scheme. 
         [0007]    While the previously described X2 messages are sent out proactively by eNBs, the OI indicator is only triggered when high-interference in the uplink direction is detected by an eNB. An overload indication will be sent to neighbor eNBs whose UEs are potentially the source of this high interference. The message contains a low, medium, or high interference level indication per each PRB. However, the question, which cell is the one responsible for the high interference is of course not a trivial question to answer. 
         [0008]    According to the 3GPP Specifications, X2 based ICIC does not include any provisioning for a decision making process, consequently, ICIC algorithms in base stations, which are originated by different vendors, may use completely different logics and criteria. This essentially limits the X2-based ICIC solution to areas where the base stations originate from a single vendor. While in existing macro deployments this constrain might still be achieved, for modern multi-RAT networks (LTE overlay over UMTS network) and HetNet networks, such a requirement of having one vendor&#39;s equipment is too restrictive, if not impossible. 
         [0009]    According to 3GPP TS 36.300, Inter-cell interference coordination is associated with managing radio resources (notably the radio resource blocks) such that inter-cell interference is kept under control. ICIC is inherently a multi-cell, radio resource management (“RRM”) function that needs to take into account information (e.g. the resource usage status and traffic load situation) obtained from various cells. Furthermore, an ICIC method may be different in the uplink and downlink. 
         [0010]    The 3GPP Release 10 introduced a new LTE network concept for the heterogeneous networks (HetNets), in contrast to previous network releases, which deal with homogeneous networks. HetNet is defined in that release as a network of eNBs with different capabilities, most importantly, different Tx-power classes. 
         [0011]    However, heterogeneous networks pose new ICIC challenges. A first ICIC challenge involves macro UE that roams about a Home eNB (HeNB) and is not part of the closed subscriber group (“CSG”). In that scenario the Macro eNB UE transmission will become uplink interference to the Home eNB authorized UEs. The second ICIC challenge is macro eNB transmission that forms downlink interference to Pico eNB center cell UE. In order to enable the use of HetNet, enhanced ICIC (eICIC) Rel. 10 requires that all members of a HetNet (Macro, Pico, HeNB) should be capable of interconnecting by using the X2 interface. 
         [0012]    Soft Fractional Frequency Reuse (“FRR”) technique implements separation of transmissions in neighbor LTE cells. The separation is performed by allocating time-frequency resources in blocks (partitions) that appear as rectangles in time-frequency plane as illustrated for example in  FIG. 1 . This concept may be demonstrated in the following example. For uplink (“UL”) transmissions, a time-frequency block X can be allocated to a group of cell edge UEs in cell A, whereas time-frequency block Y is allocated to a group of cell edge UEs in cell B. If X and Y do not overlap, this scheme may alleviate mutual interference between cells A and B. 
         [0013]    There are many FFR schemes used for HetNets that allow significant reuse of spectrum. For example in  FIGS. 2A and 2B , where A . . . D denote certain “chunks” of the UL frequency channel that are allocated for use by Macro eNBs and for use by Metro eNBs, with differentiation between cell center and cell edge UEs. Such schemes reduce interference because adjacent cells (particularly a Metro cell that can be located within a Macro cell) are using separate orthogonal parts of the spectrum. 
         [0014]    However, a solution is still required that enables automatic bandwidth partitioning for example between macro and metro eNBs, and particularly in cases where base stations produced by different vendors and located at the same geographical vicinity, are involved. 
       SUMMARY OF THE DISCLOSURE 
       [0015]    The disclosure may be summarized by referring to the appended claims. 
         [0016]    It is an object of the present disclosure to provide a method and system to enable dynamic allocation of air interface resources of the cellular network. 
         [0017]    It is another object of the present disclosure to provide a method and system to enable dynamic allocation of air interface resources of the cellular network partially by a central management entity and partially by local entities. 
         [0018]    Other objects of the present disclosure will become apparent from the following description. 
         [0019]    According to a first embodiment there is provided a method for dynamic allocation of air interface resources in a cellular network comprising at least three wireless cells, all located within a geographical proximity of each other and a plurality of mobile devices currently located within these at least three wireless cells, wherein the method comprises the steps of:
       a) determining, by a central managing entity, one or more classification rules for classifying each of the plurality of mobile devices according to s, wherein the one or more classification rules are based on the one or more members of the group that consists of: the base station which provides service to the respective mobile device; location of the respective mobile device within the wireless cell (e.g. at the cell edge, at the cell core/center), wherein the latter classification rule may be defined for example in terms of range of the signal strength, path loss, SINR, and the like;   b) providing, by the central management entity, to a group of base stations associated with the at least three cells, information that comprises:
           b.1) information that relates to the determined one or more classification rules;   b.2) information that relates to semi-static allocation of blocks of air interface resources adapted for use by one or more specific members of the group of base stations, wherein these blocks of air interface resources enable communications between base stations and mobile devices communicating therewith that match one or more classification rules, and wherein no coordination is carried out between any member of the group of base station and any other member of that group regarding the use of the blocks of air interface resources, hence these blocks of air interface resources may be considered as being resources for uncoordinated use; and   b.3) information that relates to allocation of blocks of air interface resources adapted for use by at least two members of the group of base stations, wherein these blocks of air interface resources enable communications between base stations and mobile devices communicating therewith that match one or more classification rules, and wherein coordination is carried out between at least two members of the group of base stations regarding the use of the blocks of air interface resources, hence these blocks of air interface resources may be considered as being resources for coordinated use;   
           c) exchanging messages directly between at least two members of the group of base stations in order to coordinate there-between which part (e.g. different parts) of air interface resource blocks allocated by the central management entity for coordinated use, will be used by each of these at least two members.       
 
         [0026]    The term “semi-static” as used herein throughout the specification and claims should be understood as characterization of one out of two processes being carried out in parallel, wherein the frequency at which the other of the two processes is carried out is substantially higher than the frequency at which the semi static process is carried out. In the present case, the frequency at which the process defined in step b is carried out is substantially lower than the frequency at which step c is carried out, hence step b is semi static with respect to step b. 
         [0027]    The term “base station” (BS) as used herein throughout the specification and claims should be understood as a communication entity that contains equipment for transmitting and receiving radio signals (transceivers), antennas. This term should be understood to encompass eNB for example if the cellular network is compatible with the LTE Specifications. 
         [0028]    By yet another embodiment, an air interface resource block is characterized by at least one member of the group that consists of: interval time for transmission, transmission frequency, a set of subcarriers, and the like. 
         [0029]    By still another embodiment, information related to coordinated use of particular air interface resource block, includes settings of transmission power, for example power density values or limits, for control and user planes at the base station or at the mobile device for which the air interface resource block is allocated. 
         [0030]    In accordance with another embodiment, the method further comprises a step of:
       d) repeating step c) to reestablish which part of the air interface resource blocks allocated by the central management entity for coordinated use, will be used by each of the base stations.       
 
         [0032]    Preferably, step c) is repeated every pre-defined period of time which extends up to few hundreds of msec, e.g. from about few tens of msec up to about 150 to 200 msec. 
         [0033]    In accordance with another embodiment, steps a) and b) are repeated at a frequency which is substantially less than a frequency at which step c) is repeated. 
         [0034]    By still another embodiment, the information that relates to allocation of blocks of air interface resources according to step b) is communicated to respective base stations in a form of time based periodic pattern having a length of N radio frames. In such a case, the number and properties of the blocks of air interface resources for coordinated and uncoordinated use are preferably synchronously changed by all participating base stations in every air interface frame of N consecutive air interface frames. 
         [0035]    By yet another embodiment, the cellular network is a heterogeneous cellular network (“HetNet”) comprising at least one macro cell and at least two small cells, all located within a geographical proximity of each other, wherein the group of base stations comprises at least one base station associated with a macro cell and at least two base stations each associated with a different small cell, and wherein the at least two members that exchange messages directly with each other in order to coordinate which part of the blocks of air interface resources allocated by the central management entity for coordinated use, will be used by each of these at least two base stations, are the at least two base stations associated with small cells. 
         [0036]    According to another aspect, there is provided a communication system configured to dynamically allocate air interface resources in a cellular network comprising at least three wireless cells, all located within a geographical proximity of each other, and a plurality of mobile devices currently located within these at least three wireless cells, wherein the system comprises:
       (I) a central management entity that comprises:
           (a) one or more processors operable to:
               (a.1) determine one or more classification rules for classifying each of the plurality of mobile devices, wherein the one or more classification rules are based on the one or more members of the group that consists of: the base station which provides service to the respective mobile device; location of the respective mobile device within the wireless cell (e.g. at the cell edge, at the cell core/center), wherein the latter classification rule may be defined for example in terms of range of the signal strength, path loss, SINR, and the like;   (a.2) provide to a group of base stations associated with the at least three cells, information that comprises:
                   (a.2.1) information that relates to the determined one or more classification rules;   (a.2.2) information that relates to semi-static allocation of blocks of air interface resources adapted for use by one or more specific members of the group of base stations, wherein these blocks of air interface resources would enable communications between base stations and mobile devices communicating therewith that match one or more classification rules, and wherein no coordination would be carried out between any member of the group of base station and any other member of that group regarding the use of the blocks of air interface resources; and   (a.2.3) information that relates to allocation of blocks of air interface resources adapted for use by at least two members of the group of base stations, wherein these blocks of air interface resources would enable communications between base stations and mobile devices communicating therewith that match one or more classification rules, and wherein coordination would be carried out between at least two members of the group of base stations regarding the use of the blocks of air interface resources;   
                   
               (b) one or more transmitters operable to convey the information generated by said one or more processors, towards the group of base stations; and   
           (II) a group of base stations configured for use within the at least three cells, and wherein each of the base stations comprises at least one transceiver operable to exchange information with the central management entity and with at least one member of the group of base stations, and at least one transceiver operable to exchange information with a plurality of mobile devices associated therewith, and wherein at least two members of the group of base stations are configured to exchange messages directly there-between in order to coordinate which part of the blocks of air interface resources allocated by the central management entity for coordinated use, will be used by each of these at least two base stations.       
 
         [0046]    The central management entity (e.g. a Centralized Self Optimization Network, a “cSON”, entity) may be connected either to a management system of the cellular network and/or to a management system of a plurality of small cells, thereby enabling a managing entity (e.g. the cSON) to retrieve and provide information from the management system it is connected to and/or in a synchronized manner from both systems if it is connected indeed to both. 
         [0047]    By yet another embodiment of this aspect, an air interface resource block is characterized by at least one member of the group that consists of: interval time for transmission, transmission frequency, a set of subcarriers, and the like. 
         [0048]    According to still another embodiment, the at least two members of the group of base stations are further operable to repeat the exchange of the messages in order to negotiate use of resources within the blocks of air interface resources designated for coordinated use, i.e. for reestablishing which part of air interface resource blocks allocated by the central management entity for coordinated use, will be used by each of the base stations. Preferably, these messages are repeated every pre-defined period of time, which extends up to tens of msec. 
         [0049]    In accordance with another embodiment, the one or more processors of the central management entity are further operable to re-define at least one of the one or more classification rule for classifying at least one of the plurality of mobile devices and the information provisioning to a group of base stations associated with the at least three cells, at a frequency which is substantially less than that at which the at least two members of the group of base stations exchange messages there-between. 
         [0050]    By yet another embodiment, the cellular network is a heterogeneous cellular network (“HetNet”) comprising at least one macro cell and at least two small cells, all located within a geographical proximity of each other, wherein the group of base stations comprises at least one base station associated with a macro cell and at least two base stations each associated with a different small cell, and wherein the at least two members that exchange messages directly with each other in order to coordinate which part of the blocks of air interface resources allocated by the central management entity for coordinated use, will be used by each of these at least two base stations, are the at least two base stations associated with small cells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    For a more complete understanding of the present invention, reference is now made to the following detailed description taken in conjunction with the accompanying drawings wherein: 
           [0052]      FIG. 1 —illustrates an example of a prior art allocation of time-frequency resources in blocks (partitions) that appear as rectangles in time-frequency plane; 
           [0053]      FIGS. 2A and 2B —illustrate prior art FFR schemes used for HetNets that allow significant reuse of spectrum; 
           [0054]      FIG. 3 —exemplifies a prior art case of isolated LTE small cells deployed as overlay to LTE macro deployment; 
           [0055]      FIG. 4 —exemplifies an example of LTE metro cells (small cells) deployed as a cluster with an underlying LTE macro deployment, and wherein one or more resource blocks are allocated for coordinated use by the cluster of metro cells; 
           [0056]      FIG. 5 —illustrates a system according to an embodiment of the present disclosure; 
           [0057]      FIG. 6 —exemplifies a method of carrying out an embodiment of the present disclosure in the system illustrated in  FIG. 5 ; 
           [0058]      FIG. 7 —illustrates a method of carrying out interference mitigation by using coordinated and uncoordinated air interface resource blocks according to an embodiment of the present disclosure; and 
           [0059]      FIG. 8 —illustrates a method of carrying out interference mitigation by using coordinated and uncoordinated air interface resource blocks according to an embodiment of the present disclosure where time based pattern of the length N=2 radio frames is used. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]    In this disclosure, the term “comprising” is intended to have an open-ended meaning so that when a first element is stated as comprising a second element, the first element may also include one or more other elements that are not necessarily identified or described herein, or recited in the claims. 
         [0061]    In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a better understanding of the present disclosure by way of examples. It should be apparent, however, that the present disclosure may be practiced without these specific details. 
         [0062]    Let us consider a particular example of deploying eNBs in a HetNet, which comprises small cells (i.e. metro cells), wherein the eNBs are configured to operate in one of the following two modes: 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Mode 
                 Definition 
               
               
                   
               
             
             
               
                 Mode A 
                 The MLB dSON function is disabled by the cSON over Itf-N. 
               
               
                 (Prior art) 
                 The cSON application directly manages the eNB parameters 
               
               
                   
                 over Itf-N as specified in 3GPP TS 28.657, TS 28.658, TS 
               
               
                   
                 28.659 
               
               
                 Mode B 
                 The MLB dSON function is configured and enabled by the 
               
               
                   
                 cSON over Itf-N. 
               
               
                   
                 Certain parameters may remain under the control of the cSON 
               
               
                   
               
             
          
         
       
     
         [0063]    In recent years, wireless networks operators have started to deploy their own or rely on end users to buy very small Base Stations, in order to meet the increasing demand for data traffic. This new type of cell sites, referred to herein below as “small cells” or “metro cells”, used in conjunction with wireless cells of the traditional cellular networks (macro cells). Networks that include both macro cells and metrocells are referred to herein as heterogeneous networks (HetNets). 
         [0064]    The term “small cells” as used herein and throughout the specification and claims encompass femtocells, picocells, microcells, and metrocells. Small-cell networks can also be realized by means of distributed radio technology consisting of centralized baseband units and remote radio heads. Beamforming technology (focusing a radio signal on a very specific area) can be utilized to further enhance or focus small cell coverage. A common factor in all these approaches to small cells is that they are centrally managed by mobile network operators. 
         [0065]      FIG. 3  illustrates an example of a prior art solution using the mode A configuration discussed above. In this example, isolated LTE eNodeBs are deployed as an overlay onto UMTS or LTE macro deployment, wherein in this example LTE metro cells are deployed as a cluster with an underlying LTE macro deployment. This cluster of LTE eNodeBs should preferably be made by a single vendor, in order to ensure that the X2 interface that extends between the metro eNBs can be activated and the ICIC function can be enabled. 
         [0066]    The following is a simplified example demonstrating one option of carrying out the above-discussed Mode A:
       1) The cSON collects results of measurements made by UEs (via MDT if available).   2) Once every pre-defined time interval, the cSON:
           selects a parameter for modification e.g. the distance between the UE and the serving eNB, and modify the parameter in certain direction; and   determines key indicators e.g. the total network throughput. In case of improvement, the modified value should be kept; otherwise, the change (modification) should be reversed.   
               
 
         [0071]    Now, let us assume that macro or metro eNBs are operable in accordance with the present disclosure by implementing Mode B referred to above.  FIG. 4  illustrates an example for implementing such Mode B configuration. 
         [0072]    Examples of implementing such a configuration are demonstrated in the following non-exclusive Table 2: 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Parameters 
                 Examples 
               
               
                   
               
             
             
               
                 Definition of 
                 The classification rule “CELL_EDGE1” includes 
               
               
                 classification rules 
                 UEs that are connected to eNB #123456 or eNB 
               
               
                 of UEs based on 
                 #789012 operating according to Mode B, and 
               
               
                 spatial regions in 
                 located at a distance from the serving eNB 
               
               
                 which the UEs 
                 which is greater than 30 m 
               
               
                 are located 
                 The classification rule “CELL_CENTER1” 
               
               
                   
                 includes UEs that are connected to the eNB 
               
               
                   
                 #123456 or eNB #789012 and located at distance 
               
               
                   
                 from the serving eNB which is less than 30 m 
               
               
                 Parameters that 
                 ULBlock1 = set of subcarriers with indices 
               
               
                 define air interface 
                 N1 . . . N2, over the whole Radio Frame. 
               
               
                 resource blocks in 
                 This block is for uncoordinated use. 
               
               
                 the time-frequency 
                 ULBlock2 = set of subcarriers with indices 
               
               
                 plane 
                 N3 . . . N4, over the whole Radio Frame. 
               
               
                   
                 This block is for coordinated use. 
               
               
                 Allocation of 
                 ULBlock1 is allocated for UL transmissions 
               
               
                 partitions to UEs 
                 by UEs that match the classification rule 
               
               
                 and transmit 
                 CELL_CENTER1. 
               
               
                 power limitations 
                 Maximum allowed UL transmit power density 
               
               
                   
                 associated with such UEs in ULBlock1 is M 1   
               
               
                   
                 ULBlock2 is allocated for UL transmissions by 
               
               
                   
                 UEs that are connected to the eNBs #123456 
               
               
                   
                 and #789012 and located at the spatial region 
               
               
                   
                 “CELL_EDGE1” 
               
               
                   
                 Maximum allowed UL transmit power density 
               
               
                   
                 associated with such UEs in ULBlock2 is M 2   
               
               
                   
               
             
          
         
       
     
         [0073]    In other words, the central management entity defines one or more classification rules for classifying the various mobile devices; for example CELL_EDGE1 in Table 2, transmit these rules to respective eNBs, and the eNB then applies these classification rule(s) to classify the mobile devices being in communication therewith. 
         [0074]    For the case of Mode B operation, let us consider X2-based ICIC where eNBs exchange real time information that relates to past and future events associated with interference management. This in fact exemplifies an embodiment of the present disclosure according to which an X2-based dSON is configured by the central management entity, namely the cSON, and may include any one of the following:
       RNTP—Relative Narrowband Transmit Power (a proactive signal);   OI—UL Interference Overload Indication (a reactive signal); and   HII—High Interference indication (a proactive signal).       
 
         [0078]    This information is preferably used under real time conditions. 
         [0079]    The following is a simplified example demonstrating an option of configuring the X2 ICIC by the cSON as follows:
       RNTP Threshold as defined in TS 36.423;   Ranges to UL Interference Overload Indication to define high/medium/low interference; and   Threshold to differentiate between “high” and “low” interference levels.       
 
         [0083]    Now, in accordance with the present disclosure, let us consider a method for allocating partitions (blocks) for coordinated use by a group (cluster) of eNBs operating in Mode B configuration. 
         [0084]      FIG. 5  demonstrates an embodiment of an LTE system  500  for carrying out the present disclosure in a heterogeneous network (HetNet). The system illustrated in this figure comprises a central management entity (e.g. a cSON)  510 , and three eNBs namely,  520 ,  530  and  540 . The  520  eNB is operable in a macro cell, whereas eNBs  530  and  540  are each operable in a metro cell. 
         [0085]      FIG. 6  illustrates an example of carrying out the disclosure in the system illustrated in  FIG. 5 . It should be noted that in this case, the metro eNBs may originate from same vendor while the macro eNB does not necessarily be originated from that same vendor. The central managing entity, the cSON, determines semi-statically allocation of air interface resource blocks to specific eNBs/UE categories/spatial domain(s) for non-coordinated use, and also determines allocations of certain partitions for coordinated use by two or more eNBs (step  600 ). The information about the specific allocations, as well as about the partitions which use will be coordinated by the two or more eNBs, is transferred to the eNBs  510 ,  520  and  530  (step  610 ). It should be noted however that a single partition in coordinated use may be allocated to the whole cluster of metro eNBs as shown for example in  FIG. 4 . The eNBs of the metro cells,  530  and  540  exchange X2 signals between themselves (step  620 ) and perform X2-based ICIC optimization only for the partitions allocated for coordinated use (step  630 ). Each of these partitions (allocated for coordinated use) may be used by one or by several eNBs. In the partitions allocated for non-coordinated use all three eNBs  520 ,  530  and  540  will follow the allocations of resources as set by the cSON (step  640 ), whereas in the partitions allocated for coordinated use, the relevant eNBs (namely,  530  and  540 ) will follow the allocations of resources as set by the messages interchanged by these eNBs themselves. 
         [0086]      FIG. 7  and  FIG. 8  illustrate methods of carrying out interference mitigation by using coordinated and uncoordinated air interface resource blocks according to an embodiment of the present disclosure, which is different from the prior art method illustrated in  FIG. 1 . The embodiment illustrated in  FIG. 8 , relates to the case where a time based pattern having the length N=2 radio frames, is used. 
         [0087]    In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. 
         [0088]    The present disclosure has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the disclosure in any way. The described embodiments comprise different features, not all of which are required in all embodiments of the disclosure. Some embodiments of the present disclosure utilize only some of the features or possible combinations of the features. Variations of embodiments of the present disclosure that are described and embodiments of the present disclosure comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the disclosure is limited only by the following claims.