Abstract:
An operating method of a serving Base Station (BS) in a communication system is provided). The method comprises transmitting a first message to a Mobile Station (MS), wherein the first message includes information related to a random access resource that is allocated to the MS by a target BS, and wherein the random access resource is distinguishable from other random access resources of the target BS.

Description:
PRIORITY 
     This application is a continuation application of a prior application Ser. No. 13/169,264, filed on Jun. 27, 2011, which claimed priority under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Jun. 25, 2010 and assigned Serial No. 10-2010-0060437, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for performing a handover in a wireless communication system. 
     2. Description of the Related Art 
     In a wireless communication system, basic control channels are used for communication between a Base Station (BS) and a Mobile Station (MS). The basic control channels include a downlink preamble channel for delivering information needed for system synchronization, a downlink broadcast channel for delivering system information, and an uplink Ranging CHannel (RCH) for providing information used for an MS to enter a network. 
     Ranging is a process of periodically matching an accurate timing offset and adjusting power between a BS and an MS. The MS performs ranging during network entry, for example. The RCH carries information required for ranging. Although RCHs are typically allocated periodically, additional RCHs may be allocated dynamically when a large number of MSs are expected to enter the network. 
       FIGS. 1A and 1B  illustrate exemplary frame structures to which RCHs are allocated, in a case of handover in a wireless communication system according to the related art. 
       FIG. 1A  illustrates a frame structure for a BS A, and  FIG. 1B  illustrate a frame structure for BS B. Reference numerals  101  and  121  denote downlink frames for BS A and BS B, respectively. Reference numerals  103  and  123  denote uplink frames for BS A and BS B, respectively. 
     Referring to  FIGS. 1A and 1B , BS A allocates an RCH  113  to the uplink frame  103  using a broadcast channel, SuperFrame Header (SFH)  105 . When BS A determines to dynamically allocate additional RCHs, BS A may allocate RCHs  117  and  119  to the uplink frame  103  using MAPs  109  and  111  including dynamic resource allocation information. 
     The MAPs  109  and  111  that dynamically allocate the RCHs  117  and  119  are accessible to all users and the RCHs  117  and  119  are also available to all users. 
     In a situation such as handover, BS A may allocate a specific RCH to a specific MS. If an MS is to perform handover from BS B to BS A (i.e., a target BS), BS B may allocate an RCH  115  for the MS to use within BS A by unicasting a specific message, for example, a HandOver CoMmanD (HO-CMD) message  107  to the MS. The RCH  115  is determined through negotiation with BS A or under the control of a higher-layer controller. 
     Because the HO-CMD message  107  is unicast, all other MSs within BS A cannot receive information about the RCH  115  allocated to the handover MS. 
     In the illustrated case of  FIG. 1 , there are four RCHs  113 ,  115 ,  117 , and  119 . In general, the RCHs  113 ,  115 ,  117 , and  119  are sequentially indexed from 0 to 3 in a frequency order or in a time order. The RCH indexing is performed individually by each of the BS and MSs according to their allocation information, not signaled between them. 
     Without knowledge of allocation information about the RCH  115  allocated by unicasting the HO-CMD message  107  to the handover MS, other MSs within BS A index the RCHs  113 ,  117 , and  119  sequentially with 0, 1, and 2. On the other hand, BS A indexes the RCHs  113 ,  115 ,  117 , and  119  sequentially with 0 to 3. As BS A and the MSs within BS A label the RCHs with different indexes, there may be an RCH identification mismatch between BS A and the MSs. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method and apparatus for performing a handover in a wireless communication system. 
     Another aspect of the present invention is to provide a method and apparatus for allocating a Ranging CHannel (RCH) in a wireless communication system. 
     Another aspect of the present invention is to provide a method and apparatus for, when an RCH is dynamically allocated, enabling a Base Station (BS) and a Mobile Station (MS) to identify the RCH using the same index in a wireless communication system. 
     Another aspect of the present invention is to provide a method and apparatus for, when an RCH is allocated dynamically to an MS, enabling the MS to determine whether it uses the right RCH by checking information about the position of the allocated RCH in a wireless communication system. 
     A further aspect of the present invention is to provide a format for a message used by a BS during allocation of an RCH in a wireless communication system. 
     In accordance with an aspect of the present invention, a method for supporting a handover at a source BS in a wireless communication system is provided. The method includes transmitting a HandOver CoMmanD (HO-CMD) message to an MS. The HO-CMD message includes an RCH index of an RCH allocated to the MS by a target BS and a subframe index of the allocated RCH, and the RCH index of the allocated RCH does not overlap with indexes of other RCHs of the target BS. 
     In accordance with another aspect of the present invention, a method for supporting a handover at a target BS in a wireless communication system is provided. The method includes transmitting a HandOver Response (HO-RSP) message to a source BS. The HO-RSP message includes an RCH index of an RCH allocated to an MS by the target BS and a subframe index of the allocated RCH, and the RCH index of the allocated RCH does not overlap with indexes of other RCHs of the target BS. 
     In accordance with another aspect of the present invention, a method for performing a handover at an MS in a wireless communication system is provided. The method includes receiving an HO-CMD message from a serving BS. The HO-CMD message includes an RCH index of an RCH allocated to the MS by a target BS and a subframe index of the allocated RCH, and the RCH index of the allocated RCH does not overlap with indexes of other RCHs of the target BS. 
     In accordance with another aspect of the present invention, a source BS in a wireless communication system is provided. The system includes a transmission unit for transmitting an HO-CMD message to an MS. The HO-CMD message includes an RCH index of an RCH allocated to the MS by a target BS and a subframe index of the allocated RCH, and the RCH index of the allocated RCH does not overlap with indexes of other RCHs of the target BS. 
     In accordance with another aspect of the present invention, a target BS in a wireless communication system is provided. The system includes a transmission unit for transmitting an HO-RSP message to a source BS. The HO-RSP message includes an RCH index of an RCH allocated to an MS by the target BS and a subframe index of the allocated RCH, and the RCH index of the allocated RCH does not overlap with indexes of other RCHs of the target BS. 
     In accordance with another aspect of the present invention, an MS in a wireless communication system is provided. The system includes a reception unit for receiving an HO-CMD message from a serving BS. The HO-CMD message includes an RCH index of an RCH allocated to the MS by a target BS and a subframe index of the allocated RCH, and the RCH index of the allocated RCH does not overlap with indexes of other RCHs of the target BS. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  illustrate exemplary frame structures to which Ranging CHannels (RCHs) are allocated, in case of handover in a wireless communication system according to the related art; 
         FIGS. 2A and 2B  illustrate exemplary frame structures to which dynamic RCHs are allocated in a wireless communication system according to an exemplary embodiment of the present invention; 
         FIG. 3  is a diagram illustrating a signal flow when a Mobile Station (MS) attempts to enter a Base Station (BS) in a wireless communication system according to an exemplary embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating an RCH allocation method of a BS in a wireless communication system according to an exemplary embodiment of the present invention; 
         FIG. 5  is a flowchart illustrating an RCH allocation method of a BS in a wireless communication system according to an exemplary embodiment of the present invention; 
         FIG. 6  is a flowchart illustrating an RCH allocation method, when a BS receives a ranging allocation message from another BS in a wireless communication system according to an exemplary embodiment of the present invention; and 
         FIG. 7  is a diagram illustrating a signal flow between an MS and a BS during a handover attempt according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like parts, components, and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompany drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding, but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention is provided for illustration purposes only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     While the following description is given in the context of an Institute of Electrical and Electronics Engineers (IEEE) 802.16m system, it is to be clearly understood that exemplary embodiments of the present invention are applicable to any wireless communication system requiring Ranging CHannel (RCH) allocation. 
       FIGS. 2A and 2B  illustrate exemplary frame structures to which dynamic RCHs are allocated in a wireless communication system according to an exemplary embodiment of the present invention.  FIG. 2A  illustrates a frame structure for Base Station (BS) A.  FIG. 2B  illustrate a frame structure for BS B that is serving an MS attempting handover to BS A. Reference numerals  201  and  205  denote downlink frames of BS A and BS B, respectively and reference numerals  203  and  207  denote uplink frames of BS A and BS B, respectively. 
     Referring to  FIGS. 2A and 2B , a frame is a resource allocation unit used by a BS. Frames are distinguished by frequency and time, each frame occupying part of resources available to the BS, as illustrated in  FIGS. 2A and 2B . 
     BS A allocates an RCH  221  to the uplink frame  203  using a broadcast channel, SuperFrame Header (SFH)  209 , for periodic RCH allocation. When BS A expects more MSs to enter BS A, BS A may allocate RCHs  223  and  225  dynamically using MAPs  211  and  213 . Because the RCHs  221 ,  223 , and  225  are allocated by the SFH  209  and the MAPs  211  and  213  in a broadcast manner, any MS within the service area of BS A can acquire allocation information about the RCHs  221 ,  223 , and  225  and access the RCHs  221 ,  223 , and  225  as well. 
     BS A may also allocate RCHs  227 ,  229 , and  231  to specific MSs in a unicast manner. Unicast RCH allocation refers to transmission of information about an allocated RCH by a unicast message. When BS A allocates the RCH  229  to an MS operating within the service area of BS A and when BS A allocates the RCHs  227  and  231  to MSs entering BS A through BS B due to factors such as handover, unicast RCH allocation may be performed. For example, when it is necessary to allocate a dedicated RCH to a specific MS within BS A for periodic ranging, measurement of a neighbor BS, etc., the dedicated RCH may be allocated in the unicast manner. 
     When BS A allocates an RCH to the uplink frame  203  in the unicast manner, BS A assigns an RCH index to the RCH, for identification. The RCH index assigned to the RCH does not overlap with the indexes of existing RCHs in the uplink frame  203 . Therefore, BS A and MSs within BS A can identify the unicast-allocated RCH using the same index. 
     For example, BS A may assign an index equal to or larger than the total number N of broadcast-allocated RCHs (N is 0 or a larger integer) to a unicast-allocated RCH. Given three broadcast-allocated RCHs  221 ,  223 , and  225 , they are sequentially indexed  0 ,  1 , and  2 . To avoid index overlapping, BS A always assigns index  3  or a larger index to an RCH allocated in the unicast manner. Accordingly, when BS A allocates the RCHs  227 ,  229  and  231 , the BS A may directly assign indexes  3 ,  4 , and  5  indicated by reference numerals  217 ,  219 , and  215  to the RCHs  227 ,  229 , and  231 , respectively. 
     BS A may calculate the largest M of the indexes of the existing RCHs in the uplink frame  203  (M is 0 or a larger integer) and may directly assign index (M+1) or a larger index to each unicast-allocated RCH. The three RCHs  221 ,  223 , and  225  are broadcast-allocated RCHs and have sequential indexes  0 ,  1 , and  2 . To avoid index overlapping, when BS A allocates an RCH in the unicast manner, BS A assigns index  3  or a larger index to the RCH by adding 1 to the largest of the existing RCH indexes,  2 . Accordingly, when allocating the RCHs  227 ,  229 , and  231 , BS A may directly assign indexes  3 ,  4 , and  5  to the RCHs  227 ,  229 , and  231 , respectively. The largest of the existing RCH indexes may be calculated by adding the number of the existing RCHs to (the starting RCH index- 1 ). 
       FIG. 3  is a diagram illustrating a signal flow when an MS attempts to enter a BS in a wireless communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , an MS  301  acquires synchronization to a BS  303  by receiving a synchronization signal (i.e., a preamble channel) from the BS  303  in step  305 . 
     In step  307 , the BS  303  allocates an RCH to the MS  301  and the MS  301  receives from the BS  303  a message including information about at least one ranging code available to the MS for network entry, information about the position of the allocated RCH (i.e., the frame number and/or subframe number of the RCH), and the index of the RCH. The message may be broadcast to the MS  301  on a broadcast channel, or unicast to the MS  301 . 
     The MS  301  receives the ranging code information, selects an available ranging code, and transmits the selected ranging code information in an RCH at a position indicated by the RCH position information to the BS  303  in step  309 . 
     The BS  303  notifies the MS  301  whether the network entry of the MS  301  has been accepted by transmitting a RaNGing ReSPonse (RNG-RSP) (or a Ranging ACKnowledgment (RNG-ACK)) message in step  311 . The BS  303  may allocate a plurality of RCHs to new MSs that are entering the network. When transmitting an RNG-RSP (or RNG-ACK) message to an MS, the BS  303  includes information about an RCH in which it has received a ranging code from the MS (i.e., information indicating the frame number and/or subframe number of the RCH and the index of the RCH in a frame) and information about the received ranging code in the RNG-RSP (or RNG-ACK) message. 
     As described above, if BS A transmits to an MS information about the position of an RCH allocated to the MS (i.e., the frame number, subframe number and index of the RCH) along with information about available ranging codes, the MS transmits a ranging code in the RCH and receives an RNG-RSP (or RNG-ACK) message from BS A. Since the RNG-RSP (or RNG-ACK) message indicates the RCH used by the MS (i.e., the index of the RCH used by the MS), the MS&#39;s malfunction can be prevented. 
     Now a description will be given of an example in which the RCHs  227  and  231  are allocated in the unicast manner by a HandOver CoMmanD (HO-CMD) message. 
     The RCH  227  or  231  is allocated in the unicast manner to an MS that is operating (being serviced) in another BS, BS A and attempting handover to the current BS, BS A (thus, a target BS). When the MS transmits a HandOver REQuest (HO-REQ) message to BS B due to weak signal strength of BS B, BS B notifies BS A that the MS will perform handover to BS A. The notification may be made by a message, for example, an HO-REQ message. If BS A accepts the handover of the MS, BS A allocates an RCH to the MS, assigns its index which does not overlap with the indexes of existing RCHs, and notifies BS B of the allocated RCH and the RCH index, along with an ACK message accepting the handover. BS B transmits information about the position of the RCH and the index of the RCH received from BS A to the MS in an HO-CMD message. Upon receipt of the HO-CMD message, the MS transmits a ranging code on the allocated RCH using the RCH index indicated by the HO-CMD message during network entry to BS A. After receiving an RNG-RSP (or RNG-ACK) message from BS A, the MS compares the RCH index indicated by the HO-CDM message with an RCH index indicated by the RNG-RSP (or RNG-ACK) message and thus determines whether the ranging code has been successfully transmitted to BS A according to the comparison. 
     Table 1 illustrates a format for an RNG-RSP (or RNG-ACK) message and an HO-CMD message according to an exemplary embodiment of the present invention. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Mandatory 
                   
                   
                   
                   
               
               
                 (M)/ 
                 Attribute/Attribute 
                 Size 
               
               
                 Optional(O) 
                 arrangement 
                 (bits) 
                 Value/Description 
                 Condition 
               
               
                   
               
             
             
               
                 . . . 
                   
                   
                   
                   
               
               
                 O 
                 Frame index 
                 4 
                 Indicates the frame index 
                 Shall be 
               
               
                   
                   
                   
                 of the allocated ranging 
                 included when a 
               
               
                   
                   
                   
                 opportunity 
                 dynamic RCH is 
               
               
                   
                   
                   
                   
                 allocated 
               
               
                 O 
                 Subframe index 
                 2 
                 Indicates the subframe 
                 Shall be 
               
               
                   
                   
                   
                 index of the allocated 
                 included when 
               
               
                   
                   
                   
                 ranging opportunity 
                 the dynamic 
               
               
                   
                   
                   
                   
                 RCH is allocated 
               
               
                 O 
                 Ranging 
                 3 
                 Indicates the index of the 
                 Shall be 
               
               
                   
                 Opportunity 
                   
                 ranging opportunity of 
                 included when 
               
               
                   
                 Index 
                   
                 the dynamic RCH. The 
                 Action Time 
               
               
                   
                   
                   
                 ranging opportunity index 
                 refers to the 
               
               
                   
                   
                   
                 shall not overlap with 
                 frame to which 
               
               
                   
                   
                   
                 existing ranging 
                 the dynamic 
               
               
                   
                   
                   
                 opportunity indexes by 
                 RCH is 
               
               
                   
                   
                   
                 the ABS. 
                 allocated. 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                   
               
             
          
         
       
     
     The attributes of Table 1 should be added to a message (e.g., an RNK-ACK message) that a BS unicasts to an MS operating within the BS or a message (e.g., an HO-CMD message) that another BS transmits to an MS within another BS. 
     Frame index and Subframe index specify the position of the allocated RCH and Ranging Opportunity Index indicates the index of the RCH. 
       FIG. 4  is a flowchart illustrating an RCH allocation method of a BS in a wireless communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , when the BS intends to allocate a specific RCH to a specific MS, for example, where the specific MS is to range in a non-contention-based manner or requests handover to another BS, the BS allocates the RCH to the MS in a unicast manner in step  401 . 
     In step  403 , the BS assigns an index to the allocated RCH so that the RCH index does not overlap with the indexes of existing RCHs in an uplink frame of the BS. 
     Then the BS transmits a message for RCH allocation, including the RCH index, to the specific MS or the other BS that is serving the handover-requesting MS in step  405 . 
     The message may be a ranging allocation message. The message may further include information about ranging codes available to the MS and the frame and subframe indexes of the allocated RCH. The ranging allocation message may be, for example, a HandOver ReSPonse (HO-RSP) message. 
       FIG. 5  is a flowchart illustrating an RCH allocation method of a BS in a wireless communication system according to another exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , when the BS intends to allocate a specific RCH to a specific MS, for example, when the specific MS is to range in a non-contention-based manner or requests handover to another BS, the BS allocates the RCH to the MS in a unicast manner in step  401 . The BS is aware of the handover request of the MS within the service area of the other BS by receiving an HO-REQ message from the other BS. In step  403 , the BS assigns an index to the allocated RCH so that the RCH index does not overlap with the indexes of existing RCHs in an uplink frame of the BS. 
     The BS determines whether the MS is operating in the other BS, that is, whether the MS has requested a handover in the other BS in step  501 . If the MS is operating in the other BS, the BS transmits a message including the RCH index to the other BS in step  503  so that the other BS may transmit a message (such as an HO-CMD message) including the RCH index to the MS. On the other hand, if the MS is operating within the service area of the BS, the BS transmits a message (such as a ranging allocation message) including the RCH index to the MS in step  505 . 
     The message with the RCH index may further include information about ranging codes available to the MS and the frame and subframe indexes of the allocated RCH. 
       FIG. 6  is a flowchart illustrating an RCH allocation method, when a BS receives a ranging allocation message from another BS in a wireless communication system according to an exemplary embodiment of the present invention. 
     As described before with reference to  FIG. 5 , a BS may transmit a ranging allocation message to another BS that is serving a handover-requesting MS. The operation of  FIG. 6  is performed in the case where a BS receives such a ranging allocation message. 
     Referring to  FIG. 6 , when a BS receives a message including an RCH index from another BS in step  601 , the BS transmits a message including the RCH index to an MS to which an RCH corresponding to the RCH index has been allocated in step  603 . The message including the RCH index may be an HO-CMD message. 
     Selectively, the message with the RCH index may further include information about ranging codes available to the MS and the frame and subframe indexes of the allocated RCH. 
     As illustrated in  FIGS. 4, 5, and 6 , an MS&#39;s malfunction of using an incorrect RCH can be prevented, as the MS receives a message including the index of an RCH allocated in a unicast manner. 
       FIG. 7  is a diagram illustrating a signal flow between an MS and a BS during a handover attempt according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , when an MS  705  attempts a handover from a source BS  703  to a target BS  701 , the source BS  703  notifies the target BS  701  of the handover of the MS  705 . The target BS  701  then allocates an RCH to the MS  705  in a unicast manner, assigns an index to the RCH so that the RCH index does not overlap with the indexes of existing RCHs within the target BS  701 , and transmits a ranging allocation message including the RCH index to the source BS  703  in step  707 . Upon receipt of the ranging allocation message, the source BS  703  transmits an HOP-CMD message including the RCH index to the MS  705  in step  709 . The MS  705  transmits a ranging code on the RCH corresponding to the RCH index to the target BS  701  in step  711 . Upon receipt of the ranging code, the target BS  701  transmits an RNG-RSP (or RNG-ACK) message to the MS  705  in response to the ranging code in step  713 . 
     In accordance with an exemplary embodiment of the present invention, the BS may include an RCH allocation unit for managing RCH allocations and a wireless signal processor for processing wireless transmission and reception signals. The operations of  FIGS. 4, 5, and 6  may be implemented in the RCH allocation unit of the BS. The RCH allocation unit may include a controller and a signal Input/Output (I/O) unit. The controller provides overall control to the RCH allocation methods of  FIGS. 4, 5, and 6 , and the signal I/O unit receives and outputs a plurality of messages and data from and to the controller using the wireless signal processor. 
     In accordance with an exemplary embodiment of the present invention, the MS may include a controller for controlling reception of a ranging allocation message or an HO-CMD message from a BS and controlling transmission of a ranging code to a BS, and a signal I/O unit for transmitting and receiving messages or signals to and from a BS under the control of the controller. The operations of the MS illustrated in  FIGS. 3 to 7  may be implemented in the controller and the signal I/O unit of the MS. 
     The RCH allocation frame structure of  FIG. 2 , the message flows illustrated in  FIGS. 3 and 7 , and the flowcharts illustrated in  FIGS. 4, 5, and 6 , should not be construed as limiting the scope and spirit of the present invention. The resource allocation methods of  FIGS. 3 to 7  are merely examples of operations of the RCH allocation unit (not shown) and the MS (not shown). These methods may not involve all steps or may not be performed individually by a particular device. They may be implemented by a single device or component. 
     The afore-described operations can be performed by providing a memory that stores program code in a component of a controller or an MS. Each component of the controller or the MS can perform the operations by reading the program code from the memory by means of a processor or a Central Processing Unit (CPU) and executing the program code. 
     As is apparent from the above description of the present invention, because a BS assigns an index to a dynamically allocated RCH in such a manner that the RCH index does not overlap with the indexes of existing RCHs, a malfunction that might otherwise occur due to an RCH index collision between the BS and an MS can be prevented. 
     In addition, upon receipt of a ranging code from the MS, the BS notifies the MS of an RCH in which it has received the ranging code. Accordingly, the MS can determine the position of an RCH allocated to the MS. As a consequence, a ranging procedure can be completed without a wrong RCH use-caused malfunction. 
     While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents.