Abstract:
A Dynamic Frequency Hopping Community (DFH Community) is formed from a plurality of Wireless Regional Area Network (WRAN) cells wherein each of the plurality of WRAN cells within the DFH Community is a one-hop neighbor of the leader cell. The leader cell sets and distributes a hopping pattern for use among the WRAN cells based on, in part, the number of usable channels and whether a WRAN cell is shared by two groups in the DFH Community.

Description:
RELATED APPLICATION 
     The present application relates to and claims priority from U.S. Provisional Patent Application No. 60/884,160 filed on Jan. 9, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate, in general, to systems and methods for enhanced 1-Hop Dynamic Frequency Hopping Communities, and particularly to algorithms to coordinate channel hopping and increase Dynamic Frequency Hopping Communities&#39; channel efficiency. 
     2. Relevant Background 
     Dynamic Frequency Hopping (“DFH”) is a technique that incorporates non-traditional dynamic channel allocation with slow frequency hopping. The main objective in DFH is to provide capacity improvements through the addition of interference avoidance that are higher than those provided by conventional frequency hopping while preserving interference averaging characteristics of conventional frequency hopping in order to provide robustness to rapid changes in interference. 
     Generally, the key concept behind this intelligent type of frequency hopping is to adjust or create frequency hopping patterns based on interference measurements. DFH uses slow frequency hopping and adaptively modifies the utilized frequency hopping pattern based on rapid frequency quality measurements, sometimes referred to as Quality of Service (“QoS”) measurements. This technique combines traditional frequency hopping with dynamic channel allocation, where a channel is one frequency in a frequency hop pattern. The continuous modification of frequency hop patterns is based on measurements representing an application of dynamic channel allocation to slow frequency hopping. Modifications are based on rapid interference measurements and calculations of the quality of frequencies used in a system by all Customer Premise Equipments (“CPEs”) and Base Stations (“BSs”). The target of these modifications is tracking the dynamic behavior of the channel quality as well as of interference. One significant application of DFH is found in the operation of what is referred to in the art as a Wireless Regional Area Network (“WRAN”). 
     A WRAN cell consists of a BS and the associated CPEs that communicate to the BS via a fixed point-to-multi-point radio air interface (i.e. an antenna). WRAN operations need to satisfy two apparently conflicting requirements: assure the QoS satisfaction for WRAN services while providing reliable and timely frequency spectrum sensing. Current sensing requirements state that incumbent signals shall be detected by WRAN devices with no more than a 2 second delay. Thus a WRAN cell must perform sensing on a working channel every 2 seconds. A channel that is to be sensed cannot be used for data transmission, thus a cell operating consistently on a single channel must interrupt data every 2 seconds for sensing. Such a non-hopping mode leads to periodic interruptions and can significantly decrease system throughput and impair QoS. The solution to this problem, as will be appreciated by one skilled in the art, was DFH. 
     As previously described, DFH differs from conventional frequency hopping in the way the patterns are built. Instead of using random or pre-defined repetitive hopping patterns, DFH patterns are generated for active users on the fly. In this manner the hopping patterns can be adjusted to adapt to interference changes. The basic idea behind creating the patterns is to choose the best frequency for each hop. This best frequency corresponds to the frequency that is interfered with the least. DFH thus requires continuous estimation and measurement of the interference at every frequency from every single hop of a pattern. At each hopping instant, the BS or the CPE measures the QoS of each frequency, filters the measurement to average out the instantaneous Rayleigh fading effects, and then sends the data using the ‘best’ frequency chosen according to some quality selection criteria. Typically the hopping patterns for users within the same cell are orthogonal. The performance of an established link is monitored, and upon the performance dropping below a threshold, a better hopping pattern is generated. 
     In DFH communication, components of a WRAN cell (the CPEs) hop over a set of channels. During operation on a working channel, sensing is performed in parallel on the intended next working channels. After 2 seconds, a channel switch takes place: one of the intended next working channels becomes the new working channel, and the channel previously used is vacated. Hence, an interruption is no longer required for sensing. Obviously, efficient frequency usage and mutual interference-free spectrum sensing can only be achieved if multiple neighboring overlapping WRAN cells operating in the DFH mode coordinate their hopping behavior. 
     As described a WRAN cell operating in the DFH mode uses the working (in-band) channel for data transmission and performs spectrum sensing on out-of-band channels simultaneously. This operation is referred to as Simultaneous Sensing and Data Transmissions (SSDT). Guard bands between the in-band and out-of-band channels are allocated to mitigate adjacent interference caused by data transmission to the out-of-band sensing. An out-of-band channel sensed to be vacant is considered to be validated. A WRAN system in the DFH mode thus dynamically selects one of the channels validated in a previous operation period for data transmission in the next operation period. 
     A DFH Community is a non-empty set of neighboring WRAN cells following a common protocol that supports a coordinated DFH operation in order to ensure mutual interference-free channel sensing and to minimize the channel usage, applying the DFH phase-shifting. A DFH Community has one leader and, possibly, some community members. One definition of a DFH Community, as is known to those skilled in the art, is that one-hop BS neighbors create a DFH Community meaning that each WRAN cell in a community is within one-hop of the other members of the cell. A priority value is used to elect the DFH Community leader from those members and the elected leader decides when and which channel to hop among the available channel set for each community member. The community members hop among the same available channels according to the leader&#39;s decision in a synchronized fashion. Thus the DFH Community leader is responsible for decisions about community membership, calculating the hopping patterns (phase-shifting sequences) for all members and distributing this information within the community. Members provide the leader with their neighborhood and channel availability information, i.e. information about their sensing results and observed channel usage of the neighboring WRAN cells. Within the DFH Community, hopping information does not change as long as the community is stable (i.e. no new member arrives or existing member departs). Using this method hopping collisions can be avoided and real-time inter-BS communication is not necessary. 
     DFH Community is thus a concept introducing coordination among cells. The key idea of a DFH Community is that neighboring WRAN cells form cooperating communities which choose their hopping channels and perform DFH operation in a coordinated manner. As discussed above, DFH Community leads to a better QoS and throughput behavior while requiring a modest amount of channels for hopping. A DFH Community enables coexistence of multiple WRAN cells and can also be used to coordinate channel usage of cells operating in the non-hopping mode. Moreover, coordinated channel hopping can give WRAN cells more time to do channel sensing and increase channel efficiency. 
     It has been shown that only N+1 vacant channels (i.e., channels free of both incumbents and other WRANs) are needed under certain conditions to ensure cells can operate without collisions. 
       FIG. 1  illustrates the Phase-shifting DFH operation of three, N=3, overlapping WRAN cells over four, (N+1)=4, vacant channels as is known in the prior art. Each WRAN cell shifts its DFH operation phase  110  by one Quiet Time (“QT”)  120  against the operation phase of the previous WRAN cell as shown in  FIG. 1 . For instance, WRAN 2   130  on channel D  140  shifts its operation by one QT  120  against the operation of WRAN 1   150  on channel A  160 , and WRAN 3   170  on channel C shifts by one QT  120  against that of WRAN 2   130  on channel D  140 . During a QT  120 , channel sensing is performed. This implies that a QT  120  has to be at least equal to the minimum time required for reliable channel sensing. 
     Thus a set of N overlapping cells can operate continuously using (N+1) channels (Channels A, B, C, and D in the example presented in  FIG. 1 ) as long as the length of a single transmission is larger than the product N*QT. Imposing the above explained hopping pattern of time shifted jumps is, however, only possible in case of strict coordination within the DFH Community. 
     Defining a DFH Community as a number of WRAN cells being one-hop neighbors limits channel efficiency. Redefining the community as WRAN cells that are one-hop neighbors of the leader can enhance efficiency, but such a definition presents several challenges including deciding whether to accept a one-hop neighbor WRAN cell as a community member and how to calculate the hopping mode for each community member. These and other issues are addressed by the present invention. 
     SUMMARY OF THE INVENTION 
     Systems and methods for enhanced DFH among a plurality of WRAN cells is disclosed. According to one embodiment of the present invention, a DFH Community is formed from a plurality of WRAN cells. One of these plurality of WRAN cells in the DFH Community is selected as a leader cell. According to one embodiment of the present invention, the DFH Community is formed so that each other of the plurality of WRAN cells within the DFH Community is a one-hop neighbor of the leader cell 
     To facilitate communication between the WRAN cells, a number of channels for use between each of the CPEs and BSs in the plurality of WRAN cells is determined. Using this number the leader sets and distributes a hopping pattern for use among the WRAN cells. According to one embodiment of the present invention, the number of channels needed to ensure interruption free working channels and mutual interference-free sensing among each of the plurality of WRAN cells in the DFH Community is the number of maximum cells in any community group plus the number 2. According to one embodiment of the present invention, a subset of the plurality of WRAN cells that are one-hop neighbors of each other and are each one-hop neighbors of the leader cell form a community group, and each DFH Community includes a plurality of groups. 
     According to another embodiment of the present invention, a method for DFH among a plurality of WRAN cells is disclosed wherein each WRAN cell includes a BS and a plurality of CPEs. The method forms a DFH Community from the plurality of WRAN cells by identifying one of the plurality of WRAN cells in the DFH Community as a leader cell and ensuring each other of the plurality of WRAN cells within the DFH Community is a one-hop neighbor of the leader cell. During the DFH Community formation process, the number of channels for communicating between each of the CPEs and BSs of the plurality of WRAN cells within the DFH Community is determined. Thereafter the leader cell sets a channel hopping pattern over the determined number of channels so as to provide interruption free working channels and mutual interference-free sensing among each of the plurality of WRAN cells in the DFH Community. 
     Once formed the leader of the DFH Community receives and evaluates requests from other WRAN cells seeking to join the DFH Community. The leader determines whether the set of useable channels that would be formed form the combination of the WRAN cell seeking admission to the community and those channels already utilized by the community would be sufficient to support the addition of the new cell. If the set of useable channels is less than the required number of channels, the request for admission is denied. When a sufficient number of channels exists to form an appropriate hopping pattern, an analysis is made with respect to the duration of the shortest working or operational period on each channel. When the minimum length of the working period is insufficient to support expanding the group, the request for admission is rejected. 
     The features and advantages described in this disclosure and in the following detailed description are not all-inclusive. Many additional features and advantages will be apparent to one of ordinary skill in the relevant art in view of the drawings, specification, and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes and may not have been selected to delineate or circumscribe the inventive subject matter; reference to the claims is necessary to determine such inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent, and the invention itself will be best understood, by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates the Phase-shifting DFH operation of three overlapping WRAN cells over four vacant channels as is known in the prior art; 
         FIGS. 2A and 2B  show a five WRAN cell DFH Community and six WRAN cell DFH community respectively, according to one embodiment of the present invention. 
         FIG. 3  is a flowchart of one method embodiment for determining whether a WRAN cell can join an established DFH Community according to the present invention; 
         FIG. 4  shows a comparison of two WRAN cell community group configurations in a DFH Community according to one embodiment of the present invention; 
         FIG. 5  is a flowchart of one method embodiment for labeling DFH Community groups in order to determine a hopping pattern, according to the present invention; 
         FIG. 6  is a high level depiction of a DFH Community and its correlation to a group graph representation, according to one embodiment of the present invention; 
         FIG. 7  is a flowchart of one method for determining the hopping pattern of a DFH Community having a plurality of DFH Community groups according to the present invention; 
         FIG. 8  is a high level depiction of the hopping patterns for a DFH Community according to the present invention according to a group graph label; and 
         FIG. 9  shows a hopping pattern of Group  2  shown in  FIG. 2A , according to one embodiment of the present invention. 
     
    
    
     The Figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Systems and methods for enhanced communication in a one-hop DFH Community are hereafter disclosed. Embodiments of the present invention describe a one-hop DFH Community in which each WRAN cell is within one-hop of the leader of the DFH Community. According to one embodiment of the present invention, community groups of WRAN cells are defined as composed of WRAN cells that are one-hop neighbors of each other and are not a subset of a different group. Each DFH Community can have a plurality of such groups. Groups that include at least one common WRAN cell beyond the leader cell are defined as neighbor groups. 
     As will be shown herein, the number of channels needed to ensure interference free DFH is the maximum of the number of WRAN cells in any group of the DFH Community plus the number 2 (N+2) or the maximum of the number of WRAN cells in any group of the DFH Community plus the number 1 (N+1). To describe the situation where N+2 is required, a group graph is defined including a group graph coloring scheme (represented in the Figures of the present invention using various shading techniques). According to one embodiment of the present invention, the group graph is configured by representing each group in a DFH Community as a node in the graph. When two groups within the graph have a common WRAN cell that is not the DFH Community leader, an edge is created between the two nodes corresponding to these two groups. When a group graph can be colored by two colors (each node being coupled to a different colored node), the group graph is a 2-color colorable group graph. When each group in a DFH Community has 3 WRAN cells and the group graph is a 2-color uncolorable group graph (meaning that at least one node is coupled to a node of the same color), the group graph is a strict 2-color uncolorable group graph. N+2 channels are required for the community working in DFH mode whose group graph is a strict 2-color uncolorable graph. Otherwise the N+1 channels are required for the community working in DFH mode (i.e., a 2-color colorable group graph). This combined with a channel hopping scheme as determined by the leader promotes increased channel efficiency. 
     Specific embodiments of the present invention are hereafter described in detail with reference to the accompanying Figures. Like elements in the various Figures are identified by like reference numerals for consistency. Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention. 
     DFH Communities are formed on tenet that in order for WRAN cells to overlap channels of communication must be shared and coordinated. Moreover hopping schemes, such as DFH employed by individual WRAN cells to prevent interruption of service, must be expanded to apply to the entire DFH Community. DFH Communities are therefore formed with the understanding that each community group has a hopping pattern and that each community group should use as small a number as possible of working channels. Since each WRAN must include a quiet period for sensing, the combination of the quiet period with the working period must be able to satisfy incumbent user requirements. 
       FIG. 2A   200  shows a DFH Community comprised of six WRAN cells and two community groups, according to one embodiment of the present invention. As can be seen in  FIG. 2A , each WRAN cell  215 ,  220 ,  230 ,  240 ,  250  is one-hop away from the leader cell, WRAN 0   210 . According to the definition previously presented, two community groups  260 ,  270  are present in the DFH Community  200 . Group  1   270  is comprised of WRAN 0   210 , WRAN 1   215 , WRAN 2   220 , and WRAN 3   230 . Group  2   260  is comprised of WRAN 0   210 , WRAN 4   240 , and WRAN 5   250 . According to one embodiment of the present invention, five channels would be necessary to provide an adequate channel hopping pattern since the largest group has four cells and the group graph is a 2-color colorable graph. This number is determined from the maximum number of cells of any of the groups in the DFH Community plus the number 1. In this particular example, group  1   270  has three one-hop neighbors of the leader cell WRAN 0   210  for a total of four cells in the group. Thus the minimum number of channels in the channel hopping pattern is five. 
       FIG. 2B   225  shows a DFH Community comprised of six WRAN cells and five community groups, according to one embodiment of the present invention. As can be seen in  FIG. 2B , each WRAN cell  281 ,  282 ,  283 ,  284 ,  285  is one-hop away from the leader cell, WRAN 0   280 . According to the definition previously presented, five community groups  231 ,  232 ,  233 ,  234 ,  235  are present in this DFH Community  225 . Group  1   231  is comprised of WRAN 0   280 , WRAN 1   281 , and WRAN 2   282 . Group  2   232  is comprised of WRAN 0   280 , WRAN 2   282 , and WRAN 3   283 . Group  3   233  is comprised of WRAN 0   280 , WRAN 3   283 , and WRAN 4   284 . Group  4   234  is comprised of WRAN 0   280 , WRAN 4   284 , and WRAN 5   285 . Finally group  5   235  is comprised of WRAN 0   280 , WRAN 5   285 , and WRAN 1   281 . 
     Depicted to the right of the DFH Community shown in  FIG. 2B   225  is a simplified group graph  290 . This group graph  290  is comprised of five nodes, one for each of the five groups of the DFH Community  225 : node  291  represents Group  1 , node  292  represents Group  2 , node  293  represents Group  3 , node  294  represents Group  4 , and node  295  represents Group  5 . The edges connecting the nodes show neighboring group relationships. Associated with each group is a color code of either C 1  or C 2 . As can be seen in  FIG. 2B  a two color scheme of C 1  and C 2  to color the group graph cannot be used since two neighboring nodes, Group  1   291  and Group  5   295 , have the same color C 1 . According to one embodiment of the present invention, five channels (N+2) would be necessary to provide an adequate channel hopping pattern since the largest group has three WRAN cells and the group graph is a strict 2-color uncolorable graph. This number is determined from the maximum number of WRAN cells of any of the groups in the DFH Community plus the number 2. Thus the minimum number of channels in the channel hopping pattern is five. 
     As previously discussed the present invention employs a DFH scheme to enable spatially overlapped cells. Each WRAN cell shifts its DFH operation phase by one QT against the operation phase of the previous WRAN. One requirement of a working DFH Community, according to the present invention, is that the shortest working period of the group must be adequate for the number of WRAN cells in a group given the number of usable channels. This factor is significant in the determination of whether a WRAN cell that is outside of the DFH Community is allowed to join the community. Similarly the number of channels available for the entire DFH Community must be considered when a cell is considered to be joined to the DFH Community. 
       FIG. 3  is a flowchart of one method embodiment for determining whether a WRAN cell can join an established DFH Community according to the present invention. According to one embodiment of the present invention, the leader cell decides whether a WRAN that is requesting admission to the DFH Community is accepted.  FIGS. 3 ,  5  and  7  are flowcharts illustrating methods of implementing an exemplary process for determining a hopping pattern in a DFH Community according to one embodiment of the present invention. 
     In the following description, it will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These computer program instructions may be loaded onto a computer or other programmable apparatus to produce a machine such that the instructions that execute on the computer or other programmable apparatus create means for implementing the functions specified in the flowchart block or blocks. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable apparatus to function in a particular manner such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means that implement the function specified in the flowchart block or blocks. The computer program instructions may also be loaded onto a computer or other programmable apparatus to cause a series of operational steps to be performed in the computer or on the other programmable apparatus to produce a computer implemented process such that the instructions that execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Accordingly, blocks of the flowchart illustrations support combinations of means for performing the specified functions and combinations of steps for performing the specified functions. It will also be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by special purpose hardware-based computer systems that perform the specified functions or steps, or combinations of special purpose hardware and computer instructions. 
     The process  300  begins  305  by the determination  310  of whether the DFH Community, which is assumed to be a simple community, would become a complex community upon acceptance of the new cell. A community group within the DFH Community is complex if it has more than two neighbor groups or two or more neighbor groups have more than one neighbor cell. When two groups have at least one common WRAN cell other than the leader cell, these two groups are referred to as neighbor groups. Thus if the addition of the new cell drives the community group to change from a simple classification to that of complex, the request for acceptance is rejected  315  and the process stops  395 . 
     The next query  320  is whether the joining cell will join the largest community group in the DFH Community. As the number of required channels for the DFH Community is driven by the community group with the largest number of cells, adding an additional cell to the community group having the largest number of cells will alter the required number of channels for the entire DFH Community. When the answer to the query  320  regarding whether the cell is joining the largest community group is no, the process determines whether a new group graph configuration from the addition of the new cell would remain  325  a strict two-color uncolorable group graph. 
     When the answer to the query  320  regarding whether the cell is joining the largest community group is yes, the number of usable channels required to carry out the DFH in the DFH Community is set  330  according to the community group having the largest number of WRAN cells after the new cell is accepted plus the number 1. Similarly when it is determined that the addition of the new WRAN cell will form a group graph that is a strict two-color uncolorable group graph, the number of usable channels required to carry out the DFH in the DFH Community is set  335  according to the community group having the largest number of WRAN cells after new cell is accepted plus the number 2. 
     Using the number of WRAN cells of the largest community group within the DFH Community plus the number 1 or the number 2, the shortest length of a single transmission, i.e., the shortest working period, is determined  340 . This determination, according to one embodiment of the present invention, can be found by applying algorithm 1 as constrained by inequalities 2 and 3: 
     
       
         
           
             
               
                 
                   
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     One skilled in the relevant art will recognize that M represents the number of WRAN cells of the smallest community group in which at least one cell does not belong to any other larger group. Similarly N represents the number of WRAN cells of the larger or largest community group. M ch  stands for the number of working channels of the smallest community group in which at least one cell does not belong to any other larger group. N ch  represents the number of working channels of the larger or largest community group in which at least one cell does not belong to any other larger group. Both of these numbers are determined by the leader cell. P w   sm  represents the smaller or smallest working period and P w   max  stands for the maximal working period. 
     Once calculated the shortest working period, or the shortest length of a single transmission, is compared to the number of WRAN cells in the group and the need for a quiet period to see whether this short period is long enough to meet the needs of the group  345 . When the answer to the question is in the negative, the request by the WRAN cell to join the DFH Community is rejected  315  and again the process stops  395 . Should the answer be yes, the process explores whether the joining cell has any one-hop neighbors other than the leader  350 . When the answer is no, a new community group is created  365  comprised of the new joining cell and the community leader. When the answer is yes, the process selects all community groups that have one-hop neighbors with the joining cell as candidate community groups  335 . When all of the candidate groups that have been proposed  360  meet the DFH Community requirements, the leader accepts  380  the new cell into the DFH Community. 
     However when an analysis of the proposed groups formed by the joining cells fails to identify all of the possible configurations, a new community group is formed  375  that includes the joining WRAN cell and all of the cells that are one-hop neighbors of the joining cell. This addition of a new group into the community both here and at step  365 , requires the DFH Community to consider whether, with the addition of the new group, there will be enough usable channels to conduct DFH among all of the members of the community. Recall that it is possible that the WRAN cell attempting to join the community does not have enough available channels. Thus it is possible that the addition of the new cell will not add to the number of usable channels but will force a more complex hopping scheme requiring more channels. Thus the question is posed whether with the addition of the newly created group there are more usable channels available than are required  370 . Step  370  includes two parts: first, whether the number of usable channels in the largest group after a cell joins the DFH Community is larger than the number of WRAN cells in the largest group of the new DFH Community plus the number 1 or the number 2, and second whether the number of usable channels in the group gaining a new WRAN cell after the cell joins (referred to as the joining group) is larger than the WRAN cell number of the joining group plus the number 1 or the number 2. The answer to the query  370  is yes when the answer to both of the above questions is yes. Otherwise the answer is no. An answer in the negative to this step  370  will result in the rejection  315  of the request to join the group. While an answer in the affirmative removes the last hurdle in the process and allows the leader cell to temporarily accept  380  the joining cell&#39;s request to become a member of the DFH Community (acceptance being conditional on a successful determination of a hopping mode by the leader). 
     According to one embodiment of the present invention, the determination of a hopping pattern varies depending on whether the DFH Community includes neighbor groups.  FIG. 4  shows a comparison of two WRAN cell community group configurations in a DFH Community according to one embodiment of the present invention. The DFH Community  440  on the left side of  FIG. 4  is a reproduction of the DFH Community described in  FIG. 2 . Note that each WRAN cell is one-hop from the leader cell and that no WRAN cells other than the leader are common to either group. The DFH Community depicted on the right side of  FIG. 4  shows the same number of WRAN cells and each WRAN cell is one-hop from the leader cell but WRAN 1   410  is a member of both community groups  430 ,  420 . Groups that have at least one common WRAN cell besides the leader cell are neighbor cells. In this case WRAN 1   410  is defined as the neighbor group cell as it belongs to at least two groups and is not the community leader. 
     According to one embodiment of the present invention, for community groups of a DFH Community that do not have a neighbor group cell, and thus are not neighbor groups, the determination of a hopping pattern is determined by the following algorithms. 
     
       
         
           
             
               
                 
                   
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                         jj 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   Where 
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     1 
                     ≤ 
                     m 
                     ≤ 
                     
                       N 
                       cur 
                       ch 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       N 
                       cur 
                       ch 
                     
                     ≥ 
                     
                       ( 
                       
                         
                           N 
                           cur 
                         
                         + 
                         1 
                       
                       ) 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       N 
                       max 
                       ch 
                     
                     ≥ 
                     
                       ( 
                       
                         
                           N 
                           max 
                         
                         + 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     In equations 4-7 the following variable definitions apply. 
     x m  is the channel sequence number of the m th  channel used by the current group. 
     N max  is the number of WRAN cells in the largest community group. 
     N cur  is the number of WRAN cells in the current group. 
     P w   min  is the working period in the community group having the largest number of WRAN cells. 
     P w   cur  is the working period in the current community group. 
     N max   ch  is the number of working channels in the largest community group. 
     N cur   ch  is the number of working channels in the current group. 
     T x   ij  is the phase shift time (quiet period) in working periods between group i and group j in channel x. 
     T s   ij  is the phase shift time (quiet period) in working periods between group j and group j in channel s. 
     T x   ji  is the phase shift time (quiet period) in working periods between group j and group i in channel x 
     Similarly and according to one embodiment of the present invention, for community groups of a DFH Community that do have a neighbor group cell, and thus are neighbor groups, the determination of a hopping pattern is made by the following algorithms. In such a community each neighbor cell is either labeled as an A cell or a B cell. The determination of a hopping pattern is determined by the following algorithms. 
     
       
         
           
             
               
                 
                   
                     T 
                     
                       x 
                       m 
                     
                     ij 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           T 
                           
                             x 
                             1 
                           
                           ij 
                         
                         + 
                         
                           
                             P 
                             w 
                             cur 
                           
                           * 
                           
                             ( 
                             
                               m 
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       ) 
                     
                     - 
                     
                       
                         P 
                         w 
                         i 
                       
                       * 
                       
                         ( 
                         
                           
                             x 
                             m 
                           
                           - 
                           
                             x 
                             1 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     T 
                     
                       x 
                       m 
                     
                     jk 
                   
                   = 
                   
                     
                       T 
                       
                         x 
                         1 
                       
                       jk 
                     
                     + 
                     
                       
                         P 
                         w 
                         k 
                       
                       * 
                       
                         ( 
                         
                           
                             x 
                             m 
                           
                           - 
                           
                             x 
                             1 
                           
                         
                         ) 
                       
                     
                     - 
                     
                       
                         P 
                         w 
                         cur 
                       
                       * 
                       
                         ( 
                         
                           m 
                           - 
                           1 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             T 
                             s 
                             ii 
                           
                           + 
                           
                             P 
                             w 
                             A 
                           
                         
                         ) 
                       
                       * 
                       
                         G 
                         A 
                       
                     
                     + 
                     
                       T 
                       
                         x 
                         1 
                       
                       ij 
                     
                     + 
                     
                       
                         T 
                         s 
                         jj 
                       
                       * 
                       
                         ( 
                         
                           
                             N 
                             cur 
                             ch 
                           
                           - 
                           
                             G 
                             A 
                           
                           - 
                           
                             G 
                             B 
                           
                           - 
                           3 
                         
                         ) 
                       
                     
                     + 
                     
                       
                         P 
                         w 
                         cur 
                       
                       * 
                       
                         ( 
                         
                           
                             N 
                             cur 
                             ch 
                           
                           - 
                           
                             G 
                             A 
                           
                           - 
                           
                             G 
                             B 
                           
                           - 
                           2 
                         
                         ) 
                       
                     
                     + 
                     
                       T 
                       
                         x 
                         1 
                       
                       jk 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             T 
                             s 
                             kk 
                           
                           + 
                           
                             P 
                             w 
                             B 
                           
                         
                         ) 
                       
                       * 
                       
                         G 
                         B 
                       
                     
                   
                   = 
                   
                     
                       P 
                       w 
                       min 
                     
                     * 
                     
                       ( 
                       
                         
                           N 
                           max 
                           ch 
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         MIN 
                         
                           x 
                           m 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           T 
                           
                             x 
                             m 
                           
                           ij 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           MIN 
                           
                             x 
                             m 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             T 
                             
                               x 
                               m 
                             
                             jk 
                           
                           ) 
                         
                       
                       = 
                       
                         T 
                         s 
                         ij 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   Where 
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     1 
                     ≤ 
                     m 
                     ≤ 
                     
                       ( 
                       
                         
                           N 
                           cur 
                         
                         + 
                         1 
                       
                       ) 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       N 
                       cur 
                       ch 
                     
                     ≥ 
                     
                       ( 
                       
                         
                           N 
                           cur 
                         
                         + 
                         1 
                       
                       ) 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       N 
                       max 
                       ch 
                     
                     ≥ 
                     
                       ( 
                       
                         
                           N 
                           max 
                         
                         + 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     In equations 8-11 the following variable definitions, in addition to those previously presented, apply. 
     x m  is the channel sequence number of the m th  channel used by the current group. 
     G A  is the number of cells labeled with an A. 
     G B  is the number of cells labeled with a B. 
     T s   ii  is the phase shift time (quiet period) associated with group i to which A type of WRAN cells belong. 
     T s   kk  is the phase shift time (quiet period) associated with group k to which B type of WRAN cells belong. 
     T s   jj  is the phase shift time (quiet period) in working periods between group j and group j in channel s. 
     Equations 4-11 attempt to use a smaller number of working channels in the groups having a smaller number of WRAN cells as compared to the maximum of the number of WRAN cells in any group of the DFH Community plus the number 2, or the maximum of the number of WRAN cells in any group of the DFH Community plus the number 1. For example, DFH Community  200  of  FIG. 2  requires five working channels for the DFH Community to work in frequency hopping mode. Group  1   270  will require all of these five channels since Group  1   270  includes four WRAN cells. But since Group  2   260  includes only three WRAN cells, it will use only four channels out of these five available channels. The hopping mode of Group  2   260  will be described in  FIG. 9 . 
       FIGS. 5-8  are flowcharts and high level depictions of the process for calculating a hopping pattern for a DFH Community in which, according to one embodiment of the present invention, the community is defined as a plurality of WRAN cells that are each one-hop neighbors of the leader. As one skilled in the art will appreciate, each community group within the DFH Community must coordinate with each other group to ensure that that their channel usage does not conflict with that of any other overlapping group. As the leader is the only cell with a common link to each group, it is the leader&#39;s responsibly to set the channel hopping pattern, which includes a determination of a working period, quiet period and phase shift for each group. 
     The first step in the process of determining a hopping pattern is to label each neighbor group WRAN cell in the DFH Community as either a cell A or a cell B. According to one embodiment of the present invention, neighbor cells are labeled as either A or B so that when the hopping mode is determined, the neighbor cell labeled A will be the first cell (except the leader cell) to use channel  1  and the neighbor cell labeled B will be the last cell to use channel  1 . When two neighbor cells of a group are both labeled A, these two neighbor cells are the first two cells to use channel  1 . When two neighbor cells of a group are both labeled B, these two neighbor cells are the last two cells to use channel  1 . This process is shown graphically in  FIGS. 5 and 6 . Recall that a neighbor group WRAN cell is defined as the WRAN cell which belongs to two community groups and is not the community leader. 
     Referring now to  FIG. 5 , the process  500  begins  505  with the selection of the DFH Community group having the largest number of WRAN cells  510 . From that point of reference a determination is made whether a corresponding group graph possesses a two-edge connection characteristic  520 . A group graph is a nodal representation of the DFH Community in which each group is a node and the connections between groups is termed an edge.  FIG. 6  shows a depiction of a DFH Community according to one embodiment of the present invention and a group graph representation of the same DFH Community. On the left is a DFH Community including five DFH Community groups labeled Group  1 - 5   610 ,  620 ,  630 ,  640 ,  650 . On the right side of  FIG. 6  is a group graph depiction of the same DFH Community having five nodes representing Groups  1 - 5   615 ,  625 ,  635 ,  645 ,  655 . Note that the leader cell is absent since it is a member of each group. Each of these nodes is connected by an edge  670 . Group  1   615 , for example, possesses two edges  670 . One edge  670  couples Group  1   615  to Group  2   625  and a second edge couples Group  1   615  to Group  5   655 . Thus the group graph shown in  FIG. 6  is a two-edge connected group graph. 
     Referring again to the flowchart of  FIG. 5 , upon a positive determination that the group graph is a two-edge connected graph  520 , a neighbor group is selected  530  having fewer or equal number of WRAN cells. In this case Group  1   610  (referring to  FIG. 6 ) has four WRAN cells of which two neighbor group cells, WRAN 1  and WRAN 2 , are labeled as an A and B. When a group has only one neighbor cell, the neighbor cell can be labeled as either an A or B. The process moves to Group  2   620  which has three WRAN cells of which two are neighbor group WRAN cells: WRAN  2  and WRAN 3 . Since WRAN 2  was previously labeled B, WRAN 3  is labeled A. As one skilled in the art will appreciate, the process could initially have either chosen Group  1  or Group  5  as both possess the same number of WRAN cells. 
     In the situation in which a group graph is not a two-edge connected graph, the labeling of subsequent groups is random  540 . With the labeling of Group  2   620  complete, the process shown in  FIG. 5  moves on to determine whether any groups in the DFH Community are unlabeled  550 . When the answer is no, the labeling process ends  595 . When the answer is yes, the process shown in  FIG. 5  moves on  560  to determine whether the most recent labeled group has an unlabeled neighbor group. When the answer is yes, the process moves on  565  to label the neighbor cells of the unlabeled neighbor group as an A or B. Thereafter the process  500  returns to the question of whether any group is unlabeled  550 . When the answer for  560  is no the process shown in  FIG. 5  moves on to determine  570  whether the other previously labeled groups have an unlabeled neighbor group or groups. When the answer is yes, the process moves on  575  to label the neighbor cells of the identified unlabeled neighbor group as an A or B. Thereafter the process  500  returns to the question of whether any group is unlabeled  550 . When the answer for  570  is no the process shown in  FIG. 5  moves on  580  to randomly select an unlabeled group and label its neighbor cells as an A or B. Thereafter the process  500  again returns to the question of whether any group is unlabeled  550 . 
     The process  500  shown in  FIG. 5  results in an A and B labeling of each neighbor cell of each group shown in  FIG. 6 . In this particular example the process began at Group  1   610 , the group having the largest number of WRAN cells and continued clockwise labeling each group in alternating fashion, A or B. The process  500  could have labeled the neighbor cells in a counterclockwise direction without impacting the channel hopping pattern determination process. 
     With the DFH Community group graph determined, and with a leader identified, the leader can determine the hopping pattern. When the group graph of a DFH Community is a strict 2-color uncolorable graph, a hopping shift, as shown in  FIG. 8 , is used to determine the hopping mode of each group of the DFH Community. 
       FIG. 7  shows a flowchart of one method embodiment for determining group hopping pattern according to the present invention. The process  700  begins  705  by calculating  710  the DFH Community working period. This step is accomplished by applying equation (1) as discussed previously. Once the working period is determined, the quiet period of the largest DFH Community group is set  720  by dividing the working period by the number of members (WRAN cells) in the DFH Community. 
     A query is then posed whether each DFH Community group within the DFH Community possesses channel hopping information  730 . When the answer is yes, the community leader accepts  780  the cell joining request and the process  700  ends  795 . When there are groups that do not have hopping patterns, the process  700  selects  740  the group having the largest number of WRAN cells from the groups lacking a hopping pattern. Using algorithms 4-7 or 8-11 phase shifting times are determined  750  depending on whether the group needing the hopping pattern is associated with or without a neighbor group respectively. Based on these calculations a question is raised whether the phase shifting algorithms stop successfully  760 . When they do stop the phase shift time and working period are used to calculate  770  when and to which channel a hop is to occur. With all groups having a hopping pattern the process again ends  795 . When the phase shifting algorithms fail to stop indicating an untenable configuration for the DFH Community, the process rejects the cell joining request  790  and the process terminates  795 . 
     To better understand the hopping pattern calculation, a graphical representation of the relationship between the working period and quiet time for each channel used in the DFH Community, according to one embodiment of the present invention, is shown in  FIG. 8 .  FIG. 8  shows two hopping patterns of groups in depicted in  FIG. 2 . The upper portion of  FIG. 8   810  is the hopping mode of a single group with its neighbor cells labeled A and B. In this situation a group has two neighbor cells labeled as A and B for Group  1   231 , Group  2   232 , Group  3   233 , and Group  4   234 . In this case there are five channels  810 . The working period is shown as the length of the blocks  0 , A and B ( 850 ,  830 ,  840  respectively). The period between the blocks represents the quiet period  825 . 
     The lower portion of  FIG. 8   820  is a depiction of the hopping mode of a group with two neighbor cells labeled A or B. In this scenario two neighbor cells are both labeled as an A. Note that there are five working channels  820 . The working period is shown as the length of the blocks  0 , A 1  and A 2  ( 850 ,  860 ,  870  respectively). The period between the blocks represents the quiet period  865 . 
       FIG. 9  shows a hopping pattern of Group  2   260  shown in  FIG. 2A , according to one embodiment of the present invention. As previously discussed, this particular group requires N+1 channels since this group graph is a 2-color colorable group graph. In this case Group  2   260  uses four channels out of community  200 &#39;s five working channels. The working period is shown as the length of the blocks  0 ,  4  and  5  ( 910 ,  920 ,  930  respectively). The period between the blocks represents the quiet period  915 . 
     While there have been described above the principles of the present invention in conjunction with specific DFH Community architecture, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features that are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The Applicant hereby reserves the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.