Patent Publication Number: US-2019191317-A1

Title: Coordinated Interference Mitigation in Communication Systems

Description:
BACKGROUND 
     (1) Technical Field 
     The disclosed methods and apparatus relate to radio frequency communication and more particularly to mitigating the effects of interference in a millimeter wave communication system. 
     (2) Background 
     As the use of wireless communications continues to increase, substantial progress is being made to formulate standards that govern protocols for how such communications occur. These standards are relevant to several types of communications systems, including cellular telephony, point to point communications, point to multipoint communications, short-range communications, and long-range communications using smaller cells (e.g., picocells and femto cells). Some of the industry standards, such as 802.11ax, contemplate using multiple input, multiple output (MIMO) technology to assist in increasing the system capacity and contemplate the possibility of providing service over longer ranges than the current 802.11 WiFi systems provide. In addition, a 5G communications standard is evolving to consider use of millimeter wavelength signals, such as signals that operate at frequencies in the range of 30-300 GHz. The use of smaller cells can increase the overall system capacity by allowing greater frequency reuse. In addition, providing base station sectors that are divided into subsectors further enhances the ability to increase capacity through even greater frequency reuse. The use of such advanced techniques and high frequencies pose significant challenges, such as in establishing an architecture that can support higher frequencies and provide efficient, cost effective practical solutions to rolling out such a system on a large scale. Meeting these challenges requires substantial planning and product development. 
     Already contemplated by Skyriver, a leading-edge millimeter wave (mmWave) broadband provider transforming broadband, are systems that use concepts developed for use in short range 802.11n and 802.11ac compliant systems, together with mmWave transceivers. But while the concepts used in 802.11 systems have advanced, additional advances in conforming products and systems are necessary to take full advantage of some of the new features provided in the newest forms of 802.11, such as 802.11ax. As design and implementation of next generation networks operating in mmWave frequencies is growing, specific attention should be paid to inter-cell and intr a - c  ell interference Therefore, there is currently a need to improve detection and mitigation of interference affecting communication at microwave frequencies between base stations and subscriber units attempting to communicate with the base stations. 
     SUMMARY 
     The disclosed method and apparatus provides an architecture that mitigates the effects of interference in radio frequency communication systems. In general, such systems have one or more base stations. Each base station is responsible for communicating with several subscriber units. 
     In some embodiments, a communication system includes a Coordination and Control Center (CaCC) in communication with at least one base station site. At least one of the base station sites includes a first base station in communication with a first subscriber unit of the communication system. The CaCC includes an Interference Mitigation Circuit configured to: (a) receive information regarding a source of interference affecting radio frequency communications between a first base station and a first subscriber unit of the communication system, wherein the source is at least one of a second base station or a second subscriber unit within the communication system; and (b) provide instructions to the source from the CaCC to mitigate the interference between the affected first base station and first subscriber unit. 
     In an example scenario, the first base station resides at a base station site having a plurality of base stations. The base station is responsible for communicating with subscriber units that reside within a corresponding sector of the base station site. The source of interference is: (a) a second base station within the base station site; or (b) a second subscriber unit in a different sector of the base station site. In another example scenario, the communication system includes a plurality of base station sites, each base station site having a plurality of base stations. A first base station resides within a first of the plurality of base station sites. A second base station resides within a second of the plurality of base station sites. The source of interference is: (a) the second base station; or (b) a subscriber unit within a sector of the second base station site. In each scenario, the CCaC is configured to mitigate the inference by providing the source with instructions to reschedule or re-route its transmission, or to switch to another communication frequency or sub-channel in a frequency. 
     The details, features, objects, and advantages of one or more embodiments of the disclosed method and apparatus are set forth in (or contemplated to be apparent from) the accompanying drawings, the description and claims below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a base station site and several (subscriber units within a millimeter wave (mmWave) communication system. 
         FIG. 2  is another illustration of the base station coverage area. 
         FIGS. 3A-B  are illustrations of the Earth, a radius through a base station site on the surface of the earth, and an X-Y plane tangential to the surface of the earth and perpendicular to the radius. 
         FIG. 4  is a simplified block diagram of a set of base stations within a base station site. 
         FIG. 5  is an illustration showing the base stations of a base station site. 
         FIG. 6  shows another illustration of such a system in which several base station sites and a Coordination and Control Center (CCC) are coupled to the core network. 
         FIG. 7  is an illustration of an alternative embodiment in which base stations within the same base station site are part of a wired local area network and/or a wireless local area network. 
         FIG. 8  is a simplified block diagram of one embodiment of some portions of a base station site, including some of the details of the transmit portion of a base station sector radio. 
         FIG. 9  is a simplified block schematic of the components of the RF TX chain. The RF TX chain has several inputs. 
         FIG. 10  is a simplified block diagram of one embodiment of some portions of a base station site, illustrating some of the details of the receive portion of a base station sector radio. 
         FIG. 11  is a simplified schematic of the components of the RF receive chain. 
         FIG. 12  is a simplified schematic of a base station site illustrating some of the transmit components of the sector radio in accordance with an alternative embodiment such as the base stations shown in  FIG. 7  in which a coordination control module is shared by all of the base stations. 
         FIG. 13  is a simplified schematic of the base station site of  FIG. 12  illustrating some of the receive components of the sector radio. 
         FIG. 14  is an illustration of multiple base station sites configured to communicate with a Coordination and Control Center according to some embodiments of the present disclosure. 
         FIG. 15  is a flow diagram illustrating the operation of some embodiments of the present disclosure for mitigating the effects of interference in radio frequency communication systems. 
         FIGS. 16-28  illustrate various interference scenarios in which the operation disclosed in  FIG. 15  may be performed. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments are described herein in the context of an architecture that mitigates the effects of interference in radio frequency communication systems, such as millimeter wave (mmWave), communication systems. Embodiments provided in the following description are illustrative only and not intended to limit the scope of the present disclosure. 
     In the interest of clarity, not all of the routine features of the embodiments described herein are shown and described. It will be appreciated that in any such actual implementation, numerous implementation-specific details that are not expressly disclosed may nevertheless exist in order to achieve goals such as compliance with application- and business-related constraints. In addition, the specific goals can vary from embodiment to another. 
     The disclosure is not restricted to the particular embodiments or implementations described as such. For example, references that are made to a particular means for implementing a feature, structure, operation, or other characteristic in one particular embodiment should be taken as providing support for such implementation in other disclosed embodiments. Accordingly, any particular feature, structure, operation, or other characteristic described in this specification in relation to one embodiment may be combined with other features, structures, operations, or other characteristics described in respect of any other embodiment. The appearance of the phrases “some embodiments” or variations of this phrase in various places in the specification does not necessarily refer to the same embodiment or implementation. 
     Use herein of the word “or” is intended to cover inclusive and exclusive OR conditions. In other words, A or B or C includes any or all of the following alternative combinations as appropriate for a particular usage: A alone; B alone; C alone; A and B only; A and C only; B and C only; and A and B and C. 
       FIG. 1  is an illustration of a base station site  101  and several subscriber units  103  within a millimeter wave (mmWave) communication system  100 . In some embodiments, several base stations  102  are located at each base station site  101 . The base station site  101  serves as a hub for communications to the subscriber units  103 . The system provides point-to-multipoint communications from the base station site  101  to each subscriber unit  103  over a “downlink”. In addition, the base station site  101  provides multipoint-to-point communications from each subscriber unit  103  to a base station  102  at the base station site  101  over an “uplink”. Subscriber units  103  may be located within various types of facilities, such as residential buildings, office buildings, towers of base stations within one or more mobile communications networks. A base station site coverage area  105  (i.e., the geographic area serviced by the base stations  102  within a base station site  101 , hereafter referred to as the “site coverage area” for the sake of brevity) is divided into several base station sector coverage areas  107  (hereafter referred to as “sector coverage areas” for the sake of brevity). In some embodiments, each base station  102  within the base station site  101  services one sector of the site coverage area  105 . Accordingly, each base station  102  is associated with a corresponding sector coverage area  107 . Each base station  102  is responsible for communicating with all of the subscriber units  103  within the corresponding sector coverage area  107 . In the embodiment shown in  FIG. 1 , the site coverage area  105  is divided into six such sectors coverage area  107 . For the sake of simplicity, the site coverage area  105  is shown in  FIG. 1  as a generally circular area with a radius of approximately 4 miles. Each sector coverage area  107  is shown as an essentially pie shaped region. It should be understood however that the actual site coverage area  105  may not have a uniform shape, but rather a shape that is dependent upon obstructions, terrain and other transmission channel factors. Furthermore, each sector coverage area  107  may intersect with one or more adjacent sector coverage area  107  to a greater or lesser degree than is shown in the embodiment of  FIG. 1 . Furthermore, in some embodiments, each sector coverage area  107  may be substantially different in size and shape from one or more of the other sector coverage areas  107 . 
       FIG. 2  is another illustration of the site coverage area  105 . In various embodiments of the disclosed method and apparatus, the particular number of sector coverage areas  107  may vary from that illustrated in  FIG. 1  and  FIG. 2 . In the embodiment shown in  FIG. 2 , each sector coverage area  107  is divided into four subsectors  201 . Each subsector  201  extends out from the base station  102  with an azimuth angle of approximately 15 degrees, wherein the azimuth angle is an angle on an X-Y plane approximately perpendicular to the Earth&#39;s radius through the site of the base station  102 . 
     The particular number and shape of the subsectors  201  may vary from the number shown in the embodiment illustrated in  FIG. 2 . However, having four subsectors is compatible with a system in which an 802.11 compliant MAC provides  8  spatial streams, two of which can transmitted into each subsector with each of the two being transmitted on a different polarization. In some embodiments, each of the two polarizations associated with one subsector are orthogonal. A sub-sector antenna (not shown in  FIG. 1 or 2 ) is associated with each corresponding polarization of each subsector and defines the shape and size of the subsectors  201  within the sector coverage area  107 , as will be discussed in greater detail below. Accordingly, in such embodiments, there are  8  such subsector antennas within each sector. 
     As is the case with the sector coverage areas  107 , each subsector  201  can have a substantially different size and shape from that of the other subsectors  201  within the same sector coverage area  107  or from the other subsectors  201  in each other sector coverage areas  107 . Furthermore, in some embodiments, there may be more or less than 6 sectors, each with more or less than 4 subsectors. In some embodiments, the sum of all of the azimuth angles for each sector may not be equal to 360 degrees. Accordingly, there may be some holes in the coverage where no subscriber units  103  are expected to be present, or in other embodiments, there may be an overlap in the coverage of two or more adjacent sectors. In addition, in some embodiments, the number of subsectors may vary from one sector to another and one or more subsectors may have different azimuth angles than one or more of the subsectors within the same sector or within other subsectors. 
       FIG. 3A  is an illustration of the Earth  301 , a radius  303  through a base station site  101  on the surface of the earth  301 , and an X-Y plane  305  tangential to the surface of the earth  301  and perpendicular to the radius  303 .  FIG. 3A  is oriented such that the X-axis extends outward, the Y-axis extends upward and the Z-axis extends to the left. 
       FIG. 3B  is an illustration of the Earth  301 , the X-Y plane  305  and a pair of rays  307 ,  309  emanating from the base station site  101  that define an azimuth angle of 60 degrees. The orientation of the illustration in  FIG. 3B  is rotated 90 degrees about the Y-axis with respect to the illustration of  FIG. 3A . Accordingly, in  FIG. 3B , the Y-axis extends upward, the X-axis extends to the right and the Z-axis extends outward (making the radius  303  extend outward and thus not visible in  FIG. 3B ). As can be seen in  FIG. 3B , azimuth angle lies on the X-Y plane. It should be clear that the radius of the Earth is significantly greater than the dimensions of the site coverage area  105 . Therefore, that portion of the X-Y plane  305  that is coincident with the site coverage area  105  is generally also coincident with the surface of the Earth. Furthermore, rays emanating from the base station site  101  that lie on the plane  305  are projected from the base station site  101  at an elevation angle of zero degrees. Contours of the Earth&#39;s surface, however can be taken into account when aiming the antennas. Therefore, the center of any beam transmitted from a base station  102  within the base station site  101  may be at an elevation angle other than zero degrees. 
       FIG. 4  is a simplified block diagram of a plurality of base stations  102  within a base station site  101 . In the embodiment shown in  FIG. 4 , the base station site  101  has 6 base stations  102 . Accordingly, there are 6 sector coverage areas  107  in the site coverage area  105 . Each sector coverage area  107  is serviced by a base station  102  having a coverage area with an azimuth angle of approximately  60  degrees. In some embodiments, each base station  102  has a core network interface unit (CNIU)  405 . The CNIU  405  provides a means by which the base station  102  can communicate, such as via IP Traffic  402 , with other nodes on a core network  401 . Accordingly, in some embodiments, the CNIU  405  provides access to other base stations at other base station sites or other base stations located at the same base station site  101 . 
       FIG. 5  is an illustration showing the base stations  102  of a base station site  101 . Each of the six base stations  102  are coupled to a core network  401  in accordance with some embodiments of a communication system. Only one base station site  101  is shown in  FIG. 5  for the sake of simplicity. 
       FIG. 6  shows another illustration of such a system in which several base station sites  101  and a Coordination and Control Center (CCC)  604  are coupled to the core network  401 . The CCC  604  has a CNIU  405 . The CNIU  405  allows the CCC  604  to be a node on the Core Network  401 . In  FIG. 6 , the base stations  102  and core network interface unit of only one base station site  101  are shown. The other two base station sites  101  are shown as blocks for the sake of simplicity. In some such embodiments, the CCCs  604  coordinate operations between base stations  102  within each base station site  101 . 
       FIG. 7  is an illustration of an alternative embodiment in which base stations  702  within the same base station site  101  are part of a wired local area network  704  and/or a wireless local area network  703 . Therefore, each base station  702  has a network interface unit (NIU)  701  that provides access to the local area network  704 ,  703 . A CNIU  405  is also a node on the local area network  704 ,  703 . Accordingly, each base station  702  can access the core network  401  through the local area network  704 ,  703  through one CNIU  405  that is present in the base station site  101 . 
     In either the case of the base station  102  or the base station  702 , the base station site  101  provides a means by which subscriber units  103  can be connected to devices that are part of a private network, public network or the Internet through devices (such as Internet gateways) connected to the core network. In addition, in some embodiments, the base station  102 ,  702  can provide communication links through sector radios  407  of the base station  102 ,  702  to allow two or more of the subscriber units  103  to communicate with each other through the base station  102 ,  702 . 
     It should be noted that throughout the remainder of this document, references to the base station  102  apply equally to the base station  702 . 
       FIG. 8  is a simplified block diagram of one embodiment of portions of a base station site  101 , including some of the details of the transmit portion of a base station sector radio  407 . For the sake of simplicity, only one base station sector radio  407  is shown in detail. In addition, only the transmit related components are shown in  FIG. 8 . 
     In the embodiment shown in  FIG. 8 , each base station  102  transmits mmWave signals into the sector coverage area  107  corresponding to that base station  102 . Each of the six base stations  102  has a corresponding base station sector radio  407 . In some embodiments, each such base station sector radio  407  has essentially the same architecture. However, in other embodiments, the architecture of one or more of the base station sector radios  407  may differ from the rest. In some such embodiments, each of the base station sector radios  407  has an architecture that is uniquely configured for the needs of the particular sector coverage area  107  that the radio  407  services. 
     In some embodiments of a base station  102  shown in  FIG. 8 , signals containing content to be transmitted by the radio  407  are coupled from the CNIU  405  to a MAC/Baseband/Intermediate Frequency (MBI) module  801 . In some embodiments of the disclosed method and apparatus, the MBI module  801  is capable of providing spatial division, time division and frequency division outputs  802  at an intermediate frequency (IF). That is, the MBI module  801  is capable of outputting signals  802  that carry unique information through different outputs that are coupled to spatially diverse antennas, and thus provide spatial division. 
     In addition, the MBI module  801  is capable of outputting signals  802  to each output, wherein each such signal has unique content at different times. Thus, the outputs provide time division multiplexed signals. Still further, the MBI module  801  is capable of providing unique content concurrently through each output at different frequencies, thus provide frequency division multiplexed signals. In some such embodiments, the MBI module  801  includes at least an 802.11 module, such as module capable of operating in conformance with one of the following: industry standard 802.11(n), 802.11(ac), 802.11(ax), etc. In some embodiments, the MBI module  801  implements a technique commonly referred to as multiple-input multiple-output (MIMO) to generate spatial division outputs. Each spatial division output is commonly referred to as a “spatial stream” (SS). In some embodiments, such as those that have a MBI module  801  that operates in conformance with 802.11(ac) or 802.11(ax), the MBI module  801  may have eight output ports that each output one SS  802 . A media Access Control (MAC) component of the MBI module  801  (which in some embodiments is within the 802.11 module of the MBI module  801 ) determines how the content that is coupled to the MBI module  801  is to be assigned to each SS  802 . In addition to determining which SS  802  the content is to be assigned, the MAC component  803  also determines time and frequency division allocations. That is, the MAC component  803  determines in what time slot and to which frequency the content is to be applied in each particular SS  802 . 
     In some embodiments, each SS  802  is associated with a corresponding TX input to an IF module  805 . In some such embodiments, the IF module  805  comprises a switch module  811  and several filters  807 , each filter  807  associated with a corresponding amplifier  809 . Since  FIG. 8  shows only components that are associated with the transmit function, only those TX amplifiers  809  and TX filters  807  that are in the transmit signal path are shown in  FIG. 8 . 
     Each TX output from the MBI module  801  is associated with a corresponding one of the IF module TX inputs and the corresponding TX filter  807 . The output of each TX filter  807  is coupled to the input of the corresponding TX amplifier  809 . It will be understood by those skilled in the art that the use of particular amplifiers and filters will depend upon the requirements of each particular system. Therefore, it should be understood that the configurations disclosed herein are merely provided as examples of systems. Therefore, significant variations in the amount of filtration and amplification are within the scope of the disclosed method and apparatus. 
     The output of each TX amplifier  809  is associated with, and coupled to, a corresponding TX input to a switch module  811  within the IF module  805 . The switch module  811  comprises a switch network that makes it possible to selectively connect any one input to any one output. Likewise, each output can be connected to any one input. Therefore, there is a selectable one-to-one correspondence between TX inputs and TX outputs of the switch module  811 . Other embodiments may provide a switch module that is capable of selectively connecting one or more inputs to one or more outputs. Each TX output from the switch module  811  is associated with a corresponding input to an RF transmit (TX) chain  814 . It should be noted that the switch module  811  also comprises RX inputs and RX outputs that will be discussed further below with respect to  FIG. 10  and  FIG. 11 . 
     While the MBI  801  shown in  FIG. 8  has several TX outputs, in some embodiments, the MBI  801  may have as few as two TX outputs, each associated with a corresponding one of two subsector antennas  821 . In some such embodiments, the two subsector antennas  821  are focused into the same subsector and transmit signals with different polarizations (e.g., horizontal polarization and vertical polarization). 
       FIG. 9  is a simplified block schematic of the components of the RF TX chain  814 . The RF TX chain  814  has several inputs  902 . Each input  902  is associated with a corresponding frequency converter  816 , amplifier  813 , filter  815  and output  904 . Each RF TX chain input is coupled to a corresponding IF input of the corresponding frequency converter  816 . A local oscillator  818  provides a local oscillator signal to each frequency converter  816 . Each frequency converter  816  mixes the input signal with the local oscillator signal to upconvert the IF signal to a millimeter wave frequency that is output from the frequency converter  816 . The upconverted signal output from each frequency converter is coupled to the corresponding amplifier  813 . The output of each amplifier  813  is coupled to the input of the corresponding filter  815 . 
     Referring back to  FIG. 8 , each output  904  from the RF TX chain  814  is associated with, and coupled to, a corresponding input to a subsector antenna  821 . Each input to the subsector antenna  821  is configured to form a beam directed to a corresponding subsector  201  of the sector coverage area  107  serviced by the base station sector radio  407 . In some embodiments, there are two inputs focused into the same subsector  201 . The first is applied to elements of the antenna that polarize the signal in a first polarization. The second is applied to elements of the antenna that polarize the signal in a second polarization orthogonal to the first polarization. For example, in some embodiments, a first input to the sector antenna  821  is coupled to elements that transmit signals in a beam focused upon a first subsector  201  and having a horizontal polarization. A second input to the sector antenna is coupled to elements that transmit signals in a beam focused upon a second subsector  201  and having a vertical polarization. Therefore, by selecting a particular output of the switch module  811  to which a particular input of the switch module  811  is to be coupled, the signal output from the amplifier  809  is selected for transmission on a transmission beam that is focused into the subsector  201  associated with the selected switch module output. In some embodiments, selecting a particular output further determines the polarization on which the signal will be transmitted. In the embodiment in which there are four subsectors  201 , there eight subsector antennas  821 . The subsector antennas  821  are paired such that each pair of subsector antennas  821  is focused to transmit beams into one of the four subsectors. A first subsector antenna  821  of each pair transmits signals having a first of two orthogonal polarizations (e.g., vertical or horizontal). The second subsector antenna  821  of the pair transmits signals on the second of the two orthogonal polarizations. Accordingly, each output of the switch module  811  is associated with a corresponding subsector antenna  821  focused to transmit a signal into a unique one of the four subsectors  201 . Furthermore, the combination of polarization and subsector  201  is unique for each output of the switch module  811 . 
     Ideally, in a typical 802.11 configuration, such as an 802.11(ax) configuration, each SS  802  is coupled to a different antenna to provide the spatial diversity desired to implement a MIMO transmission. In the embodiment of  FIG. 8 , a multi-user MIMO system is used in which each pair of SSs carries different content to subscriber units  103  in a different subsector  201 . At least two antennas within the receiver of each subscriber unit  103  can receive signals from the subsector antennas  821  of the transmitter that transmit beams into the subsector  201  in which the subscriber unit  103  resides. Some typical 802.11systems take advantage of MIMO techniques to increase the system throughput. Multipath channels are created by the creation of different signal paths that form as a consequence of the signals reflecting off various objects along the path between the transmitter and the receiver, creating associated different delays for each signal path. However, in accordance with some embodiments of the disclosed method and apparatus, rather than relying upon signals encountering multipath channels between the transmit antennas and the receive antennas, each SS is transmitted on a transmission beam that is focused into a unique subsector  201  of the sector coverage area  107  on a unique polarization. In some embodiments, two SSs are transmitted into the same subsector  201 . However, the two signals are transmitted on beams that have orthogonal polarizations. By virtue of the signals being transmitted through elements of the transmit antenna that are either on different polarizations or directed at different subsectors  201 , the signals will be in different channels for the purpose of the MIMO system, similar to the different spatial channels in a typical 802.11 MIMO configuration. A coordination control module  823  coordinates the assignment of SSs output from the MBI module  801  with the switch module  811  (i.e., the selection of the output to which each particular SS is coupled by the switch module  811 ). 
     In other embodiments, signals that are not completely orthogonal may be transmitted into the same subsector  201 . In such embodiments, a technique commonly known as non-orthogonal multiple access (NOMA) is used in which such signals that are not completely orthogonal are transmitted on the same frequency and at the same time into the same space, relying upon a difference in polarization (or other factor that can be used to distinguish signals), but wherein the signals are not completely orthogonal. For example, a first signal may have polarization that is between horizontal and vertical (e.g., at 45 degrees from horizontal), while other signals are either strictly horizontal, strictly vertical, or 90 degrees from the first signal. While some such signals are not orthogonal, the difference in polarization is sufficient to provide some measure of separation that provides the receiver with a limited capability to distinguish the signals from one another. Therefore, while the separation of the signals is not nearly as great as for orthogonal polarizations, there is sufficient separation to provide some advantages that, when taken together with the increase in throughput, offset the negative impact of distortion created by the cross contamination of the signals. 
     In some embodiments of the disclosed method and apparatus, the MAC component  803  is responsible for allocating resources to each subscriber unit  103 . That is, the MAC component  803  determines which SS  802  at which frequencies and at which time is to be used to transmit content to each particular subscriber unit  103 . It should be noted that in addition to providing signals with time division, frequency division and spatial division, the signals provided by the MBI module  801  may be modulated using orthogonal frequency division multiplexing (OFDM). In some cases, the content modulated on various OFDM subcarriers may be intended for reception by different subscriber units  103  (i.e., orthogonal frequency division multiple access (OFDMA)). Alternatively, different OFDM subcarriers may carry different data streams intended for the same subscriber unit  103 . In some embodiments, the MBI module  801  receives instructions from the coordination control module  823  that assist the MBI module  801  and the MAC component within the MBI module  801  to determine the manner in which the resources are to be allocated. 
     In many ways, the operation of the MAC component  803  of the disclosed method and apparatus is similar to the operation of a MAC within a conventional 802.11(n), 802.11(ac) or 802.11(ax) system. That is, the MAC component  803  need not treat the SSs  802  that are output any different from those SSs that are output from a MAC of a conventional 802.11 system. However, because SSs  802  are transmitted to the subscriber units  103  residing in different subsectors using different subsector antennas  821 , determinations of Channel State Information (SCI) by the MAC component  803  needs to be coordinated with the switch module  811  within the IF module  805 . For example, the channel from the base station  102  to a particular subscriber unit  103  depends upon the subsector  201  in which the subscriber unit  103  is located. The coordination control module  823  performs the function of controlling the switch module  811  in coordination with the MAC component  803  of the MBI module  801 . For example, in some embodiments, when the SCI is being measured for the channel from a first output of the MBI module  801  during transmission from a first subsector antenna  821 , the switch module  811  is controlled to ensure that the first output from the MBI module  801  is coupled to the first subsector antenna  821 . In some embodiments, a control signal is coupled on a line  824  from the coordination control module  823  to the MBI module  801  to allow the MBI module  801  to be coordinated with the switch module  811  during a SCI procedure. In some embodiments, the switch module  811  is controlled by a signal output on a signal line  825  from the coordination control module  823 . Similarly, each other output from the MBI module  801  is coupled to the appropriate subsector antenna  821  during measurements of the channel between the base station  102  and the subscriber unit  103  at issue. A further discussion regarding the determination of SCI for each channel is provided below. Once the SCI procedure is complete, the coordination control module  823  ensures that the signals that are output from the MBI module  801  are coupled to the appropriate subsector antenna  821  for transmission of MIMO signals from the base station  102  to each subscriber unit  103  to which the base station  102  is communicating. In some embodiments, such as the embodiment shown in  FIG. 6 , the coordination control module  823  is coupled to the MBI module  801  and also to the IF module  805 . In particular, in some embodiments, the coordination control module  823  is coupled to the switch module  811  in the IF module  805 . 
     For MIMO operations, SCI regarding the channels between the various antennas at the base station  102  and the antennas of each subscriber unit  103  must be determined. The SCI information is used by the base station to pre-code transmissions to subscriber units taking into account distortions that occur due to the nature of the transmission channel between the transmitter and the receiver. Conventions and protocols for attaining SCI are provided in the 802.11 standard. In particular, there are two protocols that are provided in 802.11 for attaining SCI. The first is referred to as “Implicit” and the second is referred to as “Explicit”. 
     In accordance with the Explicit technique for determining SCI, the base station  102  sends a “null data packet announcement” (NDPA) frame to the subscriber units. Usually, the NDPA frame contains the address of the intended subscriber units  103 , the type of feedback requested and the spatial rank of the requested feedback. The base station  102  then sends a “sounding frame” known as a “null data packet” (NDP) frame. The NDP contains a physical layer (PHY) preamble with long training fields (LTFs), short training fields (STFs) and a signal (SIG) field. The NDP contains no data. The subscriber unit  103  then analyzes the NDP and provides back a report for each receive antenna (i.e., each SS). The base station  102  then uses the report to precode further transmissions to those subscriber units  103  from which reports were received. The reports are typically relatively large and require a significant amount of bandwidth. In some embodiments, such precoding is done by a combination of the coordination control module  1023  and the MBI module  801 . In particular, in some embodiments, the MAC component  803  of the MBI module  801  applies precoding to signals output from the MBI module  801 . In some embodiments, the coordination control module  823  may be coupled to the amplifier  813 . 
     In accordance with the implicit technique for determining the SCI, the base station  102  requests the subscriber unit  103  to send the NDP frame. The base station  102  can then determine the precoding of the transmissions to the subscriber unit  103  based on the NDP frame without the report having to be communicated. This saves a substantial amount of bandwidth in the SCI procedure. In order to use the implicit technique, however, the uplink and downlink have to be reciprocal. While some differences may occur between the uplink and downlink of a mmWave system using TDD, the differences can typically be considered to be negligible when conditioning (e.g., precoding) the signals. That is, because the same frequency is used for both the uplink and the downlink, the channel characteristics will typically be the same or close enough to allow the information derived from the uplink to be used to precode signals on the downlink. 
     Accordingly, the implicit SCI procedure defined by the 802.11 standard can be used with a modification that the SSs output from the MBI module  801  have to be coordinated with the operation of the switch module  811  to ensure that the signals are transmitted to the desired subsector antennas, and thus to the intended subscriber units  103 . In addition, beamforming that is performed by adjusting the gain and phase of the signals coupled to each subsector antenna  821  must be coordinated with the operation of the MBI module  801 . The coordination control module  823  coupled to the MBI module  801  and the switch module  811  ensures the coordination of the switch module  811  and MBI module  801  during both the SCI procedure and normal operation. 
     As noted above, in addition to coordinating the SCI operations, the coordination control module  823  is also responsible for ensuring that SSs output from the MBI module  801  are routed by the switch module  811  to the appropriate feed of the appropriate subsector antenna  821  during normal operation. That is, the coordination control module  823  is responsible for ensuring that each SS output from the MBI module  801  is transmitted on the correct polarization and subsector antenna  821 . In some embodiments, the coordination control module  823  has an output that is coupled over a signal line  824  to an input of the MBI module  801 . The output from the coordination module  823  provides information that allows the MBI module  801  to determine that the SCI procedure can be performed (i.e., that the output from the MBI module  801  associated with channel being measured is coupled to the appropriate subsector antenna  821 ). 
       FIG. 10  is a simplified block diagram of one embodiment of some portions of a base station site  101 , illustrating some of the details of the receive portion of a base station sector radio  407 . For the sake of simplicity, only one base station sector radio  407  is shown in detail. In addition, only the components relevant to the receiver operation of the base station  102  are shown in  FIG. 10 . The operation of the receive sections of the base station  102  are similar to the operation of the transmit section. The signal flow however is from the subsector antenna  821  to the MBI  801 . Signals received by the subsector antennas  821  are coupled to an RF receive (RX) chain  1002 . 
       FIG. 11  is a simplified schematic of the components of the RF receive chain  1002 . Each input  1101  of the RF receive chain  1002  is associated with a corresponding amplifier  1102 , filter  1104 , frequency converter  1106  and output  1108 . Signals coupled to the RF receive chain  1002  are coupled to the input of the corresponding amplifier  1102 . The output of the amplifier  1102  is coupled to the input of the corresponding filter  1104 . The output of the filtered  1104  is coupled to the RF input of the corresponding frequency converter  1106 . A local oscillator input to the frequency converter  1106  is coupled to an RF local oscillator (LO)  1110 . The LO  1110  provides an LO signal to down convert the received RF signal to an IF frequency. The IF output of the frequency converter  1106  is then coupled to the output  1108  of the RF receive chain  1102 . 
     Referring back to  FIG. 10 , the RX outputs from the switch module  811  are each associated with a corresponding filter  1004 . Accordingly, the switch module  811  provides selectable one-to-one coupling of the outputs  1108  of the RF RX chain  1002  to the inputs of a filter  1004  within the IF RX module  805 . The output of each filter  1004  is coupled to the input of a corresponding amplifier  1006 . As noted above, the particular configuration of amplifiers and filters depends upon the requirements of the particular radio  407 . Therefore, the configuration shown in  FIG. 10  and  FIG. 11  is merely provided as an example of one particular embodiment. Other configurations in which more or less amplifiers and filters placed at the same or other places along the signal path are within the scope of the presently disclosed method and apparatus. 
       FIG. 12  is a simplified schematic of a base station site  1201  illustrating some of the transmit components of the sector radio  1207  in accordance with an alternative embodiment such as the base stations  702  shown in  FIG. 7  in which a coordination control module  1223  is shared by all of the base stations  702 . The coordination control module  1223  is responsible for coordinating the operation of the MBI modules  801  and switch modules  811  of each of base stations  702 . In some embodiments, the coordination control module  1223  is a node on the WLAN  703  (see  FIG. 7 ). The NIU  701  in each base station  702  is coupled to the MBI module  801  and IF module  805 , so the coordination control module  1223  can coordinate the routing of SSs  802  through the switch module  811  of the IF module  805  with the assignment of the SSs  802  to the outputs of the MBI module  801 . In some embodiments, the MAC component  803  of the MBI module  801  also adjusts the signals output from the MBI module  801  in response to the SCI measured during a SCI procedure. A control signal line  1224  between the NIU  701  and the MBI module  801  provides a connection through which the coordination control module  1223  can provide control signals to the MBI module  801  to coordinate the operation of the MBI module  801  with the operation of the switch module  811 . 
       FIG. 13  is a simplified schematic of the base station site  1201  of  FIG. 12  illustrating some of the receive components of the sector radio  1207 . Similar to the case described above with respect to  FIG. 12 , the coordination control module  1223  provides signals to each base station  702  to coordinate control of the MBI  801  with the switch module  811 . The signal flow through the base station radio  1207  is essentially the same as was described above with regard to the base station radio  407  of  FIG. 10  with the exception of the coordination control module  1223  providing the control signals that coordinate the operation of the MBI  801  with the operation of the switch module  811 . 
     The subsector antennas  821  within each base station sector radio  407  are a critical component of the base station  702 . In accordance with some embodiments of the disclosure, each subsector antenna  821  is designed to focus signals into one of the subsectors  201  in the site coverage area  105 . 
       FIG. 14  is an illustration of multiple base station sites  101   a - c  within a radio frequency communication system  1400 , such as a millimeter wave (mmWave) communication system. Each of the base station sites  101   a - c  is similar to the base station site  101  previously described in  FIG. 1 . Each site  101  has a site coverage area  105   a - c , respectively and one or more base stations  102   a - f . Each base station  102   a - f  is associated with a corresponding sector coverage area  107   a - f . For example, base station  102   a  is associated with sector coverage area  107   a  within the site coverage area  105   a  of base station site  101   a . One or more subscriber units  1402  reside within each sector coverage area  107 , such as subscriber units  1402   a  and  1402   b , which reside in sector coverage area  107   b  of base station  102   b.    
     In the embodiment shown in  FIG. 14 , the radio frequency communication in base station sites  101   a - c  occurs over a Line of Sight (LoS) pathway (or link). Each of the base station sites  101   a  - c  are configured to communicate with a Coordination and Control Center (CaCC)  1410 , such as the previously described Coordination and Control Center  604 . Such communications with the CaCC  1410  may be via a hardwired backhaul, fiber optic link, wireless line of sight or any other means for connecting the base stations  102  within the base station site  101  with the CaCC  1410 . 
     In some embodiments, the CaCC  1410  includes an Interference Mitigation Circuit  1415 . As described in greater detail in conjunction with  FIG. 15 , the Interference Mitigation Circuit  1415  receives information regarding a source of interference affecting radio frequency communications between a base station  102  and a subscriber unit  1402 , wherein the source is another base station  102  or a subscriber unit  1402  within the communication system  1400 . The Interference Mitigation Circuit  1415  then analyzes the received information, such as via a Statistical Analysis Module  1420 , and provides instructions to the source of the interference to mitigate its interference on the affected base station  102  or subscriber unit  1402 , such as by the providing instructions for the source to reschedule or re-route its transmission, or to switch to another frequency channel or sub-channel. 
       FIG. 15  is a flow diagram illustrating the operation of some embodiments of the present disclosure for mitigating interferences in the communication system  1400  shown in  FIG. 14 . The process begins in block  1500  in which the CaCC  1410  receives information regarding a source of interference affecting radio frequency communication between a base station and a subscriber unit in the communication system  1400 . The source is another base station  102  or subscriber unit  1402  within the communication system  1400 . As described further and in greater detail in conjunction with example scenarios A-C below, the source of interference may be a base station  102  transmitting signals that are interfering with reception by another base station  102  or a subscriber unit  1402  residing in the same or different base station site  101  as the source of the interference. For example, the source of the interference may be a base station  102   b , whose transmissions are interfering with communication between an affected base station  102   a  and its subscriber unit  1402   c , as shown in  FIG. 14 . Alternatively, the source of the interference may be a subscriber unit  1402   c  that is transmitting signals that interfere with reception by another subscriber unit  1402   a  or a base station  102   b  residing in the same or different base station site  101   a  as the source of the interference. 
     In some embodiments, the operation in block  1500  includes determining information such as: (a) the location of the interference source (e.g., the base station  102   b ); (b) the location of the affected base station (e.g., the base station  102   a ); or (c) the location of the affected subscriber unit (e.g., the subscriber unit  1402   c ). In some embodiments, determining the location of the source of the interference may be based on: (a) obtained statistical data corresponding to the source of the interference, such as from standards-based channel information (e.g., channel state information); and (b) one or more of: (1) a time-based determination (e.g., Time of Arrival, Time Difference of Arrival); (2) a power-based determination (e.g., Returned Signal Strength Indicator), or (3) an angle-based determination (e.g., Angle of Arrival). In some embodiments, the Statistical Analysis Module  1420  is configured to perform the above determination methods. In some embodiments, the Statistical Analysis Module  1420  performs other determination methods as well or instead, such as utilizing analytic tools and procedures, such as machine learning to generate a set of power-based data. In at least some embodiments, the information is attained from several sources (i.e., base stations  102  and/or subscriber units  1402 ) and used together to determine the location and nature of the source of interference. 
     It should be noted that for simplicity of illustration the CaCC  1410  is shown in  FIG. 14  as residing in a single location remote to the base stations sites  101   a - c , but it is contemplated that the CaCC  1410  may also reside partially or fully in one or more of the base station sites  101   a - c , base stations  102  or in one or more of the subscriber units  1402 . In the case in which the CaCC  1410  resides in at a base station site  101 , it may perform one or more of the above interference determinations, such as determining the locations of the source of interference, the location of the affected base stations  102  or subscriber unit  1402 , and thereafter communicate the relevant information to a remote or locally residing Interference Mitigation Circuit  1415  of the CaCC  1410 . 
     Information received in block  1500  is then used in block  1510  to generate instruction for mitigating of the interference generated by the source on communications between the affected base station and subscriber unit. The information is communicated by the CaCC  1410  to the source to perform the mitigation. In the case in which the CaCC  1410  and Interference Mitigation Circuit  1415  reside within the base station site  101  associated with the source of the interference, the CaCC  1410  need not communicate with devices outside the base station site  101  to mitigate the interference. Alternatively, if the CaCC  1410  or the Interference Mitigation Circuit  1415  reside outside the base station site  101  associated with the source of the interference  1415 , then the CaCC must communicate over longer distances. Such distances can add to the latency of the communication. Therefore, in some embodiments, a CaCC  1410  and Interference Mitigation Circuit  1415  are provided within each base station site  101 . Such CaCCs  1410  may commutate with one another over a backhaul. Different approaches may be used to mitigate the interference depending upon the situation. As described further and in greater detail in conjunction with example scenarios (A)-(C) below, approaches for mitigating the interference may include rescheduling transmissions by the source, such as by postponing transmissions to a later time at which interference is estimated by the CaCC  1410  to be lower than at other times. In addition, transmissions can be re-routed, such as by redirecting the communication via beamforming techniques from the source to a different intended receiver, such as via rerouting to an alternate path provided by the CaCC  1410 . The CaCC  1410  determines if the provided path results in reduction of interference by the source on the affected communication between the base station and the subscriber unit. Alternatively, mitigating the interference may include providing instructions by the CaCC  1410  for changing the transmissions by the source to a different frequency, or to a different sub-channel within a frequency. 
     Operations described above in  FIG. 15  will now be described in greater detail in the following example scenarios: 
     Scenario A 
     Scenario A includes examples of situations in which interference occurs due to transmissions intended for a base station  102  or a subscriber unit residing in one sector coverage area that interferes with the reception by a base station or a subscriber unit residing in another sector coverage area within the same base station site coverage area  105 . 
     Scenario Al illustrated in  FIG. 16 , shows signals transmitted from a base station  102   a  to a subscriber unit  1402   a  located in a sector coverage area  107   a  of a base station site coverage area  105  interfere with communications between another base station  102   b  and a subscriber unit  1402   b  in an adjacent sector coverage area  107   b  of the same base station site coverage area  105 . In one scenario, the interference is due to the separation angle (θ) between the transmission paths  1602  and  1604  being less than the required angular separation to prevent an overlap between the main lobe or side lobes of the transmission signals  1602 ,  1604 , or other such antenna pattern interference, in signal transmitted from the base stations  102   a  and  102   b  to their corresponding subscriber units  1402   a  and  1402   b.    
     Following the operations described in  FIG. 15 , the CaCC  1410  provides the base station  102   b  with instructions to take action to mitigate the interference, such as by rescheduling transmissions so that the base station  102   a  communicates with its subscriber unit  1402   a  at times that are different from the times when the base station  102   b  is transmitting. Alternatively, or additionally, changing the transmissions by the base station  102   a  to a different frequency or OFDMA sub-channel so to not interfere with transmissions from the base station  102   b . The CaCC  1410  instructions may also re-route communications of the base station  102   a  with the subscriber unit  1402   a  through signal paths  1702 ,  1704  that relays signals through a third subscriber unit  1402   c , as shown in  FIG. 17 . In some embodiments, re-routing may send the signal through a different base station  102  or base station site  101 . Alternatively, any combination of the above mitigation methods can be used. 
     Scenario A 2  illustrated in  FIG. 18 , shows transmissions from a subscriber unit  1402   a  to its corresponding base station  102   a  interfere with reception by a base station  102   b  attempting to receive signals from a subscriber unit  1402   b . In one such scenario, this interference can be due to the separation angle (θ) between the transmission paths  1802  and  1804  being less than a predetermined minimum angle, resulting in overlaps between the main lobe or side lobes of each transmission signal, or other such antenna pattern interference, in signals transmitted from the subscriber units  1402   a  and  1402   b  to their corresponding base stations  102   a  and  102   b . The CaCC  1410  may provide instructions to subscriber unit  1402   a  to (a) reschedule its transmission, (b) change the transmissions frequency or sub-channel, (c) re-route through a different route, such as over signal path  1902  to the subscriber unit  1402   c  and then over a signal path  1904  to base station  102   a , as shown in  FIG. 19 , or (d) any combination of the above mitigation methods. 
     Scenario A 3  illustrated in  FIG. 20 , shows transmissions from a subscriber unit  1402   a  may interfere with another subscriber unit  1402   b  in an adjacent sector coverage area  107   b  that is attempting to receive signals  2002  from the base station  102   b . The CaCC  1410  may then similarly provide instructions to the subscriber unit  1402   a  to: (a) reschedule its transmission; (b) change the transmissions frequency or sub-channel; (c) re-route transmissions through a different route, such as through paths  1902 ,  1904  through subscriber unit  1402   c  as shown in  FIG. 21 ; or (d) any combination of the above mitigation methods. 
     Example Scenarios (B) 
     These example scenarios occur when transmissions by a base station or a subscriber unit in a first base station site, interferes with receptions by another base station or a subscriber unit in a second base station site. 
     Scenario B 1  illustrated in  FIG. 22  shows transmissions by the base station  102   c  via a line of sight signal path  2202  to the subscriber unit  1402   f  in sector coverage area  107   a  interferes with reception by a subscriber unit  1402   g  of transmissions via signal path  2204  from the base station  102   g  in base station site coverage area  105   b . One reason for such interference is that the line of sight signals in the communication system  1400  are focused. Therefore, the strength of such LoS signals (relative to the strength of signals that experience R 2  roll-off when transmitted from less focused antennas) is still significant when the signal reaches a subscriber unit in the line of sight of the signal. This is due to the relatively high antenna gain of antennas used to transmit such LoS signals, as compared to the gain of antennas with wider transmission angles, such as those in cellular telephone networks. This relatively high signal strength can make it challenging to contain signals within the sector serviced by a base station  102 . That is, to ensure that a signal has sufficient power at the edge of the sector coverage area  107 , the signal transmitted to a subscriber unit  1402  will typically propagate well beyond that sector  107  (see signal  2206 , for example). Accordingly, the signal will propagate into the sector coverage areas  107  of adjacent base station sites  101 , such as sector coverage area  107   b  of the base station site  101   b . Accordingly, the signals from the base station  102   b  intended for subscriber unit  1402   g  may interfere with reception by the subscriber unit  1402   f  from the base station  102   c.    
     In Scenario B 1 , the CaCC  1410  provides instructions to the base station  102   a  to take actions to mitigate the interference on receptions by the subscriber unit  1402   g , such as via (a) rescheduling when base station  102   a  communicates with the subscriber unit  1402   a , so that it is not transmitting at the same time as base station  102   g , (b) changing the transmissions frequency or sub-channel of transmissions by base station  102   a ; (c) as illustrated in  FIG. 23 , re-route transmissions from base station  102   c , such as to subscriber unit  1402   h  over a different route such as path  2302  to the subscriber unit  1402   h  in the base station site coverage area  105 c and then over path  2304  from the subscriber unit  1402   h  to the subscriber unit  1402   f ; or (d) any combination of the above mitigation methods. 
     Scenario B 2  illustrated in  FIG. 24  shows transmissions from the base station  102   c  within the base station site  101   a  interfering with base station  102   g  in base station site  101   b  attempting to receive signals from subscriber unit  1402   g  in sector coverage area  107   g . The strength of the signal transmitted by the base station  102   c  to a subscriber unit  1402   f  may be such that it reaches the base station  102   g  with sufficient power to interfere with that base station&#39;s ability to receive signals from the subscriber unit  1402   g.    
     In Scenario B 2 , the CaCC  1410  provides instructions to the base station  102   c  to take actions to mitigate the interference with attempts by the base station  102   g  to receive signals from subscriber unit  1402   g . Such mitigation may include: (a) rescheduling of transmissions from base station  102   c  so to not occur at the same time as that of subscriber unit  1402   g ; (b) changing the transmissions frequency or sub-channel of signals transmitted from base station  102   c ; (c) as illustrated in  FIG. 25 , re-routing signals transmitted by base station  102   c  through a different route, such as over path  2302  to the subscriber unit  1402   h  and path  2304  to subscriber unit  1402   f , such that the signal propagating past the subscriber unit  1402   c  along path  2304  will not extend in the direction of the base station  102   g ; or (d) any combination of the above mitigation methods. 
     Scenario B 3  illustrated in  FIG. 26  shows transmissions  2602  from a subscriber unit  1402   f  interfering with reception by a subscriber unit  1402   g  in another site coverage area  105   b . While the transmissions  2602  from the subscriber unit  1402   f  are intended to be received by the base station  102   c , which is located in the opposite direction of the subscriber unit  1402   g , transmission  2604  via the back lobes of the transmit antenna of the subscriber unit  1402   f  might be of sufficient power to reach the subscriber unit  1402   g  and so cause interference with attempts by subscriber unit  1402   g  to receive signals from the base station  102   g . In some cases, the subscriber units  1402   f  and  1402   g  have antennas that are not directional (e.g., omni-directional antennas). In such cases, the antennas will transmit and receive signals from all directions with essentially equal power. 
     In Scenario B 3 , the CaCC  1410  provides instructions to the subscriber unit  1402   f  to take actions to mitigate the interference on receptions by the subscriber unit  1402   g , such as by (a) rescheduling of communication from the subscriber unit  1402   f  to times that do not occur at the same time as transmissions from the base station  102   g  to the subscriber unit  1402   g , (b) changing the transmissions frequency or sub-channel of transmissions from the subscriber unit  1402   f , (c) as shown in  FIG. 27 , re-routing through a different route such as path  2702  through subscriber unit  1402   h  to signal path  2704 ; or (d) any combination of the above mitigation methods. In cases in which the CaCC  1410  is instructing a subscriber unit  1402   h , the base station  102   h  that is in communication with the subscriber unit  1402   h  receives instructions from the CaCC  1410 , which are then conveyed to the subscriber unit  1402   h . It should be noted that re-routing signals  2702 ,  2704  may only work to avoid interference if the antenna pattern changes when signals are re-routed. 
     Example Scenarios (C) 
     These example scenarios occur when transmissions from one subscriber unit interferes with reception by another subscriber unit located in the same sector as the affected subscriber unit. In an example scenario shown in  FIG. 28 , transmissions  2802  from the subscriber unit  1402 d interfere with reception by subscriber unit  1402 i of transmissions  2804  from the base station  102   c . The CaCC  1410  then provides instructions to the subscriber unit  1402 d through the base station  102   c  to take actions to mitigate the interference on reception by the subscriber unit  1402 i, such as via (a) rescheduling of subscriber unit  1402 d communication times so as to not occur at the same time as that transmissions from the base station  102   c  to subscriber unit  1408 ; (b) changing the transmission frequency or sub-channel of subscriber unit  1402 d communication; (c) reroute subscriber unit  1402 d communication through a different route; or (d) any combination of the above mitigation methods. 
     It is to be understood that the foregoing description is intended to illustrate, and not to limit, the scope of the claimed invention. Accordingly, other embodiments are within the scope of the claims. Note that paragraph designations within claims are provided to make it easier to refer to such elements at other points in that or other claims. They do not, in themselves, indicate a particular required order to the elements. Further, such designations may be reused in other claims (including dependent claims) without creating a conflicting sequence. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects.