Patent Publication Number: US-8537757-B2

Title: Adaptive call admission control for use in a wireless communication system

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/414,363, filed Mar. 30, 2009, which is a continuation of U.S. application Ser. No. 11/693,546, filed Mar. 29, 2007, now U.S. Pat. No. 7,529,204, which is a continuation of U.S. application Ser. No. 11/350,464, filed Feb. 8, 2006, now U.S. Pat. No. 7,289,467, which is a divisional of U.S. application Ser. No. 10/032,044, filed Dec. 21, 2001, now U.S. Pat. No. 7,023,798, which claims priority to U.S. provisional patent application Ser. No. 60/258,428, filed Dec. 27, 2000, all entitled ADAPTIVE CALL ADMISSION CONTROL FOR USE IN A COMMUNICATION SYSTEM, all of which are incorporated herewith in their entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to communication systems and to a system and method for implementing adaptive call admission control in such systems. 
     2. Description of the Related Art 
     A wireless communication system facilitates two-way communication between a plurality of subscriber units (fixed and portable) and a fixed network infrastructure. Exemplary communication systems include mobile cellular telephone systems, personal communication systems (“PCS”), and cordless telephones. An objective of these wireless communication systems is to provide communication channels on demand between the subscriber units and their respective base stations in order to connect a subscriber unit end user with the fixed network infrastructure (usually a wire-line system). In the wireless systems having multiple access schemes, a time “frame” is used as the basic information transmission unit. Each frame is sub-divided into a plurality of time slots. Subscriber units typically communicate with their respective base station using a “duplexing” scheme thus allowing for the exchange of information in both directions of the connection. 
     Transmissions from the base station to the subscriber units are commonly referred to as “downlink” transmissions. Transmissions from the subscriber units to the base station are commonly referred to as “uplink” transmissions. Depending upon the design criteria of a given system, wireless communication systems have typically used either time division duplexing (“TDD”) or frequency division duplexing (“FDD”) methods to facilitate the exchange of information between the base station and the subscriber units. 
     SUMMARY OF THE INVENTION 
     The systems and methods have several features, no single one of which is solely responsible for its desirable attributes. Without limiting the scope as expressed by the claims which follow, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description” one will understand how the features of the system and methods provide several advantages over traditional communication systems. 
     One aspect is a communication system that is configured to control the admission of new connections and the suspension of existing connections between a base station and customer premise equipments (CPEs), wherein the base station and the CPEs are each configured to increase or decrease the robustness of their transmission modulation technique by adapting their PHY mode. The system comprises a first CPE having a first modem configured to modulate data in a communication link using a first current PHY mode and a first planned PHY mode, a second CPE having a second modem configured to modulate data in a communication link using a second current PHY mode and a second planned PHY mode, and a base station having a third modem configured to transmit and receive data to and from the first and second CPEs. The system further comprises a call admission control (CAC) module configured to determine whether to allow a new connection between the first CPE and the base station or between the second CPE and the base station based on a comparison of a total air link line rate between the first and second CPEs and the base station, wherein the total air link line rate is based on a reference PHY mode, with a bandwidth commitment value between the base station and the first and second CPEs, wherein the bandwidth commitment is based on the first and second planned PHY modes. 
     Another aspect is a method for controlling the admission of connections in a wireless communication system between a base station and associated CPEs, including a requesting CPE. The method comprises receiving a request for a new connection from a requesting CPE, summing the hard bandwidth commitments between a base station and associated CPEs, including the new connection and existing connections, based on a planned PHY mode for each connection, and determining an air link line rate between the base station and the associated CPEs based on a reference PHY mode. The method further includes if the air link line rate exceeds the hard bandwidth commitments, accepting the new connection and determining a second hard bandwidth commitments for the existing connections between the base station and the associated CPEs based on a current PHY mode for each connection, else denying the new connection. The method still further includes if the air link line rate exceeds the second hard bandwidth commitments, allocating air link resources to the new connection, else determining whether additional air link resources are available, and if additional air link resources are available, allocating the air link resources to the new connection, else suspending at least one of the existing connections between the base station and the associated CPEs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a wireless communication system including a base station and one or more CPEs. 
         FIG. 2  is an illustration of the structure of a Time Division Duplex (“TDD”) frame. 
         FIG. 3  is a block diagram of a modem. 
         FIG. 4  is a flowchart illustrating the process of adaptively adjusting a PHY mode for an uplink connection between the base station and a CPE. 
         FIG. 5  is a flowchart illustrating the process of precedence being applied to existing connections between the CPE and the base station. 
         FIG. 6  is a flowchart illustrating the process of call admission control to a new connection between a CPE and the base station. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different systems and methods. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. 
     In connection with the following description many of the components of the various systems, some of which are referred to as a “module,” can be implemented as software, firmware or a hardware component configured to perform one or more functions or processes. Hardware components can include, for example, a Field Programmable Gate Array (FPGA) or Application-Specific Integrated Circuit (ASIC). Such components or modules may reside on the addressable storage medium and configured to execute on one or more processors. Thus, a module may include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The functionality provided for in the components and modules may be combined into fewer components and modules or further separated into additional components and modules. Additionally, the components and modules may advantageously be implemented to execute on one or more computers. 
       FIG. 1  is a block diagram of an exemplary wireless communication system  100 . Alternatively, the methods and systems herein disclosed can be implemented in wired communication systems (not shown). One exemplary broadband wireless communication system is described in U.S. Pat. No. 6,016,311, by Gilbert et al., issued Jan. 18, 2000, entitled “Adaptive Time Division Duplexing Method and Apparatus for Dynamic Bandwidth Allocation within a Wireless Communication System,” hereby incorporated by reference. The system  100  includes a base station  102  and at least one customer premise equipment. The system depicted in  FIG. 1  shows three CPEs  104 ( a )-( c ). More or fewer CPEs can be used. The CPEs and the base station receive and transmit data along wireless communication links  110 ( a )-( c ),  112 ( a )-( c ). 
       FIG. 1  does not show buildings or other physical obstructions (such as trees or hills, for example), that may cause channel interference between data from communication links  110 ,  112 . The CPEs  104  and the base station  102  communicate by transmitting their data as radio frequency signals. The term channel refers to a band or range of radio frequencies of sufficient width for communication. For example, the range of frequencies from 26.500 GHz to 26.525 GHz would provide a 25 MHz wide channel. Although the following discussion uses the example of a system that transmits information within the Local Multi-Point Distribution Services (LMDS) band at frequencies of approximately 28 GHz, the invention is not so limited. Information can be transmitted at various frequencies and ranges including, for example, 10 GHz to 66 GHz using Quadrature Amplitude Modulation (QAM) symbols. The systems and methods described herein can also be used in a Multichannel Multi-point Distribution Service (MMDS) which operates below 10 GHz. In the MMDS, Orthogonal Frequency Division Multiplexing (OFDM) symbols may be transmitted between the base station and CPEs as an alternative to single carrier QAM modulation. In such a system, the methods and systems are applied to one or more of the OFDM subchannels. 
     Referring again to  FIG. 1 , the communication links  110 ( a ),  110 ( b ),  110 ( c ) are referred to as downlinks (i.e., from the base station  102  to the CPE&#39;s  104 ) and can operate on a point (base station)-to-multi-point (CPE&#39;s) basis. Transmissions to and from the base station  102  can be directional in nature, and thus limited to a particular transmission sector  106  of the base station  102 . Within a given sector  106 , CPEs  104 ( a ),  104 ( b ),  104 ( c ) receive the same transmission along their respective downlinks  110 ( a ),  110 ( b ),  110 ( c ). To distinguish between data intended for a specific CPE, the CPEs can monitor control information in their respective downlink  110 ( a ),  110 ( b ),  110 ( c ) and typically retain only the data intended for them. In communication systems that have multiple sectors, the base station  102  can include a sectored active antenna array (not shown) which is capable of simultaneously transmitting to multiple sectors. In one embodiment of the system  100 , the active antenna array transmits to four independent sectors. 
     The communication links  112 ( a ),  112 ( b ),  112 ( c ) are referred to as an uplink (i.e., from the CPEs  104  to the base station  102 ) and can operate on a point-to-point basis. Thus, in  FIG. 1 , each CPE  104 ( a ),  104 ( b ),  104 ( c ) originates its own uplink  112 ( a ),  112 ( b ),  112 ( c ). Communication with the base station  102  is bi-directional and can be multiplexed on the basis of Time Division Duplexing (TDD). For a TDD transmission from, for example, CPE  104 ( a ), CPE  104 ( a ) would send its data along communication link  112 ( a ) to the base station  102  during a preassigned time slot in a transmission frame. The specific frame structures of the uplink and downlink will be discussed further below. 
     Alternatively, the system can employ Frequency Division Duplexing (FDD). In such an FDD system, duplexing of transmissions between the base station and the CPEs is performed in the frequency domain. Different sets of frequencies are allocated for uplink and downlink transmissions. The systems and methods described herein can be used in such an FDD system. 
     Each CPE  104  is further coupled to a plurality of end users that may include both residential and business customers. Each customer can have one or more connections between the CPE and the base station. Consequently, each end user connection can have different and varying usage and bandwidth requirements. Each CPE  104 ( a )-( c ) may service several hundred or more end users, but at least one end user will be assigned to transmit and receive data via at least one connection through each CPE  104 . 
     The data transmitted along the communication links  110 ,  112  is in analog form, and thus a modem  108  is used to modulate the digital data prior to transmission.  FIG. 1  illustrates the modem  108  being located at the base station  102 , however, a similar or identical modem  108  may be used at the other end of the downlinks  110 ( a ),  110 ( b ),  110 ( c ) to demodulate the received analog data. Thus, the modems  108  in the base station and each CPE are used for uplinking data from the CPEs to the base station and for downlinking data from the base station to the CPEs. 
     The base station and CPEs can use adaptive modulation and forward error correction (FEC) schemes to communicate. Adaptive modulation, or adaptable modulation density, includes varying the bit per symbol rate modulation scheme, or modulation robustness, of downlinks and uplinks transmitted between CPEs and the base station. Examples of such modulation schemes include quadrature amplitude modulation-4 (QAM-4), QAM-16, QAM-64, and QAM-256. If QAM-4 is used, each resulting symbol represents two bits. If QAM-64 is used, each resulting symbol represents six bits. Adaptive FEC includes varying the amount of error correction data that is transmitted in the downlink and/or uplink. Channel characteristics, for example the modulation and FEC, for the downlink and/or uplink can be varied independently. For ease of explanation, the phrase “PHY mode” is used to indicate characteristics of a communication channel or link, including for example, modulation scheme and/or an FEC. 
     The PHY mode(s) planned for use in the sector  106  is normally determined as a function of the geographical relationship between the base station  102  and the CPEs, the rain region, and the implementation or modem complexity of the CPEs. Examples of rain regions include rain regions A-Q. Recommendations for modeling the rain region&#39;s effect on signal propagation can be found in Rec. ITU-R PN.837.1. Thus, a planned PHY mode may be different for the CPEs depending on the capabilities and transmission quality of each CPE  104  and base station  102  pair. For ease of explanation, the phrase “planned PHY mode” is used to indicate the planned PHY mode for a CPE  104  and base station  102  pair as described above. 
     Better environmental conditions, e.g., less distance, between some CPEs (such as CPE  104 ( c ) for example) and the base station  102  may permit the use of a less robust PHY mode by such CPEs as compared to a PHY mode used by CPEs located farther from the base station. For example, if CPE  104 ( c ) is capable of receiving QAM-64 data coupled with achieving adequate transmission quality between CPE  104 ( c ) and the base station  102 , all data transmitted between the CPE and the base station can be modulated using QAM-64. In the same system CPEs  104 ( a ),  104 ( b ), which, for example, are only capable of receiving QAM-4 data, will only transmit and receive QAM-4 data. By using different or variable PHY modes for different CPEs associated with a single base station, the communication system  100  as a whole increases its bandwidth utilization. 
     The transmission quality between the base station  102  and a CPE  104  may not only vary between each CPE and base station pair as described above, but may also vary over time, or between the uplink and downlink transmissions of a single pair (i.e. asymmetrical transmissions). For example, in  FIG. 1 , the transmission quality may significantly decrease during a rain or snow storm. When the link quality is decreased, there is an increased chance that transmitted data along communication links  110 ( a ),  110 ( b ),  110 ( c ),  112 ( a ),  112 ( b ),  112 ( c ) may be unrecognizable or lost to the receiving base station or CPE. To accommodate these time variations in link quality, the communication system  100  can dynamically adjust or “adapt” the PHY mode for each base station  102  and CPE  104 . In such an adaptive system, the bandwidth utilization of the communication system  100  further increases. 
       FIG. 2  represents a time division duplexing (“TDD”) frame and multi-frame structure for use in communication system  100 . Frame  300  includes a downlink subframe  302  and an uplink subframe  304 . The downlink subframe  302  is used by the base station  102  to transmit information to the CPEs  104 ( a )-( c ). In any given downlink subframe  302 , all, some, or none of the transmitted information is intended for a specific CPE  104 . The base station  102  may transmit the downlink subframe  302  prior to receiving the uplink subframe  304 . The uplink subframe  304  is used by the CPEs  104 ( a )-( c ) to transmit information to the base station  102 . 
     Subframes  302 ,  304  are subdivided into a plurality of physical layer slots (PS)  306 . Each PS  306  correlates with a duration of time. In  FIG. 2 , each subframe  302 ,  304  can be one-half millisecond in duration and include 400 PS for a total of 800 PS per frame  300 . Alternatively, subframes having longer or shorter durations and with more or fewer PSs can be used. Additionally, the size of the subframes can be asymmetrical and can be varied over time. 
     Each downlink subframe  302  can include a frame control header  308  and downlink data  310 . The frame control header  308  includes information for the CPEs to synchronize with the base station  102 . The frame control header  308  can include control information indicating where a PHY mode change occurs in the downlink. The frame control header  308  can also include a map of a subsequent uplink subframe  304 . This map allocates the PSs  306  in the uplink subframe  304  between the different CPEs. The frame control header  308  can further include a map of attributes of the downlink data  310 . For example, attributes may include, but are not limited to, the locations of the PSs  306  in the subframe  302  that are intended for each individual CPE. 
     The downlink data  310  is transmitted using a pre-defined PHY mode or a sequence of PHY modes with three PHY modes A, B, and C depicted in  FIG. 2  as an example. Individual or groups of PSs  306  in the downlink subframe  302  are assigned to data intended for specific CPEs  104 . For example, the base station  102  could assign PSs in one, some, or all of the PHY modes A, B, and C for transmitting data to CPE  104 ( a ). In  FIG. 2 , the data is divided into three PHY modes, where PHY mode A ( 312 ( a )) is the most robust modulation (i.e. least prone to transmission errors caused by signal interference) and while PHY mode C ( 312 ( c )) is the least robust (i.e. most prone to transmission errors caused by signal interference). In between these PHY modes is PHY mode B ( 312 ( b )). Additional PHY modes can also be used. 
     Still referring to  FIG. 2 , the uplink subframe  304  comprises uplink data  314 ( a )-( n ). The uplink subframe  304  is used by the CPEs  104 ( a )-( c ) to transmit information to the base station  102 . The subframe  304  is subdivided into a plurality of PSs  306 . Each CPE  104 ( a )-( c ) transmits its information during its allocated PS  306  or range of PSs  306 . The PSs  306  allocated for each CPE can be grouped into a contiguous block of a plurality of data blocks  314 ( a )-( n ). The CPEs use data blocks  314 ( a )-( n ) to transmit the uplink subframe  304 . The range of PSs  306  allocated to each block in the plurality of data blocks  314 ( a )-( n ) can be selected by the base station  102 . The data transmitted in each data block  314 ( a )-( n ) is modulated by the transmitting CPE. For example, CPE  104 ( a ) modulates and transmits uplink data block  314 ( a ). The same or different PHY modes can be used for each data block  314 ( a )-( n ). The data blocks  314 ( a )-( n ) can also be grouped by PHY mode. 
     During its data block, the CPE transmits with a PHY mode that is selected based on measured channel parameters from its prior transmission(s). Similarly, the base station can select a downlink PHY mode for a communication link based on measured channel parameters from its prior transmission(s). The process for selecting a PHY mode will be explained in more detail below. The measured channel parameters can be included in the uplink subframe  304  for transmission by the CPEs to the base station or can be included in the downlink subframe  302  for transmission by the base station to the CPE. Once received, the base station or CPE can utilize the channel parameters to determine if the PHY mode of the downlink subframe  302  or the uplink subframe  304  should be changed. 
     Each CPE  104  can receive all downlink transmissions that are modulated using its current PHY mode or are modulated using a more robust PHY mode than its current PHY mode. The frame control header  308  is typically modulated using the most robust PHY mode to ensure that all CPEs  104 ( a )-( c ) may receive it. Because each CPE receives the frame control header, each CPE  104  is initially synchronized with the downlink subframe  302  at the beginning of the frame  300 . The downlink subframe can be sorted by robustness, which allows each CPE to maintain synchronization during the subsequent portion of the downlink that could include data for that CPE. 
       FIG. 3  is a block diagram of a modem  108  which can be used to modulate/demodulate data in the wireless communication system  100  described above. The modem  108  is used to control the number and quality of existing and new connections between the CPEs and base station. Modems  108  are used by the base station  102  and CPEs  104  to modulate and demodulate data. For ease of description, the modem  108  will now be described with reference to the base station  102 . 
     The modem  108  can include a control section  108 ( a ) and a modem section  108 ( b ). The modem section  108 ( b ) includes a receiver module  202  and a transmitter module  204 . The control section  108 ( a ) includes a call admission control (CAC) module  206 , a Receive Signal Quality (RSQ) module  208 , a precedence module  210 , and a control module  212 . Alternatively, the functionality provided for by the control section  108 ( a ) can be separate from the modem  108 . Further, the control section  108 ( a ) components and modules may be combined into fewer components and modules or further separated into additional components and modules within the base station  102  and/or CPE  104 . 
     At a base station  102 , the transmitter module  204  converts digital data to an appropriately modulated analog signal communicated as a downlink  110 , using for example, QAM modulation and FEC. The analog signal may also be up converted to a carrier frequency prior to transmission. The receiver module  202  at the base station  102  demodulates an uplink  112 ( a ),  112 ( b ),  112 ( c ) and converts it back to digital form. When configured as a CPE  104 ( a ), the transmitter module  204  converts digital data to an appropriately modulated analog signal communicated as an uplink  112 , using for example, QAM modulation and FEC. The analog signal may also be up converted to a carrier frequency prior to transmission. The receiver module  202  at the CPE  104  demodulates a downlink  110  and converts it back to digital form. 
     The wireless communication system  100  can provide “bandwidth-on-demand” to the CPEs. Thus, the uplink can include bandwidth requests for new and existing connections from end users. The CPEs request bandwidth allocations from their respective base station  102  based upon the type and quality of service requested by the end users served by the CPE. A CPE or base station can continue an existing connection or allow a new connection depending on, for example, a user&#39;s defined quality of service, bandwidth needs, and transmission quality. Thus, each end user potentially uses a different broadband service having different bandwidth and latency requirements. Moreover, each user can select a portion(s) of their bandwidth to have variable priority levels, or precedence. 
     To this end, the type and quality of service available to the end users are variable and selectable. The amount of bandwidth dedicated to a given service can be determined by the information rate and the quality of service required by that service (and also taking into account bandwidth availability and other system parameters as will be described below). For example, T1-type continuous data services typically require a great deal of bandwidth having well controlled delivery latency. Until terminated, these services require constant bandwidth allocation for each downlink subframe  302  and uplink subframe  304  in a frame  300  (see  FIG. 2 ). In contrast, certain types of data services such as Internet Protocol data services (“TCP/IP”) are bursty, often idle (which at any one instant may require zero bandwidth), and are relatively insensitive to delay variations when active. 
     Referring again to  FIG. 3 , the Receive Signal Quality (RSQ) module  208  interfaces with the receiver module  202  and the control module  212 . The RSQ module  208  is configured to monitor signal quality of the received uplink signal. In a communication system that adapts PHY modes, the selection of a PHY mode can be based on channel parameters monitored/measured by the RSQ module  208 . These channel parameters can include the signal to noise ratio (SNR) of the modulated data at the receiver module  202  at the base station  102 . A bit error rate (BER), at the base station  102  or CPE  104 , can also be used in selecting the PHY mode. For example, when the received signal drops below a threshold value for a SNR, a more robust PHY mode can be selected by the modem  108  for the connection. Signal quality can be measured over a period of time by the RSQ module  208 , and, in response to changes in the signal quality, the control module  212  determines if the PHY mode for the transmitting CPE should be changed. The control module  212  at the base station  102  interfaces with the transmitter module  204  to control the PHY mode for the modem  108 . Further, the control module  212 , via the transmitter module  204 , can alert the transmitting CPE to change its PHY mode. Measuring signal quality over time helps avoid cyclic changes in the PHY mode due to transient changes in the communication link&#39;s quality. 
     The RSQ module at the CPE can measure signal quality for a signal that is transmitted by the base station  102  and received by the CPE. The CPE can alert the base station to change the base station&#39;s transmitting PHY mode. In one embodiment, only the modem  108  at the base station  102  includes the control module  212 . In this embodiment, each CPE measures its own signal quality and transmits its value within its uplink  112  to the base station  102 . The control module  212  is then able to monitor the signal quality of the signal received by the CPEs to determine if the downlink  110  PHY modes should be changed. 
     The call admission control (CAC) module  206  determines what CPE to base station connections are allowed at any given time. For example, the receiver module  202  can receive a request for a new connection between the CPE and base station in the uplink  112 . The CAC module determines whether to grant that request. This determination can be based on intrinsic factors relating to the new connection as well as communication system level factors. Examples of intrinsic factors are a quality of service and a type of service requested by the end user for the new connection. The extrinsic factors are external to the new connection. The extrinsic factors can include the type and quality of service for the existing connections, whether available bandwidth is allocated to the requesting CPE, the available bandwidth in the communication link, and the portion of the frame that is allocated for the uplink and downlink. An example of a type and quality of service that can be evaluated by the CAC module  206  are hard bandwidth commitments. 
     The CAC module  206  can be configured to determine whether there will be enough bandwidth to support all of the connections between the CPEs  104  and the base station  102 . For example, the CAC module  206  can determine whether there will be enough bandwidth for hard bandwidth commitments between the base station and CPEs. These hard bandwidth commitments can include, for example, constant bit rate (CBR) connections, the minimum cell rate (MCR) portion of a guaranteed frame rate (GFR) connections, and some function of sustainable cell rate (SCR) for variable bit rate (VBR) and variable bit rate real-time (VBR-rt) connections. Alternatively, hard bandwidth commitments could be the bandwidth measured, rather than calculated, that is necessary to provide the quality of service (QoS) desired for the connection. For ease of explanation, the following description uses hard bandwidth commitments as an exemplary type of connection. However, the systems and methods disclosed herein are not so limited and can be applied to any type of connection. Further, the systems and methods can be applied to one or more types of connections. 
     The CAC module  206  determines whether there is enough bandwidth to allow the new connection. This can be determined by summing the hard bandwidth commitments for each connection on each CPE  104 ( a ),  104 ( b ),  104 ( c ) (see  FIG. 1 ). Thus, each CPE will have a hard bandwidth commitment for its existing connections. All of the hard bandwidth commitments from the CPEs can then be summed to get the total hard bandwidth commitments for all of the existing connections through base station  102 . The control module  212  can perform these calculations. The CAC module  206  compares the total hard bandwidth commitments to an air link line rate. The air link line rate is the amount of bandwidth available between the CPEs and base station. If the air link line rate exceeds the total hard bandwidth commitments, the new connection is allowed. If the total hard bandwidth commitments meet or exceed the air link line rate, the CAC module  206  denies the new connection. 
     In the communication system described above, each connection between the CPE  104  and base station  102  will have a planned PHY mode. The planned PHY mode is used by the CAC module  206  in determining whether to allow the new connection. As will be explained below, the calculation of the total hard bandwidth commitments for any given sector  106  (see  FIG. 1 ) presents additional difficulties for communication systems  100  which adapt PHY modes. 
     In communication systems  100  that adapt, or vary, their PHY modes, the available bandwidth necessary for existing connections can vary. Since each PHY mode used by the base station  102  and/or CPE  104  for its communication link  110 ( a )-( c ),  112 ( a )-( c ) is adaptive, the robustness of each communication link can vary (see  FIG. 1 ). As the robustness varies, the bandwidth allocated for an existing connection or new connection will also vary. 
     In such communication systems, connections are allowed to be modulated with PHY modes that are more or less robust than the planned PHY mode. Each end user connection can dynamically select its current PHY mode. This current PHY mode can be different than the planned PHY mode that was planned for the connection. If a connection is modulated using a more robust PHY mode than the planned PHY mode, the connection will exceed its allocated bandwidth. 
     In an embodiment of a communication system  100  that adapts PHY modes, the CAC module  206  allows new connections with reference to a minimum air link line rate. The minimum air link line rate is a measure of bandwidth that would be required if all of the existing connections between the CPEs and base station were modulated using a least efficient PHY mode regardless of whether the least efficient PHY mode is actually used. The least efficient PHY mode can include, for example, QAM-4 modulation with a maximum amount of FEC overhead bits. This method ensures that during adverse weather conditions each CPE will be able to select its least efficient PHY mode and transmit its data within its assigned bandwidth without losing its connection with the base station. In this embodiment, the CAC module  206  will deny a new connection if the new connection will cause the CPE to exceed its minimum air link line rate. The CAC module  206  can determine whether to allow or deny a new connection in conjunction with the control module  212 . During spells of good weather, the CPE can select a less robust PHY mode for its current PHY mode. By selecting a less robust PHY mode, additional bandwidth between the CPE and base station would be freed up. However, the communication system  100  is constrained from taking advantage of the freed up bandwidth when the decision to allow new connections is based upon the minimum air link line rate. 
     In another embodiment of the communication system  100  that adapts PHY modes, the CAC module  206  allows the CPE to take advantage of the freed up bandwidth. The CAC module  206  limits new connections based on a comparison of the bandwidth required for the connection if it is modulated using the CPE&#39;s planned PHY mode with the available bandwidth. The available bandwidth is determined by summing the CPE&#39;s hard bandwidth commitments that would be used by the existing connections if those connections were modulated using the planned PHY mode of the CPE. If the available bandwidth is equal to or exceeds the bandwidth required for the new connection, the CAC module  206  will allow the connection. However, if the CPE operates using a less robust PHY mode than its preferred PHY mode, there is the potential that data through the CPE will be lost. 
     In the presence of adaptive PHY modes and to take advantage of the CPE&#39;s planned PHY mode, the bit rate associated with each connection&#39;s PHY mode is compared. Connections at different PHY modes (modulation and FEC) effectively have different bit rates, or air link line rates, and thus are not directly compared. One method for comparing these bit rates is to normalize the PHY modes associated with each connection. 
     Equation 1, below, can be used to normalize the bandwidth used for connections through an individual CPE. 
     
       
         
           
             
               
                 
                   
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                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     Where W CPEi  is a normalized value or weight for the entire bandwidth used by an individual CPE. W CPEi  is proportional to the equivalent bandwidth of its connections and the current modulation associated with each connection. Er is the number of bits per unit time that are transmitted by the CPE for a connection. Each connection is modulated using an associated PHY mode. The term mod is the inverse of the associated PHY mode efficiency that is used to modulate the connection. The bit/symbol rate for QAM-64 is 6, for QAM-16 is 4, and for QAM-4 is 2. For example, if during a first connection between CPE  104 ( a ) and the base station  102 , 10,000 bits/s were transmitted using QAM-4, and during a second connection between CPE  104 ( a ) and the base station, 18,000 bits/s were transmitted using QAM-64, Equation 1 would be:
 
 W   CPE104(a) =(10,000 bits/s*½ symbol/bit)+(18,000 bits/s*⅙ symbol/bit)=8,000 bits/s.
 
     The 8,000 bits/s for CPE  104 ( a ) is then added to W CPE104(b)  and W CPE104(c)  to determine a total normalized bandwidth for the CPEs in sector  106 . 
     Normalization is used to determine the effective hard bandwidth commitment usage through the modem  108 . The CAC module  206  interfaces with the control module  212  to compare the different PHY modes for the existing connections and the new connection with the available bandwidth between the base station  102  and CPEs  104 . In this embodiment, the control module  212  is configured to normalize each CPE&#39;s air link line rate. Once the control module  212  has determined the normalized value for each CPE&#39;s committed bandwidth requirements, the CAC module  206  can sum and compare them against a common air link line rate. 
     Equation 2, below, can be used by the CAC module  206  to determine the total bandwidth used, i.e. W Link −W, by all of the CPEs in the sector. 
     
       
         
           
             
               
                 
                   
                     W 
                     Link 
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                     ⁢ 
                     
                       W 
                       CPEi 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     Where W CPEi  is a normalized value or weight for the entire bandwidth used by an individual CPE in the sector. 
     For example, with PHY modes of QAM-4, QAM-16, and QAM-64, each using the same FEC, QAM-4 requires 3 times the air link resources, or bandwidth, of QAM-64 and QAM-16 requires 1.5 times the air link resources of QAM-64. In this example, the control module  212  can normalize to QAM-64. Thus, CPEs operating at QAM-64 would have their hard bandwidth commitments multiplied by a weight of 1, CPE&#39;s operating at QAM-16 would have their hard bandwidth commitments multiplied by a weight of 1.5, and CPE&#39;s operating at QAM-4 would have their hard bandwidth commitments multiplied by a weight of 3. The CAC module  206  then sums these hard bandwidth commitments and compares the total against a line rate of a communication link operating entirely at the selected normalized PHY mode, QAM-64 with the single FEC. Alternatively, the control module  212  normalizes to QAM-4 by applying weights of ⅓ to QAM-64, ½ to QAM-16, and 1 to QAM-4. The selection of QAM-64 and QAM-4, each with a single FEC, for use as a normalization PHY mode are only examples. Any PHY mode could be used to define the air link line rate for normalizing the connections between the CPEs and base station. 
     Still referring to  FIG. 3 , the precedence module  210  will now be described. The precedence module  210  interfaces with the receiver module  202  and the control module  212  to apply a priority, or precedence, to one or more connections when less bandwidth is available than required to meet the hard bandwidth commitments. This can occur when the CAC module  206  is configured as described above to limit new connections based on planned PHY modes of the CPEs but some or all of the CPEs are operating at a more robust (less efficient) current PHY mode. The precedence module  210  determines which connection(s) are to be suspended. However, before connections are suspended, the base station  102  can re-allocate bandwidth, that is not intended for hard bandwidth commitments, among the CPEs to increase the available bandwidth for hard bandwidth commitments. Alternatively or in addition to, in TDD systems, the base station  102  can adjust the portion of a downlink subframe  302  and of an uplink subframe  304  in the frame  300  (see  FIG. 2 ) to increase the available bandwidth for a CPE that requires additional bandwidth due to a change in the current PHY mode or the addition of a connection. However, if additional bandwidth is not available, the precedence module  210  selects which connections from among the CPEs are suspended. 
     Bandwidth problems can arise when one or more CPEs are using more robust PHY modes than their planned PHY modes for their connections. For example, if communication system  100  was designed for 99.99% availability, a comparison would be made between a CPE&#39;s geographical proximity to the base station and the communication system&#39;s rain region. Based on this comparison, a planned PHY mode is selected for that CPE that allows it to operate at that planned PHY mode or a less robust PHY mode the entire year except for approximately 53 minutes. If a CPE exceeds a SNR or BER threshold and transmits its uplink using a more robust PHY mode than its planned PHY mode, it will require additional bandwidth for these 53 minutes. At least two things can occur during this 53 minutes depending on whether additional air link resources in the communication system  100  are available. Should additional bandwidth be needed when only a few existing connections, between the base station  102  and CPEs  104  in sector  106 , select a more robust PHY mode, the base station  102  may be able to reallocate the available bandwidth. Thus, if the communication system is sufficiently under subscribed, the CPE  104  can use the additional air link resources it requires when using a more robust PHY mode than its planned PHY mode during the 53 minutes. If many existing connections between the base station and CPEs are subject to similar adverse environmental conditions, the base station  102  may be unable to accommodate the CPEs&#39; bandwidth requests. When the air link resources aren&#39;t available, the precedence module  210  selects which of the existing connections from the CPEs  104  ( a )-( c ) to suspend. 
     The precedence module  210  interfaces with the control module  212  to compare the bit rates for the existing connections through each CPE based on each CPE&#39;s current PHY mode. While the CAC module  206  compares the planned PHY modes of the CPEs to determine whether a new connection is allowed, the precedence module  210  compares the current PHY modes to the selected reference air link line rate to determine if a suspension should occur. The control module  212  is configured to compare the current PHY modes of the CPEs. As explained above, one method for comparing the PHY modes is normalization. Once normalized, the precedence module  210  determines if additional bandwidth between the CPEs and base station is available. If additional bandwidth is available, the precedence module  210  can determine a margin value. If additional bandwidth is not available, the precedence module  210  selects which connections are going to be suspended. 
     The precedence module  210  can be configured to suspend enough connections through the CPE that is requesting additional bandwidth until there is enough bandwidth to meet the remaining demand. The amount of outage during the year for the connections through the affected CPE  104  is planned based on the availability and rain region as discussed above. CPEs  104  located at greater distances from the base station  102  or having limited visibility of the base station would more likely be subject to the application of precedence. In this embodiment, CPE&#39;s are penalized by their geographic proximity to the base station  102 . For example, the same CPEs, those that are barely able to meet their availability numbers at their planned PHY modes, would be the first to have their hard bandwidth connections with the base station  102  suspended. These CPEs may receive the full brunt of the planned 53 minutes per year outage. In contrast, other CPEs (in particular, those barely unable to meet the availability number at the next less robust PHY mode) would have plenty of bandwidth because connections through the geographically challenged CPE&#39;s would be suspended before they need to drop to a more robust PHY mode and request additional bandwidth. 
     Alternatively, the precedence module  210  can also randomly select connections for suspension or select them in a round robin fashion. The precedence module  210  chooses connection to suspend from the entire set of connections that have hard bandwidth commitments through the CPEs in the sector  106 . The CPEs subject to potential suspension include CPEs that may still be operating at their planned PHY mode. In this embodiment, the communication system  100  as a whole, and each individual connection still meets its availability numbers since the planned outage is evenly shared. For example, if a rain fade caused the base station  102  and CPEs to lose half of their bandwidth, each connection from among all of the CPEs would, on average, see only 26 minutes outage per year rather than 53 minutes. Thus, the precedence aspect of adaptive CAC can allow you to increase system availability (26 minutes outage vs 53 minutes outage) or capacity. For example, operating a CPE  104  at a less robust PHY mode than would typically be planned for the CPE increases the system&#39;s capacity. The communication system can rely on adaptive CAC coupled with precedence to distribute the outage among all of the CPEs. This achieves the planned 53 minutes outage, but with increased modulation efficiency for the CPE operating at the less robust PHY mode. 
     Further, the precedence module  210  can use levels in conjunction with the random selection method discussed above when selecting which connections to suspend. In this embodiment, each connection between the CPEs  104 ( a )-( c ) and base station  102  is assigned a precedence level. Alternatively, each CPE is assigned a precedence level for its connections. For example, there are five levels, one through five, with precedence level one being assigned to the most important connections and precedence level five being assigned to the least important connections. The random selection of connections for suspension is applied as discussed above with reference to the second embodiment. However, instead of applying the method of the second embodiment to all connections simultaneously, the precedence module  210  applies it based on each connection&#39;s assigned precedence level. Continuing with the example above, the random selection would be initially applied to connections assigned to precedence level five. If and when the precedence level five connections are exhausted, the precedence module  210  applies the random selection process to connections assigned to precedence level four and so on until there is adequate bandwidth available for the remaining connections that have hard bandwidth commitments. Thus, individual connections can be selected to have their uplink or downlink transmissions suspended in favor of other connections. 
     Further, the precedence module  210  can allow connections to continue to operate with their current PHY mode even when a first SNR or BER threshold is exceeded. Instead, a second threshold is implemented to maintain the connection at the same PHY mode. However, the error rate associated with the connection may increase. 
       FIG. 4  is a flowchart illustrating the process of adaptively adjusting a PHY mode for a connection between the base station  102  and a CPE. This process can be implemented by a modem  108  at a base station. Alternatively, this process is performed by a modem  108  at the CPE. A specific CPE  104  can change its uplink PHY mode independent of that CPE&#39;s downlink PHY mode. The specific CPE&#39;s PHY mode can also be independent of the uplink PHY modes used by other CPEs  104  within the same sector  106 . Because the base station  102  must synchronize with each individual CPE  104  that uplinks data, the uplink quality may be different than the downlink quality with a specific CPE  104 . The base station  102  can perform the process of adaptively adjusting the uplink PHY mode used by a specific CPE  104 . As such, a similar process may be completed for each CPE  104  within the sector  106  in order to adaptively adjust each CPEs  104  uplink modulation. 
     The following description describes a process for adaptively adjusting a PHY mode for an uplink from a CPE to a base station. The same process is used for adaptively adjusting a PHY mode for a downlink from the base station to the CPE. 
     In particular, flow begins in start block  400 . Flow moves to a block  402  where a receiver module  202  at a base station  102  receives an uplink from a CPE  104 . Flow proceeds to block  404 , where the quality of the channel parameters for the uplink  112  is determined by a receive signal quality (RSQ) module  208 . The quality may be a function of the state of the transmission medium (e.g. air, foggy air, wet air, smoky air, etc.) and the ability of both the transmitting and receiving components (e.g. CPE  104  and base station  102 ) to respectively transmit and receive data. The base station  102  can determine the quality of each uplink  112 ( a )-( c ). Alternatively, the base station  102  periodically transmits channel parameter measurements, which are indicative of the quality of a CPE&#39;s uplink  112 , to that CPE  104 . The CPE  104  then uses these channel parameter measurements to determine the quality of its uplink. These channel parameter measurements can include a SNR and/or a BER measurement of the uplink  112 ( a )-( c ). For example, base station  102  can determine the quality of uplink  112 ( c ) based on a measurement by its RSQ module  208  (see  FIG. 3 ). A single SNR measurement or a series of several SNR measurements taken during a frame  300  (see  FIG. 2 ) or during multiple frames may be used to determine the uplink quality. The control module  212  can analyze multiple measurements to determine an uplink&#39;s quality. 
     Continuing to block  406 , the base station  102  or CPE  104  compares the calculated uplink quality with a current PHY mode threshold. The current PHY mode threshold can include an upper threshold and a lower threshold at which the PHY mode is changed. For example, if CPE  104 ( a ) is currently uplinking data to base station  102  using PHY mode B, the PHY mode will change when the uplink quality exceeds an upper threshold or goes below a lower threshold. 
     Next at decision block  408 , the CPE determines whether the uplink quality has decreased and crossed a PHY mode lower threshold according to the comparison made in block  406 . Continuing with the example above, if the PHY mode lower threshold associated with PHY mode B has not been crossed, flow proceeds to decision block  410  where the system determines whether the uplink quality has crossed an upper PHY mode threshold associated with PHY mode B. If the current modulation upper threshold has been exceeded, flow continues to block  412  where the PHY mode is changed to a less robust, denser modulation. For example, PHY mode C is selected for CPE  104 ( a ). The base station  102  can send a request to the CPE  104  indicating a desired uplink PHY mode change. Alternatively, the base station  102  transmits an uplink map to all CPEs  104  in the downlink subframe  302  (see  FIG. 2 ) indicating which CPEs have been allotted uplink PS&#39;s and the PS&#39;s associated PHY modes. The base station  102  indicates to an individual CPE  104  that the PHY mode has been changed by allotting uplink subframe  304  PSs to that CPE that use a less robust PHY mode. For example, if the uplink PHY mode for CPE  104 ( a ) is to be changed from PHY mode B to PHY mode C, the base station  102  assigns uplink subframe PS&#39;s which are to be modulated using PHY mode C. This uplink assignment serves as an indicator to the CPE that its uplink PHY mode has been change. Flow continues to a block  413  where the system can reallocate the newly available bandwidth. For example, the newly available bandwidth can be allocated for new or existing hard bandwidth commitments, new connections, or connections that had been previously suspended. Flow then returns to block  402  as described above. 
     Returning to decision block  410 , if the current PHY mode upper threshold has not been exceeded, flow continues to block  402  as described above. 
     Returning to decision block  408 , if the PHY mode lower threshold has been crossed, flow proceeds to a decision block  414  where the system determines whether the connections, between the CPE and base station that have a hard bandwidth commitment, are using a less robust PHY mode than the planned PHY mode for the connections. If the connection(s) is using a less robust PHY mode than its planned PHY mode, the process proceeds to block  416  where a more robust PHY mode is selected for the connection(s). If the base station determines whether the uplink quality has crossed a threshold, the base station  102  can send a request to the CPE  104  indicating a desired uplink PHY mode change. Alternatively, the base station  102  can transmit an uplink map to all CPEs  104  in the downlink subframe  302  indicating which CPEs have been allotted uplink PS&#39;s along with the PS&#39;s associated PHY modes. This allows the base station  102  to indicate to an individual CPE  104  that the PHY mode has been changed by allotting uplink subframe  304  PSs to that CPE that uses a more robust PHY mode. For example, if the uplink PHY mode for CPE  104 ( a ) is to be changed from PHY mode B to PHY mode A, the base station  102  assigns uplink subframe PS&#39;s which are to be modulated using PHY mode A. This uplink assignment serves as an indicator to CPE  104 ( a ) that its uplink PHY mode has been change. Flow then continues to block  420  where a precedence module  210  (see  FIG. 3 ) determines whether connections between the base station and the CPEs are to be suspended. Precedence will be explained with reference to  FIG. 5 . Flow then continues to block  402  as described above. 
     Returning to decision block  414 , if the connection&#39;s current PHY mode is at least as robust as its planned PHY mode, the process continues to decision block  418  where the control module  212  can replace the lower threshold associated with the current PHY mode of the connection that has the hard bandwidth commitment with a second lower threshold. The process continues to block  402  as described above except that at block  406  the RSQ module  208  and the control module  212  use the second lower threshold to compare with the measured signal quality of the connection. 
     Returning to decision block  418 , if the control module does not select the second lower threshold, the process moves to a block  420 , as described above, where the precedence module  210  (see  FIG. 3 ) determines whether connections between the base station and the CPEs are to be suspended. Precedence will be explained with reference to  FIG. 5 . Once precedence has been applied, the process returns to state  402  as described above. 
       FIG. 5  is a flowchart illustrating the process of applying precedence to existing connections between the CPEs  104  and the base station that have hard bandwidth commitments. This process can be implemented by a modem  108  at a base station. Alternatively, this process is performed by a modem  108  at the CPE. Flow begins in start block  600 . Flow moves to block  601  where a more robust PHY mode is selected for the existing connection. Flow proceeds to block  602  where the control module  212  determines an air link line rate based on a reference PHY mode. Flow moves to block  603  where the control module calculates the hard bandwidth commitments for the existing connections between the base station  102  and CPEs  104  based on the current PHY mode for each connection. Flow moves to a decision block  604  where the precedence module  210  determines whether the air link line rate determined at block  602  exceeds the hard bandwidth commitments between the CPEs and base station. If the air link line rate exceeds the hard bandwidth commitments, the process continues to a block  606  where the more robust PHY mode selected in block  601  is applied for the existing connection. Flow then returns to block  402  of  FIG. 4  where the base station  102  receives the next uplink from a CPE  104 . 
     Returning to decision block  604 , if the air link line rate does not exceed the hard bandwidth commitments, flow proceeds to a decision block  608  where the precedence module  210  determines whether additional air link resources are available. These additional air link resources can include available bandwidth in the uplink subframe  302  and available bandwidth in the downlink subframe  304  (see  FIG. 2 ). If additional air link resources are available, flow proceeds to block  606  where the more robust PHY mode is applied for the existing connection. Flow then returns to block  402  of  FIG. 4  where the base station  102  receives the next uplink from a CPE  104 . 
     Returning to decision block  608 , if additional air link resources are not available, flow moves to a block  610  where the precedence module  210  suspends existing connections between the base station  102  and the CPEs  104 . As described above, the precedence module  210  can, for example, suspend connections only between the base station and the affected CPE, randomly suspend connections between the base station and all of the CPEs in a sector  106 , or suspend connections between the base station and all of the CPEs in the sector in a round-robin fashion. Further, the precedence module  210  can randomly suspend connections between the base station and the CPEs that have a lower precedence priority than other connections. Alternatively, the precedence module  210  can suspend the connections that have a lower precedence priority in a round-robin fashion. The process moves to block  606  as described above where the more robust PHY mode is applied for the existing connection. The process then returns to block  402  of  FIG. 4  where the base station  102  receives the next uplink from a CPE  104 . 
       FIG. 6  is a flowchart illustrating the process of call admission control for a new connection between a CPE and the base station. This process can be implemented at a base station. Alternatively, this process is performed at the CPE. Flow begins in start block  500 . Flow proceeds to block  502  where the base station receiver module receives a request for a new connection. The process continues to block  504  where the CAC module  206  sums the hard bandwidth commitments between the CPEs and base station based on the planned modulations of the CPEs. Next, at a block  506 , the control module  212  determines an air link line rate for the existing connections between the base station and CPEs based on the reference PHY mode. Flow moves to a decision block  508  where the CAC module  206  determines whether the air link line rate determined at block  506  exceeds the hard bandwidth commitments determined at block  504 . If the air link line rate exceeds the hard bandwidth commitments, the process continues to a block  510  where the CAC module  206  allows the new connection. However, air link resources are not initially allocated to the connection since the connection has been allowed based on the planned PHY modes of the CPEs and base station. The CPEs and base station could be operated at a more robust PHY mode than their planned PHY mode. 
     Flow proceeds to block  512  where the control module  212  determines the hard bandwidth commitments for the existing connections between the base station  102  and CPEs  104  based on the current PHY mode for each connection. Flow moves to a decision block  514  where the precedence module  210  determines whether the air link line rate determined at block  506  exceeds the hard bandwidth commitments between the CPEs and base station determined at block  512 . If the air link line rate exceeds the hard bandwidth commitments, the process continues to a block  516  where the base station allocates air link resources to the new connection. Flow then returns to block  502  where the base station  102  receives a request for a new connection. 
     Returning to decision block  514 , if the air link line rate does not exceed the hard bandwidth commitments, flow proceeds to a decision block  518  where the precedence module  210  determines whether additional air link resources are available. These additional air link resources can include available bandwidth in the uplink subframe  302  and available bandwidth in the downlink subframe  304  (see  FIG. 2 ). If additional air link resources are available, flow proceeds to block  516  where the base station allocates air link resources to the new connection. Flow then returns to block  502  where the base station  102  receives a request for a new connection. 
     Returning to decision block  518 , if additional air link resources are not available, flow moves to a block  520  where the precedence module  210  suspends existing connections between the base station  102  and the CPEs  104 . As described above, the precedence module  210  can, for example, suspend connections only between the base station and the affected CPE, randomly suspend connections between the base station and all of the CPEs in a sector  106 , or suspend connections between the base station and all of the CPEs in the sector in a round-robin fashion. Alternatively, the new connection is accepted into a suspended state since the precedence module  210  has already determined which of the other connections are to be suspended. Further, the precedence module  210  can randomly suspend connections between the base station and the CPEs that have a lower precedence priority than other connections. Alternatively, the precedence module  210  can suspend the connections that have a lower precedence priority in a round-robin fashion. The process moves to block  516  where the base station allocates air link resources to the new connection. Flow then returns to block  502  where the base station  102  awaits a request for a new connection. 
     Returning to decision block  508 , if the air link line rate does not exceed the hard bandwidth commitments, flow proceeds to a block  522  where the CAC module  206  denies the new connection. The process then returns to block  502  to await the next request for a new connection. 
     The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention can be practiced in many ways. As is also stated above, it should be noted that the use of particular terminology when describing certain features or aspects of the embodiments should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the embodiment with which that terminology is associated. The scope of the embodiments should therefore be construed in accordance with the appended claims and any equivalents thereof.