Abstract:
Mesh Network Access Points (APs) points, including gateways and routers, are deployed over a geographic area. The APs monitor the communication channel for other carriers and transmit accordingly. The APs selectively co-transmit when other carriers are sensed, if the efficiency of the mesh network will improve. APs select a transmission rate based on observed carrier-to-interference ratios. APs use directional antennas to increase carrier-to-interference ratios and spectral efficiency. AP transmit schedules are adaptable and adjusted according to observed carrier-to-interference measurements.

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §119 
     The present Application for patent claims priority to Provisional Application No. 60/778,747 filed Mar. 3, 2006, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
     CLAIM OF PRIORITY UNDER 35 U.S.C. §120 
     The present application for patent is a Divisional and claims priority to patent application Ser. No. 11/680,065 entitled “Method and Apparatus for Increasing Spectrum use Efficiency in a Mesh Network” filed Feb. 28, 2007, now issued as U.S. Pat. No. 8,089,881, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The disclosure relates to mesh networks. More particularly, the disclosure relates to a method and apparatus for managing data flow through a mesh network. 
     2. Background 
     In recent years, there has been an increase in demand for widespread access to high speed data services. The telecommunication industry has responded to the increase in demand by offering a variety of wireless products and services. In an effort to make these products and services interoperable, the Institute for Electrical and Electronics Engineers (IEEE) has promulgated a set of wireless local area network (WLAN) standards, e.g., IEEE 802.11. The products and services that conform to these standards are frequently networked in a wireless point to multipoint configuration. In one configuration, individual wireless devices (e.g., stations) may communicate directly with an Internet access point, with each of the wireless devices sharing the available bandwidth. 
     A more efficient and resilient network can be realized through the use of a mesh network. A mesh network is a distributed network having multiple wireless mesh points. Each mesh point in the mesh network may act as a repeater capable of receiving traffic streams (TSs) and relaying the TSs to the next mesh point. A TS may proceed from an origination mesh point to a destination mesh point by hopping from mesh point to mesh point. TS routing algorithms insure that TSs are routed efficiently from their origination mesh point to their destination mesh point. TS routing algorithms may dynamically adapt to changes in the mesh network and may make the mesh network more efficient and resilient. For example, in the event a mesh point is too busy to handle the TS or a mesh point has dropped out of the mesh network, the TS routing algorithm may route the TS to the destination mesh point through other mesh points in the mesh network. 
     Mesh networks may frequently include a hierarchy of mesh points with different operating characteristics. In some mesh network architectures, clients are mesh points at the bottom of the hierarchy. Clients are individual wireless devices such as a laptop computer or a personal digital assistant. Access Points (APs) are mesh points that are a layer above the clients forming a wireless skeleton for the mesh network. Some APs are wired and are referred to as gateways since they form a bridge between the mesh network and other networks. Other APs may be wireless and are referred to as routers since they may route TSs between clients and gateways. 
     Currently, there is no generally accepted standard for routing TSs through a mesh network. Creating a standard for wireless mesh networks and dynamic routing through wireless mesh networks is one of the objectives of the IEEE 802.11(s) working group. A standard ensures that wireless devices and services conforming to the standard are interoperable. Large scale production of wireless devices and services conforming to 802.11(s) or other standards promises to increase the use and deployment of mesh networks. 
     The deployment of an amorphous and distributed communication apparatus in a small geographic area requires efficient use of the available spectrum, particularly for mesh points such as APs that carry large traffic loads. It has been recognized by those skilled in the art that apparatuses and methods that increase the productive use of the available spectrum in a mesh network are desired. 
     SUMMARY 
     A method of managing use of a communication channel in a mesh network comprising monitoring a communication channel for one or more carriers, measuring signal strength for the one or more carriers and transmitting data if the signal strength meets a condition. A method of dynamically scheduling access point communication between wireless access points comprising determining an interference condition on an available channel at a first access point and scheduling the first access point to transmit to a second access point if the interference condition meets a condition. 
     A mesh network having a plurality of network access points (APs) including gateways and routers distributed over a geographic area. The gateways may be wired APs. The routers may be unwired APs that route TSs between clients (e.g., wireless devices) and gateways. APs may dynamically assess the state of the communication channel in their neighborhood to determine, if, and when, to transmit. APs may sense at their location the received strength of carrier waves transmitted from other APs. Each AP may estimate the effect of co-transmitting with each of the observed carrier waves. If co-transmitting increases the spectral efficiency of the mesh network, the AP may transmit. The APs may also estimate the carrier to interference ratio and adjust their transmit rate accordingly. The APs may also sectorize their data links using a directional antenna. APs may also to changes in the mesh network environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, objects, and advantages of the disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, wherein: 
         FIG. 1  is a diagram of an exemplary mesh network according to an embodiment. 
         FIG. 2  is a diagram of an exemplary topography of TS flows through the exemplary mesh network of  FIG. 1  according to an embodiment. 
         FIG. 3  is a diagram of TS flow pairs in the mesh network in accordance with carrier sense multiple access (CSMA) according to an embodiment. 
         FIG. 4  is a diagram of co-transmitting TS flow pairs in the mesh network in accordance with soft CSMA according to an embodiment. 
         FIG. 5  is a diagram of TS flow pairs where synchronized transmissions may occur between the gateways and the routers according to an embodiment. 
         FIG. 6  is a diagram illustrating co-channel interference between TS flow pairs in a portion of the mesh network according to an embodiment. 
         FIG. 7  is a diagram illustrating antenna sectoring in a portion of the mesh network according to an embodiment. 
         FIG. 8  is a diagram illustrating a cluster from the mesh network using Orthogonal Frequency Division Multiple Access (OFDMA) according to an embodiment. 
         FIG. 9  is a flow diagram illustrating a method of spectral reuse for improving the efficiency of the mesh network according to an embodiment. 
         FIG. 10  is a flow diagram illustrating a method of antenna sectoring for improving the efficiency of the mesh network according to an embodiment. 
         FIG. 11  is a flow diagram illustrating a method of OFDMA for increasing the frequency reuse to reduce the spectral efficiency of the mesh network according to an embodiment. 
         FIG. 12  is a block diagram illustrating exemplary components for the apparatus and the means for apparatus for managing use of a communication channel in a mesh network according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Methods and systems that implement the embodiments of the various features of the disclosure will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the disclosure and not to limit the scope of the disclosure. Reference in the specification to “one embodiment” or “an embodiment” is intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an embodiment of the disclosure. The appearances of the phrase “in one embodiment” or “an embodiment” in various places in the specification are not necessarily all referring to the same embodiment. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. In addition, the first digit of each reference number indicates the figure in which the element first appears. 
       FIG. 1  is a diagram of an exemplary mesh network  100  according to an embodiment. In one embodiment, the mesh network  100  has twelve gateways  102  (designated by a circle around a dot) and seventy-six routers  104  (designated by a dot). A representative portion  106  includes three gateways  102  and eighteen routers  104 . 
     The gateways  102  may be wired access points (APs). The gateways  102  may form a bridge between the mesh network and other networks. For example, one or more gateways  102  may be wired to the Internet, thus bridging the mesh network  100  to the Internet. The routers  104  may be wireless APs that communicate with the gateways  102  and other routers  104 . The APs may provide a skeleton like structure for communication flow in the mesh network  100 . The APs may communicate with clients (not shown), such as laptop computers, mobile phones, and personal digital assistants. The APs may communicate with other APs over one or more communication channels. The APs may also communicate with clients over one or more communication channels. 
     TSs bound for a client may enter the mesh network  100  through the gateways  102 . TSs from a client may depart the mesh network  100  through the gateways  102 . Clients near one of the gateways  102  may establish a direct wireless link with one of the gateways  102 . Clients out of range of one of the gateways  102  may establish an indirect wireless link with a gateway  102  through one or more routers  104 . 
     The mesh network  100  of  FIG. 1  is shown as a cellular configuration. Each gateway  102  may be surrounded by six routers  104  forming a cluster. This cellular configuration is shown as an example. The topology of any mesh network may depend to a large extent on topography. For example, a mesh network deployed on a college campus may have a topology that appears to be pseudo random. The gateways  102  may be computers located in Internet wired buildings connected with antennas mounted on the roof. The routers  104  may be collocated with lamp posts providing a convenient source of power. This topology may allow a college to inexpensively extend its Internet coverage to areas of the campus far from the Internet wired buildings. Although this topology results in irregular cell shapes and sizes, the principles and ideas discussed herein apply. 
       FIG. 2  is a diagram of an exemplary topography of TS flows through the exemplary mesh network of  FIG. 1  according to an embodiment. In particular,  FIG. 2  shows exemplary TS privileges negotiated between the gateways  102  and the routers  104  of the mesh network  100 . Most of the routers  104  may have negotiated direct wireless links with the closest gateway  102 . However, some routers  104  may have negotiated an indirect wireless link  204  to a gateway  102  through another router. For indirect links  204 , TSs may hop from one router to another router. Indirect links  204  may be established for many reasons including: shadowing; preventing the establishment of a direct link with a gateway  102 ; the gateway  102  denying access to the router  104  because it is operating near capacity; and too much TS traffic in the neighborhood of the closest gateway  102 . 
     Direct links  202  and indirect links  204  between the gateways  102  and the routers  104  make the mesh network  100  resilient and adaptable. Routers  104 , unable to access the gateways  102  directly, may forward TSs to the gateways  102  through other routers. Routers  104  may also receive TSs forwarded from the gateways  102  through other routers. TS may be routed around inoperable or busy gateways  102  and routers  104 . Each gateway  102  and router  104  may also negotiate its wireless links and the data rates of those links. Each gateway  102  and router  104  may attempt to balance loads, adapt to imperfect topologies, or adapt to log normal shadowing. Each gateway  102  and router  104  may estimate or measure carrier-to-interference ratios so that it may appropriately manage its data links. 
       FIG. 3  is a diagram of TS flow pairs  301  in the mesh network  100  in accordance with CSMA according to an embodiment. In this embodiment, gateways  102  and routers  104  monitor the communication channel for the carrier waves of other gateways and routers. If a carrier wave is sensed, the gateway  102  and the router  104  avoids transmitting TSs. In this way, co-channel interference is minimized. For example, if the gateway  302  is transmitting to the router  304 , then the carrier wave of the gateway  302  may be sensed by routers  306 ,  308 ,  310 ,  312 ,  314 ,  316  and  318 . These routers may not transmit until the carrier wave of the gateway  302  is no longer sensed. 
     Many factors may affect which router senses the carrier wave of the gateway  302  during transmission. These factors may include the transmission power of the gateway  302 , the log normal shadowing, environmental conditions, etc. The carrier wave of the gateway  302  may be sensed at neighboring routers  306 ,  308 ,  310 ,  312  and  314  as well as non-neighboring routers  316  and  318 . In some embodiments, the carrier wave of the gateway  302  may be sensed by other gateways. 
     Seven TS flow pairs  301  (e.g., the gateway  302  and the router  304 ) are shown in  FIG. 3 . The TS flow pairs  301  are limited in number because each gateway or router may not sense another carrier before transmitting. The links of the network, however, are relatively efficient since it is likely that there is a relatively small amount of link-to-link co-channel interference. 
       FIG. 4  is a diagram of co-transmitting TS flow pairs  402  in the mesh network in accordance with soft CSMA according to an embodiment. The gateways  102  and the routers  104  monitor the communication channel for other gateways and routers. If a carrier wave is sensed, the gateway or router evaluates whether the spectral efficiency of the mesh network  100  may be improved if the gateway or router co-transmits over the channel. For example, the gateway  404  may have a link with the router  406 . The gateway  408  may be able to sense the carrier of the gateway  404 . The carrier of the gateway  404 , however, may be fairly well attenuated at the gateway  408 . The gateway  408  may assess the signal strength of the gateway  404  and determine that it may establish a link with the router  410 . 
     The established link may have some co-channel interference and may not be as robust as it would be without the interference. The gateway  408  may also adjust its data rate to the router  410  to compensate for any decrease in the expected signal-to-noise ratio at the router  410  due to the co-channel interference. Similarly, the gateway  402  may adjust its data rate to the router  404  to compensate for any decrease in the signal-to-noise ratio. 
     By allowing some co-channel interference, the throughput of each individual flow pair is reduced. However, the number of flow pairs in the mesh network may be increased. In this embodiment, the number of flow pairs supported by the mesh network is twelve. Thus, in this embodiment, individual link efficiency is reduced but aggregate mesh throughput is increased. It can be shown that the amount of information carried through the mesh network is proportional to:
 
N log 2 (1+γ)
 
where:
         N=the number of links; and   γ=mean carrier to interference ratio.       

     Each of the gateways and the routers may maintain a table of the sensed carrier power observed. The gateways and the routers may make a determination if co-transmitting will increase or decrease the mesh efficiency. If co-transmitting increases the mesh efficiency, they may transmit. The gateways and routers may then adjust their data rates based on estimated or measured co-channel interference to optimize the individual data rates of each of the TS flow pairs  402 . 
       FIG. 5  is a diagram of TS flow pairs  502  where synchronized transmissions may occur between the gateways  102  and the routers  104  according to an embodiment. The transmissions from the gateways  102  to the routers  104  may be timed based on the relative bearing from the gateway  102  to the router  104 . The illustration shows that all the TS flow pairs have routers  104  that are to the north-northeast of the gateways  102 . 
     The TS flow pairs  502  may be synchronized according to a time division multiple access (TDMA) scheme. For example, the first time slot may be assigned to gateways  102  transmitting to routers  104  with a relative bearing to the north-northeast. The second time slot may be assigned to the gateways  102  transmitting to the routers  104  with a relative bearing to the east. The third time slot may be assigned to the gateways  102  transmitting to the routers  104  in the south-southeast. The fourth time slot may be assigned to the gateways  102  transmitting to the routers  104  in the south-southwest. The fifth time slot may be assigned to the gateways  102  transmitting to the routers  104  in the west. The sixth time slot may be assigned to the gateways  102  transmitting to the routers  104  in the north-northwest. 
       FIG. 6  is a diagram illustrating co-channel interference between TS flow pairs in a portion of the mesh network  100  according to an embodiment. As discussed above, the efficiency of the mesh network  100  can be improved if co-channel interference is tolerated. The gateway  602  has established a link with the router  604 . Some of the radio frequency energy from this link may be sensed by both the gateway  606  and the router  608 . If the sensed carrier wave strength is low enough, the gateway  606  may establish a link with the router  608  improving the overall efficiency of the mesh network  100 . 
     For illustration purposes, if we assume that the two links shown in  FIG. 6  are the only links in the mesh network  100 , the improvement in mesh throughput capacity may be estimated using the following equation: N log 2 (1+γ) where N is the number of active links and gamma is the achieved SNR for each link (assuming to be equal for all links). For example, if the SNR of the link between the gateway  602  and the router  604  (e.g., only two devices enabled in the mesh network) is 20 dB, the spectral efficiency of the single link is N log 2 (1+γ)=1 log 2 (1+100)=6.65 bps/Hz without co-channel interference. If the gateways  602  and  606  are co-transmitting and the SNR observed on each link is 17 dB because of mutual co-channel interference, the spectral efficiency of the single link N log 2 (1+γ)=1 log 2 (1+50)=5.67 bps/Hz. However, since we have enabled two links in cluster  106 , the spectral efficient has increased to 11.35 bps/Hz because two links are transmitting. Thus, in this example, it would be prudent to establish the dual links and tolerate co-channel interference, since the information throughput is almost doubled. The throughput efficiency is almost doubled despite only a 15% decrease in the individual link efficiency as a result of co-channel interference. 
     The throughput of the mesh network  100  may also be improved if some of the gateways and/or the routers employ noise cancellation. Multiple Input Multiple Output (MIMO) spatial channels may be available on some gateways and routers. MIMO may be realized through the use of multiple element antennas. For example, a multiple element antenna may feature a 4×4, a 4×2 or a 2×2 grid of antenna elements. The gateway or the router may introduce phase delay to signals bound for the antenna element producing a directional antenna beam. The directional antenna beam may be adaptable allowing the gateway or the router to focus the beam toward a particular gateway, router or sector. The gateway or the router may also introduce phase delay to produce two or more directional beams allowing the gateway or the router to focus the beam on multiple areas of interest. 
     Some of the elements of the antenna may also be used for interference canceling. Interference canceling may be particularly effective for reducing co-channel interference. For example, the gateway  602  may sense a large carrier wave from the router  608 . If the gateway  602  has a MIMO antenna, it may use two or more of the elements of the antenna to compare arriving signals and cancel signals arriving from the direction of the router  608 , reducing or eliminating the router  608  as a source of co-channel interference. Interference canceling in effect is analogous to antenna null steering. 
       FIG. 7  is a diagram illustrating antenna sectoring in a portion of the mesh network  100  according to an embodiment. The gateways and/or the routers employ sectored antennas to improve carrier-to-interference ratios. The carrier signal strength may be increased through antenna gain and the co-channel interference may be decreased by reducing interference from co-transmitting gateways and routers. For example, the gateway  702  may have both an omni-directional antenna capability as well as a sectored antenna capability. The gateway  702  may monitor the communication channel using its omni-directional antenna until it senses a request-to-send (RTS) message. The gateway  702  may use its sectored antenna capability to establish a link with the router that sent the RTS, for example, router  704 . The router  704  may also have a sectored antenna capability further increasing the expected carrier-to-interference ratio. 
     Each antenna sector may be 120 degrees providing almost 10 dB of antenna gain over an omni-directional antenna. Each antenna sector may also be smaller providing even more gain and less co-channel interference. Antenna sectoring may conform to a mesh network sectoring scheme, for example, a first sector may have relative bearings of 0-120 degrees, a second sector may have relative bearings of 121-240 degrees and a third sector may have relative bearings of 241-359 degrees. Alternatively, the sector scheme may be ad-hoc with each of the gateways and routers independently sectoring transmit and receive space. 
       FIG. 8  is a diagram illustrating a cluster from the mesh network using Orthogonal Frequency Division Multiple Access (OFDMA) according to an embodiment. In this embodiment, the gateways and/or the routers employ OFDMA to improve carrier-to-interference ratios. Using OFDMA, a gateway, such as gateway  802 , may transmit over a frequency band  801  (e.g., between fmin  803  and fmax  805 ). The routers, however, transmit over smaller bands centered on different tones of the frequency band. For example, the router  804  may transmit on a first frequency band around a first frequency tone f 1   806 , the router  808  may transmit on a second frequency band around a second frequency tone f 2   810 , the router  812  may transmit on a third frequency band around a third frequency tone f 3   814 , the router  816  may transmit on a fourth frequency band around a fourth frequency tone f 4   818 , the router  820  may transmit on a fifth frequency band around a fifth frequency tone f 5   822 , and the router  824  may transmit on a sixth frequency band around a sixth frequency tone f 6   826 . 
       FIG. 9  is a flow diagram illustrating a method of spectral reuse for improving the efficiency of the mesh network according to an embodiment. A gateway or a router monitors the communication channel for carriers ( 902 ). If a carrier wave is sensed, the gateway or the router stores the observed carrier strength ( 904 ). The gateway or the router then determines a transmit threshold ( 906 ). The transmit threshold is based on the observed carrier strengths. The transmit threshold may be a value based on the statistical expectation that transmitting from the gateway or the router will increase the spectral efficiency of the mesh network  100 . This expectation may be calculated using the rate equation N log 2 (1+γ) as explained above. The gateway or the router may compare the sensed carrier strengths with the threshold ( 908 ). If the carrier strength is greater than the threshold, it indicates that co-transmitting from the gateway or the router at this time would result in reduced spectral efficiency and the gateway or the router should remain silent. If the carrier is less than the sensed threshold, the gateway or the router should transmit ( 910 ) even though there will be some co-channel interference since the mesh network efficiency will increase. 
       FIG. 10  is a flow diagram illustrating a method of antenna sectoring for improving the efficiency of the mesh network according to an embodiment. A gateway or a router monitors the communication channel for request-to-send messages ( 1010 ). When a request-to-send message is observed ( 1020 ), the gateway or the router calculates the bearing to the transmitter ( 1030 ). The gateway or the router focuses the antenna beam toward the sector containing the bearing ( 1040 ). The sectors may be the same for the entire mesh network or they may be unique to the individual router or mesh point. The gateway or the router monitors the link. If the link is no longer established ( 1050 ), the gateway or the router begins to monitor the communication channel ( 1010 ). 
       FIG. 11  is a flow diagram illustrating a method of OFDMA for increasing the frequency reuse to reduce the spectral efficiency of the mesh network according to an embodiment. The gateway divides the transmit frequency channel into multiple frequencies ( 1110 ). A unique frequency is assigned to each router in the neighborhood ( 1120 ). The routers establish data links centered on each of the tones. The gateway receives data from the routers modulated on each of the tones ( 1130 ). 
       FIG. 12  is a block diagram illustrating exemplary components for the apparatus and the means for apparatus for managing use of a communication channel in a mesh network according to an embodiment. The apparatus may include a module  1210  for monitoring a communication channel for one or more carriers, a module  1220  for measuring a signal strength for each of the one or more carriers, and a module  1230  for transmitting data if the signal strength meets a condition. 
     Those skilled in the art will appreciate that the various illustrative logical blocks, modules, circuits, and algorithms described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and algorithms have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processing device, a digital signal processing device (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processing device may be a microprocessing device, but in the alternative, the processing device may be any conventional processing device, processing device, microprocessing device, or state machine. A processing device may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessing device, a plurality of microprocessing devices, one or more microprocessing devices in conjunction with a DSP core or any other such configuration. 
     The apparatus, methods or algorithms described in connection with the embodiments disclosed herein may be embodied directly in hardware, software, or combination thereof. In software the methods or algorithms may be embodied in one or more instructions that may be executed by a processing device. The instructions may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processing device such the processing device can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processing device. The processing device and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processing device and the storage medium may reside as discrete components in a user terminal. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     The disclosure may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.