Abstract:
Methods and systems are provided to facilitate communication with a plurality of remote units in a communication area that includes a plurality of spatial sectors. The system includes a network controller configured to receive and transmit under IEEE 802.11 specifications, and a plurality of directional antennas coupled to the network base station. Each of directional antennas functions as an access point for a respective spatial sector. The directional antennas and the network controller are combined as a single base station apparatus in an exemplary embodiment. Further, an exemplary network controller is configured to concurrently transmit a plurality of the data frames to more than one of the spatial sectors, and to synchronize the transmissions in a manner whereby all of the concurrently transmitted frames end simultaneously.

Description:
TECHNICAL FIELD 
   The present invention generally relates to wireless communication networks, and more particularly relates to wireless networks operating under 802.11 specifications in relatively large communication areas. 
   BACKGROUND 
   An IEEE 802.11 network refers to a family of specifications developed for wireless LAN technology, and specifies an over-the-air interface between two wireless units, or between a wireless unit and a base station. Networks that include a base station to facilitate communication with and between a plurality of wireless units are increasingly being used in outdoor wireless environments. Sometimes a high number of units and/or a large communication area necessitate distribution of multiple access points about the area in which the wireless units are to be used.  FIG. 1  illustrates four access points  102  distributed about an area  100  in which communication is enabled for one or more wireless units. The access points  102  have overlapping transmitting radii  104 , and each access point  102  is wired to a central communication facility  106 . 
   Rather than distribute access points about a communication area, it is desirable to have a single, centrally located access point with an extended transmission radius that covers the entire communication area. Advantages of such a simplified network would include easier device management and reduced costs associated with hardware and outdoor cabling. 
   Implementing a large communication area network with a single access point would also impart several challenges. To start, increases in the access point transmitting power are constrained by regional regulatory requirements. Further, such increases tend to create asymmetric links since the mobile devices in the communication area may receive transmissions from the access point, but have limited transmitting power for responding to the access point. 
   One way to overcome some of the problems associated with increasing the access point transmitting power would be to increase the ability of the mobile device antennae to focus transmissions toward the access point. However, an implicit effect of antenna gain is a degree of directivity, and focused transmissions in particular directions can effectively partition the overall communication space. Additional access points may be necessary if the communication space is partitioned due to antenna gain, and to some extent this would defeat the advantages that the single access point may have initially provided. For example, multiple centrally located access points in a partitioned communication area could present a number of difficulties such as reduced system capacity due to transmission collisions and inter-sector roaming overheads using 802.11 network protocols. 
   Accordingly, it is desirable to provide a wireless communication network that operates under 802.11 specifications and covers a relatively large communication area. In addition, it is desirable for symmetry to exist in the communication links between the mobile units and a base station in such a communication network without implementing a plurality of access points. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
   BRIEF SUMMARY 
   A wireless network system is provided to facilitate communication with a plurality of remote units in a communication area that includes a plurality of spatial sectors. The system includes a network controller configured to receive and transmit under IEEE 802.11 specifications, and a plurality of directional antennas coupled to the network base station. Each of directional antennas functions as an access point for a respective spatial sector. The directional antennas and the network controller are combined as a single base station apparatus in an exemplary embodiment. Further, an exemplary network controller is configured to concurrently transmit a plurality of the data frames to more than one of the spatial sectors, and to synchronize the transmissions in a manner whereby all of the concurrently transmitted frames end simultaneously. 
   A method is also provided to facilitating communication with a plurality of remote units in a communication area that includes a plurality of spatial sectors. The method includes the step of receiving and transmitting between the remote units and a network controller under IEEE 802.11 specifications using a plurality of directional antennas that are coupled to the network controller, each directional antenna functioning as an access point for a respective spatial sector. An exemplary method further includes the steps of concurrently transmitting a plurality of the data frames to more than one of the spatial sectors, and synchronizing the concurrent transmissions in a manner whereby all of the frames end simultaneously. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  illustrates a known wireless network protocol in which four access points distributed about a communication area and wired to a central communication facility, the access points having overlapping transmitting radii; 
       FIG. 2  illustrates an exemplary network configuration in which a base station is positioned inside a sectorized communication area using a plurality of directional antennas; 
       FIG. 3  illustrates another exemplary network configuration in which a base station is positioned inside a sectorized communication area using a plurality of directional antennas, and synchronized frames are transmitted to mobile devices in different communication area sectors; and 
       FIG. 4  is a graph that illustrates how synchronized frames are coordinated to all end transmission simultaneously and thereby avoid interference between the frames and acknowledgement signals from the mobile devices. 
   

   DETAILED DESCRIPTION 
   The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
   The present invention includes a centrally located base station that coordinates transmission sectors in a communication area, and thereby increases the communication area size and the system capacity. To facilitate and coordinate sectorization, the base station employs a plurality of directional antennas to collocate transmissions between remote units and the base station. Carrier sense information for each sector is shared to prevent collisions between a transmission from a base station sector, and an omnidirectional transmission from a remote unit in a neighboring sector. Further, to increase the system capacity while avoiding interference, transmission frames from the base station to the plurality of sectors are queued in order to synchronize their transmission times. The configuration consequently allows users to cover large communication areas from a single 802.11 network base station without compromising performance and without creating a need to increase mobile unit RF power. 
     FIG. 2  illustrates a base station  16  positioned inside a sectorized communication area  10 . The base station  16  includes, among other things, a network controller coupled to send and receive transmissions using a plurality of directional antennas. The communication area  10  in the embodiment of  FIG. 2  has three spatial sectors  10   a,    10   b,    10   c,  although the number of sectors is not limited to three but is dependent on the number of directional antennas included with the base station  16 . Since the base station  16  illustrated in  FIG. 2  has three directional antennas  17   a - 17   c,  the communication area is divided into three sectors  10   a - 10   c.  Also, the illustrated embodiment has the three sectors  10   a - 10   c  divided equally for efficiency even though the size of each sector can be modified as needed. 
   Remote wireless units  12   a - 12   e  are distributed about the communication area  10 , and each unit communicates with the base station  16  using one of a plurality of directional antennas  17   a - 17   c  that are coupled to a the base station  16 . The remote units  12   a - 12   e  may be mobile or stationary, although mobile units will be primarily discussed in the hereafter-described embodiments. In an exemplary configuration the mobile units  12   a - 12   c  transmit and receive data frames by communicating with whichever directional antenna is configured to transmit in a direction that is closest to a given unit. For example, units  12   a  and  12   c  are situated in sector  10   a  and communicate with the base station  16  using the closest directional antenna  17   a.  Likewise, unit  12   b  is situated in sector  10   b  and communicates with the base station  16  using directional antenna  17   b,  and units  12   d  and  12   e  are in sector  10   c  and communicate with the base station  16  using directional antenna  17   c.  If one of the mobile devices, such as mobile unit  12   a,  is initially situated in sector  10   a  but is thereafter received with a higher receive signal strength indication (RSSI) in sector  10   c,  then future transmissions to the mobile unit  12   a  will be sent from directional antenna  17   c.    
   Although the centralized base station  16  in the embodiment illustrated in  FIG. 2  is a single apparatus that includes a plurality of directional antennas, the base station  16  can also be multiple tightly coupled devices, each having at least one directional antenna. In either case, the directional antennas  17   a - 17   c  and base station are combined as a centrally located, centralized device. Further, the remote units  12   a - 12   e  can either be mobile or stationary units. In an exemplary embodiment each of the sectors use the same frequency channel, which allows each of the antennas  17   a - 17   c  to share a common basic service set identifier (BSSID). Alternatively, different frequency channels can be used in each sector, although this requires the remote units  12   a - 12   e  to frequently scan while roaming between sectors, if the remote units are mobile, and potentially inhibits the use of nearby 802.11 cells on alternative channels. If each of the sectors use the same frequency channel, and also share a common basic BSSID, one or more of the directional antennas  17   a - 17   c  may often receive the same frame. However, using the centralized base station  16  only one of the directional antennas  17   a - 17   c  will forward the frame, or in other words, the centralized base station  16  will only forward one copy of the frame for transmission using a selected directional antenna. 
   Each of the antennas  17   a - 17   c  in  FIG. 2  provides a significant amount of functionality to the base station  16  by operating as a virtual access point for its respective sector. Since each of the antennas  17   a - 17   c  has its transmitting and receiving power focused on a sector instead of an entire radius about the base station  16 , the combined antennas  17   a - 17   c  produce a communication area that has an extended radius. By using antenna gain instead of increased transmitting power to obtain the extended communication radius, the links between the base station  16  and the mobile units  12   a - 12   e  are symmetric. Further, using antenna gain enables use of mobile units  12   a - 12   e  with relatively low, but constant, transmitting power throughout the communication area  10 . 
   Although the communication area  10  is sectorized through the use of the directional antennas  17   a - 17   c,  there are still some situations in which interference can be created from simultaneous transmissions between the mobile units  12   a - 12   e.  One such situation is illustrated in  FIG. 2  between the base station  16  and two mobile units  12   a  and  12   b  that are in different sectors but are relatively close to each other. Arrow  18  indicates that the base station is transmitting to mobile unit  12   b  using the antenna  17   b  that corresponds to the communication area sector  10   b.  Arrow  20  indicates that at the same time, the mobile unit  12   a  in communication sector  10   a  is transmitting to the base station through the corresponding antenna  17   a.  However, the mobile units transmit in all directions, and the transmission from the mobile unit  12   a  in sector  10   a  interferes with the transmission from the base station to the mobile unit  12   b  in sector  10   b,  as indicated by arrow  22 . Even if all three sectors are on the same channel, the mobile unit  12   a  in sector  10   a  is unable to carrier sense on the transmission from the base station antenna  17   b  in sector  10   b.  Similarly, the base station antenna  17   b  in sector  10   b  is unable to carrier sense on the transmission from the mobile unit  12   a  in sector  10   a.  The directional nature of the base station transmissions creates “hidden” nodes that reduce capacity, and further creates an increased probability that transmission collisions will occur throughout the communication area  10 . 
   To overcome the potential for transmission collisions, the base station device or coupled devices in an exemplary network configuration share carrier sense information in real time. For example, if the mobile unit  12   a  in sector  10   a  begins transmitting before the base station  16  begins transmitting to the mobile unit  12   b  in sector  10   b  using the corresponding directional antenna  17   b,  the base station  16  receives the transmission from the mobile unit  12   a  and asserts carrier sense, thereby holding off transmission to the mobile unit  12   b  using the directional antenna  17   b.  By holding off the transmission using the directional antenna  17   b,  the base station  16  avoids any potential transmission collisions. In another exemplary network configuration, the base station can prevent transmission collisions between the transmissions from the mobile unit  12   a  and the base station directional antenna  17   a  by, upon receiving the transmission from the mobile unit  12   a,  transmitting from the directional antennas  17   b  and  17   c  a ready-to-send (RTS)/clear-to-send (CTS) signal with a sufficiently long network allocation vector. 
   Although configuring the base station  16  to assert carrier sense and/or to transmit an RTS/CTS signal at appropriate times effectively prevents transmission collisions, the configurations alone do not fully exploit the system capacity. An exemplary base station is further configured to concurrently transmit to a plurality of sectors while avoiding transmission collisions.  FIGS. 3 and 4  illustrate this embodiment, in which the base station  16  queues frames destined for mobile units  12   a - 12   c  using the previously-described directional antennas. Periodically, the base station  16  schedules concurrent transmissions of queued frames to the intended mobile units  12   a - 12   c  in their respective sectors. The concurrent transmissions are synchronized using a suitable reservation technique such as RTS/CTS signals, content free periods (CFP), or Hybrid Control Function Controlled Channel Access (HCCF). The synchronized transmissions are enabled by the base station  16  being configured to concurrently monitor and assess carrier sense in all the sectors throughout the communication area  10  as previously discussed. 
   Synchronizing the concurrent frame transmissions prevents interference from acknowledgments sent by the mobile units  12   a - 12   e  to the base station  16  after the mobile units  12   a - 12   e  receive the frame transmissions. If frames of different length begin their transmissions at the same time, the mobile unit that receives the smallest frame will send an acknowledgement signal that potentially interferes with the base station transmission of longer frames. The graph in  FIG. 4  illustrates a way to overcome acknowledgement signal interferences with frame transmissions from the base station  16 . Instead of beginning the synchronized frame transmissions simultaneously, the base station  16  is configured to end the synchronized transmissions simultaneously and thereby assure that all the frame transmissions are completed before any of the mobile units  12   a - 12   e  transmit an acknowledgement signal. It is likely that the mobile units  12   a - 12   e  will transmit acknowledgement signals simultaneously, but such signals will still be received without interference due to the directivity of the base station antennas. 
   The exemplary wireless network configurations in the foregoing description enables the use of a single basic service set identifier (BSSID) with multiple directional antennas that function as virtual access points in a communication area having a sectorized architecture. The directional antennas collocate transmissions between mobile units and a base station, and thereby increase the communication area size and the system capacity. The 802.11 network base station shares carrier sense information for each sector to prevent transmission collisions, and queues transmission frames to increase the system capacity without compromising performance and without creating a need to increase mobile unit power. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.