Abstract:
A method synchronizes transmissions through a channel in a wireless communications network including a device and a multiple coordinators within transmission range of the device. A superframe is defined to include a beacon period, a contention access period, and a contention free period. The beacon period includes multiple slots. In each coordinator, a particular beacon slot is selected to be non-conflicting with beacon slots selected by other coordinators. Beacons are then transmitted to the device by the coordinators at time periods associated with the selected slots.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to communication systems, and more particularly to medium access control using beacons in wireless networks. 
   BACKGROUND OF THE INVENTION 
   In wireless personal area networks (WPAN), common topologies are a ‘star’ network operating in infrastructure mode and a ‘cluster’ network operating in ad hoc mode. In a star network, all devices communicate indirectly with each other via a central device called a coordinator. The coordinator receives data from transmitting devices and forwards the data to receiving devices. In a cluster network, the devices communicate directly with each other. 
   In such networks, the very first device that starts a network is called the PAN coordinator. As the network evolves, other coordinators can join the network. In this case, a joining coordinator is called a ‘child’ coordinator that joins with an already existing ‘parent’ coordinator. 
   The operation of such networks can be according to the IEEE 802.15.3 and IEEE 802.15.4 standards, see IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements “Part 15.3: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs),” 2003, and IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—“Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs),” 2003. 
   Because the signals transmitted by all devices share the same frequency channel, it is necessary to enforce a channel access methodology in order to efficiently utilize the network bandwidth. This can be done with a channel access schedule, which determines when and how terminals can access the channel. The access schedule can be broadcast periodically using a beacon, see  FIG. 1 . 
   The beacon  110  specifies network parameters, i.e., transmission rates, logical channels, network identifiers, and the channel access schedule. The period between successive beacon signals is called a superframe  100 . The beacon is followed by a contention access period (CAP)  120  and a contention free period (CFP)  130 . The beacon defines the start of the contention period, the start of the contention free period, and the access schedule for the contention free period. The beacon can also include other parameters as defined by the IEEE standard. 
   During the contention period, the devices compete with each other to gain access to the physical channel. Typically, a random access method such as Aloha or CSMA is used. After gaining access, devices transmit on the channel strictly according to the access schedule during the contention free period to guarantee interference free packet transmissions. 
   In a WPAN, there can be multiple coordinators within the same personal operating space (POS), see the IEEE 802.15.4-2003 standard, incorporated herein be reference. For a beacon enabled WPAN, the beacons sent by different coordinators can conflict with each other, directly or indirectly. 
   It absolutely necessary that the beacons  110  are received correctly for devices to operate in the WPAN. 
     FIG. 2  shows an example of direct beacon conflict. Here multiple coordinators  201 - 202  use the same physical channel, i.e., the same radio frequency band, for sending beacons  110  to device  210 . The coordinators are within transmission range  203  of each other. If the coordinators concurrently send beacons  110  such that the transmission periods for two beacons overlap  205 , then the multiple beacons collide at device  210  because CSMA/CA is not applied to beacon transmissions under the IEEE 802.15.4 standard. The direct conflict is due to the fact that a child coordinator joins the WPAN by associating a parent coordinator already in the WPAN. The direct conflict can also be caused by other conditions in which two coordinators are within each other&#39;s transmission range. 
     FIG. 3  shows an example of indirect interference. Two coordinators  301 - 302  choose the same physical channel. However, the two coordinators are out of transmission range with each other. However, device  300  is within range of both coordinators. If both coordinators concurrently send beacons  110 , the beacons collide with each other at the device  300 . This due to the fact that coordinator  301  is part of the WPAN when coordinator  302  joins. Because coordinator  302  is out of range of coordinator  301 , coordinator  302  can chose the same channel as coordinator  301 . The device  300  must be within the overlap area  310  to encounter indirect beacon conflict. This causes the device  300  to lose synchronization with its parent coordinator  301  and the device becomes an ‘orphan’ as defined by the standard. Even worse, the device cannot re-join with its parent coordinator through ‘orphan scan’ as specified in the IEEE 802.15.4-2003 standard. 
   As shown in  FIG. 4 , for a coordinator  400 , direct beacon conflict occurs for devices within transmission range of the coordinator in a direct conflict area  401 , and indirect beacon conflict occurs outside of range  401  but within double the transmission range in an indirect conflict area  402 . 
   One solution for this interference problem at the network layer is described by Lee et al., in submission IEEE P802-802.15-04-0101-00-0004b, March 2004. That solution has three explicit collision detection requirements before a beacon can be sent. 
   Another solution uses ad-hoc beacons, see Marsden et al., in IEEE P802.15-15-04-0093-00-004b, March 2004. A centralized control method uses a relay device to connect two piconet controllers (PNCs) to handle beacon collisions. Another solution is described in U.S. patent application Ser. No. 10/434,948 filed on May 8, 2003. 
   SUMMARY OF THE INVENTION 
   The invention provides a method to synchronize transmissions through a channel in a wireless communications network including a device and a multiple coordinators within transmission range of the device. 
   The invention achieves this objective by broadcasting a channel access schedule using a beacon signal within each superframe. 
   A superframe is defined to include a beacon period, a contention access period, and a contention free period. The beacon period includes multiple slots. 
   In each coordinator, a particular beacon slot in the beacon period is selected to be non-conflicting with beacon slots selected by other coordinators. Beacons are then transmitted to the device by the coordinators at time periods associated with the selected slots. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a prior art superframe including a beacon; 
       FIG. 2  is a diagram of direct conflict to be solved by the invention; 
       FIG. 3  is a diagram of indirect conflict to be solved by the invention; 
       FIG. 4  is a diagram of direct and indirect conflict ranges to be solved by the invention; and 
       FIG. 5  is a diagram of a superframe including multiple beacons according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Direct Conflict 
     FIG. 5  show a superframe  500  according to the invention. The superframe includes a beacon period  510 , a contention access period  520 , and a contention free period  530 . The length of time of the beacon period is set by a personal area network (PAN) coordinator that is the very first device to form the network. 
   For example, the PAN coordinator sends a PAN beacon  501  at the beginning of beacon period  520 . All other coordinators within its transmission range that join the WPAN later send non-conflicting beacons  502  at other times during the beacon period  510 . A coordinator that is outside of the PAN coordinator&#39;s transmission range may send its beacons at the same time as the PAN coordinator without conflict. 
   There are a number of different ways that the coordinators, except the PAN coordinator, can select a time for sending their beacons  502 . 
   A first method selects the time randomly to reduce the possibility of conflict. 
   In a second method, each coordinator includes the length of the beacon period  510 , a length of its beacon  502 , and a starting time of its beacon, with respect to the beginning of the superframe in its beacon itself when the coordinator joins the network. Then, as each additional coordinator joins the network, the additional coordinator can select its beacon to be non-conflicting with previously established beacons. This selection can also be made randomly from the remaining beacon free time within the beacon period  510 . 
   A third method partitions the beacon period into equal sized time slots, and each coordinator randomly selects a free slot for its beacon transmission time. It should be noted that the IEEE 802.15.4-2003 standard allows beacons to have various size. Therefore, the slots have to be at least as long as the longest possible beacon time, or a single beacon can occupy several consecutive beacon slots. 
   In a fourth method, a coordinator senses the channel during a selected beacon slot period for several consecutive superframes, and then selects the beacon slot if the channel remains idle during this time. 
   In a fifth method, the parent coordinator detects beacon conflicts, and broadcasts the beacon conflict to other coordinators. In response to the notifications, the child coordinators can select their beacon transmission times. 
   The parameters related to the beacon period can be specified in the MLME-START parameter list, see Table 58 of the IEEE 802.15.4-2003 standard, incorporated herein by reference. 
   Indirect Conflict 
   Solving the indirect conflict problem is more difficult because a coordinator cannot directly receive beacons from other coordinators at the indirect conflict area  402 . 
   There are two ways that the indirect conflict problem can be solved, reactively, i.e., after the conflict happens, and proactively, by avoiding conflicts in advance. 
   Reactive Indirect Conflict 
   In this method, a device that senses an indirect conflict notifies the associated coordinator of the conflict, including, perhaps, conflicting beacon parameters. In response to the notification, the coordinator can change its beacon to be non-conflicting. 
   Proactive Indirect Conflict 
   If a coordinator knows all other coordinators within its direct and indirect conflict area  402 , then beacon conflicts can be avoided. However, as specified in the IEEE 802.15.4-2003 standard, a coordinator knows only its direct neighboring coordinators, i.e., other coordinators in the direct conflict area  401 . 
   In one method, the device  300  in the overlap area  310  senses that it is within range of its parent coordinator and at least one other coordinator, by channel scanning. In response to sensing the conflict, the device notifies the conflicting coordinators by broadcasting the conflict information so the beacons can be adjusted to be non-conflicting. This can be done by having both the device and a coordinator sense for beacon request commands of coordinators and reply with the beacon information of its parent coordinator. In another method, each beacon in the beacon period  510  includes the timing parameters of some or all other beacons in the period. 
   To solve the indirect beacon conflict solution proactively, a coordinator needs to gather beacon information for all other coordinators within its direct and indirect areas in advance before collisions happen. This can be achieved by having the coordinator and all devices sense and reply to any beacon request command. In response, beacon timing information can be broadcast in a beacon information message to assist other coordinators in selecting non-conflicting beacon slots. For example, a coordinator sensing a beacon request command replies its own beacon information followed by a beacon time notification command. 
   Similar methods can be used to handle direct and indirect beacon conflicts among coordinators from different WPANs. If two WPANs are synchronized with each other, the same solution can be applied to handle beacon conflicts. If two WPANs are not synchronized with each other but have the same superframe size, the devices or the coordinators in the overlap area of the two WPAN can calculate the time relation between the two WPANs by analyzing the beacons from both WPANS. With the time relation, similar solutions can be applied. 
   If the two WPANs use different superframe sizes, the coordinator with the longest superframe maintains a table to record those time slots of the CAP and CFP allocated to beacons in the other WPAN. With such method, collisions between beacon and data frames can be avoided. Another solution is to set different transmission priorities for beacons and data frames to avoid collisions between beacons and data frames in the case that two WPANs have different superframe sizes. 
   Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.