Abstract:
A communication system is provided. In one embodiment, a communication system includes communicant nodes. Each node includes a local clock, a transmitter/receiver circuit and a control circuit. The local clock is adapted to clock operations of the communication node. The transmitter/receiver circuit is adapted to selectively transmit and receive communication signals. The synchronization circuit is adapted to synchronize the local clock with a received communication signal from another communication node to establish communications between the communication nodes. The control circuit is adapted to control the transmitter/receiver circuit and the synchronization circuit. The control circuit is also adapted to process communication signals and to direct communications between the communication nodes without re-synchronizing the local clock until the clock drift between communication nodes makes communications unreliable.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the field of wireless communications, and in particular, to systems and methods of collaboration among multiple nodes in time-synchronized communication systems. 
     BACKGROUND 
     A typical wireless communication system is composed of two or more transmitter/receiver nodes adapted to communicate with each other. Communication systems, such as cell phone systems, use frequency, time and code division multiplexing to ensure only a single transmitter is active at any given instant in time (i.e. for a given set of frequencies and codes). To accomplish a message exchange between nodes, each node is adapted to selectively switch between transmit and receive modes by local node control. 
     Wireless data communications systems, such as conventional radio frequency systems, provide data communications by modulating, or coding, data signals onto a carrier frequency(s). However, other types of wireless communication systems are carrier-less and rely on time-based coding for data communications. One such communication system that relies on time-based coding to achieve reliable data communications is Ultra Wide Band (“UWB”). 
     These UWB systems, unlike conventional radio frequency communications technology, do not use band-limited carrier frequencies to transport data. Instead UWB systems make use of a wide band energy pulse that transports data using both time-based coding and signal polarization. Time-based coding methods include pulse-position, pulse-rate or pulse-width techniques. By definition, a UWB system does not provide a common clock to the transmitting and receiving nodes. Instead, a low-drift clock is implemented in each transmitter/receiver node, providing a local reference for time-based coding and decoding. Each of these multiple clock domains is subject to short-term time drift, which will exceed the necessary tolerance for accurate UWB system operation after a predictable time period. As a result, precise time synchronization between the transmitting node and receiving node(s) is imperative in UWB systems to obtain accurate data communications. In order to precisely synchronize the receiving node(s) with the transmitting node, UWB systems typically require long preambles for each transmitted data frame. However, some applications with potential to benefit from UWB technology cannot tolerate the elapsed time resulting from preambles at the beginning of each frame or cannot be implemented if a preamble is required. Also, many potential applications for UWB technology are size and energy constrained, such as networks of unattended wireless sensors and controls, which seek to minimize transmission time and to conserve energy. 
     Existing applications employing UWB technology include short-range radar systems and high speed wireless communications characterized by large amounts of data requiring isochronous signaling, such as real-time voice and video. Generally, the signal used for a UWB application requires a preamble at the beginning of each transmitted frame to enable a receiver(s) to synchronize with the time-based coding being transmitted. The time required for transmitting the preamble, and subsequent data, imposes a minimum time between reversals in the direction of data communications between two transmitter/receiver nodes in a UWB system, which in turn restricts the scope of applications suitable for UWB implementation. Also, the energy consumed to transmit the preamble for existing applications is a significant fraction of the overall energy required to transmit the preamble and subsequent data. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the communication industries for a method to provide collaboration among two or more transmitter/receiver nodes that eliminates multiple re-synchronization preambles and minimizes energy consumption at each node. 
     SUMMARY 
     The above-mentioned problems of current communication systems are addressed by embodiments of the present invention and will be understood by reading and studying the following summary and specification. 
     In one embodiment a method of communication is provided. The method comprises transmitting a communication signal having a synchronization preamble from a first node. Setting a local clock of at least one second node pursuant to the synchronization preamble to synchronize communications between the first and at least one second node and exchanging subsequent communication signals between the first and at least one second node without additional preambles until synchronization suitable for data communication is lost. 
     In another embodiment, a method of communicating in an ultra wideband communication (UWB) system is provided. The method comprises transmitting an initial communication signal frame having a preamble that contains clock synchronization information from a first node. Receiving the initial communication signal frame with at least one second node. Synchronizing a local clock in the at least one second node pursuant to the clock synchronization information in the initial transmission signal frame. Exchanging sub-frame communication signals between the first node and at least one second node. Monitoring clock synchronization drift between the local clock in at least one second node and a local clock in the first node and when the clock synchronization drift has drifted far enough apart that synchronization suitable for UWB data communication has been lost, transmitting another initial communication signal frame having the preamble that contains the clock synchronization information to re-synchronize communication between the nodes. 
     In still another embodiment, a communication node is provided. The communication node includes a local clock, a transmitter/receiver circuit and a control circuit. The local clock is adapted to clock operations of the communication node. The transmitter/receiver circuit is adapted to selectively transmit and receive communication signals. The synchronization circuit is adapted to synchronize the local clock with a received communication signal from another communication node to establish communications between the communication nodes. The control circuit is adapted to control the transmitter/receiver circuit and the synchronization circuit. The control circuit is also adapted to process communication signals and to direct communications between the communication nodes without re-synchronizing the local clock until the clock drift between communication nodes makes communications unreliable. 
     In yet another embodiment, another communication node is provided. The communication node includes a local clock, a transmitter, a receiver, a synchronization circuit and a control circuit. The local clock is adapted to clock processes of the communication node. The transmitter is adapted to transmit communication signals. The receiver is adapted to receive communication signals. The synchronization circuit is adapted to synchronize the local clock in response to a preamble in a received initial communication signal. The control circuit is adapted to control the transmitter and the receiver. The control circuit is further adapted to direct the transmittal of sub-frame communication signals while the local clock of the communication node is synchronized with at least one other communication node in a communication system. 
     In further still another embodiment, a communication system is provided. The communication system comprises two or more communication nodes. Each node is adapted to synchronize a local clock to establish communications based on a synchronizing preamble received in an initial communication frame. Each node further is further adapted to receive and transmit communication sub-frames until communication synchronization has been lost due to clock drift. 
     In yet another embodiment an ultra wideband communication node is provided. The ultra wideband communication node comprises a means for receiving communication signals, a means for transmitting communication signals, a clocking means, a means for synchronizing the clocking means and a means for exchanging communication sub-frames. The communication signals include initial communication frames and sub-frames, wherein an initial communication frame includes a preamble, data bits and a switch point and a communication sub-frame includes only data-bits and a switch point. The means for synchronizing the clocking means is in response to a preamble received in a communication frame. The means for exchanging communication sub-frames is with at least one other communication node while clock synchronization of the clocking means has not drifted beyond a point in which data communication error is un-acceptable. 
    
    
     
       DRAWINGS 
       The present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
         FIG. 1A  is a diagram of one embodiment of a data communications system of the present invention; 
         FIG. 1B  is a diagram of another embodiment of a data communication system of the present invention; 
         FIG. 2  is communication node of one embodiment of the present invention; 
         FIG. 3  is a communication node of another embodiment of the present invention; 
         FIG. 4A  is an initial communication sub-frame of one embodiment of the present invention; 
         FIG. 4B  is a communication signal of one embodiment of the present application. 
         FIG. 5  is a graph illustrating communications of one embodiment of the present invention; 
         FIG. 6  is a flow diagram of a transmission of a communication signal of one embodiment of the present invention; and 
         FIG. 7  is a flow diagram of a receiving of a communication signal of one embodiment of the present invention. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the present invention. Reference characters denote like elements throughout Figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Embodiments of the present invention provide methods and systems for efficiently using time-synchronized communications systems. In one or more embodiments, the present invention provides methods for sub-frame synchronized signaling that avoids many of the long resynchronization periods caused by preambles at the start of each transmission frame. This method provides faster time response between network nodes. The elimination of certain resynchronization preambles also eliminates the necessity of receivers having to wait for a preamble to determine when a transmission is coming and thus, provides significant energy savings to the transmitter and receiver nodes. 
     In  FIG. 1A  an embodiment of a data communications system  100  of the present invention is illustrated. In this embodiment, the data communication system  100  includes communication node  102  and communication nodes  104 - 1  through  104 -N. The communication nodes  102  and  104 - 1  though  104 -N are adapted to communicate with each other. In particular, in the embodiment illustrated in  FIG. 1A , communication node  102  is in communication with communication nodes  104 - 1  through  104 -N. However, it will be understood that the present invention can generally apply to two or more communication nodes, any of which may be the initial transmitting node. In embodiments of the present invention, a first communication node initiates a message exchange. In the embodiment of  FIG. 1A , this is communication node  102 . A message exchange is started when communication node  102  transmits a communication signal that contains a preamble. The preamble is a known sequence of information that includes information regarding clock timing. Each receiving node uses the information in the preamble to synchronize its local clock so communication between the transmitting node and the receiving nodes can occur. In embodiments of the present invention, communication between nodes is maintained without sending an additional preamble until internal clocks in the respective nodes have drifted apart far enough that synchronization suitable for UWB data communications has been lost. 
     Referring back to  FIG. 1A , the first communication signals  110 - 1 ,  112 - 1  and  114 - 1  containing the preambles and other data to be exchanged are illustrated. The first communication signals may be referred to as synchronization communication signals. Node  102  is the transmitting node when transmitting signals  110 - 1 ,  112 - 1  and  114 - 1  and nodes  104 - 1  through  104 -N are the receiving nodes when receiving the respective first communication signals  110 - 1 ,  112 - 1  and  114 - 1 . At the end of each communication signal is an indication that the signal is complete. In one embodiment, this is referred to a switch point. The switch point provides a signal to the receiving node that it can now become a transmitting node to transmit a communication signal in response to a received signal. An example of this are signals  110 - 2 ,  112 - 2  and  114 - 2  transmitted from the respective nodes  104 - 1 ,  104 - 2  and  104 -N back to node  102  one after another in a pre-arranged sequence. The message exchange between the nodes continues like this until, as discussed above, the internal clocks in the respective nodes have drifted far enough apart that synchronization suitable for UWB data communications has been lost. The last communication signal between nodes  102  and  104 - 1  through  104 -N is illustrated as signals  110 -N,  112 -N and  114 -N in  FIG. 1A . 
     In one embodiment, the data communication system  100  is a point to point communication system where only two nodes are participating at a time. An example of this system in reference to the communication system of  FIG. 1A  is when node  120  sends a first signal  110 - 1  to node  104 - 1  and then a second signal  112 - 1  at a later time to node  104 - 2 . In another embodiment of the present invention, the data communication system  120  is arranged in a broadcast network where a transmitted signal is received by multiple nodes simultaneously. An example of this is illustrated in  FIG. 1B . In this embodiment, only a single preamble is required since only one initial transmission signal is sent to the multiple nodes. Referring to  FIG. 1B , an initial signal  130  including the single preamble is broadcast from node  122  to nodes  124 - 1  through  124 -N simultaneously. In this embodiment, a switch point in the transmitted signal is used to indicate the next transmit node. For example, in the communication system  120  of  FIG. 1B , the next node to transmit, as directed by the switch point, is node  124 - 2 . As illustrated signal  132  is transmitted from node  124 - 2  to node  122 . Further in this example, node  124 - 3  is then directed to transmit signal  134  to node  124 - 4 . 
       FIG. 2 , illustrates one embodiment of a node  200  of the present invention. Node  200  includes a data processing and UWB control circuit  202 , a UWB transmit (Tx)/receive (Rx) circuit  204  and a local clock  206 . The Tx/Rx circuit  204  includes a clock synchronization circuit. As illustrated the clock  206  is used by both the control circuit  202  and the Tx/Rx circuit  204 . The Tx/Rx circuit uses the clock, for among other things, to determine time intervals between data. The control circuit  202  uses the clock among other things, to determine when to send and pass received data. An example of a more detailed node  300  of one embodiment of the present invention is illustrated in  FIG. 3 . Node  300  includes an antenna  322 , a transmit (T)/receive (R) switch  320 , a CPU  314 , a Tx  316 , a Rx  318 , a clock synchronization circuit  310  and a local clock  312 . The CPU  314  controls the T/R switch  320 . When node  300  is transmitting the T/R switch  320  is placed in a transmitting position, and when node  300  is receiving the T/R switch is placed in a receiving position. The clock synchronization circuit  310  is used to adjust and monitor the local clock  312 . In particular, when an initial transmission with a preamble is received through the receiver  318 , the synchronization circuit  310  is used to synchronize clock  312  with the clock of the node which sent the initial transmission. After the clocks are synchronized, the clock synchronization circuit  310  monitors the drift of the clock. In one embodiment, clock drift is monitored by measuring the error rate resulting from using the local clock to detect the time-synchronous signaling from the transmitter. As the clock drifts (away from the Tx clock) the error rate increases. The error rate is measured by a detecting when a cyclic redundancy code (CRC) scheme, forward error correction method, or other error detection and correction scheme fails to successfully correct data errors. A CRC protocol uses a CRC character that is generated from each data block of data at a transmission end by a transmitting device. The value of the CRC character depends on the hexadecimal value of the number ones in the data block. The transmitting device calculates the value of the CRC character and appends it to its associated data block. At the receiving end, a receiving device makes a similar calculation on the data block and compares it with the added character (i.e. the CRC character). If there is a match, the data block is considered to be error free. If there is not a match, an error has been detected and the receiver will request a retransmission of the data block. 
     As illustrated in  FIG. 3 , the clock is further in communication with the transmitter  316  to clock the transmission of data in a transmission signal and with the receiver  318  to clock the receiving of data in a received signal. The CPU  314  is also in communication with the receiver  318  to process data in a received signal. The CPU  314  is further adapted to process data to be transmitted. There are three keys to obtaining synchronized signaling in data communications systems of embodiments for the present invention. First, synchronized signaling requires CPU  314  (or controller) of each node to obtain the accurate propagation time between nodes. Second, the nodes in communication must be using the same known frame format. Lastly, the clock drift must be monitored between data communication nodes and must be small enough to ensure time-synchronization after each sub-frame duplex or link reversal. 
     Referring to  FIG. 4A , an example of an initial signal  400  is illustrated. In particular, the initial signal in this embodiment is an initial communication frame  400 . An initial communication frame includes a preamble and a sub-frame containing data bits and a switch point. The initial communication frame  400  is sent from a transmitting node (first node) and includes preamble  402 . Preamble  402  contains information used by one or more receiving nodes for synchronization of the receiving node(s) local clock(s). After the preamble, information or data is transmitted. The information or data is generally referenced as  404 - 1  through  404 -N and in one embodiment are data communication bits. After the data  404 - 1  through  404 -N has been transmitted, a switch point is transmitted  406 . The switch point  406  indicates that the data has all been sent in this sub-frame. Moreover, in one embodiment the switch point initiates a link reversal or duplexing. Link reversal or duplexing instructs a particular receiving second node to change into a transmitting node and the transmitting node to change into a receiving node. In particular, in one embodiment, once the transmitting node has encountered a switch point, the transmitting node stops transmission, switches into receive mode and adjusts its local clock to reflect the time delay coming back from a communication signal from the particular receiving second node. Further in other embodiments, the switch point  406  is used to indicate a change in signal format. 
       FIG. 4B  illustrates a second communication sub-frame  401 . This second communication sub-frame is sent by the second node to the first node. The second communication sub-frame includes data  408 - 1  through  408 -N and a switch point. As illustrated, no preamble is required in this second communication sub-frame because the clocks in the first and the second nodes are still synchronized at this point. The communication between nodes occurs without the need for additional preambles in the frames, such as frame  401 , until the clocks of the first and second nodes have drifted out of time synchronization. Once they have drifted out of time synchronization another initial communication frame with a preamble, similar to preamble  402  of  FIG. 4A , is required to resynchronize the respective clocks. 
     This is further illustrated in graph  500  of  FIG. 5 . The embodiment of  FIG. 5 , illustrates the communication between a first node and a second node. The first node sends a frame including preamble ( 502 ). In response to the preamble ( 502 ), the second node synchronizes ( 503 ) its local clock ( 503 ). Data is then transmitted ( 504 ) by the first node in a sub-frame. The second node receives ( 505 ) the sub-frame data. A link reversal point ( 510 ) (or switch point) is then reached. The second node is than adapted to transmit ( 507 ) data and the first node is adapted to receive the data ( 506 ) in this second sub-frame. Another link reversal is encountered ( 512 ). In response to the link reversal ( 512 ) the first node is adapted to transmit ( 508 ) data and the second node is adapted to receive ( 509 ) the data in a third sub-frame. In this example, a loss of time synchronization is then encountered ( 514 ). To continue the message exchange, a preamble will have to be sent to reestablish synchronization between nodes. 
     Referring to  FIG. 6 , a transmit flow diagram  600  of one embodiment is illustrated. The transmit diagram begins by transmitting a preamble ( 602 ). A receiving node than synchronizes its local clock according to the preamble. Once the receiving node has been synchronized ( 604 ), data is transferred from the transmitting node ( 606 ). The data transfer continues ( 606 ), until a switch point is encountered ( 608 ). If a switch point has been encountered ( 608 ), a sub-frame duplex is performed (i.e. switching between transmitting and receiving function in a node) and the local clock is adjusted ( 610 ). The local clock of the transmitting node (i.e. the node being switched from a transmitting node to a receiving node) is adjusted in this embodiment to take into consideration the propagation time between nodes. If the clock drift is exceeded ( 612 ) then the node ceases data transfer and must initiate transmission of another preamble ( 602 ). 
       FIG. 7  is a receiving flow diagram  700  of one embodiment of the present invention. The receiving flow diagram  700  starts by receiving a preamble ( 702 ). In response to the preamble ( 702 ), the node synchronizes its local clock to establish communication ( 704 ). Data is then transferred to the receiving node ( 706 ). The data transfer continues ( 706 ), until a switch point is encountered ( 708 ). If a switch point has been encountered ( 708 ), a sub-frame duplex is performed (i.e. switching between transmitting and receiving function in a node) and the local clock is adjusted ( 710 ). The local clock is adjusted in this embodiment to take into consideration the propagation time between the nodes. If the clock drift is exceeded ( 712 ) then the node ceases data transfer and awaits reception of another preamble ( 702 ).