Abstract:
A method is provided for transmitting data. A first device ( 121 ) generates a first signal ( 320 ) having a first duty cycle, comprising a first gated-on portion ( 323 ) and a first gated-off portion ( 363 ) in a time slot ( 260 ); and a second device ( 125 ) generates a second signal ( 330 ) having second duty cycle, comprising a second gated-on portion ( 333 ) and a second gated-off portion ( 363 ) in the same time slot ( 260 ). The first gated-on portion ( 323 ) is generated during a first segment of the time slot ( 260 ) and the first gated-off portion ( 363 ) is generated during a second segment of the time slot ( 260 ), while the second gated-on portion ( 333 ) is generated during the second segment and the second gated-off portion ( 363 ) is generated during the first segment. The first and second duty cycles are individually below 100%, and the sum of the first and second duty cycles is below 100%.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to time division multiple access (TDMA) signal transmission schemes, including those used for ultra wideband (UWB) systems. In particular the present invention relates to TDMA signal transmission schemes in which individual assigned time slots are broken up into smaller nominal time slots, each of which is assigned to multiple devices. A particular aspect of the present invention relates to a TDMA schemes in which multiple devices transmit at lower than a 100% duty cycle and two or more devices transmit during the same nominal time slot but have their signals interleaved with each other such that they do not interfere with those of the other device or devices. Another aspect of the present invention relates to the choosing of the characteristics of the nominal time slots and the transmitted signals such that transmissions from each device do not violate power limits imposed by various regulatory agencies. 
     BACKGROUND OF THE INVENTION 
     When operating wireless networks, a problem can occur when multiple networks or multiple devices want to operate over the same channel using the same bandwidth. Some sort of scheme must be implemented to separate the networks in some way so that transmissions from one will not interfere with transmissions from the other. 
     One option for handling multiple networks or devices is to use a frequency division multiple access (FDMA) scheme. Such an implementation is often used in narrow band systems. In an FDMA scheme, different networks or devices are separated by each being assigned a different frequency band. Each network or device then gets to use its assigned portion of the spectrum and can be assured that other networks will not interfere with that assigned frequency portion. An example of this is FM radio. 
     However, this is not readily applicable to a UWB implementation since UWB by its nature uses wide frequency bands for its signals. In fact with UWB systems, it is generally advantageous to use as wide a spectrum as possible for transmissions. Because of the need to use very wide frequency bands, it is sometimes not feasible to break up the available spectrum into smaller, mutually exclusive frequency bands. 
     Another option for handling multiple networks is to use a code division multiple access (CDMA) scheme. In a CDMA scheme networks and devices transmit over the same frequency spectrum and at the same time, but signals from each are encoded using codes specially chosen to minimize their interference with each other. 
     However this kind of a scheme also has limitations. First, there are only so many codes that have the desired isolation properties needed to keep overlapping networks and devices separate, thus limiting the number of networks or devices that can operate at the same time in a given area. Second, no matter how good the code separation is, it isn&#39;t perfect. There is always some bleed over into transmissions from other networks. As a result of this, a close device of a different network can often drown out a distant device of the same network, despite the fact that the codes used by the close device are chosen to minimize interference with the other network. This can be referred to as the near-far problem. 
     Yet another option for handling multiple networks is a time division multiple access (TDMA) scheme. In a TDMA scheme, the available transmission time is broken up into multiple time slots, and each network or device is assigned one or more of the time slots. Thus, each device is given some portion of the available transmission time to use and is forced to remain silent during all other times. 
     However, this TDMA scheme forces each network or device to reduce its speed, since it isn&#39;t allowed to transmit during the entire available channel time. And as the number of overlapping networks or devices increases, transmission speed will be correspondingly reduced. For example, if there is 100 megabits per second (Mbps) capacity divided evenly over four separate networks, each network would be limited to a 25 Mbps transmission speed. 
     One way to transmit more data in a TDMA scheme is to increase the transmission power for a given network or device. In a digital system, for example, using a stronger signal means that each individual bit of data requires less time to send, enabling the device to operate at an increased data rate. A significantly increased transmit power can, therefore, compensate for time lost when other networks or devices are transmitting. 
     However, this solution of increasing transmit power has limited application to UWB systems. In the United States the Federal Communications Commission (FCC) has imposed a limit on the maximum allowable transmit power for UWB signals. And there is every reason to believe that similar agencies in other countries will impose similar restrictions. This in turn represents a limit on the maximum capacity for the combination of all available networks. 
     Therefore, if a TDMA scheme is used for UWB signals, it would be desirable to maximize the transmit power used for any given signal, while minimizing that signal&#39;s width in the time domain. And it would be desirable to achieve this result without violating the maximum signal power restrictions set up by the FCC or similar regulating agency. This would allow each network or device to maximize its data transmit rate while minimizing the portion of available transmit time that it used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages in accordance with the present invention. 
         FIG. 1  is a block diagram of a wireless network according to a disclosed embodiment of the present invention; 
         FIG. 2  is a block diagram of a TDMA scheme including superframes, time slots, and nominal slots, according to a disclosed embodiment of the present invention; 
         FIG. 3  is a signal diagram of An exemplary nominal slot according to a disclosed embodiment of the present invention; 
         FIG. 4  is a block diagram of different implementations of nominal slots, according to disclosed embodiments of the present invention; and 
         FIG. 5  is a block diagram of a local network having associated child and neighbor networks, according to a disclosed embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Wireless Network 
       FIG. 1  is a block diagram of a wireless network  100  according to a disclosed embodiment of the present invention. In this embodiment the network  100  is a wireless personal area network (WPAN), or piconet. However, it should be understood that the present invention also applies to other settings where bandwidth is to be shared among several users, such as, for example, wireless local area networks (WLAN), or any other appropriate wireless network. 
     When the term piconet is used, it refers to a network of devices connected in an ad hoc fashion, having one device act as a coordinator (i.e., it functions as a server) while the other devices (sometimes called stations) follow the time allocation instructions of the coordinator (i.e., they function as clients). The coordinator can be a designated device, or simply one of the devices chosen to function as a coordinator. One primary difference between the coordinator and non-coordinator devices is that the coordinator must be able to communicate with all of the devices in the network, while the various non-coordinator devices need not be able to communicate with all of the other non-coordinator devices. 
     As shown in  FIG. 1 , the network  100  includes a coordinator  110  and a plurality of devices  121 - 125 . The coordinator  110  serves to control the operation of the network  100 . As noted above, the system of coordinator  110  and devices  121 - 125  may be called a piconet, in which case the coordinator  110  may be referred to as a piconet coordinator (PNC). Each of the non-coordinator devices  121 - 125  must be connected to the coordinator  110  via primary wireless links  130 , and may also be connected to one or more other non-coordinator devices  121 - 125  via secondary wireless links  140 , also called peer-to-peer links. 
     In addition, although  FIG. 1  shows bi-directional links between devices, they could also be shown as unidirectional links. In this case, each bi-directional link  130 ,  140  could be shown as two unidirectional links, the first going in one direction and the second going in the opposite direction. 
     In some embodiments the coordinator  110  may be the same sort of device as any of the non-coordinator devices  121 - 125 , except with the additional functionality for coordinating the system, and the requirement that it communicate with every device  121 - 125  in the network  100 . In other embodiments the coordinator  110  may be a separate designated control unit that does not function as one of the devices  121 - 125 . 
     In some embodiments the coordinator  110  will be a device just like the non-coordinator devices  121 - 125 . In other embodiments the coordinator  110  could be a separate device dedicated to that function. Furthermore, individual non-coordinator devices  121 - 125  could include the functional elements of a coordinator  110 , but not use them, functioning as non-coordinator devices. This could be the case where any device is a potential coordinator  110 , but only one actually serves that function in a given network. 
     Each device of the network  100  may be a different wireless device, for example, a digital still camera, a digital video camera, a personal data assistant (PDA), a digital music player, or other personal wireless device. 
     The various non-coordinator devices  121 - 125  are confined to a usable physical area  150 , which is set based on the extent to which the coordinator  110  can successfully communicate with each of the non-coordinator devices  121 - 125 . Any non-coordinator device  121 - 125  that is able to communicate with the coordinator  110  (and vice versa) is within the usable area  150  of the network  100 . As noted, however, it is not necessary for every non-coordinator device  121 - 125  in the network  100  to communicate with every other non-coordinator device  121 - 125 . 
     Time Division Multiple Access (TDMA) Scheme 
     The available bandwidth in a given network  100  may be split up in time by the coordinator  110  into a series of repeated superframes. These superframes define how the available transmission time is split up among various tasks. Individual frames of data are then transferred within these superframes in accordance with the timing set forth in the superframe. 
       FIG. 2  is a block diagram of a TDMA scheme including superframes, time slots and nominal slots, according to a disclosed embodiment of the present invention. As shown in  FIG. 2 , the available transmission time  200  is broken up into a plurality of consecutive superframes  210 . Each individual superframe  210  in this embodiment includes a beacon period  220 , a contention access period (CAP)  230 , and a contention free period (CFP)  240 . The contention free period  340  is further broken up into a plurality of assigned time slots  250 . Each assigned time slot may be further divided up into a plurality of nominal time slots  260 . 
     The beacon period  220  is set aside for the coordinator  110  to send a beacon frame out to the non-coordinator devices  121 - 125  in the network  100 . Such a beacon frame will include information for organizing the operation of devices within the superframe  210 . Each non-coordinator device  121 - 125  knows how to recognize a beacon period  220  prior to joining the network  100 , and uses the beacon  220  both to identify an existing network  100  and to coordinate communication within the network  100 . 
     The beacon frame provides information required by the devices  121 - 125  in the network  100  regarding how the individual assigned time slots  250  and nominal time slots  260  will be allocated. In particular, it notes how and when devices  110 ,  121 - 125  can transmit to prevent any two devices from interfering. 
     The CAP  230  is used to transmit commands or asynchronous data across the network  100 . The CAP  230  may be eliminated in many embodiments and the system would then pass commands solely during the CFP  240 . 
     The CFP  240  includes a plurality of time slots  250 . These time slots  250  are each assigned by the coordinator  110  to one or more transmitting devices  110 ,  121 - 125  and one or more receiving devices  110 ,  121 - 125  for transmission of information between them. Generally each transmitting device will have a single associated receiver, through in some cases a single transmitter will transmit to multiple receivers at the same time. 
     The time slots  250  are provided to allow communication between devices  120 ,  121 - 125 . They do so in accordance with the information set forth in the beacon  220 . The size of the time slots  250  can vary by embodiment, but it should be large enough to transmit one or more data frames. 
     As noted above, each time slot  250  may also be broken up into multiple nominal time slots  260 . The nominal time slots  260  are set to be less than or equal to a nominal size. In one embodiment this nominal size is the length over which a regulatory body (such as the FCC) measures the power of a device  110 ,  121 - 125 . However, in alternate embodiments the nominal size may be changed to any suitable value. 
     In a situation where the length of a nominal time slot  260  is equal to that of an assigned time slot  250 , then the assigned time slot  250  would contain only a single nominal time slot  260 , making the assigned time slot  250  equivalent to the single nominal time slot  260 . 
     Although the embodiments described in this document are in the context of a WPAN (or piconet), it should be understood that the present invention also applies to other settings where bandwidth is to be shared among several users, such as, for example, wireless local area networks (WLAN), other appropriate wireless network, or any wired or wireless transmission scheme in which bandwidth must be shared. 
     The superframes  210  are fixed time constructs that are repeated in time. The specific duration of the superframe  210  is described in the beacon  220 . In fact, the beacon  220  generally includes information regarding how often the beacon  220  is repeated, which effectively corresponds to the duration of the superframe  210 . The beacon  220  also contains information regarding the network  100 , such as the identity of the transmitters and receivers assigned to each assigned time slot  250  and each nominal time slot  260 , the necessary transmission parameters for signals withing a nominal time slot  260 , and the identity of the coordinator  110 . 
     The system clock for the network  100  is preferably synchronized through the generation and reception of the beacons  220 . Each non-coordinator device  121 - 125  will store a synchronization point time upon successful reception of a valid beacon  220 , and will then use this synchronization point time to adjust its own timing. 
     Although not shown in  FIG. 2 , there may be guard times interspersed between assigned time slots  250  and between nominal time slots  260 . Guard times are used in TDMA systems to prevent two transmissions from overlapping in time because of inevitable errors in clock accuracies and differences in propagation times based on spatial positions. 
     Nominal Time Slots 
       FIG. 3  is a signal diagram of an exemplary nominal slot according to a disclosed embodiment of the present invention. The nominal time slot is formed to allow multiple devices to transmit during a single nominal time slot without any of them transmitting too much power. 
     As noted above, the FCC has imposed limits on the allowable transmit power for a given UWB device over a set measurement time T M , as well as the allowable peak-to-average ratio. In particular, they have required that the average power of a UWB transmission (using root-mean-square averaging) must be below −41.3 dBm/MHz, averaged over 1 millisecond, and that the peak-to-average power must be below 0 dBm/50 MHz peak. In other words under these rules, the output power of a UWB device is measured in 1 microsecond blocks (corresponding to 1 MHz), but the measured power is averaged over one millisecond. Furthermore, this averaged output power must be 41.3 dB below a milliwatt. In addition, the highest peak power of this transmission must be below 1 milliwatt for any 50 MHz bandwidth window across the whole of the usable bandwidth. It is expected that regulatory agencies in other countries will impose similar restrictions. 
     One way to meet this maximum power limitation is to transmit a constant UWB signal  310  at 100% duty cycle over the measurement period T M  (which is used to define the nominal time slot length in this embodiment), and set the power level of the constant UWB signal  310  such that it will not violate the power restrictions set forth by the relevant regulatory agency. However, given such a transmission scheme, the peak-to-average power ratio is likely to be relatively small compared to the regulated limit, e.g., only about 3:1. In other words, the device will not be transmitting at as high a peak-to-average ratio as it is allowed to. 
     Another way to meet the regulatory limitations is to have a first device transmit a first UWB signal  320  at a lowered duty cycle (i.e., below 100% duty cycle), including a first gated-on portion  323  (i.e., a non-zero portion including one or more wavelets) and a first gated-off portion  326  (i.e., a zeroed portion that includes no wavelets). As the duty cycle is reduced, the magnitude of the first gated-on portion  323  can be increased to maintain the same average power level (i.e., one that approaches but does not exceed the regulatory limit). The magnitude of the first gated-on portion  323  can then be increased (with a corresponding reduction in duty cycle) until any limit of peak-to-average ratio is reached. If no limit has been set on the peak-to-average ratio, then the lower limit on available duty cycle will likely be a hardware limit, i.e., how low the duty cycle can go and still have the signal function as required. 
     Now, since the first UWB signal  320  has a duty cycle lower than 100%, it includes the first gated-off portion  326  in part of the measurement time. During this first gated-off portion  326 , the current device is not transmitting anything, leaving the transmission channel empty. This allows for a secondary TDMA scheme to be used within the measurement time duration, taking advantage of this first gated-off portion  326  during which no signals are transmitted. In this secondary TDMA scheme, one or more devices are allowed to transmit during the first gated-off portion  326 . 
     For example, a second device can transmit a second UWB signal  330  also at a lowered duty cycle (i.e., below 100% duty cycle). This second UWB signal  330  will have a second gated-on portion  333  and a second gated-off portion  336 , and will also meet the regulatory limitations regarding maximum power and peak-to-average ratio. The second gated-on portion  333  can then be arranged such that it overlaps the first gated-off portion  326 , allowing for no interference between the two UWB signals  320  and  330 . 
     Then, if there is space, additional devices can transmit additional UWB signals. For example, in the embodiment of  FIG. 3 , a third device transmits a third UWB signal  340 , also at a lowered duty cycle (i.e., below 100% duty cycle). This third UWB signal  340  will have a third gated-on portion  343  and a third gated-off portion  346 , and also meets the regulatory limitations regarding maximum power and peak-to-average ratio. The third gated-on portion  343  is arranged such that it overlaps the first gated-off portion  326  and the second gated-on portion  336 , allowing for no interference between the three UWB signals  320 ,  330 , and  340 . 
     In addition, the gated-on portions  323 ,  333 , and  343  can be arranged such that guard times T G  are provided between adjacent gated-on portions. These guard times T G  can prevent two adjacent gated-on portions from overlapping in time because of inevitable errors in clock accuracies and differences in propagation times based on spatial positions. And while shown to be a uniform value in all cases in this embodiment, in alternate embodiments the value of the guard time T G  can be varied within a nominal time slot  260 . 
     In the embodiment disclosed in  FIG. 3 , three devices transmit first through third UWB signals  320 ,  330 , and  340 , respectively, during a nominal time slot  260  defined in length by a measurement time T M . Each UWB signal  320 ,  330 , and  340  is transmitted at a 25% duty cycle, providing first through third gated-on portions  323 ,  333 , and  343  having a duration equal to a reduced duty cycle time T RDC  that is ¼ of the measurement time T M . The first gated-on portion  323  is arranged such that it overlaps the second and third gated-off portions  336  and  346 ; the second gated-on portion  333  is arranged such that it overlaps the first and third gated-off portions  326  and  346 ; and the third gated-on portion  343  is arranged such that it overlaps the first and second gated-off portions  326  and  336 . A guard time T G  is provided between each gated-on portion  323 ,  333 , and  343 . 
     As a result of this, when the first through third UWB signals  320 ,  330 , and  340  are transmitted at the same time, they form a combined and interleaved UWB signal  350  filling the available transmission medium. However, since the first through third gated-on portions  323 ,  333 , and  343  are arranged such that they don&#39;t overlap, the three devices can transmit during the same nominal time slot  260  (i.e., over the same measurement time T M ) without interfering with each other. 
     And if the measurement time T M  is chosen to be less than or equal to the measurement time used by the appropriate regulatory agency to measure maximum allowable transmit power, then all of the first through third UWB signals  320 ,  330 , and  340  will be compliant with power restrictions if they keep their individual total power values below the regulatory threshold. 
     Furthermore, since the nominal time slot  260  is the smallest increment into which the available channel time is divided, if two or more devices are assigned to transmit during the same nominal time slot  260 , they are effectively transmitting at the same time since neither precludes the other from transmitting. In this way both devices can use the entire available channel bandwidth without interfering. 
     Although  FIG. 3  discloses an embodiment in which three devices each transmit during a measurement time T M  (i.e., a nominal time slot  260 ) at a 25% duty cycle, alternate embodiments can alter the number of transmitting devices and the precise duty cycles chosen. For example the nominal time slot  260  may be split between only two devices or more than three devices. 
     Furthermore, the separate UWB signals  320 ,  330 ,  340  need not have the same duty cycle. All that is required is that each device transmit at a duty cycle less than 100% (i.e., each device allows for some gated-off portion), and that the sum of all of the duty cycles be less than or equal to 100% (i.e., the total size of gated-on portions is such that they can be arranged not to overlap). And if guard times T G  are to be used, the available total duty cycle allowed for gated-on portions will have to be reduced by an appropriate amount to provide for the guard times T G . 
     It should be noted that in this embodiment the size of a nominal time slot  260  is determined by the measurement time T M  employed by the relevant regulatory agency. For example, the FCC has currently set a measurement time T M  of one millisecond. However, they could change this measurement time T M , or other agencies in other jurisdictions could use a different value. Regardless, the size of the nominal time slot  260  can be varied accordingly, with the amount of wavelets in the relevant gated-on portions  323 ,  333 ,  343  being higher or lower accordingly. 
     Alternate embodiments could employ a nominal time slot  260  greater in size than the regulatory measurement time T M . However, in order to meet the regulatory total power requirements, it will be necessary to further reduce allowable duty cycles such that no UWB transmission violates the power limits. 
     Furthermore, although  FIG. 3  shows only a repeated sine wave for each portion of the various UWB signals  310 ,  320 ,  330 , and  340 , this is by way of example only. In different embodiments the UWB signals can include different wavelet shapes, different numbers of wavelets, and the wavelets can be encoded with digital data. However, the selection of duty cycle and orientation of gated on and gated off portions will be analogous to the procedure shown with respect to  FIG. 3 . 
       FIG. 4  is a block diagram of different implementations of nominal slots, according to disclosed embodiments of the present invention. The block diagram of  FIG. 4  represents the cumulative power of a given signal by a shaded box. As a result, this diagram shows only the gated-on portions of respective UWB signals. 
     As shown in  FIG. 4 , the size and shape of gated-on portions can vary within a single nominal time slot.  FIG. 4  shows three exemplary nominal time slots  410 ,  420 , and  440 . 
     A first exemplary nominal time slot  410  includes four identical first gated-on portions  405  arranged in an interleaved fashion such that each has a guard time  415  separating it from any other first gated-on portion  405 . 
     A second exemplary nominal time slot  420  includes different first through fourth gated-on portions  405 ,  425 ,  430 , and  435 . Each of these gated-on portions  405 ,  425 ,  430 , and  435  is designed to have a duty cycle and maximum power such that it has as great a total power over the length of the nominal time slot  420  as possible, while not violating any regulatory requirements of total power or peak-to-average ratio. 
     One possible reason for the variety in the gated-on portions  405 ,  425 ,  430 , and  435  may be the evolution of UWB devices. In an early implementation it may be that the UWB signals can only be made to be a set minimum duty cycle. But as UWB technology advances, smaller duty cycles, higher power signals can be implemented. The present invention will accommodate this, since the individual UWB signals need not be identical to operate within a nominal time slot  260 . Older UWB signals can be transmitted alongside new UWB signals provided they are arranged not to overlap their gated-on portions. 
     In fact a third exemplary nominal time slot  440  shows that as duty cycles are reduced, and the gated-on portions  425  and  435  of the UWB signals are reduced in width, a greater number of UWB signals can be fit into the same nominal time slot  260 . In the third exemplary nominal time slot  440  shown in  FIG. 4 , eight separate gated-on portions  425  and  435  from eight separate UWB signals are shown. In this way eight separate devices could transmit during the same nominal time slot  260 . 
     System Operation 
     The use of nominal time slots can be particularly effective when a local network has a neighbor network or a child network to accommodate. Because they are so close in space, neighbor networks or parent/child networks must share an available channel with a local network. Under normal circumstances, this means that each of the network networks (local, neighbor, child) must accept a fraction of the available channel time in order to avoid interfering. However, by using interleaved, reduced duty cycle UWB transmissions in nominal time slots, it is possible for each network to effectively use all or most of the available channel time. 
       FIG. 5  is a block diagram of a local network having associated child and neighbor networks, according to a disclosed embodiment of the present invention. As shown in  FIG. 5 , a local network  100   a  is provided with a child network  100   b  formed around a device in the local network  100   a , and a neighbor network  100   c  formed adjacent to the local network  100   a.    
     The local network  100   a  includes a local coordinator  110   a , and first through fifth local non-coordinator devices  121   a - 125   a . As noted above with respect to  FIG. 1 , the local coordinator  110   a  communicates with each of the local non-coordinator devices  121   a - 125   a  via primary local wireless links  130   a , while the local non-coordinator devices  121   a - 125   a  communicate with each other via secondary local wireless links  140   a.    
     The child network  100   b  includes a child coordinator  110   b , and first and second child non-coordinator devices  121   b  and  122   b . In this example, the child coordinator  110   b  is the same as the third local device  123   a . As noted above with respect to  FIG. 1 , the child coordinator  110   b  communicates with the child non-coordinator devices  121   b  and  122   b  via primary child wireless links  130   b , while the child non-coordinator devices  121   b  and  122   b  communicate with each other via secondary child wireless links  140   b.    
     The neighbor network  100   c  includes a neighbor coordinator  110   c , and first and second neighbor non-coordinator devices  121   c  and  122   c . As noted above with respect to  FIG. 1 , the neighbor coordinator  110   c  communicates with the neighbor non-coordinator devices  121   c  and  122   c  via primary neighbor wireless links  130   c , while the neighbor non-coordinator devices  121   c  and  122   c  communicate with each other via secondary neighbor wireless links  140   c . In addition, the neighbor coordinator  110   c  communicates with the local coordinator  110   a  through a tertiary wireless link  560 . 
     This exemplary set of overlapping networks illustrates several situations in which interleaved, reduced duty cycle UWB signals in nominal time slots can be used to share a channel. 
     One way to share a channel is between devices in a single network. Using a conventional TDMA scheme, if first and second local devices  121   a  and  122   a  wanted to pass data, and fourth and fifth local devices  124   a  and  125   a  wanted to pass data, they would each have to do so in different assigned time slots  250  to avoid interfering with each other. However, using the reduced duty cycle transmissions described above, each device pair in the above example could be assigned a different position in a nominal time slot  260 , and the two transmissions could take place during the same assigned time slot  250 . 
     For example, the first and second local devices  121   a  and  122   a  could be assigned a first position in a nominal time slot  260  for transmitting gated-on portions of reduced duty cycle UWB signals, while the fourth and fifth local devices  124   a  and  125   a  could be assigned a second position in a nominal time slot  260  for transmitting gated-on portions of reduced duty cycle UWB signals. If the time slot were arranged as shown by way of example in  FIG. 3 , the first position could correspond to the first UWB signal  320  and the second position could correspond to the second UWB signal  330 . 
     Similarly, the channel can be shared between devices in parent and child networks. Using a conventional TDMA scheme, if first and second local devices  121   a  and  122   a  wanted to pass data, and first and second child devices  121   b  and  122   b  wanted to pass data, they would have to do so in different assigned time slots  250  to avoid interfering with each other. However, using the reduced duty cycle transmissions described above, each device pair in the above example could be assigned a different position in a nominal time slot  260 , and the two transmissions could take place during the same assigned time slot  250 . 
     As above, the first and second local devices  121   a  and  122   a  could be assigned a first position in a nominal time slot  260  for transmitting gated-on portions of reduced duty cycle UWB signals, while the first and second child devices  121   b  and  122   b  could be assigned a second position in a nominal time slot  260  for transmitting gated-on portions of reduced duty cycle UWB signals. Again, if the time slot were arranged as shown by way of example in  FIG. 3 , the first position could correspond to the first UWB signal  320  and the second position could correspond to the second UWB signal  330 . 
     Likewise, the channel can be shared between devices in neighbor networks. Using a conventional TDMA scheme, if first and second local devices  121   a  and  122   a  wanted to pass data, and first and second neighbor devices  121   c  and  122   c  wanted to pass data, they would have to do so in different assigned time slots  250  to avoid interfering with each other. However, using the reduced duty cycle transmissions described above, each device pair in the above example could be assigned a different position in a nominal time slot  260 , and the two transmissions could take place during the same assigned time slot  250 . 
     As above, the first and second local devices  121   a  and  122   a  could be assigned a first position in a nominal time slot  260  for transmitting gated-on portions of reduced duty cycle UWB signals, while the first and second neighbor devices  121   c  and  122   c  could be assigned a second position in a nominal time slot  260  for transmitting gated-on portions of reduced duty cycle UWB signals. Again, if the time slot were arranged as shown by way of example in  FIG. 3 , the first position could correspond to the first UWB signal  320  and the second position could correspond to the second UWB signal  330 . 
     In any of these cases, all that would be required to arrange a proper sharing of the channel time would be for all of the coordinators  110   a ,  110   b , and  110   c  to pass information regarding which network and which devices were assigned which positions within a nominal time slot  260 . 
     In fact, in some embodiments it would be possible to have all four transmissions (two within the local network  100   a , one within the child network  100   b , and one within the neighbor network  100   c ) take place at once, providing the nominal time slots  260  could accommodate four interleaved UWB signals. 
     Although the above examples are shown as being related to a wireless channel, alternate embodiments can apply to any situation in which a limited data channel must be shared among multiple devices or networks. 
     Also, although the above examples are shown as being used with UWB signals, this scheme is also applicable to any TDMA transmission scheme. 
     CONCLUSION 
     This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled. The various circuits described above can be implemented in discrete circuits or integrated circuits, as desired by implementation.