PATENT DOCUMENT

Publication Number: US-10644744-B2
Application Number: US-201816106354-A
Country: US
Kind Code: B2

Title: Methods and apparatus for ultra wideband multiuser interference reduction

Abstract:
Methods and apparatuses are presented to reduce multiuser interference resulting from two or more overlapping ultra wideband (UWB) transmissions by randomizing the start time of packets and/or bursts within the packets. A random offset time may be generated for a packet, and transmission of the packet may be arbitrarily delayed by that random offset time, relative to an earlier time at which the packet is prepared for transmission. A random offset time may be generated for a pulse burst within a symbol of a packet, and transmission of the burst may be delayed by that random offset time, relative to a nominal transmission window within the symbol. The burst may therefore occupy a portion of a guard period following the nominal transmission window. Either procedure, or both procedures, may be used to reduce multiuser interference between two concurrently transmitted packets by randomizing overlap occurring between the bursts.

Claims:
What is claimed is: 
     
       1. A method comprising:
 by a wireless communication device:
 generating a frame comprising a plurality of data bursts, wherein each data burst of the plurality of data bursts is associated with a respective nominal transmission window followed by a respective nominal guard window; 
 for each data burst of the plurality of data bursts, generating a respective random offset time having a value not greater than the length of the respective guard window; and 
 transmitting the frame to a receiving device, wherein each data burst of the plurality of data bursts is transmitted at a time offset by the respective offset time from the start of the respective nominal transmission window, such that transmission of the data burst is completed during the respective guard window. 
 
 
     
     
       2. The method of  claim 1 , wherein the respective random offset time is known to the receiving device. 
     
     
       3. The method of  claim 1 , wherein, for each data burst of the plurality of data bursts, the respective random offset time has a value not greater than a predetermined fraction of the respective guard window. 
     
     
       4. The method of  claim 1 , wherein generating the respective random offset comprises executing a pseudorandom number generation algorithm known to both the wireless communication device and the receiving device. 
     
     
       5. The method of  claim 1 , wherein generating the respective random offset comprises selecting a value from a sequence of values known to both the wireless communication device and the receiving device. 
     
     
       6. The method of  claim 5 , wherein the sequence of values comprises a sequence defined by a communication protocol implemented by the wireless communication device. 
     
     
       7. A wireless communication device comprising:
 at least one antenna; and 
 communication circuitry coupled to the at least one antenna and configured to:
 encode first payload data as a first burst of pulses within a first symbol comprising a nominal transmit time window followed by a nominal guard time window; 
 generate a first random offset time having a value no greater than the length of the nominal guard time window; and 
 transmit the first symbol, wherein transmitting the first symbol comprises transmitting the first burst of pulses at a time that is offset by the first random offset time from a start time of the nominal transmit time window. 
 
 
     
     
       8. The wireless communication device of  claim 7 , wherein the communication circuitry is further configured to:
 encode second payload data as a second burst of pulses within a second symbol comprising a nominal transmit time window followed by a nominal guard time window; 
 generate a second random offset time having a value no greater than the length of the nominal guard time window of the second symbol, wherein the value of the second random offset time is independent from the value of the first random offset time; and 
 transmit the second symbol, the second symbol immediately following the first symbol, wherein transmitting the second symbol comprises transmitting the second burst of pulses at a time that is offset by the second random offset time from a start time of the nominal transmit time window of the second symbol. 
 
     
     
       9. The wireless communication device of  claim 7 , wherein the communication circuitry is further configured to:
 transmit a frame comprising a plurality of symbols, the plurality of symbols comprising the first symbol, wherein each symbol of the plurality of symbols comprises a respective nominal transmit time window followed by a respective nominal guard time window, wherein each symbol comprises a respective burst of pulses offset from the respective nominal transmit time window by a respective random offset time. 
 
     
     
       10. The wireless communication device of  claim 7 , wherein the first burst of pulses occupies a portion of the nominal guard time window of the first symbol as a result of the offset. 
     
     
       11. The wireless communication device of  claim 7 , wherein the first random offset time has a value no greater than a predetermined fraction of the length of the nominal guard time window. 
     
     
       12. A method comprising:
 by a wireless communication device:
 generating a first packet comprising a plurality of data bursts, wherein each data burst of the plurality of data bursts is associated with a respective nominal transmission window followed by a respective nominal guard window, such that the generated first packet is prepared for transmission at a first time; 
 generating a first random offset; and 
 transmitting the first packet at a second time that is after the first time by the first random offset, such that transmitting the first packet concludes within the nominal guard window, wherein transmitting the first packet at the second time randomizes overlap of the data bursts of the first packet with data bursts of a concurrent packet transmitted by another device. 
 
 
     
     
       13. The method of  claim 12 , further comprising:
 generating a second packet, such that the generated second packet is prepared for transmission at a third time; 
 generating a second random offset that is independent from the first random offset; and 
 transmitting the second packet at a fourth time that is after the third time by the second random offset. 
 
     
     
       14. The method of  claim 13 , further comprising:
 receiving a notification that the first packet was not successfully received, wherein generating the second packet comprises generating a copy of the first packet for retransmission. 
 
     
     
       15. The method of  claim 12 , further comprising:
 generating a periodic series of packets at regular intervals, wherein the first packet is comprised in the periodic series of packets. 
 
     
     
       16. The method of  claim 15 , further comprising:
 transmitting each packet in the periodic series of packets at a respective time that is delayed by a respective random offset following generation of the packet. 
 
     
     
       17. The method of  claim 15 , wherein the periodic series of packets comprises audio packets. 
     
     
       18. The method of  claim 12 , wherein the first random offset has a value no greater than one symbol length. 
     
     
       19. The method of  claim 12 , wherein the wireless communication device performs the transmitting the first packet at the second time without performing a carrier-sense multiple access (CSMA) procedure. 
     
     
       20. The method of  claim 12 , wherein transmitting the first packet comprises:
 for each data burst of the plurality of data bursts, generating a respective random offset time having a value no greater than the length of the respective guard window; and 
 transmitting the first packet, wherein each data burst of the plurality of data bursts is transmitted at a time offset by the respective offset time from the start of the respective nominal transmission window, such that transmission of the data burst is completed during the respective guard window.

Description:
TECHNICAL FIELD 
     The present application relates to wireless communication, including to techniques for reducing multiuser interference in pulse-based wireless communication. 
     DESCRIPTION OF THE RELATED ART 
     Wireless communication systems are rapidly growing in usage. Further, wireless communication technology has evolved from voice-only communications to also include the transmission of data, such as Internet and multimedia content. 
     Mobile electronic devices may take the form of smart phones or tablets that a user typically carries. Wearable devices (also referred to as accessory devices) are a newer form of mobile electronic device, one example being smart watches. Additionally, low-cost low-complexity wireless devices intended for stationary or nomadic deployment are also proliferating as part of the developing “Internet of Things”. In other words, there is an increasingly wide range of desired device complexities, capabilities, traffic patterns, and other characteristics. 
     Ultra wideband (UWB) communications, such as the wireless communication protocol defined by IEEE 802.15.4, may provide low-cost, low-power, short-range communications over a very large bandwidth. For example, UWB may utilize a bandwidth in excess of 500 MHz. UWB may offer multiple advantages compared to narrower band radio communication technologies. For example, UWB communications may be less susceptible to multipath fading. Specifically, the short duration of UWB pulses makes them less susceptible to multipath effects compared to narrowband signals. As another example, UWB communications may operate with a very low (negative) signal-to-noise ratio (SNR) and exhibit excellent immunity to noise. Thus, UWB can easily co-exist with other narrowband and wideband systems. As yet another example, UWB communications may provide enhanced communication security because their signals can be buried below the noise floor of conventional receivers, due to their wide bandwidth and low SNR. This makes them difficult to detect by nefarious third-parties. 
     Because UWB was developed primarily as a low-power, short-range, low-throughput technology, previous implementations typically did not include robust solutions for avoiding multi-user interference. Accordingly, improvements in the field are desired. 
     SUMMARY 
     Embodiments are presented herein of, inter alia, systems, apparatuses, and methods for reducing multiuser interference in Ultra Wideband (UWB) communications or other pulse-based communications. 
     According to the techniques described herein, multiuser interference between two or more overlapping transmissions may be reduced by randomizing the start time of packets and/or bursts within the packets. 
     According to some embodiments, a wireless communication device may transmit a frame including a plurality of bursts, with each burst having a random time offset. 
     For example, the wireless communication device may generate a frame including a plurality of data bursts, wherein each data burst of the plurality of data bursts may be associated with a respective nominal transmission window followed by a respective nominal guard window. For each data burst of the plurality of data bursts, the wireless communication device may generate a respective random offset time having a value not greater than the length of the respective guard window. In some scenarios, for each data burst of the plurality of data bursts, the respective random offset time may have a value not greater than a predetermined fraction of the respective guard time. The wireless communication device may transmit the frame to a receiving device, wherein each data burst of the plurality of data bursts may be transmitted at a time offset by the respective offset time from the start of the respective nominal transmission window, such that transmission of the data burst is completed during the respective guard window. 
     In some scenarios, the random offsets may be known to the receiving device. For example, in some scenarios, generating the respective random offset may include executing a pseudorandom number generation algorithm known to both the wireless communication device and the receiving device. In some scenarios, generating the respective random offset may include selecting a value from a sequence of values known to both the wireless communication device and the receiving device. The sequence of values may be a sequence defined by a communication protocol implemented by the wireless communication device. 
     As another example, a wireless communication device may include at least one antenna and communication circuitry coupled to the at least one antenna. The communication circuitry may be configured to encode first payload data as a first burst of pulses within a first symbol. The first symbol may include a nominal transmit time window followed by a nominal guard time window. The communication circuitry may be further configured to generate a first random offset time having a value not greater than the length of the nominal guard time window. In some scenarios, the first random offset time may have a value not greater than a predetermined fraction of the length of the nominal guard time window. The communication circuitry may be further configured to transmit the first symbol. Transmitting the first symbol may include transmitting the first burst of pulses at a time that is offset by the first random offset time from a start time of the nominal transmit time window. 
     In some scenarios, the first burst of pulses may occupy a portion of the first nominal guard time window as a result of the offset. 
     In some scenarios, the communication circuitry may be further configured to transmit a second symbol. For example, the communication circuitry may be further configured to encode second payload data as a second burst of pulses within a second symbol including a nominal transmit time window followed by a nominal guard time window; generate a second random offset time having a value not greater than the length of the nominal guard time window of the second symbol, wherein the value of the second random offset time may be independent from the value of the first random offset time; and transmit the second symbol immediately following transmitting the first symbol, wherein transmitting the second symbol may include transmitting the second burst of pulses at a time that is offset by the second random offset time from a start time of the nominal transmit time window of the second symbol. 
     In some scenarios, the communication circuitry may be further configured to transmit a frame including a plurality of symbols, including the first symbol. Each symbol of the plurality of symbols may include a respective nominal transmit time window followed by a respective nominal guard time window, wherein each symbol may include a respective burst of pulses offset from the respective nominal transmit time window by a respective random offset time. 
     According to some embodiments, a wireless communication device may transmit one or more packets that are arbitrarily offset in time. 
     For example, a wireless communication device may generate a first packet, such that the generated first packet is prepared for transmission at a first time. The wireless communication device may generate a first random offset, and transmit the first packet at a second time that is after the first time by the first random offset. 
     In some scenarios, the first random offset may have a value not greater than one symbol length. 
     In some scenarios, the wireless communication device may perform the transmitting the first packet at the second time without performing a carrier-sense multiple access (CSMA) procedure. 
     In some scenarios, the wireless communication device may also transmit a second packet. For example, the wireless communication device may generate a second packet, such that the generated second packet is prepared for transmission at a third time; generate a second random offset that may be independent from the first random offset; and transmit the second packet at a fourth time that is after the third time by the second random offset. 
     In some scenarios, the wireless communication device may further receive a notification that the first packet was not successfully received, wherein generating the second packet may include generating a copy of the first packet for retransmission. 
     In some scenarios, the wireless communication device may generate a periodic series of packets at regular intervals, wherein the first packet is included in the periodic series of packets. In some such scenarios, the wireless communication device may further transmit each packet in the periodic series of packets at a respective time that is delayed by a respective random offset following generation of the packet. In some such scenarios, the periodic series of packets may include audio packets. 
     In some scenarios, the wireless communication device may randomize both packet start time and burst start time. 
     This summary is intended to provide a brief overview of some of the subject matter described in this document. Accordingly, it will be appreciated that the above-described features are merely examples and should not be construed to narrow the scope or spirit of the subject matter described herein in any way. Other features, aspects, and advantages of the subject matter described herein will become apparent from the following Detailed Description, Figures, and Claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present subject matter can be obtained when the following detailed description of the embodiments is considered in conjunction with the following drawings. 
         FIG. 1  illustrates an example wireless communication system, according to various exemplary embodiments described herein. 
         FIGS. 2-3  are block diagrams illustrating example wireless devices, according to various exemplary embodiments described herein. 
         FIG. 4  illustrates a waveform representing an UWB frame that can be used to perform communications, according to various exemplary embodiments described herein. 
         FIG. 5  illustrates an example symbol format for a symbol that may be included within the UWB frame of  FIG. 4 , according to various exemplary embodiments described herein. 
         FIG. 6  illustrates normalized cross-correlation between a first symbol and a second, overlapping symbol transmitted at various offsets, according to various exemplary embodiments described herein. 
         FIG. 7  shows a flow chart of an example implementation of burst start randomization, according to various exemplary embodiments described herein. 
         FIG. 8  illustrates two symbols in each of two frames transmitted according to the procedure of  FIG. 7 , according to various exemplary embodiments described herein. 
         FIG. 9  shows a chart of simulation results illustrating packet error rates for UWB wireless devices at different SNRs, according to various exemplary embodiments described herein. 
         FIG. 10  shows a chart of simulation results illustrating packet error rates for UWB wireless devices implementing burst start randomization at different SNRs, according to various exemplary embodiments described herein. 
         FIG. 11  shows a flow chart of an example implementation of packet start randomization, according to various exemplary embodiments described herein. 
     
    
    
     While the features described herein are susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to be limiting to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the subject matter as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Terminology 
     The following are definitions of terms used in this disclosure: 
     Memory Medium—Any of various types of non-transitory memory devices or storage devices. The term “memory medium” is intended to include an installation medium, e.g., a CD-ROM, floppy disks, or tape device; a computer system memory or random-access memory such as DRAM, DDR RAM, SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memory such as a Flash, magnetic media, e.g., a hard drive, or optical storage; registers, or other similar types of memory elements, etc. The memory medium may include other types of non-transitory memory as well or combinations thereof. In addition, the memory medium may be located in a first computer system in which the programs are executed, or may be located in a second different computer system which connects to the first computer system over a network, such as the Internet. In the latter instance, the second computer system may provide program instructions to the first computer for execution. The term “memory medium” may include two or more memory mediums which may reside in different locations, e.g., in different computer systems that are connected over a network. The memory medium may store program instructions (e.g., embodied as computer programs) that may be executed by one or more processors. 
     Carrier Medium—a memory medium as described above, as well as a physical transmission medium, such as a bus, network, and/or other physical transmission medium that conveys signals such as electrical, electromagnetic, or digital signals. 
     Programmable Hardware Element—includes various hardware devices including multiple programmable function blocks connected via a programmable interconnect. Examples include FPGAs (Field Programmable Gate Arrays), PLDs (Programmable Logic Devices), FPOAs (Field Programmable Object Arrays), and CPLDs (Complex PLDs). The programmable function blocks may range from fine grained (combinatorial logic or look up tables) to coarse grained (arithmetic logic units or processor cores). A programmable hardware element may also be referred to as “reconfigurable logic”. 
     Computer System—any of various types of computing or processing systems, including a personal computer system (PC), mainframe computer system, workstation, network appliance, Internet appliance, personal digital assistant (PDA), television system, grid computing system, or other device or combinations of devices. In general, the term “computer system” can be broadly defined to encompass any device (or combination of devices) having at least one processor that executes instructions from a memory medium. 
     User Equipment (UE) (or “UE Device”)—any of various types of computer systems devices which are mobile or portable and which performs wireless communications. Examples of UE devices include mobile telephones or smart phones (e.g., iPhone™, Android™-based phones), portable gaming devices (e.g., Nintendo DS™ PlayStation Portable™, Gameboy Advance™, iPhone™), laptops, wearable devices (e.g. smart watch, smart glasses), PDAs, portable Internet devices, music players, data storage devices, or other handheld devices, etc. In general, the term “UE” or “UE device” can be broadly defined to encompass any electronic, computing, and/or telecommunications device (or combination of devices) which is easily transported by a user and capable of wireless communication. 
     Wireless Device—any of various types of computer system devices which performs wireless communications. A wireless device can be portable (or mobile) or may be stationary or fixed at a certain location. A UE is an example of a wireless device. 
     Communication Device—any of various types of computer systems or devices that perform communications, where the communications can be wired or wireless. A communication device can be portable (or mobile) or may be stationary or fixed at a certain location. A wireless device is an example of a communication device. A UE is another example of a communication device. 
     Base Station—The term “Base Station” (also called “eNB”) has the full breadth of its ordinary meaning, and at least includes a wireless communication station installed at a fixed location and used to communicate as part of a wireless cellular communication system. 
     Link Budget Limited—includes the full breadth of its ordinary meaning, and at least includes a characteristic of a wireless device (e.g., a UE) which exhibits limited communication capabilities, or limited power, relative to a device that is not link budget limited, or relative to devices for which a radio access technology (RAT) standard has been developed. A wireless device that is link budget limited may experience relatively limited reception and/or transmission capabilities, which may be due to one or more factors such as device design, device size, battery size, antenna size or design, transmit power, receive power, current transmission medium conditions, and/or other factors. Such devices may be referred to herein as “link budget limited” (or “link budget constrained”) devices. A device may be inherently link budget limited due to its size, battery power, and/or transmit/receive power. For example, a smart watch that is communicating over LTE or LTE-A with a base station may be inherently link budget limited due to its reduced transmit/receive power and/or reduced antenna. Wearable devices, such as smart watches, are generally link budget limited devices. Alternatively, a device may not be inherently link budget limited, e.g., may have sufficient size, battery power, and/or transmit/receive power for normal communications over LTE or LTE-A, but may be temporarily link budget limited due to current communication conditions, e.g., a smart phone being at the edge of a cell, etc. It is noted that the term “link budget limited” includes or encompasses power limitations, and thus a power limited device may be considered a link budget limited device. 
     Processing Element (or Processor)—refers to various elements or combinations of elements. Processing elements include, for example, circuits such as an ASIC (Application Specific Integrated Circuit), portions or circuits of individual processor cores, entire processor cores, individual processors, programmable hardware devices such as a field programmable gate array (FPGA), and/or larger portions of systems that include multiple processors. 
     Wi-Fi—The term “Wi-Fi” has the full breadth of its ordinary meaning, and at least includes a wireless communication network or RAT that is serviced by wireless LAN (WLAN) access points and which provides connectivity through these access points to the Internet. Most modern Wi-Fi networks (or WLAN networks) are based on IEEE 802.11 standards and are marketed under the name “Wi-Fi”. A Wi-Fi (WLAN) network is different from a cellular network. 
     Automatically—refers to an action or operation performed by a computer system (e.g., software executed by the computer system) or device (e.g., circuitry, programmable hardware elements, ASICs, etc.), without user input directly specifying or performing the action or operation. Thus, the term “automatically” is in contrast to an operation being manually performed or specified by the user, where the user provides input to directly perform the operation. An automatic procedure may be initiated by input provided by the user, but the subsequent actions that are performed “automatically” are not specified by the user, i.e., are not performed “manually”, where the user specifies each action to perform. For example, a user filling out an electronic form by selecting each field and providing input specifying information (e.g., by typing information, selecting check boxes, radio selections, etc.) is filling out the form manually, even though the computer system must update the form in response to the user actions. The form may be automatically filled out by the computer system where the computer system (e.g., software executing on the computer system) analyzes the fields of the form and fills in the form without any user input specifying the answers to the fields. As indicated above, the user may invoke the automatic filling of the form, but is not involved in the actual filling of the form (e.g., the user is not manually specifying answers to fields but rather they are being automatically completed). The present specification provides various examples of operations being automatically performed in response to actions the user has taken. 
     Configured to—Various components may be described as “configured to” perform a task or tasks. In such contexts, “configured to” is a broad recitation generally meaning “having structure that” performs the task or tasks during operation. As such, the component can be configured to perform the task even when the component is not currently performing that task (e.g., a set of electrical conductors may be configured to electrically connect a module to another module, even when the two modules are not connected). In some contexts, “configured to” may be a broad recitation of structure generally meaning “having circuitry that” performs the task or tasks during operation. As such, the component can be configured to perform the task even when the component is not currently on. In general, the circuitry that forms the structure corresponding to “configured to” may include hardware circuits. 
     Various components may be described as performing a task or tasks, for convenience in the description. Such descriptions should be interpreted as including the phrase “configured to.” Reciting a component that is configured to perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112, paragraph six, interpretation for that component. 
       FIG. 1 —Wireless Communication System 
       FIG. 1  illustrates an exemplary (and simplified) wireless communication system  100  in which aspects of this disclosure may be implemented. It is noted that the system of  FIG. 1  is merely one example of a possible system, and embodiments of this disclosure may be implemented in any of various systems, as desired. 
     As shown, the exemplary wireless communication system includes a (“first”) wireless device  102  in communication with another (“second”) wireless device  104 . The first wireless device  102  and the second wireless device  104  may communicate wirelessly using any of a variety of wireless communication techniques, potentially including ultra wideband (UWB) communication techniques. 
     As one possibility, the first wireless device  102  and the second wireless device  104  may communicate using ultra wideband (UWB) communication technology (e.g., IEEE 802.15.4 WPAN communication) and/or other techniques based on WPAN or WLAN wireless communication. One or both of the wireless device  102  and the wireless device  104  may also be capable of communicating via one or more additional wireless communication protocols, such as any of Bluetooth (BT), Bluetooth Low Energy (BLE), WLAN (e.g., IEEE 802.11/Wi-Fi), near field communication (NFC), GSM, UMTS (WCDMA, TDSCDMA), LTE, LTE-Advanced (LTE-A), NR, 3GPP2 CDMA2000 (e.g., 1×RTT, 1×EV-DO, HRPD, eHRPD), Wi-MAX, GPS, etc. 
     The wireless devices  102 ,  104  may be any of a variety of types of wireless device. As one possibility, one or more of the wireless devices  102 ,  104  may be a substantially portable wireless user equipment (UE) device, such as a smart phone, hand-held device, a wearable device, a tablet, a motor vehicle, or virtually any type of wireless device. As another possibility, one or more of the wireless devices  102 ,  104  may be a substantially stationary device, such as a set top box, media player (e.g., an audio or audiovisual device), gaming console, desktop computer, appliance, door, or any of a variety of other types of device. 
     Each of the wireless devices  102 ,  104  may include wireless communication circuitry configured to facilitate the performance of wireless communication, which may include various digital and/or analog radio frequency (RF) components, a processor that is configured to execute program instructions stored in memory, a programmable hardware element such as a field-programmable gate array (FPGA), and/or any of various other components. The wireless device  102  and/or the wireless device  104  may perform any of the method embodiments described herein, or any portion of any of the method embodiments described herein, using any or all of such components. 
     Each of the wireless devices  102 ,  104  may include one or more antennas for communicating using one or more wireless communication protocols. In some cases, one or more parts of a receive and/or transmit chain may be shared between multiple wireless communication standards; for example, a device might be configured to communicate using either of Bluetooth or UWB using partially or entirely shared wireless communication circuitry (e.g., using a shared radio or at least shared radio components). The shared communication circuitry may include a single antenna, or may include multiple antennas (e.g., for MIMO) for performing wireless communications. Alternatively, a device may include separate transmit and/or receive chains (e.g., including separate antennas and other radio components) for each wireless communication protocol with which it is configured to communicate. As a further possibility, a device may include one or more radios or radio components which are shared between multiple wireless communication protocols, and one or more radios or radio components which are used exclusively by a single wireless communication protocol. For example, a device might include a shared radio for communicating using either of LTE or CDMA2000 1×RTT, and separate radios for communicating using each of UWB, Wi-Fi, and/or Bluetooth. Other configurations are also possible. 
     As previously noted, aspects of this disclosure may be implemented in conjunction with the wireless communication system of  FIG. 1 . For example, the wireless devices  102 ,  104  may communicate using one or more UWB wireless communication techniques or features described subsequently herein with respect to  FIGS. 4-11 . By utilizing such techniques (and/or other techniques described herein), the wireless device(s) may (at least according to some embodiments) be able to achieve reduction in multi-user interference. 
       FIGS. 2-3 —Exemplary Device Block Diagrams 
       FIG. 2  illustrates an exemplary wireless device  200  that may be configured for use in conjunction with various aspects of the present disclosure. The wireless device  200  may be one possible exemplary implementation of the device  102  illustrated in  FIG. 1 . The device  200  may be any of a variety of types of device and may be configured to perform any of a variety of types of functionality. The device  200  may be a substantially portable device or may be a substantially stationary device, potentially including any of a variety of types of device. The device  200  may be configured to perform one or more UWB wireless communication techniques or features, such as any of the techniques or features illustrated and/or described subsequently herein with respect to any or all of  FIGS. 4-11 . 
     As shown, the device  200  may include a processing element  202 . The processing element may include or be coupled to one or more memory elements. For example, the device  200  may include one or more memory media (e.g., memory  206 ), which may include any of a variety of types of memory and may serve any of a variety of functions. For example, memory  206  could be RAM serving as a system memory for processing element  202 . Other types and functions are also possible. 
     Additionally, the device  200  may include wireless communication circuitry  230 . The wireless communication circuitry may include any of a variety of communication elements (e.g., antenna for wireless communication, analog and/or digital communication circuitry/controllers, etc.) and may enable the device to wirelessly communicate using one or more wireless communication protocols. 
     Note that in some cases, the wireless communication circuitry  230  may include its own processing element (e.g., a baseband processor), e.g., in addition to the processing element  202 . For example, the processing element  202  might be an ‘application processor’ whose primary function may be to support application layer operations in the device  200 , while the wireless communication circuitry  230  might be a ‘baseband processor’ whose primary function may be to support baseband layer operations (e.g., to facilitate wireless communication between the device  200  and other devices) in the device  200 . In other words, in some cases the device  200  may include multiple processing elements (e.g., may be a multi-processor device). Other configurations (e.g., instead of or in addition to an application processor/baseband processor configuration) utilizing a multi-processor architecture are also possible. 
     The device  200  may additionally include any of a variety of other components (not shown) for implementing device functionality, depending on the intended functionality of the device  200 , which may include further processing and/or memory elements (e.g., audio processing circuitry), one or more power supply elements (which may rely on battery power and/or an external power source) user interface elements (e.g., display, speaker, microphone, camera, keyboard, mouse, touchscreen, etc.), and/or any of various other components. 
     The components of the device  200 , such as processing element  202 , memory  206 , and wireless communication circuitry  230 , may be operatively coupled via one or more interconnection interfaces, which may include any of a variety of types of interface, possibly including a combination of multiple types of interface. As one example, a USB high-speed inter-chip (HSIC) interface may be provided for inter-chip communications between processing elements. Alternatively (or in addition), a universal asynchronous receiver transmitter (UART) interface, a serial peripheral interface (SPI), inter-integrated circuit (I2C), system management bus (SMBus), and/or any of a variety of other communication interfaces may be used for communications between various device components. Other types of interfaces (e.g., intra-chip interfaces for communication within processing element  202 , peripheral interfaces for communication with peripheral components within or external to device  200 , etc.) may also be provided as part of device  200 . 
       FIG. 3  illustrates one possible block diagram of a wireless device  300 , which may be one possible exemplary implementation of the device  102  illustrated in  FIG. 1  and/or of the device  200  illustrated in  FIG. 2 . As shown, the wireless device  300  may include a system on chip (SOC)  301 , which may include portions for various purposes. For example, as shown, the SOC  301  may include processor(s)  302  which may execute program instructions for the wireless device  300 , and display circuitry  304  which may perform graphics processing and provide display signals to the display  360 . The SOC  301  may also include motion sensing circuitry  370  which may detect motion of the wireless device  300 , for example using a gyroscope, accelerometer, and/or any of various other motion sensing components. The processor(s)  302  may also be coupled to memory management unit (MMU)  340 , which may be configured to receive addresses from the processor(s)  302  and translate those addresses to locations in memory (e.g., memory  306 , read only memory (ROM)  350 , flash (e.g., NAND) memory  310 ). The MMU  340  may be configured to perform memory protection and page table translation or set up. In some embodiments, the MMU  340  may be included as a portion of the processor(s)  302 . 
     As shown, the SOC  301  may be coupled to various other circuits of the wireless device  300 . For example, the wireless device  300  may include various types of memory (e.g., including flash (e.g., NAND) memory  310 ), a connector interface  320  (e.g., for coupling to a computer system, dock, charging station, etc.), the display  360 , and wireless communication circuitry  330  (e.g., for UWB, LTE, LTE-A, CDMA2000, Bluetooth, Wi-Fi, NFC, GPS, etc.). 
     The wireless device  300  may include at least one antenna, and in some embodiments multiple antennas  335   a  and  335   b , for performing wireless communication with base stations and/or other devices. For example, the wireless device  300  may use antennas  335   a  and  335   b  to perform the wireless communication. As noted above, the wireless device  300  may in some embodiments be configured to communicate wirelessly using a plurality of wireless communication standards or radio access technologies (RATs). 
     The wireless communication circuitry  330  may include UWB Logic  332 , a Cellular Modem  334 , and additional WLAN/PAN Logic  336 . The UWB Logic  332  is for enabling the wireless device  300  to perform UWB communications, e.g., according to 802.15.4 protocols or other UWB protocols. The WLAN/PAN Logic  336  is for enabling the wireless device  300  to perform other WLAN and/or PAN communications, such as Wi-Fi and/or Bluetooth communications. The cellular modem  334  may be capable of performing cellular communication according to one or more cellular communication technologies. 
     As described herein, wireless device  300  may include hardware and software components for implementing embodiments of this disclosure. For example, one or more components of the wireless communication circuitry  330  (e.g., UWB Logic  332 ) of the wireless device  300  may be configured to implement part or all of the methods described herein, e.g., by a processor executing program instructions stored on a memory medium (e.g., a non-transitory computer-readable memory medium), a processor configured as an FPGA (Field Programmable Gate Array), and/or using dedicated hardware components, which may include an ASIC (Application Specific Integrated Circuit). 
       FIGS. 4-6 —UWB Frame Structure and Multiuser Interference 
       FIG. 4  illustrates a waveform representing an UWB frame, according to some embodiments. For example, the waveform of  FIG. 4  may represent an UWB physical layer (PHY) frame defined by IEEE 802.15.4 HRP UWB PHY. The PHY frame may alternatively be referred to as a packet or, in some embodiments, a PHY protocol data unit (PPDU). The PHY frame may include various overhead portions, such as one or more preambles and one or more headers, as well as one or more payload data portions. As illustrated, the frame may include a preamble, a start-of-frame delimiter (SFD), a PHY header (PHR), and a PHY service data unit (PSDU). The preamble may contain a sequence or information for use in frame synchronization and/or ranging, and may be, e.g., 16 symbols in length. The SFD may serve to indicate the end of the preamble and the start of the substantive portions frame, and may be used to establish frame timing. According to some embodiments, the SFD may have a fixed value, e.g., 8 symbols in length. The PHR may contain information for decoding the PSDU, such as a size of the PSDU, and may be, e.g., 16 symbols. The PSDU may contain payload data. The PSDU may, according to some embodiments, be of variable size, such as 0-1209 symbols. Other sizes are also envisioned for each portion of the frame. 
     In UWB systems such as 802.15.4 HRP UWB PHY, multiple pulses may be used to modulate information bits onto symbols. According to some embodiments, a symbol may include one or more bursts of pulses, with each burst followed by a guard time. The guard times following the bursts are employed to reduce inter-symbol interference due to multipath delay spread. 
       FIG. 5  illustrates an example symbol format, according to some embodiments. For example, the symbol  500  illustrated in  FIG. 5  may represent a data symbol, e.g., as included in the PSDU of  FIG. 4 . In various scenarios, the same symbol format or a different symbol format may be used in portions of the frame other than the PSDU. 
     As illustrated, the symbol  500  may include two burst periods: a first burst period  502  and a second burst period  506 . In some scenarios, each burst period may include a data burst including a plurality of pulses—e.g.,  16  pulses. In some scenarios, the pulses of a burst may represent a pseudorandom chip sequence to reduce multipath fading effects. In some scenarios, each symbol  500  may encode two data bits using burst position modulation and binary phase shift keying modulation (BPM-BPSK). For example, one data bit may specify one of the two burst periods ( 502  or  506 ) to carry a burst of pulses, while the other burst period may be unoccupied (e.g., no transmission is scheduled and/or transmitted). The other bit may modulate the phase of that burst. In some scenarios, other modulation schemes may be used, which may lead to differing symbol structures. For example, the symbol  500  may include other numbers of burst periods, other burst lengths, more than one occupied burst period, etc. 
     As illustrated, each burst period  502  and  506  may be followed (e.g., immediately) by a respective guard period: a first guard period  504 , following the burst period  502 , and a second guard period  508 , following the burst period  506 . No additional data is scheduled for transmission during the guard periods. In some scenarios, the guard periods  504  and  508  may be equal in length to the burst periods  502  and  506 . For example, the burst period  502  may include 16 chips, and the symbol  500  may be 64 chips in total length. In some scenarios, a chip may have a length of 2 ns, e.g., when a bandwidth of 500 MHz is used. Other chip lengths are also envisioned. 
     In some scenarios, at least a portion of an UWB frame (e.g., the PSDU) may include a sequential series of symbols, e.g., such as the symbol  500  illustrated in  FIG. 5 . Thus, a series of burst periods may occur (or be scheduled to occur) at regular intervals throughout that portion of the frame. Guard periods may similarly occur (or be scheduled to occur) at regular intervals. 
     In some scenarios, the UWB frame may include support for one or more error correction schemes, e.g., forward error correction (FEC), such as Reed-Solomon and/or convolutional error correction. Various error correction schemes may be used to correct a finite number of errors in a received communication. For example, an error correction scheme may be capable of correcting errors up to a certain percentage of received data within the communication. 
     Multiuser interference may occur when multiple UWB wireless devices access the same wireless medium. Specifically, in some scenarios, UWB wireless devices may utilize the entire bandwidth, such that multiple wireless devices are transmitting within the same frequency band, e.g., without being limited to mutually-exclusive sub-bands. In such scenarios, each wireless device may cause interference to other wireless devices. Specifically, multiuser interference occurs when pulses transmitted by one wireless device overlap completely or partially in time with the pulses transmitted by one or more other wireless devices. Such multiuser interference may result in receiving incorrect data bits at the receiving wireless device. It is therefore desirable to reduce such multiuser interference. 
     In some scenarios (e.g., in some modes of operation), multiple UWB wireless devices may access the same wireless medium asynchronously, e.g., without transmission time slots being coordinated between the wireless devices. In such scenarios, frames transmitted by two wireless devices may start at different times, such that the frames overlap in time, but are offset from each other by a random time offset. Depending on the offset, the pulses transmitted by one wireless device may or may not overlap with the pulses transmitted by the other wireless device. For example, in some scenarios, the burst periods of a first frame may coincide with the guard periods of a second, overlapping frame. In such scenarios, multiuser interference may not occur. However, in other scenarios, the burst periods of the two frames may partially or completely overlap, leading to multiuser interference. 
     The degree of interference experienced is dependent upon the offset between the bursts of the two overlapping frames. In some scenarios, a data scrambling code sequence (e.g., a pseudorandom chip sequence) may be used to encode the data to minimize cross-correlation between signals. However, because the overlapping frames are not synchronized, the scrambling code may be imperfect. Thus, cross-correlation may still occur at some offset positions. 
       FIG. 6  illustrates normalized cross-correlation between a first symbol transmitted by a first wireless device and a second, overlapping symbol transmitted by a second wireless device at various offsets, according to some embodiments. Specifically, the normalized cross-correlation represents the cross-correlation divided by the autocorrelation. 
     At an offset of 0 chips, a 16-chip data burst of the first symbol is transmitted during a guard period of the second symbol. Thus, the normalized cross-correlation is 0. No multi-user interference would occur between the first and second wireless devices in such a scenario. 
     When the offset is shifted by one chirp, the 16-chip data burst of the first symbol overlaps a 16-chip data burst of the second symbol by one chip. As illustrated, the normalized cross-correlation is non-zero in this scenario, resulting in some multiuser interference. Each increasing offset value from 1 chip up to 16 chips represents an increasing degree of overlap between the two data bursts. At an offset of 16 chips, the data burst of the first symbol and the data burst of the second symbol overlap completely. Each increasing offset value from 17 chips up to 31 chips then represents a decreasing degree of overlap between the two data bursts. At an offset of 32 chips, the data burst of the first symbol is transmitted during the next guard period of the second symbol, such that the normalized cross-correlation is again 0. 
     As illustrated, the normalized cross-correlation value may vary significantly based on symbol offset, and may change noticeably with a shift of only one chip. 
       FIGS. 7-10 —Burst Start Randomization 
     As illustrated in  FIG. 6 , the normalized cross-correlation value between symbols of two overlapping frames may vary significantly based on their relative offset, and may change noticeably with an offset shift of only one chip. In some scenarios, each of the two overlapping frames may include bursts at the same regular interval, e.g., where the two frames have symbols of the same length. Thus, each burst (or at least a significant plurality of bursts) of the first frame may have the same offset relative to respective bursts of the second, overlapping frame. If that offset has a high normalized cross-correlation, then the two frames may experience a rate of interference that exceeds the ability of the utilized error correction scheme to correct. By contrast, a different offset value may result in little or no multiuser interference, e.g., within the capacity of the utilized error correction scheme to correct. Thus, in some scenarios, reception of a frame may succeed or fail based significantly on the burst offset of that frame relative to another, overlapping frame. 
     To this end, the symbol structure of an UWB frame transmitted by a first wireless device may be varied so as to prevent a significant number of bursts within the frame from having the same offset relative to respective bursts of a second, overlapping frame transmitted by a second wireless device. 
       FIG. 7  shows a flow chart for a procedure to accomplish this goal when transmitting a frame, by randomizing burst start times within the frame, according to some embodiments. The procedure of  FIG. 7  may be performed by a wireless communication device, e.g., such as the wireless device  102  (e.g., as may be further defined by the wireless device  200  and/or the wireless device  300 ). The wireless device  102  may be referred to as the transmitting wireless device  102 . 
     At  702 , the transmitting wireless device  102  may generate a frame including a plurality of data bursts. For example, the frame may be formatted as discussed above in connection with  FIG. 4  and  FIG. 5 . Each data burst of the plurality of data bursts may be associated with a respective nominal transmission window. For example, each of the specified data bursts may be associated with a burst period, such as the burst period  502  or the burst period  506  of  FIG. 5 . Each of these burst periods may be scheduled to begin at a specific time within the frame. Thus, each burst may be associated with a respective nominal transmission window, e.g., defined as the scheduled time window of the respective burst period. These burst periods may, in some scenarios, be evenly or regularly spaced in time. 
     Each of the nominal transmission windows may be followed (e.g., immediately) by a respective guard window. For example, nominal guard windows may be defined as the scheduled time window of guard periods of a data symbol, such as the guard period  504  or the guard period  508  of  FIG. 5 . 
     At  704 , the transmitting wireless device  102  may generate a respective offset time for each data burst of the plurality of data bursts. An offset time may be a value representing a number of time units of any convenient size or form, such as seconds (or portions thereof), clock cycles, symbols (or portions thereof), etc. In some scenarios, an offset time may be generated as a random offset time. As used herein, any reference to a “random” offset time or other value may include a “pseudorandom” value, as known in the art. An offset time may be generated in any manner that allows the offset to be known to both the transmitter and the receiver. For example, in some scenarios, generating an offset time may include executing an algorithm known to both the transmitting wireless device  102  and a remote receiving wireless device, such as the wireless device  104  of  FIG. 1 , to calculate an offset time—e.g., a pseudorandom offset time. In some scenarios, generating an offset time may include selecting a value from (or generating a value based on) a set of values known to both the transmitting wireless device  102  and the receiving wireless device  104 . For example, the set of values may be a set of values stored by the wireless devices for some other purpose, such as, e.g., a scramble sequence defined by an applicable communications standard, such as 802.15.4. 
     At  706 , the transmitting wireless device  102  may transmit the frame with each of the data bursts offset (e.g., delayed) by the respective offset time. For example, if the nominal transmission window (e.g., the burst period) associated with a given data burst begins at start time T N  and the offset time for that data burst is T O , then transmission of that data burst may begin at a time T T , wherein T T =T N +T O . During the time between T N  and T T , the transmitting wireless device  102  may not transmit any pulses. 
     The offset time generated for a given data burst may have a value not greater than (e.g., less than) the length of the respective guard window. Thus, delaying the data burst by the offset time may cause transmission of the data burst to be completed during the respective guard window. 
       FIG. 8  illustrates two symbols in each of two frames transmitted according to the procedure of  FIG. 7 , according to some embodiments. As illustrated by  FIG. 8 , symbol  810 , transmitted by a first wireless device as part of a first frame, and symbol  830 , transmitted by a second wireless device as part of a second frame, are transmitted (or received) with identical start times. Thus, without the procedure of  FIG. 7 , the signals may experience high cross-correlation, e.g. as illustrated in  FIG. 6  with an offset of 16 chips, which may result in high multiuser interference. Furthermore, without the procedure of  FIG. 7 , each additional symbol within the first frame (such as symbol  820 , as shown in  FIG. 8 ) would be similarly aligned to respective symbols of the second frame (such as symbol  840 , shown in  FIG. 8 ), to the extent that the frames overlap in time. Thus, both frames may be received with high error rates. In some scenarios, the error rates for the frames may be so high as to prevent decoding of the frames. 
     However, each of the symbols illustrated in  FIG. 8  has been transmitted according to the procedure of  FIG. 7 . Each of the symbol  810  and the symbol  830  includes a first burst period  801 , a first guard period  802 , a second burst period  803 , and a second guard period  804 . Similarly, each of the symbol  820  and the symbol  840  includes a third burst period  805 , a third guard period  806 , a fourth burst period  807 , and a fourth guard period  808 . However, each symbol includes a burst that is offset from its respective burst period. 
     For example, symbol  810  includes a burst  812 . The burst  812  is associated with a nominal transmission window defined by the first burst period  801 , followed by a guard window  814  defined by the first guard period  802 . However, transmission of the burst  812  is delayed by offset time  811 . Thus, transmission of the burst  812  begins after the start of the first burst period  801  and is completed after the end of the first burst period  801 . Specifically, transmission of the burst period  812  is completed during the guard window  814 . 
     As illustrated, symbol  810  further includes a burst  816 . The burst  816  is associated with a nominal transmission window defined by the second burst period  803 , followed by a guard window  818  defined by the second guard period  804 . However, transmission of the burst  816  is delayed by offset time  813 . Thus, transmission of the burst  816  begins after the start of the second burst period  803  and is completed after the end of the second burst period  803 . Specifically, transmission of the burst  816  is completed during the guard window  818 . 
     In some scenarios, the offset time  811  and the offset time  813  may be independently generated, and may thus have different (or independent) values. In other scenarios, such as the scenario illustrated in  FIG. 8 , one offset time may be generated for symbol  810 , and may be used for both the offset time  811  and the offset time  813 . 
     In some scenarios, the symbol  810  may include only a single burst, associated with either the first burst period  801  or the second burst period  803 , e.g., in accordance with BPM-BPSK modulation. For example, the burst may be sent either as the burst  812  or as the burst  816 , while no burst is sent in the other position. In such scenarios, a single offset time may be generated for the symbol  810 , and may be used to offset the single burst, regardless of the position in which the burst is transmitted. 
     As illustrated, symbol  820  is transmitted with a format similar to that of symbol  810 . For example, symbol  820  includes a burst  822 . The burst  822  is associated with a nominal transmission window defined by the third burst period  805 , followed by a guard window  824  defined by the third guard period  806 . However, transmission of the burst  822  is delayed by offset time  821 . Thus, transmission of the burst  822  begins after the start of the third burst period  805  and is completed after the end of the third burst period  805 . Specifically, transmission of the burst period  822  is completed during the guard window  824 . 
     As illustrated, symbol  820  further includes a burst  826 . The burst  826  is associated with a nominal transmission window defined by the fourth burst period  807 , followed by a guard window  828  defined by the fourth guard period  808 . However, transmission of the burst  826  is delayed by offset time  823 , which may be the same as or independent of offset time  821 . Thus, transmission of the burst  826  begins after the start of the fourth burst period  807  and is completed after the end of the fourth burst period  807 . Specifically, transmission of the burst  826  is completed during the guard window  828 . 
     Like symbol  810 , symbol  820  may, in some scenarios, include only one burst, e.g., in either the position of the burst  822  or the position of the burst  826 . 
     The offset time(s) of the symbol  820  may be distinct (e.g., independently generated) relative to the offset time(s) of the symbol  810 . 
     As illustrated, the symbols  830  and  840  have the same structure as symbols  810  and  820 , which may be more briefly reiterated as follows. The symbol  830  includes a burst  832 . The burst  832  is associated with a nominal transmission window defined by the first burst period  801 , followed by a guard window  834  defined by the first guard period  802 . However, transmission of the burst  832  is delayed by offset time  831 . 
     As illustrated, symbol  830  further includes a burst  836 . The burst  836  is associated with a nominal transmission window defined by the second burst period  803 , followed by a guard window  838  defined by the second guard period  804 . Transmission of the burst  836  is delayed by offset time  833 . 
     The symbol  840  includes a burst  842 . The burst  842  is associated with a nominal transmission window defined by the third burst period  805 , followed by a guard window  844  defined by the third guard period  806 . Transmission of the burst  842  is delayed by offset time  841 . 
     As illustrated, symbol  840  further includes a burst  846 . The burst  846  is associated with a nominal transmission window defined by the fourth burst period  807 , followed by a guard window  848  defined by the fourth guard period  808 . Transmission of the burst  846  is delayed by offset time  843 , which may be the same as or independent of offset time  841 . 
     The offset time(s) of the symbol  840  may be distinct (e.g., independently generated) relative to the offset time(s) of the symbol  830 . 
     The offset time  831  may also be independent of the offset time  811 . For example, each of the offset time  811  and the offset time  831  may be a randomly generated value. Thus, the burst  812  and the burst  832  are unlikely to be transmitted beginning at the same time. Specifically, the two bursts may have a random shift in time relative to each other. As can be seen from  FIG. 6 , such a random offset may result in cross-correlation becoming either better or worse. 
     However, subsequent symbols  820  and  840  also utilize offset times that are independent from each other, and are also independent of the offset times of the preceding symbols. Thus, the cross-correlation between the bursts of the two frames varies randomly from symbol to symbol. As a result, the cross-correlation between the bursts will average out over the course of the frames. In scenarios in which a data scrambling code sequence has been used to encode the data, it may be expected that the average cross-correlation should be low enough to allow for correction of any errors received due to cross-correlation between the two frames. 
     As may be seen from the illustration of  FIG. 8 , the offset time for each burst may theoretically be any value not greater than (e.g., less than) the following guard window. However, the more the burst impinges on the guard window, the more susceptible the burst becomes to multipath interference. Thus, in some scenarios, the offset times may have a lower maximum value, such as a predetermined fraction of the length of the guard window. For example, in some scenarios, the offset times may be selected from values ranging from 0 to 25% of the length of the guard window. For example, if each guard window of a symbol has a length of 16 chips, then the offset times may be any value from 0 to 4 chips. In other scenarios, the offset times may have a larger range, such as from 0 to ⅓, from 0 to 50%, or from 0 to 75% of the length of the guard window. Because multipath interference is a function of distance between the transmitter and the receiver, the range used for selecting offset times may, in some scenarios, be selected based on the distance over which transmitters is expected to operate for the given technology. 
     The burst start randomization procedure of  FIG. 7  may result in significantly lower SNR requirements for UWB multiple user communications. For example, in test scenarios utilizing an 802.15.4 system, improvement several dB may be achieved for an 8-user system.  FIG. 9  and  FIG. 10  illustrate such improvements. 
       FIG. 9  shows a chart of simulation results illustrating packet error rates for UWB wireless devices at different SNRs. Specifically,  FIG. 9  shows simulation results within a traditional 802.15.4 HRP UWB PHY system (e.g., without the burst start randomization procedure disclosed herein). The curve  902  shows the packet error rate at a receiver for various SNR values for a single transmitting wireless device, e.g., as a baseline performance metric. Each of the curves  904  and  906  shows the packet error rate at a receiver attempting to receive a signal from one transmitting wireless device within a system including 8 transmitting wireless devices having different timing. Specifically, the curve  904  shows a scenario in which the channel impulse response (CIR) is modeled such that the signals from each of the transmitting wireless devices is received with the same signal strength. The curve  906  shows a scenario in which the CIR is modeled such that the desired signal is received with a slightly greater signal strength than the interfering signals. 
     As illustrated in  FIG. 9 , both the curve  904  and the curve  906  demonstrate significant interference, even at high SNR. 
       FIG. 10  shows a chart of simulation results illustrating the same system as  FIG. 9 , but with burst start randomization. The curves  902 ,  904 , and  906  from  FIG. 9  are shown in  FIG. 10 , as well, for comparison. As in  FIG. 9 , these curves represent simulation results without burst start randomization. Each of the curves  1004 ,  1006 , and  1008  shows the packet error rate at a receiver attempting to receive a signal from one transmitting wireless device within a system including 8 transmitting wireless devices having different timing, but with each of the 8 transmitting wireless devices implementing burst start randomization. Specifically, the curve  1004  shows a scenario in which the CIR is modeled such that the signals from each of the transmitting wireless devices is received with the same signal strength. The curve  1006  shows a scenario in which the CIR is modeled such that the desired signal is received with a slightly greater signal strength than the interfering signals. The curve  1008  shows a scenario in which the CIR is modeled such that the various interfering signals are received with differing signal strengths. 
     As illustrated in  FIG. 10 , the curve  1004  reflects significantly lower packet error rates than the curve  904  within the same channel model, particularly at high SNR values. Similarly, the curve  1006  reflects significantly lower packet error rates than the curve  906  within the same channel model. This illustrates improvements that may be realized by implementing burst start randomization. 
       FIGS. 11-12 —Packet Start Randomization 
     In some applications, PHY frames may be transmitted regularly with a high duty-cycle. For example, if UWB communication is utilized for audio streaming, a payload packet (e.g., an audio packet) may arrive at the PHY for transmission at regular intervals, e.g., 500 μs intervals. 
     In some applications, a high degree of multiuser interference may be expected when a plurality of wireless devices each perform such regular, high-duty-cycle communications. For example, a local environment may include a dense population of users simultaneously utilizing UWB communications for audio streaming between a respective media device (e.g., the user&#39;s smartphone) and the respective user&#39;s wireless headphones. Because the audio packets are generated regularly, according to a common technology, each wireless device may transmit frames with the same periodicity, e.g., 500 μs. As a result, any overlap between a first frame transmitted by a first wireless device and a second frame transmitted by a second wireless device (e.g., if the two frames overlap such that their bursts coincide in time), may be repeated for subsequent frames. For example, if the first wireless device transmits the first frame, followed by a third frame 500 μs later, and the second wireless device transmits the second frame, followed by a fourth frame 500 μs later, then the third frame will overlap with the fourth frame in exactly the same manner that the first frame overlapped with the second frame. Thus, a similar level of multiuser interference may be observed on every frame, which may be extremely detrimental if that level of user interference is particularly high. 
     A similar problem may occur in connection with ACK/NACK messages, regardless of whether a communication has a high degree of regularity. For example, if a first message transmitted by a first wireless device overlaps with a second message transmitted by a second wireless device, such that a high degree of multiuser interference results, then both messages may fail to be received by their respective target receivers. In response, each of the target receivers may transmit a NACK message, or other message indicating that the message was not successfully received. In response to the respective NACK messages, the first wireless device may transmit a third frame including a retransmission of the payload of the first frame, and the second wireless device may transmit a fourth frame including a retransmission of the payload of the second frame. Because the first wireless device and the second wireless device are using the same communications technology, the time delay required to receive a NACK and retransmit may be the same for both devices. Thus, the third frame will overlap with the fourth frame in exactly the same manner that the first frame overlapped with the second frame. Thus, the third frame and the fourth frame may also fail. Further retransmissions may suffer from the same problem. 
     To this end, transmission of an UWB frame transmitted by a first wireless device may be delayed so as to prevent a significant number of frames from having the same offset relative to respective frames transmitted by a second wireless device. 
       FIG. 11  shows a flow chart for a procedure to accomplish this goal when transmitting a series of frames, by randomizing packet start times, according to some embodiments. The procedure of  FIG. 11  may be performed by a wireless communication device, e.g., such as the wireless device  102  (e.g., as may be further defined by the wireless device  200  and/or the wireless device  300 ). The wireless device  102  may be referred to as the transmitting wireless device  102 . 
     At  1102 , the transmitting wireless device  102  may generate a first packet. The first packet may refer to either a payload packet (e.g., a MAC packet or an audio packet) or may refer to a PHY packet (e.g., a PPDU). For example, the packet may include a frame as illustrated in  FIG. 4  or a payload packet encoded as a part thereof, such as the PSDU. The first packet may be prepared for transmission at a first time. For example, the first packet may be saved in a memory, register, buffer, etc., and may be in a completed state, such that it is ready to be transmitted. The first time may represent the earliest time at which the first packet is in such a state that the wireless device is capable of transmitting it. 
     At  1104 , the transmitting wireless device  102  may generate a first offset, which may be a value representing a number of time units of any convenient size or form, such as seconds (or portions thereof), clock cycles, symbols (or portions thereof), etc. In some scenarios, the first offset may be generated as a random offset (to include a “pseudorandom offset). In some scenarios, the first offset may be generated in any manner that allows the offset to be known to both the transmitter and the receiver. In other scenarios, the first offset may be known only to the transmitter. For example, in some scenarios, generating the first offset may include executing an algorithm known to the transmitting wireless device  102  and possibly also known to a remote receiving wireless device, such as the wireless device  104  of  FIG. 1 , to calculate an offset—e.g., a pseudorandom offset. In some scenarios, generating the first offset may include selecting a value from (or generating a value based on) a set of values known to the transmitting wireless device  102  and possibly also known to the receiving wireless device  104 . For example, the set of values may be a set of values stored by the wireless devices for some other purpose, such as, e.g., a scramble sequence defined by an applicable communications standard, such as 802.15.4. 
     At  1106 , the transmitting wireless device  102  may transmit the first packet at a second time. The second time may follow the first time by the amount of time identified by the first offset. In this way, transmission of the first packet may be offset (e.g., randomly) in time relative to the first time. 
     At  1108 , the transmitting wireless device  102  may receive from the receiving wireless device  104  a message indicating whether the first packet was successfully received. For example, the message may include an ACK message (or other message) indicating successful receipt of the first packet or a NACK message (or other message) indicating that receipt of the first packet was unsuccessful. 
     At  1110 , the transmitting wireless device  102  may generate a second packet. The second packet may be similar in form to the first packet. The second packet may be prepared for transmission at a third time. For example, the second packet may be saved in a memory, register, buffer, etc., and may be in a completed state, such that it is ready to be transmitted. The third time may represent the earliest time at which the second packet is in such a state that the wireless device is capable of transmitting it. 
     In some scenarios, generation of the second packet may be responsive to, or influenced by, the message received at  1108 . For example, if the message received at  1108  includes a NACK message, then the transmitting wireless device may, in response, determine to retransmit the first packet or a payload portion thereof. Therefore, in such scenarios, the second packet may be or include the first packet or a payload portion thereof. For example, generating the second packet may include generating a copy of the first packet for retransmission. 
     As another example, if the message received at  1108  includes an ACK message, then the transmitting wireless device may, in response, determine not to retransmit the first packet or a payload portion thereof, and/or may decide to transmit a different packet. Therefore, in such scenarios, generating the second packet may include generating an entirely new packet, such as a new payload packet (e.g., MAC packet or audio packet). 
     At  1112 , the transmitting wireless device  102  may generate a second offset. The second offset may be generated in a same or similar manner as the first offset, and may be of a same or similar form. The second offset may be different than, or independent of, the first offset. 
     At  1114 , the transmitting wireless device  102  may transmit the second packet at a fourth time. The fourth time may follow the third time by the amount of time identified by the second offset. In this way, transmission of the second packet may be offset (e.g., randomly) in time relative to the third time. Further, transmission of the second packet may be offset in time relative to transmission of the first packet. Thus, if another wireless device is concurrently transmitting packets at intervals foreseeably similar to the interval between the first time and the third time, it is unlikely that those packets will overlap both the first packet and the second packet to the same extent. 
     It should be appreciated that various steps of the procedure of  FIG. 11  may be performed in different order or may be omitted. For example, the first offset may be generated before or after generation of the first packet, such that  1102  and  1104  may be reversed in order. Similarly, the second offset may be generated before or after generation of the second packet, such that  1112  may occur at any point prior to  1114 . As another example, in some scenarios,  1108  may be omitted. As yet another example, in some scenarios, the transmitting wireless device  102  may transmit only a single packet, such that  1110 - 1114  may be omitted. 
     Each offset may be effectively any value. However, as the goal of delaying transmission by an offset is to shift bursts of the packet so as to align randomly with bursts of any other packet concurrently transmitted by another device, the effect of the delay may be cyclic with a period of one symbol length. Thus, in some scenarios, an offset (e.g., the first offset and/or the second offset) may have a value not greater than (e.g., less than) a symbol length of the packet (e.g., a data symbol length). In other scenarios, the offsets may have a higher or lower maximum value. As can be seen from  FIG. 6 , an offset of even one chip may have a significant effect on the multiuser interference between two overlapping frames. 
     In some scenarios, offsetting transmission start time of a packet may include offsetting transmission start time of an entire frame. In other scenarios, e.g., where the packet is a payload packet, offsetting transmission start time of the packet may include initiating transmission of the frame without offset, but offsetting a start time of the packet within the frame. 
     In some scenarios, a frame may include more than one packet. In such scenarios, both packets may be offset by a single offset, or each packet may be offset within the frame by a different, or independent, offset. 
     It should be noted that the first offset and the second offset are distinct from backoff times as known in the art, and are not a part of a backoff procedure. For example, the transmitting wireless device  102  may, in some scenarios, not perform a carrier-sense multiple access (CSMA) procedure (e.g., as known in the art) and/or an enhanced distributed channel access (EDCA) procedure (e.g., as known in the art) at any of the first time, the second time, the third time, and/or the fourth time. In some scenarios, the transmitting wireless device  102  does not perform a CSMA and/or EDCA procedure at any time. More generally, the transmitting wireless device  102  may not perform any procedure to determine whether the transmission medium is occupied or available before transmitting. 
     In other scenarios, the transmitting wireless device  102  may perform one or more procedures (e.g., CSMA/EDCA procedures) to determine whether the transmission medium is occupied or available before transmitting, and may implement any of various collision avoidance (CA) schemes, e.g., as known in the art, such as use of backoff counters. Further, the packet start randomization procedure of  FIG. 11  may then be implemented when (e.g., in response to) the medium is found to be available (e.g., unoccupied). For example, within the context of the procedure of  FIG. 11 , the first packet may, in some scenarios, be considered to be prepared for transmission only once a CSMA-CA or EDCA scheme determines that the first packet may be transmitted, e.g., because the medium is available. Thus, the first time defined at  1102  may be considered to be the time at which the CSMA-CA/EDCA scheme determines that the first packet may be transmitted, e.g., after any applicable backoff period. In keeping with  1106 , transmission of the packet may then be further delayed by the first offset. In this manner, the packet start randomization procedure of  FIG. 11  may provide additional protection against so-called “hidden terminal” problems. 
     It should be appreciated that the packet start randomization procedure of  FIG. 11  may be used in conjunction with the burst start randomization procedure of  FIG. 7 . Either or both procedures may also be used in conjunction with other methods of reducing and/or overcoming multiuser interference, such as CSMA-CA/EDCA, retransmission, etc. 
     Although the devices, systems, and methods described herein have been described with reference to UWB communications, it should be appreciated that they are equally applicable to any pulse-based communication technology or protocol. Reference to UWB communications or protocols is not intended to be limiting. 
     In addition to the above-described exemplary embodiments, further embodiments of the present disclosure may be realized in any of various forms. For example, some embodiments may be realized as a computer-implemented method, a computer-readable memory medium, or a computer system. Other embodiments may be realized using one or more custom-designed hardware devices such as ASICs. Still other embodiments may be realized using one or more programmable hardware elements such as FPGAs. 
     In some embodiments, a device (e.g., a wireless device  102  or  104 ) may include means for performing each step of a method, e.g., any of the method embodiments described herein, or, any combination of the method embodiments described herein, or, any subset of any of the method embodiments described herein, or, any combination of such subsets. 
     In some embodiments, a non-transitory computer-readable memory medium may be configured so that it stores program instructions and/or data, where the program instructions, if executed by a computer system, cause the computer system to perform a method, e.g., any of the method embodiments described herein, or, any combination of the method embodiments described herein, or, any subset of any of the method embodiments described herein, or, any combination of such subsets. 
     In some embodiments, a device (e.g., a wireless device  102  or  104 ) may be configured to include a processor (or a set of processors) and a memory medium, where the memory medium stores program instructions, where the processor is configured to read and execute the program instructions from the memory medium, where the program instructions are executable to implement any of the various method embodiments described herein (or, any combination of the method embodiments described herein, or, any subset of any of the method embodiments described herein, or, any combination of such subsets). The device may be realized in any of various forms. 
     Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

Metadata:
Filing Date: 20180821
Publication Date: 20200505
Grant Date: 20200505
Priority Date: 20180821
Inventors: YU, XIAOMING
MARQUEZ, Alejandro J.
SEN, INDRANIL S.
AGBOH, PETER M.
YANG, SHANG-TE
REDDY, VUSTHLA SUNIL
Assignee: APPLE INC
CPC Classifications: [{"code": "H04B7/06", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B1/71635", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W74/0808", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B1/719", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04J3/0635", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J3/0658", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B1/7103", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04B1/7163", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B2201/7097", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B1/71632", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B7/0613", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B1/7163", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B2201/7097", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B1/7103", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04B7/0613", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B7/06", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B1/71635", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J3/0658", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J3/0635", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B1/71632", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W74/0808", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B1/7163", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 69586641