PATENT DOCUMENT

Publication Number: US-11917517-B2
Application Number: US-202117379458-A
Country: US
Kind Code: B2

Title: Power-efficient synchronization for long-range device-to-device communications

Abstract:
A communications network may include user equipment (UE) devices. When a first UE device becomes off-grid and has an emergency message, the first UE device may transmit device-to-device signals for receipt by a second UE device. The signals may include one or more preambles that precede the emergency message and that are used by the second UE device to perform synchronization with the first UE device. The second UE device may periodically activate a receiver during a series of windows to listen for the signals, may receive a preamble sequence during one of the windows, and may synchronize its timing to the first UE device for receipt of the emergency message based on the preamble sequence. This may allow the UE devices to be synchronized for transfer of the emergency message over long distances while consuming a minimal amount of power on the second UE device.

Claims:
What is claimed is: 
     
       1. An electronic device comprising:
 one or more antennas; 
 a transmitter configured to transmit radio-frequency signals to an additional electronic device using the one or more antennas; and 
 one or more processors configured to
 transmit, using the transmitter, a message in the radio-frequency signals, and 
 transmit, using the transmitter, a series of preambles in the radio-frequency signals prior to the message, wherein each preamble in the series of preambles identifies, for the additional electronic device, a different respective time offset between an end of that respective preamble and transmission of the message. 
 
 
     
     
       2. The electronic device of  claim 1 , wherein each of the preambles in the series of preambles has a preamble length and the preambles in the series of preambles are separated by gaps during which the transmitter is inactive. 
     
     
       3. The electronic device of  claim 2 , wherein the one or more processors is further configured to:
 re-transmit the message in the radio-frequency signals; and 
 transmit an additional series of preambles in the radio-frequency signals after transmitting the message and prior to re-transmitting the message, wherein the preambles in the additional series of preambles are separated by additional gaps during which the transmitter is inactive, and the additional gaps are separated from the re-transmitted message by the respective time offsets identified by the series of preambles. 
 
     
     
       4. The electronic device of  claim 1 , wherein each preamble in the series of preambles is shorter than a receiver window length of the additional electronic device. 
     
     
       5. The electronic device of  claim 1 , wherein each preamble in the series of preambles comprises an M-sequence or a Zadoff Chu sequence. 
     
     
       6. The electronic device of  claim 1 , wherein the electronic device is configured to use the one or more antennas to communicate with a wireless base station in an on-grid state and the electronic device is configured to transmit the radio-frequency signals in an off-grid state in which the electronic device is unable to connect to any wireless base stations. 
     
     
       7. The electronic device of  claim 1 , wherein the transmitter is configured to transmit the series of preambles in the radio-frequency signals at a first frequency and is configured to transmit the message in the radio-frequency signals at a second frequency that is different from the first frequency. 
     
     
       8. The electronic device of  claim 1 , wherein the electronic device is a first user equipment (UE) device, the additional electronic device is a second UE device, and the radio-frequency signals comprise device-to-device (D2D) signals. 
     
     
       9. An electronic device comprising:
 one or more antennas; 
 a transmitter configured to transmit radio-frequency signals to a user equipment device using the one or more antennas; and 
 one or more processors configured to
 transmit, using the transmitter, a message in the radio-frequency signals, and 
 transmit, using the transmitter, a preamble in the radio-frequency signals prior to the message that identifies, for the user equipment device, a time offset between the preamble and the message wherein the preamble comprises a repeating sequence and the one or more processors is configured to apply a cover code over the repeating sequence that varies across a length of the preamble. 
 
 
     
     
       10. The electronic device of  claim 9 , wherein the radio-frequency signals comprise device-to-device (D2D) signals. 
     
     
       11. The electronic device of  claim 9 , wherein the cover code comprises a binary cover code. 
     
     
       12. An electronic device comprising:
 one or more antennas; 
 a receiver configured to receive radio-frequency signals from an additional electronic device using the one or more antennas; and 
 one or more processors configured to
 periodically activate the receiver during a series of receiver (RX) windows, 
 receive a preamble sequence in the radio-frequency signals during an RX window of the series of RX windows, 
 deactivate the receiver after the RX window, 
 re-activate the receiver during a message listening window that begins after a time offset has elapsed since an end of the preamble sequence, the time offset being based on the preamble sequence received during the RX window, and 
 receive a message in the radio-frequency signals during the message listening window. 
 
 
     
     
       13. The electronic device of  claim 12 , wherein the one or more processors is further configured to identify the time offset based on a cover code applied to the preamble sequence. 
     
     
       14. The electronic device  10 , wherein the RX window has a length that is at least twice a length of the preamble sequence. 
     
     
       15. The electronic device of  claim 12 , wherein the one or more processors is further configured to relay the message to an emergency services provider. 
     
     
       16. The electronic device of  claim 12 , wherein the one or more processors is further configured to deactivate the receiver between the RX windows in the series of RX windows. 
     
     
       17. The electronic device of  claim 12 , wherein the receiver is configured to receive the preamble sequence at a first frequency and is configured to receive the message at a second frequency that is different from the first frequency. 
     
     
       18. The electronic device of  claim 12 , wherein the one or more processors is further configured to receive the message using a frequency hopping pattern identified, by the electronic device, based on the preamble sequence. 
     
     
       19. The electronic device of  claim 12 , wherein the one or more processors is configured to extend a duration of the RX window in which the preamble sequence is received until the one or more processors detects receipt of an end of the preamble sequence. 
     
     
       20. The electronic device of  claim 12 , wherein the electronic device is a first user equipment (UE) device, the additional electronic device is a second UE device, and the radio-frequency signals comprise device-to-device (D2D) signals.

Description:
FIELD 
     This disclosure relates generally to wireless communications, including wireless communications performed by user equipment devices. 
     BACKGROUND 
     Communications systems often include user equipment and wireless base stations. The wireless base stations have corresponding coverage areas. When the user equipment is located within a coverage area, radio-frequency signals are exchanged between the user equipment and a wireless base station to convey wireless data. 
     In practice, there arise situations where the user equipment is no longer within the coverage areas of the wireless base stations. In these situations, the user equipment is unable to convey wireless data with the wireless base stations. However, scenarios may still arise where the user equipment needs to send wireless data to a recipient while the user equipment is located outside of the coverage areas of the wireless base stations. 
     SUMMARY 
     A communications network may include user equipment (UE) devices and external communications equipment such as wireless base stations or access points. A first UE device may wirelessly communicate with the rest of the network via the external communications equipment while on-grid. When the first UE device becomes off-grid, the first UE device may still occasionally need to send messages such as emergency messages to a second UE device. However, going off-grid may cause timing on the first UE device to drift with respect to timing on the second UE device. 
     When the first UE device is off-grid and has an emergency message or other data to transmit, the first UE device may transmit device-to-device (D2D) signals for receipt by the second UE device. The D2D signals may include one or more preambles that precede the emergency message and that are used by the second UE device to perform time and frequency synchronization with the first UE device. The second UE device may periodically activate a receiver during a series of receiver (RX) windows to listen for the D2D signals. The second UE device may deactivate the receiver between the RX windows to conserve power. The second UE device may receive a preamble sequence during one of the RX windows. The second UE device may synchronize its timing to the timing of the first UE device for subsequent receipt of the emergency message based on the preamble sequence received during the RX window. 
     The one or more preambles may, for example, include a series of N preambles. Each preamble in the series of N preambles may identify a respective time offset between that preamble and the subsequent transmission of the emergency message. If desired, the series of N preambles may be divided into a first set of odd-numbered preambles and a second set of even-numbered preambles that precede respective transmissions of the emergency message. In other examples, the one or more preambles may include an extended preamble. The second HE device may receive a portion of the extended preamble during the RX window. The second UE device may keep its receiver active after the RX window and until an end of the extended preamble is received. The second UE device may then re-activate the receiver after a predetermined time offset has elapsed since the end of the extended preamble for receiving the emergency message. If desired, the extended preamble may be provided with a binary cover code that varies across a length of the extended preamble. In these examples, the second UE device may identify the time offset based on the binary cover code and may deactivate the receiver after the RX window to conserve power. These techniques may allow the first and second UE devices to be synchronized for transfer of the emergency message over long distances while one or both of the UE devices is off-grid, all while consuming a minimal amount of power on the second UE device. 
     An aspect of the disclosure provides a user equipment device. The user equipment device may include one or more antennas. The user equipment device may include a transmitter configured to transmit device-to-device (D2D) signals to an additional user equipment device using the one or more antennas. The user equipment device may include one or more processors. The one or more processors may be configured to transmit a message in the D2D signals. The one or more processors may be configured to transmit a preamble in the D2D signals prior to the message that identifies, for the additional user equipment device, a time offset between the preamble and the message. 
     An aspect of the disclosure provides a user equipment device. The user equipment device may include one or more antennas. The user equipment device may include a receiver configured to receive device-to-device (D2D) signals from an additional user device using the one or more antennas. The user equipment device may include one or more processors. The one or more processors may be configured to periodically activate the receiver during a series of receiver (RX) windows. The one or more processors may be configured to receive a preamble sequence in the D2D signals during an RX window of the series of RX windows. The one or more processors may be configured to deactivate the receiver after the RX window. The one or more processors may be configured to identify a time offset based the preamble sequence received during the RX window. The one or more processors may be configured to re-activate the receiver during a message listening window that begins once the identified time offset has elapsed since an end of the preamble sequence. The one or more processors may be configured to receive a message in the D2D signals during the message listening window. 
     An aspect of the disclosure provides a method of operating a first user equipment device to receive, from a second user equipment device, device-to-device (D2D) signals that include an emergency message preamble and an emergency message following the emergency message preamble. The method can include periodically activating a receiver on the first user equipment device during a series of receiver (RX) windows. The method can include receiving a portion of the emergency message preamble in the D2D signals during an RX window of the series of RX windows. The method can include keeping the receiver active until an end of the emergency message preamble has been received and then deactivating the receiver. The method can include re-activating the receiver during an emergency message listening window that begins after a predetermined time offset has elapsed since the end of the emergency message preamble. The method can include receiving the emergency message in the D2D signals during the emergency message listening window. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of an illustrative communications network having user equipment devices and external communications equipment such as a wireless base station or access point in accordance with some embodiments. 
         FIG.  2    is a block diagram of an illustrative user equipment device in accordance with some embodiments. 
         FIG.  3    is a flow chart of illustrative operations involved in using a first user equipment device to transmit device-to-device signals to a second user equipment device while minimizing power consumption at the second user equipment device in accordance with some embodiments. 
         FIG.  4    includes timing diagrams showing how an illustrative first user equipment device may transmit device-to-device signals that include a series of preambles for time-synchronizing an illustrative second user equipment device to the first user equipment device in accordance with some embodiments. 
         FIG.  5    is a flow chart of illustrative operations involved in using a first user equipment device to transmit device-to-device signals and in using a second user equipment device to synchronize receipt of a message transmitted by the first user equipment device based on a series of preambles in the device-to-device signals in accordance with some embodiments. 
         FIG.  6    is a timing diagram showing how an illustrative first user equipment device may divide a series of preambles between multiple transmissions of a message in device-to-device signals in accordance with some embodiments. 
         FIG.  7    includes timing diagrams showing how an illustrative first user equipment device may transmit device-to-device signals that include an extended preamble for time-synchronizing an illustrative second user equipment device to the first user equipment device in accordance with some embodiments. 
         FIG.  8    is a flow chart of illustrative operations involved in using a first user equipment device to transmit device-to-device signals and in using a second user equipment device to synchronize receipt of a message transmitted by the first user equipment device based on an extended preamble in the device-to-device signals in accordance with some embodiments. 
         FIG.  9    is a flow chart of illustrative operations involved in using a first user equipment device to transmit device-to-device signals and in using a second user equipment device to synchronize receipt of a message transmitted by the first user equipment device based on an extended preamble with a cover code in the device-to-device signals in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a schematic diagram of an illustrative communications system  26  (sometimes referred to herein as communications network  26 ) for conveying wireless data between communications terminals. Communications system  26  may include network nodes (e.g., communications terminals). The network nodes may include user equipment (UE) such as one or more UE devices  10 . The network nodes may also include external communications equipment (e.g., communications equipment other than UE devices  10 ) such as external communications equipment  12 . External communications equipment  12  may include a wireless base station or a wireless access point, for example. UE devices  10  and external communications equipment  12  may communicate with each other using wireless communications links. If desired, UE devices  10  may wirelessly communicate with external communications equipment  12  without passing communications through any other intervening network nodes in communications system  26  (e.g., UE devices  10  may communicate directly with external communications equipment  12  over-the-air). 
     Communications system  26  may form a part of a larger communications network that includes network nodes coupled to external communications equipment  12  via wired and/or wireless links. The larger communications network may include one or more wired communications links (e.g., communications links formed using cabling such as ethernet cables, radio-frequency cables such as coaxial cables or other transmission lines, optical fibers or other optical cables, etc.), one or more wireless communications links (e.g., short range wireless communications links that operate over a range of inches, feet, or tens of feet, medium range wireless communications links that operate over a range of hundreds of feet, thousands of feet, miles, or tens of miles, and/or long range wireless communications links that operate over a range of hundreds or thousands of miles, etc.), communications gateways, wireless access points, base stations, switches, routers, servers, modems, repeaters, telephone lines, network cards, line cards, portals, user equipment (e.g., computing devices, mobile devices, etc.), etc. The larger communications network may include communications (network) nodes or terminals coupled together using these components or other components (e.g., some or all of a mesh network, relay network, ring network, local area network, wireless local area network, personal area network, cloud network, star network, tree network, or networks of communications nodes having other network topologies), the Internet, combinations of these, etc. UE devices  10  may send data to and/or may receive data from other nodes or terminals in the larger communications network via external communications equipment  12  (e.g., external communications equipment  12  may serve as an interface between user equipment devices  10  and the rest of the larger communications network). Some or all of the communications network may, if desired, be operated by a corresponding network operator or service provider. 
     External communications equipment  12  may include one or more antennas that provides wireless coverage for UE devices  10  located within a corresponding geographic area or region such as cell  14 . The size of cell  14  may correspond to the maximum transmit power level of external communications equipment  12  and the over-the-air attenuation characteristics for radio-frequency signals conveyed by external communications equipment  12 , for example. When a UE device  10  is located within cell  14 , the UE device may communicate with external communications equipment  12  over a wireless link. To support the wireless link, external communications equipment  12  may transmit radio-frequency signals in a downlink (DL) direction from external communications equipment  12  to the UE device and/or the UE device may transmit radio-frequency signals in an uplink (UL) direction from the UE device to external communications equipment  12 . In the example of  FIG.  1   , a first UE device  10  such as UE device  10 - 1  may be located within cell  14 . UE device  10 - 1  may therefore communicate with external communications equipment  12  over a corresponding wireless link. Radio-frequency signals  16  may be conveyed between UE device  10 - 1  and external communications equipment  12  to support the wireless link. 
     In practice, situations may arise where UE device  10 - 1  is outside of the coverage area of external communications equipment  12  and the coverage area for any other wireless access points or base stations in communications system  26 . For example, UE device  10 - 1  may move to a location  18 , as shown by arrow  24 . Location  18  is outside of cell  14  and outside of the coverage area of any other wireless access points or base stations in communications system  26 . While at location  18 , UE device  10 - 1  may sometimes be referred to as being “off-grid.” UE device  10 - 1  may also be off-grid (e.g., outside of cell  14  and outside of the coverage area of any other wireless access points or base stations in communications system  26 ) when external communications equipment  12  is inactive, disabled, or otherwise unavailable to UE device  10 - 1  (e.g., due to a power outage or other disability at external communications equipment  12 , due to a disaster or other emergency situation, due to network load balancing that excludes UE device  10 - 1 , due to access to the rest of the communications network  26  being blocked or denied to UE device  10 - 1 , due to intervening obstacles, terrain, or weather blocking UE device  10 - 1  from conveying radio-frequency signals with external communications equipment  12 , etc.). Conversely, UE device  10 - 1  may sometimes be referred to as being “on-grid” when UE device  10 - 1  is within a coverage area such as coverage area  14  and is able to convey wireless data with the rest of the network (e.g., communications system  26 ) via external communications equipment  12 . 
     When UE device  10 - 1  is located off-grid, UE device  10 - 1  may still need to provide wireless data such as message data, voice data, video data, or other data to a communications terminal in communications system  26  or to another UE device such as UE device  10 - 2 . For example, the user of UE device  10 - 1  may encounter an emergency while off-grid and may need to use UE device  10 - 1  to send an emergency message to the authorities (e.g., emergency services) and/or another person to alert the authorities and/or another person to the user&#39;s situation and/or to call for help. 
     While off-grid, UE device  10 - 1  may still be able to convey radio-frequency signals with other UE devices such as UE device  10 - 2  (e.g., over a wireless device-to-device (D2D) link). UE device  10 - 1  may have its own coverage area  20  (e.g., extending around location  18  when UE device  10 - 1  is at location  18 ). The size of coverage area  20  is determined by the maximum transmit power level of UE device  10 - 1  and the over-the-air attenuation characteristics for radio-frequency signals transmitted by UE device  10 - 1 . When the user needs to send an emergency message while off-grid, UE device  10 - 1  may transmit radio-frequency signals  22  that include an emergency message or other wireless data. When there is another UE device such as UE device  10 - 2  within coverage area  20 , UE device  10 - 2  may receive radio-frequency signals  22  and thus the emergency message transmitted by UE device  10 - 1 . UE device  10 - 2  may then alert emergency services and/or may provide assistance to the user of UE device  10 - 1 . In situations where UE device  10 - 2  is located within cell  14  (e.g., whereas UE device  10 - 1  is located outside of cell  14 ), UE device  10 - 2  may additional or alternatively relay the emergency message transmitted by UE device  10 - 1  to other network nodes such as network nodes operated by emergency services (e.g., a “911” service in the United States) or to other users. Radio-frequency signals  22  are D2D signals and may therefore sometimes be referred to herein as D2D signals  22 . D2D signals  22  may form a corresponding wireless D2D communications link between UE device  10 - 1  and UE device  10 - 2 . Implementations in which D2D signals  22  include an emergency message transmitted by UE device  10 - 1  are merely illustrative and described herein as an example. In general, D2D signals  22  may include any desired data (e.g., message data, voice data, application data, video data, etc.) for transmission to UE device  10 - 2 . UE device  10 - 2  may also transmit D2D signals to UE device  10 - 1  (e.g., the D2D link may be a bidirectional link). 
       FIG.  2    is a block diagram of an illustrative user equipment device  10  (e.g., one or both of UE devices  10 - 1  and  10 - 2  of  FIG.  1   ). UE device  10  is an electronic device and may therefore sometimes be referred to herein simply as device  10 . UE device  10  may be a computing device such as a laptop computer, a desktop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wristwatch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, a wireless internet-connected voice-controlled speaker, a home entertainment device, a remote control device, a gaming controller, a peripheral user input device, a wireless base station or access point, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     As shown in  FIG.  2   , UE device  10  may include components located on or within an electronic device housing such as housing  50 . Housing  50 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, metal alloys, etc.), other suitable materials, or a combination of these materials. In some situations, parts or all of housing  50  may be formed from dielectric or other low-conductivity material (e.g., glass, ceramic, plastic, sapphire, etc.). In other situations, housing  50  or at least some of the structures that make up housing  50  may be formed from metal elements. 
     UE device  10  may include control circuitry  28 . Control circuitry  28  may include storage such as storage circuitry  30 . Storage circuitry  30  may include hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Storage circuitry  30  may include storage that is integrated within UE device  10  and/or removable storage media. 
     Control circuitry  28  may include processing circuitry such as processing circuitry  32 . Processing circuitry  32  may be used to control the operation of UE device  10 . Processing circuitry  32  may include on one or more processors, microprocessors, microcontrollers, digital signal processors, host processors, baseband processor integrated circuits, application specific integrated circuits, central processing units (CPUs), graphics processing units (GPUs), etc. Control circuitry  28  may be configured to perform operations in UE device  10  using hardware (e.g., dedicated hardware or circuitry), firmware, and/or software. Software code for performing operations in UE device  10  may be stored on storage circuitry  30  (e.g., storage circuitry  30  may include non-transitory (tangible) computer readable storage media that stores the software code). The software code may sometimes be referred to as program instructions, software, data, instructions, or code. Software code stored on storage circuitry  30  may be executed by processing circuitry  32 . 
     Control circuitry  28  may be used to run software on UE device  10  such as satellite navigation applications, internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, etc. To support interactions with external communications equipment, control circuitry  28  may be used in implementing communications protocols. Communications protocols that may be implemented using control circuitry  28  include internet protocols, wireless local area network (WLAN) protocols (e.g., IEEE 802.11 protocols—sometimes referred to as Wi-Fi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol or other wireless personal area network (WPAN) protocols, IEEE 802.11ad protocols (e.g., ultra-wideband protocols), cellular telephone protocols (e.g., 3G protocols, 4G (LTE) protocols, 3GPP Fifth Generation (5G) New Radio (NR) protocols, etc.), antenna diversity protocols, satellite navigation system protocols (e.g., global positioning system (GPS) protocols, global navigation satellite system (GLONASS) protocols, etc.), antenna-based spatial ranging protocols, or any other desired communications protocols. Each communications protocol may be associated with a corresponding radio access technology (RAT) that specifies the physical connection methodology used in implementing the protocol. 
     UE device  10  may include input-output circuitry  36 . Input-output circuitry  36  may include input-output devices  38 . Input-output devices  38  may be used to allow data to be supplied to UE device  10  and to allow data to be provided from UE device  10  to external devices. Input-output devices  38  may include user interface devices, data port devices, and other input-output components. For example, input-output devices  38  may include touch sensors, displays (e.g., touch-sensitive and/or force-sensitive displays), light-emitting components such as displays without touch sensor capabilities, buttons (mechanical, capacitive, optical, etc.), scrolling wheels, touch pads, key pads, keyboards, microphones, cameras, buttons, speakers, status indicators, audio jacks and other audio port components, digital data port devices, motion sensors (accelerometers, gyroscopes, and/or compasses that detect motion), capacitance sensors, proximity sensors, magnetic sensors, force sensors (e.g., force sensors coupled to a display to detect pressure applied to the display), temperature sensors, etc. In some configurations, keyboards, headphones, displays, pointing devices such as trackpads, mice, and joysticks, and other input-output devices may be coupled to UE device  10  using wired or wireless connections (e.g., some of input-output devices  38  may be peripherals that are coupled to a main processing unit or other portion of UE device  10  via a wired or wireless link). 
     Input-output circuitry  36  may include wireless circuitry  34  to support wireless communications. Wireless circuitry  34  (sometimes referred to herein as wireless communications circuitry  34 ) may include one or more antennas  40 . Wireless circuitry  34  may also include one or more radios  44 . Radio  44  may include circuitry that operates on signals at baseband frequencies (e.g., baseband circuitry) and radio-frequency transceiver circuitry such as one or more radio-frequency transmitters  46  and one or more radio-frequency receivers  48 . Transmitter  46  may include signal generator circuitry, modulation circuitry, mixer circuitry for upconverting signals from baseband frequencies to intermediate frequencies and/or radio frequencies, amplifier circuitry such as one or more power amplifiers, digital-to-analog converter (DAC) circuitry, control paths, power supply paths, switching circuitry, filter circuitry, and/or any other circuitry for transmitting radio-frequency signals using antenna(s)  40 . Receiver  48  may include demodulation circuitry, mixer circuitry for downconverting signals from intermediate frequencies and/or radio frequencies to baseband frequencies, amplifier circuitry (e.g., one or more low-noise amplifiers (LNAs)), analog-to-digital converter (ADC) circuitry, control paths, power supply paths, signal paths, switching circuitry, filter circuitry, and/or any other circuitry for receiving radio-frequency signals using antenna(s)  40 . The components of radio  44  may be mounted onto a single substrate or integrated into a single integrated circuit, chip, package, or system-on-chip (SOC) or may be distributed between multiple substrates, integrated circuits, chips, packages, or SOCs. 
     Antenna(s)  40  may be formed using any desired antenna structures for conveying radio-frequency signals. For example, antenna(s)  40  may include antennas with resonating elements that are formed from loop antenna structures, patch antenna structures, inverted-F antenna structures, slot antenna structures, planar inverted-F antenna structures, helical antenna structures, monopole antennas, dipoles, hybrids of these designs, etc. Filter circuitry, switching circuitry, impedance matching circuitry, and/or other antenna tuning components may be adjusted to adjust the frequency response and wireless performance of antenna(s)  40  over time. If desired, two or more of antennas  40  may be integrated into a phased antenna array (sometimes referred to herein as a phased array antenna) in which each of the antennas conveys radio-frequency signals with a respective phase and magnitude that is adjusted over time so the radio-frequency signals constructively and destructively interfere to produce a signal beam in a given pointing direction. 
     The term “convey radio-frequency signals” as used herein means the transmission and/or reception of the radio-frequency signals (e.g., for performing unidirectional and/or bidirectional wireless communications with external wireless communications equipment). Antenna(s)  40  may transmit the radio-frequency signals by radiating the radio-frequency signals into free space (or to free space through intervening device structures such as a dielectric cover layer). Antenna(s)  40  may additionally or alternatively receive the radio-frequency signals from free space (e.g., through intervening devices structures such as a dielectric cover layer). The transmission and reception of radio-frequency signals by antennas  40  each involve the excitation or resonance of antenna currents on an antenna resonating element in the antenna by the radio-frequency signals within the frequency band(s) of operation of the antenna. 
     Each radio  44  may be coupled to one or more antennas  40  over one or more radio-frequency transmission lines  42 . Radio-frequency transmission lines  42  may include coaxial cables, microstrip transmission lines, stripline transmission lines, edge-coupled microstrip transmission lines, edge-coupled stripline transmission lines, transmission lines formed from combinations of transmission lines of these types, etc. Radio-frequency transmission lines  42  may be integrated into rigid and/or flexible printed circuit boards if desired. One or more radio-frequency lines  42  may be shared between multiple radios  44  if desired. Radio-frequency front end (RFFE) modules may be interposed on one or more radio-frequency transmission lines  42 . The radio-frequency front end modules may include substrates, integrated circuits, chips, or packages that are separate from radios  44  and may include filter circuitry, switching circuitry, amplifier circuitry, impedance matching circuitry, radio-frequency coupler circuitry, and/or any other desired radio-frequency circuitry for operating on the radio-frequency signals conveyed over radio-frequency transmission lines  42 . 
     Radio  44  may transmit and/or receive radio-frequency signals within corresponding frequency bands at radio frequencies (sometimes referred to herein as communications bands or simply as “bands”). The frequency bands handled by radio  44  may include wireless local area network (WLAN) frequency bands (e.g., Wi-Fi® (IEEE 802.11) or other WLAN communications bands) such as a 2.4 GHz WLAN band (e.g., from 2400 to 2480 MHz), a 5 GHz WLAN band (e.g., from 5180 to 5825 MHz), a Wi-Fi® 6E band (e.g., from 5925-7125 MHz), and/or other Wi-Fi® bands (e.g., from 1875-5160 MHz), wireless personal area network (WPAN) frequency bands such as the 2.4 GHz Bluetooth® band or other WPAN communications bands, cellular telephone frequency bands (e.g., bands from about 600 MHz to about 5 GHz, 3G bands, 4G LTE bands, 5G New Radio Frequency Range 1 (FR1) bands below 10 GHz, 5G New Radio Frequency Range 2 (FR2) bands between 20 and 60 GHz, etc.), other centimeter or millimeter wave frequency bands between 10-300 GHz, near-field communications frequency bands (e.g., at 13.56 MHz), satellite navigation frequency bands (e.g., a GPS band from 1565 to 1610 MHz, a Global Navigation Satellite System (GLONASS) band, a BeiDou Navigation Satellite System (BDS) band, etc.), ultra-wideband (UWB) frequency bands that operate under the IEEE 802.15.4 protocol and/or other ultra-wideband communications protocols, communications bands under the family of 3GPP wireless communications standards, communications bands under the IEEE 802.XX family of standards, industrial, scientific, and medical (ISM) bands such as an ISM band between around 900 MHz and 950 MHz or other ISM bands below or above 1 GHz, one or more unlicensed bands, one or more bands reserved for emergency and/or public services, and/or any other desired frequency bands of interest. Wireless circuitry  34  may also be used to perform spatial ranging operations if desired. 
     Transmitter  46  may transmit radio-frequency signals over antenna(s)  40  when transmitter  46  is active (e.g., enabled). Transmitter  46  does not transmit radio-frequency signals over antenna(s)  40  when transmitter  46  is inactive (e.g., disabled or not actively transmitting sign). Similarly, receiver  48  may receive radio-frequency signals over antenna(s)  40  when receiver  48  is active (e.g., enabled). Receiver  48  does not receive radio-frequency signals over antenna(s)  40  when receiver  48  is inactive (e.g., disabled). Control circuitry  28  may control transmitter  46  to be active or inactive at any given time. Control circuitry  28  may also control receiver  48  to be active or inactive at any given time. Control circuitry  28  may activate or deactivate transmitter  46  and/or receiver  48  at different times as dictated by a communications protocol governing radio  44  and/or based on instructions provided by a user and/or from other software running on control circuitry  28 , for example. Control circuitry  28  may configure transmitter  46  to be inactive by powering off transmitter  46 , by providing control signals to switching circuitry on power supply or enable lines for transmitter  46 , by providing control signals to control circuitry on transmitter  46 , and/or by providing control signals to switching circuitry within transmitter  46 , for example. When transmitter  46  is inactive, some or all of transmitter  46  may be inactive (e.g., disabled or powered off) or transmitter  46  may remain powered on but without transmitting radio-frequency signals over antenna(s)  40 . Similarly, control circuitry  28  may configure receiver  48  to be inactive by powering off receiver  48 , by providing control signals to switching circuitry on power supply or enable lines for receiver  48 , by providing control signals to control circuitry on receiver  48 , and/or by providing control signals to switching circuitry within receiver  48 , for example. When receiver  48  is inactive, some or all of receiver  48  may be disabled (e.g., powered off) or receiver  48  may remain powered on but without actively receiving radio-frequency signals incident upon antenna(s)  40 . Transmitter  46  and receiver  48  may consume more power on UE device  10  when active than when inactive (e.g., a battery on UE device  10  may drain more rapidly while transmitter  46  and receiver  48  are active than while transmitter  46  or receiver  48  are inactive). 
     The example of  FIG.  2    is merely illustrative. While control circuitry  28  is shown separately from wireless circuitry  34  in the example of  FIG.  1    for the sake of clarity, wireless circuitry  34  may include processing circuitry (e.g., one or more processors) that forms a part of processing circuitry  32  and/or storage circuitry that forms a part of storage circuitry  30  of control circuitry  28  (e.g., portions of control circuitry  28  may be implemented on wireless circuitry  34 ). As an example, control circuitry  28  may include baseband circuitry (e.g., one or more baseband processors), digital control circuitry, analog control circuitry, and/or other control circuitry that forms part of radio  44 . The baseband circuitry may, for example, access a communication protocol stack on control circuitry  28  (e.g., storage circuitry  30 ) to: perform user plane functions at a PHY layer, MAC layer, RLC layer, PDCP layer, SDAP layer, and/or PDU layer, and/or to perform control plane functions at the PHY layer, MAC layer, RLC layer, PDCP layer, RRC, layer, and/or non-access stratum layer. If desired, the PHY layer operations may additionally or alternatively be performed by radio-frequency (RF) interface circuitry in wireless circuitry  34 . 
     When UE device  10 - 1  of  FIG.  1    is off-grid, UE device  10 - 1  should still be reachable in case the user of UE device  10 - 1  encounters an emergency or otherwise needs to transmit wireless data to another UE device such as UE device  10 - 2 . To maximize the likelihood that another UE device such as UE device  10 - 2  will be able to receive D2D signals  22 , UE device  10 - 1  should be able to transmit D2D signals  22  over a relatively long distance (e.g., it may be desirable for UE device  10 - 1  to have as large a coverage area  20  as possible). This distance (e.g., the radius of coverage area  20 ) may be as far as hundreds of meters, a few km, several km, or dozens of km, for example. UE device  10 - 1  may maximize the range of D2D signals  22  by transmitting at relatively high transmit power levels (e.g., a maximum transmit power level) and for a relatively long amount of time. 
     In general, UE device  10 - 1  may transmit D2D signals  22  at any desired frequencies (e.g., frequencies in an ISM band, an unlicensed band, a band reserved for emergency/public services, etc.). If desired, UE device  10 - 1  may transmit D2D signals  22  at relatively low frequencies such as frequencies in a frequency band below 1 GHz, below 2 GHz, below 3 GHz, below 950 MHz, etc. This may serve to minimize over-the-air signal attenuation for D2D signals  22 , thereby maximizing the size of coverage area  20 . The wireless circuitry  34  on UE device  10 - 1  may include a dedicated radio  44  for transmitting D2D signals  22  or the radio that transmits D2D signals  22  may also transmit other signals associated with other communications protocols or RATs (e.g., a single radio  44  on UE device  10 - 1  may convey both WLAN signals and D2D signals  22 , a single radio  44  on UE device  10 - 1  may convey both cellular telephone signals and D2D signals  22 , etc.). 
     At the same time, even when UE device  10 - 2  is located within coverage area  20 , UE device  10 - 2  is only able to correctly recover wireless data (e.g., an emergency message) in D2D signals  22  ( a ) when the receiver  48  on UE device  10 - 2  is active and (b) when UE device  10 - 2  is time-synchronized with UE device  10 - 1 . While UE device  10 - 2  can keep its receiver  48  active at all times to listen for any D2D signals  22  that happen to be transmitted, this would consume an excessive amount of power in UE device  10 - 2 , causing UE device  10 - 2  to drain its battery relatively quickly. Keeping receiver  48  active at all times is particularly power-inefficient because off-grid UE devices such as UE device  10 - 1  only need to transmit emergency messages or other wireless data in D2D signals  22  on rare occasions. In addition, while UE devices can synchronize to each other using signals from external communications equipment  12  when located within cell  14  (e.g., the base station can configure sleeping patterns and paging cycles to allow the devices to sleep when able to save power), UE devices that are off-grid such as UE device  10 - 1  and UE device  10 - 2  are not previously synchronized to each other or to a time reference. Even if the UE devices are time-synchronized at one point in time (e.g., while both UE devices are on-grid), the timing for UE device  10 - 1  can drift with respect to the timing for UE device  10 - 2  once one or both of the UE devices go off-grid. As such, simple paging mechanisms may be insufficient to allow UE device  10 - 2  to correctly receive and recover wireless data in D2D signals  22 . It would therefore be desirable to provide power efficient systems and methods for the time-synchronized transmission of D2D signals  22  by UE device  10 - 1  and reception of D2D signals  22  by UE device  10 - 2  while at least UE device  10 - 1  is off-grid. 
     Some communications protocols such as Wi-Fi incorporate time synchronization methods that are configured for much shorter ranges than those associated with coverage area  20 . These short-range protocols are unsuitable for conveying D2D signals  22  because the protocols would consume an excessive amount of power on UE devices  10 - 1  and/or  10 - 2 , thereby draining battery at an excessive rate. In addition, these short-range protocols also operate under control of external communications equipment such as a wireless access point (AP) and follow the timing of the AP, but there is no AP available when the UE devices are off-grid. Other protocols like Wi-Fi neighborhood aware network (NAN) protocols do not support the desired long ranges for D2D signals  22  and expend excessive energy on maintaining synchronization so as not to be suitable for rare communications events like emergency message transmission. 
     In order to allow UE device  10 - 2  to minimize power consumption while listening for potential D2D signals  22 , UE device  10 - 2  may periodically activate its receiver  48  during receiver (RX) windows, during which the receiver is able to receive D2D signals  22  (e.g., where the receiver is inactive between the RX windows). When the duration of the RX windows is short, there is a high likelihood that any transmission of D2D signals  22  will arrive at UE device  10 - 2  while the receiver is inactive—thereby preventing proper recovery of the data in D2D signals  22  by UE device  10 - 2 . When the duration of the RX windows is long, there is a greater likelihood that a transmission of D2D signals  22  will arrive at UE device  10 - 2 . However, longer RX windows cause UE device  10 - 2  to consume an excessive amount of power, in effect “searching” for D2D signals  22  that are only rarely transmitted by another UE device. It would instead be more power efficient for UE device  10 - 1  to expend power searching for a potential receiver device such as UE device  10 - 2  than vice versa. UE device  10 - 1  may search for a potential receiver device by transmitting one or more preambles before transmitting the desired data (e.g., an emergency message) for reception at UE device  10 - 2 . 
       FIG.  3    is a flow chart of illustrative operations involved in conveying D2D signals  22  between UE device  10 - 1  and UE device  10 - 2  in a time-synchronized and power efficient manner. Operations  52 - 56  and  66  of  FIG.  3    may be performed by UE device  10 - 1 . Operations  60 - 66  of  FIG.  3    may be performed by UE device  10 - 2 . 
     At operation  52 , UE device  10 - 1  may communicate with the network (e.g., network nodes or terminals of communications system  26 ) via external communications equipment  12  while on-grid (e.g., while within cell  14  of  FIG.  1   ). When UE device  10 - 1  becomes off-grid, processing may proceed to operation  54 . 
     At operation  60 , UE device  10 - 2  may periodically activate its receiver  48  during a series of RX windows. UE device  10 - 2  may listen for D2D signals  22  during the RX windows. The receiver  48  on UE device  10 - 2  may be inactive between the RX windows to conserve power. 
     At operation  54 , while UE device  10 - 1  is off-grid, UE device  10 - 1  may identify wireless data for transmission to another UE device such as UE device  10 - 2  using D2D signals  22 . UE device  10 - 1  may, for example, identify an emergency message to transmit in D2D signals  22 . UE device  10 - 1  may generate the emergency message in response to a user input provided to a software application running on UE device  10 - 1  (e.g., in response to a user pressing an emergency call, emergency message, or “SOS” button displayed on a touch screen display, in response to a user activating an emergency response software application, etc.), in response to an application call by a software application running on UE device  10 - 1 , in response to sensor data gathered by one or more sensors on UE device  10 - 1  (e.g., accelerometer data or other sensor data indicating that UE device  10 - 1  has been in an accident, fallen a great distance, been subject to a forceful impact, etc.), and/or in response to any other desired trigger condition. The emergency message may include information identifying the location of UE device  10 - 1  (e.g., based on GPS data or other location information gathered by UE device  10 - 1 ), user information, a message drafted by the user, and/or any other desired information to alert another person or emergency services of the user&#39;s situation or to call for help, as examples. 
     At operation  56 , UE device  10 - 1  may activate its transmitter  46  and may transmit D2D signals  22  that include one or more preambles followed by the emergency message (e.g., D2D signals  22  may include the emergency message preceded by the one or more preambles). If desired, UE device  10 - 1  may periodically repeat this transmission, as shown by loopback path  58 , to maximize the probability that another UE device such as UE device  10 - 2  will receive D2D signals  22 . The emergency message may be separated from the end of the one or more preambles by a predetermined time period or offset time. This predetermined time period or offset time may be known to UE device  10 - 2  if desired (e.g., from software or other data on both UE device  10 - 1  and  10 - 2 , from a communications protocol governing D2D signals  22 , etc.). 
     At operation  62 , UE device  10 - 2  may receive at least a portion of one or more of the preambles transmitted by UE device  10 - 1  in D2D signals  22  during one of the RX windows during which the receiver  48  on UE device  10 - 2  is active. The control circuitry  28  on UE device  10 - 2  may process the received preamble to synchronize timing with UE device  10 - 1 . For example, UE device  10 - 2  may identify (e.g., determine, calculate, compute, generate, produce, etc.) timing for an emergency message listening window during which UE device  10 - 1  will transmit the emergency message (e.g., the emergency message listening window may begin at an initial time that is separated from the end of the one or more preambles by a predetermined time period or offset time). This may serve to time-synchronize UE device  10 - 2  to UE device  10 - 1  so UE device  10 - 2  will be able to correctly recover the emergency message transmitted by UE device  10 - 1 . If desired, UE device  10 - 2  may deactivate receiver  48  after identifying this timing and/or after receipt of the one or more preambles to conserve power. 
     At operation  64 , the control circuitry  28  on UE device  10 - 2  may re-activate its receiver  48  during the emergency message listening window (e.g., based on the timing identified at operation  62 ). The receiver  48  on UE device  10 - 2  may receive the emergency message transmitted during the emergency message listening window. UE device  10 - 2  may perform any desired subsequent processing based on the received emergency message. For example, UE device  10 - 2  may alert or inform a user of UE device  10 - 2  about the emergency message and/or its contents, may transmit UL signals to external communications equipment  12  informing the network of the emergency message (e.g., when UE device  10 - 2  is located within cell  14 ), may transmit additional D2D signals to another UE device to inform that UE device of the emergency message (e.g., UE device  10 - 2  may provide the emergency message to external communications equipment  12  or other portions of the network via a relay of one or more additional UE devices, the last of which is located within cell  14 ), etc. 
     If desired, processing may proceed from operations  56  and  64  to optional operation  66 . At optional operation  66 , UE device  10 - 1  and UE device  10 - 2  may continue to communicate unidirectionally or bidirectionally using D2D signals (e.g., where the communications are time-synchronized and frequency-synchronized based on the emergency preambles processed by UE device  10 - 2  at operation  62 ). The example of  FIG.  3    is merely illustrative. One or more iterations of operation  56  may be performed by UE device  10 - 1  concurrently with the performance of operations  62  and/or  64  by UE device  10 - 2 . 
       FIG.  4    includes timing diagrams for one example in which the one or more preambles transmitted by UE device  10 - 1  (e.g., at operation  56  of  FIG.  3   ) include a series of preambles that identify respective time offsets for synchronizing the timing of UE device  10 - 2  to the timing of UE device  10 - 1 . 
     Timing diagram  68  of  FIG.  4    plots transmit (TX) timing for UE device  10 - 1  in transmitting D2D signals  22 . Timing diagram  70  of  FIG.  4    plots receive (RX) timing for UE device  10 - 2  in receiving D2D signals  22 . As shown by timing diagram  68 , UE device  10 - 1  may begin to transmit a series of N preambles  72  at time TA, each of which is labeled by a corresponding index n from 1 to N (e.g., a first preamble  72 - 1 , a second preamble  72 - 2  following preamble  72 - 1 , an Nth preamble  72 -N following an (N−1)th preamble, etc.). UE device  10 - 1  may then begin to transmit emergency message  74  after transmission of the series of N preambles  72  (e.g., after a predetermined offset time such as at time TC). 
     Each preamble  72  may be a sequence of bits having a predetermined (temporal) preamble length T that allows UE device  10 - 2  to detect the presence of a signal and to allow the receiver on UE device  10 - 2  to synchronize to the transmit timing for the transmitter in UE device  10 - 1 . Preamble length T may be less than or equal to 1 ms, as an example. The duration of emergency message  74  is generally much longer than preamble length T. Preambles  72  may sometimes also be referred to herein as preamble sequences  72  or synchronization sequences  72 . Each preamble  72  may be a known signal (e.g., of a format known to both UE device  10 - 1  and UE device  10 - 2 ) having relatively strong correlation properties to allow UE device  10 - 2  to easily detect and distinguish the preamble from background noise. Each preamble  72  may be a Zadoff Chu sequence or an M-sequence, as two examples. 
     Each preamble  72  in the series of N preambles may include or otherwise identify respective offset information OFS n . Offset information OFS n  may identify a time offset ΔTn between the corresponding preamble and the time at which UE device  10 - 1  is going to begin transmitting emergency message  74  (time TC in the example of  FIG.  4   ). For example, preamble  72 - 1  may include offset information OFS 1  that identifies a time offset ΔT 1  from the end of preamble  72 - 1  to the beginning of emergency message  74  (e.g., time offset ΔT 1  may be the time duration between time TA+T and time TC), preamble  72 - 2  may include offset information OFS 2  that identifies a time offset ΔT 2  from the end of preamble  72 - 2  to time TC, preamble  72 -N may include offset information OFS N  that identifies a time offset ΔTN from the end of preamble  72 -N to time TC, etc. The series of N preambles  72  may be offset from emergency message  74  (in time) by offset time ΔTN. 
     As shown by timing diagrams  68  and  70 , UE device  10 - 1  may initially be out-of-sync with respect to UE device  10 - 2  by an a priori time offset  78 . For example, a time t 0  as kept by UE device  10 - 2  may have drifted from the corresponding time t 0  as kept by UE device  10 - 1  by a priori time offset  78  since one or both of the UE devices went off-grid. UE devices  10 - 1  and  10 - 2  may retain coarse time synchronization via an internal/network standard/protocol (e.g., UTC) time (e.g., to within a maximum supported time uncertainty  82 , which is equal to T*N). This internal standard time may be periodically adjusted/corrected whenever the UE device communicates with external communications equipment  12  or receives satellite navigation signals if desired (e.g., to synchronize time t 0  at each UE device to external communications equipment  12  and/or a satellite navigation system). Preambles  72  may allow UE device  10 - 2  to fine-tune its synchronization within maximum supported time uncertainty  82  to allow UE device  10 - 2  to correctly receive emergency message  74  while consuming a minimal amount of power. 
     When no time reference is available, a device will drift by D ppm so that at some later time, the relative offset between devices may prevent synchronization within maximum supported time uncertainty  82 . As one possible countermeasure, UE device  10 - 1  may occasionally transmit a global clock beat preamble while off-grid that allows all nearby off-grid devices such as UE device  10 - 2  to adjust its time t 0  to match time t 0  on UE device  10 - 1  (e.g., so UE device  10 - 2  is coarsely synchronized to UE device  10 - 1  in case UE device  10 - 1  later needs to transmit an emergency message). Such a global clock beat preamble may be transmitted once an hour with some random offset, as one example. 
     UE device  10 - 2  may periodically activate its receiver  48  during RX windows  76 , one of which is shown in  FIG.  4   . Each RX window  76  may be separated in time from the preceding and subsequent RX window by RX window periodicity P (not shown in  FIG.  4    for the sake of clarity). RX window  76  may have (temporal) RX window length L. RX window length L may be greater than preamble length T to ensure that UE device  10 - 2  is able to fully receive at least one of the preambles  72  in the series of N preambles  72 . RX window length L may, for example, be equal to 2*T, 1.5*T, 3*T, 4*T, more than 4*T, etc. 
     As shown by timing diagram  70 , UE device  10 - 2  may receive preamble  72 - 1  during RX window  76 . The control circuitry  28  on UE device  10 - 2  may process the received preamble  72 - 1  to identify time offset ΔT 1  from the offset information OFS 1  of preamble  72 - 1  (e.g., while processing operation  62  of  FIG.  3   ). UE device  10 - 2  may then have knowledge that it will receive emergency message  74  after time offset ΔT 1  elapses after the end of preamble  72 - 1 , thereby synchronizing the timing of UE device  10 - 2  to the timing of UE device  10 - 1 . UE device  10 - 2  may deactivate its receiver  48  after RX time window  76  has ended to conserve power. UE device  10 - 2  may reactivate receiver  48  after time offset ΔT 1  has elapsed since the end of preamble  72 - 1  (e.g., during an emergency message listening window  80  beginning at time TC). UE device  10 - 2  may then receive emergency message  74  during emergency message listening window  80 . UE device  10 - 2  may correctly recover the emergency message  74  from D2D signals  22  because UE device  10 - 2  has been time-synchronized to UE device  10 - 1  using preambles  72 . By transmitting a series of preambles  72 , each of which identifies a respective time offset ΔTn, UE device  10 - 2  will be able to synchronize its timing to UE device  10 - 1  (e.g., for receipt of emergency message  74  at a common time TC now shared by both UE devices) regardless of where RX window  76  happens to land in the series of N preambles  72  transmitted by UE device  10 - 1 . This may allow UE devices  10 - 1  and  10 - 2  to time-synchronize for receipt of a message in D2D signals  22  (e.g., emergency message  74 ) despite one or both devices being off-grid and therefore subject to potential clocking drift between the devices, while consuming a minimal amount of power on UE device  10 - 2 . 
     Each preamble  72  may, for example, be the same type of sequence but having different variants for each index n (e.g., where UE device  10 - 2  identifies offset information OFS n  and thus time offset ΔTn based on the particular variant of the sequence that is received during its RX window  76 ). Each variant may have strong autocorrelation properties with low cross-correlation between different variants (e.g., between variants or different preambles in the series of N preambles  72 ). Zadoff Chu sequences or M-sequences are two non-limiting examples of sequences that may fit these criteria. Depending on how many sequence variants are feasible (e.g., how large N can feasibly be), each variant may carry further information beyond time offset. For example, each variant may identify a frequency to use or a frequency hopping pattern indication to use for the subsequent receipt of emergency message  74 . UE device  10 - 2  may process the preamble  72  received during its RX window  76  to identify the frequency or frequency hopping pattern indication to use for receiving emergency message  74 . UE device  10 - 2  may then use that frequency or a frequency hopping pattern corresponding to the frequency hopping pattern indication when receiving emergency message  74  (e.g., within emergency message listening window  80 ). If desired, sequence variants may be used to convey further information besides beam hopping patterns. For example, sequence variants may be used to distinguish emergency from other paging types (e.g., where UE device  10 - 1  searches for another UE device  10 - 2  in their contact list to chat), limiting the number of potential responders to UE device  10 - 2  (e.g., by asking for devices that have internet connectivity), for dissemination of clock synchronization information, etc. 
     Detecting the particular variant of the preamble sequence received during RX window  76  (e.g., detecting the particular preamble  72  received during RX window  76 ) may carry the risk of a false positive detection. However, if the false positive rate is relatively low, only a small amount of energy will be consumed by UE device  10 - 2  in an unneeded emergency message reception attempt (e.g., activating the receiver on UE device  10 - 2  during emergency message listening window  80  does not excessively drain the battery on UE device  10 - 2 ). Preambles  72  may give UE device  10 - 2  an indication of the presence of an emergency message that is about to be transmitted while allowing for time and frequency synchronization between UE device  10 - 1  and  10 - 2 . Emergency message  74  may include a cyclic redundancy check (CRC) or other measures that make it almost impossible for UE device  10 - 2  to decode a false positive emergency message. If desired, UE device  10 - 2  may leave its RX window open after receipt of a preamble  72  so UE device  10 - 2  receives all subsequent preambles  72 . This may serve to further reduce the presence of false positives, for example. 
       FIG.  5    is a flow chart showing how UE devices  10 - 1  and  10 - 2  may time-synchronize for the transfer of emergency message  74  based on the series of N preambles  72  shown in  FIG.  4   . Operations  90 - 94  of  FIG.  5    may be performed by UE device  10 - 1  (e.g., while processing operation  56  of  FIG.  3   ). Operations  96 - 102  of  FIG.  5    may be performed by UE device  10 - 2  (e.g., while processing operations  62  and  64  of  FIG.  3   ). 
     At operation  90  of  FIG.  5   , wireless circuitry  34  on UE device  10 - 1  may generate the series of N preambles  72  for transmission. Each preamble  72  may include respective offset information OFS n , each of which identifies a corresponding time offset ΔTn from the end of that preamble until the transmission of emergency message  74  at time TC. 
     At operation  92 , UE device  10 - 1  may begin transmitting the series of N preambles to search for a UE device within its coverage area  20  ( FIG.  1   ). 
     At operation  96 , the receiver  48  on UE device  10 - 2  (e.g., a UE device in coverage area  20 ) may receive one of the N preambles  72  transmitted by UE device  10 - 1  during one of its RX windows  76  when the receiver  48  on UE device  10 - 2  is active. For example, UE device  10 - 2  may receive preamble  72 - 1  during the RX window  76  shown in  FIG.  4   . 
     At operation  98 , the control circuitry  28  on UE device  10 - 2  may identify (e.g., calculate, generate, determine, produce, compute, etc.) the time offset ΔTn associated with the received preamble  72  (e.g., control circuitry  28  may identify offset information OFS n  and may identify time offset ΔTn based the identified offset information OFS n ). In the example of  FIG.  4   , UE device  10 - 2  may identify time offset ΔT 1  based on the offset information OFS 1  in preamble  72 - 1 . UE device  10 - 2  may identify the time offset according to the communications protocol governing D2D signals  22  or any other standard or scheme that is known to both UE devices  10 - 1  and  10 - 2  (e.g., as implemented using one or more software applications running on both UE devices  10 - 1  and  10 - 2 ). 
     At operation  100 , UE device  10 - 2  may deactivate its receiver  48  (e.g., when the RX window has ended). 
     At operation  102 , UE device  10 - 2  may reactivate its receiver  48  after the identified time offset ΔTn has elapsed since the end of the preamble received during RX window  76  (e.g., during emergency message listening window  80  beginning after ΔTn has elapsed since the end of the preamble). UE device  10 - 2  may then receive emergency message  74  during emergency message listening window  80  (sometimes referred to herein as emergency message listening period  80 ). In the example of  FIG.  4   , UE device  10 - 2  may reactivate its receiver  48  at time TC, which is separated in time from the end of preamble  72 - 1  by time offset ΔT 1 . UE device  10 - 2  may perform any desired subsequent processing based on the received emergency message  74 . 
     As an example, preamble length T may be 25 ms. A relatively long preamble length such as 25 ms may allow for reliable detection of the preambles at a maximum distance. If RX window length L is 2*T and the preambles are consecutive, the duration of at least one full preamble length T will fit inside RX window  76 . With a RX window periodicity P=5 seconds and an active duty cycle of 1%, for example, RX window length L will be 50 ms. With no preamble repetitions (i.e., with N=1), the maximum time offset would need to be +/−12.5 ms to still have one full preamble in the RX window. With a relative time drift of +/−10 ppm between UE device  10 - 1  and UE device  10 - 2 , the UE devices would have drifted too far to time-synchronize after only about 20 minutes. Longer drift times between UE device  10 - 1  and UE device  10 - 2  are supported by using UE device  10 - 1  to perform transmitter-side searching for a receiver rather than using the receiver to search for a transmitter (e.g., by using UE device  10 - 1  to transmit a series of N preambles  72  where N is greater than one). For example, if N=4, the maximum time offset would need to be +/−(N/2)*L. When RX window length L is 50 ms and the drift is +/−10 ppm, the UE devices may still be time-synchronized using the transmitted preambles as long as 1.4 hours after going off-grid. Increasing N can further increase this time. In-device coexistence time alignment needs may be another motivation for the receiver other than clock drift, if desired. For example, time gaps of varying lengths may be associated with in-device coexistence schemes where the transmitter needs to protect other ongoing TX/RX operations (having arbitrary durations or transmit patterns) from the preamble transmission. 
     Because UE device  10 - 1  might be located relatively far from UE device  10 - 2 , the transmitter  46  on UE device  10 - 1  may transmit for relatively long time periods and with relatively high transmit power levels. In some scenarios, these high transmit power levels may prevent the transmitter from continuously transmitting multiple sequential preambles  72  (e.g., because the power amplifier(s) in the transmitter may need time to cool off between transmissions). To mitigate these issues, UE device  10 - 1  may transmit a first subset of the series of N preambles  72  prior to a first transmission of emergency message  74  and then may transmit a second subset of the series of N preambles  72  prior to a subsequent second transmission of emergency message  74 . 
       FIG.  6    is a timing diagram showing one example of how UE device  10 - 1  may transmit different subsets of the series of N preambles  72  prior to different transmissions of emergency message  74  (e.g., to allow cool-off time for the power amplifier(s) in transmitter  46 ). As shown in  FIG.  6   , UE device  10 - 1  may transmit a first set of the N preambles  72  such as the odd-numbered preambles in the series of preambles. For example, UE device  10 - 1  may transmit the first preamble  72 - 1  from the series of N preambles  72  at time TA. UE device  10 - 1  may then forego transmission of the second preamble from the series of N preambles  72  (e.g., for preamble length T) after transmission of the first preamble  72 - 1 . UE device  10 - 1  may then transmit the third preamble  72 - 3  from the series of N preambles  72 , may forego transmission of the fourth preamble, etc. At time TC, UE device  10 - 1  may transmit emergency message  74 . Foregoing transmission of the even-numbered preambles may allow the power amplifier(s) in UE device  10 - 1  to cool off between transmission of the odd-numbered preambles. 
     After the transmission of emergency message  74  (e.g., after RX window periodicity P has elapsed since time TA), UE device  10 - 1  may then transmit the even-numbered preambles from the series of N preambles  72  prior to re-transmitting emergency message  74 . For example, at time TA′, UE device  10 - 1  may forego transmission of the first preamble from the series of N preambles  72  (e.g., for preamble length T). After preamble length T has passed (e.g., at time TA′+T), UE device  10 - 1  may transmit the second preamble  72 - 2  from the series of N preambles  72 . UE device  10 - 2  may then forego transmission of the third preamble from the series of N preambles  72  (e.g., for preamble length T) after transmission of the second preamble  72 - 2 . UE device  10 - 1  may then transmit the fourth preamble  72 - 4  from the series of N preambles  72 , may forego transmission of the fifth preamble, etc. At time TC′, UE device  10 - 1  may then re-transmit emergency message  74 . This may allow UE device  10 - 2  to receive one of the preambles  72  prior to a transmission of emergency message  74  that is offset from the received preamble  72  by the time offset identified by that preamble  72 , while allowing the power amplifier(s) on UE device  10 - 1  to cool off between transmissions at relatively high transmit power levels. 
     The example of  FIG.  6    is merely illustrative and, in general, the first set and second sets of preambles may include any desired preambles from the series of N preambles  72  (e.g., the first set of preambles may include every other pair of sequential preambles  72 , every other trio of sequential preambles  72 , etc.). UE device  10 - 1  may divide the series of N preambles  72  into any desired number of sets (e.g., more than two sets) for transmission prior to any desired number of re-transmissions of emergency message  74 . The example of  FIGS.  4 - 6    in which UE device  10 - 1  transmits a series of multiple preambles  72  is merely illustrative. If desired, UE device  10 - 1  may instead transmit a single long (extended) preamble prior to transmitting emergency message  74 . 
       FIG.  7    includes timing diagrams for one example in which UE device  10 - 1  transmits a single extended preamble prior to transmitting emergency message  74 . Timing diagram  104  of  FIG.  7    plots TX timing for UE device  10 - 1  in transmitting D2D signals  22 . Timing diagram  106  of  FIG.  7    plots RX timing for UE device  10 - 2  in receiving D2D signals  22 . 
     As shown by timing diagram  104 , UE device  10 - 1  may begin to transmit a single extended preamble  108  at time TA. Extended preamble  108  may sometimes also be referred to herein as extended preamble sequence  108  or extended synchronization sequence  108 . Extended preamble  108  may have a duration extending from time TA until time TD. Extended preamble  108  may be a continuous preamble sequence (e.g., having a continuous series of bits). As an example, the sequence may be a repetition of the same sub-sequence, such as a repetition of a Zadoff Chu sequence (e.g., where each symbol lasts 1/14 ms) for hundreds of ms (e.g., extended preamble  108  may be hundreds of ms long). 
     As shown by timing diagram  106 , the receiver on UE device  10 - 2  may be active during an RX window  76  that falls between times TA and TD. UE device  10 - 2  may therefore receive the portion (subset or subsequence) of extended preamble  108  falling within RX window  76 . If desired, once UE device  10 - 2  has received a portion of extended preamble  108  within RX window  76 , UE device  10 - 2  may keep its receiver  48  active after the end of RX window  76  until UE device  10 - 2  has received the end of extended preamble  108  (e.g., during an extended RX window  76 E that extends the active time of the receiver from RX window length L until time TD). Once UE device  10 - 2  has received the end of extended preamble  108 , UE device  10 - 2  may deactivate its receiver for predetermined time offset  110  (e.g., UE device  10 - 2  may deactivate receiver  48  in response to receiving the end of extended preamble  108 ). 
     After predetermined time offset  110  (e.g., at time TC), UE device  10 - 1  may transmit emergency message  74  and UE device  10 - 2  may re-activate its receiver (e.g., during receiver listening window  80 ) for receiving emergency message  74 . Predetermined time offset  110  may be known to both UE device  10 - 1  and UE device  10 - 2  according to the communications protocol governing D2D signals  22 , any other standard or scheme that is known to both UE devices  10 - 1  and  10 - 2  (e.g., as implemented using one or more software applications running on both UE devices  10 - 1  and  10 - 2 ), identified by information conveyed in extended preamble  108  itself, etc. In this way, the end of extended preamble  108  may serve as the time-synchronization trigger to synchronize the timing of UE device  10 - 2  with the timing of UE device  10 - 1  for receipt of emergency message  74 , even if the UE devices had previous drifted apart by a priori time offset  78 . Keeping the receiver on UE device  10 - 2  active during extended RX window  76 E may allow UE device  10 - 2  to re-confirm the initial detection of extended preamble  108  during RX window  76 , for example. 
       FIG.  8    is a flow chart of illustrative operations involved in transmitting and receiving an emergency message  74  that is time-synchronized based on extended preamble  108  of  FIG.  7   . Operations  120 - 124  of  FIG.  8    may be performed by UE device  10 - 1  (e.g., while processing operation  56  of  FIG.  3   ). Operations  126 - 132  of  FIG.  8    may be performed by UE device  10 - 2  (e.g., while processing operations  62  and  64  of  FIG.  3   ). 
     At operation  120  of  FIG.  8   , UE device  10 - 1  may generate extended preamble  108  for transmission. 
     At operation  122 , UE device  10 - 1  may begin transmitting extended preamble  108  to search for a UE device within its coverage area  20  ( FIG.  1   ). 
     At operation  126 , the receiver  48  on UE device  10 - 2  (e.g., a UE device in coverage area  20 ) may receive a portion (subset or subsequence) of the extended preamble  108  transmitted by UE device  10 - 1  during one of its RX windows  76  when the receiver on UE device  10 - 2  is active. 
     At operation  128 , UE device  10 - 2  may keep its receiver  48  active after RX window  76  has elapsed (e.g., during extended RX window  76 E of  FIG.  7   ). UE device  10 - 2  may continue to receive extended preamble  108  (during extended RX window  76 E) until UE device  10 - 2  receives the end of extended preamble  108 . 
     At operation  130 , once UE device  10 - 2  has received the end of extended preamble  108 , UE device  10 - 2  may disable its receiver  48  to conserve power. 
     At operation  124 , after predetermined time offset  110  has elapsed since the end of extended preamble  108  was transmitted, UE device  10 - 1  may transmit emergency message  74 . 
     At operation  132 , after predetermined time offset  110  has elapsed since the end of extended preamble  108  was received, UE device  10 - 2  may reactivate its receiver  48  for emergency message listening window  80  (e.g., beginning at time TC). UE device  10 - 2  may then receive and decode emergency message  74  during emergency message listening window  80  (e.g., because UE device  10 - 2  has now been time-synchronized with UE device  10 - 1  using extended preamble  108 ). UE device  10 - 2  may perform any desired subsequent processing based on emergency message  74 . 
     Using extended preamble  108  to time-synchronize UE device  10 - 2  to UE device  10 - 1  may allow UE device  10 - 2  to implement RX windows  76  having arbitrary RX window lengths L. For example, RX window length L does not need to be at least 2× the size of preamble length T ( FIG.  4   ) to guarantee that a complete sequence is captured. RX window length L may, for example, be equal to the preamble length T of each of the preambles  72  in the series of N preambles  72  of  FIG.  6    if desired. This may allow UE device  10 - 2  to further reduce power consumption relative to examples where a series of N preambles  72  is used for time-synchronization. If desired, the receiver  48  on UE device  10 - 2  may dynamically adapt or adjust RX window length L over time (e.g., to make detection more or less sensitive and/or to consume more or less power based on the current battery capacity or power consumption on UE device  10 - 2 ). If desired, different types of UE device  10 - 2  may use different RX window lengths L to account for different antenna/receiver performance. UE device  10 - 2  may detect the end of extended preamble  108  by exploiting the fact that by time TD the sequence has already been detected at UE device  10 - 2  (e.g., using coherent detection), by exploiting the fact that the end can only appear at fixed positions of the (sub)sequence, etc. Usage of different sequence types (e.g., to convey limited information such as information for identifying one or more frequencies for receipt of emergency message  74  or identifying predetermined time offset  110 ) is still possible using extended preamble  108  (e.g., by selection of Zadoff Chu sequence root, M-sequence index, etc.). 
     The example of  FIGS.  7  and  8    in which UE device  10 - 2  keeps its receiver  48  active during extended RX window  76 E is merely illustrative. If desired, UE device  10 - 2  may deactivate its receiver  48  after the RX window  76  in which a portion of extended preamble  108  was received. For example, as shown in  FIG.  7   , UE device  10 - 2  may deactivate its receiver  48  at the end of RX window  76 . In this example, time-synchronization may be performed based on where in the extended preamble  108  RX window  76  falls. Extended preamble  108  may therefore include a sequence that allows UE device  10 - 2  to detect where the portion of extended preamble  108  received during RX window  76  is located within extended preamble  108  as a whole. 
     For example, UE device  10 - 1  may generate extended preamble  108  as a continuous non-repeating sequence that allows locating position of any subsequence of extended preamble  108  in the continuous non-repeating sequence. The transmitter in UE device  10 - 1  may produce such a sequence by applying a binary cover code (or a non-binary cover code) on top of an underlying sequence, where the cover code varies across the length of extended preamble  108 . The continuous non-repeating sequence may be, for example, a repeated underlying Zadoff Chu sequence that has been provided with a binary cover code that varies across the length of extended preamble  108 . 
     Once UE device  10 - 2  has received a portion (subsequence) of extended preamble  108  during its RX window  76 , UE device  10 - 2  may process the received portion (subsequence) to identify a time offset between the end of RX window  76  and time TC, when UE device  10 - 1  will transmit emergency message  74 . In the example of  FIG.  7   , UE device  10 - 2  may process the subsequence of extended preamble  108  received during RX window  76  to identify time offset  112  (e.g., a time offset that includes the duration that would have been occupied by extended window  76 E and predetermined time offset  110 ) from the end of RX window  76  until the beginning of emergency message listening window  80 . The non-repeating structure of extended preamble  108  (e.g., as provided by the cover code) may allow UE device  10 - 2  to determine or identify the temporal location of the subsequence within extended preamble  108 , thereby allowing UE device  10 - 2  to identify time offset  112  (e.g., given the already predetermined time offset  110  between times TD and TC). UE device  10 - 2  may then reactivate its receiver after the identified time offset  112  has elapsed since the end of RX window  76  to receive emergency message  74 . In other words, UE device  10 - 2  may synchronize its timing for the receipt of emergency message  74  based on the subsequence of extended preamble  108  received during RX window  76 . Synchronizing timing using extended preamble  108  with an overlying cover code in this way may allow UE device  10 - 2  to minimize power consumption (e.g., because UE device  10 - 2  can deactivate its receiver after RX window  76 ) without needing to expend resources detecting the end of extended preamble  108 , for example. 
       FIG.  9    is a flow chart of illustrative operations involved in transmitting and receiving an emergency message  74  that is time-synchronized based on an extended preamble  108  of  FIG.  7    having an overlying cover code. Operations  140 - 144  of  FIG.  9    may be performed by UE device  10 - 1  (e.g., while processing operation  56  of  FIG.  3   ). Operations  146 - 152  of  FIG.  9    may be performed by UE device  10 - 2  (e.g., while processing operations  62  and  64  of  FIG.  3   ). 
     At operation  140  of  FIG.  9   , UE device  10 - 1  may generate extended preamble  108  for transmission. Extended preamble  108  may include a cover code such as a binary cover code over an underlying repeated sequence such as a Zadoff Chu sequence or M-sequence. The cover code may vary across the length of extended preamble  108 . 
     At operation  142 , UE device  10 - 1  may begin transmitting extended preamble  108  to search for a UE device within its coverage area  20  ( FIG.  1   ). 
     At operation  146 , the receiver  48  on UE device  10 - 2  (e.g., a UE device in coverage area  20 ) may receive a portion (subsequence) of the extended preamble  108  having an overlying cover code transmitted by UE device  10 - 1  during one of its RX windows  76 . 
     At operation  148 , UE device  10 - 2  may identify time offset  112  between the end of RX window  76  and emergency message listening window  80  based on the cover code of the subsequence of extended preamble  108  received during the RX window. 
     At operation  150 , UE device  10 - 2  may deactivate its transmitter (e.g., once RX window  76  has ended) to conserve power. Operations  148  and  150  may be performed concurrently or operation  150  may be performed prior to operation  148  if desired. 
     At operation  144 , after predetermined time offset  110  has elapsed since the end of extended preamble  108  was transmitted, UE device  10 - 1  may transmit emergency message  74 . 
     At operation  152 , after the identified time offset  112  has elapsed since the end of RX window  76 , UE device  10 - 2  may reactivate its receiver  48  for RX window listening window  80  (e.g., beginning at time TC). UE device  10 - 2  may then receive and decode emergency message  74  during RX window listening window  80  (e.g., because UE device  10 - 2  has now been time-synchronized with UE device  10 - 1  using the subsequence of extended preamble  108  received during RX window  76 ). UE device  10 - 2  may perform any desired subsequent processing based on emergency message  74 . 
     If desired, UE device  10 - 2  may transmit D2D signals to UE device  10 - 1  that include an acknowledgement that UE device  10 - 2  has received emergency message  74 . If UE device  10 - 1  does not receive an acknowledgement for its emergency message, one reason could be that its timing hypothesis has drifted out of the N*T maximum supported time uncertainty  82  compared to all other potential receivers. In this case, during a subsequent period (e.g., during subsequent iterations of path  58  of  FIG.  3   ), UE device  10 - 1  may increase the number N of preambles  72  or the length of extended preamble  108  transmitted before subsequent emergency message transmissions. Additionally or alternatively, UE device  10 - 1  may shift its time to hypothesis forwards in a first RX window period, backwards in the next RX window period, and even further forwards/backwards in subsequent RX window periods to ultimately hit an RX window  76  of the receiver in UE device  10 - 2 . If desired, UE device  10 - 1  may transmit the series of N preambles  72  and/or extended preamble  108  at a different frequency than emergency message  74  (e.g., the preamble(s) may be transmitted on a common channel whereas frequency hopping is used for transmitting emergency message  74 ). 
     The example of  FIGS.  1 - 9    in which UE device  10 - 1  transmits emergency message  74  at time TC is merely illustrative. In general, UE device  10 - 1  may transmit any desired data (e.g., message data, application data, control data, video data, voice data, image data, web data, etc.) instead of emergency message  74  (e.g., the synchronization operations described herein may be used to synchronize UE device  10 - 2  to UE device  10 - 1  for transfer of any desired wireless data using D2D signals  22 ). Emergency message  74  may therefore sometimes simply be referred to as message  74 . Message  74  may contain emergency payload content (emergency message payload content) as one example. As another example, message  74  may include a header and a control message for CRC purposes (e.g., to exclude false positive preamble detections) and to boot strap subsequent (e.g., bidirectional) communications. Message  74  may include other information if desired. The examples herein described as being related to a binary cover code are merely illustrative and, in general other cover codes such as non-binary cover codes may be used. 
     Device  10  may gather and/or use personally identifiable information. It is well understood that the use of personally identifiable information should follow privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining the privacy of users. In particular, personally identifiable information data should be managed and handled so as to minimize risks of unintentional or unauthorized access or use, and the nature of authorized use should be clearly indicated to users. 
     The methods and operations described above in connection with  FIGS.  1 - 9    (e.g., the operations of  FIGS.  3 - 9   ) may be performed by the components of device  10  using software, firmware, and/or hardware (e.g., dedicated circuitry or hardware). Software code for performing these operations may be stored on non-transitory computer readable storage media (e.g., tangible computer readable storage media) stored on one or more of the components of device  10  (e.g., storage circuitry  30  of  FIG.  2   ). The software code may sometimes be referred to as software, data, instructions, program instructions, or code. The non-transitory computer readable storage media may include drives, non-volatile memory such as non-volatile random-access memory (NVRAM), removable flash drives or other removable media, other types of random-access memory, etc. Software stored on the non-transitory computer readable storage media may be executed by processing circuitry on one or more of the components of device  10  (e.g., processing circuitry  18  of  FIG.  1   , etc.). The processing circuitry may include microprocessors, central processing units (CPUs), application-specific integrated circuits with processing circuitry, or other processing circuitry. 
     If desired, an apparatus may be provided that includes means to perform one or more elements or any combination of elements of one or more methods or processes described herein. 
     If desired, one or more non-transitory computer-readable media may be provided that include instructions to cause an electronic device, upon execution of the instructions by one or more processors of the electronic device, to perform one or more elements or any combination of elements of one or more methods or processes described herein. 
     If desired, an apparatus may be provided that includes logic, modules, or circuitry to perform one or more elements or any combination of elements of one or more methods or processes described herein. 
     If desired, an apparatus may be provided that includes one or more processors and one or more non-transitory computer-readable storage media comprising instructions that, when executed by the one or more processors, cause the one or more processors to perform one or more elements or any combination of elements of one or more methods or processes described herein. 
     If desired, a signal (e.g., a signal encoded with data), datagram, information element (IE), packet, frame, segment, PDU, or message may be provided that includes or performs one or more elements or any combination of elements of one or more methods or processes described herein. 
     If desired, an electromagnetic signal may be provided that carries computer-readable instructions, where execution of the computer-readable instructions by one or more processors causes the one or more processors to perform one or more elements or any combination of elements of one or more methods or processes described herein. 
     If desired, a computer program may be provided that includes instructions, where execution of the program by a processing element causes the processing element to carry out one or more elements or any combination of elements of one or more methods or processes described herein. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20210719
Publication Date: 20240227
Grant Date: 20240227
Priority Date: 20210719
Inventors: ELLENBECK, JAN
BADIC, BILJANA
DREWES, CHRISTIAN
Assignee: APPLE INC
CPC Classifications: [{"code": "H04W4/90", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04B1/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W40/22", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/14", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/28", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W92/18", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W56/0025", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W4/90", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W76/14", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W88/04", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W4/90", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W52/0216", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W8/005", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W76/28", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W92/18", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y02D30/70", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W76/28", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/14", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B1/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W40/22", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W92/18", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 81748828