Patent Description:
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.

<CIT> discusses techniques for supporting narrowband device-to-device wireless communication, including possible techniques for performing discovery in an off grid radio system. A wireless device may determine a number of synchronization signal repetitions to use for a narrowband device-to-device transmission. The wireless device may perform the transmission, including transmitting the determined number of synchronization signal repetitions. The transmission may include an indication of the number of synchronization signal repetitions used in the transmission.

<CIT> discusses a method and an apparatus for performing device-to-device (D2D) relay communication. The method comprises the steps of: receiving a first signal that is beamformed and including counter information and a message, the counter information including at last one counter comparison value established for each direction and at least one counter value; and comparing the at least one counter comparison value established for mutually corresponding directions and the at least one counter value to determine at least one transmission direction, and beamforming a second signal for relaying the message for each of the at least one transmission direction and transmitting same, wherein the second signal includes the counter information in which a counter value corresponding to a transmission direction from among the at least one counter value is modified by a predetermined value, when transmitted in one direction among the at least one transmission direction.

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 UE 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.

There is provided a user equipment device as set out in claim <NUM>. The user equipment device includes one or more antennas. The user equipment device includes 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 includes one or more processors. The one or more processors are configured to periodically activate the receiver during a series of receiver (RX) windows. The one or more processors are 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 are configured to deactivate the receiver after the RX window. The one or more processors are configured to identify a time offset based the preamble sequence received during the RX window. The one or more processors are 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 are configured to receive a message in the D2D signals during the message listening window. There is provided a method of operating a user equipment device as set out in claim <NUM>.

<FIG> is a schematic diagram of an illustrative communications system <NUM> (sometimes referred to herein as communications network <NUM>) for conveying wireless data between communications terminals. Communications system <NUM> may include network nodes (e.g., communications terminals). The network nodes may include user equipment (UE) such as one or more UE devices <NUM>. The network nodes may also include external communications equipment (e.g., communications equipment other than UE devices <NUM>) such as external communications equipment <NUM>. External communications equipment <NUM> may include a wireless base station or a wireless access point, for example. UE devices <NUM> and external communications equipment <NUM> may communicate with each other using wireless communications links. If desired, UE devices <NUM> may wirelessly communicate with external communications equipment <NUM> without passing communications through any other intervening network nodes in communications system <NUM> (e.g., UE devices <NUM> may communicate directly with external communications equipment <NUM> over-the-air).

Communications system <NUM> may form a part of a larger communications network that includes network nodes coupled to external communications equipment <NUM> 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 <NUM> may send data to and/or may receive data from other nodes or terminals in the larger communications network via external communications equipment <NUM> (e.g., external communications equipment <NUM> may serve as an interface between user equipment devices <NUM> 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 <NUM> may include one or more antennas that provides wireless coverage for UE devices <NUM> located within a corresponding geographic area or region such as cell <NUM>. The size of cell <NUM> may correspond to the maximum transmit power level of external communications equipment <NUM> and the over-the-air attenuation characteristics for radio-frequency signals conveyed by external communications equipment <NUM>, for example. When a UE device <NUM> is located within cell <NUM>, the UE device may communicate with external communications equipment <NUM> over a wireless link. To support the wireless link, external communications equipment <NUM> may transmit radio-frequency signals in a downlink (DL) direction from external communications equipment <NUM> 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 <NUM>. In the example of <FIG>, a first UE device <NUM> such as UE device <NUM>-<NUM> may be located within cell <NUM>. UE device <NUM>-<NUM> may therefore communicate with external communications equipment <NUM> over a corresponding wireless link. Radio-frequency signals <NUM> may be conveyed between UE device <NUM>-<NUM> and external communications equipment <NUM> to support the wireless link.

In practice, situations may arise where UE device <NUM>-<NUM> is outside of the coverage area of external communications equipment <NUM> and the coverage area for any other wireless access points or base stations in communications system <NUM>. For example, UE device <NUM>-<NUM> may move to a location <NUM>, as shown by arrow <NUM>. Location <NUM> is outside of cell <NUM> and outside of the coverage area of any other wireless access points or base stations in communications system <NUM>. While at location <NUM>, UE device <NUM>-<NUM> may sometimes be referred to as being "off-grid. " UE device <NUM>-<NUM> may also be off-grid (e.g., outside of cell <NUM> and outside of the coverage area of any other wireless access points or base stations in communications system <NUM>) when external communications equipment <NUM> is inactive, disabled, or otherwise unavailable to UE device <NUM>-<NUM> (e.g., due to a power outage or other disability at external communications equipment <NUM>, due to a disaster or other emergency situation, due to network load balancing that excludes UE device <NUM>-<NUM>, due to access to the rest of the communications network <NUM> being blocked or denied to UE device <NUM>-<NUM>, due to intervening obstacles, terrain, or weather blocking UE device <NUM>-<NUM> from conveying radio-frequency signals with external communications equipment <NUM>, etc.). Conversely, UE device <NUM>-<NUM> may sometimes be referred to as being "on-grid" when UE device <NUM>-<NUM> is within a coverage area such as coverage area <NUM> and is able to convey wireless data with the rest of the network (e.g., communications system <NUM>) via external communications equipment <NUM>.

When UE device <NUM>-<NUM> is located off-grid, UE device <NUM>-<NUM> 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 <NUM> or to another UE device such as UE device <NUM>-<NUM>. For example, the user of UE device <NUM>-<NUM> may encounter an emergency while off-grid and may need to use UE device <NUM>-<NUM> 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's situation and/or to call for help.

While off-grid, UE device <NUM>-<NUM> may still be able to convey radio-frequency signals with other UE devices such as UE device <NUM>-<NUM> (e.g., over a wireless device-to-device (D2D) link). UE device <NUM>-<NUM> may have its own coverage area <NUM> (e.g., extending around location <NUM> when UE device <NUM>-<NUM> is at location <NUM>). The size of coverage area <NUM> is determined by the maximum transmit power level of UE device <NUM>-<NUM> and the over-the-air attenuation characteristics for radio-frequency signals transmitted by UE device <NUM>-<NUM>. When the user needs to send an emergency message while off-grid, UE device <NUM>-<NUM> may transmit radio-frequency signals <NUM> that include an emergency message or other wireless data. When there is another UE device such as UE device <NUM>-<NUM> within coverage area <NUM>, UE device <NUM>-<NUM> may receive radio-frequency signals <NUM> and thus the emergency message transmitted by UE device <NUM>-<NUM>. UE device <NUM>-<NUM> may then alert emergency services and/or may provide assistance to the user of UE device <NUM>-<NUM>. In situations where UE device <NUM>-<NUM> is located within cell <NUM> (e.g., whereas UE device <NUM>-<NUM> is located outside of cell <NUM>), UE device <NUM>-<NUM> may additional or alternatively relay the emergency message transmitted by UE device <NUM>-<NUM> to other network nodes such as network nodes operated by emergency services (e.g., a "<NUM>" service in the United States) or to other users. Radio-frequency signals <NUM> are D2D signals and may therefore sometimes be referred to herein as D2D signals <NUM>. D2D signals <NUM> may form a corresponding wireless D2D communications link between UE device <NUM>-<NUM> and UE device <NUM>-<NUM>. Implementations in which D2D signals <NUM> include an emergency message transmitted by UE device <NUM>-<NUM> are merely illustrative and described herein as an example. In general, D2D signals <NUM> may include any desired data (e.g., message data, voice data, application data, video data, etc.) for transmission to UE device <NUM>-<NUM>. UE device <NUM>-<NUM> may also transmit D2D signals to UE device <NUM>-<NUM> (e.g., the D2D link may be a bidirectional link).

<FIG> is a block diagram of an illustrative user equipment device <NUM> (e.g., one or both of UE devices <NUM>-<NUM> and <NUM>-<NUM> of <FIG>). UE device <NUM> is an electronic device and may therefore sometimes be referred to herein simply as device <NUM>. UE device <NUM> 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'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>, UE device <NUM> may include components located on or within an electronic device housing such as housing <NUM>. Housing <NUM>, 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 <NUM> may be formed from dielectric or other low-conductivity material (e.g., glass, ceramic, plastic, sapphire, etc.). In other situations, housing <NUM> or at least some of the structures that make up housing <NUM> may be formed from metal elements.

UE device <NUM> may include control circuitry <NUM>. Control circuitry <NUM> may include storage such as storage circuitry <NUM>. Storage circuitry <NUM> 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 <NUM> may include storage that is integrated within UE device <NUM> and/or removable storage media.

Control circuitry <NUM> may include processing circuitry such as processing circuitry <NUM>. Processing circuitry <NUM> may be used to control the operation of UE device <NUM>. Processing circuitry <NUM> 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 <NUM> may be configured to perform operations in UE device <NUM> using hardware (e.g., dedicated hardware or circuitry), firmware, and/or software. Software code for performing operations in UE device <NUM> may be stored on storage circuitry <NUM> (e.g., storage circuitry <NUM> 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 <NUM> may be executed by processing circuitry <NUM>.

Control circuitry <NUM> may be used to run software on UE device <NUM> 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 <NUM> may be used in implementing communications protocols. Communications protocols that may be implemented using control circuitry <NUM> include internet protocols, wireless local area network (WLAN) protocols (e.g., IEEE <NUM> 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 <NUM>. 11ad protocols (e.g., ultra-wideband protocols), cellular telephone protocols (e.g., <NUM> protocols, <NUM> (LTE) protocols, 3GPP Fifth Generation (<NUM>) 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 <NUM> may include input-output circuitry <NUM>. Input-output circuitry <NUM> may include input-output devices <NUM>. Input-output devices <NUM> may be used to allow data to be supplied to UE device <NUM> and to allow data to be provided from UE device <NUM> to external devices. Input-output devices <NUM> may include user interface devices, data port devices, and other input-output components. For example, input-output devices <NUM> 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 <NUM> using wired or wireless connections (e.g., some of input-output devices <NUM> may be peripherals that are coupled to a main processing unit or other portion of UE device <NUM> via a wired or wireless link).

Input-output circuitry <NUM> may include wireless circuitry <NUM> to support wireless communications. Wireless circuitry <NUM> (sometimes referred to herein as wireless communications circuitry <NUM>) may include one or more antennas <NUM>. Wireless circuitry <NUM> may also include one or more radios <NUM>. Radio <NUM> 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 <NUM> and one or more radio-frequency receivers <NUM>. Transmitter <NUM> 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) <NUM>. Receiver <NUM> 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) <NUM>. The components of radio <NUM> 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) <NUM> may be formed using any desired antenna structures for conveying radio-frequency signals. For example, antenna(s) <NUM> 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) <NUM> over time. If desired, two or more of antennas <NUM> 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) <NUM> 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) <NUM> 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 <NUM> 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 <NUM> may be coupled to one or more antennas <NUM> over one or more radio-frequency transmission lines <NUM>. Radio-frequency transmission lines <NUM> 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 <NUM> may be integrated into rigid and/or flexible printed circuit boards if desired. One or more radio-frequency lines <NUM> may be shared between multiple radios <NUM> if desired. Radio-frequency front end (RFFE) modules may be interposed on one or more radio-frequency transmission lines <NUM>. The radio-frequency front end modules may include substrates, integrated circuits, chips, or packages that are separate from radios <NUM> 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 <NUM>.

Radio <NUM> 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 <NUM> may include wireless local area network (WLAN) frequency bands (e.g., Wi-Fi® (IEEE <NUM>) or other WLAN communications bands) such as a <NUM> WLAN band (e.g., from <NUM> to <NUM>), a <NUM> WLAN band (e.g., from <NUM> to <NUM>), a Wi-Fi® 6E band (e.g., from <NUM>-<NUM>), and/or other Wi-Fi® bands (e.g., from <NUM>-<NUM>), wireless personal area network (WPAN) frequency bands such as the <NUM> Bluetooth® band or other WPAN communications bands, cellular telephone frequency bands (e.g., bands from about <NUM> to about <NUM>, <NUM> bands, <NUM> LTE bands, <NUM> New Radio Frequency Range <NUM> (FR1) bands below <NUM>, <NUM> New Radio Frequency Range <NUM> (FR2) bands between <NUM> and <NUM>, etc.), other centimeter or millimeter wave frequency bands between <NUM>-<NUM>, near-field communications frequency bands (e.g., at <NUM>), satellite navigation frequency bands (e.g., a GPS band from <NUM> to <NUM>, 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 <NUM>. <NUM> protocol and/or other ultra-wideband communications protocols, communications bands under the family of 3GPP wireless communications standards, communications bands under the IEEE <NUM>. XX family of standards, industrial, scientific, and medical (ISM) bands such as an ISM band between around <NUM> and <NUM> or other ISM bands below or above <NUM>, 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 <NUM> may also be used to perform spatial ranging operations if desired.

Transmitter <NUM> may transmit radio-frequency signals over antenna(s) <NUM> when transmitter <NUM> is active (e.g., enabled). Transmitter <NUM> does not transmit radio-frequency signals over antenna(s) <NUM> when transmitter <NUM> is inactive (e.g., disabled or not actively transmitting sign). Similarly, receiver <NUM> may receive radio-frequency signals over antenna(s) <NUM> when receiver <NUM> is active (e.g., enabled). Receiver <NUM> does not receive radio-frequency signals over antenna(s) <NUM> when receiver <NUM> is inactive (e.g., disabled). Control circuitry <NUM> may control transmitter <NUM> to be active or inactive at any given time. Control circuitry <NUM> may also control receiver <NUM> to be active or inactive at any given time. Control circuitry <NUM> may activate or deactivate transmitter <NUM> and/or receiver <NUM> at different times as dictated by a communications protocol governing radio <NUM> and/or based on instructions provided by a user and/or from other software running on control circuitry <NUM>, for example. Control circuitry <NUM> may configure transmitter <NUM> to be inactive by powering off transmitter <NUM>, by providing control signals to switching circuitry on power supply or enable lines for transmitter <NUM>, by providing control signals to control circuitry on transmitter <NUM>, and/or by providing control signals to switching circuitry within transmitter <NUM>, for example. When transmitter <NUM> is inactive, some or all of transmitter <NUM> may be inactive (e.g., disabled or powered off) or transmitter <NUM> may remain powered on but without transmitting radio-frequency signals over antenna(s) <NUM>. Similarly, control circuitry <NUM> may configure receiver <NUM> to be inactive by powering off receiver <NUM>, by providing control signals to switching circuitry on power supply or enable lines for receiver <NUM>, by providing control signals to control circuitry on receiver <NUM>, and/or by providing control signals to switching circuitry within receiver <NUM>, for example. When receiver <NUM> is inactive, some or all of receiver <NUM> may be disabled (e.g., powered off) or receiver <NUM> may remain powered on but without actively receiving radio-frequency signals incident upon antenna(s) <NUM>. Transmitter <NUM> and receiver <NUM> may consume more power on UE device <NUM> when active than when inactive (e.g., a battery on UE device <NUM> may drain more rapidly while transmitter <NUM> and receiver <NUM> are active than while transmitter <NUM> or receiver <NUM> are inactive).

The example of <FIG> is merely illustrative. While control circuitry <NUM> is shown separately from wireless circuitry <NUM> in the example of <FIG> for the sake of clarity, wireless circuitry <NUM> may include processing circuitry (e.g., one or more processors) that forms a part of processing circuitry <NUM> and/or storage circuitry that forms a part of storage circuitry <NUM> of control circuitry <NUM> (e.g., portions of control circuitry <NUM> may be implemented on wireless circuitry <NUM>). As an example, control circuitry <NUM> 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 <NUM>. The baseband circuitry may, for example, access a communication protocol stack on control circuitry <NUM> (e.g., storage circuitry <NUM>) 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 <NUM>.

When UE device <NUM>-<NUM> of <FIG> is off-grid, UE device <NUM>-<NUM> should still be reachable in case the user of UE device <NUM>-<NUM> encounters an emergency or otherwise needs to transmit wireless data to another UE device such as UE device <NUM>-<NUM>. To maximize the likelihood that another UE device such as UE device <NUM>-<NUM> will be able to receive D2D signals <NUM>, UE device <NUM>-<NUM> should be able to transmit D2D signals <NUM> over a relatively long distance (e.g., it may be desirable for UE device <NUM>-<NUM> to have as large a coverage area <NUM> as possible). This distance (e.g., the radius of coverage area <NUM>) may be as far as hundreds of meters, a few km, several km, or dozens of km, for example. UE device <NUM>-<NUM> may maximize the range of D2D signals <NUM> 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 <NUM>-<NUM> may transmit D2D signals <NUM> 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 <NUM>-<NUM> may transmit D2D signals <NUM> at relatively low frequencies such as frequencies in a frequency band below <NUM>, below <NUM>, below <NUM>, below <NUM>, etc. This may serve to minimize over-the-air signal attenuation for D2D signals <NUM>, thereby maximizing the size of coverage area <NUM>. The wireless circuitry <NUM> on UE device <NUM>-<NUM> may include a dedicated radio <NUM> for transmitting D2D signals <NUM> or the radio that transmits D2D signals <NUM> may also transmit other signals associated with other communications protocols or RATs (e.g., a single radio <NUM> on UE device <NUM>-<NUM> may convey both WLAN signals and D2D signals <NUM>, a single radio <NUM> on UE device <NUM>-<NUM> may convey both cellular telephone signals and D2D signals <NUM>, etc.).

At the same time, even when UE device <NUM>-<NUM> is located within coverage area <NUM>, UE device <NUM>-<NUM> is only able to correctly recover wireless data (e.g., an emergency message) in D2D signals <NUM> (a) when the receiver <NUM> on UE device <NUM>-<NUM> is active and (b) when UE device <NUM>-<NUM> is time-synchronized with UE device <NUM>-<NUM>. While UE device <NUM>-<NUM> can keep its receiver <NUM> active at all times to listen for any D2D signals <NUM> that happen to be transmitted, this would consume an excessive amount of power in UE device <NUM>-<NUM>, causing UE device <NUM>-<NUM> to drain its battery relatively quickly. Keeping receiver <NUM> active at all times is particularly power-inefficient because off-grid UE devices such as UE device <NUM>-<NUM> only need to transmit emergency messages or other wireless data in D2D signals <NUM> on rare occasions. In addition, while UE devices can synchronize to each other using signals from external communications equipment <NUM> when located within cell <NUM> (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 <NUM>-<NUM> and UE device <NUM>-<NUM> 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 <NUM>-<NUM> can drift with respect to the timing for UE device <NUM>-<NUM> once one or both of the UE devices go off-grid. As such, simple paging mechanisms may be insufficient to allow UE device <NUM>-<NUM> to correctly receive and recover wireless data in D2D signals <NUM>. It would therefore be desirable to provide power efficient systems and methods for the time-synchronized transmission of D2D signals <NUM> by UE device <NUM>-<NUM> and reception of D2D signals <NUM> by UE device <NUM>-<NUM> while at least UE device <NUM>-<NUM> 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 <NUM>. These short-range protocols are unsuitable for conveying D2D signals <NUM> because the protocols would consume an excessive amount of power on UE devices <NUM>-<NUM> and/or <NUM>-<NUM>, 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 <NUM> 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 <NUM>-<NUM> to minimize power consumption while listening for potential D2D signals <NUM>, UE device <NUM>-<NUM> may periodically activate its receiver <NUM> during receiver (RX) windows, during which the receiver is able to receive D2D signals <NUM> (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 <NUM> will arrive at UE device <NUM>-<NUM> while the receiver is inactive - thereby preventing proper recovery of the data in D2D signals <NUM> by UE device <NUM>-<NUM>. When the duration of the RX windows is long, there is a greater likelihood that a transmission of D2D signals <NUM> will arrive at UE device <NUM>-<NUM>. However, longer RX windows cause UE device <NUM>-<NUM> to consume an excessive amount of power, in effect "searching" for D2D signals <NUM> that are only rarely transmitted by another UE device. It would instead be more power efficient for UE device <NUM>-<NUM> to expend power searching for a potential receiver device such as UE device <NUM>-<NUM> than vice versa. UE device <NUM>-<NUM> 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 <NUM>-<NUM>.

<FIG> is a flow chart of illustrative operations involved in conveying D2D signals <NUM> between UE device <NUM>-<NUM> and UE device <NUM>-<NUM> in a time-synchronized and power efficient manner. Operations <NUM>-<NUM> and <NUM> of <FIG> may be performed by UE device <NUM>-<NUM>. Operations <NUM>-<NUM> of <FIG> may be performed by UE device <NUM>-<NUM>.

At operation <NUM>, UE device <NUM>-<NUM> may communicate with the network (e.g., network nodes or terminals of communications system <NUM>) via external communications equipment <NUM> while on-grid (e.g., while within cell <NUM> of <FIG>). When UE device <NUM>-<NUM> becomes off-grid, processing may proceed to operation <NUM>.

At operation <NUM>, UE device <NUM>-<NUM> may periodically activate its receiver <NUM> during a series of RX windows. UE device <NUM>-<NUM> may listen for D2D signals <NUM> during the RX windows. The receiver <NUM> on UE device <NUM>-<NUM> may be inactive between the RX windows to conserve power.

At operation <NUM>, while UE device <NUM>-<NUM> is off-grid, UE device <NUM>-<NUM> may identify wireless data for transmission to another UE device such as UE device <NUM>-<NUM> using D2D signals <NUM>. UE device <NUM>-<NUM> may, for example, identify an emergency message to transmit in D2D signals <NUM>. UE device <NUM>-<NUM> may generate the emergency message in response to a user input provided to a software application running on UE device <NUM>-<NUM> (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 <NUM>-<NUM>, in response to sensor data gathered by one or more sensors on UE device <NUM>-<NUM> (e.g., accelerometer data or other sensor data indicating that UE device <NUM>-<NUM> 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 <NUM>-<NUM> (e.g., based on GPS data or other location information gathered by UE device <NUM>-<NUM>), user information, a message drafted by the user, and/or any other desired information to alert another person or emergency services of the user's situation or to call for help, as examples.

At operation <NUM>, UE device <NUM>-<NUM> may activate its transmitter <NUM> and may transmit D2D signals <NUM> that include one or more preambles followed by the emergency message (e.g., D2D signals <NUM> may include the emergency message preceded by the one or more preambles). If desired, UE device <NUM>-<NUM> may periodically repeat this transmission, as shown by loopback path <NUM>, to maximize the probability that another UE device such as UE device <NUM>-<NUM> will receive D2D signals <NUM>. 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 <NUM>-<NUM> if desired (e.g., from software or other data on both UE device <NUM>-<NUM> and <NUM>-<NUM>, from a communications protocol governing D2D signals <NUM>, etc.).

At operation <NUM>, UE device <NUM>-<NUM> may receive at least a portion of one or more of the preambles transmitted by UE device <NUM>-<NUM> in D2D signals <NUM> during one of the RX windows during which the receiver <NUM> on UE device <NUM>-<NUM> is active. The control circuitry <NUM> on UE device <NUM>-<NUM> may process the received preamble to synchronize timing with UE device <NUM>-<NUM>. For example, UE device <NUM>-<NUM> may identify (e.g., determine, calculate, compute, generate, produce, etc.) timing for an emergency message listening window during which UE device <NUM>-<NUM> 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 <NUM>-<NUM> to UE device <NUM>-<NUM> so UE device <NUM>-<NUM> will be able to correctly recover the emergency message transmitted by UE device <NUM>-<NUM>. If desired, UE device <NUM>-<NUM> may deactivate receiver <NUM> after identifying this timing and/or after receipt of the one or more preambles to conserve power.

At operation <NUM>, the control circuitry <NUM> on UE device <NUM>-<NUM> may re-activate its receiver <NUM> during the emergency message listening window (e.g., based on the timing identified at operation <NUM>). The receiver <NUM> on UE device <NUM>-<NUM> may receive the emergency message transmitted during the emergency message listening window. UE device <NUM>-<NUM> may perform any desired subsequent processing based on the received emergency message. For example, UE device <NUM>-<NUM> may alert or inform a user of UE device <NUM>-<NUM> about the emergency message and/or its contents, may transmit UL signals to external communications equipment <NUM> informing the network of the emergency message (e.g., when UE device <NUM>-<NUM> is located within cell <NUM>), may transmit additional D2D signals to another UE device to inform that UE device of the emergency message (e.g., UE device <NUM>-<NUM> may provide the emergency message to external communications equipment <NUM> or other portions of the network via a relay of one or more additional UE devices, the last of which is located within cell <NUM>), etc..

If desired, processing may proceed from operations <NUM> and <NUM> to optional operation <NUM>. At optional operation <NUM>, UE device <NUM>-<NUM> and UE device <NUM>-<NUM> 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 <NUM>-<NUM> at operation <NUM>). The example of <FIG> is merely illustrative. One or more iterations of operation <NUM> may be performed by UE device <NUM>-<NUM> concurrently with the performance of operations <NUM> and/or <NUM> by UE device <NUM>-<NUM>.

<FIG> includes timing diagrams for one example in which the one or more preambles transmitted by UE device <NUM>-<NUM> (e.g., at operation <NUM> of <FIG>) include a series of preambles that identify respective time offsets for synchronizing the timing of UE device <NUM>-<NUM> to the timing of UE device <NUM>-<NUM>.

Timing diagram <NUM> of <FIG> plots transmit (TX) timing for UE device <NUM>-<NUM> in transmitting D2D signals <NUM>. Timing diagram <NUM> of <FIG> plots receive (RX) timing for UE device <NUM>-<NUM> in receiving D2D signals <NUM>. As shown by timing diagram <NUM>, UE device <NUM>-<NUM> may begin to transmit a series of N preambles <NUM> at time TA, each of which is labeled by a corresponding index n from <NUM> to N (e.g., a first preamble <NUM>-<NUM>, a second preamble <NUM>-<NUM> following preamble <NUM>-<NUM>, an Nth preamble <NUM>-N following an (N-<NUM>)th preamble, etc.). UE device <NUM>-<NUM> may then begin to transmit emergency message <NUM> after transmission of the series of N preambles <NUM> (e.g., after a predetermined offset time such as at time TC).

Each preamble <NUM> may be a sequence of bits having a predetermined (temporal) preamble length T that allows UE device <NUM>-<NUM> to detect the presence of a signal and to allow the receiver on UE device <NUM>-<NUM> to synchronize to the transmit timing for the transmitter in UE device <NUM>-<NUM>. Preamble length T may be less than or equal to <NUM>, as an example. The duration of emergency message <NUM> is generally much longer than preamble length T. Preambles <NUM> may sometimes also be referred to herein as preamble sequences <NUM> or synchronization sequences <NUM>. Each preamble <NUM> may be a known signal (e.g., of a format known to both UE device <NUM>-<NUM> and UE device <NUM>-<NUM>) having relatively strong correlation properties to allow UE device <NUM>-<NUM> to easily detect and distinguish the preamble from background noise. Each preamble <NUM> may be a Zadoff Chu sequence or an M-sequence, as two examples.

Each preamble <NUM> in the series of N preambles may include or otherwise identify respective offset information OFSn. Offset information OFSn may identify a time offset ΔTn between the corresponding preamble and the time at which UE device <NUM>-<NUM> is going to begin transmitting emergency message <NUM> (time TC in the example of <FIG>). For example, preamble <NUM>-<NUM> may include offset information OFS<NUM> that identifies a time offset ΔT1 from the end of preamble <NUM>-<NUM> to the beginning of emergency message <NUM> (e.g., time offset ΔT1 may be the time duration between time TA+T and time TC), preamble <NUM>-<NUM> may include offset information OFS<NUM> that identifies a time offset ΔT2 from the end of preamble <NUM>-<NUM> to time TC, preamble <NUM>-N may include offset information OFSN that identifies a time offset ΔTN from the end of preamble <NUM>-N to time TC, etc. The series of N preambles <NUM> may be offset from emergency message <NUM> (in time) by offset time ΔTN.

As shown by timing diagrams <NUM> and <NUM>, UE device <NUM>-<NUM> may initially be out-of-sync with respect to UE device <NUM>-<NUM> by an a priori time offset <NUM>. For example, a time t<NUM> as kept by UE device <NUM>-<NUM> may have drifted from the corresponding time t<NUM> as kept by UE device <NUM>-<NUM> by a priori time offset <NUM> since one or both of the UE devices went off-grid. UE devices <NUM>-<NUM> and <NUM>-<NUM> may retain coarse time synchronization via an internal/network standard/protocol (e.g., UTC) time (e.g., to within a maximum supported time uncertainty <NUM>, which is equal to T*N). This internal standard time may be periodically adjusted/corrected whenever the UE device communicates with external communications equipment <NUM> or receives satellite navigation signals if desired (e.g., to synchronize time t<NUM> at each UE device to external communications equipment <NUM> and/or a satellite navigation system). Preambles <NUM> may allow UE device <NUM>-<NUM> to fine-tune its synchronization within maximum supported time uncertainty <NUM> to allow UE device <NUM>-<NUM> to correctly receive emergency message <NUM> 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 <NUM>. As one possible countermeasure, UE device <NUM>-<NUM> may occasionally transmit a global clock beat preamble while off-grid that allows all nearby off-grid devices such as UE device <NUM>-<NUM> to adjust its time t<NUM> to match time t<NUM> on UE device <NUM>-<NUM> (e.g., so UE device <NUM>-<NUM> is coarsely synchronized to UE device <NUM>-<NUM> in case UE device <NUM>-<NUM> 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 <NUM>-<NUM> may periodically activate its receiver <NUM> during RX windows <NUM>, one of which is shown in <FIG>. Each RX window <NUM> may be separated in time from the preceding and subsequent RX window by RX window periodicity P (not shown in <FIG> for the sake of clarity). RX window <NUM> may have (temporal) RX window length L. RX window length L may be greater than preamble length T to ensure that UE device <NUM>-<NUM> is able to fully receive at least one of the preambles <NUM> in the series of N preambles <NUM>. RX window length L may, for example, be equal to <NUM>*T, <NUM>*T, <NUM>*T, <NUM>*T, more than <NUM>*T, etc..

As shown by timing diagram <NUM>, UE device <NUM>-<NUM> may receive preamble <NUM>-<NUM> during RX window <NUM>. The control circuitry <NUM> on UE device <NUM>-<NUM> may process the received preamble <NUM>-<NUM> to identify time offset ΔT1 from the offset information OFS<NUM> of preamble <NUM>-<NUM> (e.g., while processing operation <NUM> of <FIG>). UE device <NUM>-<NUM> may then have knowledge that it will receive emergency message <NUM> after time offset ΔT1 elapses after the end of preamble <NUM>-<NUM>, thereby synchronizing the timing of UE device <NUM>-<NUM> to the timing of UE device <NUM>-<NUM>. UE device <NUM>-<NUM> may deactivate its receiver <NUM> after RX time window <NUM> has ended to conserve power. UE device <NUM>-<NUM> may reactivate receiver <NUM> after time offset ΔT1 has elapsed since the end of preamble <NUM>-<NUM> (e.g., during an emergency message listening window <NUM> beginning at time TC). UE device <NUM>-<NUM> may then receive emergency message <NUM> during emergency message listening window <NUM>. UE device <NUM>-<NUM> may correctly recover the emergency message <NUM> from D2D signals <NUM> because UE device <NUM>-<NUM> has been time-synchronized to UE device <NUM>-<NUM> using preambles <NUM>. By transmitting a series of preambles <NUM>, each of which identifies a respective time offset ΔTn, UE device <NUM>-<NUM> will be able to synchronize its timing to UE device <NUM>-<NUM> (e.g., for receipt of emergency message <NUM> at a common time TC now shared by both UE devices) regardless of where RX window <NUM> happens to land in the series of N preambles <NUM> transmitted by UE device <NUM>-<NUM>. This may allow UE devices <NUM>-<NUM> and <NUM>-<NUM> to time-synchronize for receipt of a message in D2D signals <NUM> (e.g., emergency message <NUM>) 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 <NUM>-<NUM>.

Each preamble <NUM> may, for example, be the same type of sequence but having different variants for each index n (e.g., where UE device <NUM>-<NUM> identifies offset information OFSn and thus time offset ΔTn based on the particular variant of the sequence that is received during its RX window <NUM>). 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 <NUM>). 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 <NUM>. UE device <NUM>-<NUM> may process the preamble <NUM> received during its RX window <NUM> to identify the frequency or frequency hopping pattern indication to use for receiving emergency message <NUM>. UE device <NUM>-<NUM> may then use that frequency or a frequency hopping pattern corresponding to the frequency hopping pattern indication when receiving emergency message <NUM> (e.g., within emergency message listening window <NUM>). 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 <NUM>-<NUM> searches for another UE device <NUM>-<NUM> in their contact list to chat), limiting the number of potential responders to UE device <NUM>-<NUM> (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 <NUM> (e.g., detecting the particular preamble <NUM> received during RX window <NUM>) 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 <NUM>-<NUM> in an unneeded emergency message reception attempt (e.g., activating the receiver on UE device <NUM>-<NUM> during emergency message listening window <NUM> does not excessively drain the battery on UE device <NUM>-<NUM>). Preambles <NUM> may give UE device <NUM>-<NUM> 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 <NUM>-<NUM> and <NUM>-<NUM>. Emergency message <NUM> may include a cyclic redundancy check (CRC) or other measures that make it almost impossible for UE device <NUM>-<NUM> to decode a false positive emergency message. If desired, UE device <NUM>-<NUM> may leave its RX window open after receipt of a preamble <NUM> so UE device <NUM>-<NUM> receives all subsequent preambles <NUM>. This may serve to further reduce the presence of false positives, for example.

<FIG> is a flow chart showing how UE devices <NUM>-<NUM> and <NUM>-<NUM> may time-synchronize for the transfer of emergency message <NUM> based on the series of N preambles <NUM> shown in <FIG>. Operations <NUM>-<NUM> of <FIG> may be performed by UE device <NUM>-<NUM> (e.g., while processing operation <NUM> of <FIG>). Operations <NUM>-<NUM> of <FIG> may be performed by UE device <NUM>-<NUM> (e.g., while processing operations <NUM> and <NUM> of <FIG>).

At operation <NUM> of <FIG>, wireless circuitry <NUM> on UE device <NUM>-<NUM> may generate the series of N preambles <NUM> for transmission. Each preamble <NUM> may include respective offset information OFSn, each of which identifies a corresponding time offset ΔTn from the end of that preamble until the transmission of emergency message <NUM> at time TC.

At operation <NUM>, UE device <NUM>-<NUM> may begin transmitting the series of N preambles to search for a UE device within its coverage area <NUM> (<FIG>).

At operation <NUM>, the receiver <NUM> on UE device <NUM>-<NUM> (e.g., a UE device in coverage area <NUM>) may receive one of the N preambles <NUM> transmitted by UE device <NUM>-<NUM> during one of its RX windows <NUM> when the receiver <NUM> on UE device <NUM>-<NUM> is active. For example, UE device <NUM>-<NUM> may receive preamble <NUM>-<NUM> during the RX window <NUM> shown in <FIG>.

At operation <NUM>, the control circuitry <NUM> on UE device <NUM>-<NUM> may identify (e.g., calculate, generate, determine, produce, compute, etc.) the time offset ΔTn associated with the received preamble <NUM> (e.g., control circuitry <NUM> may identify offset information OFSn and may identify time offset ΔTn based the identified offset information OFSn). In the example of <FIG>, UE device <NUM>-<NUM> may identify time offset ΔT1 based on the offset information OFS<NUM> in preamble <NUM>-<NUM>. UE device <NUM>-<NUM> may identify the time offset according to the communications protocol governing D2D signals <NUM> or any other standard or scheme that is known to both UE devices <NUM>-<NUM> and <NUM>-<NUM> (e.g., as implemented using one or more software applications running on both UE devices <NUM>-<NUM> and <NUM>-<NUM>).

At operation <NUM>, UE device <NUM>-<NUM> may deactivate its receiver <NUM> (e.g., when the RX window has ended).

At operation <NUM>, UE device <NUM>-<NUM> may reactivate its receiver <NUM> after the identified time offset ΔTn has elapsed since the end of the preamble received during RX window <NUM> (e.g., during emergency message listening window <NUM> beginning after ΔTn has elapsed since the end of the preamble). UE device <NUM>-<NUM> may then receive emergency message <NUM> during emergency message listening window <NUM> (sometimes referred to herein as emergency message listening period <NUM>). In the example of <FIG>, UE device <NUM>-<NUM> may reactivate its receiver <NUM> at time TC, which is separated in time from the end of preamble <NUM>-<NUM> by time offset ΔT1. UE device <NUM>-<NUM> may perform any desired subsequent processing based on the received emergency message <NUM>.

As an example, preamble length T may be <NUM>. A relatively long preamble length such as <NUM> may allow for reliable detection of the preambles at a maximum distance. If RX window length L is <NUM>*T and the preambles are consecutive, the duration of at least one full preamble length T will fit inside RX window <NUM>. With a RX window periodicity P = <NUM> seconds and an active duty cycle of <NUM>%, for example, RX window length L will be <NUM>. With no preamble repetitions (i.e., with N = <NUM>), the maximum time offset would need to be +/- <NUM> to still have one full preamble in the RX window. With a relative time drift of +/- <NUM> ppm between UE device <NUM>-<NUM> and UE device <NUM>-<NUM>, the UE devices would have drifted too far to time-synchronize after only about <NUM> minutes. Longer drift times between UE device <NUM>-<NUM> and UE device <NUM>-<NUM> are supported by using UE device <NUM>-<NUM> to perform transmitter-side searching for a receiver rather than using the receiver to search for a transmitter (e.g., by using UE device <NUM>-<NUM> to transmit a series of N preambles <NUM> where N is greater than one). For example, if N = <NUM>, the maximum time offset would need to be +/- (N/<NUM>)*L. When RX window length L is <NUM> and the drift is +/- <NUM> ppm, the UE devices may still be time-synchronized using the transmitted preambles as long as <NUM> 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 <NUM>-<NUM> might be located relatively far from UE device <NUM>-<NUM>, the transmitter <NUM> on UE device <NUM>-<NUM> 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 <NUM> (e.g., because the power amplifier(s) in the transmitter may need time to cool off between transmissions). To mitigate these issues, UE device <NUM>-<NUM> may transmit a first subset of the series of N preambles <NUM> prior to a first transmission of emergency message <NUM> and then may transmit a second subset of the series of N preambles <NUM> prior to a subsequent second transmission of emergency message <NUM>.

<FIG> is a timing diagram showing one example of how UE device <NUM>-<NUM> may transmit different subsets of the series of N preambles <NUM> prior to different transmissions of emergency message <NUM> (e.g., to allow cool-off time for the power amplifier(s) in transmitter <NUM>). As shown in <FIG>, UE device <NUM>-<NUM> may transmit a first set of the N preambles <NUM> such as the odd-numbered preambles in the series of preambles. For example, UE device <NUM>-<NUM> may transmit the first preamble <NUM>-<NUM> from the series of N preambles <NUM> at time TA. UE device <NUM>- <NUM> may then forego transmission of the second preamble from the series of N preambles <NUM> (e.g., for preamble length T) after transmission of the first preamble <NUM>-<NUM>. UE device <NUM>-<NUM> may then transmit the third preamble <NUM>-<NUM> from the series of N preambles <NUM>, may forego transmission of the fourth preamble, etc. At time TC, UE device <NUM>-<NUM> may transmit emergency message <NUM>. Foregoing transmission of the even-numbered preambles may allow the power amplifier(s) in UE device <NUM>-<NUM> to cool off between transmission of the odd-numbered preambles.

After the transmission of emergency message <NUM> (e.g., after RX window periodicity P has elapsed since time TA), UE device <NUM>-<NUM> may then transmit the even-numbered preambles from the series of N preambles <NUM> prior to re-transmitting emergency message <NUM>. For example, at time TA', UE device <NUM>-<NUM> may forego transmission of the first preamble from the series of N preambles <NUM> (e.g., for preamble length T). After preamble length T has passed (e.g., at time TA'+T), UE device <NUM>-<NUM> may transmit the second preamble <NUM>-<NUM> from the series of N preambles <NUM>. UE device <NUM>-<NUM> may then forego transmission of the third preamble from the series of N preambles <NUM> (e.g., for preamble length T) after transmission of the second preamble <NUM>-<NUM>. UE device <NUM>-<NUM> may then transmit the fourth preamble <NUM>-<NUM> from the series of N preambles <NUM>, may forego transmission of the fifth preamble, etc. At time TC', UE device <NUM>-<NUM> may then re-transmit emergency message <NUM>. This may allow UE device <NUM>-<NUM> to receive one of the preambles <NUM> prior to a transmission of emergency message <NUM> that is offset from the received preamble <NUM> by the time offset identified by that preamble <NUM>, while allowing the power amplifier(s) on UE device <NUM>-<NUM> to cool off between transmissions at relatively high transmit power levels.

The example of <FIG> 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 <NUM> (e.g., the first set of preambles may include every other pair of sequential preambles <NUM>, every other trio of sequential preambles <NUM>, etc.). UE device <NUM>-<NUM> may divide the series of N preambles <NUM> 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 <NUM>. The example of <FIG> in which UE device <NUM>-<NUM> transmits a series of multiple preambles <NUM> is merely illustrative. If desired, UE device <NUM>-<NUM> may instead transmit a single long (extended) preamble prior to transmitting emergency message <NUM>.

<FIG> includes timing diagrams for one example in which UE device <NUM>-<NUM> transmits a single extended preamble prior to transmitting emergency message <NUM>. Timing diagram <NUM> of <FIG> plots TX timing for UE device <NUM>-<NUM> in transmitting D2D signals <NUM>. Timing diagram <NUM> of <FIG> plots RX timing for UE device <NUM>-<NUM> in receiving D2D signals <NUM>.

As shown by timing diagram <NUM>, UE device <NUM>-<NUM> may begin to transmit a single extended preamble <NUM> at time TA. Extended preamble <NUM> may sometimes also be referred to herein as extended preamble sequence <NUM> or extended synchronization sequence <NUM>. Extended preamble <NUM> may have a duration extending from time TA until time TD. Extended preamble <NUM> 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 <NUM>/<NUM>) for hundreds of ms (e.g., extended preamble <NUM> may be hundreds of ms long).

As shown by timing diagram <NUM>, the receiver on UE device <NUM>-<NUM> may be active during an RX window <NUM> that falls between times TA and TD. UE device <NUM>-<NUM> may therefore receive the portion (subset or subsequence) of extended preamble <NUM> falling within RX window <NUM>. If desired, once UE device <NUM>-<NUM> has received a portion of extended preamble <NUM> within RX window <NUM>, UE device <NUM>-<NUM> may keep its receiver <NUM> active after the end of RX window <NUM> until UE device <NUM>-<NUM> has received the end of extended preamble <NUM> (e.g., during an extended RX window 76E that extends the active time of the receiver from RX window length L until time TD). Once UE device <NUM>-<NUM> has received the end of extended preamble <NUM>, UE device <NUM>-<NUM> may deactivate its receiver for predetermined time offset <NUM> (e.g., UE device <NUM>-<NUM> may deactivate receiver <NUM> in response to receiving the end of extended preamble <NUM>).

After predetermined time offset <NUM> (e.g., at time TC), UE device <NUM>-<NUM> may transmit emergency message <NUM> and UE device <NUM>-<NUM> may re-activate its receiver (e.g., during receiver listening window <NUM>) for receiving emergency message <NUM>. Predetermined time offset <NUM> may be known to both UE device <NUM>-<NUM> and UE device <NUM>-<NUM> according to the communications protocol governing D2D signals <NUM>, any other standard or scheme that is known to both UE devices <NUM>-<NUM> and <NUM>-<NUM> (e.g., as implemented using one or more software applications running on both UE devices <NUM>-<NUM> and <NUM>-<NUM>), identified by information conveyed in extended preamble <NUM> itself, etc. In this way, the end of extended preamble <NUM> may serve as the time-synchronization trigger to synchronize the timing of UE device <NUM>-<NUM> with the timing of UE device <NUM>-<NUM> for receipt of emergency message <NUM>, even if the UE devices had previous drifted apart by a priori time offset <NUM>. Keeping the receiver on UE device <NUM>-<NUM> active during extended RX window 76E may allow UE device <NUM>-<NUM> to re-confirm the initial detection of extended preamble <NUM> during RX window <NUM>, for example.

<FIG> is a flow chart of illustrative operations involved in transmitting and receiving an emergency message <NUM> that is time-synchronized based on extended preamble <NUM> of <FIG>. Operations <NUM>-<NUM> of <FIG> may be performed by UE device <NUM>-<NUM> (e.g., while processing operation <NUM> of <FIG>). Operations <NUM>-<NUM> of <FIG> may be performed by UE device <NUM>-<NUM> (e.g., while processing operations <NUM> and <NUM> of <FIG>).

At operation <NUM> of <FIG>, UE device <NUM>-<NUM> may generate extended preamble <NUM> for transmission.

At operation <NUM>, UE device <NUM>-<NUM> may begin transmitting extended preamble <NUM> to search for a UE device within its coverage area <NUM> (<FIG>).

At operation <NUM>, the receiver <NUM> on UE device <NUM>-<NUM> (e.g., a UE device in coverage area <NUM>) may receive a portion (subset or subsequence) of the extended preamble <NUM> transmitted by UE device <NUM>-<NUM> during one of its RX windows <NUM> when the receiver on UE device <NUM>-<NUM> is active.

At operation <NUM>, UE device <NUM>-<NUM> may keep its receiver <NUM> active after RX window <NUM> has elapsed (e.g., during extended RX window 76E of <FIG>). UE device <NUM>-<NUM> may continue to receive extended preamble <NUM> (during extended RX window 76E) until UE device <NUM>-<NUM> receives the end of extended preamble <NUM>.

At operation <NUM>, once UE device <NUM>-<NUM> has received the end of extended preamble <NUM>, UE device <NUM>-<NUM> may disable its receiver <NUM> to conserve power.

At operation <NUM>, after predetermined time offset <NUM> has elapsed since the end of extended preamble <NUM> was transmitted, UE device <NUM>-<NUM> may transmit emergency message <NUM>.

At operation <NUM>, after predetermined time offset <NUM> has elapsed since the end of extended preamble <NUM> was received, UE device <NUM>-<NUM> may reactivate its receiver <NUM> for emergency message listening window <NUM> (e.g., beginning at time TC). UE device <NUM>-<NUM> may then receive and decode emergency message <NUM> during emergency message listening window <NUM> (e.g., because UE device <NUM>-<NUM> has now been time-synchronized with UE device <NUM>-<NUM> using extended preamble <NUM>). UE device <NUM>-<NUM> may perform any desired subsequent processing based on emergency message <NUM>.

Using extended preamble <NUM> to time-synchronize UE device <NUM>-<NUM> to UE device <NUM>-<NUM> may allow UE device <NUM>-<NUM> to implement RX windows <NUM> having arbitrary RX window lengths L. For example, RX window length L does not need to be at least 2X the size of preamble length T (<FIG>) 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 <NUM> in the series of N preambles <NUM> of <FIG> if desired. This may allow UE device <NUM>-<NUM> to further reduce power consumption relative to examples where a series of N preambles <NUM> is used for time-synchronization. If desired, the receiver <NUM> on UE device <NUM>-<NUM> 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 <NUM>-<NUM>). If desired, different types of UE device <NUM>-<NUM> may use different RX window lengths L to account for different antenna/receiver performance. UE device <NUM>-<NUM> may detect the end of extended preamble <NUM> by exploiting the fact that by time TD the sequence has already been detected at UE device <NUM>-<NUM> (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 <NUM> or identifying predetermined time offset <NUM>) is still possible using extended preamble <NUM> (e.g., by selection of Zadoff Chu sequence root, M-sequence index, etc.).

The example of <FIG> and <FIG> in which UE device <NUM>-<NUM> keeps its receiver <NUM> active during extended RX window 76E is merely illustrative. If desired, UE device <NUM>-<NUM> may deactivate its receiver <NUM> after the RX window <NUM> in which a portion of extended preamble <NUM> was received. For example, as shown in <FIG>, UE device <NUM>-<NUM> may deactivate its receiver <NUM> at the end of RX window <NUM>. In this example, time-synchronization may be performed based on where in the extended preamble <NUM> RX window <NUM> falls. Extended preamble <NUM> may therefore include a sequence that allows UE device <NUM>-<NUM> to detect where the portion of extended preamble <NUM> received during RX window <NUM> is located within extended preamble <NUM> as a whole.

For example, UE device <NUM>-<NUM> may generate extended preamble <NUM> as a continuous non-repeating sequence that allows locating position of any subsequence of extended preamble <NUM> in the continuous non-repeating sequence. The transmitter in UE device <NUM>-<NUM> 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 <NUM>. 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 <NUM>.

Once UE device <NUM>-<NUM> has received a portion (subsequence) of extended preamble <NUM> during its RX window <NUM>, UE device <NUM>-<NUM> may process the received portion (subsequence) to identify a time offset between the end of RX window <NUM> and time TC, when UE device <NUM>-<NUM> will transmit emergency message <NUM>. In the example of <FIG>, UE device <NUM>-<NUM> may process the subsequence of extended preamble <NUM> received during RX window <NUM> to identify time offset <NUM> (e.g., a time offset that includes the duration that would have been occupied by extended window 76E and predetermined time offset <NUM>) from the end of RX window <NUM> until the beginning of emergency message listening window <NUM>. The non-repeating structure of extended preamble <NUM> (e.g., as provided by the cover code) may allow UE device <NUM>-<NUM> to determine or identify the temporal location of the subsequence within extended preamble <NUM>, thereby allowing UE device <NUM>-<NUM> to identify time offset <NUM> (e.g., given the already predetermined time offset <NUM> between times TD and TC). UE device <NUM>-<NUM> may then reactivate its receiver after the identified time offset <NUM> has elapsed since the end of RX window <NUM> to receive emergency message <NUM>. In other words, UE device <NUM>-<NUM> may synchronize its timing for the receipt of emergency message <NUM> based on the subsequence of extended preamble <NUM> received during RX window <NUM>. Synchronizing timing using extended preamble <NUM> with an overlying cover code in this way may allow UE device <NUM>-<NUM> to minimize power consumption (e.g., because UE device <NUM>-<NUM> can deactivate its receiver after RX window <NUM>) without needing to expend resources detecting the end of extended preamble <NUM>, for example.

<FIG> is a flow chart of illustrative operations involved in transmitting and receiving an emergency message <NUM> that is time-synchronized based on an extended preamble <NUM> of <FIG> having an overlying cover code. Operations <NUM>-<NUM> of <FIG> may be performed by UE device <NUM>-<NUM> (e.g., while processing operation <NUM> of <FIG>). Operations <NUM>-<NUM> of <FIG> may be performed by UE device <NUM>-<NUM> (e.g., while processing operations <NUM> and <NUM> of <FIG>).

At operation <NUM> of <FIG>, UE device <NUM>-<NUM> may generate extended preamble <NUM> for transmission. Extended preamble <NUM> 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 <NUM>.

At operation <NUM>, the receiver <NUM> on UE device <NUM>-<NUM> (e.g., a UE device in coverage area <NUM>) may receive a portion (subsequence) of the extended preamble <NUM> having an overlying cover code transmitted by UE device <NUM>-<NUM> during one of its RX windows <NUM>.

At operation <NUM>, UE device <NUM>-<NUM> may identify time offset <NUM> between the end of RX window <NUM> and emergency message listening window <NUM> based on the cover code of the subsequence of extended preamble <NUM> received during the RX window.

At operation <NUM>, UE device <NUM>-<NUM> may deactivate its transmitter (e.g., once RX window <NUM> has ended) to conserve power. Operations <NUM> and <NUM> may be performed concurrently or operation <NUM> may be performed prior to operation <NUM> if desired.

At operation <NUM>, after the identified time offset <NUM> has elapsed since the end of RX window <NUM>, UE device <NUM>-<NUM> may reactivate its receiver <NUM> for RX window listening window <NUM> (e.g., beginning at time TC). UE device <NUM>-<NUM> may then receive and decode emergency message <NUM> during RX window listening window <NUM> (e.g., because UE device <NUM>-<NUM> has now been time-synchronized with UE device <NUM>-<NUM> using the subsequence of extended preamble <NUM> received during RX window <NUM>). UE device <NUM>-<NUM> may perform any desired subsequent processing based on emergency message <NUM>.

If desired, UE device <NUM>-<NUM> may transmit D2D signals to UE device <NUM>-<NUM> that include an acknowledgement that UE device <NUM>-<NUM> has received emergency message <NUM>. If UE device <NUM>-<NUM> 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 <NUM> compared to all other potential receivers. In this case, during a subsequent period (e.g., during subsequent iterations of path <NUM> of <FIG>), UE device <NUM>-<NUM> may increase the number N of preambles <NUM> or the length of extended preamble <NUM> transmitted before subsequent emergency message transmissions. Additionally or alternatively, UE device <NUM>-<NUM> may shift its time t<NUM> 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 <NUM> of the receiver in UE device <NUM>-<NUM>. If desired, UE device <NUM>-<NUM> may transmit the series of N preambles <NUM> and/or extended preamble <NUM> at a different frequency than emergency message <NUM> (e.g., the preamble(s) may be transmitted on a common channel whereas frequency hopping is used for transmitting emergency message <NUM>).

The example of <FIG> in which UE device <NUM>-<NUM> transmits emergency message <NUM> at time TC is merely illustrative. In general, UE device <NUM>-<NUM> 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 <NUM> (e.g., the synchronization operations described herein may be used to synchronize UE device <NUM>-<NUM> to UE device <NUM>-<NUM> for transfer of any desired wireless data using D2D signals <NUM>). Emergency message <NUM> may therefore sometimes simply be referred to as message <NUM>. Message <NUM> may contain emergency payload content (emergency message payload content) as one example. As another example, message <NUM> 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 <NUM> 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 <NUM> may gather and/or use personally identifiable information.

The methods and operations described above in connection with <FIG> (e.g., the operations of <FIG>) may be performed by the components of device <NUM> 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 <NUM> (e.g., storage circuitry <NUM> of <FIG>). 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 <NUM> (e.g., processing circuitry <NUM> of <FIG>, 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 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 methods or processes described herein.

If desired, an apparatus may be provided that includes logic, modules, or circuitry to perform 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 includes instructions that, when executed by the one or more processors, cause the one or more processors to perform 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 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 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 methods or processes described herein.

In accordance with an embodiment, a user equipment device is provided that includes one or more antennas, a transmitter configured to transmit device-to-device (D2D) signals to an additional user equipment device using the one or more antennas and one or more processors configured to transmit a message in the D2D signals, and 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.

The one or more processors may be further configured to transmit a series of preambles in the D2D signals prior to the message, the series of preambles may include the preamble and each preamble in the series of preambles may identify a respective time offset between the preamble and the message.

Each of the preambles in the series of preambles may have a preamble length and the preambles in the series of preambles may be separated by gaps during which the transmitter is inactive.

The one or more processors may be further configured to re-transmit the message in the D2D signals and transmit an additional series of preambles in the D2D signals after transmitting the message and prior to re-transmitting the message, the preambles in the additional series of preambles may be separated by additional gaps during which the transmitter is inactive, and the additional gaps may be separated from the re-transmitted message by the respective time offsets identified by the series of preambles.

The preamble may be shorter than a receiver window length of the additional user equipment device.

The preamble may include a repeating sequence and the one or more processors may be configured to apply a cover code over the repeating sequence that varies across a length of the preamble.

The preamble may include an M-sequence or a Zadoff Chu sequence.

The user equipment device may be configured to use the one or more antennas to communicate with a wireless base station in an on-grid state and the user equipment device is configured to transmit the D2D signals in an off-grid state in which the user equipment device is unable to connect to any wireless base stations.

The transmitter may be configured to transmit the preamble in the D2D signals at a first frequency and is configured to transmit the message in the D2D signals at a second frequency that is different from the first frequency.

In accordance with an embodiment, a user equipment device is provided that includes one or more antennas, a receiver configured to receive device-to-device (D2D) signals from an additional user 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 D2D signals during an RX window of the series of RX windows, deactivate the receiver after the RX window, identify a time offset based the preamble sequence received during the RX window, 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, and receive a message in the D2D signals during the message listening window.

The one or more processors may be further configured to identify a cover code applied to the preamble sequence, and identify the time offset based on the cover code.

The RX window may have a length that is at least twice a length of the preamble sequence.

The one or more processors may be further configured to relay the message to an emergency services provider.

The one or more processors may be further configured to deactivate the receiver between the RX windows in the series of RX windows.

The receiver may be configured to receive the preamble sequence at a first frequency and may be configured to receive the message at a second frequency that is different from the first frequency.

The one or more processors may be further configured to identify a frequency hopping pattern based on the preamble sequence and receive the message using the identified frequency hopping pattern.

The one or more processors may be 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.

In accordance with an embodiment, 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 is provided that includes periodically activating a receiver on the first user equipment device during a series of receiver (RX) windows, receiving a portion of the emergency message preamble in the D2D signals during an RX window of the series of RX windows, keeping the receiver active until an end of the emergency message preamble has been received and then deactivating the receiver, 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 and receiving the emergency message in the D2D signals during the emergency message listening window.

The method may include coarsely synchronizing timing of the receiver to timing of the second user equipment device using a network time protocol or cellular telephone signals received at the first user equipment device.

The method may include coarsely synchronizing timing of the receiver to timing of the second user equipment device using satellite navigation signals received at the first user equipment device.

Claim 1:
A user equipment device (<NUM>-<NUM>) comprising:
one or more antennas (<NUM>);
a receiver (<NUM>) configured to receive device-to-device, D2D, signals (<NUM>) from an additional user device (<NUM>-<NUM>) using the one or more antennas (<NUM>); and
one or more processors (<NUM>) configured to:
periodically activate (<NUM>) the receiver (<NUM>) during a series of receiver, RX, windows,
receive (<NUM>) a preamble sequence in the D2D signals (<NUM>) during an RX window of the series of RX windows,
deactivate the receiver (<NUM>) after the RX window,
identify a time offset based the preamble sequence received during the RX window,
re-activate (<NUM>) the receiver (<NUM>) during a message listening window that begins once the identified time offset has elapsed since an end of the preamble sequence, and
receive a message in the D2D signals (<NUM>) during the message listening window.