Patent Publication Number: US-10772056-B2

Title: Wakeup radio (WUR) packet preamble design

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/652,146, entitled “Wake-up Radio (WUR) Preamble Format Design,” filed on Apr. 3, 2018, the disclosure of which is hereby expressly incorporated herein by reference in its entirety. 
    
    
     FIELD OF TECHNOLOGY 
     The present disclosure relates generally to wireless communication systems, and more particularly to formats of packets for communication systems employing wakeup radios (WURs). 
     BACKGROUND 
     Wireless local area networks (WLANs) have evolved rapidly over the past decade, and development of WLAN standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.11 Standard family has improved single-user peak data throughput. For example, the IEEE 802.11b Standard specifies a single-user peak throughput of 11 megabits per second (Mbps), the IEEE 802.11a and 802.11g Standards specify a single-user peak throughput of 54 Mbps, the IEEE 802.11n Standard specifies a single-user peak throughput of 600 Mbps, and the IEEE 802.11ac Standard specifies a single-user peak throughput in the gigabits per second (Gbps) range. Future standards promise to provide even greater throughput, such as throughputs in the tens of Gbps range. 
     Some WLANs include low cost wireless devices, such as wireless sensors, that do not require high data rates. To reduce operating costs, it may be useful for such wireless devices to be battery operated or otherwise power constrained. Power saving techniques for reducing power consumption are used with such power-constrained wireless devices. For example, a WLAN network interface of a power-constrained wireless device is put into to a low power state (e.g., a sleep state) for periods of time in order to decrease power consumption of the wireless device. When the wireless device is ready to transmit data to an access point, the WLAN network interface is transitioned to an active state so that the data can be transmitted. After the WLAN network interface transmits the data, the WLAN network interface transitions back to the low power state. 
     A WLAN network interface of a power-constrained wireless device may “wake up” periodically to listen for transmissions from the access point to determine whether the access point has data to transmit to the wireless device. However, such periodic “wake ups” by the WLAN network interface consume power even when the access point has no data to transmit to the wireless device. Therefore, to further reduce power consumption, some wireless devices employ a low power wakeup radio (LP-WUR) that consumes much less power as compared to the WLAN network interface. For example, the LP-WUR does not include any transmitter circuitry and may be capable of only receiving very low data rate transmissions. When the access point is ready to transmit data to the wireless device, the access point transmits a wakeup radio (WUR) wakeup packet (referred to herein simply as a “wakeup packet”) addressed to the wireless device. In response to receiving the wakeup packet and determining that the wakeup packet is addressed to the wireless device, the LP-WUR wakes up the WLAN network interface so that the WLAN network interface is ready to receive data from the access point. 
     SUMMARY 
     In an embodiment, a method is performed by a first communication device and is for transmitting a wakeup packet configured to cause a wakeup radio of a second communication device to cause a wireless local area network (WLAN) network interface device of the second communication device to transition from a low power state to an active state. The method includes: generating, at the first communication device, a first portion of the wakeup packet, wherein the first portion of the wakeup packet corresponds to a WLAN legacy preamble of the wakeup packet, and wherein the first portion spans a first frequency bandwidth; generating, at the first communication device, a second portion of the wakeup packet, wherein the second portion of the wakeup packet spans a second bandwidth that is less than the first bandwidth, and wherein: the second portion of the wakeup packet is configured to cause the wakeup radio of the second communication device to cause the WLAN network interface device of the second communication device to transition from the low power state to the active state, generating the second portion of the wakeup packet includes i) generating a sync portion having a plurality of sync symbols, and ii) generating a wakeup packet body. The method also includes transmitting, by the first communication device, the wakeup packet. 
     In another embodiment, an apparatus comprises: a network interface device associated with a first communication device, wherein the network interface device comprises one or more integrated circuit (IC) devices. The one or more IC devices are configured to: generate a wireless local area network (WLAN) legacy preamble of a wakeup packet, wherein the wakeup packet is configured to cause a wakeup radio of a second communication device to cause a WLAN network interface device of the second communication device to transition from a low power state to an active state. The one or more IC devices are also configured to: generate a first portion of a wakeup packet, wherein the first portion of the wakeup packet corresponds to a wireless local area network (WLAN) legacy preamble of the wakeup packet, and wherein the first portion spans a first frequency bandwidth. The one or more IC devices are further configured to: generate a second portion of the wakeup packet, wherein the second portion of the wakeup packet spans a second bandwidth that is less than the first bandwidth, and wherein: the second portion of the wakeup packet is configured to cause a wakeup radio of a second communication device to cause a WLAN network interface device of the second communication device to transition from a low power state to an active state, and generating the second portion of the wakeup packet includes i) generating a sync portion having a plurality of sync symbols, and ii) generating a wakeup packet body. Additionally, the one or more IC devices are further configured to transmit the wakeup packet. 
     In yet another embodiment, a method is performed by a wakeup radio (WUR) of a communication device and is for processing a wakeup packet configured to cause the WUR of the communication device to cause a wireless local area network (WLAN) network interface device of the communication device to transition from a low power state to an active state. The wakeup packet includes i) a first portion having a WLAN legacy preamble that spans a first frequency bandwidth, and ii) a second portion that follows in time the first portion and that spans a second bandwidth narrower than the first bandwidth. The second portion includes i) a sync portion having a plurality of sync symbols, and ii) a wakeup packet body. The method includes: calculating, at the WUR, one or more correlations corresponding to one or more sync symbols included in the sync portion; detecting, at the WUR, the sync portion of the wakeup packet using the one or more correlations; determining, at the WUR, a symbol timing using the one or more correlations; and processing, at the WUR, the wakeup packet body based on the determined symbol timing. 
     In still another embodiment, an apparatus comprises: a wakeup radio WUR associated with a wireless local area network (WLAN) network interface device. The WUR comprises one or more integrated circuit (IC) devices configured to: process a wakeup packet. The wakeup packet includes i) a first portion having a WLAN legacy preamble that spans a first frequency bandwidth, and ii) a second portion that follows in time the first portion and that spans a second bandwidth narrower than the first bandwidth. The second portion includes i) the sync portion having a plurality of sync symbols, and ii) a wakeup packet body. Processing the wakeup packet includes calculating one or more correlations corresponding to one or more sync symbols included in the sync portion, detecting the sync portion of the wakeup packet using the one or more correlations, determining a symbol timing using the one or more correlations, and processing the wakeup packet body based on the determined symbol timing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of an example wireless local area network (WLAN) having a client station with a low power wakeup radio (LP-WUR), according to an embodiment. 
         FIG. 1B  is a block diagram of an example wireless network interface device of an access point included in the WLAN of  FIG. 1A , according to an embodiment. 
         FIG. 1C  is a block diagram of an example wireless network interface device of the client station included in the WLAN of  FIG. 1A , according to an embodiment. 
         FIG. 1D  is a block diagram of an example LP-WUR in the WLAN of  FIG. 1A , according to an embodiment. 
         FIG. 2  is a diagram of a wakeup packet, according to an embodiment. 
         FIG. 3A  is a diagram of an example payload of the wakeup packet of  FIG. 2 , according to an embodiment. 
         FIG. 3B  is a diagram of another example payload of the wakeup packet of  FIG. 2 , according to another embodiment. 
         FIG. 4  is a diagram of an example wakeup radio (WUR) sync portion of a wakeup packet, according to another embodiment. 
         FIG. 5A  is a diagram of an example WUR sync portion selection device that is included in one or more of the communication devices of  FIGS. 1A-C , according to an embodiment. 
         FIG. 5B  is a diagram of another example WUR sync portion selection device that is included in one or more of the communication devices of  FIGS. 1A-C , according to another embodiment. 
         FIG. 6  is a flow diagram of an example method for generating wakeup packets, according to an embodiment. 
         FIG. 7  is a block diagram of an example WUR preamble detector, according to an embodiment. 
         FIG. 8  is a block diagram of another example WUR preamble detector, according to another embodiment. 
         FIG. 9  is a block diagram of another example WUR preamble detector, according to another embodiment. 
         FIG. 10  is a flow diagram of an example method for processing a wakeup packet, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Techniques for generating and processing packets are described below in the context of low power wakeup radios merely for explanatory purposes. In other embodiments, packet generation and processing techniques are utilized in other types of wireless communication systems such as personal area networks (PANs), mobile communication networks such as cellular networks, metropolitan area networks (MANs), satellite communication networks, etc., that use a narrower bandwidth than WLANs. 
       FIG. 1A  is a block diagram of an example WLAN  110 , according to an embodiment. The WLAN  110  includes an access point (AP)  114  that comprises a host processor  118  coupled to a wireless network interface device  122 . The wireless network interface device  122  is coupled to a plurality of antennas  126 . Although three antennas  126  are illustrated in  FIG. 1A , the AP  114  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of antennas  126  in other embodiments. As will be described in more detail below, the wireless network interface device  122  is configured to generate and transmit a wakeup packet that can be decoded by low power wakeup radios (LP-WURs) in the WLAN  110 . 
     The host processor  118  is configured to executed machine readable instructions stored in a memory device (not shown), according to an embodiment. The host processor  118  is implemented on an integrated circuit (IC), according to an embodiment. The wireless network interface device  122  is implemented on one or more ICs. The host processor  118  is implemented on one IC and the wireless network interface device  122  is implemented on one or more other, different ICs, according to an embodiment. The host processor  118  is implemented on a first IC and the wireless network interface device  122  is implemented on at least the same first IC and optionally on one or more second ICs, according to an embodiment. 
     The WLAN  110  also includes one or more client stations  134 . Although three client stations  134  are illustrated in  FIG. 1A , the WLAN  110  includes other suitable numbers (e.g., 1, 2, 4, 5, 6, etc.) of client stations  134  in various embodiments. The client station  134 - 1  includes a host processor  138  coupled to a wireless network interface device  142 . The wireless network interface device  142  is coupled to one or more antennas  146 . Although three antennas  146  are illustrated in  FIG. 1A , the client station  134 - 1  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of antennas  146  in other embodiments. 
     The wireless network interface device  142  is configured to go into a low power state in which the wireless network interface device  142  consumes significantly less power as compared to an active state of the wireless network interface device  142 . The wireless network interface device  142  is capable of wirelessly receiving and transmitting via the one or more antennas  146  while in the active state. In an embodiment, the wireless network interface device  142  is incapable of wirelessly receiving and transmitting via the one or more antennas  146  while in the low power state. 
     The client station  134 - 1  also includes a LP-WUR  150  coupled to the wireless network interface device  142  and to at least one of the antennas  146 . The LP-WUR  150  is configured to use very low power (e.g., less than 100 microwatts or another suitable amount of power). The LP-WUR  150  is configured to use significantly less power (e.g., less than 20%, less than 10%, less than 5%, less than 2%, less than 1%, etc) than the wireless network interface device  142  while the wireless network interface device  142  is in the active state, according to an embodiment. 
     The LP-WUR  150  is configured to receive and decode wakeup packets transmitted by the AP  114  and received via one or more of the antennas  146 . The LP-WUR  150  is configured to determine whether a received wakeup packet includes an address (e.g., a media access control (MAC) address, an association identifier (AID), or another suitable network address) corresponding to the client station  134 - 1 , according to an embodiment. The LP-WUR  150  is configured to generate a wakeup signal in response to determining that a received wakeup packet includes the address corresponding to the client station  134 - 1 . An address corresponding to the client station  134 - 1  includes one or more of i) a unicast address corresponding to the client station  134 - 1 , ii) a multicast address corresponding to a group of client stations that includes the client station  134 - 1 , and/or iii) a broadcast address that corresponds to all client stations, in various embodiments. 
     When the wireless network interface device  142  is in the low power state and receives the wakeup signal from the LP-WUR  150 , the wireless network interface device  142  is configured to transition to the active power state in response to the wakeup signal, according to an embodiment. For example, when the wireless network interface device  142  is in the low power state and receives the wakeup signal from the LP-WUR  150 , the wireless network interface device  142  responsively transitions to the active power state to become ready to transmit and/or receive, according to an embodiment. 
     The host processor  138  is configured to executed machine readable instructions stored in a memory device (not shown), according to an embodiment. The host processor  138  is implemented on an IC, according to an embodiment. The wireless network interface device  142  is implemented on one or more ICs. The host processor  138  is implemented on one IC and the wireless network interface device  142  is implemented on one or more other, different ICs, according to an embodiment. The host processor  138  is implemented on a first IC and the wireless network interface device  142  is implemented on at least the same first IC and optionally on one or more second ICs, according to an embodiment. 
     The LP-WUR  150  is implemented on one IC and the wireless network interface device  142  is implemented on one or more other, different ICs, according to an embodiment. The LP-WUR  150  is implemented on a first IC and the wireless network interface device  142  is implemented on at least the same first IC and optionally on one or more second ICs, according to an embodiment. 
     In an embodiment, each of the client stations  134 - 2  and  134 - 3  has a structure that is the same as or similar to the client station  134 - 1 . For example, one or both of the client stations  134 - 2  and  134 - 3  includes a respective LP-WUR, according to an embodiment. As another example, one or both of the client stations  134 - 2  and  134 - 3  does not include a LP-WUR, according to another embodiment. Each of the client stations  134 - 2  and  134 - 3  has the same or a different number of antennas (e.g., 1, 2, 3, 4, 5, etc.). For example, the client station  134 - 2  and/or the client station  134 - 3  each have only two antennas (not shown), according to an embodiment. 
       FIG. 1B  is a block diagram of the network interface device  122  of the AP  114  of  FIG. 1A , according to an embodiment. The network interface  122  includes a MAC layer processor  160  coupled to a physical layer (PHY) processor  164 . The PHY processor  164  includes a plurality of transceivers  168  coupled to the plurality of antennas  126 . Although three transceivers  168  and three antennas  126  are illustrated in  FIG. 1B , the PHY processor  164  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  168  coupled to other suitable numbers of antennas  126  in other embodiments. In some embodiments, the AP  114  includes a higher number of antennas  126  than transceivers  168 , and the PHY processor  164  is configured to use antenna switching techniques. 
     The network interface  122  is implemented using one or more ICs configured to operate as discussed below. For example, the MAC layer processor  160  may be implemented, at least partially, on a first IC, and the PHY processor  164  may be implemented, at least partially, on a second IC. As another example, at least a portion of the MAC layer processor  160  and at least a portion of the PHY processor  164  may be implemented on a single IC. For instance, the network interface  122  may be implemented using a system on a chip (SoC), where the SoC includes at least a portion of the MAC layer processor  160  and at least a portion of the PHY processor  164 . 
     In various embodiments, the MAC layer processor  160  and/or the PHY processor  164  of the AP  114  are configured to generate data units, and process received data units, that conform to a WLAN communication protocol such as a communication protocol conforming to the IEEE 802.11 Standard or another suitable wireless communication protocol. For example, the MAC layer processor  160  may be configured to implement MAC layer functions, including MAC layer functions of the WLAN communication protocol, and the PHY processor  164  may be configured to implement PHY functions, including PHY functions of the WLAN communication protocol. For instance, the MAC layer processor  160  may be configured to generate MAC layer data units such as MAC service data units (MSDUs), MAC protocol data units (MPDUs), etc., and provide the MAC layer data units to the PHY processor  164 . The PHY processor  164  may be configured to receive MAC layer data units from the MAC layer processor  160  and encapsulate the MAC layer data units to generate PHY data units such as PHY protocol data units (PPDUs) for transmission via the antennas  126 . Similarly, the PHY processor  164  may be configured to receive PHY data units that were received via the antennas  126 , and extract MAC layer data units encapsulated within the PHY data units. The PHY processor  164  may provide the extracted MAC layer data units to the MAC layer processor  160 , which then processes the MAC layer data units. 
     In connection with generating one or more radio frequency (RF) signals for transmission, the PHY processor  164  is configured to process (which may include modulating, filtering, etc.) data corresponding to a PPDU to generate one or more digital baseband signals, and convert the digital baseband signal(s) to one or more analog baseband signals, according to an embodiment. Additionally, the PHY processor  164  is configured to upconvert the one or more analog baseband signals to one or more RF signals for transmission via the one or more antennas  138 . 
     In connection with receiving one or more RF signals, the PHY processor  164  is configured to downconvert the one or more RF signals to one or more analog baseband signals, and to convert the one or more analog baseband signals to one or more digital baseband signals. The PHY processor  164  is further configured to process (which may include demodulating, filtering, etc.) the one or more digital baseband signals to generate a PPDU. 
     The PHY processor  164  includes amplifiers (e.g., a low noise amplifier (LNA), a power amplifier, etc.), a radio frequency (RF) downconverter, an RF upconverter, a plurality of filters, one or more analog-to-digital converters (ADCs), one or more digital-to-analog converters (DACs), one or more discrete Fourier transform (DFT) calculators (e.g., a fast Fourier transform (FFT) calculator), one or more inverse discrete Fourier transform (IDFT) calculators (e.g., an inverse fast Fourier transform (IFFT) calculator), one or more modulators, one or more demodulators, etc. 
     The PHY processor  164  is configured to generate one or more RF signals that are provided to the one or more antennas  126 . The PHY processor  164  is also configured to receive one or more RF signals from the one or more antennas  126 . 
     The MAC processor  160  is configured to control the PHY processor  164  to generate one or more RF signals by, for example, providing one or more MAC layer data units (e.g., MPDUs) to the PHY processor  164 , and optionally providing one or more control signals to the PHY processor  164 , according to some embodiments. In an embodiment, the MAC processor  160  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a RAM, a read ROM, a flash memory, etc. In an embodiment, the MAC processor  160  includes a hardware state machine. 
       FIG. 1C  is a block diagram of the network interface device  142  of the client station  134 - 1  of  FIG. 1A , according to an embodiment. The network interface  142  includes a MAC layer processor  172  coupled to a PHY processor  174 . The PHY processor  174  includes a plurality of transceivers  178  coupled to the one or more antennas  146 . Although three transceivers  178  and three antennas  126  are illustrated in  FIG. 1C , the PHY processor  174  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  178  coupled to other suitable numbers of antennas  146  in other embodiments. In some embodiments, the client station  134 - 1  includes a higher number of antennas  146  than transceivers  178 , and the PHY processor  174  is configured to use antenna switching techniques. 
     The network interface  142  is implemented using one or more ICs configured to operate as discussed below. For example, the MAC layer processor  172  may be implemented, at least partially, on a first IC, and the PHY processor  174  may be implemented, at least partially, on a second IC. As another example, at least a portion of the MAC layer processor  172  and at least a portion of the PHY processor  174  may be implemented on a single IC. For instance, the network interface  142  may be implemented using a system on a chip (SoC), where the SoC includes at least a portion of the MAC layer processor  172  and at least a portion of the PHY processor  174 . 
     In various embodiments, the MAC layer processor  172  and the PHY processor  174  of the client device  134 - 1  are configured to generate data units, and process received data units, that conform to the WLAN communication protocol or another suitable communication protocol. For example, the MAC layer processor  172  may be configured to implement MAC layer functions, including MAC layer functions of the WLAN communication protocol, and the PHY processor  174  may be configured to implement PHY functions, including PHY functions of the WLAN communication protocol. The MAC layer processor  172  may be configured to generate MAC layer data units such as MSDUs, MPDUs, etc., and provide the MAC layer data units to the PHY processor  174 . The PHY processor  174  may be configured to receive MAC layer data units from the MAC layer processor  172  and encapsulate the MAC layer data units to generate PHY data units such as PPDUs for transmission via the one or more antennas  146 . Similarly, the PHY processor  174  may be configured to receive PHY data units that were received via the one or more antennas  146 , and extract MAC layer data units encapsulated within the PHY data units. The PHY processor  174  may provide the extracted MAC layer data units to the MAC layer processor  172 , which then processes the MAC layer data units. 
     As discussed above, the network interface device  142  is configured to transition between an active state and a low power state. When the wireless network interface device  142  is in the low power state and receives the wakeup signal from the LP-WUR  150 , the wireless network interface device  142  is configured to transition to the active power state in response to the wakeup signal, according to an embodiment. 
     The PHY processor  174  is configured to downconvert one or more RF signals received via the one or more antennas  146  to one or more baseband analog signals, and convert the analog baseband signal(s) to one or more digital baseband signals, according to an embodiment. The PHY processor  174  is further configured to process the one or more digital baseband signals to demodulate the one or more digital baseband signals and to generate a PPDU. The PHY processor  174  includes amplifiers (e.g., an LNA, a power amplifier, etc.), an RF downconverter, an RF upconverter, a plurality of filters, one or ADCs, one or more DACs, one or more DFT calculators (e.g., a fast Fourier transform (FFT) calculator), one or more IDFT calculators (e.g., an inverse fast Fourier transform (IFFT) calculator), one or more modulators, one or more demodulators, etc. 
     The PHY processor  174  is configured to generate one or more RF signals that are provided to the one or more antennas  146 . The PHY processor  174  is also configured to receive one or more RF signals from the one or more antennas  146 . 
     The MAC processor  172  is configured to control the PHY processor  174  to generate one or more RF signals by, for example, providing one or more MAC layer data units (e.g., MPDUs) to the PHY processor  174 , and optionally providing one or more control signals to the PHY processor  174 , according to some embodiments. In an embodiment, the MAC processor  172  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a RAM, a read ROM, a flash memory, etc. In an embodiment, the MAC processor  172  includes a hardware state machine. 
       FIG. 1D  is a block diagram of the LP-WUR  150  of the client station  134 - 1  of  FIG. 1A , according to an embodiment. The LP-WUR  150  includes radio frequency (RF)/analog front-end circuitry  184  coupled to at least one of the antennas  146 . The RF/analog front-end circuitry  184  includes one or more amplifiers (e.g., a low noise amplifier (LNA)), an RF downconverter, one or more filters, and one or more analog-to-digital converters (ADCs). In an embodiment, the RF/analog front-end circuitry  184  is configured to downconvert an RF signal to a baseband analog signal, and convert the analog baseband signal to a digital baseband signal. 
     The RF/analog front-end circuitry  184  is coupled to digital baseband circuitry  188 . The digital baseband circuitry  188  is configured to process the digital baseband signal to determine whether the digital baseband signal corresponds to a wakeup packet. The digital baseband circuitry  188  includes a demodulator that demodulates data from the digital baseband signal to generate an information signal corresponding to information included in a wakeup packet. 
     The digital baseband circuitry  188  is coupled to logic circuitry  192 . The logic circuitry  192  is configured to process the information signal to determine whether a wakeup packet includes an address (e.g., a MAC address, an AID, or another suitable network address) corresponding to the client station  134 - 1 , according to an embodiment. The logic circuitry  192  is configured to generate the wakeup signal in response to determining that a received wakeup packet includes the address corresponding to the client station  134 - 1 . 
       FIG. 2  is a block diagram of a wakeup packet  200  used in the example WLAN  110  of  FIG. 1 , according to an embodiment. The network interface  122  of the AP  114  is configured to generate and transmit the wakeup packet  200 , according to an embodiment. The network interface  142  of the client station  134 - 1  is also configured to generate and transmit the wakeup packet  200 , e.g., to prompt another client station  134  to wake up from a low power state, according to another embodiment. 
     The LP-WUR  150  of the client station  134 - 1  is configured to receive, detect, and decode the wakeup packet  200 , according to an embodiment. 
     The wakeup packet  200  includes an 802.11 preamble portion  204  and a payload  208 . The 802.11 preamble portion  204  enables IEEE 802.11 stations (e.g., wireless communication devices that are configured to operate according to the IEEE 802.11 Standard) to detect the wakeup packet  200  and determine a length of the wakeup packet  200  for the purpose of reducing transmissions by IEEE 802.11 stations that will collide with the wakeup packet  200 , according to an embodiment. 
     The 802.11 preamble portion  204  includes a legacy 802.11 preamble  210 , which corresponds to a legacy preamble defined by the IEEE 802.11 Standard, according to an embodiment. The legacy 802.11 preamble  210  includes a legacy short training field (L-STF)  212 , a legacy long training field (L-LTF)  216 , a legacy signal field (L-SIG)  220 . The L-STF  212  includes signals designed for packet detection and automatic gain control (AGC) training. The L-LTF  216  includes signals designed for channel estimation and synchronization. The L-SIG  220  includes information regarding the wakeup packet  200 , including length information (e.g., in a length subfield (not shown)) that can be used by IEEE 802.11 stations to determine when the wakeup packet  200  will end. 
     In other embodiments, the wakeup packet  200  includes a legacy preamble (different than the legacy 802.11 preamble  210 ) that enables stations that conform to a different suitable wireless communication protocol (e.g., other than the IEEE 802.11 Standard) to detect the wakeup packet  200  and determine a length of the wakeup packet  200  for the purpose of reducing transmissions by such stations that will collide with the wakeup packet  200 , according to an embodiment. 
     In an embodiment, the 802.11 preamble portion  204  also includes a binary phase shift keying (BPSK) modulated field (BPSK-modulated field)  224  that follows the legacy 802.11 preamble  210 . In an embodiment, the BPSK-modulated field  224  is a repetition of the L-SIG  220 . In an embodiment, the BPSK-modulated field  224  is identical to at least a portion of the L-LTF  216 . In other embodiments, the BPSK-modulated field  224  includes any other suitable signal and/or information. In an embodiment, the BPSK-modulated field  224  does not convey any useful information to recipient communication devices. In another embodiment, the BPSK-modulated field  224  does convey useful information to recipient communication devices. For example, in an embodiment, wakeup packet data (e.g., which includes a network address corresponding to an intended client station or stations) is encoded within/on a set of OFDM symbols that includes the BPSK-modulated field  224  and the payload  208 . In some embodiments, the BPSK-modulated field  224  is omitted from the wakeup packet  200 . 
     The payload  208  includes a wakeup preamble  228 . In an embodiment, the wakeup preamble  228  includes signals that enable LP-WURs such as the LP-WUR  150  to detect the payload  208  of the wakeup packet  200  and to synchronize to the payload  208  of the wakeup packet  200 . The payload  208  also includes a wakeup packet data portion  232 . In an embodiment, the wakeup packet data portion  232  includes an address (e.g., a MAC address, an AID, or another suitable network address) corresponding to a client station (or client stations) to which the wakeup packet  200  is intended. Referring now to  FIG. 1D , the digital baseband circuitry  188  is configured to detect the wakeup packet  200  at least by detecting the wakeup preamble  228 , according to an embodiment. The logic circuitry  192  is configured to process the wakeup packet body  232  to determine whether the wakeup packet body  232  includes an address (e.g., a MAC address, an AID, or another suitable network address) corresponding to the client station  134 - 1 . 
     In an embodiment, the legacy 802.11 preamble  210  spans a first frequency bandwidth, and the wakeup preamble  228  and the wakeup packet data portion  232  span a second frequency bandwidth that is narrower than the first frequency bandwidth. For example, the first frequency bandwidth is 20 MHz and the second frequency bandwidth is a narrower bandwidth such as approximately 4 MHz (e.g. 4.06 MHz), or another suitable narrower bandwidth such as 1 MHz, 2 MHz, 5 MHz, 10 MHz, etc. 
       FIG. 3A  is a diagram of an example payload portion  300  of a wakeup packet, such as the wakeup packet  200  of  FIG. 2 , according to an embodiment. The payload portion  300  is used as the payload  208  of the wakeup packet  200  of  FIG. 2 , according to an embodiment.  FIG. 3A  is described in the context of the wakeup packet  200  of  FIG. 2  for explanatory purposes. In other embodiments, however, the payload portion  300  is included in another suitable wakeup packet different that the wakeup packet  200  of  FIG. 2 . 
     The payload portion  300  includes a WUR preamble  304  and the wakeup packet data portion  232  ( FIG. 2 ). The WUR preamble  304  includes a WUR sync portion  308  and a WUR PHY header portion  312 . The WUR sync portion  308  is used by a wakeup radio (e.g., the LP-WUR  150  of  FIG. 1 ) for one or more of carrier sensing, detection of the payload portion  300 , synchronization to the payload portion  300 , etc. The WUR PHY header portion  312  includes information regarding the payload portion  300  that is used by a wakeup radio (e.g., the LP-WUR  150  of  FIG. 1 ) to process the wakeup packet data portion  232  ( FIG. 2 ). The information regarding the payload portion  300  in the WUR PHY header portion  312  includes PHY parameter signaling, such as one or more of a data rate at which the wakeup packet data portion  232  is encoded, a modulation schemed used for the wakeup packet data portion  232 , a coding scheme used for the wakeup packet data portion  232 , a length/duration of the wakeup packet data portion  232 , etc. 
     In an embodiment, the WUR preamble  304  is modulated/encoded according to a first, lowest data rate defined by a communication protocol, whereas the wakeup packet data portion  232  is modulated/encoded according to a second data rate defined by the communication protocol, where the second data rate is selected from a plurality of data rates defined by the communication protocol, which includes one or more data rates that are higher than the first data rate. The WUR PHY header portion  312  includes information that indicates the second data rate, according to an embodiment. 
       FIG. 3B  is a diagram of another example payload portion  350  of a wakeup packet, such as the wakeup packet  200  of  FIG. 2 , according to another embodiment. The payload portion  350  is used as the payload  208  of the wakeup packet  200  of  FIG. 2 , according to an embodiment.  FIG. 3B  is described in the context of the wakeup packet  200  of  FIG. 2  for explanatory purposes. In other embodiments, however, the payload portion  350  is included in another suitable wakeup packet different that the wakeup packet  200  of  FIG. 2 . 
     The payload portion  350  includes a WUR sync portion  354  and a WUR PHY header portion  358 . The WUR sync portion  354  is used by a wakeup radio (e.g., the LP-WUR  150  of  FIG. 1 ) for one or more of carrier sensing, detection of the payload portion  300 , synchronization to the payload portion  300 , etc. The WUR sync portion  354  also indicates one or more PHY parameters, such as one or more of a data rate at which the wakeup packet data portion  232  is encoded, a modulation schemed used for the wakeup packet data portion  232 , a coding scheme used for the wakeup packet data portion  232 , etc., and is used by a wakeup radio (e.g., the LP-WUR  150  of  FIG. 1 ) to process the WUR PHY header portion  358  and/or the wakeup packet data portion  232  ( FIG. 2 ). For example, the WUR sync portion  354  may include a sync pattern selected from a plurality of sync patterns, wherein sync patterns from among the plurality of sync patterns indicate one or more of respective data rates, respective modulation schemes, respective coding rates, etc. 
     The WUR PHY header portion  358  includes information regarding the payload portion  300  that is used by a wakeup radio (e.g., the LP-WUR  150  of  FIG. 1 ) to process the wakeup packet data portion  232  ( FIG. 2 ). The information regarding the payload portion  300  in the WUR PHY header portion  358  includes PHY parameter signaling, such as one or more of a data rate at which the wakeup packet data portion  232  is encoded, a modulation schemed used for the wakeup packet data portion  232 , a coding scheme used for the wakeup packet data portion  232 , a length/duration of the wakeup packet data portion  232 , etc. 
     In an embodiment, the WUR sync portion  354  is modulated/encoded according to the first, lowest data rate defined by the communication protocol, whereas the WUR PHY header portion and the wakeup packet data portion  232  are modulated/encoded according to a second data rate defined by the communication protocol, where the second data rate is selected from a plurality of data rates defined by the communication protocol, which includes one or more data rates that are higher than the first data rate. The WUR sync portion  354  indicates the second data rate, according to an embodiment. 
     In some embodiments in which the WUR sync portion  354  indicates one or more PHY parameters, such as one or more of a data rate at which the wakeup packet data portion  232  is encoded, a modulation schemed used for the wakeup packet data portion  232 , a coding scheme used for the wakeup packet data portion  232 , etc., the WUR PHY header portion  358  is omitted from the payload portion  350 . 
       FIG. 4  is a diagram of an example WUR sync portion  400 , according to an embodiment. The WUR sync portion  400  is used in a WUR packet such as those described above with reference to  FIGS. 2, 3A, and 3B , or another suitable WUR packet, in various embodiments. 
     The WUR sync portion  400  includes N sync sequences  404 , where N is a suitable integer greater than two. Each sync sequence  404  is sometimes referred to herein as a sync symbol  404 . 
     Each sync sequence  404  has a length of K samples corresponding to a suitable sampling rate, wherein K is a suitable integer greater than two. Generally, as the value of N increases, a carrier sensing (CS) property of the sync sequence  404  improves and a symbol timing acquisition (ST) property of the sync sequence  404  improves, at least in some embodiments. Generally, as the value of K increases, a correlation property of the sync sequence  404  improves, at least in some embodiments. Generally, as N and K increase, the length of the WUR sync portion  400  increases, which adversely affects efficiency because more channel time is consumed transmitting the WUR sync portion  400  rather than user data, at least in some embodiments. 
     In some embodiments, the WUR sync portion  400  is modulated according to on-off keying (OOK). For example, in some embodiments, each sync sequence  404  is selected from a set consisting of two sync symbols: i) a predetermined sequence corresponding to “On” (O), and ii) a zero energy sequence (e.g., a sequence of zeros) corresponding to “Off” (F). Thus, as an illustrative example in which N is four, the WUR sync portion  400  comprises OOOF. As another illustrative example in which N is four, the WUR sync portion  400  comprises OFOF. As an illustrative example in which N is five, the WUR sync portion  400  comprises OOOOF. As another illustrative example in which N is five, the WUR sync portion  400  comprises OOFOF. As an illustrative example in which N is six, the WUR sync portion  400  comprises OOOOOF. As another illustrative example in which N is six, the WUR sync portion  400  comprises OFOFOF. 
     In other embodiments, each sync sequence  404  is selected from a set consisting of three sync symbols: i) a predetermined sequence corresponding to “On” (O), ii) a zero energy sequence (e.g., a sequence of zeros) corresponding to “Off” (F), and iii) the predetermined sequence phase rotated by 180 degrees corresponding to “Negative On” (M). Thus, as an illustrative example in which N is five, the WUR sync portion  400  comprises OOMOF. As another illustrative example in which N is five, the WUR sync portion  400  comprises OOMOM. 
     In some embodiments, the WUR sync portion  400  is modulated according to OOK combined with Manchester coding. For example, in some embodiments, each sync sequence  404  is selected from a set consisting of two sync symbols: i) a first sequence comprising a) a first occurring half (in time) set to a predetermined sequence, and b) a second occurring half (in time) set to a zero energy sequence (e.g., a sequence of zeros) “On” (O); and ii) a second sequence comprising a) a first occurring half (in time) set to a zero energy sequence (e.g., a sequence of zeros), and b) a second occurring half (in time) set to a predetermined sequence “Off” (F). Thus, as an illustrative example in which N is four, the WUR sync portion  400  comprises OOOF. As another illustrative example in which N is four, the WUR sync portion  400  comprises OFOF. As an illustrative example in which N is five, the WUR sync portion  400  comprises OOOOF. As another illustrative example in which N is five, the WUR sync portion  400  comprises OOFOF. As an illustrative example in which N is six, the WUR sync portion  400  comprises OOOOOF. As another illustrative example in which N is six, the WUR sync portion  400  comprises OFOFOF. 
     In some embodiments, each sync sequence  404  is selected from a set consisting of two sync symbols: i) a predetermined sequence (SYNC), and ii) the predetermined sequence phase rotated by 180 degrees (−SYNC), wherein the predetermined sequence (SYNC) comprises a plurality of modulated subsymbols. For instance, each subsymbol is modulated according to a suitable modulation scheme such as BPSK, quadrature phase shift keying (QPSK), etc., according to various embodiments. 
     The predetermined sequence (SYNC) is selected to have an autocorrelation function that resembles an impulse function (e.g., a relatively high center peak as compared to heights of side lobes, where the center peak is also relatively narrow, e.g., as compared to a height of the center peak). In an embodiment, the predetermined sequence (SYNC) is selected to have a zero direct current (DC) component. In an embodiment, an exhaustive search technique is used to determine a predetermined sequence (SYNC) with suitable autocorrelation and DC component properties. In one illustrative embodiment, the predetermined sequence (SYNC) is:
         [−1 −1 1 −1 1 −1 1 1 −1 −1 1 1 1 1 −1 −1]
 
In other embodiments, the predetermined sequence (SYNC) is another suitable sequence.
       

     As an illustrative embodiment in which N is four, the WUR sync portion  400  comprises [SYNC SYNC SYNC −SYNC]. As another illustrative embodiment in which N is six, the WUR sync portion  400  comprises [SYNC SYNC SYNC SYNC −SYNC −SYNC]. 
     In some embodiments, each sync sequence  404  comprises one or more Golay sequences Ga. In some embodiments, each sync sequence  404  comprises one or more Golay sequences Ga, and one or more Golay sequences Gb, wherein the Golay sequence Gb is a complementary sequence to the Golay sequence Ga. Generally, the two complementary Golay sequences Ga and Gb have correlation properties suitable for detection at a receiving device. For example, the complementary Golay sequences Ga and Gb may be selected so that the sum of corresponding out-of-phase aperiodic autocorrelation coefficients of the sequences Ga and Gb is zero. In some embodiments, the complementary sequences Ga and Gb have a zero or almost-zero periodic cross-correlation. In another aspect, the sequences Ga and Gb may have aperiodic cross-correlation with a narrow main lobe and low-level side lobes, or aperiodic auto-correlation with a narrow main lobe and low-level side lobes. 
     In an embodiment, Ga (or, Ga and Gb) is/are selected to have a smallest DC component. In an embodiment, Ga is selected to have a duration of 2 microseconds. In another embodiment, Ga (or, Ga and Gb) are selected to have a duration of 2 microseconds. In another embodiment, Ga (or, Ga and Gb) are selected to have a duration of 4 microseconds. 
     In some embodiments utilizing Golay sequences, each sync sequence  404  is selected from a set consisting of two sync symbols: i) Ga, and ii) Ga rotated by 180 degrees (−Ga). For example, in an embodiment, the WUR sync portion  400  comprises a plurality of consecutive sequences Ga, and a last occurring sync sequence  404  comprising −Ga. As an illustrative embodiment in which N is four, the WUR sync portion  400  comprises [Ga Ga Ga −Ga]. As an illustrative embodiment in which N is five, the WUR sync portion  400  comprises [Ga Ga Ga Ga −Ga]. 
     In some embodiments utilizing Golay sequences, each sync sequence  404  is selected from a set consisting of two sync symbols: i) Ga, and ii) Gb. For example, in an embodiment, the WUR sync portion  400  comprises a plurality of consecutive sequences Ga, and a last occurring sync sequence  404  comprising Gb. As an illustrative embodiment in which N is four, the WUR sync portion  400  comprises [Ga Ga Ga Gb]. As an illustrative embodiment in which N is five, the WUR sync portion  400  comprises [Ga Ga Ga Ga Gb]. 
     In some embodiments utilizing Golay sequences, each sync sequence  404  is selected from a set consisting of two sync symbols: i) [Ga Gb], and ii) [−Ga −Gb]. For example, in an embodiment, the WUR sync portion  400  comprises one or more consecutive sequences [Ga Gb], and a last occurring sync sequence  404  comprising [−Ga −Gb]. As an illustrative embodiment in which N is three, the WUR sync portion  400  comprises [Ga Gb Ga Gb −Ga −Gb]. 
     In some embodiments utilizing Golay sequences, each sync sequence  404  is selected from a set consisting of two sync symbols: i) [Ga Ga Gb Gb], and ii) [−Ga −Ga −Gb −Gb]. For example, in an embodiment, the WUR sync portion  400  comprises one or more consecutive sequences [Ga Ga Gb Gb], and a last occurring sync sequence  404  comprising [−Ga −Ga −Gb −Gb]. As an illustrative embodiment in which N is three, the WUR sync portion  400  comprises [Ga Ga Gb Gb −Ga −Ga −Gb −Gb]. 
     Referring again to  FIGS. 3A and 3B , in some embodiments in which a WUR sync portion (e.g., the WUR sync portion  308 , the WUR sync portion  354 ) of a wakeup packet signals one or more PHY parameters corresponding to the data portion  232  (e.g., a data rate, a modulation scheme, a coding scheme, a length/duration, etc.), different WUR sync portion contents correspond to different PHY modes, wherein the different sync portion contents are included in a set of candidate WUR sync portions. In such embodiments, a candidate WUR sync portion is selected from the set of candidate WUR sync portions according to the PHY mode that is to be used to transmit the data portion  232  of the wakeup packet. 
       FIG. 5A  is a diagram of a WUR sync portion selection device  500 , according to an embodiment. The WUR sync portion selection device  500  is included in a network interface such as the network interface  122  of  FIGS. 1A and 1B , in an embodiment. The WUR sync portion selection device  500  is included in the PHY processor  164 , in an embodiment. 
     The WUR sync portion selection device  500  includes a multiplexer  504 . A plurality of candidate WUR sync portions are provided to a plurality of inputs of the multiplexer  504 . In an embodiment, candidate WUR sync portions are stored in respective registers, and the registers are coupled to respective inputs of the multiplexer  504 . A control signal is provided to a control input of the multiplexer  504 . The control signal indicates a PHY mode that is to be used to transmit the data portion  232  of the wakeup packet. The control signal causes the multiplexer  504  to couple a selected one of the multiplexer inputs to an output of the multiplexer  504 . 
     In other embodiments, the WUR sync portion selection device  500  is implemented using a hardware state machine. For example, the hardware state machine is coupled to a memory device that stores the plurality of candidate WUR sync portions in respective locations of the memory device, in an illustrative embodiment. The control signal causes the hardware state machine to select one of the locations of the memory device to read out a selected WUR sync portion from the memory device. 
     In other embodiments, the WUR sync portion selection device  500  is implemented using a processor executing machine readable instructions. For example, the processor is coupled to a memory device that stores the plurality of candidate WUR sync portions in respective locations of the memory device, in an illustrative embodiment. The machine readable instructions, when executed by the processor, cause the processor to select one of the locations of the memory device to read out a selected WUR sync portion from the memory device. 
     Referring again to  FIGS. 3A and 3B , in some embodiments in which a WUR sync portion (e.g., the WUR sync portion  308 , the WUR sync portion  354 ) of a wakeup packet signals one or more PHY parameters corresponding to the data portion  232  (e.g., a data rate, a modulation scheme, a coding scheme, a length/duration, etc.), different SYNC symbols correspond to different PHY modes, wherein the different SYNC symbols are included in a set of candidate SYNC symbols. In such embodiments, a candidate SYNC symbol is selected from the set of candidate SYNC symbols according to the PHY mode that is to be used to transmit the data portion  232  of the wakeup packet. The selected SYNC portion is then used to generate the WUR sync portion. 
       FIG. 5B  is a diagram of a SYNC symbol selection device  550 , according to an embodiment. The SYNC symbol selection device  550  is included in a network interface such as the network interface  122  of  FIGS. 1A and 1B , in an embodiment. The SYNC symbol selection device  500  is included in the PHY processor  164 , in an embodiment. 
     The SYNC symbol selection device  500  includes a multiplexer  554 . A plurality of candidate SYNC symbols are provided to a plurality of inputs of the multiplexer  554 . In an embodiment, candidate SYNC symbols are stored in respective registers, and the registers are coupled to respective inputs of the multiplexer  554 . A control signal is provided to a control input of the multiplexer  554 . The control signal indicates a PHY mode that is to be used to transmit the data portion  232  of the wakeup packet. The control signal causes the multiplexer  554  to couple a selected one of the multiplexer inputs to an output of the multiplexer  504 . 
     In other embodiments, the SYNC symbol selection device  550  is implemented using a hardware state machine. For example, the hardware state machine is coupled to a memory device that stores the plurality of candidate SYNC symbols in respective locations of the memory device, in an illustrative embodiment. The control signal causes the hardware state machine to select one of the locations of the memory device to read out a selected SYNC symbol from the memory device. 
     In other embodiments, the SYNC symbol selection device  500  is implemented using a processor executing machine readable instructions. For example, the processor is coupled to a memory device that stores the plurality of candidate SYNC symbols in respective locations of the memory device, in an illustrative embodiment. The machine readable instructions, when executed by the processor, cause the processor to select one of the locations of the memory device to read out a selected SYNC symbol from the memory device. 
       FIG. 6  is a flow diagram of an example method  600  for generating wakeup packets, according to an embodiment. In some embodiments, the network interface device  122  of  FIG. 1  is configured to implement the method  600 . The method  600  is described in the context of the network interface device  122  merely for explanatory purposes and, in other embodiments, the method  600  is implemented by another suitable device, such as the network interface device  142  or another suitable network interface device. 
     At block  604 , the network interface device  122  generates (e.g., the PHY processor  164  generates) a first portion of a wakeup packet. The first portion corresponds to a legacy PHY preamble corresponding to a communication protocol. The first portion spans a first bandwidth. In an embodiment, the legacy PHY preamble is a legacy 802.11 preamble corresponding to the protocol specified by the IEEE 802.11n Standard. The legacy PHY preamble corresponds to other communication protocols as well, such as the protocol specified by the IEEE 802.11ac Standard, the protocol specified by the IEEE 802.11ax Standard (now under development), etc., in some embodiments. 
     At block  608 , the network interface device  122  generates a second portion of the wakeup packet. The second portion spans a second bandwidth that is less than the first bandwidth. The second portion of the wakeup packet is configured to prompt one or more wakeup radios at one or more respective communication devices to prompt one or more respective network interfaces to transition from a low power state to an active state. According to an embodiment, the second portion of the wakeup packet does not conform to the communication protocol to which the legacy PHY preamble conforms. 
     In some embodiments, generating the second portion of the wakeup packet includes generating a sync portion having a plurality of sync symbols. In an embodiment, the sync portion is modulated according to OOK, and each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) a zero energy sequence. In another embodiment in which the sync portion is modulated according to OOK, each sync symbol is selected from a set consisting of i) a non-zero energy sequence, ii) the non-zero energy sequence phase rotated by 180 degrees, and iii) a zero energy sequence. 
     In an embodiment, the sync portion is modulated according to a Manchester code, and each sync symbol is selected from a set consisting of i) a first sync symbol comprising a) a first occurring non-zero energy portion, and b) a second occurring zero energy portion; and ii) a second sync symbol comprising a) a first occurring zero energy portion, and b) a second occurring non-zero energy portion. 
     In another embodiment, each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) the non-zero energy sequence phase rotated by 180 degrees. 
     In another embodiment, each sync symbol comprises one or more Golay codes. 
     Generating the second portion of the wakeup packet further includes generating a wakeup packet body. 
     In an embodiment, the second portion of the wakeup packet is not configured to be decoded and processed by a network interface device that can decode and process data units conforming to a standard within the IEEE 802.11 Standard family. 
     At block  612 , the network interface device  122  transmits the wakeup packet. 
     In some embodiments, the method  600  further includes determining, at the network interface device  122 , a PHY mode according to which the wakeup packet body is to be transmitted; and selecting the sync portion from a set of candidate sync portions according to the selected PHY mode, wherein the selected sync portion indicates the PHY mode. 
     In some embodiments, the method  600  further includes determining, at the network interface device  122 , a PHY mode according to which the wakeup packet body is to be transmitted; and selecting the sync symbol from a set of candidate sync symbol according to the selected PHY mode, wherein the selected sync symbol indicates the PHY mode. 
     Referring again to  FIGS. 1B and 4 , the LP-WUR  150  (e.g., the digital baseband circuitry  188 ) includes a WUR preamble detector that is configured to i) detect a WUR preamble portion (e.g., the WUR preamble portion  400 ) in a wakeup packet (sometimes referred to herein as carrier sensing (CS)), and ii) determine a symbol timing (ST) of the wakeup packet using the WUR preamble portion. The WUR preamble detector includes one or more correlators (e.g., autocorrelators, cross-correlators, etc.) that compute one or more correlations corresponding to sync symbols in the WUR preamble portion, according to an embodiment. For example, the one or more correlators are configured to detect sync symbols in the WUR preamble portion, according to an embodiment. The WUR preamble detector is configured to use one or more outputs of one or more correlators to detect a pattern of sync symbols in a received signal that matches a pattern of sync symbols in the WUR preamble portion. Detection of the pattern of sync symbols in the received signal indicates that the received signal includes the WUR preamble portion, according to an embodiment. 
     In some embodiments, the WUR preamble detector is configured to use one or more outputs of one or more correlators to a symbol timing of sync symbols in a received signal that includes the WUR preamble portion. For example, the WUR preamble detector is configured to estimate centers of peaks in the one or more outputs of the one or more correlators, and to use the estimated centers of the peaks to determine the symbol timing of sync symbols in the received signal, according to an embodiment. 
     In some embodiments in which the sync symbols include zero power sync symbols (e.g., OOK-modulated WUR preamble portions) or portions of sync symbols corresponding to a zero power sync signal (e.g., Manchester code-modulated WUR preamble portions), the WUR preamble detector is configured to measure one or more energy levels at one or more expected locations of zero power signals in the WUR sync portion. For example, the WUR preamble detector includes one or more energy level measurement circuits, and the WUR preamble detector uses outputs of the one or more energy level measurement circuits to detect energy levels that fall below a threshold at one or more expected locations of zero power signals in the WUR sync portion. In some embodiments, the WUR preamble detector uses outputs of the one or more energy level measurement circuits to detect one or more transitions at expected locations in the WUR sync portion where the WUR sync portion is configured to transition from a non-zero energy signal to a zero energy signal, or vice versa. 
     For example, with OOK-modulated WUR preamble portions, the WUR preamble detector includes one or more correlators (e.g., autocorrelators, cross-correlators, etc.) that compute one or more correlations corresponding to O (and/or M) locations in the WUR preamble portion, according to an embodiment. Also with OOK-modulated WUR preamble portions, the WUR preamble detector includes one or more energy detectors that generate one or more energy measurements corresponding to F locations in the WUR preamble portion, according to an embodiment. 
     For example, the WUR preamble detector compares the one or more correlations to one or more respective first thresholds to generate one or more correlation peak detection signals, according to an embodiment. As another example, the WUR preamble detector compares the one or more energy measurements to one or more respective second thresholds to generate one or more energy detection signals, according to an embodiment. As an illustrative embodiment, the WUR preamble detector compares an energy measurement to two second thresholds (an O second threshold and an F second threshold) to generate an energy detection signal that indicates when a signal energy as i) exceeded the O second threshold (corresponding to an expected O location), and ii) then fallen below the F second threshold (corresponding to an expected F location), according to an embodiment. 
       FIG. 7  is a block diagram of an example WUR preamble detector  700 , according to an embodiment. The WUR preamble detector  700  is for use with the OOK-modulated WUR preamble portion  400  when the OOK-modulated WUR preamble portion  400  comprises five symbols: OOOOF. 
     The WUR preamble detector  700  comprises an autocorrelation calculator  704  coupled to three delay elements  708 , which are coupled together in series. An output of the delay element  708 - 1  corresponds to an autocorrelation between a fourth occurring symbol in the OOK-modulated WUR preamble portion  400 , and a third occurring symbol in the OOK-modulated WUR preamble portion  400 . An output of the delay element  708 - 2  corresponds to an autocorrelation between the third occurring symbol in the OOK-modulated WUR preamble portion  400 , and a second occurring symbol in the OOK-modulated WUR preamble portion  400 . An output of the delay element  708 - 3  corresponds to an autocorrelation between the second occurring symbol in the OOK-modulated WUR preamble portion  400 , and a first occurring symbol in the OOK-modulated WUR preamble portion  400 . 
     The WUR preamble detector  700  also comprises an energy measurement device  712 . An output of the energy measurement device  712  corresponds to an energy measurement of a fifth occurring symbol in the OOK-modulated WUR preamble portion  400 . 
     The WUR preamble detector  700  also comprises logic  720 , which is coupled to the output of the delay element  708 - 1 , the output of the delay element  708 - 2 , the output of the delay element  708 - 3 , and the output of the energy measurement device  712 . In an embodiment, the logic  720  includes three respective first comparators that are configured to compare the output of the delay element  708 - 1 , the output of the delay element  708 - 2 , and the output of the delay element  708 - 3  to respective first thresholds. For example, when the logic  720  determines the output of the delay element  708 - 1 , the output of the delay element  708 - 2 , and the output of the delay element  708 - 1  all exceed the respective first thresholds, this indicates that the received signal includes four consecutive O symbols. 
     In another embodiment, the logic  720  includes an adder that sums the output of the output of the delay element  708 - 1 , the output of the delay element  708 - 2 , and the output of the delay element  708 - 3 . The logic  720  also includes one first comparator that compares an output of the adder to one first threshold. For example, when the logic  720  determines the output of the adder exceeds the first threshold, this indicates that the received signal includes four consecutive O symbols. 
     In an embodiment, the logic  720  includes two second comparators that compare the output of the energy measurement device  712  to two second thresholds (an O second threshold and an F second threshold). When the logic  720  determines that the output of the energy measurement device  712  i) first exceeds the O second threshold (corresponding to the fourth symbol in the preamble), and ii) then falls below the F second threshold (corresponding to the fifth location in the preamble), this indicates that the received signal includes an O symbol followed by an F symbol, according to an embodiment. 
     When the logic determines i) that the received signal includes four consecutive O symbols corresponding to the first, second, third, and fourth symbol locations in the preamble (OOOOF), and ii) that the received signal includes an O symbol (at the fourth symbol location) followed by an F symbol (at the fifth symbol location), the logic  720  determines that the preamble (OOOOF) has been detected, generates a carrier sense (CS) signal to indicate that the preamble (OOOOF) has been detected. 
     Additionally, the logic  720  estimates the centers of peaks in one or more of the output of the autocorrelation calculator  704 , the output of the delay element  708 - 1 , the output of the delay element  708 - 2 , and the output of the delay element  708 - 3  to estimate the centers of symbols in the preamble (OOOOF), according to an embodiment. The logic  720  uses the estimated centers of symbols in the preamble (OOOOF) to generate a symbol timing (ST) signal that indicates the centers of symbols in the preamble (OOOOF), according to an embodiment. 
     In an embodiment, the logic  720  comprises hardware circuitry (e.g., one or more comparator circuits, one or more adder circuits, one or more state machine circuits, etc.) configured to implement the functions described above. In another embodiment, the logic  720  is implemented using a processor executing machine readable instructions stored in a memory device, wherein the machined readable instructions, when executed by the processor, cause the processor to implement the functions described above. In another embodiment, the logic  720  comprises a combination of i) hardware circuitry (e.g., one or more comparator circuits, one or more adder circuits, one or more state machine circuits, etc.) and ii) a processor executing machined readable instructions, that together implement the functions described above. 
     Although the example WUR preamble detector  700  includes a single autocorrelator  704 , the WUR preamble detector  700  includes multiple autocorrelators corresponding to multiple locations in the WUR sync portion at which two consecutive O symbols occur, in another embodiment. In other embodiments, the autocorrelator  704  (and/or one or more other autocorrelators) in the WUR preamble detector  700  are replaced with one or more cross-correlators that are configured to cross-correlate the received signal with one or more local copies of sync symbols in the WUR sync portion. 
     With Manchester code encoded WUR preamble portions, the WUR preamble detector includes one or more correlators (e.g., autocorrelators, cross-correlators, etc.) that compute one or more correlations corresponding to locations in the WUR preamble portion at which a non-zero energy signal is present, according to an embodiment. Also with Manchester code encoded WUR preamble portions, the WUR preamble detector includes one or more energy detectors that generate one or more energy measurements corresponding to locations in the WUR sync portion at which the zero energy signal is present, according to an embodiment. 
     In embodiments in which the WUR sync portion is modulated using a techniques other than OOK or Manchester code encoding, the WUR preamble detector includes one or more correlators (e.g., autocorrelators, cross-correlators, etc.) that compute one or more correlations corresponding to locations of sync symbols in the WUR sync portion. For example, if the WUR sync portion includes two consecutive sync symbols with a same signal, the WUR preamble detector uses an autocorrelation to detect two consecutive sync symbols in a received signal, according to an embodiment. Additionally or alternatively, the WUR preamble detector uses a cross-correlation to detect a sync symbol in a received signal, according to an embodiment. 
       FIG. 8  is a block diagram of another example WUR preamble detector  800 , according to another embodiment. The WUR preamble detector  800  is for use with a Golay code-modulated WUR preamble portion  400  when the Golay code-modulated WUR preamble portion  400  comprises four symbols: Ga Ga Ga Gb. 
     The WUR preamble detector  800  comprises a first cross-correlation calculator  804  coupled to three delay elements  808 , which are coupled together in series. The first cross-correlation calculator  804  is configured to compute a cross-correlation between a received signal and the Golay code Ga. An output of the delay element  808 - 1  corresponds to a cross-correlation between a third occurring symbol in the WUR preamble portion  400  and Ga. An output of the delay element  808 - 2  corresponds to a cross-correlation between a second occurring symbol in the WUR preamble portion  400  and Ga. An output of the delay element  808 - 3  corresponds to a cross-correlation between a first occurring symbol in the WUR preamble portion  400  and Ga. 
     The WUR preamble detector  800  also comprises a second cross-correlation calculator  812 . The second cross-correlation calculator  812  is configured to compute a cross-correlation between the received signal and the Golay code Gb. An output of the second cross-correlation calculator  812  corresponds to a cross-correlation between a fourth occurring symbol in the WUR preamble portion  400  and Gb. 
     The WUR preamble detector  800  also comprises logic  820 , which is coupled to the output of the delay element  808 - 1 , the output of the delay element  808 - 2 , the output of the delay element  808 - 3 , and the output of the second cross-correlation calculator  812 . In an embodiment, the logic  820  includes three respective first comparators that are configured to compare the output of the delay element  808 - 1 , the output of the delay element  808 - 2 , and the output of the delay element  808 - 3  to respective first thresholds. For example, when the logic  820  determines the output of the delay element  808 - 1 , the output of the delay element  808 - 2 , and the output of the delay element  808 - 3  all exceed the respective first thresholds, this indicates that the received signal includes three consecutive Ga symbols. 
     In another embodiment, the logic  820  includes an adder that sums the output of the output of the delay element  808 - 1 , the output of the delay element  808 - 2 , and the output of the delay element  808 - 3 . The logic  820  also includes one first comparator that compares an output of the adder to one first threshold. For example, when the logic  820  determines the output of the adder exceeds the first threshold, this indicates that the received signal includes three consecutive Ga symbols. 
     In an embodiment, the logic  820  includes a second comparator that compares the output of the second cross-correlation calculator  812  to a second threshold. When the logic  820  determines that the output of the second cross-correlation calculator  812  exceeds the second threshold, this indicates that the received signal includes the Gb symbol, according to an embodiment. 
     In another embodiment, the logic  820  includes an adder that sums the output of the output of the delay element  808 - 1 , the output of the delay element  808 - 2 , the output of the delay element  808 - 3 , and the output of the second cross-correlation calculator  812 . The logic  820  also includes one comparator that compares an output of the adder to a threshold. For example, when the logic  820  determines the output of the adder exceeds the threshold, this indicates that the received signal includes three consecutive Ga symbols followed by a Gb symbol. 
     When the logic determines i) that the received signal includes three consecutive Ga symbols corresponding to the first, second, and third symbol locations in the preamble, and ii) that the received signal includes a Gb symbol at the fourth symbol location, the logic  820  determines that the preamble (Ga Ga Ga Gb) has been detected, and generates a carrier sense (CS) signal to indicate that the preamble (Ga Ga Ga Gb) has been detected. 
     Additionally, the logic  820  estimates the centers of peaks in one or more of the output of the first cross-correlation calculator  804 , the output of the delay element  808 - 1 , the output of the delay element  808 - 2 , the output of the delay element  808 - 3 , and the output of the second cross-correlation calculator  812  to estimate the centers of symbols in the preamble, according to an embodiment. The logic  820  uses the estimated centers of symbols in the preamble to generate a symbol timing (ST) signal that indicates the centers of symbols in the preamble, according to an embodiment. 
     In an embodiment, the logic  820  comprises hardware circuitry (e.g., one or more comparator circuits, one or more adder circuits, one or more state machine circuits, etc.) configured to implement the functions described above. In another embodiment, the logic  820  is implemented using a processor executing machine readable instructions stored in a memory device, wherein the machined readable instructions, when executed by the processor, cause the processor to implement the functions described above. In another embodiment, the logic  820  comprises a combination of i) hardware circuitry (e.g., one or more comparator circuits, one or more adder circuits, one or more state machine circuits, etc.) and ii) a processor executing machined readable instructions, that together implement the functions described above. 
     Although the example WUR preamble detector  800  includes a single first cross-correlation calculator  804 , the WUR preamble detector  800  includes multiple first cross-correlation calculators  804  corresponding to multiple locations in the WUR sync portion at which Ga symbols occur, in another embodiment. In other embodiments, the first cross-correlation calculator  804  is replaced with one or more autocorrelators that are configured to generate an autocorrelation of the received signal to detect two consecutive Ga symbols in the received signal. 
       FIG. 9  is a block diagram of another example WUR preamble detector  900 , according to another embodiment. The WUR preamble detector  900  is for use with a Golay code-modulated WUR preamble portion  400  when the Golay code-modulated WUR preamble portion  400  comprises four symbols: Ga Ga Gb Gb. 
     The WUR preamble detector  900  comprises a first cross-correlation calculator  904  coupled to a delay element  908 . The first cross-correlation calculator  904  is configured to compute a cross-correlation between a received signal and the Golay code Ga. An output of the delay element  908  corresponds to a cross-correlation between a first occurring symbol and a second occurring symbol the WUR preamble portion  400  and Ga. 
     The WUR preamble detector  800  also comprises a second cross-correlation calculator  912 . The second cross-correlation calculator  912  is configured to compute a cross-correlation between the received signal and the Golay code Gb. An output of the second cross-correlation calculator  912  corresponds to a cross-correlation between a third occurring symbol and a fourth occurring symbol in the WUR preamble portion  400  and Gb. 
     The WUR preamble detector  900  also comprises an adder  916 , which is coupled to the output of the delay element  908 , and the output of the second cross-correlation calculator  912 . An output of the adder  916  is coupled to a buffer  918 . In an embodiment, the buffer  918  is configured to store at least K outputs of the adder  916 . When the received signal includes the WUR preamble portion  400  Ga Ga Gb Gb, the output of the adder  916  will a first sample equal to approximately 2*K, K−1 second samples of zeros (or near zero), and then a third sample equal to approximately 2*K. 
     The WUR preamble detector  900  also comprises logic  920 , which is coupled to the output of the buffer  918 . In an embodiment, the logic  920  is configured to detect when the output of the adder  916  includes the first sample equal to approximately 2*K, K−1 second samples of zeros (or near zero), and then the third sample equal to approximately 2*K. In response, the logic  920  generates a carrier sense (CS) signal to indicate that the preamble (Ga Ga Gb Gb) has been detected. Additionally, the logic  920  uses the first sample equal to approximately 2*K, K−1 second samples of zeros (or near zero), and then the third sample equal to approximately 2*K to generate a symbol timing (ST) signal that indicates the centers of symbols in the preamble, according to an embodiment. 
     In an embodiment, the logic  920  comprises hardware circuitry (e.g., one or more comparator circuits, one or more state machine circuits, etc.) configured to implement the functions described above. In another embodiment, the logic  920  is implemented using a processor executing machine readable instructions stored in a memory device, wherein the machined readable instructions, when executed by the processor, cause the processor to implement the functions described above. In another embodiment, the logic  920  comprises a combination of i) hardware circuitry (e.g., one or more comparator circuits, one or more state machine circuits, etc.) and ii) a processor executing machined readable instructions, that together implement the functions described above. 
     As discussed above, different WUR sync portions and/or different sync symbols are utilized to indicate different PHY modes of a wakeup packet, at least in some embodiments. Thus, in some embodiments, the LP-WUR  150  includes multiple WUR preamble detectors to detect different WUR sync portions corresponding to the different PHY modes. For example, which one of the multiple WUR preamble detectors detects a WUR preamble indicates a PHY mode of the wakeup packet, according to an embodiment. 
     In other embodiments, the WUR preamble detector includes different cross-correlators corresponding to different sync symbols, and the WUR preamble detector uses the different cross-correlators to detect WUR sync portions using techniques such as described above. For example, which one of the cross-correlators detects a sync symbol in a WUR preamble indicates a PHY mode of the wakeup packet, according to an embodiment. 
       FIG. 10  is a flow diagram of an example method  1000  for processing wakeup packets, according to an embodiment. In some embodiments, the LP-WUR  150  of  FIGS. 1A and 1D  is configured to implement the method  1000 . The method  1000  is described in the context of the LP-WUR  150  merely for explanatory purposes and, in other embodiments, the method  1000  is implemented by another network interface device. 
     At block  1004 , the LP-WUR  150  calculates (e.g., the digital baseband circuitry  188  calculates) one or more correlations on a received signal. The one or more correlations correspond to one or more sync symbols included in a sync portion of a wakeup packet. The wakeup packet includes i) a first portion having a WLAN legacy preamble that spans a first frequency bandwidth, and ii) a second portion that follows in time the first portion and that spans a second bandwidth narrower than the first bandwidth. The second portion includes i) the sync portion having a plurality of sync symbols, and ii) a wakeup packet body. 
     At block  1008 , the LP-WUR  150  detects (e.g., the digital baseband circuitry  188  detects) the sync portion of the wakeup packet using the one or more correlations. 
     At block  1012 , the LP-WUR  150  determines (e.g., the digital baseband circuitry  188  determines) a symbol timing using the one or more correlations. 
     At block  1016 , the LP-WUR  150  processes (e.g., the digital baseband circuitry  188  processes) the wakeup packet body based on the determined symbol timing. 
     In some embodiments, the sync portion is modulated according to OOK; each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) a zero energy sequence; calculating the one or more correlations comprises: generating, at the WUR, one or more respective autocorrelations between expected adjacent sync symbols corresponding to the non-zero energy sequence, and measuring, at the WUR, one or more energy levels of expected sync symbols corresponding to the zero energy sequence; detecting the WUR preamble comprises using the one or more respective generated autocorrelations and the one or more measured energy levels; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     In an embodiment, determining the symbol timing further comprises using the one or more measured energy levels. 
     In some embodiment, the sync portion is modulated according to on-off keying (OOK); each sync symbol is selected from a set consisting of i) a non-zero energy sequence, ii) the non-zero energy sequence phase rotated by 180 degrees, and iii) a zero energy sequence; the method further comprises: generating, at the WUR, one or more respective correlations at expected sync symbols corresponding to the non-zero energy sequence and the non-zero energy sequence phase rotated by 180 degrees, and measuring, at the WUR, one or more energy levels at expected sync symbols corresponding to the zero energy sequence; detecting the WUR preamble comprises using the one or more respective generated correlations and the one or more measured energy levels; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     In an embodiment, determining the symbol timing further comprises using the one or more measured energy levels. 
     In some embodiments, the sync portion is modulated according to a Manchester code; each sync symbol is selected from a set consisting of i) a first sync symbol comprising a) a first occurring non-zero energy portion, and b) a second occurring zero energy portion; and ii) a second sync symbol comprising a) a first occurring zero energy portion, and b) a second occurring non-zero energy portion; 
     the method further comprises: generating, at the WUR, one or more respective correlations at expected non-zero energy portions, and measuring, at the WUR, one or more energy levels at expected zero energy portions; detecting the WUR preamble comprises using the one or more respective generated correlations and the one or more measured energy levels; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     In an embodiment, determining the symbol timing further comprises using the one or more measured energy levels. 
     In some embodiments, each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) the non-zero energy sequence phase rotated by 180 degrees; the method further comprises: generating, at the WUR, one or more respective correlations regarding the WUR preamble; detecting the WUR preamble comprises using the one or more respective generated correlations; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     In some embodiments, each sync symbol comprises one or more Golay codes; the method further comprises: generating, at the WUR, one or more respective correlations regarding the WUR preamble; detecting the WUR preamble comprises using the one or more respective generated correlations; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     In some embodiments, the method further comprises: determining, at the WUR, a matching sync portion from a set of candidate sync portions that is included in the WUR preamble; and determining, at the WUR, a physical layer (PHY) mode according to which the wakeup packet body was transmitted based on the determined matching sync portion; wherein processing the wakeup packet body is performed in accordance with the determined PHY mode. 
     In some embodiments, the method further comprises: determining, at the WUR, a matching sync symbol from a set of candidate sync symbols that is included in the WUR preamble; and determining, at the WUR, a physical layer (PHY) mode according to which the wakeup packet body was transmitted based on the determined matching sync symbol; wherein processing the wakeup packet body is performed in accordance with the determined PHY mode. 
     Embodiment 1 
     A method, performed by a first communication device, for transmitting a wakeup packet configured to cause a wakeup radio of a second communication device to cause a wireless local area network (WLAN) network interface device of the second communication device to transition from a low power state to an active state, the method comprising: generating, at the first communication device, a first portion of the wakeup packet, wherein the first portion of the wakeup packet corresponds to a WLAN legacy preamble of the wakeup packet, and wherein the first portion spans a first frequency bandwidth; generating, at the first communication device, a second portion of the wakeup packet, wherein the second portion of the wakeup packet spans a second bandwidth that is less than the first bandwidth, and wherein: the second portion of the wakeup packet is configured to cause the wakeup radio of the second communication device to cause the WLAN network interface device of the second communication device to transition from the low power state to the active state, generating the second portion of the wakeup packet includes i) generating a sync portion having a plurality of sync symbols, and ii) generating a wakeup packet body; and transmitting, by the first communication device, the wakeup packet. 
     Embodiment 2 
     The method of Embodiment 1, wherein generating the sync portion comprises: modulating the sync portion according to on-off keying (OOK); and selecting each sync symbol from a set consisting of i) a non-zero energy sequence, and ii) a zero energy sequence. 
     Embodiment 3 
     The method of Embodiment 1, wherein generating the sync portion comprises: modulating the sync portion according to on-off keying (OOK); and selecting each sync symbol from a set consisting of i) a non-zero energy sequence, ii) the non-zero energy sequence phase rotated by 180 degrees, and iii) a zero energy sequence. 
     Embodiment 4 
     The method of Embodiment 1, wherein generating the sync portion comprises: modulating the sync portion according to a Manchester code; and selecting each sync symbol from a set consisting of i) a first sync symbol comprising a) a first occurring non-zero energy portion, and b) a second occurring zero energy portion; and ii) a second sync symbol comprising a) a first occurring zero energy portion, and b) a second occurring non-zero energy portion. 
     Embodiment 5 
     The method of Embodiment 1, wherein generating the sync portion comprises: selecting each sync symbol from a set consisting of i) a non-zero energy sequence, and ii) the non-zero energy sequence phase rotated by 180 degrees. 
     Embodiment 6 
     The method of Embodiment 1, wherein: each sync symbol comprises one or more Golay codes. 
     Embodiment 7 
     The method of any of Embodiments 1-6, further comprising: determining, at the first communication device, a physical layer (PHY) mode according to which the wakeup packet body is to be transmitted; and selecting, at the first communication device, the sync portion from a set of candidate sync portions according to the selected PHY mode, wherein the selected sync portion indicates the PHY mode. 
     Embodiment 8 
     The method of any of Embodiments 1-6, further comprising: determining, at the first communication device, a PHY mode according to which the wakeup packet body is to be transmitted; and selecting, at the first communication device, a sync symbol to use for the sync portion from a set of candidate sync symbols according to the selected PHY mode, wherein the selected sync symbol indicates the PHY mode. 
     Embodiment 9 
     An apparatus, comprising: a network interface device associated with a first communication device, wherein the network interface device comprises one or more integrated circuit (IC) devices configured to: generate a wireless local area network (WLAN) legacy preamble of a wakeup packet, wherein the wakeup packet is configured to cause a wakeup radio of a second communication device to cause a WLAN network interface device of the second communication device to transition from a low power state to an active state, generate a first portion of a wakeup packet, wherein the first portion of the wakeup packet corresponds to a wireless local area network (WLAN) legacy preamble of the wakeup packet, and wherein the first portion spans a first frequency bandwidth, generate a second portion of the wakeup packet, wherein the second portion of the wakeup packet spans a second bandwidth that is less than the first bandwidth, and wherein: the second portion of the wakeup packet is configured to cause a wakeup radio of a second communication device to cause a WLAN network interface device of the second communication device to transition from a low power state to an active state, and generating the second portion of the wakeup packet includes i) generating a sync portion having a plurality of sync symbols, and ii) generating a wakeup packet body; wherein the one or more IC devices are further configured to transmit the wakeup packet. 
     Embodiment 10 
     The apparatus of Embodiment 9, wherein the one or more IC devices are further configured to: modulate the sync portion according to on-off keying (OOK); and select each sync symbol from a set consisting of i) a non-zero energy sequence, and ii) a zero energy sequence. 
     Embodiment 11 
     The apparatus of Embodiment 9, wherein the one or more IC devices are further configured to: modulate the sync portion according to on-off keying (OOK); and select each sync symbol from a set consisting of i) a non-zero energy sequence, ii) the non-zero energy sequence phase rotated by 180 degrees, and iii) a zero energy sequence. 
     Embodiment 12 
     The apparatus of Embodiment 9, wherein the one or more IC devices are further configured to: modulate the sync portion according to a Manchester code; and select each sync symbol from a set consisting of i) a first sync symbol comprising a) a first occurring non-zero energy portion, and b) a second occurring zero energy portion; and ii) a second sync symbol comprising a) a first occurring zero energy portion, and b) a second occurring non-zero energy portion. 
     Embodiment 13 
     The apparatus of Embodiment 9, wherein the one or more IC devices are further configured to: select each sync symbol from a set consisting of i) a non-zero energy sequence, and ii) the non-zero energy sequence phase rotated by 180 degrees. 
     Embodiment 14 
     The apparatus of Embodiment 9, wherein: each sync symbol comprises one or more Golay codes. 
     Embodiment 15 
     The apparatus of any of Embodiments 9-14, wherein the one or more IC devices are further configured to: determine a physical layer (PHY) mode according to which the wakeup packet body is to be transmitted; and select the sync portion from a set of candidate sync portions according to the selected PHY mode, wherein the selected sync portion indicates the PHY mode. 
     Embodiment 16 
     The apparatus of any of Embodiments 9-14, wherein the one or more IC devices are further configured to: determine a PHY mode according to which the wakeup packet body is to be transmitted; and select a sync symbol to use for the sync portion from a set of candidate sync symbols according to the selected PHY mode, wherein the selected sync symbol indicates the PHY mode. 
     Embodiment 17 
     A method, performed by a wakeup radio (WUR) of a communication device, for processing a wakeup packet configured to cause the WUR of the communication device to cause a wireless local area network (WLAN) network interface device of the communication device to transition from a low power state to an active state, wherein the wakeup packet includes i) a first portion having a WLAN legacy preamble that spans a first frequency bandwidth, and ii) a second portion that follows in time the first portion and that spans a second bandwidth narrower than the first bandwidth, and wherein the second portion includes i) a sync portion having a plurality of sync symbols, and ii) a wakeup packet body, the method comprising: calculating, at the WUR, one or more correlations corresponding to one or more sync symbols included in the sync portion; detecting, at the WUR, the sync portion of the wakeup packet using the one or more correlations; determining, at the WUR, a symbol timing using the one or more correlations; and processing, at the WUR, the wakeup packet body based on the determined symbol timing. 
     Embodiment 18 
     The method of Embodiment 17, wherein: the sync portion is modulated according to on-off keying (OOK); each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) a zero energy sequence; calculating the one or more correlations comprises: generating, at the WUR, one or more respective autocorrelations between expected adjacent sync symbols corresponding to the non-zero energy sequence, and measuring, at the WUR, one or more energy levels of expected sync symbols corresponding to the zero energy sequence; detecting the sync portion comprises using the one or more respective generated autocorrelations and the one or more measured energy levels; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     Embodiment 19 
     The method of Embodiment 18, wherein: determining the symbol timing further comprises using the one or more measured energy levels. 
     Embodiment 20 
     The method of Embodiment 17, wherein: the sync portion is modulated according to on-off keying (OOK); each sync symbol is selected from a set consisting of i) a non-zero energy sequence, ii) the non-zero energy sequence phase rotated by 180 degrees, and iii) a zero energy sequence; the method further comprises: generating, at the WUR, one or more respective correlations at expected sync symbols corresponding to the non-zero energy sequence and the non-zero energy sequence phase rotated by 180 degrees, and measuring, at the WUR, one or more energy levels at expected sync symbols corresponding to the zero energy sequence; detecting the sync portion comprises using the one or more respective generated correlations and the one or more measured energy levels; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     Embodiment 21 
     The method of Embodiment 20, wherein: determining the symbol timing further comprises using the one or more measured energy levels. 
     Embodiment 22 
     The method of Embodiment 17, wherein: the sync portion is modulated according to a Manchester code; each sync symbol is selected from a set consisting of i) a first sync symbol comprising a) a first occurring non-zero energy portion, and b) a second occurring zero energy portion; and ii) a second sync symbol comprising a) a first occurring zero energy portion, and b) a second occurring non-zero energy portion; the method further comprises: generating, at the WUR, one or more respective correlations at expected non-zero energy portions, and measuring, at the WUR, one or more energy levels at expected zero energy portions; detecting the sync portion comprises using the one or more respective generated correlations and the one or more measured energy levels; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     Embodiment 23 
     The method of Embodiment 22, wherein: determining the symbol timing further comprises using the one or more measured energy levels. 
     Embodiment 24 
     The method of Embodiment 17, wherein: each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) the non-zero energy sequence phase rotated by 180 degrees; the method further comprises: generating, at the WUR, one or more respective correlations regarding the sync portion; detecting the sync portion comprises using the one or more respective generated correlations; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     Embodiment 25 
     The method of Embodiment 17, wherein: each sync symbol comprises one or more Golay codes; the method further comprises: generating, at the WUR, one or more respective correlations regarding the sync portion; detecting the sync portion comprises using the one or more respective generated correlations; and determining the symbol timing comprises using the one or more respective generated autocorrelations. 
     Embodiment 26 
     The method of any of Embodiments 17-25, further comprising: determining, at the WUR, a matching sync portion from a set of candidate sync portions that is included in the sync portion; and determining, at the WUR, a physical layer (PHY) mode according to which the wakeup packet body was transmitted based on the determined matching sync portion; wherein processing the wakeup packet body is performed in accordance with the determined PHY mode. 
     Embodiment 27 
     The method of any of Embodiments 17-25, further comprising: determining, at the WUR, a matching sync symbol from a set of candidate sync symbols that is included in the sync portion; and determining, at the WUR, a physical layer (PHY) mode according to which the wakeup packet body was transmitted based on the determined matching sync symbol; wherein processing the wakeup packet body is performed in accordance with the determined PHY mode. 
     Embodiment 28 
     An apparatus, comprising: a wakeup radio WUR associated with a wireless local area network (WLAN) network interface device, wherein the WUR comprises one or more integrated circuit (IC) devices configured to: process a wakeup packet, wherein the wakeup packet includes i) a first portion having a WLAN legacy preamble that spans a first frequency bandwidth, and ii) a second portion that follows in time the first portion and that spans a second bandwidth narrower than the first bandwidth, and wherein the second portion includes i) the sync portion having a plurality of sync symbols, and ii) a wakeup packet body. Processing the wakeup packet includes: calculating one or more correlations corresponding to one or more sync symbols included in the sync portion, detecting the sync portion of the wakeup packet using the one or more correlations, determining a symbol timing using the one or more correlations, and processing the wakeup packet body based on the determined symbol timing. 
     Embodiment 29 
     The apparatus of Embodiment 28, wherein: the sync portion is modulated according to on-off keying (OOK); each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) a zero energy sequence; the one or more IC devices are configured to: generate one or more respective autocorrelations between expected adjacent sync symbols corresponding to the non-zero energy sequence, and measure one or more energy levels of expected sync symbols corresponding to the zero energy sequence, detect the sync portion using the one or more respective generated autocorrelations and the one or more measured energy levels, and determine the symbol timing using the one or more respective generated autocorrelations. 
     Embodiment 30 
     The apparatus of Embodiment 29, wherein the one or more IC devices are configured to: determine the symbol timing using the one or more measured energy levels. 
     Embodiment 31 
     The apparatus of Embodiment 28, wherein: the sync portion is modulated according to on-off keying (OOK); each sync symbol is selected from a set consisting of i) a non-zero energy sequence, ii) the non-zero energy sequence phase rotated by 180 degrees, and iii) a zero energy sequence; the one or more IC devices are configured to: generate one or more respective correlations at expected sync symbols corresponding to the non-zero energy sequence and the non-zero energy sequence phase rotated by 180 degrees, measure one or more energy levels at expected sync symbols corresponding to the zero energy sequence, detect the sync portion using the one or more respective generated correlations and the one or more measured energy levels, and determine the symbol timing using the one or more respective generated autocorrelations. 
     Embodiment 32 
     The apparatus of Embodiment 31, wherein the one or more IC devices are configured to: determine the symbol timing using the one or more measured energy levels. 
     Embodiment 33 
     The apparatus of Embodiment 28, wherein: the sync portion is modulated according to a Manchester code; each sync symbol is selected from a set consisting of i) a first sync symbol comprising a) a first occurring non-zero energy portion, and b) a second occurring zero energy portion; and ii) a second sync symbol comprising a) a first occurring zero energy portion, and b) a second occurring non-zero energy portion; the one or more IC devices are configured to: generate one or more respective correlations at expected non-zero energy portions, measure one or more energy levels at expected zero energy portions, detect the sync portion using the one or more respective generated correlations and the one or more measured energy levels, and determine the symbol timing using the one or more respective generated autocorrelations. 
     Embodiment 34 
     The apparatus of Embodiment 33, wherein the one or more IC devices are configured to: determine the symbol timing using the one or more measured energy levels. 
     Embodiment 35 
     The apparatus of Embodiment 28, wherein: each sync symbol is selected from a set consisting of i) a non-zero energy sequence, and ii) the non-zero energy sequence phase rotated by 180 degrees; the one or more IC devices are configured to: generate one or more respective correlations regarding the sync portion, detect the sync portion using the one or more respective generated correlations, and determine the symbol timing using the one or more respective generated autocorrelations. 
     Embodiment 36 
     The apparatus of Embodiment 28, wherein: each sync symbol comprises one or more Golay codes; the one or more IC devices are configured to: generate one or more respective correlations regarding the sync portion, detect the sync portion comprises using the one or more respective generated correlations, and determine the symbol timing using the one or more respective generated autocorrelations. 
     Embodiment 37 
     The apparatus of any of Embodiments 28-36, wherein the one or more IC devices are configured to: determine a matching sync portion from a set of candidate sync portions that is included in the second portion of the wakeup packet; determine a physical layer (PHY) mode according to which the wakeup packet body was transmitted based on the determined matching sync portion; and process the wakeup packet body in accordance with the determined PHY mode. 
     Embodiment 38 
     The apparatus of any of Embodiments 28-36, wherein the one or more IC devices are configured to: determine a matching sync symbol from a set of candidate sync symbols that is included in the second portion of the wakeup packet; determine a physical layer (PHY) mode according to which the wakeup packet body was transmitted based on the determined matching sync symbol; and process the wakeup packet body in accordance with the determined PHY mode. 
     At least some of the various blocks, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. The software or firmware instructions may include machine readable instructions that, when executed by one or more processors, cause the one or more processors to perform various acts. 
     When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), a programmable logic device (PLD), etc. 
     While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the invention.