Patent Publication Number: US-11044669-B2

Title: Wakeup radio (WUR) preamble design

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No., 15/985,480, entitled “Wakeup Radio (WUR) Preamble Design,” filed on May 21, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/511,516, entitled “Wakeup Radio (WUR) Preamble Design,” filed on May 26, 2017. Both of the applications referenced above are incorporated herein by reference in their entireties. 
    
    
     FIELD OF TECHNOLOGY 
     The present disclosure relates generally to wireless communication systems, and more particularly to wireless communication systems utilizing low power wakeup radios to implement power saving features. 
     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 wake up 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 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 includes: generating, at the first communication device, a first portion of a wakeup packet, wherein the first portion of the wakeup packet corresponds to a legacy physical layer protocol (PHY) preamble corresponding to a communication protocol, wherein the first portion of the wakeup packet includes a first orthogonal frequency division multiplexing (OFDM) symbol that spans a first bandwidth and is modulated according to a modulation scheme; generating, at the first communication device, a second OFDM symbol, wherein the second OFDM symbol spans the first bandwidth and is modulated according to the modulation scheme; generating, at the first communication device, a second portion of the wakeup packet, wherein the second portion of the wakeup packet corresponds to a second bandwidth that is less than the first bandwidth, and wherein the second portion of the wakeup packet does not conform to the communication protocol; and transmitting, by the first communication device, the wakeup packet, including transmitting, in time, the second OFDM symbol between the first portion of the wakeup packet and the second portion of the wakeup packet, and wherein the modulation of the second OFDM symbol according to the modulation scheme signals to third communication devices operating according to the communication protocol that the wakeup packet does not conform to the communication protocol. 
     In another embodiment, an apparatus comprises a network interface device associated with a first communication device. The network interface device includes one or more integrated circuits (ICs) configured to: generate a first portion of a wakeup packet, wherein the first portion of the wakeup packet corresponds to a legacy PHY preamble corresponding to a communication protocol, wherein the first portion of the wakeup packet includes a first OFDM symbol that spans a first bandwidth and is modulated according to a modulation scheme, generate a second OFDM symbol, wherein the second OFDM symbol spans the first bandwidth and is modulated according to the modulation scheme, generate a second portion of the wakeup packet, wherein the second portion of the wakeup packet corresponds to a second bandwidth that is less than the first bandwidth, and wherein the second portion of the wakeup packet does not conform to the communication protocol. The one or more ICs are also configured to transmit the wakeup packet, including transmitting, in time, the second OFDM symbol between the first portion of the wakeup packet and the second portion of the wakeup packet. The modulation of the second OFDM symbol according to the modulation scheme signals to third communication devices operating according to the communication protocol that the wakeup packet does not conform to the communication protocol. 
    
    
     
       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 wake up 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 block diagram of a prior physical layer protocol (PHY) data unit. 
         FIG. 3  is a diagram of a modulation scheme used to modulate fields in the prior art PHY data unit of  FIG. 2 . 
         FIG. 4  is a block diagram of another prior art PHY data unit. 
         FIG. 5  is a diagram of a modulation scheme used to modulate fields in the prior art PHY data unit of  FIG. 4 . 
         FIG. 6  is a block diagram of an example wakeup packet, according to an embodiment. 
         FIG. 7  is a block diagram of another example wakeup packet, according to another embodiment. 
         FIG. 8  is a diagram of a modulation scheme used to modulate fields in the example wakeup packet of  FIG. 7 , according to an embodiment. 
         FIG. 9  is a flow diagram of an example method for generating wakeup packets, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Low power wakeup techniques described below are discussed in the context of wireless local area networks (WLANs) that utilize protocols the same as or similar to protocols defined by the 802.11 Standard from the Institute of Electrical and Electronics Engineers (IEEE) merely for explanatory purposes. In other embodiments, however, the same or similar power saving 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. 
     In embodiments described below, an access point (AP) is configured to transmit a wakeup request packet to one or more client stations, causing the one or more client stations to transition from a low power state to an active state. In various embodiments, frame formats for wakeup packets are configured to avoid compatibility issues in communication networks that include multiple devices operating according to different communication protocols. 
       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 wake up 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 operate over a smaller bandwidth (e.g., less than 50%, less than 25%, less than 20%, less than 10%) than the operating bandwidth of the wireless network interface device  142  while the wireless network interface device  142  is in the active state, in an embodiment. For instance, in an embodiment, the LP-WUR  150  operates over a communication channel that is approximately 4 MHz wide (e.g. 4.06 MHz wide) while the wireless network interface device  142 , in an active state, operates over a wider communication channel that is 20 MHz wide. In an embodiment, the LP-WUR  150  operates over a communication channel that is centered within the communication channel corresponding to the operating bandwidth of the wireless network interface device  142 . In another embodiment however, the LP-WUR  150  is configured to operate over a bandwidth that is equal or substantially equal to an operating bandwidth of the wireless network interface device  142  while the wireless network interface device  142  is in the active state. In an embodiment, the wireless network interface device  142 , in an active state, is further configured to operate over a communication channel that is wider than 20 MHz (e.g., 40 MHz, 80 MHz, 160 MHz, etc.). 
     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 . 
     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 device  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 device  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 device  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  130  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  130  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 signals RF signals, the PHY processor  130  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  130  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  130  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 device  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 device  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 device  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 station  134 - 1  are configured to generate data units, and process received data units, that conform to the 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  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 . In various embodiments and/or scenarios, the network address included in the wakeup packet comprises a unicast address, a multicast address, or a broadcast address. For example, a broadcast network address generally corresponds to all client stations having an LP-WUR, according to an embodiment. As another example, a multicast network address corresponds to the client station  134 - 1  if the client station  134 - 1  has been assigned to a group of client stations (e.g., by the AP  114 ) that is associated with the multicast network address, according to an embodiment. As yet another example, a unicast network address may be assigned to the client station  134 - 1  at time of manufacture, assigned by the AP  114  when the client station  134  becomes associated with the network  110 , etc., according to various embodiments. 
       FIG. 2  is a diagram of a prior art PHY data unit  200  that conforms to the IEEE 802.11n Standard. 
     The PHY data unit  200  includes a PHY preamble  204  preamble and a PHY data portion  208 . The PHY preamble  204  includes a legacy preamble portion  212 , which includes a legacy short training field (L-STF)  216 , a legacy long training field (L-LTF)  220 , and a legacy signal field (L-SIG)  224 . The L-STF  216  includes a defined signal (e.g., defined by the communication protocol) generally used for packet detection, initial synchronization, automatic gain control, etc. The L-LTF  220  includes a defined signal (e.g., defined by the communication protocol) generally used for channel estimation, fine synchronization, etc. The L-SIG  224  includes certain PHY parameters regarding the PHY data unit  200 , such as a coding rate and a length, which together can be used to estimate a duration of the PHY data unit  200 , according to an embodiment. 
     A format of the legacy portion  212  conforms to the IEEE 802.11a Standard such that a communication device that conforms to the IEEE 802.11a Standard and/or the IEEE 802.11g Standard (e.g., a legacy device), for example, but does not conform to the IEEE 802.11n Standard can use the legacy preamble  212  portion to detect the PHY data unit  200  and estimate a length of the PHY data unit  200 . For example, the legacy device can detect the PHY data unit  200  by detecting the L-STF  216 , for example, and can estimate a duration of the PHY data unit  200  using the L-SIG  224 , for example. 
     The PHY preamble  204  also includes a high throughput signal field (HT-SIG)  228 , a high throughput short training field (HT-STF)  232 , and M high throughput long training fields (HT-LTFs)  236 , where M is an integer generally determined by a number of spatial streams used to transmit the data unit  200  in a multiple input multiple output (MIMO) channel configuration. In particular, according to the IEEE 802.11n Standard, the data unit  200  includes two HT-LTFs  212  if the data unit  200  is transmitted using two spatial streams, and four HT-LTFs  212  if the data unit  200  is transmitted using three or four spatial streams. An indication of the particular number of spatial streams being utilized is included in the HT-SIG field  208 . 
       FIG. 3  is a set of diagrams illustrating a modulation  300  of the L-SIG  206 , the HT-SIG 1   208 - 1 , and the HT-SIG 2   208 - 2  of the data unit  200  of  FIG. 2 , as defined by the IEEE 802.11n Standard. The L-SIG  206  corresponds to a first orthogonal frequency division multiplexing (OFDM) symbol, and individual tones of the first OFDM symbol are modulated using binary phase shift keying (BPSK). The HT-SIG 1   208 - 1  corresponds to a second OFDM symbol, and the HT-SIG 2   208 - 2  corresponds to a third OFDM symbol. Individual tones of the second OFDM symbol and the third OFDM symbol are modulated according to quaternary-BPSK (Q-BPSK). In other words, the modulation of tones of the HT-SIG 1   208 - 1  and HT-SIG 2   208 - 2  is rotated by 90 degrees as compared to the modulation of tones of the L-SIG  206 . 
       FIG. 4  is a diagram of a prior art PHY data unit  400  that conforms to the IEEE 802.11ac Standard. 
     The PHY data unit  400  includes a PHY preamble  404  preamble and a PHY data portion  408 . The PHY preamble  404  includes the legacy preamble portion  212  discussed above. A communication device that conforms to the IEEE 802.11a Standard, the IEEE 802.11g Standard, and/or the IEEE 802.11n Standard (e.g., a legacy device), for example, but does not conform to the IEEE 802.11ac Standard can use the legacy preamble  212  portion to detect the PHY data unit  400  and estimate a length of the PHY data unit  400 . For example, the legacy device can detect the PHY data unit  400  by detecting the L-STF  216 , for example, and can estimate a duration of the PHY data unit  200  using the L-SIG  224 , for example. 
     The preamble  404  also includes a first very high throughput signal (VHT-SIGA) field  412  (which includes a first OFDM symbol  412 - 1  and a second OFDM symbol  412 - 2 ), a very high throughput short training field (VHT-STF)  416 , M very high throughput long training fields (VHT-LTFs)  420 , where M is an integer, and a second very high throughput signal field (VHT-SIG-B)  424 . 
       FIG. 5  is a set of diagrams illustrating modulation of the L-SIG  406  and the VHT-SIGA  412 , as defined by the IEEE 802.11ac Standard. Unlike the OFDM symbol  228 - 1  in  FIG. 3 , tones of the OFDM symbol  412 - 1  are modulated according to BPSK, which is the same as the modulation of tones of the L-SIG  224 . On the other hand, tones of the OFDM symbol  412 - 2  are modulated using Q-BPSK, e.g., the modulation of the OFDM symbol  412 - 2  is rotated by 90 degrees as compared to the modulation of the L-SIG  406 . 
       FIGS. 2-4  merely illustrate some PHY data unit formats that the AP  114  and the client station  134  are configured to generate and transmit in a WLAN  110 . In other embodiments, the AP  114  and the client station  134  are configured to generate and transmit PHY data units conforming to other standards within the IEEE 802.11 Standard family and/or PHY data units conforming to one or more different communication protocols. 
     A communication device that receives PHY data units (e.g., data units conforming to a communication protocol within the IEEE 802.11 Standard family) may need to, in certain embodiments, distinguish between PHY data units conforming to different communication protocols (e.g., IEEE 802.11a Standard, IEEE 802.11n Standard, IEEE 802.11ac Standard, etc.) in order to appropriately process the received PHY data unit. In an embodiment, detection of a digital modulation scheme (e.g., binary phase shift keying (BPSK), quadrature binary phase shift keying (Q-BPSK), etc.), at one or more fields within a PHY preamble of the received PHY data unit, is used for distinguishing between different communication protocols at a receiver. A communication device may be configured to detect the modulation (e.g., BPSK, Q-BPSK, etc.) of one or more fields in the PHY preamble of the received PHY data unit and classify the received PHY data unit as conforming to a particular communication protocol or not conforming to a particular communication protocol. 
     According to an embodiment, the network interface device  142  of the client station  134  is capable of detecting and decoding PHY data units conforming to the IEEE 802.11n Standard. Thus, referring now to  FIG. 2 , when the network interface device  142  receives the PHY data unit  200 , and the network interface device  142  detects the BPSK modulation in the L-SIG  206  and the Q-BPSK modulation in the HT-SIGs  208 . Based on the detected modulation of the L-SIG  206  and the HT-SIGs  208 , the network interface device  142  properly classifies the received PHY data unit  200  as conforming to an IEEE 802.11n Standard. When the network interface device  142  receives a PHY data unit and determines that an L-SIG field of the PHY preamble is modulated using BPSK, and that a field following the L-SIG field is modulated using a digital modulation scheme that is not Q-BPSK (e.g., BPSK), the network interface device  142  determines that the data unit does not conform to the IEEE 802.11n Standard. 
     For instance, referring now to  FIG. 4 , when the network interface device  142  receives the PHY data unit  400  and determines that the L-SIG  406  is BPSK modulated and a field immediately following the L-SIG  406  (i.e. VHT-SIGA 1   408 - 1 ) is also BPSK modulated, the network interface device  142  determines that the PHY data unit  400  does not conform to the IEEE 802.11n Standard. Further, if the network interface device  142  is also capable of detecting and decoding data units conforming to the IEEE 802.11ac Standard, the network interface device  142  determines that a next field (i.e., VHT-SIGA 2  field  408 - 2 ) is Q-BPSK modulated and classifies the PHY data unit  400  as conforming to an IEEE 802.11ac Standard. 
     The above examples refer to the network interface device  142  at a client station  134  utilizing detected modulation schemes at one or more fields in a received data unit to classify a received data unit as conforming to a particular communication protocol or not conforming to a particular communication protocol. While the above examples refer to PHY data units conforming to a standard within the IEEE 802.11 Standard family, in other embodiments, similar techniques can be utilized for classifying data units as conforming or not conforming to other suitable communication protocols. 
     Various mechanisms are used to prevent multiple communication devices from simultaneously transmitting respective data units over a same communication channel. In an embodiment, communication devices in a wireless network (e.g., WLAN  110 ) avoid simultaneously transmitting over a communication channel by using a clear channel assessment (CCA) technique to determine whether the channel is idle (i.e., available for data transmission) or if the communication channel is busy due to another communication device transmitting in the communication channel, in an embodiment. For instance, a communication device (e.g., the AP  114 , the client station  134 , etc., or a network interface device  122  in the AP  114 , a network interface device  142  in the client station  134 , etc.) measures a received signal energy level in the communication channel to determine whether the channel is idle. In an embodiment, the communication device, compares the received signal energy level to a threshold value and, if the received signal energy level is lower than the threshold value, determines that the communication channel is idle and available for data transmission. 
     On the other hand, if the received signal energy level is determined to exceed the threshold value, the communication device determines that the communication channel is busy. In an embodiment, the threshold value is set based on whether or not the received signal corresponds to a packet that conforms to a recognized communication protocol. For instance, a communication device compares a received signal energy level to a lower threshold value when it determines that the received signal corresponds to a packet that conforms to the recognized communication protocol, and compares the received signal energy level to a higher threshold value when the received signal is determined to not be a packet that conforms to the recognized communication protocol. In an embodiment, the communication device determines whether the received signal corresponds to a packet that conforms to the recognized communication protocol based on whether the communication device detects the L-STF  216  in the received signal. In an embodiment, the communication device determines whether the received signal corresponds to a packet that conforms to the recognized communication protocol further based on whether the communication device determines that received signal includes the L-SIG  224  with valid data. 
     The IEEE 802.11 Standard provides backward compatibility features that permit communication devices that conform to different versions of the IEEE 802.11 Standard to interoperate. For example, the PHY data units such as the PHY data unit  200  of  FIG. 2  and the PHY data unit  400  of  FIG. 4  include the legacy preamble  212 . The legacy preamble  212  permits legacy devices to detect the PHY data unit  200 / 400  and estimate a duration of the PHY data unit  200 / 400  even though the legacy devices are not able to decode other portions of the PHY data unit  200 / 400 . When the legacy device detects the presence and estimates the duration of the PHY data unit  200 / 400 , the legacy device generally refrains from transmitting for the estimated duration of the PHY data unit  200 / 400 , and thus avoids interfering with the PHY data unit  200 / 400 . 
     Additionally, the IEEE 802.11 Standard helps communication devices that conform to different versions of the IEEE 802.11 Standard to interoperate by designing PHY preambles that permit detecting to which version of the protocol PHY data units conform. 
     For instance, a legacy device that conforms to the IEEE 802.11n Standard (and does not conform to the IEEE 802.11ac Standard) analyzes a modulation of an OFDM symbol that follows the L-SIG  224  in a received PHY data unit. If the OFDM symbol that follows the L-SIG  224  is BPSK modulated, the legacy device determines that the received PHY data unit does not conform to the IEEE 802.11n Standard. On the other hand, if the OFDM symbol that follows the L-SIG  224  is Q-BPSK modulated, the legacy device determines that the received PHY data unit does conform to the IEEE 802.11n Standard. 
     A device that conforms to the IEEE 802.11ac Standard also analyzes a modulation of an OFDM symbol that follows the L-SIG  224  in a received PHY data unit. If the OFDM symbol that follows the L-SIG  224  is Q-BPSK modulated, the device determines that the received PHY data unit conforms to the IEEE 802.11n Standard. On the other hand, if the OFDM symbol that follows the L-SIG  224  is BPSK modulated, the legacy device determines that the received PHY data unit does conform to the IEEE 802.11n Standard, e.g., the received PHY data may conform to the IEEE 802.11a Standard or the IEEE 802.11ac Standard. The device that conforms to the IEEE 802.11ac Standard then analyzes a modulation of a next OFDM symbol. If the next OFDM symbol is BPSK modulated, the device determines that the received PHY data unit does not conform to the IEEE 802.11ac Standard. On the other hand, if the next OFDM symbol is Q-BPSK modulated, the device determines that the received PHY data unit conforms to the IEEE 802.11ac Standard. 
       FIG. 6  is a block diagram of a wakeup packet  600  used in the example WLAN  110  of  FIG. 1 , according to an embodiment. The network interface device  122  of the AP  114  is configured to generate and transmit the wakeup packet  600 , according to an embodiment, e.g., to prompt the network interface  142  of the client station  134  to transition from a low power state to an active state, according to an embodiment. The network interface device  142  of the client station  134 - 1  is also configured to generate and transmit the wakeup packet  600 , e.g., to prompt the network interface  142  of another client station  134  to wake up from the low power state, according to another embodiment. 
     The wakeup packet  600  includes the legacy 802.11 preamble  212  (discussed above) and a payload  608 . The legacy 802.11 preamble  212  enables IEEE 802.11 stations (e.g., wireless communication devices that are configured to operate according to an IEEE 802.11 Standard) to detect the wakeup packet  600  and estimate a duration of the wakeup packet  600  for the purpose of reducing transmissions by the IEEE 802.11 stations that will collide with the wakeup packet  600 , according to an embodiment. 
     In other embodiments, the wakeup packet  600  includes a legacy preamble (different than the legacy 802.11 preamble  212 ) 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  600  and estimate a duration of the wakeup packet  600  for the purpose of reducing transmissions by such stations that will collide with the wakeup packet  600 , according to an embodiment. 
     The payload  608  includes a wakeup preamble  624  and a wakeup packet body  628 . In an embodiment, the wakeup preamble  624  includes signals that enable LP-WURs such as the LP-WUR  150  to detect the payload  608  of the wakeup packet  620  and to synchronize to the payload  608  of the wakeup packet  620 . In an embodiment, the wakeup packet body  628  includes a network address (e.g., a MAC address, an AID, or another suitable network address) corresponding to a client station (or a group of client stations) for which the wakeup packet  600  is intended. Referring now to  FIG. 1D , the digital baseband circuitry  188  is configured to detect the wakeup packet  600  at least by detecting the wakeup preamble  624 , according to an embodiment. The logic circuitry  192  is configured to process the wakeup packet body  628  to determine whether the wakeup packet body  628  includes a network address (e.g., a MAC address, an AID, or another suitable network address) corresponding to the client station  134 - 1 . 
     In an embodiment, the wakeup packet  600  is generated and transmitted (by network interface device  122  of the AP  114 , etc.) using orthogonal frequency domain multiplexing (OFDM) modulation. In one such embodiment, each of the fields of the wakeup packet  600  corresponds to one or more OFDM symbols. For example, the L-SIG  224  corresponds to one OFDM symbol. 
     In an embodiment, network interface devices (such as the network interface device  142 ) of client stations are configured to receive, detect, and decode at least the legacy 802.11 preamble  212  of the wakeup packet  600 . In an embodiment, the legacy 802.11 preamble  212  is detected and decoded at i) wireless network interface devices of communication devices that do not include a LP-WUR, and/or ii) at wireless network interface devices of communication devices that include a LP-WUR and whose wireless network interface devices have already transitioned to an active state from a low power state (based on a prior wakeup packet, for example). 
     In an embodiment, the payload  608  of the wakeup packet  200  does not conform to a communication protocol that can be decoded and processed by a wireless network interface device such as the network interface device  142 . In one such embodiment, the network interface device  142  is not configured to (or fails to) decode and process at least a portion of the wakeup preamble  624  and the wakeup packet body  628  of the wakeup packet  600 . 
     The LP-WUR  150  of the client station  134 - 1  is not capable of detecting and/or decoding the payload legacy preamble  212  of the wakeup packet  600 , according to an embodiment. In other embodiments, however, the LP-WUR  150  of the client station  134 - 1  is configured to additionally receive, detect, and decode the legacy 802.11 preamble  212 , or a portion thereof. 
     The payload  608  corresponds to a PPDU defined by a communication protocol (sometime referred to herein as a “wakeup packet communication protocol”) which is different than the communication protocol according to which the network interface device  142  is configured to operate (sometime referred to herein as a “WiFi communication protocol”), according to an embodiment. In an embodiment, the LP-WUR  150  cannot process and/or cannot correctly decode the legacy 802.11 preamble  212  (or at least a portion thereof), and the network interface device  142  cannot process and/or cannot correctly decode the payload  608  (or at least a portion thereof). 
     In an embodiment, the payload  608  of the wakeup packet  600  occupies a smaller bandwidth (e.g., less than 50%, less than 25%, less than 20%, less than 10%) than the bandwidth of the legacy 802.11 preamble  212 . For instance, in an embodiment, the legacy preamble  212  spans a 20 MHz bandwidth and the payload  608  spans 4 MHz. In an embodiment, the wakeup packet  600  is generated and transmitted (by network interface device  122  of the AP  114 , network interface device  142  of the client station  134 , etc.) using OFDM and the legacy 802.11 preamble  212  is transmitted using a number of subcarriers that is greater than a number of subcarriers used for transmission of the payload  608 . For instance, in an embodiment, the legacy 802.11 preamble  212  is transmitted over 52 subcarriers within a 20 MHz wide communication channel, whereas the payload  608  is transmitted over 13 subcarriers that span only a portion (e.g., a center 4 MHz portion) of the wider 20 MHz communication channel. In an embodiment, during the transmission of the payload  608 , the remaining subcarriers corresponding to the wider 20 MHz communication channel are not used for transmission of any data and, therefore, do not include any transmitted signals. 
     A lower occupied bandwidth of a payload  608  of the wakeup packet  600  may lead to spurious detection of a communication protocol at a legacy wireless network interface device such as a wireless network interface device that is configured to i) operate according to the IEEE 802.11n Standard and ii) is not configured to operated according to the IEEE 802.11ac Standard (hereinafter referred to as a “legacy 11n wireless network interface device”). For instance, as described above with respect to the wakeup packet  600 , the payload  608  is transmitted over a narrower communication channel (e.g., over a lower number of subcarriers) as compared to the legacy 802.11 preamble  212  that is transmitted over the wider 20 MHz communication channel. However, a legacy 11n wireless network interface device is configured to operate over the entirety of the wider communication channel. As a result, the legacy 11n wireless network interface device may improperly detect the modulation of an OFDM symbol in the payload  608  that follows the L-SIG  224 . As merely an example, following the reception of the L-SIG  224 , the legacy 11n wireless network interface device will continue to process signals over the entirety of 20 MHz communication channel, including ambient noise over subcarriers that do not include any data. The legacy 11n wireless network interface then attempts to determine a modulation of the OFDM symbol that follows the L-SIG  224  in order to determine the communication protocol corresponding to the received wakeup packet  600 . Ambient noise that is received along with the payload  608  may lead to incorrect detection of the communication protocol at the legacy 11n network interface device. 
     In an embodiment, based on the detected modulations of L-SIG  224  and the OFDM symbol that immediately follows the L-SIG  224  (including the ambient noise), the legacy 11n network interface device may sometimes incorrectly classify the wakeup packet  600  as conforming to the IEEE 802.11n Standard. However, since the payload  608  of the wakeup packet  600  does not conform to the IEEE 802.11 Standard, the legacy 11n network interface device will likely detect an error in data that the legacy 11n network interface device decodes. 
     For instance, in an embodiment, the legacy 11n network interface device may incorrectly detect a modulation of the OFDM symbol that immediately follows the L-SIG  224  as Q-BPSK, and thus classify the wakeup packet  600  as conforming to the IEEE 802.11n Standard. The legacy 11n network interface device will then assume that the two OFDM symbols that follow the L-SIG  224  corresponds to the HT-SIG field  208  ( FIG. 2 ), and thus processes the two OFDM symbols that follow the L-SIG  224  accordingly. The legacy 11n network interface device will then likely detect a cyclic redundancy check (CRC) error in the data decoded from the two OFDM symbols (which the legacy 11n network interface device assumes are the HT-SIG field  208  ( FIG. 2 )) that follow the L-SIG  224 . In response to the detected CRC error, the network interface device  142  stops receiving and processing the PHY data unit  600  and determines whether the communication channel should be considered as idle before an end of the duration of the PHY data unit  600 . 
     For example, the IEEE 802.11n Standard specifies that if a CRC error is detected in the HT-SIG field  228 , the legacy 11n network interface device should monitor the received signal energy level and compares the received signal energy level to a threshold. If the received signal energy level falls below the threshold, the IEEE 802.11n Standard specifies that the legacy 11n network interface device should consider the communication channel as being idle, even before the duration of the packet has ended. Because the payload  608  of the wakeup packet  600  does not span the entire 20 MHz channel, there is a likelihood that the legacy 11n network interface device will detect the received signal energy level as falling below the threshold during reception of the payload  608 . Thus, there is a likelihood that, when the legacy 11n network interface device receives the wakeup packet  600 , the legacy 11n network interface device will determine that the communication channel is idle prior to an end of the wakeup packet  600 , and may thus initiate transmission of a legacy 11n packet during transmission of the wakeup packet  600 . Such a transmission of a legacy 11n packet during transmission of the wakeup packet  600  may collide with the wakeup packet  600  and interfere with the ability of LP-WURs to detect and/or correctly process the wakeup packet  600 . 
     To reduce the chance of a legacy 11n network interface device incorrectly classifying the wakeup packet  600  as conforming to the IEEE 802.11n Standard, an additional field may be included between the legacy 802.11 preamble  212  and the wakeup preamble  624 . In an embodiment, the presence of this additional field “spoofs” a recipient legacy 11n network interface device into classifying the wakeup packet as not conforming to the IEEE 802.11n Standard. 
       FIG. 7  is a diagram of an example wakeup packet  700  used in the example WLAN  110  of  FIG. 1 , according to an embodiment. The network interface device  122  of the AP  114  is configured to generate and transmit the wakeup packet  700 , according to an embodiment. The network interface device  142  of the client station  134 - 1  is also configured to generate and transmit the wakeup packet  700 , e.g., to prompt another client station  134  to wake up from a low power state, according to another embodiment. The wakeup packet  700  is similar to the wakeup packet  600  and like-numbered elements are not discussed for reasons of brevity. 
     A payload  710  of the of the wakeup packet  700  includes an extra field  712  immediately following the legacy preamble  212  and occupying a bandwidth that is equal to the bandwidth of the legacy preamble  212 . For instance, in an embodiment, the legacy preamble  212  is transmitted to span a 20 MHz communication channel and the extra field  712  is also transmitted to span the same 20 MHz communication channel. In an embodiment, the extra field  712  is modulated using a digital modulation technique that is different from Q-BPSK. For instance, as shown in  FIG. 8 , the L-SIG  224  and the extra field  712  are both modulated using BPSK. 
     In an embodiment, the extra field  712  is a repeated L-SIG  224 . In an embodiment, the extra field  712  is identical to at least a portion of the L-LTF  220 . In other embodiments, the extra field  712  includes any other suitable signal and/or information. In an embodiment, the extra field  172  does not convey any useful information to recipient communication devices. In another embodiment, the extra field  172  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 extra field  172  and the wakeup packet body  628 . 
     In an embodiment, bits in the extra field  712  are scrambled. For instance, in an embodiment, the bits of the extra field  712  are scrambled using a scrambling technique that conforms to a standard within the IEEE 802.11 Standard family (e.g. IEEE 802.11a Standard, IEEE 802.11n Standard, etc.). In another embodiment, a different scrambling technique is utilized for the bits in the extra field  712 . 
     A legacy 11n network interface device that receives the wakeup packet  700  detects the L-SIG  224  as being BPSK modulated. Following this, the legacy 11n network interface device detects the extra field  712  as being modulated using a digital modulation technique that is different from Q-BPSK and therefore classifies the wakeup packet  700  as not conforming to an IEEE 802.11n Standard. For example, the legacy 11n network interface device may assume that the wakeup packet  700  conforms to the IEEE 802.11a Standard. When the network interface device  142  detects a CRC error in an attempt to decode and process the payload  710  of the wakeup packet  700 , the network interface device  142  stops processing the wakeup packet  700  and considers the communication channel as busy until an estimated duration (determined using the L-SIG  224 ) of the wakeup packet  700  has ended. Thus, the legacy 11n network interface device will not initiate a transmission during the transmission of the wakeup packet  700 . 
     The presence of the extra field  712  with a defined modulation and that spans a same wider communication channel as the legacy 802.11 preamble  604  enables the legacy 11n network interface device to unambiguously classify the wakeup packet  700  as not conforming to the IEEE 802.11n Standard, according to an embodiment. 
     In an embodiment, the wakeup packet  700  is generated and transmitted by network interface device  122  of the AP  114 , the network interface device  142  of the client station  134 , etc., using OFDM modulation. In one such embodiment, each of the fields of the wakeup packet  700  correspond to one or more OFDM symbols. In an embodiment, for example, the L-SIG  224  corresponds to one OFDM symbol and the extra field  712  corresponds to another OFDM symbol. 
     In an embodiment, the LP-WUR  150  of the client station  134 - 1  is configured to receive, detect, and decode only the wakeup preamble  624  and the wakeup packet body  628  of the wakeup packet  700 , according to an embodiment. In other embodiments, however, the LP-WUR  150  of the client station  134 - 1  is configured to additionally receive, detect, and decode the extra field  712 . 
     In an embodiment, where the LP-WUR  150  is configured to decode the extra field  712 , the LP-WUR  150  operates over the entire bandwidth of the 20 MHz communication channel. For instance, in an embodiment, the LP-WUR  150  is configured to receive signals that span the entire 20 MHz communication channel that is used to transmit the extra field  712 . In one such embodiment, the extra field  712  further includes signals that enable the LP-WUR  150  to perform receiver gain adjustment and/or synchronization. In an embodiment where the extra field  712  includes signals for receiver gain adjustment and/or synchronization, a duration of the wakeup preamble  624  may be shortened as compared to the wakeup packet  600  ( FIG. 6 ) that does not include the extra field  712 . 
     While the wakeup packet  700  has been described in the context of the IEEE 802.11 Standard family, in other embodiments, a similar technique is used to “spoof” communication devices in networks utilizing other communication protocols. For instance, one or more extra fields with particular modulation schemes may be inserted into a packet that “spoofs” a receiving communication device into classifying the packet as not conforming to a communication protocol that has disadvantageous characteristics (e.g., that may lead to an inadvertent determination that a communication channel is idle during transmission of a packet). 
       FIG. 9  is a flow diagram of an example method  900  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  900 . The method  900  is described, however, in the context of the network interface device  122  merely for explanatory purposes and, in other embodiments, the method  900  is implemented by another suitable device, such as the network interface device  142 . 
     At block  904 , the network interface device  122  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 includes a first OFDM symbol that spans a first bandwidth and is modulated according to a modulation scheme. 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  908 , the network interface device  122  generates a second OFDM symbol, wherein the second OFDM symbol spans the first bandwidth and is modulated according to the modulation scheme. In an embodiment, the second OFDM symbol corresponds to an extra symbol that follows legacy PHY preamble, such as the extra symbol  712  as described above with reference to  FIG. 7 . The second OFDM symbol is also modulated according to the BPSK modulation scheme, in an embodiment. 
     At block  912 , 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. The second portion of the wakeup packet does not conform to the communication protocol to which the legacy PHY preamble conforms. In an embodiment, the second portion of the wakeup packet corresponds to the wakeup preamble  624  and wakeup packet body  628  of the wakeup packet  700  as described above. 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  916 , the network interface device  122  transmits the wakeup packet. The transmitted wakeup packet includes the second OFDM symbol that is transmitted, in time, between the first portion and the second portion of the wakeup packet. The modulation of the second OFDM symbol according to the modulation scheme signals to communication devices operating according to the communication protocol that the wakeup packet does not conform to the communication protocol. For instance, modulation of both the L-SIG and modulation of the second OFDM symbol following L-SIG according to a BPSK modulation scheme signals to a legacy 11n network interface device that the wakeup packet does not conform to the IEEE 802.11n Standard. 
     Although the techniques described above were discussed in the context of wakeup packets that can be decoded by LP-WURs, similar techniques can be utilized in other types of wireless communication systems that may coexist with IEEE 802.11n/ac/ax wireless networks having legacy 11n wireless network interface device, in other embodiments. For example, other narrow band wireless communication networks in the 2.4 GHz and 5 GHz bands may utilize a packet structure similar to those discussed above so that legacy 802.11n network interface devices unambiguously classify such packets as not conforming to the IEEE 802.11n Standard, according to other embodiments. 
     Embodiment 1 
     A method, comprising: generating, at the first communication device, a first portion of a wakeup packet, wherein the first portion of the wakeup packet corresponds to a legacy physical layer protocol (PHY) preamble corresponding to a communication protocol, wherein the first portion of the wakeup packet includes a first orthogonal frequency division multiplexing (OFDM) symbol that spans a first bandwidth and is modulated according to a modulation scheme; generating, at the first communication device, a second OFDM symbol, wherein the second OFDM symbol spans the first bandwidth and is modulated according to the modulation scheme; generating, at the first communication device, a second portion of the wakeup packet, wherein the second portion of the wakeup packet corresponds to a second bandwidth that is less than the first bandwidth, wherein the second portion of the wakeup packet is configured to prompt one or more wakeup radios at one or more respective second communication devices to prompt one or more respective network interfaces to transition from a low power state to an active state, and wherein the second portion of the wakeup packet does not conform to the communication protocol; and transmitting, by the first communication device, the wakeup packet, including transmitting, in time, the second OFDM symbol between the first portion of the wakeup packet and the second portion of the wakeup packet, and wherein the modulation of the second OFDM symbol according to the modulation scheme signals to third communication devices operating according to the communication protocol that the wakeup packet does not conform to the communication protocol. 
     Embodiment 2 
     The method of Embodiment 1, wherein the first OFDM symbol corresponds to a legacy signal (L-SIG) field that includes information indicating a duration of the wakeup packet. 
     Embodiment 3 
     The method of Embodiment 2, wherein the second OFDM symbol is a repetition of the first OFDM symbol. 
     Embodiment 4 
     The method of any of Embodiments 1-3, wherein: the first portion of the wakeup packet includes a third OFDM symbol corresponding to a legacy long training field (L-LTF), and the second OFDM symbol is a repetition of the third OFDM symbol. 
     Embodiment 5 
     The method of Embodiment 1, wherein the second OFDM symbol of the wakeup packet does not convey any useful information to the one or more second communication devices. 
     Embodiment 6 
     The method of Embodiment 1, wherein the second OFDM symbol includes a plurality of information bits intended for the one or more second communication devices. 
     Embodiment 7 
     The method of Embodiment 6, further comprising scrambling the plurality of information bits. 
     Embodiment 8 
     The method of any of Embodiments 1-7, wherein the second portion of the wakeup packet includes at least a portion of a network identifier corresponding to the one or more second communication devices. 
     Embodiment 9 
     The method of Embodiment 8, wherein the network identifier is a multicast address corresponding to the one or more second communication devices. 
     Embodiment 10 
     The method of any of Embodiments 1-7, wherein the second portion of the wakeup packet includes at least a portion of a network identifier corresponding to a broadcast address. 
     Embodiment 11 
     The method of Embodiment 1, wherein generating the second OFDM symbol includes generating the second OFDM symbol to include signals that enable the one or more low power wakeup radios at the respective one or more second communication devices to perform at least one of i) receiver gain adjustment, and ii) synchronization. 
     Embodiment 12 
     The method of any of Embodiments 1-11, wherein the first OFDM symbol and the second OFDM symbol are modulated according to a binary phase shift keying (BPSK) modulation scheme. 
     Embodiment 13 
     An apparatus, comprising: a network interface device associated with a first communication device, wherein the network interface device includes one or more integrated circuits (ICs) configured to: generate a first portion of a wakeup packet, wherein the first portion of the wakeup packet corresponds to a legacy physical layer protocol (PHY) preamble corresponding to a communication protocol, wherein the first portion of the wakeup packet includes a first orthogonal frequency division multiplexing (OFDM) symbol that spans a first bandwidth and is modulated according to a modulation scheme, generate a second OFDM symbol, wherein the second OFDM symbol spans the first bandwidth and is modulated according to the modulation scheme, generate a second portion of the wakeup packet, wherein the second portion of the wakeup packet corresponds to a second bandwidth that is less than the first bandwidth, wherein the second portion of the wakeup packet is configured to prompt one or more wakeup radios at one or more respective second communication devices to prompt one or more respective network interfaces to transition from a low power state to an active state, and wherein the second portion of the wakeup packet does not conform to the communication protocol, and transmit the wakeup packet, including transmitting, in time, the second OFDM symbol between the first portion of the wakeup packet and the second portion of the wakeup packet, and wherein the modulation of the second OFDM symbol according to the modulation scheme signals to third communication devices operating according to the communication protocol that the wakeup packet does not conform to the communication protocol. 
     Embodiment 14 
     The apparatus of Embodiment 13, wherein the first OFDM symbol corresponds to a legacy signal (L-SIG) field that includes information indicating a duration of the wakeup packet. 
     Embodiment 15 
     The apparatus of Embodiment 14, wherein the one or more ICs are configured to generate the second OFDM symbol as a repetition of the first OFDM symbol. 
     Embodiment 16 
     The apparatus of any of Embodiments 13-15, wherein the one or more ICs are configured to: generate the first portion of the wakeup packet to include a third OFDM symbol corresponding to a legacy long training field (L-LTF), and generate the second OFDM symbol as a repetition of the third OFDM symbol. 
     Embodiment 17 
     The apparatus of Embodiment 13, wherein the second OFDM symbol of the wakeup packet does not convey any useful information to the one or more second communication devices. 
     Embodiment 18 
     The apparatus of Embodiment 13, wherein the second OFDM symbol includes a plurality of information bits intended for the one or more second communication devices. 
     Embodiment 19 
     The apparatus of Embodiment 18, wherein the one or more ICs are configured to scramble the plurality of information bits. 
     Embodiment 20 
     The apparatus of any of Embodiments 13-19, wherein the one or more ICs are configured to generate the second portion of the wakeup packet to include at least a portion of a network identifier corresponding to the one or more second communication devices. 
     Embodiment 21 
     The apparatus of Embodiment 20, wherein the network identifier is a multicast address corresponding to the one or more second communication devices. 
     Embodiment 22 
     The apparatus of claim any of Embodiments 13-19, wherein the one or more ICs are configured to generate the second portion of the wakeup packet to include at least a portion of a network identifier corresponding to a broadcast address. 
     Embodiment 23 
     The apparatus of Embodiment 13, wherein the one or more ICs are further configured to generate the second OFDM symbol to include signals that enable the one or more low power wakeup radios at the respective one or more second communication devices to perform at least one of i) receiver gain adjustment, and ii) synchronization. 
     Embodiment 24 
     The apparatus of any of Embodiments 13-23, wherein the one or more ICs are configured to modulate the first OFDM symbol and the second OFDM symbol according to a binary phase shift keying (BPSK) modulation scheme. 
     Embodiment 25 
     A method, comprising: generating, at the first communication device, a first portion of a packet intended for one or more second communication devices, wherein the first portion of the packet corresponds to a legacy physical layer protocol (PHY) preamble corresponding to a first communication protocol, wherein the first portion of the packet includes a first orthogonal frequency division multiplexing (OFDM) symbol that spans a first bandwidth and is modulated according to a modulation scheme; generating, at the first communication device, a second OFDM symbol, wherein the second OFDM symbol spans the first bandwidth and is modulated according to the modulation scheme; generating, at the first communication device, a second portion of the packet, wherein the second portion of the packet conforms to a second communication protocol configured for communications on one or more channel bandwidths that are narrower than the first bandwidth, wherein the second portion of the packet corresponds to a second bandwidth that is less than the first bandwidth, and wherein the second portion of the packet does not conform to the first communication protocol; and transmitting, by the first communication device, the packet, including transmitting, in time, the second OFDM symbol between the first portion of the packet and the second portion of the packet, and wherein the modulation of the second OFDM symbol according to the modulation scheme signals to third communication devices operating according to the first communication protocol that the packet does not conform to the first communication protocol. 
     Embodiment 26 
     The method of Embodiment 25, wherein the first OFDM symbol corresponds to a legacy signal (L-SIG) field that includes information indicating a duration of the packet. 
     Embodiment 27 
     The method of Embodiment 26, wherein the second OFDM symbol is a repetition of the first OFDM symbol. 
     Embodiment 28 
     The method of any of Embodiments 25-27, wherein: the first portion of the packet includes a third OFDM symbol corresponding to a legacy long training field (L-LTF), and the second OFDM symbol is a repetition of the third OFDM symbol. 
     Embodiment 29 
     The method of Embodiment 25, wherein the second OFDM symbol of the packet does not convey any useful information to the one or more second communication devices. 
     Embodiment 30 
     The method of Embodiment 25, wherein the second OFDM symbol includes a plurality of information bits intended for the one or more second communication devices. 
     Embodiment 31 
     The method of Embodiment 30, further comprising scrambling the plurality of information bits. 
     Embodiment 32 
     The method of any of Embodiments 25-31, wherein the second portion of the packet includes at least a portion of a network identifier corresponding to the one or more second communication devices. 
     Embodiment 33 
     The method of Embodiment 32, wherein the network identifier is a multicast address corresponding to the one or more second communication devices. 
     Embodiment 34 
     The method of any of Embodiments 25-31, wherein the second portion of the packet includes at least a portion of a network identifier corresponding to a broadcast address. 
     Embodiment 35 
     The method of Embodiment 25, wherein generating the second OFDM symbol includes generating the second OFDM symbol to include signals that enable one or more network interfaces at the respective one or more second communication devices to perform at least one of i) receiver gain adjustment, and ii) synchronization. 
     Embodiment 36 
     The method of any of Embodiments 25-35, wherein the first OFDM symbol and the second OFDM symbol are modulated according to a binary phase shift keying (BPSK) modulation scheme. 
     Embodiment 37 
     An apparatus, comprising: a network interface device associated with a first communication device. The network interface device includes one or more integrated circuits (ICs) configured to: generate a first portion of a packet intended for one or more second communication devices, wherein the first portion of the packet corresponds to a legacy physical layer protocol (PHY) preamble corresponding to a first communication protocol, wherein the first portion of the packet includes a first orthogonal frequency division multiplexing (OFDM) symbol that spans a first bandwidth and is modulated according to a modulation scheme; generate a second OFDM symbol, wherein the second OFDM symbol spans the first bandwidth and is modulated according to the modulation scheme; generate a second portion of the packet, wherein the second portion of the packet conforms to a second communication protocol configured for communications on one or more channel bandwidths that are narrower than the first bandwidth, wherein the second portion of the packet corresponds to a second bandwidth that is less than the first bandwidth, and wherein the second portion of the packet does not conform to the first communication protocol; and transmit the packet, including transmitting, in time, the second OFDM symbol between the first portion of the packet and the second portion of the packet, and wherein the modulation of the second OFDM symbol according to the modulation scheme signals to third communication devices operating according to the first communication protocol that the packet does not conform to the first communication protocol. 
     Embodiment 38 
     The apparatus of Embodiment 37, wherein the first OFDM symbol corresponds to a legacy signal (L-SIG) field that includes information indicating a duration of the packet. 
     Embodiment 39 
     The apparatus of Embodiment 38, wherein the one or more ICs are configured to generate the second OFDM symbol as a repetition of the first OFDM symbol. 
     Embodiment 40 
     The apparatus of any of Embodiments 37-39, wherein the one or more ICs are configured to: generate the first portion of the packet to include a third OFDM symbol corresponding to a legacy long training field (L-LTF), and generate the second OFDM symbol as a repetition of the third OFDM symbol. 
     Embodiment 41 
     The apparatus of Embodiment 37, wherein the second OFDM symbol of the packet does not convey any useful information to the one or more second communication devices. 
     Embodiment 42 
     The apparatus of Embodiment 37, wherein the second OFDM symbol includes a plurality of information bits intended for the one or more second communication devices. 
     Embodiment 43 
     The apparatus of Embodiment 42, wherein the one or more ICs are configured to scramble the plurality of information bits. 
     Embodiment 44 
     The apparatus of any of Embodiments 37-43, wherein the one or more ICs are configured to generate the second portion of the packet to include at least a portion of a network identifier corresponding to the one or more second communication devices. 
     Embodiment 45 
     The apparatus of Embodiment 44, wherein the network identifier is a multicast address corresponding to the one or more second communication devices. 
     Embodiment 46 
     The apparatus of claim any of Embodiments 37-43, wherein the one or more ICs are configured to generate the second portion of the packet to include at least a portion of a network identifier corresponding to a broadcast address. 
     Embodiment 47 
     The apparatus of Embodiment 13, wherein the one or more ICs are further configured to generate the second OFDM symbol to include signals that enable one or more network interface devices at the respective one or more second communication devices to perform at least one of i) receiver gain adjustment, and ii) synchronization. 
     Embodiment 24 
     The apparatus of any of Embodiments 37-47, wherein the one or more ICs are configured to modulate the first OFDM symbol and the second OFDM symbol according to a binary phase shift keying (BPSK) modulation scheme. 
     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.