Patent Publication Number: US-2023155869-A1

Title: Methods and apparatus for generation of physical layer protocol data units for vehicular environments

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 17/334,638, now U.S. Pat. No. 11,558,226, entitled “Methods and Apparatus for Generation of Physical Layer Protocol Data Units for Vehicular Environment,” filed on May 28, 2021, which is a continuation of U.S. patent application Ser. No. 16/179,320, now U.S. Pat. No. 11,032,118, entitled “Methods and Apparatus for Generation of Physical Layer Protocol Data Units for Vehicular Environments”, filed on Nov. 2, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/595,452, entitled “Enhanced 802.11p PHY Design,” filed on Dec. 6, 2017. All of the applications referenced above are incorporated by reference herein in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to communication networks and, more particularly, to physical layer protocol data units. 
     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. 
     The IEEE 802.11p Standard specifies protocols for wireless access in vehicular environments (WAVE). Future WAVE standards are aimed at delivering improved car-to-car or car-to-infrastructure connectivity, infotainment features, etc. 
     SUMMARY 
     In an embodiment, a method for wireless communication in a vehicular communication network includes: selecting, at a communication device, a frequency bandwidth via which a physical layer (PHY) protocol data unit (PPDU) will be transmitted in the vehicular communication network, wherein the frequency bandwidth is selected from a set of permissible frequency bandwidths that includes a 10 MHz bandwidth and a 20 MHz bandwidth; selecting, at a communication device, a same compression mode for long training fields (LTFs) included in a PHY preamble and one or more PHY midambles of the PPDU; generating, at a communication device, the PPDU i) according to a downclocking ratio of 1/2, and ii) based on an orthogonal frequency division multiplexing (OFDM) numerology defined by an IEEE 802.11ac Standard. The PPDU is generated to span the selected frequency bandwidth and to include a PHY preamble having a legacy preamble portion and a non-legacy preamble portion. In response to the selected frequency bandwidth being 10 MHz, the PPDU is generated according to the downclocking ratio of 1/2 and based on the OFDM numerology defined by the IEEE 802.11ac Standard for 20 MHz PPDUs. In response to the selected frequency bandwidth being 20 MHz, the PPDU is generated according to the downclocking ratio of 1/2 and based on the OFDM numerology defined by the IEEE 802.11ac Standard for 40 MHz PPDUs. The method also includes transmitting, by the communication device, the PPDU in the vehicular communication network. Generating the PPDU includes generating the PPDU to include i) the PHY preamble having one or more first LTFs generated according to the selected same compression mode, and ii) the one or more PHY midambles each having one or more respective second LTFs generated according to the same selected compression mode. 
     In another embodiment, a wireless communication device comprises a wireless network interface device having one or more integrated circuit (IC) devices. The one or more IC devices are configured to: select a frequency bandwidth via which a physical layer (PHY) protocol data unit (PPDU) will be transmitted in the vehicular communication network, wherein the frequency bandwidth is selected from a set of permissible frequency bandwidths that includes a 10 MHz bandwidth and a 20 MHz bandwidth; select a same compression mode for long training fields (LTFs) to be included in a PHY preamble of the PPDU and one or more PHY midambles of the PPDU; generate the PPDU i) according to a downclocking ratio of 1/2, and ii) based on an orthogonal frequency division multiplexing (OFDM) numerology defined by an IEEE 802.11ac Standard. The PPDU is generated to span the selected frequency bandwidth. In response to the selected frequency bandwidth being 10 MHz, the PPDU is generated according to the downclocking ratio of 1/2 and based on the OFDM numerology defined by the IEEE 802.11ac Standard for 20 MHz PPDUs. In response to the selected frequency bandwidth being 20 MHz, the PPDU is generated according to the downclocking ratio of 1/2 and based on the OFDM numerology defined by the IEEE 802.11ac Standard for 40 MHz PPDUs. The one or more IC devices are further configured to: generate the PPDU to include i) the PHY preamble having one or more first LTFs generated according to the selected same compression mode, and ii) the one or more PHY midambles each having one or more respective second LTFs generated according to the same selected compression mode; and control the wireless network interface device to transmit the PPDU in the vehicular communication network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of an example wireless local area network (WLAN), according to an embodiment. 
         FIG.  2    is a diagram of an example physical layer (PHY) protocol data unit, according to an embodiment. 
         FIG.  3    is a diagram of another example PHY protocol data unit, according to another embodiment. 
         FIG.  4    is a diagram of another example PHY protocol data unit, according to another embodiment. 
         FIG.  5    is a diagram of another example PHY protocol data unit, according to another embodiment. 
         FIG.  6    is a diagram of another example PHY protocol data unit, according to another embodiment. 
         FIG.  7    is a block diagram of an example PHY processor for generating PHY protocol data units, according to an embodiment. 
         FIG.  8    is a table illustrating the effect of downclocking on bandwidth and subcarrier frequency spacing of a PHY protocol data unit, according to an embodiment. 
         FIG.  9 A  is a diagram of an example PHY protocol data unit having PHY midambles, according to an embodiment. 
         FIG.  9 B  is a diagram of an example PHY midamble of the PHY protocol data unit of  FIG.  9 A , according to an embodiment. 
         FIG.  10    is a flow diagram of an example method for generating and transmitting a PHY data unit, according to an embodiment. 
         FIG.  11    is a flow diagram of another example method for generating and transmitting a PHY data unit, according to another embodiment. 
         FIG.  12    is a flow diagram of another example method for generating and transmitting a PHY data unit, according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Generation, transmission, and reception of physical layer (PHY) data units, as described below, are discussed in the context of wireless local area networks (WLANs) that utilize protocols 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, PHY data unit generation/transmission/reception 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. 
       FIG.  1    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 network interface device  122 . The network interface device  122  includes a medium access control (MAC) processor  126  and a PHY processor  130 . The PHY processor  130  includes a plurality of transceivers  134 , and the transceivers  134  are coupled to a plurality of antennas  138 . Although three transceivers  134  and three antennas  138  are illustrated in  FIG.  1   , the AP  114  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  134  and antennas  138  in other embodiments. In some embodiments, the AP  114  includes a higher number of antennas  138  than transceivers  134 , and antenna switching techniques are utilized. 
     The network interface device  122  is implemented using one or more integrate circuits (ICs) configured to operate as discussed below. For example, the MAC processor  126  may be implemented, at least partially, on a first IC, and the PHY processor  130  may be implemented, at least partially, on a second IC. As another example, at least a portion of the MAC processor  126  and at least a portion of the PHY processor  130  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 processor  126  and at least a portion of the PHY processor  130 . 
     In an embodiment, the host processor  118  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a random access memory (RAM), a read-only memory (ROM), a flash memory, etc. In an embodiment, the host processor  118  may be implemented, at least partially, on a first IC, and the network interface device  122  may be implemented, at least partially, on a second IC. As another example, the host processor  118  and at least a portion of the network interface device  122  may be implemented on a single IC. 
     In various embodiments, the MAC processor  126  and/or the PHY processor  130  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, a future version of the IEEE 802.11 Standard, or another suitable wireless communication protocol. For example, the MAC processor  126  may be configured to implement MAC layer functions, including MAC layer functions of the WLAN communication protocol, and the PHY processor  130  may be configured to implement PHY functions, including PHY functions of the WLAN communication protocol. For instance, the MAC processor  126  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  130 . The PHY processor  130  may be configured to receive MAC layer data units from the MAC processor  126  and encapsulate the MAC layer data units to generate PPDUs such as PHY protocol data units (PPDUs) for transmission via the antennas  138 . Generating PHY protocol data units includes generating PHY preambles and PHY midambles of the PHY protocol data units, at least in some embodiments. Similarly, the PHY processor  130  may be configured to receive PPDUs that were received via the antennas  138 , and extract MAC layer data units encapsulated within the PPDUs. The PHY processor  130  is configured to process PHY preambles and PHY midambles of the PHY protocol data units to perform functions such as one or more of synchronization, automatic gain control (AGC) adjustment, channel estimation, etc., at least in some embodiments. The PHY processor  130  may provide the extracted MAC layer data units to the MAC processor  126 , which processes the MAC layer data units. 
     The PHY processor  130  is configured to downconvert one or more radio frequency (RF) signals received via the one or more antennas  138  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  130  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  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  130  is configured to generate one or more RF signals that are provided to the one or more antennas  138 . The PHY processor  130  is also configured to receive one or more RF signals from the one or more antennas  138 . 
     The MAC processor  126  is configured to control the PHY processor  130  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  130 , and optionally providing one or more control signals to the PHY processor  130 , according to some embodiments. In an embodiment, the MAC processor  126  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  126  includes a hardware state machine. 
     The WLAN  110  includes a plurality of client stations  154 . Although three client stations  154  are illustrated in  FIG.  1   , the WLAN  110  includes other suitable numbers (e.g., 1, 2, 4, 5, 6, etc.) of client stations  154  in various embodiments. The client station  154 - 1  includes a host processor  158  coupled to a network interface device  162 . The network interface device  162  includes a MAC processor  166  and a PHY processor  170 . The PHY processor  170  includes a plurality of transceivers  174 , and the transceivers  174  are coupled to a plurality of antennas  178 . Although three transceivers  174  and three antennas  178  are illustrated in  FIG.  1   , the client station  154 - 1  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  174  and antennas  178  in other embodiments. In some embodiments, the client station  154 - 1  includes a higher number of antennas  178  than transceivers  174 , and antenna switching techniques are utilized. 
     The network interface device  162  is implemented using one or more ICs configured to operate as discussed below. For example, the MAC processor  166  may be implemented on at least a first IC, and the PHY processor  170  may be implemented on at least a second IC. As another example, at least a portion of the MAC processor  166  and at least a portion of the PHY processor  170  may be implemented on a single IC. For instance, the network interface device  162  may be implemented using an SoC, where the SoC includes at least a portion of the MAC processor  166  and at least a portion of the PHY processor  170 . 
     In an embodiment, the host processor  158  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a RAM, a ROM, a flash memory, etc. In an embodiment, the host processor  158  may be implemented, at least partially, on a first IC, and the network interface device  162  may be implemented, at least partially, on a second IC. As another example, the host processor  158  and at least a portion of the network interface device  162  may be implemented on a single IC. 
     In various embodiments, the MAC processor  166  and the PHY processor  170  of the client device  154 - 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 processor  166  may be configured to implement MAC layer functions, including MAC layer functions of the WLAN communication protocol, and the PHY processor  170  may be configured to implement PHY functions, including PHY functions of the WLAN communication protocol. The MAC processor  166  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  170 . The PHY processor  170  may be configured to receive MAC layer data units from the MAC processor  166  and encapsulate the MAC layer data units to generate PPDUs such as PPDUs for transmission via the antennas  178 . Generating PHY protocol data units includes generating PHY preambles and PHY midambles of the PHY protocol data units, at least in some embodiments. Similarly, the PHY processor  170  may be configured to receive PPDUs that were received via the antennas  178 , and extract MAC layer data units encapsulated within the PPDUs. The PHY processor  170  is configured to process PHY preambles and PHY midambles of the PHY protocol data units to perform functions such as one or more of synchronization, AGC adjustment, channel estimation, etc., at least in some embodiments. The PHY processor  170  may provide the extracted MAC layer data units to the MAC processor  166 , which processes the MAC layer data units. 
     The PHY processor  170  is configured to downconvert one or more RF signals received via the one or more antennas  178  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  170  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  170  includes amplifiers (e.g., an LNA, a power amplifier, etc.), an RF downconverter, an RF upconverter, a plurality of filters, one or more ADCs, one or more DACs, one or more DFT calculators (e.g., an FFT calculator), one or more IDFT calculators (e.g., an IFFT calculator), one or more modulators, one or more demodulators, etc. 
     The PHY processor  170  is configured to generate one or more RF signals that are provided to the one or more antennas  178 . The PHY processor  170  is also configured to receive one or more RF signals from the one or more antennas  178 . 
     The MAC processor  166  is configured to control the PHY processor  170  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  170 , and optionally providing one or more control signals to the PHY processor  170 , according to some embodiments. In an embodiment, the MAC processor  166  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a RAM, a ROM, a flash memory, etc. In an embodiment, the MAC processor  166  includes a hardware state machine. 
     In an embodiment, each of the client stations  154 - 2  and  154 - 3  has a structure that is the same as or similar to the client station  154 - 1 . Each of the client stations  154 - 2  and  154 - 3  has the same or a different number of transceivers and antennas. For example, the client station  154 - 2  and/or the client station  154 - 3  each have only two transceivers and two antennas (not shown), according to an embodiment. 
     PPDUs are sometimes referred to herein as packets. MPDUs are sometimes referred to herein as frames. 
     In an embodiment, the WLAN  110  corresponds to a vehicular communication environment in which the AP  114  is located in a fixed location proximate to a roadway, and the client stations  154  are located in vehicles travelling on the roadway. In some embodiments corresponding to a vehicular communication environment, client stations  154  in different vehicles directly communicate with one another. In such environments, Doppler effects are more pronounced as compared to home or office WLAN environments. Additionally, longer range communications may be required in such environments as compared to home or office environments. Described below are example PPDU generation/transmission/reception techniques that are useful for environments that require longer ranges and/or in which Doppler effects are more pronounced, as compared to typical existing home or office WLANs environments. 
       FIG.  2    is a diagram of an example PPDU  200  that the network interface device  122  ( FIG.  1   ) is configured to generate and transmit to one or more client stations  154 , according to an embodiment. The network interface device  162  ( FIG.  1   ) may also be configured to generate/transmit PPDUs the same as or similar to the PPDU  200  to the AP  114  or to another client station  154 . In an embodiment, the PPDU  200  has a format similar to a PPDU format defined by the IEEE 802.11a Standard. 
     The PPDU  200  includes a PHY preamble  204  having a short training field (STF)  208 , generally used for packet detection, initial synchronization, and automatic gain control (AGC) adjustment, and a long training field (LTF)  212 , generally used for channel estimation and fine synchronization. The PPDU  200  also includes a signal field (SIG)  206  that includes PHY parameters corresponding to the data unit  200 , such as an indication of a length of the data unit  200 , and an indication of a coding rate used to generate and transmit the PPDU  200 , for example. The PPDU  200  also includes a data portion  220 . The data portion  220  includes one or more MPDUs, MSDUs, an aggregate MPDU (A-MPDU), etc. In some scenarios, the PPDU  200  may omit the data portion  220 . 
     Each of the STF  208 , the LTF  212 , the SIG  216 , and the data portion  220  comprises one or more orthogonal frequency division multiplexing (OFDM) symbols. As merely an illustrative example, the SIG  216  comprises one OFDM symbol. 
     In an embodiment, the PPDU  200  is downclocked with respect to a PPDU format defined by the IEEE 802.11a Standard. In an embodiment, the PPDU  200  spans a frequency bandwidth equal to or less than 20 MHz (e.g., a bandwidth of the PPDU is selectable from a set of bandwidths that includes 20 MHz, 10 MHz, and 5 MHz). In other embodiments, the bandwidth of the PPDU  200  is selectable from a set of bandwidths that additionally or alternatively includes bandwidths other than 20 MHz, 10 MHz, and 5 MHz (e.g., 1 MHz, 2 MHz, etc.). In an embodiment, the PPDU  200  employs an OFDM tone spacing that is different than a tone spacing defined by the IEEE 802.11a Standard. 
       FIG.  3    is a diagram of an example PPDU  300  that the network interface device  122  ( FIG.  1   ) is configured to generate and transmit to one or more client stations  154 , according to an embodiment. The network interface device  162  ( FIG.  1   ) may also be configured to generate/transmit PPDUs the same as or similar to the PPDU  300  to the AP  114  or to another client station  154 . In an embodiment, the PPDU  300  is similar to a PPDU format defined by the IEEE 802.11n Standard. 
     The PPDU  300  includes a PHY preamble  304  having a legacy STF (L-STF)  308 , a legacy LTF (L-LTF)  312 , a legacy SIG (L-SIG)  316 , a high throughput (HT) signal field (HT-SIG)  320  including a first HT signal subfield (HT-SIG 1 )  320 - 1  and a second HT signal subfield (HT-SIG 2 )  320 - 2 , an HT short training field (HT-STF)  324 , and N data HT long training fields (HT-LTFs)  328 , where N is a positive integer generally corresponding to a number of spatial streams used to transmit the PPDU  300  in a multiple input multiple output (MIMO) channel configuration. In an embodiment, N generally corresponds to (e.g., is greater than or equal to) a number of spatial streams via which the PPDU  300  will be transmitted. The HT-SIG  320  includes PHY parameters corresponding to the data unit  300 , such as an indication of a modulation and coding scheme (MCS), a number of spatial streams, etc. used to generate and transmit the PPDU  300 , for example. The PPDU  300  also includes a data portion  332 . The data portion  332  includes one or more MPDUs, MSDUs, an aggregate MPDU (A-MPDU), etc. In some scenarios, the PPDU  300  may omit the data portion  332 . 
     Each of the L-STF  308 , the L-LTF  312 , the L-SIG  316 , the HT-SIG  320 , the HT-STF  324 , the HT-LTFs  328 , and the data portion  332  comprises one or more OFDM symbols. As merely an illustrative example, each of the HT-SIG 1   320 - 1  and the HT-SIG 2   320 - 2  comprises a respective single OFDM symbol. As another example, each HT-LTF  328  comprises a single OFDM symbol. 
     In an embodiment, the PPDU  300  is downclocked with respect to a PPDU format defined by the IEEE 802.11n Standard. In an embodiment, the PPDU  300  spans a frequency bandwidth equal to or less than 20 MHz (e.g., a bandwidth of the PPDU is selectable from a set of bandwidths that includes 20 MHz, 10 MHz, and 5 MHz). In other embodiments, the bandwidth of the PPDU  300  is selectable from a set of bandwidths that additionally or alternatively includes bandwidths other than 20 MHz, 10 MHz, and 5 MHz (e.g., 1 MHz, 2 MHz, etc.). In an embodiment, the PPDU  300  employs an OFDM tone spacing that is different than a tone spacing defined by the IEEE 802.11n Standard. 
       FIG.  4    is a diagram of an example PPDU  400  that the network interface device  122  ( FIG.  1   ) is configured to generate and transmit to one or more client stations  154 , according to an embodiment. The network interface device  162  ( FIG.  1   ) may also be configured to transmit PPDUs the same as or similar to the PPDU  400  to the AP  114  or to another client station  154 . In an embodiment, the PPDU  400  is similar to a PPDU format defined by the IEEE 802.11n Standard. 
     The PPDU  400  includes a PHY preamble  404  having an HT Greenfield short training field (HT-GF-STF)  408 , an HT-LTF  412 , an HT-SIG  416  including an HT-SIG 1   416 - 1  and an HT-SIG 2   416 - 2 , and N further HT-LTFs  420 , where N is an integer which generally corresponds to a number of spatial streams used to transmit the PPDU  400  in a MIMO channel configuration. The HT-SIG  416  includes PHY parameters associated with the data unit  400 , such as an indication of an MCS, an indication of a number of spatial streams, etc. used to generate and transmit the PPDU  400 , for example. The PPDU  400  also includes a data portion  424 . The data portion  424  includes one or more MPDUs, MSDUs, an A-MPDU, etc. In some scenarios, the PPDU  400  may omit the data portion  424 . 
     Each of the HT-GF-STF  408 , the HT-LTF  412 , the HT-SIG  416 , the HT-LTFs  420  and the data portion  424  comprises one or more OFDM symbols. As merely an illustrative example, each of the HT-SIG 1   416 - 1  and the HT-SIG 2   416 - 2  comprises a respective single OFDM symbol. As another example, each HT-LTF  420  comprises a single OFDM symbol. In an embodiment, the content of HT-SIG  416  is redefined as compared to the IEEE 802.11n Standard to signal PHY information regarding the PPDU  400  that is not provided for in the HT-SIG 1  and HT-SIG 2  fields defined by the IEEE 802.11n Standard. 
     In an embodiment, the PPDU  400  is downclocked with respect to a PPDU format defined by the IEEE 802.11n Standard. In an embodiment, the PPDU  400  spans a frequency bandwidth equal to or less than 20 MHz (e.g., a bandwidth of the PPDU is selectable from a set of bandwidths that includes 20 MHz, 10 MHz, and 5 MHz). In other embodiments, the bandwidth of the PPDU  400  is selectable from a set of bandwidths that additionally or alternatively includes bandwidths other than 20 MHz, 10 MHz, and 5 MHz (e.g., 1 MHz, 2 MHz, etc.). In an embodiment, the PPDU  400  employs an OFDM tone spacing that is different than a tone spacing defined by the IEEE 802.11n Standard. 
       FIG.  5    is a diagram of an example PPDU  500  that the network interface device  122  ( FIG.  1   ) is configured to generate and transmit to one or more client stations  154 , according to an embodiment. The network interface device  162  ( FIG.  1   ) may also be configured to transmit PPDUs the same as or similar to the PPDU  500  to the AP  114  or to another client station  154 . In an embodiment, the PPDU  500  is similar to a PPDU format defined by the IEEE 802.11ac Standard. 
     The data unit  500  includes a PHY preamble  504  having an L-STF  508 , an L-LTF  512 , an L-SIG  516 , first very high throughput (VHT) signal field (VHT-SIG-A)  520  including a VHT-SIG-A 1   520 - 1  and a VHT-SIG-A 2   520 - 2 , a VHT short training field (VHT-STF)  524 , N VHT long training fields (VHT-LTFs)  528 , where N is an integer which generally corresponds to a number of spatial streams used to transmit the PPDU  500  in a MIMO channel configuration, and a second VHT signal field (VHT-SIG-B)  536 . The data unit  500  also includes a data portion  536 . The data portion  536  includes one or more MPDUs, MSDUs, an A-MPDU, etc. In some scenarios, the PPDU  500  may omit the data portion  536 . 
     Each of the L-STF  508 , the L-LTF  512 , the L-SIG  516 , the VHT-SIG-A  520 , the VHT-STF  524 , the N VHT-LTFs  528 , the VHT-SIG-B  532 , and the data portion  536  comprises one or more OFDM symbols. As merely an illustrative example, each of the VHT-SIG-A 1   520 - 2  and the VHT-SIG-A 2  comprises a respective single OFDM symbol. As another example, each VHT-LTF  528  comprises a single OFDM symbol. In an embodiment, the content of VHT-SIG-A  520  is redefined as compared to the IEEE 802.11ac Standard to signal PHY information regarding the PPDU  500  that is not provided for in the VHT-SIG-A field defined by the IEEE 802.11ac Standard. In an embodiment, the content of VHT-SIG-B  532  is redefined as compared to the IEEE 802.11ac Standard to signal PHY information regarding the PPDU  500  that is not provided for in the VHT-SIG-B field defined by the IEEE 802.11ac Standard. In an embodiment, the VHT-SIG-B  532  is omitted. 
     In an embodiment, the PPDU  500  is downclocked with respect to a PPDU format defined by the IEEE 802.11ac Standard. In an embodiment, the PPDU  500  spans a frequency bandwidth less than 20 MHz (e.g., a bandwidth of the PPDU is selectable from a set of bandwidths that includes 20 MHz, 10 MHz, and 5 MHz). In other embodiments, the bandwidth of the PPDU  500  is selectable from a set of bandwidths that additionally or alternatively includes bandwidths other than 20 MHz, 10 MHz, and 5 MHz (e.g., 1 MHz, 2 MHz, etc.). In an embodiment, the PPDU  500  employs an OFDM tone spacing that is different than a tone spacing defined by the IEEE 802.11ac Standard. 
       FIG.  6    is a diagram of an example PPDU  600  that the network interface device  122  ( FIG.  1   ) is configured to generate and transmit to one or more client stations  154 , according to an embodiment. The network interface device  162  ( FIG.  1   ) may also be configured to transmit PPDUs the same as or similar to the PPDU  600  to the AP  114  or to another client station  154 . 
     The PPDU  600  includes a PHY preamble  604  including an L-STF  608 , an L-LTF  612 , an L-SIG  616 , a repeated L-SIG field (RL-SIG)  620 , a high efficiency (HE) signal-A field (HE-SIG-A)  624 , an HE signal-B field (HE-SIG-B)  628 , an HE short training field (HE-STF)  230 , and N HE long training fields (HE-LTFs)  235 , where N is a suitable positive integer. In an embodiment, N generally corresponds to (e.g., is greater than or equal to) a number of spatial streams via which the PPDU  600  will be transmitted. The PPDU  600  also includes a PHY data portion  640  and a packet extension (PE) field  644 . The PHY data portion  640  includes one or more MPDUs, MSDUs, an A-MPDU, etc. In some scenarios, the PPDU  600  may omit the PHY data portion  640 . In some embodiments, the PPDU  600  may omit one or more fields corresponding to the preamble PHY preamble  604 . In some embodiments, the PHY preamble  604  includes additional fields not illustrated in  FIG.  6   . In an embodiment, the PPDU  600  is a downclocked version of a PPDU format defined by a current draft of the IEEE 802.11ax Standard (e.g., IEEE P802.11ax™/D2.2, “Draft Standard for Information technology—Telecommunications and information exchange between systems Local and metropolitan area networks—Specific requirements, Part 11: Wireless LAN Medium Access Control, (MAC) and Physical Layer (PHY) Specifications, Amendment 6: Enhancements for High Efficiency WLAN,” IEEE Computer Society, (February 2018), or IEEE P802.11ax™/D2.2, “Draft Standard for Information technology—Telecommunications and information exchange between systems Local and metropolitan area networks—Specific requirements, Part 11: Wireless LAN Medium Access Control, (MAC) and Physical Layer (PHY) Specifications, Amendment 6: Enhancements for High Efficiency WLAN,” IEEE Computer Society, (October 2017)). 
     Each of the L-STF  608 , the L-LTF  612 , the L-SIG  616 , the RL-SIG  620 , the HE-SIG-A  624 , the HE-SIG-B  628 , the HE-STF  632 , the N HE-LTFs  636 , and the data portion  640  comprises one or more OFDM symbols. As merely an illustrative example, the HE-SIG-A  624  comprises two OFDM symbols. As another example, each HE-LTF  636  comprises a single OFDM symbol. In an embodiment, the content of HE-SIG-A  624  is redefined as compared to the current draft of the IEEE 802.11ax Standard to signal PHY information regarding the PPDU  600  that is not provided for in the HE-SIG-A field defined by the current draft of IEEE 802.11ax Standard. In an embodiment, the content of HE-SIG-B  628  is redefined as compared to the current draft of the IEEE 802.11ax Standard to signal PHY information regarding the PPDU  600  that is not provided for in the HE-SIG-B field defined by the current draft of the IEEE 802.11ax Standard. In an embodiment, the HE-SIG-B  628  is omitted. 
     In an embodiment, the PPDU  600  is downclocked with respect to a PPDU format defined by the current draft of the IEEE 802.11ax Standard. In an embodiment, the PPDU  600  spans a frequency bandwidth less than 20 MHz (e.g., a bandwidth of the PPDU is selectable from a set of bandwidths that includes 20 MHz, 10 MHz, and 5 MHz). In other embodiments, the bandwidth of the PPDU  600  is selectable from a set of bandwidths that additionally or alternatively includes bandwidths other than 20 MHz, 10 MHz, and 5 MHz (e.g., 1 MHz, 2 MHz, etc.). In an embodiment, the PPDU  600  employs an OFDM tone spacing that is different than a tone spacing defined by the current draft of the IEEE 802.11ax Standard. 
       FIG.  7    is a block diagram illustrating an example PHY processor  700  for generating PPDUs, according to an embodiment. In an embodiment, the PHY processor  700  corresponds to the PHY processor  130  and/or the PHY processor  170  as described above with respect to  FIG.  1   . In various embodiments and/or scenarios, the PHY processing unit  700  generates PPDUs such as one or more of the PPDUs  200 ,  300 ,  400 ,  500 , and  600 , for example. In other embodiments and/or scenarios, the PHY processing unit  700  generates PPDUs having suitable formats other than the PPDUs  200 ,  300 ,  400 ,  500 , and  600 . 
     The PHY processing unit  700  includes a scrambler  704  that generally scrambles an information bit stream to reduce the occurrence of long sequences of ones or zeros. An FEC encoder  708  encodes scrambled information bits to generate encoded data bits. In one embodiment, the FEC encoder  708  includes a binary convolutional code (BCC) encoder. In another embodiment, the FEC encoder  708  includes a binary convolutional encoder followed by a puncturing block. In yet another embodiment, the FEC encoder  708  includes a low density parity check (LDPC) encoder. 
     An interleaver  712  interleaves encoded bits (i.e., changes the order of the bits) to prevent long sequences of adjacent noisy bits from entering a decoder at the receiver. A constellation mapper  716  maps the interleaved sequence of bits to constellation points corresponding to different subcarriers of an OFDM symbol. More specifically, for each spatial stream, the constellation mapper  716  translates every bit sequence of length log 2 (M) into one of M constellation points. In an embodiment, the constellation mapper  716  optionally further implements dual carrier modulation (DCM) whereby a same bit sequence is mapped to two constellation points corresponding to two different subcarriers. 
     A space-time block coding (STBC) unit  720  applies STBC to map N SS  spatial streams to N STS  space-time streams. A cyclic shift diversity (CSD) unit  724  inserts a cyclic shift into all but one of the spatial streams to prevent unintentional beamforming. The output of the CSD unit  724  is provided to a spatial mapping unit  728  that maps the N STS  space-time streams to N TX  transmit chains. An inverse discrete Fourier transform (IDFT) unit  732  operates on each transmit chain and converts constellation points corresponding to a block of subcarriers to a time-domain signal corresponding to an OFDM symbol. A guard interval (GI) insertion and windowing unit  736  prepends, to each OFDM symbol, a circular extension of the OFDM symbol and smooths the edges of each OFDM symbol to increase spectral decay. An analog and radio frequency (RF) unit  740  upconverts a baseband signal corresponding to the OFDM symbols to generate an RF signal for transmission over the communication channel. 
     In various embodiments and scenarios, the PPDUs  200 ,  300 ,  400 ,  500 , and  600  have a 20 MHz frequency bandwidth and are transmitted over a 20 MHz channel. In other embodiments and/or scenarios, the PPDUs  200 ,  300 ,  400 ,  500 , and  600  have a frequency bandwidth smaller than 20 MHz (as discussed above) and are transmitted over a channel that is narrower than 20 MHz. 
     In various embodiments, different numbers of subcarriers are used for a given frequency bandwidth, and thus different subcarrier spacings are employed in the different embodiments. 
     For instance, in an embodiment, the PPDU  300  is transmitted over a 20 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz, which corresponds to a same clock rate as used in the IEEE 802.11n Standard (i.e., corresponding to a downclocking ratio of one). In another embodiment, the PPDU  300  is transmitted over a 20 MHz channel with 128 subcarriers (e.g., corresponding to a size 128 FFT), corresponding to a subcarrier frequency spacing of 156.25 kHz, which corresponds to a 2-times (2×) downclock (i.e., a downclocking ratio of 1/2) as compared to the clock rate used in the IEEE 802.11n Standard. In another embodiment, the PPDU  300  is transmitted over a 20 MHz channel with 256 subcarriers (e.g., corresponding to a size 256 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4-times (4×) downclock (i.e., a downclocking ratio of 1/4) as compared to the clock rate used in the IEEE 802.11n Standard. 
     In an embodiment, the PPDU  400  is transmitted over a 20 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz, which corresponds to a same clock rate as used in the IEEE 802.11n Standard (i.e., corresponding to a downclocking ratio of one). In another embodiment, the PPDU  400  is transmitted over a 20 MHz channel with 128 subcarriers (e.g., corresponding to a size 128 FFT), corresponding to a subcarrier frequency spacing of 156.25 kHz, which corresponds to a 2× downclock as compared to the clock rate used in the IEEE 802.11n Standard. In another embodiment, the PPDU  400  is transmitted over a 20 MHz channel with 256 subcarriers (e.g., corresponding to a size 256 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× downclock as compared to the clock rate used in the IEEE 802.11n Standard. 
     In an embodiment, the PPDU  500  is transmitted over a 20 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz, which corresponds to a same clock rate as used in the IEEE 802.11ac Standard (i.e., corresponding to a downclocking ratio of one). In another embodiment, the PPDU  500  is transmitted over a 20 MHz channel with 128 subcarriers (e.g., corresponding to a size 128 FFT), corresponding to a subcarrier frequency spacing of 156.25 kHz, which corresponds to a 2× downclock as compared to the clock rate used in the IEEE 802.11ac Standard. In another embodiment, the PPDU  500  is transmitted over a 20 MHz channel with 256 subcarriers (e.g., corresponding to a size 256 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× downclock as compared to the clock rate used in the IEEE 802.11ac Standard. 
     In an embodiment, the PPDU  600  is transmitted over a 20 MHz channel with 256 subcarriers (e.g., corresponding to a size 256 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a same clock rate as used in the draft IEEE 802.11ax Standard (i.e., corresponding to a downclocking ratio of one). 
     In an embodiment, the PPDU  300  is transmitted over a 10 MHz channel with 32 subcarriers (e.g., corresponding to a size 32 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz. In an embodiment, the PPDU  300  is transmitted over a 10 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 156.25 kHz, which corresponds to a 2× clock rate as used in the IEEE 802.11n Standard. In another embodiment, the PPDU  300  is transmitted over a 10 MHz channel with 128 subcarriers (e.g., corresponding to a size 128 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× downclock as compared to the clock rate used in the IEEE 802.11n Standard. 
     In an embodiment, the PPDU  400  is transmitted over a 10 MHz channel with 32 subcarriers (e.g., corresponding to a size 32 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz. In an embodiment, the PPDU  400  is transmitted over a 10 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 156.25 kHz, which corresponds to a 2× clock rate as used in the IEEE 802.11n Standard. In another embodiment, the PPDU  400  is transmitted over a 10 MHz channel with 128 subcarriers (e.g., corresponding to a size 128 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× downclock as compared to the clock rate used in the IEEE 802.11n Standard. 
     In an embodiment, the PPDU  500  is transmitted over a 10 MHz channel with 32 subcarriers (e.g., corresponding to a size 32 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz. In an embodiment, the PPDU  500  is transmitted over a 10 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 156.25 kHz, which corresponds to a 2× clock rate as used in the IEEE 802.11ac Standard. In another embodiment, the PPDU  500  is transmitted over a 10 MHz channel with 128 subcarriers (e.g., corresponding to a size 128 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× downclock as compared to the clock rate used in the IEEE 802.11ac Standard. 
     In an embodiment, the PPDU  600  is transmitted over a 10 MHz channel with 256 subcarriers (e.g., corresponding to a size 256 FFT), corresponding to a subcarrier frequency spacing of 30.0625 kHz, which corresponds to a 2× downclock compared to the clock rate used in the draft IEEE 802.11ax Standard. In an embodiment, the PPDU  600  is transmitted over a 10 MHz channel with 128 subcarriers (e.g., corresponding to a size 128 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz. 
     In an embodiment, the PPDU  300  is transmitted over a 5 MHz channel with 16 subcarriers (e.g., corresponding to a size 16 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz. In an embodiment, the PPDU  300  is transmitted over a 5 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× clock rate as used in the IEEE 802.11n Standard. 
     In an embodiment, the PPDU  400  is transmitted over a 5 MHz channel with 16 subcarriers (e.g., corresponding to a size 16 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz. In an embodiment, the PPDU  400  is transmitted over a 5 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× clock rate as used in the IEEE 802.11n Standard. 
     In an embodiment, the PPDU  500  is transmitted over a 5 MHz channel with 16 subcarriers (e.g., corresponding to a size 16 FFT), corresponding to a subcarrier frequency spacing of 312.5 kHz. In an embodiment, the PPDU  500  is transmitted over a 5 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz, which corresponds to a 4× clock rate as used in the IEEE 802.11ac Standard. 
     In an embodiment, the PPDU  600  is transmitted over a 5 MHz channel with 256 subcarriers (e.g., corresponding to a size 256 FFT), corresponding to a subcarrier frequency spacing of 19.53125 kHz, which corresponds to a 4× downclock compared to the clock rate used in the draft IEEE 802.11ax Standard. In an embodiment, the PPDU  600  is transmitted over a 5 MHz channel with 64 subcarriers (e.g., corresponding to a size 64 FFT), corresponding to a subcarrier frequency spacing of 78.125 kHz. 
     In some embodiments, the network interface  122  is configured to transmit/receive PPDUs at different frequency bandwidths such as all of 20 MHz, 10 MHz, and 5 MHz, or only two of 20 MHz, 10 MHz, and 5 MHz. In some embodiments, the network interface  162  is configured to transmit/receive PPDUs at different frequency bandwidths such as all of 20 MHz, 10 MHz, and 5 MHz, or only two of 20 MHz, 10 MHz, and 5 MHz. 
     In some embodiments, the network interface  122  is additionally or alternatively configured to transmit/receive PPDUs at frequency bandwidths such as both of 2 MHz and 1 MHz, or only one of 2 MHz and 1 MHz. In some embodiments, the network interface  162  is additionally or alternatively configured to transmit/receive PPDUs at frequency bandwidths such as both of 2 MHz and 1 MHz, or only one of 2 MHz and 1 MHz. 
     In an embodiment, a PPDU corresponding to a lower channel bandwidth may be generated using a format and a process flow that are substantially similar to a format and a process flow of a PPDU corresponding to a higher channel bandwidth except with a reduced subcarrier frequency spacing. In an embodiment, this is accomplished by the use of downclocking in a manner such that OFDM symbols of the PPDU corresponding to the lower channel bandwidth have a longer symbol duration than corresponding OFDM symbols of the PPDU corresponding to the higher channel bandwidth. A longer symbol duration allows a reduced subcarrier frequency spacing, thereby reducing a channel bandwidth necessary for transmission. In an embodiment, the downclocking may be accomplished in the IDFT unit  732  of the PHY processor  700 . 
     As merely an example, a format and process flow of a first PPDU corresponding to a 40 MHz channel may be used to generate a second PPDU corresponding to a 20 MHz channel, except that OFDM symbols of the second PPDU are downclocked by 1/2 (i.e., OFDM symbol duration doubled) as compared to OFDM symbols of the first PPDU. In an embodiment wherein the first PPDU corresponds to a 40 MHz channel with 128 subcarriers (i.e., 312.5 kHz subcarrier frequency spacing), downclocking OFDM symbols of the first PPDU by 2× results in OFDM symbols of the second PPDU corresponding to a 20 MHz channel with a subcarrier frequency spacing of 156.25 kHz. In various embodiments, OFDM symbols of PPDUs corresponding to channel bandwidths of 20 MHz, 40 MHz, 80 MHz, etc. may be downclocked with appropriate ratios to generate PPDUs corresponding to channel bandwidths of 5 MHz, 10 MHz, 20 MHz, etc.  FIG.  8    is a table  800  outlining some ways in which a PPDU including an OFDM symbol corresponding to a first channel bandwidth  804  and a first subcarrier frequency spacing  808  may be downclocked with a downclocking ratio  812  to generate a PPDU including an OFDM symbol corresponding to a lower second channel bandwidth  816  and a lower second subcarrier frequency spacing  820 . 
     In various embodiments, different guard interval durations may be used to generate OFDM symbols corresponding to the PPDUs described above. In an embodiment, a guard interval duration may be selected from a set comprising of 0.8 μs, 1.6 μs, 2.4 μs, 3.2 μs, etc., and the selected guard interval duration may be used to generate one or more OFDM symbols corresponding to the above PPDUs. In an embodiment, a guard interval indication field in a signal field (e.g., the HT-SIG  320 / 416 , the VHT-SIG  520 , HE-SIG-A  624 , etc.) of a PPDU may be used to indicate the selected guard interval duration used for generating one or more OFDM symbols (e.g., in a data portion of the PPDU). 
     A set of parameters for transmitting a PPDU such as described above is sometimes referred to as an OFDM numerology. An OFDM numerology is a set of parameters that includes at least i) a channel bandwidth, and ii) a number of OFDM subcarriers (e.g., corresponding to an FFT size or a discrete Fourier transform (DFT) size). The channel bandwidth and the number of OFDM subcarriers for a particular OFDM numerology implies a particular OFDM subcarrier spacing, and thus an OFDM numerology may also be considered to include an OFDM subcarrier spacing parameter, in an embodiment. In some embodiments, an OFDM numerology defined by a communication protocol includes different numbers of OFDM subcarriers for different portions of a PPDU. For example, the communication protocol may specify that a PHY data portion utilizes a first number of OFDM subcarriers, whereas the communication protocol may specify that the PHY preamble utilizes a second number of OFDM subcarriers, according to an embodiment. 
     In various embodiments, multiple spatial streams may be used to transmit PPDUs described above. In an embodiment, a number of spatial streams N SS  field in a signal field (e.g., the HT-SIG  320 / 416 , the VHT-SIG  520 , HE-SIG-A  624 , etc.) of a PPDU may be used to indicate a number of spatial streams corresponding to the PPDU. 
     Packets (e.g., the PPDUs described above) generated by communication devices such as the network interface device  122  and/or the network interface device  162  sometimes include both a PHY preamble and one or more PHY midambles to allow a receiving device to update channel estimation, adjust synchronization, etc., during reception of a single packet. For example, if the packet is relatively long, in situations such as when a channel is particularly noisy, in situations such as when relative movement between a transmitting device and a receiving device causes a Doppler effect, and/or in situations such as when movement of objects in the environment causes a Doppler effect, etc., refining a channel estimate and/or synchronization more than once during the reception of a single packet unit may be beneficial. A transmitting device generates the PHY preamble and one or more PHY midambles so as to allow a receiving device to estimate a communication channel between the transmitting device and the receiving device and/or to refine synchronization during reception of the packet, and accordingly demodulate a data payload of the packet. 
     Accordingly, one or more PHY midambles are included within a data portion of the packet, e.g., each PHY midamble is adjacent to data from the payload at both a beginning of the PHY midamble and an end of the PHY midamble. Each PHY midamble includes one or more OFDM symbols that include training information (sometimes referred to as calibration information), at least a portion of which may be used for one or more of retraining channel training or estimation, refining frequency synchronization or carrier frequency offset (CFO) adjustment, calculating a steering matrix or other parameters used in beamforming, adapting to a communication link, AGC adjustment, etc. Embodiments of PHY midambles described herein may have particular applicability to communication systems in which a packet duration is longer than durations of channel coherence, such as in wireless systems that use shorter wavelengths, and/or in communication systems that experience a Doppler effect. 
       FIG.  9 A  is a diagram of an example PPDU  900 , according to an embodiment. In an embodiment, the network interface device  122  ( FIG.  1   ) is configured to generate and transmit the PPDU  900 , and the network interface device  162  ( FIG.  1   ) is configured to receive and process the PPDU  900 . In some embodiments, the network interface device  162  ( FIG.  1   ) is configured to generate and transmit the PPDU  900 , and the network interface device  122  ( FIG.  1   ) is configured to receive and process the PPDU  900 . The PPDU  900  may occupy a 20 MHz bandwidth or another suitable bandwidth. Data units similar to the PPDU  900  occupy other suitable bandwidths such as 1 MHz, 2 MHz, 5 MHz, 10 MHz, for example, or other suitable bandwidths, in other embodiments. 
     The PPDU  900  includes a PHY preamble  904  and a PHY data portion  908 . In various embodiments, elements of the PPDU  900  are similar to the PPDU  200 / 300 / 400 / 500 / 600  described above, wherein the PHY preamble  904  corresponds to the PHY preamble  204 / 304 / 404 / 504 / 604  and the data portion  908  corresponds to the data portion  220 / 332 / 424 / 536 / 640 . 
     The PHY data portion  908  includes a data payload comprising a plurality of OFDM data symbols partitioned into a plurality of data portions  916 , between which are located one or more PHY midambles  920 . A number of OFDM data symbols, in the data portion  916 , between two consecutive PHY midambles  920  is referred to herein as a midamble periodicity M MA . Within the PHY data portion  908 , a PHY midamble  920  is inserted after every M MA  OFDM data symbols. In an embodiment, a last data portion  916 -M may have a number of OFDM symbols L that is different from M MA . In another embodiment, however, L is equal to M MA . In an embodiment, a number of the data portions  916  and a number of the PHY midambles  920  are identical. In another embodiment, a number of data portions  916  and a number of PHY midambles  920  are different from each other. For instance, in an embodiment, the number of PHY midambles  920  is one less than the number of data portions  916 . 
     In an embodiment, each PHY midamble  920  includes N LTF  LTFs, where N LTF  is a suitable positive integer. In an embodiment, LTFs included in the PHY midamble  920  are identical to (e.g., have content that is the same as), or similar to, the LTFs included in the PHY preamble  904 . For instance, in an embodiment wherein the PPDU  900  corresponds to the PPDU  600 , each PHY midamble  920  includes fields that are identical (e.g., formatted, encoded, modulated, etc., in a manner that is identical) to HE-LTFs  636 . In another embodiment wherein the PPDU  900  corresponds to the PPDU  500 , each PHY midamble  920  includes fields that are identical (e.g., formatted, encoded, modulated, etc., in a manner that is identical) to VHT-LTFs  528 . In another embodiment wherein the PPDU  900  corresponds to the PPDU  400 , each PHY midamble  920  includes fields that are identical (e.g., formatted, encoded, modulated, etc., in a manner that is identical) to HT-LTFs  420 . In another embodiment wherein the PPDU  900  corresponds to the PPDU  300 , each PHY midamble  920  includes fields that are identical (e.g., formatted, encoded, modulated, etc., in a manner that is identical) to HT-LTFs  328 . In another embodiment wherein the PPDU  900  corresponds to the PPDU  200 , each PHY midamble  920  includes a field that is identical (e.g., formatted, encoded, modulated, etc., in a manner that is identical) to the LTF  212 . In another embodiment, however, each PHY midamble additionally or alternatively includes LTFs that are different from LTFs included in the PHY preamble  904 . 
       FIG.  9 B  is a diagram of an example PHY midamble  950  that is included in the PPDU  900  of  FIG.  9 A  (e.g., each PHY midamble  920  includes the PHY midamble  950 ), according to an embodiment. 
     The PHY midamble  950  consists of N LTF  LTFs  954 , where N LTF  is a suitable positive integer. In an embodiment, N LTF  generally corresponds to (e.g., is greater than or equal to) a number of spatial streams (N STS ) via which the PPDU  900  will be transmitted. In an embodiment, a number of LTFs (N LTF )  954  in the PHY midamble  950  is equal to a number of LTFs in the PHY preamble (i.e., the PHY preamble  904 ). In another embodiment, however, the number of LTFs  954  in the PHY midamble  950  is different from a number of LTFs in the PHY preamble (i.e., the PHY preamble  904 ). In an embodiment, each LTF  954  corresponds to a single OFDM symbol. 
     In other embodiments, the PHY midamble  950  includes one or more of: a short training field, a signal field, or another suitable field used in PHY preambles. 
     In some embodiments, M MA  corresponding to a PPDU is determined based on one or more parameters such as: communication protocol corresponding to the PPDU, format of the PPDU, transmission channel bandwidth of the PPDU, subcarrier frequency spacing, etc. In some embodiments, a corresponding set comprising of one or more values of M MA  is defined for each combination of one or more of the above parameters. For instance, for generation and transmission of a PPDU, a set of one or more values of M MA  is determined based on one or more above parameters of the PPDU, and a particular value of M MA  is selected from the determined set. The selected M MA  is used for generating the PHY midambles  920  in the data portion  908  of the PPDU, as described above with reference to  FIG.  9   . 
     In an embodiment, a value of M MA  is selected from a set of valid midamble periodicity values. In an embodiment, the set consists of two values. In another embodiment, the set consists of four values. 
     In an embodiment, a set of one or more values of M MA  is defined corresponding to a default subcarrier spacing value and scaled to determine corresponding one or more values of M MA  corresponding to other subcarrier spacing values. For instance, in an embodiment, a PPDU corresponding to a default subcarrier frequency spacing of 312.5 kHz is generated using an M MA  selected from a set {16 40 80 160}, a PPDU corresponding to a subcarrier frequency spacing of 156.25 kHz (i.e. N=2) is generated using an M MA  selected from a set {8 20 40 80}, a PPDU corresponding to a subcarrier frequency spacing of 78.125 kHz (i.e. N=4) is generated using an M MA  selected from a set {4 10 20 40}, and a PPDU corresponding to a subcarrier frequency spacing of 39.0625 kHz (i.e. N=8) is generated using an M MA  selected from a set {2 5 10 20}. 
     In another embodiment, respective sets of one or more values of M MA  are defined for corresponding subcarrier spacings. In another embodiment, respective sets of one or more values of M MA  are defined for corresponding channel frequency bandwidths. 
     LTFs included in the PHY preamble  904  and/or the PHY midamble  920  are encoded with training sequence values. In an embodiment, the training sequence values correspond to a predefined sequence comprising of +1s and −1s that are encoded on different subcarriers of OFDM symbols corresponding to the LTFs. In an embodiment, LTFs included in the PHY preamble  904  and/or the PHY midamble  920  may support different compression modes corresponding to a different number of subcarriers including a training sequence value. For instance, in various embodiments, not all subcarriers in an OFDM symbol corresponding to an LTF include a non-zero training sequence value. For example, in some embodiments, only every n-th subcarrier is set to a non-zero value, whereas the remaining subcarriers in between are set to zero. This results in a time-domain sequence that repeats n times. In an embodiment, only one of the instances of the time-domain sequence is transmitted, resulting in a 1/n compression of the time duration of the LTF. In an embodiment, every fourth subcarrier of an OFDM symbol corresponding to an LTF is configured to include a non-zero training sequence value whereas the remaining subcarriers in between are set to zero, and only one instance of the resulting time-domain sequence is transmitted (sometimes referred to as a “1×LTF compression mode”). In another embodiment, every second subcarrier of an OFDM symbol corresponding to an LTF is configured to include a non-training sequence value (sometimes referred to as a “2×LTF compression mode”) whereas the remaining subcarriers in between are set to zero, and only one instance of the resulting time-domain sequence is transmitted. In another embodiment, every subcarrier of an OFDM symbol corresponding to an LTF is configured to include a non-training sequence value (sometimes referred to as a “4×LTF compression mode”). In such embodiments, the 4×LTF compression mode has an LTF time duration that is four times an LTF time duration of the 1×LTF compression mode, and twice an LTF time duration of the 2×LTF compression mode; the 2×LTF compression mode has an LTF time duration that is twice the LTF time duration of the 1×LTF compression mode. 
     In an embodiment, LTFs included in the PHY preamble  904  and LTFs included in the PHY midambles  920  are formatted using corresponding different compression modes. In other embodiments, however, LTFs included in the PHY preamble  904  and LTFs included in the PHY midambles  920  are formatted using a same compression mode. 
     In an embodiment, a communication protocol defines different joint LTF compression/guard interval (GI) duration modes. For example, in some embodiments, only certain GI durations can be used with certain LTF compression modes, in an embodiment. In an embodiment, a signal field in the PHY preamble  904  of the PPDU  900  includes a subfield that indicates a LTF compression/GI duration mode that is used in the PPDU  900 , e.g., indicates an LTF compression used for LTFs in the preamble and/or the midambles  920  and a GI duration used at least in connection with the OFDM data symbols  916 . 
     In an embodiment, a signal field in the PHY preamble  904  of the PPDU  900  includes one or more fields that indicate a presence of the PHY midambles  920 , and/or the midamble periodicity M MA  corresponding to the PPDU  900 . In an embodiment wherein the PHY preamble  904  corresponds to the PHY preamble  304 / 404 , the HT-SIG  320 /HT-SIG  416  includes one or more fields that indicate a presence of the PHY midambles  920  and/or the midamble periodicity M MA . In an embodiment wherein the PHY preamble  904  corresponds to the PHY preamble  504 , the VHT-SIG-A  520  includes one or more fields that indicate a presence of the PHY midambles  920  and/or the midamble periodicity M MA . In an embodiment wherein the PHY preamble  904  corresponds to the PHY preamble  604 , the HE-SIG-A  624  includes one or more fields that indicate a presence of the PHY midambles  920  and/or the midamble periodicity M MA . In an embodiment, a presence of the PHY midambles  920  need not be signaled if the PPDU  900  is configured to compulsorily include the PHY midambles  920 . 
     In various embodiment, PPDUs discussed above with reference to  FIGS.  2 - 6    include a signal field, in a PHY preamble, that is smaller (e.g., includes a lesser number of bits) than signal fields discussed above (i.e., the HT-SIG  320 / 416 , the VHT-SIG  520 , and HE-SIG-A  624 ). For instance, in at least some embodiments, less information in the signal field is required than information in signal fields defined by the IEEE 802.11n, 802.11ac, and 802.11ax standards discussed above (i.e., the HT-SIG  320 / 416 , the VHT-SIG  520 , and HE-SIG-A  624 ). In some such embodiments, the HT-SIG  320 / 416 , the VHT-SIG  520 , or HE-SIG-A  624  may be replaced with a shorter signal (SIG) field that includes information such as GI duration, MCS, midamble periodicity M MA , number of LTFs, LTF compression mode, whether LDPC is utilized, whether BCC is utilize, whether DCM is utilized, whether STBC is utilized, a number of spatial streams N SS , etc., corresponding to the PPDU. In an embodiment, the shorter SIG field comprises of a single OFDM symbol. In other such embodiments, the HT-SIG  320 / 416 , the VHT-SIG  520 , or HE-SIG-A  624  is modified (as compared to the IEEE 802.11n Standard, the IEEE 802.11ac Standard, and the current draft of the IEEE 802.11ax Standard) to omit one or more subfields that are not required by the communication protocol used in the network  110  ( FIG.  1   ), and/or to include subfields that are not defined by the IEEE 802.11n Standard, the IEEE 802.11ac Standard, or the current draft of the IEEE 802.11ax Standard, for the HT-SIG  320 / 416 , the VHT-SIG  520 , or HE-SIG-A  624 . 
     In some embodiments, PPDUs such as discussed above are generated according to an extended range (ER) mode that provides a greater transmission mode and/or improves receiver sensitivity. For example, an ER mode uses one or more of i) DCM, ii) bit repetition, etc., and/or is restricted to using one or more MCSs that provide more robust modulation/encoding and lower data rates as compared to other MCSs that can be used in one or more non-ER modes, according to some embodiments. In embodiments that provide an ER mode, the signal field in the PHY preamble  304 , the PHY preamble  404 , the PHY preamble  504 , PHY preamble  604 , the PHY preamble  904 , etc., includes a subfield that indicates whether the ER mode is used in connection with the corresponding PPDU. 
       FIG.  10    is a flow diagram of an example method  1000  for generating and transmitting a PPDU in a vehicular communication network, according to an embodiment. With reference to  FIG.  1   , the method  1000  is implemented by a network interface device such as the network interface device  122  or the network interface device  162 , in various embodiments. For example, in one such embodiment, a PHY processor such as the PHY processor  130  or the PHY processor  170  is configured to implement at least a portion of the method  1000 . According to another embodiment, a MAC processor such as the MAC processor  126  or the MAC processor  166  is also configured to implement a portion of the method  1000 . 
     At block  1004 , one or more PHY parameters for the PPDU are selected. 
     In an embodiment, block  1004  includes selecting a downclocking ratio. In an embodiment, the downclocking ratio is selected based on a selected frequency bandwidth for the PPDU. In an embodiment, the downclocking ratio is selected from a set that consists of 1, 1/2, and 1/4. In an embodiment, the downclocking ratio 1 corresponds to a 20 MHz bandwidth, the downclocking ratio 1/2 corresponds to a 10 MHz bandwidth, and the downclocking ratio 1/4 corresponds to a 5 MHz bandwidth. In another embodiment, the downclocking ratios 1, 1/2, and/or 1/4 corresponds to other suitable bandwidths. In another embodiment, the downclocking ratio is selected from a set that additionally or alternatively includes downclocking ratios other than 1, 1/2, or 1/4, such as 1/5, 1/10, 1/20, etc. 
     In an embodiment, block  1004  includes selecting a frequency bandwidth for the PPDU from a set of frequency bandwidths defined by a communication protocol. In an embodiment, the set of frequency bandwidths includes at least two frequency bandwidths. In another embodiment, the set of frequency bandwidths includes at least three frequency bandwidths. In an embodiment, the set of frequency bandwidths includes at least 20 MHz, and 10 MHz. In an embodiment, the set of frequency bandwidths includes at least 20 MHz, 10 MHz, and 5 MHz. In an embodiment, the set of frequency bandwidths includes at least 10 MHz, and 5 MHz. In an embodiment, the set of frequency bandwidths includes at least 5 MHz and 2 MHz. In an embodiment, the set of frequency bandwidths includes at least 5 MHz and 1 MHz. In other embodiments, the set of frequency bandwidths additionally or alternatively includes one or more suitable frequency bandwidths other than 20 MHz, 10 MHz, 5 MHz, 2 MHz, and 1 MHz. 
     In an embodiment, all of the frequency bandwidths in the set of frequency bandwidths defined by the communication protocol correspond to a same OFDM subcarrier spacing. In an embodiment, the OFDM subcarrier spacing is 312.5 kHz. In another embodiment, the OFDM subcarrier spacing is 156.25 kHz. In another embodiment, the OFDM subcarrier spacing is 78.125 kHz. In another embodiment, the OFDM subcarrier spacing is 39.0625 kHz. In another embodiment, the OFDM subcarrier spacing is a suitable OFDM subcarrier spacing other than 312.5 kHz, 156.25 kHz, 78.125 kHz, and 39.0625 kHz. 
     In an embodiment, at least two of the frequency bandwidths in the set of frequency bandwidths defined by the communication protocol correspond to a first OFDM subcarrier spacing, while at least one of the frequency bandwidths in the set of frequency bandwidths correspond to a second OFDM subcarrier spacing. In an embodiment, the second OFDM subcarrier spacing is 1/N multiplied by the first OFDM subcarrier spacing, where N is a suitable positive integer less than 100. In an embodiment, N is an even positive integer less than 100. In an embodiment, N is two. In another embodiment, N is four. In another embodiment, N is eight. In another embodiment, N is sixteen. In an embodiment, the first OFDM subcarrier spacing is 312.5 kHz, and the second OFDM subcarrier spacing is 156.25 kHz. In another embodiment, the first OFDM subcarrier spacing is 156.25 kHz, and the second OFDM subcarrier spacing is 78.125 kHz. In another embodiment, the first OFDM subcarrier spacing is 78.125 kHz, the second OFDM subcarrier spacing is 39.0625 kHz. In another embodiment, the first OFDM subcarrier spacing and the second OFDM subcarrier are suitable OFDM subcarrier spacings other than 312.5 kHz, 156.25 kHz, 78.125 kHz, and 39.0625 kHz. 
     In an embodiment, at least three of the frequency bandwidths in the set of frequency bandwidths defined by the communication protocol correspond to different subcarriers spacings: a first OFDM subcarrier spacing, a second OFDM subcarrier spacing, and a third OFDM subcarrier spacing. In an embodiment, the second OFDM subcarrier spacing is 1/N multiplied by the first OFDM subcarrier spacing, and the third OFDM subcarrier spacing is 1/N multiplied by the second OFDM subcarrier spacing, where N is a suitable positive integer less than 100. In an embodiment, N is an even positive integer less than 100. In an embodiment, N is two. In another embodiment, N is four. In another embodiment, N is eight. In another embodiment, N is sixteen. In an embodiment, the first OFDM subcarrier spacing is 312.5 kHz, the second OFDM subcarrier spacing is 156.25 kHz, and the third OFDM subcarrier spacing is 78.125 kHz. In another embodiment, the first OFDM subcarrier spacing is 156.25 kHz, the second OFDM subcarrier spacing is 78.125 kHz, and the third OFDM subcarrier spacing is 39.0625 kHz. In another embodiment, the first OFDM subcarrier spacing and the second OFDM subcarrier are suitable OFDM subcarrier spacings other than 312.5 kHz, 156.25 kHz, 78.125 kHz, and 39.0625 kHz. 
     In an embodiment, block  1004  includes selecting a guard interval (GI) duration for the PPDU from a set of GI durations defined by a communication protocol. In an embodiment, the set of GI durations includes at least four GI durations. In an embodiment, the set of GI durations is {0.8 μs, 1.6 μs, 2.4 μs, 3.2 μs}. In another embodiment, the set of frequency bandwidths includes at least 0.8 μs, 1.6 μs, 2.4 μs, and 3.2 μs. In another embodiment, the set of frequency bandwidths includes at least three GI durations. In other embodiments, the set of GI durations additionally or alternatively includes one or more suitable frequency GI durations other than 0.8 μs, 1.6 μs, 2.4 μs, and 3.2 μs. In some embodiments, the communication protocol defines only a single GI duration and block  1004  does not include selecting a GI duration for the PPDU. 
     In some embodiments, the PPDU includes a PHY preamble having one or more training fields such as LTFs or other suitable training fields, and the communication protocol specifies a set of compression modes for the training fields such as described above. In an embodiment, block  1004  includes selecting a compression mode for training fields in the PPDU from a set of training field compression modes defined by the communication protocol. In some embodiments, the communication protocol does not define multiple training field compression modes and block  1004  does not include selecting a training field compression mode for the PPDU. 
     In an embodiment, the communication protocol defines a set of GI duration, training field compression mode tuples, and block  1004  includes selecting a GI duration, training field compression mode tuple for the PPDU from a set of set of GI duration, training field compression mode tuples defined by the communication protocol. In some embodiments, the communication protocol does not define multiple GI duration, training field compression mode tuples, and block  1004  does not include selecting a GI duration, training field compression mode tuple for the PPDU. 
     In an embodiment, block  1004  includes determining that one or more PHY midambles are to be included in the PPDU. In an embodiment in which different OFDM subcarrier spacings are permissible by the communication protocol (e.g., different frequency bandwidths correspond to different OFDM subcarrier spacings), block  1004  also includes selecting a set of permissible midamble periodicity values, from among a plurality of sets of permissible midamble periodicity values, based on the selected frequency bandwidth. For example, different sets of permissible midamble periodicity values correspond to different OFDM subcarrier spacings, in an embodiment. In an embodiment, block  1004  includes selecting a midamble periodicity value from the selected set of permissible midamble periodicity values. In some embodiments, the communication protocol defines only a single set of permissible midamble periodicity values, and block  1004  does not include selecting a set of permissible midamble periodicity values. In some embodiments, the communication protocol requires that all PPDUs require one or more PHY midambles, and block  1004  does not include determining whether one or more PHY midambles are to be included in the PPDU. In some embodiments, the communication protocol does not permit PHY midambles, and block  1004  does not include determining whether one or more PHY midambles are to be included in the PPDU. 
     In an embodiment, each PHY midamble includes one or more LTFs. In an embodiment, block  1004  includes selecting an LTF compression mode for LTFs in the one or midambles separately from selecting an LTF compression mode for LTFs in the PHY preamble. In another embodiment, the LTF compression mode for the one or midambles must be the same as the LTF compression mode for LTFs in the PHY preamble, and block  1004  does not include selecting an LTF compression mode for LTFs in the one or midambles separately from selecting an LTF compression mode for LTFs in the PHY preamble. 
     In an embodiment, block  1004  includes determining a number of spatial streams for transmitting the PPDU. In another embodiment, the communication protocol only permits transmitting using a single spatial stream, and block  1004  does not include determining a number of spatial streams for transmitting the PPDU. 
     In an embodiment, block  1004  includes determining whether to use STBC for the PPDU. In another embodiment, the communication protocol does not permit using STBC, and block  1004  does not include determining whether to use STBC for the PPDU. In another embodiment, the communication protocol requires STBC to be used for all PPDUs, and block  1004  does not include determining whether to use STBC for the PPDU. 
     In an embodiment, block  1004  includes determining whether to use DCM for the PPDU. In another embodiment, the communication protocol does not permit using DCM, and block  1004  does not include determining whether to use DCM for the PPDU. In another embodiment, the communication protocol requires using DCM for all PPDUs, and block  1004  does not include determining whether to use DCM for the PPDU. 
     In an embodiment, block  1004  includes determining whether to use an extended range mode for the PPDU. In another embodiment, the communication protocol does not define an extended range mode, and block  1004  does not include determining whether to use the extended range mode for the PPDU. 
     In an embodiment, block  1004  includes determining whether to use LDPC for the PPDU. In another embodiment, the communication protocol does not permit using LDPC, and block  1004  does not include determining whether to use LDPC for the PPDU. In another embodiment, the communication protocol requires using LDPC for all PPDUs, and block  1004  does not include determining whether to use LDPC for the PPDU. 
     At block  1008 , the PPDU is generated in accordance with the one or more PHY parameters determined and/or selected at block  1004 . In an embodiment, block  1008  includes generating the PPDU i) according to the downclocking ratio selected at block  1004 , and ii) as a downclocked version of a PPDU format defined by the IEEE 802.11n Standard. In an embodiment, generating the PPDU as a downclocked version of a PPDU format defined by the IEEE 802.11n Standard includes generating the PPDU to include a PHY preamble with one or more modified signal fields as compared to signal fields defined by the IEEE 802.11n Standard. 
     In another embodiment, block  1008  includes generating the PPDU i) according to the downclocking ratio selected at block  1004 , and ii) as a downclocked version of a PPDU format defined by the IEEE 802.11ac Standard. In an embodiment, generating the PPDU as a downclocked version of a PPDU format defined by the IEEE 802.11ac Standard includes generating the PPDU to include a PHY preamble with one or more modified signal fields as compared to signal fields defined by the IEEE 802.11ac Standard. 
     In an embodiment, block  1008  includes generating the PPDU i) according to the downclocking ratio selected at block  1004 , and ii) as a downclocked version of a PPDU format defined by the current draft of the IEEE 802.11ax Standard. In an embodiment, generating the PPDU as a downclocked version of a PPDU format defined by the current draft of the IEEE 802.11ax Standard includes generating the PPDU to include a PHY preamble with one or more modified signal fields as compared to signal fields defined by the current draft of the IEEE 802.11ax Standard. 
     In an embodiment, block  1008  includes generating the PPDU to include a PHY preamble, where the PHY preamble indicates at least some of the one or more PHY parameters determined and/or selected at block  1004 . In an embodiment, block  1008  includes generating the PHY preamble to indicate all of the one or more PHY parameters determined and/or selected at block  1004 . In an embodiment, generating the PHY preamble includes generating a signal field of the PHY preamble, and wherein the signal field is generated to indicate at least some of the one or more PHY parameters determined and/or selected at block  1004 . In an embodiment, block  1008  includes generating the signal field of the PHY preamble to indicate all of the one or more PHY parameters determined and/or selected at block  1004 . In an embodiment, the signal field is a single contiguous field that is immediately adjacent to i) a training field in the PHY preamble, and ii) another training field in the PHY preamble or a PHY data portion of the PPDU; and the entire signal field is modulated on a single OFDM symbol. 
     At block  1012 , the PPDU is transmitted in the vehicular communication network. 
       FIG.  11    is a flow diagram of another example method  1100  for generating and transmitting a PPDU in a vehicular communication network, according to an embodiment. With reference to  FIG.  1   , the method  1100  is implemented by a network interface device such as the network interface device  122  or the network interface device  162 , in various embodiments. For example, in one such embodiment, a PHY processor such as the PHY processor  130  or the PHY processor  170  is configured to implement at least a portion of the method  1100 . According to another embodiment, a MAC processor such as the MAC processor  126  or the MAC processor  166  is also configured to implement a portion of the method  1100 . 
     The method  1100  is a more specific embodiment of the method  1000  of  FIG.  10   . In some embodiments, the method  1100  is combined with one or more aspects of the method  1000  of  FIG.  10   . 
     At block  1104 , a frequency bandwidth via which a PPDU will be transmitted in a vehicular communication network is selected. In an embodiment, the frequency bandwidth is selected from a set of permissible frequency bandwidths. 
     At block  1108 , a set of permissible midamble periodicity values is selected from among a plurality of sets of permissible midamble periodicity values based on the frequency bandwidth selected at block  1104 . 
     At block  1112 , a midamble periodicity value is selected from the set of permissible midamble periodicity values selected at block  1108 . 
     In an embodiment, the set of permissible frequency bandwidths includes a first frequency bandwidth and a second frequency bandwidth; the first frequency bandwidth corresponds to a first OFDM subcarrier spacing, and the second frequency bandwidth corresponds to a second OFDM subcarrier spacing that is different than the first OFDM subcarrier spacing; the plurality of sets of permissible midamble periodicity values includes a first set of permissible midamble periodicity values corresponding to the first OFDM subcarrier spacing and a second set of permissible midamble periodicity values corresponding to the second OFDM subcarrier spacing; and the first set of permissible midamble periodicity values is different than the second set of permissible midamble periodicity values. 
     In an embodiment, the second OFDM subcarrier spacing is 1/N of the first OFDM subcarrier spacing; N is a positive integer; and respective values in the second set of permissible midamble periodicity values are 1/N multiplied by respective values in the first set of permissible midamble periodicity values. 
     In an embodiment, the first frequency bandwidth is 20 MHz; the second frequency bandwidth is 10 MHz; and N is two. 
     In another embodiment, the first frequency bandwidth is 10 MHz; the second frequency bandwidth is 5 MHz; and N is two. 
     In an embodiment, the first OFDM subcarrier spacing is 78.125 kHz; and the second OFDM subcarrier spacing is 39.0625 kHz. 
     In another embodiment, the first OFDM subcarrier spacing is 156.25 kHz; and the second OFDM subcarrier spacing is 78.125 kHz. 
     In an embodiment, the set of permissible frequency bandwidths further includes a third frequency bandwidth; the third frequency bandwidth corresponds to a third OFDM subcarrier spacing that is different than the first OFDM subcarrier spacing and the second OFDM subcarrier spacing; the plurality of sets of permissible midamble periodicity values further includes a third set of permissible midamble periodicity values corresponding to the third OFDM subcarrier spacing; and the third set of permissible midamble periodicity values is different than the first set of permissible midamble periodicity values and the second set of permissible midamble periodicity values. In an embodiment, the first frequency bandwidth is 20 MHz; the second frequency bandwidth is 10 MHz; and the third frequency bandwidth is 5 MHz. 
     In an embodiment, the set of permissible frequency bandwidths includes a first frequency bandwidth, a second frequency bandwidth, and a third frequency bandwidth. 
     At block  1116 , the PPDU is generated to include i) a PHY preamble and ii) a PHY data portion that includes one or more PHY midambles according to the selected midamble periodicity. Block  1116  includes generating the PHY preamble to include an indication of the selected midamble periodicity, and generating the PPDU to span the selected frequency bandwidth. 
     At block  1120 , the PPDU is transmitted in the vehicular communication network. 
     In other embodiments, the method  1100  further includes selecting a downclocking ratio based on the bandwidth selected at block  1104 ; and block  1116  includes generating the PPDU i) according to the downclocking ratio, and ii) as a downclocked version of a PPDU format defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the current draft of the IEEE 802.11ax Standard. 
     In other embodiments, the method  1100  further includes selecting a GI duration value from a set of at least four permissible GI duration values; and block  1116  includes: generating the PPDU according to the selected GI duration value, and generating the PHY preamble to include an indication of the selected GI duration value. 
     In an embodiment, block  1116  includes generating the PHY preamble to indicate the PHY midamble periodicity selected at block  1112 . In an embodiment, generating the PHY preamble includes generating a signal field of the PHY preamble, and wherein the signal field is generated to indicate the PHY midamble periodicity selected at block  1112 . In an embodiment, the signal field is a single contiguous field that is immediately adjacent to i) a training field in the PHY preamble, and ii) another training field in the PHY preamble or a PHY data portion of the PPDU; and the entire signal field is modulated on a single OFDM symbol. 
       FIG.  12    is a flow diagram of another example method  1200  for generating and transmitting a PPDU in a vehicular communication network, according to an embodiment. With reference to  FIG.  1   , the method  1200  is implemented by a network interface device such as the network interface device  122  or the network interface device  162 , in various embodiments. For example, in one such embodiment, a PHY processor such as the PHY processor  130  or the PHY processor  170  is configured to implement at least a portion of the method  1200 . According to another embodiment, a MAC processor such as the MAC processor  126  or the MAC processor  166  is also configured to implement a portion of the method  1200 . 
     The method  1200  is a more specific embodiment of the method  1000  of  FIG.  10   . In some embodiments, the method  1200  is combined with one or more aspects of the method  1000  of  FIG.  10   . In some embodiments, the method  1200  is combined with one or more aspects of the method  1100  of  FIG.  11   . 
     At block  1204 , a frequency bandwidth via which a PPDU will be transmitted in a vehicular communication network is selected. In an embodiment, the frequency bandwidth is selected from a set of permissible frequency bandwidths. In an embodiment, the set of permissible frequency bandwidths consists of three bandwidths: 20 MHz, 10 MHz, and 5 MHz. In another embodiment, the set of three permissible frequency bandwidths includes one or more bandwidths that are different than 20 MHz, 10 MHz, and 5 MHz. In another embodiment, the set of permissible frequency bandwidths includes at least three bandwidths, e.g., 20 MHz, 10 MHz, and 5 MHz, and/or one or more other suitable bandwidths. In another embodiment, the set of permissible frequency bandwidths consists of two bandwidths. 
     At block  1208 , an OFDM numerology corresponding to a particular bandwidth is selected from a set of multiple OFDM numerologies corresponding to respective bandwidths. In an embodiment, the multiple OFDM numerologies in the set are defined by one or more of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard. 
     In some embodiments, block  1208  is omitted. 
     At block  1212 , a downclocking ratio is selected based on the frequency bandwidth selected at block  1204  and the bandwidth corresponding to the OFDM numerology selected at block  1208 . As merely an illustrative example, if the bandwidth selected at block  1204  is 10 MHz and the particular bandwidth corresponding to the selected numerology is 20 MHz, the downclocking ratio is selected as 1/2. As another merely illustrative example, if the bandwidth selected at block  1204  is 5 MHz and the particular bandwidth corresponding to the selected numerology is 20 MHz, the downclocking is selected as 1/4. 
     In embodiments that omit block  1208 , the downclocking ratio is selected (at block  1212 ) based on the frequency bandwidth selected at block  1204 . 
     At block  1216 , the PPDU is generated i) based on the selected OFDM numerology and ii) according to the selected downclocking ratio. The PPDU is generated to span the selected frequency bandwidth. In embodiments that omit block  1208 , the PPDU is generated i) based on a particular OFDM numerology and ii) according to the selected downclocking ratio. In an embodiment, the particular OFDM numerology is defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard. In an embodiment, the PPDU is generated as a downclocked version of a PPDU format defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard. 
     At block  1220 , the PPDU is transmitted in the vehicular communication network. 
     Embodiment 1: A method for wireless communication, the method comprising: selecting, at a communication device, a frequency bandwidth via which a physical layer (PHY) protocol data unit (PPDU) will be transmitted in a vehicular communication network, wherein the frequency bandwidth is selected from a set of permissible frequency bandwidths; selecting, at a communication device, a downclocking ratio for generating the PPDU to span the selected frequency bandwidth, wherein the downclocking ratio is selected based on the selected frequency bandwidth; generating, at a communication device, the PPDU i) according to the selected downclocking ratio, and ii) based on an orthogonal frequency division multiplexing (OFDM) numerology defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard, wherein the PPDU is generated to span the selected frequency bandwidth; and transmitting, by the communication device, the PPDU in the vehicular communication network. 
     Embodiment 2: The method of embodiment 1, further comprising: selecting, at the communication device, the OFDM numerology from a set of multiple OFDM numerologies defined by one or more of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard. 
     Embodiment 3: The method of embodiment 2, wherein: the downclocking ratio is selected further based on the selected OFDM numerology. 
     Embodiment 4: The method of any of embodiments 1-3, wherein: the PPDU is generated as a downclocked version of a PPDU format defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard. 
     Embodiment 5: The method of any of embodiments 1-4, wherein: the frequency bandwidth is selected from a set consisting of 20 MHz, 10 MHz, and 5 MHz; and the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4. 
     Embodiment 6: The method of any of embodiments 1-5, further comprising: selecting, at the communication device, a set of permissible midamble periodicity values, from among a plurality of sets of permissible midamble periodicity values, based on the selected frequency bandwidth; and selecting, at the communication device, a midamble periodicity value from the selected set of permissible midamble periodicity values; wherein generating the PPDU comprises generating the PPDU to include i) a PHY preamble and ii) a PHY data portion that includes one or more PHY midambles according to the selected midamble periodicity, wherein the PHY preamble is generated to include an indication of the selected midamble periodicity. 
     Embodiment 7: The method of embodiment 6, wherein generating the PPDU includes generating the PHY preamble to include an indication of the selected midamble periodicity value. 
     Embodiment 8: The method of embodiment 7, wherein generating the PPDU includes generating the PHY preamble to further include an indication of the selected frequency bandwidth. 
     Embodiment 9: The method of embodiment 7, wherein: the indication of the selected midamble periodicity is included in a signal field of the PHY preamble; and the entire signal field is modulated onto a single OFDM symbol. 
     Embodiment 10: The method of any of embodiments 1-9, wherein: the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4; generating the PPDU according to the downclocking ratio 1 comprises generating the PPDU to have to a first orthogonal frequency division multiplexing (OFDM) subcarrier spacing; generating the PPDU according to the downclocking ratio 1/2 comprises generating the PPDU to have to a second OFDM subcarrier spacing that is one half of the first OFDM subcarrier spacing; and generating the PPDU according to the downclocking ratio 1/4 comprises generating the PPDU to have to a third OFDM subcarrier spacing that is one quarter of the first OFDM subcarrier spacing. 
     Embodiment 11: The method of embodiment 10, wherein: the first OFDM subcarrier spacing is 156.25 kHz; the second OFDM subcarrier spacing is 78.125 kHz; and the third OFDM subcarrier spacing is 39.0625 kHz. 
     Embodiment 12: The method of embodiment 10, wherein: the first OFDM subcarrier spacing is 312.5 kHz; the second OFDM subcarrier spacing is 156.25 kHz; and the third OFDM subcarrier spacing is 78.125 kHz. 
     Embodiment 13: The method of any of embodiments 1-12, further comprising: selecting, at the communication device, a guard interval (GI) duration value from a set of at least four permissible GI duration values; wherein generating the PPDU includes: generating the PPDU according to the selected GI duration value, and generating the PPDU to include a PHY preamble having an indication of the selected GI duration value. 
     Embodiment 14: An apparatus, comprising: a network interface device having one or more integrated circuit (IC) devices configured to: select a frequency bandwidth via which a physical layer (PHY) protocol data unit (PPDU) will be transmitted in a vehicular communication network, wherein the frequency bandwidth is selected from a set of permissible frequency bandwidths; select a downclocking ratio for generating the PPDU to span the selected frequency bandwidth, wherein the downclocking ratio is selected based on the selected frequency bandwidth; generate the PPDU i) according to the selected downclocking ratio, and ii) based on an orthogonal frequency division multiplexing (OFDM) numerology defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard, wherein the PPDU is generated to span the selected frequency bandwidth; and transmit the PPDU in the vehicular communication network. 
     Embodiment 15: The apparatus of embodiment 14, wherein the one or more IC devices are further configured to: select the OFDM numerology from a set of multiple OFDM numerologies defined by one or more of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard. 
     Embodiment 16: The apparatus of embodiment 15, wherein the one or more IC devices are further configured to: select the downclocking ratio further based on the selected OFDM numerology. 
     Embodiment 17: The apparatus of any of embodiments 14-16, wherein the one or more IC devices are further configured to: generate the PPDU as a downclocked version of a PPDU format defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard. 
     Embodiment 18: The apparatus of any of embodiments 14-17, wherein: the one or more IC devices are configured to select the frequency bandwidth from a set consisting of 20 MHz, 10 MHz, and 5 MHz; and the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4. 
     Embodiment 19: The apparatus of any of embodiments 14-18, wherein the one or more IC devices are further configured to: select a set of permissible midamble periodicity values, from among a plurality of sets of permissible midamble periodicity values, based on the selected frequency bandwidth; select a midamble periodicity value from the selected set of permissible midamble periodicity values; and generate the PPDU to include i) a PHY preamble and ii) a PHY data portion that includes one or more PHY midambles according to the selected midamble periodicity, wherein the PHY preamble is generated to include an indication of the selected midamble periodicity. 
     Embodiment 20: The apparatus of embodiment 19, wherein the one or more IC devices are further configured to generate the PHY preamble to include an indication of the selected midamble periodicity value. 
     Embodiment 21: The apparatus of embodiment 20, wherein the one or more IC devices are further configured to generate the PHY preamble to further include an indication of the selected frequency bandwidth. 
     Embodiment 22: The apparatus of embodiment 20, wherein the one or more IC devices are further configured to: include the indication of the selected midamble periodicity in a signal field of the PHY preamble; and modulate the entire signal field onto a single OFDM symbol. 
     Embodiment 23: The apparatus of any of embodiments 14-22, wherein: the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4; generating the PPDU according to the downclocking ratio 1 comprises generating the PPDU to have to a first orthogonal frequency division multiplexing (OFDM) subcarrier spacing; generating the PPDU according to the downclocking ratio 1/2 comprises generating the PPDU to have to a second OFDM subcarrier spacing that is one half of the first OFDM subcarrier spacing; and generating the PPDU according to the downclocking ratio 1/4 comprises generating the PPDU to have to a third OFDM subcarrier spacing that is one quarter of the first OFDM subcarrier spacing. 
     Embodiment 24: The apparatus of embodiment 23, wherein: the first OFDM subcarrier spacing is 156.25 kHz; the second OFDM subcarrier spacing is 78.125 kHz; and the third OFDM subcarrier spacing is 39.0625 kHz. 
     Embodiment 25: The apparatus of embodiment 23, wherein: the first OFDM subcarrier spacing is 312.5 kHz; the second OFDM subcarrier spacing is 156.25 kHz; the third OFDM subcarrier spacing is 78.125 kHz. 
     Embodiment 26: The apparatus of any of embodiments 14-25, wherein the one or more IC devices are further configured to: select a guard interval (GI) duration value from a set of at least four permissible GI duration values; generate the PPDU according to the selected GI duration value; and generate the PPDU to have a PHY preamble that includes an indication of the selected GI duration value. 
     Embodiment 27: A method for wireless communication, the method comprising: selecting, at a communication device, a downclocking ratio for generating a physical layer (PHY) protocol data unit (PPDU) to be transmitted in a vehicular communication network; generating, at a communication device, the PPDU i) according to the selected downclocking ratio, and ii) as a downclocked version of a PPDU format defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard; transmitting, by the communication device, the PPDU in the vehicular communication network. 
     Embodiment 28: The method of embodiment 27, further comprising: selecting, at the communication device, a frequency bandwidth via which the PPDU will be transmitted in the vehicular communication network, wherein the frequency bandwidth is selected from a set of permissible frequency bandwidths; wherein generating the PPDU comprises generating the PPDU to span the selected frequency bandwidth. 
     Embodiment 29: The method of embodiment 28, wherein: the downclocking ratio is selected based on the selected frequency bandwidth. 
     Embodiment 30: The method of embodiment 29, wherein: the frequency bandwidth is selected from a set consisting of 20 MHz, 10 MHz, and 5 MHz; the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4; the downclocking ratio 1 corresponds to the frequency bandwidth 20 MHz; the downclocking ratio 1/2 corresponds to the frequency bandwidth 10 MHz; and the downclocking ratio 1/4 corresponds to the frequency bandwidth 5 MHz. 
     Embodiment 31: The method of any of embodiments 28-30, further comprising: selecting, at the communication device, a set of permissible midamble periodicity values, from among a plurality of sets of permissible midamble periodicity values, based on the selected frequency bandwidth; and selecting, at the communication device, a midamble periodicity value from the selected set of permissible midamble periodicity values; wherein generating the PPDU comprises generating the PPDU to include i) a PHY preamble and ii) a PHY data portion that includes one or more PHY midambles according to the selected midamble periodicity, wherein the PHY preamble is generated to include an indication of the selected midamble periodicity. 
     Embodiment 32: The method of embodiment 31, wherein generating the PPDU includes generating the PHY preamble to include an indication of the selected midamble periodicity value. 
     Embodiment 33: The method of embodiment 31, wherein generating the PPDU includes generating the PHY preamble to further include an indication of the selected frequency bandwidth. 
     Embodiment 34: The method of embodiment 31, wherein: the indication of the selected midamble periodicity is included in a signal field of the PHY preamble; and the entire signal field is modulated onto a single OFDM symbol. 
     Embodiment 35: The method of any of embodiments 27-34, wherein: the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4; generating the PPDU according to the downclocking ratio 1 comprises generating the PPDU to have to a first orthogonal frequency division multiplexing (OFDM) subcarrier spacing; generating the PPDU according to the downclocking ratio 1/2 comprises generating the PPDU to have to a second OFDM subcarrier spacing that is one half of the first OFDM subcarrier spacing; and generating the PPDU according to the downclocking ratio 1/4 comprises generating the PPDU to have to a third OFDM subcarrier spacing that is one quarter of the first OFDM subcarrier spacing. 
     Embodiment 36: The method of embodiment 35, wherein: the first OFDM subcarrier spacing is 156.25 kHz; the second OFDM subcarrier spacing is 78.125 kHz; and the third OFDM subcarrier spacing is 39.0625 kHz. 
     Embodiment 37: The method of embodiment 35, wherein: the first OFDM subcarrier spacing is 312.5 kHz; the second OFDM subcarrier spacing is 156.25 kHz; and the third OFDM subcarrier spacing is 78.125 kHz. 
     Embodiment 38: The method of any of embodiments 27-37, further comprising: selecting, at the communication device, a guard interval (GI) duration value from a set of at least four permissible GI duration values; wherein generating the PPDU includes: generating the PPDU according to the selected GI duration value, and generating the PPDU to include a PHY preamble having an indication of the selected GI duration value. 
     Embodiment 39: An apparatus, comprising: a network interface device having one or more integrated circuit (IC) devices configured to: select a downclocking ratio for generating a physical layer (PHY) protocol data unit (PPDU) to be transmitted in a vehicular communication network, generate the PPDU i) according to the selected downclocking ratio, and ii) as a downclocked version of a PPDU format defined by one of a) the IEEE 802.11n Standard, b) the IEEE 802.11ac Standard, and c) the IEEE 802.11ax Standard, and transmit the PPDU in the vehicular communication network. 
     Embodiment 40: The apparatus of embodiment 39, wherein the one or more IC devices are further configured to: select a frequency bandwidth via which the PPDU will be transmitted in the vehicular communication network, wherein the frequency bandwidth is selected from a set of permissible frequency bandwidths; wherein generating the PPDU comprises generating the PPDU to span the selected frequency bandwidth. 
     Embodiment 41: The apparatus of embodiment 40, wherein the one or more IC devices are further configured to: select the downclocking ratio based on the selected frequency bandwidth. 
     Embodiment 42: The apparatus of embodiment 41, wherein: the one or more IC devices are configured to select the frequency bandwidth from a set consisting of 20 MHz, 10 MHz, and 5 MHz; the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4; the downclocking ratio 1 corresponds to the frequency bandwidth 20 MHz; the downclocking ratio 1/2 corresponds to the frequency bandwidth 10 MHz; and the downclocking ratio 1/4 corresponds to the frequency bandwidth 5 MHz. 
     Embodiment 43: The apparatus of any of embodiments 40-42, wherein the one or more IC devices are further configured to: select a set of permissible midamble periodicity values, from among a plurality of sets of permissible midamble periodicity values, based on the selected frequency bandwidth; select a midamble periodicity value from the selected set of permissible midamble periodicity values; and generate the PPDU to include i) a PHY preamble and ii) a PHY data portion that includes one or more PHY midambles according to the selected midamble periodicity, wherein the PHY preamble is generated to include an indication of the selected midamble periodicity. 
     Embodiment 44: The apparatus of embodiment 43, wherein the one or more IC devices are further configured to generate the PHY preamble to include an indication of the selected midamble periodicity value. 
     Embodiment 45: The apparatus of embodiment 44, wherein the one or more IC devices are further configured to generate the PHY preamble to further include an indication of the selected frequency bandwidth. 
     Embodiment 46: The apparatus of embodiment 44, wherein the one or more IC devices are further configured to: include the indication of the selected midamble periodicity in a signal field of the PHY preamble; and modulate the entire signal field onto a single OFDM symbol. 
     Embodiment 47: The apparatus of any of embodiments 39-46, wherein: the downclocking ratio is selected from a set consisting of 1, 1/2, and 1/4; generating the PPDU according to the downclocking ratio 1 comprises generating the PPDU to have to a first orthogonal frequency division multiplexing (OFDM) subcarrier spacing; generating the PPDU according to the downclocking ratio 1/2 comprises generating the PPDU to have to a second OFDM subcarrier spacing that is one half of the first OFDM subcarrier spacing; and generating the PPDU according to the downclocking ratio 1/4 comprises generating the PPDU to have to a third OFDM subcarrier spacing that is one quarter of the first OFDM subcarrier spacing. 
     Embodiment 48: The apparatus of embodiment 47, wherein: the first OFDM subcarrier spacing is 156.25 kHz; the second OFDM subcarrier spacing is 78.125 kHz; and the third OFDM subcarrier spacing is 39.0625 kHz. 
     Embodiment 49: The apparatus of embodiment 47, wherein: the first OFDM subcarrier spacing is 312.5 kHz; the second OFDM subcarrier spacing is 156.25 kHz; the third OFDM subcarrier spacing is 78.125 kHz. 
     Embodiment 60: The apparatus of any of embodiments 39-49, wherein the one or more IC devices are further configured to: select a guard interval (GI) duration value from a set of at least four permissible GI duration values; generate the PPDU according to the selected GI duration value; and generate the PPDU to have a PHY preamble that includes an indication of the selected GI duration value. 
     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 the processor, cause the processor to perform various acts. 
     When implemented in hardware, the hardware may comprise one or more of discrete components, one or more integrated circuits (ICs), 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.