Patent Publication Number: US-11656349-B1

Title: Trigger frame for NDP ranging

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/977,643, now U.S. Pat. No. 10,928,505, entitled “Null Data Packet (NDP) Announcement Frame and Trigger Frame for NDP Ranging,” filed on May 11, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/505,443, entitled “Null Data Packet Announcement (NDPA) and Trigger for EFTM Null Data Packet (NDP) Ranging,” filed on May 12, 2017. Both of the applications identified above are expressly incorporated herein by reference in their entireties. 
    
    
     FIELD OF TECHNOLOGY 
     The present disclosure relates generally to wireless communication systems, and more particularly to communication exchanges between wireless communication devices for ranging measurements among the wireless communication devices. 
     BACKGROUND 
     Wireless local area networks (WLANs) have evolved rapidly over the past decade, and development of WLAN standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.11 Standard family has improved single-user peak data throughput. For example, the IEEE 802.11b Standard specifies a single-user peak throughput of 11 megabits per second (Mbps), the IEEE 802.11a and 802.11g Standards specify a single-user peak throughput of 54 Mbps, the IEEE 802.11n Standard specifies a single-user peak throughput of 600 Mbps, and the IEEE 802.11ac Standard specifies a single-user peak throughput in the gigabits per second (Gbps) range. Future standards promise to provide even greater throughput, such as throughputs in the tens of Gbps range. 
     Some mobile communication devices include a WLAN network interface and satellite positioning technology, such as global positioning system (GPS) technology. GPS technology in mobile communication devices is useful for navigating to a desired location, for example. However, GPS technology does not typically provide accurate location information when a GPS receiver is not in direct sight of a GPS satellite, and thus GPS technology is often not useful for providing location information while a mobile communication device is within a building such as an airport, a shopping mall, etc., within a tunnel, etc. 
     Techniques for determining a position of a communication device using WLAN technology are now under development. For example, a distance between a first communication and a second communication device is determined by measuring a time of flight of WLAN transmissions between the first communication device and the second communication device, and the determined distance. Similarly, distances between the first communication device and multiple third communication devices are determined. Then, the determined distances are used to estimate a location of the first communication device by employing, for example, a triangulation technique. For a first communication device having multiple antennas, an angle of departure (AoD) of a WLAN transmission can be determined. Similarly, for a second communication device having multiple antennas, an angle of arrival (AoA) of the WLAN transmission from the first communication device can be determined. The AoD and the AoA, along with the determined distances, can be also be used for estimating the location of the first communication device. 
     SUMMARY 
     In an embodiment, a method for performing a multi-user (MU) ranging measurement procedure includes: generating, at a first communication device, a trigger frame for use in a MU ranging measurement exchange session with a plurality of second communication devices, wherein the trigger frame includes trigger type information for indicating a type of frame exchange to which the trigger frame corresponds, wherein the trigger frame is generated to include trigger type information that indicates the trigger frame is for prompting an uplink (UL) MU null data packet (NDP) transmission as part of the MU ranging measurement exchange session; transmitting, by the first communication device, the trigger frame as part of the ranging measurement exchange session with the plurality of second communication devices; receiving, at the first communication device, an UL MU NDP transmission from the plurality of second communication devices as part of the ranging measurement exchange session with the plurality of second communication devices, wherein the UL MU NDP transmission is responsive to the trigger frame; transmitting, by the first communication device, a downlink (DL) NDP as part of the ranging measurement exchange session with the plurality of second communication devices; and receiving, at the first communication device, an UL MU feedback transmission that includes a plurality of feedback packets from the plurality of second communication devices, wherein the plurality of feedback packets includes feedback information regarding the MU ranging measurement exchange session. 
     In another embodiment, an apparatus comprises: a network interface device associated with a first communication device. The network interface device includes one or more integrated circuit (IC) devices configured to: generate a trigger frame for use in a MU ranging measurement exchange session with a plurality of second communication devices, wherein the trigger frame includes trigger type information for indicating a type of frame exchange to which the trigger frame corresponds, wherein the trigger frame is generated to include trigger type information that indicates the trigger frame is for prompting an UL MU NDP transmission as part of the MU ranging measurement exchange session; transmit the trigger frame as part of the ranging measurement exchange session with the plurality of second communication devices; receive the UL MU NDP transmission from the plurality of second communication devices as part of the ranging measurement exchange session with the plurality of second communication devices, wherein the UL MU NDP transmission is responsive to the trigger frame; transmit a DL NDP as part of the ranging measurement exchange session with the plurality of second communication devices; and receive an UL MU feedback transmission that includes a plurality of feedback packets from the plurality of second communication devices, wherein the plurality of feedback packets includes feedback information regarding the MU ranging measurement exchange session. 
     In yet another embodiment, a method for performing a MU ranging measurement procedure includes: receiving, at a first communication device, a trigger frame from a second communication device, wherein the trigger frame includes trigger type information for indicating a type of frame exchange to which the trigger frame corresponds; processing, at the first communication device, the trigger frame, including determining that the trigger frame corresponds to a MU ranging measurement exchange session based on determining that the trigger type information indicates the trigger frame is for prompting an UL MU NDP transmission as part of the MU ranging measurement exchange session; in response to determining that the trigger type information indicates the trigger frame is for prompting the UL MU NDP transmission as part of the MU ranging measurement exchange session, transmitting, by the first communication device, an UL NDP as part of the UL MU NDP transmission; receiving, at the first communication device, a DL NDP as part of the MU ranging measurement exchange session; generating, at the first communication device, feedback information regarding the MU ranging measurement exchange session; and transmitting, by the first communication device, the feedback information in a feedback packet as part of an UL MU feedback transmission as part of the MU ranging measurement exchange session. 
     In still another embodiment, an apparatus comprises: a network interface device associated with a first communication device. The network interface device includes one or more IC devices that are configured to: receive a trigger frame from a second communication device, wherein the trigger frame includes trigger type information for indicating a type of frame exchange to which the trigger frame corresponds; process the trigger frame, including determining that the trigger frame corresponds to an MU ranging measurement exchange session based on determining that trigger type information indicates the trigger frame is for prompting an UL MU NDP transmission as part of the MU ranging measurement exchange session; in response to determining that the trigger type information indicates the trigger frame is for prompting the UL MU NDP transmission as part of the MU ranging measurement exchange session, transmit an UL NDP as part of the UL MU NDP transmission; receive a DL NDP as part of the MU ranging measurement exchange session; generate feedback information regarding the MU ranging measurement exchange session; and transmit the feedback information in a feedback packet as part of an UL MU feedback transmission as part of the MU ranging measurement exchange session. 
    
    
     
       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 A  is a diagram of an example multi-user (MU) ranging measurement exchange in an MU ranging measurement procedure, according to an embodiment. 
         FIG.  2 B  is a timing diagram of the example MU ranging measurement exchange of  FIG.  2 A , according to an embodiment. 
         FIG.  3 A  is a diagram of an example single-user (SU) ranging measurement exchange in an SU ranging measurement procedure, according to an embodiment. 
         FIG.  3 B  is a timing diagram of the example SU ranging measurement exchange of  FIG.  3 A , according to the embodiment. 
         FIG.  4    is a diagram of another example SU ranging measurement exchange in an SU ranging measurement procedure, according to an embodiment. 
         FIG.  5    is a diagram of another example SU ranging measurement exchange in an SU ranging measurement procedure, according to an embodiment. 
         FIG.  6 A  is an example frame format of a trigger frame, according to an embodiment. 
         FIG.  6 B  is an example format of a common information field within a trigger frame, according to an embodiment. 
         FIG.  6 C  is an example format of a user information field within a trigger frame, according to an embodiment. 
         FIG.  7 A  is an example frame format of a null data packet announcement (NDPA) frame, according to an embodiment. 
         FIG.  7 B  is an example format of a client station information field within an NDPA frame, according to an embodiment. 
         FIG.  8 A  is another example format of an NDPA frame, according to an embodiment. 
         FIG.  8 B  is an example format of a client station information field within an NDPA frame, according to an embodiment. 
         FIG.  9    is a flow diagram of an example method for performing a ranging measurement procedure, according to an embodiment. 
         FIG.  10    is a flow diagram of another example method for performing a ranging measurement procedure, according to an embodiment. 
         FIG.  11    is a flow diagram of another example method for performing a MU ranging measurement procedure, according to an embodiment. 
         FIG.  12    is a flow diagram of another example method for performing a MU ranging measurement procedure, according to an embodiment 
     
    
    
     DETAILED DESCRIPTION 
     Frame formats for ranging measurement procedures and ranging measurement techniques described below are discussed in the context of wireless local area networks (WLANs) that utilize protocols the same as or similar to protocols defined by the 802.11 Standard from the Institute of Electrical and Electronics Engineers (IEEE) merely for explanatory purposes. In other embodiments, however, ranging measurement 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), 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  122  includes a medium access control (MAC) processor  126  and a physical layer (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  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  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 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  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 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 PHY data units such as PHY protocol data units (PPDUs) for transmission via the antennas  138 . Similarly, the PHY processor  130  may be configured to receive PHY data units that were received via the antennas  138 , and extract MAC layer data units encapsulated within the PHY data units. 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  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  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  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 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  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 PHY data units such as PPDUs for transmission via the antennas  178 . Similarly, the PHY processor  170  may be configured to receive PHY data units that were received via the antennas  178 , and extract MAC layer data units encapsulated within the PHY data units. 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. 
       FIG.  2 A  is a diagram of an example multi-user (MU) ranging measurement exchange  200  in an MU ranging measurement procedure, according to an embodiment. The diagram  200  is described in the context of the example network  110  merely for explanatory purposes. In some embodiments, signals illustrated in  FIG.  2 A  are generated by other suitable communication devices in other suitable types of wireless networks. 
     The MU ranging measurement exchange  200  corresponds to an AP-initiated MU ranging measurement exchange, according to an embodiment. The MU ranging measurement exchange  200  includes an uplink (UL) null data packet (NDP) frame exchange  204 , a downlink (DL) NDP transmission portion  208 , a DL feedback (FB) frame exchange  210 , and an UL FB frame exchange  212 . In an embodiment, the uplink UL NDP frame exchange  204 , the DL NDP transmission portion  208 , the DL FB frame exchange  210 , and the UL FB frame exchange  212  occur within a single transmit opportunity period (TXOP). In another embodiment, the uplink UL NDP frame exchange  204 , the DL NDP transmission portion  208 , the DL FB frame exchange  210 , and the UL FB frame exchange  212  do not occur within a single TXOP. For example, the uplink UL NDP frame exchange  204  and the DL NDP transmission portion  208  occur within a single TXOP, whereas the DL FB frame exchange  210  and the UL FB frame exchange  212  occur after the single TXOP (e.g., in another TXOP or in multiple other TXOPs). 
     In the UL NDP exchange  204 , a first communication device (e.g., the AP  114 ) transmits a DL PPDU  216  that includes a trigger frame to cause a group of multiple second communication devices (e.g., client stations  154 ) to simultaneously transmit, as part of an uplink (UL) MU transmission  220 , UL null data packets (NDPs)  224 . In an embodiment, the trigger frame in the PPDU  216  is a type of trigger frame specifically for initiating an MU ranging measurement exchange such as the MU ranging measurement exchange  200 . The trigger frame in the PPDU  216  causes multiple client stations  154  to begin simultaneously transmitting the UL MU transmission  220  a defined time period after an end of the PPDU  216 . In an embodiment, the defined time period is a short interframe space (SIFS) as defined by the IEEE 802.11 Standard. In other embodiments, another suitable time period is utilized. 
     In an embodiment, the UL MU transmission  220  includes an UL MU multiple input, multiple output (MIMO) transmission having two or more UL NDPs  224  from multiple client stations  154 , e.g., STA1, STA2, STA3, and STA4. The two or more of the UL NDPs  224  are transmitted within a same frequency band via different spatial streams (e.g., MU-MIMO). In an embodiment in which the UL MU transmission includes an UL MU-MIMO transmission, the AP  114  uses a P matrix to demap the different NDPs from the different spatial streams. In another embodiment, the UL MU transmission  220  includes an UL orthogonal frequency division multiple access (OFDMA) transmission having two or more UL NDPs  224  from multiple client stations  154 , e.g., STA1, STA2, STA3, and STA4, in different respective frequency bandwidth portions. In yet another embodiment, three or more UL NDP packets  224  transmitted using a combination of UL MU-MIMO and UL OFDMA, where at least two NDPs are transmitted using MU-MIMO in a same frequency bandwidth portion via different spatial streams, and at least one NDP is transmitted in at least one other different frequency bandwidth portion. The UL NDPs  224  include PHY preambles having one or more short training fields (STFs), one or more long training fields (LTFs) and one or more signal fields, in an embodiment. The UL NDPs  224  omit data portions. 
     When transmitting the UL NDPs  224 , each client station  154  records a time t 1,k  at which the client station  154  began transmitting a particular portion of the UL NDP  224 , where k is an index indicating the particular client station  154 , e.g. the time at which the client station  154  began transmitting particular training fields in the UL NDP  224 , e.g., HE-LTFs. Similarly, when the AP  114  receives each UL NDP  224 , the AP  114  records a time t 2,k  at which the AP  114  began receiving a particular portion of the UL NDP  224 , e.g. the time at which the AP  114  began receiving particular training fields in the UL NDP  224 , e.g., HE-LTFs. 
     In some embodiments, when transmitting the UL NDPs  224 , each of at least some of the client stations  154  (e.g., client stations  154  with multiple antennas  174 ) records an angle of departure, AoD 1,k , at which the UL NDP  224  left the antennas  178  of the client station  154 . Similarly, when the AP  114  receives each UL NDP  224 , the AP  114  records an angle of arrival, AoA 1,k , at which the UL NDP  224  arrived at the antennas  138  of the AP  114 . 
       FIG.  2 B  is a timing diagram of the example MU ranging measurement exchange  200  of  FIG.  2 A . As illustrated in  FIG.  2 B , each client station  154  records the time t 1,k  at which the client station  154  began transmitting the UL NDP  224 , and records the AoD 1,k  at which the UL NDP  224  left the antennas  178  of the client station  154 . Additionally, the AP  114  records the time t 2,k  at which the AP  114  began receiving each UL NDP  224 , and the AoA 1,k , at which each UL NDP  224  arrived at the antennas  138  of the AP  114 . 
     Referring now to  FIGS.  2 A and  2 B , responsive to the UL MU transmission  220 , the AP  114  begins transmitting a DL PPDU  228  that includes an NDP announcement (NDPA) frame a defined time period after an end of the UL MU transmission  220 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The NDPA frame in the PPDU  228  is configured to cause the client stations  154  to be prepared to receive an NDP from the AP  114 , according to an embodiment. 
     The AP  114  generates a DL PPDU  232  and begins transmitting the DL PPDU  232  a defined time period after an end of the DL PPDU  228 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The DL PPDU  232  is a MU PPDU that includes DL NDPs  236  to respective client stations  154 . In another embodiment, the AP  114  transmits a single DL NDP  236  using a SU DL transmission (e.g., with a broadcast destination address) to the client stations  154 , where all receiving STAs share a same group of HE LTF(s) for measurement. In another embodiment, the AP  114  transmits a single DL NDP  236  using a SU DL transmission (e.g., with a broadcast destination address) to the client stations  154 , where each receiving client station  154  decodes a respective group of HE LTF(s) for a respective measurement. The DL NDP(s)  236  include PHY preamble(s) having one or more STFs, one or more LTFs and one or more signal fields, in an embodiment. The DL NDP(s)  236  omit data portions. In an embodiment, different DL NDPs  236  are transmitted in different frequency bandwidth portions (e.g., OFDMA). In some embodiments, two or more of the DL NDPs  236  are transmitted within a same frequency band (e.g., two or more of the DL NDPs  236  span the same frequency band) using different spatial streams (e.g., the two or more DL NDPs  236  are transmitted using MU-MIMO). In another embodiment, a single DL NDP  236  is broadcast to the client stations  154 . 
     When transmitting the DL NDP(s)  236 , the AP  114  records a time t 3,k  at which the AP  114  began transmitting a particular portion of the DL NDP(s)  236 , e.g. the time at which the AP  114  began transmitting a particular training field portion in the DL NDP(s)  236 , e.g., HE-LTFs. Similarly, when each client station  154  receives the corresponding DL NDP  236 , the client station  154  records a time t 4,k  at which the client station  154  began receiving a particular portion of the DL NDP  236 , e.g. the time at which the client station  154  began receiving particular training fields in the DL NDP  236 , e.g., HE-LTFs. As illustrated in  FIG.  2 B , the AP  114  records the time t 3,k  at which the AP  114  began transmitting the DL NDP  236 , and the client station  154  records the time t 4,k  at which the client station  154  began receiving the DL NDP  236 . 
     In some embodiments, when transmitting the DL NDP  236 , the AP  114  records an AoD 2,k  at which the DL NDP  236  left the antennas  138  of the AP  114 . Similarly, when the client station  154  receives the DL NDP  236 , the client station  154  records an AoA 2,k  at which the DL NDP  236  arrived at the antennas  178  of the client station  154 . 
     In some embodiments, the MU ranging measurement exchange  200  omits the DL PPDU  228 . For example, the AP  114  begins transmitting the DL PPDU  232  a defined time period after an end of the UL MU transmission  220 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. 
     The DL FB exchange  210  includes a DL PPDU  240  (which may be a DL OFDMA transmission or a DL MU-MIMO transmission) having FB frames  244  for multiple client stations  154 , e.g., STA1, STA2, STA3, and STA4. The FB frames  244  are illustrated in  FIG.  2 A  as being transmitted in different frequency bandwidth portions. In some embodiments, two or more of the FB frames  244  are transmitted within a same frequency band (e.g., two or more of the FB frames  244  span the same frequency band) using different spatial streams (e.g., the two or more FB frames  244  are transmitted using MU-MIMO). 
     In some embodiments, the DL PPDU  240  is transmitted a defined time period after an end of the DL PPDU  232 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. In other embodiments, the DL PPDU  240  is transmitted after some delay. As discussed above, in some embodiments, the DL PPDU  240  is not transmitted within a same TXOP as the DL PPDU  232 . 
     The FB frames  244  respectively include the recorded times t 2,k  and t 3,k . In some embodiments, each of one or more FB frames  244  respectively includes the recorded angles AoA 1,k  and AoD 2,k . In some embodiments, the FB frames  244  optionally also include respective channel estimate information determined by the AP  114  based on reception of the UL NDPs  224 . 
     After receipt of the FB frames  244 , one or more of the client stations  154  respectively calculate one or more respective of times-of-flight between the AP  114  and the one or more client stations  154  using the recorded times t 1,k , t 2,k , t 3,k , and t 4,k , according to an embodiment. Any suitable technique, including currently known techniques, may be utilized to calculate a time-of-flight using the recorded times t 1,k , t 2,k , t 3,k , and t 4,k . Respective distances between the AP  114  and the client stations  154  may be calculated using the calculated times-of-flight, e.g., by respectively multiplying the times-of-flight by the speed of light, according to an embodiment. 
     In some embodiments, one or more of the client stations  154  calculates estimated positions of one or more of the client stations using the calculated times-of-flight. For example, the client station  154 - 1  uses triangulation techniques to calculate an estimated positions of the client station  154 - 1  using the calculated time-of-flight. In some embodiments, the client station  154 - 1  calculates an estimated position of the client station also using the recorded angles AoD 1,k , AoA 1,k , AoD 2,k , and AoA 2,k . For example, the recorded angles AoD 1,k , AoA 1,k , AoD 2,k , and AoA 2,k  are used as part of a triangulation algorithm for determining a position of the client station  154 - 1 . 
     Responsive to receipt of the FB frames  244 , the client stations  154  generate an UL MU transmission  250  (which may be an UL OFDMA transmission or an UL MU MIMO transmission) that includes respective ACK frames  254  from respective client stations, according to an embodiment. The client stations  154  transmit as part of the UL MU transmission  250  a defined time period after an end of the DL transmission  240 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The ACK frames  254  are illustrated in  FIG.  2 A  as being transmitted in different frequency bandwidth portions. In some embodiments, two or more of the ACK frames  254  are transmitted within a same frequency band (e.g., two or more of the ACK frames  254  span the same frequency band) using different spatial streams (e.g., the two or more ACK frames  254  are transmitted using MU-MIMO). In another embodiment, the client station  154  do not transmit ACK frames  254  even after receiving and correctly decoding the DL FB frame  244 . 
     In an embodiment, the AP  114  transmits a DL PPDU  260  a defined time period after an end of the UL MU transmission  250 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The PPDU  260  includes a trigger frame to cause the group of client stations  154  to simultaneously transmit, as part of an UL MU transmission  264 , uplink PPDUs  268  that include ranging measurement feedback. The trigger frame in the PPDU  260  causes multiple client stations  154  to begin simultaneously transmitting the UL MU transmission  264  a defined time period after an end of the PPDU  260 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. 
     The UL MU transmission  264  (which may be an UL OFDMA transmission or an UL MU-MIMO transmission) includes UL PPDUs  268  from multiple client stations  154 , e.g., STA1, STA2, STA3, and STA4. The UL PPDUs  268  are illustrated in  FIG.  2 A  as being transmitted in different frequency bandwidth portions. In some embodiments, two or more of the UL PPDUs  268  are transmitted within a same frequency band (e.g., two or more of the UL PPDUs  268  span the same frequency band) using different spatial streams (e.g., the two or more UL PPDUs  268  are transmitted using MU-MIMO). 
     The UL PPDUs  268  correspond to uplink ranging measurement feedback packets. The PPDUs  268  respectively include the recorded times t 1,k  and t 4,k . In some embodiments, each of one or more PPDUs  268  respectively includes the recorded angles AoD 1,k  and AoA 2,k . In some embodiments, the PPDUs  268  optionally also include respective channel estimate information determined by the client station  154  based on reception of the DL NDP  236 . 
     After receipt of the PPDUs  268 , the AP  114  calculates respective of times-of-flight between the AP  114  and the client stations  154  using the recorded times t 1,k , t 2,k , t 3,k , and t 4,k , according to an embodiment. Any suitable technique, including currently known techniques, may be utilized to calculate a time-of-flight using the recorded times t 1,k , t 2,k , t 3,k , and t 4,k . Respective distances between the AP  114  and the client stations  154  may be calculated using the calculated times-of-flight, e.g., by respectively multiplying the times-of-flight by the speed of light, according to an embodiment. 
     In some embodiments, the AP  114  calculates estimated positions of one or more of the client stations using the calculated times-of-flight. For example, the AP  114  uses triangulation techniques to calculate estimated positions of one or more of the client stations using the calculated times-of-flight. In some embodiments, the AP  114  calculates estimated positions of one or more of the client stations also using the recorded angles AoD 1,k , AoA 1,k , AoD 2,k , and AoA 2,k . For example, the recorded angles AoD 1,k , AoA 1,k , AoD 2,k , and AoA 2,k  are used as part of a triangulation algorithm for determining positions of communication devices. 
     In other embodiments, each of one or more of the client stations  154  calculates a respective estimated position of the client station  154  using the calculated times-of-flight. For example, the client station  154  uses triangulation techniques to calculate an estimated position of the client station  154  using the calculated times-of-flight. In some embodiments, the client station  154  calculates an estimated position of the client station  154  also using the recorded angles AoD 1,k , AoA 1,k , AoD 2,k , and AoA 2,k . For example, the recorded angles AoD 1,k , AoA 1,k , AoD 2,k , and AoA 2,k  are used as part of a triangulation algorithm for determining a position of the client station  154 . 
     In another embodiment, the order, in time, of the DL FB exchange  210  and the UL FB exchange  212  is reversed, and the UL FB exchange  212  occurs before the DL FB exchange  210 . In some embodiments, the DL FB exchange  210  is omitted. In some embodiments, the UL FB exchange  212  is omitted. 
     As discussed above, DL FB PPDUs  244  may include, in addition to recorded times t 2,k  and t 3,k , one or more of i) the recorded angles AoA 1,k , ii) the recorded angles AoD 2,k , and iii) channel estimate information determined by the AP  114  based on reception of the UL NDPs  224 . In some embodiments, channel estimate information can be conveyed in different granularities. For example, in some embodiments, one respective channel measurement is provided for each OFDM tone, or one respective channel measurement is provided for each group of n OFDM tones, where n is an integer greater than one. Sending one respective channel measurement for each group of n OFDM tones requires less total channel estimate information to be conveyed across the wireless channel medium, as opposed to sending one respective channel measurement for each OFDM tone. In some embodiments, a channel measurement can be quantized to different numbers of bits. For instance, a channel measurement can represented using m bits, where m is a positive integer chosen from a suitable set of different positive integers corresponding to different quantization granularities. Sending channel measurements that are each represented using m bits requires less total channel estimate information to be conveyed across the wireless channel medium, as opposed to sending channel measurements that are each represented using m+2 bits, for example. Thus, different granularities channel estimate information correspond to different value(s) of one or both of n and m, according to an embodiment. 
     In some embodiments, one or more client stations  154  (e.g., one or more MAC processors in the client stations  154  (e.g., the MAC processor  166 )) determine that the AP  114  is to include, in one or more of the DL FB PPDUs  244 , one or more of i) recorded angle(s) AoA 1,k , ii) recorded angle(s) AoD 2,k , and iii) channel estimate information determined by the AP  114  based on reception of the UL NDPs  224 . In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines the granularity(ies) of channel estimate information to be included in one or more of the UL FB PPDUs  268 . In some embodiments, one or more client stations  154  (e.g., one or more MAC processors in the client stations  154  (e.g., the MAC processor  166 )) generates one or more MAC frames that include information configured to cause the AP  114  to include, in one or more of the DL FB PPDUs  244 , one or more of i) recorded angle(s) AoA 1,k , ii) recorded angle(s) AoD 2,k , and iii) channel estimate information determined by the AP  114  based on reception of the UL NDPs  224 . In some embodiments, contents of the sounding feedback, e.g., time stamp(s), AoA, AoD, channel estimation information, etc., is decided during an NDP sounding negotiation that occurs prior to the MU ranging measurement exchange  200 . 
     Also as discussed above, UL FB PPDUs  268  may include, in addition to recorded times t 1,k  and t 4,k , one or more of i) the recorded angles AoD 1,k , ii) the recorded angles AoA 2,k , and iii) channel estimate information determined by client stations  154  based on reception of the DL NDP(s)  236 . 
     In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines that one or more client stations  154  are to include, in one or more of the UL FB PPDUs  268 , one or more of i) recorded angle(s) AoD 1,k , ii) recorded angle(s) AoA 2,k , and iii) channel estimate information determined by client station(s)  154  based on reception of the DL NDPs  236 . In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines the granularity(ies) of channel estimate information to be included in one or more of the UL FB PPDUs  268 . In some embodiments, the AP  114  (e.g., the MAC processor  126 ) generates one or more MAC frames that include information configured to cause one or more of the client stations  154  to include, in one or more of the UL FB PPDUs  268 , one or more of i) recorded angle(s) AoD 1,k , ii) recorded angle(s) AoA 2,k , and iii) channel estimate information determined by client station(s)  154  based on reception of the DL NDPs  236 . In some embodiments, contents of the sounding feedback, e.g., time stamp(s), AoA, AoD, channel estimation information, etc., is decided during an NDP sounding negotiation that occurs prior to the MU ranging measurement exchange  200 . In some embodiments, contents of the sounding feedback is specified in a trigger frame (e.g., trigger frame included in PPDU  216  and/or PPDU  260 ) or an NDP Announcement frame (e.g., NDPA frame included in PPDU  228 ). 
     If the one or more of the client stations  154  are to include, in one or more of the UL FB PPDUs  268 , channel estimate information, the one or more MAC frames may include information that indicates the granularity(ies) of the channel estimate information to be included in one or more of the UL FB PPDUs  268 , according to some embodiments. The AP  114  then transmits the one or more MAC frames prior to the MU ranging measurement exchange  200 . In some embodiments, granularity(ies) of the channel estimation information is decided during an NDP sounding negotiation that occurs prior to the MU ranging measurement exchange  300 . In some embodiments, granularity(ies) of the channel estimation information is specified in a trigger frame (e.g., trigger frame included in PPDU  216  and/or PPDU  260 ) or an NDP Announcement frame (e.g., NDPA frame included in PPDU  228 ). 
       FIG.  3 A  is a diagram of an example single-user (SU) ranging measurement exchange  300  in an SU ranging measurement procedure, according to an embodiment. The diagram  300  is described in the context of the example network  110  merely for explanatory purposes. In some embodiments, signals illustrated in  FIG.  3 A  are generated by other suitable communication devices in other suitable types of wireless networks. 
     The SU ranging measurement exchange  300  corresponds to a client-initiated SU ranging measurement exchange, according to an embodiment. The SU ranging measurement exchange  300  includes an UL NDP transmission portion  304 , a DL NDP transmission portion  308 , and a DL feedback transmission portion  312 . In an embodiment, the uplink UL NDP transmission portion  304 , the DL NDP transmission portion  308 , and the DL feedback portion  312  occur within a single TXOP. In another embodiment, the uplink UL NDP transmission portion  304 , the DL NDP transmission portion  308 , and the DL feedback transmission portion  312  do not occur within a single TXOP. For example, the uplink UL NDP transmission portion  304  and the DL NDP transmission portion  308  occur within a single TXOP, whereas the DL feedback transmission portion  312  occurs outside of the single TXOP (e.g., in another TXOP). 
     In the UL NDP transmission portion  304 , a first communication device (e.g., the client station  154 ) transmits a PPDU  316  that includes an SU UL NDPA having information indicating the initiation of an SU ranging measurement exchange. In an embodiment, the SU UL NDPA in the PPDU  316  is a type of NDPA frame specifically for initiating an SU ranging measurement exchange such as the SU ranging measurement exchange  300 . The SU UL NDPA in the PPDU  316  causes the AP  114  to be ready to receive an NDP as part of an SU ranging measurement exchange. 
     The client station  154  then begins transmitting an NDP  320  a defined time period after an end of the PPDU  316 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. 
     The UL NDP  320  includes a PHY preamble having one or more STFs, one or more LTFs and one or more signal fields, in an embodiment. The UL NDP  320  omits a data portion. 
     When transmitting the UL NDP  320 , the client station  154  records a time t 1  at which the client station  154  began transmitting a particular portion of the UL NDP  320 , e.g. the time when the client station  154  began transmitting an HE/VHT LTF portion of the UL NDP  320 . Similarly, when the AP  114  receives the UL NDP  320 , the AP  114  records a time t 2  at which the AP  114  began receiving the particular portion of the UL NDP  320 , e.g. the time when the client station  154  began AP  114  began receiving the HE/VHT LTF portion of the UL NDP  320 . 
     In some embodiments, when transmitting the UL NDP  320 , the client station  154  (e.g., a client station  154  with multiple antennas  174 ) records an angle of departure, AoD 1 , at which the UL NDP  320  left the antennas  178  of the client station  154 . Similarly, when the AP  114  receives the UL NDP  320 , the AP  114  records an angle of arrival, AoA 1 , at which the UL NDP  320  arrived at the antennas  138  of the AP  114 . In some embodiment, after receiving the DL FB  312 , the client station  154  transmits UL FB to the AP  114  for the AP  114  to calculate a range and/or position. 
       FIG.  3 B  is a timing diagram of the example MU ranging measurement exchange  300  of  FIG.  3 A . As illustrated in  FIG.  3 B , the client station  154  records the time t 1  at which the client station  154  began transmitting a particular portion of the UL NDP  320  (e.g. the time when the client station  154  began transmitting an HE/VHT LTF portion of the UL NDP  320 ), and records the AoD 1  at which the UL NDP  320  left the antennas  178  of the client station  154 . Additionally, the AP  114  records the time t 2  at which the AP  114  began receiving the particular portion of the UL NDP  320  (e.g. the time when the client station  154  began transmitting an HE/VHT LTF portion of the UL NDP  320 ), and the AoA 1 , at which each UL NDP  320  arrived at the antennas  138  of the AP  114 . 
     Referring now to  FIGS.  3 A and  3 B , the AP  114  generates a DL NDP  328  and, responsive to the UL NDP  320 , the AP  114  begins transmitting the DL NDP  328  a defined time period after an end of the UL NDP  320 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The DL NDP  328  includes a PHY preamble having one or more STFs, one or more LTFs and one or more signal fields, in an embodiment. The DL NDP  328  omits a data portion. 
     When transmitting the DL NDP  328 , the AP  114  records a time t 3  at which the AP  114  began transmitting a particular portion of the DL NDP  328 , e.g. the time when the AP  114  began transmitting an HE/VHT LTF portion of the DL NDP  328 . Similarly, when the client station  154  receives the DL NDP  328 , the client station  154  records a time t 4  at which the client station  154  began receiving the particular portion of the DL NDP  328 , e.g. the time when the client station  154  began receiving the HE/VHT LTF portion of the DL NDP  328 . As illustrated in  FIG.  3 B , the AP  114  records the time t 3  at which the AP  114  began transmitting the particular portion of the DL NDP  328  (e.g. the time when the AP  114  began transmitting an HE/VHT LTF portion of the DL NDP  328 ), and the client station  154  records the time t 4  at which the client station  154  began receiving the DL NDP  328  (e.g. the time when the client station  154  began receiving the HE/VHT LTF portion of the DL NDP  328 ). 
     In some embodiments, when transmitting the DL NDP  328 , the AP  114  records an AoD 2  at which the DL NDP  328  left the antennas  138  of the AP  114 . Similarly, when the client station  154  receives the DL NDP  328 , the client station  154  records an AoA 2  at which the DL NDP  328  arrived at the antennas  178  of the client station  154 . 
     In another embodiment, responsive to the UL NDP  320 , the AP  114  begins transmitting a DL PPDU (not shown) that includes an NDPA frame a defined time period after an end of the UL NDP  320 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The NDPA frame is configured to cause the client stations  154  to be prepared to receive the DL NDP  328  from the AP  114 , according to an embodiment. The AP  114  then begins transmitting the DL NDP  328  a defined time period after an end of the DL PPDU that includes the NDPA frame. In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. 
     In an embodiment, the AP  114  transmits a DL PPDU  332  a defined time period after an end of the DL NDP  328 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The DL PPDU  332  corresponds to a downlink ranging measurement feedback packet that includes a FB frame. The FB frame in the DL PPDU  332  includes the recorded times t 2  and t 3 . In some embodiments, the FB frame in the DL PPDU  332  respectively includes the recorded angles AoA 1  and AoD 2 . In some embodiments, the FB frame in the DL PPDU  332  optionally also includes respective channel estimate information determined by the AP  114  based on reception of the UL NDP  1120 . 
     After receipt of the DL PPDU  332 , the client station  154  calculates a time-of-flight between the AP  114  and the client station  154  using the recorded times t 1 , t 2 , t 3 , and t 4 , according to an embodiment. Any suitable technique, including currently known techniques, may be utilized to calculate a time-of-flight using the recorded times t 1 , t 2 , t 3 , and t 4 . A distance between the AP  114  and the client station  154  may be calculated using the calculated times-of-flight, e.g., by respectively multiplying the times-of-flight by the speed of light, according to an embodiment. 
     In some embodiments, the client station  154  calculates an estimated position of the client station using the calculated time-of-flight. For example, the client station  154  uses triangulation techniques to calculate an estimated position of the client station  154  using the calculated time-of-flight. In some embodiments, the client station  154  calculates an estimated positions of the client station also using the recorded angles AoD 1 , AoA 1 , AoD 2 , and AoA 2 . For example, the recorded angles AoD 1 , AoA 1 , AoD 2 , and AoA 2  are used as part of a triangulation algorithm for determining positions of communication devices. 
     Responsive to receipt of the DL PPDU  332 , the client station  154  generates an UL PPDU  336  that includes an ACK frame, according to an embodiment. The client station  154  transmits the UL PPDU  336  a defined time period after an end of the DL PPDU  332 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. 
     In some embodiments, the DL feedback PPDU  332  is in a single TXOP with the UL NDP transmission portion  304  and the DL NDP transmission portion  308 , but includes feedback information for another SU ranging measurement exchange (not shown) that occurred prior to the UL NDP transmission portion  304  and the DL NDP transmission portion  308 . 
     The client station  154  does not generate and transmit the UL PPDU  336  even when the client station  154  successfully receives the DL PPDU  332 , according to an embodiment. Thus, in some embodiments, the UL PPDU  336  is omitted from the procedure  300 . For example, if the network interface device  162  determines (e.g., the MAC processor  166  determines) that the network interface device  162  did not successfully receive the FB frame in the PPDU  332 , the network interface device  162  determines (e.g., the MAC processor  166  determines) that the SU ranging measurement exchange  300  is to be repeated. 
     As discussed above, the FB frame in the PPDU  332  may include, in addition to recorded times t 2  and t 3 , one or more of i) the recorded angles AoA 1 , ii) the recorded angles AoD 2 , and iii) channel estimate information determined by the AP  114  based on reception of the UL NDP  320 . 
     In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines which feedback information is to be included in the PPDU  332  (e.g., one or more of i) recorded angle AoA 1 , ii) recorded angle AoD 2 , and iii) channel estimate information determined by the AP  114  based on reception of the UL NDP  320 . In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines the granularity of channel estimate information to be included in the DL FB PPDU  332 . 
     In some embodiments, the client station  154  (e.g., the MAC processor  166 ) determines which information (e.g., the recorded angle AoA 1 , the recorded angle AoD 2 , channel estimate information, channel estimate information at a particular granularity, etc.) the AP  114  should include within the DL FB PPDU  332 . In some embodiments, the client station  154  includes an indication(s) of the requested information in the UL NDPA in the PPDU  316 . Upon receiving the UL NDPA in the PPDU  316 , the AP  114  determines which information to include in the DL FB PPDU  332 . 
     In some embodiments, the client station  154  (e.g., the MAC processor  166 ) generates one or more MAC frames that include information configured to cause the AP  114  to include, in the DL FB PPDU  332 , the requested information. The client station  154  then transmits the one or more MAC frames prior to the SU ranging measurement exchange  300 . Upon receiving the one or more MAC frames prior to the SU ranging measurement exchange  300 , the AP  114  determines which information to include in the DL FB PPDU  332 . 
     In some embodiments, the AP  114  and the client station  154  negotiate whether the AP  114  is to include, in the DL FB PPDU  332 , one or more of i) the recorded angle AoA 1 , ii) the recorded angle AoD 2 , iii) channel estimate information, and/or iv) the granularity of the channel estimate information prior to the SU ranging measurement exchange  300 . For example, in an embodiment, negotiating includes the client station  154  (e.g., the MAC processor  166 ) generating one or more MAC frames with information indicating requested types of information to be included and/or requested granularities, and the client station  154  transmits the one or more MAC frames to the AP  114  prior to the SU ranging measurement exchange  300 . Similarly, in an embodiment, negotiating includes the AP  114  (e.g., the MAC processor  126 ) generating one or more MAC frames with information indicating proposed types of information to be included and/or requested granularities, and the AP  114  transmits the one or more MAC frames to the client station  154  prior to the SU ranging measurement exchange  300 . 
     In an embodiment, the DL FB PPDU  332  is not transmitted the defined time period after the DL NDP  328 , but rather a delay occurs between the DL NDP  328  and the DL FB portion  312 . 
     Although  FIG.  3    was described in the context of a ranging measurement exchange between the client station  154  and the AP  114 , a similar ranging measurement exchange is performed between two client stations  154 , in an embodiment. Similarly, the roles of the client station  154  and the AP  114  in the ranging measurement exchange  300  are reversed, in another embodiment. 
       FIG.  4    is a diagram of another example SU ranging measurement exchange  400  in another SU ranging measurement procedure, according to an embodiment. The diagram  400  is described in the context of the example network  110  merely for explanatory purposes. In some embodiments, signals illustrated in  FIG.  4    are generated by other suitable communication devices in other suitable types of wireless networks. 
     The SU ranging measurement exchange  400  corresponds to a client station-initiated SU ranging measurement exchange, according to an embodiment. The SU ranging measurement exchange  400  is similar to the SU ranging measurement exchange  300  of  FIG.  3 A , but the DL FB portion  312  does not begin SIFS after the DL NDP  328  as in the SU ranging measurement exchange  300  of  FIG.  3 A . Additionally, a DL NDP portion  404  includes a DL PPDU  408  having a DL NDPA. The AP  114  begins transmitting the DL PPDU  408  a defined time period (e.g., SIFS or another suitable time period) after an end of the UL NDP  320 . The AP  114  begins transmitting the DL NDP  328  a defined time period (e.g., SIFS or another suitable time period) after an end of the DL PPDU  408 . In an embodiment, the PPDU  408  having the DL NDPA is omitted. 
     In an embodiment, the UL NDP portion  304  and the DL NDP portion  404  occur within a first TXOP  416 , whereas the DL FB portion  312  occurs within a second TXOP  420 . 
     In some embodiments, the client station  154  does not transmit the UL ACK PPDU  336  even when the client station  154  successfully receives the DL FB PPDU  332 , according to an embodiment. Thus, in some embodiments, the UL ACK PPDU  336  is omitted from the procedure  400 . 
       FIG.  5    is a diagram of an example SU ranging measurement exchange  500  in an SU ranging measurement procedure, according to another embodiment. The diagram  500  is described in the context of the example network  110  merely for explanatory purposes. In some embodiments, signals illustrated in  FIG.  5    are generated by other suitable communication devices in other suitable types of wireless networks. 
     The SU ranging measurement exchange  500  corresponds to an AP-initiated SU ranging measurement exchange, according to an embodiment. The SU ranging measurement exchange  500  includes a DL NDP transmission portion  504 , an UL NDP transmission portion  508 , and an UL feedback transmission portion  512 . In an embodiment, the DL NDP transmission portion  504  and the UL NDP transmission portion  508  occur within a TXOP  516 , and the UL FB portion  512  occurs with another TXOP  520 . 
     In the DL NDP transmission portion  504 , a first communication device (e.g., the AP  114 ) transmits a PPDU  516  that includes an SU DL NDPA having information indicating the initiation of an SU ranging measurement exchange. In an embodiment, the SU DL NDPA in the PPDU  516  is a type of NDPA frame specifically for initiating an SU ranging measurement exchange such as the SU ranging measurement exchange  500 . The SU DL NDPA in the PPDU  516  causes a second communication device (e.g., the client station  154 - 1 ) to be ready to receive an NDP as part of an SU ranging measurement exchange. 
     The AP  114  then begins transmitting a DL NDP  520  a defined time period after an end of the PPDU  516 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. 
     The DL NDP  520  includes a PHY preamble having one or more STFs, one or more LTFs and one or more signal fields, in an embodiment. The DL NDP  520  omits a data portion. 
     When transmitting the DL NDP  520 , the AP  114  records a time t 1  at which the AP  114  began transmitting the DL NDP  520 . Similarly, when the client station  154 - 1  receives the DL NDP  520 , the client station  154 - 1  records a time t 2  at which the client station  154 - 1  began receiving the DL NDP  520 . 
     In some embodiments, when transmitting the DL NDP  520 , the AP  114  records an angle of departure, AoD 1 , at which the DL NDP  520  left the antennas  138  of the AP  114 . Similarly, when the client station  154 - 1  receives the DL NDP  520 , the client station  154 - 1  records an angle of arrival, AoA 1 , at which the DL NDP  520  arrived at the antennas  178  of the client station  154 - 1 . 
     The client station  154 - 1  generates an UL NDP  528  and, responsive to the DL NDP  520 , the client station  154 - 1  begins transmitting the UL NDP  528  a defined time period after an end of the DL NDP  520 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The UL NDP  528  includes a PHY preamble having one or more STFs, one or more LTFs and one or more signal fields, in an embodiment. The UL NDP  528  omits a data portion. 
     When transmitting the UL NDP  528 , the client station  154 - 1  records a time t 3  at which the client station  154 - 1  began transmitting the UL NDP  528 . Similarly, when the AP  114  receives the UL NDP  528 , the AP  114  records a time t 4  at which the AP  114  began receiving the UL NDP  528 . 
     In some embodiments, when transmitting the UL NDP  528 , the client station  154 - 1  records an AoD 2  at which the UL NDP  528  left the antennas  178  of the client station  154 - 1 . Similarly, when the AP  114  receives the UL NDP  528 , the AP  114  records an AoA 2  at which the UL NDP  528  arrived at the antennas  138  of the AP  114 . 
     In another embodiment, responsive to the DL NDP  520 , the client station  154 - 1  begins transmitting an UL PPDU (not shown) that includes an NDPA frame a defined time period after an end of the DL NDP  520 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. The NDPA frame is configured to cause the AP  114  to be prepared to receive the UL NDP  528  from the client station  154 - 1 , according to an embodiment. The client station  154 - 1  then begins transmitting the UL NDP  528  a defined time period after an end of the UL PPDU that includes the NDPA frame. In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. 
     In an embodiment, the client station  154 - 1  transmits an UL PPDU  532  during the TXOP  520 . The PPDU  532  corresponds to an uplink ranging measurement feedback packet. The PPDU  532  includes the recorded times t 2  and t 3 . In some embodiments, the PPDU  532  respectively includes the recorded angles AoA 1  and AoD 2 . In some embodiments, the PPDU  532  optionally also includes respective channel estimate information determined by the client station  154 - 1  based on reception of the DL NDP  520 . 
     After receipt of the PPDU  532 , the AP  114  calculates a time-of-flight between the AP  114  and the client station  154  using the recorded times t 1 , t 2 , t 3 , and t 4 , according to an embodiment. Any suitable technique, including currently known techniques, may be utilized to calculate a time-of-flight using the recorded times t 1 , t 2 , t 3 , and t 4 . A distance between the AP  114  and the client station  154  may be calculated using the calculated times-of-flight, e.g., by respectively multiplying the times-of-flight by the speed of light, according to an embodiment. 
     In some embodiments, the AP  114  calculates an estimated position of the client station  154  using the calculated time-of-flight. For example, the AP  114  uses triangulation techniques to calculate an estimated position of the client station  154  using the calculated time-of-flight. In some embodiments, the AP  114  calculates an estimated position of the client station  154  also using the recorded angles AoD 1 , AoA 1 , AoD 2 , and AoA 2 . For example, the recorded angles AoD 1 , AoA 1 , AoD 2 , and AoA 2  are used as part of a triangulation algorithm for determining positions of communication devices. 
     Responsive to receipt of the PPDU  532 , the AP  114  generates a DL PPDU (not shown) that includes an ACK frame, according to an embodiment. The AP  114  transmits the DL PPDU that includes the ACK frame a defined time period after an end of the UL PPDU  332 . In an embodiment, the defined time period is SIFS. In other embodiments, another suitable time period is utilized. In another embodiment, the AP  114  does not generate and transmit the DL PPDU that includes the ACK frame even when the AP  114  successfully receives the UL PPDU  532 , according to an embodiment. Thus, in some embodiments, the DL PPDU that includes the ACK frame is omitted from the procedure  500 . 
     As discussed above, the UL FB in the PPDU  532  may include, in addition to recorded times t 2  and t 3 , one or more of i) the recorded angles AoA 1 , ii) the recorded angles AoD 2 , and iii) channel estimate information determined by the client station  154 - 1  based on reception of the DL NDP  520 . 
     In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines which information (e.g., the recorded angle AoA 1 , the recorded angle AoD 2 , channel estimate information, channel estimate information at a particular granularity, etc.) the client station  154 - 1  should include within the UL FB PPDU  532 . In some embodiments, the AP  114  includes an indication(s) of the requested information in the DL NDPA in the PPDU  516 . Upon receiving the DL NDPA in the PPDU  516 , the client station  154 - 1  determines which information to include in the UL FB PPDU  532 . 
     In some embodiments, the AP  114  (e.g., the MAC processor  136 ) generates one or more MAC frames that include information configured to cause the client station  154 - 1  to include, in the UL FB PPDU  532 , the requested information. The AP  114  then transmits the one or more MAC frames prior to the SU ranging measurement exchange  500 . Upon receiving the one or more MAC frames prior to the SU ranging measurement exchange  500 , the client station  154 - 1  determines which information to include in the DL FB PPDU  532 . 
     In some embodiments, the AP  114  and the client station  154  negotiate whether the client station  154 - 1  is to include, in the DL FB PPDU  532 , one or more of i) the recorded angle AoA 1 , ii) the recorded angle AoD 2 , iii) channel estimate information, and/or iv) the granularity of the channel estimate information prior to the SU ranging measurement exchange  500 . For example, in an embodiment, negotiating includes the AP  114  (e.g., the MAC processor  126 ) generating one or more MAC frames with information indicating requested types of information to be included and/or requested granularities, and the AP  114  transmits the one or more MAC frames to the client station  154 - 1  prior to the SU ranging measurement exchange  500 . Similarly, in an embodiment, negotiating includes the client station  154 - 1  (e.g., the MAC processor  166 ) generating one or more MAC frames with information indicating proposed types of information to be included and/or requested granularities, and the client station  154 - 1  transmits the one or more MAC frames to the AP  114  prior to the SU ranging measurement exchange  500 . 
     Although  FIG.  5    was described in the context of a ranging measurement exchange between the client station  154  and the AP  114 , a similar ranging measurement exchange is performed between two client stations  154 , in an embodiment. Similarly, the roles of the client station  154  and the AP  114  in the ranging measurement exchange  500  are reversed, in another embodiment. 
       FIG.  6 A  is a diagram of an example frame format of a trigger frame  600  for use in an MU ranging measurement exchange, according to an embodiment. Referring now to  FIG.  2 A , the trigger frame  600  is the trigger frame included in the PPDU  216  in the UL NDP frame exchange  204 , according to an embodiment. In an embodiment, the trigger frame  600  is generated by the MAC processor  126  of the network interface  122 .  FIG.  6 A  indicates example lengths (e.g., in terms of octets) of fields of the trigger frame  600 . In other embodiments, length(s) one or more of the fields has another suitable number length. In some embodiments, one or more of the fields are omitted and/or one or more additional fields are included. 
     The trigger frame  600  includes a frame control field  602 , a duration field  604 , a receiver address (RA) field  606 , a transmitter address (TA) field  608 , a common information field  610 , multiple user information fields  612 , a padding field  614  and a frame check sequence (FCS) field  616 . 
     The frame control field  602  includes information that indicates that frame  600  is a trigger frame configured to prompt a plurality of other communication devices to simultaneously transmit as part of an uplink MU transmission (e.g., OFDMA and/or MU-MIMO). The duration field  604  includes information that indicates a length of a transmit opportunity period (TXOP) during which the MU ranging measurement exchange will take place, in an embodiment. The RA field  606  includes an address corresponding to the multiple client stations  154  that are the target recipients of the trigger frame  600 . For instance, the RA field  606  indicates a broadcast address or a multicast address corresponding to the multiple client stations  154 , in various embodiments. The TA field  608  includes an address corresponding to the AP  114  transmitting the trigger frame  600 . The common information field  610  includes information that is common to the multiple client stations  154 . Each of the user information fields  612  includes information specific to a corresponding client station  154 . For instance, in an embodiment, the user information field 1  612 - 1  indicates information specific to client station  154 - 1 , the user information field 2  612 - 2  indicates information specific to client station  154 - 2 , etc. The padding field  614  includes padding bits for the trigger frame  600 , if any. The FCS field  616  includes an error detecting code that enables a receiving device to determine whether the trigger frame  600  was received without any errors. 
       FIG.  6 B  is a diagram of an example format of the common information field  610  of the trigger frame  600 , according to an embodiment.  FIG.  6 B  indicates example lengths (e.g., in bits) of subfields of the common information field  610 . In other embodiments, one or more of the subfields has another suitable length. In some embodiments, one or more of the subfields are omitted and/or one or more additional subfields are included. 
     The common information field  610  includes a trigger type subfield  622 , a length subfield  624 , a bandwidth (BW) subfield  628 , a guard interval (GI) and long training field (LTF) type subfield  630 , an MU-MIMO LTF mode subfield  632 , a number of high efficiency long training field (HE-LTF) symbols subfield  634 , an AP transmit power subfield  638 , a packet extension subfield  640 , a high efficiency signal-A (HE-SIG-A) reserved subfield  644 , a trigger dependent common information subfield  648 , and reserved subfields  626 ,  636 ,  642 , and  646 . In an embodiment, the trigger dependent common information subfield  648  is omitted from the common information field  610 . 
     The trigger type subfield  622  includes information indicating that the trigger frame  600  is a type of trigger frame specifically for initiating an MU ranging measurement exchange, such as the MU ranging measurement exchange  200 , and/or specifically for prompting client stations  154  to transmit NDPs as part of an UL MU-MIMO transmission for an MU ranging measurement exchange such as the MU ranging measurement exchange  200 . In an embodiment, the value of the trigger type subfield  622  is selected from among a plurality of values corresponding to a plurality of trigger variants defined by a communication protocol (e.g., the IEEE 802.11 Standard). Different trigger variants correspond to different type of information being solicited in an UL MU transmission and/or the UL MU transmission being part of different types of procedures, in some embodiments. In an illustrative embodiment, the plurality of trigger variants defined by the communication protocol include any suitable combination of two or more of the following: i) a basic trigger for soliciting an UL MU transmission having basic user data from communication devices, ii) a beamforming report poll trigger for soliciting an UL MU transmission having beamforming training feedback from communication devices, iii) an MU request-to-send trigger for soliciting an UL MU transmission having clear-to-send (CTS) frames from communication devices, iv) a buffer status report poll (BSRP) trigger for soliciting an UL MU transmission having information regarding how much user data communication devices have to send to the AP  114 , v) an EFTM trigger for initiating an MU ranging measurement exchange, such as the MU ranging measurement exchange  200 , and/or specifically for soliciting an UL MU-MIMO transmission of NDPs for an MU ranging measurement exchange such as the MU ranging measurement exchange  200 , etc. 
     The length subfield  624  includes a value indicating a length the NDPs to be transmitted in the UL MU-MIMO transmission responsive to the trigger frame  660 . In an embodiment, the value of the length subfield  624  corresponds to a value that client stations  154  will include in a legacy signal (L-SIG) field included in UL NDPs  224  that will be transmitted by multiple client stations  154  responsive to the trigger frame  600 . The BW subfield  628  includes a value indicating a bandwidth corresponding to the UL NDPs  224 . The GI and LTF subfield  630  includes a value indicating i) a GI duration to be used for generating OFDM symbols corresponding to the UL NDPs  224 , and ii) parameters corresponding to the generation of LTF fields to be included the UL NDPs  224 . The MU-MIMO LTF mode subfield  632  includes a value indicating an LTF mode (e.g., single stream pilot HE-LTF mode or masked HE-LTF sequence mode) of the UL NDPs  224 . The number of HE-LTF symbols subfield  634  includes a value indicating the number of HE-LTF symbols to be included in UL NDPs  224 . The AP transmit power subfield  638  includes a value indicating a combined average power per 20 MHz bandwidth of all transmit antennas used to transmit the trigger frame from the AP  114 . The packet extension subfield  640  includes a value indicating a duration of PPDU extension (PE) fields to be added by the client stations  154  at the end of the UL NDPs  224 . The HE-SIG-A reserved subfield includes a value indicating to which values the client stations  154  should set reserved bits in the HE-SIG-A2 subfield of the UL NDPs  224 . In another embodiment, the subfield  632  and/or subfield  640  are reserved. 
       FIG.  6 C  is a diagram of an example format of a user information field  612  of the trigger frame  600 , in an embodiment.  FIG.  6 C  indicates example lengths (e.g., in bits) of subfields of the user information field  612 . In other embodiments, one or more of the subfields has another suitable length. In some embodiments, one or more of the subfields are omitted and/or one or more additional subfields are included. 
     The user information field  612  includes an association identifier (AID) 12 subfield  660 , a reserved subfield  662 , a spatial stream (SS) allocation subfield  664 , a target receive signal strength indicator (RSSI) subfield  666 , and a reserved subfield  668 . In another embodiment, the user information field  612  also includes a trigger dependent user information subfield, e.g., after the reserved subfield  668 . 
     The AID12 subfield  660  includes an identifier of the client station  154  for which the user information field  612  is intended. In an embodiment, the AID 12 subfield  660  includes 12 least significant bits of an AID assigned to the client station  154  by the AP  114 . The SS allocation subfield  664  includes a value indicating which spatial streams the client station  154  is to use for transmitting a corresponding UL NDP  224 . The target RSSI subfield  666  includes a value indicating a target receive signal power of the UL NDP  224  to be transmitted by the client station  154 . 
     As discussed above, the multiple client stations  154  transmit ranging measurement feedback corresponding to the MU ranging measurement exchange. For instance, in an embodiment, the multiple client stations transmit ranging measurement feedback in UL PPDUs  268  of an UL MU transmission  264 , as described above with respect to MU ranging measurement exchange  200 . In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines that one or more client stations  154  are to include in ranging measurement feedback packets, such as the UL PPDUs  268 , one or more of i) recorded time(s) t 1,k , ii) recorded time(s) t 4,k , iii) recorded angle(s) AoD 1,k , iv) recorded angle(s) AoA 2,k , and v) channel estimate information determined by client station(s)  154  based on reception of the DL NDP(s)  236 . In some embodiments, the AP  114  (e.g., the MAC processor  126 ) determines the granularity(ies) of channel estimate information to be included in the ranging measurement feedback packets. 
     In an embodiment, the information to be included in the ranging measurement feedback transmitted by the multiple client stations  154  is determined by the AP  114  and indicated in a DL transmission to the multiple client stations  154 . For instance, the trigger frame  600  in a DL transmission (for example, DL PPDU  216 ) includes (e.g., in one or more of subfield  626 , subfield  636 , subfield  642 , subfield  646 ; in another subfield (not shown in  FIG.  6 B ) formed by consolidating reserved bits from two or more of subfield  626 , subfield  636 , subfield  642 , subfield  646 ; in one or both of subfield  662  and subfield  668 ; in another subfield (not shown in  FIG.  6 C ) formed by consolidating reserved bits from subfield  662  and subfield  668 ) information indicating whether one or more client stations  154  are to include, in ranging measurement feedback packets, one or more of i) recorded time(s) t 1,k , ii) recorded time(s) t 4,k , iii) recorded angle(s) AoD 1,k , iv) recorded angle(s) AoA 2,k , and v) channel estimate information determined by client station(s)  154  based on reception of the DL NDP(s)  236 . In an embodiment where the trigger frame  600  indicates that one or more client stations  154  are to include, in ranging measurement feedback packets, the channel estimate information, the trigger frame  600  further includes an indication of the granularity(ies) of channel estimate information to be included in the ranging measurement feedback packets. 
     In other embodiments, an indication of what ranging measurement feedback information is to be included in the ranging measurement feedback is included in one or both of the trigger dependent common information subfield  648  and/or the trigger dependent user information subfield (e.g., in the user information field  612 ). 
     In other embodiments, however, the trigger frame  600  does not indicate the information to be included by the one or more client stations  154  in ranging measurement feedback packets. For instance, in some such embodiments, the information to be included is determined and indicated to the one or more client stations  154  by the AP  114  during an NDP sounding negotiation that occurs prior to the MU ranging measurement exchange. In another embodiment, the information to be included is indicated to the one or more client stations  154  by the AP  114  in an NDPA frame, such as the NDPA frame included in the DL PPDU  228 . 
       FIG.  7 A  is a diagram of an example frame format of a NDPA frame  700  for use in an MU and/or SU ranging measurement exchange, according to an embodiment. Referring now to  FIG.  2 A , the NDPA frame  700  is the NDPA frame included in the PPDU  228  in the DL NDP transmission portion  208 , according to an embodiment. Referring now to  FIG.  3 A , the NDPA frame  700  is the NDPA frame included in the PPDU  316  in the UL NDP transmission portion  304 , according to an embodiment. Referring now to  FIG.  4   , the NDPA frame  700  is the NDPA frame included in the PPDU  408  in the DL NDP transmission portion  404 , according to an embodiment. Referring now to  FIG.  5   , the NDPA frame  700  is the NDPA frame included in the PPDU  516  in the DL NDP transmission portion  504 , according to an embodiment. 
     In an embodiment, the NDPA frame  700  is generated by the MAC processor  126  of the network interface  122 . In another embodiment, the NDPA frame  700  is generated by the MAC processor  166  of the network interface  162 .  FIG.  7 A  indicates example lengths (e.g., in terms of octets) of fields of the NDPA frame  700 . In other embodiments, length(s) one or more of the fields has another suitable number length. In some embodiments, one or more of the fields are omitted and/or one or more additional fields are included. 
     In an embodiment, the NDPA frame  700  is similar to an NDPA frame format defined for the IEEE 802.11ac Standard. 
     The NDPA frame  700  includes a frame control field  702 , a duration field  704 , an RA field  706 , a TA field  708 , a sounding dialog token field  710 , one or more station (STA) information fields  712 , and a frame check sequence (FCS) field  714 . 
     In an embodiment, the frame control field  702  indicates that the frame  700  is an NDPA frame. In one such embodiment, a bit (e.g., bit B0) of the sounding dialog token field  710  is used to indicate that the NDPA frame  700  corresponds to an NDP ranging measurement exchange. In another embodiment, the frame control field  702 , by itself, indicates that the frame  700  is an NDPA frame corresponding to an NDP ranging measurement exchange. In another embodiment, another bit of the sounding dialog token field  710  is used to additionally indicate whether the NDPA frame announces a NDP frame transmission in an SU NDP sounding sequence or an MU NDP sounding sequence. 
     In an embodiment, the duration field  704  includes a value indicating an estimation of a time required for the ranging measurement exchange, in an embodiment. The RA field  706  includes an address corresponding to one or more client stations  154  that are the target recipient of the NDPA frame  700 . For instance, in an MU ranging measurement exchange, the RA field  706  includes a broadcast address or a multicast address corresponding to multiple client stations  154  that are intended recipients of the NDPA frame  700 . In a SU ranging measurement exchange, the RA field  706  includes an address of a client station  154  or an AP  114  that is an intended recipient of the NDPA frame  700 . The TA field includes an address corresponding to the communication device transmitting the NDPA frame  700 . In an embodiment, a bit (e.g., bit B1) of the sounding dialog token field  710  is used to indicate that the NDPA frame  700  corresponds to an NDPA frame defined by the IEEE 802.11ac Standard. The remaining bits of the sounding dialog token field  710  are set to a value that is used to identify the NDPA frame  700  as corresponding to an NDP ranging measurement exchange, in an embodiment. The FCS field  714  includes an error detecting code that enables a receiving device to determine whether the trigger frame  700  was received without any errors. 
     In an embodiment, the number of STA information fields  712  is used to distinguish an NDPA frame as employed in an MU ranging measurement exchange from an NDPA frame as employed in an SU ranging measurement exchange. For instance, the presence of a single STA information field  712  indicates that the NDPA frame  700  corresponds to an SU ranging measurement exchange and the presence of multiple STA information fields  712  indicates that the NDPA frame  700  corresponds to an MU ranging measurement exchange. In an embodiment, the RA field  706  is used to distinguish an NDPA frame as employed in an MU ranging measurement exchange from an NDPA frame as employed in an SU ranging measurement exchange. For instance, the presence of a unicast address in the RA field  706  indicates that the NDPA frame  700  corresponds to an SU ranging measurement exchange, and the presence of a broadcast/multicast address in the RA field  706  indicates that the NDPA frame  700  corresponds to an MU ranging measurement exchange. 
       FIG.  7 B  is a diagram of an example format of an STA information field  712  of the NDPA frame  700 , in an embodiment. The AID12 subfield  722  includes an identifier (e.g., a 12-bit AID) of the communication device (i.e., AP  114  or client station  154 ) for which the STA information field  712  is intended. The subfield  724  is reserved. 
     The feedback type subfield  726  includes information indicating which feedback information is to be included in a ranging measurement feedback packet transmitted by a communication device addressed in the AID12 subfield  722 . For instance, in the MU ranging measurement exchange  200 , the NDPA frame  700  (having the feedback type subfield  726 ) is included in the DL PPDU  228 , and the subfield  726  indicates whether a client station  154  is to include, in a corresponding ranging measurement feedback packet  268 , one or more of i) a recorded time t 1,k , ii) a recorded time t 4,k , iii) a recorded angle AoD 1,k , iv) a recorded angle AoA 2,k , and v) a channel estimate information determined by the client station  154  based on reception of the DL NDP(s)  236 , according to an embodiment. 
     In the SU ranging measurement exchange  300  or  400 , the feedback type subfield  726  (in an NDPA frame  700 ) included in the PPDU  316  indicates whether the AP  114  is to include, in the downlink ranging measurement feedback packet  332 , one or more of i) a recorded time t 2 , ii) a recorded time t 3 , iii) a recorded angle AoD 2 , iv) a recorded angle AoA 1 , and v) a channel estimate information determined by the AP  114  based on reception of the UL NDP  320 , according to an embodiment. Similarly, in the SU ranging measurement exchange  500 , the feedback type subfield  726  (in an NDPA frame  700 ) included in the PPDU  516  indicates whether a client station  154  is to include, in corresponding uplink ranging measurement feedback packet  532 , one or more of i) a recorded time t 2 , ii) a recorded time t 3 , iii) a recorded angle AoD 2 , iv) a recorded angle AoA 1 , and v) a channel estimate information determined by the client station  154  based on reception of the DL NDP  520 , according to an embodiment. 
     In an embodiment in which the feedback type subfield  726  indicates that a communication device is to include, in a corresponding ranging measurement feedback packet, the channel estimate information, the feedback type subfield  726  further includes an indication of the granularity of the channel estimate information to be included in the corresponding ranging measurement feedback packet. 
     In other embodiments, however, the NDPA frame  700  does not indicate information to be included in a ranging measurement feedback packet transmitted by a communication device addressed in the AID12 subfield  722  and the feedback type field  726  is omitted. For instance, in some such embodiments, the information to be included is determined and indicated to the communication during an NDP sounding negotiation that occurs prior to the MU or SU ranging measurement exchange. In another embodiment, the feedback information to be included is indicated in another suitable field of the NDPA frame  700 . 
       FIG.  8 A  is a diagram of another example frame format of a NDPA frame  800  for use in an MU and/or SU ranging measurement exchange, according to an embodiment. Referring now to  FIG.  2 A , the NDPA frame  800  is the NDPA frame included in the PPDU  228  in the DL NDP transmission portion  208 , according to an embodiment. Referring now to  FIG.  3 A , the NDPA frame  800  is the NDPA frame included in the PPDU  316  in the UL NDP transmission portion  304 , according to an embodiment. Referring now to  FIG.  4   , the NDPA frame  800  is the NDPA frame included in the PPDU  408  in the DL NDP transmission portion  404 , according to an embodiment. Referring now to  FIG.  5   , the NDPA frame  800  is the NDPA frame included in the PPDU  516  in the DL NDP transmission portion  504 , according to an embodiment. 
     In an embodiment, the NDPA frame  800  is generated by the MAC processor  126  of the network interface  122 . In another embodiment, the NDPA frame  800  is generated by the MAC processor  166  of the network interface  162 .  FIG.  8 A  indicates example lengths (e.g., in terms of octets) of fields of the NDPA frame  800 . In other embodiments, length(s) one or more of the fields has another suitable number length. In some embodiments, one or more of the fields are omitted and/or one or more additional fields are included. 
     In an embodiment, the NDPA frame  800  is similar to an NDPA frame format as described in draft 2.2 of the IEEE 802.11ax Standard, dated February 2018. 
     The NDPA frame  800  is similar to the NDPA frame  700 , and like-numbered elements are not discussed in detail for purposes of brevity. The NDPA frame  800  includes one or more STA information fields  812 . 
       FIG.  8 B  is a diagram of an example format of the STA information field  812 , according to an embodiment. The AID11 subfield  822  includes an identifier of the communication device (i.e., AP  114  or client station  154 ) for which the STA information field  812  is intended. In an embodiment, the AID11 subfield  822  includes an 11-bit AID. 
     Subfields  824  and  828  are reserved. A disambiguation field  826  includes information to help prevent a communication device operating according to a different communication protocol from improperly processing the NDPA frame  800 . A feedback type subfield  830  includes information that indicates which feedback information is to be included in a ranging measurement feedback packet transmitted by a communication device addressed in the AID11 subfield  822 . The feedback type subfield  830  is similar to the feedback type subfield  726  and is not described in detail for purposes of brevity. 
     In some embodiments, the NDPA frame  800  does not indicate information to be included in a ranging measurement feedback packet transmitted by a communication device addressed in the AID11 subfield  822  and the feedback type field  830  is omitted. For instance, in some such embodiments, the information to be included is determined and indicated to the communication during an NDP sounding negotiation that occurs prior to the MU or SU ranging measurement exchange. In another such embodiment, the information to be included is indicated in another field of the NDPA frame  800 . 
       FIG.  9    is a flow diagram of an example method  900  for performing a ranging measurement procedure, according to an embodiment. In some embodiments, the network interface device  162  of  FIG.  1    is configured to implement the method  900 . The method  900  is described, however, in the context of the network interface device  162  merely for explanatory purposes and, in other embodiments, the method  900  is implemented by another suitable device. For instance, in an embodiment, the method the network interface device  122  of  FIG.  1    is configured to implement the method  900 . 
     At block  904 , the network interface device  162  generates an NDPA frame, wherein the NDPA frame is generated to indicate that a first NDP will be transmitted by the network interface device  162  following the transmission of the NDPA frame, and further includes information that indicates that the NDPA frame is part of a ranging measurement exchange session. For instance, in an embodiment, a frame control field within the NDPA frame indicates that the NDPA frame is a part of a ranging measurement exchange session. In an embodiment, the NDPA frame indicates what feedback information to be included in a feedback packet transmitted by another communication device as part of the ranging measurement exchange session. In an embodiment, the NDPA frame is generated according to formats described with respect to  FIG.  7    and/or  FIG.  8   . In another embodiment, the NDPA frame is generated according to another suitable format. 
     At block  908 , the network interface device  162  transmits the NDPA frame to another communication device as part of the ranging measurement exchange session. For instance, the network interface device transmits the NDPA frame in a PPDU, such as a PPDU  316  described above, to the other communication device. As another example, the network interface device transmits the NDPA frame in a PPDU, such as a PPDU  408  described above, to the other communication device. As another example, the network interface device transmits the NDPA frame in a PPDU, such as a PPDU  516  described above, to the other communication device. 
     At block  912 , after transmitting the NDPA frame, the network interface device  162  transmits a first NDP as part of the ranging measurement exchange session. In an embodiment, the network interface device  162  transmits an UL NDP, such as the UL NDP  320  described above, to the other communication device following the transmission of the NDPA frame. As another example, the network interface device transmits a DL NDP, such as the DL NDP  328  or the DL NDP  520  described above. 
     At block  916 , the network interface device  162  receives a second NDP from the other communication device as part of the ranging measurement exchange session. In an embodiment, the network interface device  162  receives a DL NDP, such as the DL NDP  328 , from the other communication device following the transmission of the first NDP frame. As another example, the network interface device receives an UL NDP, such as the UL NDP  320  or the UL NDP  528  described above. 
     At block  920 , the network interface device  162  receives a feedback packet from the other communication device as part of the ranging measurement exchange session, wherein the feedback packet includes feedback information regarding the ranging measurement exchange session. In an embodiment, the feedback packet includes information that was previously indicated in the NDPA frame. In an embodiment, the feedback packet includes information determined based on the reception of the first NDP packet at the other communication device and the transmission of the second NDP packet from the other communication device. 
       FIG.  10    is a flow diagram of an example method  1000  for performing a ranging measurement procedure, according to an embodiment. In some embodiments, the network interface device  122  of  FIG.  1    is configured to implement the method  1000 . The method  1000  is described, however, in the context of the network interface device  122  merely for explanatory purposes and, in other embodiments, the method  1000  is implemented by another suitable device. For instance, in an embodiment, the method the network interface device  162  of  FIG.  1    is configured to implement the method  1000 . 
     At block  1004 , the network interface device  122  receives an NDPA frame from another communication device. The NDPA frame indicates that an NDP transmission from the other communication device will follow the NDPA frame and further indicates that the NDPA frame is part of a ranging measurement exchange session. In an embodiment, the NDPA frame indicates what feedback information is to be included in a feedback packet transmitted by the network interface device  122 . In an embodiment, the received NDPA frame is formatted according to the formats described with respect to the NDPA frame  700  or the NDPA frame  800 . In another embodiment, the network interface device  162  receives the NDPA frame. 
     At block  1008 , the network interface device  122  processes information in the received NDPA frame and determines that the NDPA frame is part of a ranging measurement exchange session. For instance, in an embodiment, the network interface device  122  processes information in a frame control field and/or a sounding dialog token field within the NDPA frame to determine that the NDPA frame is a part of a ranging measurement exchange session. 
     At block  1012 , the network interface device  122  receives a first NDP, from the other communication device, as part of the ranging measurement exchange session. For instance, the network interface device  122  receives an NDP  320  as described above. As another example, the network interface device  162  receives a DL NDP, such as the DL NDP  328  or the DL NDP  520  described above. 
     At block  1016 , the network interface device  122  transmits a second NDP, to the other communication device, as part of the ranging measurement exchange session. For instance, the network interface device  122  transmits an NDP  328  as described above. As another example, the network interface device  162  transmits an UL NDP, such as the UL NDP  320  or the UL NDP  528  described above. 
     At block  1020 , the network interface device  122  generates a feedback packet, wherein the feedback packet includes feedback information regarding the ranging measurement exchange session. In an embodiment, the feedback packet includes information that was previously indicated in the NDPA frame, such as the NDPA frame received at block  1004 . In an embodiment, the feedback packet includes information determined based on the reception of the first NDP packet at the network interface device  122  and the transmission of the second NDP packet by the network interface device  122 . 
     At block  1024 , the network interface device  122  transmits the feedback packet as generated at block  1020 . 
       FIG.  11    is a flow diagram of an example method  1100  for performing a MU ranging measurement procedure, according to an embodiment. In some embodiments, the network interface device  122  of  FIG.  1    is configured to implement the method  1100 . The method  1100  is described, however, in the context of the network interface device  122  merely for explanatory purposes and, in other embodiments, the method  1100  is implemented by another suitable device. 
     At block  1104 , the network interface device  122  generates a trigger frame for MU ranging measurement exchange session with a plurality of other communication devices. The trigger frame includes a trigger type field for indicating a type of frame exchange to which the trigger frame corresponds, wherein a value in the trigger type field indicates that the trigger frame is for prompting an uplink MU NDP transmission as part of the MU ranging measurement exchange session. In an embodiment, the trigger frame further indicates what feedback information is to be included in a feedback packet transmitted by one or more of the plurality of other communication devices. In an embodiment, the trigger frame is generated according to the format described with respect trigger frame  600 . 
     At block  1108 , the network interface device  122  transmits the trigger frame to the plurality of other communication devices as part of the MU ranging measurement exchange session. For instance, in an embodiment, the network interface device  122  transmits the trigger frame to the client stations  154 . 
     At block  1112 , the network interface device  122  receives an uplink MU NDP transmission from the plurality of other communication devices, as part of the MU ranging measurement exchange session, wherein the uplink MU NDP transmission is responsive to the trigger frame. In an embodiment, the uplink MU NDPs are transmitted using MU-MIMO by the plurality of other communication devices, for example, a plurality of client stations  154 . 
     At block  1116 , the network interface device  122  transmits a downlink NDP to the plurality of other communication devices as part of the MU ranging measurement exchange session. 
     At block  1120 , the network interface device  122  receives an uplink MU feedback transmission that includes a plurality of feedback packets from the plurality of other communication devices, wherein the feedback packets include feedback information regarding the MU ranging measurement exchange session. In an embodiment, the feedback packets includes feedback information that was previously indicated in the trigger frame, such as the NDPA frame transmitted at block  1104 . In an embodiment, the feedback packets includes information determined based on the transmission of the uplink MU NDP packet from the plurality of other communication devices and the reception of the downlink NDP packet at the plurality of other communication devices. 
       FIG.  12    is a flow diagram of an example method  1200  for performing a MU ranging measurement procedure, according to an embodiment. In some embodiments, the network interface device  162  of  FIG.  1    is configured to implement the method  1200 . The method  1200  is described, however, in the context of the network interface device  162  merely for explanatory purposes and, in other embodiments, the method  1200  is implemented by another suitable device. 
     At block  1204 , the network interface device  162  receives a trigger frame from another communication device, wherein the trigger frame includes a trigger type field for indicating a type of frame exchange to which the trigger frame corresponds. In an embodiment, the trigger frame includes a trigger type field that indicates that the trigger frame corresponds to an MU ranging measurement exchange session and is for prompting an uplink MU NDP transmission. In an embodiment, the trigger frame further indicates what feedback information is to be included in a feedback packet to be transmitted by the network interface device  162 . In an embodiment, the received trigger frame is formatted according to the format described with respect to trigger frame  600 . 
     At block  1208 , the network interface device  162  processes the trigger frame received at block  1204  and determines that the trigger frame corresponds to an MU ranging measurement exchange session based on determining that a value in the trigger type field indicates that the trigger frame is for prompting an uplink MU NDP transmission as part of the MU ranging measurement exchange session. 
     At block  1212  and in response to determining that the value in the trigger type field indicates that the trigger frame is for prompting the uplink MU NDP transmission as part of the MU ranging measurement exchange session, the network interface device  162  transmits an uplink NDP as part of the uplink MU NDP transmission. In an embodiment, the uplink NDP is transmitted using MU-MIMO as part of an uplink MU NDP transmission from multiple communication devices. For instance, the network interface device  162  transmits an NDP  224  as described above in the context of the MU ranging measurement exchange  200 . 
     At block  1216 , the network interface device  162  receives a downlink NDP as part of the MU ranging measurement exchange session. For instance, the network interface device  162  receives an NDP  236  as described above in the context of the MU ranging measurement exchange  200 . 
     At block  1220 , the network interface device  162  generates feedback information, wherein the feedback information corresponds to the MU ranging measurement exchange session. In an embodiment, the feedback information includes information that was previously indicated, by the trigger frame, to be included in a feedback packet. In an embodiment, the feedback information includes information determined based on the transmission of the uplink NDP packet by the network interface device  162  and the reception of the downlink NDP packet at the network interface device  162 . 
     At block  1224 , the network interface device  162  transmits the generated feedback information in a feedback packet as part of an uplink MU feedback transmission in the MU ranging exchange session. 
     At least some of the various blocks, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. The software or firmware instructions may include machine readable instructions that, when executed by one or more processors, cause the one or more processors to perform various acts. 
     When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), a programmable logic device (PLD), etc. 
     While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the invention.