Patent Publication Number: US-2021185725-A1

Title: Dynamic backoff selection for wlan airtime fairnness

Description:
TECHNICAL FIELD 
     This disclosure relates generally to wireless networks, and more specifically, to medium access contention operations on a shared wireless medium. 
     DESCRIPTION OF THE RELATED TECHNOLOGY 
     A wireless local area network (WLAN) may be formed by one or more access points (APs) that provide a shared wireless communication medium for use by a number of client devices also referred to as stations (STAs). The basic building block of a WLAN conforming to the Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of standards is a Basic Service Set (BSS), which is managed by an AP. Each BSS is identified by a Basic Service Set Identifier (BSSID) that is advertised by the AP. An AP periodically broadcasts beacon frames to enable any STAs within wireless range of the AP to establish or maintain a communication link with the WLAN. 
     Many wireless networks use random channel access mechanisms to control access to a shared wireless medium. In these wireless networks, wireless communication devices (including APs and STAs) contend with each other to gain access to the wireless medium. The wireless communication device that wins the contention operation becomes the owner of a transmission opportunity (TXOP) and may use the wireless medium for a duration of the TXOP. Other wireless communication devices are generally not permitted to transmit during the TXOP to prevent interference with transmissions from the TXOP owner. 
     SUMMARY 
     The systems, methods and devices of this disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. 
     One innovative aspect of the subject matter described in this disclosure can be implemented in a method for wireless communication. The method may be performed by a wireless communication device, and may include initiating a countdown of a backoff counter associated with a medium access contention operation for transmitting uplink (UL) data on a shared wireless medium, detecting a presence of downlink (DL) data on the shared wireless medium, determining whether the wireless communication device is an intended recipient of the DL data, and adjusting the backoff counter based on the determination. In some implementations, adjusting the backoff counter based on the determination includes resetting the backoff counter to an initial value in response to determining that the wireless communication device is an intended recipient of the DL data. In some other implementations, adjusting the backoff counter based on the determination includes not adjusting the backoff counter in response to determining that the wireless communication device is not an intended recipient of the DL data. The method may also include stopping the countdown of the backoff counter in response to detecting the presence of the DL data, and continuing, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. 
     In some implementations, the method may also include receiving an indication of a contention window size to be used for one or more UL medium access contention operations, selecting a random backoff number from the indicated contention window size, and setting an initial value of the backoff counter based on the selected random backoff number. In some other implementations, the method may also include receiving, from an access point (AP), one or more beacon frames including the indicated contention window size. 
     In some implementations, the indicated contention window size may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, or a level of contention on the shared wireless medium. In addition, or in the alternative, the indicated contention window size may be based on a relationship between DL throughput and UL throughput on the shared wireless medium. 
     Another innovative aspect of the subject matter described in this disclosure can be implemented in a wireless communication device. In some implementations, the wireless communication device may include at least one modem, at least one processor communicatively coupled with the at least one modem, and at least one memory communicatively coupled with the at least one processor. The memory stores processor-readable code that, when executed by the at least one processor in conjunction with the at least one modem, causes the wireless communication device to perform one or more operations. In some implementations, the one or more operations may include initiating a countdown of a backoff counter associated with a medium access contention operation for transmitting uplink (UL) data on a shared wireless medium, detecting a presence of downlink (DL) data on the shared wireless medium, determining whether the wireless communication device is an intended recipient of the DL data, and adjusting the backoff counter based on the determination. In some implementations, adjusting the backoff counter based on the determination includes resetting the backoff counter to an initial value in response to determining that the wireless communication device is an intended recipient of the DL data. In some other implementations, adjusting the backoff counter based on the determination includes not adjusting the backoff counter in response to determining that the wireless communication device is not an intended recipient of the DL data. The one or more operations may also include stopping the countdown of the backoff counter in response to detecting the presence of the DL data, and continuing, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. 
     In some implementations, the one or more operations may also include receiving an indication of a contention window size to be used for one or more UL medium access contention operations, selecting a random backoff number from the indicated contention window size, and setting an initial value of the backoff counter based on the selected random backoff number. In some other implementations, the one or more operations may also include receiving, from an access point (AP), one or more beacon frames including the indicated contention window size. The indicated contention window size may be proportional to a number of devices associated with the AP. 
     In some implementations, the indicated contention window size may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, or a level of contention on the shared wireless medium. In addition, or in the alternative, the indicated contention window size may be based on a relationship between DL throughput and UL throughput on the shared wireless medium. 
     Another innovative aspect of the subject matter described in this disclosure can be implemented in a method for wireless communication. The method may be performed by a wireless communication device, and may include receiving an indication of a contention window size to be used for one or more medium access contention operations, selecting a random backoff number from the indicated contention window size, setting an initial value of the backoff counter based on the selected random backoff number, and contending for medium access, based on a countdown of the backoff counter, to transmit uplink (UL) data on the shared wireless medium. In some implementations, the indicated contention window size may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, or a level of contention on the shared wireless medium. The method may also include receiving, from an access point (AP), one or more beacon frames including the indicated contention window size. 
     In some implementations, contending for medium access may include initiating the countdown of the backoff counter, detecting a presence of downlink (DL) data on the shared wireless medium, determining whether the wireless communication device is an intended recipient of the DL data, and adjusting the backoff counter based on the determination. In some implementations, adjusting the backoff counter based on the determination includes resetting the backoff counter to an initial value in response to determining that the wireless communication device is an intended recipient of the DL data. In some other implementations, adjusting the backoff counter based on the determination includes not adjusting the backoff counter in response to determining that the wireless communication device is not an intended recipient of the DL data. In addition, or in the alternative, the method may also include stopping the countdown of the backoff counter in response to detecting the presence of the DL data, and continuing, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. 
     Another innovative aspect of the subject matter described in this disclosure can be implemented in a wireless communication device. In some implementations, the wireless communication device may include at least one modem, at least one processor communicatively coupled with the at least one modem, and at least one memory communicatively coupled with the at least one processor. The memory stores processor-readable code that, when executed by the at least one processor in conjunction with the at least one modem, causes the wireless communication device to perform one or more operations. In some implementations, the one or more operations may include receiving an indication of a contention window size to be used for one or more medium access contention operations, selecting a random backoff number from the indicated contention window size, setting an initial value of the backoff counter based on the selected random backoff number, and contending for medium access, based on a countdown of the backoff counter, to transmit uplink (UL) data on the shared wireless medium. In some implementations, the indicated contention window size may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, or a level of contention on the shared wireless medium. The one or more operations may also include receiving, from an access point (AP), one or more beacon frames including the indicated contention window size. 
     In some implementations, contending for medium access may include initiating the countdown of the backoff counter, detecting a presence of downlink (DL) data on the shared wireless medium, determining whether the wireless communication device is an intended recipient of the DL data, and adjusting the backoff counter based on the determination. In some implementations, adjusting the backoff counter based on the determination includes resetting the backoff counter to an initial value in response to determining that the wireless communication device is an intended recipient of the DL data. In some other implementations, adjusting the backoff counter based on the determination includes not adjusting the backoff counter in response to determining that the wireless communication device is not an intended recipient of the DL data. In addition, or in the alternative, the one or more operations may also include stopping the countdown of the backoff counter in response to detecting the presence of the DL data, and continuing, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. 
         FIG. 1  shows a pictorial diagram of an example wireless communication network. 
         FIG. 2A  shows an example protocol data unit (PDU) usable for communications between an access point (AP) and a number of stations (STAs). 
         FIG. 2B  shows an example field in the PDU of  FIG. 2A . 
         FIG. 3A  shows an example physical layer (PHY) preamble usable for communications between an AP and each of a number of STAs. 
         FIG. 3B  shows another example PHY preamble usable for communications between an AP and each of a number of stations. 
         FIG. 4  shows an example physical layer convergence protocol (PLCP) protocol data unit (PPDU) usable for communications between an AP and a number of STAs. 
         FIG. 5  shows a block diagram of an example wireless communication device. 
         FIG. 6A  shows a block diagram of an example AP. 
         FIG. 6B  shows a block diagram of an example STA. 
         FIG. 7  shows a timing diagram illustrating the transmissions of communications according to some implementations. 
         FIG. 8  shows a timing diagram illustrating the transmissions of communications according to some implementations. 
         FIG. 9  shows a sequence diagram illustrating the transmissions of communications according to other implementations. 
         FIG. 10  shows a flowchart illustrating an example process for wireless communication according to some implementations. 
         FIG. 11A  shows a flowchart illustrating an example process for wireless communication according to some implementations. 
         FIG. 11B  shows a flowchart illustrating an example process for wireless communication according to some implementations. 
         FIG. 11C  shows a flowchart illustrating an example process for wireless communication according to some implementations. 
         FIG. 12  shows a flowchart illustrating an example process for wireless communication according to some implementations. 
         FIG. 13  shows a block diagram of an example wireless communication device according to some implementations. 
         FIG. 14  shows a block diagram of another example wireless communication device according to some implementations. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The following description is directed to some particular implementations for the purposes of describing innovative aspects of this disclosure. However, a person having ordinary skill in the art will readily recognize that the teachings herein can be applied in a multitude of different ways. The described implementations can be implemented in any device, system, or network that is capable of transmitting and receiving radio frequency (RF) signals according to one or more of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, the IEEE 802.15 standards, the Bluetooth® standards as defined by the Bluetooth Special Interest Group (SIG), or the Long Term Evolution (LTE), 3G, 4G, or 5G (New Radio (NR)) standards promulgated by the 3rd Generation Partnership Project (3GPP), among others. The described implementations can be implemented in any device, system, or network that is capable of transmitting and receiving RF signals according to one or more of the following technologies or techniques: code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrier FDMA (SC-FDMA), single-user (SU) multiple-input multiple-output (MIMO), and multi-user (MU) MIMO. The described implementations also can be implemented using other wireless communication protocols or RF signals suitable for use in one or more of a wireless personal area network (WPAN), a wireless local area network (WLAN), a wireless wide area network (WWAN), or an internet of things (TOT) network. 
     Various implementations relate generally to medium access contention operations on a shared wireless medium. Some implementations more specifically relate to maintaining a balance between downlink (DL) and uplink (UL) throughput in a wireless network based on one or more of detection of an increase in UL traffic relative to DL traffic in the wireless network, detection of an increase in the level of contention in the wireless network, or detection of an increase in the number of collisions in the wireless network. 
     In some implementations, a wireless communication device contending for medium access to transmit UL data on a shared wireless medium may detect a presence of downlink (DL) data on the shared wireless medium, and may adjust its backoff counter based on whether the wireless communication device is an intended recipient of the DL data. The wireless communication device may not adjust the backoff counter when it is not the intended recipient of the DL data, and the wireless communication device may adjust the backoff counter when it is one of the intended recipients of the DL data. In some implementations, not adjusting the backoff counter may include stopping the countdown of the backoff counter in response to detecting the presence of the DL data, and continuing, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. In some implementations, adjusting the backoff counter may include resetting the backoff counter to an initial value in response to receiving the DL data. 
     In other implementations, an AP may indicate a contention window size to be used by one or more associated wireless communication devices for medium access contention operations. The indicated contention window size may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, or a level of contention on the shared wireless medium. For example, in response to detecting an increase in the number of actively associated wireless communication devices, the AP may indicate a larger contention window size from which the wireless communication devices are to randomly select their backoff numbers. 
     Particular implementations of the subject matter described in this disclosure can be implemented to realize one or more of the following potential advantages. By resetting the backoff counter to an initial value in response to receiving DL data, rather than pausing the backoff counter, a wireless communication device may have a longer backoff period during one or more subsequent medium access contention operations on a wireless medium, thereby decreasing a likelihood of the wireless communication device obtaining a transmit opportunity (TXOP) to transmit UL data on the wireless medium during the one or more subsequent medium access contention operations. Reducing or at least delaying UL transmissions from wireless communication devices that also received DL data may decrease the amount of UL traffic relative to the amount of DL traffic, and thereby reduce asymmetries between UL and DL throughput on the wireless medium. As such, implementations of the subject matter described in this disclosure may be used to increase the symmetry between UL and DL throughput on the wireless medium, for example, to increase the number of traffic flows having substantially symmetrical UL and DL traffic (such as voice and video calls) that can be supported by a wireless network that includes a given number of associated wireless communication devices (such as STAs). 
     In addition, the ability to indicate or modify a contention window size to be used by one or more wireless communication devices for medium access contention operations may allow the AP to at least partially determine or adjust the contention backoff periods of the wireless communication devices, and therefore at least partially determine or adjust a likelihood of the wireless communication devices winning a particular medium access contention operation and transmitting UL data during a corresponding TXOP. For example, in response to detecting an imbalance between UL and DL throughput, the AP may indicate a larger contention window size to be used by the wireless communication devices. Increasing the range of numbers from which the wireless communication devices randomly select backoff numbers for their backoff counters may result in an increased average backoff counter value, and as such, may decrease a likelihood of the wireless communication devices obtaining a TXOP for UL transmissions in a given medium access contention operation. 
       FIG. 1  shows a block diagram of an example wireless communication network  100 . According to some aspects, the wireless communication network  100  can be an example of a wireless local area network (WLAN) such as a Wi-Fi network (and will hereinafter be referred to as WLAN  100 ). For example, the WLAN  100  can be a network implementing at least one of the IEEE 802.11 family of wireless communication protocol standards (such as that defined by the IEEE 802.11-2016 specification or amendments thereof including, but not limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az, 802.11ba, and 802.11be). The WLAN  100  may include numerous wireless communication devices such as an access point (AP)  102  and multiple stations (STAs)  104 . While only one AP  102  is shown, the WLAN network  100  also can include multiple APs  102 . 
     Each of the STAs  104  also may be referred to as a mobile station (MS), a mobile device, a mobile handset, a wireless handset, an access terminal (AT), a user equipment (UE), a subscriber station (SS), or a subscriber unit, among other possibilities. The STAs  104  may represent various devices such as mobile phones, personal digital assistants (PDAs), other handheld devices, netbooks, notebook computers, tablet computers, laptops, display devices (for example, TVs, computer monitors, navigation systems, among others), music or other audio or stereo devices, remote control devices (“remotes”), printers, kitchen or other household appliances, key fobs (for example, for passive keyless entry and start (PKES) systems), among other possibilities. 
     A single AP  102  and an associated set of STAs  104  may be referred to as a basic service set (BSS), which is managed by the respective AP  102 .  FIG. 1  additionally shows an example coverage area  108  of the AP  102 , which may represent a basic service area (BSA) of the WLAN  100 . The BSS may be identified to users by a service set identifier (SSID), as well as to other devices by a basic service set identifier (BSSID), which may be a medium access control (MAC) address of the AP  102 . The AP  102  periodically broadcasts beacon frames (“beacons”) including the BSSID to enable any STAs  104  within wireless range of the AP  102  to “associate” or re-associate with the AP  102  to establish a respective communication link  106  (hereinafter also referred to as a “Wi-Fi link”), or to maintain a communication link  106 , with the AP  102 . For example, the beacons can include an identification of a primary channel used by the respective AP  102  as well as a timing synchronization function for establishing or maintaining timing synchronization with the AP  102 . The AP  102  may provide access to external networks to various STAs  104  in the WLAN via respective communication links  106 . 
     To establish a communication link  106  with an AP  102 , each of the STAs  104  is configured to perform passive or active scanning operations (“scans”) on frequency channels in one or more frequency bands (for example, the 2.4 GHz, 5 GHz, 6 GHz, or 60 GHz bands). To perform passive scanning, a STA  104  listens for beacons, which are transmitted by respective APs  102  at a periodic time interval referred to as the target beacon transmission time (TBTT) (measured in time units (TUs) where one TU may be equal to 1024 microseconds (μs)). To perform active scanning, a STA  104  generates and sequentially transmits probe requests on each channel to be scanned and listens for probe responses from APs  102 . Each STA  104  may be configured to identify or select an AP  102  with which to associate based on the scanning information obtained through the passive or active scans and to perform authentication and association operations to establish a communication link  106  with the selected AP  102 . The AP  102  assigns an association identifier (AID) to the STA  104  at the culmination of the association operations, which the AP  102  uses to track the STA  104 . 
     As a result of the increasing ubiquity of wireless networks, a STA  104  may have the opportunity to select one of many BSSs within range of the STA or to select among multiple APs  102  that together form an extended service set (ESS) including multiple connected BSSs. An extended network station associated with the WLAN  100  may be connected to a wired or wireless distribution system that may allow multiple APs  102  to be connected in such an ESS. As such, a STA  104  can be covered by more than one AP  102  and can associate with different APs  102  at different times for different transmissions. Additionally, after association with an AP  102 , a STA  104  also may be configured to periodically scan its surroundings to find a more suitable AP  102  with which to associate. For example, a STA  104  that is moving relative to its associated AP  102  may perform a “roaming” scan to find another AP  102  having more desirable network characteristics such as a greater received signal strength indicator (RSSI) or a reduced traffic load. 
     In some cases, STAs  104  may form networks without APs  102  or other equipment other than the STAs  104  themselves. One example of such a network is an ad hoc network (or wireless ad hoc network). Ad hoc networks may alternatively be referred to as mesh networks or peer-to-peer (P2P) networks. In some cases, ad hoc networks may be implemented within a larger wireless network such as the WLAN  100 . In such implementations, while the STAs  104  may be capable of communicating with each other through the AP  102  using communication links  106 , STAs  104  also can communicate directly with each other via direct wireless links  110 . Additionally, two STAs  104  may communicate via a direct communication link  110  regardless of whether both STAs  104  are associated with and served by the same AP  102 . In such an ad hoc system, one or more of the STAs  104  may assume the role filled by the AP  102  in a BSS. Such a STA  104  may be referred to as a group owner (GO) and may coordinate transmissions within the ad hoc network. Examples of direct wireless links  110  include Wi-Fi Direct connections, connections established by using a Wi-Fi Tunneled Direct Link Setup (TDLS) link, and other P2P group connections. 
     The APs  102  and STAs  104  may function and communicate (via the respective communication links  106 ) according to the IEEE 802.11 family of wireless communication protocol standards (such as that defined by the IEEE 802.11-2016 specification or amendments thereof including, but not limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az, 802.11ba, and 802.11be). These standards define the WLAN radio and baseband protocols for the PHY and medium access control (MAC) layers. The APs  102  and STAs  104  transmit and receive wireless communications (hereinafter also referred to as “Wi-Fi communications”) to and from one another in the form of physical layer convergence protocol (PLCP) protocol data units (PPDUs). The APs  102  and STAs  104  in the WLAN  100  may transmit PPDUs over an unlicensed spectrum, which may be a portion of spectrum that includes frequency bands traditionally used by Wi-Fi technology, such as the 2.4 GHz band, the 5 GHz band, the 60 GHz band, the 3.6 GHz band, and the 900 MHz band. Some implementations of the APs  102  and STAs  104  described herein also may communicate in other frequency bands, such as the 6 GHz band, which may support both licensed and unlicensed communications. The APs  102  and STAs  104  also can be configured to communicate over other frequency bands such as shared licensed frequency bands, where multiple operators may have a license to operate in the same or overlapping frequency band or bands. 
     Each of the frequency bands may include multiple sub-bands or frequency channels. For example, PPDUs conforming to the IEEE 802.11n, 802.11ac, and 802.11ax standard amendments may be transmitted over the 2.4 and 5 GHz bands, each of which is divided into multiple 20 MHz channels. As such, these PPDUs are transmitted over a physical channel having a minimum bandwidth of 20 MHz, but larger channels can be formed through channel bonding. For example, PPDUs may be transmitted over physical channels having bandwidths of 40 MHz, 80 MHz, 160 MHz, or 320 MHz by bonding together multiple 20 MHz channels. 
     Each PPDU is a composite structure that includes a PHY preamble and a payload in the form of a PLCP service data unit (PSDU). The information provided in the preamble may be used by a receiving device to decode the subsequent data in the PSDU. In instances in which PPDUs are transmitted over a bonded channel, the preamble fields may be duplicated and transmitted in each of the multiple component channels. The PHY preamble may include both a legacy portion (or “legacy preamble”) and a non-legacy portion (or “non-legacy preamble”). The legacy preamble may be used for packet detection, automatic gain control, and channel estimation, among other uses. The legacy preamble also may generally be used to maintain compatibility with legacy devices. The format of, coding of, and information provided in the non-legacy portion of the preamble is based on the particular IEEE 802.11 protocol to be used to transmit the payload. 
       FIG. 2A  shows an example protocol data unit (PDU)  200  usable for wireless communication between an AP and a number of STAs. For example, the PDU  200  can be configured as a PPDU. As shown, the PDU  200  includes a PHY preamble  202  and a PHY payload  204 . For example, the preamble  202  may include a legacy portion that itself includes a legacy short training field (L-STF)  206 , which may consist of two binary phase shift keying (BPSK) symbols, a legacy long training field (L-LTF)  208 , which may consist of two BPSK symbols, and a legacy signal field (L-SIG)  210 , which may consist of two BPSK symbols. The legacy portion of the preamble  202  may be configured according to the IEEE 802.11a wireless communication protocol standard. The preamble  202  also may include a non-legacy portion including one or more non-legacy fields  212 , for example, conforming to an IEEE wireless communication protocol, such as the IEEE 802.11ac, 802.11ax, 802.11be, or later wireless communication protocol standards. 
     The L-STF  206  generally enables a receiving device to perform automatic gain control (AGC) and coarse timing and frequency estimation. The L-LTF  208  generally enables a receiving device to perform fine timing and frequency estimation and also to estimate of the wireless channel. The L-SIG  210  generally enables a receiving device to determine a duration of the PDU and to use the determined duration to avoid transmitting on top of the PDU. For example, the L-STF  206 , the L-LTF  208 , and the L-SIG  210  may be modulated according to a BPSK modulation scheme. The payload  204  may be modulated according to a BPSK modulation scheme, a quadrature BPSK (Q-BPSK) modulation scheme, a quadrature amplitude modulation (QAM) scheme, or another appropriate modulation scheme. The payload  204  may generally carry higher layer data, for example, in the form of medium access control (MAC) protocol data units (MPDUs) or aggregated MPDUs (A-MPDUs). 
       FIG. 2B  shows an example L-SIG  210  in the PDU  200  of  FIG. 2A . The L-SIG  210  includes a data rate field  222 , a reserved bit  224 , a length field  226 , a parity bit  228 , and a tail field  230 . The data rate field  222  indicates a data rate (note that the data rate indicated in the data rate field  222  may not be the actual data rate of the data carried in the payload  204 ). The length field  226  indicates a length of the packet in units of, for example, symbols or bytes. The parity bit  228  may be used to detect bit errors. The tail field  230  includes tail bits that may be used by the receiving device to terminate operation of a decoder (for example, a Viterbi decoder). The receiving device may utilize the data rate and the length indicated in the data rate field  222  and the length field  226  to determine a duration of the packet in units of, for example, microseconds (μs) or other time units. 
       FIG. 3A  shows another example PDU  300  usable for wireless communication between an AP and a number of STAs. The PDU  300  includes a PHY preamble including a legacy portion  302  and a non-legacy portion  304 . The PDU  300  may further include a PHY payload  306  after the preamble, for example, in the form of a PSDU including a DATA field  322 . The legacy portion  302  of the preamble includes L-STF  308 , L-LTF  310 , and L-SIG  312 . The non-legacy portion  304  of the preamble and the DATA field  322  may be formatted as a Very High Throughput (VHT) preamble and frame, respectively, in accordance with the IEEE 802.11ac amendment to the IEEE 802.11 wireless communication protocol standard. The non-legacy portion  304  includes a first VHT signal field (VHT-SIG-A)  314 , a VHT short training field (VHT-STF)  316 , a number of VHT long training fields (VHT-LTFs)  318 , and a second VHT signal field (VHT-SIG-B)  320  encoded separately from VHT-SIG-A  314 . Like the L-STF  308 , L-LTF  310 , and L-SIG  312 , the information in VHT-SIG-A  314  may be duplicated and transmitted in each of the component 20 MHz channels in instances involving the use of a bonded channel. 
     VHT-STF  316  may be used to improve AGC estimation in a MIMO transmission. VHT-LTFs  318  may be used for MIMO channel estimation and pilot subcarrier tracking. The preamble may include one VHT-LTF  318  for each spatial stream the preamble is transmitted on. VHT-SIG-A  314  may indicate to VHT-compatible APs  102  and STAs  104  that the PPDU is a VHT PPDU. VHT-SIG-A  314  includes signaling information and other information usable by STAs  104  to decode VHT-SIG-B  320 . VHT-SIG-A  314  may indicate a bandwidth (BW) of the packet, the presence of space-time block coding (STBC), the number N STS  of space-time streams per user, a Group ID indicating the group and user position assigned to a STA, a partial association identifier that may combine the AID and the BSSID, a short guard interval (GI) indication, a single-user/multi-user (SU/MU) coding indicating whether convolutional or LDPC coding is used, a modulation and coding scheme (MCS), an indication of whether a beamforming matrix has been applied to the transmission, a cyclic redundancy check (CRC), and a tail. VHT-SIG-B  320  may be used for MU transmissions and may contain the actual data rate and MPDU or A-MPDU length values for each of the multiple STAs  104 , as well as signaling information usable by the STAs  104  to decode data received in the DATA field  322 , including, for example, an MCS and beamforming information. 
       FIG. 3B  shows another example PDU  350  usable for wireless communication between an AP and a number of STAs. The PDU  350  may be used for MU-OFDMA or MU-MIMO transmissions. The PDU  350  includes a PHY preamble including a legacy portion  352  and a non-legacy portion  354 . The PDU  350  may further include a PHY payload  356  after the preamble, for example, in the form of a PSDU including a DATA field  374 . The legacy portion  352  includes L-STF  358 , L-LTF  360 , and L-SIG  362 . The non-legacy portion  354  of the preamble and the DATA field  374  may be formatted as a High Efficiency (HE) WLAN preamble and frame, respectively, in accordance with the IEEE 802.11ax amendment to the IEEE 802.11 wireless communication protocol standard. The non-legacy portion  354  includes a repeated legacy signal field (RL-SIG)  364 , a first HE signal field (HE-SIG-A)  366 , a second HE signal field (HE-SIG-B)  368  encoded separately from HE-SIG-A  366 , an HE short training field (HE-STF)  370 , and a number of HE long training fields (HE-LTFs)  372 . Like the L-STF  358 , L-LTF  360 , and L-SIG  362 , the information in RL-SIG  364  and HE-SIG-A  366  may be duplicated and transmitted in each of the component 20 MHz channels in instances involving the use of a bonded channel. In contrast, HE-SIG-B  368  may be unique to each 20 MHz channel and may target specific STAs  104 . 
     RL-SIG  364  may indicate to HE-compatible STAs  104  that the PPDU is an HE PPDU. An AP  102  may use HE-SIG-A  366  to identify and inform multiple STAs  104  that the AP has scheduled UL or DL resources for them. HE-SIG-A  366  may be decoded by each HE-compatible STA  104  served by the AP  102 . HE-SIG-A  366  includes information usable by each identified STA  104  to decode an associated HE-SIG-B  368 . For example, HE-SIG-A  366  may indicate the frame format, including locations and lengths of HE-SIG-Bs  368 , available channel bandwidths, modulation and coding schemes (MCSs), among other possibilities. HE-SIG-A  366  also may include HE WLAN signaling information usable by STAs  104  other than the number of identified STAs  104 . 
     HE-SIG-B  368  may carry STA-specific scheduling information such as, for example, per-user MCS values and per-user RU allocation information. In the context of DL MU-OFDMA, such information enables the respective STAs  104  to identify and decode corresponding RUs in the associated data field. Each HE-SIG-B  368  includes a common field and at least one STA-specific (“user-specific”) field. The common field can indicate RU distributions to multiple STAs  104 , indicate the RU assignments in the frequency domain, indicate which RUs are allocated for MU-MIMO transmissions and which RUs correspond to MU-OFDMA transmissions, and the number of users in allocations, among other possibilities. The common field may be encoded with common bits, CRC bits, and tail bits. The user-specific fields are assigned to particular STAs  104  and may be used to schedule specific RUs and to indicate the scheduling to other WLAN devices. Each user-specific field may include multiple user block fields (which may be followed by padding). Each user block field may include two user fields that contain information for two respective STAs to decode their respective RU payloads in DATA field  374 . 
       FIG. 4  shows an example PPDU  400  usable for communications between an AP  102  and a number of STAs  104 . As described above, each PPDU  400  includes a PHY preamble  402  and a PSDU  404 . Each PSDU  404  may represent (or “carry”) one or more MAC protocol data units (MPDUs)  416 . For example, each PSDU  404  may carry an aggregated MPDU (A-MPDU)  406  that includes an aggregation of multiple A-MPDU subframes  408 . Each A-MPDU subframe  406  may include an MPDU frame  410  that includes a MAC delimiter  412  and a MAC header  414  prior to the accompanying MPDU  416 , which comprises the data portion (“payload” or “frame body”) of the MPDU frame  410 . Each MPDU frame  410  may also include a frame check sequence (FCS) field  418  for error detection (for example, the FCS field may include a cyclic redundancy check (CRC)) and padding bits  420 . The MPDU  416  may carry one or more MAC service data units (MSDUs)  430 . For example, the MPDU  416  may carry an aggregated MSDU (A-MSDU)  422  including multiple A-MSDU subframes  424 . Each A-MSDU subframe  424  contains a corresponding MSDU  430  preceded by a subframe header  428  and in some cases followed by padding bits  432 . 
     Referring back to the MPDU frame  410 , the MAC delimiter  412  may serve as a marker of the start of the associated MPDU  416  and indicate the length of the associated MPDU  416 . The MAC header  414  may include a number of fields containing information that defines or indicates characteristics or attributes of data encapsulated within the frame body  416 . The MAC header  414  includes a duration field indicating a duration extending from the end of the PPDU until at least the end of an acknowledgment (ACK) or Block ACK (BA) of the PPDU that is to be transmitted by the receiving wireless communication device. The use of the duration field serves to reserve the wireless medium for the indicated duration and enables the receiving device to establish its network allocation vector (NAV). The MAC header  414  also includes a number of fields indicating addresses for the data encapsulated within the frame body  416 . For example, the MAC header  414  may include a combination of a source address, a transmitter address, a receiver address, or a destination address. The MAC header  414  may further include a frame control field containing control information. The frame control field may specify a frame type, for example, a data frame, a control frame, or a management frame. 
     Access to the shared wireless medium is generally governed by a distributed coordination function (DCF). With a DCF, there is generally no centralized master device allocating time and frequency resources of the shared wireless medium. On the contrary, before a wireless communication device, such as an AP  102  or a STA  104 , is permitted to transmit data, it must wait for a particular time and then contend for access to the wireless medium. In some implementations, the wireless communication device may be configured to implement the DCF through the use of carrier sense multiple access (CSMA) with collision avoidance (CA) (CSMA/CA) techniques and timing intervals. Before transmitting data, the wireless communication device may perform a clear channel assessment (CCA) and determine that the appropriate wireless channel is idle. The CCA includes both physical (PHY-level) carrier sensing and virtual (MAC-level) carrier sensing. Physical carrier sensing (or packet detection (PD)) is accomplished via a measurement of the received signal strength of a valid frame, which is then compared to a value to determine whether the channel is busy. For example, if the received signal strength of a detected preamble is above the value, the medium is considered busy. Physical carrier sensing also includes energy detection (ED). Energy detection involves measuring the total energy the wireless communication device receives regardless of whether the received signal represents a valid frame. If the total energy detected is above a value, the medium is considered busy. Virtual carrier sensing is accomplished via the use of a network allocation vector (NAV), an indicator of a time when the medium may next become idle. The NAV is reset each time a valid frame is received that is not addressed to the wireless communication device. The NAV effectively serves as a time duration that must elapse before the wireless communication device may contend for access even in the absence of a detected symbol or even if the detected energy is below the value. 
     As described above, the DCF is implemented through the use of time intervals. These time intervals include the slot time (or “slot interval”) and the inter-frame space (IFS). The slot time is the basic unit of timing and may be determined based on one or more of a transmit-receive turnaround time, a channel sensing time, a propagation delay, and a MAC processing time. Measurements for channel sensing are performed for each slot. All transmissions may begin at slot boundaries. Example varieties of IFS include: the short IFS (SIFS), the distributed IFS (DIFS), the extended IFS (EIFS), or the arbitration IFS (AIFS). For example, the DIFS may be defined as the sum of the SIFS and two times the slot time. The values for the slot time and IFS may be provided by a suitable standard specification, such as one of the IEEE 802.11 family of wireless communication protocol standards (such as that defined by the IEEE 802.11-2016 specification or amendments thereof including, but not limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az, 802.11ba, and 802.11be). 
     When the NAV reaches 0, the wireless communication device performs physical carrier sensing. If the channel remains idle for the appropriate IFS (for example, a DIFS), the wireless communication device initiates a backoff timer, which represents a duration of time that the device must sense the medium to be idle before it is permitted to transmit. The backoff timer is decremented by one slot each time the medium is sensed to be idle during a corresponding slot interval. If the channel remains idle until the backoff timer expires, the wireless communication device becomes the holder (or “owner”) of a transmit opportunity (TXOP) and may begin transmitting. The TXOP is the duration of time the wireless communication device can transmit frames over the channel after it has won contention for the wireless medium. If, on the other hand, one or more of the carrier sense mechanisms indicate that the channel is busy, a MAC controller within the wireless communication device will not permit transmission. 
     Each time the wireless communication device generates a new PPDU for transmission in a new TXOP, it randomly selects a new backoff timer duration. The available distribution of numbers that may be randomly selected for the backoff timer is referred to as the contention window (CW). If, when the backoff timer expires, the wireless communication device transmits the PPDU, but the medium is still busy, there may be a collision. Additionally, if there is otherwise too much energy on the wireless channel resulting in a poor signal-to-noise ratio (SNR), the communication may be corrupted or otherwise not successfully received. In such instances, the wireless communication device may not receive a communication acknowledging the transmitted PDU within a timeout interval. The MAC may then increase the CW exponentially, for example, doubling it, and randomly select a new backoff timer duration from the CW before each attempted retransmission of the PPDU. Before each attempted retransmission, the wireless communication device may wait a duration of DIFS and, if the medium remains idle, proceed to initiate the new backoff timer. There are different CW and TXOP durations for each of the four access categories (ACs): voice (AC_VO), video (AC_VI), background (AC_BK), and best effort (AC_BE). This enables particular types of traffic to be prioritized in the network. 
     As described above, APs  102  and STAs  104  can support multi-user (MU) communications; that is, concurrent transmissions from one device to each of multiple devices (for example, multiple simultaneous downlink (DL) communications from an AP  102  to corresponding STAs  104 ), or concurrent transmissions from multiple devices to a single device (for example, multiple simultaneous uplink (UL) transmissions from the corresponding STAs  104  to the AP  102 ). To support the MU transmissions, the APs  102  and the STAs  104  may utilize multi-user multiple-input, multiple-output (MU-MIMO) and multi-user orthogonal frequency division multiple access (MU-OFDMA) techniques. 
     In MU-OFDMA schemes, the available frequency spectrum of the wireless channel may be divided into multiple resource units (RUs) each including a number of different frequency subcarriers (“tones”). Different RUs may be allocated or assigned by an AP  102  to different STAs  104  at particular times. The sizes and distributions of the RUs may be referred to as an RU allocation. In some implementations, RUs may be allocated in 2 MHz intervals, and as such, the smallest RU may include 26 tones consisting of 24 data tones and 2 pilot tones. Consequently, in a 20 MHz channel, up to 9 RUs (such as 2 MHz, 26-tone RUs) may be allocated (because some tones are reserved for other purposes). Similarly, in a 160 MHz channel, up to 74 RUs may be allocated. Larger 52-tone, 106-tone, 242-tone, 484-tone, and 996-tone RUs also may be allocated. Adjacent RUs may be separated by a null subcarrier (such as a DC subcarrier), for example, to reduce interference between adjacent RUs, to reduce receiver DC offset, and to avoid transmit center frequency leakage. 
     For UL MU transmissions, an AP  102  can transmit a trigger frame to initiate and synchronize an UL MU-OFDMA or an UL MU-MIMO transmission from multiple STAs  104  to the AP  102 . Such trigger frames may thus enable multiple STAs  104  to send UL traffic to the AP  102  concurrently in time. A trigger frame may address one or more STAs  104  through respective association identifiers (AIDs) and may assign each AID (and thus, each STA  104 ) one or more RUs that can be used to send UL traffic to the AP  102 . The AP also may designate one or more random access (RA) RUs that unscheduled STAs  104  may contend for. 
       FIG. 5  shows a block diagram of an example wireless communication device  500 . In some implementations, the wireless communication device  500  can be an example of a device for use in a STA such as one of the STAs  104  described above with reference to  FIG. 1 . In some implementations, the wireless communication device  500  can be an example of a device for use in an AP such as the AP  102  described above with reference to  FIG. 1 . The wireless communication device  500  is capable of transmitting (or outputting for transmission) and receiving wireless communications (for example, in the form of wireless packets). For example, the wireless communication device can be configured to transmit and receive packets in the form of physical layer convergence protocol (PLCP) protocol data units (PPDUs) and medium access control (MAC) protocol data units (MPDUs) conforming to an IEEE 802.11 wireless communication protocol standard, such as that defined by the IEEE 802.11-2016 specification or amendments thereof including, but not limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az, 802.11ba, and 802.11be. 
     The wireless communication device  500  can be, or can include, a chip, system on chip (SoC), chipset, package, or device that includes one or more modems  502 , for example, a Wi-Fi (IEEE 802.11 compliant) modem. In some implementations, the one or more modems  502  (collectively “the modem  502 ”) additionally include a WWAN modem (for example, a 3GPP 4G LTE or 5G compliant modem). In some implementations, the wireless communication device  500  also includes one or more radios  504  (collectively “the radio  504 ”). In some implementations, the wireless communication device  506  further includes one or more processors, processing blocks, or processing elements  506  (collectively “the processor  506 ”), and one or more memory blocks or elements  508  (collectively “the memory  508 ”). 
     The modem  502  can include an intelligent hardware block or device such as, for example, an application-specific integrated circuit (ASIC) among other possibilities. The modem  502  is generally configured to implement a PHY layer. For example, the modem  502  is configured to modulate packets and to output the modulated packets to the radio  504  for transmission over the wireless medium. The modem  502  is similarly configured to obtain modulated packets received by the radio  504  and to demodulate the packets to provide demodulated packets. In addition to a modulator and a demodulator, the modem  502  may further include digital signal processing (DSP) circuitry, automatic gain control (AGC), a coder, a decoder, a multiplexer, and a demultiplexer. For example, while in a transmission mode, data obtained from the processor  506  is provided to a coder, which encodes the data to provide encoded bits. The encoded bits are then mapped to points in a modulation constellation (using a selected MCS) to provide modulated symbols. The modulated symbols may then be mapped to a number N SS  of spatial streams or a number N STS  of space-time streams. The modulated symbols in the respective spatial or space-time streams may then be multiplexed, transformed via an inverse fast Fourier transform (IFFT) block, and subsequently provided to the DSP circuitry for Tx windowing and filtering. The digital signals may then be provided to a digital-to-analog converter (DAC). The resultant analog signals may then be provided to a frequency upconverter, and ultimately, the radio  504 . In implementations involving beamforming, the modulated symbols in the respective spatial streams are precoded via a steering matrix prior to their provision to the IFFT block. 
     While in a reception mode, digital signals received from the radio  504  are provided to the DSP circuitry, which is configured to acquire a received signal, for example, by detecting the presence of the signal and estimating the initial timing and frequency offsets. The DSP circuitry is further configured to digitally condition the digital signals, for example, using channel (narrowband) filtering, analog impairment conditioning (such as correcting for I/Q imbalance), and applying digital gain to ultimately obtain a narrowband signal. The output of the DSP circuitry may then be fed to the AGC, which is configured to use information extracted from the digital signals, for example, in one or more received training fields, to determine an appropriate gain. The output of the DSP circuitry also is coupled with the demodulator, which is configured to extract modulated symbols from the signal and, for example, compute the logarithm likelihood ratios (LLRs) for each bit position of each subcarrier in each spatial stream. The demodulator is coupled with the decoder, which may be configured to process the LLRs to provide decoded bits. The decoded bits from all of the spatial streams are then fed to the demultiplexer for demultiplexing. The demultiplexed bits may then be descrambled and provided to the MAC layer (the processor  506 ) for processing, evaluation, or interpretation. 
     The radio  504  generally includes at least one radio frequency (RF) transmitter (or “transmitter chain”) and at least one RF receiver (or “receiver chain”), which may be combined into one or more transceivers. For example, the RF transmitters and receivers may include various DSP circuitry including at least one power amplifier (PA) and at least one low-noise amplifier (LNA), respectively. The RF transmitters and receivers may, in turn, be coupled to one or more antennas. For example, in some implementations, the wireless communication device  500  can include, or be coupled with, multiple transmit antennas (each with a corresponding transmit chain) and multiple receive antennas (each with a corresponding receive chain). The symbols output from the modem  502  are provided to the radio  504 , which then transmits the symbols via the coupled antennas. Similarly, symbols received via the antennas are obtained by the radio  504 , which then provides the symbols to the modem  502 . 
     The processor  506  can include an intelligent hardware block or device such as, for example, a processing core, a processing block, a central processing unit (CPU), a microprocessor, a microcontroller, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a programmable logic device (PLD) such as a field programmable gate array (FPGA), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. The processor  506  processes information received through the radio  504  and the modem  502 , and processes information to be output through the modem  502  and the radio  504  for transmission through the wireless medium. For example, the processor  506  may implement a control plane and MAC layer configured to perform various operations related to the generation and transmission of MPDUs, frames, or packets. The MAC layer is configured to perform or facilitate the coding and decoding of frames, spatial multiplexing, space-time block coding (STBC), beamforming, and OFDMA resource allocation, among other operations or techniques. In some implementations, the processor  506  may generally control the modem  502  to cause the modem to perform various operations described above. 
     The memory  504  can include tangible storage media such as random-access memory (RAM) or read-only memory (ROM), or combinations thereof. The memory  504  also can store non-transitory processor- or computer-executable software (SW) code containing instructions that, when executed by the processor  506 , cause the processor to perform various operations described herein for wireless communication, including the generation, transmission, reception, and interpretation of MPDUs, frames, or packets. For example, various functions of components disclosed herein, or various blocks or steps of a method, operation, process, or algorithm disclosed herein, can be implemented as one or more modules of one or more computer programs. 
       FIG. 6A  shows a block diagram of an example AP  602 . For example, the AP  602  can be an example implementation of the AP  102  described with reference to  FIG. 1 . The AP  602  includes a wireless communication device (WCD)  610  (although the AP  602  may itself also be referred to generally as a wireless communication device as used herein). For example, the wireless communication device  610  may be an example implementation of the wireless communication device  500  described with reference to  FIG. 5 . The AP  602  also includes multiple antennas  620  coupled with the wireless communication device  610  to transmit and receive wireless communications. In some implementations, the AP  602  additionally includes an application processor  630  coupled with the wireless communication device  610 , and a memory  640  coupled with the application processor  630 . The AP  602  further includes at least one external network interface  650  that enables the AP  602  to communicate with a core network or backhaul network to gain access to external networks including the Internet. For example, the external network interface  650  may include one or both of a wired (for example, Ethernet) network interface and a wireless network interface (such as a WWAN interface). Ones of the aforementioned components can communicate with other ones of the components directly or indirectly, over at least one bus. The AP  602  further includes a housing that encompasses the wireless communication device  610 , the application processor  630 , the memory  640 , and at least portions of the antennas  620  and external network interface  650 . 
       FIG. 6B  shows a block diagram of an example STA  604 . For example, the STA  604  can be an example implementation of the STA  104  described with reference to  FIG. 1 . The STA  604  includes a wireless communication device  615  (although the STA  604  may itself also be referred to generally as a wireless communication device as used herein). For example, the wireless communication device  615  may be an example implementation of the wireless communication device  500  described with reference to  FIG. 5 . The STA  604  also includes one or more antennas  625  coupled with the wireless communication device  615  to transmit and receive wireless communications. The STA  604  additionally includes an application processor  635  coupled with the wireless communication device  615 , and a memory  645  coupled with the application processor  635 . In some implementations, the STA  604  further includes a user interface (UI)  655  (such as a touchscreen or keypad) and a display  665 , which may be integrated with the UI  655  to form a touchscreen display. In some implementations, the STA  604  may further include one or more sensors  675  such as, for example, one or more inertial sensors, accelerometers, temperature sensors, pressure sensors, or altitude sensors. Ones of the aforementioned components can communicate with other ones of the components directly or indirectly, over at least one bus. The STA  604  further includes a housing that encompasses the wireless communication device  615 , the application processor  635 , the memory  645 , and at least portions of the antennas  625 , UI  655 , and display  665 . 
     As mentioned above, wireless communication devices may contend with each other for access to a shared wireless medium. The IEEE 802.11 standards define a distributed coordination function (DCF) in which wireless communication devices use carrier sensing techniques to determine that the wireless medium has been idle for a period of time before attempting to transmit data on the wireless medium. Many wireless communication devices employ an Enhanced Distributed Channel Access (EDCA) mechanism for medium access contention operations. The EDCA mechanism is an example of a listen-before-talk (LBT) channel access mechanism, and may prevent multiple devices from accessing the wireless medium at the same time by arbitrating access to the wireless medium using randomly selected numbers representing periods of time during which the wireless medium is to remain idle before a given wireless communication device may transmit on the wireless medium. 
     As the number of wireless communication devices associated with an AP increases, the likelihood of collisions on the wireless medium also increases, which may decrease throughput of the wireless network. The ability to provide a certain quality-of-service (QoS) in a wireless network may depend on the throughput of the wireless network. Decreases in either UL throughput or DL throughput of the wireless network may reduce an AP&#39;s ability to guarantee certain levels of QoS for time-critical traffic flows (such as voice and video calls). Further, the presence of legacy devices that do not support multiple-access communication schemes (such as Orthogonal Frequency-Division Multiple Access (OFDMA) modulation schemes) in a wireless network may decrease DL throughput to a greater extent than UL throughput, and may therefore exacerbate imbalances between UL and DL throughput of the wireless network. Imbalances between UL and DL throughput may limit or restrict the number of bi-directional symmetric traffic flows (such as voice and video calls) that can be supported by a wireless network for a given number of associated devices. 
     Implementations of the subject matter described in this disclosure may be used to maintain a balance between UL and DL throughput in a contention-based wireless network in response to one or more of an increase in UL traffic relative to DL traffic in the wireless network, an increase in the level of contention in the wireless network, an increase in the number of wireless communication devices actively associated with an AP, or an increase in the number of collisions in the wireless network. In some implementations, a wireless communication device contending for medium access to transmit UL data on a shared wireless medium may detect a presence of downlink (DL) data and adjust its backoff counter based on whether the wireless communication device is an intended recipient of the DL data. The wireless communication device may not adjust the backoff counter when it is not the intended recipient of the DL data, and may adjust the backoff counter when it is one of the intended recipients of the DL data. In some implementations, not adjusting the backoff counter may include stopping the countdown of the backoff counter in response to detecting the presence of the DL data, and continuing, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. In some implementations, adjusting the backoff counter may include resetting the backoff counter to an initial value in response to receiving the DL data. 
     By resetting the backoff counter to an initial value in response to receiving DL data, rather than pausing the backoff counter, a wireless communication device may have a longer backoff period during one or more subsequent medium access contention operations on a wireless medium, thereby decreasing a likelihood of the wireless communication device obtaining a transmit opportunity (TXOP) to transmit UL data on the wireless medium during the one or more subsequent medium access contention operations. Reducing or delaying UL transmissions from wireless communication devices that also received DL data may decrease the amount of UL traffic relative to the amount of DL traffic, thereby increasing symmetry between UL and DL throughput on the wireless medium. In this manner, implementations of the subject matter described in this disclosure may increase the number of traffic flows having substantially symmetrical UL and DL traffic (such as voice and video calls) that can be supported by a wireless network that includes a given number of associated wireless communication devices (such as STAs). 
     In other implementations, an AP may indicate a contention window size to be used by one or more associated wireless communication devices for medium access contention operations. The indicated contention window size may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, an increase in the number of wireless communication devices actively associated with an AP, or a level of contention on the shared wireless medium. For example, in response to an increase in the number of active wireless communication devices associated with an AP, the AP may indicate a larger contention window size from which wireless communication devices contending for medium access are to randomly select their backoff numbers. 
     The ability to indicate or modify a contention window size to be used by one or more wireless communication devices contending for medium access may allow the AP to at least partially determine or adjust the backoff periods during which the contending devices must wait before transmitting on the wireless medium. In some implementations, the AP may cause or instruct the wireless communication devices to increase their backoff periods (or at least a maximum duration of their backoff periods) when the imbalance between DL and UL throughput is greater than a value. In this manner, when the DL throughput is at a relatively low level (such as compared with the UL throughput), the AP may delay or decrease the likelihood of UL transmissions from wireless communication devices that received DL data. 
       FIG. 7  shows a timing diagram  700  illustrating the transmissions of communications according to some implementations. The communications may relate to medium access contention operations. In some implementations, the wireless communication device D 1  described with reference to  FIG. 7  may operate as or within a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively. In some other implementations, the wireless communication device D 1  may operate as or within an AP, such as one of the APs  102  and  602  described above with reference to  FIGS. 1 and 6A , respectively. 
     In some implementations, the device D 1  and one or more other wireless devices (not shown for simplicity) may contend for medium access using an EDCA mechanism, which may be implemented through the use of CSMA/CA and timing intervals such as SIFS, DIFS, EIFS, and AIFS. For example, the device D 1  may randomly select or generate a backoff number from a range of numbers defined by a contention window (CW), and may set its backoff counter to an initial value based on the randomly selected backoff number. The size of the contention window may be initially set to a minimum value (CW min ), for example, such that the backoff number is randomly selected from a range of numbers between 0 and CW min . 
     The device D 1  may sense the wireless medium, and decrement its backoff counter by one slot each time the wireless medium is continuously idle for an appropriate IFS period (such as a DIFS period). When the backoff counter reaches zero, the device D 1  may become the owner of a TXOP and transmit UL data on the wireless medium for a duration of the TXOP. If there is a collision on the wireless medium, the device D 1  may use an exponential backoff procedure in which the CW size is doubled for each subsequent medium access contention operation. When the contention window reaches a maximum value (CW max ), the contention window size remains at CW max  until one of the contending devices wins access to the shared wireless medium. The one or more other wireless devices contending for medium access follow a similar procedure and decrement their backoff counters from randomly selected backoff numbers between 0 and CW min  each time the wireless medium is sensed to be idle for the appropriate IFS period. 
     With reference to  FIG. 7 , at time t 0 , a sensing period  710  begins during which the device D 1  senses or determines whether the wireless medium is idle or busy. The device D 1  senses that the wireless medium is busy between times t 0  and t 1 , and may defer medium access contention operations. The wireless medium becomes free at time t 1 , and remains idle until at least time t 2 . The device D 1  senses that the wireless medium has been continuously idle for a DIFS period between times t 1  and t 2 , decrements its backoff counter by one slot, and enters a contention period  720  at time t 2 . 
     During the contention period  720 , the device D 1  may contend with the one or more other wireless devices for medium access. The device D 1  and each of the one or more other wireless devices waits for a period of time determined by their respective randomly selected backoff numbers before attempting to transmit on the wireless medium. Each of the randomly selected backoff numbers may correspond to one of a number of slot times ST 1 -ST N  within a contention window  725 , and may indicate a backoff period for a corresponding one of the contending devices. The contending device that selects the lowest backoff number has the shortest backoff period, and “wins” the medium access contention operation. For the example of  FIG. 7 , the device D 1  selected the lowest backoff number (which corresponds to the earliest one of the slot times ST 1 -ST N ), and becomes the owner of a TXOP  730  on the wireless medium. The device D 1  may transmit UL data  735  on the wireless medium for a duration of the TXOP  730  between times t 3  and t 4 . 
       FIG. 8  shows a timing diagram  800  illustrating the transmissions of communications according to some implementations. The communications may relate to medium access contention operations performed by an AP and three wireless communication devices D 1 -D 3  associated with the AP. In some implementations, the devices D 1 -D 3  may be examples of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively, and the AP may be an example of the AP  102  and  602  described above with reference to  FIGS. 1 and 6A , respectively. Although three contending devices D 1 -D 3  are shown in the example of  FIG. 8 , the medium access contention operations described herein may be performed by any suitable number of wireless communication devices. 
     The AP and devices D 1 -D 3  each select a random backoff number between 0 and CW min , and initialize their backoff counter to the selected backoff number. For the example of  FIG. 8 , the AP selects a random backoff number of 1, the device D 1  selects a random backoff number of 4, the device D 2  selects a random backoff number of 2, and the device D 3  selects a random backoff number of 6. 
     At time t 0 , a first contention period  810  begins. After sensing that the wireless medium has been continuously idle for a DIFS period between times t 0  and t 1 , each of the AP and the devices D 1 -D 3  initiates a countdown of its respective backoff counter, for example, by decrementing its backoff counter by one slot, and contends with the other ones of the devices for medium access during a first contention window (CW) between times t 1  and t 2 . The AP&#39;s backoff counter is decremented from 1 to 0, device D 1 &#39;s backoff counter is decremented from 4 to 3, device D 2 &#39;s backoff counter is decremented from 2 to 1, and device D 3 &#39;s backoff counter is decremented from 6 to 5. 
     The AP selected the lowest backoff number, and as such, its backoff counter reaches zero before the backoff counters of devices D 1 -D 3 . As a result, the AP wins the medium access contention operation, and becomes the owner of a first TXOP  812  on the wireless medium. The AP transmits DL data  814  to the device D 1  on the wireless medium during the first TXOP  812  between times t 3  and t 4 . In some implementations, the DL data  814  may be a single-user (SU) packet, a multi-user (MU) packet, or an aggregated packet. In various implementations, the DL data  814  may be transmitted using any suitable format. After the DL transmission, the AP may select a new random backoff number between 0 and CW min . For the example of  FIG. 8 , the AP selects a new backoff number of 2, and sets its backoff counter to an initial value of 2 accordingly. 
     During the first TXOP  812 , each of the devices D 1 -D 3  detects a presence of the DL data  814  on the wireless medium, and determines whether it is an intended recipient of the DL data  814 . The devices D 2  and D 3  are not intended recipients of the DL data  814 , and may stop the countdown of their backoff counters based on the detection of the DL data  814 . The device D 1  determines that it is the intended recipient of the DL data  814 , and adjusts its backoff counter based on receiving the DL data  814 . In some implementations, the device D 1  may adjust its backoff counter by resetting the backoff counter to its initial value (such as the backoff number randomly selected by the device D 1 ). For the example of  FIG. 8 , the device D 1  resets its backoff counter to 4. 
     Resetting device D 1 &#39;s backoff counter to its initial value of 4, rather than stopping the countdown of the backoff counter at 3, based on the reception of the DL data  814  may delay or reduce a likelihood of UL transmissions from the device D 1 , which may not only decrease UL traffic on the wireless medium but also increase the likelihood of the AP winning contention operations and transmitting DL data on the wireless medium. 
     After a SIFS period between times t 4  and t 5 , a second contention period  820  begins. Sensing that the wireless medium has been continuously idle for a DIFS period between times t 5  and t 6 , the AP and devices D 1 -D 3  decrement their respective backoff counters by one slot, and contend with each other for medium access during a second CW between times t 6  and t 7 . The AP&#39;s backoff counter is decremented from 2 to 1, device D 1 &#39;s backoff counter is decremented from 4 to 3, device D 2 &#39;s backoff counter is decremented from 1 to 0, and device D 3 &#39;s backoff counter is decremented from 5 to 4. 
     Device D 2 &#39;s backoff counter reaches zero before the backoff counters of the AP, device D 1 , and device D 3 . Thus, device D 2  wins the medium access contention operation, and becomes the owner of a second TXOP  822  on the wireless medium. The device D 2  transmits UL data  824  during the second TXOP  822  between times t 8  and t 9 , and randomly selects a new backoff number between 0 and CW min . For the example of  FIG. 8 , the device D 2  selects a new backoff number of 6, and sets its backoff counter to the new backoff number of 6. 
     After a SIFS period between times t 9  and t 10 , a third contention period  830  begins. Sensing that the wireless medium has been continuously idle for a DIFS period between times t 10  and t 11 , the AP and devices D 1 -D 3  decrement their respective backoff counters by one slot, and contend with each other for medium access during a third CW between times t 11  and t 12 . The AP&#39;s backoff counter is decremented from 1 to 0, device D 1 &#39;s backoff counter is decremented from 3 to 2, device D 2 &#39;s backoff counter is decremented from 6 to 5, and device D 3 &#39;s backoff counter is decremented from 4 to 3. 
     In this example, the AP selected the lowest backoff number, and its backoff counter reaches zero before the backoff counters of devices D 1 -D 3 . Thus, the AP again wins the medium access contention operation, and becomes the owner of a third TXOP  832  on the wireless medium. The AP transmits DL data  834  to the device D 3  during the third TXOP  832  between times t 13  and t 14 . In some implementations, the DL data  834  may be an SU packet, an MU packet, or an aggregated packet. After the DL transmission, the AP randomly selects a new backoff number between 0 and CW min . For the example of  FIG. 8 , the AP selects a new backoff number of 5, and sets its backoff counter to the new backoff number of 5. 
     During the third TXOP  832 , each of the devices D 1 -D 3  detects a presence of the DL data  834  on the wireless medium, and determines whether it is an intended recipient of the DL data  834 . The devices D 1  and D 2  are not intended recipients of the DL data  834 , and may stop the countdown of their backoff counters based on the detection of the DL data  834 . The device D 3  determines that it is the intended recipient of the DL data  834 , and adjusts its backoff counter based on receiving the DL data  834 . As described above, in some implementations, the device D 3  may adjust its backoff counter by resetting the backoff counter to its initial value of 6. Resetting device D 3 &#39;s backoff counter to its initial value of 6, rather than stopping the countdown of the backoff counter at 3, based on the reception of the DL data  834  may delay or reduce a likelihood of UL transmissions from the device D 3 , which may not only decrease UL traffic on the wireless medium but also increase the likelihood of the AP winning contention operations and transmitting DL data on the wireless medium. 
       FIG. 9  shows a sequence diagram  900  illustrating the transmissions of communications according to some implementations. The communications may relate to medium access contention operations performed by an AP and a wireless communication device D 1  associated with the AP. In some implementations, the device D 1  may be an example of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively, and the AP may be an example of the AP  102  and  602  described above with reference to  FIGS. 1 and 6A , respectively. Although only one wireless communication device D 1  is shown in the example of  FIG. 9 , the medium access contention operations described herein may be performed by any suitable number of wireless communication devices. 
     At time t 1 , the AP may detect a decrease in DL throughput of an associated wireless network. The decrease in DL throughput may be caused by any number of conditions or factors including (but not limited to) an increase in the number of collisions on the wireless medium, an increase in the number of active wireless communication devices associated with the AP, an increase in the amount of traffic on the wireless medium, or an increase in the level of contention on the wireless medium. In some implementations, the AP may select (or adjust) the size of the contention window used by the device D 1  for randomly selecting backoff numbers based on the detected decrease in DL throughput (such as by increasing the contention window size). 
     At time t 2 , the AP may transmit an indication of the selected contention window size to be used by the device D 1 . In some implementations, the indication may be included within an information element (IE) of one or more beacon frames transmitted by the AP. In some other implementations, the indication may be included within another suitable frame or packet transmitted by the AP. 
     The device D 1  receives the indication at time t 3 , and selects a random backoff number from a range of numbers defined by the contention window size indicated by the AP. The device D 1  may set an initial value of its backoff counter to the randomly selected backoff number, and may begin sensing the wireless medium to determine whether the wireless medium is busy or idle. 
     If the AP transmits DL data to the device D 1  (such as at time t A ), the device D 1  may reset its backoff counter to its initial state, as described with reference to  FIG. 8 . If the AP transmits DL data that is not addressed to the device D 1 , the device D 1  may pause or stop the countdown of its backoff counter. 
     At time t 4 , the device D 1  contends with the AP (and other nearby wireless communication devices, not shown for simplicity) for medium access based on a backoff number randomly selected from the contention window size indicated by the AP. At time t 5 , the device D 1  becomes the owner of a TXOP on the wireless medium, and transmits UL data to the AP. 
       FIG. 10  shows a flowchart illustrating an example process  1000  for wireless communication according to some implementations. The process  1000  may be performed by a wireless communication device such as the wireless communication device  500  described above with reference to  FIG. 5 . In some implementations, the process  1000  may be performed by a wireless communication device operating as or within a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively, or as one of the wireless communication devices D 1 -D 3  described above with reference to  FIGS. 7, 8, and 9 . 
     In some implementations, in block  1002 , the wireless communication device initiates a countdown of a backoff counter associated with a medium access contention operation for transmitting uplink (UL) data on a shared wireless medium. In block  1004 , the wireless communication device detects a presence of downlink (DL) data on the shared wireless medium. In block  1006 , the wireless communication device determines whether the wireless communication device is an intended recipient of the DL data. In block  1008 , the wireless communication device adjusts the backoff counter based on the determination. 
     In some implementations, adjusting, in block  1008 , the backoff counter based on the determination in block  1006  includes resetting the backoff counter to an initial value in response to determining, in block  1006 , that the wireless communication device is an intended recipient of the DL data. In some implementations, adjusting, in block  1008 , the backoff counter based on the determination in block  1006  includes not adjusting the backoff counter in response to determining, in block  1006 , that the wireless communication device is not an intended recipient of the DL data. 
       FIG. 11A  shows a flowchart illustrating an example process  1100  for wireless communication according to some implementations. The process  1100  may be performed by a wireless communication device such as the wireless communication device  500  described above with reference to  FIG. 5 . In some implementations, the process  1100  may be performed by a wireless communication device operating as or within a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively, or as one of the wireless communication devices D 1 -D 3  described above with reference to  FIGS. 7, 8, and 9 . For example, the process  1100  may be one implementation of adjusting, in block  1008  of the process  1000 , the backoff counter in response to determining, in block  1006  of the process  1000 , that the wireless communication device is not an intended recipient of the DL data. 
     In some implementations, in block  1102 , the wireless communication device stops the countdown of the backoff counter in response to detecting the presence of the DL data (for example, as detected in block  1004  of the process  1000 ). In block  1104 , the wireless communication device continues, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. 
       FIG. 11B  shows a flowchart illustrating an example process  1110  for wireless communication according to some implementations. The process  1110  may be performed by a wireless communication device such as the wireless communication device  500  described above with reference to  FIG. 5 . In some implementations, the process  1110  may be performed by a wireless communication device operating as or within a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively, or as one of the wireless communication devices D 1 -D 3  described above with reference to  FIGS. 7, 8, and 9 . For example, the process  1110  may be one implementation of initiating the countdown of the backoff counter in block  1002  of  FIG. 10 . 
     In some implementations, in block  1112 , the wireless communication device receives an indication of a contention window size to be used for one or more UL medium access contention operations. In block  1114 , the wireless communication device selects a random backoff number from the indicated contention window size. In block  1116 , the wireless communication device sets an initial value of the backoff counter based on the selected random backoff number. 
       FIG. 11C  shows a flowchart illustrating an example process  1120  for wireless communication according to some implementations. The process  1120  may be performed by a wireless communication device such as the wireless communication device  500  described above with reference to  FIG. 5 . In some implementations, the process  1120  may be performed by a wireless communication device operating as or within a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively, or as one of the wireless communication devices D 1 -D 3  described above with reference to  FIGS. 7, 8, and 9 . For example, the process  1120  may be performed prior to initiating the countdown of the backoff counter in block  1002  of  FIG. 10 . 
     In some implementations, in block  1122 , the wireless communication device receives, from an access point (AP), one or more beacon frames including the indication. The indication may be included within or appended to any suitable portion of the beacon frames. In some implementations, the indication of the contention window size may be included within an information element (IE) of one or more beacon frames. 
     In some implementations, the indicated contention window size may be based on the number of devices associated with the AP. In some other implementations, the indicated contention window size may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, or a level of contention on the shared wireless medium. In addition, or in the alternative, the indicated contention window size may be based on a relationship between UL and DL throughput on the shared wireless medium. For example, when the UL throughput decreases relative to the DL throughput (such as because of an increase in the number of wireless communication devices associated with the AP), the contention window size may be increased to reduce the likelihood of UL transmissions from the associated wireless communication devices during one or more subsequent TXOPs, which as described with reference to  FIG. 9  may reduce imbalances between UL and DL throughput. 
       FIG. 12  shows a flowchart illustrating an example process  1200  for wireless communication according to some implementations. The process  1200  may be performed by a wireless communication device such as the wireless communication device  500  described above with reference to  FIG. 5 . In some implementations, the process  1200  may be performed by a wireless communication device operating as or within a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively, or as one of the wireless communication devices D 1 -D 3  described above with reference to  FIGS. 7, 8, and 9 . 
     In some implementations, in block  1202 , the wireless communication device receives an indication of a contention window size to be used for one or more medium access contention operations. In block  1204 , the wireless communication device selects a random backoff number from the indicated contention window size. In block  1206 , the wireless communication device sets an initial value of the backoff counter based on the selected random backoff number. In block  1208 , the wireless communication device contends for medium access, based on a countdown of the backoff counter, to transmit uplink (UL) data on the shared wireless medium. 
     In some implementations, the indicated contention window size in block  1202  may be based on one or more of a number of collisions on the shared wireless medium, an amount of traffic on the shared wireless medium, or a level of contention on the shared wireless medium. In addition, or in the alternative, the indicated contention window size may be based on a relationship between DL throughput and UL throughput on the shared wireless medium. For example, when the UL throughput decreases relative to the DL throughput (such as because of an increase in the number of wireless communication devices associated with the AP), the contention window size may be increased to reduce the likelihood of UL transmissions from the associated wireless communication devices during one or more subsequent TXOPs, which as described with reference to  FIG. 9  may reduce imbalances between UL and DL throughput. In various implementations, in block  1202 , the wireless communication device receives, from an access point (AP), one or more beacon frames including the indication. 
     In some implementations, contending for medium access in block  1208  may be implemented using the example processes  1000  and  1100  of  FIG. 10  and  FIG. 11A , respectively. 
       FIG. 13  shows a block diagram of an example wireless communication device  1300  according to some implementations. In some implementations, the wireless communication device  1300  is configured to perform one or more of the processes  1000 ,  1100 ,  1110 , and  1120  described above with reference to  FIGS. 10, 11A, 11B, and 11C , respectively. The wireless communication device  1300  may be an example implementation of the wireless communication device  500  described above with reference to  FIG. 5 . For example, the wireless communication device  1300  can be a chip, SoC, chipset, package, or device that includes at least one processor and at least one modem (for example, a Wi-Fi (IEEE 802.11) modem or a cellular modem). In some implementations, the wireless communication device  1300  can be a device for use in a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively. In some other implementations, the wireless communication device  1300  can be a STA that includes such a chip, SoC, chipset, package, or device as well as at least one transmitter, at least one receiver, and at least one antenna. 
     The wireless communication device  1300  includes a module for initiating a countdown  1302 , a module for detecting a presence of DL data  1304 , a module for determining an intended recipient of DL data  1306 , a module for adjusting the backoff counter  1308 , a module for stopping the countdown  1310 , and a module for continuing the countdown  1312 . Portions of one or more of the modules  1302 ,  1304 ,  1306 ,  1308 ,  1310 , and  1312  may be implemented at least in part in hardware or firmware. For example, the module for detecting a presence of DL data  1304  and the module for determining an intended recipient of DL data  1306  may be implemented at least in part by a modem (such as the modem  502 ). In some implementations, at least some of the modules  1304 ,  1306 ,  1308 ,  1310 , and  1312  are implemented at least in part as software stored in a memory (such as the memory  508 ). For example, portions of one or more of the modules  1304 ,  1306 ,  1308 ,  1310 , and  1312  can be implemented as non-transitory instructions (or “code”) executable by a processor (such as the processor  506 ) to perform the functions or operations of the respective module. 
     The wireless communication device  1300  may contend for access to a shared wireless medium. The module for initiating a countdown  1302  is configured to initiate a countdown of a backoff counter associated with a medium access contention operation for transmitting uplink (UL) data on a shared wireless medium. The module for detecting a presence of DL data  1304  is configured to detect a presence of downlink (DL) data on the shared wireless medium. The module for determining an intended recipient of DL data  1306  is configured to determine whether the wireless communication device  1300  is an intended recipient of the DL data. The module for adjusting the backoff counter  1308  is configured to reset the backoff counter to an initial value in response to determining that the wireless communication device  1300  is an intended recipient of the DL data, and to not adjust the backoff counter in response to determining that the wireless communication device  1300  is not an intended recipient of the DL data. The module for stopping the countdown  1310  is configured to stop the countdown of the backoff counter in response to detecting the presence of the DL data. The module for continuing the countdown  1312  is configured to continue, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. 
       FIG. 14  shows a block diagram of an example wireless communication device  1400  according to other implementations. In some implementations, the wireless communication device  1400  is configured to perform one or more of the processes  1000 ,  1100 , and  1200  described above with reference to  FIGS. 10, 11A, and 12 , respectively. The wireless communication device  1400  may be an example implementation of the wireless communication device  500  described above with reference to  FIG. 5 . For example, the wireless communication device  1400  can be a chip, SoC, chipset, package, or device that includes at least one processor and at least one modem (for example, a Wi-Fi (IEEE 802.11) modem or a cellular modem). In some implementations, the wireless communication device  1400  can be a device for use in a STA, such as one of the STAs  104  and  604  described above with reference to  FIGS. 1 and 6B , respectively. In some other implementations, the wireless communication device  1400  can be a STA that includes such a chip, SoC, chipset, package, or device as well as at least one transmitter, at least one receiver, and at least one antenna. 
     The wireless communication device  1400  includes a module for receiving an indication  1402 , a module for selecting a random backoff number  1404 , a module for setting an initial value of the backoff counter  1406 , a module for contending for medium access  1408 , a module for stopping the countdown  1410 , and a module for continuing the countdown  1412 . Portions of one or more of the modules  1402 ,  1404 ,  1406 ,  1408 ,  1410 , and  1412  may be implemented at least in part in hardware or firmware. For example, the module for receiving an indication  1402  and the module for contending for medium access  1408  may be implemented at least in part by a modem (such as the modem  502 ). In some implementations, at least some of the modules  1404 ,  1406 ,  1408 ,  1410 , and  1412  are implemented at least in part as software stored in a memory (such as the memory  508 ). For example, portions of one or more of the modules  1404 ,  1406 ,  1408 ,  1410 , and  1412  can be implemented as non-transitory instructions (or “code”) executable by a processor (such as the processor  506 ) to perform the functions or operations of the respective module. 
     The wireless communication device  1400  may contend for access to a shared wireless medium. The module for receiving an indication  1402  is configured to receive indications of a contention window size that may be included in one or more beacon frames transmitted by an AP. The module for selecting a random backoff number  1404  is configured to randomly select a backoff number from a range of numbers defined by a contention window. The module for setting an initial value of the backoff counter  1406  is configured to initialize a value of the backoff counter based on the randomly selected backoff number. The module for contending for medium access  1408  is configured to perform medium access contention operations for transmitting UL data on a shared wireless medium. The module for stopping the countdown  1410  is configured to stop the countdown of the backoff counter in response to detecting the presence of DL data on the wireless medium. The module for continuing the countdown  1412  is configured to continue, during a next medium access contention operation, the countdown of the backoff counter from where it was stopped. 
     As used herein, a phrase referring to “at least one of” or “one or more of” a list of items refers to any combination of those items, including single members. For example, “at least one of: a, b, or c” is intended to cover the possibilities of: a only, b only, c only, a combination of a and b, a combination of a and c, a combination of b and c, and a combination of a and b and c. 
     The various illustrative components, logic, logical blocks, modules, circuits, operations and algorithm processes described in connection with the implementations disclosed herein may be implemented as electronic hardware, firmware, software, or combinations of hardware, firmware or software, including the structures disclosed in this specification and the structural equivalents thereof. The interchangeability of hardware, firmware and software has been described generally, in terms of functionality, and illustrated in the various illustrative components, blocks, modules, circuits and processes described above. Whether such functionality is implemented in hardware, firmware or software depends upon the particular application and design constraints imposed on the overall system. 
     Various modifications to the implementations described in this disclosure may be readily apparent to persons having ordinary skill in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein. 
     Additionally, various features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable subcombination. As such, although features may be described above as acting in particular combinations, and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flowchart or flow diagram. However, other operations that are not depicted can be incorporated in the example processes that are schematically illustrated. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the illustrated operations. In some circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.