Patent Publication Number: US-2023141728-A1

Title: Apparatus and methods for tb ppdu alignment for multi-link triggered uplink access in a wireless network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to co-pending U.S. patent application Ser. No. 17/082,762, Attorney Docket Number MEDT-0072-05U00US, with filing date Oct. 28, 2020, and provisional patent application Ser. No. 62/927,755, Attorney Docket Number MUSI-19-0166PUS, with filing date Oct. 30, 2019, and provisional patent application Ser. No. 62/968,266, Attorney Docket Number MUSI-20-0019PUS, with filing date Jan. 31, 2020, provisional patent application Ser. No. 62/992,228, Attorney Docket Number MUSI-20-0064PUS, with filing date Mar. 20, 2020, and provisional patent application Ser. No. 63/001,602, Attorney Docket Number MUSI-20-0068PUS, with filing date Mar. 30, 2020, and provisional patent application Ser. No. 63/013,586, Attorney Docket Number MUSI-20-0079PUS, with filing date Apr. 22, 2020, which are hereby incorporated by reference in their entirety. 
    
    
     FIELD 
     Embodiments of the present invention generally relate to the field of wireless communications. More specifically, embodiments of the present invention relate to systems and methods for triggered uplink access for a multi-link operation in a wireless network. 
     BACKGROUND 
     Modern electronic devices typically send and receive data with other electronic devices wirelessly using Wi-Fi, and many of these devices are “dual band” devices that include at least two wireless transceivers capable of operating in different frequency bands, e.g., 2.4 GHz, 5 GHz, and 6 GHz. In most cases, a wireless device will communicate over only a single band at a time. For example, older and low-power devices, e.g., battery powered devices, often operate on the 2.4 GHz band. Newer devices and devices that require greater bandwidth often operate on the 5 GHz band. The availability of the 6 GHz band is a recent advancement and can provide higher performance, lower latency, and faster data rates. 
     However, in some cases, the use of a single band may not satisfy the bandwidth needs of certain devices. Therefore, some developing approaches to wireless communication increase communication bandwidth by operating on multiple bands concurrently (technically called link aggregation or multi-link operation). 
     When a wireless device sends and receives data simultaneously in a multi-link operation, in-device coexistence (IDC) interference can cause substantial performance loss between the multiple wireless links. The IDC interference caused by simultaneous transmission and reception over a 2.4 GHz band and 5 GHz band is typically negligible. However, the IDC interference caused by simultaneous transmission and reception over a 5 GHz band and 6 GHz band can substantially impact performance and lead to frequent packet loss. 
     Accordingly, for avoiding the IDC interference, a wireless device should not transmit and receive frames on multi-link simultaneously. However, because the wireless device cannot easily determine when the transmission of a frame will end, it is currently difficult to synchronize transmissions on multiple wireless links to ensure that the wireless device does not transmit and receive frames on multiple wireless links at the same time. Accordingly, multi-link operations are subject to IDC interference that can substantially impact performance do to overlapping (unaligned) frame transmissions over multiple wireless links. 
     Uplink transmission (UL) involving multiple non-access point (AP) station (STA)s transmitting simultaneously to an AP station is introduced in the IEEE 802.11ax standard called Triggered Uplink Access (TUA). This mechanism uses an 802.11 MAC frame called a Trigger Frame sent by the AP station. An HE-enabled AP station sends a trigger frame for initiating orthogonal frequency division multiple access (OFDMA) or multi-user multiple-input multiple-output (MU-MIMO) transmissions prior to the transmission of other non-AP stations. The trigger frame identifies non-AP stations participating in the UL MU transmissions and assigns resource units (RUs) to the non-AP stations. Each non-AP station receiving the trigger frame sends a trigger-based (TB) physical layer conformance procedure (PLCP) protocol data unit (PPDU) back to the AP using the RUs assigned to the respective station. Therefore, a method of performing constrained multi-link operation using triggered uplink access that prevents IDC interference caused by the transmission of a TB PPDU while a trigger frame is already being received on another wireless link of the non-AP STA is needed. 
     SUMMARY 
     Accordingly, embodiments of the present invention provide methods of TB uplink transmission for constrained multi-link operations that substantially prevent interference caused by the simultaneous transmission and reception of data (e.g., a PPDU) by a wireless device (e.g., a wireless STA or AP). 
     According to one embodiment, a method is disclosed that includes transmitting a frame from a first affiliated wireless access point (AP) of a multi-band wireless AP for receipt by a wireless station (STA), where the transmitting performed over a first wireless link, and the frame solicits a response frame, determining that a second affiliated wireless AP of the multi-band wireless AP is scheduled to transmit a trigger frame to the wireless STA over a second wireless link, the trigger frame includes a carrier sense (CS) required field having a predetermined value, and transmitting the trigger frame from the second affiliated wireless AP to the wireless STA over the second wireless link, where the response frame solicited by the frame is scheduled for transmission after the predetermined value has expired following an end of the physical layer conformance procedure (PLCP) protocol data units (PPDUs) containing the trigger frame. 
     According to some embodiments, the method includes determining that a difference between a starting time of a PPDU transmitted by the wireless STA in response to the first trigger frame and an ending time of a PPDU carrying the second trigger frame is equal to or greater than a period defined as: aSIFSTime+aSignalExtension−aRxTxTurnaroundTime. 
     According to some embodiments, the method includes including determining that a difference between a starting time of a PPDU transmitted by the wireless STA in response to the first trigger frame and an ending time of a PPDU carrying the second trigger frame is equal to or greater than 12 μs. 
     According to some embodiments, the wireless STA includes a non-simultaneous transmit and receive (NSTR) wireless STA. 
     According to some embodiments, the first wireless link includes a 6 GHz wireless link and the second wireless link includes a 5 GHz wireless link. 
     According to another embodiment, a method of data transmission in a wireless network is disclosed. The method includes receiving a first trigger frame and a second trigger frame from a wireless access point (AP) at a wireless station (STA) over a first wireless link and a second wireless link, respectively, where the first trigger frame and the second trigger frame solicit data frames that overlap, and transmitting a first data frame and a second data frame responsive to the first trigger frame and the second trigger frame over the first wireless link and the second wireless link, respectively, where at least a portion the first data frame and the second data frame overlap in time. 
     According to some embodiments, the method includes calculating a difference between an ending time of a first physical layer conformance procedure (PLCP) protocol data unit (PPDU) carrying the first trigger frame and an ending time of a second PPDU carrying the second trigger frame. The first data frame and the second data frame are transmitted when the difference between the ending time of the first PPDU and the ending time of the second PPDU is greater than a predetermined value, and the predetermines value includes at least one of: 4 μs; 8 μs; and 16 μs. 
     According to some embodiments, the first PPDU and the second PPDU are formatted using one of: a non-high throughput (HT); an HT; a very high throughput (VHT); and an extremely high throughput (EHT) format. 
     According to some embodiments, the method includes adjusting the ending time of the first PPDU by adjusting a MAC padding of the first PPDU. 
     According to some embodiments, the adjusting the ending time of the first PPDU substantially mitigates in-device coexistence interference. 
     According to some embodiments, the method includes adjusting the ending time of the first PPDU by adjusting a PHY padding of the first PPDU. 
     According to some embodiments, the method includes adjusting the ending time of the first PPDU by adjusting a packet extension (PE) frame of the first PPDU. 
     According to some embodiments, the method includes calculating a difference between an ending time of a first physical layer conformance procedure (PLCP) protocol data unit (PPDU) carrying the first trigger frame and an ending time of a second PPDU carrying the second trigger frame. The first data frame and the second data frame are transmitted when the difference between the ending time of the first PPDU and the ending time of the second PPDU is greater than a short interframe space (SIFS)−(10%×aSlotTime), and aSlotTime is determined according to a specification of the wireless network. 
     According to some embodiments, the first wireless link includes a 6 GHz wireless link and the second wireless link includes a 5 GHz wireless link. 
     According to some embodiments, the wireless STA is a non-simultaneous transmit and receive (NSTR) wireless STA. 
     According to another embodiment, a wireless communication device is disclosed, including a first transceiver and a second transceiver configured to send and receive data over the wireless network, and a processor operable to cause the first transceiver to transmit a first trigger frame for receipt by a wireless station (STA) over a first wireless link, where the first trigger frame includes a carrier sense (CS) required field having a predetermined value, determine that the second transceiver of the multi-band wireless AP is scheduled to transmit a second trigger frame to the wireless STA over a second wireless link, and cause the second transceiver to transmit the second trigger frame to the wireless STA over the second wireless link. The first trigger frame and the second trigger frame solicit physical layer conformance procedure (PLCP) protocol data units (PPDUs) that overlap in time. 
     According to some embodiments, the processor is further operable to determine that a difference between a starting time of a PPDU transmitted by the wireless STA in response to the frame and an ending time of the PPDU containing the trigger frame is equal to or greater than a period defined as: aSIFSTime+aSignalExtension−aRxTxTurnaroundTime. 
     According to some embodiments, the processor is further operable to determine that a difference between a starting time of a PPDU transmitted by the wireless STA in response to the trigger frame and an ending time of the PPDU containing the trigger frame is equal to or greater than 12 μs. 
     According to some embodiments, the wireless STA includes a non-simultaneous transmit and receive (NSTR) wireless STA. 
     According to some embodiments, the first wireless link includes a 6 GHz wireless link and the second wireless link includes a 5 GHz wireless link. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
         FIG.  1    is a block diagram of an exemplary synchronous multi-link transmission according to embodiments of the present invention. 
         FIG.  2    is a block diagram depicting an exemplary constrained multi-link operation using triggered uplink access resulting in IDC interference caused by simultaneous transmission and reception of data by a wireless STA according to embodiments of the present invention. 
         FIG.  3    is a block diagram of an exemplary constrained multi-link operation using triggered uplink access for preventing IDC interference caused by simultaneous transmission and reception of data by a wireless STA according to embodiments of the present invention. 
         FIG.  4    is a block diagram of an exemplary constrained multi-link operation using triggered uplink access resulting in interference leakage caused by a wireless STA sensing a channel during a SIFS according to embodiments of the present invention. 
         FIG.  5    is a block diagram of an exemplary constrained multi-link operation using triggered uplink access for preventing interference leakage caused by a wireless STA sensing a channel during a SIFS according to embodiments of the present invention. 
         FIG.  6    is a block diagram of an exemplary constrained multi-link operation using triggered uplink access resulting in a reception error due to interference leakage between wireless links according to embodiments of the present invention. 
         FIG.  7    is a block diagram of an exemplary constrained multi-link operation using triggered uplink access for preventing interference leakage according to embodiments of the present invention. 
         FIG.  8    is a flowchart depicting an exemplary sequence of computer-implemented steps for performing constrained multi-link triggered uplink access to prevent IDC interference according to embodiments of the present invention. 
         FIG.  9    is a flowchart depicting an exemplary sequence of computer-implemented steps for performing constrained multi-link triggered uplink access to prevent a wireless link from entering a busy state due to interference leakage according to embodiments of the present invention. 
         FIG.  10    is a flowchart depicting an exemplary sequence of computer-implemented steps for performing constrained multi-link triggered uplink access to prevent interference leakage between a wireless links according to embodiments of the present invention. 
         FIG.  11    is a block diagram depicting an exemplary computer system platform upon which embodiments of the present invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to several embodiments. While the subject matter will be described in conjunction with the alternative embodiments, it will be understood that they are not intended to limit the claimed subject matter to these embodiments. On the contrary, the claimed subject matter is intended to cover alternative, modifications, and equivalents, which may be included within the spirit and scope of the claimed subject matter as defined by the appended claims. 
     Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. However, it will be recognized by one skilled in the art that embodiments may be practiced without these specific details or with equivalents thereof. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects and features of the subject matter. 
     Portions of the detailed description that follow are presented and discussed in terms of a method. Although steps and sequencing thereof are disclosed in a figure herein (e.g.,  FIGS.  8 - 10   ) describing the operations of this method, such steps and sequencing are exemplary. Embodiments are well suited to performing various other steps or variations of the steps recited in the flowchart of the figure herein, and in a sequence other than that depicted and described herein. 
     Some portions of the detailed description are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits that can be performed on computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. A procedure, computer-executed step, logic block, process, etc., is here, and generally, conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout, discussions utilizing terms such as “accessing,” “configuring,” “coordinating,” “storing,” “transmitting,” “retransmitting,” “authenticating,” “identifying,” “requesting,” “reporting,” “determining,” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     SYNCHRONIZATION OF EHT MULTI-LINK 
     Triggered Uplink Access 
     As used herein, the term “EHT” may refer generally to a recent generation of wireless communication (Wi-Fi) known as Extremely High Throughput (EHT) and is defined according to the IEEE 802.11be standards. The term station (STA) refers generally to an electronic device capable of sending and receiving data over Wi-Fi that is not operating as an access point (AP). 
     Multi-link operations can provide higher network throughput and improved network flexibility compared to traditional techniques for wireless communication. Embodiments of the present invention substantially prevent interference caused by the simultaneous transmission and reception of data in a constrained multi-link operation of a wireless network. Multi-link operations can be performed between multi-link devices (MLDs) that include multiple affiliated STAs (non-AP STAs or AP-STAs) operating on different wireless links. 
     With regard to  FIG.  1   , an exemplary wireless communication system  100  including a multi-band cooperative AP  105  and a multi-band cooperative STA  155  are depicted according to embodiments of the present invention. The multi-band cooperative AP  105  and multi-band cooperative STA  155  are examples of MLDs. The multi-band cooperative AP  105  includes a 5 GHz transceiver  110  and a 2.4 GHz transceiver  115 . Other types of transceivers that operate on different bands, such as 6 GHz and above, can also be used by the multi-band cooperative AP  105  according to embodiments of the present invention. The transceivers  110  and  115  of AP  105  exchange data and information with cooperative management unit  120  that coordinates information sent and/or received by transceivers  110  and  115 . The AP  105  can be considered to include two AP instances or “affiliated” APs for communicating using 5 GHz transceiver  110  and a 2.4 GHz transceiver  115 . 
     The multi-band cooperative STA  155  includes a 5 GHz transceiver  160  and a 2.4 GHz transceiver  165 . Other types of transceivers that operate on different bands, such as 6 GHz and above, can also be used by the multi-band cooperative STA  155  according to some embodiments of the present invention. The transceivers  160  and  165  of STA  155  exchange data and information with cooperative management unit  170  that coordinates information sent and received by transceivers  160  and  165  using 5 GHz band wireless communication and 2.4 GHz band wireless communication, respectively, although any well-known wireless communication band (e.g., 6 GHz) can be used. The STA  155  can be considered to include two STA instances or “affiliated” STAs for communicating using 5 GHz transceiver  160  and a 2.4 GHz transceiver  165 . 
     The multi-band cooperative AP  105  and the multi-band cooperative STA  155  have simultaneous transmission and reception capabilities for communicating using different wireless bands. The transmitters operating on the different bands can perform independent clear channel assessments (CCAs) using joint or intendent transmissions. Moreover, full duplex communication can be enabled by independent multi-band operation using FDD mode. 
     STA  155  transmitting frames using multiple bands simultaneously can mitigate delay and improve peak throughput of STA  155 . However, in some cases, transmitting frames using multiple bands simultaneously can degrade the performance of the basic service set (BSS) comprising STA  155 . Therefore, the AP  105  can control which STAs are granted multi-band channel access, and the access can be terminated by the AP at any time, for example, based on changing network conditions or requirements. 
     Depending on certain conditions, such as traffic load, a non-AP STA may use fewer than all supported/available links in order to reduce energy consumption. Moreover, a non-AP STA may apply an independent power management for each link, and the AP can provide the TID-to-link mapping information for each link. Depending the Quality of Service (QoS) policy of the Basic Service Set (BSS), an AP may allocate the traffic to different links based on traffic type, such as voice, video, data, etc. For example, frames belonging to a first Traffic Identifier (TID 1) can be allocated to a first link, and frames belonging to a second Traffic Identifier (TID 2) can be allocated to a second link. In this case, the AP may provide the TID-to-link mapping information for both links to the wireless STA, where some data can only be sent on the first link, and other data can only be sent on the second link. 
     Data transmitted over a first wireless link, such as the 5 GHz wireless link provided by 5 GHz transceiver  110  or  160 , can be retransmitted over a different wireless link. For example, if a data transmission over the 5 GHz wireless link is sent unsuccessfully (e.g., no acknowledgment received), the data can be retransmitted over the 2.4 GHz wireless link provided by 2.4 GHz transceiver  115 / 165 . The data can be retransmitted over the second link when an unscheduled channel switch to another channel occurs, or when transmitted a delay-sensitive packet over multiple links. Moreover, the data transmission (e.g., a PPDU) can be originally encoded for transmission on a first wireless link (e.g., a 2.4 GHz or 5 GHz wireless link), and the retransmitted data can be prepared for transmission according to embodiments of the present invention described herein for encrypting data for retransmission in a multi-link environment. 
       FIG.  2    depicts an exemplary constrained multi-link operation  200  using triggered uplink access resulting in IDC interference caused by simultaneous transmission and reception of data by a wireless STA (e.g., a non-AP wireless STA or a multi-link device (MLD)) according to embodiments of the present invention. In  FIG.  2   , wireless AP  205  transmits trigger frame  215  to a wireless STA  210  soliciting an uplink response frame over a 5 GHz wireless link  220 . AP  205  transmits trigger frame  225  to a wireless STA  210  soliciting an uplink response frame over a 6 GHz wireless link  230  before the transmission of trigger frame  215  to wireless STA  210  is complete. EHT TB PPDU  235  is transmitted by wireless STA  210  (responsive to trigger frame  215 ) before STA  210  has completed reception of trigger frame  225  resulting in IDC interference  240  caused by simultaneous transmission and reception of data in a multi-link operation. According to embodiments of the present invention, wireless APs are restricted from scheduling a PPDU carrying a trigger frame to an STA while the STA is transmitting an EHT TB PPDU responsive to a trigger frame on a separate wireless link within a prescribed margin to prevent IDC interference  240 . In the example of  FIG.  2   , the multi-band AP  205  is a MLD including multiple affiliated APs (e.g., a first affiliated AP operating on 6 GHz link  220  and a second affiliated AP operating on 5 GHz link  230 ), and the multi-band STA  210  is a MLD including multiple affiliated STAs (e.g., a first affiliated STA operating on 6 GHz link  220  and a second affiliated STA operating on 5 GHz link  230 ). 
       FIG.  3    depicts an exemplary constrained multi-link operation  300  using triggered uplink access for preventing IDC interference caused by simultaneous transmission and reception of data by a multi-band wireless STA according to embodiments of the present invention. In  FIG.  3   , wireless multi-band AP  305  transmits trigger frame  315  to a multi-band wireless STA  310  soliciting an uplink response frame over a 5 GHz wireless link  320 . AP  305  transmits trigger frame  325  to a wireless STA  310  soliciting an uplink response frame over a 6 GHz wireless link  330  before the transmission of trigger frame  315  to wireless STA  310  is complete. TB PPDU  335  is transmitted by wireless STA  310  (responsive to trigger frame  315 ) before STA  310  has completed reception of trigger frame  325  resulting in IDC interference. According the example of  FIG.  3   , wireless AP  305  can schedule a PPDU carrying a trigger frame to STA  310  when the difference between the ending times of the respective PPDU transmissions (e.g., trigger frames  315  and  325 ) is be less than the short interframe space (SIFS)−(10%×aSlotTime), where aSlotTime is a fixed/predefined value. As depicted in  FIG.  3   , because the minimum inter-frame space  340  is not less than SIFS  345 , the transmission  315  and reception  335  by AP  305  advantageously do not overlap. The starting and ending time alignment in  FIG.  3    can be achieved using MAC padding  345 , PHY padding  350 , and/or PE  355 , for example. The SIFS time can be calculated as: SIFS =aSIFSTime+aSignalExtension. According to some embodiments, aSIFSTime is fixed at 10 μs at 5 GHz and 16 μs at 2.4, and the value of aSignalExtension is 0 μs for 5 GHz band and 6 μs for 2.4 GHz bands. The solicited PPDU can be sent in any well-known PPDU format, such as non-HT, HT, VHT, HE, EHT, etc. 
     According to one embodiment, the starting times and ending times of the multi-link transmissions are synchronized within a margin equal to SIFSTime+aSignalExtension using, for example, MAC padding, PHY padding, and/or packet extension (PE). 
     According to some embodiments, the margin is fixed at 16 μs. Specifically, the value of aSIFSTime is 10 μs for 2.4 GHz bands and 16 μs for 5 GHz bands, and the value of aSignalExtension is and 6 μs for 2.4 GHz bands and 0 μs for 5 GHz bands. 
     With regard to  FIG.  4   , an exemplary constrained multi-link operation  400  for triggered uplink access resulting in interference leakage caused by a wireless STA sensing a channel during a SIFS is depicted according to embodiments of the present invention. In  FIG.  4   , wireless multi-band AP  405  transmits trigger frame  415  to a multi-band wireless STA  410  soliciting an uplink response frame over a 5 GHz wireless link  420  and trigger frame  425  over a 6 GHz wireless link  430 . TB PPDU  435  is transmitted by wireless STA  410  (responsive to trigger frame  415 ) and the difference between the ending time of trigger frame  415  and the starting time of TB PPDU  435  is less than SIFS−(10%×aSlotTime). STA  410  senses the 6 GHz channel  430  during SIFS  440  which changes the carrier sense (CS) of the 5 GHz when the IDC interference is greater than the energy detection (ED) threshold. In this case, STA  410  will not respond to trigger frame  425  when the CS required field of trigger frame  425  is set to 1. 
     With regard to  FIG.  5   , an exemplary constrained multi-link operation  500  using triggered uplink access for preventing interference leakage caused by a wireless STA sensing a channel during a SIFS is depicted according to embodiments of the present invention. In  FIG.  5   , wireless AP  505  transmits a trigger frame  515  to a wireless STA  510  soliciting an uplink response frame over a 5 GHz wireless link  520  and a trigger frame  525  over a 6 GHz wireless link  530 . The TB PPDU  535  is transmitted by wireless STA  510  (responsive to trigger frame  515 ). The carrier sense (CS) required fields of the trigger frames  515  and  525  are set to 1. Accordingly, AP  505  determines if the difference  540  between the ending time of trigger frame  525  and the starting time of TB PPDU  535  is greater than or equal to the aCCATime prior to transmission. If the difference  540  is greater than or equal to the aCCATime, STA  510  can perform channel sensing during the SIFS and respond with a solicited TB PPDU  535  without causing interference. Otherwise, if the difference  540  between the ending time of trigger frame  525  and the starting time of TB PPDU  535  is less than the aCCATime, the transmission is restricted. According to some embodiments, the aCCATime is 4 μs or 8 μs. According to some embodiments, the aCCATime of a wireless STA is signaled by the wireless STA to the associated wireless AP. 
     According to some embodiments, a wireless STA does not respond to a trigger frame having a CS required field set to 1 if the TB PPDUs sent over different links are not exactly synchronized on OFDM symbol level. According to some embodiments, the STA reports any interference leakage from a first link that causes the CS status of a second link to switch to a busy state (when the interference is greater than the ED threshold). For concurrent TB PPDU transmission on the first link and the second link, the AP provides OFDM symbol alignment between the PPDU containing the trigger frame and the responsive TB PPDU. In other word, the ending times of PPDUs containing the trigger frames are aligned within the OFDM symbol duration (e.g., 4 us), and therefore the starting times of the responsive TB PPDUs are also aligned within the OFDM symbol duration (e.g., 4 us). 
     According to some embodiments, when a wireless AP cannot meet a synchronization requirement (e.g., the difference between ending times of PPDUs is less than SIFS−(10%×aSlotTime)), the wireless AP does not simultaneously trigger multiple TB PPDU transmissions that require CS. Accordingly, the wireless AP cannot simultaneously send more than one PPDUs containing a trigger frame with CS required set to 1. Nevertheless, the AP can simultaneously send more than one PPDU containing a trigger frame when one trigger frame includes a CS required field set to 1, and the other trigger frame includes a CS required field set to 0. 
     In some cases TB PPDUs sent on multi-link use different TXVECTOR parameters (e.g., GI or LTF type) making it difficult to synchronize the starting times and ending times of the transmissions on multiple links. To relax the requirements for multi-link operations, the starting times and ending times of the transmissions on multiple links can be synchronized within the margin of (aSIFSTime+aSignalExtension)/2 using MAC padding, PHY padding, or packet extension (PE) at the wireless STA. The transmission opportunity (TXOP) holder can adjust an inter-frame space (IFS) between the SIFS and the SIFS+SIFS/2 such that the IFS is different for each link. In these embodiments, the AP does not send a trigger frame having the CS required equal to 1 to the wireless STA on the second link if the wireless AP determines that the wireless STA may transmit a frame on the first wireless link during a SIFS after the soliciting trigger frame. 
     According to some embodiments, a wireless AP performing TB multi-link access is restricted from sending a Trigger frame with the CS Required subfield set to 1 to a wireless STA when a PPDU from an STA affiliated with the same wireless STA is scheduled for transmission within a predetermined margin after receiving the PPDU containing the Trigger frame. According to some embodiments, the margin is determined according to (aSIFSTime+aSignalExtention−aRxTxTurnaroundTime), and the aRxTxTurnaroundTime can equal 4 μs. For example, a wireless AP affiliated with a multi-band AP does not transmit a Trigger frame with the CS Required subfield set to 1 to an STA affiliated with a multi-band STA when a PPDU from another STA affiliated with the same multi-band STA is scheduled for transmission before a timer with a value of 12 μs (aSIFSTime+aSignalExtension−aRxTxTurnaroundTime, where aRxTxTurnaroundTime is equal to 4 μs) has expired. In this case the wireless AP and wireless STA follow CS Required rules defined according to the IEEE 802.11ax specification (e.g., 26.5.2 UL MU operation). 
     According to some embodiments, an AP can allocate traffic to a different link based on the QoS policy of a basic service set (BSS). For example, when frames belonging to traffic identifier (TID) 1 are allocated to link 1, and frames belonging to TID 2 are allocated to link 2, an HE TB PPDU sent on multi-links carries frames belonging to the QoS policy. For example, if the wireless AP allocates frames belonging to TID 1 to link 1, the HE TB PPDU sent over link 1 only carries frames belonging to TID 1. When MAC padding is used, the wireless STA can aggregate frames allocated to other links as determined by the wireless AP. The AP can indicate in the trigger frame whether a wireless STA can aggregate frames allocated to other links. 
     With regard to  FIG.  6   , an exemplary constrained multi-link operation  600  using triggered uplink access resulting in a reception error due to interference leakage is between wireless links is depicted according to embodiments of the present invention. In  FIG.  6   , wireless AP  605  transmits trigger frame  615  to a wireless STA  610  soliciting an uplink response frame over a 6 GHz wireless link  620  and trigger frame  625  over a 6 GHz wireless link  630 . EHT TB PPDU  635  is transmitted by wireless STA  610  responsive to trigger frame  615 . The length of an EHT TB PPDUs (e.g., EHT TB PPDUs  635  and  640 ) is determined according to the respective trigger frame sent on the link associated with the EHT TB PPDU. In  FIG.  6   , the difference between the ending times of EHT TB PPDUs is greater than SIFS−(10%×aSlotTime) resulting in interference leakage, and the wireless STA fails to decode response frame  645 . 
     With regard to  FIG.  7    an exemplary constrained multi-link operation  700  using triggered uplink access for preventing interference leakage is depicted according to embodiments of the present invention. In  FIG.  7   , wireless AP  705  transmits trigger frame  715  to a wireless STA  710  soliciting an uplink response frame over a 5 GHz wireless link  720 , and transmits trigger frame  725  over a 6 GHz wireless link  730 . TB PPDU  735  is transmitted by wireless STA  710  responsive to trigger frame  715  and TB PPDU  740  is transmitted by wireless STA  710  responsive to trigger frame  725 . The UL Length subfield values in trigger frames  715  and  725  are set so that the difference between the ending times of TB PPDUs  735  and  740  is less than or equal to SIFS−(10%×aSlotTime). According to some embodiments, SIFS−(10%×aSlotTime) is a maximum bound for ending time alignment of the PPDUs, although of course tighter synchronization requirement can be applied (e.g., SIFS−(10%×aSlotTime)−aCCATime or 8 μs). 
       FIG.  8    is a flowchart  800  depicting an exemplary sequence of computer-implemented steps for performing constrained multi-link triggered uplink access to prevent IDC interference according to embodiments of the present invention. 
     At step  805 , first and second trigger frames are received from a wireless access point (AP) at a wireless station (STA) over first and second wireless links, respectively. 
     At step  810 , a difference between an ending time of the PPDU carrying the first trigger frame and an ending time of the PPDU carrying the second trigger frame is calculated. 
     At step  815 , first and second data frames are transmitted in PPDUs responsive to the first and second trigger frames over the first and second wireless links, respectively, when the difference between the ending time of the PPDU carrying the first trigger frame and the ending time of the PPDU carrying the second trigger frame is less than the predetermined margin. According to some embodiments, the predetermined margin is calculated as: SIFS−(10%×aSlotTime). According to some embodiments, SIFS−(10%×aSlotTime) is a maximum bound for the ending time alignment of the PPDUs, although of course tighter synchronization requirement can be applied (e.g., SIFS−(10%×aSlotTime)−aCCATime or 8 μs). According to some embodiments, the ending time of the PPDUs is adjusted using MAC padding, PHY padding, and/or PE, for example. 
       FIG.  9    is a flowchart  900  depicting an exemplary sequence of computer-implemented steps for performing constrained multi-link triggered uplink access to prevent a wireless link from entering a busy state due to interference leakage according to embodiments of the present invention. 
     At step  905 , a first trigger frame is transmitted in a PPDU from a wireless access point (AP) to a wireless station (STA) over first wireless link, in a multi-link operation. The wireless AP and the wireless STA are MLDs that can communicate over multiple wireless links. The first trigger frame include a CS required fields having a value of 1 indicating that channel sensing is performed before transmission of a PPDU by a wireless STA transmitted in response to the trigger frame. 
     At step  910 , the wireless AP determines that the AP is scheduled to transmit a second trigger frame in a PPDU over a second wireless link. The second trigger frame is sent by an AP instance affiliated with the wireless AP to an STA instance affiliated with the wireless STA. 
     At step  915 , the second trigger frame is transmitted by the wireless AP (e.g., an AP affiliated with the wireless AP) when a difference between a starting time of a PPDU transmitted by the wireless STA in response to the first trigger frame and an ending time of the second trigger frame is greater than or equal to a predetermined margin. According to some embodiments, the predetermined margin is set according to an aCCATime defined by the wireless AP. For example, the aCCATime can be determined according to aSIFSTime+aSignalExtension−aRxTxTurnaroundTime, where aRxTxTurnaroundTime is equal to 4 μs. For example, the predetermined margin can be 12 μs. According to some embodiments, the ending time of the PPDUs is adjusted using MAC padding, PHY padding, and/or PE, for example. 
       FIG.  10    is a flowchart  1000  depicting an exemplary sequence of computer-implemented steps for performing constrained multi-link triggered uplink access to prevent interference leakage between a first wireless link and a second wireless link. according to embodiments of the present invention. 
     At step  1005 , first and second trigger frames are received from a wireless access point (AP) at a wireless station (STA) over first and second wireless links, respectively. 
     At step  1010 , a difference between an ending time of the PPDU carrying the first data frame and an ending time of the PPDU carrying the second data frame is calculated. 
     At step  1015 , a UL length field of the first and/or second PPDU is adjusted so that the difference between the ending times of the responsive PPDUs is less than or equal to SIFS−(10%×aSlotTime). 
     At step  1020 , the first and second data frames are transmitted in PDDUs responsive to the first and second trigger frames over the first and second wireless links, respectively. The ending times of the data frames are substantially aligned within the predetermined margin to advantageously prevent channel leakage interference caused by the reception of a subsequent response frame (e.g., ACK or Block ACK) at the wireless STA. According to some embodiments, the ending time of the PPDUs is adjusted using MAC padding, PHY padding, and/or PE, for example. 
     At step  1025 , the response frame is received responsive to the wireless AP receiving the first data frame and interference leakage is substantially prevented. 
     EXEMPLARY COMPUTER CONTROLLED SYSTEM 
     Embodiments of the present invention are drawn to electronic systems that perform multi-link operations in a wireless network. The multi-link operations can include constrained multi-link operations configured to prevent or mitigate IDC interference, for example, by aligning the starting/ending times of received/transmitted frames within a predetermined margin so that simultaneous transmission and reception is substantially prevented. The frame timing can be aligned using MAC padding, PHY padding, and/or packet extension (PE), for example. The following discussion describes one such exemplary electronic system or computer system that can be used as a platform for implementing embodiments of the present invention. The exemplary computer system  1112  can be a wireless access point or a wireless station, for example. 
     In the example of  FIG.  11   , the exemplary computer system or wireless device includes a central processing unit (such as a processor or a CPU)  1101  for running software applications and optionally an operating system. Read-only memory  1102  and random access memory  1103  store applications and data for use by the CPU  1101 . Data storage device  1104  provides non-volatile storage for applications and data and may include fixed disk drives, removable disk drives, flash memory devices, and CD-ROM, DVD-ROM or other optical storage devices. The optional user inputs  1106  and  1107  comprise devices that communicate inputs from one or more users to the computer system  1112  (e.g., mice, joysticks, cameras, touch screens, and/or microphones). 
     A communication or network interface  1108  includes a plurality of transceivers and allows the computer system  1112  to communicate with other computer systems, networks, or devices via an electronic communications network, including wired and/or wireless communication and including an Intranet or the Internet (e.g., 802.11 wireless standard). The network interface  1108  can perform multi-link operations (e.g., multi-link packet scheduling and channel access) using multiple wireless links to improve network throughput, for example. According to embodiments of the present invention, the communication or network interface  1108  can operate multiple transceivers simultaneously. The communication or network interface  1108  and can include a multi-band (e.g., dual-band) interface that can operate in multiple bands simultaneously, such as 2.4 GHz, 5 GHz, and/or 6 GHz. 
     The optional display device  1110  may be any device capable of displaying visual information in response to a signal from the computer system  1112  and may include a flat panel touch sensitive display, for example, and may be remotely disposed. The components of the computer system  1112 , including the CPU  1101 , memory  1102 / 1103 , data storage  1104 , user input devices  1106 , and graphics subsystem  1105  may be coupled via one or more data buses. 
     Some embodiments may be described in the general context of computer-executable instructions, such as program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Typically the functionality of the program modules may be combined or distributed as desired in various embodiments. 
     Embodiments of the present invention are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the following claims.