Patent Publication Number: US-2023156864-A1

Title: Physical protocol data unit transmission method employed by wireless fidelity multi-link device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. application Ser. No. 17/027,735, filed on Sep. 22, 2020, which claims the benefit of U.S. Provisional Application No. 62/909,831, filed on Oct. 3, 2019. The contents of these applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to wireless communications, and more particularly, to a physical protocol data unit (PPDU) transmission method employed by a wireless fidelity (WiFi) multi-link device (MLD). 
     In a WiFi multi-link operation (MLO), there exist several links between two MLDs, including one access point (AP) and one non-AP station (STA), that occupy different radio-frequency (RF) bands. These links can operate independently to increase the overall throughput and/or to improve the connection stability. Regarding a synchronous mode of MLO, an MLD may synchronize its transmit (TX) timing and/or receive (RX) timing of multiple links. However, each link has its own capacity that is based on several parameters, including bandwidth (BW), number of spatial streams (NSS), modulation and coding mechanism (MCS), etc. Capacities of links can be very different. Hence, for the same upper layer content to be transmitted over these links, physical protocol data unit (PPDU) lengths may be very different, thus resulting in PPDU ending time misalignment. 
     The PPDU ending time misalignment may occur in some scenarios. In a first scenario, duplicate transmission is performed over a high capacity link and a low capacity link. Due to link capacity difference, the same contents transmitted over different links result in very different PPDU lengths. In a second scenario, a low capacity link&#39;s PPDU goes first and a high capacity link&#39;s PPDU goes later. Due to link capacity difference and transmission timing difference, the data left on the high capacity link is not enough to fulfill the PPDU length requirement to be synchronized with the low capacity link&#39;s PPDU. In a third scenario, the PPDU takes 80% of data on the first link, and the 20% rest data transmitted later on the second link results in a much shorter PPDU due to the fact that uncertain second link transmission makes the transmitter to load more data to the early coming transmission opportunity. 
     Thus, there is a need for an innovative PPDU transmission design which can deal with any of the above-mentioned scenarios to help a WiFi MLD to achieve PPDU ending time alignment of multiple links. 
     SUMMARY 
     One of the objectives of the claimed invention is to provide a physical protocol data unit (PPDU) transmission method employed by a wireless fidelity (WiFi) multi-link device (MLD). For example, the proposed PPDU transmission method can achieve PPDU ending time alignment of multiple links under any of the above-mentioned scenarios. 
     According to a first aspect of the present invention, an exemplary physical protocol data unit (PPDU) transmission method is disclosed. The exemplary PPDU transmission method includes: setting parameters of each of a plurality of links for enabling the plurality of links to have different capacity for PPDU transmission, wherein parameters of one of the plurality of links are different from parameters of another of the plurality of links, and highest capacity supported by said one of the plurality of links is higher than highest capacity supported by said another of the plurality of links; aligning an ending time instant of transmission of a first PPDU transmitted via said one of the plurality of links with an ending time instant of transmission of a second PPDU transmitted via said another of the plurality of links through setting, by a wireless fidelity (WiFi) multi-link device (MLD), a content that is carried by the first PPDU transmitted via said one of the plurality of links, wherein the ending time instant of transmission of the first PPDU and the ending time instant of transmission of the second PPDU are aligned with mismatch between the ending time instant of transmission of the first PPDU and the ending time instant of transmission of the second PPDU falling within a predefined tolerance range; and transmitting PPDUs via the plurality of links, wherein one PPDU is transmitted via each of the plurality of links, and the PPDUs comprise the first PPDU and the second PPDU. 
     According to a second aspect of the present invention, an exemplary wireless fidelity (WiFi) multi-link device (MLD) is disclosed. The exemplary WiFi MLD is arranged to perform operations, including: setting parameters of each of a plurality of links for enabling the plurality of links to have different capacity for PPDU transmission, wherein parameters of one of the plurality of links are different from parameters of another of the plurality of links, and highest capacity supported by said one of the plurality of links is higher than highest capacity supported by said another of the plurality of links; aligning an ending time instant of transmission of a first PPDU transmitted via said one of the plurality of links with an ending time instant of transmission of a second PPDU transmitted via said another of the plurality of links through setting a content that is carried by the first PPDU transmitted via said one of the plurality of links, wherein the ending time instant of transmission of the first PPDU and the ending time instant of transmission of the second PPDU are aligned with mismatch between the ending time instant of transmission of the first PPDU and the ending time instant of transmission of the second PPDU falling within a predefined tolerance range; and transmitting PPDUs via the plurality of links, wherein one PPDU is transmitted via each of the plurality of links, and the PPDUs comprise the first PPDU and the second PPDU. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating a wireless fidelity (WiFi) system according to an embodiment of the present invention. 
         FIG.  2    is a diagram illustrating a first physical protocol data unit (PPDU) ending time alignment mechanism according to an embodiment of the present invention. 
         FIG.  3    is a diagram illustrating a second PPDU ending time alignment mechanism according to an embodiment of the present invention. 
         FIG.  4    is a diagram illustrating a third PPDU ending time alignment mechanism according to an embodiment of the present invention. 
         FIG.  5    is a diagram illustrating a fourth PPDU ending time alignment mechanism according to an embodiment of the present invention. 
         FIG.  6    is a diagram illustrating a fifth PPDU ending time alignment mechanism according to an embodiment of the present invention. 
         FIG.  7    is a diagram illustrating a sixth PPDU ending time alignment mechanism according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terms are used throughout the following description and claims, which refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not in function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
       FIG.  1    is a diagram illustrating a wireless fidelity (WiFi) system according to an embodiment of the present invention. For brevity and simplicity, it is assumed that the WiFi system  100  includes one access point (AP)  102  and two non-AP stations (STAs)  104  and  106 . In this embodiment, AP  102  and non-AP STAs  104  and  106  are wireless fidelity (WiFi) multi-link devices (MLDs) that support multi-link operation (MLO). For example, AP  102  may own K links L 1 -L K  at different channels (radio frequency bands), may communicate with non-AP STA  104  via N links L 11 -L 1N  (which are selected from links L 1 -L K ), and may communicate with non-AP STA  106  via M links L 21 -L 2M  (which are selected from links L 1 -L K ), where K, M and N are positive integers, N is not smaller than 2, M is not smaller than 2, and K is not smaller than any of M and N. In some embodiments of the present invention, non-AP STAs  104  and  106  may be dual-radio STAs (M=N=2), and AP  102  may be a tri-band AP (K=3). For example, links owned by AP  102  may include a channel in 5 GHz, a channel in 6 GHz, and a channel in 2.4 GHz. However, this is for illustrative purposes only, and is not meant to be a limitation of the present invention. Any WiFi system using the proposed physical protocol data unit (PPDU) alignment mechanism falls within the scope of the present invention. In addition to MLO, the AP  102  may support other features such as multi-User multiple-input multiple-output (MU-MIMO) and orthogonal frequency-division multiple access (OFDMA) for multi-user transmission. 
     In certain cases, the PPDUs&#39; starting time instants may not be synchronized due to the fact that transmission opportunity (TXOP) timing of multiple links may be different and/or the transmitter needs to check clear channel assessment (CCA) for a certain period before medium access. The proposed PPDU alignment mechanism is capable of synchronizing ending time instants of transmission of PPDUs transmitted over multiple links to help the acknowledgement (ACK) receiving and the follow-up transmission to be synchronized. In this present invention, ending time instants of transmission of PPDUs transmitted over multiple links may be regarded as aligned with each other when mismatch between the ending time instants falls within a predefined tolerance range. For example, the predefined tolerance range may be set by short interframe space (SIFS)±aSlotTime*10%. In practice, the predefined tolerance range may be adjusted, depending upon actual design considerations. 
       FIG.  2    is a diagram illustrating a first PPDU ending time alignment mechanism according to an embodiment of the present invention. In accordance with the first PPDU ending time alignment mechanism, one WiFi MLD (e.g., AP  102  shown in  FIG.  1   ) aligns parameters of a plurality of links (e.g., links L 11 -L 1N  or links L 21 -L 2M ) for constraining the links to have the same capacity for PPDU transmission, and transmits PPDUs via the links, where highest capacity supported by one of the links is different from highest capacity supported by another of the links, and each of the PPDUs is generated and transmitted under the same parameters. For example, the parameters may include BW, NSS, and MCS. In this embodiment, one WiFi MLD (e.g., AP  102 ) communicates with another WiFi MLD (e.g., Non-AP STA  104  or Non-AP STA  106 ) via a first link (denoted by “Link 1”) and a second link (denoted by “Link 2”), where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, and best MCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by the second link is higher than the highest capacity supported by the first link. 
     In accordance with the first PPDU ending time alignment mechanism, the PPDU transmission for each of the first link and the second link may use BW=40 MHz, NSS=1, and best MCS=MCS3. The first PPDU ending time alignment mechanism may be applicable to the duplicate transmission scenario with synchronized PPDU starting time. As shown in  FIG.  2   , one PPDU  202  is transmitted via the first link, and one PPDU  204  is transmitted via the second link, where the WiFi MLD (e.g., AP  102 ) transmits the PPDUs  202  and  204  to the same user (e.g., non-AP STA  104  or non-AP STA  106 ), and the PPDU  204  is a duplicate of the PPDU  202  such that the PPDUs  202  and  204  carry the same content. As shown in  FIG.  2   , the ending time instants of transmission of PPDUs  202  and  204  are aligned. 
     Since the highest capacity supported by the second link is higher than the highest capacity supported by the first link and parameters of the first link and the second link are constrained to be the same for PPDU transmission, the available capacity of the second link is not fully utilized by PPDU transmission. As a result, links capable of allowing more data transmission may lose efficiency. As shown in  FIG.  2   , a half of the capacity supported by the second link is unused during a transmission period of the PPDU  204 . To address this issue, the present invention proposes a second PPDU ending time alignment mechanism. 
       FIG.  3    is a diagram illustrating a second PPDU ending time alignment mechanism according to an embodiment of the present invention. In accordance with the second PPDU ending time alignment mechanism, one WiFi MLD (e.g., AP  102  shown in  FIG.  1   ) aligns parameters of a plurality of links (e.g., links L 11 -L 1N  or links L 21 -L 2M ) for constraining the links to have the same capacity for PPDU transmission, and transmits PPDUs via the links, where highest capacity supported by one of the links is different from highest capacity supported by another of the links, each of the PPDUs is generated and transmitted under the same parameters (which may include BW, NSS, and MCS), and more than one PPDU is transmitted via one link that supports higher capacity. For example, one WiFi MLD (e.g., AP  102 ) communicates with another WiFi MLD (e.g., non-AP STA  104  or non-AP STA  106 ) via a first link (denoted by “Link 1”) and a second link (denoted by “Link 2”), where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, and best MCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by the second link is higher than the highest capacity supported by the first link. 
     In accordance with the second PPDU ending time alignment mechanism, the PPDU transmission for each of the first link and the second link may use BW=40 MHz, NSS=1, and best MCS=MCS3. The second PPDU ending time alignment mechanism may be applicable to the duplicate transmission scenario with synchronized PPDU starting time. As shown in  FIG.  3   , one PPDU  302  is transmitted via the first link, and one PPDU  304  and one duplicate PPDU  306  (which is a duplicate of PPDU  304 ) are transmitted via the second link. In this embodiment, the WiFi MLD (e.g., AP  102 ) transmits the PPDUs  302 ,  304 , and  306  to the same user (e.g., non-AP STA  104  or non-AP STA  106 ), and the PPDU  304  is a duplicate of the PPDU  302  such that the PPDUs  302 ,  304 , and  306  carry the same content. As shown in  FIG.  3   , the ending time instants of transmission of PPDUs  302 ,  304 , and  306  are aligned. Duplicated transmission in extra BW of a high capacity link (e.g., second link) further increases transmission robustness. In addition, extra signaling about the duplicate PPDU in the extra BW of the high capacity link (e.g., second link) may be required. 
       FIG.  4    is a diagram illustrating a third PPDU ending time alignment mechanism according to an embodiment of the present invention. In accordance with the third PPDU ending time alignment mechanism, one WiFi MLD (e.g., AP  102  shown in  FIG.  1   ) sets parameters of each of links for enabling the links to have different capacity for PPDU transmission, sets a content that is carried by a first PPDU transmitted via one of the links to ensure that an ending time instant of transmission of the first PPDU transmitted via one of the links and an ending time instant of transmission of a second PPDU transmitted via another of the links are aligned, and transmits PPDUs (which include the first PPDU and the second PPDU) via the links. Parameters of each link may include BW, NSS, and MCS. In this embodiment, parameters of one of the links are different from parameters of another of the links, highest capacity supported by one of the links is higher than highest capacity supported by another of the links, and one PPDU is transmitted via each of the links. Each link may use its best parameters to generate and transmit the PPDU. For example, one WiFi MLD (e.g., AP  102 ) communicates with another WiFi MLD (e.g., non-AP STA  104  or non-AP STA  106  shown in  FIG.  1   ) via a first link (denoted by “Link 1”) and a second link (denoted by “Link 2”), where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, and best MCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by the second link is higher than the highest capacity supported by the first link. The first link is configured to use best parameters, including BW=40 MHz, NSS=1, and best MCS=MCS3, for PPDU transmission, and the second link is configured to use best parameters, including BW=80 MHz, NSS=1, and best MCS=MCS7, for PPDU transmission. 
     Since the capacity provided by the second link is higher than the capacity provided by the first link, partial duplication can be enabled to align PPDU lengths for achieving PPDU ending time alignment. The third PPDU ending time alignment mechanism may be applicable to all of the aforementioned scenarios. As shown in  FIG.  4   , one PPDU  402  is transmitted via the first link, and one PPDU  404  is transmitted via the second link, where the WiFi MLD (e.g., AP  102 ) transmits PPDUs  402  and  404  to the same user (e.g., non-AP STA  104  or non-AP STA  106 ), and the PPDU  404  transmitted via the second link includes a partial PPDU  406  (which includes one or more media access control protocol data units (MPDUs)  407 ), one or more duplicate MPDUs  408 , and optional padding bits  410 . The duplicate MPDU(s)  408  may be derived from duplication of MPDU(s)  407 , and one or more duplicate MPDUs may be obtained from the same source MPDU. The optional padding bits  410  may be set by 0&#39;s. In this embodiment, the duplicate MPDU(s)  408  are appended to the partial PPDU  406 , and the optional padding bits  410  are appended to the duplicate MPDU(s)  408 . In an alternative design, source MPDU(s)  407  and duplicate MPDU(s)  408  may be interleaved in the PPDU  404  to increase diversity. As shown in  FIG.  4   , the ending time instants of transmission of PPDUs  402  and  404  are aligned. Signaling of MPDU duplication may be required, and MPDUs with the same content can be decoded individually or jointly to reduce error possibility. It should be noted that PPDU starting time alignment is not necessarily required by the third PPDU ending time alignment mechanism. 
       FIG.  5    is a diagram illustrating a fourth PPDU ending time alignment mechanism according to an embodiment of the present invention. In accordance with the fourth PPDU ending time alignment mechanism, one WiFi MLD (e.g., AP  102  shown in  FIG.  1   ) sets parameters of links for enabling the links to have different capacity for PPDU transmission, and transmits PPDUs via the links. Parameters of each link may include BW, NSS, and MCS. In this embodiment, parameters of one of the links are different from parameters of another of the links, highest capacity supported by one of the links is higher than highest capacity supported by another of the links, one PPDU is transmitted via one link, and multiple PPDUs are transmitted via another link to ensure that an ending time instant of transmission of one PPDU on one link and an ending time instant of transmission of multiple PPDUs on another link are aligned. 
     Each link may use its best parameters to generate and transmit the PPDU. For example, one WiFi MLD (e.g., AP  102 ) communicates with another WiFi MLD (e.g., non-AP STA  104 ) via multiple links including a first link (denoted by “Link 1”) and a second link (denoted by “Link 2”), and further communicates with yet another WiFi MLD (e.g., non-AP STA  106 ) via multiple links including the second link (denoted by “Link 2”), where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, and best MCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by the second link is higher than the highest capacity supported by the first link. The first link is configured to use best parameters, including BW=40 MHz, NSS=1, and best MCS=MCS3, for PPDU transmission, and the second link is configured to use best parameters, including BW=80 MHz, NSS=1, and best MCS=MCS7, for PPDU transmission. 
     Since the capacity provided by the second link is higher than the capacity provided by the first link, the second link is allowed to transmit multiple PPDUs during a transmission period of one PPDU that is transmitted via the first link. The fourth PPDU ending time alignment mechanism may be applicable to all of the aforementioned scenarios. As shown in  FIG.  5   , one PPDU  502  is transmitted via the first link, and one PPDU  504  and at least one padding PPDU  506  are transmitted via the second link, where the WiFi MLD (e.g., AP  102 ) transmits the PPDUs  502  and  504  to the same user (e.g., non-AP STA  104 ), and transmits padding PPDU(s)  506  to a different user (e.g., non-AP STA  106 ). The multiple PPDUs  504  and  506  transmitted via the second link are separated by SIFS to ensure medium occupation. As shown in  FIG.  5   , an ending time instant of transmission of PPDU  502  and an ending time instant of transmission of multiple PPDUs  504  and  506  (particularly, an ending time instant of transmission of the last one of PPDUs  504  and  506 ) are aligned. It should be noted that PPDU starting time alignment is not necessarily required by the fourth PPDU ending time alignment mechanism. 
     In a case where the fourth PPDU ending time alignment mechanism is applied to the duplicate transmission scenario, the PPDU  504  is a duplicate of the PPDU  502 , and an ending time instant of transmission of the PPDU  504  is earlier than an ending time instant of transmission of the PPDU  502 . If immediate acknowledgement (ACK) is required by duplicate transmission and an ACK message is sent over the second link, the ACK message receiving interferes with transmission of padding PPDU(s)  506 . To address this issue, the present invention proposes sending the ACK message over the first link that occupies the longer period to avoid interfering with transmission of the padding PPDU(s)  506  on the second link. Alternatively, a block ACK mechanism may be adopted to collect acknowledgement later after the transmission, and/or signaling can be adopted to delay the acknowledgement until the end of transmission. 
       FIG.  6    is a diagram illustrating a fifth PPDU ending time alignment mechanism according to an embodiment of the present invention. In accordance with the fifth PPDU ending time alignment mechanism, one WiFi MLD (e.g., AP  102  shown in  FIG.  1   ) sets parameters of links for enabling the links to have different capacity for PPDU transmission, sets a content that is carried by a first PPDU transmitted via one of the links to ensure that an ending time instant of transmission of the first PPDU transmitted via one of the links and an ending time instant of transmission of a second PPDU transmitted via another of the links are aligned, and transmits PPDUs (which include the first PPDU and the second PPDU) via the links. Parameters of each link may include BW, NSS, and MCS. In this embodiment, parameters of one of the links are different from parameters of another of the links, highest capacity supported by one of the links is higher than highest capacity supported by another of the links, and one PPDU is transmitted via each of the links. Each link may use its best parameters to generate and transmit PPDU. For example, one WiFi MLD (e.g., AP  102 ) communicates with another WiFi MLD (e.g., non-AP STA  104  or non-AP STA  106 ) via a first link (denoted by “Link 1”) and a second link (denoted by “Link 2”), where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, and best MCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by the second link is higher than the highest capacity supported by the first link. The first link is configured to use best parameters, including BW=40 MHz, NSS=1, and best MCS=MCS3, for PPDU transmission, and the second link is configured to use best parameters, including BW=80 MHz, NSS=1, and best MCS=MCS7, for PPDU transmission. 
     Since the capacity provided by the second link is higher than the capacity provided by the first link, different MPDUs can be allowed in the second link to align the PPDU lengths for achieving PPDU ending time alignment. The fifth PPDU ending time alignment mechanism may be applicable to the duplication transmission scenario with no PPDU starting time alignment. As shown in  FIG.  6   , one PPDU  602  is transmitted via the first link, and one PPDU  604  is transmitted via the second link, where the WiFi MLD (e.g., AP  102 ) transmits PPDUs  602  and  604  to the same user (e.g., non-AP STA  104  or non-AP STA  106 ), and the PPDU  604  transmitted via the second link includes a partial PPDU  606  (which includes one or more media access control protocol data units (MPDUs)  607 ), and one or more other MPDUs  608  that are not duplicate(s) of MPDU(s)  607 . The MPDU(s)  608  are appended to the partial PPDU  606 . As shown in  FIG.  6   , an ending time instant of transmission of PPDU  602  and an ending time instant of transmission of PPDU  604  are aligned. The partial PPDU  606  may be a duplicate of the PPDU  602 . Hence, the MPDU(s)  608  carried in the second link are different from the MPDU(s) carried in the first link. In some embodiments of the present invention, the PPDU  604  may include optional padding bits (not shown) that are appended to the MPDU(s)  608  to make the PPDU length aligned. 
       FIG.  7    is a diagram illustrating a sixth PPDU ending time alignment mechanism according to an embodiment of the present invention. In accordance with the sixth PPDU ending time alignment mechanism, one WiFi MLD (e.g., AP  102  shown in  FIG.  1   ) sets parameters of links for enabling the links to have different capacity for PPDU transmission, sets a content that is carried by a first PPDU transmitted via one of the links to ensure that an ending time instant of transmission of the first PPDU transmitted via one of the links and an ending time instant of transmission of a second PPDU transmitted via another of the links are aligned, and transmits PPDUs (which include the first PPDU and the second PPDU) via the links. Parameters of each link may include BW, NSS, and MCS. In this embodiment, parameters of one of the links are different from parameters of another of the links, highest capacity supported by one of the links is higher than highest capacity supported by another of the links, and one PPDU is transmitted via each of the links. Each link may use its best parameters to generate and transmit the PPDU. For example, one WiFi MLD (e.g., AP  102 ) communicates with another WiFi MLD (e.g., non-AP STA  104 ) via multiple links including a first link (denoted by “Link 1”) and a second link (denoted by “Link 2”), and communicates with yet another WiFi MLD (e.g., non-AP STA  106 ) via multiple links including the second link (denoted by “Link 2”), where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, and best MCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by the second link is higher than the highest capacity supported by the first link. The first link is configured to use best parameters, including BW=40 MHz, NSS=1, and best MCS=MCS3, for PPDU transmission, and the second link is configured to use best parameters, including BW=80 MHz, NSS=1, and best MCS=MCS7, for PPDU transmission. 
     Since the capacity provided by the second link is higher than the capacity provided by the first link, the second link can transmit a multi-user (MU) PPDU under a condition that the PPDU ending time alignment is met. The sixth PPDU ending time alignment mechanism may be applicable to all of the aforementioned scenarios. As shown in  FIG.  7   , one PPDU  702  is transmitted via the first link, and one MU PPDU  704  is transmitted via the second link. The MU PPDU  704  accommodates data of different users (e.g., different WiFi MLDs) in a plurality of resource units (RUs), where each RU is a group of subcarriers (tones). For example, the MU PPDU  704  carries user data D_RU1 in a first RU of the MU PPDU  704  and user data D_RU2 in a second RU of the MU PPDU  704 , where the user data D_RU1 is transmitted to one user (e.g., non-AP STA  104 ), and the user data D_RU2 is transmitted to another user (e.g., non-AP STA  106 ). As shown in  FIG.  7   , an ending time instant of transmission of PPDU  702  and an ending time instant of transmission of MU PPDU  704  are aligned. In some embodiments of the present invention, the PPDU  702  and the user data D_RU1 included in the MU PPDU  704  may be transmitted to the same user (e.g., non-AP STA  104  or non-AP STA  106 ). In addition, padding in RUs for multiple users may be required. It should be noted that PPDU starting time alignment is not necessarily required by the sixth PPDU ending time alignment mechanism. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.