Patent Publication Number: US-11658768-B2

Title: Wireless fidelity multi-link device with dynamic operation mode switch and associated method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 63/048,733, filed on Jul. 7, 2020 and incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to wireless communications, and more particularly, to a wireless fidelity (Wi-Fi) multi-link device (MLD) with dynamic operation mode switch and an associated method. 
     In a Wi-Fi multi-link operation, there exists several links between two Wi-Fi MLDs, including one access point (AP) and one non-AP station (STA), that occupy different radio-frequency (RF) bands. One Wi-Fi MLD can perform channel access (e.g., enhanced distributed channel access (EDCA)) on multiple wireless links independently. Specifically, these wireless links can operate independently to increase the overall throughput and/or to improve the connection stability. For example, one Wi-Fi MLD may have one spatial stream specified on one wireless link. If a wireless link can use more resources to transmit or receive during a specific period, the performance of the wireless link can be improved. Thus, there is a need for an innovative design which is capable of dynamically changing operation mode parameter(s) of a Wi-Fi MLD. 
     SUMMARY 
     One of the objectives of the claimed invention is to provide a wireless fidelity (Wi-Fi) multi-link device (MLD) with dynamic operation mode switch and an associated method. 
     According to a first aspect of the present invention, an exemplary wireless fidelity (Wi-Fi) multi-link device (MLD) is disclosed. The exemplary MLD includes a transmit (TX) circuit, a receive (RX) circuit, and a control circuit. The control circuit is arranged to control the RX circuit to receive a first frame under an operation mode parameter with a first setting, control the TX circuit to transmit a second frame responsive to the first frame under the operation mode parameter with the first setting, and after the second frame is transmitted, control the RX circuit to receive at least one physical layer protocol data unit (PPDU) under the operation mode parameter with a second setting, wherein the second setting is different from the first setting. 
     According to a second aspect of the present invention, an exemplary wireless fidelity (Wi-Fi) multi-link device (MLD) is disclosed. The MLD includes a transmit (TX) circuit, a receive (RX) circuit, and a control circuit. The control circuit is arranged to control the TX circuit to transmit a first frame under an operation mode parameter with a first setting, control the RX circuit to receive a second frame responsive to the first frame under the operation mode parameter with the first setting, and after the second frame is received, control the TX circuit to transmit at least one physical layer protocol data unit (PPDU) under the operation mode parameter with a second setting, wherein the second setting is different from the first setting. 
     According to a third aspect of the present invention, an exemplary wireless communication method employed by a wireless fidelity (Wi-Fi) multi-link device (MLD) is disclosed. The exemplary wireless communication method includes: controlling a receive (RX) circuit to receive a first frame under a first operation mode parameter with a first setting; controlling a transmit (TX) circuit to transmit a second frame responsive to the first frame under the first operation mode parameter with the first setting; after the second frame is transmitted, controlling the RX circuit to receive at least one physical layer protocol data unit (PPDU) under the first operation mode parameter with a second setting, wherein the second setting is different from the first setting; controlling the TX circuit to transmit a third frame under a second operation mode parameter with a third setting; controlling the RX circuit to receive a fourth frame responsive to the third frame under the second operation mode parameter with the third setting; and after the fourth frame is received, controlling the TX circuit to transmit at least one PPDU under the second operation mode parameter with a fourth setting, wherein the fourth setting is different from the third setting. 
     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 (Wi-Fi) communication system according to an embodiment of the present invention. 
         FIG.  2    is a diagram illustrating a first dynamic operation mode switch scenario according to an embodiment of the present invention. 
         FIG.  3    is a diagram illustrating a second dynamic operation mode switch scenario according to an embodiment of the present invention. 
         FIG.  4    is a diagram illustrating a third dynamic operation mode switch scenario according to an embodiment of the present invention. 
         FIG.  5    is a diagram illustrating a fourth dynamic operation mode switch scenario according to an embodiment of the present invention. 
         FIG.  6    is a diagram illustrating a fifth dynamic operation mode switch scenario according to an embodiment of the present invention. 
         FIG.  7    is a diagram illustrating a sixth dynamic operation mode switch scenario according to an embodiment of the present invention. 
         FIG.  8    is a diagram illustrating a seventh dynamic operation mode switch scenario according to an embodiment of the present invention. 
         FIG.  9    is a diagram illustrating additional signaling provided by a channel quality indicator feedback according to an embodiment of the present invention. 
         FIG.  10    is a diagram illustrating an alternative design implemented on the basis of the dynamic operation mode switch scenario shown in  FIG.  2   . 
         FIG.  11    is a diagram illustrating an alternative design implemented on the basis of the dynamic operation mode switch scenario shown in  FIG.  3   . 
         FIG.  12    is a diagram illustrating an alternative design implemented on the basis of the dynamic operation mode switch scenario shown in  FIG.  4   . 
         FIG.  13    is a diagram illustrating an alternative design implemented on the basis of the dynamic operation mode switch scenario shown in  FIG.  7   . 
         FIG.  14    is a diagram illustrating an alternative design implemented on the basis of the dynamic operation mode switch scenario shown in  FIG.  8   . 
     
    
    
     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 (Wi-Fi) communication system according to an embodiment of the present invention. The Wi-Fi communication system  100  has a plurality of Wi-Fi multi-link devices (MLDs)  102  and  104 , including an access point (AP) and a non-AP station (STA). For example, the Wi-Fi MLD  104  may be an AP, and the Wi-Fi MLD  102  may be a non-AP STA. For brevity and simplicity, only two Wi-Fi MLDs are shown in  FIG.  1   . In practice, the Wi-Fi communication system  100  is allowed to have more than two Wi-Fi MLDs, including an AP and more than one non-AP STA in the same basic service set (BSS). By way of example, but not limitation, both of Wi-Fi MLDs  102  and  104  may be in compliance with IEEE 802.11be standard. 
     As shown in  FIG.  1   , the Wi-Fi MLD  102  includes a processor  112 , a memory  114 , a control circuit  116 , a receive (RX) circuit  118 , a transmit (TX) circuit  120 , and multiple antennas  122 . The memory  114  is arranged to store a program code. The processor  112  is arranged to load and execute the program code to manage the Wi-Fi MLD  102 . The control circuit  116  is arranged to control wireless communications with the Wi-Fi MLD  104 . Considering a case where the Wi-Fi MLD  102  is a non-AP STA and the Wi-Fi MLD  104  is an AP, the control circuit  116  controls the TX circuit  120  to deal with uplink (UL) traffic between AP and non-AP STA, and controls the RX circuit  118  to deal with downlink (DL) traffic between AP and non-AP STA. Due to inherent characteristics of Wi-Fi MLD  102 , the TX circuit  120  includes multiple TX chains, and the RX circuit  118  includes multiple RX chains. In this embodiment, the Wi-Fi MLD  102  is equipped with dynamic operation mode switch functionality. 
     Regarding DL traffic between AP and non-AP STA, the control circuit  116  is arranged to control the RX circuit  118  to receive a first frame under an operation mode parameter with a first setting (e.g., default setting), control the TX circuit  120  to transmit a second frame responsive to the first frame under the operation mode parameter with the first setting, and after the second frame is transmitted, control the RX circuit  118  to receive at least one DL physical layer protocol data unit (PPDU) under the operation mode parameter with a second setting (e.g., maximum setting or non-default setting) during a remaining period of a transmission opportunity (TXOP), wherein the second setting is different from the first setting. In addition, the control circuit  116  is further arranged to control the RX circuit  118  to resume the operation mode parameter with the first setting (e.g., default setting) at an end of the TXOP. 
     In one exemplary mode switch design, the first frame may be a request to send (RTS) frame, and the second frame may be a clear to send (CTS) frame responsive to the RTS frame, where the RTS frame does not carry any indication of operation mode parameter change, and the CTS frame may or may not carry indication of operation mode parameter change that specifies the use of the second setting. When the CTS frame does not carry any indication of operation mode parameter change, the CTS frame may act as indication of using the maximum setting for the operation mode parameter (i.e., second setting=maximum setting). 
     In another exemplary mode switch design, the first frame may be an RTS frame, and the second frame may be a CTS frame responsive to the RTS frame, where the CTS frame does not carry any indication of operation mode parameter change, and the RTS frame may or may not carry indication of operation mode parameter change that specifies the use of the second setting. When the RTS frame does not carry any indication of operation mode parameter change, the RTS frame may act as indication of using the maximum setting for the operation mode parameter (i.e., second setting=maximum setting). 
     In yet another exemplary mode switch design, the first frame may be an inquiry frame transmitted after RTS/CTS handshaking is done, and the second frame may be a response frame responsive to the inquiry frame, where the response frame is particularly designed to carry indication of operation mode parameter change that specifies the use of the second setting (e.g., maximum setting or non-default setting). 
     Regarding UL traffic between AP and non-AP STA, the control circuit  116  is arranged to control the TX circuit  120  to transmit a first frame under an operation mode parameter with a first setting (e.g., default setting), control the RX circuit  118  to receive a second frame responsive to the first frame under the operation mode parameter with the first setting, and after the second frame is received, control the TX circuit  120  to transmit at least one UL PPDU under the operation mode parameter with a second setting (e.g., maximum setting or non-default setting) during a remaining period of a TXOP, wherein the second setting is different from the first setting. In addition, the control circuit  116  is further arranged to control the TX circuit  120  to resume the operation mode parameter with the first setting (e.g., default setting) at an end of the TXOP. 
     The Wi-Fi MLD  102  can have different operation modes on each link established between Wi-Fi MLDs  102  and  104 . In some embodiments of the present invention, the operation mode parameter allowed to be dynamically switched may be the number of spatial streams (NSS), the bandwidth, the decoding capability (e.g., modulation and coding scheme (MCS)), the maximum media access control protocol data unit (MPDU) length, the maximum aggregate media access control service data unit (A-MSDU) length, or the maximum aggregate media access control protocol data unit (A-MPDU) length exponent. For example, a default bandwidth may be set by 80 megahertz (MHz), and a maximum bandwidth may be set by 160 MHz. For another example, a default decoding capability may cover MCS0-MCS3 only, and a maximum decoding capability may cover all MCSs. For yet another example, the default maximum MPDU length may be set by 8K on each link, and the maximum MPDU length may be set by 11K. 
     For better understanding of technical features of the proposed dynamic operation mode switch scheme, several examples are provided as below. 
       FIG.  2    is a diagram illustrating a first dynamic operation mode switch scenario according to an embodiment of the present invention. In this embodiment, the Wi-Fi MLD  104  may be an AP, the Wi-Fi MLD  102  may be a non-AP STA, and the Wi-Fi MLD  102  on a link may set a default receive (RX) NSS=1 and a maximum RX NSS=4. On each link, the Wi-Fi MLD  102  operates under a default mode listening to the channel. The control circuit  116  controls the RX circuit  118  to receive an RTS frame from Wi-Fi MLD (which is an AP)  104 , where the RTS frame is sent to initialize a TXOP. After the RX circuit  118  receives the RTS frame on a link, the control circuit  116  controls the TX circuit  120  to respond with a CTS frame on the same link, where the RTS/CTS mechanism claims the TXOP for frame exchange. The RTS frame is received based on a default operation mode parameter (e.g., RX NSS=1). In this embodiment, the CTS frame carries indication IND of operation mode parameter change that specifies the use of RX NSS=2. In other words, the Wi-Fi MLD  102  will control the RX circuit  118  to switch another RX chain to the link for enhancing the RX NSS capability. After the CTS frame is transmitted by the TX circuit  120 , the control circuit  116  controls the RX circuit  118  to operate under a changed operation mode parameter (e.g., RX NSS=2) and other unchanged parameters, and receive one or more DL PPDUs on the link during a remaining period of the TXOP. Hence, during the remaining period of the TXOP, DL PPDU(s) can be received by RX NSS=2. At an end of the TXOP, the control circuit  116  controls the RX circuit  118  to resume the default operation mode parameter (e.g., RX NSS=1) on the link. It should be noted that, if the CTS frame does not carry any indication of operation mode parameter change, the maximum setting (e.g., RX NSS=4) is inferred from such a CTS frame. 
       FIG.  3    is a diagram illustrating a second dynamic operation mode switch scenario according to an embodiment of the present invention. In this embodiment, the Wi-Fi MLD  104  may be an AP, the Wi-Fi MLD  102  may be a non-AP STA, and the Wi-Fi MLD  102  on a link may set a default RX NSS=1 and a maximum RX NSS=4. On each link, the Wi-Fi MLD  102  operates under a default mode listening to the channel. The control circuit  116  controls the RX circuit  118  to receive an RTS frame from Wi-Fi MLD (which is an AP)  104 , where the RTS frame is sent to initialize a TXOP. After the RX circuit  118  receives the RTS frame on a link, the control circuit  116  controls the TX circuit  120  to respond with a CTS frame on the same link, where the RTS/CTS mechanism claims a TXOP for frame exchange. After the CTS frame is transmitted by the TX circuit  120 , the RX circuit  118  receives an inquiry frame sent from Wi-Fi MLD (which is an AP)  104 , where the inquiry frame is used to make an inquiry about possible operation mode change for the following period of the TXOP on the link. The TX circuit  120  sends back a response frame after the inquiry frame is received by the RX circuit  118 . The RTS frame and the inquiry frame are both received based on a default operation mode parameter (e.g., RX NSS=1). In this embodiment, the response frame carries indication IND of operation mode parameter change that specifies the use of RX NSS=4. In other words, the Wi-Fi MLD  102  will control the RX circuit  118  to switch other RX chains to the link for achieving its maximum RX NSS capability. After the response frame is transmitted by the TX circuit  120 , the control circuit  116  controls the RX circuit  118  to operate under a changed operation mode parameter (e.g., RX NSS=4) and other unchanged parameters, and receive one or more DL PPDUs on the link during a remaining period of the TXOP. Hence, during the remaining period of the TXOP, DL PPDU(s) can be received by RX NSS=4. At an end of the TXOP, the control circuit  116  controls the RX circuit  118  to resume the default operation mode parameter (e.g., RX NSS=1) on the link. 
       FIG.  4    is a diagram illustrating a third dynamic operation mode switch scenario according to an embodiment of the present invention. In this embodiment, the Wi-Fi MLD  104  may be an AP, the Wi-Fi MLD  102  may be a non-AP STA, and the Wi-Fi MLD  102  on a link may set a default bandwidth (BW)=80 MHz and a maximum BW=160 MHz. On each link, the Wi-Fi MLD  102  operates under a default mode listening to the channel. The control circuit  116  controls the RX circuit  118  to receive an RTS frame from Wi-Fi MLD (which is an AP)  104 , where the RTS frame is sent to initialize a TXOP, and also carries indication IND of operation mode parameter change that specifies the use of BW=160 MHz. After the RTS frame is received by the RX circuit  118 , the control circuit  116  controls the TX circuit  120  to respond with a CTS frame on the same link, where the RTS/CTS mechanism claims a TXOP for frame exchange, and the CTS frame also carries indication IND′ about accepting the request of operation mode parameter change or rejecting the request of operation mode parameter change. The RTS frame is received based on the default operation mode parameter (e.g., BW=80 MHz). If the control circuit  116  accepts the request of operation mode parameter change, the control circuit  116  controls the RX circuit  118  to change the BW to 160 MHz on the link. In other words, the control circuit  116  controls the RX circuit  118  to operate under a changed operation mode parameter (e.g., BW=160 MHz) and other unchanged parameters, and receive one or more DL PPDUs on the link during a remaining period of the TXOP. Hence, during the remaining period of the TXOP, DL PPDU(s) can be received by BW=160 MHz. At an end of the TXOP, the control circuit  116  controls the RX circuit  118  to resume the default operation mode parameter (e.g., BW=80 MHz) on the link. 
       FIG.  5    is a diagram illustrating a fourth dynamic operation mode switch scenario according to an embodiment of the present invention. In this embodiment, the Wi-Fi MLD  104  may be an AP, the Wi-Fi MLD  102  may be a non-AP STA, and the Wi-Fi MLD  102  on a link may set a default transmit (TX) NSS=1 and a maximum TX NSS=4. On each link, the Wi-Fi MLD  102  operates under a default mode listening to the channel. The control circuit  116  controls the TX circuit  120  to transmit an RTS frame on a link to initialize a TXOP. The RTS frame is transmitted based on the default operation mode parameter (e.g., TX NSS=1). In this embodiment, the RTS frame carries indication IND of operation mode parameter change that specifies the use of TX NSS=2. In other words, the Wi-Fi MLD  102  will control the TX circuit  120  to switch another TX chain to the link for enhancing the TX NSS capability. The RX circuit  118  receives a CTS frame responsive to the RTS frame from Wi-Fi MLD (which is an AP)  104  on the same link, where the RTS/CTS mechanism claims a TXOP for frame exchange. After the CTS frame is received by the RX circuit  118 , the control circuit  116  controls the TX circuit  120  to operate under a changed operation mode parameter (e.g., TX NSS=2) and other unchanged parameters, and transmit one or more UL PPDUs on the link during a remaining period of the TXOP. Hence, during the remaining period of the TXOP, UL PPDU(s) can be transmitted by TX NSS=2. At an end of the TXOP, the control circuit  116  controls the TX circuit  120  to resume the default operation mode parameter (e.g., TX NSS=1) on the link. 
       FIG.  6    is a diagram illustrating a fifth dynamic operation mode switch scenario according to an embodiment of the present invention. In this embodiment, the Wi-Fi MLD  104  may be an AP, the Wi-Fi MLD  102  may be a non-AP STA, and the Wi-Fi MLD  102  on a link may set a default TX NSS=1 and a maximum TX NSS=4. On each link, the Wi-Fi MLD  102  operates under a default mode listening to the channel. The control circuit  116  controls the TX circuit  120  to transmit an RTS frame on a link, and then controls the RX circuit  118  to receive a CTS frame responsive to the RTS frame from Wi-Fi MLD (which is an AP)  104  on the same link, where the RTS/CTS mechanism claims a TXOP for frame exchange. After the CTS frame is received by the RX circuit  118 , the control circuit  116  controls the TX circuit  120  to transmit an announcement frame that is used to announce operation mode change for the following period of the TXOP on the link. In this embodiment, the announcement frame carries indication IND of operation mode parameter change that specifies the use of TX NSS=2. In other words, the Wi-Fi MLD  102  will control the TX circuit  120  to switch another TX chain to the link for enhancing the TX NSS capability. The RTS frame and the announcement frame are both transmitted based on the default operation mode parameter (e.g., TX NSS=1). After the RX circuit  118  receives a response frame responsive to the announcement frame from the Wi-Fi MLD (which is an AP)  104 , the control circuit  116  controls the TX circuit  120  to operate under a changed operation mode parameter (e.g., TX NSS=2) and other unchanged parameters, and transmit one or more UL PPDUs on the link during a remaining period of the TXOP. Hence, during the remaining period of the TXOP, UL PPDU(s) can be transmitted by TX NSS=2. At an end of the TXOP, the control circuit  116  controls the TX circuit  120  to resume the default operation mode parameter (e.g., TX NSS=1) on the link. 
     As mentioned above, the CTS frame/response frame is capable of carrying indication of operation mode change. In some embodiments of the present invention, more signaling can be exchanged during the initialization of the period so that efficiency during that period can be further improved. For example, since RX chain switch may require additional sounding to ensure the RX quality, the CTS frame/response frame may further carry a sounding request. For another example, the CTS frame/response frame may further carry an RX chain bitmap that is used for indicating which antennas are used on a certain link for an RX operation and is also used for allowing a sender (which transmits the RTS frame/inquiry frame) to use suitable beamforming parameters derived from previous sounding. 
       FIG.  7    is a diagram illustrating a sixth dynamic operation mode switch scenario according to an embodiment of the present invention. In this embodiment, the Wi-Fi MLD  104  may be an AP, the Wi-Fi MLD  102  may be a non-AP STA, and the Wi-Fi MLD  102  on a link may set a default RX NSS=1 and a maximum RX NSS=4. On each link, the Wi-Fi MLD  102  operates under a default mode listening to the channel. After the RX circuit  118  receives an RTS frame from Wi-Fi MLD (which is an AP)  104  on a link, the control circuit  116  controls the TX circuit  120  to respond with a CTS frame on the same link, where the RTS/CTS mechanism claims a TXOP for frame exchange. The RTS frame is received based on the default operation mode parameter (e.g., RX NSS=1). In this embodiment, the CTS frame carries indication IND of operation mode parameter change that specifies the use of RX NSS=2, and also carries a sounding request SR. After the CTS frame is received by the Wi-Fi MLD (which is an AP)  104 , the Wi-Fi MLD  104  accepts the sounding request SR, and then initiates a sounding process by sending a null data packet announcement (NDPA) frame and a null data packet (NDP) frame, and the Wi-Fi MLD (which is a non-AP STA)  102  replies with a channel quality indicator (CQI) feedback (FB) during the sounding process. After the CTS frame is transmitted by the TX circuit  120 , the control circuit  116  controls the RX circuit  118  to operate under a changed operation mode parameter (e.g., RX NSS=2) and other unchanged parameters, and receive one or more DL PPDUs on the link during a remaining period of the TXOP. Hence, during the remaining period of the TXOP, DL PPDU(s) can be received by RX NSS=2. At an end of the TXOP, the control circuit  116  controls the RX circuit  118  to resume the default operation mode parameter (e.g., RX NSS=1) on the link. 
       FIG.  8    is a diagram illustrating a seventh dynamic operation mode switch scenario according to an embodiment of the present invention. In this embodiment, the Wi-Fi MLD  104  may be an AP, the Wi-Fi MLD  102  may be a non-AP STA, and the Wi-Fi MLD  102  on a link may set a default RX NSS=1 and a maximum RX NSS=4. On each link, the Wi-Fi MLD  102  operates under a default mode listening to the channel. After the RX circuit  118  receives an RTS frame from Wi-Fi MLD (which is an AP)  104  on a link, the control circuit  116  controls the TX circuit  120  to respond with a CTS frame on the same link, where the RTS/CTS mechanism claims a TXOP for frame exchange. The RTS frame is received based on the default operation mode parameter (e.g., RX NSS=1). In this embodiment, the CTS frame carries indication IND of operation mode parameter change that specifies the use of RX NSS=2, and also carries an RX chain bitmap BMP. After the CTS frame received by the Wi-Fi MLD (which is an AP)  104 , the Wi-Fi MLD  104  refers to the RX chain bitmap BMP to decide to do sounding, and initiates a sounding process by sending an NDPA frame and an NDP frame, where the RX chain bitmap BMP indicates the RX NSS employed by Wi-Fi MLD (which is a no-AP STA)  102 . The Wi-Fi MLD  102  replies with a CQI FB during the sounding process, where the CQI FB is based on the RX bitmap BMP carried in the CTS frame. After the CTS frame is transmitted by the TX circuit  120 , the control circuit  116  controls the RX circuit  118  to operate under a changed operation mode parameter (e.g., RX NSS=2) and other unchanged parameters, and receive one or more DL PPDUs on the link during a remaining period of the TXOP. Hence, during the remaining period of the TXOP, DL PPDU(s) can be received by RX NSS=2. At an end of the TXOP, the control circuit  116  controls the RX circuit  118  to resume the default operation mode parameter (e.g., RX NSS=1) on the link. 
       FIG.  9    is a diagram illustrating additional signaling provided by a channel quality indicator feedback according to an embodiment of the present invention. For any sounding process (NDPA+NDP+CQI FB) applied on any link of a multi-link operation (MLO), CQI FB can also carry the RX chain bitmap BMP. In this way, sounding overhead reduction can be achieved if the same RX chains are used on some links in the future. 
     In above embodiments, an AP MLD uses an RTS frame as an initial Control frame of a frame exchange sequence. However, these are for illustrative purposes only, and are not meant to be limitations of the present invention. In some embodiments of the present invention, the initial Control frame of the frame exchange sequence that is initiated by the AP MLD may be a trigger frame, such as a multi-user request to send (MU-RTS) frame or a buffer status report poll (BSRP) frame.  FIGS.  10 - 14    illustrate additional dynamic operation mode switch scenarios. In a case where the AP MLD (e.g., one of Wi-Fi MLDs  102  and  104  shown in  FIG.  1   ) sends a trigger frame TF being an MU-RTS frame, the non-AP MLD (e.g., the other of Wi-Fi MLDs  102  and  104  shown in  FIG.  1   ) may respond with a response frame RF being a CTS frame after receiving the MU-RTS frame. In another case where the AP MLD (e.g., one of Wi-Fi MLDs  102  and  104  shown in  FIG.  1   ) sends a trigger frame TF being a BSRP frame, the non-AP MLD (e.g., the other of Wi-Fi MLDs  102  and  104  shown in  FIG.  1   ) may respond with a response frame RF being a buffer status report (BSR) frame after receiving the BSRP frame. The operation mode change signaling design employed by the RTS/CTS mechanism may also be employed by the MU-RTS/CTS mechanism and the BSRP/BSR mechanism. Since a person skilled in the art can readily understand details of the dynamic operation mode switch examples shown in  FIGS.  10 - 14    after reading above paragraphs directed to  FIGS.  2 - 4  and  7 - 8   , similar description is omitted here for brevity. It should be noted that one Wi-Fi MLD of the present invention may support one or more of the dynamic operation mode switch scenarios mentioned above. 
     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.