Patent Publication Number: US-2022232414-A1

Title: Method and apparatus for radio link flow control

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to wireless communication technology, and more particularly to radio link flow control in a wireless communication system. 
     BACKGROUND 
     A wireless communication system may include a base station (hereinafter referred to as “BS”) which can be in communication with a user equipment (hereinafter referred to as “UE”). UE may include a mobile device (e.g., a cell phone, a tablet, a laptop, an internet-of-things (IoT) device, etc.). Quality of a communication link or channel between a BS and an UE may deteriorate due to various factors, for example but is not limited to, blockage by building(s), relatively long distance between the BS and the UE, etc. One of several solutions to resolve this problem may include deployment of relay nodes (hereinafter referred to as “RNs”) in the wireless communication system to enhance and/or expand coverage of the BS, as disclosed in the 3rd Generation Partnership Project (3GPP). 
     A BS, which communicates with an UE through one or more RNs, can be called as a donor BS. These RNs together with the donor BS may form a backhaul link allowing the UE to reach the donor BS through one or more RNs. Signals from the UE may also be simply transmitted from one RN directly to the donor BS. An Integrated Access and Backhaul (IAB) architecture, which may be evolved from RNs deployment in 3GPP, is being developed to support multi-hop relay in a New Radio (NR) communication network. 
     Nevertheless, an RN may experience congestion under certain conditions, and therefore a new solution is required to indicate congestion and control the congestion in the communication network. 
     SUMMARY OF THE DISCLOSURE 
     Some embodiments of the present disclosure provides a method, including: receiving, at a communication device, a configuration message including a threshold to configure the communication device; and transmitting, from the communication device, a congestion indication if an occupied buffer size of the communication device is equal to or greater than the threshold. 
     Another embodiment of the present disclosure provides a method, including: receiving, from a communication device, a congestion indication via a medium access control (MAC) unit or a message in adaptation layer. 
     Yet another embodiment of the present disclosure provides an apparatus. According to some embodiments of the present disclosure, the apparatus includes: at least one non-transitory computer-readable medium having computer executable instructions stored therein; at least one receiving circuitry; at least one transmitting circuitry; and at least one processor coupled to the at least one non-transitory computer-readable medium, the at least one receiving circuitry and the at least one transmitting circuitry, wherein the at least one non-transitory computer-readable medium and the computer executable instructions are configured to, with the at least one processor, cause the apparatus to perform a method according to some embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the advantages and features of the disclosure can be obtained, a description of the disclosure is rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. These drawings depict only exemplary embodiments of the disclosure and are not therefore to be considered limiting of its scope. 
         FIG. 1  illustrates a schematic wireless communication system according to some embodiments of the present disclosure; 
         FIG. 2  illustrates a schematic wireless communication system according to some embodiments of the present disclosure; 
         FIG. 3  illustrates a schematic wireless communication system according to some embodiments of the present disclosure; 
         FIG. 4  illustrates a schematic wireless communication system according to some embodiments of the present disclosure; 
         FIG. 5  illustrates a schematic wireless communication system according to some embodiments of the present disclosure; 
         FIG. 6  illustrates an exemplary method of congestion indication according to some embodiments of the present disclosure; 
         FIG. 7A  illustrates an exemplary MAC control element according to some embodiments of the present disclosure; 
         FIG. 7B  illustrates an exemplary MAC control element according to some embodiments of the present disclosure; 
         FIG. 7C  illustrates an exemplary MAC control element according to some embodiments of the present disclosure; 
         FIG. 7D  illustrates an exemplary MAC control element according to some embodiments of the present disclosure; and 
         FIG. 8  illustrates an example block diagram of an apparatus according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description of the appended drawings is intended as a description of preferred embodiments of the present disclosure and is not intended to represent the only form in which the present disclosure may be practiced. It should be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the present disclosure. 
       FIG. 1  illustrates a wireless communication system  100  according to some embodiments of the present disclosure. 
     Referring to  FIG. 1 , a wireless communication system  100  may include some nodes (e.g., BS  110  and RN  120 ) and some UEs (e.g., UE  130 A and UE  130 B). Although, for simplicity, merely two nodes are illustrated in  FIG. 1 , it is contemplated that wireless communication system  100  may also include more or fewer nodes in some other embodiments of the present disclosure. Although, for simplicity, merely two UEs are illustrated in  FIG. 1 , it is also contemplated that wireless communication system  100  may include more or fewer UEs in some other embodiments of the present disclosure. 
     The BS  110 , which communicates with a Core Network (CN)  150 , may operate or work under the control of a Mobility Management Entity (MME)  140 . The core network may include a Home Subscriber Server (HSS) (not illustrated in  FIG. 1 ), which communicatively coupled with the MME. The BS  110  may operate, for example based on a standard protocol such as Long-Term Evolution (LTE), LTE-Advanced (LTE-A), New Radio (NR), or other suitable protocol(s). For example, the BS  110  may include an eNB or a gNB, and may define one or more cells (e.g., cell  111 ). The RN  120  may include a relay node or an integrated access and backhaul node (IAB node). UE  130 A may include, for example, but is not limited to, a computing device, a wearable device, a mobile device, an IoT device, etc. UE  130 B may include a device that is the same or similar to UE  130 A. UE  130 B may also include a device different from UE  130 A. Persons skilled in the art should understand that as technology develops and advances, the terminologies described in the present disclosure may change, but should not affect or limit the principles and spirit of the present disclosure. 
     BS  110  (or Donor BS) may establish radio connections with UE  130 B and RN  120  through an Access Link (AL) and a Backhaul Link (BL) based on protocol Layer-1 (Physical Layer) to Layer-3 (Radio Resource Control (RRC) Layer), respectively. 
     In some embodiments of the present disclosure, RN  120  may establish a radio connection with UE  130 A through an RN-access link (AL 1 ) based on protocol Layer-1 to Layer-3. In some other embodiments of the present disclosure, the RN  120  may establish a radio connection with the UE  130 A through the AL 1  based on protocol Layer-1 to Layer-2. 
     Although  FIG. 1  merely shows that the Donor BS  110  is connected to a single UE for simplicity, it is contemplated that the Donor BS  110  may provide or establish connections with multiple UEs. Similarly, although  FIG. 1  merely shows that the RN  120  is connected to a single UE for simplicity, it is contemplated that the RN  120  may also provide or establish connections with multiple UEs. 
     Deployment of RN(s) helps to enhance and/or extend coverage of a BS by a backhaul link. Evolved Universal Terrestrial Radio Access Network (E-UTRAN) supports relaying by having an RN wirelessly connect to an eNB serving the RN, called Donor eNB (DeNB), via a modified version of the Evolved Universal Terrestrial Radio Access (E-UTRA) radio interface, i.e., the BL, also referred to as the Un interface. The radio interface that provides radio protocol connection between RN and the UE is referred to as the Uu interface. Relay function and use of RN/DeNB entities in a network is transparent to the operations of the connected UEs. 
     As mentioned above, 3GPP is envisioning an IAB architecture for 5G (NR) communication networks supporting multi-hop relays. In other words, an IAB node may hop through one or more IAB nodes before reaching the IAB Donor. A single hop may be considered a special instance of multiple hops. Multi-hop backhauling is relatively beneficial because it provides a relatively greater coverage extension compared to single-hop backhauling. In a relatively high frequency radio communication system (e.g., radio signals transmitted in frequency bands over 6 GHz), relatively narrow or less signal coverage may benefit from multi-hop backhauling techniques. Multi-hop backhauling further enables backhauling around obstacles (e.g., buildings in urban environments for in-clutter deployments). 
     The maximum number of hops in RN deployment may depend on various factors, for example but is not limited to, frequency, cell density, propagation environment, traffic load, or other factors. These factors are expected to change over time. Therefore, from the perspective of network architecture, it is desirable to ensure the flexibility in hop count. On the other hand, as the number of hops increases, scalability issues may arise. For example, performance may degrade or network load may increase to an unacceptable level. 
       FIG. 2  illustrates a wireless communication system  200  according to some embodiments of the present disclosure. 
     Referring to  FIG. 2 , the wireless communication system  200  can include a Donor node (e.g., IAB Donor  210 ), some IAB nodes (e.g., IAB node  220 A, IAB node  220 B, IAB node  220 C, and IAB node  220 D), and some UEs (e.g., UE  230 A and UE  230 B). Although merely one Donor node is illustrated in  FIG. 2  for simplicity, it is contemplated that wireless communication system  200  may include more Donor node(s) in some other embodiments of the present disclosure. Similarly, although merely four IAB nodes are illustrated in  FIG. 2  for simplicity, it is contemplated that wireless communication system  200  may include more or fewer IAB nodes in some other embodiments of the present disclosure. Although merely two UEs are illustrated in  FIG. 2  for simplicity, it is contemplated that wireless communication system  200  may include more or fewer UEs in some other embodiments of the present disclosure. 
     IAB node  220 A can be directly connected to IAB Donor  210 . IAB node  220 B can reach IAB Donor  210  by hopping through IAB node  220 A. IAB node  220 A is a parent IAB node of IAB node  220 B. In other words, IAB node  220 B may be a child IAB node of IAB node  220 A. 
     IAB node  220 C and IAB node  220 D can reach IAB Donor  210  by hopping through IAB node  220 B and IAB node  220 A. IAB node  220 A and IAB node  220 B may be upstream IAB nodes of IAB node  220 C and IAB node  220 D, and IAB node  220 B may be a parent IAB node of IAB node  220 C and IAB node  220 D. IAB node  220 B, IAB node  220 C, IAB node  220 D may be downstream IAB nodes of IAB node  220 A. 
     UE  230 A can be directly connected to IAB node  220 C, and UE  230 B can be directly connected to IAB node  220 B. In other words, UE  230 A and UE  230 B may be served by IAB node  220 C and IAB node  220 B, respectively. IAB node  220 C, IAB node  220 D, UE  230 A, and UE  230 B may be downstream nodes of IAB node  220 B. IAB node  220 C, IAB node  220 D, and UE  230 B may be child nodes of IAB node  220 B. The radio link between IAB node  220 B and IAB node  220 C, the radio link between IAB node  220 B and IAB node  220 D, and the radio link between IAB node  220 B and UE  230 B are referred to as downstream links of the IAB node  220 B. 
     Each of IAB node  220 A, IAB node  220 B, IAB node  220 C, and IAB node  220 D may be directly connected to one or more UE(s) in accordance with some other embodiments of the present disclosure. 
     Each of IAB node  220 A, IAB node  220 B, IAB node  220 C, and IAB node  220 D may be directly connected to one or more IAB node(s) in accordance with some other embodiments of the present disclosure. 
       FIG. 3  illustrates a wireless communication system  300  according to some embodiments of the present disclosure. 
     Referring to  FIG. 3 , the wireless communication system  300  may include IAB donor  310 , IAB node  320 A, IAB node  320 B, UE  330 A, UE  330 B, UE  330 C and a Next-Generation Core (NGC)  350 . 
     Each of the IAB node  320 A and IAB node  320 B may include a Distributed Unit (DU) and a Mobile Termination (MT). In the context of this disclosure, MT is referred to as a function resided in an IAB node that terminates the radio interface layers of the backhaul Uu interface toward an IAB donor or other IAB nodes. The IAB nodes may be connected to an upstream IAB node or a BS (e.g., an IAB donor) via the MT function. The IAB nodes may be connected to UEs and a downstream IAB node via the DU. 
     IAB node  320 A may be connected to an upstream IAB node  320 B via MT  322 A function. IAB node  320 A may be connected to UE  330 A via the DU  321 A. IAB node  320 B may be connected to an upstream IAB node or IAB donor  310  via MT  322 B function. IAB node  320 B may be connected to UE  330 B via DU  321 B. IAB node  320 B may be connected to downstream IAB node  320 A via DU  321 B. 
     Still referring to  FIG. 3 , the BS (e.g., IAB donor  310 ) may include at least one DU to support UEs and MTs of downstream IAB nodes. One DU of a BS can support at least one cell. One cell can be supported by only one DU of a BS or DU of an IAB node. 
     A Central Unit (CU)  311  included in the IAB donor  310  controls the DUs of all IAB nodes (e.g., IAB node  320 A and IAB node  320 B) and the DU resided in the IAB donor  310 . The DU(s) and the CU of an IAB donor may be co-located or may be located in different positions. The DU(s) and the CU of the IAB donor are connected via F 1  interface. In other words, the F 1  interface provides a means for interconnecting the CU and the DU(s) of an IAB donor. The F 1  Application Protocol (FLAP) supports the functions of the F 1  interface by certain F 1 AP signaling procedures. 
     In some embodiments of the present disclosure, each of the DU of the JAB donor  310 , IAB node  320 A and IAB node  320 B may host adaptation layer, Radio Link Control (RLC) layer, Medium Access Control (MAC) layer and Physical layer (PHY). The adaptation layer may be configured by the CU of a BS. The adaptation layer performs many functions including routing and bearer mapping (e.g., mapping of backhaul RLC channels), and may include a Transmit (TX) component and a Receive (RX) part. 
     Referring back to  FIG. 2 , the IAB nodes (e.g., IAB node  220 A, IAB node  220 B, IAB node  220 C, and IAB node  220 D) may include respective MTs and DUs (not illustrated in  FIG. 2 ), and the IAB Donor  210  may include at least one DU and a CU (not illustrated in  FIG. 2 ). 
     Still referring to  FIG. 2 , link capacity of the downlink between IAB node  220 A and IAB node  220 B may be relatively greater than the link capacity of the downlink between IAB node  220 B and each of its child nodes (e.g., IAB node  220 C, IAB node  220 D, and UE  230 B). In this scenario, the ingress data rate into IAB node  220 B scheduled by IAB node  220 A may be relatively higher than the egress data rate out of IAB node  220 B scheduled by IAB node  220 B to its child nodes. This may result in downlink data congestion at IAB node  220 B, and may cause packet retransmission, or even packet being discarded, under some circumstances. Various flow control techniques have been developed to address this issue. In data communications, flow control may be used to manage data transmission rate between two nodes to control or relieve congestion. 
     An end-to-end follow control technique can be used to relieve congestion. In such solution, if congestion happens at a communication device, the communication device may report its congestion status to the BS to which the communication device is connected. After receiving the congestion status report, the BS may alleviate the congestion at the communication device by, for example, allocating more resources to the communication device. 
     A hop-by-hop follow control technique can be used to relieve congestion. In such solution, if congestion happens at a communication device, the communication device may report the congestion status to its parent node (e.g., another upstream communication device or a BS) and its child nodes (e.g., another downstream communication device or an UE). After receiving the congestion status report, the parent node may try to resolve this issue by, for example, transmitting data to the communication device (which is undergoing congestion) at a relatively lower data rate, or transmitting the received congestion status report to the BS. After receiving the congestion status report, the child nodes of the communication device (which is undergoing congestion) may, for example, schedule a relatively faster data egress rate so as to alleviate or mitigate the congestion at the communication device (which is undergoing congestion). 
       FIG. 4  illustrates a wireless communication system  400  according to some embodiments of the present disclosure. 
     In  FIG. 4 , communication devices  420 A,  420 B,  420 C and  420 D are accessing BS  410 , and UE  430 A is connected to communication device  420 C, and UE  430 B is connected to communication device  420 B. For example, the communication device  420 A may function as IAB node  220 A shown in  FIG. 2 , the communication device  420 B may function as IAB node  220 B shown in  FIG. 2 , the communication device  420 C may function as IAB node  220 C shown in  FIG. 2 , the communication device  420 D may function as IAB node  220 D shown in  FIG. 2 , and the BS  410  may function as IAB donor  210  shown in  FIG. 2 . 
     An end-to-end follow control technique can be used to control congestion in the wireless communication system  400 . For example, as shown in  FIG. 4 , when congestion occurs at the communication device  420 B, the communication device  420 B may transmit a congestion indication to the BS  410 . In other words, the communication device  420 B may directly transmit a congestion indication to the BS  410  without relaying by the communication device  420 A. After receiving the congestion indication, the BS may alleviate the congestion at the communication device by, for example, allocating more resources to the communication device. 
     For example, as shown in  FIG. 4 , when congestion occurs at the communication device  420 B toward the downstream child node  420 C, the communication device  420 B may directly transmit a congestion indication to the BS  410 . Subsequent to the receipt of the congestion indication from the communication device  420 B, the BS  410  may handle the congestion problem. For example, more resource(s) may be allocated to the communication device  420 B. 
       FIG. 5  illustrates a wireless communication system  500  according to some embodiments of the present disclosure. In  FIG. 5 , communication devices  520 A,  520 B,  520 C and  520 D are accessing BS  510 , and UE  530 A is connected to communication device  520 C, and UE  530 B is connected to communication device  520 B. For example, the communication device  520 A may function as IAB node  220 A shown in  FIG. 2 , the communication device  520 B may function as IAB node  220 B shown in  FIG. 2 , the communication device  520 C may function as IAB node  220 C shown in  FIG. 2 , the communication device  520 D may function as IAB node  220 D shown in  FIG. 2 , and the BS  510  may function as IAB donor  210  shown in FIG.  2 . 
     A hop-by-hop follow control technique can be used to control congestion in the wireless communication system  500 . For example, as shown in  FIG. 5 , when congestion occurs at the communication device  520 B, the communication device  520 B may transmit a congestion indication to the parent node (e.g., communication device  520 A) of the communication device  520 B. The communication device  520 A may transmit the received congestion indication to the BS  510  (denoted by a dotted line). The communication device  520 A may not transmit the received congestion indication to the BS  510 . The communication device  520 B may transmit a congestion indication to the child node (e.g., communication device  520 C) of the communication device  520 B. 
     Embodiments of the present disclosure propose technical solutions for transmitting a congestion indication, which can facilitate flow control in the new generation communication systems, such as 5G communication systems. The proposed technical solutions can be applied to various flow control techniques, such as the end-to-end and hop-by-hop follow control techniques as described above. More details on the embodiments of the present disclosure will be illustrated in the following text in combination with the appended drawings. 
       FIG. 6  illustrates an exemplary method  600  of congestion indication according to some embodiments of the present disclosure. 
     At step  602 , a communication device, which may include the IAB node  220 B shown in  FIG. 2 , the communication device  420 B shown in  FIG. 4 , or the communication device  520 B shown in  FIG. 5 , can receive a configuration message including a threshold for triggering a congestion indication report from a BS, which may include the IAB donor  210  shown in  FIG. 2 , the BS  410  shown in  FIG. 4 , or the BS  510  shown in  FIG. 5 . 
     In some embodiments of the present disclosure, the threshold may be configured per downstream link of the communication device. 
     For example, referring back to  FIG. 2 , the IAB node  220 B may have three downstream links, e.g., the radio link between IAB node  220 B and IAB node  220 C, the radio link between IAB node  220 B and IAB node  220 D, and the radio link between IAB node  220 B and UE  230 B. The IAB donor  210  may configure a respective threshold for each of the three downstream links. One threshold configured for one downstream link may be different from another threshold configured for another downstream link. One threshold configured for one downstream link may be the same to another threshold configured for another downstream link. 
     In some embodiments of the present disclosure, the threshold may be configured to be applied to all downstream links of the communication device. For example, referring to  FIG. 2 , the IAB donor  210  may configure one uniform or identical threshold for all of the three downstream links of the IAB node  220 B. 
     Referring to  FIG. 6 , at step  604 , the communication device may determine whether the occupied buffer size of the communication device is equal to or greater than the threshold. If it is determined that the occupied buffer size of the communication device is equal to or greater than the threshold, the communication device may transmit a congestion indication at step  606  to the BS, to a parent node of the communication device, to a child node of the communication device, to both the BS and the parent node of the communication device, to both the parent node of the communication device and the child node of the communication device, to both the BS and the child node of the communication device, or to all the BS, the parent node of the communication device and the child node of the communication device, depending on which flow control technique is employed. 
     For example, referring back to  FIG. 4 , the communication device  420 B may determine that the occupied buffer size of the communication device  420 B is equal to or greater than the threshold configured by the BS  410 . Then, the communication device  420 B may transmit a congestion indication to the BS  410 . In some embodiments of the present disclosure, the congestion indication may be transmitted to the BS via RRC signaling or F 1 AP signaling. 
     For example, referring to  FIG. 5 , the communication device  520 B may determine that the occupied buffer size of the communication device  520 B is equal to or greater than the threshold configured by the BS  510 . Then, the communication device  520 B may transmit a congestion indication to the parent node (e.g., communication device  520 A) of the communication device  520 B. The communication device  520 B may transmit a congestion indication to the child node (e.g., communication device  520 C) of the communication device  520 B which is experiencing the congestion. 
     In some embodiments of the present disclosure, the congestion indication may be transmitted to the parent node, the child node, or both the parent node and the child node, via a MAC unit or a message in adaptation layer. 
     In some embodiments of the present disclosure, the congestion indication may be included in a MAC Control Element (CE) of a MAC Protocol Data Unit (PDU). In these embodiments, information or data may be included in the MAC header of the MAC PDU to indicate the MAC CE including the congestion indication. For example, in the MAC header of a MAC PDU, there may be one corresponding field (e.g., Logical Channel ID (LCID) field) indicating the type of each MAC CE. A dedicated LCID value may be assigned to indicate the MAC CE including the congestion indication. 
     In some embodiments of the present disclosure, in the case that the congestion indication is transmitted to the child node via the MAC PDU, the header of the MAC PDU may include the dedicated LCID. In some embodiments of the present disclosure, the MAC CE corresponding to the dedicated LCID may be empty. The child node would know which MAC CE is for congestion indication based on the dedicated LCID. 
     The formats of the MAC CE transmitted to the parent node will be described in detail in the following text with respect to  FIGS. 7A-7D . 
     Referring back to  FIG. 6 , in some embodiments of the present disclosure, the congestion indication may include the identities of the child nodes of the communication device which are experiencing congestion. The identity of a node may be a cell identity of the node (e.g., a physical cell identity (PCI)), an identity of a DU of the node, a Cell Radio Network Temporary Identifier (C-RNTI), or any other ID(s) that can be used to identify the node. 
     For example, referring back to  FIG. 2 , the IAB node  220 B may include a corresponding buffer for each downstream link. If it is determined that the occupied amount of the buffer for the radio link between IAB node  220 B and IAB node  220 C is equal to or greater than the threshold configured by the IAB donor  210 , which may be determined that IAB node  220 C is experiencing congestion, the congestion indication to be transmitted by the IAB node  220 B may include an identity of IAB node  220 C. 
     Similarly, if it is determined that the occupied amount of the buffer for the radio link between IAB node  220 B and UE  230 B is equal to or greater than the threshold configured by the IAB donor  210 , which may be determined that UE  230 B is experiencing congestion, the congestion indication to be transmitted by the IAB node  220 B may include an identity of UE  230 B. 
     Referring to  FIG. 6 , in some embodiments of the present disclosure, the congestion indication may include the occupied buffer size of the communication device. In some embodiments, the occupied buffer size of the communication device may be the number of bits, bytes, or the like occupied by data in an RLC transmission buffer of the communication device. In some embodiments, the RLC transmission buffer may refer to the RLC transmission buffer in the DU of the communication device. 
     For example, referring back to  FIG. 3 , the occupied buffer size of the IAB node  320 A may include the number of bytes occupied by data in an RLC transmission buffer in the DU  321 A of the IAB node  320 A. 
     Referring to  FIG. 6 , in some embodiments of the present disclosure, prior to transmitting the congestion indication at step  606 , the communication device may transmit a message including a total buffer size of the communication device. In some embodiments, the total buffer size of the communication device is transmitted during a setup procedure of the communication device. In some embodiments, the total buffer size of the communication device is included in the capability Information Element (IE) of the communication device. 
     For example, referring back to  FIG. 4 , the communication device  420 B may transmit the total buffer size of the communication device  420 B to the BS  410 . 
     For example, referring to  FIG. 5 , the communication device  520 B may transmit the total buffer size of the communication device  520 B to its parent node (e.g., communication device  520 A). 
     After receiving the congestion indication from the communication device (e.g., the communication device  420 B in  FIG. 4  or the communication device  520 B in  FIG. 5 ), the BS (e.g., the BS  410  in  FIG. 4 ) or the parent node (e.g., the communication device  520 A in  FIG. 5 ) of the communication device may determine the remaining buffer size of the communication device by subtracting the occupied buffer size of the communication device from the total buffer size of the communication device. 
     It is beneficial to know the remaining buffer size since it may indicate whether the buffer at the communication device will overflow or not, and the BS may determine whether the Quality of Service (QoS) requirement of a certain flow will be met or not. 
     Referring to  FIG. 6 , in some embodiments of the present disclosure, the communication device may determine the remaining buffer size by subtracting the occupied buffer size of the communication device from the total buffer size of the communication device. In these embodiments, the congestion indication may include the remaining buffer size of the communication device. In these embodiments, total buffer size of the communication device may not be required to be transmitted beforehand. 
     In some embodiments of the present disclosure, the total buffer size of the communication device may be the size of the RLC transmission buffer of the communication device. In some embodiments, the RLC transmission buffer may refer to the RLC transmission buffer in the DU of the communication device. 
     In some embodiments of the present disclosure, the total buffer size of the communication device may be a sum of the size of the RLC transmission buffer of the communication device and the size of an RLC reception buffer of the communication device. In some embodiments, the RLC transmission buffer may refer to the RLC transmission buffer in the DU of the communication device, and the RLC reception buffer may refer to the RLC reception buffer in an MT of the communication device. 
     Still referring to  FIG. 6 , the communication device may determine, at step  608 , whether the content of the congestion indication is changed or not. If it is determined that the content of the congestion indication is changed, the communication device may transmit, at step  612 , the congestion indication to the BS, to a parent node of the communication device, to a child node of the communication device, to both the BS and the parent node of the communication device, to both the parent node of the communication device and the child node of the communication device, to both the BS and the child node of the communication device, or to all the BS, the parent node of the communication device and the child node of the communication device, depending on which flow control technique is employed. 
     In some embodiments of the present disclosure, the change of the content of the congestion indication may include a change of the congestion state of a child node of the communication device. 
     For example, referring back to  FIG. 2 , the IAB node  220 B may transmit a relatively earlier congestion indication including the identity of the IAB node  220 C if it determines that the occupied amount of the buffer for the radio link between the IAB node  220 B and the IAB node  220 C is equal to or greater than a corresponding threshold configured by the IAB donor  210 . 
     After a period of time, the congestion on the radio link between the IAB node  220 B and the IAB node  220 C may be solved. That is, the congestion state of the IAB node  220 C may be changed from congestion to non-congestion. The IAB node  220 B may then transmit an updated congestion indication. 
     In another example, the relatively earlier congestion indication may not include the identity of the IAB node  220 C. However, after a period of time, the congestion state of the IAB node  220 C may be changed from non-congestion to congestion. For example, the occupied amount of the buffer for the radio link between the IAB node  220 B and the IAB node  220 C may now be equal to or greater than the corresponding threshold configured by the IAB donor  210 . The IAB node  220 B may then transmit an updated congestion indication. 
     Referring to  FIG. 4 , the communication device  420 B may transmit the updated congestion indication to the BS  410 . 
     Referring to  FIG. 5 , the communication device  520 B may transmit the updated congestion indication to its parent node (e.g., communication device  520 A) and/or relevant child nodes (e.g., communication device  520 C). 
     Referring to  FIG. 6 , in some embodiments of the present disclosure, in response to transmitting the congestion indication at step  606 , the communication device may start a timer (T 1 ). Upon the expiry of the Timer T 1 , the communication device may determine whether the congestion at the communication device is solved; and if not, the communication device may transmit a congestion indication to the BS, to a parent node of the communication device, to a child node of the communication device, to both the BS and the parent node of the communication device, to both the parent node of the communication device and the child node of the communication device, to both the BS and the child node of the communication device, or to all the BS, the parent node of the communication device and the child node of the communication device, depending on which flow control technique is employed. For example, the communication device may determine that the congestion at the communication device is not solved if the occupied buffer size of the communication device is equal to or greater than a threshold configured by the BS. 
       FIGS. 7A-7D  illustrate exemplary formats of MAC CE according to embodiments of the present disclosure. 
     As described above, in some embodiments of the present disclosure, the congestion indication may include an identity of a child node of a communication device which is experiencing congestion. In some embodiments of the present disclosure, the congestion indication may include buffer size information of the communication device associated with the child node (e.g., the occupied buffer size, the remaining buffer size, or both). The MAC CE may be used to carry the congestion indication. 
     Although, the node identity shown in  FIGS. 7A-7D  includes 8 bits for simplicity, it is contemplated that the node identity may include more or less bits in some other embodiments of the present disclosure. Although, for simplicity, the buffer size information in  FIGS. 7C and 7D  includes 4 bits, it is contemplated that the buffer size information may include more or fewer bits in some other embodiments of the present disclosure. In some other embodiments of the present disclosure, the exemplary formats of MAC CE shown in  FIGS. 7A-7D  can be varied or changed of interest. 
       FIG. 7A  illustrates an exemplary MAC CE  700 A according to some embodiments of the present disclosure. The MAC CE  700 A may be used to indicate an identity of only one child node of a communication device which is experiencing congestion. Specifically, as shown in  FIG. 7A , the MAC CE  700 A is octet aligned and may include field  702 A. Thus, the 8-bit field  702 A is used to indicate the node identity. 
     For example, referring back to  FIG. 5 , the communication device  520 B may transmit to its parent node (e.g., communication device  520 A) a MAC CE including the identity of one of its child nodes which is experiencing congestion (e.g., communication device  520 C). In the case that there is more than one child node of the communication device  520 B which is experiencing congestion, for example, in addition to communication device  520 C, UE  530 B is also experiencing congestion, the communication device  520 B may transmit an additional MAC CE including the identity of UE  530 B to the communication device  520 A in a different MAC PDU. 
       FIG. 7B  illustrates an exemplary MAC CE  700 B according to some embodiments of the present disclosure. The MAC CE  700 B may be used to indicate an identity of each of the child nodes of a communication device which are experiencing congestion. 
     Specifically, as shown in  FIG. 7B , the MAC CE  700 B is octet aligned and includes 4 bytes, the first, second, third, and fourth bytes being respectively referred to as “Oct 1”, “Oct 2”, “Oct 3”, “Oct 4” in the figure. 
     The MAC CE  700 B may include field  704 B, field  702 B- 1 , field  702 B- 2 , field  702 B- 3 , and field  706 B. Field  704 B may include 4 bits, each of the field  702 B- 1 , field  702 B- 2 , and field  702 B- 3  may include 8 bits, and field  706 B may include 4 bits. Thus, the field  702 B- 1  occupies 4 bits of byte “Oct 1” and 4 bits of byte “Oct 2”; the field  702 B- 2  occupies 4 bits of byte “Oct 2” and 4 bits of byte “Oct 3”; the field  702 B- 3  occupies 4 bits of byte “Oct 3” and 4 bits of byte “Oct 4”. 
     Field  702 B- 1 , field  702 B- 2 , and field  702 B- 3  may be used to indicate respective identities of the child nodes which are experiencing congestion, field  704 B may be used to indicate the number of node identities included in the MAC CE  700 B, field  706 B may be reserved for future use and may be set to a value of “0”. The value of the number of node identities included in the MAC CE  700 B may be equal to or greater than “0”. In the case of  FIG. 7B , the MAC CE  700 B includes three node identities, and field  704 B may thus be set to “0011” (equal to the value of “3” in decimal format). 
     In some embodiments of the present disclosure, field  704 B may be set to “0000” (equal to the value of “0” in decimal format). This suggests that all downstream links of the communication device are not in congestion. In these embodiments, the MAC CE  700 B may not include any node identities. That is, the MAC CE  700 B may not include field  702 B- 1 , field  702 B- 2 , and field  702 B- 3 . 
     Although  FIG. 7B  shows that the field, i.e., field  704 B, used to indicate the number of node identities includes 4 bits for simplicity, it is contemplated that this field may include more or fewer bits in some other embodiments of the present disclosure. Although  FIG. 7B  shows that MAC CE  700 B includes three node identities for simplicity, it is contemplated that MAC CE  700 B may include more or fewer node identities in some other embodiments of the present disclosure. Although  FIG. 7B  shows that MAC CE  700 B starts with field  704 B followed by field  702 B- 1 , field  702 B- 2 , field  702 B- 3 , and field  706 B for explanation, MAC CE  700 B may include other formats in some other embodiments of the subject application. 
     For example, referring back to  FIG. 5 , assuming that in addition to the communication device  520 C, UE  530 B and the communication device  520 D are also experiencing congestion (not shown), the communication device  520 B may transmit to its parent node (e.g., communication device  520 A) a MAC CE including an identity of each of these three child nodes which are experiencing congestion (i.e., the communication device  520 C, the communication device  520 D, and UE  530 B). 
       FIG. 7C  illustrates an exemplary MAC CE  700 C according to some embodiments of the present disclosure. Similarly to  FIG. 7A , the MAC CE  700 C may be used to indicate an identity of only one child node of a communication device which is experiencing congestion. However, the MAC CE  700 C may further include the buffer size information of the communication device associated with the child node identified therein. 
     Specifically, as shown in  FIG. 7C , the MAC CE  700 C is octet aligned and includes 2 bytes, the first and second bytes being respectively referred to as “Oct 1” and “Oct 2” in the figure. 
     The MAC CE  700 C may include field  702 C, field  708 C, and field  706 C. Field  702 C may include 8 bits, field  708 C may include 4 bits, and field  706 C may include 4 bits. Thus, the field  702  occupies all 8 bits of byte “Oct 1”; the field  708 C occupies 4 bits of byte “Oct 2”; and field  706 C occupies 4 bits of byte “Oct 2”. 
     Field  702 C may be used to indicate the node identity, field  708 C may be used to indicate the buffer size information, and field  706 C may be reserved for future use and may be set to a value of “0”. 
     The buffer size information may include an occupied buffer size of the communication device associated with the child node, a remaining buffer size of the communication device associated with the child node, or both. 
     In the case that the buffer size information includes both the occupied buffer size and the remaining buffer size, the field  708 C may include two sub-fields (not shown). Each of the two sub-fields may include 2 bits, and may be used to indicate a respective one of the occupied buffer size and the remaining buffer size. Although, for simplicity, each of the two sub-fields of field  708 C includes 2 bits, it is contemplated that the sub-fields may include more or fewer bits in some other embodiments of the present disclosure. 
     Although  FIG. 7C  shows that MAC CE  700 C starts with field  702 C followed by field  708 C and field  706 C, the MAC CE  700 C may include other format(s) in accordance with some other embodiments of the subject application. 
     For example, referring to  FIG. 5 , the communication device  520 B may transmit to its parent node (e.g., communication device  520 A) a MAC CE including the identity of one of its child nodes which is experiencing congestion (e.g., communication device  520 C). The MAC CE also includes the buffer size information of the communication device  520 B associated with the communication device  520 C. The buffer size information may include an occupied buffer size of the communication device  520 B associated with the communication device  520 C, the remaining buffer size of the communication device  520 B associated with the communication device  520 C, or both. 
     In the case that there is more than one child node of the communication device  520 B which is experiencing congestion, for example, in addition to communication device  520 C, UE  530 B is also experiencing congestion, the communication device  520 B may transmit an additional MAC CE including the identity of UE  530 B to the communication device  520 A and the corresponding buffer size information in a different MAC PDU. 
       FIG. 7D  illustrates an exemplary MAC CE  700 D according to some embodiments of the present disclosure. Similarly to  FIG. 7B , the MAC CE  700 D may be used to indicate an identity of each of the child nodes of a communication device which are experiencing congestion. However, the MAC CE  700 D may further include respective buffer size information associated with each of the child nodes. 
     Specifically, as shown in  FIG. 7D , the MAC CE  700 D is octet aligned and includes 4 bytes, the first, second, third, and fourth bytes being respectively referred to as “Oct 1”, “Oct 2”, “Oct 3”, “Oct 4” in the figure. 
     The MAC CE  700 D may include field  704 D, field  702 D- 1 , field  708 D- 1 , field  702 D- 2 , field  708 D- 2 , and field  706 D. Field  704 D may include 4 bits, each of the field  702 D- 1  and field  702 D- 2  may include 8 bits, each of the field  708 D- 1  and field  708 D- 2  may include 4 bits, and field  706 D may include 4 bits. Thus, the field  704 D occupies 4 bits of byte “Oct 1”; the field  702 D- 1  occupies 4 bits of byte “Oct 1” and 4 bits of byte “Oct 2”; the field  708 D- 1  occupies 4 bits of byte “Oct 2”; the field  702 D- 2  occupies all 8 bits of byte “Oct 3”; the field  708 D- 2  occupies 4 bits of byte “Oct 4”; and the field  706 D occupies 4 bits of byte “Oct 4”. 
     Field  702 D- 1  and field  702 D- 2  may be used to indicate respective identities of the child nodes which are experiencing congestion, field  708 D- 1  and field  708 D- 2  may be used to indicate respective buffer size information associated with the child nodes, field  704 D may be used to indicate the number of node identities included in the MAC CE  700 D, and field  706 D may be reserved for future use and may be set to a value of “0”. In the case of  FIG. 7D , the MAC CE  700 D includes two node identities, and field  704 D may thus be set to “0010” (equal to the value of “2” in decimal format). 
     Similar to field  704 B in  FIG. 7B , field  704 D may be set to “0000” (equal to the value of “0” in decimal format). This suggests that all downstream links of the communication device are not in congestion. In these embodiments, the MAC CE  700 D may not include any node identities. That is, the MAC CE  700 D may not include field  702 D- 1  and field  702 D- 2 . 
     The buffer size information indicated in each of the field  708 D- 1  and field  708 D- 2  may include respective occupied buffer size, respective remaining buffer size, or both. In the case that the buffer size information includes both the occupied buffer size and the remaining buffer size, similar to field  708 C in  FIG. 7C , each of the field  708 D- 1  and field  708 D- 2  may include two sub-fields (not shown), which may be used to indicate a respective one of the occupied buffer size and the remaining buffer size. 
     Although  FIG. 7D  shows that the field, i.e., field  704 D, used to indicate the number of node identities includes 4 bits for simplicity, it is contemplated that this field may include more or fewer bits in some other embodiments of the present disclosure. Although  FIG. 7D  shows that MAC CE  700 D includes two node identities for simplicity, it is contemplated that MAC CE  700 D may include more or fewer node identities in some other embodiments of the present disclosure. Although  FIG. 7D  shows that MAC CE  700 D starts with field  704 D followed by field  702 D- 1 , field  708 D- 1 , field  702 D- 2 , field  708 D- 2 , and field  706 D, the MAC CE  700 D may include other format(s) in accordance with some other embodiments of the subject application. 
     For example, referring back to  FIG. 5 , assuming that in addition to the communication device  520 C, UE  530 B is also experiencing congestion (not shown), the communication device  520 B may transmit to its parent node (e.g., communication device  520 A) a MAC CE including an identity of each of these two child nodes which are experiencing congestion (i.e., the communication device  520 C and UE  530 B). For example, in the MAC CE, field  704 D may be set to “0010”, field  702 D- 1  may include the identity of the communication device  520 C, field  708 D- 1  may include the buffer size information of the communication device  520 B associated with the communication device  520 C, field  702 D- 2  may include the identity of the UE  530 B, field  708 D- 2  may include the buffer size information of the communication device  520 B associated with the UE  530 B, and field  706 D may be set to “0000”. 
       FIG. 8  illustrates an example block diagram of an apparatus  800  according to some embodiments of the present disclosure. 
     As shown in  FIG. 8 , the apparatus  800  may include at least one non-transitory computer-readable medium (not illustrated in  FIG. 8 ), a receiving circuitry  802 , a transmitting circuitry  804 , and a processor  806  coupled to the non-transitory computer-readable medium (not illustrated in  FIG. 8 ), the receiving circuitry  802  and the transmitting circuitry  804 . The apparatus  800  may be a BS, a communication device (e.g., an IAB node), or an UE. 
     Although in this figure, elements such as processor  806 , transmitting circuitry  804 , and receiving circuitry  802  are described in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. In some embodiments of the present disclosure, the receiving circuitry  802  and the transmitting circuitry  804  are combined into a single device, such as a transceiver. In certain embodiments of the present disclosure, the apparatus  800  may further include an input device, a memory, and/or other components. 
     In some embodiments of the present disclosure, the non-transitory computer-readable medium may have stored thereon computer-executable instructions to cause a processor to implement the method with respect to the communication device as described above. For example, the computer-executable instructions, when executed, cause the processor  806  interacting with receiving circuitry  802  and transmitting circuitry  804 , so as to perform the steps with respect to the IAB nodes depicted in  FIGS. 2 and 3  and the communication devices depicted in  FIGS. 4-6 . 
     In some embodiments of the present disclosure, the non-transitory computer-readable medium may have stored thereon computer-executable instructions to cause a processor to implement the method with respect to the BS as described above. For example, the computer-executable instructions, when executed, cause the processor  806  interacting with receiving circuitry  802  and transmitting circuitry  804 , so as to perform the steps with respect to the IAB donors depicted in  FIGS. 2 and 3  and the BSs depicted in  FIGS. 4-6 . 
     Those having ordinary skill in the art would understand that the steps of a method described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. Additionally, in some aspects, the steps of a method may reside as one or any combination or set of codes and/or instructions on a non-transitory computer-readable medium, which may be incorporated into a computer program product. 
     While this disclosure has been described with specific embodiments thereof, it is evident that many alternatives, modifications, and variations may be apparent to those skilled in the art. For example, various components of the embodiments may be interchanged, added, or substituted in the other embodiments. Also, all of the elements of each figure are not necessary for the operation of the disclosed embodiments. For example, one of ordinary skill in the art of the disclosed embodiments would be enabled to make and use the teachings of the disclosure by simply employing the elements of the independent claims. Accordingly, embodiments of the disclosure as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the disclosure. 
     In this document, the terms “includes”, “including”, or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a”, “an”, or the like does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that includes the element. Also, the term “another” is defined as at least a second or more. The term “having” and the like, as used herein, are defined as “including”.