Patent Publication Number: US-2023156732-A1

Title: Transmission apparatus and transmission method of resource assignment information

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally pertains to wireless communications and, more particularly, to a method for formatting and transmitting resource assignment information in a wireless communications system. 
     2. Description of the Related Art 
     The IEEE (Institute of Electrical and Electronics Engineers) 802.11 Working Group is developing 802.11ax HE (High Efficiency) WLAN (Wireless Local Area Network) air interface in order to achieve a very substantial increase in the real-world throughput achieved by users in high density scenarios. OFDMA (Orthogonal Frequency Division Multiple Access) multiuser transmission has been envisioned as one of the most important features in 802.11ax. 
     OFDM (Orthogonal Frequency Division Multiplexing) is a multiplexing technique that subdivides a system bandwidth into a plurality of orthogonal frequency subcarriers. In OFDM system, an input data stream is divided into several parallel substreams with a lower data rate (accordingly, increased symbol duration), and the substreams are modulated with respective orthogonal subcarriers and are transmitted. The increased symbol duration improves the robustness of OFDM system with respect to the channel delay spread. Further, introduction of a CP (Cyclic Prefix) is able to completely remove intersymbol interference so far as the CP duration is longer than the channel delay spread. Further, OFDM modulation may be realized by an efficient IFFT (Inverse Fast Fourier Transform) that makes a plurality of subcarriers usable with little complexity. In OFDM system, time and frequency resources are defined by OFDM symbols in a time domain and subcarriers in a frequency domain. OFDMA is a multiple access scheme that performs multiple operations of data streams to and from the plurality of users over the time and frequency resources of the OFDM system. 
     Studies are underway to perform frequency scheduling for OFDMA multiuser transmission in 802.11ax. According to frequency scheduling, a radio communication access point apparatus (hereinafter simply “access point”) adaptively assigns subcarriers to a plurality of radio communication station apparatuses (i.e., terminal apparatus, herein-after simply “stations”) based on reception qualities of frequency bands of the stations (also called as “STAs”). This makes it possible to obtain a maximum multiuser diversity effect and perform communication quite efficiently. 
     Frequency scheduling is generally performed based on a Resource Unit (RU). A RU comprises a plurality of consecutive subcarriers. The RUs are assigned by an access point (AP) to each of a plurality of STAs with which the AP communicates. The resource assignment result of frequency scheduling performed by the AP shall be reported to the STAs as resource assignment information. However, unlike other OFDMA based mobile communication standards such as LTE (Long Term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access), 802.11ax is packet oriented and does not support control channels for transmitting resource assignment information. IEEE Std 802.11ac-2013 is an example of related art. 
     SUMMARY 
     As flexibility in frequency scheduling increases, more signaling bits are needed to report the resource assignment information to STAs. This results in an increase of the overhead for reporting resource assignment information. So there is a relationship of trade-off between flexibility in frequency scheduling and overhead for reporting resource assignment information. A challenge is how to achieve flexible frequency scheduling while reducing an increase of the overhead for reporting resource assignment information. 
     In one general aspect, the techniques disclosed here feature: a transmission apparatus of the present disclosure comprising a signal generator which, in operation, generates a transmission signal that includes a legacy preamble, a non-legacy preamble and a data field, wherein the non-legacy preamble comprises a first signal field and a second signal field, the second signal field including a resource assignment subfield that indicates a plurality of resource unit (RU) assignments in a frequency domain and a plurality of user-specific subfields, each carrying per-user allocation information, and wherein a single RU is allocated to each of the plurality of RU assignments and a start tone index of second one of the plurality of RU assignments is larger than an end tone index of its preceding assignment; and a transmitter which, in operation, transmits the generated transmission signal. 
     With the transmission apparatus and transmission method of resource assignment information of the present disclosure, it is possible to achieve flexible frequency scheduling while suppressing an increase of the overhead for reporting resource assignment information. 
     It should be noted that general or specific disclosures may be implemented as a system, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a diagram illustrating an example format of PPDU according to the prior art; 
         FIG.  2    shows a diagram illustrating an example OFDMA structure of the data field in case of CBW=20 MHz according to the prior art; 
         FIG.  3    shows a diagram illustrating an example OFDMA structure of the data field in case of CBW=40 MHz according to the prior art; 
         FIG.  4    shows a diagram illustrating an example OFDMA structure of the data field in case of CBW=80 MHz according to the prior art; 
         FIG.  5    shows a diagram illustrating an example of continuous resource allocation in the data field according to the prior art; 
         FIG.  6    shows a diagram illustrating an example of non-continuous resource allocation in the data field according to the prior art; 
         FIG.  7    shows a diagram illustrating an example of resource assignment according to a first embodiment of the present disclosure; 
         FIG.  8 A  shows a diagram illustrating a first example of resource assignment indication according to the first embodiment of the present disclosure; 
         FIG.  8 B  shows a diagram illustrating a second example of resource assignment indication according to the first embodiment of the present disclosure; 
         FIG.  8 C  shows a diagram illustrating a third example of resource assignment indication according to the first embodiment of the present disclosure; 
         FIG.  9    shows a diagram illustrating an example of resource assignment according to a second embodiment of the present disclosure; 
         FIG.  10 A  shows a diagram illustrating a first example of resource assignment indication according to the second embodiment of the present disclosure; 
         FIG.  10 B  shows a diagram illustrating a second example of resource assignment indication according to the second embodiment of the present disclosure; 
         FIG.  10 C  shows a diagram illustrating a third example of resource assignment indication according to the second embodiment of the present disclosure; 
         FIG.  11    shows a diagram illustrating an example of resource assignment according to a third embodiment of the present disclosure; 
         FIG.  12 A  shows a diagram illustrating a first example of resource assignment indication according to the third embodiment of the present disclosure; 
         FIG.  12 B  shows a diagram illustrating a second example of resource assignment indication according to the third embodiment of the present disclosure; 
         FIG.  13    shows a diagram illustrating a signaling of the RU type and position information according to the third embodiment of the present disclosure; 
         FIG.  14    shows a diagram illustrating example information content of HE-SIG-A and HE-SIG-B according to the present disclosure; 
         FIG.  15    shows a diagram illustrating an example sequence of executing OFDMA transmission according to the present disclosure; 
         FIG.  16    shows a diagram illustrating an example format of Assignment Set ID Management frame according to the present disclosure; 
         FIG.  17    shows a block diagram illustrating an example configuration of AP according to the present disclosure; 
         FIG.  18    shows a block diagram illustrating an example configuration of STA according to the present disclosure; 
         FIG.  19    shows a diagram illustrating an example of resource assignment according to a fourth embodiment of the present disclosure; 
         FIG.  20 A  shows a diagram illustrating a first example of resource assignment indication according to the fourth embodiment of the present disclosure; 
         FIG.  20 B  shows a diagram illustrating a second example of resource assignment indication according to the fourth embodiment of the present disclosure; 
         FIG.  21    shows a diagram illustrating another example of information content of HE-SIG-A and HE-SIG-B according to the present disclosure; 
         FIG.  22    shows a diagram illustrating an example structure of HE-SIG-B according to the present disclosure; 
         FIG.  23    shows a flow chart illustrating a method for distributing resource assignment information into HE-SIG-B field according to the present disclosure; 
         FIG.  24    shows a diagram illustrating a first example format of the HE-SIG-B1 or the HE-SIG-B2 in case of CBW=80 MHz; 
         FIG.  25    shows a diagram illustrating a second example format of the HE-SIG-B1 or the HE-SIG-B2 in case of CBW=80 MHz; and 
         FIG.  26    shows a diagram illustrating a third example format of the HE-SIG-B1 or the HE-SIG-B2 in case of CBW=80 MHz. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present disclosure will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations has been omitted for clarity and conciseness. 
     Underlying Knowledge Forming Basis of the Present Disclosure 
       FIG.  1    illustrates an example format of PPDU (Physical layer Protocol Data Unit)  100  according to the prior art (see IEEE802.11-15/0132r5, Specification Framework for TGax, May 2015 and IEEE802.11-15/0621r2, Design Principles for HE Preamble, May 2015). The PPDU  100  comprises a legacy preamble  110 , a non-legacy preamble (i.e., High Efficiency (HE) preamble)  120  and a data field  130 . 
     The data field  130  carries the payload for one or more STAs. For a specific STA in terms of single user transmission or a specific group of STAs in terms of multiuser MIMO transmission, the payload is carried on a designated resource in units of Resource Unit (RU) spanning a plurality of OFDM symbols. A RU may have different types depending on the number of constituent subcarriers per RU. OFDM symbols in the data field  130  shall use a DFT period of 12.8 μs and subcarrier spacing of 78.125 kHz. The number of subcarriers per OFDM symbol depends on a size of channel bandwidth (CBW). For example, in case of CBW=80 MHz, the number of subcarriers per OFDM symbol is 1024. Therefore for a specific type of RU, the maximum number of RUs per OFDM symbol depends on a size of CBW as well. 
       FIG.  2    illustrates an example OFDMA structure of the data field  130  in case of CBW=20 MHz according to the prior art (see IEEE802.11-15/0132r5, Specification Framework for TGax, May 2015 and IEEE802.11-15/0330r5, OFDMA Numerology and Structure, May 2015). The 20 MHz OFDMA supports four types of RUs. The Type I RU comprises 26 consecutive tones and has a bandwidth of about 2 MHz. The Type II RU comprises 52 consecutive tones and has a bandwidth of about 4.1 MHz. The Type III RU comprises 106 consecutive tones and has a bandwidth of about 8.3 MHz. The Type IV RU comprises 242 consecutive tones and has a bandwidth of about 18.9 MHz. The maximum number of Type I RUs, Type II RUs, Type III RUs and Type IV RUs which the 20 MHz OFDMA is able to support is nine, four, two and one, respectively. A mix of different types of RUs can be accommodated in the 20 MHz OFDMA. For example, the 20 MHz OFDMA may be divided into one Type III RU  202 , three Type I RUs  204 ,  208  and  210  as well as one Type II RU  206 . 
       FIG.  3    illustrates an example OFDMA structure of the data field  130  in case of CBW=40 MHz according to the prior art (see IEEE802.11-15/0132r5, Specification Framework for TGax, May 2015 and IEEE802.11-15/0330r5, OFDMA Numerology and Structure, May 2015). In addition to Type I RU, Type II RU, Type III RU and Type IV RU, the 40 MHz OFDMA also supports Type V RU, which comprises 484 consecutive tones and has a bandwidth of about 37.8 MHz. The maximum number of Type I RUs, Type II RUs, Type III RUs, Type IV RUs and Type V RUs which the 40 MHz OFDMA is able to support is eighteen, eight, four, two and one, respectively. Similar to the 20 MHz OFDMA, a mix of different types of RUs can also be accommodated in the 40 MHz OFDMA. 
       FIG.  4    illustrates an example OFDMA structure of the data field  130  in case of CBW=80 MHz according to the prior art (see IEEE802.11-15/0132r5, Specification Framework for TGax, May 2015 and IEEE802.11-15/0330r5, OFDMA Numerology and Structure, May 2015). In addition to Type I RU, Type II RU, Type III RU, Type IV RU and Type V RU, the 80 MHz OFDMA also supports Type VI RU, which comprises 996 consecutive tones and has a bandwidth of about 77.8 MHz. The maximum number of Type I RUs, Type II RUs, Type III RUs, Type IV RUs, Type V RUs and Type VI RUs which the 80 MHz OFDMA is able to support is thirty-seven, sixteen, eight, four, two and one, respectively. Similar to the 20 MHz or 40 MHz OFDMA, a mix of different types of RUs can also be accommodated in the 80 MHz OFDMA. 
     Similar to the 80 MHz OFDMA, the 80+80 MHz OFDMA or 160 MHz OFDMA also supports six types of RU, i.e., Type I RU, Type II RU, Type III RU, Type IV RU, Type V RU and Type VI RU. The maximum number of Type I RUs, Type II RUs, Type III RUs, Type IV RUs, Type V RUs and Type VI RUs which the 80+80 MHz OFDMA or 160 MHz OFDMA is able to support is seventy-four, thirty-two, sixteen, eight, four and two, respectively. Similar to the 20 MHz, 40 MHz or 80 MHz OFDMA, a mix of different types of RUs can also be accommodated in the 80+80 MHz OFDMA or 160 MHz OFDMA. 
     Note that use of a Type IV RU in context of 20 MHz OFDMA implies a non-OFDMA configuration, which refers to a case where OFDMA is not used in the data field  130  of  FIG.  1   . That is, the entire bandwidth of operation is scheduled for single user transmission or multiuser MIMO transmission. Similarly, use of a Type V RU in context of 40 MHz OFDMA or a Type VI RU in context of 80 MHz OFDMA implies a non-OFDMA configuration. In particular, use of two Type VI RUs in context of 160 MHz or 80+80 MHz OFDMA implies a non-OFDMA configuration. 
     Both continuous resource allocation and non-continuous resource allocation are possible in OFDMA frequency scheduling. 
       FIG.  5    illustrates an example of continuous resource allocation in the data field  130  according to the prior art (see IEEE802.11-15/0330r5, OFDMA Numerology and Structure, May 2015). As shown in  FIG.  5   , a single RU is allocated to a specific STA in terms of single user transmission or a specific group of STAs in terms of multiuser MIMO transmission in one assignment. 
       FIG.  6    illustrates an example of non-continuous resource allocation in the data field  130  according to the prior art (see IEEE802.11-15/0586r1, Frequency Diversity Options in OFDMA, May 2015). In non-continuous resource allocation, more than one RUs which may be not continuous in the frequency domain can be allocated in one assignment for the purpose of achieving frequency diversity effect. For example, three non-consecutive RUs  602 ,  604 , and  606  are allocated in one assignment. 
     With reference to  FIG.  1   , the legacy preamble  110  comprises a L-STF (Legacy Short Training Field)  112 , a L-LTF (Legacy Long Training Field)  114  and a L-SIG (Legacy SIGnal field)  116  in order to keep backward compatibility with legacy standard 802.11a/g/n/ac. The L-STF  112  is used for start-of-packet detection, AGC (Automatic Gain Control) setting, initial frequency offset estimation and initial time synchronization. The L-LTF  114  is used for further fine frequency offset estimation and time synchronization. The L-LTF  114  is also used to generate channel estimates for receiving and equalizing the L-SIG  116 , HE-SIG-A (High Efficiency SIGnal A field)  122  and HE-SIG-B (High Efficiency SIGnal B field)  124 . 
     The HE preamble  120  comprises a first signal field (i.e., HE-SIG-A)  122 , a second signal field (i.e., HE-SIG-B)  124 , a HE-STF  126  and a HE-LTF  128 . The HE-STF  126  is used to retrain AGC. The HE-LTF  128  comprises a plurality of HE-LTF symbols and is used to generate MIMO (Multiple Input Multiple Output) channel estimates for receiving and equalizing the data field  130 . If the PPDU  100  is a DL OFDMA PPDU, both the HE-SIG-A  122  and the HE-SIG-B  124  contain resource assignment information and user specific information which are used for each scheduled STA to decode its payload in the data field  130  at designated resource (see IEEE802.11-15/0621r2, Design Principles for HE Preamble, May 2015). If the PPDU  100  is a UL OFDMA PPDU, the HE-SIG-A  122  and HE-SIG-B  124  may contain neither resource assignment information nor user specific information since such information is preset by an AP and sent to scheduled STAs via a trigger frame which is carried in the data field of a previously transmitted DL PPDU (see IEEE802.11-15/0574r0, SIG Structure for UL PPDU, May 2015). Note that both HE-SIG-A  122  and HE-SIG-B  124  shall use a DFT period of 3.2 μs and subcarrier spacing of 312.5 kHz in 802.11ax. 
     Next, various embodiments for resource assignment in frequency scheduling will be explained in further details. 
     First Embodiment 
       FIG.  7    illustrates an example of resource assignment according to a first embodiment of the present disclosure. The first embodiment is applicable to continuous resource allocation where one or more RUs that are consecutive in the frequency domain are allocated in one assignment. In this example, there are eleven assignments in the 80 MHz OFDMA. Each assignment, which is referenced by an assignment index, is addressed to either a specific STA in terms of single user transmission or a specific group of STAs in terms of multiuser MIMO transmission. 
     According to the first embodiment, the first assignment has a predetermined start position (e.g., the start tone index of a first RU (e.g.,  202  as shown in  FIG.  2   ) which is known according to the size of CBW and the type of the first RU). And a start tone index of a subsequent assignment is next to the end tone index of its preceding assignment (i.e., there is no gap between consecutive assignments). The total number of assignments may be negotiated in advance between an Access Point (AP) and one or more station apparatus (STAs) or signaled to each STA in the HE-SIG-A field of DL PPDU or the trigger frame explicitly. However, assume that all available RUs are allocated, a STA can determine that an assignment is the last assignment if a last RU (e.g.,  210  as shown in  FIG.  2   ) is allocated in this assignment. Consequently, signaling of the total number of assignments can be omitted 
     According to the first embodiment, the start position of the first assignment is predetermined and the start position of a subsequent assignment can be determined from the end position of its preceding assignment. Therefore, it is enough to report the allocation bandwidth for each assignment. As a result, the overhead due to reporting resource assignment information for each assignment can be minimized. 
     According to the first embodiment, the resource assignment information includes a plurality of resource assignment indications, each of which corresponds to a particular assignment. 
       FIG.  8 A  illustrates a first example of resource assignment indication for one assignment according to the first embodiment of the present disclosure. The resource assignment indication for one assignment contains the number of allocated RUs and the type of each of allocated RUs, from which the allocation bandwidth for the assignment can be derived. 
       FIG.  8 B  illustrates a second example of resource assignment indication for one assignment according to the first embodiment of the present disclosure. In this example, only the same type of RUs can be allocated in one assignment. The resource assignment indication for the assignment contains the number of allocated RUs and the type of allocated RUs, from which the allocation bandwidth for the assignment can be derived. 
       FIG.  8 C  illustrates a third example of resource assignment indication for one assignment according to the first embodiment of the present disclosure. In this example, only a single RU can be allocated in one assignment. The resource assignment indication for the assignment contains the type of allocated RU only, from which the allocation bandwidth for the assignment can be derived. 
     In the above mentioned examples of the first embodiment, the number of allocated RUs and the RU type are indicated separately by using bit signalings. 
     According to the first embodiment, a two-bit signaling shown in Table 1 can be used to indicate the number of allocated RUs. According to Table 1, one RU to four RUs can be allocated in one assignment. 
                                 TABLE 1                       Signaling   Number of           bits   allocated RUs                          00   1           01   2           10   3           11   4                        
Additionally, a three-bit signaling shown in Table 2 can be used to indicate the RU type as follows:
 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Signaling 
                   
               
               
                   
                 bits 
                 RU Type 
               
               
                   
                   
               
             
            
               
                   
                 000 
                 Type I RU 
               
               
                   
                 001 
                 Type II RU 
               
               
                   
                 010 
                 Type III RU 
               
               
                   
                 011 
                 Type IV RU 
               
               
                   
                 100 
                 Type V RU 
               
               
                   
                 101 
                 Type VI RU 
               
               
                   
                 110, 111 
                 Reserved 
               
               
                   
                   
               
            
           
         
       
     
     For example, the type of the RU (Type II RU) allocated in the first assignment as shown in  FIG.  7    can be indicated by “001”. 
     According to the first embodiment, in case of 20 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type of allocated RUs shall be set to Type IV. In case of 40 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type of allocated RUs shall be set to Type V. In case of 80 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type of allocated RUs shall be set to Type VI. In case of 80+80 MHz or 160 MHz non-OFDMA transmission, the number of allocated RUs shall be set to two and the type of each of allocated RUs shall be set to Type VI. In this way, STA shall be able to determine whether an incoming DL PPDU  100  is an OFDMA PPDU or a non-OFDMA PPDU according to the resource assignment information without any dedicated signaling for such purpose. 
     Second Embodiment 
       FIG.  9    illustrates an example of resource assignment according to a second embodiment of the present disclosure. The second embodiment is also applicable to continuous resource allocation where one or more RUs that are consecutive in the frequency domain can be allocated in one assignment. In this example, there are ten assignments in the 80 MHz OFDMA. Each assignment is addressed to either a specific STA in terms of single user transmission or a specific group of STAs in terms of multiuser MIMO transmission. 
     According to the second embodiment, a start position of the first assignment may be variable and a gap may exist between consecutive assignments. In this embodiment, the start tone index of an assignment is always larger than the end tone index of its preceding assignment. The total number of assignments may be negotiated in advance between an AP and one or more STAs or signaled to each STA in the HE-SIG-A field of DL PPDU or the trigger frame explicitly. 
     According to the second embodiment, the start position of the first assignment is variable and the start position of a subsequent assignment cannot be derived only from the end position of its preceding assignment. Therefore, in addition to allocation bandwidth, it is necessary to report start position for each assignment. 
     According to the second embodiment, the resource assignment information includes a plurality of resource assignment indications, each of which corresponds to a particular assignment. 
       FIG.  10 A  illustrates a first example of resource assignment indication for one assignment according to the second embodiment of the present disclosure. The resource assignment indication for one assignment contains the assignment offset, the number of allocated RUs and the type of each of allocated RUs. As illustrated in  FIG.  9   , for the first assignment, the assignment offset  902  is relative to the start tone index of the first Type I RU. For each of the remaining assignments, the assignment offset (e.g.,  904 ) is relative to the end tone index of its preceding assignment. The start position for a subsequent assignment can be determined according to the assignment offset and the end tone index of its preceding assignment. Further, the allocation bandwidth for the assignment can be determined according to the number of allocated RUs and the type of each of allocated RUs. 
       FIG.  10 B  illustrates a second example of resource assignment indication for one assignment according to the second embodiment of the present disclosure. In this example, only the same type of RUs can be allocated in one assignment. The resource assignment indication for the assignment contains the assignment offset, the number of allocated RUs and the type of allocated RUs. The start position for the assignment can be determined according to the assignment offset and the end tone index of its preceding assignment. Further, the allocation bandwidth for the assignment can be determined according to the number of allocated RUs and the type of allocated RUs. 
       FIG.  10 C  illustrates a third example of resource assignment indication for one assignment according to the second embodiment of the present disclosure. In this example, only a single RU can be allocated in one assignment. The resource assignment indication for the assignment contains the assignment offset and the type of allocated RU. The start position for the assignment can be determined according to the assignment offset and the end tone index of its preceding assignment. Further, the allocation bandwidth for the assignment can be determined according to the type of allocated RU. 
     If reception quality of a RU is very poor for all scheduled STAs, the AP may not allocate the RU to them. This RU with poor reception quality is not used for resource assignment and becomes a gap between two assignments in this embodiment. The number of unused RUs that form a gap can be one or plural. As a result, the second embodiment provides more flexibility in frequency scheduling than the first embodiment. The overhead of reporting resource assignment information will slightly increase compared to the first embodiment. However, such overhead increase is not so significant. 
     In the above mentioned examples of the second embodiment, the assignment offset, the number of allocated RUs and the RU type are indicated separately by using bit signalings. 
     According to the second embodiment, if the assignment offset is not larger than three Type I RUs, a two-bit signaling shown in Table 3 can be used to indicate the assignment offset in units of the smallest RU (i.e., Type I RU). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Signaling 
                   
               
               
                   
                 bits 
                 Assignment offset 
               
               
                   
                   
               
             
            
               
                   
                 00 
                 no offset 
               
               
                   
                 01 
                 an offset of one Type I RU 
               
               
                   
                 10 
                 an offset of two Type I RUs 
               
               
                   
                 11 
                 an offset of three Type I RUs 
               
               
                   
                   
               
            
           
         
       
     
     For example, for the first assignment as shown in  FIG.  9   , the assignment offset  902  (e.g., an offset of two Type I RUs) can be indicated by “10”. 
     Two-bit signaling shown in Table 1 can be used to indicate the number of allocated RUs. An alternative two-bit signaling is shown in Table 4. According to Table 4, zero RU to three RUs can be allocated in an assignment. When no RU is allocated in an assignment, the assignment is called a “dummy assignment” with zero RU allocation. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Signaling 
                 Number of 
               
               
                   
                 bits 
                 allocated RUs 
               
               
                   
                   
               
             
            
               
                   
                 00 
                 0 
               
               
                   
                 01 
                 1 
               
               
                   
                 10 
                 2 
               
               
                   
                 11 
                 3 
               
               
                   
                   
               
            
           
         
       
     
     Two-bit signaling shown in Table 4 makes it possible to indicate an offset that is larger than three Type I RUs. For example, if there is an offset of five Type I RUs between a first assignment and a second assignment, this offset can be indicated by inserting a “dummy assignment” with zero RU allocation. More specifically, the “dummy assignment” located between the first assignment and the second assignment has an offset of three RUs and the second assignment has an offset of two RUs. Then, total offset will be five Type I RUs in this case. In addition, two-bit signaling shown in Table 4 can also make it possible to omit an explicit signaling of the total number of assignments. For example, if no last RU(s) (e.g.,  210  as shown in  FIG.  2   ) is allocated to any STA, a “dummy assignment” with zero RU allocation, which has some offset can be used to indicate such unused resource (RU). In this case, the STA is able to determine that the dummy assignment is the last assignment. 
     According to the second embodiment, in case of 20 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type of allocated RU shall be set to Type IV. In case of 40 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type of allocated RU shall be set to Type V. In case of 80 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type of allocated RU shall be set to Type VI. In case of 80+80 MHz or 160 MHz non-OFDMA transmission, the number of allocated RUs shall be set to two and the type of each of allocated RUs shall be set to Type VI. In this way, STA shall be able to determine whether incoming DL PPDU  100  is an OFDMA PPDU or a non-OFDMA PPDU according to the resource assignment information without any dedicated signaling for such purpose. 
     Third Embodiment 
       FIG.  11    illustrates an example of resource assignment according to a third embodiment of the present disclosure. The third embodiment is applicable to both continuous resource allocation and non-continuous resource allocation where one or more RUs which may not be consecutive in the frequency domain can be allocated in an assignment. The third embodiment enables even more flexibility in frequency scheduling than the first embodiment and the second embodiment. In this example, there are ten assignments in the 80 MHz OFDMA. Each assignment is addressed to either a specific STA in terms of single user transmission or a specific group of STAs in terms of multiuser MIMO transmission. 
     According to the third embodiment, the total number of assignments may be negotiated in advance between an AP and one or more STAs, or signaled to each STA in the HE-SIG-A field of DL PPDU or the trigger frame explicitly. 
     According to the third embodiment, the resource assignment information includes a plurality of resource assignment indications, each of which corresponds to a particular assignment. 
       FIG.  12 A  illustrates a first example of resource assignment indication for one assignment according to the third embodiment of the present disclosure. For each assignment, the resource assignment indication contains the number of allocated RUs and the type and position information of each of allocated RUs. 
       FIG.  12 B  illustrates a second example of resource assignment indication for one assignment according to the third embodiment of the present disclosure. In this example, only a single RU can be allocated in one assignment. For the assignment, the resource assignment indication contains the type and position information of allocated RU. 
     According to the third embodiment, the type and position of an allocated RU are jointly signaled in a single signaling field. That is, a single signaling field can be used to indicate both position and type of each of allocated RUs.  FIG.  13    illustrates a signaling of the RU type and position information according to the third embodiment of the present disclosure. Encoding of the RU type and position information is performed for RUs which 20 MHz OFDMA can support, followed by encoding for additional RUs which 40 MHz OFDMA can support, encoding for additional RUs which 80 MHz OFDMA can support, and encoding for additional RUs which 160 MHz and 80+80 MHz OFDMA can support in this order. 
     In the HE preamble of DL PPDU, assignment information regarding RUs of 20 MHz OFDMA is allocated first, followed by assignment information regarding additional RUs of 40 MHz OFDMA, assignment information regarding additional RUs of 80 MHz OFDMA, and assignment information regarding additional RUs of 160 MHz OFDMA in this order. This provides a technical advantage that a receiver of the resource assignment information (i.e., STA) that only supports CBW=20 MHz has to decode only a first part (i.e., assignment information regarding RUs of 20 MHz OFDMA) of the resource assignment information, and it can disregard the remaining part of the resource assignment information. Similarly, a STA that supports CBW=40 MHz has to decode only a first and second parts (i.e., assignment information regarding RUs of 20 MHz OFDMA and 40 MHz OFDMA) of the resource assignment information. Further, a STA that supports CBW=80 MHz has to decode a first, second and third parts (i.e., assignment information regarding RUs of 20 MHz OFDMA, 40 MHz OFDMA and 80 MHz OFDMA) of the resource assignment information. Lastly, a STA that supports CBW=160 MHz has to decode the resource assignment information as a whole. In this way, decoding workload at a STA supporting a smaller channel bandwidth (CBW) can be significantly lowered. 
     According to the signaling of the RU type and position information illustrated in  FIG.  13   , in one embodiment, an eight-bit signaling is used to indicate the type and position of an allocated RU. So, the overhead of reporting resource assignment information further increases compared to the second embodiment. Alternatively, signaling whose length is variable depending on CBW may be used. In more details, four-bit signaling, six-bit signaling, seven-bit signaling and eight-bit signaling can be used when CBW=20 MHz, CBW=40 MHz, CBW=80 MHz and CBW=80+80 MHz or 160 MHz, respectively. As a result, an increase of the overhead of reporting resource assignment information due to much more flexible frequency scheduling is reduced. For example, the type and position information of the RU allocated to the first assignment of 80 MHz OFDMA as illustrated in  FIG.  11    can be indicated by “0001010”. 
     According to the signaling of the RU type and position information illustrated in  FIG.  13   , in order to decode the type and position of each of the allocated RUs, a STA supporting CBW up to 20 MHz only needs to maintain a four-bit look up table Likewise, a STA supporting CBW up to 40 MHz only needs to maintain a six-bit look up table and a STA supporting CBW up to 80 MHz only needs to maintain a seven-bit look up table. As a result, the memory required for decoding the type and position information of each of allocated RUs is minimized for STAs with different PHY capabilities in terms of supported CBW. 
     According to the third embodiment, in case of 20 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type and position of allocated RU shall be set to the first Type IV RU. In case of 40 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type and position of allocated RU shall be set to the first Type V RU. In case of 80 MHz non-OFDMA transmission, the number of allocated RUs shall be set to one and the type and position of allocated RU shall be set to the first Type VI RU. In case of 80+80 MHz or 160 MHz non-OFDMA transmission, the number of allocated RUs shall be set to two and the type and position of allocated RUs shall be set to the first Type VI RU and the second Type VI RU, respectively. Consequently, STA shall be able to determine whether incoming DL PPDU  100  is an OFDMA PPDU or a non-OFDMA PPDU according to the resource assignment information without any dedicated signaling for such purpose. 
     &lt;HE SIG Field&gt; 
       FIG.  14    illustrates an example of information content of HE-SIG-A  122  and HE-SIG-B  124  of DL PPDU  100  according to the present disclosure. Common control information is included in both the HE-SIG-A for non-OFDMA transmission and the HE-SIG-A for OFDMA transmission. According to the present disclosure, the information contained in the HE-SIG-A  122  for non-OFDMA transmission differs from the HE-SIG-A  122  for OFDMA transmission. In case of non-OFDMA transmission, in addition to the common control information, the HE-SIG-A field  122  contains resource assignment information and user specific information for single user transmission or multiuser MIMO transmission. The HE-SIG-B field  124  does not exist in case of non-OFDMA transmission in the data field  130 . In case of OFDMA transmission in the data field  130 , in addition to the common control information, the HE-SIG-A field  122  contains resource assignment indication and user specific information for the first assignment, and the HE-SIG-B field  124  contains resource assignment indication and user specific information for each of the remaining assignments. 
     According to the present disclosure, common control information includes CBW and GI (Guard Interval), etc. The user specific information is required for each scheduled STA to decode its payload, e.g., Group ID, Nsts (i.e., the number of space-time streams) and MCS (Modulation and Coding Scheme), etc. 
     According to the present disclosure, common control information further includes an assignment set ID that maps a plurality of resource assignments indicated by resource assignment information to scheduled STAs, which will be detailed later. As a result, after decoding HE-SIG-A  122  of a DL PPDU  100 , if a STA determines that it is not addressed by the PPDU  100 , it will ignore the remaining of the PPDU  100  and reduce its power consumption. 
     According to the present disclosure, the common control information may further include an Allocation Defined flag in conjunction with the assignment set ID. Assume a first DL PPDU and a subsequent second DL PPDU are associated with the same assignment set ID. The Allocation Defined flag of the second DL PPDU shall be set if the resource assignment information contained in the first DL PPDU can be reused by the second DL PPDU. In that case, the resource assignment information for the second DL PPDU can be omitted, and thus signaling overhead can be reduced. 
     According to the present disclosure illustrated in  FIG.  14   , the HE-SIG-A  122  contains similar information for non-OFDMA transmission and OFDMA transmission in the data field  130 . This would reduce implementation complexity of STA. 
     According to the present disclosure illustrated in  FIG.  14   , when non-OFDMA transmission is performed in the data field  130 , the HE-SIG-B  124  does not exist. As a result, STAs need not to decode HE-SIG-B  124 , which leads to a reduced power consumption of STAs. 
     &lt;Radio Communication System&gt; 
       FIG.  15    illustrates an example sequence of executing OFDMA transmission in a radio communication system according to the present disclosure. The radio communication system comprises an AP  1502  and a plurality of STAs (e.g.,  1504 ) which are associated with AP  1502 . AP  1502  performs frequency scheduling using the plurality of RUs in the radio communication system. 
     Prior to initiation of DL OFDMA transmission, AP  1502  determines possible combinations of STAs that can be addressed by a DL OFDMA PPDU by assigning STAs to DL assignment sets and to specific assignment indices within those sets. One assignment set is identified by an assignment set ID and refers to a plurality of STAs and a plurality of assignment indices where each of the plurality of assignment indices is addressed to one or more of the plurality of STAs. For example, one assignment set comprises two STAs (STA1 and STA2) and two assignments where the first assignment is addressed to STA1 and the second assignment is addressed to STA2. Then AP  1502  transmits an Assignment Set ID Management frame  1510  to STA  1504  to assign or change its assignment indices corresponding to one or more DL assignment sets of which STA  1504  is a member. 
     Prior to initiation of UL OFDMA transmission, AP  1502  determines the possible combinations of STAs that transmit a UL OFDMA PPDU by assigning STAs to UL assignment sets and to specific assignment indices within those sets. Then AP  1502  transmits an Assignment Set ID Management frame  1512  to STA  1504  to assign or change its assignment indices corresponding to one or more UL assignment sets of which STA  1504  is a member. 
       FIG.  16    illustrates an example format of Assignment Set ID Management frame  1510  or  1512  according to the present disclosure. The frame  1510  comprises a Directionality field  1622 , a Membership Status Array field  1624  and an Assignment Index Array field  1626 . The Directionality field  1622  indicates whether OFDMA assignment sets are for DL or UL. STA  1504  may be assigned to multiple sets by setting multiple subfields of the Membership Status Array field  1624  to  1  in the frame  1510 . An assignment index in each assignment set of which STA  1504  is a member is indicated by the associated subfield in the Assignment Index Array field  1626  in the frame  1510 . For each Set ID, AP  1502  may assign the same assignment index to multiple STAs. STA  1504  shall have only one assignment index in each set of which it is a member. 
     According to the present disclosure, the AP  1502  may transmit the Assignment Set ID management frames to STA  1504  when it associates with the AP  1502 . In addition, the AP  1502  may transmit the Assignment Set ID management frames to STA  1504  periodically or if necessary. 
     If only a specific combination of STAs is allowed to communicate with the AP  1502  in an OFDMA transmission for a period of time, a simple management frame can be used instead of the Assignment Set ID management frame to indicate an assignment index for each STA. In this case, the assignment set ID in the HE-SIG-A of DL PPDU or the trigger frame can be omitted. 
     If AP  1502  has buffered data addressed to STA  1504 , AP  1502  selects a DL assignment set of which STA  1504  is a member and determines DL resource required to transmit the data addressed to STA  1504  based on the data size and QoS (Quality of Service) requirement. Then AP  1502  transmits a DL OFDMA PPDU  1514  which includes the data addressed to STA  1504 , assignment set ID of the selected DL assignment set as well as other control information (e.g., resource assignment information) which is required by STA  1504  to decode its data inside the DL OFDMA PPDU  1514 . Note that when a subsequent DL OFDMA PPDU which includes the same assignment set ID as the DL OFDMA PPDU  1514  is transmitted, if the resource assignment information contained in the DL OFDMA PPDU  1514  can be reused by the subsequent DL OFDMA PPDU, the Allocation Defined flag in the subsequent DL OFDMA PPDU shall be set and then resource assignment information needs not to be included in the subsequent DL OFDMA PPDU. 
     If STA  1504  has buffered data addressed to AP  1502 , STA  1504  may perform ADDTS Request/Response frame exchange  1516  with AP  1502  to request transmission bandwidth for its data. ADDTS Request frame may also include information on RUs, for example, channel quality information to show which RUs are preferable or not preferable for the STA  1504 . Then AP  1502  selects a UL assignment set of which STA  1504  is a member and determines UL resource according to the requested transmission bandwidth by STA  1504 . After that, AP  1502  transmits a trigger frame  1518  to STA  1504  which includes assignment set ID of the selected UL assignment set as well as other control information (e.g., resource assignment information) which is required by STA  1504  to transmit its data. Note that when a subsequent trigger frame which includes the same assignment set ID as the trigger frame  1518  is transmitted, if the resource assignment information contained in the trigger frame  1518  can be reused by the subsequent trigger frame, the Allocation Defined flag in the subsequent trigger frame shall be set and then resource assignment information needs not to be included in the subsequent trigger frame. The trigger frame may also include UL transmission power control information and UL transmission duration information. After receiving the trigger frame  1518 , STA  1504  transmits a UL OFDMA PPDU  1520  to send its data using the designated resource accordingly. STA  1504  may control its transmission power based on the transmission power control information so that, at the AP  1502 , large variation between reception power from each STA can be avoided. 
     &lt;Configuration of an Access Point&gt; 
       FIG.  17    is a block diagram illustrating example configuration of AP  1502  according to the present disclosure. The AP  1502  comprises a controller  1702 , a scheduler  1704 , a message generator  1708 , a message processor  1706 , a PHY processor  1710  and an antenna  1712 . The controller  1702  is a MAC protocol controller and controls general MAC protocol operations. 
     For DL OFDMA transmission, scheduler  1704  performs frequency scheduling under the control of the controller  1702  based on channel quality indicators (CQIs) from STAs and assigns data for STAs to RUs. Examples of a CQI-based scheduling method include the Max CIR method and the proportional-fairness method. Scheduler  1704  also outputs the resource assignment results to the message generator  1708 . The message generator  1708  generates corresponding common control information, resource assignment information, user specific information and data for scheduled STAs, which are formulated by the PHY processor  1710  into an OFDMA PPDU and transmitted through the antenna  1712 . The resource assignment information can be configured according to the above mentioned embodiments. On the other hand, the message processor  1706  analyzes the received CQIs from STAs through the antenna  1712  under the control of the controller  1702  and provides them to scheduler  1704  and controller  1702 . These CQIs are received quality information reported from the STAs. Further, each STA can measure received quality on a per RU basis using the received SNR, received SIR, received SINR, received CINR, received power, interference power, bit error rate, throughput and MCS whereby a predetermined error rate can be achieved. Furthermore, the CQI may also be referred to as “CSI” (Channel State Information). 
     For UL OFDMA transmission, scheduler  1704  performs frequency scheduling under the control of the controller  1702  based on transmission bandwidth request from STAs and assigns resource for scheduled STAs for UL data transmission. At the same time, scheduler  1704  may also perform time scheduling to determine duration of UL OFDMA frame or transmission opportunity (TXOP) in which STAs have a right to perform UL OFDMA frame exchanges. Scheduler  1704  also outputs the resource assignment results to the message generator  1708 . The message generator  1708  generates a trigger frame including common control information, resource assignment information and user specific information, which is formulated by the PHY processor  1710  into a DL PPDU and transmitted through the antenna  1712 . On the other hand, the message processor  1706  analyzes the received transmission bandwidth request from STAs through the antenna  1712  and provides them to scheduler  1704  and controller  1702 . The antenna  1712  can be comprised of one antenna port or a combination of a plurality of antenna ports. 
     &lt;Configuration of a STA&gt; 
       FIG.  18    is a block diagram illustrating example configuration of STA  1504  according to the present disclosure. STA  1504  comprises a controller  1802 , a message generator  1804 , a message processor  1806 , a PHY processor  1808  and an antenna  1810 . The controller  1802  is a MAC protocol controller and controls general MAC protocol operations. The antenna  1810  can be comprised of one antenna port or a combination of a plurality of antenna ports. 
     For UL OFDMA transmission, the message processor  1806  analyzes the received trigger frame from AP  1502  through the antenna  1810  and provides common control information, resource assignment information and user specific information to controller  1802 . The resource assignment information can be configured according to the above mentioned embodiments. The message generator  1804  generates data under the control of the controller  1802 , which are formulated by the PHY processor  1808  under the control of the controller  1802  into an UL OFDMA PPDU in such a way that the data is transmitted at the designated resource. The UL OFDMA PPDU is transmitted through the antenna  1810 . 
     For DL OFDMA transmission, the message processor  1806  estimates channel quality from the received DL PPDU through the antenna  1810  and provides them to controller  1802 . The message generator  1804  generates CQI message, which is formulated by the PHY processor  1808  into an UL PPDU and transmitted through the antenna  1810 . 
     Fourth Embodiment 
       FIG.  19    illustrates an example of resource assignment according to a fourth embodiment of the present disclosure. The fourth embodiment is applicable to continuous resource allocation where one or more RUs that are consecutive in the frequency domain can be allocated in one assignment. In this example, there are nine assignments (#0 to #9) in the 80 MHz OFDMA. Each assignment is addressed to either a specific STA in terms of single user transmission or a specific group of STAs in terms of multiuser MIMO transmission. 
     According to the fourth embodiment, the total number of assignments may be negotiated in advance between an AP and one or more STAs or may be explicitly signaled to each STA in the HE-SIG-A field of DL PPDU or the trigger frame. 
     Unlike the first and second embodiments where the start tone index of an assignment is always larger than the end tone index of its preceding assignment, there is no such restriction in the fourth embodiment. The start tone index and the end tone index of an assignment can be smaller than the first tone index of another preceding assignment. As a result, the scheduling flexibility is improved in the fourth embodiment. 
     According to the fourth embodiment, the resource assignment information includes a plurality of resource assignment indications, each of which corresponds to a particular assignment. 
       FIG.  20 A  illustrates a first example of resource assignment indication for one assignment according to the fourth embodiment of the present disclosure. The resource assignment indication for one assignment contains the number of allocated RUs, the position and type of the first allocated RU and the type of each of remaining allocated RUs. In other words, each resource assignment indication contains position and type information of the first RU only and type information of each of the remaining RUs. The start position for an assignment can be determined according to the position of the first allocated RU. Further, the allocation bandwidth for the assignment can be determined according to the number of allocated RUs and the type of each of allocated RUs. 
       FIG.  20 B  illustrates a second example of resource assignment indication for one assignment according to the fourth embodiment of the present disclosure. In this example, only the same type of RUs can be allocated in one assignment. The resource assignment indication for the assignment contains the number of allocated RUs and the position and type of the first allocated RU. The start position for an assignment can be determined according to the position of the first allocated RU. Further, the allocation bandwidth for the assignment can be determined according to the number of allocated RUs and the type of the first allocated RU. 
     Two-bit signaling shown in Table 1 can be used to indicate the number of allocated RUs, and three-bit signaling shown in Table 2 can be used to indicate the RU type. The type and position of the first allocated RU can be jointly signalled in a single signaling field as illustrated in  FIG.  13   . 
     [HE SIG Field] 
       FIG.  21    illustrates another example of information content of HE-SIG-A  122  and HE-SIG-B  124  of DL PPDU according to the present disclosure. According to the present disclosure, the HE-SIG-B field  124  does not exist in the DL PPDU in case of single user transmission. In case of multiuser transmission, the HE-SIG-B field  124  exists in the DL PPDU and contains resource assignment information (i.e., resource assignment indication for each assignment), followed by user specific information for each assignment. The HE-SIG-B field  124  is encoded on a per 20 MHz subband basis. For CBW=40 MHz, 80 MHz, 160 MHz or 80+80 MHz, the number of 20 MHz subbands carrying different content is two. 
     An example structure of the HE-SIG-B field  124  in  FIG.  21    in case of CBW=80 MHz is illustrated in  FIG.  22   . The HE-SIG-B field  124  comprises two portions: HE-SIG-B1  2202  and HE-SIG-B2  2204 . The HE-SIG-B1  2202  is transmitted over the first 20 MHz subband channel  2222  and a duplicate of the HE-SIG-B  2202  is transmitted over the third 20 MHz subband channel  2226  while the HE-SIG-B2  2204  is transmitted over the second 20 MHz subband channel  2224  and a duplicate of the HE-SIG-B2  2204  is transmitted over the fourth 20 MHz subband channel  2228 . 
     According to the present disclosure, resource assignment indication for one assignment that is fully located within a 20 MHz subband channel should be carried in one of the HE-SIG-B1  2202  and HE-SIG-B2  2204  that is transmitted over the same 20 MHz subband channel. In more details, the HE-SIG-B1  2202  should carry resource assignment indications for the assignments (e.g.,  2212 ) that are fully located within the first 20 MHz subband channel  2222  or the third 20 MHz subband channel  2226 . The HE-SIG-B2  2204  should carry resource assignment indications for the assignments (e.g.,  2218 ) that are fully located within the second 20 MHz subband channel  2224  or the fourth 20 MHz subband channel  2228 . In this way, even if control signaling in a 20 MHz subband channel (e.g.,  2222  or  2226 ) is corrupted due to interference, the DL PPDU in another 20 MHz subband channel (e.g.,  2224  or  2228 ) can be decoded correctly. 
     According to the present disclosure, for the assignments (e.g.,  2216 ) that span across two or more neighboring 20 MHz subband channels, the corresponding resource assignment indications can be carried either in the HE-SIG-B1  2202  or in the HE-SIG-B2  2204  such that data amount of the HE-SIG-B1  2202  and data amount of the HE-SIG-B2  2204  become similar in size. Since smaller one of the HE-SIG-B1 and the HE-SIG-B2 will be appended padding bits until their payload sizes become the same, the padding efficiency of HE-SIG-B field can be improved or maximized according to this embodiment. 
       FIG.  23    is a flow chart illustrating a method for distributing resource assignment information into the HE-SIG-B field according to the present disclosure. The method shown in  FIG.  23    starts at Step  2302 . At Step  2304 , resource assignment indications for the assignments that are fully located in any 20 MHz subband channel over which the HE-SIG-B1 is transmitted are included (i.e., mapped) in the HE-SIG-B1. At Step  2306 , resource assignment indications for the assignments that are fully located in any 20 MHz subband channel over which the HE-SIG-B2 is transmitted are included (i.e., mapped) in the HE-SIG-B2. Note that the sequential order of Step  2304  and Step  2306  may be interchangeable. At Step  2308 , resource assignment indications for the assignments that span across two or more neighboring 20 MHz subband channels are included (i.e., mapped) in either the HE-SIG-B1 or the HE-SIG-B2 so that data amount of the HE-SIG-B1 and data amount of the HE-SIG-B2 become similar in size. This method stops at Step  2310 . 
     Take the following case as an example: 
     CBW=40 MHz; 
     Four assignments: A1, A2, A3 and A4; 
     Assignment A1 contains one or more RUs that are located in the lower 20 MHz subband channel over which the HE-SIG-B1 is transmitted; 
     Each of assignments A2 and A3 contains one or more RUs that are located in the upper 20 MHz subband channel over which the HE-SIG-B2 is transmitted; 
     Assignment A4 contains one or more RUs that span across both of the lower and the upper 20 MHz subband channels; and 
     Assume that resource assignment indication for each of four assignments requires the similar number of information bits. 
     According to the method illustrated in  FIG.  23   , resource assignment indications for the above four assignments should be distributed into the HE-SIG-B as follows: 
     Resource assignment indication for assignment A1 is signaled in the HE-SIG-B1; 
     Resource assignment indications for assignments A2 and A3 are signaled in the HE-SIG-B2; and 
     Resource assignment indication for assignment A4 is signaled in the HE-SIG-B1. 
     By distributing resource assignment indications between the HE-SIG-B1 and the HE-SIG-B2, data amount of the HE-SIG-B1 and data amount of the HE-SIG-B2 become similar in size, thus improving padding efficiency in the HE-SIG-B field. 
     [HE-SIG-B Field] 
       FIG.  24    illustrates a first example format of the HE-SIG-B1  2202  or the HE-SIG-B2  2204  in  FIG.  22    in case of CBW=80 MHz. The HE-SIG-B1  2202  or the HE-SIG-B2  2204  comprises a common field  2410  and a user-specific field  2450 . The common field  2410  comprises a first resource assignment subfield  2412 , a second resource assignment subfield  2414 , a CRC (Cyclic Redundancy Check) subfield  2418  and a tail bits subfield. 
     In context of the HE-SIG-B1  2202 , the first resource assignment subfield  2412  contains a RU arrangement pattern index which indicates a specific RU arrangement in the frequency domain (including MU-MIMO (Multiuser Multiple Input Multiple Output) related information) for the first 20 MHz subband channel  2222  in  FIG.  22   . The mapping of RU arrangement pattern indices and the corresponding RU arrangement patterns is predetermined. An example mapping of RU arrangement pattern indices and the corresponding RU arrangement patterns is shown in Table 5. Note that RUs are arranged from lower frequency to higher frequency in the frequency domain within a 20 MHz subband channel and Type I RUs and Type II RUs can be used for SU-MIMO transmission only. 
     
       
         
           
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 RU 
                   
               
               
                 Arrange- 
                   
               
               
                 ment 
                   
               
               
                 Pattern 
                   
               
               
                 Index 
                 RU Arrangement Pattern 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 0 
                 9 Type I RUs 
               
               
                 1 
                 1 Type II RU, followed by 7 Type I RUs 
               
               
                 2 
                 2 Type I RUs, followed by 1 Type II RU and 5 Type I RUs 
               
               
                 3 
                 5 Type I RUs, followed by 1 Type II RU and 2 Type I RUs 
               
               
                 4 
                 7 Type I RUs, followed by 1 Type II RU 
               
               
                 5 
                 2 Type II RUs, followed by 5 Type I RUs 
               
               
                 6 
                 1 Type II RU, followed by 3 Type I RUs, 1 Type II RU and 
               
               
                   
                 2 Type I RUs 
               
               
                 7 
                 1 Type II RU, followed by 5 Type I RUs and 1 Type II RU 
               
               
                 8 
                 2 Type I RUs, followed by 1 Type II RU, 1 Type I RU, 
               
               
                   
                 1 Type II RU and 2 Type I RUs 
               
               
                 9 
                 2 Type I RUs, followed by 1 Type II RU, 3 Type I RUs and 
               
               
                   
                 1 Type II RU 
               
               
                 10 
                 5 Type I RUs, followed by 2 Type II RUs 
               
               
                 11 
                 2 Type II RUs, followed by 1 Type I RU, 1 Type II RU and 
               
               
                   
                 2 Type I RUs 
               
               
                 12 
                 2 Type II RUs, followed by 3 Type I RUs and 1 Type II RU 
               
               
                 13 
                 1 Type II RU, followed by 3 Type I RUs and 2 Type II RUs 
               
               
                 14 
                 2 Type I RUs, followed by 1 Type II RU, 1 Type I RU and 
               
               
                   
                 2 Type II RUs 
               
               
                 15 
                 2 Type II RUs, followed by 1 Type I RU and 2 Type II RUs 
               
               
                 16 
                 1 Type III RU for SU-MIMO transmission, followed by 5 
               
               
                   
                 Type I RUs 
               
               
                 17 
                 1 Type III RU for SU-MIMO transmission, followed by 3 
               
               
                   
                 Type I RUs and 1 Type II RU 
               
               
                 18 
                 1 Type III RU for SU-MIMO transmission, followed by 1 
               
               
                   
                 Type I RU, 1 Type II RU and 2 Type I RUs 
               
               
                 19 
                 1 Type III RU for SU-MIMO transmission, followed by 1 
               
               
                   
                 Type I RU and 2 Type II RUs 
               
               
                 20 
                 1 Type III RU for SU-MIMO transmission, followed by 1 
               
               
                   
                 Type I RU and 1 Type III RU for SU-MIMO transmission 
               
               
                 21 
                 5 Type I RUs, followed by 1 Type III RU for SU-MIMO 
               
               
                   
                 transmission 
               
               
                 22 
                 1 Type II RU, followed by 3 Type I RUs and 1 Type III RU 
               
               
                   
                 for SU-MIMO transmission 
               
               
                 23 
                 2 Type I RUs, followed by 1 Type II RU, 1 Type I RU and  
               
               
                   
                 1 Type III RU for SU-MIMO transmission 
               
               
                 24 
                 2 Type II RUs, followed by 1 Type I RU and 1 Type III RU 
               
               
                   
                 for SU-MIMO transmission 
               
               
                 25 
                 5 Type I RUs, followed by 1 Type III RU for MU-MIMO 
               
               
                   
                 transmission with 2 users multiplexed 
               
               
                 26 
                 5 Type I RUs, followed by 1 Type III RU for MU-MIMO 
               
               
                   
                 transmission with 3 users multiplexed 
               
               
                 27 
                 5 Type 1 RUs, followed by 1 Type ill RU for MU-MIMO 
               
               
                   
                 transmission with 4 users multiplexed 
               
               
                 28 
                 5 Type 1 RUs, followed by 1 Type III RU for MU-MIMO 
               
               
                   
                 transmission with 5 users multiplexed 
               
               
                 29 
                 5 Type 1 RUs, followed by 1 Type III RU for MU-MIMO 
               
               
                   
                 transmission with 6 users multiplexed 
               
               
                 30 
                 5 Type I RUs, followed by 1 Type III RU for MU-MIMO 
               
               
                   
                 transmission with 7 users multiplexed 
               
               
                 31 
                 5 Type I RUs, followed by 1 Type III RU for MU-MIMO 
               
               
                   
                 transmission with 8 users multiplexed 
               
               
                 . 
                 . 
               
               
                 . 
                 . 
               
               
                 . 
                 . 
               
               
                   
               
            
           
         
       
     
     With reference to Table 1, for example, the first resource assignment subfield  2412  may contain a RU arrangement pattern index  25  to indicate a specific RU arrangement for the first 20 MHz subband channel where five Type I RUs followed by one Type III RU in the frequency domain, and each of five Type I RUs is used for SU-MIMO (Single User Multiple Input Multiple Output) transmission while the Type III RU is used for MU-MIMO transmission with two users multiplexed. The second resource assignment subfield  2414  indicates the RU arrangement in the frequency domain and MU-MIMO related information for the third 20 MHz subband channel  2226  in  FIG.  22   . 
     In context of the HE-SIG-B2  2204 , the first resource assignment subfield  2412  indicates the RU arrangement in the frequency domain and MU-MIMO related information for the second 20 MHz subband channel  2224  in  FIG.  22   . The second resource assignment subfield  2414  indicates the RU arrangement in the frequency domain and MU-MIMO related information for the fourth 20 MHz subband channel  2228  in  FIG.  22   . It should be noted that the RU arrangement signaled by the first resource assignment subfield  2412  and the second resource assignment subfield  2414  does not involve the center Type I RU  402  as illustrated in  FIG.  4   , which is located between two adjacent 20 MHz subband channels. 
     The user-specific field  2450  comprises a plurality of BCC (Binary Convolutional Coding) blocks  2460 . Each of the BCC blocks  2460  except the last BCC block  2460 -N comprises a first user-specific subfield, a second user-specific subfield, a CRC subfield and a tail bits subfield. The last BCC block  2460 -N may comprise a single user-specific subfield. Each of user-specific subfields in the user-specific field  2450  carries per-user allocation information (e.g., STA identifier for addressing and the information necessary for decoding the PPDU  100  such as the number of spatial streams and modulation and coding scheme, etc). For each RU assigned for SU-MIMO transmission, there is only a single corresponding user-specific subfield. For each RU assigned for MU-MIMO transmission with K users multiplexed, there are K corresponding user-specific subfields. The ordering of user-specific subfields in the user-specific field  2450  is compliant with the RU arrangement signaled by the first resource assignment subfield  2412  and the second resource assignment subfield  2414 . 
     According to the present disclosure, one of the user-specific subfields of the user-specific field  2450  in each of the HE-SIG-B1  2022  and the HE-SIG-B2  2024  is used to carry per-user allocation information for the center Type I RU  402  as illustrated in  FIG.  4   . The user-specific subfield for the center Type I RU shall be located at a predetermined position in the user-specific field  2450 . For example, the user-specific subfield for the center Type I RU is the last user-specific subfield  2470  in the user-specific field  2450 . 
     According to the present disclosure, the number of the user-specific subfields in the user-specific field  2450  except the user-specific subfield for the center Type I RU can be derived from the first resource assignment subfield  2412  and the second resource assignment subfield  2414  in the common field  2410 . 
     In case of CBW=160 MHz or 80+80 MHz, there is a center Type I RU that is located between two adjacent 20 MHz subband channels for every 80 MHz. As a result, there are two center Type-I RUs in total in case of CBW=160 MHz or 80+80 MHz. In this case, according to the present disclosure, two of the user-specific subfields of the user-specific field  2450  in each of the HE-SIG-B1  2022  and the HE-SIG-B2  2024  are used to carry per-user allocation information for the two center Type I RUs, respectively. Each of the two user-specific subfields for the center Type I RUs shall be located at a predetermined position in the user-specific field  2450 . For example, the user-specific subfield for a first center Type I RU is the last user-specific subfield in the user-specific field  2450  while the user-specific subfield for a second center Type I RU is the second last user-specific subfield in the user-specific field  2450 . 
       FIG.  25    illustrates a second example format of the HE-SIG-B1  2202  or the HE-SIG-B2  2204  in  FIG.  22    in case of CBW=80 MHz. The HE-SIG-B1  2202  or the HE-SIG-B2  2204  comprises a common field  2510  and a user-specific field  2550 . The common field  2510  comprises a first resource assignment subfield  2512 , a second resource assignment subfield  2514 , a presence of allocation information for center RU subfield  2516 , a CRC subfield  2518  and a tail bits subfield. The user-specific field  2550  comprises a plurality of BCC blocks  2560 . Each of the BCC blocks  2560  except the last BCC block  2560 -N comprises a first user-specific subfield, a second user-specific subfield, a CRC subfield and a tail bits subfield. The last BCC block  2560 -N may comprise a single user-specific subfield. Each of the user-specific subfields in the user-specific field  2450  carries per-user allocation information. 
     The first resource assignment subfield  2512 , the second resource assignment subfield  2514  and each of user-specific subfields are defined in the same way as their respective counterparts in  FIG.  24   . 
     According to the present disclosure, the presence of allocation information for center RU subfield  2516  in the common field  2510  is used to indicate whether there is a user-specific subfield for the center Type I RU in the user-specific field  2550 . If a user-specific subfield for the center Type I RU is present in the user-specific field  2550 , its position in the user-specific field  2550  shall be predetermined. For example, the user-specific subfield for the center Type I RU is the last user-specific subfield  2570  in the user-specific field  2550 . 
     According to the present disclosure, the number of user-specific subfields in the user-specific field  2550  can be derived from the first resource assignment subfield  2512 , the second resource assignment subfield  2514  and the presence of allocation information for center RU subfield  2516  in the common field  2510 . 
     Compared with the first example format of the HE-SIG-B1  2202  or the HE-SIG-B2  2204  as illustrated in  FIG.  24    where the user-specific subfield for the center Type I RU is included in both the HE-SIG-B1  2202  and the HE-SIG-B2  2204 , the second example format as illustrated in  FIG.  25    enables more flexible arrangement of user-specific subfield for the center Type I RU in the HE-SIG-B1  2202  and the HE-SIG-B2  2204 . For one example, the user-specific subfield for the center Type I RU may be included in either of the HE-SIG-B1  2202  and the HE-SIG-B2  2204  for the purpose of keeping load balancing between the HE-SIG-B1  2202  and the HE-SIG-B2  2204  and improving channel efficiency. In other words, the user-specific subfield for the center Type I RU may be included in either of the HE-SIG-B1  2202  and the HE-SIG-B2  2204  so that the difference in terms of the number of user-specific subfields between the HE-SIG-B1  2202  and the HE-SIG-B2  2204  is minimized. For another example, the user-specific subfield for the center Type I RU may be included in both of the HE-SIG-B1  2202  and the HE-SIG-B2  2204  for the purpose of improving reliability for decoding the user-specific subfield for the center Type I RU. 
     In case of CBW=160 MHz or 80+80 MHz, the presence of allocation information for center RU subfield  2516  in the common field  2510  needs to indicate whether there is a user-specific subfield for each of the two center Type I RUs in the user-specific field  2550 . If the user-specific subfield for only one of the two center Type I RUs is present in the user-specific field  2550 , its position in the user-specific field  2550  shall be predetermined. For example, the user-specific subfield for the center Type I RU is the last user-specific subfield in the user-specific field  2550 . If the user-specific subfield for each of the two center Type I RUs is present in the user-specific field  2550 , the two user-specific subfields for the center Type I RUs shall be located at the predetermined positions in the user-specific field  2550 . For example, the user-specific subfield for a first center Type I RU is the last user-specific subfield in the user-specific field  2550  while the user-specific subfield for a second center Type I RU is the second last user-specific subfield in the user-specific field  2550 . 
       FIG.  26    illustrates a third example format of the HE-SIG-B1  2202  or the HE-SIG-B2  2204  in  FIG.  22    in case of CBW=80 MHz. The HE-SIG-B1  2202  or the HE-SIG-B2  2204  comprises a common field  2610  and a user-specific field  2650 . The common field  2610  comprises a first resource assignment subfield  2612 , a second resource assignment subfield  2614 , a CRC subfield  2618  and a tail bits subfield. The user-specific field  2650  comprises a plurality of BCC blocks  2660 . Each of BCC blocks  2660  except the last BCC block  2660 -N comprises a first user-specific subfield, a second user-specific subfield, a CRC subfield and a tail bits subfield. The last BCC block  2660 -N may comprise a single user-specific subfield. Each of user-specific subfields in the user-specific field  2650  carries per-user allocation information. 
     The first resource assignment subfield  2612 , the second resource assignment subfield  2614  and each of user-specific subfields are defined in the same way as their respective counterparts in  FIG.  24   . 
     According to the present disclosure, whether the CRC subfield  2618  in the common field  2610  is masked by a predefined binary sequence (i.e., whether a XOR (Exclusive OR) is applied to the CRC subfield  2618  and a predefined binary sequence) is used to indicate whether there is a user-specific subfield for the center Type I RU in the user-specific field  2650 . For example, if the CRC subfield  2618  in the common field  2610  is not masked with a predefined binary sequence, there is no user-specific subfield for the center Type I RU in the user-specific field  2650 . Otherwise there is a user-specific subfield for the center Type I RU in the user-specific field  2650 . 
     Alternatively, instead of the CRC subfield  2618  in the common field  2610 , whether the CRC subfield of a specific BCC block in the user-specific field  2650  is masked by a predefined binary sequence is used to indicate whether there is a user-specific subfield for the center Type I RU in the user-specific field  2650 . For example, if the CRC subfield  2666  of the first BCC block  2660 - 1  is not masked by a predefined binary sequence, there is no user-specific subfield for the center Type I RU in the user-specific field  2650 . Otherwise there is a user-specific subfield for the center Type I RU in the user-specific field  2650 . 
     If a user-specific subfield for the center Type I RU is present in the user-specific field  2650 , its position in the user-specific field  2650  shall be predetermined. For example, the user-specific subfield for the center Type I RU is the last user-specific subfield  2670  in the user-specific field  2650 . 
     According to the present disclosure, the number of user-specific subfields in the user-specific field  2650  except the user-specific subfield for the center Type I RU can be derived from the first resource assignment subfield  2612  and the second resource assignment subfield  2614  in the common field  2610 . 
     Compared with the second example format of the HE-SIG-B1  2202  or the HE-SIG-B2  2204  as illustrated in  FIG.  25   , the third example format as illustrated in  FIG.  26    does not need a signaling subfield in the common field to signal the presence of user-specific subfield for the center Type I RU in the user-specific field. In other words, the signaling bits required by the third example format is reduced compared with the second example format. 
     In case of CBW=160 MHz or 80+80 MHz, whether the CRC subfield  2618  in the common field  2610  (or the CRC subfield  2666  in the user-specific field  2650 ) is masked by one of the three predefined binary sequences is used to indicate whether there is a user-specific subfield for each of the two center Type I RUs in the user-specific field  2650 . For example, if the CRC subfield  2618  in the common field  2610  (or the CRC subfield  2666  in the user-specific field  2650 ) is not masked by one of three predefined binary sequences, there is no user-specific subfield for the center Type I RU in the user-specific field  2650 . If the CRC subfield  2618  in the common field  2610  (or the CRC subfield  2666  in the user-specific field  2650 ) is masked by a first predefined binary sequence, there is a user-specific subfield for a first center Type I RU in the user-specific field  2650 . If the CRC subfield  2618  in the common field  2610  (or the CRC subfield  2666  in the user-specific field  2650 ) is masked by a second predefined binary sequence, there is a user-specific subfield for a second center Type I RU in the user-specific field  2650 . If the CRC subfield  2618  in the common field  2610  (or the CRC subfield  2666  in the user-specific field  2650 ) is masked by a third predefined binary sequence, there is a user-specific subfield for each of the two center Type I RUs in the user-specific field  2650 . If the user-specific subfield for only one of the two center Type I RUs is present in the user-specific field  2650 , its position in the user-specific field  2650  shall be predetermined. For example, the user-specific subfield for the center Type I RU is the last user-specific subfield in the user-specific field  2650 . If the user-specific subfield for each of the two center Type I RUs is present in the user-specific field  2650 , the two user-specific subfields for the center Type I RUs shall be located at the predetermined positions in the user-specific field  2650 . For example, the user-specific subfield for a first center Type I RU is the last user-specific subfield in the user-specific field  2650 ; while the user-specific subfield for a second center Type I RU is the second last user-specific subfield in the user-specific field  2650 . 
     In the foregoing embodiments, the present disclosure is configured with hardware by way of example, but the disclosure may also be provided by software in cooperation with hardware. 
     In addition, the functional blocks used in the descriptions of the embodiments are typically implemented as LSI devices, which are integrated circuits. The functional blocks may be formed as individual chips, or a part or all of the functional blocks may be integrated into a single chip. The term “LSI” is used herein, but the terms “IC,” “system LSI,” “super LSI” or “ultra LSI” may be used as well depending on the level of integration. 
     In addition, the circuit integration is not limited to LSI and may be achieved by dedicated circuitry or a general-purpose processor other than an LSI. After fabrication of LSI, a field programmable gate array (FPGA), which is programmable, or a reconfigurable processor which allows reconfiguration of connections and settings of circuit cells in LSI may be used. 
     Should a circuit integration technology replacing LSI appear as a result of advancements in semiconductor technology or other technologies derived from the technology, the functional blocks could be integrated using such a technology. Another possibility is the application of biotechnology and/or the like. 
     This disclosure can be applied to a method for formatting and transmitting resource assignment information in a wireless communications system.