Patent Publication Number: US-2018054803-A1

Title: Base station apparatus and terminal apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a technique of transmitting a transmission frame to a terminal apparatus by using at least one of multiple radio resources. 
     BACKGROUND ART 
     IEEE (The Institute of Electrical and Electronics Engineers Inc.) has defined IEEE 802.11ac that achieves a further increase in the speed of IEEE 802.11 that is a wifeless LAN (Local Area Network) standard. Currently, as a succeeding standard of IEEE 802.11ac, activities for standardizing IEEE 802.11ax (hereinafter also referred to as “802.11ax”) have started. Also in the standardization of 802.11ax, a study for improving throughput per user in an environment in which wireless LAN devices are located overcrowded has progressed with rapid, widespread use of wireless LAN devices. 
     A wireless LAN system is a system that makes a determination about whether transmission is to be performed, on the basis of carrier sense (CS: Carrier Sense). When a reception interference level obtained through carrier sense is lower than a threshold, the wireless LAN system determines that it is possible to perform transmission. When interference power higher than the threshold is received, transmission is avoided. 
     In the standardization of IEEE 802.11ax, introduction of DL-OFDMA in which a frequency band is divided so that the frequency bands obtained through division are allocated to multiple wireless LAN devices for transmission has been studied. In DL-OFDMA, data addressed to multiple terminal apparatuses may be transmitted in a multiplex manner. Therefore, compared with OFDM of the related art, effects of reduction in transmission waiting time and header, and the like are expected. In the standardization of IEEE 802.11ax an environment in which wireless LAN devices are located overcrowdedly is assumed. Therefore, it is expected that an effect of multiplex access using DL-OFDMA is conspicuous. 
     In application of DL-OFDMA to wireless LAN systems, the configuration of a transmission frame has been an issue. Data addressed to a terminal apparatus may be different in size from data addressed to another terminal apparatus. A transmission frame in DL-OFDMA has to be constructed in accordance with data of the maximum size. Therefore, the sizes, which are other than the maximum size, of pieces of data addressed to terminal apparatuses need to match the size, which is the maximum, of data addressed to a terminal apparatus. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2014-212579 
     Non Patent Literature 
     NPL 1: IEEE 802.11-14/1209r1 Multiple RF Operation for 802.11ax OFDMA 
     SUMMARY OF INVENTION 
     Technical Problem 
     In NPL 1, a method of constructing a transmission frame in DL-OFDMA has been proposed. In the proposed method, padding is performed on pieces of data that have sizes other than the maximum size and that are addressed to terminal apparatuses. Padding causes the sizes of pieces of data addressed to the terminal apparatuses to be apparently equal to one another. Therefore, DL-OFDMA transmission frames may be constructed. However, the method described in NPL 1 causes redundant areas produced through padding to be set, resulting in concern about degradation in frequency efficiency. 
     In NPL 1, as a second method, a system in which, when the sizes of pieces of data addressed to terminal apparatuses are different from one another, a timing of transmission of an Ack that is an acknowledgement from a terminal apparatus is changed for the terminal apparatus has been proposed. In the method described in NPL 1, since setting of a redundant area as in padding is not performed, degradation in frequency efficiency may be avoided. However, in the method described in NPL 1, a base station needs to perform a reception operation in a channel adjacent to a channel in which the base station apparatus performs a transmission operation, resulting in concern about an adverse effect such as interference between the adjacent channels. 
     In PTL 1, a method in which data addressed to multiple terminal apparatuses is multiplexed in the time direction so that the lengths of transmission frames are adjusted has been proposed. However, in the method described in PTL 1, in resources of the same frequency, space, code, or the like, temporal multiplexing is performed. For example, as typified by wireless LAN systems, after a terminal apparatus completes adequate reception of a transmission frame, the terminal apparatus transmits an acknowledgement immediately. Therefore, terminal apparatuses need to make an arrangement about how to transmit acknowledgements. 
     The present invention is made to address such issues, and its object is to provide a base station apparatus and a terminal apparatus that efficiently use radio resources and that may construct preferable transmission frames which enable a transmission time of the frames to be reduced, in a transmission system in which data addressed to multiple terminal apparatuses is multiplexed. 
     Solution to Problem 
     To attain the above-described object, the present invention takes the following measures. That is, a base station apparatus of the present invention includes a base station apparatus transmitting a transmission frame to a terminal apparatus by using at least one of radio resources. The base station apparatus includes a physical-layer frame generating unit and a radio transmission unit. The physical-layer frame generating unit divides a transmission frame addressed to the terminal apparatus into a plurality of transmission frames, and generates physical layer frames in such a manner that the transmission frames obtained through division are transmitted in a plurality of radio resources. The radio transmission unit transmits the generated physical layer frames to the terminal apparatus in the plurality of radio resources. 
     Thus, a transmission frame addressed to a terminal apparatus is divided into multiple transmission frames. Physical layer frames are generated so that the transmission frames obtained through the division are transmitted in multiple radio resources. Therefore, the radio resources may be efficiently used, and a transmission time of the transmission frame may be reduced. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the present invention, radio resources may be efficiently used, and a transmission time of a transmission frame may be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an exemplary management range  3101  of a wireless communication system according to the present embodiment. 
         FIG. 2  is a diagram illustrating an exemplary apparatus configuration of a base station apparatus  1101 . 
         FIG. 3  is a diagram illustrating an exemplary apparatus configuration of a terminal apparatus  2100 . 
         FIG. 4  is a diagram illustrating exemplary subchannels allocated on the frequency axis. 
         FIG. 5  is a diagram illustrating exemplary DL-MU transmission performed when a frame-length adjusting unit  11013   b  does not operate (when a physical-layer frame generating unit  11013   a  is connected to a radio transmission unit  11013   c ). 
         FIG. 6  is a diagram illustrating exemplary DL-MU transmission performed when the frame-length adjusting unit  11013   b  operates. 
         FIG. 7  is a diagram illustrating other exemplary DL-MU transmission performed when the frame-length adjusting unit  11013   b  operates. 
         FIG. 8  is a diagram illustrating exemplary first resource allocation information used in the case of the example in  FIG. 6 . 
         FIG. 9  is a diagram illustrating exemplary DL-MU transmission performed when the frame-length adjusting unit  11013   b  operates. 
         FIG. 10  is a diagram illustrating an exemplary management range  3201  of a wireless communication system according to the present embodiment. 
         FIG. 11  is a diagram illustrating an exemplary apparatus configuration of a base station apparatus  1201 . 
         FIG. 12  is a diagram illustrating an exemplary apparatus configuration of a terminal apparatus  2200 . 
         FIG. 13  is a diagram illustrating exemplary UL-MU transmission performed when a frame-length adjusting unit  12012  operates. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A communication system according to the present embodiment includes a radio transmitting apparatus (access point, base station apparatus: Access point, base station apparatus), and multiple radio receiving apparatuses (stations, terminal apparatuses: Stations, terminal apparatuses). A network including a base station apparatus and terminal apparatuses is designated as a basic service set (BSS: Basic service set, management range). Base station apparatuses and terminal apparatuses are also collectively designated as wireless LAN apparatuses. 
     A base station apparatus and terminal apparatuses in a BSS communicate with one another on the basis of CSMA/CA (Carrier sense multiple access with collision avoidance). In the present embodiment, the infrastructure mode in which a base station apparatus communicates with multiple terminal apparatuses is used. However, the method of the present embodiment may be also performed in the ad hoc mode in which terminal apparatuses directly communicate with one another. In the ad hoc mode, a terminal apparatus serves as a base station apparatus, and forms a BSS. A BSS in the ad hoc mode is also designated as an IBSS (Independent Basic Service Set). In the description below, a terminal apparatus forming an IBSS in the ad hoc mode is regarded as a base station apparatus. 
     In an IEEE 802.11 system, each apparatus is capable of transmitting transmission frames that are of multiple frame types and that have a common frame format. A transmission frame is defined in each of the physical (Physical: PHY) layer, the medium access control (Medium access control: MAC) layer, and the logical link control (LLC: Logical Link Control) layer. 
     A transmission frame in the PHY layer is designated as a physical protocol data unit (PPDU: PHY protocol data unit, physical layer frame). A PPDU includes a physical layer header (PHY header) including header information for performing signal processing in the physical layer, and a physical service data unit (PSDU: PHY service data unit, MAC layer frame) that is a data unit processed in the physical layer. A PSDU may include an aggregated MPDU (A-MPDU: Aggregated MPDU) in which multiple MAC protocol data units (MPDUs: MAC protocol data units) that serve as a retransmission unit in a radio section are aggregated. 
     A PHY header includes reference signals, such as a short training field (STF: Short training field) used for detection, synchronization, and the like of signals, and a long fining field (LTF: Long training field) used to obtain channel information for data demodulation, and a control signal, such as a signal (Signal: SIG) including control information for data demodulation. An STF is classified according to a corresponding standard into legacy-STF (L-STF: Legacy-STF), high throughput-STF (HT-STF: High throughput-STF), very high throughput-STF (VHT-STF: Very high throughput-STF), and the like. Similarly, an LTF and a SIG are classified into L-LTF, HT-LTF, VHT-LTF, L-SIG, HT-SIG, and VHT-SIG. VHT-SIG is further classified into VHT-SIG-A and VHT-SIG-B. 
     A PPDU is modulated according to a corresponding standard. For example, according to the IEEE 802.11n standard, a PPDU is modulated into an orthogonal frequency division multiplexing (OFDM: Orthogonal frequency division multiplexing) signal. 
     An MPDU includes a MAC layer header (MAC header) including header information for performing signal processing in the MAC layer, a MAC service data unit (MSDU: MAC service data unit) or frame body that is a data unit processed in the MAC layer, and a frame check unit (Frame check sequence: FCS) for checking if the frame has an error. Multiple MSDUs may be aggregated as an aggregated MSDU (A-MSDU: Aggregated MSDU). 
     The frame types of a transmission frame in the MAC layer are broadly classified into three types: a management frame for managing an association state and the like between apparatuses; a control frame for managing a communication state between apparatuses; and a data frame including actual transmission data. Each of the three types is further classified into multiple subframe types. A control frame includes an acknowledge (Ack: Acknowledge) frame, a request-to-send (RTS: Request to send) frame, and a clear-to-send (CTS: Clear to send) frame. A management frame includes a beacon (Beacon) frame, a probe request (Probe request) frame, a probe response (Probe response) frame, an authentication (Authentication) frame, an association request (Association request) frame, and an association response (Association response) frame. A data frame includes a data (Data) frame and a polling (CF-poll) frame. Each apparatus may grasp the frame type and the subframe type of a received frame by reading information in a frame control field included in the MAC header. 
     An Ack may include a Block Ack. A Block Ack may be used to transmit an acknowledgement for multiple MPDUs. 
     A beacon frame includes a field (Field) for describing an interval (Beacon interval) in which a beacon is transmitted, and a field (Field) for describing information (SSID: Service set identifier and the like) for identifying a base station apparatus. A base station apparatus is capable of broadcasting a beacon frame periodically to a BSS. A terminal apparatus is capable of grasping a base station apparatus around the terminal apparatus by receiving a beacon frame. An operation in which a terminal apparatus grasps a base station apparatus on the basis of a beacon frame broadcasted by the base station apparatus is designated as passive scanning (Passive scanning). In contrast, an operation in which a terminal apparatus searches for a base station apparatus by broadcasting a probe request frame in a BSS is designated as active scanning (Active scanning). A base station apparatus is capable of transmitting a probe response frame as a response to the probe request frame, and information described in the probe response frame is equivalent to information in a beacon frame. 
     After a terminal apparatus recognizes a base station apparatus, the terminal apparatus performs an association process on the base station apparatus. The association process is classified into an authentication (Authentication) procedure and an association (Association) procedure. The terminal apparatus transmits an authentication frame (authentication request) to the base station apparatus with which establishment of association is to be made. When the base station apparatus receives the authentication frame, the base station apparatus transmits, to the terminal apparatus, an authentication frame (authentication response) including a status code indicating whether or not authentication of the terminal apparatus has been successfully performed. The terminal apparatus reads the status code described in the authentication frame so as to determine whether or not the base station apparatus has given permission for authentication of the terminal apparatus. The base station apparatus and the terminal apparatus are capable of receiving/transmitting multiple authentication frames. 
     Subsequent to the authentication procedure, the terminal apparatus transmits an association request frame in order to perform the association procedure on the base station apparatus. When the base station apparatus receives the association request frame, the base station apparatus determines whether or not association with the terminal apparatus is to be permitted, and transmits an association response frame in order to notify the determination result. In the association response frame, an association identification number (AID: Association identifier) for identifying the terminal apparatus is described in addition to a status code indicating whether or not the association process has been permitted. The base station apparatus is capable of managing multiple terminal apparatuses by setting different AIDS to the respective terminal apparatuses which have been given association permission by the base station apparatus. 
     After the association process, the base station apparatus and the terminal apparatus perform actual data transmission. In an IEEE 802.11 system, distributed coordination function (DCF: Distributed Coordination Function), point coordination function (PCF: Point Coordination Function), and functions (enhanced distributed channel access (EDCA: Enhanced distributed channel access), hybrid coordination function (HCF: Hybrid coordination function), and the like) obtained by enhancing these are defined. A description will be made below by taking, as an example, a case in which a base station apparatus transmits signals to terminal apparatuses in DCF. 
     In DCF, prior to communication, a base station apparatus and a terminal apparatus perform carrier sensing (CS: Carrier sense) for checking the usage of a radio channel around the base station apparatus and the terminal apparatus. For example, when the base station apparatus that serves as a transmitting station receives a signal higher than a predetermined clear channel assessment level (CCA level: Clear channel assessment level) in the radio channel, the base station apparatus postpones transmission of a transmission frame in the radio channel. Hereinafter, a state in which a signal of the CCA level or larger is detected in the radio channel is designated as the busy (Busy) state. A state in which a signal of the CCA level or larger is not detected is designated as the idle (Idle) state. Thus, CS performed on the basis of the power (received power level) of a signal that is actually received by each apparatus is designated as physical carrier sensing (physical CS). The CCA level is also designated as a carrier sense level (CS level) or a CCA threshold (CCA threshold: CCAT). When the base station apparatus and the terminal apparatus detect a signal of the CCA level or larger, the base station apparatus and the terminal apparatus start demodulating at least a signal in the PHY layer. 
     The base station apparatus performs carrier sensing during an inter frame space (IFS: Inter frame space) according to the type of a transmission frame that is to be transmitted, and determines whether the radio channel is in the busy state or the idle state. A period in which the base station apparatus performs carrier sensing is different depending on the frame type and the subframe type of a transmission frame that is to be transmitted by the base station apparatus. In an IEEE 802.11 system, multiple IFSs having different periods are defined. The defined IFSs are a short inter frame space (SIFS: Short IFS) used for a transmission frame given the highest priority, a polling inter frame space (PCF IFS: PIFS) used for a transmission frame given relatively high priority, a distributed-coordination inter frame space (DCF IFS: DIFS) used for a transmission frame given the lowest priority, and the like. When a base station apparatus transmits a data frame in DCF, the base station apparatus uses the DIFS. 
     After waiting lust for a DIFS, the base station apparatus further waits just for a random backoff time for preventing frame collision. In an IEEE 802.11 system, a random backoff time designated as a contention window (CW: Contention window) is used. In CSMA/CA, as a precondition, a transmission frame transmitted by a certain transmitting station is received by a receiving station without interference from a different transmitting station. Therefore, when transmitting stations transmit transmission frames at the same timing, the frames collide with each other, resulting in a state in which the receiving station fails to successfully receive the frames. Accordingly, before start of transmission, each transmitting station waits just for a time that is randomly set, so that frame collision is avoided. When the base station apparatus determines, through carrier sense, that a radio channel is in the idle state, the base station apparatus starts countdown of a CW. When the CW becomes zero, the base station apparatus then obtains transmission right, and may transmit a transmission frame to the terminal apparatus. In countdown of a CW, when the base station apparatus determines that the radio channel is in the busy state through carrier sense, the base station apparatus stops countdown of the CW. When the radio channel enters the idle state, subsequent to the above-description IFS, the base station apparatus restarts countdown of the remaining CW. 
     The terminal apparatus that serves as a receiving station receives the transmission frame, reads the PHY header of the transmission frame, and demodulates the received transmission frame. The terminal apparatus may recognize whether or not the transmission frame is addressed to the terminal apparatus, by reading the MAC header of the demodulated signal. Alternatively, the terminal apparatus may determine the destination of the transmission frame on the basis of information described in the PHY header (for example, the group identification number (GID: Group identifier) described in a VHT-SIG-A). 
     In the case where the terminal apparatus determines that the received transmission frame is addressed to the terminal apparatus and where the transmission frame has been demodulated without an error, the terminal apparatus has to transmit an ACK frame that indicates that the frame has been successfully received, to the base station apparatus that is a transmitting station. The ACK frame is one of the transmission frames of the highest priority which are transmitted after waiting only for an SIFS period (without waiting for a random backoff time). Upon reception of the ACK frame transmitted from the terminal apparatus, the base station apparatus ends a series of communication processes. When the terminal apparatus fails to successfully receive the frame, the terminal apparatus does not transmit an ACK. Therefore, after the base station apparatus transmits the frame, when the base station apparatus has not received an ACK frame from a receiving station for a certain period (an SIFS+the length of an ACK frame), the base station apparatus regards the communication as a failure, and ends the communication. Thus, end of one communication operation (also designated as a burst) in an IEEE 802.11 system is always determined by determining whether or not an ACK frame has been received, except for special cases, such as a case of transmission of a broadcast signal such as a beacon frame and a case of use of fragmentation for dividing transmission data. 
     When the terminal apparatus determines that the received transmission frame is not addressed to the terminal apparatus, the terminal apparatus sets a network allocation vector (NAV: Network allocation vector) on the basis of the length (Length) of the transmission frame which is described in the PHY header or the like. The terminal apparatus does not try to communicate for a period that is set in the NAV. That is, the terminal apparatus performs the same operation as in the case where it is determined that the radio channel is in the busy state through physical CS, for the period that is set in the NAV. Therefore, the communication control using an NAV is also designated as virtual carrier sense (virtual CS). In addition to the case in which an NAV is set on the basis of information described in the PHY header, the NAV is also set by using a request-to-send (RTS: Request to send) frame or a clear-to-send (CTS: Clear to send) frame which is introduced to solve the hidden node problem. 
     In DCF, each apparatus performs carrier sensing, and autonomously obtains transmission right. In contrast, in PCF, control station designated as a point coordinator (PC: Point coordinator) controls transmission right of each apparatus in a BSS. Typically, a base station apparatus serves as a PC, and obtains transmission right of the terminal apparatuses in a BSS. 
     A communication period in PCF includes a contention free period (CFP: Contention free period) and a contention period (CP: Contention period). During a CP, communication is performed on the basis of DCF described above, and a PC controls transmission right in a CFP. A base station apparatus that serves as a PC broadcast a beacon frame describing a CFP period (CFP Max duration) and the like, in a BSS prior to communication in PCF. A PIFS is used in transmission of a beacon frame broadcasted upon start of transmission in PCF, and the beacon frame is transmitted without waiting for a CW. A terminal apparatus receiving the beacon frame sets the CEP period described in the beacon frame to an NAV. After that, until the NAV has elapsed or a signal (for example, a data frame including CF-end) for broadcasting end of the CFP in the BSS is received, only when the terminal apparatus receives a signal (for example, a data frame including CF-poll) for signaling acquisition of transmission right which is transmitted from the PC, the terminal apparatus may obtain transmission right. In a CFP period, packet collision does not occur in the same BSS. Therefore, each terminal apparatus does not wait for a random backoff time used in DCF. 
     First Embodiment 
       FIG. 1  is a diagram illustrating an exemplary management range  3101  of a wireless communication system according to the present embodiment. The management range  3101  includes a base station apparatus  1101  and terminal apparatuses  2101  to  2104 . The example in  FIG. 1  includes four terminal apparatuses. The method according to the present embodiment may be implemented as long as the management range  3101  includes two or more terminal apparatuses. Hereinafter, the terminal apparatuses  2101  to  2104  are also referred to as terminal apparatuses  2100 . The base station apparatus  1101  may perform multi-user transmission (Multi-user Transmission) on multiple terminal apparatuses  2100 . Examples of multi-user transmission include OFDMA (Orthogonal Frequency Division Multiple Access), MU-MIMO (Multi User Multiple Input Multiple Output), and CDMA (Code Division Multiple Access). A description will be made below under the assumption that the base station apparatus  1101  performs OFDMA. The present invention may employ another multi-user transmission scheme. Each of the terminal apparatuses  2100  is capable of receiving a transmission frame for multi-user transmission (hereinafter referred to as an “MU frame”) which is generated by the base station apparatus  1101 . The terminal apparatus  2100  has a function of selecting data addressed to the terminal apparatus  2100  from the received MU frame. A method in which the terminal apparatus  2100  obtains information that is used to select data addressed to the terminal apparatus  2100  from a MU frame and that is about where the data addressed to the terminal apparatus  2100  is located in the MU frame will be described below. 
     When a terminal apparatus  2100  successfully receives data transmitted by the base station apparatus  1101 , the terminal apparatus  2100  transmits an ACK frame addressed to the base station apparatus  1101 . The base station apparatus  1101  receives the ACK frame transmitted by the terminal apparatus  2100 , and thereby recognizes completion of transmission of the data. 
     Hereinafter, an operation in which the base station apparatus  1101  performs multi-user transmission to the multiple terminal apparatuses  2100  is referred to as DL-MU transmission. In DL-MU transmission, the multiple terminal apparatuses  2100  prepare transmission of ACK frames. The method in which the multiple terminal apparatuses  2100  transmit ACK frames addressed to the base station apparatus  1101  at the same time is referred to as UL-MU transmission. 
     The UL-MU transmission is not limited to the above-described method. An operation in which the multiple terminal apparatuses  2100  transmit their respective transmission frames in a certain radio resource in a multiplex way and in which the base station apparatus  1101  receives the multiplexed transmission frame may be referred to as UL-MU transmission. 
     The base station apparatus  1101  has a function of DL-MU transmission. In the present embodiment, the lengths of the physical layer headers for the terminal apparatuses  2100  may be different from one another. For example, in the specification of IEEE 802.11, the length of a physical layer header may depend on the number of transmission streams. The present invention may be carried out even when the lengths of physical layer headers for the terminal apparatuses  2100  are different from one another. 
       FIG. 2  is a diagram illustrating an exemplary apparatus configuration of the base station apparatus  1101 . The base station apparatus  1101  has a configuration including a higher layer unit  11011 , a carrier sensing unit  11012 , a transmission unit  11013 , a receiving unit  11014 , and an antenna unit  11015 . 
     The higher layer unit  11011  is connected to other networks, and has a function of notifying the carrier sensing unit  11012  of information associated with a transmission frame. A description will be made below under the assumption that a transmission frame is defined in the MAC layer. Alternatively, a transmission frame according to the present embodiment may be defined in the LLC layer, the physical layer, or a higher layer. 
     The carrier sensing unit  11012  has a function of making a determination about whether transmission is to be performed, on the basis of carrier sense. In the present embodiment, when the base station apparatus  1101  performs OFDMA transmission, the carrier sensing unit  11012  may perform carrier sensing on multiple channels. A method of performing carrier sensing on multiple channels and a method of OFDMA transmission will be described below. 
     The transmission unit  11013  includes a physical-layer frame generating unit  11013   a,  a frame-length adjusting unit  11013   b,  and a radio transmission unit  11013   c.  The physical-layer frame generating unit  11013   a  has a function of generating a physical layer frame from a transmission frame transmitted from the carrier sensing unit  11012 . The physical-layer frame generating unit  11013   a  performs error correction coding, modulation, precoding-filter multiplication, and the like on the transmission frame. The physical-laver frame generating unlit  11013   a  notifies the frame-length adjusting unit  11013   b  of the generated physical layer frame. 
     The frame-length adjusting unit  11013   b  has a function of generating an MU frame suitable for DL-MU transmission. Operations of the frame-length adjusting unit  11013   b  will be described in detail below. 
     The radio transmission unit  11013   c  converts the MU frame generated by the frame-length adjusting unit  11013   b  into a signal in a radio frequency (RF: Radio Frequency) band, and generates a radio frequency signal. The processes performed by the radio transmission unit  11013   c  include digital-analog conversion, filtering, and frequency conversion from a base band to an RF band. 
     The receiving unit  11014  includes a radio receiving unit  11014   a  and a signal demodulating unit  11014   b.  The receiving unit  11014  has a function of calculating a received power level from an RF band signal received by the antenna unit  11015 . However, the method of calculating a received power level is not limiting. The receiving unit  11014  notifies the carrier sensing unit  11012  of information about the calculated received power level. The carrier sensing unit  11012  may make a determination about whether transmission is to be performed, on the basis of the information about a received power level which is transmitted by the receiving unit  11014 . 
     The radio receiving unit  11014   a  has a function of converting an RF band signal received by the antenna unit  11015  into a base band signal and generating a physical layer signal (for example, a physical layer frame). The processes performed by the radio receiving unit  11014   a  include a frequency conversion process from an RF band to a base band, filtering, and analog-digital conversion. 
     The signal demodulating unit  11014   b  has a function of demodulating the physical layer signal generated by the radio receiving unit  11014   a.  The processes performed by the signal demodulating unit  11014   b  include channel equalization, de-mapping, and error correction decoding. The signal demodulating unit  11014   b  may extract, for example, information included in the physical layer header, information included in the MAC header, and information included in the transmission frame from the physical layer signal. The signal demodulating unit  11014   b  may notify the higher layer unit  11011  of the extracted information. The signal demodulating unit  11014   b  may extract one or some of the information included in the physical layer header, the information included in the MAC header, and the information included in the transmission frame. 
     The antenna unit  11015  has a function of transmitting a radio frequency signal generated by the radio transmission unit  11013   c  through a wireless space to the terminal apparatuses  2100 . The antenna unit  11015  has a function of receiving radio frequency signals transmitted from the terminal apparatuses  2100 . When the base station apparatus  1101  performs carrier sensing, the antenna unit  11015  has a function of receiving a signal in the channel in the wireless space. 
       FIG. 3  is a diagram illustrating an exemplary apparatus configuration of a terminal apparatus  2100 . The terminal apparatus  2100  includes a higher layer unit  21001 , a carrier sensing unit  21002 , a transmission unit  21003 , a receiving unit  21004 , and an antenna unit  21005 . 
     The higher layer unit  21001  is connected to other networks, and has a function of notifying the carrier sensing unit  21002  of information associated with a transmission frame. 
     The carrier sensing unit  21002  has a function of making a determination about whether transmission is to be performed, on the basis of carrier sense. The transmission unit  21003  includes a physical-layer frame generating unit  21003   a  and a radio transmission unit  21003   b.    
     The physical-layer frame generating unit  21003   a  has a function of generating a physical layer frame from a transmission frame transmitted from the carrier sensing unit  21002 . The physical-layer frame generating unit  21003   a  performs error correction coding, modulation, precoding-filter multiplication, and the like on the transmission frame. The physical-layer frame generating unit  21003   a  notifies the radio transmission unit  21003   b  of the generated physical layer frame. 
     The radio transmission unit  21003   b  converts the physical layer frame generated by the physical-layer frame generating unit  21003   a  into a signal in a radio frequency (RF: Radio Frequency) band, and generates a radio frequency signal. The processes performed by the radio transmission unit  21003   b  include digital-analog conversion, filtering, and frequency conversion from a base band to an RF band. 
     The receiving unit  21004  includes a radio receiving unit  21004   a  and a signal demodulating unit  21004   b.  The receiving unit  21004  has a function of calculating a received power level from an RF band signal received by the antenna unit  21005 . However, the method of calculating a received power level is not limiting. The receiving unit  21004  notifies the carrier sensing unit  21002  of information about the calculated received power level. The carrier sensing unit  21002  may make a determination about whether transmission is to be performed, on the basis of the information about a received power level which is transmitted by the receiving unit  21004 . 
     The radio receiving unit  21004   a  has a function of converting an RF band signal received by the antenna unit  21005  into a base band signal and generating a physical layer signal (for example, a physical layer frame, an MU frame, and the like). The processes performed by the radio receiving unit  21004   a  include a frequency conversion process from an RF band to a base band, filtering, and analog-digital conversion. 
     The signal demodulating unit  21004   b  has a function of demodulating the physical layer signal generated by the radio receiving unit  21004   a.  The processes performed by the signal demodulating unit  21004   b  include channel equalization, de-mapping, and error correction decoding. The signal demodulating unit  21004   b  may extract, for example, information included in the physical layer header, information included in the MAC header, and information included in the transmission frame from the physical layer signal. The signal demodulating unit  21004   b  may notify the higher layer unit  21001  of the extracted information. The signal demodulating unit  21004   b  may extract one or some of the information included in the physical layer header, the information included in the MAC header, and the information included in the transmission frame. 
     The signal demodulating unit  21004   b  has a function of demodulating an MU frame transmitted by the base station apparatus  1101 . A method of demodulating an MU frame will he described below. The antenna unit  21005  has a function of transmitting a radio frequency signal generated by the radio transmission unit  21003   b  through a wireless space to the base station apparatus  1101 . The antenna unit  21005  has a function of receiving a radio frequency signal transmitted from the base station apparatus  1100 . When the terminal apparatus  2100  performs carrier sensing, the antenna unit  21005  has a function of receiving a signal in the channel in the wireless space. 
       FIG. 4  is a diagram illustrating exemplary subchannels allocated on the frequency axis.  FIG. 4  illustrates an example in which subchannels  401  to  404  are allocated on the frequency axis. The subchannels  401  to  404  are collectively referred to as subchannels  400 . In OFDMA transmission, different terminal apparatuses  2100  are assigned to the respective subchannels  400 , achieving DL-MU transmission. 
     The IEEE 802.11 standard supports multiple 20-MHz channels. For example, the subchannels  400  may correspond to the respective 20-MHz channels supported by the IEEE 802.11 standard. In this example, this corresponds to a state in which the base station apparatus  1101  assigns the different terminal apparatuses  2100  to the respective 20-MHz channels. In this case, it is preferable that the base station apparatus  1101  make a determination about whether transmission is to be performed, on the basis of carrier sense on each 20-MHz channel. The method in which the base station apparatus  1101  makes determinations about whether transmission is to be performed, on the basis of carrier sense on multiple 20-MHz channels is not limiting. After the base station apparatus  1101  calculates a received power level individually on each subchannel  400 , the base station apparatus  1101  may perform carrier sensing, or may perform carrier sensing on the basis of the average received power level obtained by averaging the received power levels of all of the subchannels  400 . 
     For example, the base station apparatus  1101  may divide a 20-MHz channel supported by the IEEE 802.11 standard, and may assign the terminal apparatuses  2100  to the respective subchannels  400 . In this example, each of the subchannels  400  has a band width of 20 MHz/4=5 MHz. In this case, after the base station apparatus  1101  calculates a received power level only for a single 20-MHz channel, the base station apparatus  1101  may perform carrier sensing, or may perform carrier sensing for every 5 MHz. The base station apparatus  1101  may divide a 20-MHz channel into units having a band width other than 5 MHz, or does not necessarily divide a 20-MHz channel evenly. 
     The method in which the base station apparatus  1101  assigns the subchannels to the terminal apparatuses  2100  is not limited to the above-described method. For example, two or more certain subchannels  400  may be assigned to the same terminal apparatus  2100 . For example, the base station apparatus  1101  may assign the subchannels  401  to  402  to the terminal apparatus  2101 , the subchannel  403  to the terminal apparatus  2102 , and the subchannel  404  to the terminal apparatus  2103 . 
       FIG. 5  is a diagram illustrating exemplary DL-MU transmission performed when the frame-length adjusting unit  11013   b  operates so as to connect the physical-layer frame generating unit  11013   a  to the radio transmission unit  11013   c  (or when the physical-layer frame generating unit  11013   a  is equivalently connected to the radio transmission unit  11013   c ). As illustrated in the example in  FIG. 5 , when the sizes of PPDUs  411  to  414  (hereinafter also referred to as “PPDUs  410 ” collectively) transmitted to the terminal apparatuses are different from one another, there arises a problem of transmission timing of Acks. The terminal apparatuses  2100  have to transmit Acks  421  to  424  (hereinafter also referred to as “Acks  420 ” collectively) after the base station apparatus  1101  completes the DL-MU transmission. In the example in  FIG. 5 , the DL-MU transmission period of the base station apparatus  1101  is until completion of transmission of the PPDU  411 . Therefore, after completion of transmission of the PPDU  411 , the terminal apparatuses  2100  wait for a given period (for example, an SIFS period). Then, the terminal apparatuses  2100  transmit the Acks  420 . Therefore, it is preferable that the base station apparatus  1101  adjust the lengths of the DL-MU frames in order that the frequency efficiency is improved. One or all of the PPDUs  410  may constitute an A-MPDU. 
     The method of transmitting the Acks  420  which is illustrated in  FIG. 5  corresponds to UL-MU transmission. The method of transmitting Acks which is performed by the terminal apparatuses  2100  according to the present embodiment is not necessarily UL-MU transmission. For example, the Acks  420  may be transmitted in different time slots (time division transmission). 
       FIG. 6  is a diagram illustrating exemplary DL-MU transmission performed when the frame-length adjusting unit  11013   b  adjusts DL-MU frames. The frame-length adjusting unit  11013   b  generates PPDUs  431  to  435  (hereinafter also referred to as “PPDUs  430 ” collectively). For example, the PPDU  431  and the PPDU  435  are two PPDUs for the same terminal apparatus  2100 . In this example, the subchannel  402  includes PPDUs for two different terminal apparatuses in the DL-MU transmission period. In the example in  FIG. 6 , it is expected that the frame-length adjusting function of the frame-length adjusting unit will cause a DL-MU transmission period of the base station apparatus  1101  to be reduced. Therefore, the terminal apparatuses  2100  may transmit Acks at a timing earlier than the timing in the example in  FIG. 5 . The PPDU  435  may have a configuration that does not include a part or all of the physical layer header. 
     As illustrated in  FIG. 6 , the base station apparatus  1101  may set a waiting time (for example, an SIFS, a PIFS, an RIFS, a DIFS, an AIFS, or another waiting time) between the PPDU  432  and the PPDU  435 , or may continuously transmit the PPDU  432  and the PPDU  435  without waiting for a waiting time. 
     The present invention may be described as follows. The base station apparatus  1101  transmits the PPDU  431  addressed to one of the terminal apparatuses  2100  (for example, the terminal apparatus  2101 ) in a first radio resource (for example, the subchannel  401 ). A section in which the PPDU  431  is transmitted is also referred to as a first frame section. The base station apparatus  1101  transmits the PPDU  435  addressed to the terminal apparatus  2101  by using a second radio resource (for example, the subchannel  402 ). A section in which the PPDU  435  is transmitted is also referred to as a second frame section. 
     The base station apparatus  1101  transmits a physical layer frame including the first frame section by using the first radio resource, and transmits a physical layer frame including the second frame section by using the second radio resource, achieving reduction of a DL-MU frame. Preferably, in order to adequately receive physical layer frames including the first frame section and the second frame section, the terminal apparatus  2101  has some or all of information about the first radio resource, information about the second radio resource, information about the first frame section, and information about the second frame section. That is, the base station apparatus  1101  may transmit, to the terminal apparatus  2101 , some or all of the information about the first radio resource, the information about the second radio resource, the information about the first frame section, and the information about the second frame section. Further, the base station apparatus  1101  may also transmit a physical layer frame including a third frame section by using a third radio resource. That is, the base station apparatus  1101  may transmit physical layer frames including multiple frame sections by using multiple radio resources. The base station apparatus  1101  has a function of performing DL-MU transmission by using multiple radio resources. 
       FIG. 7  is a diagram illustrating other exemplary DL-MU transmission performed when the frame-length adjusting unit  11013   b  operates. The frame-length adjusting unit  11013   b  generates PPDUs  451  to  455  (hereinafter also referred to as “PPDUs  450 ”). For example, the PPDU  451  and the PPDU  455  are two PPDUs for the same terminal apparatus  2100 . The PPDU  455  is a channel aggregated PPDU generated by aggregating the subchannels  401  to  402  (Channel Aggregation). in the example in  FIG. 7 , it is expected that the frame-length adjusting function of the frame-length adjusting unit will cause a DL-MU transmission period of the base station apparatus  1101  to be reduced. Therefore, the terminal apparatuses  2100  may transmit Acks at a timing earlier than the timing in the example illustrated in  FIG. 5 . 
     The examples in  FIGS. 6 and 7  indicate that each of the PPDU  431  and the PPDU  451  having a large PPDU length is offloaded to the subchannel  402  including a respective one of the PPDU  432  and the PPDU  452  having a small PPDU length, enabling a DL-MU transmission period to he reduced. 
     A terminal apparatus  2100  may transmit an Ack in a subchannel in which a PPDU addressed to the terminal apparatus  2100  is received. For example, in the example in  FIG. 6 , it is preferable that a terminal apparatus  2100  receiving the PPDU  431  and the PPDU  435  transmit an Ack  441  to the base station apparatus  1101 . 
     In the example in  FIG. 7 , a terminal apparatus  2100  having received the PPDU  451  and the PPDU  455  may complete the reception operation by notifying the base station apparatus  1101  of an Ack  461 . 
     The method of transmitting an Ack is not particularly limiting in the present embodiment. For example, a terminal apparatus  2100  may transmit an Ack in one of subchannels  400  in which PPDUs just after physical layer headers are received. 
     Another method in the example in  FIG. 7  will be described. For example, a terminal apparatus  2100  receiving the PPDU  451  and the PPDU  455  may be instructed to perform a receiving operation on the subchannel  401  and the subchannel  402  in a DL-MU transmission period. The terminal apparatus  2100  may extract only PPDUs addressed to the terminal apparatus  2100  on the basis of first resource allocation information. 
     The present invention may be interpreted as follows. The base station apparatus  1101  transmits the PPDU  451  addressed to one of the terminal apparatuses  2100  (for example, the terminal apparatus  2101 ) in a first radio resource (for example, the subchannel  401 ). A section in which the PPDU  451  is transmitted is also referred to as a first frame section. The base station apparatus  1101  transmits the PPDU  455  addressed to the terminal apparatus  2101  by using a second radio resource (for example, the subchannel  402 ). A section in which the PPDU  455  is transmitted is also referred to as a second frame section. 
     The base station apparatus  1101  transmits a physical layer frame including the first frame section by using the first radio resource, and transmits a physical layer frame including the second frame section by using the second radio resource, achieving reduction of a DL-MU frame. It is preferable that, in order to adequately receive physical layer frames including the first frame section and the second frame section, the terminal apparatus  2101  have all or some of the information about the first radio resource, the information about the second radio resource, the information about the first frame section, and the information about the second frame section. That is, the base station apparatus  1101  may transmit, to the terminal apparatus  2101 , all or some of the information about the first radio resource, the information about the second radio resource, the information about the first frame section, and the information about the second frame section. 
     As illustrated in  FIG. 7 , the base station apparatus  1101  may set a waiting time (for example, an SIFS, a PIFS, an RIFS, a DIFS, an AIFS, or another waiting time) between the PPDU  451  and the PPDU  455  and between the PPDU  452  and the PPDU  455 , or may continuously transmit the PPDU  451  and the PPDU  455 , and the PPDU  452  and the PPDU  455  without a waiting time. 
     As described above, it is preferable that the terminal apparatuses  2100  have information about allocation of radio resources (for example, the information about the first radio resource and the information about the second radio resource), and information about adjustment of frame lengths performed by the frame-length adjusting unit  11013   b  (for example, the information about the first frame section and the information about the second frame section). The base station apparatus  1101  may generate the information about adjustment of frame lengths. The base station apparatus  1101  generates information about allocation of the radio resources of the PPDUs  430  or the PPDUs  450  on the basis of the PPDUs  430  or the PPDUs  450  generated by the frame-length adjusting unit  11013   b.  Preferably, the base station apparatus  1101  notifies the terminal apparatuses  2100  of the first resource allocation information. The first resource allocation information may include all or part of the information about allocation of the radio resources and the information about adjustment of frame lengths. 
     The base station apparatus  1101  may also generate two or more pieces of information about a frame section. The frame-length adjusting unit  11013   b  may also define a DL-MU frame length by using the two or more pieces of information about a frame section. 
       FIG. 8  is a diagram illustrating exemplary first resource allocation information in the example in  FIG. 6 . In  FIG. 8 , PPDUs  431   a  to  435   a  (hereinafter also referred to as “PPDUs  430   a ”) are information elements including information about destination terminal apparatuses for the PPDUs  431  to  435 . Acks  441   a  to  444   a  (hereinafter also referred to as “Acks  440   a ”) are information about terminal apparatuses transmitting Acks  441  to  444 . For example, each of the PPDUs  430   a  and the Acks  440   a  may be a MAC address of a corresponding terminal apparatus or a GIG. Other than this, as information about a terminal apparatus, an AID (Association Identifier), a PAID (Partial AID), or the like is used. An AID is an identifier that is independently set for a connecting terminal apparatus by a base station apparatus, and has a length of 16 bit. A PAID is a reduced identifier of 9 bit obtained by performing a determined hash function on an AID. The information about a terminal apparatus may be an identifier other than the above-described examples. 
     In the example in  FIG. 8 , t represents time, and a time of t=0 corresponds to the start time of a DL-MU transmission. A time of t=t a  corresponds to a time at which the base station apparatus  1101  start transmission of the PPDU  435 . Information at a time of t=t a  is information (for example, the information about the first frame section, the information about the second frame section) about a time at which the base station apparatus  1101  changes, in the DL-MU transmission period, the destination of a transmission PPDU (in the example in  FIG. 8 , in the subchannel  402 , information about a destination terminal apparatus is changed from the PPDU  432   a  to the PPDU  435   a ). 
     A time of t=t ack  and the Acks  440   a  may be explicitly notified, or may be implicitly notified. For example, a terminal apparatus  2100  may regard a time of t=t ack  as a time at which the terminal apparatus  2100  completes reception of PPDUs addressed to the terminal apparatus  2100 . In this case, a terminal apparatus  2100  corresponding to the PPDU  432   a  may erroneously set a time of t=t ack . For example, even after a terminal apparatus  2100  has received a PPDU addressed to the terminal apparatus  2100 , when the terminal apparatus  2100  continues to receive a signal having a high received power level, a time at which reception of the signal having a high received power level is completed may be set as a time of t=t ack . For example, the base station apparatus  1101  may insert information about a DL-MU transmission period in the MAC headers in the PPDUs  430 . 
     Information about the Acks  440   a  transmitted by the terminal apparatuses  2100  may be explicitly notified to the terminal apparatuses  2100  by the base station apparatus  1101 , or may be implicitly notified. It may be understood that, as an exemplary method of implicit notification, for example, a PPDU  430   a  at a time of t=0 and the corresponding Ack  440   a  indicate the same terminal apparatus  2100 . That is, each of the terminal apparatuses  2100  may transmit an Ack by using a subchannel in which any of the PPDUs  430  is received at a time of t=0. 
     In the example in  FIG. 6 , a period between the PPDU  432  and the PPDU  435  may be provided or may not be provided. 
     In the example in  FIG. 7 , the PPDU  451  has a band width different from that of the PPDU  455 . Therefore, in the example in  FIG. 7 , it is preferable that the first resource allocation information include the information about the first radio resource and the information about the second radio resource. 
     As described above, the base station apparatus  1101  notifies the terminal apparatuses  2100  of the first resource allocation information. However, the first resource allocation information may not include all of the information elements illustrated in  FIG. 8 . The terminal apparatuses  2100  may implicitly obtain some of the first resource allocation information elements. 
     The base station apparatus  1101  may include the first resource allocation information in information elements of a beacon, a probe response, an authentication response, and an association response, or may include the first resource allocation information in the physical layer header, the MAC header, or an MSDU in a transmission frame. The base station apparatus  1101  may divide the first resource allocation information for transmission. 
       FIG. 9  is a diagram illustrating exemplary DL-MU transmission per when the frame-length adjusting unit  11013   b  operates. The frame-length adjusting unit generates PPDUs  471  to  479  and a PPDU  479   a  (hereinafter also referred to as “PPDUs  470 ” collectively). For example, the base station apparatus  1101  may generate the PPDU  471 , the PPDU  473 , and the PPDU  474  in accordance with the shortest PPDU  472 . After the generated PPDUs  471  to  474  are transmitted by using the respective subchannels  400 , the terminal apparatuses  2100  wait for a certain time so as not to perform transmission. Then, each of the terminal apparatuses  2100  transmits a corresponding one of Acks  481  to  484  (hereinafter also referred to as “Acks  480 ” collectively). 
     In the example in  FIG. 9 , a terminal apparatus  2100  that has received the PPDU  472  in the subchannel  402  transmits the Ack  481  in the subchannel  402 . The terminal apparatuses  2100  that have received the PPDU  471 , the PPDU  473 , and the PPDU  474  in the subchannel  401 , the subchannel  403 , and the subchannel  404  are capable of determining that PPDUs  470  addressed to the terminal apparatuses  2100  remain in the DL-MU transmission period, and not transmitting Acks. The terminal apparatuses  2100  are also capable of multiplexing the other Acks on the Ack  481  by using UL-MU transmission. 
     Subsequently, after the base station apparatus  1101  successfully receives the Ack  481 , the base station apparatus  1101  waits for a certain period (for example, an SIFS period). Then the base station apparatus  1101  may transmit PPDUs  475  to  477 . In the DCF mode, typically, in the case where a wireless LAN apparatus having received an Ack wants to transmit the next PPDU, after the wireless LAN apparatus waits for a DIFS or AIFS period, the wireless LAN apparatus has to make a transition to backoff. In the example in  FIG. 9 , after the base station apparatus  1101  receives the Ack  481 , the base station apparatus  1101  waits for an SIFS period. Then the base station apparatus  1101  transmits the PPDUs  475  to  477 . Thus, reduction of a DL-MU transmission period is expected. 
     In the example in  FIG. 9 , after the base station apparatus  1101  receives the Ack  481 , the base station apparatus  1101  uses the available subchannel  402  to transmit the PPDU  475  obtained through aggregation of the subchannels  401  to  402 . The base station apparatus  1101  generates the PPDU  475  obtained through aggregation of the subchannels  401  to  402 . Thus, improvement in frequency efficiency of the subchannel  402  is expected. 
     Subsequently, after a terminal apparatus  2100  having received the PPDU  476  waits for a certain time, the terminal apparatus  2100  transmits the Ack  482 . After receiving the Ack  482 , the base station apparatus  1101  waits for a certain time. Then, the base station apparatus  1101  transmits the PPDU  478  and the PPDU  479 . The PPDU  478  is a PPDU obtained through aggregation of the subchannels  401  to  403 . 
     In the example in  FIG. 9 , after a terminal apparatus  2100  having received she PPDU  479  waits for a certain time, the terminal apparatus  2100  transmits the Ack  483 . After receiving the Ack  483 , the base station apparatus  1101  waits for a certain time, and then transmits the PPDU  479   a.  The PPDU  479   a  is a PPDU obtained through aggregation of the subchannels  400 . 
     Finally, after a terminal apparatus  2100  having received the PPDU  479   a  waits for a certain time, the terminal apparatus  2100  transmits the Ack  484 . 
     In the example in  FIG. 9 , the terminal apparatuses  2100  do not aggregate some or all of the subchannels  400 , and transmit the Acks  480  only by using a single subchannel. The terminal apparatuses  2100  may transmit the Acks  480  obtained through aggregation of some or all of the subchannels  400 . The terminal apparatuses  2100  may notify the base station apparatus  1101  of function information about whether or not the terminal apparatuses  2100  have a function of receiving a DL-MU frame generated by the frame-length adjusting unit  11013   b.    
     As described above, the base station apparatus  1101  adjusts frame lengths in DL-MU transmission, achieving reduction of a DL-MU transmission period. Thus, frequency efficiency of the wireless communication system may be improved. 
     Second Embodiment 
       FIG. 10  is a diagram illustrating an exemplary management range  3201  of a wireless communication system according to the present embodiment. The management range  3201  includes a base station apparatus  1201  and terminal apparatuses  2201  to  2204 . In the example in  FIG. 10 , the management range  3201  includes four terminal apparatuses. However, the method according to the present embodiment may be implemented as long as the management range  3201  includes two or more terminal apparatuses  2100 . Hereinafter, the terminal apparatuses  2201  to  2204  are referred to as terminal apparatuses  2100 . 
     The wireless communication system according to the present embodiment may perform UL-MU transmission. That is, the base station apparatus  1201  may receive a frame (UL-MU frame) that is obtained through multiplexing in a radio resource in UL transmission and that is transmitted by multiple terminal apparatuses  2200 . 
     A description will be made below under the assumption that the management range  3201  performs UL-OFDMA. However, the method of the present invention is not limited to UL-OFDMA. 
     In UL-MU transmission, the base station apparatus  1201  may notify the multiple terminal apparatuses  2200  of a timing of start of UL-MU transmission. The notification of a timing of start of UL-MU transmission enables the multiple terminal apparatuses  2200  to perform transmission at the same time. However, the terminal apparatuses  2200  may be provided with different respective pieces of hardware. Therefore, a transmission time may be varied due to deviation of clock timing or the like. In order that the base station apparatus  1201  determines a timing of start of UL-MU transmission, the base station apparatus  1201  needs to grasp the number of transmission frames (or a payload, a data amount, and the like) contained in the multiple terminal apparatuses  2200 . A method in which the base station apparatus  1201  grasps the number of transmission frames contained by the multiple terminal apparatuses  2200  will be described below. 
       FIG. 11  is a diagram illustrating an exemplary apparatus configuration of the base station apparatus  1201 . The base station apparatus  1201  has a configuration including a higher layer unit  12011 , a frame-length adjusting unit  12012 , a carrier sensing unit  12013 , a transmission unit  12014 , a receiving unit  12015 , and an antenna unit  12016 . The higher layer unit  12011  is connected to other networks, and has a function of notifying the carrier sensing unit  11012  of information associated with a transmission frame. 
     The frame-length adjusting unit  12012  has a function of determining the structure of a UL-MU frame suitable for UL-MU transmission. The frame-length adjusting unit  12012  generates first resource allocation information including information about the structure of a UL-MU frame. A method of determining the structure of a UL-MU frame will be described. 
     The frame-length adjusting unit  12012  may have a function of generating a transmission frame for notifying the terminal apparatuses  2200  of a timing of start of UL-MU transmission. Hereinafter, a transmission frame for notifying the terminal apparatuses  2200  of a timing of start of UL-MU transmission is referred to as a timing frame or UL-MU Poll. The timing frame may include information associated with a UL-MU transmission time, or the base station apparatus  1201  and the multiple terminal apparatuses  2200  may agree that, after receiving a timing frame, the multiple terminal apparatuses wait for a certain time, and then UL-MU transmission is started. In the latter case, a format similar to the format of a control frame or a management frame which is defined in the IEEE 802.11 standard may be used for the timing frame. 
     The carrier sensing unit  12013  has a function of making a determination about whether transmission is to be performed, on the basis of carrier sense. In the present embodiment, the carrier sensing unit  12013  may perform carrier sensing on multiple channels. 
     The transmission unit  12014  includes a physical-layer frame generating unit  12014   a  and a radio transmission unit  12014   b.    
     The physical-layer frame generating unit  12014   a  has a function of generating a physical layer frame from a transmission frame transmitted from the carrier sensing unit  12013 . The physical-layer frame generating unit  12014   a  performs error correction coding, modulation, precoding-filter multiplication, and the like on a transmission frame. The physical-layer frame generating unit  12014   a  notifies the radio transmission unit  12014   b  of the generated physical layer frame. 
     The radio transmission unit  12014   b  converts the UL-MU frame generated by the physical-layer frame generating unit  12014   a  into a signal in a radio frequency (RF: Radio Frequency) band, and generates a radio frequency signal. The processes performed by the radio transmission unit  12014   b  include digital-analog conversion, filtering, and frequency conversion from a base band to an RF band. 
     The receiving unit  12015  includes a radio receiving unit  12015   a  and a signal demodulating unit  12015   b.  The receiving unit  12015  has a function of calculating a received power level from an RF band signal received by the antenna unit  12016 . However, the method of calculating a received power level is not limiting. The receiving unit  12015  notifies the carrier sensing unit  12013  of information about the calculated received power level. The carrier sensing unit  12013  may make a determination about whether transmission is to be performed, on the basis of the information about a received power level which is transmitted by the receiving unit  12015 . 
     The radio receiving unit  12015   a  has a function of converting an RF band signal received by the antenna unit  12016  into a base band signal and generating a physical layer signal (for example, a physical layer frame). The processes performed by the radio receiving unit  12015   a  include a frequency conversion process from an RF band to a base band, filtering, and analog-digital conversion. 
     The signal demodulating unit  12015   b  has a function of demodulating the physical layer signal generated by the radio receiving unit  12015   a.  The processes performed by the signal demodulating unit  12015   b  include channel equalization, de-mapping, and error correction decoding. The signal demodulating unit  12015   b  may extract, for example, information included in the physical layer header, information included in the MAC header, and information included in the transmission frame, from the physical layer signal. The signal demodulating unit  12015   b  may notify the higher layer unit  12011  of the extracted information. The signal demodulating unit  12015   b  may extract one or some of the information included in the physical layer header, the information included in the MAC header, and the information included in the transmission frame. 
     The antenna unit  12016  has a function of transmitting a radio frequency signal generated by the radio transmission unit  12014   b,  through a wireless space to the terminal apparatuses  2200 . The antenna unit  12016  also has a function of receiving radio frequency signals transmitted from the terminal apparatuses  2200 . When the base station apparatus  1201  performs carrier sensing, the antenna unit  12016  also has a function of receiving a signal in the channel in the wireless space. 
       FIG. 12  is a diagram illustrating an exemplary apparatus configuration of a terminal apparatus  2200 . The terminal apparatus  2200  includes a higher layer unit  22001 , a carrier sensing unit  22002 , a transmission unit  22003 , a receiving unit  22004 , and an antenna unit  22005 . 
     The higher layer unit  22001  is connected to other networks, and has a function of notifying the carrier sensing unit  22002  of information about a transmission frame. 
     The carrier sensing unit  22002  has a function of making a determination about whether transmission is to be performed, on the basis of carrier sense. 
     The transmission unit  22003  includes a physical-layer frame generating unit  22003   a  and a radio transmission unit  22003   b.    
     The physical-layer frame generating unit  22003   a  has a function of generating a physical layer frame from a transmission frame transmitted from the carrier sensing unit  22002 . The physical-layer frame generating unit  22003   a  performs error correction coding, modulation, precoding-filter multiplication, and the like on a transmission frame. The physical-layer frame generating unit  22003   a  notifies the radio transmission unit  22003   b  of the generated physical layer frame. The physical-layer frame generating unit  22003   a  may construct a physical layer frame on the basis of the first resource allocation information transmitted from the base station apparatus  1201 . Operations performed by the physical-layer frame generating unit  22003   a  will be described in detail below. 
     The radio transmission unit  22003   b  converts the physical layer frame generated by the physical-layer frame generating unit  22003   a  into a signal in a radio frequency (RF: Radio Frequency) band, and generates a radio frequency signal. The processes performed by the radio transmission unit  22003   b  include digital-analog conversion, filtering, and frequency conversion from a base band to an RF band. 
     The receiving unit  22004  includes a radio receiving unit  22004   a  and a signal demodulating unit  22004   b.  The receiving unit  22004  has a function of calculating a received power level from an RF band signal received by the antenna unit  22005 . However, the method of calculating a received power level is not limiting. The receiving unit  22004  notifies the carrier sensing unit  22002  of information about the calculated received power level. The carrier sensing unit  22002  may make a determination about whether transmission is to be performed, on the basis of the information about a received power level which is transmitted by the receiving unit  22004 . 
     The radio receiving unit  22004   a  has a function of converting an RF band signal received by the antenna unit  22005  into a base band signal and generating a physical layer signal (for example, a physical layer frame or an MU frame). The processes performed by the radio receiving unit  22004   a  include a frequency conversion process from an RF band to a base band, filtering, and analog-digital conversion. 
     The signal demodulating unit  22004   b  has a function of demodulating the physical layer signal generated by the radio receiving unit  22004   a.  The processes performed by the signal demodulating unit  22004   b  include channel equalization, de-mapping, and error correction decoding. The signal demodulating unit  22004   b  may extract, for example, information included in the physical layer header, information included in the MAC header, and information included in the transmission frame from the physical layer signal. The signal demodulating unit  22004   b  may notify the higher layer unit  22001  of the extracted information. The signal demodulating unit  22004   b  may extract one or some of the information included in the physical layer header, the information included in the MAC header, and the information included in the transmission frame. 
     The antenna unit  22005  has a function of transmitting a radio frequency signal generated by the radio transmission unit  22003   b  through a wireless space to the base station apparatus  1201 . The antenna unit  22005  also has a function of receiving a radio frequency signal transmitted from the base station apparatus  1201 . When the terminal apparatus  2200  performs carrier sensing, the antenna unit  22005  also has a function of receiving a signal in the channel in the wireless space. 
     Subchannels used in the wireless communication system according to the present embodiment are similar to the subchannels  400  according to the first embodiment, and will not be described. 
       FIG. 13  is a diagram illustrating exemplary UL-MU transmission performed when the frame-length adjusting unit  12012  adjusts the lengths of UL-MU frames. A flow of UL-MU transmission will be described below on the basis of the example in  FIG. 13 . A shaded frame indicates a frame transmitted by the base station apparatus  1201 . A method of adjusting frame lengths which is performed by the frame-length adjusting unit  12012  is not limited to the example in  FIG. 13 . 
     In the example in  FIG. 13 , the base station apparatus  1201  and the terminal apparatuses  2200  wait just for an SIFS so as not to perform transmission when the base station apparatus  1201  and the terminal apparatuses  2200  are to transmit frames. The base station apparatus  1201  and the terminal apparatuses  2200  according to the present embodiment may set an SIFS, a PIFS, an RIFS, a DIFS, an AIFS, or another waiting time as a transmission waiting time used in participation in UL-MU transmission, or do not necessarily set a waiting time (or may set the waiting time to 0). 
     UL-MU Polls  2500  and Frame Infos  2520  may include information for notifying the terminal apparatuses  2200  of radio resources used in transmission of control frames and management frames in UL-MU transmission. The example in  FIG. 13  is described under the assumption that the terminal apparatuses  2200  transmit Acks  2520  and Acks  2530  in a multiplexing manner in frequency resource. However, Acks  2510  and the Acks  2520  may be multiplexed in time resource. 
     The base station apparatus  1201  obtains information about payloads of the multiple terminal apparatuses  2200 , and determines whether or not UL-MU transmission is to be performed. The base station apparatus  1201  having determined that UL-MU transmission is to be performed transmits UL-MU Polls  2501  to  2504  (hereinafter also referred to as the “UL-MU Polls  2500 ”) to the multiple terminal apparatuses  2200 . The UL-MU Polls  2500  are frames that enable the base station apparatus  1201  to notify the terminal apparatuses  2200  of start of a UL-MU transmission period. The UL-MU Polls  2500  may be skipped. When the base station apparatus  1201  skips the UL-MU Polls  2500 , the base station apparatus  1201  may notify the terminal apparatuses  2200  of start of a UL-MU transmission period by using Frame Infos  2521  to  2524  (hereinafter also referred to as the “Frame Infos  2520 ”). 
     The terminal apparatuses  2200  having received the UL-MU Polls  2500  notify the base station apparatus  1201  of an Ack  2511 . 
     Subsequently, the base station apparatus  1201  notifies the terminal apparatuses  2200  of the Frame Infos  2520 . The Frame Infos  2500  may include the first resource allocation information generated by the frame-length adjusting unit  12012 . The first resource allocation information may include information about generation of physical layer frames, such as a modulating method, a coding method, and a precoding filter generating method which are used by the terminal apparatuses  2200 . 
     The terminal apparatuses  2200  having received the Frame Infos  2520  notify the base station apparatus  1201  of Acks  2531  to  2534  (hereinafter also referred to as “Acks  2530 ”). The terminal apparatuses  2200  may skip the Acks  2530 . When the terminal apparatuses  2200  skip the Acks  2530 , the terminal apparatuses  2200  transmit PPDUs  2541  to  2545  (hereinafter also referred to as “PPDUs  2540 ”) to the base station apparatus  1201 . 
     The terminal apparatuses  2200  generate the PPDUs  2540  on the basis of the first resource allocation information transmitted from the base station apparatus  1201 . When the first resource information includes information about generation of physical layer frames in addition to information about frame lengths and information about resources to be used, the physical-layer frame generating unit  22003   a  generates the PPDUs  2540  according to the first resource allocation information. The terminal apparatuses  2200  start transmission of the PPDUs  2540  on the basis of a timing of start of UL-MU transmission which is transmitted by the base station apparatus  1201 . 
     The terminal apparatuses  2200  having received the PPDUs  2540  notify the multiple terminal apparatuses of Acks  2551  to  2554  (hereinafter also referred to as “Acks  2550 ”), and ends the UL-MU transmission. 
     The example in  FIG. 13  is described under the assumption that the base station apparatus  1201  uses DL-MU transmission to transmit the UL-MU Polls  2500 , the Frame Infos  2520 , and the Acks  2550 . However, the base station apparatus  1201  does not necessarily perform DL-MU transmission. For example, the base station apparatus  1201  may temporally divide the UP-MU Polls  2500 , the Frame Infos  2520 , and the Acks  2550  for transmission, or may use multicasting (transmission means for notifying multiple terminal apparatuses of the same information). The terminal apparatuses  2200  may notify the base station apparatus  1201  of function information about whether or not the terminal apparatuses  2200  have a function of generating a physical layer frame, for example, according to the first resource allocation information. 
     As described above, the terminal apparatuses  2200  adjust frame lengths in UL-MU transmission, achieving reduction of a UL-MU transmission period. Thus, frequency efficiency of the wireless communication system may be improved. 
     In addition to the embodiments described above, the following aspects may be employed. 
     (A) A base station apparatus of the present invention is applied to a communication system that controls transmission occasions in an autonomous and distributed manner, and communicates with a terminal apparatus. The base station apparatus includes a physical-layer frame generating unit and a radio unit. The physical-layer frame generating unit generates a physical layer frame which is addressed to the terminal apparatus and which includes a first frame section transmitted in a first radio resource and a second frame section transmitted in a second radio resource. The radio unit transmits the physical layer frame. 
     (B) The base station apparatus of the present invention is characterized by signaling information about a function of generating the physical layer frame including the first frame section and the second frame section, to the terminal apparatus. 
     (C) The base station apparatus of the present invention is characterized in that the physical-frame generating unit multiplexes a different physical layer frame on the physical layer frame. The different physical layer frame is addressed to a test apparatus different from the terminal apparatus. 
     (D) The base station apparatus of the present invention is characterized by including a frame-length adjusting unit that determines the lengths of the first frame section and the second frame section on the basis of the frame length of a physical layer frame addressed to a terminal apparatus different from the terminal apparatus. 
     (E) The base station apparatus of the present invention is characterized by signaling, to the terminal apparatus, information indicating the first frame section and the second frame section. 
     (F) The base station apparatus of the present invention characterized by signaling, to the terminal apparatus, information indicating that a function of generating the physical layer frame including the first frame section and the second frame section is not included. 
     (G) A terminal apparatus of the present invention is applied to a communication system that controls transmission occasions in an autonomous and distributed manner, and communicates with a base station apparatus. The terminal apparatus includes a receiving unit that receives a first frame section on the basis of information indicating the first frame section which is signaled by the base station apparatus. 
     (H) The terminal apparatus of the present invention is characterized in that the receiving unit receives the first frame section and a second frame section on the basis of information about a function of generating a physical layer frame including the second frame section, in addition to information about a function of generating a physical layer frame including the first frame section. 
     As described above, according to the present embodiments, efficient use of radio resources and reduction of the transmission time of transmission frames may be achieved. 
     Programs operated in the base station apparatus and the terminal apparatuses according to the present invention are programs (programs causing a computer to operate) controlling a CPU and the like so that the functions of the above-described embodiments of the present invention are implemented. Information handled in these apparatuses is temporarily stored in a RAM when the information is to be processed. After that, the information is stored in various ROMs and HDDs, and is read by the CPU when necessary for modification and writing. A recording medium storing the programs may be any of a semiconductor medium (for example, a ROM or a nonvolatile memory card), an optical recording medium (for example, a DVD, an MO, an MD, a CD, or a BD), a magnetic recording medium (for example, a magnetic tape or a flexible disk), or the like. By executing the programs having been loaded, not only are the functions of the above-described embodiments implemented, but also processing may be performed in collaboration with an operating system, other application programs, or the like on the basis of instructions of the programs, achieving the functions of the present invention. 
     When the programs are to be distributed in a market, the programs may be distributed by storing the programs in a portable recording medium, or may be transferred to a server computer connected over a network such as the Internet. In this case, a storage device of the server computer is also encompassed in the present invention. Some or all of the terminal apparatuses and the base station apparatus according to the above-described embodiments may be implemented typically as LSIs that are integrated circuits. The functional blocks of a receiving apparatus may be made into individual chips, or some or all of the functional blocks may be integrated into one chip. When the functional blocks are made into integrated circuits, an integrated-circuit controller for controlling these is added. 
     Ways of fabricating an integrated circuit are not limited to an LSI. A dedicated circuit or a general-purpose processor may be used in the implementation. In addition, when a technique of fabricating an integrated circuit replaced by an LSI emerges with advance of the semiconductor technique, the integrated circuit produced in the technique may be used. 
     The invention of the subject application is not limited to the above-described embodiments. A terminal apparatus provided by the invention of the subject application is not limited to application to a mobile station apparatus. It goes without saying that the invention may be applied to fixed or non-moving electronic equipment that is installed indoors or outdoors, such as an AV apparatus, a kitchen apparatus, a cleaning/washing machine, an air conditioner, office equipment, a vending machine, other living appliances, and the like. 
     The embodiments of the present invention are described in detail with reference to the drawings. The specific configuration is not limited to the embodiments. A design or the like in a scope without departing from the gist of the invention is also encompassed in the claims. 
     This international application claims the priority of Japanese Patent Application No. 2015-040590, filed Mar. 2, 2015, which is hereby incorporated by reference herein in the entirety of Japanese Patent Application No. 2015-040590. 
     REFERENCE SIGNS LIST 
       401 - 404  subchannel 
       1101  base station apparatus 
       1201  base station apparatus 
       2100  terminal apparatus 
       2101 - 2104  terminal apparatus 
       2200  terminal apparatus 
       2201 - 2204  terminal apparatus 
       3101  management range 
       3201  management range 
       11011  higher layer unit 
       11012  carrier sensing unit 
       11013  transmission unit 
       11013   a  physical-layer frame generating unit 
       11013   b  frame-length adjusting unit 
       11013   c  radio transmission unit 
       11014  receiving unit 
       11014   a  radio receiving unit 
       11014   b  signal demodulating unit 
       11015  antenna unit 
       12011  higher layer unit 
       12012  frame-length adjusting unit 
       12013  carrier sensing unit 
       12014  transmission unit 
       12014   a  physical-layer frame generating snit 
       12014   h  radio transmission unit 
       12015  receiving unit 
       12015   a  radio receiving unit 
       12015   b  signal demodulating unit 
       12016  antenna unit 
       21001  higher layer unit 
       21002  carrier sensing unit 
       21003  transmission unit 
       21003   a  physical-layer frame generating unit 
       21003   b  radio transmission unit 
       21004  receiving unit 
       21004   a  radio receiving unit 
       21004   b  signal demodulating unit 
       21005  antenna unit 
       22001  higher layer unit 
       22002  carrier sensing unit 
       22003  transmission unit 
       22003   a  physical-layer frame generating unit 
       22003   b  radio transmission unit 
       22004  receiving unit 
       22004   a  radio receiving unit 
       22004   b  signal demodulating snit 
       22005  antenna unit