JP2007208522A - Wireless communication method and system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance the throughput during transmission from a plurality of wireless terminals to a wireless base station by achieving a frame structure which can implement SDMA transmission. <P>SOLUTION: A wireless terminal STA1 specified by a wireless base station AP is constituted to transmit to the AP a first SDMA transmission frame including a synchronous field SYNC, a frame structure designation field SIG, an AGC field STF1, a channel estimation field LTF1, and a data field DATA1. The other wireless terminal STA2 is constituted to transmit to the AP a second SDMA transmission frame including an AGC field STF2, a channel estimation field LTF2, and a data field DATA2. The channel estimation fields LTF1 and LTF2 are transmitted at different timing. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a radio communication system composed of one radio base station and a plurality of radio terminals, and more particularly to a radio communication method and system using Space Division Multiple Access (SDMA) transmission for transmission of a specific frame.

  IEEE802.11 (ISO / IEC 8802-11: 1999 (E) ANSI /) that employs Carrier Sense Multiple Access (CSMA) as a wireless communication system that performs communication between a wireless base station and a wireless terminal IEEE Std 802.11, 1999 edition) standard wireless LAN is widely known and has already been put into practical use as IEEE 802.11a / b / g. In the IEEE 802 committee, for example, task group n (TGn) or the like is studying to further increase the speed of the IEEE 802.11 wireless LAN. In TGn, application of multi-input multi-output (MIMO) technology using a plurality of transmission units and a plurality of reception units is being studied. MIMO technology can improve reachability and throughput without sacrificing bandwidth even in a multipath environment or in the presence of interference.

  On the other hand, in order to improve throughput, an aggregation technique for transmitting a plurality of frames as a single frame is also being studied. Aggregation can reduce overhead and increase throughput. However, the object of aggregation is only frames transmitted from the same wireless terminal, and aggregation cannot be applied to frames transmitted from a plurality of wireless terminals to the wireless base station.

Space division multiple access (SDMA) is known as a multiplexing technique for increasing transmission efficiency by transmitting data to a wireless base station at the same time using the same frequency to a wireless base station. (Cited document 1).
JP 2003-52079 A

  SDMA is effective in improving the slew rate when a plurality of wireless terminals transmit data to a wireless base station, but Patent Document 1 does not show a specific frame configuration for realizing SDMA. . That is, in order to realize SDMA, processing for estimating the response (channel response) of each propagation path from a plurality of wireless terminals to a wireless base station, that is, channel estimation is essential. It does not show how to perform channel estimation.

  On the other hand, even if SDMA becomes widespread in the future, for the time being, SDMA-compatible wireless terminals and non-SDMA-compatible wireless terminals may coexist. A system in which such a SDMA compatible wireless terminal and a non-SDMA compatible wireless terminal coexist has not been studied. In particular, in a CSMA system such as an IEEE 802.11 wireless LAN, the transmission / reception timing and the frame length of a transmission frame are different for each wireless terminal, so it is considered difficult to coexist between an SDMA compatible wireless terminal and a non-SDMA compatible wireless terminal. .

  An object of the present invention is to improve the throughput at the time of transmission from an uplink, that is, a plurality of wireless terminals to a wireless base station, by a frame configuration capable of realizing SDMA.

  It is another object of the present invention to provide an SDMA frame configuration in which an SDMA compatible wireless terminal and a non-SDMA compatible wireless terminal can coexist.

  According to an aspect of the present invention, the first radio terminal specified by the radio base station is synchronized with the radio base station, a synchronization field for receiving synchronization with a signal transmitted from the radio base station Transmitting a first transmission frame including a first AGC field for controlling the first channel, a first channel estimation field for estimating a channel to and from the radio base station, and a first data field to the radio base station; The second radio terminal has a second AGC field for controlling a reception gain for a signal transmitted from the radio base station, a second channel estimation field for estimating a channel with the radio base station, and A second transmission frame including a second data field is transmitted to the radio base station. Here, the first channel estimation field and the second channel estimation field are transmitted at different timings.

  In this way, by shifting the transmission timing of the channel estimation field from each wireless terminal during SDMA transmission, the wireless base station can perform channel estimation with each wireless terminal necessary for SDMA transmission.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiment, an IEEE 802.11 wireless LAN will be described as an example, but the present invention is not limited to this.

(First embodiment)
FIG. 1 shows a schematic configuration of a wireless system according to an embodiment of the present invention. Here, it is assumed that three wireless terminals STA1, STA2, and STA3 are connected to one wireless base station (also referred to as an access point) AP connected to a network NW such as the Internet.

  FIG. 2 shows a configuration of a wireless terminal based on the MIMO technology as a specific example of the wireless terminals STA1, STA2, and STA3. The receiving unit 13 and the transmitting unit 14 are connected to the plurality of antennas 11 via the transmission / reception switching unit 12. Here, an example in which the number of antennas 11, reception units 13, and transmission units 14 is four is shown, but the present invention is not limited to this.

  The reception unit 13 receives a signal (referred to as a reception frame) transmitted from the radio base station AP and received by the antenna 11 from the transmission / reception switching unit 12, and performs demodulation processing on the reception frame. The reception frame analysis unit 15 performs analysis of the reception frame, for example, destination determination of the reception frame and frame type determination. The transmission frame generation unit 16 generates a transmission frame and sends it to the transmission unit 14. The transmission unit 14 performs modulation processing on the transmission frame. The modulated transmission frame is supplied to the antenna 11 via the transmission / reception switching unit 12 and transmitted to the radio base station AP by the antenna 11.

  3 and 4 show an example of frame exchange when communication is performed between the radio base station AP and the radio terminals STA1, STA2, and STA3 by SDMA transmission. The SDMA transmission in this embodiment is particularly suitable for transmitting a data field having a relatively small amount of data such as an acknowledgment (Ack) signal and a voice signal.

  According to the example of FIG. 3, first, in the downlink, the radio base station AP transmits polling frames to the plurality of radio terminals STA1, STA2, and STA3 to perform polling. In the uplink, the radio terminals STA1, STA2, and STA3 simultaneously transmit data frames Data1, Data2, and Data3 by SDMA after a predetermined time ΔT has elapsed since receiving the polling frame from the radio base station AP.

  According to the example of FIG. 4, in the downlink, the radio base station AP transmits data frames to a plurality of radio terminals STA1, STA2, and STA3. In the uplink, the wireless terminals STA1, STA2, and STA3 simultaneously transmit the acknowledgment signals Ack1, Ack2, and Ack3 by SDMA after a certain time ΔT has elapsed since receiving the data frame from the wireless base station AP.

  3 and 4 corresponds to SIFS (Short Interframe Space), which is a time interval between frames defined by the MAC protocol specification, for example, in the case of IEEE 802.11 wireless LAN.

  In this way, the radio base station AP transmits a polling frame and a data frame, and the radio terminals STA1, STA2, and STA3 perform SDMA transmission using them as a trigger. As a transmission method when the radio base station AP transmits the polling frame shown in FIG. 3 or the data frame shown in FIG. 4, a normal single input / single output (SISO) may be used. MIMO or Space Division Multiplexing (SDM) may be used and is not particularly limited. The transmission of the acknowledgment signal may be a BlockAck (also referred to as Group Ack) method in which acknowledgment of a plurality of received data frames is performed in one frame.

  Next, the details of the configuration of a transmission frame (referred to as an SDMA transmission frame) when the radio terminals STA1, STA2, and STA3 perform transmission by SDMA toward the radio base station AP will be described. FIG. 5 shows an example of the frame structure of such an SDMA frame. Here, STA1 is assumed to be a terminal designated by the radio base station AP among the radio terminals STA1, STA2, and STA3. The designation of the radio terminal STA1 by the radio base station AP may be explicit or indirect.

  The frames transmitted from the wireless terminals STA1, STA2, and STA3 in the SDMA transmission frame in FIG. 5 correspond to the data frames Data1, Data2, and Data3 in FIG. 3, respectively, or the delivery confirmation signals Ack1, Ack2, and Ack3 in FIG. To do. Data fields DATA1, DATA2, and DATA3 in FIG. 6 correspond to, for example, a frame body in IEEE 802.11.

  The radio terminal STA1 designated by the radio base station AP first transmits a synchronization field SYNC for synchronization such as time synchronization (timing synchronization) and frequency synchronization with the AP, and then signal length information and transmission rate information. A frame configuration instruction field SIG including Here, the signal length information indicates the total signal length (time length) of STF1 (STF2, STF3), LTF1, LTF2, LTF3, and DATA1 (DATA2, DATA3) in FIG. The transmission rate information indicates a transmission rate at which the wireless terminals STA1, STA2, and STA3 transmit an SDMA transmission frame. The frame configuration instruction field SIG may include information such as a guard interval and the number of LTFs described later as information other than the signal length and transmission rate information.

  After all the radio terminals STA1, STA2, and STA3 transmit the frame configuration instruction field SIG, the radio base station AP optimally adjusts the gain for the received signal by auto gain control (AGC). For example, a short training field (STF) (hereinafter referred to as AGC field) is simultaneously transmitted.

  Thereafter, only the radio terminal STA1 transmits a signal for channel estimation, for example, a long training field (LTF) (hereinafter referred to as channel estimation field) 1. While the wireless terminal STA1 transmits the channel estimation field LTF1, the other wireless terminals STA2 and STA3 do not transmit anything. The wireless terminal STA2 transmits LTF2 after the wireless terminal STA1 transmits LTF1, and then the wireless terminal STA3 transmits LTF3 after the wireless terminal STA2 transmits LTF2.

  Finally, after the wireless terminal STA3 transmits the channel estimation field LTF3, all the wireless terminals STA1, STA2, and STA3 simultaneously transmit the data fields DATA1, DATA2, and DATA3 by SDMA. Here, the data fields DATA1, DATA2, and DATA3 are, for example, the payloads of the data frames Data1, Data2, and Data3 described with reference to FIG. 3 or the payloads of the delivery confirmation signals Ack1, Ack2, and Ack3 described with reference to FIG.

  According to the frame configuration of FIG. 5, by shifting the transmission timing of the channel estimation fields LTF1, LTF2, and LTF3 from the radio terminals STA1, STA2, and STA3 in time, the radio base station AP requires the STA1, STA2 required for SDMA transmission. , Channel estimation between each of the STA3 and the AP becomes possible. That is, if channel estimation fields are simultaneously transmitted from a plurality of wireless terminals, these are mixed and received at the wireless base station at the same time, so channel estimation for each channel cannot be performed. If the channel estimation fields LTF1, LTF2, and LTF3 are transmitted at different timings as shown in FIG. 5, such a problem is solved.

  Further, the AGC fields STF1, STF2, and STF3 from the radio terminals STA1, STA2, and STA3 are transmitted at the same time, so that the radio terminals STA1, STA2, and STA3 have the data fields DATA1, DATA2, and DATA3 in the radio base station AP. Can be measured at the same time, so that the AGC for the data fields DATA1, DATA2, and DATA3 can be effectively functioned.

  In FIG. 5, the reason why the SDMA transmission frame includes the frame configuration instruction field SIG and the reason why the SIG is transmitted after the synchronization field SYNC enable coexistence of the SDMA compatible wireless terminal and the non-SDMA compatible wireless terminal in the wireless communication system. Because. The synchronization field SYNC and the subsequent frame configuration instruction field SIG are common to all wireless terminals, and can be understood even for wireless terminals that do not support SDMA. Therefore, the non-SDMA compatible wireless terminal interprets the signal length included in the SIG in the SDMA transmission frame transmitted from the SDMA compatible wireless terminal, and the signal length period is transmitted regardless of the carrier sense of the SDMA transmission frame. Stop. Thus, frame collision can be prevented even when carrier sense is not performed correctly.

  When Orthogonal Frequency-Division Multiplexing (OFDM) is used as a transmission frame multiplexing scheme, a guard interval (GI) for multipath countermeasures is added to the OFDM symbol. If the guard interval length is large, the multipath tolerance is enhanced, but the guard interval is an overhead and the throughput is lowered. For this reason, it is desirable to make the guard interval length variable according to the wireless environment to be used.

  For example, in the IEEE 802.11n wireless LAN, which is being standardized by the above-described TGn and the like as a high-throughput version of the IEEE 802.11 wireless LAN, it is considered to prepare two types of guard interval lengths. . When performing SDMA transmission in a wireless communication system that supports a plurality of types of guard interval lengths, a large guard interval length is selected. As a result, the difference in propagation distance between the radio base station AP and the radio terminals STA1, STA2, and STA3 is absorbed, and the accuracy of timing control when transmitting the SDMA transmission frame in each radio terminal STA1, STA2, and STA3 is eased. Therefore, it is effective for simplification and cost reduction of the apparatus.

  Next, the time from when the wireless terminal STA1 transmits the channel estimation field LTF1 to the transmission of the data field DATA1, the time until the wireless terminal STA2 transmits the AGC field STF2 and the transmission of the channel estimation field LTF2, and A method for determining the time from transmission of LTF2 to transmission of data field DATA2 will be described.

When the wireless terminal STA1 performs SDMA transmission by transmitting a polling frame transmitted from the wireless base station AP, the number of destination addresses described in the polling frame is the maximum SDMA multiplexing number (data frame multiplexing number). When the wireless terminal STA1 performs SDMA transmission by receiving a data frame transmitted from the wireless base station AP, the number of destination addresses described in the data frame is the maximum SDMA multiplexing number (delivery confirmation signal multiplexing number). Therefore, the wireless terminals STA1, STA2, and STA3 determine the maximum SDMA multiplexing number by analyzing the destination address of the received frame in the received frame analysis unit 15.
The radio base station AP transmits to each radio terminal so that each radio terminal can determine whether to transmit the synchronization field SYNC and the frame configuration instruction field SIG as the STA1 or not to transmit the synchronization field and the frame configuration instruction field SIG as other than the STA1. On the other hand, it is necessary to notify whether to operate as STA1 or other than STA1. Even if it is not explicitly notified in this way, the radio base station AP needs to provide each radio terminal with information that can determine whether the radio terminal operates as the STA1 or other than the STA1. Examples are shown below.

  In order to serve as a trigger for the wireless terminal to perform SDMA transmission, the wireless base station AP transmits a frame having a frame configuration as shown in FIG. 6 (hereinafter referred to as an SDMA trigger frame). In FIG. 6, the source address (Source Address; SA) is the MAC address of the radio base station AP that is the sender of the SDMA trigger frame, and the destination addresses (Destination Address; DA) 1, DA2, and DA3 are the SDMA trigger frame. This is the MAC address of the destination wireless terminal. Here, an example of transmitting SDMA trigger frames addressed to three wireless terminals is shown, but the present invention is not limited to this. Payloads 1, 2, and 3 are payloads addressed to three wireless terminals. Control information for determining which wireless terminal is addressed by separating the three payloads 1, 2, and 3 is also necessary, but the description thereof is omitted here. When the SDMA trigger frame is a polling frame, there is no need for a payload.

  When configuring the SDMA trigger frame of FIG. 6, the radio base station AP adds the MAC address of the radio terminal expected to operate as the STA1 as the destination address DA1, immediately after the source address SA, that is, at the beginning of the destination address group. . Similarly, the MAC address of the wireless terminal that expects the operation as STA2 (for example, LTF2 is transmitted after LTF1 as shown in FIG. 5) is set as the destination address DA2, and similarly the operation as STA3 (for example, as shown in FIG. 5). The destination MAC address of the wireless terminal that expects LTF3 to be transmitted after LTF2) is assumed to be the destination address DA3.

  As described above, when the radio base station AP transmits the SDMA trigger frame having the frame configuration as shown in FIG. 6, an operation method (STA1, STA2, or STA3 in SDMA transmission) for each radio terminal. ). Here, the method of notifying the operation method in the SDMA transmission of each wireless terminal according to the setting order of the destination addresses DA1, DA2, DA3 is shown, but another method may be used.

  For example, a 1-bit SDMA operation identifier is added to the MAC address, the SDMA operation identifier of the MAC address of the wireless terminal that is expected to operate as STA1 is set to “1”, and the MAC of the wireless terminal that is expected to perform operations other than STA1 A method of setting “0” to the SDMA operation identifier of the address may be used.

  An LTF transmission timing instruction field and a data transmission timing instruction field may be provided, and for example, a waiting time from when the AGC field STF2 is transmitted until the channel estimation field LTF2 is transmitted may be notified by the LTF transmission timing instruction field.

  The data transmission timing indication field may notify the waiting time from the transmission of LTF2 to the transmission of data, or the waiting time from the transmission of STF2 to the transmission of the data field.

  FIG. 7 shows another example of the frame configuration of the SDMA trigger frame. Using the destination address DA of the entire MAC header as a broadcast address or multicast address, an individual MAC header for each wireless terminal is added after the DA, and each wireless terminal is assigned to the destination addresses DA1, DA2, and DA3 of the individual MAC headers 1, 2, and 3. A MAC address is added. Here, the transmission source address SA is described in the entire MAC header, but an SA may be added to each of the individual MAC headers 1, 2, and 3.

  The wireless base station AP sets the MAC address of the wireless terminal that is expected to operate as the STA1 to DA1, and the data to payload 1. The radio base station AP further sets DA2 and DA3 as MAC addresses of wireless terminals expected to operate as STA2 and STA3, and sets payload 2 and payload 3 as data fields, respectively. As described above, in the SDMA trigger frame of FIG. 7, the operation method of the wireless terminal is notified by the frame configuration, that is, the transmission order of the individual MAC headers 1, 2, 3 addressed to the wireless terminals STA1, STA2, STA3.

  As another SDMA trigger frame transmission method, an LTF transmission timing instruction field and a data transmission timing instruction field are provided in the individual MAC header as in the method of FIG. 6, and the AGC field STF2 is used by using, for example, the LTF transmission timing instruction field. May be notified of the waiting time from the transmission of the channel estimation field to the transmission of the channel estimation field LTF2. By using the data transmission timing indication field, the waiting time from the transmission of the channel estimation field LTF2 to the transmission of the data field or the waiting time from the transmission of the STF2 to the transmission of the data field may be notified. Good.

  In this way, by using the SDMA trigger frame having the frame structure as shown in FIG. 6 or FIG. 7, the radio base station AP notifies each radio terminal of the operation method in SDMA transmission, so that each radio terminal performs its own operation. Since the method can be grasped, SDMA transmission becomes possible.

(Second Embodiment)
In the example of frame exchange shown in FIG. 3 and FIG. 4 of the first embodiment, a method of selecting a wireless terminal when the wireless base station AP polls a plurality of wireless terminals, or a plurality of wireless base stations AP There is no particular restriction on how to select a wireless terminal when transmitting data to the wireless terminal. In the second embodiment, a wireless terminal that performs simultaneous polling to the wireless terminal by the wireless base station AP and performs data transmission by SDMA, and a wireless terminal that simultaneously performs data transmission and performs Ack transmission by SDMA, A description will be given of a method in which only terminals grouped as the same group with reference to reception power of signals transmitted by those wireless terminals in the wireless base station AP will be described.

When receiving a signal transmitted by SDMA, the radio base station AP has a large input dynamic range, that is, a quantization bit, in order to enable reception even if the difference in reception power of transmission signals from each radio terminal is large. A large number of analog-to-digital converters (ADC) are required. For example, in the case of a 10-bit ADC, if the margin for receiving an OFDM signal corresponding to MIMO transmission is 4 bits, the effective input dynamic range is 36 dB (= 20 log ² (10−4)). Therefore, when the reception power of a transmission signal from a certain wireless terminal STAa is −30 dBm, the transmission signal from the wireless terminal STAb whose reception power is 36 dB lower than that of the wireless terminal STAa is the quantum at the time of analog-digital conversion. It becomes buried in noise and reception becomes difficult. In order to ensure the input dynamic range of the ADC, the number of quantization bits may be increased. However, an ADC having a large number of quantization bits is generally expensive and increases the cost of the radio base station.

  In view of this, wireless terminals having relatively close received power at the wireless base station AP are grouped as the same group, and only the wireless terminals in the same group are simultaneously subjected to SDMA transmission. As a more specific example, grouping is performed such that the difference between the maximum received power and the minimum received power of transmission signals from the wireless terminals in the same group is less than a threshold (for example, 30 dB). The grouping threshold needs to be smaller than the input dynamic range of the ADC.

  If the difference in the number of wireless terminals in each group is large, it is natural that SDMA becomes inefficient. Therefore, it is desirable to perform grouping in consideration of making the number of wireless terminals as uniform as possible. A wireless terminal having a large amount of transmission data (particularly a wireless terminal having a large number of transmission frames) can perform efficient high-speed transmission by using the IEEE 802.11n aggregation function and the MIMO function. Therefore, it is more effective to group wireless terminals having a small transmission data size and a small number of transmission data frames into the same group.

  As described above, according to the second embodiment, grouping of radio terminals is performed based on the reception power of the transmission signal from each radio terminal in the radio base station AP, and SDMA transmission is performed for the radio terminals in the same group. . As a result, it is not necessary to increase the input dynamic range (number of bits) of the ADC in the radio base station AP, and SDMA transmission can be performed while reducing the cost of the apparatus.

(Third embodiment)
In the second embodiment, wireless terminals that simultaneously perform SDMA are grouped to solve the problem of the dynamic range of received power of transmission signals from wireless terminals in the wireless base station AP. On the other hand, in the third embodiment, a method for solving a similar problem by performing transmission power control by a wireless terminal will be described.

  FIG. 8 shows an example in which the wireless terminal STA1 is far away from the wireless base station AP and the wireless terminal STA2 is located relatively close to the wireless base station AP. For example, it is assumed that the signal attenuation rate from the radio terminal STA1 to the radio base station AP is 80 dB, and the signal attenuation rate from the radio terminal STA2 to the radio base station AP is 40 dB.

  When both the wireless terminals STA1 and STA2 perform SDMA transmission with a maximum transmission power of 16 dBm, the wireless base station AP receives a signal from the STA1 with a reception power of −64 dBm and a signal from the STA2 with a reception power of −24 dBm. Is done. As described in the second embodiment, for example, if the backoff is set to 4 bits using a 10-bit ADC, the dynamic range is 36 dB, and thus the signal from the wireless terminal STA1 is buried in the quantization noise. .

  In the present embodiment, this problem is solved by the radio terminal controlling the transmission power based on the received power of the packet from the radio base station AP. For example, FIG. 9 shows a transmission power control method in a radio terminal when SDMA is started by polling of the radio base station AP as shown in FIG.

  First, the radio base station AP transmits a polling packet for causing the radio terminals STA1 and STA2 to start SDMA transmission at a maximum power of 16 dBm. When the transmission powers of the wireless terminals STA1 and STA2 are both 16 dBm and the power attenuation between the STA1 and STA2 and the AP is reversible, the STA1 and STA2 receive -64 dBm and -24 dBm, respectively. A polling packet arrives. Since the method for estimating the received power at the wireless terminal is known, a description thereof will be omitted.

  Here, it is assumed that the standard of transmission power control in the wireless terminal is “set reception power at the wireless base station AP to −54 dBm”. In the example of FIG. 9, since the reception power of the polling packet received by the wireless terminal STA1 is −64 dBm, the wireless terminal STA1 can reach the wireless base station AP with the reception power of −54 dBm. The transmission power must be increased. However, since it is impossible to increase the transmission power beyond the maximum value, the wireless terminal STA1 performs transmission with the maximum power of 16 dBm.

  On the other hand, the wireless terminal STA2 receives the polling packet from the wireless base station AP at −24 dBm. Since communication between the wireless terminal and the wireless base station AP uses the same channel, this means that when the wireless terminal STA2 transmits a packet with the maximum transmission power, it reaches the wireless base station AP with a reception power of −24 dBm. To do. Here, as described above, since the transmission power control standard in the wireless terminal is determined as “the reception power at the wireless base station AP is −54 dBm”, the wireless terminal STA2 reduces the transmission power by 30 dB and transmits at −26 dBm. I do.

  By performing such control, in the radio base station AP, a packet transmitted from the radio terminal STA1 during SDMA transmission is received at -64 dBm, and a packet transmitted from the radio terminal STA2 is received at -54 dBm. This is a value that is well within the input dynamic range of the ADC, and it is possible to perform digital signal processing with high accuracy when the transmission signals from the radio terminals STA1 and STA2 are separated.

  In the transmission power control of the wireless terminal, the transmission power may be calculated from the calculation result as described above, or a table storing the reception power from the wireless base station AP and the transmission power of the wireless terminal in association with each other is provided. The transmission power control may be performed with reference to this table. The packet for measuring the received power from the radio base station AP is not limited to the polling packet.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described with reference to FIG. FIG. 10 shows a frame configuration in which the estimation accuracy based on the channel estimation field is further improved. The radio terminal STA1 designated by the radio base station AP and the other radio terminals STA2 and STA3 simultaneously transmit frames to perform SDMA. It is an example in the case of realization.

  The radio terminal STA1 designated by the radio base station AP first transmits a synchronization field SYNC, and then transmits a frame configuration instruction field SIG including a signal length and transmission rate information. The frame configuration instruction field SIG may include, for example, information such as a guard interval and the number of channel estimation fields LTF described later as information other than the signal length and transmission rate information.

  Next, after transmitting the frame configuration instruction field SIG, the wireless terminal STA1 sequentially transmits the AGC field STF1 and the channel estimation field LTF1. Until the wireless terminal STA1 completes transmission of the channel estimation field LTF1, the other wireless terminals STA2 and STA3 do not transmit anything.

  Next, when the wireless terminal STA1 completes the transmission of the LTF1, the wireless terminal STA2 sequentially transmits the AGC field STF2 and the channel estimation field LTF2. When the wireless terminal STA2 completes transmission of the LTF2, the wireless terminal STA3 sequentially transmits the AGC field STF3 and the channel estimation field LTF3. Finally, the wireless terminals STA1, STA2, and STA3 simultaneously transmit data fields DATA1, DATA2, and DATA3.

  As described above, in the present embodiment, not only the transmission timing of the channel estimation field LTF but also the transmission timing of the AGC field STF are shifted in time when frames are transmitted from the wireless terminals STA1, STA2, and STA3. As a result, channel estimation for space division can be performed in the radio base station AP, and since AGC for the channel estimation field LTF can be effectively functioned, more accurate channel estimation is possible.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described with reference to FIG. FIG. 11 shows a frame configuration that improves the estimation accuracy by the channel estimation field and enables reception power measurement when data is multiplexed. This is an example in which the wireless terminal STA1 designated by the wireless base station AP and the other wireless terminals STA2 and STA3 simultaneously transmit frames to realize SDMA.

  The radio terminal STA1 designated by the radio base station AP first transmits a synchronization field SYNC, and then transmits a frame configuration instruction field SIG including a signal length and transmission rate information. The frame configuration instruction field SIG may include information such as a guard interval and the number of LTFs described later as information other than the signal length and transmission rate information.

  Next, after transmitting the frame configuration instruction field SIG, the wireless terminal STA1 sequentially transmits the AGC field STF1a and the channel estimation field LTF1. Here, the AGC field STF1a is a signal for the radio base station AP to adjust the received signal strength of the channel estimation field LTF by AGC. Until the wireless terminal STA1 completes transmission of the channel estimation field LTF1, the other wireless terminals STA2 and STA3 do not transmit anything.

  Next, the wireless terminals STA1, STA2, and STA3 transmit the AGC fields STF1b, STF2b, and STF3b at the same time when the STA2 and STA3 complete the transmission of the AGC field and the channel estimation field (details will be described later). Here, the AGC fields STF1b, STF2b, STF3b are signals for the radio base station AP to adjust the received signal strength of the data field by AGC. Finally, the wireless terminals STA1, STA2, and STA3 simultaneously transmit data fields DATA1, DATA2, and DATA3.

  The radio terminal STA2 transmits the AGC field STF2a at the timing when the transmission of the channel estimation field LTF1 from the radio terminal STA1 is completed, and then transmits the channel estimation field LTF2. The wireless terminal STA3 transmits the AGC field STF3a at the timing when the transmission of the channel estimation field LTF2 from the wireless terminal STA2 is completed, and then transmits the channel estimation field LTF3. Here, the AGC fields STF2a and STF3a are signals for the radio base station AP to adjust the received signal strength of the channel estimation field LTF by AGC, similarly to the STF 1a.

  As described above, in the present embodiment, similarly to the previous embodiment, the transmission timing of the channel estimation field LTF from each of the radio terminals STA1, STA2, and STA3 is shifted in time, so that the radio base station AP performs space division. Channel estimation can be performed with high accuracy.

  Further, in the present embodiment, the AGC field is divided into first sub AGC fields STF1a, STF2a, STF3a for AGC of the channel estimation fields LTF1, LTF2, and LTF3, and second sub AGC fields STF1b for the AGC of the data field. Separately from STF2b and STF3b, the transmission timing of STF1a, STF2a, and STF3a is shifted in time, and the transmission timing of STF1b, STF2b, and STF3b is simultaneously improved, thereby improving the reception performance of the data field in the radio base station AP. be able to.

(Sixth embodiment)
Next, a sixth embodiment of the present invention will be described with reference to FIG. FIG. 12 shows a frame configuration of an SDMA transmission frame that does not require control such as temporarily stopping transmission during frame transmission. This is an example in which the wireless terminal STA1 designated by the wireless base station AP and the other wireless terminals STA2 and STA3 simultaneously transmit frames to realize SDMA transmission.

  The radio terminal STA1 designated by the radio base station AP first transmits a synchronization field SYNC, and then transmits a frame configuration instruction field SIG including a signal length and transmission rate information. The frame configuration instruction field SIG may include information such as a guard interval and the number of LTFs described later as information other than the signal length and transmission rate information.

  Next, the wireless terminal STA1 sequentially transmits the AGC field STF1 and the channel estimation fields LTF1a, LTF1b, and LTF1c after transmitting the frame configuration instruction field SIG. The channel estimation fields LTF1a, LTF1b, and LTF1c have different frequencies. When multicarrier transmission such as OFDM transmission is used for communication between the radio base station AP and the radio terminal, the channel estimation fields LTF1a, LTF1b, and LTF1c are transmitted using a plurality of frequencies. For example, in the case of OFDM transmission, subcarriers of frequencies f1, f4, and f7 are used for LTF1a, subcarriers of frequencies f2, f5, and f8 are used for LTF1b, and subcarriers of frequencies f3, f6, and f9 are used for LTF1c. Use.

  On the other hand, the wireless terminal STA2 sequentially transmits the AGC field STF2 and the channel estimation fields LTF2b, LTF2c, and LTF2a from the timing when the transmission of the frame configuration instruction field SIG from the wireless terminal STA1 is completed. Similarly, the wireless terminal STA3 transmits the wireless terminal STA3. The AGC field STF3 and the channel estimation fields LTF3c, LTF3a, and LTF3b are sequentially transmitted from the timing when transmission of the frame configuration instruction field SIG from is completed. Therefore, after transmitting the frame configuration instruction field SIG, the wireless terminals STA1, STA2, and STA3 first transmit the AGC fields STF1, STF2, and STF3 at the same time, and then transmit the first channel estimation fields LTF1a, LTF2b, and LTF3c. The second channel estimation fields LTF1b, LTF2c, and LTF3a are transmitted at the same time, and then the third channel estimation fields LTF1c, LTF2a, and LTF3b are transmitted at the same time. Finally, data fields DATA1, DATA2, and DATA3 are simultaneously transmitted from the wireless terminals STA1, STA2, and STA3.

  Here, the channel estimation fields LTF2a, LTF2b, and LTF2c and LTF3a, LTF3b, and LTF3c transmitted from the wireless terminals STA2 and STA3 use different frequencies in the same manner as the LTF1a, LTF1b, and LTF1c. Accordingly, the channel estimation fields LTF1a, LTF2b, and LTF3c transmitted from the wireless terminals STA1, STA2, and STA3 at the same time are separated in terms of frequency. Similarly, the channel estimation fields LTF1b, LTF2c, LTF3a, and the channel estimation field are separated. The LTF 1c, LTF 2a, and LTF 3b are also separated in frequency. Such frequency separation of the channel estimation fields simultaneously transmitted from the wireless terminals STA1, STA2, and STA3 is called tone interleaving. By doing so, the radio base station AP can correctly perform channel estimation.

  According to the present embodiment, since each of the radio terminals STA1, STA2, and STA3 transmits both the channel estimation field and the data field at the same time, the AGC of both the channel estimation field and the data field functions effectively. Accordingly, the radio base station AP can perform channel estimation for space division with high accuracy and improve data reception performance. Furthermore, according to the present embodiment, even during frame transmission, complicated control such as temporarily stopping transmission is not necessary.

(Seventh embodiment)
Next, a seventh embodiment of the present invention will be described with reference to FIG. In the sixth embodiment, as shown in FIG. 12, only the radio terminal STA1 designated by the radio base station AP transmits the synchronization field SYNC and the frame configuration instruction field SIG, and the other radio terminals STA2 and STA3 Transmission was performed from the AGC field. On the other hand, in the seventh embodiment, as shown in FIG. 13, all wireless terminals STA1, STA2, and STA3 transmit the synchronization field SYNC and the frame configuration instruction field SIG.

  As described in the first embodiment, in the previous embodiments, only the radio terminal STA1 designated by the radio base station AP transmits the synchronization field SYNC and the frame configuration instruction field SIG, so that the non-SDMA compatible terminal Even if it exists, frame collision can be avoided. Therefore, even when all the wireless terminals STA1, STA2, and STA3 transmit the channel estimation field and the data field at the same time as in the sixth embodiment, they are transmitted from a single wireless terminal STA1 as in the seventh embodiment. It is desirable to transmit the same synchronization field SYNC and frame configuration instruction field SIG from a plurality of STAs, which is essentially the same as in the case of performing the above.

  More specifically, in the seventh embodiment, the synchronization field SYNC and the frame configuration instruction field SIG are transmitted from the wireless terminal STA1 as in the previous embodiments. From the wireless terminals STA2 and STA3, a signal obtained by cyclically shifting the synchronization field SYNC and the frame configuration instruction field SIG transmitted from the wireless terminal STA1 on the time axis is transmitted. However, SYNC and SIG transmitted from the wireless terminal STA2 and SYNC and SIG transmitted from the wireless terminal STA3 have different cyclic shift amounts. By transmitting the cyclically shifted signals from the wireless terminals STA2 and STA3 in this way, it is possible to reduce the amount of interference between signals transmitted simultaneously from the wireless terminals STA1, STA2 and STA3. Therefore, it is possible to prevent the hidden wireless terminal problem that occurs due to the signal from the wireless terminals STA1, STA2, and STA3 not reaching the base station AP.

  Here, the cyclic shift amounts of the synchronization field SYNC and the frame configuration instruction field SIG in the wireless terminals STA1, STA2, and STA3 are set as follows. That is, for example, the cyclic shift amounts of the wireless terminals STA1, STA2, and STA3 are set in the order of setting of the destination addresses shown in FIG. 6, or the cyclic shift amounts of STA1, STA2, and STA3 are described in the individual MAC header of FIG. To do. For example, if the method shown in FIG. 6 is used, the cyclic shift amount is 0 for the wireless terminal STA1, the cyclic shift amount is 50 nsec for the wireless terminal STA2, and the cyclic shift amount is 100 nsec for the wireless terminal STA3. . In this case, the relationship between the order of the destination address and the cyclic shift amount may be defined on a one-to-one basis, or the upper limit value of the cyclic shift amount is defined in advance, and the number of wireless terminals that perform SDMA transmission (in this example, An individual cyclic shift amount may be calculated by dividing the upper limit in 3).

  According to the present embodiment, power fluctuations in a section in which the radio base station AP receives the synchronization field SYNC and the frame configuration instruction field SIG, and a section in which the AGC field STF, the channel estimation field LTF, and the data field DATA are received. The difference of becomes smaller. Therefore, it is possible to shorten the AGC processing time started when the AGC field STF is received, and the input dynamic range required for the ADC of the receiver of the radio base station AP can be reduced.

(Eighth embodiment)
Next, an eighth embodiment of the present invention will be described with reference to FIGS. In the present embodiment, a method of obtaining the same effect as the tone interleaving described in the sixth embodiment by multiplexing the AGC field using an orthogonal matrix will be described.

Equation (1) represents an orthogonal matrix when the spatial multiplexing number is 2, and Equation (2) represents an orthogonal matrix when the spatial multiplexing number is 4.

  FIG. 14 and FIG. 15 show examples of arrangement of channel estimation fields LTF that are orthogonalized by using the orthogonal matrices of the equations (1) and (2), respectively. FIG. 16 shows the channel response h1 of the channel (propagation path) from the radio terminal STA1 to the antenna with the radio base station AP, and the channel response h2 of the channel from the radio terminal STA2 to the antenna with the radio base station AP.

  When the spatial multiplexing number is 2, the received signal r1 in the first channel estimation field section of FIG. 14 in the radio base station AP is r1 = (h1 × LTF1) + (h2 × LTF1), and the second channel estimation field. The reception signal r2 in the period is r2 = (h1 × LTF1) − (h2 × LTF1). Therefore, h1 × LTF1 = (r1 + r2) / 2 and h2 × LTF1 = (r1−r2) / 3 can be obtained by processing the received signals r1 and r2 in the radio base station AP.

  Here, since the channel estimation fields LTF1 and LTF2 are known signals, the radio base station AP can perform channel estimation, that is, estimation of channel responses h1 and h2, thereby realizing SDMA transmission. Similarly, when the spatial multiplexing number shown in FIG. 15 is 4, channel estimation can be performed by separating the channel estimation field in the radio base station AP, and SDMA transmission can be realized.

  For example, in the case of the example of FIG. 14, since the same channel estimation field LTF1 is transmitted from the wireless terminals STA1 and STA2 in the first channel estimation field section, there is a possibility that a beamforming effect occurs in which transmission frames interfere with each other. . In order to avoid this, it is desirable that the LTF1 transmitted from the wireless terminal STA2 is cyclically shifted with respect to the LTF1 transmitted from the wireless terminal STA1.

  According to the present embodiment, since the channel estimation field LTF transmitted by each wireless terminal is basically the same frequency, the tone interleaving method described in the sixth embodiment using channel estimation fields of different frequencies in each wireless terminal. There is an advantage in that the amount of memory for holding the pattern of the channel estimation field can be reduced.

(Ninth embodiment)
The wireless terminal can also use MIMO transmission when performing SDMA transmission. Here, a MIMO transmission method in the case of the SDMA transmission frame configuration shown in FIG. 12 according to the sixth embodiment will be described. FIG. 17 shows an example in which the wireless terminal STA1 designated by the wireless base station AP and another wireless terminal STA2 simultaneously transmit frames to realize SDMA by MIMO transmission. 12 differs from FIG. 12 in that terminal STA1 transmits two data streams DATA1-1 and DATA1-2 as data fields in parallel by a plurality of (two in this example) transmission units. In this case, the MIMO multiplexing number is 2.

  Here, in FIG. 17, the wireless terminal STA1 transmits the synchronization field SYNC and the frame configuration instruction field SIG by only one transmission unit, and does not perform MIMO transmission. In general, when signals of the same frequency are transmitted using a plurality of transmission units, directional beams are formed by the transmission signals from the respective transmission units interfering with each other. When a directional beam is formed, the null point where the electric field suddenly drops is also directed in the same direction, so that it is almost impossible to perform reception in the direction of the null point. If a directional beam is formed at the time of transmission by the wireless terminal STA1, if the wireless base station AP exists in the direction of the null point, the wireless base station AP can receive the synchronization field SYNC and the frame configuration instruction field SIG. As a result, the wireless terminals STA1 and STA2 cannot perform SDMA transfer.

  On the other hand, as shown in FIG. 17, if the synchronization field SYNC and the frame configuration instruction field SIGN, at least the synchronization field SYNC, are transmitted by only one transmission unit, a directional beam is not formed. It is possible to ensure that the SYNC is received by the radio base station AP.

  The MIMO transmission can be similarly realized even with the SDMA transmission frame configurations shown in the first to fifth embodiments. In addition, although an example in which the wireless terminal STA1 that transmits the synchronization field SYNC frame performs the MIMO transmission is shown here, wireless terminals other than the STA1 can also perform the MIMO transmission. However, when performing MIMO transmission, it is desirable to perform the following control.

  In the first embodiment, when the radio base station AP performs polling and the radio terminals STA1, STA2 and STA3 perform SDMA transmission, the radio base station AP notifies the radio terminal capable of SDMA transmission only by the MAC address. Was. However, when performing MIMO transmission, even if there is only one wireless terminal that performs SDMA transmission (for example, STA2), the same SDMA transmission frame as when there are the same number of wireless terminals as the MIMO multiplexing number Must be configured. For example, when a wireless terminal transmits two data streams by MIMO transmission, the same SDMA transmission frame configuration as that in the case of two wireless terminals is used.

  In this case, the radio base station AP has to transmit a signal that triggers SDMA transmission in a format that indicates that MIMO transmission is included. For example, by entering two identical MAC addresses in the destination address, a method for indicating that the number of MIMO multiplexing of the wireless terminal of the corresponding MAC address is 2, or in addition to the destination address, the number of MIMO multiplexing (transmitted in parallel) There is a method of providing a field for notifying the number of data streams to be transmitted).

  When performing MIMO transmission, the wireless terminal STA1 may transmit not only the data streams DATA1-1 and DATA1-2 but also the synchronization field SYNC and the frame configuration instruction field SIG in parallel from a plurality of transmission units as shown in FIG. Good. As shown in FIG. 13, not only the wireless terminal STA1 but also the wireless terminal STA2 transmits the SDMA transmission frame from the synchronization field SYNC and the frame configuration instruction field SIGN, in addition to the data stream, the synchronization field SYNC and the frame configuration. The instruction field SIG can be transmitted in parallel from a plurality of transmission units.

(Tenth embodiment)
In the embodiments so far, the example in which the radio base station AP transmits the SDMA transmission trigger frame has been described. In the tenth embodiment, an example will be described in which SDMA transmission is started using frame transmission from a wireless terminal as a trigger.

  FIG. 19 shows an example of frame exchange between the radio base station AP and the radio terminal in such a case. First, in the uplink, the radio terminal STA1 transmits an RTS (Request To Send) frame RTS1 having the radio base station AP as a destination address, and in response to the response, the radio base station AP transmits a CTS (Clear To Send) frame CTS1 in the downlink. Send. Thereafter, the wireless terminal STA1 transmits a data frame Data1. On the other hand, if the wireless terminals STA2 and STA3 other than the STA1 receive the frame RTS1 or the frame CTS1 from the wireless base station AP and determine that SDMA transmission is possible, the wireless terminals STA2 and STA3 perform SDMA transmission together with the wireless terminal STA1.

  Specifically, the transmission of the AGC field STF, the channel estimation field LTF, and the data field DATA is started at the timing when the wireless terminal STA1 completes the transmission of the synchronization field SYNC and the frame configuration instruction field SIG as in the previous embodiments. Thus, SDMA transmission is realized. Detailed transmission timings of the AGC field STF, the channel estimation field LTF, and the data field DATA are the same as those in the previous embodiments, and are omitted here.

  In order for the wireless terminals STA2 and STA3 to determine that SDMA transmission is possible, the frame RTS1 transmitted by the wireless terminal STA1 is multiplexed with SDMA availability information or SDMA transmission as information for prompting other wireless terminals to perform SDMA. A method of adding maximum multiplexing number information for indicating the maximum number possible is conceivable. There is also a method of adding the above-described SDMA availability information and maximum multiplexing number information to the frame CTS1 returned as a response to the frame RTS1 without any particular restriction on the frame RTS1 itself transmitted by the wireless terminal STA1.

  However, when there are a large number of wireless terminals that can perform SDMA transmission by connecting to the wireless base station AP, wireless terminals that exceed the maximum multiplexing number may perform SDMA transmission. That is, at the timing of transmitting the AGC field STF and the channel estimation field LTF, there is a possibility that a plurality of wireless terminals transmit the STF and LTF at the same time. This is similar to a phenomenon in which a collision occurs in an access method called a Slotted-Aloha method.

  As a method of reducing such a collision, (a) the probability of transmitting STF or LTF for data that can be transmitted by SDMA is smaller as the maximum multiplexing number is smaller, that is, the probability of determining to perform SDMA transmission is reduced (for example, Monotonously decreasing method), (b) a method in which data that can be transmitted by SDMA is limited to a case where the frame length is the same as the data transmitted by the wireless terminal STA1, and (c) data that can be transmitted by SDMA is transmitted by the wireless terminal STA1. A method of limiting to a frame shorter than data is conceivable.

  For example, information indicating the frame length of the data frame Data1 is added to the frame RTS1 of FIG. When the wireless terminal STA1 transmits a data frame Data having a short packet length such as a VoIP (Voice over IP) frame, other wireless terminals also transmit a frame shorter than Data1 such as a VoIP frame by SDMA, and normal data Long frames such as frames are not transmitted by SDMA transmission. There is also a method in which quality of service (QoS) type information is added to a frame transmitted by the wireless terminal STA1, and only SDMA transmission is permitted only for data that requires the same QoS. In order to ensure that the number of wireless terminals performing SDMA transmission does not exceed the maximum multiplexing number, there is a method in which the wireless base station AP adds the MAC address of the wireless terminal that permits SDMA transmission to the CTS1 frame.

  FIG. 20 shows another example of frame exchange between the radio base station AP and the radio terminal when SDMA transmission is started using frame transmission of the radio terminal STA1 as a trigger. First, the wireless terminal STA1 transmits a CTS frame CTS1 having its own address as a destination address. Thereafter, the wireless terminal STA1 transmits a data frame Data1. When the other wireless terminals STA2 and STA3 receive the CTS frame CTS1 and determine that SDMA transmission is possible, they perform SDMA transmission with the wireless terminal STA1. Specifically, as in the previous embodiments, the AGC field STF, the channel estimation field LTF, and the data field DATA are transmitted when the wireless terminal STA1 completes the transmission of the synchronization field SYNC and the frame configuration instruction field SIG. By starting, SDMA transmission is realized.

  In the first to ninth embodiments, SDMA is started by using polling from the radio base station AP as a trigger. Therefore, the wireless terminals STA2 and STA3 can calculate the timing of the AGC field STF at which they start transmission from the polling time. On the other hand, in the tenth embodiment, since SDMA transmission is started using frame transmission from the wireless terminal STA1 as a trigger, it is necessary to estimate the timing at which STA1 completes transmission of the frame configuration instruction field SIG. This estimation method is as follows.

  FIG. 21 shows details of the synchronization field SYNC and the frame configuration instruction field SIG in FIG. The synchronization field SYNC is composed of L-STF and L-LTF, and L-STF is composed of 10 L-STSs. In this embodiment, a format conforming to the existing IEEE802.11a standard, which is one of the wireless LAN standards, is used. However, the present invention can also be applied to a wireless packet as studied in the above-described TGn.

  FIG. 22 shows one transmission / reception unit of the wireless terminal in this embodiment. As illustrated in FIG. 22, the wireless terminal includes a wireless reception unit 21, a packet detection unit 22, an L-LTF detection unit 23, a transmission timing control unit 24, a baseband transmission unit 25, and a wireless transmission unit 26.

  Hereinafter, a method for estimating the transmission completion timing of the synchronization field SYNC and the frame configuration instruction field SIG transmitted by the wireless terminal STA1 in the wireless terminal STA2 will be described using FIG. 21 and FIG. The wireless terminal STA2 is assumed to be as shown in FIG.

  The synchronization field SYNC and the frame configuration instruction field SIG transmitted from the wireless terminal STA1 and received by the wireless terminal STA2 are subjected to wireless reception processing (for example, down-conversion, filtering, and analog-digital conversion) by the wireless reception unit 21 in FIG. The received signal (digital baseband signal) obtained as a result is input to the packet detector 22. The packet detection unit 22 detects the output of the matched filter that matches the power increase or L-STS sequence, and detects the packet from the received signal. When the packet detection unit 22 detects a packet, the packet detection unit 22 outputs an operation start command to the L-LTF detection unit 23.

  The L-LTF detection unit 23 detects the boundary between the L-STF and the L-LTF as follows. The L-LTF unit detector 23 calculates the autocorrelation of the received signal. For example, since L-STF is a repetition of L-STS, when autocorrelation is calculated in a section of L-STS length, the autocorrelation value increases in a section in which L-STF is received. On the other hand, since L-LTF transmits a signal different from L-STS and L-STF, the autocorrelation value becomes small. Therefore, in the section where the autocorrelation value is lower than a certain value, the probability that it is the boundary between the L-STF and the L-LTF increases. In this way, the L-LTF detector 23 can detect the boundary between the L-STF and the L-LTF.

  The L-LTF detection unit 23 outputs information indicating the boundary between the L-STF and the L-LTF (referred to as boundary information) to the transmission timing control unit 24. The transmission timing control unit 24 determines the packet transmission timing based on the input boundary information. For example, when transmitting the packet shown in FIG. 10, the wireless terminal STA2 should start transmission of the AGC field STF2 at the timing when the wireless terminal STA1 has transmitted the frame configuration instruction field SIG. Since the wireless terminal STA2 already knows the timing of the boundary between the L-STF and the L-LTF as described above, the transmission timing control unit 24 determines the L-STF from the timing of the boundary between the L-STF and L-LTF. The timing when SIG finishes transmitting is calculated. For example, when L-LTF is 8 μsec and SIG is 8 μsec, the wireless terminal STA2 may transmit the AGC field STF2 after 16 μsec from the timing of the boundary between the L-STF and L-LTF.

  The transmission timing control unit 24 issues a command to the baseband transmission unit at the timing thus calculated, and starts transmission of the AGC field STF2 and subsequent fields. In order to transmit the AGC field STF2, an instruction may be issued only when there is a command from a MAC (Media-Access Control) layer (not shown) as to whether SDMA transmission is currently possible.

  Thus, according to this embodiment, even when there is no polling control from the radio base station AP, the radio terminal STA2 or STA3 can know the timing for transmitting the AGC field STF2 or STF3. Therefore, it is possible to avoid destroying the radio packet by starting transmission of the AGC field STF2 or STF3 at an incorrect timing.

  Detailed transmission timings of the AGC field STF, the channel estimation field LTF, and the data field DATA are the same as those described in the embodiments so far, and are omitted here. In order to determine that other wireless terminals can perform SDMA transmission, a method of adding SDMA enable / disable information and maximum multiplexing number information to the CTS frame CTS1 as described above can be considered.

(Eleventh embodiment)
In the embodiment described so far, the frame length of the SDMA transmission frame transmitted from each wireless terminal is not particularly referred to, and each figure used for the description shows an example in which these frame lengths are equal. However, the frame length of the SDMA transmission frame transmitted from each wireless terminal is not necessarily the same. Therefore, in the present embodiment, a method for realizing SDMA transmission when the frame lengths of SDMA transmission frames transmitted from the respective wireless terminals are different will be described. When this embodiment is applied, SDMA transmission in which the frame length of the SDMA transmission frame transmitted from each wireless terminal is different is possible in all the embodiments described so far.

  First, a case where the SDMA transmission frame from another radio terminal STA2 or STA3 is shorter than the SDMA transmission frame from the radio terminal STA1 will be described. Normally, the radio base station AP determines that SDMA transmission continues for L hours from the result of information exchange with the radio terminal STA1. Information exchange here means, for example, signal length information included in the frame configuration instruction field SIG in the SDMA transmission frame transmitted by the wireless terminal STA1, or transmission permission time (transmission) added to the polling frame transmitted by the wireless base station AP. Permission signal length). The radio base station AP includes a packet length estimation unit to be described later, and even when the SDMA transmission frame transmitted by the radio terminal STA2 or STA3 is shorter than the SDMA transmission frame transmitted by the radio terminal STA1, the radio base station STA2 and It becomes possible to accurately decode the SDMA transmission frame transmitted from the STA3.

  FIG. 23 illustrates the transmission / reception unit of the radio base station AP in the present embodiment. As shown in FIG. 23, the radio base station AP includes radio reception units 31A and 31B, fast Fourier transform (FFT) units 32A and 32B, signal separation unit 33, demapping units 34A and 34B, packet length estimation unit 35, and Viterbi. Decoding units 36A and 36B are provided. In the present embodiment, for simplicity, the radio base station AP includes two radio reception units 31A and 31B, and only the radio terminals STA1 and STA2 transmit an SDMA transmission frame.

  Hereinafter, a method for estimating the packet length of the SDMA transmission frame from the wireless terminals STA2 and STA3 in the packet length estimation unit 35 will be described.

  Reception signals from the radio reception units 31A and 31B are separated into signals for each subcarrier in the FFT units 32A and 32B. The signal separated for each subcarrier is input to the signal separation unit 33, and the signals of the SDMA transmission frames from the radio terminals STA1 and STA2 are further separated. Since the separation algorithm can use a known MIMO technique, description thereof is omitted. The separated signals are input to the demapping units 34A and 34B, respectively.

  The demapping units 34A and 34B calculate soft decision values from the positions of the received signal points and the predicted signal points of the SDMA transmission frame transmitted from the wireless terminals STA2 and STA3. The soft decision values calculated by the demapping units 34A and 34B are input to the Viterbi decoding units 36A and 36B, respectively. The soft decision value output from the demapping unit 34B is sent to the packet length estimation unit 35.

  Referring to FIG. 24, when using quadrature phase shift keying (QPSK (Quadrature Phase Shift Keying)) for transmission of the SDMA frame, the signal separated in the signal separation unit 33, that is, the modulation point of each data subcarrier is It appears at any point on the IQ plane as shown by the white circle in Fig. 24. When the noise is large, the received signal points are dispersed around the position of the white circle, if there is an SDMA transmission frame from the wireless terminal STA2. If it ends with a symbol, the signal appears to be zero, that is, near the intersection of the I axis and the Q axis in Fig. 24. The packet length estimation unit 35 indicates that the signal point appears near the intersection of the I axis and the Q axis. Thus, it can be estimated that the packet of the SDMA transmission frame from the wireless terminal STA2 has ended, in which the output of the data subcarrier is used for the estimation of the packet length. It is also possible to use the output of the pilot subcarriers transmitted in BPSK (Binary Phase Shift Keying) shown in FIG. 24.

  When the packet length estimation unit 35 finds that the packet of the SDMA transmission frame from the wireless terminal STA2 has ended, the packet length estimation unit 35 decodes the signal of the packet of the SDMA transmission frame from the STA2 to viterbi decoding unit A command is issued to 36B to notify the symbol that terminates Viterbi decoding.

  As described above, according to the present embodiment, even when the frame length of the SDMA transmission frame transmitted from the wireless terminal STA2 is shorter than the frame length of the SDMA transmission frame transmitted from the wireless terminal STA1, the wireless base station AP It becomes possible to accurately demodulate the SDMA transmission frame by estimating the frame length.

(Twelfth embodiment)
FIG. 25 shows still another example of the SDMA transmission frame configuration as the twelfth embodiment of the present invention. In the frame configuration in FIG. 25, second frame configuration instruction fields SIG-S1, SIG-S2, and SIG-S3 are added to the frame configuration shown in FIG. That is, the second frame configuration instruction fields SIG-S1, SIG-S2, and SIG-S3 are transmitted immediately before the data fields DATA1, DATA2, and DATA3 (after transmission of the channel estimation field LTF), and are respectively at least DATA1, DATA2, and Includes the signal length of DATA3.

  The signal length included in each of the frame configuration instruction field SIG and the second frame configuration instruction field SIG-S1 transmitted by the wireless terminal STA1 is basically equivalent information, but the SIG is a signal transmitted to the SDMA non-compliant wireless terminal. Sent to notify the length. Therefore, when the SDMA non-compliant radio terminal is not connected to the same radio communication system, the SIG may be removed, and the entire contents of the SIG may be arranged at the SIG-S position. Even when a non-SDMA compatible wireless terminal is connected to the same wireless communication system, the presence / absence of the frame configuration instruction field SIG is dynamically determined depending on whether the non-SDMA compatible wireless terminal is connected or not connected. You may switch to.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the components without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

1 is a schematic diagram of a wireless communication system according to an embodiment of the present invention. A block diagram showing a configuration example of a wireless terminal in the same embodiment The figure which shows the example of a frame exchange between the wireless base station and wireless terminal in SDMA transmission The figure which shows the other example of frame exchange between the wireless base station and wireless terminal in SDMA transmission The figure which shows the structural example of a SDMA transmission frame The figure which shows the example of the SDMA trigger frame transmitted from a radio base station The figure which shows the other example of the SDMA trigger frame transmitted from a wireless base station The figure which shows the other structural example of the frame for SDMA transmission The figure which shows the other structural example of an SDMA transmission frame The figure which shows the other structural example of an SDMA transmission frame The figure which shows the other structural example of an SDMA transmission frame The figure which shows the other structural example of an SDMA transmission frame The figure which shows the other example of frame exchange between the wireless base station and wireless terminal in SDMA transmission The figure which shows the other example of frame exchange between the wireless base station and wireless terminal in SDMA transmission The figure which shows the other structural example of an SDMA transmission frame The figure which shows the channel response of the channel from wireless terminal STA1, STA2 to wireless base station AP The figure which shows the other structural example of an SDMA transmission frame The figure which shows the other structural example of an SDMA transmission frame The figure which shows the other example of frame exchange between the wireless base station and wireless terminal in SDMA transmission The figure which shows the other example of frame exchange between the wireless base station and wireless terminal in SDMA transmission The figure which shows the detail of the synchronous field SYNC and frame structure instruction | indication field SIG in FIG. The block diagram which shows the structure of the transmission / reception part of a wireless terminal Block diagram showing the configuration of the transceiver unit of the radio base station The figure which shows IQ plane of QPSK and BPSK for demonstrating the estimation operation | movement of the packet length estimation part in FIG. The figure which shows the other structural example of an SDMA transmission frame

Explanation of symbols

AP: wireless base station;
STA1, STA2, STA3 ... wireless terminals;
11 ... Antenna;
12 ... transmission / reception switching unit;
13 ... receiving part;
14: Transmitter;
15 ... Received frame analysis unit;
16 ... transmission frame forming unit;
21 ... Wireless receiver;
22 ... packet detection unit;
23 ... L-LTF detector;
24... Transmission timing control unit;
25 ... baseband transmission unit;
26: wireless transmission unit;
31A, 31B ... wireless receiver;
32A, 32B ... FFT section;
33: Signal separation unit;
34A, 34B ... demapping unit;
35 ... packet length estimation unit;
36A, 36B ... Viterbi decoding unit

Claims (25)

A synchronization field for synchronizing with the radio base station from a first radio terminal designated by the radio base station, and a first AGC field for controlling a reception gain for a signal transmitted from the radio base station Transmitting a first transmission frame including a first channel estimation field and a first data field for estimating a channel with the radio base station to the radio base station;
A second AGC field for controlling a reception gain for a signal transmitted from the radio base station from a second radio terminal, a second channel estimation field for estimating a channel with the radio base station, and Transmitting a second transmission frame including two data fields to the radio base station,
The wireless communication method configured to transmit the first transmission frame and the second transmission frame at different timings of the first channel estimation field and the second channel estimation field. In a radio communication system having a plurality of radio terminals including one radio base station and a first radio terminal and a second radio terminal,
A synchronization field for synchronizing with the radio base station, a first AGC field for controlling a reception gain for a signal transmitted from the radio base station, and a channel estimation with the radio base station are performed. A first radio terminal designated by the radio base station for transmitting a first transmission frame including a first channel estimation field and a first data field for the radio base station;
A second AGC field for controlling a reception gain for a signal transmitted from the radio base station; a second channel estimation field for estimating a channel between the radio base station and a second data field; A second wireless terminal for transmitting a transmission frame to the wireless base station,
The wireless communication system configured to transmit the first transmission frame and the second transmission frame at different timings of the first channel estimation field and the second channel estimation field.   The first transmission frame is configured to transmit a frame configuration indication field having signal length information indicating a length from the first AGC field to the first data field after transmission of the synchronization field. The wireless communication system described.   The wireless communication system according to claim 2, wherein the first transmission frame and the second transmission frame are configured such that the first AGC field and the second AGC field are transmitted at the same timing.   The wireless communication system according to claim 2, wherein the first transmission frame and the second transmission frame are configured so that the first AGC field and the second AGC field are transmitted at different timings.   The wireless communication system according to any one of claims 2 to 5, wherein the first transmission frame and the second transmission frame are configured such that the first data field and the second data field are transmitted at the same timing. .   The first transmission frame and the second transmission frame include first and second sub-AGC fields for performing AGC of the first channel estimation field and the second channel estimation field as the first AGC field and the second AGC field, The wireless communication system according to claim 2, wherein the third and fourth sub-AGC fields for performing AGC of the first data field and the second data field are transmitted.   The first transmission frame and the second transmission frame are configured such that the first and second sub AGC fields are transmitted at different timings, and the third and fourth sub AGC fields are transmitted at the same timing. The wireless communication system according to claim 7. )
The first transmission frame and the second transmission frame are configured to sequentially transmit a plurality of sub-estimation fields having different frequencies as the first channel estimation field and the second channel estimation field, and are transmitted simultaneously. The wireless communication system of claim 2, wherein the fields are configured to have different frequencies.   The wireless communication system according to claim 1, wherein the second transmission frame is configured to transmit a second synchronization field obtained by cyclically shifting the synchronization field simultaneously with the synchronization field.   The first transmission frame is configured to transmit a frame configuration indication field having signal length information after transmission of the synchronization field, and the second transmission frame cyclically transmits the synchronization field simultaneously with the synchronization field. 2. The shifted second synchronization field is transmitted, and the second frame configuration indication field is generated by cyclically shifting the frame configuration indication field simultaneously with the frame configuration indication field. Wireless communication system.   The first transmission frame and the second transmission frame are configured to sequentially transmit a plurality of orthogonal sub-estimation fields as the first channel estimation field and the second channel estimation field, respectively. Wireless communication system.   3. The first transmission frame is configured such that the second data field is transmitted after the first channel estimation field is transmitted and after at least a time longer than the time length of the first channel estimation field has elapsed. The wireless communication system according to 1.   The wireless communication system according to claim 13, wherein the time interval is an integral multiple of a time length of the first channel estimation field.   The wireless communication system according to claim 13, wherein the time interval is an integral multiple of a time length of the first channel estimation field and a time length of the first AGC field.   The transmission timing of the second AGC field and the second channel estimation field in the second transmission frame is set between time intervals between the second channel estimation field and the second data field. Wireless communication system.   In the first transmission frame, transmission start times of the first AGC field and the first channel estimation field are set between time intervals that are integral multiples of the time lengths of the first channel estimation field and the second channel estimation field. The wireless communication system according to claim 2, wherein the second transmission frame has a transmission start time of the second AGC field set equal to a transmission start time of the second AGC field.   The radio base station is configured to simultaneously transmit a polling frame to the first radio terminal and the second radio terminal, and the first radio terminal and the second radio terminal transmit the polling frame to the first radio terminal and the second radio terminal, respectively. The wireless communication system according to claim 3, wherein the wireless communication system is configured to simultaneously transmit data frames as the first transmission frame and the second transmission frame after a predetermined time from reception.   The radio base station is configured to simultaneously transmit a data frame to the first radio terminal and the second radio terminal, and the first radio terminal and the second radio terminal transmit the data frame. The wireless communication system according to claim 2, wherein the wireless communication system is configured to simultaneously transmit a delivery confirmation frame as the first transmission frame and the second transmission frame after a predetermined time from reception.   The radio base station is configured to transmit a trigger frame that notifies the plurality of radio terminals as to which one of the first radio terminal and the second radio terminal operates. Wireless communication system.   The radio base station groups the plurality of radio terminals based on reception power of signals transmitted from the plurality of radio terminals, and sets the radio terminals in the same group as the first radio terminal and the second radio terminal. The wireless communication system of claim 2, configured to select as   The first radio terminal and the second radio terminal may use the first transmission frame and the second radio terminal so that reception powers of the first transmission frame and the second transmission frame in the radio base station become set values. The wireless communication system according to claim 2, further comprising transmission power control means for controlling transmission power of the transmission frame.   The first wireless terminal includes a plurality of transmission units, and the first data field is transmitted by a plurality of transmission units, and the synchronization field is configured to be transmitted by a single transmission unit. The wireless communication system according to claim 2.   The first wireless terminal transmits an RTS (Request To Send) frame with the wireless base station as a destination address, and receives a CTS (Clear To Send) frame in response to the RTS frame from the wireless base station Thereafter, the second wireless terminal is configured to transmit a data frame as the first transmission frame, and the second wireless terminal receives the frame RTS frame or the CTS frame and then transmits the data frame as the second transmission frame. The wireless communication system according to claim 2, configured to transmit.   The first wireless terminal transmits a data frame as the first transmission frame after transmitting a CTS (Clear To Send) frame having a local address as a destination address, and the second wireless terminal The wireless communication system according to claim 2, configured to transmit a data frame as the second transmission frame after receiving a CTS frame.

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