Interoperability between different wireless LAN systems

BACKGROUND OF THE INVENTION

Field of the Invention

0001The present invention relates to a wireless LAN system for transmitting a signal for occupying a radio medium.

Related Background of the Invention

0002Conventionally, a wireless LAN system in compliance with the IEEE 802.11a (hereinafter, referred to simply as "11a") standard having a maximum transmission speed of 54 Mbit/s is proposed, in addition to a wireless LAN system in compliance with the IEEE 802.11b (hereinafter, referred to simply as "11b") standard having a transmission speed of 11 Mbit/s. Moreover, the IEEE 802.11g (hereinafter, referred to simply as "11g") standard having both features of the IEEE 802.11b and IEEE 802.11a is proposed. Hereinafter, an access point and a station in a wireless LAN system in compliance with these standards are referred to as a "legacy terminal", a legacy access point as a "legacy AP", and a legacy station as a "legacy STA."

0003In recent years, a transmission speed as high as 100 Mbit/s is required in a wireless LAN system. A method is proposed as one of methods for fulfilling the need, in which the transmission speed is increased by using a frequency band n times (n is a natural number) that of a legacy terminal. Hereinafter, an access point and a station in a wireless LAN system in which such a high-speed transmission is performed are referred to as an "HT (High Throughput) terminal", an HT access point as an "HTAP", and an HT station as an "HTSTA". In <figref idref="f0040">Fig.40</figref>, the HT terminal includes a frequency band f 1 used by the legacy terminal and uses, for example, a frequency band fh having a band width three times that of the legacy terminal. At present, communication by an HT terminal and a legacy terminal using the same frequency band, that is, the coexistence of different wireless LAN systems is under examination.

0004Conventionally, examples of coexistence of different wireless LAN systems using the same frequency band include the coexistence of the 11b system and the 11g system. This wireless LAN system employs such a configuration as shown in <figref idref="f0041">Fig.41</figref>. In the configuration, an access point 100 of 11g controls both of a station 101 of 11g and a station 102 of 11b.

0005In the case of this configuration, it becomes possible for the station 101 of 11g to perform a high-speed transmission by transmitting a data frame in the OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme, but the station 102 of 11b recognizes the OFDM signals as interfering signals from other systems because the OFDM signals are not the signals of 11b. Then, depending on a signal level, there may be a case where the station 102 of 11b transmits a data frame in the CCK (Complementary Code Keying) modulation scheme. In such a case, a collision of packets occurs and the throughput is deteriorated.

0006In order to avoid this, as shown in <figref idref="f0043">Fig.43</figref>, before transmitting an OFDM signal, the station 101 of 11g transmits a CTS (Clear To Send: a signal to notify the completion of reception) in the DSSS (Direct Sequence Spread Spectrum) modulation scheme, which can be demodulated by the station 102 of 11b and a station, which is not shown, in compliance with the IEEE802.11 (hereinafter, referredto simply as "11"). Due to this, an NAV (Network Allocation Vector: suppression of transmission by a virtual carrier sense) is set to the station 102 of 11b and the station of 11, which is not shown, thereby a collision is avoided.

0007Here, the frame format of CTS employs a configuration shown in <figref idref="f0044">Fig.44</figref>. That is, the frame format consists of a frame control section 103, a duration section 104, and a receiver address 105 as an MAC (Media Access Control) header, and an FCS (Frame Check Sequence) section 106. Other terminals that have received such a CTS refrain from performing transmission by setting an NAV in the interval described in the duration section 104. Due to this, a collision is avoided.

0008Also in the case where the station 102 of 11b has already performed transmission, it is possible for the station 101 of 11g to demodulate or recognize the signal transmitted from the station 102 of 11b, therefore, a collision can be avoided. In other words, as shown in <figref idref="f0045">Fig.45</figref>, the station 101 of 11g demodulates (or recognizes) the data frame (CCK) transmitted from the station 102 of 11b and sets a backoff to avoid a collision, then transmits an OFDM signal. <ul id="ul0001" list-style="none" compact="compact"><li>[Non-patent document 1] "System Capacity Comparison with various High Data Rate Wireless LAN Systems", Satoru Hori et al., The 2002 IEICE (Institute of Electronics, Information and Communication Engineers) General Conference, B-5-250</li><li>[Non-patent document 2] "802.11 High-speed Wireless LAN Textbook", Hideaki Matsue et al., 2003, IDG Japan</li><li>[Non-patent document 3] IEEE Std 802.11g-2003</li></ul>

0009<patcit id="pcit0001" dnum="US20020181426A"><text>US 2002/0181426</text></patcit> discloses an interference suppression method, wherein an 802.11 source station transmits a signal with the duration field other than that required for the transmission to prevent transmission by other stations during known sequences. Thus, the source station uses the duration field to spoof the actual time the medium will be occupied, to stations within range of the signal. A station within range of the transmitted signal will check the duration field of the transmitted signal, and update the station's network allocation vector. Thus, the station will not transmit because the station's network allocation vector indicates that the medium is in use, even though the station maybe unable to hear the carrier. Accordingly, spoofed stations may, for example, 1) delay transmission until a more critical transmission has completed, 2) allow unknown or foreign protocol to have preferential use of the medium, 3) prevent interference from hidden stations, and 4) allow sharing of the medium by overlapping basic service sets.

SUMMARY OF THE INVENTION

0010When an HTAP performs communication with an HTSTA, as shown in <figref idref="f0046">Fig. 46</figref>, it can be expected that communication is performed using an HT transmission for all of the beacon (HT beacon) as a notification signal transmitted at fixed intervals, the control frame (HT control frame), and the data frame (HT data frame), that is, using a frequency band wider than that of a legacy terminal, due to this, a high speed transmission is realized. Hereinafter, a series of procedure shown in <figref idref="f0046">Fig.46</figref> is referred to as an "HT sequence". The downward arrow in the diagram shows the transmission and reception of a signal from HTAP to HTSTA and the upward arrow shows the transmission and reception of a signal from HTSTA to HTAP. "HT ACK" shows a signal for confirming the completion of reception to the "HT control frame" or "HT DATA frame".

0011However, as shown in <figref idref="f0047">Fig. 47</figref>, when HT terminals 110 to 112 and legacy terminals 113 and 114 perform communication in the same frequency band, it is not possible to mutually recognize signals transmitted from each other. Because of this, the HT terminals 110 to 112 and the legacy terminals 113 and 114 recognize the transmitted signals from other terminals as interfering signals from other systems and therefore a collision of packets occurs frequently. As a result, the throughput is deteriorated. In other words, there arises a problem of considerable deterioration in communication speed between legacy terminals and between HT terminals.

0012The above-mentioned circumstances being taken into account, the present invention has been developed and an object thereof is to provide a high-speed wireless LAN system capable of avoiding a collision of packets and maintaining the original transmission speed.

0013According to the present invention there is provided a transmitter applied to a second radio communication system using a second frequency band which contains all or part of the first frequency band and is wider than a first frequency band used in a first radio communication system, comprising: an occupation signal generation section for generating an occupation signal compatible with said first radio communication system and notifying said first radio communication system that said second radio communication system occupies a radio medium; and a transmission section for performing communication in said second radio communication system after transmitting said occupation signal using every frequency in said second frequency band; said transmission section transmits a start signal for notifying that the period during which communication is performed in said second radio communication system starts.

0015The present invention is defined by appended independent claims 1, 9, 10, 11 and 12.

BRIEF DESCRIPTION OF THE DRAWINGS

0016<ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Fig.1</figref> is a block diagram of a transmitter of an HTSTA;</li><li><figref idref="f0002">Fig.2</figref> is a block diagram of a transmitter of a legacy STA;</li><li><figref idref="f0003">Fig.3</figref> is a block diagram of a transmitter of an HTAP;</li><li><figref idref="f0004">Fig.4</figref> is a block diagram of a symbol repeating circuit of the transmitter of the HTAP;</li><li><figref idref="f0005">Fig.5</figref> is a diagram showing a signal transmitted by the transmitter of the HTAP;</li><li><figref idref="f0006">Fig.6</figref> is a diagram showing a signal transmitted by the transmitter of the HTAP;</li><li><figref idref="f0007">Fig.7</figref> is a block diagram of a receiver of the HTSTA;</li><li><figref idref="f0008">Fig.8</figref> is a block diagram of a receiver of the HTSTA;</li><li><figref idref="f0009">Fig.9</figref> is a block diagram of a thinning circuit of the receiver of the HTSTA;</li><li><figref idref="f0010">Fig. 10</figref> is a diagram showing how an HTAP suppresses transmission for a legacy AP;</li><li><figref idref="f0011">Fig. 11</figref> is a diagram showing how packets collide with each other when an HT terminal and a legacy terminal coexist;</li><li><figref idref="f0012">Fig.12</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0013">Fig.13</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0014">Fig.14</figref> is a diagram showing an MAC frame format ;</li><li><figref idref="f0015">Fig.15</figref> is a diagram showing an MAC frame format;</li><li><figref idref="f0016">Fig.16</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0017">Fig.17</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0018">Fig.18</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0019">Fig.19</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0020">Fig.20</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0021">Fig.21</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0022">Fig.22</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0023">Fig.23</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0024">Fig.24</figref> is a diagram showing a configuration of data transmitted by an HTAP;</li><li><figref idref="f0025">Fig.25</figref> is a diagram showing a configuration of data transmitted by an HTAP and a legacy AP;</li><li><figref idref="f0026">Fig.26</figref> is a diagram showing a configuration of data transmitted by an HTAP and a legacy AP;</li><li><figref idref="f0027">Fig.27</figref> is a diagram showing a configuration of data transmitted by an HTAP and a legacy AP;</li><li><figref idref="f0028">Fig.28</figref> is a block diagram of an HTSTA;</li><li><figref idref="f0029">Fig.29</figref> is a block diagram of an HTSTA;</li><li><figref idref="f0030">Fig.30</figref> is a diagram showing an interval in which an HTAP and a legacy AP can perform transmission;</li><li><figref idref="f0031">Fig.31</figref> is a diagram showing an interval in which an HTAP and a legacy AP can perform transmission;</li><li><figref idref="f0032">Fig.32</figref> is a diagram showing a CTS frame format;</li><li><figref idref="f0033">Fig.33</figref> is a flow chart showing operations when an HTSTA receives a legacy CTS;</li><li><figref idref="f0034">Fig.34</figref> is a block diagram showing a general configuration of a transmitter of an HT terminal used in a 2.4 GHz band;</li><li><figref idref="f0035">Fig.35</figref> is a block diagram showing a detailed configuration of a DSSS signal transmitting circuit section;</li><li><figref idref="f0036">Fig.36</figref> is a block diagram showing a general configuration of a receiver of the HT terminal used in a 2.4 GHz band;</li><li><figref idref="f0037">Fig.37</figref> is a block diagram showing a detailed configuration of a DSSS signal reception circuit section 363;</li><li><figref idref="f0038">Fig. 38</figref> is a diagram showing a frequency band width used by an HT terminal and a frequency band width used when a signal compatible with a legacy terminal is transmitted in a 5 GHz band;</li><li><figref idref="f0039">Fig. 39</figref> is a diagram showing a frequency band width used by an HT terminal and a frequency band width used when a signal compatible with a legacy terminal is transmitted in a 2.4 GHz band;</li><li><figref idref="f0040">Fig.40</figref> is a diagram showing a frequency band used by an HT terminal and a legacy terminal;</li><li><figref idref="f0041">Fig.41</figref> is a diagram showing a general configuration in a conventional wireless LAN system;</li><li><figref idref="f0042">Fig.42</figref> is a diagram showing an interval of data transmitted in the conventional wireless LAN system;</li><li><figref idref="f0043">Fig.43</figref> is a diagram showing an interval of data transmitted in the conventional wireless LAN system;</li><li><figref idref="f0044">Fig.44</figref> is a diagram showing a CTS frame format;</li><li><figref idref="f0045">Fig.45</figref> is a diagram showing an interval of data transmitted in the conventional wireless LAN system;</li><li><figref idref="f0046">Fig.46</figref> is a diagram showing an HT sequence; and</li><li><figref idref="f0047">Fig.47</figref> is a diagram showing how packets collide with each other when an HT terminal and a legacy terminal coexist.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0017In the wireless LAN system according to the present invention, an HT terminal uses a frequency band n times (n is a natural number) that of a legacy terminal. In an example shown in <figref idref="f0040">Fig.40</figref>, the HT terminal uses a frequency band the band width of which is three times that of the legacy terminal. Due to this, a high-speed transmission can be realized between HT terminals.

0018An HTAP has the compatibility with a legacy terminal at every frequency (the entire band) at which communication is performed with an HTSTA and transmits an occupation signal for notifying the occupation of the radio medium in a form (for example, CTS) that the legacy terminal can demodulate. Due to this, the HTAP logically occupies the time for performing an HT sequence with respect to the legacy terminal. By the way, the notification signal means a beacon and the occupation signal means a CTS. Moreover, as will be described later, it is also possible to use a CF-poll or an MAC frame as an occupation signal instead of the CTS.

(First arrangement)

0019<figref idref="f0001">Fig.1</figref> a block diagram showing a configuration of a transmitter of an HTSTA applied to a wireless LAN system according to a first arrangement and <figref idref="f0002">Fig.2</figref> is a block diagram showing a configuration of a transmitter of a legacy STA applied to a legacywireless LAN system. In <figref idref="f0001">Fig. 1</figref>, in a transmitter 1 of the HTSTA, transmission data are inputted in an MAC circuit 2 and a transmitting/receiving method of a frame, which is a transmission/reception unit of data, a frame format, etc., are defined. An Forward Error Correction Coder 3 performs error correction coding for a signal inputted from the MAC circuit 2. A modulator 4 performs modulation such as BPSK (Binary PSK) or QPSK (Quadrature PSK) for a signal inputted from the Forward Error Correction Coder 3. An IFFT circuit 5 converts a signal inputted from the modulator 4 from a frequency signal into a time signal. A filter circuit 6 allows only the signals in a desired band among signals inputted from the IFFT circuit 5 to pass for output and the output signals are transmitted through an antenna 7. The Forward Error Correction Coder 3, the modulator 4, the IFFT circuit 5, and the filter circuit 6 compose a transmitting circuit 1a.

0020As shown in <figref idref="f0002">Fig.2</figref>, also in transmitter 10 of the legacy STA, an MAC circuit 11, an Forward Error Correction Coder 12, a modulator 13, an IFFT circuit 14, a filter circuit 15, and an antenna 16 function substantially in the same manner as those in the transmitter 1 of the HTSTA. The Forward Error Correction Coder 12, the modulator 13, the IFFT circuit 14, and the filter circuit 15 compose a transmitting circuit 10a.

0021However, the IFFT circuit 5 in the transmitter 1 of the HTSTA uses an IFFT circuit the number of points of which is n times (here, a case where n=3 is shown, for example) that of the IFFT circuit 14 in the transmitter 10 of the legacy STA. Moreover, the filter circuit 6 in the transmitter 1 of the HTSTA uses a filter circuit the pass band width of which is n times (here, a case where n=3 is shown, for example) that of the filter circuit 15 in the transmitter 10 of the legacy STA. Due to this, in the HTSTA, a high-speed transmission using a band width n times that of the legacy STA is made possible.

0022<figref idref="f0003">Fig.3</figref> is a block diagram showing a configuration of a transmitter of an HTAP applied to the wireless LAN system according to the first arrangement. In a transmitter 20 of the HTAP, an MAC circuit 21 is controlled by a control circuit 21a to define a transmitting/receiving method of a frame, a frame format, etc., for transmission data. An Forward Error Correction Coder 22 performs error correction coding for a signal inputted from the MAC circuit 21. A modulator 23 performs modulation such as BPSK (Binary PSK) or QPSK (Quadrature PSK) for a signal inputted from the Forward Error Correction Coder 22. A switching device 24 is controlled by the control circuit 21a to switch a signal inputted from the modulator 23 to a symbol repeating circuit 25 or an IFFT circuit 26 for output.

0023The symbol repeating circuit 25 repeatedly outputs transmission signal data corresponding to an occupation signal generated in the MAC circuit 21, to which error correction coding and modulation processing have been performed, n times (n is a natural number) inpredeterminedsymbol units. Specifically, the symbol is repeated n times for the signal inputted from the switching device 24 and the signal is outputted to the IFFT circuit 26. The symbol repeating circuit 25 repeats an input signal n times in OFDM symbol units (here, a case where n=3 is shown, for example).

0024The IFFT circuit 26 converts a signal inputted from the switching device 24 or the symbol repeating circuit 25 from a frequency signal into a time signal. A filter circuit 27 allows only the signals in a desired band among the output signals of the IFFT circuit 26 for output and the output signals are transmitted through an antenna 28. The Forward Error Correction Coder 22, the modulator 23, the switching device 24, the symbol repeating circuit 25, the IFFT circuit 26, and the filter 27 compose a transmitting circuit 20a. When outputting the output from the modulator 23 directly to the IFFT circuit 26 by means of the switching device 24, the transmitter 20 of this HTAP can transmit an HT signal using a band width n times that of the legacy terminal as shown in <figref idref="f0005">Fig.5</figref>. When outputting the output from the modulator 23 to the IFFT circuit 26 via the symbol repeating circuit 25 by means of the switching device 24, it becomes possible to transmit a signal having a shape of three identical legacy signals piled up along the frequency axis as shown in <figref idref="f0006">Fig.6</figref>.

0025In the above description, a configuration is employed, in which the symbol repeating circuit 25 is provided between the modulator 23 and the IFFT circuit 26, but the configuration is not limited to this. Other configurations may be employed as long as the same functions as those in this configuration are provided.

0026In the description given below, a part consisting of the transmitting circuit 20a excluding the symbol repeating circuit 25, the MAC circuit 21, and the control circuit 21a is defined as a transmission section 300. A part consisting of the transmitting circuit 20a including the symbol repeating circuit 25, the MAC circuit 21, and the control circuit 21a is defined as an occupation signal generation section 301. In other words, it is assumed that a signal compatible with a legacy terminal is transmitted by directing transmission of a legacy signal from the control circuit 21a included in the occupation signal generation section 301 and outputting transmission signal data from the MAC circuit 21 to the transmitting circuit 20a, and a signal compatible with an HT terminal is transmitted by directing transmission of an HT signal from the control circuit 21a included in the transmission section 300 and outputting transmission signal data from the MAC circuit 21 to the transmitting circuit 20a.

0027<figref idref="f0007">Fig.7</figref> is a block diagram showing a configuration of a receiver of the HTSTA applied to the wireless LAN system according to the first arrangement. Here, a case where the receiver is applied to a wireless LAN system including HT beacons in an HT sequence is explained. In other words, in the case of a wireless LAN system including HT beacons in an HT sequence, only the HT signals need to be demodulated. As shown in <figref idref="f0007">Fig.7</figref>, in a receiver 30 of the HTSTA, a radio signal received by an antenna 31 undergoes a process in which only the signals in a desired band are allowed to pass by a filter circuit 32. The output signal of the filter circuit 32 is converted from a time signal into a frequency signal by an FFT circuit 33. A demodulator 34 converts a signal inputted from the FFT circuit 33 into a bit string based on the signal point. An Forward Error Correction Decoder 35 performs error correction decoding for a signal inputted from the demodulator 34. An MAC circuit 36 reads out the transmitting/receiving method of a frame, the frame format, etc. , and outputs the received data. The filter circuit 32, the FFT circuit 33, the demodulator 34, and the Forward Error Correction Decoder 35 compose a receiving circuit 30a.

0028On the other hand, a receiver of the HTSTA applied to the wireless LAN system, in which HT beacons are not included in the HT sequence and legacy beacons need to be demodulated, employs such a configuration as shown in <figref idref="f0008">Fig.8</figref>. That is, in a receiver 40, a radio signal received by an antennal 41 undergoes a process in which only the signals in a desired band are allowed to pass by a filter circuit 42. The output signal of the filter circuit 42 is converted from a time signal into a frequency signal by an FFT circuit 43. A switching device 44 switches the output signal of the FFT circuit 43 to a thinning circuit 45 or a demodulator 46 for output. The thinning circuit 45 cuts out the first one n-th of each OFDM symbol for the signal inputted from the switching device 44, as shown in <figref idref="f0009">Fig. 9</figref> (here, a case where n=3 is shown, for example). The demodulator 46 converts the signal inputted from the switching device 44 or the signal inputted from the thinning circuit 45 intoabit string based on the signal point. An Forward Error Correction Decoder 47 performs error correction decoding for the signal inputted from the demodulator 46. An MAC circuit 48 reads out the frame transmission/reception method, the frame format, etc., based on the signal inputted from the Forward Error Correction Decoder 47 and outputs the received data. The filter circuit 42, the FFT circuit 43, the switching device 44, the thinning circuit 45, the demodulator 46, and the Forward Error Correction Decoder 47 compose a receiving circuit 40a.

0029In the above description, a configuration in which the thinning circuit 45 is interposed between the FFT circuit 43 and the demodulator 46 is employed, but the configuration is not limited to this. Other configurations may be employed as long as the same functions as those in this configuration are provided.

0030Next, the operation in the wireless LAN system according to the first arrangement is explained. <figref idref="f0010">Fig.10</figref> shows how an HTAP suppresses transmission for a legacy AP. As shown in <figref idref="f0010">Fig.10</figref>, when an HTAP and a legacy AP coexist in the same frequency band, the HTAP transmits a legacy beacon, a legacy CTS, and an HT sequence (HT beacon, HT data) in this order. Due to this, it is possible to set an NAV for the legacy AP and the HT sequence can be prevented from a collision of packets. Moreover, it also becomes possible to notify the legacy AP of the presence of another AP in the frequency currently used by transmitting the legacy beacon. Here, in the case of a 5 GHz band, the legacy beacon and the legacy CTS mean a beacon of 11a format and a CTS of 11a format. In the case of a 2.4 GHz band, they mean a beacon of 11 format and a CTS of 11 format.

0031As shown in <figref idref="f0011">Fig. 11</figref>, it may also be possible to employ a method in which an HTAP 110 simply transmits a CTS immediately before transmitting HT data and occupies the interval for transmitting the HT data. However, if only this method is employed, it is expected that, for example, transmission of a signal from a legacy STA 114 is started after the transmission of the HT data is completed and the signal and a CTS transmitted by the HTAP 110 collide with each other.

0032Therefore, as shown in <figref idref="f0010">Fig. 10</figref>, it is preferable to occupy the radio medium all the time in every band. By employing this method, it is possible to avoid a collision of packets between the HT terminal and the legacy terminal. In other words, by occupying the radio medium all the time, the possibility is eliminated that the legacy terminal performs transmission during the period of transmission of packets by the HTAP and, by transmitting a signal notifying that the HTAP occupies the radio medium, it is unlikely that the signal from the legacy terminal exists before the HT sequence. In particular, it is difficult for the HT terminal to demodulate (or recognize) the signal transmitted by the legacy terminal, therefore, the fact that no transmission signal from the legacy terminal exists before the HT sequence is really essential in order to avoid a collision.

0033The legacy CTS transmitted by the HTAP shown in <figref idref="f0010">Fig. 10</figref> may be a legacy CF-poll as shown in <figref idref="f0012">Fig. 12</figref>.

0034Moreover, the legacy CTS transmitted by the HTAP may be a normal MAC frame that specifies the period to protect in the duration as shown in <figref idref="f0013">Fig. 13</figref>. An MAC frame format in this case is shown in <figref idref="f0014">Fig. 14</figref>. However, as shown in <figref idref="f0015">Fig. 15</figref>, the type and subtype included in the frame control have to be those other than the PS-Poll. This is because in the case of the PS-Poll, the duration interval indicates the AID rather than the duration (refer to IEEE 802.11-1999 7.1.3.1.2 Table - Valid type/subtype combinations).

0035As shown in <figref idref="f0016">Fig.16</figref>, HT beacons may be included in an HT sequence. As described above, when the HT beacons are included in the HT sequence, the HT terminal has an advantage of being not required to demodulate the legacy information such as the legacy beacon or the legacy CTS.

0036As shown in <figref idref="f0017">Fig. 17</figref>, HT beacons may not be included in an HT sequence. In this case, the parameter about the HT transmission (HT sequence), namely information about the band width to be used etc. is transmitted/received in the legacy beacon.

0037As describe above, according to the wireless LAN system in the first arrangement, since the legacy beacon and the legacy CTS are transmitted as an occupation signal before the HT sequence, it is possible to make the legacy terminal recognize these signals and notify that the HT terminal occupies the radio medium. Moreover, since transmission is performed by the HT terminal after the occupation signal is transmitted using every frequency in the frequency band n times that of the legacy terminal, it is possible to perform transmission by the HT terminal while making the legacy terminal suppress transmission and occupying the radio medium. Due to this, a collision of packets can be avoided and the deterioration of throughput can be prevented. As a result, it becomes possible to exhibit the original transmission speed between HT terminals.

(Second arrangement)

0038In a wireless LAN system according to a second arrangement, as in the wireless LAN system according to the first arrangement, a signal for notifying that the radio medium is occupied is transmitted and the period of time during which an HT sequence is performed is occupied logically with respect to a legacy terminal. Here, in the case where a legacy AP and an HTAP coexist in the same frequency band, as in the first arrangement, transmission of both the legacy beacon and the legacy CTS prior to the HT sequence leads to an increase in overhead. Therefore, in the second arrangement, the legacy beacon is not transmitted but the legacy CTS and the HT sequence (HT beacon, HT data) are transmitted in this order, as shown in <figref idref="f0018">Fig. 18</figref>. In this case also, as shown in <figref idref="f0019">Fig.19</figref>, the legacy CF-poll may be used instead of the legacy CTS.

0039As shown in <figref idref="f0020">Fig.20</figref>, the normal MAC frame that specifies the period to protect in the duration maybeused. However, as explained in <figref idref="f0014">Fig. 14</figref> and <figref idref="f0015">Fig. 15</figref>, the type and subtype included in the frame control have to be those other than the PS-Poll (refer to IEEE 802.11 - 1999 7.1.3.1.2 Table 1 - Valid type/subtype combinations). In this case, as shown in <figref idref="f0021">Fig.21</figref>, the HT beacons have to be included in the HT sequence.

0040As described above, according to the wireless LAN system in the second arrangement, it becomes possible to improve the throughput without increasing overhead.

(First embodiment)

0041In a wireless LAN system according to a first embodiment, coexistence of the HT terminal and the legacy terminal is realized. In other words, the HT terminal occupies a part of period of the radio medium but does not occupy another part, thereby the coexistence with the legacy terminal is realized.

0042In the first embodiment, the time of duration in a legacy MAC frame transmitted by the HT terminal is set to x% (x% is equal to or less than 100%) of the interval from the end of the legacy MAC frame to the next beacon. It is possible to adjust the ratio of communication time of the HT terminal to that of the legacy terminal in accordance with this ratio (the ratio of the part of the radio medium occupied by the HT terminal to the whole). <figref idref="f0022">Fig.22</figref> shows a state in which the HT terminal occupies 100% of the interval from the end of the legacy MAC frame to the next beacon. <figref idref="f0023">Fig. 23</figref> shows a state in which the HT terminal occupies, for example, 70% of the interval from the end of the legacy MAC frame to the next beacon and the legacy terminal occupies the rest, that is, 30% thereof. Due to this, the coexistence of the HT terminal and the legacy terminal is made possible. By the way, the ratio (x%) may be a fixed value or variable. Hereinafter, it is assumed that when x=100%, the radio medium is occupied perfectly and when x&lt;100, a part of period of the radio medium is occupied.

0043As shown in <figref idref="f0024">Fig.24</figref> and <figref idref="f0025">Fig.25</figref>, it may also possible to transmit the legacy MAC frame and the HT sequence (HT beacon, HT data) in this order without transmitting the legacy beacon and set the time of the duration in the legacy MAC frame to x% (x% is equal to or less than 100%) of the interval from the end of the legacy MAC frame to the next legacy MAC frame.

0044In the explanation described above, a case where the setting of the duration is performed in the legacy MAC frame is shown, but the CTS or the legacy CF-Poll may be used instead of the legacy MAC frame.

0045Next, it is necessary for an HTSTA to know an HT sequence interval in which HT transmission is possible, therefore, a method for notifying the interval of the HT sequence from an HTAP to the HTSTA is described below. As shown in <figref idref="f0026">Fig.26</figref>, an HT beacon in which information about the HT sequence interval is described is included in the HT sequence. On the other hand, as shown in <figref idref="f0027">Fig.27</figref>, an HT beacon in which information about the HT sequence interval is described may be included in the HT sequence and an HT interval end signal may be included at the rear of the HT sequence. Due to this, it becomes possible for the HTSTA that receives the HT beacon to grasp the interval during which HT transmission is possible.

0046As shown in <figref idref="f0026">Fig.26</figref>, in the case of the wireless LAN system in which the HT beacon describing information about the HT sequence interval is included in the HT sequence, the HTSTA employs such a configuration as shown in <figref idref="f0028">Fig.28</figref>. In other words, a radio signal received by an antenna 50 is demodulated in a receiving circuit 51. An HT interval start judgment circuit 52 judges the start of the HT interval based on the HT beacon included in the demodulated data outputted from the receiving circuit 51. A counter circuit 53 starts a counter based on the output signal of the HT interval start judgment circuit 52. This counter counts the time elapsed from the start of the HT interval. An HT interval end judgment circuit 54 judges the end of the HT interval together with the output of the counter circuit 53 according to information about the HT interval included in the HT beacon. A transmission control circuit 55 controls a transmitting circuit 57 based on the output signal of the HT interval start judgment circuit 52 and the output signal of the HT interval end judgment circuit 54. An MAC circuit 56 defines the transmission/reception method, the format, etc., of a frame that is a transmission/reception unit of data for transmission data and reads out the transmission/reception method of the frame, the frame format, etc., from the received data. The transmitting circuit 57 undergoes the control of the transmission control circuit 55 and the MAC circuit 56, converts the transmission data into a radio signal, and transmits the signal through an antenna 58. Due to such a configuration, it becomes possible for the HTSTA to grasp the start and end of the HT sequence interval. However, receiving circuit 51 is assumed to be the one to adopt the same composition as receiving circuit 40a shown in <figref idref="f0008">Figure 8</figref>. Moreover, transmitting circuit 57 is assumed to be the one to adopt the same composition as transmission circuit 20a shown in <figref idref="f0003">Figure 3</figref>. Above-mentioned antenna 50, receiving circuit 51, HT interval start judgment circuit 52, counter circuit 53, HT interval end judgment circuit 54, transmission control circuit 55, and MAC circuit 56 compose receiver 59a.

0047As shown in <figref idref="f0027">Fig.27</figref>, in the case of the wireless LAN system in which the HT beacon describing information about the HT sequence interval is included in the HT sequence and the HT interval end signal is included at the rear of the HT sequence, the HTSTA employs such a conf igurat ion as shown in <figref idref="f0029">Fig. 29</figref>. Here, since both the signals indicating the start and the end of the HT sequence are transmitted, it is possible for the HTSTA to easily grasp the HT sequence interval. Therefore, the configuration is the same as that shown in <figref idref="f0028">Fig.28</figref>, fromwhich the counter circuit 53 is removed.

0048In <figref idref="f0029">Fig.29</figref>, at the HTSTA, the HT interval start judgment circuit 52 judges the start of the HT interval based on the HT beacon included in the demodulated data outputted from the receiving circuit 51. The HT interval end judgment circuit 54 judges the end of the HT interval by receiving the HT interval end signal. The transmission control circuit 55 controls the transmitting circuit 57 based on the output signal of the HT interval start judgment circuit 52 and the output signal of the HT interval end judgment circuit 54. Due to this, it is possible for the HTSTA to grasp the start of the HT interval by the HT beacon included at the top of the HT sequence, grasp the end of the HT interval by the HT interval end signal included at the rear of the HT sequence, and know the interval during which HT transmission is possible. Above-mentioned antenna 50, receiving circuit 51, HT interval start judgment circuit 52, HT interval end judgment circuit 54, transmission control circuit 55, and MAC circuit 56 compose receiver 59b.

0049As described above, according to the wireless LAN system in the first embodiment, it becomes possible to reserve a period during which the legacy terminal performs transmission by changing the occupation period during which the radio medium is occupied. Therefore, while the HT terminal occupies 100% of the radio medium, the transmission by the legacy terminal is suppressed, a collision of packets is avoided, and the deterioration of throughput is prevented. On the other hand, while the HT terminal does not occupy the radio medium, since the legacy terminal can perform transmission, the coexistence of the HT terminal and the legacy terminal becomes possible. Moreover, since the HT beacon describing information about the HT sequence interval is included in the HT sequence, it is possible for the HTSTA to clearly grasp the start of the period during which transmission is performed. Moreover, since the HTAP transmits the HT interval end signal, it becomes possible for the HTSTA to clearly grasp the end of the period during which transmission is performed without measuring the elapsed time from the start of the period. Since the start and end of the above-mentioned period can be clearly grasped by the receiving side, it becomes possible to easily configure a system in which the occupation period during which the HTAP and HTSTA occupy a part of the period of the radio medium is changed.

(Third arrangement)

0050In a wireless LAN system according to a third arrangement, an occupation signal for the occupation of the radio medium is transmitted intermittently in order to realize the coexistence of the HT terminal and the legacy terminal. The wording "to transmit intermittently" means to transmit or not to transmit an occupation signal at fixed time intervals. For example, in the case of the frame configuration shown in <figref idref="f0013">Fig.13</figref>, while the legacy beacon is transmitted L times (L is a natural number), the legacy MAC frame is transmitted only m times (m is a natural number, equal to or less than L) and not transmitted otherwise. In the case where only the legacy MAC frame is transmitted as in the frame configuration shown in <figref idref="f0019">Fig. 19</figref>, if the legacy MAC frame is transmitted P times (P is a natural number), the legacy MAC frame is not transmitted in the next frame. Due to this, in the interval during which the legacy MAC frame is not transmitted, it becomes possible for the legacy terminal to transmit a signal. Therefore, it is possible to adjust the ratio of the communication time between the HT terminal and the legacy terminal, as shown in <figref idref="f0030">Fig.30</figref>. In other words, if it is assumed in <figref idref="f0030">Fig.30</figref> that the interval during which the HTAP transmits the legacy MAC frame to occupy the radio medium is represented by (1) and the interval during which the HTAP does not transmit the legacy MAC frame nor occupy the radio medium is represented by (2) , it becomes possible for the legacy terminal to perform transmission in the interval (3).

0051Moreover, even when the HTAP transmits the legacy MAC frame to occupy the radio medium as shown in <figref idref="f0031">Fig.31</figref>, if it is assumed that x% of the interval (4) is occupied, it is possible for the legacy terminal to perform transmission in the rest of, that is, (100 - x%) of the interval (4) in the interval (5) . In <figref idref="f0031">Fig. 31</figref>, since the HTAP does not transmit the legacy MAC frame nor occupy the radio medium in the interval (2), it becomes possible for the legacy terminal to perform transmission in the interval (3).

0052It is also possible to use the legacy CTS or the legacy CF -poll instead of the legacy MAC frame.

0053As described above, according to the wireless LAN system in the third arrangement, the occupation signal such as the legacy MAC frame is transmitted intermittently. Therefore, when the occupation signal is transmitted, the HT terminal occupies the radio medium, transmission is suppressed at the legacy terminal and a collision of packets is avoided, and the deterioration of throughput is prevented at the HT terminal. On the other hand, when the occupation signal is not transmitted, since the HT terminal does not occupy the radio medium, it becomes possible for the legacy terminal to perform transmission. Due to this, it becomes possible for the HT terminal and the legacy terminal to coexist.

(Fourth arrangement)

0054In a wireless LAN system according to a Fourth arrangement, in order to realize the coexistence of the HT terminal and the legacy terminal, the communication time reserved by the HT terminal is adjusted, and the coexistence is realized. In the fourth arrangement, for example, in the case of the frame configuration shown in <figref idref="f0022">Fig. 22</figref> and <figref idref="f0023">Fig. 23</figref>, the number of times the HT sequence occupies x% of the interval from the end of the legacy MAC frame to the next legacy beacon is set to m times (m is a natural number, equal to or less than L) out of L times (L is a natural number) of transmission of the legacy beacon. Then, the HT sequence occupies y% (y% is equal to or less than 100%) of the interval from the end of the legacy MAC frame to the next legacy beacon in the rest of, that is, (L - m) times.

0055In the above explanation, a method is shown for switching the HT sequences in the two kinds of ratios (x% and y% of the interval from the end of the legacy MAC frame to the next beacon) during the L-time transmission of the legacy beacon, but the number of kinds of ratios is not limited to two but i kinds (i is a positive integer, equal to 2 or greater) of ratios are acceptable. Moreover, as shown in <figref idref="f0024">Fig.24</figref> and <figref idref="f0025">Fig.25</figref>, it is also possible to adjust the interval from the end of the legacy MAC frame to the next legacy MAC frame in the above-mentioned ratios.

0056Due to this, the ratio of the communication time between the HT terminal and the legacy terminal is adjusted and it becomes possible to realize the coexistence of both.

Fifth arrangement

0057In the arrangements and embodiment described above, the method is shown, in which the HTAP transmits the legacy CTS to set the NAV for the legacy terminal and suppress transmission, thereby the transmission time of the HT sequence is logically reserved.

0058However, when the HTSTA can receive the legacy CTS, it is necessary for the HTSTA to be capable of recognizing that the purpose of the legacy CTS is to reserve the time for the HT sequence.

0059Because of this, in a wireless LAN system according to a fifth arrangement, the HTAP notifies the HTSTA that the occupation of the medium by CTS is for the occupation for the HT sequence by setting the receiver address included in the legacy CTS transmitted in order to reserve the HT sequence to a specific value. The receiver address constitutes identification information.

0060On the other hand, the HTSTA may employ such a configuration as shown in <figref idref="f0007">Fig.7</figref> or <figref idref="f0008">Fig.8</figref>. In other words, the HTSTA employs a configuration including the MAC circuit 36 or 48 as a recognition section for recognizing the receiver address, and the receiving circuit 30a or 40a as a reception section for demodulating a received signal and outputting the received data when the receiver address is recognized as a specific address, or stopping demodulation of the received signal when the receiver address is not recognized as the specific address.

0061<figref idref="f0032">Fig.32</figref> is a diagram showing a CTS frame format. As shown in <figref idref="f0032">Fig.32</figref>, the CTS frame consists of a frame control section 60 as an MAC header, a duration section 61, a receiver address 62, and an FCS section 63. Here, when the receiver address 62 is a specific address, it is possible for the HTSTA to recognize that the received CTS is a CTS for occupying the time for the HT sequence. In other words, since there exists an address that is not used normally in the MAC address, it is possible to make the HTSTA recognize that the received CTS is for the occupation by the HT terminal by using the address.

0062Next, the operation when the HTSTA receives the legacy CTS is explained below using the flow chart shown in <figref idref="f0033">Fig.33</figref>. When the HTSTA receives the legacy CTS (step S1), the MAC circuit 36 or 48 judges whether the receiver address included in the legacy CTS is a specific address (step S2). When the result is that the address is a specific address, it is judged that the legacy CTS is from the HTAP and preparation is started for demodulating the HT signal (step S3). Then, the HT signal is demodulated and when the HT interval ends (step S4), the procedure returns to step S1, in which the legacy CTS is waited for to be received again. On the other hand, if the receiver address included in the legacy CTS is not a specific address in step S2 , it is judged that the CTS is from the legacy terminal and the receiving circuit 30a or 40a is stopped (step S5). Then, after the interval of the NAV set by the CTS elapses, the receiving circuit 30a or 40a is activated (step S6) , the procedure returns to step S1, in which the legacy CTS is waited for to be received again.

0063As described above, according to the wireless LAN system in the fifth embodiment, since the legacy CTS at the HT terminal uses a specific value for the receiver address as identification information for recognizing to be a signal for occupying the radio medium, it is possible for the HT terminal (STA) to recognize that the CTS is transmitted in order to notify the legacy terminal of the occupation of the radio medium. Due to this, it is possible for the HTSTA to judge whether the data transmitted after the CTS is transmitted to the HTSTA. When the judgment result is that the data transmitted after the CTS is transmitted to the HTSTA, the data is demodulated, or when the result is that the data transmitted after the CTS is not transmitted to the HTSTA, the receiving operation is stopped. Due to this, unnecessary operations are not performed and saving power consumption becomes possible.

0064In the wireless LAN system according to the first embodiment and second to fourth arrangements, when a chance for transmission is given to the legacy terminal, it is not easy to perfectly avoid a collision of the HT signal and the legacy signal if the legacy terminal transmits a very long packet. However, when the packet that the legacy terminal transmits is short, it is possible to considerably reduce the possibility of the collision.

Sixth arrangement

0065In the first to fifth arrangements and in the first embodiment described above, the use in a 5 GHz band is assumed as a specific example. That is, it is assumed that the legacy terminal uses the IEEE802.11a scheme (OFDM scheme). In contrast to this, in a sixth arrangement, a transmitter and a receiver of the HT terminal are explained on the assumption of the use in a 2.4 GHz band, that is, the legacy terminal uses the IEEE802.11 scheme (DSSS scheme).

0066<figref idref="f0034">Fig.34</figref> is a block diagram showing a general configuration of a transmitter of an HT terminal used in a 2.4 GHz band. With this transmitter 340, transmission data are inputted to an MAC circuit 341 and the MAC circuit 341 outputs transmission signal data to a DSSS signal transmitting circuit section 342 when transmitting a signal compatible with the legacy terminal, and a radio signal is transmitted through a switching section 343 and an antenna 344.

0067On the other hand, when communication is performed with another HT terminal using a frequency band wider than that of the legacy terminal, the MAC circuit 341 outputs the transmission signal data to an HT transmitting circuit section 345 and a radio signal is transmitted through the switching section 343 and the antenna 344.

0068It is assumed that the HT transmitting circuit section 345 shown in <figref idref="f0034">Fig.34</figref> employs the same configuration as that of the transmitting circuit 20a shown in <figref idref="f0003">Fig.3</figref>.

0069In the following explanation, the part constituted of the HT transmitting circuit section 345 and the MAC circuit 341 is defined as a transmission section 400. The part constituted of the DSSS signal transmitting circuit section 342 and the MAC circuit 341 is defined as an occupation signal generation section 401. In other words, it is assumed that a signal compatible with the legacy terminal is transmitted by outputting transmission signal data from the MAC circuit 341 to the DSSS signal transmitting circuit 342 included in the occupation signal generation section 401, and a signal compatible with the HT terminal is transmitted by outputting transmission signal data from the MAC circuit 341 to the HT transmitting circuit 345 included in the transmission section 400.

0070<figref idref="f0035">Fig. 35</figref> is a block diagram showing a detailed configuration of the DSSS signal transmitting circuit section 342 in <figref idref="f0034">Fig.34</figref>. The transmission signal data outputted from the MAC circuit 341 shown in <figref idref="f0034">Fig.34</figref> undergoes a scramble process (put into a random state by a specific pattern) in a scramble processing section 351 shown in <figref idref="f0035">Fig.35</figref> and then the data is modulation-processed into binary data in a DQPSK / DBPSK modulation section 352. After signal spreading is performed for the modulation-processed data using a spread code 353a in a spread section 353, the band is limited to 22 MHz in a filter section 354. The data outputted from the filter section 354 is inputted to an adder 355, a first frequency conversion section 356a, and a second frequency conversion section 356b, respectively. The first frequency conversion section 356a outputs the data the center frequency of which has been shifted by 20 MHz to the adder 355 and the second frequency conversion section 356b outputs the data the center frequency of which has been shifted by 40 MHz to the adder 355. The adder 355 adds up the data inputted from the filter section 354, the data inputted from the first frequency conversion section 356a, and the data inputted from the second frequency conversion section 356b. Due to this, a radio signal can be generated, in which three DSSS signals are arranged at 20 MHz intervals when viewed in the direction of frequency.

0071<figref idref="f0036">Fig. 36</figref> is a block diagram showing a general configuration of a receiver of an HT terminal used in a 2.4 GHz band. A radio signal is received by an antenna 361 and when the signal is compatible with the legacy terminal, a switching section 362 outputs the signal to a DSSS signal reception circuit section 363 in which the DSSS signal is demodulated, and then the demodulated signal data is outputted to an MAC circuit 364.

0072On the other hand, when communication is performed with the HT terminal using a frequency band wider than that of the legacy terminal, the signal is demodulated by an HT reception circuit section 365 and the demodulated signal data is outputted to the MAC circuit 364.

0073It is assumed that the HT reception circuit section 365 employs the same configuration as that of the receiving circuit 40a shown in <figref idref="f0008">Fig.8</figref>.

0074<figref idref="f0037">Fig. 37</figref> is a block diagram showing a detailed configuration of the DSSS signal reception circuit section 363 in <figref idref="f0036">Fig.36</figref>. From among the radio signals outputted from the antenna 361 shown in <figref idref="f0036">Fig.36</figref>, only the signals in the 22 MHz band required by a filter section 371 shown in <figref idref="f0037">Fig. 37</figref> are extracted and an inverse spreading process is performed for the signals by applying a spread code 372a by an inverse spread section 372. The output from the inverse spread section 372 is demodulated into binary data by a delay wave detection section 373 and undergoes a descramble process (the random state by the specific pattern on the transmitting side is released) in a descramble processing section 374, and the obtained reception signal data are outputted to the MAC circuit 364 shown in <figref idref="f0036">Fig.36</figref>.

0075The "frequency band width" in the first embodiment can be thought as follows. That is, in a 5 GHz band, the frequency band width used by the HT terminal is n times (n=3 in the diagram) that of the legacy terminal as shown in <figref idref="f0038">Fig.38</figref>. In contrast to this, in a 2.4 GHz band, while the frequency band width used by the HT terminal is n times 20 MHz, the frequency band width used by the legacy terminal is 22 MHz, as shown in <figref idref="f0039">Fig. 39</figref>, therefore, it is not possible to define that the frequency band width used by the HT terminal is n times the frequency band used by the legacy terminal.

0076However, it can be said that <figref idref="f0038">Fig.38</figref> and <figref idref="f0039">Fig.39</figref> are identical to each other in that the occupation signal is a "signal compatible with the legacy terminal in the use of every frequency in a frequency band fh used by the HT terminal."

0077Therefore, in a 2.4 GHz band also, the same effect as that shown in the first to fifth arrangements can be obtained by using an occupation signal compatible with the legacy terminal shown in <figref idref="f0039">Fig.39</figref>.

[Explanation of Reference Numerals]

0078<dl id="dl0001" compact="compact"><dt>20</dt><dd>HTAP transmitter</dd><dt>20a</dt><dd>transmitting circuit</dd><dt>24</dt><dd>switching device</dd><dt>25</dt><dd>symbol repeating circuit</dd><dt>30</dt><dd>receiver</dd><dt>30a</dt><dd>receiving circuit</dd><dt>36</dt><dd>MAC circuit</dd><dt>40</dt><dd>receiver</dd><dt>40a</dt><dd>receiving circuit</dd><dt>45</dt><dd>thinning circuit</dd><dt>46</dt><dd>demodulator</dd><dt>48</dt><dd>MAC circuit</dd><dt>50</dt><dd>antenna</dd><dt>51</dt><dd>receiving circuit</dd><dt>52</dt><dd>HT interval start judgment circuit</dd><dt>53</dt><dd>counter circuit</dd><dt>54</dt><dd>HT interval end judgment circuit</dd><dt>55</dt><dd>transmission control circuit</dd><dt>59a</dt><dd>receiver</dd><dt>59b</dt><dd>receiver</dd><dt>300</dt><dd>transmission section</dd><dt>301</dt><dd>occupation signal generation section</dd><dt>340</dt><dd>transmitter</dd><dt>356a</dt><dd>first frequency conversion section</dd><dt>356b</dt><dd>second frequency conversion section</dd><dt>355</dt><dd>adder</dd><dt>400</dt><dd>transmission section</dd><dt>401</dt><dd>occupation signal generation section</dd></dl>