Abstract:
A wireless transmitting device for use in communication with a wireless receiving device with a wireless packet, includes a plurality of antennas; and a signal generator configured to generate a signal for the wireless packet being transmitted, the wireless packet comprising a short-preamble sequence, a first long-preamble sequence, a signal field, an AGC preamble sequence, and transmitted in parallel via the plurality of antennas, a second long-preamble sequence, and a data field conveying data, wherein the first signal field includes information at least one of (a) information for notifying transmission of the AGC preambles, (b) information for notifying transmission of the second signal field, the AGC preambles and the data and (c) information for notifying transmission of the AGC preambles and the data using the plurality of antennas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-107881, filed Mar. 31, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a wireless transmitting device and wireless receiving device for respectively transmitting and receiving wireless signals in mobile communication system like a wireless LAN, using a wireless packet including a preamble and data, and a wireless transmission method and wireless receiving method for use in the devices.  
         [0004]     2. Description of the Related Art  
         [0005]     The Institute of Electrical and Electronics Engineers (IEEE) is now defining a wireless LAN standard called IEEE 802.11n, which aims to achieve a high throughput of 100 Mbps or more. It is very possible that IEEE 802.11n will employ a technique, called multi-input multi-output (MIMO), for using a plurality of antennas in a transmitter and receiver. IEEE 802.11n is required to coexist with the standard IEEE 802.11a where OFDM (Orthogonal Frequency Division Multiplex) is used. So, it is required that IEEE 802.11n wireless transmitting device and receiving device have so called backwards compatibility.  
         [0006]     A proposal presented by Jan Boer et al. in “Backwards Compatibility”, IEEE 802.11-03/714r0, introduces a wireless preamble for MIMO. In this proposal, a short-preamble sequence used for time synchronization, frequency synchronization and automatic gain control (AGC), a long-preamble sequence used to estimate a channel impulse response, a signal field indicating a modulation scheme used in the wireless packet, and another signal field for IEEE 802.11n are firstly transmitted from a single particular transmit antenna. Subsequently, long-preamble sequences are transmitted from the other three transmit antennas. After finishing the transmission of the preamble, transmission data is transmitted from all the antennas.  
         [0007]     From the short-preamble to the first signal field, the proposed preamble is identical to the preamble stipulated in IEEE 802.11a where single transmit antenna is assumed. Therefore, when wireless receiving devices that conform to IEEE 802.11a receive a wireless packet containing the Boer&#39;s proposed preamble, they recognize that the packet is based on IEEE 802.11a. Thus, the proposed preamble conforming to both IEEE 802.11a and IEEE 802.11n enables IEEE 802.11a and IEEE 802.11n to coexist.  
         [0008]     Generally, in wireless receiving devices, demodulation of a received signal is performed by digital signal processing. Therefore, an analog-to-digital (A/D) converter is provided in the devices for quantizing a received analog signal. A/D converters have an input dynamic range (an allowable level range of analog signals to be converted). Accordingly, it is necessary to perform automatic gain control (AGC) for adjusting the levels of received signals within the input dynamic range of the A/D converter.  
         [0009]     Since the estimation of a channel impulse response using the above-mentioned long preamble sequences is performed by digital signal processing, AGC must be performed using the signal transmitted before the long-preamble sequence. In the Boer&#39;s preamble, AGC is performed using a short-preamble sequence transmitted before the long-preamble sequence from a particular transmit antenna. That is, the receiving level of the short-preamble sequence is measured, and AGC is performed so that the receiving level falls within the input dynamic range of the A/D converter. By virtue of AGC using the short-preamble sequence, the long-preamble sequence and data transmitted from the particular transmit antenna can be received correctly. If all the antennas are arranged apart, the receiving levels of signals transmitted from the antennas are inevitably different from each other. Therefore, when a wireless receiving device receives long-preamble sequences transmitted from the other three transmit antennas, or data transmitted from all the antennas, their receiving levels may be much higher or lower than the level acquired by AGC using the short-preamble sequence transmitted from the particular transmit antenna. When the receiving level exceeds the upper limit of the input dynamic range of the A/D converter, the output of the A/D converter is saturated. On the other hand, when the receiving level is lower than the lower limit of the input dynamic range of the A/D converter, the output of the A/D converter suffers a severe quantization error. In either case, the A/D converter cannot perform appropriate conversion, which adversely influences the processing after A/D conversion.  
         [0010]     Further, data is transmitted from all the antennas. Therefore, during data transmission, the range of variations in receiving level is further increased, which worsens the above-mentioned saturation of the A/D converter output and/or the quantization error therein, thereby significantly degrading the receiving performance.  
         [0011]     As described above, in the Boer&#39;s proposed preamble, AGC is performed at the receive side using only the short-preamble sequence transmitted from a single transmit antenna, which makes it difficult to deal with variations in receiving level that may occur when signals transmitted from the other antennas in MIMO mode are received.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     The first aspect of the present invention provides a wireless transmitting device for use in communication with a wireless receiving device with a wireless packet, comprising: a plurality of antennas; and a signal generator configured to generate a signal for the wireless packet being transmitted, the wireless packet comprising a short-preamble sequence used for a first automatic gain control (1 st  AGC) at the wireless receiving device, a first long-preamble sequence used for an estimation of a channel impulse response between the wireless transmitting device and the wireless receiving device, a signal field used for conveying information regarding a length of the wireless packet, an AGC preamble sequence used for a second automatic gain control (2 nd  AGC) which is performed after the first automatic gain control at the receiving device, and transmitted in parallel via the plurality of antennas, a second long-preamble sequence used for estimation of a channel impulse response between the wireless transmitting device and the wireless receiving device, and a data field conveying data, wherein the first signal field includes at least one of (a) a reserve bit for notifying transmission of the AGC preambles, (b) a reserve bit for notifying transmission of the second signal field, the AGC preambles and the data and (c) a reserve bit for notifying transmission of the AGC preambles and the data using the plurality of antennas.  
         [0013]     The second aspect of the present invention provides a wireless receiving device comprising: a receiving unit configured to receive a wireless packet to generate a received signal, the packet comprising a short-preamble sequence, a first long-preamble sequence, a first signal field having a reserve bit, and a second signal field, which are sequentially transmitted from one of the plurality of antennas, the signal also including a plurality of AGC preambles and data signals transmitted in parallel via the plurality of antennas after the second signal field is transmitted; a variable-gain amplifier which amplifies the received signal; and a gain controller which controls, upon receiving the reserve bit, a gain of the variable-gain amplifier using the AGC preambles.  
         [0014]     The third aspect of the present invention provides a wireless receiving device comprising: a receiving unit configured to receive a wireless packet to generate a received signal, the packet comprising a short-preamble sequence, a first long-preamble sequence, a first signal field having a reserve bit, and a second signal field, which are sequentially transmitted from at least one of the plurality of antennas, the signal also including a plurality of AGC preambles and data signals transmitted in parallel via the plurality of antennas after the second signal field is transmitted; a variable-gain amplifier which amplifies the received signal; a gain controller which controls a gain of the variable-gain amplifier using the AGC preambles; and a start controller which controls the gain controller to start a gain control operation thereof, in response to reception of the reserve bit. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0015]      FIG. 1  is a view illustrating a format for a wireless packet including the AGC preambles for wireless communication used in an embodiment of the invention;  
         [0016]      FIG. 2  is a view illustrating a wireless packet conforming to IEEE 802.11a;  
         [0017]      FIG. 3  is a block diagram illustrating the configuration of a wireless transmitting device according to the embodiment;  
         [0018]      FIG. 4  is a block diagram illustrating the configuration of a wireless receiving device according to the embodiment;  
         [0019]      FIG. 5  is a block diagram illustrating a configuration example of a receiving unit incorporated in the device of  FIG. 4 ;  
         [0020]      FIG. 6  is a block diagram illustrating an example of a digital demodulator incorporated in the device of  FIG. 4 ;  
         [0021]      FIG. 7  is a graph illustrating the distribution of the receiving power of short preambles and data in the prior art; and  
         [0022]      FIG. 8  is a graph illustrating the distribution of the receiving power of short preambles and data in the embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Embodiments of the invention will be described in detail with reference to the accompanying drawings.  
         [0024]      FIG. 1  shows a format for a wireless packet employed in a first embodiment of the invention. This format is a physical layer protocol data unit format for the MIMO mode and provides interoperability and coexistence with IEEE 802.11a wireless stations.  
         [0025]     As seen from  FIG. 1 , a preamble includes a physical layer convergence protocol (PLCP) signal transmitted from an antenna Tx 1 . The PLCP signal includes a short-preamble sequence  101 , first long-preamble sequence  102 , first signal field (SIGNAL)  103  and second signal field (SIGNAL  2 )  104 . The short-preamble sequence  101  contains several unit preambles SP. The long-preamble sequence  102  contains the unit preambles LP having respective predetermined lengths. The unit preambles of LP are longer than those of SP.  
         [0026]     The short-preamble sequence  101 , first long-preamble sequence  102  and first signal field  103  conform to IEEE 802.11a, while the second signal field  104  is necessary for the new wireless LAN standard IEEE 802.11n. First signal field  103  conforming to IEEE 802.11a may be called “legacy signal field”. Since the second signal field  104  is provided for new high throughput wireless LAN standard, it may be called “high throughput signal field”. A guard interval GI is inserted between the short-preamble sequence  101  and the long-preamble sequence  102 .  
         [0027]     After the PLCP signal, AGC preambles  105 A to  105 D that are transmitted in parallel from a plurality of antennas Tx 1  to Tx 4  are positioned. The AGC preambles  105 A to  105 D are transmitted simultaneously from a plurality of antennas Tx 1  to Tx 4 . The AGC preambles  105 A to  105 D are used to enable the receiving device to perform fine AGC when performing MIMO communication. These preambles are unique to perform fine tune the AGC for reception of MIMO mode in accordance with IEEE 802.11n. Therefore, the AGC preambles  105 A to  105 D may be called “high throughput short trainings field”. On the other hand, since the short-preamble sequence  101  conforms to IEEE 802.11a, being used for coarse AGC operation, it may be called “legacy short training field”.  
         [0028]     After the AGC preambles  105 A to  105 D, second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D are positioned. In the embodiment, the same signal sequences are used as the AGC preambles  105 A to  105 D. However, different signal sequences may be used as the AGC preambles  105 A to  105 D. A guard interval GI is inserted between each pair of adjacent ones of the unit preambles LP that form the second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D. The second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D sequences are in the orthogonal relationship in this embodiment. But, they are not limited in the orthogonal relationship. The number of unit preambles LP  106 - 109  for each transmit antenna is equal to the number of transmit antennas in MIMO mode. In order to distinguish between two kinds of long-preamble sequences, first long-preamble sequence  102  conforming to IEEE 802.11a may be called “legacy long training field”. Since the second long preambles sequences  106 - 109  are provided for new high throughput wireless LAN standard, it may be called “high throughput long training field”.  
         [0029]     After each of the second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D, a field for transmission data (DATA)  110 A to  110 C transmitted from the antennas Tx 1  to Tx 4 , respectively, is positioned. The second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D are transmitted simultaneously from a plurality of antennas Tx 1  to Tx 4  respectively. Before the first signal field  103 , the second signal field  104  will be described. The second signal field  104  contains identification information indicating that the radio packet shown in  FIG. 1  conforms to IEEE 802.11n different from IEEE 802.11a. In other words, the second signal field  104  indicates that the second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D are to be received next, and that the number of symbols included in the second long-preamble sequences. The field  104  also indicates that a modulation and coding scheme (MCS) which is the combination of the modulation and coding schemes of the transmission data  110 A to  110 D. The coding scheme indicates the coding rate of a convolution code as an error correction signal.  
         [0030]     The first signal field  103  will now be described in detail. The field  103  contains information indicating the modulation scheme and radio packet length of the following transmission data  110 A to  110 D. As mentioned above, in the radio communication preamble signal shown in  FIG. 1 , the PLCP signal zone, in particular, the radio packet zone ranging from the short-preamble sequence  101  to the first signal field  103 , conforms to IEEE 802.11a.  
         [0031]      FIG. 2  shows a wireless packet based on IEEE 802.11a. In this case, from a single transmission antenna Tx 1 , a short-preamble sequence x 11  used for time synchronization, frequency synchronization and AGC, a long-preamble sequence x 12  for channel response estimation, and a signal field x 13  including a field indicating the modulation scheme and length of the wireless packet are transmitted. After that, transmission data items x 14  and x 15  are transmitted from the antenna Tx 1 .  
         [0032]     The first signal field  103  shown in  FIG. 1  is similar to the signal field x 13  of the wireless packet based on IEEE 802.11a and shown in  FIG. 2 . As shown in detail in  FIG. 1 , the first signal field  103  comprises a rate section (RATE)  131  indicating a modulation and coding Scheme (MCS) of a data signal in a wireless packet based on IEEE 802.11a, a reserve bit (R)  132  reserved for future standard extension, and a packet length section (LENGTH)  133  indicating the length of the wireless packet. The field  103  also comprises a parity section (P)  134  that performs parity checking of information ranging from the rate section  131  to the packet length section  133 , and a signal tail section (SIGNAL TAIL)  135  for terminating a convolution code. These sections are combined by OFDM multiplexing and transmitted from the transmission antenna Tx 1 .  
         [0033]     Accordingly, if the wireless device conforms to IEEE 802.11a, it can perform normal receiving operations within the wireless packet zone indicated by the packet length section  133 . Namely, the wireless packet is protected from being destroyed by another wireless transmission device, which conforms to IEEE 802.11a, starting transmission within the signal zone following the first signal field  103 .  
         [0034]     The reserve bit  132  is not necessary for wireless device conforming to IEEE 802.11a and hence ignored at the receiver device. The embodiment controls, using the reserve bit  132 , the operation of a wireless device based on a standard other than IEEE 802.11a, i.e., for example, IEEE 802.11n. Specifically, for example, the reserve bit  132  (a) beforehand notifies the transmission of the AGC preambles  105 A to  105 D, and (b) indicates the transmission of a wireless packet corresponding to IEEE 802.11n shown in  FIG. 1 . Further, the reserve bit  132  (c) beforehand notifies the transmission of the AGC preambles  105 A to  105 D and data items  110 A to  110 D performed by a plurality of transmission antennas  205 A to  205 D, and (d) notifies the transmission of the second signal field  104 .  
         [0035]     The notification (a) includes indirect notification of the transmission of the AGC preambles  105 A to  105 D by beforehand notifying the transmission of the second signal field  104 . The wireless packet corresponding to IEEE 802.11n, recited in (b), indicates a wireless packet that includes the short-preamble sequence  101 , first long-preamble sequence  102 , first signal field  103 , second signal field  104 , AGC preambles  105 A to  105 D, second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D, and data items  110 A to  110 D. That is, the wireless packet includes signals transmitted from a plurality of transmission antennas and combined by multiplexing using MIMO.  
         [0036]     If transmission is performed with the reserve bit  132  set to, for example, “1”, a wireless device conforming to IEEE 802.11n receives and demodulates the reserve bit  132 , thereby recognizing the reception of a wireless packet corresponding to IEEE 802.11n. More specifically, the reserve bit  132  can indicate the reception of the wireless packet shown in  FIG. 1 , and indicate that the second signal field and AGC preambles  105 A to  105 D will be received after the reserve bit  132 .  
         [0037]     Referring now to  FIG. 3 , the wireless transmitting device according to the embodiment will be described. Firstly, digital modulator  203  forms a signal for wireless packet by combining transmission data  201  and the above-described preamble outputted from a memory  202 . The thus-obtained signal for wireless packet is sent to transmitting units  204 A to  204 D, where they are subjected to processing needed for transmission, for example, digital-to-analog (D/A) conversion, frequency conversion into a radio frequency (RF) band (up-conversion) and power amplification. Thereafter, the resultant signal is sent to a plurality of antennas  205 A to  205 D corresponding to the antennas Tx 1  to Tx 4  described with reference to  FIG. 1 , where an RF signal is sent from each transmit antenna  205 A to  205 D to the wireless receiving device shown in  FIG. 4 . In the description below, the antennas Tx 1  to Tx 4  shown in  FIG. 1  are referred to as the antennas  205 A to  205 D, respectively.  
         [0038]     In the embodiment, the PLCP signal shown in  FIG. 1 , which includes the short-preamble sequence  101 , first long-preamble sequence  102 , first signal field  103  and second signal field  104 , is transmitted from the transmit antenna  205 A of the transmission unit  204 A shown in  FIG. 2 . The AGC preambles  105 A to  105 D, second long-preamble sequences  106 A to  109 A,  106 B to  109 B,  106 C to  109 C and  106 D to  109 D, which are positioned after the PLCP signal as shown in  FIG. 1 , and the data  110 A to  110 D are transmitted across all the transmit antennas  205 A to  205 D shown in  FIG. 3 . In the wireless receiving device shown in  FIG. 4 , a plurality of receiving antennas  301 A to  301 D receive RF signals transmitted from the wireless transmitting device shown in  FIG. 3 . The wireless receiving device may have one receiving antenna or multiple receiving antennas. The RF signals received by the receiving antennas  301 A to  301 D are sent to receiving units  302 A to  302 D, respectively. The receiving units  302 A to  302 D each perform various types of receiving processing, such as frequency conversion (down-conversion) from the RF band to BB (baseband), automatic gain control (AGC), analog-to-digital conversion, etc., thereby generating a baseband signal.  
         [0039]     The baseband signals from the receiving units  302 A to  302 D are sent to channel impulse response estimation units  303 A to  303 D and digital demodulator  304 . These units  303 A to  303 D estimate the impulse responses of the respective propagation paths between the wireless transmitting device of  FIG. 3  and the wireless receiving device of  FIG. 4 . The channel impulse response estimation units  303 A to  303 D will be described later in detail. The digital demodulator  304  demodulates the baseband signals based on the estimated channel impulse response provided by units  303 A to  303 D, thereby generating received data  305  corresponding to the transmission data  201  shown in  FIG. 3 .  
         [0040]     More specifically, the digital demodulator  304  has an equalizer of the channel impulse response at its input section. The equalizer performs equalization for correcting the received signal distorted in the propagation path, based on the estimated channel impulse response. The digital demodulator  304  also demodulates the equalized signal at appropriate timing determined by the time synchronization, thereby reproducing data. The receiving units  302 A to  302 D shown in  FIG. 4  will now be described.  FIG. 5  shows the configuration of the receiving unit  302 A in detail. Since the other receiving units  302 B to  302 D have the same configuration as the unit  302 A, only the receiving unit  302 A will be described. The RF received signal received by the receiving antenna  301 A is down-converted by a down-converter  401  into a baseband signal. At this time, The RF signal may be directly converted into a baseband signal, or may be firstly converted into an intermediate frequency (IF) signal and then into a baseband signal.  
         [0041]     The baseband signal generated by the down-converter  401  is sent to a variable gain amplifier  402 , where it is subjected to perform AGC, i.e., signal level adjustment. The signal output from the variable gain amplifier  402  is sampled and quantized by an A/D converter  403 . The digital signal output from the A/D converter  403  is sent to the outside of the receiving unit  302  and to a gain controller  404 . The gain controller  404  performs gain calculation based on the digital signal output from the A/D converter  403 , and controls the gain of the variable gain amplifier  402 . The specific procedure for the gain control will be described later.  
         [0042]     The operation of the wireless receiving device shown in  FIGS. 4 and 5  executed for receiving the wireless packet including the preamble whose format is shown in  FIG. 1  is as follows. Firstly, the wireless receiving device receives a short-preamble sequence  101  transmitted from the transmit antenna  205 A of  FIG. 3 , and then performs packet edge detection, time synchronization, auto frequency control (AFC) and AGC, using a baseband signal corresponding to the short-preamble sequence  101 . AFC is also called frequency synchronization. Packet edge detection, time synchronization and AFC can be performed using known techniques, therefore no description will be given thereof. Only AGC will be explained below.  
         [0043]     The baseband signal corresponding to the short-preamble sequence  101  is amplified by the variable gain amplifier  402  in accordance with a predetermined initial gain value. The signal output from the variable gain amplifier  402  is input to the gain controller  404  via the A/D converter  403 . The gain controller  404  calculates a gain from the level of the received signal corresponding to the short-preamble sequence  101 , which is acquired after A/D conversion, and controls the gain of the variable gain amplifier  402  in accordance with the calculated gain.  
         [0044]     Assume here that the level of the baseband signal corresponding to the short-preamble sequence  101 , which is acquired before A/D conversion, is X. If level X is high, the baseband signal input to the A/D converter  403  exceeds the upper limit of the input dynamic range of the A/D converter  403 . As a result, the signal (digital signal) output from the A/D converter  403  is saturated and degraded the quality of signal reception. On the other hand, if level X is extremely low, the signal output from the A/D converter  402  (i.e., the digital signal acquired by A/D conversion) suffers a severe quantization error. Thus, when level X L is very high or low, the A/D converter  403  cannot perform appropriate conversion, thereby significantly degrading the quality of signal reception.  
         [0045]     To overcome this problem, the gain controller  404  controls the gain of the variable gain amplifier  402  so that the level X of the baseband signal corresponding to the short-preamble sequence  101 , is adjusted to a target value Z. If the input baseband signal has such a very high level as makes the output of the A/D converter  403  limited to its upper limit level, or if it has a very low level, the gain of the variable gain amplifier  402  may not appropriately be controlled by one control process. In this case, gain control is performed repeatedly. As a result, the level of the baseband signal input to the A/D converter  403  can be adjusted to a value that falls within the input dynamic range of the A/D converter  403 . Thus, the gain of the variable gain amplifier  402  is appropriately controlled using the baseband signal corresponding to the short-preamble sequence  101 , thereby performing appropriate A/D conversion to avoid a reduction in the quality of signal reception.  
         [0046]     In the above-described embodiment, the reception level needed for calculating the gain of the variable gain amplifier  402  is measured using a digital signal output from the A/D converter  403 . However, such level measurement can be executed using an analog signal acquired before A/D conversion. Furthermore, the reception level may be measured in the IF band or RF band, instead of BB.  
         [0047]     The wireless receiving device receives a first long-preamble sequence  102  transmitted from the transmit antenna  205 A, and performs the estimation of channel impulse response, i.e., estimates the response (frequency transfer function) of the propagation path between the wireless transmitting device to the wireless receiving device, using a baseband signal corresponding to the long-preamble sequence  102 . Since the signal transmitted from the transmit antenna  205 A has already been subjected to AGC as described above, the level of an input to the A/D converter  403  is appropriately adjusted when the estimation of channel impulse response is performed. Accordingly, concerning the signal transmitted from the transmit antenna  205 A, a highly accurate digital signal is acquired from the A/D converter  403 . The estimation of channel impulse can be performed accurately with the acquired digital signal.  
         [0048]     The wireless receiving device receives a first signal field  103  transmitted from the transmit antenna  205 A, and demodulates a baseband signal corresponding to the first signal field  103 , using the digital demodulator  304  and the above-mentioned channel estimation result. As shown in  FIG. 1 , the first signal field  103  contains the rate section  131  indicating the MCS of a data signal following preamble data, and the packet length section  133  indicating the length of the wireless packet. In the wireless packet zone recognized from the packet length section  133  of the first signal field  103 , the wireless receiving device causes the digital demodulator  304  to continue decoding processing.  
         [0049]     Referring to  FIG. 6 , the digital demodulator  304  shown in  FIG. 4  will be described in detail. The digital demodulator  304  receives signals  500  from the receiving units  302 A to  302 D shown in  FIG. 4 . The digital demodulator  304  comprises a fast Fourier transform (FFT) unit  501 , symbol timing controller  502 , de-mapping unit  503 , error correction unit  504 , signal decoder  505  and AGC start controller  506 .  
         [0050]     The symbol timing controller  502  performs symbol synchronization included in timing synchronization, using the input short-preamble sequence  101 , long-preamble sequence  102 , etc. Specifically, the end of each symbol in the wireless packet shown in  FIG. 1  is recognized. Since symbol synchronization is performed by a known method, no detailed description will be given of the method.  
         [0051]     The FFT unit  501  performs FFT on the input signal  500  in accordance with the timing recognized by the symbol timing controller  502 , thereby performing channel response estimation using the first long-preamble sequence  102 . Propagation path estimation is also a known technique, therefore no description will be given thereof.  
         [0052]     After that, the FFT unit  501  performs FFT on the input signal  500  in synchronism with the first signal field  103 . The output of the FFT unit  501  is input to the error correction unit  504  after it is converted into a binary-value sequence by the de-mapping unit  503 . The output of the error correction unit  504  is output as received data  305  from the digital demodulator  304  to the signal decoder  505 . Alternatively, the output of the de-mapping unit  503  can be directly input to the signal decoder  505 , without using the error correction unit.  
         [0053]     The signal decoder  505  is provided for decoding the first signal field  103 . When the signal decoder  505  decodes the reserve bit  132  in the first signal field  103  and detects that it is a preset value, e.g. “1”, it recognizes that the AGC preambles  105 A to  105 D will be received soon, and informs the AGC start controller  506  of this, i.e., a previous notice of reception of the AGC preambles. Upon receiving the previous notice, the AGC start controller  506  supplies an AGC start command to the gain controller  404  shown in  FIG. 5 , thereby causing the gain controller  404  to start gain control.  
         [0054]     After receiving the second signal field  104  from the transmission antenna  205 A, the wireless receiving device receives the AGC preambles  105 A to  105 D from the transmission antennas  205 A to  205 D. The AGC preambles  105 A to  105 D are transmitted from the transmission antenna  205 A that has transmitted so far the previous signals, and from the transmission antennas  205 B to  205 D that have not yet transmitted any signals. Accordingly, the AGC preambles  105 A to  105 D are received with different received-signal levels, which differs from the signals (first short-preamble sequence  101 , second long-preamble sequence  102 , first signal  103  and second signal  104 ) transmitted with the almost same received-signal level from the transmission antenna  205 A.  
         [0055]     At this time, the AGC start controller  506  already has the previous notice of the reception of the AGC preambles  105 A to  105 D issued by the signal decoder  505 . Therefore, it supplies, based on symbol timing information from the symbol timing controller  502 , the receiving units  302 A to  302 D with another AGC start command when the AGC preambles pass through the A/D converter  403  in  FIG. 5 . Upon receiving the AGC start command, the receiving units  302 A to  302 D again perform AGC using the AGC preambles  105 A to  105 D. As a result, the signals supplied from the transmission antennas  205 A to  205 D, i.e., the signals transmitted through MIMO channels, can be appropriately adjusted and input to the respective A/D converters  403 .  
         [0056]     The second AGC start command may be issued after the second signal field  104  is decoded. However, in the embodiment, the second AGC start command is issued after the reserve bit  132  of the first signal field  103  is decoded. This enables a sufficient time to be held before AGC is actually started in response to the AGC start signal. Specifically, a margin can be imparted by the time required to decode the second signal field  104 . Accordingly, compared to the case where the AGC start command is output after the second signal field  104  is decoded, the speed of decoding can be reduced and hence more inexpensive LSIs can be provided. Further, since AGC using the AGC preambles  105 A to  105 D can be performed within a longer time than in the case where the AGC start command is output after the second signal field  104  is decoded, high-quality signals can be received under the control using appropriate AGC values. In other words, gain control for the variable-gain amplifier  402  is performed again using the signal levels acquired after baseband signals corresponding to the AGC preambles  105 A to  105 D are A/D converted as shown in  FIG. 4 .  
         [0057]     In the preamble proposed by Jan Boer, which is described before, AGC is performed only using a short-preamble sequence (legacy short preamble), transmitted from a single transmit antenna. AGC is performed using a reception level with which the signal transmitted from the antenna where the short-preamble sequence transmits. When a wireless receiving device receives signals transmitted from other three antennas, the device executes gain control by using the acquired gain.  
         [0058]      FIG. 7  is a graph illustrating the distribution of the receiving power of a short preamble and data, acquired when Jan Boer&#39;s proposed preamble is utilized. The channel is in a multipath environment with a delay spread of 50 nsec (the duration for one data symbol is 4 μsec). As is evident from this figure, the ratio of the receiving level of short preamble (legacy short preamble) to the receiving level of the data varies significantly.  
         [0059]     In, for example, region A in  FIG. 7 , the short preamble is received with a high receiving level, although the receiving level of data is low. Accordingly, if AGC is adjusted in accordance with the receiving power of the short preamble, the receiving power of the data is lower than the receiving power of the short preamble, resulting in a quantization error in the A/D converter  403 . In region B in  FIG. 7 , the short preamble is received with a low receiving level, although the receiving level of data is high. Accordingly, if AGC is adjusted in accordance with the receiving power of the short preamble, the output of the A/D converter when data is input is saturated. Thus, it is understood that since, in the conventional scheme, the receiving power ratio of data to the short-preamble is not constant; the receiving characteristic is degraded because of a quantization error or saturation in the output of the A/D converter.  
         [0060]     On the other hand, in the embodiment, all antennas  205 A to  205 D that transmit data signals transmit AGC preambles  105 A to  105 D, respectively.  FIG. 8  shows the distribution of the receiving power of the short-preambles and data, according to the embodiment. The channel environment is the same as in the case of  FIG. 7 .  
         [0061]     As shown in  FIG. 8 , the receiving power of the AGC preambles is substantially proportional to that of the data  110 A to  110 D. This indicates that the input level of the A/D converter is adjusted so appropriate that the receiving accuracy is remarkably enhanced as compared to the  FIG. 7 .  
         [0062]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.