Abstract:
A wireless transmitting device includes a first and a second antennas, a first and a second transmitters connected to the first and the second antenna, respectively, signal provide unit provide a short preamble sequence, a first and a second signal fields to the first transmitter, and provide an AGC preamble sequence, a data and a long-preamble sequence to estimate a channel response to the first and the second transmitters, and a controller to power on the transmitters at different timings, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-173138, filed Jun. 10, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a wireless transmitting device and a method for transmitting by using a wireless packet including a preamble signal.  
         [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 preamble of wirelss packet 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, a wireless transmitting device uses a power amplifier for amplifying a transmission signal. Right after the power amplifier is powered on, it generates distortions in an output signal. The power amplifier needs a certain extent of time until the output signal achieve a specified level after it is powered on. A long-preamble sequence included in the wireless packet so as to estimate a channel response is extremely degrades receiving performance of the wireless transmitting device if the long-preamble sequence is not received in a low distortion state. Therefore, it is required to transmit the long-preamble sequence so as to avoid the distortion thereof as much as possible, however, Jan Boer, et al., have not disclosed any measures therefore.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     An object of the invention is to provide a wireless transmitting device and a method capable of estimating a channel response on a receiving side by preventing a signal especially used for estimating the channel response being transmitted with distortions when a plurality of transmission antennas conduct transmission like a multi-input/multi-output (MIMO).  
         [0010]     The first aspect of the present invention provides a wireless transmitting device for use in communication with a wireless receiving device, comprises a first antenna; at least one second antenna; a first and a second transmitter connected to the first antenna and the second antenna, respectively; a signal provide unit configured to provide a short preamble sequence, a first signal field and a second signal field to the first transmitter, and provide an automatic gain control (AGC) preamble sequence, a data and a long-preamble sequence to estimate a channel response, to the first transmitter and the second transmitter; and a controller to power on the first transmitter and the second transmitter at different timings, respectively.  
         [0011]     The second aspect of the present invention provides a wireless transmitting device for use in communication with a wireless receiving device, comprises a first antenna; at least one second antenna; a first and a second transmitter connected to the first antenna and the second antenna, respectively; a signal provide unit configured to provide a short preamble sequence, a first signal field and a second signal field to the first transmitter, and provide an automatic gain control (AGC) preamble sequence, and a long-preamble sequence to estimate data and a channel response, to the first transmitter and the second transmitter; and a controller to power on the first transmitter in time with transmission of the short preamble sequence from the first antenna and power on the second transmitter during transmission of the AGC preamble sequence.  
         [0012]     The third aspect of the present invention provides a wireless transmitting method comprises transmitting a short preamble sequence, a first signal field and a second signal field from a first antenna by using a first transmitter; transmitting an automatic gain control (AGC) preamble sequence, a data and a long-preamble sequence to estimate a channel response, from the first antenna and the second antenna after transmission of the second signal field from the first antenna by using the first transmitter and a second transmitter; powering on the first transmitter in time with transmission of the short preamble sequence from the first antenna; and powering on the second transmitter after transmission of the second signal field from the first antenna.  
         [0013]     The fourth aspect of the present invention provides a wireless transmitting method comprises transmitting a short preamble sequence, a first signal field and a second signal field from a first antenna by using a first transmitter; transmitting an automatic gain control (AGC) preamble sequence, a data and a long-preamble sequence to estimate a channel response, from the first antenna and the second antenna after transmission of the second signal field from the first antenna by using the first transmitter and a second transmitter; powering on the first transmitter in time with transmission of the short preamble sequence from the first antenna; and powering on the second transmitter after transmission of the second signal field from the first antenna. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0014]      FIG. 1  is a view showing a wireless packet including a preamble signal, variations in an output from a power amplifier and automatic gain control (AGC) start timing on a receiving side according to an embodiment of the invention;  
         [0015]      FIG. 2  is a view showing subcarrier arrangements of AGC preambles in  FIG. 1 ;  
         [0016]      FIG. 3  is a view showing a wireless packet based on the IEEE 802.11a;  
         [0017]      FIG. 4  is a block diagram of a wireless transmitting device according to the embodiment of the invention;  
         [0018]      FIG. 5  is a block diagram of the wireless receiving device according to the embodiment of the invention;  
         [0019]      FIG. 6  is a block diagram showing a specific example of a receiving unit in  FIG. 5 ; and  
         [0020]      FIG. 7  is a block diagram showing a specific example of a digital demodulator in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Embodiments of the invention will be described in detail with reference to the accompanying drawings.  
         [0022]      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 IEEE802.11a wireless stations.  
         [0023]     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.  
         [0024]     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 .  
         [0025]     After the PLCP signal, AGC preambles  105 A and  105 B that are transmitted in parallel from a plurality of antennas Tx 1  and Tx 2  are positioned. The AGC preambles  105 A and  105 B are transmitted simultaneously from a plurality of antennas Tx 1  and Tx 2 . The AGC preambles  105 A and  105 B 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 IEEE802.11n. Therefore, the AGC preambles  105 A and  105 B may be called “high throughput short trainings field (HTS)”. 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”.  
         [0026]     A second long-preamble sequence  106 A- 109 A and  103 B- 109 B are arranged after the AGC preambles  105 A and  105 B, respectively. In this embodiment, sequence shown in  FIG. 2  is used as the AGC preambles  105 A and  105 B.  FIG. 2  shows subcarrier arrangements in the frequency domains of the AGC preambles  105 A and  105 B. Subcarriers indicated in black in  FIG. 2  indicate that the subcarriers have values, for example, 1 or −1, in the AGC preambles  105 A and  105 B, and subcarriers indicated in white in  FIG. 2  indicate that the subcarriers have zero values in the AGC preambles  105 A and  105 B. In  FIG. 2 , there are 52 subcarriers except for zero subcarriers indicated at centers with hatched lines.  
         [0027]     Each transmission antenna Tx 1  and Tx 2  uses six subcarriers (indicated in black), respectively, and the number of subcarriers used at the antennas Tx 1  and Tx 2  is designed to become twelve in total in this example. The number of subcarriers used in the short preamble sequence  101  is also twelve and the subcarriers used therein are the same as those used in the AGC preambles  105 A and  105 B in this example. It is possible that the number of subcarriers used in the short preamble sequence  101  is not same as those used in the AGC preambles  105 A and  105 B. In this example, the number of subcarriers used in the short preamble sequence  101  is also twelve and the subcarriers used therein are the same as those used in the AGC preambles  105 A and  105 B. Accordingly, if the subcarriers in the frequency domains shown in  FIG. 2  are converted into waveforms in the time domains, periods of the AGC preambles  105 A and  105 B become same as that of the short preamble sequence  101 .  
         [0028]     If the subcarriers used in the short preamble sequence  101  is the same as those used in the AGC preambles  105 A and  105 B, periods of the AGC preambles  105 A and  105 B become different length of that of the short preamble sequence  101 .  
         [0029]     Although the case that the number of the transmission antennas is two is explained in this embodiment, the number of the transmission antennas is not limited to this case. For example, in the case that the number of the transmission antennas is three, each transmission antenna uses four subcarriers, respectively.  
         [0030]     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 and  106 B to  109 B. The second long-preamble sequences  106 A to  109 A and  106 B to  109 B 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”.  
         [0031]     After each of the second long-preamble sequences  106 A to  109 A and  106 B to  109 B, a field for transmission data (DATA)  110 A and  110 B transmitted from the antennas Tx 1  and Tx 2 , respectively, is positioned. The second long-preamble sequences  106 A to  109 A and  106 B to  109 B are transmitted simultaneously from a plurality of antennas Tx 1  and Tx 2  respectively.  
         [0032]     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 and  106 B to  109 B 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 and  110 B. The coding scheme indicates the coding rate of a convolution code as an error correction signal.  
         [0033]     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 and  110 B. 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.  
         [0034]      FIG. 3  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 a first 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 .  
         [0035]     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. 3 . 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 .  
         [0036]     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 .  
         [0037]     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 and  105 B, 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 and  105 B and data items  110 A and  110 B performed by a plurality of transmission antennas Tx 1  and Tx 2 , and (d) notifies the transmission of the second signal field  104 .  
         [0038]     The notification (a) includes indirect notification of the transmission of the AGC preambles  105 A and  105 B 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 and  105 B, second long-preamble sequences  106 A to  109 A and  106 B to  109 B, and data items  110 A and  110 B. That is, the wireless packet includes signals transmitted from a plurality of transmission antennas and combined by multiplexing using MIMO.  
         [0039]     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 and  105 B will be received after the reserve bit  132 .  
         [0040]     It is required for timing of AGC on the receiving side to respond receiving of a second signal field  104  and of the AGC preambles  105 A and  105 B.  
         [0041]     Thereafter, the wireless transmitting device according to the embodiment of the invention for transmitting the wireless packets shown in  FIG. 1  and  FIG. 2  will be explained by referring to  FIG. 4 .  
         [0042]     At first, a timing controller  201  with a transmission command received thereby outputs a read command to a baseband-signal processing unit  202 . The baseband-signal processing unit  202  with the read command received thereby reads the preamble shown in  FIG. 1  by referring to a memory  203 . The baseband-signal processing unit  202  constitutes the whole of the wireless packet shown in  FIG. 1  by modulating and encoding transmission data transmitted from further high order layer.  
         [0043]     A baseband-signal of the wireless packets output from the baseband-signal processing unit  202  are respectively transferred to wireless transmission units  204 A and  204 B for the antennas Tx 1  and Tx 2  shown in  FIG. 1 , prescribed processing (for example, up conversion, filtering) is performed therefore here, then, transferred to power amplifiers  205 A and  205 B. The power amplifiers  205 A and  205 B perform power amplification to signals input from the wireless transmission units  204 A and  204 B to supply the amplified outputs to the antennas Tx 1  and Tx 2 , respectively. As a result, the antennas Tx 1  and Tx 2  transmit the wireless packets shown in  FIG. 1 , respectively.  
         [0044]     In the embodiment, the transmission antenna Tx 1  in  FIG. 4  transmits PLCP signals in a range from the 2.0 short preamble sequence  101  up to a first long-preamble sequence  102 , a first signal field  103  and a second signal field  104 . The transmission antennas Tx 1  and Tx 2  in  FIG. 4  transmit the AGC preambles  105 A and  105 B, the second long-preamble sequence  106 A- 109 A and  106 B- 109 B and data  110 A and  110 B.  
         [0045]     The timing controller  201  outputs a command to the power amplifier  205 A just before the transmission of the short preamble sequence  101  and powers on the power amplifier  205 A. The timing controller  201  also powers on the power amplifier  205 B in time with transmission of the AGC preambles  105 A and  105 B. In the embodiment shown in  FIG. 1 , just after the signal of the AGC preambles  105  is arrived at the power amplifier  205 B, the timing controller  201  powers on the power amplifier  205 B.  
         [0046]      FIG. 1  schematically shows variations in output level of the power amplifiers  205 A and  505 B with powering on them. That is, the output level of the power amplifier  205 A for the antenna Tx 1  remains in low level until the antenna Tx 1  transmits the short preamble sequence  101 , and rises to a high level almost the same time of the transmission of the short preamble sequence  101 . On the other hand, the output level of the power amplifier  205 B for the antenna Tx 2  is switched from the low level to the high level in time with transmission of the AGC preambles  105 A and  105 B.  
         [0047]     The timing to switch the output level of the power amplifier  205 A for the antenna Tx 1  from the low level to the high level can be advanced or delayed in comparison with the timing shown in  FIG. 1 . Specifically, there is no problem if the distortion of the output signal from the power amplifier  205 A is in an extent not to affect frame detection and the first AGC using the short preamble sequence  101  mentioned later.  
         [0048]     In contrary, in the case that the power amplifier  205 B is powered on before transmission of the AGC preamble  105 A, specifically, during the transmission of the second signal field  104 , there is a possibility for the power amplifier  205 B to cause distortions of the transmission signal from the antenna Tx 1  because a large current flows in a wireless circuit. Accordingly, the power-on timing for the power amplifier  205 B is optimal after transmitting the second signal field  101 , in other words, during transmission of the AGC preambles  105 A and  105 S, and further preferably, it is preferable to set the timing so as not to affect a second AGC using the AGC preambles  105 A and  105 B.  
         [0049]     In the wireless receiving device shown in  FIG. 5 , 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.  
         [0050]     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. 4  and the wireless receiving device of  FIG. 5 . 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 shown in  FIG. 4 .  
         [0051]     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. 5  will now be described.  FIG. 6  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.  
         [0052]     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.  
         [0053]     The operation of the wireless receiving device shown in  FIG. 5  and  FIG. 6  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 Tx 1  of  FIG. 4 , 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.  
         [0054]     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.  
         [0055]     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.  
         [0056]     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.  
         [0057]     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.  
         [0058]     Since the power amplifier  205 A is powered on in time with transmission of the sort preamble sequence  101 , distortions are generated in a first half of the short preamble sequence  101 . However, the first half of the short preamble sequence  101  is used for detection of the wireless packet and the AGC on the receiving side, so that no problem occurs if a quantity of distortions is generated in the first half of the short preamble sequence  101 .  
         [0059]     The AGC setting is conducted repeatedly. Since a second half of the short preamble sequence  101  can correctly conduct the AGC by using signals without distortions owing to affection of power-on, the occurrence of the distortions in the first half of the short preamble sequence  101  does not cause any specific trouble against the AGC.  
         [0060]     The wireless receiving device receives a first long-preamble sequence  102  transmitted from the transmit antenna Tx 1 , 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 Tx 1  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 Tx 1 , 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.  
         [0061]     The wireless receiving device receives a first signal field  103  transmitted from the transmit antenna Tx 1 , 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.  
         [0062]     Referring to  FIG. 7 , the digital demodulator  304  shown in  FIG. 5  will be described in detail. The digital demodulator  304  receives signals  500  from the receiving units  302 A to  302 D shown in  FIG. 5 . 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 .  
         [0063]     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.  
         [0064]     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.  
         [0065]     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.  
         [0066]     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 and  105 B 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. 6 , thereby causing the gain controller  404  to start gain control.  
         [0067]     After receiving the second signal field  104  from the transmission antenna Tx 1 , the wireless receiving device receives the AGC preambles  105 A and  105 B from the transmission antennas Tx 1  and Tx 2 . The AGC preambles  105 A and  105 B are transmitted from the transmission antenna Tx 1  that has transmitted so far the previous signals, and from the transmission antenna Tx 2  that has not yet transmitted any signals. Accordingly, the AGC preambles  105 A and  105 B 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 Tx 1 .  
         [0068]     At this time, the AGC start controller  506  already has the previous notice of the reception of the AGC preambles  105 A and  105 B 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. 6 . Upon receiving the AGC start command, the receiving units  302 A to  302 D again perform AGC using the AGC preambles  105 A and  105 B. As a result, the signals supplied from the transmission antennas Tx 1  and Tx 2 , i.e., the signals transmitted through MIMO channels, can be appropriately adjusted and input to the respective A/D converters  403 .  
         [0069]     In the explanation described above, though a target value Z used for AGC at first time and a target value used for AGC at second time are set to the same value, different values may be set for the first time and the second time AGC. Thereby, it becomes possible for signals transmitted from a single antenna and also signals simultaneously transmitted from a plurality of antennas to be respectively conducted A/D conversion with high precision.  
         [0070]     The FFT is used to decode the first signal field  103 . The FFT is principally can not be started before the first signal field  103  is wholly received. Processing of the FFT and error correction decoding of the first signal field  103  are applied during reception of the second signal field  104 .  
         [0071]     Taking a current large-scale integration (LSI) technique into account, the timing to know a decoding result of the first signal field  103  is set in a period when a wireless receiving device has already been receiving the AGC preambles  105 A and  105 B. The timing to start the second time AGC is also set in a period when the wireless receiving device has already been receiving the AGC preambles  105 A and  105 B. At a lower section in  FIG. 1 , timing at which a wireless communication device starts the AGC is drawn. According to the drawing, aspects to conduct the first time AGC during the receiving of the short preamble sequence  101  and start the second time AGC during reception of the AGC preambles  105 A and  105 B are exhibited.  
         [0072]     As shown in  FIG. 1 , if the power amplifier  205 B for the antenna Tx 2  is powered on at the first halves of the AGC preambles  105 A and  105 B, the output from the power amplifier  205 B has already converged at the timing when the wireless receiving device decodes the first signal field  103 , thereby, the power-on of the power amplifier  205 B does not affect the second AGC. That is, it is not necessary for the power amplifier  205 B to be powered on at a timing of transmission of the AGC preamble  105 A, in other words, at a boundary between the second signal field  104  and the AGC preamble  105 A (in the case of  FIG. 1 , powered on at the boundary), and it is required for the power  205 B to be adjusted so that the distortion of the power amplifier  205 B is converged until the wireless communication device will start the second AGC.  
         [0073]     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 the first AGC using the AGC preambles  105 A and  105 B 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 and  105 B are A/D converted as shown in  FIG. 5 .  
         [0074]     In this embodiment, the indicator of the IEEE802.11n is in the first signal field  103 , however, the indicator can be implemented in the second signal filed  104 . In this case, the indicator should be recognized by the receiver before the error correction, because the time from the indicator to the second AGC preamble is shorter than that when the indicator is in the first signal field  103 . The second AGC start command will be issued after the recognizing the second signal field.  
         [0075]     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.