High-speed communications technologies have developed remarkably. Particularly, a communication method using a spread spectrum wireless signal has been widely used in the field of data communication and mobile communication so as to allow for (i) efficient use of radio waves and (ii) noise resistance.
Since a general wireless LAN apparatus serves as a wireless terminal, there has been a demand for miniaturization and portability of the wireless LAN apparatus. Therefore, the wireless LAN apparatus needs to be driven by a battery or the like, and is expected to operate with low power consumption. Normally, when a reverse spread signal reaches a certain level or higher during a reception standby state, the wireless LAN apparatus determines to start a reception operation. Then, the wireless LAN apparatus switches from the reception standby state to a reception state so as to start the reception operation. On the other hand, in response to a transmission request sent from a terminal, the wireless LAN apparatus confirms that no other terminals are carrying out transmission, and then starts transmission. During the reception, the wireless LAN apparatus needs to operate a large number of flip-flop circuits such as a shift register for the purpose of, for example, (i) despreading spread spectrum data, (ii) detecting amplitude per symbol, and (iii) operating an integrating circuit in a synchronization process. Therefore, the wireless LAN apparatus consumes a large amount of power during the reception. Further, even in the reception standby state, the wireless LAN apparatus needs to operate its RF section, its reverse spread demodulation section, its amplitude detection section, and the like for the purpose of detecting the level of the reverse spread signal. Therefore, the power consumption cannot be greatly reduced even in the reception standby state. Thus, although the wireless LAN apparatus is in the reception standby state for most of the time, most of its circuits are always operated. Therefore, the wireless LAN apparatus consumes as much power in the reception standby state as it does during the reception, i.e., wastes power.
Further, Japanese Unexamined Patent Publication No. 333638/2003 (Tokukai 2003-333638; published on Nov. 21, 2003) discloses cell detection carried out in accordance with a result of a correlation arithmetic process carried out with respect to a received signal with the use of a spread code used at a transmitting end. However, also in this case, even in the reception standby state, it is necessary to supply power to a circuit for carrying out the correlation detection with respect to the received signal. Therefore, the effect of reducing the power consumption is small.
Power supply and clock signal supply to such a reception circuit (digital circuit component) can be stopped in cases where the wireless circuit is provided with means for detecting the received signal in an analog manner. This makes it possible to reduce the power consumption. Generally adopted as the method for detecting the received signal in the analog manner is a method for generating, at an IF stage (intermediate frequency stage), a received signal level signal indicating a received signal level substantially proportional to the reception power. A typical example of the received signal level signal indicating the received signal level is an RSSI (Received Signal Strength Indicator). In accordance with the received signal level signal thus generated, a judgment is carried out whether or not the signal exists at the receiver input. When the signal is detected, it is judged that some sort of signal has been received by an antenna. An A/D converter and the reception circuit provided after the A/D converter are operated only in such a case. In this way, it becomes possible to reduce power that is to be consumed by the A/D converter and the digital circuit component (component constituting the reception circuit in this case).
FIG. 5 is a functional block diagram showing an example of an arrangement of each of a transmitting section and a receiving section of a conventional wireless LAN apparatus proposed based on the idea described above and adopting the direct-sequence spread spectrum technique. (See Japanese Unexamined Patent Publication No. 307428/1996 (Tokukaihei 8-307428; published on Nov. 22, 1996).) The communication apparatus (wireless LAN apparatus) 100 shown in FIG. 5 includes an antenna 102, an RF section 103 of the receiving system, a received signal level determination section 115, a power supply/clock control section 117, an A/D converter 105, a despreading demodulation section 107, an amplitude detection section 111, a synchronization integrating section 121, a synchronization detection section 123, and an information demodulation section 113. The RF section 103 carries out frequency conversion of an RF signal contained in an electric wave picked up by the antenna 102, into a baseband signal. The received signal level determination section 115 makes, by using a comparator provided therein, a comparison between (i) a received signal level signal generated in the RF section 103 and (ii) a threshold value C shown in FIG. 7, and determines whether or not the reception operation is to be started. The power supply/clock control section 117 controls power supply and a clock for each of the blocks. The A/D converter 105 carries out A/D conversion with respect to an output of the RF section 103. The despreading demodulation section 107 reverse-spreads a spread signal. The amplitude detection section 111 calculates an amplitude value of an output of the despreading demodulation section 107. The synchronization integrating section 121 integrates an output of the amplitude detection section 111 per symbol. The synchronization detection section 123 generates a synchronization signal in accordance with an output of the synchronization integrating section 121. The information demodulation section 113 carries out information demodulation by using (a) the output of the amplitude detection section 111 and (b) the synchronization signal sent from the synchronization detection section 123. The information demodulation section 113 supplies an output signal to a terminal apparatus 125. In FIG. 5, the transmitting system is omitted.
The following explains operations of the apparatus shown in FIG. 5. In the reception standby state, only the RF section 103 of the receiving system, the received signal level determination section 115, and the power supply/clock control section 117 are operated (reception standby mode). The power supply/clock control section 117 stops the power from being supplied to an RF section (not shown) of the transmitting system, and stops the operation clock signal from being supplied to the other circuits (i.e., an information modulation section and a spread modulation section (both not shown) of the transmitting system, the A/D converter 105, the despreading demodulation section 107, the amplitude detection section 111, the synchronization integrating section 121, the synchronization detection section 123, and the information demodulation section 113), so that the circuits completely stop operating. Those circuits whose operation is stopped by stopping the supply of the operation clock signal may be arranged such that the operation of the circuits is stopped by stopping the power from being supplied to the circuits.
In such a reception standby mode, the RF section 103 of the receiving system sends a received signal level signal SG101 to the received signal level determination section 115. The received signal level determination section 115 makes, by using the comparator provided therein, a comparison between (i) the received signal level signal SG101 and (ii) the threshold value C which is shown in FIG. 7 and which has been designated, for example, by the terminal apparatus 125. In cases where the received signal level signal SG101 has a value greater than the threshold value C, the received signal level determination section 115 determines to start the reception operation. Then, the power supply/clock control section 117 operates the other circuits necessary for the reception operation (i.e., the A/D converter 105, the despreading demodulation section 107, the amplitude detection section 111, the synchronization integrating section 121, the synchronization detection section 123, and the information demodulation section 113) (reception mode). When the reception operation is finished, the power supply/clock control section 117 causes only the RF section 103 of the receiving system, the received signal level determination section 115, and the power supply/clock control section 117 to be operated (reception standby mode).
The communication apparatus 100 starts transmission in the following manner. That is, upon receiving a transmission request from the terminal apparatus 125, the communication apparatus 100 confirms that the communication apparatus 100 is not carrying out the reception operation, and then starts the transmission by causing the circuits necessary for the transmission (i.e., the information modulation section, the spread modulation section, and the RF section of the transmitting system (all not shown)) to be operated. When the transmission is finished, the power supply/clock control section 117 causes only the RF section 103 of the receiving system, the received signal level determination section 115, and the power supply/clock control section 117 to be operated. By providing such a low power consumption mode, the power consumption can be reduced.
In the wireless apparatus, the received signal level signal normally has a characteristic shown in FIG. 7. In FIG. 7, the horizontal axis represents reception power (received power) available at an end of the antenna, and the vertical axis represents a value of the received signal level signal such as the RSSI. Further, the dotted line L1 represents the minimum value of reception sensitivity, i.e., the weakest reception power of a signal that can be received by the wireless apparatus. Furthermore, the threshold value C is usually set at a value greater than the minimum value of the received signal level signal for the purpose of preventing a malfunction from occurring due to the influence of noise. Therefore, the reception power range represented by R3 of FIG. 7 is a range of the reception power of the signal that can be received. On the other hand, a wireless terminal that starts the reception operation in cases where the received signal level signal has a value not less than the threshold value C starts the reception operation only when the reception power is greater than the reception power indicated by the dotted line L2, which is a vertical line extending down from the intersection of the characteristic graph with the dotted line indicating the threshold value C. Therefore, the range of the reception power of the signal that can be received by the wireless apparatus is the range represented by R2. That is, the range of the reception power of the signal that can be received becomes narrower by R1, i.e., a difference between R3 and R2. Particularly, since the spread spectrum (SS) technique is capable of demodulating even a signal indicating a level substantially equal to that of noise, the difference R1 between R3 and R2 becomes larger. Generally, as the wireless terminal gets further away from a transmitting terminal, the reception power becomes smaller. Therefore, the communication distance is shortened by a distance corresponding to R1, i.e., the difference between the two ranges of the reception power of the signal that can be received by the wireless terminal which starts the reception operation in cases where the received signal level signal has a value not less than the threshold value C, although the wireless terminal allows reduction of power that is to be consumed in the reception standby state. That is, since the conventional wireless LAN apparatus is provided with the low power consumption mode, the range of the reception power of the signal that can be received becomes narrow. This causes such a problem that the communication distance becomes short.