JP4914334B2 - Wireless communication system - Google Patents

Wireless communication system Download PDF

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JP4914334B2
JP4914334B2 JP2007321162A JP2007321162A JP4914334B2 JP 4914334 B2 JP4914334 B2 JP 4914334B2 JP 2007321162 A JP2007321162 A JP 2007321162A JP 2007321162 A JP2007321162 A JP 2007321162A JP 4914334 B2 JP4914334 B2 JP 4914334B2
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lowering
frame
signal
wireless communication
signal quality
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JP2009147551A (en
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真由子 上野
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株式会社リコー
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  The present invention relates to a wireless communication system, and more particularly, to a wireless communication system including a method for reliably detecting CCA in a wireless communication system including a modulation unit that modulates a frame.

  In recent years, wireless LANs based on 802.11 standards have rapidly spread to restaurants, such as stations, airports, hotels, and cafes, as well as businesses and general households, because of the convenience of moving terminals freely. . In the future, various wireless communication networks such as wireless LAN, WirelessUSB, and Bluetooth are expected to be installed in various devices in order to improve the efficiency in the office and the convenience of living. However, unlike wired communication, which cannot be intercepted without connecting a cable, wireless communication has problems such as wiretapping, unauthorized access, and spoofing due to leakage of radio waves, and ensuring security is always an issue. In particular, in public wireless LAN services that enable high-speed wireless communication at restaurants such as stations, airports, hotels, and cafes that are rapidly spreading in recent years, since an unspecified number of users always enter and exit, eavesdropping and spoofing, etc. Is likely to be attacked. Therefore, realization of strong security and reliable authentication technology is required.

Further, in the technique disclosed in Patent Document 1, the wireless transmission output is lowered, thereby minimizing the reach of radio waves and making it difficult for others to intercept radio waves, thereby enhancing the security of radio communication. . FIG. 10 shows a general wireless LAN AP (access point) 41, a wireless communication terminal 40 connected to the AP 41, a wireless communication terminal 42 of the other party, and a range in which radio waves of the respective wireless communication devices can reach. FIG. FIG. 11 is a diagram illustrating a range in which radio waves output from the AP 41 and the wireless communication terminal 40 reach when the AP 41 and the radio communication terminal 40 communicate with each other by minimizing the range in which the radio waves reach by the conventional wireless LAN. is there. Many of the wireless communication devices have a CCA (Clear Channel Assessment) function that indicates the usage status of the wireless channel. The CCA is detected when a use state of a radio channel is confirmed and a reception signal having a certain power level or higher or a signal that can be demodulated is received. In a wireless communication apparatus using a CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance) access method such as 802.11, CCA is confirmed before data transmission, and when CCA is detected, The channel is determined to be in use and cannot transmit data.
Further, in the technique disclosed in Patent Document 2 filed by the same applicant, the range in which radio waves reach is narrowed by adding a noise signal to transmission data.
JP 2006-33654 A Japanese Patent Application No. 2006-136703

  However, in the environment shown in FIG. 10, the prior art disclosed in Patent Document 1 allows the AP 41, the wireless communication terminal 40, and the other party wireless communication terminal 42 to receive each other's transmission data. CCA detection can be performed correctly. However, in the environment shown in FIG. 11, the other-party wireless communication terminal 42 is located outside the range (area A) within which the transmission radio wave during communication between the AP 41 and the wireless communication terminal 40 can reach. CCA cannot be detected while the communication terminal 40 is communicating. Therefore, although the AP 41 and the wireless communication terminal 40 are communicating, another person's wireless communication terminal 42 transmits data to the AP 41, causing a problem that frames collide and throughput is reduced. Further, although the frequency of collision of frames can be reduced by using RTS / CTS, since it takes extra time to exchange RTS signals and CTS signals, throughput is reduced. As described above, reducing the radio transmission output and minimizing the reach of radio waves has the problem that other wireless communication terminals connected to the same AP cannot perform CCA detection correctly and throughput is reduced. It was.

  In the prior art disclosed in Patent Document 2, CCA detection in 802.11a is detected when an 802.11a preamble is detected and when a signal with a power level of −62 dBm or more is detected. That is, a CCA is not detected for a signal having a power level of less than -62 dBm unless an 802.11a preamble is detected. If the noise signal is added to the transmission data, the signal quality of the preamble is deteriorated, so that it is difficult to detect the preamble. Therefore, there arises a problem that CCA detection becomes difficult when a signal having a power level of less than -62 dBm is received. Furthermore, in WirelessUSB, detection of CCA is not maintained unless detection of a WirelessUSB preamble and a PLCP (Physical Layer Convergence Protocol) header is performed at a power level equal to or higher than the minimum reception sensitivity (−80.8 dBm). For this reason, when a noise signal is added to transmission data and the signal quality deteriorates, there is a problem that the CCA cannot be detected correctly because the preamble and PLCP header cannot be detected.

As described above, in the conventional wireless communication device, if the communication range is limited to ensure security, CCA detection is not performed correctly in other wireless communication devices, frames collide, and throughput decreases. was there.
The present invention has been made in view of such a problem. In a wireless communication apparatus that performs wireless communication, CCA is reliably detected by reducing signal quality only during a partial period of a modulated frame, and CCA detection is performed. It is an object of the present invention to provide a wireless communication apparatus that can prevent a decrease in throughput due to a failure and can ensure security.

In order to solve such a problem, the present invention provides a wireless communication system comprising modulation means for modulating a frame, and transmitting and receiving a frame modulated by the modulation means to and from a counterpart wireless communication apparatus. A reduction permission signal generating unit for generating a reduction permission signal for designating a signal reduction period for reducing the signal quality of a partial period of the frame modulated by the modulation means, and the signal quality of the frame modulated by the modulation means Signal quality lowering means for reducing the signal quality, wherein the signal quality lowering means lowers the signal quality of the frame based on the lowering permission signal generated from the lowering permission signal generating unit.
According to a second aspect of the present invention, the lowering permission signal generated from the lowering permission signal generating unit is asserted during a period other than the preamble constituting the modulated frame.
A third aspect of the present invention is characterized in that the lowering permission signal generated from the lowering permission signal generation unit is asserted during a period of a data payload constituting the modulated frame.

According to a fourth aspect of the present invention, the signal quality reduction means includes quantization error addition means for adding a quantization error by reducing the number of effective bits of the modulated frame, and the signal quality reduction means includes The quantization error is added to a partial period of the frame modulated by the quantization error adding means.
According to a fifth aspect of the present invention, the signal quality reduction unit further includes a noise signal generation unit that generates a noise signal, and the signal quality reduction unit is generated by the noise signal generation unit during a partial period of a modulated frame. A noise signal is added.
According to a sixth aspect of the present invention, the lowering permission signal generated from the lowering permission signal generating unit is asserted so as to lower the signal quality in a partial period of the modulated frame only when a frame including security information is transmitted. It is characterized by that.

  According to the present invention, in a wireless communication apparatus that performs wireless communication, CCA is reliably detected by reducing the signal quality only during a partial period of a modulated frame, thereby preventing a decrease in throughput due to CCA detection failure, and security. Can be provided.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the components, types, combinations, shapes, relative arrangements, and the like described in this embodiment are merely illustrative examples and not intended to limit the scope of the present invention only unless otherwise specified. .
FIG. 1 is a diagram illustrating a configuration example of a wireless communication apparatus according to the present invention. Only the modulation function unit related to the present invention is shown, and the function units related to reception and transmission are omitted. Radio communication apparatus 100 designates antenna 1, RF unit 2 that transmits an RF signal to antenna 1, DAC unit 3 that converts a digital signal into an analog signal, modulation unit 5 that performs frame modulation, and a signal reduction period A reduction permission signal generation unit 6 for reducing the signal quality in a partial period of the modulated frame.

  The lowering permission signal generation unit 6 is set with an arbitrary period for lowering the signal quality, and generates a lowering permission signal as the modulated frame is transmitted. 1 shows a first embodiment of the present invention. 1 shows a first embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the modulated frame 10 and the lowering permission signal 14. The signal quality is lowered in the period 15 in which the lowering permission signal 14 is asserted, here, in the shaded portion 13 of the modulated frame 10. The reduction permission signal generation unit 6 may be provided in the modulation unit 5 or may be provided in a higher layer than the modulation unit 5. The frame is modulated by the modulation unit 5, and the signal quality reduction unit 4 reduces the signal quality in the partial period 15 of the modulated frame 10 based on the reduction permission signal 14 generated by the reduction permission signal generation unit 6. The signal is transmitted from the antenna 1 through the DAC unit 3 and the RF unit 2.

  2 shows a second embodiment of the present invention. FIG. 3 is a diagram showing an example of a frame format in 802.11a. A frame in 802.11a includes a PLCP preamble 11, a SIGNAL 12 having information such as a transmission rate and a frame data length, and data 13. The decrease permission signal generation unit 6 deasserts the decrease permission signal 14 during the PLCP preamble period of the modulated frame 10, and asserts the decrease permission signal 14 during the SIGNAL 13 and the data period. The signal quality during the period when the lowering permission signal 14 is asserted is lowered by the signal quality lowering means 4. In the CCA detection in 802.11a, a signal having a power level of −62 dBm or more is detected when a preamble is detected. Since the signal quality of the PLCP preamble 11 does not deteriorate, the detection range of CCA detection by the preamble is not narrowed. On the other hand, since the signal quality of the data portion 13 of the modulated frame is deteriorated, the range in which data can be received becomes narrow.

  A third embodiment of the present invention will be described. FIG. 4 is a diagram showing an example of a frame format in WirelessUSB. A frame in WirelessUSB includes a PLCP preamble 16, a PLCP header 17 including a PHY header and a MAC header, and a PSDU (PLCP Service Data Unit) 18. The lowering permission signal generation unit 6 deasserts the lowering permission signal 14 during the period of the PLCP preamble 16 and the PLCP header 17 of the modulated frame, and does not decrease the signal quality, but only the PSDU 18 asserts the lowering permission signal, and the signal quality Reduce. In WirelessUSB, if the power level is not less than the minimum reception sensitivity (−80.8 dBm) and the PLCP preamble 16 and the PLCP header 17 cannot be detected, the CCA is not held. Therefore, the range in which CCA can be detected is maintained by not degrading the signal quality of the PLCP preamble 16 and the PLCP header 17. On the other hand, since the signal quality of the PSDU 18 of the modulated frame is degraded, the range in which data can be received becomes narrow. Of course, the entire PSDU may be reduced in signal quality, or only the frame payload 18a may be included without including the FCS 18b, the tail 18c, and the padding bit 18d. It is also effective to reduce the signal quality of only the data without reducing the signal quality of the PLCP preamble 16 and SIGNAL in the 802.11a frame. This is because the SIGNAL includes transmission rate and frame length information, so that the CCA can be reliably held for the length of the frame by receiving the SIGNAL.

FIG. 5 is an example of a configuration diagram illustrating a wireless communication system according to the embodiment of the present invention. As shown in FIG. 5, the wireless communication system 110 includes an AP (access point) 21, a wireless communication terminal 20, and another person wireless communication terminal 22. When the AP 21 performs transmission, a circle 23 is a range where data can be received, and a circle 24 is a range where CCA detection is possible.
FIG. 6 is a diagram showing the propagation characteristics of radio waves in free space at 5.2 GHz. As shown in FIG. 6, the vertical axis indicates free space propagation loss (dB) and the horizontal axis indicates distance (m), and a 5.2 GHz radio wave attenuates by about 47 dB when separated by 1 m. Therefore, for example, in a wireless communication system that performs communication with a transmission power of +10 dBm, in a communication method in which a packet error of about 10% occurs with respect to a received signal of −73 dBm, if the signal quality is further reduced by 36 dB, the data The range that can be received is 1 m. In addition, the loss of the antenna and the loss due to obstacles such as walls are not considered. On the other hand, since the CCA detection range is not narrowed, CCA can be detected in a range of 65 m.

  FIG. 7 is a diagram illustrating a configuration example of a wireless communication apparatus according to another embodiment of the present invention. The same components will be described with the same reference numerals as in FIG. The signal quality degradation unit 4 has a quantization error addition unit 4a. Assuming that the DAC unit 3 in FIG. 7 is a 10-bit DAC, a quantization error is added by fixing the lower 6 bits of the frame modulated by the signal quality degradation unit 4 to 1 (or 0). As a result, the signal quality of about 36 dB can be lowered, and the range in which data can be received can be narrowed to 1 m. Furthermore, when it is desired to narrow the range in which data can be received from 1 m to 75 cm, the signal quality must be further reduced by 3 dB. The quantization error adding unit 4a can reduce the signal quality only about every 6 dB.

  FIG. 8 is a diagram illustrating a configuration example of a wireless communication apparatus according to another embodiment of the present invention. The same components will be described with the same reference numerals as in FIG. The signal quality lowering unit 4 includes a noise signal generating unit 4b in addition to the quantization error adding unit 4a. The noise signal generation unit 4b generates a pseudo random noise signal using, for example, a PN code. The generated noise signal is added to a partial period of the frame signal modulated by the adder 4c. Here, by adding noise of 3 dB, the range in which data can be received can be narrowed from 1 m to 75 cm. The range in which CCA can be detected at that time remains unchanged at 65 m. Here, it is assumed that the communication system generates a packet error of about 10% with respect to a received signal of −73 dBm. However, the received signal level at which a packet error of 10% occurs varies depending on the transmission speed. Therefore, it is necessary to change the amount of quantization error and noise signal added depending on the transmission speed of the data signal to be transmitted. Here, it is assumed that the transmission power is +10 dBm. Of course, the transmission power may be changed in accordance with the range in which CCA is desired to be detected. At that time, it is necessary to change the amount of quantization error or noise signal added depending on the transmission power.

  FIG. 9 is a diagram showing the CCA detection range and the data receivable range when the signal quality during the partial period of the modulated frame is stopped being lowered. As shown in FIG. 5, when the signal quality of a modulated frame in a partial period is lowered, the range in which data can be received is narrowed, and the convenience of freely moving the wireless communication device is impaired. End up. Therefore, the signal quality of a partial period of a modulated frame is reduced only when transmitting security information such as authentication information such as user ID, password, MAC address, and SSID and encryption information such as an encryption key, as shown in FIG. When communicating at a short distance of a wireless communication apparatus that wants to transmit security information and transmitting data that does not include security information, the signal quality during a partial period of the modulated frame is not degraded, and the wireless communication at a long distance as shown in FIG. The communication device can also receive data.

It is a figure which shows the structural example of the radio | wireless communication apparatus of this invention. It is the figure which showed the relationship between the modulated flame | frame 10 and the fall permission signal 14. FIG. It is a figure which shows an example of the frame format in 802.11a. It is a figure which shows an example of the frame format in WirelessUSB. It is a block diagram which shows the radio | wireless communications system which concerns on embodiment of this invention. It is the figure which showed the propagation characteristic of the electromagnetic wave in the free space in 5.2 GHz. It is a figure which shows the structural example of the radio | wireless communication apparatus which concerns on other embodiment of this invention. It is a figure which shows the structural example of the radio | wireless communication apparatus which concerns on other embodiment of this invention. It is the figure which showed the CCA detection range and data receivable range when it stops stopping the signal quality of the partial period of the modulated flame | frame. It is a block diagram which shows the radio | wireless communications system in a certain environment. It is a block diagram which shows the radio | wireless communications system in a certain environment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Antenna, 2 RF part, 3 DAC part, 4 Signal quality reduction part, 5 Modulation part, 6 Reduction permission signal generation part, 10 Modulated frame, 11 PLCP preamble, 12 SIGNAL, 13 Data, 100 Wireless communication apparatus

Claims (6)

  1. A wireless communication system comprising modulation means for modulating a frame, and transmitting and receiving a frame modulated by the modulation means to and from a counterpart wireless communication device,
    A lowering permission signal generating unit for generating a lowering permission signal for designating a signal lowering period for lowering the signal quality of a partial period of the frame modulated by the modulation unit;
    Signal quality lowering means for lowering the signal quality of the frame modulated by the modulating means,
    The radio communication system, wherein the signal quality lowering unit lowers the signal quality of the frame based on a lower permission signal generated from the lower permission signal generating unit.
  2.   The wireless communication system according to claim 1, wherein the lowering permission signal generated from the lowering permission signal generation unit is asserted in a period other than a preamble constituting the modulated frame.
  3.   The wireless communication system according to claim 1, wherein the lowering permission signal generated from the lowering permission signal generating unit is asserted during a period of a data payload constituting the modulated frame.
  4. The signal quality lowering means includes quantization error adding means for adding a quantization error by reducing the number of effective bits of the modulated frame,
    The wireless communication system according to claim 1, wherein the signal quality lowering unit adds the quantization error to a partial period of the frame modulated by the quantization error adding unit.
  5. The signal quality lowering means further comprises a noise signal generating means for generating a noise signal,
    The radio communication system according to claim 1 or 4, wherein the signal quality lowering unit adds the noise signal generated by the noise signal generating unit to a partial period of a modulated frame.
  6.   The degradation permission signal generated from the degradation permission signal generation unit is asserted so as to degrade the signal quality in a partial period of the modulated frame only when transmitting a frame including security information. The wireless communication system according to claim 1.
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