Radio communication system, device and method

0001The present invention relates to a radio communication system, a radio communication device, and a radio communication method of a radio wave communication system using microwaves. Illustrative embodiments of the present invention relate to a radio communication system, a radio communication device, and a radio communication method for realizing communication operation with low power consumption between devices at a relatively short distance.

0002More particularly, illustrative embodiments of the present invention relate to a radio communication system, a radio communication device, and a radio communication method that perform data communication by a backscatter system using transmission of an unmodulated carrier from a reading device side, and absorption and reflection of the received radio wave on the basis of an operation of terminating an antenna on a transmitting device side, and particularly to a radio communication system, a radio communication device, and a radio communication method that eliminate or at least reduce effects of transmitter noise on the reading device side to improve reception sensitivity and increase a communication distance.

0003One example of radio communication means applicable in only a limited area is RFID. RFID is a system including a tag and a reader, in which system the reader reads information stored in the tag in a non-contact manner. While the system is also referred to as an "ID system," a "data carrier system" and the like, a universally common name for the system is an RFID system. The RFID system may be abbreviated to RFID. Incidentally, the RFID system is an "identification system using high frequencies (radio waves)." Methods of communication between a tag and a reader/writer include for example an electromagnetic coupling type, an electromagnetic induction type, and a radio frequency communication type (see for example, <nplcit id="ncit0001" npl-type="b"><text>Klaus Finkenzeller (Translated from the 3rd German edition by Rachel Waddington, Swadlincote, UK) "Fundamentals and Applications in Contactless Smart Cards and Identification" (Wiley &amp; Sons LTD</text></nplcit>))

0004An RFID tag is a device including unique identifying information, and has an operation characteristic of oscillating a radio wave at a modulation frequency corresponding to the identifying information in response to reception of a radio wave of a specific frequency. On the basis of the oscillation frequency of the RFID tag, a reading device side can identify the RFID tag. Hence, a system using RFID makes it possible to identify an article, an owner, and the like using a unique ID written in the RFID tag. The RFID system is now used in many systems including for example a system for monitoring the entering and leaving of a room, an article identifying system in distribution, a bill payment system in restaurants and the like, and a system for preventing takeout before payment in stores selling CDs, software and the like.

0005A radio identification device of a small size can be produced by packaging an IC chip having a transmission and reception function and a memory function, a source for driving the chip, and an antenna (see for example, Japanese Patent Laid-Open No. <patcit id="pcit0001" dnum="JPHEI6123773B"><text>Hei 6-123773</text></patcit>). According to this radio identification device, it is possible to transmit various data on an article or the like to receiving means of the IC chip via the antenna and store the output in a memory, and also read the data in the memory and supply the data to the outside by radio via the antenna as required. Hence, the presence and position of the article or the like can be checked and traced quickly and easily.

0006An RFID system includes an RFID tag and a tag reader. When the tag receives an unmodulated wave f_o transmitted from the tag reader, the unmodulated wave f_o is rectified and converted into direct-current power, and the direct-current power can be used as operating power of the tag. The tag side performs an operation of terminating an antenna according to a bit image of transmission data, and thus uses absorption and reflection of the received radio wave to represent the data. Specifically, when data is 1, the tag terminates the antenna by an antenna impedance to absorb the radio wave from the tag reader. When data is 0, the tag reflects the radio wave from the tag reader by setting a terminal of the antenna in an open state. A signal of the same frequency as that of the signal transmitted from the tag reader is returned by the reflection of a backscatter system. A communication method of representing data by a pattern of absorption and reflection of the thus arrived radio wave is referred to as a "backscatter system." Thus, the tag can transmit information therewithin to the reader side without a power supply.

0007Conventionally, a radio communication system of the backscatter system is limited in communication range to a relatively short distance, and is thus often applied to identification and authentication of an article, a person and the like, as is typified by the RFID tag.

0008On the other hand, the RFID tag generally has no power supply, and is supplied with power from the radio wave from the reader. This power is supplied from a battery within the device, whereby radio data transmission with low power consumption by the backscatter system can be realized. That is, when a communication distance is limited, radio communication of the backscatter system has a characteristic of being able to establish a radio transmission line with a very low power consumption. Recently, with improvement of packaging technology, IC chips having a memory function have appeared, and also memories of the IC chips have been increasing in capacity. There is hence a desire to not only communicate relatively short data such as identifying and authenticating information but also adopt the communication of the backscatter system for general data transmission. For example, the communication of the backscatter system is useful in transmitting images from a digital camera or a portable telephone to a PC, a printer, a TV or the like.

0009The communication system based on the backscatter system performs data communication using absorption and reflection of the received radio wave on the basis of the operation of terminating the antenna as a fundamental operation. Generally, frequency of a carrier from the reader and center frequency of the reflected wave are the same, and the reader side performs transmission and reception at the same frequency.

0010In such a case, a receiving unit is affected by the transmission frequency that goes around into the receiving unit from a transmitting side, and needs to process the reflected wave having a weak power. That is, the receiving unit is easily affected by a DC offset and transmitter noise, thus making it difficult to increase a transmission distance. In addition, a modulation system in the backscatter system is generally an ASK modulation system or a PSK modulation system in most cases, thus making it difficult to increase speed.

0011<figref idref="f0005">FIG. 7</figref> shows an example of configuration of a radio communication system of a previously proposed backscatter system.

0012Reference numeral 500 denotes a radio transmission device on a mobile device side. Reference numeral 510 denotes a radio transmission and reception device on a reader side. Suppose that data transmission is performed by the backscatter system from the radio transmission device 500 to the radio transmission and reception device 510.

0013The radio transmission device 500 is connected to an application unit 503 such as a digital camera or the like. Similarly, the radio transmission and reception device 510 is connected to an application unit 519 such as a printer or the like.

0014The radio transmission and reception device 510 includes an antenna 511, a circulator 512 for separating a transmitting wave and a received wave from each other, a receiving unit 514, a local oscillator 513 shared for transmission and reception by the receiving unit 514 and a transmitting unit 517, and a baseband processing unit 518. Suppose in the example shown in the figure that the receiving unit 514 and the transmitting unit 517 both use a direct conversion system. Further, the receiving unit 514 includes a quadrature demodulation unit 515 and an AGC amplifier 516. An unmodulated carrier is transmitted to the radio transmission device 500 by turning on the transmitting unit 517 by the baseband processing unit 518 and thereby transmitting frequency f_o of the local oscillator 513 from the antenna 511 via the circulator 512.

0015The transmitted unmodulated carrier f_o reaches the radio transmission device 500. The radio transmission device 500 includes an antenna 501 and a backscatter modulator 502. The backscatter modulator 502 performs backscatter ASK, PSK, or QPSK modulation according to transmission data of the application unit 503. The modulation can be easily performed by on/off operation of a diode, a GaAs switch or the like. Thus, a modulated wave eventually reflected from the antenna 501 is generated with the center frequency f_o of the unmodulated carrier as a center.

0016In the radio transmission and reception device 510, the backscattered modulated wave having the center frequency f_o is received by the antenna 511, the circulator 512, and the receiving unit 514. The quadrature demodulation unit 515 is supplied with the frequency f_o of the local oscillator 513, performs direct conversion reception, and generates an I' signal and a Q' signal of a baseband signal.

0017The I' signal and the Q' signal of the baseband signal are amplified to a desired level by the AGC amplifier 516 in a succeeding stage. Thereby an I signal and a Q signal of the baseband signal are obtained. The I signal and the Q signal of the baseband signal are supplied to the baseband processing unit 518. The baseband processing unit 518 performs demodulation, and then supplies received data and a received clock to the application unit 519.

0018The unmodulated carrier f_o from the transmitting unit 517 is emitted from the antenna 511 via the circulator 512, and also goes around into the receiving unit 514 side. This component going around into the receiving unit 514 side can be reduced to a degree by the circulator 512. However, a value of the reduction is not infinite, and an isolation of about 20 dB is an actual value.

0019<figref idref="f0005">FIG. 7</figref> also shows a frequency spectrum on the reader side. Reference numeral 520 denotes a frequency spectrum at an input terminal of the quadrature demodulation unit 515. Reference numeral 522 denotes the modulated wave reflected by backscatter, for example a BPSK modulated wave. Reference numeral 521 denotes the unmodulated carrier. When the modulated signal 522 is small, the unmodulated carrier 521 has a larger value.

0020This unmodulated carrier f_o enters the quadrature demodulation unit 515 to be mixed with the local frequency f_o of the local oscillator 513. Consequently a high direct-current voltage is generated. This forms a DC offset, which produces a great adverse effect on operation of the quadrature demodulation unit 515. Thus, the very small modulated signal becomes distorted to be difficult to demodulate, thereby constituting a major cause of an impediment to increasing a transmission distance.

0021As one method for solving such a problem, there is a method of shifting reception frequency f_o by a predetermined center frequency f_s in one of a positive direction and a negative direction, and returning a reflected wave on a tag side. In this case, frequency of the reflected wave received on a tag reader side is not the same as transmission frequency. Therefore effects of a DC offset and transmitter noise are avoided, so that the reflected wave can be received with high sensitivity. Thus a transmission distance can be increased.

0022For example, a method of first performing QPSK modulation using a subcarrier and then performing ASK or PSK modulation by the backscatter system as secondary modulation is proposed (see for example, Japanese Patent Laid-Open No. <patcit id="pcit0002" dnum="JPHEI10209914B"><text>Hei 10-209914</text></patcit>).

0023<figref idref="f0003">FIG. 5</figref> shows an example of configuration of an RFID system in which a tag side shifts reception frequency f_o by a predetermined center frequency f_s in one of a positive direction and a negative direction, and returns a reflected wave.

0024Reference numeral 100 denotes a radio transmission device on a mobile device side. Reference numeral 110 denotes a radio transmission and reception device on a reader side. Suppose that data transmission is performed by the backscatter system from the radio transmission device 100 to the radio transmission and reception device 110. The radio transmission device 100 is connected to an application unit 105 such as a digital camera or the like. Similarly, the radio transmission and reception device 110 is connected to an application unit 119 such as a printer or the like.

0025The radio transmission and reception device 110 includes an antenna 111, a circulator 112 for separating a transmitting wave and a received wave from each other, a receiving unit 114, a local oscillator 115 for the receiving unit 114, a transmitting unit 116, a local oscillator 117 for the transmitting unit 116, and a baseband processing unit 118. Suppose in this case that the receiving unit 114 and the transmitting unit 116 both use a direct conversion system.

0026An unmodulated carrier is transmitted to the radio transmission device 100 by turning on the transmitting unit 116 by the baseband processing unit 118 and transmitting frequency f_o of the local oscillator 117 from the antenna 111 via a band-pass filter 113 and the circulator 112. The transmitted unmodulated carrier f_o reaches the radio transmission device 100. The band-pass filter 113 is provided to reduce effects of transmitter noise on the receiving unit 114.

0027The radio transmission device 100 includes an antenna 101, a backscatter modulator 102, a subcarrier QPSK modulator 103, and a subcarrier oscillator 104.

0028The subcarrier QPSK modulator 103 performs QPSK modulation at a subcarrier frequency f_s. Data to be subjected to the QPSK modulation is received from the application unit 105 as transmission data (TXDATA) and a transmission clock (TXCLK).

0029Generally QPSK modulation requires a 90° phase shifter. However, when QPSK modulation is performed by a digital circuit, the 90° phase shifter can be easily created from a clock of four times f_s. Also, an analog delay line may be used.

0030A generated QPSK modulated wave having a center frequency f_s is subjected to ASK modulation by the backscatter modulator 102. The backscatter modulation can be easily performed by using a diode, a GaAs switch or the like (known). Thus, the QPSK modulated wave eventually reflected from the antenna 101 is generated in both sidebands of the frequency f_o of the unmodulated carrier, that is, two bands of center frequencies f_o + f_s and f_o - f_s.

0031In the example shown in <figref idref="f0003">FIG. 5</figref>, f_o + f_s of the modulated wave divided into both sidebands is used. The modulated wave of f_o - f_s can be removed by using a band-pass filter 106 inserted between the antenna 101 and the backscatter modulator 102, for example. However, a loss from insertion of the band-pass filter 106 occurs twice, causing a decrease in reflection efficiency. In addition, the insertion of the band-pass filter 106 increases device cost.

0032In the radio transmission and reception device 110, the backscattered modulated wave of f_o + f_s is received by the antenna 111, the circulator 112, and the receiving unit 114.

0033The receiving unit 114 performs direct conversion reception at the frequency f_o + f_s of the local oscillator 115. The QPSK modulated wave is converted into baseband signals I and Q. The baseband signals I and Q are sent to the baseband processing unit 118.

0034The baseband processing unit 118 performs QPSK demodulation processing (carrier synchronization and symbol synchronization), thereby generates received data RXDATA and a received clock RXCLK, and then supplies the received data RXDATA and the received clock RXCLK to the application unit 119.

0035However, the above-described method of thus shifting the unmodulated carrier of the frequency f_o from the tag reader side by f_s on the tag side and returning the reflected wave has the following problems. <ol id="ol0001" compact="compact"><li>(1) The reflected modulated wave appears in a state of being divided into both sidebands shifted by the subcarrier frequency to a plus side and a minus side from a center of the unmodulated carrier from the reader. Since the actually necessary modulated wave is on only one side, the other side needs to be cut off by the filter. However, when the filter is used in the backscatter system, a loss from the insertion of the filter occurs in both ways, causing a decrease in reflection efficiency. In addition, an increase in cost of the filter is a problem.</li><li>(2) Energy of the reflected wave is divided into both sidebands. Thus, when only one side is used, the energy allocated to the unused other side constitutes a power loss, thus causing a decrease in power of the reflected wave. For example, it is considered that the power of the reflected wave is decreased by at least 3 dB. The backscatter system using ASK further increases the decrease.</li></ol>

0036<figref idref="f0004">FIG. 6</figref> shows a spectrum of the reflected wave in the RFID system shown in <figref idref="f0003">FIG. 5</figref>. Suppose that the backscatter modulation system is ASK.

0037Reference numeral 200 denotes a returned component of the unmodulated carrier of the frequency f_o transmitted from the radio transmission and reception device 110. Reference numeral 201 denotes the QPSK modulated wave of the center frequency f_o + f_s. Reference numeral 202 denotes the QPSK modulated wave of the center frequency f_o - f_s.

0038As shown in the figure, the unmodulated carrier transmitted from the radio transmission and reception device 110 is divided into the components 200, 201, and 202 and then reflected. Therefore the modulated signal on one side has a low level. That is, the level of the originally very weak reflected wave is further lowered, which is one cause of decrease in communication distance.

0039It is desirable to provide a radio communication system, a radio communication device, and a radio communication method that are excellent in suitably performing data communication by a backscatter system using transmission of an unmodulated carrier from a reading device side, and absorption and reflection of the received radio wave on the basis of an operation of terminating an antenna on a transmitting device side.

0040It is also desirable to provide a radio communication system, a radio communication device, and a radio communication method that are excellent in eliminating or at least reducing effects of transmitter noise on the reading device side to improve reception sensitivity and increase a communication distance.

0041It is also desirable to provide a radio communication system, a radio communication device, and a radio communication method that are excellent in eliminating or at least reducing effects of a DC offset and transmitter noise on the tag reader side to improve the reception sensitivity and increase the communication distance by shifting an unmodulated carrier of frequency f_o from the tag reader side by f_s on the tag side and returning a reflected wave.

0042It is also desirable to provide a radio communication system, a radio communication device, and a radio communication method that are excellent in making it possible to improve the reception sensitivity and increase the communication distance by improving reflection efficiency and preventing or at least reducing a power loss of the reflected wave when shifting the unmodulated carrier of frequency f_o from the tag reader side by fs on the tag side and returning the reflected wave.

0043<patcit id="pcit0003" dnum="EP0779520A2"><text>EP 0 779 520 A2</text></patcit> describes a Frequency Division Multiple (FDMA) duplex radio communication system.

0044<patcit id="pcit0004" dnum="US20020142747A1"><text>US 2002/0142747 A1</text></patcit> describes a frequency conversion circuit. <patcit id="pcit0005" dnum="US5861781A"><text>US 5,861,781</text></patcit> describes a single sideband double quadrature modulator.

0045The present invention has been made in view of the above problems. Various respective aspects of the present invention are set out in the appended claims.

0046According to an embodiment of the present invention, there is provided a radio communication system for performing data communication by a backscatter system, using absorption and reflection of a received radio wave according to claim 6.

0047The system herein refers to a logical set of a plurality of apparatuses (or functional modules for realizing specific functions) regardless of whether each apparatus or functional module is present within the same casing.

0048The radio communication system according to the embodiment of the present invention is applicable to an RFID system in which a tag can perform data communication without a power supply by the backscatter system. The data transmitting device corresponds to a tag, and the data reading device corresponds to a tag reader.

0049In an ordinary RFID system, frequency of a carrier from a reader and center frequency of a reflected wave are the same, and since the reader side performs transmission and reception at the same frequency, a receiving unit of the tag reader needs to process the reflected wave having a weak power while being affected by a DC offset and transmitter noise.

0050On the other hand, the radio communication system according to the embodiment of the present invention eliminates or at least reduces effects of a DC offset and transmitter noise on the tag reader side by shifting the unmodulated carrier of the frequency f_o from the tag reader side by f_s on the tag side and then returning the reflected wave.

0051It is thereby possible to improve reception sensitivity and increase transmission distance.

0052In addition, when the unmodulated carrier is shifted by fs on the tag side and the reflected wave is returned, the reflected modulated wave appears in a state of being divided into both sidebands shifted by the subcarrier frequency to a plus side and a minus side from a center of the unmodulated carrier. Thus there are problems of a power loss due to division of energy of the reflected wave into both sidebands and an increase in cost due to cutting of one side by a filter.

0053On the other hand, according to the present invention, the data transmitting device subjects transmission data to primary modulation with a center frequency fs, and performs secondary modulation by multiplying a signal resulting from the primary modulation by the unmodulated carrier. At this time, the data transmitting device generates a modulated signal of a reflected wave in which one of frequencies fo + fs and fo - fs is suppressed, and then transmits the modulated signal of the reflected wave. It is therefore possible to improve reflection efficiency and prevent or at least reduce a power loss of the reflected wave, and thus improve reception sensitivity and increase transmission distance. In addition, since a reflected wave on one side of two divided sidebands does not need to be removed by a filter, an increase in cost can be prevented.

0054The data transmitting device subjects transmission data to primary modulation by one of a BPSK system and a QPSK system using a center frequency fs. The data transmitting device thereby generates primary modulated transmission data I. Also, the data transmitting device generates primary modulated transmission data Q by giving a phase difference of 90 degrees to the primary modulated transmission data I.

0055Further, the data transmitting device generates a modulated signal of a reflected wave in which one of frequencies fo + fs and fo - fs is suppressed by multiplying the primary modulated transmission data by an I and a Q of the unmodulated carrier for quadrature modulation by the backscatter system.

0056The reflected modulated wave of one of the frequencies fo + fs and fo - fs can be selectively generated by changing a combination in which the inputs I and Q of the unmodulated carrier for quadrature modulation and the primary modulated transmission data I and Q resulting from the primary modulation are multiplied together.

0057For example, by multiplying together the input I of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data I and multiplying together the input Q of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data Q, a reflected wave of a frequency fo - fs can be returned to the data reading device. Alternatively, by multiplying together the input I of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data Q and multiplying together the input Q of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data I, a reflected wave of a frequency fo + fs can be returned to the data reading device.

0058The data transmitting device may change the center frequency fs for the primary modulation.

0059When a clock for the transmission data is obtained by dividing the frequency f_s for the primary modulation, synchronization is easily achieved because of clock unification.

0060According to the present invention, it is possible to provide a radio communication system, a radio communication device, and a radio communication method that are excellent in making it possible to improve reception sensitivity and increase communication distance by separating transmission frequency on a reading device side from reception frequency returned as a reflected wave from a transmitting device side.

0061In addition, it is possible to provide a radio communication system, a radio communication device, and a radio communication method that are excellent in eliminating or at least reducing effects of a DC offset and transmitter noise on the tag reader side to improve the reception sensitivity and increase the communication distance by shifting an unmodulated carrier of frequency f_o from the tag reader side by f_s on the tag side and returning a reflected wave.

0062In addition, it is possible to provide a radio communication system, a radio communication device, and a radio communication method that are excellent in making it possible to improve the reception sensitivity and increase the communication distance by improving reflection efficiency and preventing or at least reducing a power loss of the reflected wave when shifting the unmodulated carrier of frequency f_o from the tag reader side by f_s on the tag side and returning the reflected wave. Further, since the tag reader side does not need to remove a reflected wave on one side of two divided sidebands by a filter, an increase in cost can be prevented.

0063Other and further objects, features, and advantages of the present invention will become apparent from more detailed description on the basis of accompanying drawings and embodiments of the present invention to be described later.

0064The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a diagram schematically showing a configuration of a radio communication device according to one embodiment of the present invention;</li><li><figref idref="f0001">FIG. 2</figref> is a diagram showing a modification of the radio communication device as an RFID tag shown in <figref idref="f0001">FIG. 1</figref>;</li><li><figref idref="f0002">FIG. 3</figref> is a diagram showing a concrete configuration of a radio communication device according to an embodiment of the present invention;</li><li><figref idref="f0002">FIG. 4</figref> is a diagram showing a spectrum of a reflected wave in the radio communication device shown in <figref idref="f0002">FIG. 3</figref>;</li><li><figref idref="f0003">FIG. 5</figref> is a diagram showing an example of configuration of an RFID system in which a tag side shifts reception frequency f_o by a predetermined center frequency f_s in one of a positive direction and a negative direction, and returns a reflected wave;</li><li><figref idref="f0004">FIG. 6</figref> is a diagram showing a spectrum of a reflected wave in the RFID system shown in <figref idref="f0003">FIG. 5</figref>; and</li><li><figref idref="f0005">FIG. 7</figref> is a diagram showing an example of configuration of a radio communication system of a conventional backscatter system.</li></ul>

0065Preferred embodiments of the present invention will hereinafter be described in detail with reference to the drawings.

0066Embodiments of the present invention relate to an RFID system in which a tag can perform data communication without a power supply by a backscatter system.

0067In an ordinary RFID system, frequency of a carrier from a reader and center frequency of a reflected wave are the same, and since the reader side performs transmission and reception at the same frequency, a receiving unit of the tag reader is affected by a DC offset and transmitter noise.

0068On the other hand, a radio communication system according to an embodiment of the present invention eliminates or at least reduces effects of a DC offset and transmitter noise on a tag reader side by shifting an unmodulated carrier of a frequency f_o from the tag reader side by f_s on a tag side and then returning a reflected wave.

0069<figref idref="f0001">FIG. 1</figref> schematically shows a configuration of a radio communication device according to one embodiment of the present invention. The radio communication device shown in the figure corresponds to a tag in an RFID system. The radio communication device receives an unmodulated carrier of a frequency f_o transmitted from a tag reader, and shifts the carrier by a frequency f_s by a backscatter system. Also, the radio communication device generates a modulated signal of a reflected wave in which one of both sidebands f_o + f_s and f_o - f_s is suppressed, and then transmits the modulated signal.

0070As shown in <figref idref="f0001">FIG. 1</figref>, the radio communication device includes a frequency generating unit 11, a primary modulating unit 12, and a secondary modulating unit 13.

0071The frequency generating unit 11 generates a center frequency f_s for primary modulation. The frequency generating unit 11 may change the center frequency f_s for primary modulation.

0072The primary modulating unit 12 subjects transmission data to primary modulation by a BPSK or a QPSK system using the center frequency f_s. The primary modulating unit 12 thereby generates an I-axis signal of the transmission data. Further, the primary modulating unit 12 generates a Q-axis signal of the transmission data by giving a phase difference of 90 degrees to the I-axis signal of the transmission data.

0073The secondary modulating unit 13 subjects the primary modulated transmission data to secondary modulation. The secondary modulating unit in the present embodiment is formed by a quadrature modulator for quadrature modulation of the unmodulated carrier of the frequency f_o transmitted from the tag reader by the backscatter system. That is, the secondary modulating unit 13 multiplies an I-axis signal and a Q-axis signal of the unmodulated carrier input thereto by the I-axis signal and the Q-axis signal of the above-described primary modulated transmission data, respectively.

0074Generally, when two different frequencies f_o and f_s are subjected to a frequency operation, frequency components f_o + f_s and f_o - f_s in both sidebands of f_o are generated, and one can be a wave interfering with the other. On the other hand, as described above, by multiplying the I-axis signals and the Q-axis signals of the frequency signals and thus performing quadrature modulation, it is possible to generate a modulated signal of a reflected wave in which one of the frequencies f_o + f_s and f_o - f_s is suppressed. That is, it is possible to generate only a signal of a desired frequency component, and thus eliminate or at least reduce the interfering wave.

0075As a result of this, it is possible to improve reflection efficiency and prevent or at least reduce a power loss of the reflected wave, improve reception sensitivity, and increase a transmission distance. In addition, since the tag reader side does not need to remove a reflected wave on one side of the two divided sidebands by a filter, an increase in cost can be prevented.

0076<figref idref="f0001">FIG. 2</figref> shows a modification of the radio communication device as an RFID tag shown in <figref idref="f0001">FIG. 1</figref>. The radio communication device shown in the figure further includes a switching unit 14.

0077The switching unit 14 changes a combination in which an I-axis signal and a Q-axis signal of an unmodulated carrier and an I-axis signal and a Q-axis signal of primary modulated transmission data as described above are multiplied together.

0078By changing the combination in which the inputs I and Q of the unmodulated carrier and the primary modulated transmission data I and Q are multiplied together, it is possible to selectively generate a modulated signal of a reflected wave in which one of frequencies f_o + f_s and f_o - f_s is suppressed.

0079Specifically, by multiplying together the input I of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data I and multiplying together the input Q of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data Q, a reflected wave signal of a frequency f_o - f_s can be returned to the tag reader. Alternatively, by multiplying together the input I of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data Q and multiplying together the input Q of the unmodulated carrier to be quadrature-modulated and the primary modulated transmission data I, a reflected wave signal of a frequency f_o + f_s can be returned to the tag reader.

0080<figref idref="f0002">FIG. 3</figref> shows a concrete configuration of a radio communication device according to an embodiment of the present invention. The radio communication device shown in the figure operates as a tag in an RFID system, and is capable of QPSK radio communication by the backscatter system with reflection efficiency improved and an unwanted spectrum suppressed. This radio communication device is characterized in that an image rejection type backscatter modulator is used to extract a frequency component on only one side of a modulated wave.

0081Reference numeral 300 denotes an image rejection type backscatter modulator. This backscatter modulator 300 includes an antenna 301, a splitter/synthesizer 302, high-frequency switches 303 and 305, and a λ/8 phase shifter 304. Generally, the high-frequency switches 303 and 305 are formed by a diode, a GaAs switch or the like, and the phase shifter 304 is formed by a strip line or the like. The parts indicated by reference numerals 301 to 305 form a quadrature modulation unit.

0082A signal received by the antenna 301 is branched into two signals by the splitter/synthesizer 302. One high-frequency switch 303 is turned on to terminate the antenna 301 by an antenna impedance and thus absorb the radio wave from a tag reader, and the switch is turned off to reflect the radio wave from the tag reader. That is, the high-frequency switch 303 creates total reflection in an open state and a short state according to data, and thus operates as a BPSK modulator with a phase difference of 180 degrees.

0083The other high-frequency switch 305 similarly absorbs or reflects the radio wave from the tag reader by being turned on/off. However, since the lambda λ/8 phase shifter 304 is inserted at an intermediate point, the high-frequency switch 305 operates as a BPSK modulator advanced in phase in both ways by a total of lambda λ/4, that is, 90°.

0084Thus, the two high-frequency switches 303 and 305 and the phase shifter 304 can form a backscatter type quadrature modulator. Suppose in this case that a signal controlled by the high-frequency switch 303 is an I-axis signal and that a signal controlled by the high-frequency switch 305 is a Q-axis signal. Primary modulated transmission data is given to the I and the Q to thereby form a backscatter type QPSK modulator. However, a shift of frequency of the reflected wave cannot be realized. Therefore a function of shifting the frequency by f_s is required.

0085Reference numerals 306 to 311 in <figref idref="f0002">FIG. 3</figref> generate an I-axis signal and a Q-axis signal of primary modulated transmission data for shifting the frequency by f_s.

0086First, a S/P converter unit 309 subjects transmission data (TX DATA) and a transmission clock (TX CLK) to a serial-to-parallel conversion including Gray coding. Suppose that the output is P1 and P2. Specifically, transmission data 00 is converted into (P1 = 0, P2 = 0); transmission data 01 is converted into (P1 = 0, P2 = 1); transmission data 10 is converted into (P1 = 1, P2 = 1); and transmission data 11 is converted into (P1 = 1, P2 = 1).

0087A shift clock oscillator 310 is required to shift the frequency by f_s. This oscillator oscillates at f_s or four times f_s. An output of the shift clock oscillator 310 is divided into two signals of 0° and 90° in a 90° phase shifter 308.

0088The 90° phase shifter 308 can be easily created from a clock of four times f_s when the 90° phase shifter 308 is formed by a digital circuit. Also, an analog delay line may be used without use of a clock of four times f_s. The two signals different from each other in phase by 90° are input to a QPSK modulator 306.

0089The QPSK modulator 306 generates for example four signals of f_s different from each other in phase by 90° from the two signals from the phase shifter 308. The QPSK modulator 306 generates a digital QPSK signal by selecting the four signals according to the transmission data P1 and P2. Let this digital QPSK signal be an I-axis signal 312. Further, let a Q-axis signal 313 be a signal obtained by delaying the phase of the I-axis signal 312 by 90° by a 90° phase shifter 307.

0090The modulation system of the QPSK modulator 306 is controlled by a modulation switching signal (MOD CONT) 314 so that switching can be performed between modulation systems QPSK and BPSK. For example, MOD CONT = 0 at the time of BPSK modulation, and MOD CONT = 1 at the time of QPSK modulation.

0091The I-axis signal 312 and the Q-axis signal 313 described above are subjected to backscatter modulation by the above-described quadrature modulation unit. Thereby a spectrum in which one side of QPSK spectra occurring on both sides of a carrier frequency is attenuated can be obtained.

0092Letting A (= ±1) and B (= ±1) be the P1 and P2 data for calculation, X = 2πf_s, and Y = 2πf_o, the QPSK modulated I-axis signal 312 and the Q-axis signal 313 delayed in phase by 90° with respect to the I-axis signal 312 can be expressed as follows:<maths id="math0001" num="(1)"><math display="block"><mi mathvariant="normal">I</mi><mo mathvariant="normal">=</mo><mi>AcosX</mi><mo mathvariant="normal">+</mo><mi>BsinX</mi><mo mathvariant="normal">,</mo><mi mathvariant="normal">Q</mi><mo mathvariant="normal">=</mo><mi>AsinX</mi><mo mathvariant="normal">-</mo><mi>BcosX</mi></math><img file="EP1596320B1_D0001.tif" /></maths>

0093The above signals are subjected to quadrature modulation by the quadrature modulation unit using the unmodulated carrier f_o from the tag reader. A modulated wave signal reflected by the antenna 301 is as follows:<maths id="math0002" num="(2)"><math display="block"><mtable columnalign="left"><mtr><mtd><mfenced><mi>A cos X</mi><mo mathvariant="normal">+</mo><mi>B sin X</mi></mfenced><mo mathvariant="normal">×</mo><mi>cos Y</mi><mo mathvariant="normal">+</mo><mfenced><mi>A sin X</mi><mo mathvariant="normal">+</mo><mi>B cos X</mi></mfenced><mo mathvariant="normal">×</mo><mi>sin Y</mi></mtd></mtr><mtr><mtd><mo mathvariant="normal">=</mo><mi>A cos X cos Y</mi><mo mathvariant="normal">+</mo><mi>B sin X cos Y</mi><mo mathvariant="normal">+</mo><mi>A sin X sin Y</mi><mo mathvariant="normal">-</mo><mi>B cos X sin Y</mi></mtd></mtr><mtr><mtd><mfrac><mn mathvariant="normal">1</mn><mn mathvariant="normal">2</mn></mfrac><mrow><mo mathvariant="normal">{</mo><mtable><mtr><mtd><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced></mtd></mtr><mtr><mtd><mo mathvariant="normal">-</mo><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced></mtd></mtr></mtable></mrow></mtd></mtr><mtr><mtd><mo mathvariant="normal">=</mo><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced></mtd></mtr><mtr><mtd><mspace width="1em" /></mtd></mtr></mtable></math><img file="EP1596320B1_D0002.tif" /></maths>

0094The above equation indicates that the reflected modulated wave appears on only a f_o - f_s side of both sidebands of f_o. In addition, by connecting the I and the Q in an opposite manner, it is possible to generate the modulated wave on only a f_o + f_s side.

0095On the other hand, when modulation is performed on only the I side, the modulated wave at a terminal of the antenna 301 is as follows:<maths id="math0003" num="(3)"><math display="block"><mtable columnalign="left"><mtr><mtd><mfrac><mn mathvariant="normal">1</mn><mn mathvariant="normal">2</mn></mfrac><mfenced open="{" close="}"><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced></mfenced></mtd></mtr><mtr><mtd><mo mathvariant="normal">=</mo><mfrac><mn mathvariant="normal">1</mn><mn mathvariant="normal">2</mn></mfrac><mfenced open="{" close="}"><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Y</mi></mfenced></mfenced><mo mathvariant="normal">+</mo><mfrac><mn mathvariant="normal">1</mn><mn mathvariant="normal">2</mn></mfrac><mfenced open="{" close="}"><mi>A cos</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced><mo mathvariant="normal">+</mo><mi>B sin</mi><mo>⁢</mo><mfenced><mi mathvariant="normal">X</mi><mo mathvariant="normal">-</mo><mi mathvariant="normal">Y</mi></mfenced></mfenced></mtd></mtr></mtable></math><img file="EP1596320B1_D0003.tif" /></maths>

0096It is indicated that in this case, the modulated wave occurs in both sidebands of f_o - f_s and f_o + f_s. It is also indicated that signal level is halved.

0097The above example of calculation relates to QPSK modulation. However, when in <figref idref="f0002">FIG. 3</figref>, P2 is set to zero at all times and P1 is set as transmission data (TX DATA), and BPSK modulation is made to be performed by setting MOD CONT = 0, the above equation (2) results in Acos(X - Y), and thus a BPSK signal on the f_o - f_s side is similarly obtained.

0098Reference numeral 311 in <figref idref="f0002">FIG. 3</figref> denotes a frequency divider. The frequency divider 311 divides frequency of the shift clock oscillator 310. By using 1, 2, 4 or the like as a frequency dividing ratio n and using an output signal of the frequency divider for the transmission clock, it is possible to generate an I-axis signal and a Q-axis signal in synchronism at an EXOR output of the 90° phase shifter 307 and the QPSK modulator 306. However, a hazard occurs because a data change point and a clock change point become the same. A measure against this is not directly related to the subject matter of the present invention, and therefore will not be described herein.

0099When the clock for the transmission data is obtained by dividing the frequency f_s for primary modulation, synchronization can be easily achieved because of clock unification.

0100<figref idref="f0002">FIG. 4</figref> shows a spectrum of the thus generated reflected wave.

0101Reference numeral 401 denotes the QPSK modulated wave at a center frequency f_o - f_s. Reference numeral 400 denotes a leakage component of the QPSK modulated wave at a center frequency f_o, and reference numeral 402 denotes a leakage component of the QPSK modulated wave at a center frequency f_o + f_s. Both the components are caused by an imbalance of orthogonality at each of f_o and f_s, and are not caused ideally.

0102The transmitting and receiving device 110 shown in <figref idref="f0003">FIG. 5</figref> can be used as it is as the tag reader for receiving and demodulating the above-described f_o + f_s modulated wave, and therefore description thereof will not be made in order to avoid repetition of the description.

0103By the above method, it is possible to realize a radio transmission device for PSK or QPSK modulation by the backscatter system which device improves reflection efficiency and suppresses an unwanted spectrum.

0104The present invention has been explained above in detail with reference to specific embodiments thereof. It is obvious, however, that modifications and substitutions in the embodiments may be made by those skilled in the art without departing from the scope of the present invention as defined in the appended claims.