CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2020-211422, filed on Dec. 21, 2020, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a wireless communication apparatus and a wireless communication method.

BACKGROUND

With the 5th Generation Mobile Communication System (5G), use of millimeter waves with a wavelength of 1 to 10 mm, for example, for data communication is discussed. The use of millimeter waves makes it possible to perform data communication with a wider frequency bandwidth, and it is expected to improve the data rate. On the other hand, millimeter waves have a characteristic of being greatly attenuated when propagating through a space. Therefore, in communication using millimeter waves, beamforming may be performed by using an array antenna including a large number of antenna elements, for example.

The “beamforming” means to form a beam with a high gain in a predetermined direction by providing a phase shifter to each of a plurality of antenna elements to set a phase difference in signals transmitted from the respective antenna elements. Relatively expensive variable phase shifters are provided to the respective antenna elements for beamforming using the phase shifters, so that the cost for the array antenna increases. Thus, as a radar and the like using an array antenna, for example, there is a type that performs frequency scanning mode beamforming that forms a beam by giving a phase difference in the signals of the respective antenna elements by a frequency shift. With the frequency scanning mode beamforming, it is possible to form a beam in a predetermined direction by providing mixers for up-conversion or down-conversion to the respective antenna elements and adjusting the frequencies of oscillation signals supplied to the mixers from an oscillator such as a local voltage controlled oscillator (VCO), for example.

With the frequency scanning mode beamforming, however, a frequency shift according to the beam direction is given to the signals transmitted and received at the respective antenna elements so that the frequencies of the transmitted and received signals vary depending on the beam directions. Therefore, with a wireless communication apparatus in which the frequency of the signals used for communication is defined, it is difficult to switch the beam to a predetermined direction only by providing the mixers to the respective antenna elements. Therefore, considered is a wireless communication apparatus in which the mixers are provided to the respective antenna elements and, at the same time, an intermediate frequency (IF) mixer is provided to a circuit of a previous stage where the antenna elements are branched. With such a configuration, the intermediate frequency mixer can apply, to the signals, a frequency shift opposite from the frequency shift applied by the mixers of the respective antenna elements. Therefore, the frequencies of the signals to be transmitted and received can be made constant regardless of the beam directions.

Patent document 1: Japanese Laid-open Patent Publication No. 2003-060424

However, with the wireless communication apparatus that performs beamforming by the frequency scanning mode as described above, the circuit scale increases. Specifically, since image components having image frequencies different from a desired frequency are generated in the output from the intermediate frequency mixer and the mixers of the respective antenna elements so that a band pass filter (BPF) for removing the image components is provided, thereby increasing the circuit scale.

For example, a case of upconverting a signal with an intermediate frequency fIFby a mixer of an antenna element will be considered. It is assumed that an oscillation signal with a local frequency fLOis supplied to the mixer of the antenna element from a local VCO, for example. At this time, when a signal IF with the intermediate frequency fIFis input to the mixer and upconverted, a signal RF2with a radio frequency fRF2(=fLO+fIF) as a desired frequency and an image component RF1with a radio frequency fRF1(=fLO−fIF) with the local frequency fLOof an oscillation signal LO being the center frequency are output as illustrated inFIG. 5, for example.

In such a case, a BPF that removes the image component RF1while allowing the signal RF2with the radio frequency fRF2to transmit therethrough is provided in a latter stage of the mixer so that the image component RF1is not emitted from the antenna element. That is, the BPF for removing the image component is provided to each of the antenna elements.

Furthermore, in a case where the intermediate frequency mixer is provided for canceling the frequency shift in the mixers of the respective antenna elements, the image component also appears in the output of the intermediate frequency mixer. Therefore, the BPF for removing the image component from the output of the intermediate frequency mixer is also provided in the previous state where the antenna elements are branched. Since the frequency shift at the mixer of each of the antenna elements changes depending on the beam direction, the frequency of the signal for canceling the frequency shift may also change. As a result, a BPF with a variable transmission passband is used as the BPF for removing the image component from the output of the intermediate frequency mixer. Therefore, an increase in the circuit scale due to the added BPFs is more prominent.

SUMMARY

According to an aspect of an embodiment, a wireless communication apparatus includes: a plurality of antenna elements; a processor that outputs an in-phase signal and a quadrature signal; a splitter that distributes the in-phase signal output from the processor to the antenna elements, and distributes the quadrature signal output from the processor to the antenna elements; and a plurality of quadrature modulators that are provided to the respective antenna elements, and that generate transmission signals of a radio frequency to be transmitted from the respective antenna elements by performing quadrature modulation on the in-phase signals and the quadrature signals distributed by the splitter. The processor is configured to execute a process including applying a first frequency shift according to a transmission beam direction to a first oscillation signal that is supplied to the quadrature modulators and used for frequency conversion into the radio frequency, digitally modulating transmission data to generate the in-phase signal and the quadrature signal, and performing complex multiplication to the generated in-phase signal and the generated quadrature signal with a second oscillation signal to which a second frequency shift opposite from the first frequency shift is applied.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

