Microwave/millimeter wave band communication system is a LOS communication system in which a transmitting/receiving station is physically fixed, and demand for it as mobile communication infrastructure is dramatically increasing in recent years. As a result, an increase in mobile communication traffic calls for larger transmission capacity. Means for increasing transmission capacity to address such demand include the use of polarization multiplexing, increasing modulation levels, or bandwidth expansion, however, utilizing all of these means will still not deal with the anticipated increase in traffic sufficiently, and in addition to these, the development of a technology that is able to further increase the communication capacity is expected.
As such a technology, the MIMO transmission system using a plurality of transmit/receive antennas has attracted attention. This technology has hitherto been developed mainly for applications to non-line-of-sight mobile communication systems such as mobile phones or wireless LANs (Local Area Networks), however, more recently, applications to line-of-sight fixed wireless communication systems using micro- and millimeter waves have been considered and it has gained attention as a technology that addresses the demand for larger capacity caused by the increase in traffic.
Non-Patent Literature (NPL) 1 discloses the principle of MIMO (referred to as “LOS-MIMO” hereinafter) transmission in line-of-sight fixed wireless communication. Non-Patent Literatures 1 and 2 show that the difference in transmission delay is adjusted by appropriately positioning a plurality of transmit and receive antennas, and the amount of the phase rotation of the carrier caused by the adjusted difference in transmission delay contributes to the improvement of the signal-to-noise ratio, increasing the communication capacity. For instance, in a case where two transmit antennas and two receive antennas are provided, the communication capacity is expected to increase twofold, compared with ordinary single-input single-output (SISO) transmission having one transmit antenna and one receive antenna. Such line-of-sight MIMO communication is distinguished from the non-line-of-sight MIMO (referred to as “NLOS-MIMO” hereinafter) communication in mobile communication or wireless LAN actively and effectively utilizing a multiplex transmission wave in which many signals overlap and vary over time due to the reflection, diffraction, and disturbance of radio waves.
Non-Patent Literature 3 describes a LOS-MIMO demodulation method in which signal separation means related to 2×2 LOS-MIMO having two transmit antennas and two receive antennas is integrated with a time domain equalizer that compensates for inter-symbol interference caused by fading.
Patent Literatures 1 and 2 describe means for realizing LOS-MIMO communication in line-of-sight fixed wireless communication systems utilizing a microwave or millimeter wave band. In the configurations in these documents, means for separating and extracting a desired signal from received signals multiplexed by LOS-MIMO transmission and ordinary demodulation means in SISO transmission conventionally using a single transmit antenna and a single receive antenna are deployed in tandem. The signal separating/extracting means related to LOS-MIMO is realized by estimating parameters representing a LOS-MIMO transmission path model by transmitting a pilot signal sequence comprising several symbols that form orthogonal patterns and by performing reverse conversion of multiplexing that has occurred in the MIMO transmission path with signal processing.
In addition, documents that constitute background art include Patent Literatures 3 and 4. How they relate to the present invention will be described later.
[PTL 1]
    Japanese Patent No. 5317021B[PTL 2]    Japanese Patent No. 5322274B[PTL 3]    International Publication No. WO2013/161801A1[PTL 4]    Japanese Patent Kokai Publication No. JP2010-119070A[NPL 1]    P. F. Driessen and G. J. Foschini, “On the Capacity Formula for Multiple Input—Multiple Output Wireless Channels: A Geometric Interpretation,” IEEE Transactions on Communications, Vol. 47, No. 2, pp. 173-176, February 1999.[NPL 2]    I. Sarris and A. R. Niz, “Maximum MIMO Capacity in Line-of-Sight,” IEEE International Conference on Information, Communications and Signal Processing (ICICS), Proceedings, pp. 1236-1240, December 2005.[NPL 3]    T. Ingason, H. Liu, M. Coldrey, A. Wolfgang, and J. Hansryd, “Impact of Frequency Selective Channels on a Line-of-Sight MIMO Microwave Radio Link,” IEEE Vehicular Technology Conference (VTC), Proceedings, May 2010.