In mobile communication, various frequency efficiency enhancing technologies have been used in order to support more users and achieve a higher transmission rate. One among them is a multi-input multi-output technology that increases a channel capacity by using multiple transmitters and receivers. It is known that the multi-input multi-output technology shows an excellent performance in a rich scattering environment, but the performance of the multi-input multi-output technology deteriorates under a line-of-sight environment. However, a wider bandwidth has been required according to a demand for high-speed communication, and as a result, there is a trend that a transmitting frequency increases up to a millimeter wave band. Due to a characteristic of the millimeter wave, since the millimeter wave is primarily used in a line-of-sight environment having strong linearity, it is not easy to apply the multi-input multi-output technology thereto. However, by recent researches, when transmitting and receiving antennas are appropriately separated from each other, it is demonstrated that orthogonality can be maintained among channels and various performance verification using the demonstration is made.
FIG. 1 is a diagram for describing a general line-of-sight 2×2 multi-input multi-output system.
As illustrated in FIG. 1, when the 2×2 system is assumed, a distance Da/Db between one-side antenna and one-side antenna is proportional to a link distance R and a wavelength λ. An example of the distance Da/Db between one-side antenna and one-side antenna in which a signal phase difference between the antennas becomes 90° according to the link distance Rat 18.7 GHz is shown in [Table 1]. For example, in the link distance R of 1 Km, as the distance Da/Db between one-side antenna and one-side antenna, 2.828 m is required.
TABLE 1Distance between one-side antenna and one-side antenna requiredaccording to link distanceDistance90° 5 m20cm31.25 m  50cm125 m1m500 m2m1000 m 2.828m
In general, it can be seen that an 18.7-GHz fixed wireless communication system may have a lot of problems in actual installation on reality in which a link distance of several kms is required. In addition, when a radio link is configured by using the same product, there is a problem in that a distance between the antennas according to the distance needs to be continuously controlled.