L. I. Ponomarev, A. I. Skorodumov

The possibilities of the spectral efficiency enhancement of perspective cellular wireless communication systems (CWCS) by means of MIMO systems for arrangement of several spatial independent channels are analyzed. The structure of CWCS with multielement transmitting and receiving antenna arrays is presented. The spectrum decomposition of energy matrix into eigenvectors made it possible to determine the full signal power at the receiving point via eigenvalue of radio channels. The main theoretical ratios are presented analytically. Meeting the conditions of signal power maximum in receiving antenna at the given power of radiated signal in accordance with the Shannon theorem for "white" noisy channel the expression for maximum spectrum efficiency of a single-channel CWCS is obtained. It is established that in case of homogeneous space the self directional pattern of a radio channel corresponds to the directional pattern of adaptive antenna in free space. For the arrangement of M independent radio channels the expression determining the spectral efficiency of CWCS with MIMO antennas is obtained. The limit value of spectral efficiency maximum is reached in case of equality of eigenvalue of radio channels. Then the reward in the spectral efficiency of CWCS with MIMO antennas comparing to a single-channel scheme is dependent only on the number of channels and S/N (signal/noise) ratio. Multipath signal transmission goes with significant power attenuation due to obstacle return and diffraction that leads to substantial decrease of radio channels eigenvalue. The analysis of dependency of maximum spectral efficiency on the number of channels shows that its increase more then half as large is possible only if S/N ratio is more than 10 dB. Radio channels inequivalence and interference from different inputs decrease the spectral efficiency of cellular wireless communication and the influence of interference can substantially reduce the number of active channels down to one. The research of the main principles of CWCS with MIMO antennas and determination of potentialities of the spectral efficiency enhancement of CWCS is conducted for diffracting and reflecting models of multipath signal transmission. It is proved that application of MIMO antennas increases the spectral efficiency of CWCS when the directional patterns of antennas are self directional patterns of equivalent radio channels and are formed with regard to spatial non-homogenous radio pathways parameters, antenna array elements interaction effects and interference level from different inputs. Due to antenna array elements interaction effect there is such distance between them whereby the spectral efficiency reaches maximum. Eigenvalues of radio channels substantially and differently depend on mutual position of a base station, terminal station and a screen that limit the possibility of the spectral efficiency enhancement especially when operated at long distances and in motion. Two methods of antenna systems construction with multichannel and single-channel schemes of signal processing are suggested. It is proved that the multichannel signal processing scheme should be applied when eigenvalues of radio channels are almost the same. In this regard the spectral efficiency of multichannel CWCS increases in proportion to M where M is the number of independent channels and coefficient of proportionality depends on S/N (signal/noise) ratio. If the eigenvalues of radio channels differ three times as much or more and one of them is marked by the highest eigenvalue then the multichannel scheme of signal processing becomes ineffective and the single beam with the direction of maximum corresponding to a radio channel with the highest eigenvalue is required. In this case the single-channel scheme of signal processing along with the definite choice of directional patterns of receiving and transmitting antennas provides the same spectral efficiency as compared to multichannel scheme. Self directional patterns designed for dual-channel diffractive and four-channel reflecting models of multipath signal transmission are introduced. The research results made it possible to determine the principles of application and construction of MIMO antennas with optimal spatial filtering.