N-port DL-MIMO Antenna System Realization Using Systematically Designed Mode Matching and Mode Decomposition Network

A major issue in the design of small N-port Multiple Input - Multiple Output (MIMO) antennas is to achieve approximate Hermitian match over sufficient bandwidth. Radiation quality factors of individual radiation modes differ significantly, as is typical for small antennas. For MIMO operation the bandwidth is limited by the mode with the highest radiation quality factor and is often insufficient to match the antenna in both Uplink (UL) and Downlink (DL) portions of a band simultaneously. However, a trade-off solution where N orthogonal radiation modes are mapped to N separate external feed ports of which a first subset is assigned to the DL frequency band and used for DL reception and a second subset is assigned to the UL frequency band exists. The maximum attainable bandwidth of each radiation mode can thus be exploited. A systematic design procedure for Mode Matching Network (MMN) and Mode Decomposition Network (MDN) which supports this approach is shown. The MMN design is based on minimization of the total multi-port reflectance or minimization of a subset of modal reflectances over different frequency bands. This network matches radiation modes of the antenna. A subsequent MDN maps modes to separate external ports. The design of a MDN uses Givens rotations of the matrix containing in its columns the eigenvectors of the radiation matrix. Separation of the matching and decomposition functionality and the optimum sequence of MMN and MDN allows for low-loss realization of a DL-MIMO or UL-MIMO antenna system.