Millimeter-wave to microwave MIMO relays (M4R) for 5G building penetration communications

5G communication systems promise high data rates over Gbits/S and minimum delay in data transfer. Millimeter wave band, due to its abundant spectrum that is available, is considered to be the key enabling technique for such links. However, the high propagation loss and poor penetration through obstacles of electromagnetic waves at millimeter wave (mmWave) makes it difficult for outdoor-to-indoor communication. On the other hand, signals in the microwave band can easily penetrate through buildings, but its single channel capacity is limited by the narrow available bandwidth. In this article, we propose a new method of mmWave to microwave MIMO relay (M4R) which combines the broad bandwidth of mmWave link and the better penetration and the more abundant spatial channels in the microwave band to form high speed wireless links. The essential idea is to use frequency translational relay units in RF to connect frequency multiplexing mmWave channels and MIMO microwave channels to realize seamless pathway of information flow and avoid bottleneck in data traffic. System principle and link budget of M4R is discussed in this article and shows that M4R may significantly improve the link performance and maximize the channel capacity for high speed outdoor to indoor communications.