High spectral-efficiency multiple-antenna wireless using single-antenna terminals

Multiple-antenna wireless - also called MIMO provides order-of-magnitude improvements in throughput and it was originally conceived as a point-to-point scheme where the throughput increases linearly with the minimum of the number of transmit or receive antennas. Recent theoretical and practical developments in MIMO obviate the former need for equipping terminals with multiple-element receive arrays. Instead a basestation is equipped with a multiple-element transmit array sends multiple messages, simultaneously and selectively, to multiple autonomous terminals, where each terminal has only a single antenna. The major complication of multiple messaging is that it requires the basestation array to possess complete and timely knowledge of the complex forward-link propagation matrix. Time-division duplex (TDD) systems exploits reciprocity to obtain an effective transfer of the forward propagation matrix by having the terminals transmit pilots to the array over the reverse channel. Frequency-division duplex (FDD) systems require the terminals first to estimate the forward channel from pilots transmitted by the array over the forward channel, and then to transfer this channel state information (CSI) to the basestation array over the reverse link. The large amount of information to transmit, combined with the need to transmit the CSI in near real-time and the inherent delays associated with conventional channel coding, has inhibited the consideration of FDD for fast-changing propagation environments. We show that FDD CSI transfer can be accomplished simply and efficiently through uncoded analog linear modulation. Compared with TDD CSI transfer, the FDD CSI transfer provides the same quality of CSI using typically only two-to four-times the number of reverse link symbols that are used by TDD.