An Uplink Multiuser MIMO Decomposition without Feedback or Precoding for Frequency Flat Fading Channels

Methods to decompose downlink (DL) multiuser (MU) MIMO signals for both frequency flat or selective fading channels have been proposed at the expense of feedback and precoding. The paper proposes a method to decompose the uplink (UL) MU MIMO signal (for both asynchronous and synchronous MU transmissions) into parallel single-user (SU) MIMOs without feedback or precoding. The base station (BS) decomposes the MU MIMO signal by cancelling, for each user, its MU interference through multiplying the received signal with the left null space of the MU interference MIMO space. The decomposed SU MIMO has a reduced size because each user's signal can only be received undistorted in the left null space of its MU interference MIMO space. The paper proves in theory that the distribution of the decomposed reduced-size SU MIMO channel has the same distribution as that of an independent SU MIMO channel with the same size. In addition, the paper presents simulation results on the performance of MU MIMO using our decomposition method and the capacity of uplink MU-MIMO systems, which match the theoretical analyses. The decomposition method can be extended to frequency selective fading channels similarly but on a per subcarrier basis.