Modulation for MIMO Networks with Several Users
  In a recent work, a capacity-achieving scheme for the common-message two-user MIMO broadcast channel, based on single-stream coding and decoding, was described. This was obtained via a novel joint unitary triangularization which is applied to the corresponding channel matrices. In this work, the triangularization is generalized, to any (finite) number of matrices, allowing multi-user applications. To that end, multiple channel uses are jointly treated, in a manner reminiscent of space-time coding. As opposed to the two-user case, in the general case there does not always exist a perfect (capacity-achieving) solution. However, a nearly optimal scheme (with vanishing loss in the limit of large blocks) always exists. Common-message broadcasting is but one example of communication networks with MIMO links which can be solved using an approach coined "Network Modulation"; the extension beyond two links carries over to these problems. 
Anatoly Khina and Ayal Hitron and Uri Erez
cs.IT math.IT