Design Criteria for Omnidirectional STBC in Massive MIMO Systems

In this letter, we study the design criteria for omnidirectional space time block codes (STBCs) in downlink massive multiple-input multiple-output (MIMO) systems. To this end, we first derive a tighter upper bound on the average error probability and then use it to formulate two design criteria, namely the rank criterion and the sum determinant criterion (SDC). The rank criterion is similar to that for conventional MIMO, but it is obtained here by invoking the new upper bound for massive MIMO. The SDC is more accurate than its conventional MIMO counterpart since it takes into account the sum of the determinants of the codeword difference matrices, as opposed to the minimum of these determinants. In addition, the SDC provides a useful metric for the performance comparison and design of STBCs. The effectiveness of the proposed SDC for omnidirectional STBC is validated by simulations for massive MIMO systems.