CSI Reference Signal Designs for Enabling Closed-Loop MIMO Feedback

Obtaining reliable channel state information (CSI) in future wireless standards is critical for the operation of downlink closed-loop techniques such as SU-MIMO, MU-MIMO, and coordinated multipoint (CoMP). However, this CSI is difficult to obtain especially when considering the potentially strong interference from neighboring sectors/base stations on the downlink reference symbols (RSs, aka pilot symbols) used to obtain the CSI. This paper describes three possible CSI RS designs for enabling such closed- loop feedback in light of the strong interference environment and discusses some tradeoffs between the designs. First, two code-division multiplexing (CDM) designs are proposed where one has nine orthogonal sequences and the other has three orthogonal sequences. Next a new CSI-RS design is described which uses mutually unbiased bases (MUB) to provide a 6 dB quasi-orthogonal gain over neighboring cells. The MUB design is shown in system-level simulation to have performance close to a design with 9 orthogonal sequences while providing a finer frequency-domain sampling than the 9- orthogonal approach.