A MIMO Relay With Delayed Feedback Can Improve DoF in $K$- User MISO Interference Channel With No CSIT.

This paper studies the impact of a multiple-input–multiple-output (MIMO) relay on the $K$- user multiple-input–single-output (MISO) interference channel (IC) when the relay has <bold>delayed </bold> feedback (i.e., delayed channel state information (CSI) or delayed channel output feedback) from each receiver, yet each transmitter has no knowledge about CSI. We first develop a new <bold>relay-aided retrospective interference alignment</bold> (r-RIA) and characterize the <bold>optimal</bold> sum degrees of freedom (DoFs) for the $K$- user MISO IC with a MIMO relay by providing a new matching outer bound. It is shown that the sum DoF does not decrease, even in the absence of direct links between transmitters and receivers, as long as $K$ is sufficiently large. We next show that a MIMO relay can provide a sum-DoF gain in the $K$- user MISO IC with delayed feedback to the relay in the absence of CSI at transmitter (CSIT), whereas relaying is shown to be <bold>not</bold> useful with <bold>instantaneous</bold> CSIT from the DoF perspective. [ABSTRACT FROM PUBLISHER]