Optimally bridging the gap from delayed to perfect CSIT in the K-user MISO BC

Abstract

This work 1 derives the optimal Degrees-of-Freedom (DoF) of the K-User MISO Broadcast Channel (BC) with delayed Channel-State Information at the Transmitter (CSIT) and with additional current noisy CSIT where the channel estimation error scales in P −α for α ∈ [0, 1]. The optimal sum DoF takes the simple form (1 − α)K/HK + αK where HK K k=1 1 k .This optimal performance is the result of a novel scheme which deviates from existing efforts as it digitally combines interference, decodes symbols of any order in the MAT alignment [1], and utilizes a hierarchical quantizer whose output is distributed across rounds in a way that minimizes unwanted interference. These jointly deliver, for the first time, the elusive DoF-optimal combining of MAT and ZF.

Optimal DoF of the K-User Broadcast Channel with Delayed and Imperfect Current CSIT

“…Although the main motivation for the our scheme is clearly the case of K > 2, this simple setting contains all the key components and allows to present in detail all the steps, with lighter notations than in the general K-user case. Note that the K = 3-user case is described in the conference version of this work [29] and can serve as an additional example to gain insights before dealing with the general case 3 .…”

“…] (see (29) and ( 31)), but by induction, these data symbols are known at user k, and thus their interference can be removed. Consequently, user k is able to decode its corresponding…”