Toward the Performance Versus Feedback Tradeoff for the Two-User MISO Broadcast Channel

Abstract

For the two-user MISO broadcast channel with imperfect and delayed channel state information at the transmitter (CSIT), the work explores the tradeoff between performance on the one hand, and CSIT timeliness and accuracy on the other hand. The work considers a broad setting where communication takes place in the presence of a random fading process, and in the presence of a feedback process that, at any point in time, may provide CSIT estimates -of some arbitrary accuracy -for any past, current or future channel realization. This feedback quality may fluctuate in time across all ranges of CSIT accuracy and timeliness, ranging from perfectly accurate and instantaneously available estimates, to delayed estimates of minimal accuracy. Under standard assumptions, the work derives the degrees-offreedom (DoF) region, which is tight for a large range of CSIT quality. This derived DoF region concisely captures the effect of channel correlations, the accuracy of predicted, current, and delayed-CSIT, and generally captures the effect of the quality of CSIT offered at any time, about any channel.The work also introduces novel schemes which -in the context of imperfect and delayed CSIT -employ encoding and decoding with a phase-Markov structure. The results hold for a large class of block and non-block fading channel models, and they unify and extend many prior attempts to capture the effect of imperfect and delayed feedback. This generality also allows for consideration of novel pertinent settings, such as the new periodically evolving feedback setting, where a gradual accumulation of feedback bits progressively improves CSIT as time progresses across a finite coherence period.

On the Two-User MISO Broadcast Channel With Alternating CSIT: A Topological Perspective

“…In the second example -where CSIT has a periodic structure -the two phases are jointly performed in the same communication block, and this block is repeated many times, in a block Markov manner where the multicasting phase in one block is designed to aid for the overloading phase from the previous block. This sequence follows closely from the scheme in [14] that considered a similar setting without though any topology considerations (α = 1).…”

“…Such overload-multicast strategy was explored in different settings, including in [30] for the heterogeneous parallel channel with delayed CSIT. It is worth noting that one of the main differences between the new schemes, and the older schemes by Tandon et al [15] as well as the schemes from the general CSIT setting in [14] -and by extension, the difference between the new schemes here and other blockMarkov related schemes [41]- [45] (see also [46], [47]) -relates the new schemes' ability to properly capitalize on the inherent weakness of a link in order to (often optimally) reduce interference in at least one direction.…”

“…In addition to its direct relevance to cellular downlink communications, the MISO BC has attracted much attention for the critical role played in this setting by the feedback mechanism through which channel state information at the transmitter (CSIT) is typically acquired. Interesting insights into the dependence of the capacity limits of the MISO BC on the timeliness and quality of feedback, have been found through degrees of freedom (DoF) characterizations under perfect CSIT [1], no CSIT [2]- [5], compound CSIT [6]- [8], delayed CSIT [9], CSIT comprised of channel coherence patterns [10], mixed CSIT [11]- [14], and alternating CSIT [15]. Other related work can be found in [16]- [30].…”

Fundamental Limits of Cache-Aided Wireless BC: Interplay of Coded-Caching and CSIT Feedback

“…The range of interest 1 is α ∈ [0, 1]. We also assume availability of delayed CSIT (as in for example [6], as well as in a variety of subsequent works [2], [3], [7]- [14], see also [15]- [18] as well as [19]- [21]) where now the delayed estimates of any channel, can be received without error but with arbitrary delay, even if this delay renders this CSIT completely obsolete. As it is argued in [2], this mixed CSI model (partial current CSIT, and delayed CSIT) nicely captures different realistic settings that might involve channel correlations and an ability to improve CSI as time progresses.…”

Robust full-dimension MIMO transmission based on limited feedback angular-domain CSIT

“…On the other hand, CSI inaccuracies also result from the delay of the feedback path, leading to a mismatch of the CSI experienced during the quantization phase and the CSI valid during transmission. Several theoretical studies exist that evaluate the performance degradation due to imperfect and outdated CSIT for a variety of transmission scenarios [24][25][26][27].…”