IMAGE-NET
Gestion d'interférence pour les réseaux favorables à l'environnement

Recent breakthroughs: Performance vs. Feedback in Multiuser Communications

In many multiuser wireless communications scenarios, good feedback is a crucial ingredient that facilitates improved performance.  While being useful, perfect feedback is also hard and time-consuming to obtain.  With this challenge as a starting point, the main work up to now in this project, seeks to address the simple yet elusive and fundamental question of "HOW MUCH QUALITY of feedback, AND WHEN, must one send to achieve a certain performance in specific settings of multiuser communications".  This applies towards exploring the effect of reduced and imperfect CSIT in interference management, as well as towards designing of interference avoidance algorithms that are robust to imperfect CSIT.  We have recently explored the theoretical limits of the Multiple-Input Single-Output broadcast channel (MISO BC), with imperfect and delayed channel state information at the transmitter (CSIT), and we explored the tradeoff between performance, and CSIT timeliness and quality.

Our work considered 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, provides CSIT estimates - of some arbitrary quality - for any past, current or future channel realization. Under standard assumptions, the work derives the degrees-of-freedom (DoF) region, which is optimal for a large regime of sufficiently good (but potentially imperfect) delayed CSIT.  This region concisely captures the effect of channel correlations, the quality of predicted, current, and delayed-CSIT, as well as concisely captures the effect of the quality of CSIT offered at any time, about any channel.

Our general approach allows for insight on pertinent questions such as:

  • How much CSIT quality (delayed, current or predicted) allows for a certain DoF performance?
  • Can imperfect delayed CSIT achieve the same optimality that was previously attributed to perfect delayed CSIT?
  • When is delayed feedback unnecessary?
  • Is predicted CSIT useful in terms of the DoF performance?
  • Can symmetric feedback offer DoF benefit over the asymmetric feedback?
  • What is the impact of imperfect global CSIR (imperfect receiver estimates of the channel of the other receiver)?
  • Can a communication scheme achieve both full diversity and full DoF for the setting with only delayed CSIT?

We have also derived novel schemes which - in the context of imperfect and delayed CSIT - introduce here for the first time, encoding and decoding with a phase-Markov structure.

The schemes maintain their performance 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.  The theory and the schemes allow 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.

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Image-Net ANR Project.