Abstract

In many wireless networks, link strengths are affected by many topological factors such as different distances, shadowing and inter-cell interference, thus resulting in some links being generally stronger than other links. From an information theoretic point of view, accounting for such topological aspects has remained largely unexplored, despite strong indications that such aspects can crucially affect transceiver and feedback design, as well as the overall performance. The work here takes a step in exploring this interplay between topology, feedback and performance. This is done for the two user broadcast channel with random fading, in the presence of a simple two-state topological setting of statistically strong vs. weaker links, and in the presence of a practical ternary feedback setting of alternating channel state information at the transmitter (alternating CSIT) where for each channel realization, this CSIT can be perfect, delayed, or not available. In this setting, the work derives generalized degrees-of-freedom bounds and exact expressions, that capture performance as a function of feedback statistics and topology statistics. The results are based on novel topological signal management (TSM) schemes that account for topology in order to fully utilize feedback. This is achieved for different classes of feedback mechanisms of practical importance, from which we identify specific feedback mechanisms that are best suited for different topologies. This approach offers further insight on how to split the effort --- of channel learning and feeding back CSIT --- for the strong versus for the weaker link. Further intuition is provided on the possible gains from topological spatio-temporal diversity, where topology changes in time and across users.

Highlights

  • INTRODUCTIONA vector Gaussian broadcast channel, known as the Gaussian MISO BC (multiple-input single-output broadcast channel) is comprised of a transmitter with multiple antennas that wishes to send independent messages to different receivers, each equipped with a single antenna

  • A vector Gaussian broadcast channel, known as the Gaussian MISO BC is comprised of a transmitter with multiple antennas that wishes to send independent messages to different receivers, each equipped with a single antenna

  • In wireless communication settings, the link strengths are affected by many topological factors, such as propagation path loss, shadow fading and inter-cell interference [17], which lead to statistically unequal channel gains, with some links being much weaker or stronger than others

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Summary

INTRODUCTION

A vector Gaussian broadcast channel, known as the Gaussian MISO BC (multiple-input single-output broadcast channel) is comprised of a transmitter with multiple antennas that wishes to send independent messages to different receivers, each equipped with a single antenna. In wireless communication settings, the link strengths are affected by many topological factors, such as propagation path loss, shadow fading and inter-cell interference [17], which lead to statistically unequal channel gains, with some links being much weaker or stronger than others. Accounting for these topological aspects, by going beyond the DoF framework into the generalized degrees of freedom (GDoF) framework, is the focus of the topological perspective that we seek here. The work here combines considerations of topology with considerations of feedback timeliness and quality, and addresses questions on performance bounds, on encoding designs that account for topology and feedback, on feedback and channel learning mechanisms that adapt to topology, and on handling and even exploiting fluctuations in topology

SYSTEM MODEL FOR THE TOPOLOGICAL BC
Conventions and structure
OUTER BOUNDS
PRACTICAL FEEDBACK SCHEMES OVER A FIXED
Delayed CSIT and fixed topology
CONCLUSIONS
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