177 research outputs found
On the Vector Broadcast Channel with Alternating CSIT: A Topological Perspective
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.Comment: Shorter version will be presented at ISIT 201
On the Fundamental Feedback-vs-Performance Tradeoff over the MISO-BC with Imperfect and Delayed CSIT
This work considers the multiuser multiple-input single-output (MISO)
broadcast channel (BC), where a transmitter with M antennas transmits
information to K single-antenna users, and where - as expected - the quality
and timeliness of channel state information at the transmitter (CSIT) is
imperfect. Motivated by the fundamental question of how much feedback is
necessary to achieve a certain performance, this work seeks to establish bounds
on the tradeoff between degrees-of-freedom (DoF) performance and CSIT feedback
quality. Specifically, this work provides a novel DoF region outer bound for
the general K-user MISO BC with partial current CSIT, which naturally bridges
the gap between the case of having no current CSIT (only delayed CSIT, or no
CSIT) and the case with full CSIT. The work then characterizes the minimum CSIT
feedback that is necessary for any point of the sum DoF, which is optimal for
the case with M >= K, and the case with M=2, K=3.Comment: An initial version of this paper has been reported as Research Report
No. RR-12-275 at EURECOM, December 7, 2012. This paper was submitted in part
to the ISIT 201
MISO Broadcast Channel with Delayed and Evolving CSIT
The work considers the two-user MISO broadcast channel with gradual and
delayed accumulation of channel state information at the transmitter (CSIT),
and addresses the question of how much feedback is necessary, and when, in
order to achieve a certain degrees-of-freedom (DoF) performance. Motivated by
limited-capacity feedback links that may not immediately convey perfect CSIT,
and focusing on the block fading scenario, we consider a progressively
increasing CSIT quality as time progresses across the coherence period (T
channel uses - evolving current CSIT), or at any time after (delayed CSIT).
Specifically, for any set of feedback quality exponents a_t, t=1,...,T,
describing the high-SNR rates-of-decay of the mean square error of the current
CSIT estimates at time t<=T (during the coherence period), the work describes
the optimal DOF region in several different evolving CSIT settings, including
the setting with perfect delayed CSIT, the asymmetric setting where the quality
of feedback differs from user to user, as well as considers the DoF region in
the presence of a imperfect delayed CSIT corresponding to having a limited
number of overall feedback bits. These results are supported by novel
multi-phase precoding schemes that utilize gradually improving CSIT.
The approach here naturally incorporates different settings such as the
perfect-delayed CSIT setting of Maddah-Ali and Tse, the imperfect current CSIT
setting of Yang et al. and of Gou and Jafar, the asymmetric setting of Maleki
et al., as well as the not-so-delayed CSIT setting of Lee and Heath.Comment: Submitted to Transactions on Information Theory - November 2012 18
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