12 research outputs found
Optimal DoF Region of the Two-User MISO-BC with General Alternating CSIT
In the setting of the time-selective two-user multiple-input single-output
(MISO) broadcast channel (BC), recent work by Tandon et al. considered the case
where - in the presence of error-free delayed channel state information at the
transmitter (delayed CSIT) - the current CSIT for the channel of user 1 and of
user 2, alternate between the two extreme states of perfect current CSIT and of
no current CSIT.
Motivated by the problem of having limited-capacity feedback links which may
not allow for perfect CSIT, as well as by the need to utilize any available
partial CSIT, we here deviate from this `all-or-nothing' approach and proceed -
again in the presence of error-free delayed CSIT - to consider the general
setting where current CSIT now alternates between any two qualities.
Specifically for and denoting the high-SNR asymptotic
rates-of-decay of the mean-square error of the CSIT estimates for the channel
of user~1 and of user~2 respectively, we consider the case where for any two positive current-CSIT quality exponents
. In a fast-fading setting where we consider communication over
any number of coherence periods, and where each CSIT state is present
for a fraction of this total duration, we focus on the
symmetric case of , and derive
the optimal degrees-of-freedom (DoF) region. The result, which is supported by
novel communication protocols, naturally incorporates the aforementioned
`Perfect current' vs. `No current' setting by limiting .
Finally, motivated by recent interest in frequency correlated channels with
unmatched CSIT, we also analyze the setting where there is no delayed CSIT
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
Space-Time Encoded MISO Broadcast Channel with Outdated CSIT: An Error Rate and Diversity Performance Analysis
Studies of the MISO Broadcast Channel (BC) with delayed Channel State
Information at the Transmitter (CSIT) have so far focused on the sum-rate and
Degrees-of-Freedom (DoF) region analysis. In this paper, we investigate for the
first time the error rate performance at finite SNR and the
diversity-multiplexing tradeoff (DMT) at infinite SNR of a space-time encoded
transmission over a two-user MISO BC with delayed CSIT. We consider the
so-called MAT protocol obtained by Maddah-Ali and Tse, which was shown to
provide 33% DoF enhancement over TDMA. While the asymptotic DMT analysis shows
that MAT is always preferable to TDMA, the Pairwise Error Probability analysis
at finite SNR shows that MAT is in fact not always a better alternative to
TDMA. Benefits can be obtained over TDMA only at very high rate or once
concatenated with a full-rate full-diversity space-time code. The analysis is
also extended to spatially correlated channels and the influence of transmit
correlation matrices and user pairing strategies on the performance are
discussed. Relying on statistical CSIT, signal constellations are further
optimized to improve the error rate performance of MAT and make it insensitive
to user orthogonality. Finally, other transmission strategies relying on
delayed CSIT are discussed
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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