562 research outputs found
Multiuser Scheduling in a Markov-modeled Downlink using Randomly Delayed ARQ Feedback
We focus on the downlink of a cellular system, which corresponds to the bulk
of the data transfer in such wireless systems. We address the problem of
opportunistic multiuser scheduling under imperfect channel state information,
by exploiting the memory inherent in the channel. In our setting, the channel
between the base station and each user is modeled by a two-state Markov chain
and the scheduled user sends back an ARQ feedback signal that arrives at the
scheduler with a random delay that is i.i.d across users and time. The
scheduler indirectly estimates the channel via accumulated delayed-ARQ feedback
and uses this information to make scheduling decisions. We formulate a
throughput maximization problem as a partially observable Markov decision
process (POMDP). For the case of two users in the system, we show that a greedy
policy is sum throughput optimal for any distribution on the ARQ feedback
delay. For the case of more than two users, we prove that the greedy policy is
suboptimal and demonstrate, via numerical studies, that it has near optimal
performance. We show that the greedy policy can be implemented by a simple
algorithm that does not require the statistics of the underlying Markov channel
or the ARQ feedback delay, thus making it robust against errors in system
parameter estimation. Establishing an equivalence between the two-user system
and a genie-aided system, we obtain a simple closed form expression for the sum
capacity of the Markov-modeled downlink. We further derive inner and outer
bounds on the capacity region of the Markov-modeled downlink and tighten these
bounds for special cases of the system parameters.Comment: Contains 22 pages, 6 figures and 8 tables; revised version including
additional analytical and numerical results; work submitted, Feb 2010, to
IEEE Transactions on Information Theory, revised April 2011; authors can be
reached at [email protected]/[email protected]/[email protected]
On the Required Number of Antennas in a Point-to-Point Large-but-Finite MIMO System: Outage-Limited Scenario
This paper investigates the performance of the point-to-point
multiple-input-multiple-output (MIMO) systems in the presence of a large but
finite numbers of antennas at the transmitters and/or receivers. Considering
the cases with and without hybrid automatic repeat request (HARQ) feedback, we
determine the minimum numbers of the transmit/receive antennas which are
required to satisfy different outage probability constraints. Our results are
obtained for different fading conditions and the effect of the power amplifiers
efficiency on the performance of the MIMO-HARQ systems is analyzed. Moreover,
we derive closed-form expressions for the asymptotic performance of the
MIMO-HARQ systems when the number of antennas increases. Our analytical and
numerical results show that different outage requirements can be satisfied with
relatively few transmit/receive antennas.Comment: Under review in IEEE Transactions on Communication
Turbo Packet Combining for Broadband Space-Time BICM Hybrid-ARQ Systems with Co-Channel Interference
In this paper, efficient turbo packet combining for single carrier (SC)
broadband multiple-input--multiple-output (MIMO) hybrid--automatic repeat
request (ARQ) transmission with unknown co-channel interference (CCI) is
studied. We propose a new frequency domain soft minimum mean square error
(MMSE)-based signal level combining technique where received signals and
channel frequency responses (CFR)s corresponding to all retransmissions are
used to decode the data packet. We provide a recursive implementation algorithm
for the introduced scheme, and show that both its computational complexity and
memory requirements are quite insensitive to the ARQ delay, i.e., maximum
number of ARQ rounds. Furthermore, we analyze the asymptotic performance, and
show that under a sum-rank condition on the CCI MIMO ARQ channel, the proposed
packet combining scheme is not interference-limited. Simulation results are
provided to demonstrate the gains offered by the proposed technique.Comment: 12 pages, 7 figures, and 2 table
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