210,978 research outputs found
Capacity of a POST Channel with and without Feedback
We consider finite state channels where the state of the channel is its
previous output. We refer to these as POST (Previous Output is the STate)
channels. We first focus on POST() channels. These channels have binary
inputs and outputs, where the state determines if the channel behaves as a
or an channel, both with parameter . %with parameter We
show that the non feedback capacity of the POST() channel equals its
feedback capacity, despite the memory of the channel. The proof of this
surprising result is based on showing that the induced output distribution,
when maximizing the directed information in the presence of feedback, can also
be achieved by an input distribution that does not utilize of the feedback. We
show that this is a sufficient condition for the feedback capacity to equal the
non feedback capacity for any finite state channel. We show that the result
carries over from the POST() channel to a binary POST channel where the
previous output determines whether the current channel will be binary with
parameters or . Finally, we show that, in general, feedback may
increase the capacity of a POST channel
Communicating over Filter-and-Forward Relay Networks with Channel Output Feedback
Relay networks aid in increasing the rate of communication from source to
destination. However, the capacity of even a three-terminal relay channel is an
open problem. In this work, we propose a new lower bound for the capacity of
the three-terminal relay channel with destination-to-source feedback in the
presence of correlated noise. Our lower bound improves on the existing bounds
in the literature. We then extend our lower bound to general relay network
configurations using an arbitrary number of filter-and-forward relay nodes.
Such network configurations are common in many multi-hop communication systems
where the intermediate nodes can only perform minimal processing due to limited
computational power. Simulation results show that significant improvements in
the achievable rate can be obtained through our approach. We next derive a
coding strategy (optimized using post processed signal-to-noise ratio as a
criterion) for the three-terminal relay channel with noisy channel output
feedback for two transmissions. This coding scheme can be used in conjunction
with open-loop codes for applications like automatic repeat request (ARQ) or
hybrid-ARQ.Comment: 15 pages, 8 figures, to appear in IEEE Transactions on Signal
Processin
Multiuser Switched Diversity Scheduling Schemes
Multiuser switched-diversity scheduling schemes were recently proposed in
order to overcome the heavy feedback requirements of conventional opportunistic
scheduling schemes by applying a threshold-based, distributed, and ordered
scheduling mechanism. The main idea behind these schemes is that slight
reduction in the prospected multiuser diversity gains is an acceptable
trade-off for great savings in terms of required channel-state-information
feedback messages. In this work, we characterize the achievable rate region of
multiuser switched diversity systems and compare it with the rate region of
full feedback multiuser diversity systems. We propose also a novel proportional
fair multiuser switched-based scheduling scheme and we demonstrate that it can
be optimized using a practical and distributed method to obtain the feedback
thresholds. We finally demonstrate by numerical examples that
switched-diversity scheduling schemes operate within 0.3 bits/sec/Hz from the
ultimate network capacity of full feedback systems in Rayleigh fading
conditions.Comment: Accepted at IEEE Transactions on Communications, to appear 2012,
funded by NPRP grant 08-577-2-241 from QNR
Using Channel Output Feedback to Increase Throughput in Hybrid-ARQ
Hybrid-ARQ protocols have become common in many packet transmission systems
due to their incorporation in various standards. Hybrid-ARQ combines the normal
automatic repeat request (ARQ) method with error correction codes to increase
reliability and throughput. In this paper, we look at improving upon this
performance using feedback information from the receiver, in particular, using
a powerful forward error correction (FEC) code in conjunction with a proposed
linear feedback code for the Rayleigh block fading channels. The new hybrid-ARQ
scheme is initially developed for full received packet feedback in a
point-to-point link. It is then extended to various different multiple-antenna
scenarios (MISO/MIMO) with varying amounts of packet feedback information.
Simulations illustrate gains in throughput.Comment: 30 page
The Degrees of Freedom of the MIMO Y-channel
The degrees of freedom (DoF) of the MIMO Y-channel, a multi-way communication
network consisting of 3 users and a relay, are characterized for arbitrary
number of antennas. The converse is provided by cut-set bounds and novel
genie-aided bounds. The achievability is shown by a scheme that uses
beamforming to establish network coding on-the-fly at the relay in the uplink,
and zero-forcing pre-coding in the downlink. It is shown that the network has
min{2M_2+2M_3,M_1+M_2+M_3,2N} DoF, where M_j and N represent the number of
antennas at user j and the relay, respectively. Thus, in the extreme case where
M_1+M_2+M_3 dominates the DoF expression and is smaller than N, the network has
the same DoF as the MAC between the 3 users and the relay. In this case, a
decode and forward strategy is optimal. In the other extreme where 2N
dominates, the DoF of the network is twice that of the aforementioned MAC, and
hence network coding is necessary. As a byproduct of this work, it is shown
that channel output feedback from the relay to the users has no impact on the
DoF of this channel.Comment: 5 pages, 4 figures, ISIT 201
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