370 research outputs found
Capacity Limits of Multiuser Multiantenna Cognitive Networks
Unlike point-to-point cognitive radio, where the constraint imposed by the
primary rigidly curbs the secondary throughput, multiple secondary users have
the potential to more efficiently harvest the spectrum and share it among
themselves. This paper analyzes the sum throughput of a multiuser cognitive
radio system with multi-antenna base stations, either in the uplink or downlink
mode. The primary and secondary have and users, respectively, and their
base stations have and antennas, respectively. We show that an uplink
secondary throughput grows with if the primary is a
downlink system, and grows with if the primary is an
uplink system. These growth rates are shown to be optimal and can be obtained
with a simple threshold-based user selection rule. Furthermore, we show that
the secondary throughput can grow proportional to while simultaneously
pushing the interference on the primary down to zero, asymptotically.
Furthermore, we show that a downlink secondary throughput grows with in the presence of either an uplink or downlink primary system. In
addition, the interference on the primary can be made to go to zero
asymptotically while the secondary throughput increases proportionally to . Thus, unlike the point-to-point case, multiuser cognitive radios can
achieve non-trivial sum throughput despite stringent primary interference
constraints.Comment: 32 pages, 6 figure
Degrees of Freedom of the Broadcast Channel with Hybrid CSI at Transmitter and Receivers
In general, the different links of a broadcast channel may experience
different fading dynamics and, potentially, unequal or hybrid channel state
information (CSI) conditions. The faster the fading and the shorter the fading
block length, the more often the link needs to be trained and estimated at the
receiver, and the more likely that CSI is stale or unavailable at the
transmitter. Disparity of link fading dynamics in the presence of CSI
limitations can be modeled by a multi-user broadcast channel with both
non-identical link fading block lengths as well as dissimilar link CSIR/CSIT
conditions. This paper investigates a MISO broadcast channel where some
receivers experience longer coherence intervals (static receivers) and have
CSIR, while some other receivers experience shorter coherence intervals
(dynamic receivers) and do not enjoy free CSIR. We consider a variety of CSIT
conditions for the above mentioned model, including no CSIT, delayed CSIT, or
hybrid CSIT. To investigate the degrees of freedom region, we employ
interference alignment and beamforming along with a product superposition that
allows simultaneous but non-contaminating transmission of pilots and data to
different receivers. Outer bounds employ the extremal entropy inequality as
well as a bounding of the performance of a discrete memoryless multiuser
multilevel broadcast channel. For several cases, inner and outer bounds are
established that either partially meet, or the gap diminishes with increasing
coherence times.Comment: 36 pages, 8 figures, submitted to IEEE Transactions on Information
Theor
Coherence Disparity in Broadcast and Multiple Access Channels
Individual links in a wireless network may experience unequal fading
coherence times due to differences in mobility or scattering environment, a
practical scenario where the fundamental limits of communication have been
mostly unknown. This paper studies broadcast and multiple access channels where
multiple receivers experience unequal fading block lengths, and channel state
information (CSI) is not available at the transmitter(s), or for free at any
receiver. In other words, the cost of acquiring CSI at the receiver is fully
accounted for in the degrees of freedom. In the broadcast channel, the method
of product superposition is employed to find the achievable degrees of freedom.
We start with unequal coherence intervals with integer ratios. As long as the
coherence time is at least twice the number of transmit and receive antennas,
these degrees of freedom meet the upper bound in four cases: when the
transmitter has fewer antennas than the receivers, when all receivers have the
same number of antennas, when the coherence time of one receiver is much
shorter than all others, or when all receivers have identical block fading
intervals. The degrees of freedom region of the broadcast under identical
coherence times was also previously unknown and is settled by the results of
this paper. The disparity of coherence times leads to gains that are distinct
from those arising from other techniques such as spatial multiplexing or
multi-user diversity; this class of gains is denoted coherence diversity. The
inner bounds are further extended to the case of multiple receivers
experiencing fading block lengths of arbitrary ratio or alignment. Also, in the
multiple access channel with unequal coherence times, achievable and outer
bounds on the degrees of freedom are obtained.Comment: 45 pages, 13 figures, submitted to IEEE Transactions on Information
Theor
Community Detection with Side Information: Exact Recovery under the Stochastic Block Model
The community detection problem involves making inferences about node labels
in a graph, based on observing the graph edges. This paper studies the effect
of additional, non-graphical side information on the phase transition of exact
recovery in the binary stochastic block model (SBM) with nodes. When side
information consists of noisy labels with error probability , it is
shown that phase transition is improved if and only if
. When side information consists
of revealing a fraction of the labels, it is shown that phase
transition is improved if and only if . For a
more general side information consisting of features, two scenarios are
studied: (1)~ is fixed while the likelihood of each feature with respect to
corresponding node label evolves with , and (2)~The number of features
varies with but the likelihood of each feature is fixed. In each case, we
find when side information improves the exact recovery phase transition and by
how much. The calculated necessary and sufficient conditions for exact recovery
are tight except for one special case. In the process of deriving inner bounds,
a variation of an efficient algorithm is proposed for community detection with
side information that uses a partial recovery algorithm combined with a local
improvement procedure
Diversity Order in ISI Channels with Single-Carrier Frequency-Domain Equalizers
This paper analyzes the diversity gain achieved by single-carrier
frequency-domain equalizer (SC-FDE) in frequency selective channels, and
uncovers the interplay between diversity gain , channel memory length ,
transmission block length , and the spectral efficiency . We specifically
show that for the class of minimum means-square error (MMSE) SC-FDE receivers,
for rates full diversity of is achievable,
while for higher rates the diversity is given by .
In other words, the achievable diversity gain depends not only on the channel
memory length, but also on the desired spectral efficiency and the transmission
block length. A similar analysis reveals that for zero forcing SC-FDE, the
diversity order is always one irrespective of channel memory length and
spectral efficiency. These results are supported by simulations.Comment: 30 pages, 6 figures, to appear in the IEEE Transactions on Wireless
Communication
Coherent Product Superposition for Downlink Multiuser MIMO
In a two-user broadcast channel where one user has full CSIR and the other
has none, a recent result showed that TDMA is strictly suboptimal and a product
superposition requiring non-coherent signaling achieves DoF gains under many
antenna configurations. This work introduces product superposition in the
domain of coherent signaling with pilots, demonstrates the advantages of
product superposition in low-SNR as well as high-SNR, and establishes DoF gains
in a wider set of receiver antenna configurations. Two classes of decoders,
with and without interference cancellation, are studied. Achievable rates are
established by analysis and illustrated by simulations.Comment: IEEE Trans. Wireless Communications, 201
Ergodic Fading MIMO Dirty Paper and Broadcast Channels: Capacity Bounds and Lattice Strategies
A multiple-input multiple-output (MIMO) version of the dirty paper channel is
studied, where the channel input and the dirt experience the same fading
process and the fading channel state is known at the receiver (CSIR). This
represents settings where signal and interference sources are co-located, such
as in the broadcast channel. First, a variant of Costa's dirty paper coding
(DPC) is presented, whose achievable rates are within a constant gap to
capacity for all signal and dirt powers. Additionally, a lattice coding and
decoding scheme is proposed, whose decision regions are independent of the
channel realizations. Under Rayleigh fading, the gap to capacity of the lattice
coding scheme vanishes with the number of receive antennas, even at finite
Signal-to-Noise Ratio (SNR). Thus, although the capacity of the fading dirty
paper channel remains unknown, this work shows it is not far from its dirt-free
counterpart. The insights from the dirty paper channel directly lead to
transmission strategies for the two-user MIMO broadcast channel (BC), where the
transmitter emits a superposition of desired and undesired (dirt) signals with
respect to each receiver. The performance of the lattice coding scheme is
analyzed under different fading dynamics for the two users, showing that
high-dimensional lattices achieve rates close to capacity.Comment: Accepted in IEEE Transactions on Wireless Communication
Recovering a Single Community with Side Information
We study the effect of the quality and quantity of side information on the
recovery of a hidden community of size in a graph of size . Side
information for each node in the graph is modeled by a random vector with the
following features: either the dimension of the vector is allowed to vary with
, while log-likelihood ratio (LLR) of each component with respect to the
node label is fixed, or the LLR is allowed to vary and the vector dimension is
fixed. These two models represent the variation in quality and quantity of side
information. Under maximum likelihood detection, we calculate tight necessary
and sufficient conditions for exact recovery of the labels. We demonstrate how
side information needs to evolve with in terms of either its quantity, or
quality, to improve the exact recovery threshold. A similar set of results are
obtained for weak recovery. Under belief propagation, tight necessary and
sufficient conditions for weak recovery are calculated when the LLRs are
constant, and sufficient conditions when the LLRs vary with . Moreover, we
design and analyze a local voting procedure using side information that can
achieve exact recovery when applied after belief propagation. The results for
belief propagation are validated via simulations on finite synthetic data-sets,
showing that the asymptotic results of this paper can also shed light on the
performance at finite
Diversity of MIMO Linear Precoding
Linear precoding is a relatively simple method of MIMO signaling that can
also be optimal in certain special cases. This paper is dedicated to high-SNR
analysis of MIMO linear precoding. The Diversity-Multiplexing Tradeoff (DMT) of
a number of linear precoders is analyzed. Furthermore, since the diversity at
finite rate (also known as the fixed-rate regime, corresponding to multiplexing
gain of zero) does not always follow from the DMT, linear precoders are also
analyzed for their diversity at fixed rates. In several cases, the diversity at
multiplexing gain of zero is found not to be unique, but rather to depend on
spectral efficiency. The analysis includes the zero-forcing (ZF), regularized
ZF, matched filtering and Wiener filtering precoders. We calculate the DMT of
ZF precoding under two common design approaches, namely maximizing the
throughput and minimizing the transmit power. It is shown that regularized ZF
(RZF) or Matched filter (MF) suffer from error floors for all positive
multiplexing gains. However, in the fixed rate regime, RZF and MF precoding
achieve full diversity up to a certain spectral efficiency and zero diversity
at rates above it. When the regularization parameter in the RZF is optimized in
the MMSE sense, the structure is known as the Wiener precoder which in the
fixed-rate regime is shown to have diversity that depends not only on the
number of antennas, but also on the spectral efficiency. The diversity in the
presence of both precoding and equalization is also analyzed.Comment: 46 pages, 10 figure
Relay-Assisted Interference Channel: Degrees of Freedom
This paper investigates the degrees of freedom of the interference channel in
the presence of a dedicated MIMO relay. The relay is used to manage the
interference at the receivers. It is assumed that all nodes including the relay
have channel state information only for their own links and that the relay has
M (greater than or equal to K) antennas in a K-user network. We pose the
question: What is the benefit of exploiting the direct links from the source to
destinations compared to a simpler two-hop strategy. To answer this question,
we first establish the degrees of freedom of the interference channel with a
MIMO relay, showing that a K-pair network with a MIMO relay has K/2 degrees of
freedom. Thus, appropriate signaling in a two-hop scenario captures the degrees
of freedom without the need for the direct links. We then consider more
sophisticated encoding strategies in search of other ways to exploit the direct
links. Using a number of hybrid encoding strategies, we obtain non-asymptotic
achievable sum-rates. We investigate the case where the relay (unlike other
nodes) has access to abundant power, showing that when sources have power P and
the relay is allowed power proportional to O(P^2), the full degrees of freedom
K are available to the network.Comment: 7 double-column pages, 3 figures, accepted in IEEE Transactions on
Information Theor
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