5,257 research outputs found
Randomly Spread CDMA: Asymptotics via Statistical Physics
This paper studies randomly spread code-division multiple access (CDMA) and
multiuser detection in the large-system limit using the replica method
developed in statistical physics. Arbitrary input distributions and flat fading
are considered. A generic multiuser detector in the form of the posterior mean
estimator is applied before single-user decoding. The generic detector can be
particularized to the matched filter, decorrelator, linear MMSE detector, the
jointly or the individually optimal detector, and others. It is found that the
detection output for each user, although in general asymptotically non-Gaussian
conditioned on the transmitted symbol, converges as the number of users go to
infinity to a deterministic function of a "hidden" Gaussian statistic
independent of the interferers. Thus the multiuser channel can be decoupled:
Each user experiences an equivalent single-user Gaussian channel, whose
signal-to-noise ratio suffers a degradation due to the multiple-access
interference. The uncoded error performance (e.g., symbol-error-rate) and the
mutual information can then be fully characterized using the degradation
factor, also known as the multiuser efficiency, which can be obtained by
solving a pair of coupled fixed-point equations identified in this paper. Based
on a general linear vector channel model, the results are also applicable to
MIMO channels such as in multiantenna systems.Comment: To be published in IEEE Transactions on Information Theor
On the Capacity of Multilevel NAND Flash Memory Channels
In this paper, we initiate a first information-theoretic study on multilevel
NAND flash memory channels with intercell interference. More specifically, for
a multilevel NAND flash memory channel under mild assumptions, we first prove
that such a channel is indecomposable and it features asymptotic equipartition
property; we then further prove that stationary processes achieve its
information capacity, and consequently, as its order tends to infinity, its
Markov capacity converges to its information capacity; eventually, we establish
that its operational capacity is equal to its information capacity. Our results
suggest that it is highly plausible to apply the ideas and techniques in the
computation of the capacity of finite-state channels, which are relatively
better explored, to that of the capacity of multilevel NAND flash memory
channels.Comment: Submitted to IEEE Transactions on Information Theor
An Overview on Application of Machine Learning Techniques in Optical Networks
Today's telecommunication networks have become sources of enormous amounts of
widely heterogeneous data. This information can be retrieved from network
traffic traces, network alarms, signal quality indicators, users' behavioral
data, etc. Advanced mathematical tools are required to extract meaningful
information from these data and take decisions pertaining to the proper
functioning of the networks from the network-generated data. Among these
mathematical tools, Machine Learning (ML) is regarded as one of the most
promising methodological approaches to perform network-data analysis and enable
automated network self-configuration and fault management. The adoption of ML
techniques in the field of optical communication networks is motivated by the
unprecedented growth of network complexity faced by optical networks in the
last few years. Such complexity increase is due to the introduction of a huge
number of adjustable and interdependent system parameters (e.g., routing
configurations, modulation format, symbol rate, coding schemes, etc.) that are
enabled by the usage of coherent transmission/reception technologies, advanced
digital signal processing and compensation of nonlinear effects in optical
fiber propagation. In this paper we provide an overview of the application of
ML to optical communications and networking. We classify and survey relevant
literature dealing with the topic, and we also provide an introductory tutorial
on ML for researchers and practitioners interested in this field. Although a
good number of research papers have recently appeared, the application of ML to
optical networks is still in its infancy: to stimulate further work in this
area, we conclude the paper proposing new possible research directions
Capacity Analysis for Gaussian and Discrete Memoryless Interference Networks
Interference is an important issue for wireless communication systems where multiple
uncoordinated users try to access to a common medium. The problem is even more
crucial for next-generation cellular networks where frequency reuse becomes ever more
intense, leading to more closely placed co-channel cells. This thesis describes our attempt to understand the impact of interference on communication performance as well as optimal ways to handle interference. From the theoretical point of view, we examine how interference affects the fundamental performance limits, and provide insights on how interference should be treated for various channel models under different operating
conditions. From the practical design point of view, we provide solutions to improve the
system performance under unknown interference using multiple independent receptions
of the same information.
For the simple two-user Gaussian interference channel, we establish that the simple
Frequency Division Multiplexing (FDM) technique suffices to provide the optimal sum-
rate within the largest computable subregion of the general achievable rate region for a
certain interference range.
For the two-user discrete memoryless interference channels, we characterize different
interference regimes as well as the corresponding capacity results. They include one-
sided weak interference and mixed interference conditions. The sum-rate capacities are
derived in both cases. The conditions, capacity expressions, as well as the capacity achieving schemes are analogous to those of the Gaussian channel model. The study
also leads to new outer bounds that can be used to resolve the capacities of several new
discrete memoryless interference channels.
A three-user interference up-link transmission model is introduced. By examining how
interference affects the behavior of the performance limits, we capture the differences
and similarities between the traditional two-user channel model and the channel model
with more than two users. If the interference is very strong, the capacity region is just
a simple extension of the two-user case. For the strong interference case, a line segment
on the boundary of the capacity region is attained. When there are links with weak
interference, the performance limits behave very differently from that of the two-user
case: there is no single case that is found of which treating interference as noise is
optimal. In particular, for a subclass of Gaussian channels with mixed interference, a
boundary point of the capacity region is determined. For the Gaussian channel with
weak interference, sum capacities are obtained under various channel coefficients and
power constraint conditions. The optimalities in all the cases are obtained by decoding
part of the interference.
Finally, we investigate a topic that has practical ramifications in real communication
systems. We consider in particular a diversity reception system where independently
copies of low density parity check (LDPC) coded signals are received. Relying only on
non-coherent reception in a highly dynamic environment with unknown interference, soft-decision combining is achieved whose performance is shown to improve significantly over existing approaches that rely on hard decision combining
Wiretap and Gelfand-Pinsker Channels Analogy and its Applications
An analogy framework between wiretap channels (WTCs) and state-dependent
point-to-point channels with non-causal encoder channel state information
(referred to as Gelfand-Pinker channels (GPCs)) is proposed. A good sequence of
stealth-wiretap codes is shown to induce a good sequence of codes for a
corresponding GPC. Consequently, the framework enables exploiting existing
results for GPCs to produce converse proofs for their wiretap analogs. The
analogy readily extends to multiuser broadcasting scenarios, encompassing
broadcast channels (BCs) with deterministic components, degradation ordering
between users, and BCs with cooperative receivers. Given a wiretap BC (WTBC)
with two receivers and one eavesdropper, an analogous Gelfand-Pinsker BC (GPBC)
is constructed by converting the eavesdropper's observation sequence into a
state sequence with an appropriate product distribution (induced by the
stealth-wiretap code for the WTBC), and non-causally revealing the states to
the encoder. The transition matrix of the state-dependent GPBC is extracted
from WTBC's transition law, with the eavesdropper's output playing the role of
the channel state. Past capacity results for the semi-deterministic (SD) GPBC
and the physically-degraded (PD) GPBC with an informed receiver are leveraged
to furnish analogy-based converse proofs for the analogous WTBC setups. This
characterizes the secrecy-capacity regions of the SD-WTBC and the PD-WTBC, in
which the stronger receiver also observes the eavesdropper's channel output.
These derivations exemplify how the wiretap-GP analogy enables translating
results on one problem into advances in the study of the other
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