6,563 research outputs found
The Ergodic Capacity of Phase-Fading Interference Networks
We identify the role of equal strength interference links as bottlenecks on
the ergodic sum capacity of a user phase-fading interference network, i.e.,
an interference network where the fading process is restricted primarily to
independent and uniform phase variations while the channel magnitudes are held
fixed across time. It is shown that even though there are cross-links,
only about disjoint and equal strength interference links suffice to
determine the capacity of the network regardless of the strengths of the rest
of the cross channels. This scenario is called a \emph{minimal bottleneck
state}. It is shown that ergodic interference alignment is capacity optimal for
a network in a minimal bottleneck state. The results are applied to large
networks. It is shown that large networks are close to bottleneck states with a
high probability, so that ergodic interference alignment is close to optimal
for large networks. Limitations of the notion of bottleneck states are also
highlighted for channels where both the phase and the magnitudes vary with
time. It is shown through an example that for these channels, joint coding
across different bottleneck states makes it possible to circumvent the capacity
bottlenecks.Comment: 19 page
Interference Mitigation in Large Random Wireless Networks
A central problem in the operation of large wireless networks is how to deal
with interference -- the unwanted signals being sent by transmitters that a
receiver is not interested in. This thesis looks at ways of combating such
interference.
In Chapters 1 and 2, we outline the necessary information and communication
theory background, including the concept of capacity. We also include an
overview of a new set of schemes for dealing with interference known as
interference alignment, paying special attention to a channel-state-based
strategy called ergodic interference alignment.
In Chapter 3, we consider the operation of large regular and random networks
by treating interference as background noise. We consider the local performance
of a single node, and the global performance of a very large network.
In Chapter 4, we use ergodic interference alignment to derive the asymptotic
sum-capacity of large random dense networks. These networks are derived from a
physical model of node placement where signal strength decays over the distance
between transmitters and receivers. (See also arXiv:1002.0235 and
arXiv:0907.5165.)
In Chapter 5, we look at methods of reducing the long time delays incurred by
ergodic interference alignment. We analyse the tradeoff between reducing delay
and lowering the communication rate. (See also arXiv:1004.0208.)
In Chapter 6, we outline a problem that is equivalent to the problem of
pooled group testing for defective items. We then present some new work that
uses information theoretic techniques to attack group testing. We introduce for
the first time the concept of the group testing channel, which allows for
modelling of a wide range of statistical error models for testing. We derive
new results on the number of tests required to accurately detect defective
items, including when using sequential `adaptive' tests.Comment: PhD thesis, University of Bristol, 201
Error exponent of amplify and forward relay networks in presence of I.I.D. interferers
© 2014 IEEE. In this paper, we derive the random coding error exponent of amplify-and-forward (AF) relay networks in presence of arbitrary number of independent and identically distributed (i.i.d.) interferers both at the relay and the destination. Multiuser networks are common examples of interference limited networks. We derive the ergodic capacity of the network and present simulation results on the performance of the network where we compare the capacity and error exponent performance of interference limited networks with noise limited networks. Numerical results show that noise limited networks outperform interference limited networks even when only a very few interferers exist in the network
Asymptotic analysis of precoded small cell networks
International audienceIn this paper, we study precoded MIMO based small cell networks. We derive the theoretical sum-rate capacity, when multi-antenna base stations transmit precoded information to its multiple single-antenna users in the presence of inter- cell interference from neighboring cells. Due to an interference limited scenario, increasing the number of antennas at the base stations does not yield necessarily a linear increase of the capacity. We assess exactly the effect of multi-cell interference on the capacity gain for a given interference level. We use recent tools from random matrix theory to obtain the ergodic sum-rate capacity, as the number of antennas at the base station, number of users grow large. Simulations confirm the theoretical claims and also indicate that in most scenarios the asymptotic derivations applied to a finite number of users give good approximations of the actual ergodic sum-rate capacity
Performance Analysis of SSK-NOMA
In this paper, we consider the combination between two promising techniques:
space-shift keying (SSK) and non-orthogonal multiple access (NOMA) for future
radio access networks. We analyze the performance of SSK-NOMA networks and
provide a comprehensive analytical framework of SSK-NOMA regarding bit error
probability (BEP), ergodic capacity and outage probability. It is worth
pointing out all analysis also stand for conventional SIMO-NOMA networks. We
derive closed-form exact average BEP (ABEP) expressions when the number of
users in a resource block is equal to i.e., . Nevertheless, we analyze the
ABEP of users when the number of users is more than i.e., , and derive
bit-error-rate (BER) union bound since the error propagation due to iterative
successive interference canceler (SIC) makes the exact analysis intractable.
Then, we analyze the achievable rate of users and derive exact ergodic capacity
of the users so the ergodic sum rate of the system in closed-forms. Moreover,
we provide the average outage probability of the users exactly in the
closed-form. All derived expressions are validated via Monte Carlo simulations
and it is proved that SSK-NOMA outperforms conventional NOMA networks in terms
of all performance metrics (i.e., BER, sum rate, outage). Finally, the effect
of the power allocation (PA) on the performance of SSK-NOMA networks is
investigated and the optimum PA is discussed under BER and outage constraints
Capacity limits of bursty interference channels
Mención Internacional en el tÃtulo de doctorThis dissertation studies the effects of interference burstiness in the transmission
of data in wireless networks. In particular, we investigate the effects of
this phenomenon on the largest data rate at which one can communicate with a
vanishing small probability of error, i.e., on channel capacity. Specifically, we study
the capacity of two different channel models as described in the next sections.
Linear deterministic bursty interference channel.
First, we consider a two-user linear deterministic bursty interference channel (IC),
where the presence or absence of interference is modeled by a block- independent
and identically distributed (IID) Bernoulli process that stays constant for a
duration of T consecutive symbols (this is sometimes referred to as a coherence
block) and then changes independently to a new interference state. We assume
that the channel coefficients of the communication and interference links remain
constant during the whole message transmission. For this channel, we consider
both its quasi-static setup where the interference state remains constant during
the whole transmission of the codeword (which corresponds to the case whether
the blocklength N is smaller than T) and its ergodic setup where a codeword
spans several coherence blocks. For the quasi-static setup, we follow the seminal
works by Khude, Prabhakaran and Viswanath and study the largest sum rate of
a coding strategy that provides reliable communication at a basic (or worstcase)
rate R and allows an increased (opportunistic) rate ΔR in absence of interference.
For the ergodic scenario, we study the largest achievable sum rate as commonly
considered in the multi-user information theory literature. We study how (noncausal)
knowledge of the interference state, referred to as channel state information
(CSI), affects the sum capacity. Specifically, for both scenarios, we derive converse
and achievability bounds on the sum capacity for (i) local CSI at the receiverside
only; (ii) when each transmitter and receiver has local CSI, and (iii) global CSI
at all nodes, assuming both that interference states are independent of each other
and that they are fully correlated. Our bounds allow us to identify regions and
conditions where interference burstiness is beneficial and in which scenarios global CSI improves upon local CSI. Specifically, we show the following:
• Exploiting burstiness: For the quasi-static scenario we have shown that
in presence of local CSI, burstiness is only beneficial if the interference
region is very weak or weak. In contrast, for global CSI, burstiness is
beneficial for all interference regions, except the very strong interference
region, where the sum capacity corresponds to that of two parallel channels
without interference. For the ergodic scenario, we have shown that, under
global CSI, burstiness is beneficial for all interference regions and all possible
values of p. For local CSI at the receiver-side only, burstiness is beneficial for
all values of p and for very weak and weak interference regions. However, for
moderate and strong interference regions, burstiness is only of clear benefit
if the interference is present at most half of the time.
• Exploiting CSI: For the quasi-static scenario, local CSI at the transmitter is
not beneficial. This is in stark contrast to the ergodic scenario, where local
CSI at the transmitter-side is beneficial. Intuitively, in the ergodic scenario
the input distributions depend on the realizations of the interference states.
Hence, adapting the input distributions to these realizations increases the
sum capacity. In contrast, in the quasi-static case, the worst-case scenario
(presence of interference) and the best-case scenario (absence of interference)
are treated separately. Hence, there is no difference to the case of having
local CSI only at the receiver side. Featuring global CSI at all nodes yields
an increased sum rate for both the quasi-static and the ergodic scenarios.
The joint treatment of the quasi-static and the ergodic scenarios allows us to
thoroughly compare the sum capacities of these two scenarios. While the converse
bounds for the quasi-static scenario and local CSI at the receiver-side appeared
before in the literature, we present a novel proof based on an information density
approach and the Verd´u-Han lemma. This approach does not only allow for
rigorous yet clear proofs, it also enables more refined analyses of the probabilities
of error that worst-case and opportunistic messages can be decoded correctly.
For the converse bounds in the ergodic scenario, we use Fano’s inequality as the
standard approach to derive converse bounds in the multi-user information theory literature.
Bursty noncoherent wireless networks.
The linear deterministic model can be viewed as a rough approximation of a
fading channel, which has additive and multiplicative noise. The multiplicative
noise is referred to as fading. As we have seen in the previous section, the linear
deterministic model provides a rough understanding of the effects of interference
burstiness on the capacity of the two-user IC. Now, we extend our analysis to a
wireless network with a very large number of users and we do not approximate
the fading channel by a linear deterministic model. That is, we consider a memoryless
flat-fading channel with an infinite number of interferers. We incorporate
interference burstiness by an IID Bernoulli process that stays constant during the
whole transmission of the codeword.
The channel capacity of wireless networks is often studied under the assumption
that the communicating nodes have perfect knowledge of the fading coefficients in
the network. However, it is prima-facie unclear whether this perfect knowledge
of the channel coefficients can actually be obtained in practical systems. For
this reason, we study in this dissertation the channel capacity of a noncoherent
model where the nodes do not have perfect knowledge of the fading coefficients.
More precisely, we assume that the nodes know only the statistics of the channel
coefficients but not their realizations. We further assume that the interference
state (modeling interference burstiness) is known non-causally at the receiver-side
only. To the best of our knowledge, one of the few works that studies the capacity
of noncoherent wireless networks (without considering interference burstiness)
is by Lozano, Heath, and Andrews. Inter alia, Lozano et al. show that in the
absence of perfect knowledge of the channel coefficients, if the channel inputs
are given by the square-root of the transmit power times a power-independent
random variable, and if interference is always present (hence, it is non-bursty),
then the achievable information rate is bounded in the signal-to-noise ratio (SNR).
However, the considered inputs do not necessarily achieve capacity, so one may
argue that the information rate is bounded in the SNR because of the suboptimal
input distribution. Therefore, in our analysis, we allow the input distribution
to change arbitrarily with the SNR. We analyze the asymptotic behavior of the
channel capacity in the limit as the SNR tends to infinity. We assume that all
nodes (transmitting and interfering) use the same codebook. This implies that
each node is transmitting at the same rate, while at the same time it keeps the analysis tractable. We demonstrate that if the nodes do not cooperate and if the
variances of the path gains decay exponentially or slower, then the achievable
information rate remains bounded in the SNR, even if the input distribution
is allowed to change arbitrarily with the transmit power, irrespective of the
interference burstiness. Specifically, for this channel, we show the following:
• The channel capacity is bounded in the SNR. This suggests that noncoherent
wireless networks are extremely power inefficient at high SNR.
• Our bound further shows that interference burstiness does not change the
behavior of channel capacity. While our upper bound on the channel capacity
grows as the channel becomes more bursty, it remains bounded in the SNR.
Thus, interference burstiness cannot be exploited to mitigate the power
inefficiency at high SNR.
Possible strategies that could mitigate the power inefficiency of noncoherent
wireless networks and that have not been explored in this thesis are cooperation
between users and improved channel estimation strategies. Indeed,
coherent wireless networks, in which users have perfect knowledge of the
fading coefficients, have a capacity that grows to infinity with the SNR.
Furthermore, for such networks, the most efficient transmission strategies,
such as interference alignment, rely on cooperation. Our results suggest that
these two strategies may be essential to obtain an unbounded capacity in the SNR.Programa Oficial de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan CarlosPresidente: Ignacio SantamarÃa Caballero.- Secretario: David RamÃrez GarcÃa, David.- Vocal: Paul de Kerre
Interference Analysis for Vehicle-to-Vehicle Communications at 28 GHz
High capacity and ultra-reliable vehicular communication are going to be important aspects of beyond 5G communication networks. However, the vehicular communication problem becomes complex at a large scale when vehicles are roaming on the road, while simultaneously communicating with each other. Moreover, at higher frequencies (like 28 GHz), the dynamics of vehicular communication completely shift towards unpredictability and low-reliability. These factors may result in high packet error and a large amount of interference, resulting in regular disruptions in communications. A thorough understanding of performance variations is the key to moving towards the next generation of vehicular networks. With this intent, this article aims to provide a comprehensive interference analysis, wherein the closed-form expressions of packet error probability (PEP) and ergodic capacity are derived. Using the expression of the PEP, diversity analysis is provided which unveils the impact of channel nonlinearities on the performance of interference-constrained vehicular networks. The insights provided here are expected to pave the way for reliable and high capacity vehicular communication networks
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Analysis of millimeter wave ad hoc networks
Over the coming few years, the next-generation of wireless networks will be standardized and defined. Ad hoc networks, which operate without expensive infrastructure, are desirable for use cases such as military networks or disaster relief. Millimeter wave (mmWave) technology may enable high speed ad hoc networks. Directional antennas and building blockage limit the received interference power while the huge bandwidth enables high data rates. For this reason, understanding the interference and network performance of mmWave ad hoc networks is crucial for next-generation network design.
In my first contribution, I derive the SINR complementary cumulative distribution function (CCDF) for a random single-hop mmWave ad hoc network. These base results are used to further give insights in mmWave ad hoc networks. The SINR distribution is used to compute the transmission capacity of a mmWave ad hoc network using a Taylor bound. The CDF of the interference to noise ratio (INR) is also derived which shows that mmWave ad hoc networks are line-of-sight interference limited. I extend my work in the second contribution to include general clustered Poisson point processes to derive insights in the effect of different spatial interference patterns. Using the developed framework, I derive the ergodic rate of both spatially uniform and cluster mmWave ad hoc networks. I develop scaling trends for the antenna array size to keep the ergodic rate constant. The impact of beam alignment is computed in the final part of the contribution. Finally, I account for the overhead of beam alignment in mmWave ad hoc networks. The final contribution leverages the first two contributions to derive the expected training time a mmWave ad hoc network must perform before data transmission occurs. The results show that the optimal conditions for minimizing the training time are different than the optimal conditions for maximizing rate.Electrical and Computer Engineerin
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