79 research outputs found
Coverage and Rate Analysis for Co-Existing RF/VLC Downlink Cellular Networks
This paper provides a stochastic geometry framework to perform the coverage
and rate analysis of a typical user in co-existing VLC and RF networks covering
a large indoor area. The developed framework can be customized to capture the
performance of a typical user in various network configurations such as (i)
RF-only, in which only small base-stations (SBSs) are available to provide the
coverage to a user, (ii) VLC-only, in which only optical BSs (OBSs) are
available to provide the coverage to a user, (iii) opportunistic RF/VLC, where
a user selects the network with maximum received signal power, and (iv) hybrid
RF/VLC, where a user can simultaneously utilize the available resources from
both RF and VLC networks. The developed model for VLC network precisely
captures the impact of the field-of-view (FOV) of the photo-detector (PD)
receiver on the number of interferers, distribution of the aggregate
interference, association probability, and the coverage of a typical user.
Closed-form approximations are presented for special cases of practical
interest and for asymptotic scenarios such as when the intensity of SBSs
becomes very low. The derived expressions enable us to obtain closed-form
solutions for various network design parameters (such as intensity of OBSs and
SBSs, transmit power, and/or FOV) such that the number of active users can be
distributed optimally among RF and VLC networks. Also, we optimize the network
parameters in order to prioritize the association of users to VLC network.
Finally, simulations are carried out to verify the derived analytical
solutions. Important trade-offs between height and intensity of OBSs are
highlighted to optimize the performance of a user in VLC networks
Ambient RF Energy Harvesting in Ultra-Dense Small Cell Networks: Performance and Trade-offs
In order to minimize electric grid power consumption, energy harvesting from
ambient RF sources is considered as a promising technique for wireless charging
of low-power devices. To illustrate the design considerations of RF-based
ambient energy harvesting networks, this article first points out the primary
challenges of implementing and operating such networks, including
non-deterministic energy arrival patterns, energy harvesting mode selection,
energy-aware cooperation among base stations (BSs), etc. A brief overview of
the recent advancements and a summary of their shortcomings are then provided
to highlight existing research gaps and possible future research directions. To
this end, we investigate the feasibility of implementing RF-based ambient
energy harvesting in ultra-dense small cell networks (SCNs) and examine the
related trade-offs in terms of the energy efficiency and
signal-to-interference-plus-noise ratio (SINR) outage probability of a typical
user in the downlink. Numerical results demonstrate the significance of
deploying a mixture of on-grid small base stations (SBSs)~(powered by electric
grid) and off-grid SBSs~(powered by energy harvesting) and optimizing their
corresponding proportions as a function of the intensity of active SBSs in the
network.Comment: IEEE Wireless Communications, to appea
Multi-tier Drone Architecture for 5G/B5G Cellular Networks: Challenges, Trends, and Prospects
Drones (or unmanned aerial vehicles [UAVs]) are expected to be an important
component of fifth generation (5G)/beyond 5G (B5G) cellular architectures that
can potentially facilitate wireless broadcast or point-to-multipoint
transmissions. The distinct features of various drones such as the maximum
operational altitude, communication, coverage, computation, and endurance impel
the use of a multi-tier architecture for future drone-cell networks. In this
context, this article focuses on investigating the feasibility of multi-tier
drone network architecture over traditional single-tier drone networks and
identifying the scenarios in which drone networks can potentially complement
the traditional RF-based terrestrial networks. We first identify the challenges
associated with multi-tier drone networks as well as drone-assisted cellular
networks. We then review the existing state-of-the-art innovations in drone
networks and drone-assisted cellular networks. We then investigate the
performance of a multi-tier drone network in terms of spectral efficiency of
downlink transmission while illustrating the optimal intensity and altitude of
drones in different tiers numerically. Our results demonstrate the specific
network load conditions (i.e., ratio of user intensity and base station
intensity) where deployment of drones can be beneficial (in terms of spectral
efficiency of downlink transmission) for conventional terrestrial cellular
networks
Integral Approximations for Coverage Probability
This letter gives approximations to an integral appearing in the formula for
downlink coverage probability of a typical user in Poisson point process (PPP)
based stochastic geometry frameworks of the form . Four different approximations are studied. For
systems that are interference-limited or noise-limited, conditions are
identified when the approximations are valid. For intermediate cases, we
recommend the use of Laplace approximation. Numerical results validate the
accuracy of the approximations
Accuracy of Distance-Based Ranking of Users in the Analysis of NOMA Systems
We characterize the accuracy of analyzing the performance of a NOMA system
where users are ranked according to their distances instead of instantaneous
channel gains, i.e., product of distance-based path-loss and fading channel
gains. Distance-based ranking is analytically tractable and can lead to
important insights. However, it may not be appropriate in a multipath fading
environment where a near user suffers from severe fading while a far user
experiences weak fading. Since the ranking of users in a NOMA system has a
direct impact on coverage probability analysis, impact of the traditional
distance-based ranking, as opposed to instantaneous signal power-based ranking,
needs to be understood. This will enable us to identify scenarios where
distance-based ranking, which is easier to implement compared to instantaneous
signal power-based ranking, is acceptable for system performance analysis. To
this end, in this paper, we derive the probability of the event when
distance-based ranking yields the same results as instantaneous signal
power-based ranking, which is referred to as the accuracy probability. We
characterize the probability of accuracy considering Nakagami-m fading channels
and three different spatial distribution models of user locations in NOMA. We
illustrate the impact of accuracy probability on uplink and downlink coverage
probability
Analysis of SINR Outage in Large-Scale Cellular Networks Using Campbell's Theorem and Cumulant Generating Functions
The signal-to-noise-plus-interference ratio (SINR) outage probability is one
of the key performance parameters of a wireless cellular network, and its
analytical as well as numerical evaluation has occupied many researchers.
Recently, the introduction of stochastic geometric modeling of cellular
networks has brought the outage problem to the forefront again. A popular and
powerful approach is to exploit the available moment generating function (or
Laplace transform) of received signal and interference, whenever it exists, by
applying the Gil-Pelaez inversion formula. However, with the stochastic
geometric modeling, the moment generating function may either be too
complicated to exist in closed-form or at worst may not exist. Toward this end,
in this paper, we study two alternate ways of evaluating the SINR outage. In
the first case, we emphasize the significance of calculating cumulants over
moments and exploit the fact that the cumulants of point processes are easily
calculable using Campbell's theorem. The SINR outage is then analytically
characterized by Charlier expansion based on Gaussian and Student's
-distributions and their associated Hermite and Krishnamoorthy polynomials.
In the second case, we exploit the saddle point method, which gives a
semi-analytical method of calculating the SINR outage, whenever the cumulant
generating function of received signal and interference exists. For the purpose
of demonstration, we apply these techniques on a downlink cellular network
model where a typical user experiences a coordinated multi-point transmission,
and the base stations are modeled by homogeneous Poisson point process. For the
convenience of readers, we also provide a brief overview of moments, cumulants,
their generating functions, and Campbell's theorem, without invoking measure
theory. Numerical results illustrate the accuracy of the proposed mathematical
approaches
Meta Distribution of the SIR in Large-Scale Uplink and Downlink NOMA Networks
We develop an analytical framework to derive the meta distribution and
moments of the conditional success probability (CSP), which is defined as
{success probability for a given realization of the transmitters}, in
large-scale co-channel uplink and downlink non-orthogonal multiple access
(NOMA) networks with one NOMA cluster per cell. The moments of CSP translate to
various network performance metrics such as the standard success or
signal-to-interference ratio (SIR) coverage probability (which is the -st
moment), the mean local delay (which is the -st moment in a static network
setting), and the meta distribution (which is the complementary cumulative
distribution function of the conditional success probability and can be
approximated by using the -st and -nd moments). For uplink NOMA, to make
the framework tractable, we propose two point process models for the spatial
locations of the interferers by utilizing the base station (BS)/user pair
correlation function. We validate the proposed models by comparing the second
moment measure of each model with that of the actual point process for the
inter-cluster (or inter-cell) interferers obtained via simulations. For
downlink NOMA, we derive closed-form solutions for the moments of the CSP,
success (or coverage) probability, average local delay, and meta distribution
for the users. As an application of the developed analytical framework, we use
the closed-form expressions to optimize the power allocations for downlink NOMA
users in order to maximize the success probability of a given NOMA user with
and without latency constraints. Closed-form optimal solutions for the transmit
powers are obtained for two-user NOMA scenario. We note that maximizing the
success probability with latency constraints can significantly impact the
optimal power solutions for low SIR thresholds and favour orthogonal multiple
access (OMA)
Saddle Point Approximation for Outage Probability Using Cumulant Generating Functions
This letter proposes the use of saddle point approximation (SPA) to evaluate
the outage probability of wireless cellular networks. Unlike traditional
numerical integration-based approaches, the SPA approach relies on cumulant
generating functions (CGFs) and eliminates the need for explicit numerical
integration. The approach is generic and can be applied to a wide variety of
distributions, given that their CGFs exist. We illustrate the usefulness of SPA
on channel fading distributions such as Nakagami-, Nakagami- (Hoyt), and
Rician distributions. Numerical results validate the accuracy of the proposed
SPA approach.Comment: 4 pages, 4 figures, submitted to IEEE Wireless Communications Letter
Meta Distribution of SIR in Dual-Hop Internet-of-Things (IoT) Networks
This paper characterizes the meta distribution of the downlink
signal-to-interference ratio (SIR) attained at a typical Internet-of-Things
(IoT) device in a dual-hop IoT network. The IoT device associates with either a
serving macro base station (MBS) for direct transmissions or associates with a
decode and forward (DF) relay for dual-hop transmissions, depending on the
biased received signal power criterion. In contrast to the conventional success
probability, the meta distribution is the distribution of the conditional
success probability (CSP), which is conditioned on the locations of the
wireless transmitters. The meta distribution is a fine-grained performance
metric that captures important network performance metrics such as the coverage
probability and the mean local delay as its special cases. Specifically, we
derive the moments of the CSP in order to calculate analytic expressions for
the meta distribution. Further, we derive mathematical expressions for special
cases such as the mean local delay, variance of the CSP, and success
probability of a typical IoT device and typical relay with different offloading
biases. We take in consideration in our analysis the association probabilities
of IoT devices. Finally, we investigate the impact of increasing the relay
density on the mean local delay using numerical results
Downlink Power Control in Two-Tier Cellular Networks with Energy-Harvesting Small Cells as Stochastic Games
Energy harvesting in cellular networks is an emerging technique to enhance
the sustainability of power-constrained wireless devices. This paper considers
the co-channel deployment of a macrocell overlaid with small cells. The small
cell base stations (SBSs) harvest energy from environmental sources whereas the
macrocell base station (MBS) uses conventional power supply. Given a stochastic
energy arrival process for the SBSs, we derive a power control policy for the
downlink transmission of both MBS and SBSs such that they can achieve their
objectives (e.g., maintain the signal-to-interference-plus-noise ratio (SINR)
at an acceptable level) on a given transmission channel. We consider a
centralized energy harvesting mechanism for SBSs, i.e., there is a central
energy storage (CES) where energy is harvested and then distributed to the
SBSs. When the number of SBSs is small, the game between the CES and the MBS is
modeled as a single-controller stochastic game and the equilibrium policies are
obtained as a solution of a quadratic programming problem. However, when the
number of SBSs tends to infinity (i.e., a highly dense network), the
centralized scheme becomes infeasible, and therefore, we use a mean field
stochastic game to obtain a distributed power control policy for each SBS. By
solving a system of partial differential equations, we derive the power control
policy of SBSs given the knowledge of mean field distribution and the available
harvested energy levels in the batteries of the SBSs.Comment: IEEE Transactions on Communications, 201
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