722 research outputs found
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
On User Association in Multi-Tier Full-Duplex Cellular Networks
We address the user association problem in multi-tier in-band full-duplex
(FD) networks. Specifically, we consider the case of decoupled user association
(DUA) in which users (UEs) are not necessarily served by the same base station
(BS) for uplink (UL) and downlink (DL) transmissions. Instead, UEs can
simultaneously associate to different BSs based on two independent weighted
path-loss user association criteria for UL and DL. We use stochastic geometry
to develop a comprehensive modeling framework for the proposed system model
where BSs and UEs are spatially distributed according to independent point
processes. We derive closed-form expressions for the mean rate utility in FD,
half-duplex (HD) DL, and HD UL networks as well as the mean rate utility of
legacy nodes with only HD capabilities in a multi-tier FD network. We formulate
and solve an optimization problem that aims at maximizing the mean rate utility
of the FD network by optimizing the DL and UL user association criteria. We
investigate the effects of different network parameters including the spatial
density of BSs and power control parameter. We also investigate the effect of
imperfect self-interference cancellation (SIC) and show that it is more severe
at UL, where there exist minimum required SIC capabilities for BSs and UEs for
which FD networks are preferable to HD networks; otherwise, HD networks are
preferable. In addition, we discuss several special cases and provide
guidelines on the possible extensions of the proposed framework. We conclude
that DUA outperforms coupled user association (CUA) in which UEs associate to
the same BS for both UL and DL transmissions
Outage Analysis of Uplink Two-tier Networks
Employing multi-tier networks is among the most promising approaches to
address the rapid growth of the data demand in cellular networks. In this
paper, we study a two-tier uplink cellular network consisting of femtocells and
a macrocell. Femto base stations, and femto and macro users are assumed to be
spatially deployed based on independent Poisson point processes. We consider an
open access assignment policy, where each macro user based on the ratio between
its distances from its nearest femto access point (FAP) and from the macro base
station (MBS) is assigned to either of them. By tuning the threshold, this
policy allows controlling the coverage areas of FAPs. For a fixed threshold,
femtocells coverage areas depend on their distances from the MBS; Those closest
to the fringes will have the largest coverage areas. Under this open-access
policy, ignoring the additive noise, we derive analytical upper and lower
bounds on the outage probabilities of femto users and macro users that are
subject to fading and path loss. We also study the effect of the distance from
the MBS on the outage probability experienced by the users of a femtocell. In
all cases, our simulation results comply with our analytical bounds
Modeling, Analysis and Design for Carrier Aggregation in Heterogeneous Cellular Networks
Carrier aggregation (CA) and small cells are two distinct features of
next-generation cellular networks. Cellular networks with small cells take on a
very heterogeneous characteristic, and are often referred to as HetNets. In
this paper, we introduce a load-aware model for CA-enabled \textit{multi}-band
HetNets. Under this model, the impact of biasing can be more appropriately
characterized; for example, it is observed that with large enough biasing, the
spectral efficiency of small cells may increase while its counterpart in a
fully-loaded model always decreases. Further, our analysis reveals that the
peak data rate does not depend on the base station density and transmit powers;
this strongly motivates other approaches e.g. CA to increase the peak data
rate. Last but not least, different band deployment configurations are studied
and compared. We find that with large enough small cell density, spatial reuse
with small cells outperforms adding more spectrum for increasing user rate.
More generally, universal cochannel deployment typically yields the largest
rate; and thus a capacity loss exists in orthogonal deployment. This
performance gap can be reduced by appropriately tuning the HetNet coverage
distribution (e.g. by optimizing biasing factors).Comment: submitted to IEEE Transactions on Communications, Nov. 201
Downlink Coordinated Multi-Point with Overhead Modeling in Heterogeneous Cellular Networks
Coordinated multi-point (CoMP) communication is attractive for heterogeneous
cellular networks (HCNs) for interference reduction. However, previous
approaches to CoMP face two major hurdles in HCNs. First, they usually ignore
the inter-cell overhead messaging delay, although it results in an irreducible
performance bound. Second, they consider the grid or Wyner model for base
station locations, which is not appropriate for HCN BS locations which are
numerous and haphazard. Even for conventional macrocell networks without
overlaid small cells, SINR results are not tractable in the grid model nor
accurate in the Wyner model. To overcome these hurdles, we develop a novel
analytical framework which includes the impact of overhead delay for CoMP
evaluation in HCNs. This framework can be used for a class of CoMP schemes
without user data sharing. As an example, we apply it to downlink CoMP
zero-forcing beamforming (ZFBF), and see significant divergence from previous
work. For example, we show that CoMP ZFBF does not increase throughput when the
overhead channel delay is larger than 60% of the channel coherence time. We
also find that, in most cases, coordinating with only one other cell is nearly
optimum for downlink CoMP ZFBF.Comment: 27 pages, 8 figure
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
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
Joint Resource Allocation for eICIC in Heterogeneous Networks
Interference coordination between high-power macros and low-power picos
deeply impacts the performance of heterogeneous networks (HetNets). It should
deal with three challenges: user association with macros and picos, the amount
of almost blank subframe (ABS) that macros should reserve for picos, and
resource block (RB) allocation strategy in each eNB. We formulate the three
issues jointly for sum weighted logarithmic utility maximization while
maintaining proportional fairness of users. A class of distributed algorithms
are developed to solve the joint optimization problem. Our framework can be
deployed for enhanced inter-cell interference coordination (eICIC) in existing
LTE-A protocols. Extensive evaluation are performed to verify the effectiveness
of our algorithms.Comment: Accepted by Globecom 201
User-Centric Interference Nulling in Downlink Multi-Antenna Heterogeneous Networks
In heterogeneous networks (HetNets), strong interference due to spectrum
reuse affects each user's signal-to-interference ratio (SIR), and hence is one
limiting factor of network performance. In this paper, we propose a
user-centric interference nulling (IN) scheme in a downlink large-scale HetNet
to improve coverage/outage probability by improving each user's SIR. This IN
scheme utilizes at most maximum IN degree of freedom (DoF) at each macro-BS to
avoid interference to uniformly selected macro (pico) users with
signal-to-individual-interference ratio (SIIR) below a macro (pico) IN
threshold, where the maximum IN DoF and the two IN thresholds are three design
parameters. Using tools from stochastic geometry, we first obtain a tractable
expression of the coverage (equivalently outage) probability. Then, we analyze
the asymptotic coverage/outage probability in the low and high SIR threshold
regimes. The analytical results indicate that the maximum IN DoF can affect the
order gain of the outage probability in the low SIR threshold regime, but
cannot affect the order gain of the coverage probability in the high SIR
threshold regime. Moreover, we characterize the optimal maximum IN DoF which
optimizes the asymptotic coverage/outage probability. The optimization results
reveal that the IN scheme can linearly improve the outage probability in the
low SIR threshold regime, but cannot improve the coverage probability in the
high SIR threshold regime. Finally, numerical results show that the proposed
scheme can achieve good gains in coverage/outage probability over a maximum
ratio beamforming scheme and a user-centric almost blank subframes (ABS)
scheme.Comment: Transactions on Wireless Communications (under revision). arXiv admin
note: text overlap with arXiv:1504.0528
MmWave UAV Networks with Multi-cell Association: Performance Limit and Optimization
This paper aims to exploit the fundamental limits on the downlink coverage
and spatial throughput performances of a cellular network comprised of a tier
of unmanned aerial vehicle (UAV) base stations (BSs) using the millimeter wave
(mmWave) band and a tier of ground BSs using the ultra high frequency (UHF)
band. To reduce handover signaling overhead, the ground BSs take charge of
control signaling delivery whereas the UAVs are in charge of payload data
transmission so that users need to be simultaneously associated with a ground
BS and a UAV in this network with a control-data plane-split architecture. We
first propose a three-dimensional (3D) location distribution model of the UAVs
using stochastic geometry which is able to generally characterize the positions
of the UAVs in the sky. Using this 3D distribution model of UAVs, two
performance metrics, i.e., multi-cell coverage probability and volume spectral
efficiency, are proposed. Their explicit low-complexity expressions are derived
and their upper limits are found when each of the UAVs and ground BSs is
equipped with a massive antenna array. We further show that the multi-cell
coverage probability and the volume spectral efficiency can be maximized by
optimally deploying and positioning the UAVs in the sky and thereby their
fundamental maximal limits are found. These important analytical findings are
validated by numerical simulations.Comment: 17pages, 1 table, 8 figures, journal submissio
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