683 research outputs found
D2D Enhanced Heterogeneous Cellular Networks with Dynamic TDD
Over the last decade, the growing amount of UL and DL mobile data traffic has
been characterized by substantial asymmetry and time variations. Dynamic
time-division duplex (TDD) has the capability to accommodate to the traffic
asymmetry by adapting the UL/DL configuration to the current traffic demands.
In this work, we study a two-tier heterogeneous cellular network (HCN) where
the macro tier and small cell tier operate according to a dynamic TDD scheme on
orthogonal frequency bands. To offload the network infrastructure, mobile users
in proximity can engage in D2D communications, whose activity is determined by
a carrier sensing multiple access (CSMA) scheme to protect the ongoing
infrastructure-based and D2D transmissions. We present an analytical framework
to evaluate the network performance in terms of load-aware coverage probability
and network throughput. The proposed framework allows to quantify the effect on
the coverage probability of the most important TDD system parameters, such as
the UL/DL configuration, the base station density, and the bias factor. In
addition, we evaluate how the bandwidth partition and the D2D network access
scheme affect the total network throughput. Through the study of the tradeoff
between coverage probability and D2D user activity, we provide guidelines for
the optimal design of D2D network access.Comment: 15 pages; 9 figures; submitted to IEEE Transactions on Wireless
Communication
Dynamic Uplink-Downlink Optimization in TDD-based Small Cell Networks
Dynamic Time-division duplex (TDD) can provide efficient and flexible
splitting of the common wireless cellular resources between uplink (UL) and
downlink (DL) users. In this paper, the UL/DL optimization problem is
formulated as a noncooperative game among the small cell base stations (SCBSs)
in which each base station aims at minimizing its total UL and DL flow delays.
To solve this game, a self-organizing UL/DL resource configuration scheme for
TDD-based small cell networks is proposed. Using the proposed scheme, an SCBS
is able to estimate and learn the UL and DL loads autonomously while optimizing
its UL/DL configuration accordingly. Simulations results show that the proposed
algorithm achieves significant gains in terms of packet throughput in case of
asymmetric UL and DL traffic loads. This gain increases as the traffic
asymmetry increases, reaching up to 97% and 200% gains relative to random and
fixed duplexing schemes respectively. Our results also show that the proposed
algorithm is well- adapted to dynamic traffic conditions and different network
sizes, and operates efficiently in case of severe cross-link interference in
which neighboring cells transmit in opposite directions.Comment: In the IEEE 11th International Symposium on Wireless Communication
Systems (ISWCS) 201
Harvest the potential of massive MIMO with multi-layer techniques
Massive MIMO is envisioned as a promising technology for 5G wireless networks
due to its high potential to improve both spectral and energy efficiency.
Although the massive MIMO system is based on innovations in the physical layer,
the upper layer techniques also play important roles in harvesting the
performance gains of massive MIMO. In this article, we begin with an analysis
of the benefits and challenges of massive MIMO systems. We then investigate the
multi-layer techniques for incorporating massive MIMO in several important
network deployment scenarios. We conclude this article with a discussion of
open and potential problems for future research.Comment: IEEE Networ
On the Fundamental Characteristics of Ultra-Dense Small Cell Networks
In order to cope with the forecasted 1000x increase in wireless capacity
demands by 2030, network operators will aggressively densify their network
infrastructure to reuse the spectrum as much as possible. However, it is
important to realise that these new ultra-dense small cell networks are
fundamentally different from the traditional macrocell or sparse small cell
networks, and thus ultra-dense networks (UDNs) cannot be deployed and operated
in the same way as in the last 25 years. In this paper, we systematically
investigate and visualise the performance impacts of several fundamental
characteristics of UDNs, that mobile operators and vendors should consider when
deploying UDNs. Moreover, we also provide new deployment and management
guidelines to address the main challenges brought by UDNs in the future.Comment: To appear in IEEE Network Magazine. 1536-1276 copyright 2015 IEEE.
Personal use is permitted, but republication/redistribution requires IEEE
permission. Please find the final version in IEEE from the link:
http://ieeexplore.ieee.org/document/xxxxxxx/. Digital Object Identifier:
10.1109/TNM.2017.xxxxxx
Towards 1 Gbps/UE in Cellular Systems: Understanding Ultra-Dense Small Cell Deployments
Todays heterogeneous networks comprised of mostly macrocells and indoor small
cells will not be able to meet the upcoming traffic demands. Indeed, it is
forecasted that at least a 100x network capacity increase will be required to
meet the traffic demands in 2020. As a result, vendors and operators are now
looking at using every tool at hand to improve network capacity. In this epic
campaign, three paradigms are noteworthy, i.e., network densification, the use
of higher frequency bands and spectral efficiency enhancement techniques. This
paper aims at bringing further common understanding and analysing the potential
gains and limitations of these three paradigms, together with the impact of
idle mode capabilities at the small cells as well as the user equipment density
and distribution in outdoor scenarios. Special attention is paid to network
densification and its implications when transitioning to ultra-dense small cell
deployments. Simulation results show that network densification with an average
inter site distance of 35 m can increase the cell- edge UE throughput by up to
48x, while the use of the 10GHz band with a 500MHz bandwidth can increase the
network capacity up to 5x. The use of beamforming with up to 4 antennas per
small cell base station lacks behind with cell-edge throughput gains of up to
1.49x. Our study also shows how network densifications reduces multi-user
diversity, and thus proportional fair alike schedulers start losing their
advantages with respect to round robin ones. The energy efficiency of these
ultra-dense small cell deployments is also analysed, indicating the need for
energy harvesting approaches to make these deployments energy- efficient.
Finally, the top ten challenges to be addressed to bring ultra-dense small cell
deployments to reality are also discussed
Over-the-Air Time Synchronization for URLLC: Requirements, Challenges and Possible Enablers
Ultra-reliable and low-latency communications (URLLC) is an emerging feature
in 5G and beyond wireless systems, which is introduced to support stringent
latency and reliability requirements of mission-critical industrial
applications. In many potential applications, multiple sensors/actuators
collaborate and require isochronous operation with strict and bounded jitter,
e.g., \SI{1}{\micro\second}. To this end, network time synchronization becomes
crucial for real-time and isochronous communication between a controller and
the sensors/actuators. In this paper, we look at different applications in
factory automation and smart grids to reveal the requirements of device-level
time synchronization and the challenges in extending the high-granularity
timing information to the devices. Also, we identify the potential over-the-air
synchronization mechanisms in 5G radio interface, and discuss the needed
enhancements to meet the jitter constraints of time-sensitive URLLC
applications
Joint Downlink Cell Association and Bandwidth Allocation for Wireless Backhauling in Two-Tier HetNets with Large-Scale Antenna Arrays
The problem of joint downlink cell association (CA) and wireless backhaul
bandwidth allocation (WBBA) in two-tier cellular heterogeneous networks
(HetNets) is considered. Large-scale antenna array is implemented at the macro
base station (BS), while the small cells within the macro cell range are
single-antenna BSs and they rely on over-the-air links to the macro BS for
backhauling. A sum logarithmic user rate maximization problem is investigated
considering wireless backhauling constraints. A duplex and spectrum sharing
scheme based on co-channel reverse time-division duplex (TDD) and dynamic soft
frequency reuse (SFR) is proposed for interference management in two-tier
HetNets with large-scale antenna arrays at the macro BS and wireless
backhauling for small cells. Two in-band WBBA scenarios, namely, unified
bandwidth allocation and per-small-cell bandwidth allocation scenarios, are
investigated for joint CA-WBBA in the HetNet. A two-level hierarchical
decomposition method for relaxed optimization is employed to solve the
mixed-integer nonlinear program (MINLP). Solutions based on the General
Algorithm Modeling System (GAMS) optimization solver and fast heuristics are
also proposed for cell association in the per-small-cell WBBA scenario. It is
shown that when all small cells have to use in-band wireless backhaul, the
system load has more impact on both the sum log-rate and per-user rate
performance than the number of small cells deployed within the macro cell
range. The proposed joint CA-WBBA algorithms have an optimal load approximately
equal to the size of the large-scale antenna array at the macro BS. The cell
range expansion (CRE) strategy, which is an efficient cell association scheme
for HetNets with perfect backhauling, is shown to be inefficient when in-band
wireless backhauling for small cells comes into play.Comment: IEEE Transactions on Wireless Communications, to appea
Energy efficient D2D communications in dynamic TDD systems
Network-assisted device-to-device communication is a promising technology for
improving the performance of proximity-based services. This paper demonstrates
how the integration of device-to-device communications and dynamic
time-division duplex can improve the energy efficiency of future cellular
networks, leading to a greener system operation and a prolonged battery
lifetime of mobile devices. We jointly optimize the mode selection,
transmission period and power allocation to minimize the energy consumption
(from both a system and a device perspective) while satisfying a certain rate
requirement. The radio resource management problems are formulated as
mixed-integer nonlinear programming problems. Although they are known to be
NP-hard in general, we exploit the problem structure to design efficient
algorithms that optimally solve several problem cases. For the remaining cases,
a heuristic algorithm that computes near-optimal solutions while respecting
practical constraints on execution times and signaling overhead is also
proposed. Simulation results confirm that the combination of device-to-device
and flexible time-division-duplex technologies can significantly enhance
spectrum and energy-efficiency of next generation cellular systems.Comment: Submitted to IEEE Journal of Selected Areas in Communication
User Selection and Power Allocation in Full Duplex Multi-Cell Networks
Full duplex (FD) communications has the potential to double the capacity of a
half duplex (HD) system at the link level. However, in a cellular network, FD
operation is not a straightforward extension of half duplex operations. The
increased interference due to a large number of simultaneous transmissions in
FD operation and realtime traffic conditions limits the capacity improvement.
Realizing the potential of FD requires careful coordination of resource
allocation among the cells as well as within the cell. In this paper, we
propose a distributed resource allocation, i.e., joint user selection and power
allocation for a FD multi-cell system, assuming FD base stations (BSs) and HD
user equipment (UEs). Due to the complexity of finding the globally optimum
solution, a sub-optimal solution for UE selection, and a novel geometric
programming based solution for power allocation, are proposed. The proposed
distributed approach converges quickly and performs almost as well as a
centralized solution, but with much lower signaling overhead. It provides a
hybrid scheduling policy which allows FD operations whenever it is
advantageous, but otherwise defaults to HD operation. We focus on small cell
systems because they are more suitable for FD operation, given practical
self-interference cancellation limits.With practical self-interference
cancellation, it is shown that the proposed hybrid FD system achieves nearly
two times throughput improvement for an indoor multi-cell scenario, and about
65% improvement for an outdoor multi-cell scenario compared to the HD system.Comment: 15 pages, to be published in IEEE Transactions on Vehicular
Technology, 2016. arXiv admin note: text overlap with arXiv:1412.870
Dynamic Joint Uplink and Downlink Optimization for Uplink and Downlink Decoupling-Enabled 5G Heterogeneous Networks
The concept of user-centric and personalized service in the fifth generation
(5G) mobile networks encourages technical solutions such as dynamic asymmetric
uplink/downlink resource allocation and elastic association of cells to users
with decoupled uplink and downlink (DeUD) access. In this paper we develop a
joint uplink and downlink optimization algorithm for DeUD-enabled wireless
networks for adaptive joint uplink and downlink bandwidth allocation and power
control, under different link association policies. Based on a general model of
inter-cell interference, we propose a three-step optimization algorithm to
jointly optimize the uplink and downlink bandwidth allocation and power
control, using the fixed point approach for nonlinear operators with or without
monotonicity, to maximize the minimum level of quality of service satisfaction
per link, subjected to a general class of resource (power and bandwidth)
constraints. We present numerical results illustrating the theoretical findings
for network simulator in a real-world setting, and show the advantage of our
solution compared to the conventional proportional fairness resource allocation
schemes in both the coupled uplink and downlink (CoUD) access and the novel
link association schemes in DeUD.Comment: 17 pages, 8 figure
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