115 research outputs found
MISO in Ultra-Dense Networks: Balancing the Tradeoff between User and System Performance
With over-deployed network infrastructures, network densification is shown to
hinder the improvement of user experience and system performance. In this
paper, we adopt multi-antenna techniques to overcome the bottleneck and
investigate the performance of single-user beamforming, an effective method to
enhance desired signal power, in small cell networks from the perspective of
user coverage probability (CP) and network spatial throughput (ST).
Pessimistically, it is proved that, even when multi-antenna techniques are
applied, both CP and ST would be degraded and even asymptotically diminish to
zero with the increasing base station (BS) density. Moreover, the results also
reveal that the increase of ST is at the expense of the degradation of CP.
Therefore, to balance the tradeoff between user and system performance, we
further study the critical density, under which ST could be maximized under the
CP constraint. Accordingly, the impact of key system parameters on critical
density is quantified via the derived closed-form expression. Especially, the
critical density is shown to be inversely proportional to the square of antenna
height difference between BSs and users. Meanwhile, single-user beamforming,
albeit incapable of improving CP and ST scaling laws, is shown to significantly
increase the critical density, compared to the single-antenna regime.Comment: for journal submissio
Limitation of SDMA in Ultra-Dense Small Cell Networks
Benefitting from multi-user gain brought by multi-antenna techniques, space
division multiple access (SDMA) is capable of significantly enhancing spatial
throughput (ST) in wireless networks. Nevertheless, we show in this letter
that, even when SDMA is applied, ST would diminish to be zero in ultra-dense
networks (UDN), where small cell base stations (BSs) are fully densified. More
importantly, we compare the performance of SDMA, single-user beamforming
(SU-BF) (one user is served in each cell) and full SDMA (the number of served
users equals the number of equipped antennas). Surprisingly, it is shown that
SU-BF achieves the highest ST and critical density, beyond which ST starts to
degrade, in UDN. The results in this work could shed light on the fundamental
limitation of SDMA in UDN
The Impact of Antenna Height Difference on the Performance of Downlink Cellular Networks
Capable of significantly reducing cell size and enhancing spatial reuse,
network densification is shown to be one of the most dominant approaches to
expand network capacity. Due to the scarcity of available spectrum resources,
nevertheless, the over-deployment of network infrastructures, e.g., cellular
base stations (BSs), would strengthen the inter-cell interference as well, thus
in turn deteriorating the system performance. On this account, we investigate
the performance of downlink cellular networks in terms of user coverage
probability (CP) and network spatial throughput (ST), aiming to shed light on
the limitation of network densification. Notably, it is shown that both CP and
ST would be degraded and even diminish to be zero when BS density is
sufficiently large, provided that practical antenna height difference (AHD)
between BSs and users is involved to characterize pathloss. Moreover, the
results also reveal that the increase of network ST is at the expense of the
degradation of CP. Therefore, to balance the tradeoff between user and network
performance, we further study the critical density, under which ST could be
maximized under the CP constraint. Through a special case study, it follows
that the critical density is inversely proportional to the square of AHD. The
results in this work could provide helpful guideline towards the application of
network densification in the next-generation wireless networks.Comment: conference submission - Mar. 201
Correlations of Interference and Link Successes in Heterogeneous Cellular Networks
In heterogeneous cellular networks (HCNs), the interference received at a
user is correlated over time slots since it comes from the same set of randomly
located BSs. This results in the correlations of link successes, thus affecting
network performance. Under the assumptions of a K-tier Poisson network,
strongest-candidate based BS association, and independent Rayleigh fading, we
first quantify the correlation coefficients of interference. We observe that
the interference correlation is independent of the number of tiers, BS density,
SIR threshold, and transmit power. Then, we study the correlations of link
successes in terms of the joint success probability over multiple time slots.
We show that the joint success probability is decided by the success
probability in a single time slot and a diversity polynomial, which represents
the temporal interference correlation. Moreover, the parameters of HCNs have an
important influence on the joint success probability by affecting the success
probability in a single time slot. Particularly, we obtain the condition under
which the joint success probability increases with the BS density and transmit
power. We further show that the conditional success probability given prior
successes only depends on the path loss exponent and the number of time slots.Comment: 26 pages, 6 figure
Effect of Densification on Cellular Network Performance with Bounded Pathloss Model
In this paper, we investigate how network densification influences the
performance of downlink cellular network in terms of coverage probability (CP)
and area spectral efficiency (ASE). Instead of the simplified unbounded
pathloss model (UPM), we apply a more realistic bounded pathloss model (BPM) to
model the decay of signal power caused by pathloss. It is shown that network
densification indeed degrades CP when the base station (BS) density
is sufficiently large. This is inconsistent with the result derived using UPM
that CP is independent of . Moreover, we shed light on the impact of
ultra-dense deployment of BSs on the ASE scaling law. Specifically, it is
proved that the cellular network ASE scales with rate , i.e., first increases with and then diminishes
to be zero as goes to infinity.Comment: submitted to IEEE Commun. Let
Network Densification in 5G: From the Short-Range Communications Perspective
Besides advanced telecommunications techniques, the most prominent evolution
of wireless networks is the densification of network deployment. In particular,
the increasing access points/users density and reduced cell size significantly
enhance spatial reuse, thereby improving network capacity. Nevertheless, does
network ultra-densification and over-deployment always boost the performance of
wireless networks? Since the distance from transmitters to receivers is greatly
reduced in dense networks, signal is more likely to be propagated from long- to
short-range region. Without considering short-range propagation features,
conventional understanding of the impact of network densification becomes
doubtful. With this regard, it is imperative to reconsider the pros and cons
brought by network densification. In this article, we first discuss the
short-range propagation features in densely deployed network and verify through
experimental results the validity of the proposed short-range propagation
model. Considering short-range propagation, we further explore the fundamental
impact of network densification on network capacity, aided by which a concrete
interpretation of ultra-densification is presented from the network capacity
perspective. Meanwhile, as short-range propagation makes interference more
complicated and difficult to handle, we discuss possible approaches to further
enhance network capacity in ultra-dense wireless networks. Moreover, key
challenges are presented to suggest future directions.Comment: submitted to IEEE Commun. Ma
Modeling and Analysis of SCMA Enhanced D2D and Cellular Hybrid Network
Sparse code multiple access (SCMA) has been recently proposed for the future
wireless networks, which allows non-orthogonal spectrum resource sharing and
enables system overloading. In this paper, we apply SCMA into device-to-device
(D2D) communication and cellular hybrid network, targeting at using the
overload feature of SCMA to support massive device connectivity and expand
network capacity. Particularly, we develop a stochastic geometry based
framework to model and analyze SCMA, considering underlaid and overlaid mode.
Based on the results, we analytically compare SCMA with orthogonal
frequency-division multiple access (OFDMA) using area spectral efficiency (ASE)
and quantify closed-form ASE gain of SCMA over OFDMA. Notably, it is shown that
system ASE can be significantly improved using SCMA and the ASE gain scales
linearly with the SCMA codeword dimension. Besides, we endow D2D users with an
activated probability to balance cross-tier interference in the underlaid mode
and derive the optimal activated probability. Meanwhile, we study resource
allocation in the overlaid mode and obtain the optimal codebook allocation
rule. It is interestingly found that the optimal SCMA codebook allocation rule
is independent of cellular network parameters when cellular users are densely
deployed. The results are helpful in the implementation of SCMA in the hybrid
system.Comment: submitted to IEEE Trans. Commu
Access Points in the Air: Modeling and Optimization of Fixed-Wing UAV Network
Fixed-wing unmanned aerial vehicles (UAVs) are of great potential to serve as
aerial access points (APs) owing to better aerodynamic performance and longer
flight endurance. However, the inherent hovering feature of fixed-wing UAVs may
result in discontinuity of connections and frequent handover of ground users
(GUs). In this work, we model and evaluate the performance of a fixed-wing UAV
network, where UAV APs provide coverage to GUs with millimeter wave backhaul.
Firstly, it reveals that network spatial throughput (ST) is independent of the
hover radius under real-time closest-UAV association, while linearly decreases
with the hover radius if GUs are associated with the UAVs, whose hover center
is the closest. Secondly, network ST is shown to be greatly degraded with the
over-deployment of UAV APs due to the growing air-to-ground interference under
excessive overlap of UAV cells. Finally, aiming to alleviate the interference,
a projection area equivalence (PAE) rule is designed to tune the UAV beamwidth.
Especially, network ST can be sustainably increased with growing UAV density
and independent of UAV flight altitude if UAV beamwidth inversely grows with
the square of UAV density under PAE
Cognitive Learning of Statistical Primary Patterns via Bayesian Network
In cognitive radio (CR) technology, the trend of sensing is no longer to only
detect the presence of active primary users. A large number of applications
demand for more comprehensive knowledge on primary user behaviors in spatial,
temporal, and frequency domains. To satisfy such requirements, we study the
statistical relationship among primary users by introducing a Bayesian network
(BN) based framework. How to learn such a BN structure is a long standing
issue, not fully understood even in the statistical learning community.
Besides, another key problem in this learning scenario is that the CR has to
identify how many variables are in the BN, which is usually considered as prior
knowledge in statistical learning applications. To solve such two issues
simultaneously, this paper proposes a BN structure learning scheme consisting
of an efficient structure learning algorithm and a blind variable
identification scheme. The proposed approach incurs significantly lower
computational complexity compared with previous ones, and is capable of
determining the structure without assuming much prior knowledge about
variables. With this result, cognitive users could efficiently understand the
statistical pattern of primary networks, such that more efficient cognitive
protocols could be designed across different network layers.Comment: This paper has been refreshed with a new versio
On throughput capacity for a class of buffer-limited MANETs
Available throughput performance studies for mobile ad hoc networks (MANETs)
suffer from two major limitations: they mainly focus on the scaling law study
of throughput, while the exact throughput of such networks remains largely
unknown; they usually consider the infinite buffer scenarios, which are not
applicable to the practical networks with limited buffer. As a step to address
these limitations, this paper develops a general framework for the exact
throughput capacity study of a class of buffer-limited MANETs with the two-hop
relay. We first provide analysis to reveal how the throughput capacity of such
a MANET is determined by its relay-buffer blocking probability (RBP). Based on
the Embedded Markov Chain Theory and Queuing Theory, a novel theoretical
framework is then developed to enable the RBP and closed-form expression for
exact throughput capacity to be derived. We further conduct case studies under
two typical transmission scheduling schemes to illustrate the applicability of
our framework and to explore the corresponding capacity optimization as well as
capacity scaling law. Finally, extensive simulation and numerical results are
provided to validate the efficiency of our framework and to show the impacts
brought by the buffer constraint
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